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MARINE DIESEL ENGINES. NOTES FOR BE(Marine Engineering) CADETS. MARINE DIESEL ENGINE PRINCIPLE AND PRACTICE. Notes prepared By: Prof. K. Venkataraman. CEng, FIMarE, MIE.
1. Engine Classification: ENGINE: Any machine, which produces power, is called an engine. HEAT ENGINE: Any engine, which produces power or work from a supply of heat, is called Heat Engine. The heat can be supplied by burning, i.e. by combustion of fuel. EXTERNAL COMBUSTION ENGINE: If the combustion of fuel takes place outside the engine, it is called an external combustion engine, e.g. steam engine, steam turbine, etc. INTERNAL COMBUSTION ENGINE: If the combustion of fuel takes place within the engine itself, it is called an Internal Combustion Engine. Fuel economy, simplicity, and low operational costs make it more p opular than external combustion engines. CLASSIFICATION OF INTERNAL COMBUSTION ENGINES: Internal combustion engines can be classified according to different criteria as follows: 1. According to ignition System.
a) Compression Ignition Engine (C. I. Engine) Engine) In this type of engine, the heat hea t of the compressed air itself ignites the fuel. No o ther means of ignition are required, e.g. Diesel Engine. In a Compression Ignition Engine, e.g. Diesel Engine, a piston reciprocates in a cylinder. At downward stroke of piston, air enters the cylinder. At upward stroke o f piston air is compressed. Due to compression pressure and temperature of air becomes quite high (over 35 bar and 500*C respectively). Finely atomised fuel oils sprayed into such compressed air ignite spontaneously and produce power. b) Spark Ignition Engine (S. I. Engine) In this type of engine (Otto engine), the fuel is ignited by the spark produced by a high-tension electrical circuit. In spark ignition Engine, liquid gasoline is sprayed or drawn through a nozzle or jet into the air stream going to the working cylinder. A combination of mild heating and reduction of pressure partially vapourises the gasoline. Proportionate mixing of air and gasoline vapour is done in carburetor. Mixture enters the cylinder where at a suitable time, an electric spark ignites the mixture, which burns then quickly and produces power. Spark Ignition Engine Versus Compression Ignition Engines Similarities. 1. Both Both are Intern Internal al Combu Combust stion ion Engi Engines nes.. 2. Both Both run run on on liq liqui uid d fue fuels ls..
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Dissimilarities. S. I. Engine. C. I. Engine. 1. Ignition system required 1. Not required. 2. Draws air and fuel into the system. 2. Draws only air into the cylinder. 3. Compresses air and fuel together. 3. Compresses air only. 4. Fuel is mixed with air at before compression 4. Fuel is mixed with air at the end of compression. starts. 5. As too much compression of air and fuel 5. Only air can be compresses without pre-ignition p re-ignition mixture causes pre ignition and detonation and detonation, so compression ratio can be high permissible compression ratio is not high(about 7). (about 16). 6. Efficiency, being proportional to compression, 6. Higher efficiencies can be obtained due to is limited due to less compression ratios. possible higher compression ratio. 7. Uses highly volatile liquid fuels so that it can 7. Uses less volatile liquid fuels mix with air at low temperature. 8. Fuel used is costly. 8. Cheaper fuel can be used 9. More fuel is used for same power. 9. Less fuel consumption.
10. Lighter in weight. 11. Initial cost less. 12. Smooth operation.
10. Heavier and stronger engines due to higher pressures involved. 11. Initial cost high. 12. Certain roughness in operation encountered, especially in high-speed engines at light loads.
2. According to Operating Cycles. (a) OTTO CYCLE (Constant Volume Combustion Cycle). It is the ideal air standard cycle for Petrol engine, the gas engine and the high-speed oil engine. The engines based on this cycle have high thermal efficiency but noisiness results particularly at higher power due to higher pressures in the cylinders. (b) DIESEL CYCLE (Constant Pressure Combustion Cycle). It is the ideal Air standard cycle for Diesel Engine, espec ially suitable for low speed Diesel Engine but not for high speed Diesel Engine. (The thermal efficiency is lower than Otto cycle engines but engines run smoothly due to lower pressures in the cylinder. (c) DUAL COMBUSTION CYCLE (Constant Pressure and Constant Volume Combustion Cycle). Modern Diesel Engines do not operate op erate purely on constant pressure combustion cycle but some part of combustion process takes place at constant volume while the rest is completed at constant pressure. In general, this cycle resembles Constant volume combustion Cycle more than constant pressure combustion cycle. It is suitable for modern Medium and High Speed Diesel Engines. The thermal efficiency is more than Diesel Cycle but less than Otto cycle. Also noise level is in between the two. This is a more practical engine. 3. According to Strokes/Cycle. In an engine, the following events form a cycle: a) Fillin Filling g the engine engine cyli cylinder nder with with fres fresh h air. air.
3 b) Compressin Compressing g the air so much that inject injected ed fuel ignited ignited readily readily by coming coming in contact with with hot air and burns efficiently. c) Comb Combus usti tion on of fuel fuel.. d) Expa Expans nsio ion n of hot hot gas gases es.. e) Emptying Emptying the products products of combus combustion tion from from the the cylinder cylinder.. Depending on how many strokes of piston are required in completing this cycle, the engines can be divided into two classes: 1. Four Stroke Engine An engine, which needs 4 strokes of the piston (2 in and 2 out) to complete one cycle, is called Fourstroke engine. 2. Two Stroke Engine An engine that needs only o nly 2 strokes of the piston (1 in and 1 out) to complete one cycle is called Twostroke engine. 4. According to Piston Action: (a) Single Single Acting Acting Engin Enginee
One end of the cylinder and one face of the piston are used to develop power. The working face is at the end, which is away from crankshaft. Generally, single acting vertical engines develop power on the down stroke. (b) Double Acting Engine
4 Both ends of the cylinder and both faces of the piston are used to develop power on the upward as well as on the downward stroke. c) Opposed Piston Engines.
Two pistons travel in opposite directions. The combustion space is in the middle of the cylinder between the pistons. There are two crankshafts. The upper pistons drive one, the lower pistons the other. Each piston is single acting. 5. According to Piston Connection. (a) Trunk Piston Type.
The piston is connected directly to the upper end of the connecting rod. A horizontal pin (Gudgeon Pin) within piston is encircled by the upper end of the connecting rod. This construction is quite common, especially in small and medium size engines. (b) Cross Head Type. The piston fastens to a vertical piston rod whose lower end is attached to a ‘cross head’, which slides up and down in guides. The crosshead carries a crosshead pin, which is encircled by the upper end of the
5 connecting rod. This more complicated construction is common in double acting engines and large slow speed single acting engines.
Crosshead type Engine arrangement. Comparison between Trunks Piston Versus Cross Head Engine .
Most medium and small size engines use trunk pistons. Resulting side thrust causes the piston to press against the cylinder wall, first on one side, then on the other. At the top of stroke, when the gas pressure is greatest, side thrust is negligible (due to small connecting rod angle). So most of wear takes place at the middle of stroke: making piston skirt increases thrust-bearing area, and hence reduces wear. In medium and small size engines, due to lower gas pressure, units’ side pressure is so small that neither piston nor liner wears much. In crosshead engines, crosshead takes the side thrust, which will be high in large engines. So, crosshead engines have the following advantages: 1. 2. 3. 4.
Easier lubrication. Reduced liner wear. Uniformly distributed clearance around piston. Simpler piston construction because the ‘Gudgeon pin’ and its bearing are eliminated.
However these advantages of cross head engines are offset by: 1. Greater complication. 2. Added weight. 3. Added height. 4. Careful adjustments.
6 6. According to Cylinder Arrangement a) Cylinder-in-Line Arrangement This is the simplest and most common arrangement, with all cylinders arranged vertically in line. This construction is used for engines having up to 12 cylinders. The arrangement is shown in figure below.
(b) V - Arrangement: If an engine has more than eight cylinders, it becomes difficult to make a sufficiently rigid frame and crankshaft with an inline arrangement. Also engine becomes quite long and takes up considerable space. So V-arrangement is used for engines with more cylinders, (generally 8, 12, 16) giving about half-length of engine, more rigid and stiff crankshaft, less manufacturing and installing cost. Angle between two ‘Banks’ is kept from 30* to 120* (most commonly 40*, 75*), as shown in the figure.
7 (c) Flat Arrangement. It is a V-engine with angle between the banks increased to 180*. Generally, it is used in trucks, buses, rail cars, etc. where there is little headroom. Arrangement is shown in the following figure.
(d) Radial Arrangement. In a radial engine, all the cylinders are set in a circle and all point towards the centre of the circle. The connecting rods of all the pistons work on a single crankpin, which rotates around the centre of the circle. Such a radial engine occupies little floor space. By attaching the connecting rods to a master disk surrounding the crankpin, up to 12 cylinders have been made to work on a single crankpin. The arrangement is shown in the figure below.
7. According to Method Of Fuel Injection. (a) Air Injection Engine The fuel is injected into the cylinder by a blast of high compressed air. It was commonly used on early diesel engines. Being too heavy and complicated, this system is now obsolete. (b) Airless (or Solid or Mechanical) Injection Engine. Fuel is injected into the cylinder, through the fuel valve, by high-pressure fuel pump. At present, it is being used for all types and sizes of diesel engines.
8 8. According to method of Charging.
(a) Natural aspirated Engine. The vacuum is created when the piston moves away from the combustion space draws in the fresh charge. (b) Supercharged Engine. The charge is admitted into the cylinder at a higher than atmospheric pressure. This high pressure is produced by a pump or blower or exhaust gas turbocharger. 9. According to Fuel Used. (a) Heavy Oil Engine. It can burn fuels of high viscosities, e.g. 1500 sec. Redwood No. 1 or 350 sec. Redwood No. 1.
(b) Diesel Oil Engine. This uses diesel oil. (c) Gasoline Engine. This uses gasoline as fuel. It can also use ‘kerosene’. As the 'perfect mixture' of fuel and air is led to cylinder for compression, compression ratio is limited to 7 to avoid self- ignition, power loss, knocking, etc. (d) Gas Burning Engine. It uses gaseous fuels at higher compression. Three wa ys have been adopted to burn gas at higher compression. The engines are named accordingly as follows: i) Gas Diesel Engines They compress air alone. At the end of compression, they inject the gas at high pressure into the cylinder just at the moment it is to fire. With gas, a small amount of ‘pilot oil’ is also admitted to assist the ignition and to cause smooth and prompt ignition. ii) Dual Fuel Engine Admit the gas and air at the same time and compress the gas/air mixture at diesel compression ratio. At the end of compression, they inject fuel oil, which the high temperature of the gas-air mixture ignites to fire the mixture. Using ‘lean mixture’ unlike to ‘perfect mixture’ of gasoline engine prevents selfignition. iii) High Compression, Spark Ignited Gas Engines. Like dual fuel engines, they compress a mixture of gas and air to high pressure, preventing self-ignition by using a ‘lean mixture’ but they use spark, instead of oil, for ignition. 10. According to Speed.
1. Slow Speed Engines: 100 to 150 r.p.m. 2. Medium Speed Engines: 300 to 1000 r.p.m. 3. High Speed Engines: More than 1000 r.p.m.
9 The following table compares the various aspects of Slow Speed, Medium Speed and High Speed Engines: Slow Speed. Medium Speed . High Speed. 1. No Gearing. Gearing Necessary. Four Gearing Necessary. Four 2. Four or Two Stroke Stroke better. Stroke only. acceptable. Slow Speed. Medium Speed. High Speed. 3. Poor quality fuel Better Fuel required. Distillate Fuel only. acceptable. Diesel oil/Gas oil. 4. Crankcase can be Trunk Piston type. Trunk Piston Type. separated from combustion zone. 5. Less noise and vibration. More noise. Most vibration and noise. 6. Less fatigue failure. More. Most. 7. Fewer stresses due to More. Most. heavier scantling. 8. Heavy and Large Size. Compact. Extremely Compact. 9. More head-room required. Less. Least. 10 Heavy Lifting Gear Light parts easy to handle. Lighter parts can be handled . required for heavy parts. manually. Engine r.p.m. is Engine r.p.m. limited by Engine r.p.m. limited by 11 limited by propeller piston speed. piston speed. . efficiency. Can have long strokes. Small strokes. Smallest stroke. Large bore cylinders. Small bore. Smallest. 12 Heavy & large pistons. Light and small piston. Lightest & smallest piston. . Round section ‘I’ section connecting rod. ‘I’ section connecting rod. 13 connecting rod. . Failures less and easier to More and difficult to Most and very difficult to 14 manage. manage. manage. . 15 . 16 . 11. According to Bore/Stroke Ratio:
a) Square Engine: If bore/stroke is about one, crankshaft web dimensions become less compared to journal and crankpin. b) Over Square Engines (Short Stroke) If bore/stroke > 1, web dimensions (less height, more thickness) are such that webs will be weak. So generally over square engines are not used. (c) Long Stroke Engines.
10 Generally, engines have stroke/bore >1. This gives crankshafts of good strength. Most common ratio is stroke/bore = 2. 0: 2.2. (d) Super-long Stroke Engines. To have better propeller efficiency and better combustion even with lower grade fuels, lower r.p.m. engines with longer strokes are gaining popularity. These engines have stroke/bore ratio = 3. 12. According to Use. Engine can be named as Marine, Auto, Tractor, Locomotive, Aero-engines, and Rocket Engines according to their use. On ships they can be called Main Engines if used for propulsion or Auxiliary Engines, if used for generation of electricity. The Diesel Engines used in Marine power plants are termed as ‘Marine Diesel Engines’.
The Diesel Engines find the following application on board merchant ships. 1. Main Propulsion. 2. Electric Power generation. 3. Emergency Pumps (e.g. fire pump). 4. Life Boat. 5. Emergency Generator. 6. Emergency Air Compressor REASONS FOR WIDE USE OF DIESEL ENGINES IN MARINE POWER PLANTS .
1. Small fuel consumption: Diesel Engine is one of the most efficient heat engines. Hence it gives more power with less fuel. It is an engine of high economy. 2. Cheap fuel: Diesel engine uses fuel costing very less as compared to other engines. 3. Economy at light loads: Diesel Engine is not only efficient when it is fully loaded, but also when it is partly loaded. 4. Greater Safety: Diesel fuel is non-explosive and less flammable at normal temperatures an d pressures. It requires special effort to make it start to burn. This feature makes it very attractive in the marine trade, because it would be much safer carrying diesel oil on board ships. Diesel exhaust gases are less poisonous than other engines, because they contain less carbon monoxide. 5. Ignition System is not required: Diesel engines do not require battery or magneto running them. 6. More power can be produced due to more compression allowed. 7. Diesel Engine is more robust and stronger. 8. Economy in small sizes: As great contrast to steam power plant, a small diesel engine has nearly as good an economy as a large one. This makes it possible to enlarge a diesel engine plant with additional units as the load grows. At all stages of growth, the efficiency is high. 9. Sustained economy in service: Again in contrast to a steam power plant, diesel efficiency falls off very little during thousands of hours of use between overhauls. 10. Lightness and compactness: Diesel engine plants have less weight and space per unit power. It is therefore well suited to portable and mobile installations. 11. Independence of water supply:
11 A diesel engine requires very less water in contrast to steam plants. 12. Quick Starting. A cold diesel engine can be started instantly and made to carry its full load in few minutes. It is therefore ideal for supplying emergency power. 13. Easily in Maneuvering: A diesel engine can be made to run at full power in either direction. 14. Economy in Labour. No fire room force is needed. 15. Freedom from nuisance: There are no ashes to be disposed of, no noisy and dusty coal handling and pulverising equipment to maintain, no smoke, and noise can be easily eliminated. Due to above mentioned reasons, Diesel engines are quite popular on board ships. These reasons can very well be regarded as the advantages of Diesel Engines over other prime movers such as gasoline engines, gas turbines, steam engines, steam turbines and hydraulic turbines. However, Diesel engines also have certain disadvantages, which can be listed as following : 1. Cost: Diesel engines, because of the higher pressures at which, they work, require sturdier construction, better materials and closer fits than gasoline engines. Therefore, they co st more to build. 2. Weight: Because of sturdier construction, weight per power is more than gasoline engines. 3. Attendance: A diesel engine requires more attention than an electric motor running on purchased current. It also requires more attention per unit of power produced than a large steam turbine. 4. Fuel Cost: Oil used in Diesel engines is costlier than coal. Hence, steam power plants using coal as fuel are cheaper in operation. RECENT TRENDS: The Diesel Engines is at present acknowledged to be the best prime mover in a wider range of marine applications than any other engine. Due to higher efficiency, lower specific fuel consumption and capability to use cheaper fuel, Diesel engine is preferred to spark ignition Engine, gas turbine and steam turbine for moderate power applications. However, small pleasure boats are still powered popularly by spark Ignition engines and very large ships e.g. Tankers and VLCCS are still powered by steam turbines. Gas turbines are popular on naval vessels. However, Diesel engines are making in rails in heavily these fields too. Even, U.S. merchant ships dominated heavily by steam propulsion are more and more embracing diesel engines.
In Diesel engines also, there is tough competition between medium and slow speed engines. However, the recent trend is towards having very slow speed super-long stroke engines e.g. SULZER/ RTA: M.A.N-B & W/ LMC, due to significant improvement in propulsion efficiency and specific fuel consumption at low speeds as well as their ability to burn very poor grade fuels which are available now-a-days. The worse quality of fuels available and increase in the cost of oil has led to renewed interest in coalfired ships. Keeping in view the limited world reserves of o il, coal fired ships seem to provide a good alternative in 2000’s but at present the position of Diesel engines remains unchallenged. **************************** Kv************************************
12 End of Engine Classification/ BIT/AMET/BE/Motor/KV/May 2003.
Engine Cycles & Timing Diagrams. Introduction . We will discuss the full series of the separate steps or events, which follow each other while a diesel engine is in operation. We will also discuss the timing diagrams of Two Stroke Cycle & Four Stroke Cycle Engines.
1. Various Steps or Events of a Cycle. First Step. Air is introduced into the cylinder because no n o fuel will burn without air. Burning or combustion is a process of uniting fuel or combustible with the oxygen in air. The process is chemical reaction which means that fuel & oxygen, in uniting, change into new substances. Second Step. The air must be squeezed or compressed to a high pressure. Two reasons for compressing the air are to get high temperature and high pressure there by higher power. In a diesel engine the air is compressed so much that it becomes hot and a nd in fact, it will be hot enough en ough to ignite oil that is sprayed into it. Third Step. The fuel is injected into the cylinder in the form of fine spray after the air has been compressed and thus heated to a high temperature. It must be in the form of fine spray so that a cloud of oil droplets will spread through all the air in the cylinder. Fourth Step. Combustion takes place after the oil is sprayed in the cylinder. This will generate a large amount of heat. The gaseous mixture gets hotter and grows larger or expands due higher pressure. It pushes on the piston, which in turn transmits the force through the connecting rod to the crank of the crankshaft. This will make the crankshaft revolve. Fifth & Last Step. When the piston has finished its preceding power stroke and the gases in the cylinder have lost their pressure, the spent gases must be exhausted. A cycle is a full series of separate steps or events, which follow each other. For a Four Stroke Cycle Engine, a complete cycle requires four stroke of the piston. For a Two Stroke Cycle Engine, a complete cycle requires two stroke of the piston. “Four Stroke Cycle” & “Two Stroke Cycle” engines are abbreviations, which do not really make any statement other than what is stated above. 4-Stroke Cycle Engine. In a 4-stroke engine, the engine eng ine crankshaft makes two revolutions for each working cycle.
13 The four corresponding piston strokes are as follows: 1. Suction Stroke, 2.Compression Stroke, 3. Power Stroke & 4. Exhaust Stroke. The engine has air inlet and exhaust valves. By the opening and closing of these valves in proper sequence, the piston can be b e made to perform its main function of o f transmitting power to the crank. In addition to that the piston also performs p erforms subsidiary functions of drawing air into the cylinder, compressing the air and subsequent expulsion of exhaust gases.
Four Stroke Cycle Engine:
A, B & C: SUCTION STROKE. (A): Piston at top of stroke, inlet valve open, air intake begins: (B): Piston descending, air being taken in. (C): Piston at the bottom of the stroke, all valves closed, air intake full and completed.
14 D: COMPRESSION STROKE.
(D): Piston rising. All valves closed. Air being compressed.
E & F POWER STROKE. (E): Piston at the top of the stroke. Inlet and exhaust valves closed. Injector spraying oil into hot air. (F): Piston descending. All valves closed. Hot high pressure gas forcing the piston down.
G & H EXHAUST STROKE.
(G): Piston at the bottom of the stroke. Exhaust valve opens. (H): Piston rising. Exhaust valve opens. Exhaust gas being driven out of the cylinder.
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TIMING DIAGRAM AND POWER CARD OF FOUR STROKE CYCLE :
16 The timing diagram shows the closing and opening of the valves. The working cycle is illustrated as a ‘P - V’ diagram (pressure-volume). The line ‘l – l’ represents atmospheric line. The piston is considered to have just moved over the ‘top dead centre’ and is on its way down. The air inlet is already open and because of the partial vacuum created when the piston moves towards its bottom position, fresh air is sucked into the cylinder. This process is represented in the 'p-v' diagram by the line ‘1-2’ which is termed suction line. This movement of the piston is called 'Suction Stroke". After the piston has moved over bottom dead centre, the suction valve closes and the volume of air in the cylinder is compressed during the course of the up stroke of the piston. This is represented by the line ‘2-3’ in the above diagram and termed as compression line. This movement o f piston is compression stroke. The ignition takes place at point 3 and combustion continues for the duration of fuel injection, ending at point 4. After this combustion products expand to point 5 when the exhaust valve opens. Power is produced between point ‘4 – 5’. The pressure drops in the cylinder to the exhaust line from 5 to 6. The exhaust valve remains open till after piston passes over the top dead center. The combustible gases are expelled. The line 6 to 1 represents this. The pressure is slightly above atmosphere, because of the resistance in the exhaust pipe. This stroke is 'exhaust stroke'. A 4-stroke engine requires two complete revolutions of the crankshaft to finish working cycle. This means inlet, exhaust & fuel valve must only function once for every two revolutions of the crankshaft. In order to activate those valves in the correct sequence, it is necessary to operate them from a shaft, which rotates at half the speed of the crankshaft. This is called camshaft. Two Stroke Cycle Engine :
In this engine, two of the strokes necessary to complete working cycle in a 4-stroke engine are eliminated. The remaining strokes are as follows: --- Compression Stroke. --- Power Stroke The working cycle is illustrated by 'p-v" diagram in the next page. The compression takes place by 1 to 2. The combustion process and expansion take place as described for a 4-stroke engine. At point 4, the exhaust valve at top of the cylinder opens (uniflow scavenging). At point 5, the piston exposes the ports in the cylinder wall. The result being that fresh air, known as scavenge air, flows into the cylinder & flushes out exhaust gases. Piston covers the port in cylinder wall at 6 an d the exhaust valve closes at point 1. The compression beings a new working cycle. The pressure of scavenge air is little higher than the atmospheric air. 2-stroke engines carry out useful work for each revolution of the crankshaft. This means fuel and exhaust must function each revolution. The camshaft must rotate at same speed of the engine crankshaft. REASONS FOR TIMING. EXHAUST AND INLET VALVE TIMING. These deal with the expulsion of the burnt gases and recharging the cylinders with fresh air. The overall efficiency of an engine depends largely upon getting the exhaust gases out or scavenging them.
17 The exhaust value is opened after the piston has traveled about 80% of the working stroke. By that time it has done its useful work, the energy has been spent. The opening of the valve will allow a large part of the exhaust gases to be blown out of the cylinder. The cylinder pressure equalises with the pressure in the exhaust line during this period. This is referred to as the ‘blown down’ period. When the piston moves upwards the piston movement expel exhaust gases. Two Stroke Timing Diagram (Uniflow Type).
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Two Stroke Engine – ‘Pressure/Volume’ Diagram (Uniflow Type).
In a 4-stroke engine, towards the end of exhaust stroke and beginning of section stroke, both exhaust and inlet values are open. This is called ‘overlap period’ . This would further help in achieving an efficient scavenging. Exhaust valve closes after the piston has moved over the top dead centre. The inlet valve remains and the down ward movement of the piston lowers the pressure in the cylinder and thereby atmospheric air is drawn in. The air in the inlet passages to the inlet valve will gain a high velocity and in turn kinetic energy. Use is made of this effect to keep the air inlet value open until the piston is past bottom dead centre. The air then continues to flow into the cylinder until its k inetic energy is lost and airflow ceases. The inlet value is closed now . In two stroke engines, the events described above as taking place will have to be carried out in about 120* of the crank movement. It will require the assistance of low-pressure air. The speed of op ening of valve or part has to be rapid so that the pressure of gas falls quickly. It would be easier now for scavenge air to rush in and get the gases out. We have to briefly discuss the combustion process to understand fuel timing. Combustion takes place in three distinct stages: 1. Ignition Delay Period, during which some fuel has been admitted but has not yet been ignited. This is the stage during which fuel is atomised, vapourised, mixed with air and raised in temperature. 2. Rapid or Uncontrolled Combustion, following ignition. The pressure rise is rapid during this stage. 3. Controlled Combustion. The first stage or the delay period exerts great influence on both engine design and performance. The pressure reached during ‘rapid or uncontrolled combustion’ will depend upon delay period. The longer the delay, more rapid and higher is the pressure rise. It is because more fuel will be present in the cylinder before rate of burning comes under control (in the 3rd stage). This will cause rough running and ‘diesel knock’. But at the same time, there must be certain amount of delay period for proper mixing. One of the main factors that affect delay period is ‘fuel timing’. If it is too early, the delay period is more because the pressure and temperature are low in the cylinder. If the injection is too late, the fuel will burn during the expansion stroke. The pressure rise in the cylinder will drop considerably, reducing
19 the efficiency. The exhaust temperature will be high and may cause overheating of the engine in severe cases. So, an optimum angle has to be introduced to get best effect. It depends on delay period. The injection is earlier on higher speed engines. Four-stroke or Two-stroke cycle will make no difference on the point at which injection begins. Four Stroke Timing Diagrams :
Four Stroke Un-supercharged.
Four Stroke Supercharged.
Two Stroke Timing Diagram:
Two Stroke Uniflow Engine.
Two Stroke Loop Scavenging Engine.
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Uniflow Scavenge Engine. Loop Scavenge Engine . (Exhaust Valve). ******************************** Kv*********************************** End of Engine Cycles & Timing Diagrams/ BIT/AMET/BE/Motor/Kv/May 2003.
Engine Indicator and Indicator Diagrams: An engine indicator is used to record pressure/volume or indicator diagrams taken off engines, the areas of these indicator diagrams represent the work done per cycle of one unit. There are two types of engine indicators: 1. Mechanical type: This records indicator diagrams on paper. a) Can record pressure within the engine cylinder at any part of the engine cycle. b) Not considered reliable of engine speed more than 150 rpm. c) Small lightweight models can be used for engines with speeds up to 350 rpm. d) Mean indicated pressure (m.e.p) from an indicator power diagram. 2. Pressure indicator type - this measures maximum combustion pressure only. a) Also known as maximum pressure indicator. b) Compression pressure is recorded with fuel cut off. c) No engine speed limitation. d) Often used on medium speed engines. e) Does not record indicator diagrams on paper. Mean Effective Pressure and Indicated Power:
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Power Indicator Diagram. Referring to the Indicated diagram (Power card), the area of the diagram divided by its length represents the mean pressure effectively pushing the piston forward and transmitting useful energy to the crank in one cycle. This, expressed in N/m2, is termed the indicated mean effective pressure (pm).
Power is the rate of doing work (basic unit is the Watt) or: 1 Watt = 1 J/s = Nm/s Let: pm = mean effective pressure (N/m2). A = area of piston (m2). L = length of stroke (m). N = Number of power stroke per second. Then: Average force (N) on piston = pm x A newtons. Work done (J) in one power stroke = Pm x A x L newton-metres = joules. Work per second (J/s = W) = pm x A x L x n watts of power, Therefore: Indicated power = pmALn.
This is the power indicated in one cylinder. The total power of a multi-cylinder engine is that multiplied by the number of cylinders, if the mean effective pressure is the same for all cylinders. Construction and Working Principle of Indicator:
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Engine Indicator. An engine indicator consists of a small bore cylinder containing a short stroke piston which is subjected to the same varying pressure that takes place inside the engine cylinder during one cycle of operations. This is done by connecting the indicator cylinder to the top of the engine cylinder in the case of singleacting engines, or through change over cocks and pipes leading to the top and bottom ends of the engine cylinder in the case of double-acting engines. The gas pressure pushes the indicator piston up against the resistance of a spring, a choice of specially scaled springs of different stiffness being available to suit the operating pressures within the cylinder and a reasonable height of diagram. A spindle connects the indicator piston to a system of small levers designed to produce a vertical straight-line motion at the pencil on the end of the pencil lever, parallel (but magnified abo ut six times) to the motion of the indicator piston. The “pencil” is often a brass point, or stylus, this is brought to press lightly on specially prepared indicator paper which is scrapped around a cylindrical drum and clipped to it. The drum, which has a built-in recoil spring, is actuated in a semi-rotary manner by a cord wrapped around a groove in the bottom of it; a hook at its lower end to a reduction lever system from the engine crosshead attaches the cord, passing over a guide pulley. Instead of the lever system from the crosshead, many engines are fitted with a special cam and tappet gear to reproduce the stroke of the engine piston to a small scale. The drum therefore turns part of a revolution when the engine piston moves down, and turns back again when the engine piston moves up, thus the pencil or stylus on the end of the indicator lever draws a diagram which is a record of the pressure in the engine cylinder during one complete cycle.
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Line Diagram of Engine Indicator.
Above figures show an engine indicator which is suitable for taking indicator diagrams of steam reciprocating engines and internal combustion engines up to rotational speeds of about 300 rev/min. In this type, the pressure scale spring is anchored at its bottom end to the framework, and the top of the piston spindle bears upwards on the top coil of the spring, the upward motion of the indicator piston thus stretches the spring. Types of Indicator Diagrams : Four types of indicator diagrams or cards can be obtained from a slow-running diesel engine: 1. POWER CARD:
This is taken with the indicator drum in phase with piston movement. The area within this diagram represents the work done during the cycle to scale. This may be used to calculate the power produced after obtaining the indicated mean effective pressure of the unit.
24 2. COMPRESSION DIAGRAM :
This is taken in a similar manner to the power card but with the fuel shut off from the cylinder. The height of this diagram shows maximum compression pressure. If compression and expansion line coincide, it shows that the indicator is correctly synchronized with the engine. 3. DRAW CARD or OUT-OF-PHASE DIAGRAM:
Taken in a similar manner to the power card with fuel pump engaged but with the indicator drum 90* out of phase with piston stroke. This illustrates more clearly the pressure changes during fuel combustion. 4. LIGHT SPRING DIAGRAM :
Taken similar to power card and in phase with the engine stroke, but this diagram is taken with light compression spring fitted to the indicator. This shows clearly pressure changes during exhaust and scavenge in enlarged scale. This can be used to find any defects during those operations.
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TWO-STROKE CYCLE.
26
Typical Power Card with Out Of Phase Card taken on the same Diagram.
Trace of a power card taken over a full cycle with the card ‘opened’ out so that the compression curve appears to the left of the vertical (tdc) line and the combustion and expansion occurring to the right of the same line. This is common way for electronic monitors to record events in the cylinder, again relevant pressures and angles may be well recorded on the print out.
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• • •
Card taken by Electronic Device. Typical print taken from an electronic measuring device. Pressure and their relevant angles are automatically printed on to the card. Very useful for checking engine performance.
-------------: Early Injection. T.D.C -------------: Normal Injection. -------------: Late Injection. -------------: Late Injection with After Burning . ***************************** Kv************************************
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3. Combustion. This is an exothermic reaction (one in which heat is liberated by the action) between a fuel and oxygen. Liquid fuels consist of carbon, & hydrogen, in the form of hydrocarbons, with small quantities of sulphur & traces of other metallic Impurities such as vanadium. A typical fuel analysis, by mass would be: C = 5%, H2 = 12%, S = 3%, with a C.V. of 44000 KJ/Kg. (19000 BTU/lb.) The oxygen is obtained from the air, which can be considered to contain 77% nitrogen & 23% oxygen by mass. The nitrogen plays no active part in the combustion process but it is necessary as it acts as a moderator. With pure oxygen, the combustion would be violent & difficult to control & it would produce very high temperatures, creating cooling, metallurgical & lubrication problems. The reactions, which occur, are: 2H2 + O2 ----------- 2H 2O – liberating 142 MJ/kg. H2. C + O2 -------------- CO 2 – liberating 33 MJ/kg. C. S + O2 --------------- SO 2 – liberating 9.25 MJ/kg. S. 2C + O2 --------------2CO – liberating 10 MJ/kg. C. Combustion will only occur within limits in the air/fuel mixture. If too much air is supplied all the fuel will be burnt but the excess of oxygen & nitrogen will carry away heat. If too little air is supplied incomplete combustion will occur, when all the hydrogen will be burnt but only part of the carbon, with the remainder only burning to carbon monoxide or not burning at all. In diesel engine practice it is usual to supply between 100 & 200% excess air by mass, though 15% is sufficient for a steady flow combustion process (boiler). This difference has two reasons: 1. As the combustion proceeds in the diesel engine, the fuel finds less & less air to combine with in a boiler air is constantly being fed in. 2. More air is needed in the diesel engine as it lowers the maximum temperature, allowing Cast iron to be used. Combustion Process. Fuel is injected into the clearance volume towards the end of the compression stroke, as a fine mist of very small droplets, which have a surface area many times that of the accumulated fuel charge. These droplets are rapidly heated by the hot compressed air, which has a temperature of between 550* to 650*C, causing vaporisation. The vapour mixes with air and when the mixture exceeds the spontaneous ignition temperature, (S.I.T.) combustion begins. The process can be divided into four phases : 1. Injection delay. 2. Ignition delay. 3. Constant volume combustion. 4. Direct burning. Injection delay : A time lag of about 0.005 seconds occurs between trapping the fuel charge in the pump barrel and starting injection into the engine cylinder. This is due to: a) Elasticity of high-pressure fuel lines & system. b) Slight compressibility of the fuel charge. c) Leakage past the pump plunger & injector needle. d) Opening delay of the pump discharge valve & injector needle.
29 In a slow speed engine the lag period accounts for up to 5* of crank movement. In a high speed engine it may account for 20* or more and because of point (a) it is necessary to use fuel lines of similar length for all cylinders, when the fuel pumps are grouped together. Ignition Delay. Ignition delay is another short period of time delay, which is sufficient to account for several degrees of crank angle. Several factors are involved: a) Spreading and penetrating of the fuel in to the clearance volume space. b) Heating of the fuel to cause vaporization & then exceeding the fuels’ spontaneous ignition temperature. c) Mixing of the fuel & air in the clearance volume space before detonation. Constant Volume Combustion. Ignition occurs at T.D.C. when the fuel charge, which has entered during the ignition delay period, burns rapidly causing a sharp rise in cylinder pressure with little movement of the piston occurring. Modern four stroke engines may attain 100 bar; at this point where as a two stroke engines are likely to operate with pressures of 75 to 98 bar. Direct Burning. The remainder of the fuel burns as it enters the cylinder and mixes with air. The excess air and combustion gases prevent high temperatures and rapid combustion so the pressure remains about constant. Injection and combustion should cease simultaneously at the end of this period. Factors Affecting Combustion. In order to attain good combustion it is essential that: a) Sufficient air is supplied. b) Compression is high enough to give a temperature above the spontaneous ignition temperature. c) Good mixing of the air and fuel is obtained. All of these give problems. The factors affecting combustion are: 1. Atomisation. 2. Penetration. 3. Turbulence.
1. Atomisation. The rate of heat absorption and burning depends upon the surface area of the fuel particles. As this must be rapid it follows that the surface area needs to be big & this is achieved by breaking up the fuel into small droplets. The amount of the fuel pressure, diameter of injector nozzle holes and the viscosity of the fuel, affect the process. 2. Penetration. To use all the air in the combustion space it is necessary to give the fuel particles sufficient energy to enable them to penetrate to the extremes of the space. This is controlled by the fuel pressure, the size of the particle & the length to diameter ratio of the nozzle hole (From 2:1 to 5:1). The latter also controls the angle of spray. 3. Turbulence. To aid mixing of fuel with air and atomisation, friction between the fuel & air is needed. Friction is a function of the relative velocity between the fuel particle and the air, and may be obtained by either of two methods. a) Fuel seeks air. b) Air seeks fuel.
30 a) The air is static or slow moving and the mixing energy is obtained from the fuel particles. Injection pressures of 200 to around 1000 bars are needed from multi-holed nozzle injectors. Advantages are, simplicity, economy and easier for cold starting the engine. The latter because little air movement means reduced heat loss to the cold liner and piston crown (also assists in the burning of heavy fuel). Disadvantages are in producing and sealing high fuel pressures. b) The air is made to swirl rapidly at the end of the compression stroke by using a pre-designed combustion chamber. Single holed nozzles and lower fuel pressures are used, 70-100 bars. Advantages are simplicity of injection, equipment and rapid combustion (useful in high speed engines). Disadvantages are complicated combustion chambers and high rate of heat loss to surroundings. Causes difficulties in cold starting, sometimes needing cylinder combustion space heating system. In practice, a combination is often used minimum fuel pressures being used with a small degree of swill produced by vaned inlet valves or tangentially cut scavenge ports. Quantity of swirl causes half the liner circumference to be traversed during combustion. Combustion Faults. Detonation. The combustion process is regarded as a controlled explosion with a flame front speed of ab out 25 m/s. However if combustion conditions are not correct d ouble ignition may occur and a ‘detonation’ may result. The latter occurs when the mixture is rapidly compressed by an initial ignition and the remaining mixture is overheated and burns almost instantaneously (Flame speed 2000 m/s). The detonation can set up very high pressures, temperatures and causes vibration of the cylinder and piston. It also reduces the efficiency of the engine as energy is absorbed producing the vibration. After burning. This occurs when combustion extends into the expansion period after the injector has closed. It is caused by poor ignition qualities or very poor atomization and produces high exhaust pressures and temperatures. Injection timing. Early injection produces high firing pressures; late injection produces low firing pressures and high exhaust pressures. In both cases the engine power is reduced. All these faults could be seen very clearly in indicator cards of each unit. Ideal Combustion. To obtain maximum thermal efficiency, the combustion process should be carried out as close to the Otto cycle as practically possible. This means, the rate of rise of pressure should be as rapid as possible, without exceeding the designed mechanical and thermal loading. To achieve maximum mean effective pressure the fuel remaining after the initial period of rapid rise, should be burned at a rate which will hold the cylinder pressure constant, at the maximum design value until the fuel is burned. Some of those factors affecting the ideal combustion can be considered as follows.
Injection timing. Using jerk injection system, it has been found that the shortest delay period occurs when it includes T.D.C. 1. Early injection results in increased delay since the pressure and temperature are still rising, so auto injection energy has not been reached. 2. Late injection causes increased delay since the piston is accelerating away from the cylinder head and temperature and pressure fall rapidly. In each case, the rate of pressure rise is increased due to the large quantity of the fuel in the combustion space before the chemical reaction is initiated. The reaction, which follows involves a massive amoun t of fuel and approximates to detonation.
31 This results in ‘Diesel knock’, the effects of which are d etermined objectionable. Many engines are timed later than that which gives maximum mean effective pressure to reduce the rate of pressure rise and the maximum pressure. This however involves some sacrifice in efficiency and power output. Engine R.P.M. Since the delay period is determined mainly by the fuel characteristics, it follows that delay tends to be independent of engine speed. The delay angle however will vary with engine speed and have considerable influence on the pressure / crank angle diagram. In each case – 10 deg. BTDC & 20deg. BTDC the delay angle is increased with increase in speed.
- - - - - - -: High Speed. -----------: Slow Speed. Other factors influenced by engine speed may include . 1. Fuel spray characteristics (since fuel pumps are engine driven and pressure and temperature in cylinder affect secondary atomisation). 2. Volumetric Efficiency (since the piston speed & valve opening characteristics influence the gas exchange process). 3. Combustion chamber wall temperature (since rate of heat input & rate of heat conduction determines the wall temp). Fuel / Air Ratio. As fuel is being injected there will be local fuel-air ratios varying from infinity near the injector to zero where fuel vapour has not yet reached. Provided the vapourisation is not complete before injection commences the amount of fuel injected would have no direct effect on the delay period. However, with reduced Fuel /Air ratio, combustion temperatures are lowered, which reduces the cylinder wall temperature. With some engines, this may have the effect of increasing the delay period.
Varying Fuel/Air Ratio Diagram.
32 From the above diagram it may be seen that: 1. The delay period is not effected. 2. There is little reduction in rate of pressure rise. 3. Provided that only a small proportion of the fuel is injected during the delay, it will have limited effect, on the maximum pressure. Combustion can take place with extremely low Fuel/Air ratios, probably due to burning taking place close to the injector where the local F/A is high enough for stable reactions to occur. Turbulence. The turbulence effect is probably associated more with the mixing process rather than with propagation of chemical reactions. Turbulence takes place possibly in two ways: 1. Primary turbulence: Due to the way in which the air enters the cylinder. In large diesel engines this is produced by the angularity of the inlet ports, near the end of compression, when the air density is high, the effective swirl will be greatly reduced. 2. Secondary turbulence: Squish, is produced, by the shape of the piston crown and cylinder head. The air is made to move readily inward and across the path of the automised fuel. This may help to secure short second and third combustion stages. Turbulence after complete combustion, say due to detonation, can break down the cool insulating layer of gas near the cylinder head wall, which will: 1. Reaches cylinder wall temperature locally (Hot spot). 2. Increases heat loss to cooling water. 3. Breaks down the oil film on the cylinder walls. Promotes micro seizure and service wear. Compression ratio. The compression ratio determines the air pressure and the temperature at the moment of fuel injection and will have a considerable influence on the degree of secondary automisation, the delay period, and the rate of rise of maximum pressure. Increasing the compression ratio alone, in the range used for diesel engines, has only a marginal effect on the power developed and cycle efficiency. High compression ratio, do however increase cylinder friction loss, ring leakage, and starting torque requirements. With highly pressure charged engines, the cylinder air charge is increased which allows more fuel to be burnt, but if working close to the Otto cycle the maximum pressure can be high. To limit the maximum pressure and therefore maximum stress the engine is designed to operate with lowest compression ratio consistent with satisfactory running and starting. Turbocharging . This has the tendency of raising both the pressure and temperature at the point of fuel injection. This is beneficial in reducing the delay period and the rate of pressure rise. The degree of supercharging is limited not so much by combustion considerations but by durability and reliability of the components concerned in stressing the high maximum cylinder pressure and high heat flow rates. Air inlet and Jacket water Temperatures : Increasing both of above: 1. Reduce the delay period. 2. Reduce rate of pressure rise. 3. Reduce heat flow to the cylinder coolant. 4. Reduce power developed due to reduced air mass. 5. Increases cylinder wall temperature. 6. Increases cycle efficiency due to reduced heat loss. Increasing the air inlet temperature has the effect of down rating the engine and lowering the smoke thresh-hold. In each case this is due to the reduced air mass. This effect can be pronounced when operating in the tropics, where both air and sea temperatures are high. One should keep in mind that, while operating in cold climates, where sea and air temperatures are low, the inlet air temperature should not be brought down too low, as humidity in the air may cause corrosive damage to cylinder liners. *********************** End of Combustion/BIT/AMET/BE/Motor/Kv/May 2003********************************
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4. Marine Diesel Structural Parts:
MAIN STRUCTURE OF MODERN LARGE POWER SLOW SPEED DIESEL ENGINE. Type: MAN/B&W: ‘MC’ Type Engine.
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Bed Plate: The bedplate is a substantial, rigid structure which forms the base on which the engine is built. It is supported by the ship structure through the dou ble bottom arrangement, but this support does no t reduce the rigidity needed & in fact with some modern vessels, the hull is left flexible and the b edplate stiffened so that a simple four-point attachment to the h ull can be used. This reduces the distortions developed in the bedplate when hull deflection occurs. Forces applied to the bedplates: 1. Firing load from cylinders. 2. Side thrust from guide faces. 3. Unbalanced inertia forces in the running gear. 4. Weight of engine structure & running gear. 5. Torque reaction from propeller. 6. Hull deflections due to hogging, sagging, racking. 7. Vibration due to torque variations, shock loading. 8. Thermal stresses due to atmospheric and lubricating oil temperature changes. 9. Inertia & gyroscopic forces due to ship's movement in heavy seas. In addition to withstanding forces due to the above causes,, the bedplate should provide. 1. An oil tight chamber to contain the oil splash & spray of the forced lubricating oil system. 2. A drainage grid to filter out large particles before they enter the oil sump or drain tank. 3. A housing for the thrust bearing. Having provided for all the above the bedplate should also be small & light to keep the overall size and mass of the engine to a minimum.
Basic Structure: The bedplate consists of longitudinal and transverse girders as shown below:
Longitudinal Girders may be single or double plate construction.
Single plate type.
Double plate type.
35 Their purpose is to maintain longitudinal alignment by providing sufficient rigidity to withstand the hogging & sagging of the hull structure & provide a stiff support for attachment of the transverse girders. The double plate form is stiffer but more complex than the single plate and makes access for holding down arrangements more difficult. The single plate form is becoming more popular with the use of box bedplates and similar construction of columns. Transverse Girders are deep plates lying between the longitudinals and fitted with pockets to carry the main bearings. A deep plate is needed to give sufficient stiffness (“I”-value) to resist the firing load without bending. Inadequate stiffness will cause distortions of the bearing po cket, which will ‘nip’ the main bearings, gripping the crankshaft journal and causing ‘wiping’ of the bearing. The girders may be of single or double plate construction with a flat plate on the top to give a landing for the ‘A’ frames or equivalent.
The double plate arrangement provides the greatest strength & stiffness but holes must be cut in the plate to allow access for welding and inspection. These holes must be large enough to allow easy entry by a welder and can, seriously weaken a double plate arrangement for a small engine. To restore strength & stiffness a tube may be welded through the girder holes. Depending upon the material used, the attachment of the transverse girders to the longitudinal girders may differ most are welded but some may be bolted if the girder is cast as this reduces repair difficulties, allows stress relieving of the girder only and lessens risk of distortions. Types of Bedplates. The two most common types are: 1. Box type. 2. Trestle type.
1. Sulzer, B & W, MAN and Doxford, all use the box or flat bottom type as it can be mounted directly to the tank top plating (via chocks) and is suitable for fabricated construction. 2. G.M.T. & Mitsubishi are examples of engines still using the trestle type. This type provides a deep and therefore stiff transverse section. To accommodate this deep suction however the bedplate must be seated on special built up stools in the double bottom structure or a special well must be left in the double bottom structure. Both complicate the double bottom structure. If the ‘well’ is used an added attraction is a reduction in engine height.
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Flat or Box type Bedplate Construction.
Tie-bolt.
Trestle type Bedplate Construction.
37 Bedplate Materials.
1. Fabricated mild steel. Slow speed engines Sulzer, Doxford. 2. Cast iron. Medium speed engine (small). 3. Composite type. Fabricated mild steel longitudinal girders and cast steel transverse girders. Engines that use above are B & W, G.M.T. Mitsubishi, MAN. Sulzer. 1. The all welded form of construction gives the lightest bedplate (about 25% less than C.I.) with the greatest strength against shock loads & the highest guarantee of manufacture. It is also the easiest to repair. However it possesses poor vibration damping c haracteristics & due to the multitude of welds is liable to cracking. To ensure freedom from distortion the welding sequence must be correct and after welding the bedplate requires stress relieving by heating to 600*C and holding for 1 hour/inch (25 mm) of plate thickness. Normal plate thickness is 1½” - 2” (35-50 mm). The size of the bedplate is controlled by lifting equipment available and the size of the stress-relieving furnace. Because of these factors plates are normally made in at least two parts. Transverse girders are normally cut from a single plate and supporting ribs welded on below the bearing pockets. Pockets are usually of cast steel. Examples: M.A.N. & SUIZER Engines.
2. Cast Iron: It is never used for large bedplates any more as the quality of generator of a defect free casting is not good enough. Frequently used for small engines however. The main advantage is the materials ability to absorb vibration (not shook), which limits vibration transmission through the engine & reduces the frequency of cracking in the bedplate. Any cracks are difficult to repair & require a ‘Metalock’ type repair, which canno t be effected by ship's staff. The material has a low tensile strength and is usually supported by tie-bolts. Examples: Only small medium or highspeed engines use this type of bedplate.
3. Composite construction involves fabricated mild steel for the longitudinal girders and cast steel for the transverse girders. This system has the advantage of a continuous transverse girder with the bearing pocket integral. Strengthening ribs can be cast in and the complete unit stress relieved before bolting or welding to the longitudinal girders. The cast steel must be of wieldable quality, up to 0.23% C. The steel has a higher resistance to cracking compared to fabricated mild steel due to the irregular grain flow and lack of welds. Examples: B & W, Doxford, G.M.T., Mitsubishi engines. The following surfaces of the bedplate must be machined :
1. 2. 3. 4.
Top face: For attachment of ‘A’ frames. Bottom seating face: For chocks, tie-bolt heads and oil sump pan. End face: For thrust block housing, turning gear & end chocks. Side face: For side chocks and Entablature cover plates.
Faults found in Bedplates: 1. 2. 3. 4.
Cracks. Oil leaks. Loose chocks. Loose ‘A’ frames.
38 1. Cracks usually occur:
i) ii) iii) iv) v)
Under bearing pockets on fabricated mild steel bedplates. Radially around tie bolt & frame boltholes. Between longitudinal and transverse girders. Around ‘lightening’ holes. At the base of serrated seating for main bearing keeps.
Causes may be:
a) b) c) d) e)
Bearing wear & therefore overloading. Slack tie bolts. Vibration. Poor welding or stress relieving. Stress risers on welds -(Coarse welds should be ground).
Repair : For mild steel & cast steel crack chipped out and welded, but care should be taken to ensure a minimum distortion by determining the optimum welding sequence.
For Cast Iron the crack should be arrested by drilling a small hole, sketch or photograph the crack for future assessment. The crack could be “Metallocked’ or supported by a mild steel doubling plate, bolted on, if serious. 2. Oil leaks: i) Sump pan. ii) Doors and casings. iii) Crank case relief valves. iv) Bedplate cracks. 3. Chocks may fret if the holding down bolts get lack and due to the movement of bed plate chocks ‘bed’ into the tank top. As a temporary measure the chock should be shimmed up and the bolt hardened down and as soon as possible the chock should be removed, the tank top faced up by grinding and a new, thicker chock prepared and re-bedded. Bedplate inspection. 1. Cracks. 2. Corrosion. This may be due to moisture or acidic compounds in the oil. If the bedplate has been painted, remove flaking paint and cheek for pitting. After that do not repaint.
39 3. Cleanliness. Check for sludge and carbon building up in corners, under bearings, behind bearingcover studs, etc. 4. Loose connections - bolted transverse girders, A-frames, oil pipes, sump grids, chocks and holding down bolts. 5. Oil leaks - through cracked welds, loose sumps, leaking seals. 6. Faulty welding - on new engines - under cutting, blowholes, slgg; etc. 7. Faulty castings - porosity, blowholes, inclusions etc.
MAN Engine Bedplate.
Engine Bedplate sketch.
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Bedplate The bedplate acts as the main strength member, maintains correct alignment and supports the weight of the components. it must be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure. In addition it may also collect lubricating oil. In slow speed engine design, it consists of a deep longitudinal box section with stiffening in the form of members and webs. Transverse members are fitted between each throw of the crankshaft. These support the main bearing saddles and Tie -rod connection. They are attached to the structure by substantial butt welds. To reduce the engine height the sump of the bedplate may be sunken allowing it to fitted into a recess in the ships structure. Plate and weld preparation is required with welds of the double butt type if possible. Regular internal inspection of the parts especially the transverse girder is required for fatigue cracking. Tie bolts should be checked for tighteness. Box girders-A box girder is stronger and more rigid then I or H section girder of the same c.s.a. From the simple beam bending equation we have; M /I = s /y = E/R M=Bending moment I=2nd moment of area of the cross section s =Stress y=distance from the axis of bending to the outer face E= modulus of elasticity R -radius of curvature of the bending. This can be arranged into s = (M/I) . y
It can be seen that for the same bending moment on a symmetrical shape of same size, the stress is reduced on the increasing 2nd moment of area. The second moment of area increase with moving of material away from the axis of bending towards the extremes of the section. Because of this the commonest way of construction a fabricated bedplate is by creating two box section girders and tie them using transverse girders.
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The advent of the small bore slow speed has seen the use of single side bedplates. A box section is then created by using a box section crankcase structure rather than the more traditional A-frame.This has the advantages of reducing width as well as weight and increasing the amount of fabrication so reducing assembly times.
Due to the weight penalty, the use of cast iron is generally limited to smaller units where fabrication becomes impractical. However, cast iron has internal resilience allowing it to dampen down vibrations, this has led to its usage on some medium speed installations, especially passenger carriers, where noise and vibration suppression is important. .
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The most highly loaded pat of a bedplate is the transverse girder. Classification societies require that residual stress is removed after construction. The transverse girder acts as a simple beam with the forces of combustion acting on the piston passing down through the bearing. The forces acting on the head are passed through the Tie rods.
It can be seen that to reduce the bending moment the tie rods have to be brought closer to the crankshaft. The limit to this is the securing arrangement required for the main bearing keep. One method is to use two instead of one bolts which can be made of smaller diameter. Sulzer use an alternative and very successful method in the form of jacking bolts. These jack against the bottom of the A-frame.
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.
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ENGINE CHOCKS. These are needed between the bedplate and tank top to ensure that any variations in the surface of the tank top does not cause misalignment. Up to 200 chocks per engine may be fitted. They also permit any chaffing or fretting to be repaired by adjustment of individual chocks and any subsequent distortions after fitting (due to settlement) to be corrected. End chocks are fitted at each end of the long girder to position the engine, absorb collision loads and in the case of the integral thrust block, absorb propeller thrust & propeller excited vibrations. Side chocks are needed to absorb side loads due to components of unbalanced reciprocating forces and thermal expansion. They also prevent chaffing of the supporting chocks and tank top and also help the holding down bolts resist the lateral forces when the vessel is rolling. Chocks are usually made of cast iron or steel. Cast Iron chocks are popular because: 1. Easy to form. 2. High compressive strength & low malleability. The chock retains its shape under load reducing the chance of bolt slackening & therefore bolt fracture. Unfortunately this also means that the chock is ha rd and liable to ‘bed’ into the tank top or bedplate. It is also brittle and therefore liable to fracture under exce ssive impact loads, hence minimum chock thickness should not be less than 30 mm. Steel is used to reduce those problems and allow easier fitting. Steel chocks should be used for clearances less than 30 mm. Epoxy resin is increasing in popularity and now widely used for small, medium & large engines. The compound has the following advantages: i) Elimination of fitting & machining. ii) Increased support as large areas of the bedplate can be used. iii) Elimination of breakage, fretting and slackness. iv) Improved resilience, which absorbs vibrations, reduces noise and gives greater ductility. The compound is suitable for any bedplate, which can be fitted with a sealing dam to contain the compound while it is setting (may take up to 24 hrs with some heating, around 16*C necessary). It can be used on new engines or as a replacement on old engines. Where the chock is deep, steel rollers are added to the resin chock to increase strength & Durability. Fitting of engine chocks:
Process is more or less similar for cast iron, steel or resin chocks. 1. Bedplate is aligned on the tank top using temporary chocks, jackscrews or wedges using the sagging wires pilgrim or optical alignment method. 2. Crankshaft is budded & deflections taken after the engine is fully built up and the vessel is floating in even keel with all transmission shafts in place. 3. Metal chocks are machined slightly oversize and then hand filed and scrapped. It is bedded in its place and fitted. Minimum 70 to 80% bedding is required. For bedding purpose the chock could be tapered up to 1/100 from outside to inside. 4. For Resin Chocks the surfaces are cleaned, a dam prepared around the chock area, holding down bolts placed in position and greased and all surfaces sprayed with a releasing agen t. Resin is mixed and poured into position. When solid, temporary support can be removed and after 24 hours, holding down bolts tensioned. A 1μm per mm of chock thickness is allowed for shrinkage. 5. Crankshaft deflections are retaken to confirm alignment. The deflection reading should be the same at the end of fitting the chocks as it was when taken before fitting as per step 2.
45 A third material, rubber is used for some installations; usually high speed diesel engines in small vessels. These are resilient mountings and fitted to reduce vibration transmission from engine to hull or vice versa. Very careful selection of the right size and stiffness must be made in order to obtain the optimum operation and sufficient flexibility must be arranged in all connections to the engine to prevent any restrictions. This includes the output shaft coupling. Generally flexible hoses connect all pipelines and for engine exhaust pipe line a good metal exhaust bellow is fitted. 4 to 8 mountings are normal. Some points regarding Epoxy Resin: Resin chocking is a recent development in engine chocking arrangement. It has been widely used now for large, medium and small engines. It has following advantages: 1. Reliable permanent alignment without machining foundation, bedplates or chocks. 2. Provides uniform precise mounting for superior retention of critical alignment. 3. Resists degradation by fuels, lubricants, eliminates corrosion in chock area. 4. Non-fretting. 5. Reduces noise levels, maintaining the alignment and hold down bolt tension. 6. The modulus of resin helps to maintain crankshaft deflection and machinery alignment during hull flexure or distortion.
MOST IMPORTANT This is a liquid; it conforms to all irregularities in the fitting surface, providing a p recise contact fit between machinery bases and foundations (after solidification). Properties of the chock after it has cured: i) Compressive strength: 1330 kg/cm2. ii) Tensile strength: 350 kg/cm2. iii) Shear strength: 380 kg/cm2. iv) Heat distortion temperature: 93*C.
Bedplate holding down bolts. Holding down bolts may be fitted or clear. If collision & side chooks are used the bolts are usually clear. If not the bolts at the flywheel end are fitted, remainder clear, to ensure the coupling to output shaft is not strained. Bearing faces of bolt heads & nuts must be normal to the bolt shank & parallel to each other to prevent any bending stresses. If necessary the bedplate & tank top may be machined.
Traditional holding down bolt arrangement.
46 Procedure for fitting the above holding down bolt. a) Harden the stud into screwed tank top to achieve watertight seal on the conical face. b) Tighten lower nut, tack weld or caulk over thread for locking. c) Tighten upper nut. Modern method of holding down bolt arrangement. The traditional method suffers the problem of fretting cast iron chocks and bolt failure particularly under slow speed diesel machinery. In modern days to e liminate the above problem long elastic bolts with extended collars are used. These bolts possessed high resilience and are highly stressed when tightened. As such when strained while in service, there will be less reversal of stress which results in reduced possibility of fatigue failure. When these bolts are tightened and slightly stretched, the bedplate, chock and tank top seating are under compression. When holding down bolts come under strain while in service, the parts under compression expand and the mating surfaces of the chocks remain in contact with the bedplate and tank top seating. Fretting is hence avoided. Their cost is considerable and an additional £40,000 for the bolts of a 6-cylinder engine is typical. Epoxy Resin chock. ‘Chockfast’ system needs only simple bolts and nuts to give permanent engine security. It is claimed that the use of pour able resin chocks overcomes bolt stretching, slack nuts and bolt failure, while also offering considerable economies when erecting the engine since perfect matching takes place between engine bed-plate and the unmachined tank-top seating.
Line diagram for holding down bolt and chock.
47 Modern slow speed main engine bed plate arrangement over ships structure.
Bedplate holding down arrangement for the above engine with long bolt.
48 Earlier engines bedplate and holding down arrangement showing main chock, side chock, end chock and hydraulic stretching tool.
Chocking arrangement with tall bolts and washer system of holding down bolts.
49 Inspection requirement pertaining to holding down bolts and engine chocks .
Holding down bolts are strained while in service and thus required to be tightened up occasionally if troubles with bedplates is to be prevented. Even the most imperceptible movement of the bed plate will cause fretting to occur on the bedded mating surfaces of the bed plate, chock and foundation plates. If fretting occurs in areas covering a number of adjacent chocks, the crankshaft may be seriously damaged through misalignment. New installations should have the bolts checked after a few running hours and at least every six months after that. A record should be kept. These holding down bolts should be checked fully if the vessel had met with an accident, such as grounding near engine room, fire in engine room or near the engine room and collision.
Bedplate Inspection 1. Cracks (split around the parts mentioned earlier). 2. Faulty welding - on new engines (under cutting, blow holes, slag etc.). 3. Faulty castings - porosity, blowholes, inclusions etc. 4. Corrosion. 5. Cleanliness - sludge and carbon build up in corners, under bearings, cover studs etc. 6. Loose connections - bolted transverse girders, A-frames, oil pipes, chocks and holding down bolts. 7. Oil leakage.
*************************************** Kv****************************************** End of Bedplate/chocks/BIT/AMET/BE/KV/May 2003.
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Engine Frames and Cylinder Blocks: Engine Frames . These fit between the bedplate and cylinder block beam. They are sometimes referred to as the entablature. They serve the following functions. a) Support the cylinder blocks, turbo-chargers, camshaft and driving gear, scavenge belt etc. b) Provide a facing for the girders & absorb the guide forces. c) Develop an oil tight easing, for forced lubricating oil system, & support pipes & walkways. ‘A’ – Frames. In old engines the frames were of cast iron and made hollow to reduce weight without reducing rigidity. The frames or columns were held in compression by tie-bolts. These frames were later fabricated from mild steel tube and plate with guides of cast iron bolted onto the frames. This type of arrangement uses individual frames at each transverse girder position of the bedplate with the longitudinal spaces between frames filled by plates bolted to the frames. The structure is strong and rigid in the transverse plane but relatively flexible longitudinally. This makes oil tight fixing of the side covers difficult unless very heavy covers or longitudinal stiffness are used. It also produces a weak structure if exposed to internal pressure from a crankcase explosion and will allow alignment of the cylinder blocks to the b edplate to vary in relation to ship movement. The ‘A’-frame construction is now being abandoned in favor of longitudinal girder construction. Improved methods of prefabrication which can b e relied upon to produce large, distortion free units has allowed longitudinal girders to be manufactured so that the longitudinal stiffness of the structure can be increased without altering the transverse stiffness. This also contributes to the bedplate stiffness and reduces effects of hull hogging and sagging. ‘MAN’ engine manufactures claim that the b edplate only contributes 17% to the overall stiffness compared to 60% for the traditional ‘A’-frame construction.
In the ‘Sulzer’ engine the fabricated longitudinals form a sandwich by enclosing a cast iron centerpiece at each transverse girder spaces. The cast iron centerpiece forms the crosshead guides. The structure is bolted together. In the ‘B&W’ engine the entablature retains the ‘A’ transverse section but both longitudinals and transverse components are fabricated into a bo x form. The guide faces are bolted to the transverse components. The entablature is formed in two pieces connected at the camshaft drive position at the middle of the engine. In the ‘MAN’ engine, regular box shaped fabrications are used, again with longitudinal and transverse sections welded together to form a single unit. The layer sizes (more than 700 mm bore) have the box divided into 2 on the horizontal plane. The upper box has openings on the back into which the cast iron guide faces are bolted. In the ‘Doxford-J’ engine a continuous girder is fabricated for the guide side of the framework with the columns at each main bearing position welded to the longitudinal. The front of the engine is left more open to allow easy access to the running gear. Apart from increased stiffness which reduces: i) Misalignment, ii) Bearing distortion, iii) Vibration, The structure is more oil tight, as fewer joints are required & the structure ‘works’ less. It is also easier to build the engine & ensure equivalent alignment when the engine is reassembled in the ship.
51
M.A.N. Engine Bedplate, Lower frame, Upper frame and Cylinder jacket.
52
Transverse section of Sulzer Engine, showing all internal bolts and fittings.
53
‘Doxford’ Engine Structural Arrangement.
‘B&W’ Engine Structural arrangement.
Tie Bolts. These are fitted to relieve the frames of tensile stress. The bolts are mounted between the transverse girder of the bedplate and the upper face of the cylinder jacket. As this in variably makes the bolt very long it is sometimes fitted in two lengths joined at the base of the cylinder jacket. Hydraulic tightening tensions the bolt and this pre-tensioning should be sufficient to keep the frames in compression throughout the engine cycle. This produces a substantial tensile stress in the bolts requiring them to be checked frequently. Transmission of Firing Load. In most single acting engines, apart from ‘Opposed Piston Engines’, the long tie bolts transmit the main gas loads from the cylinders. Two bolts are fitted to each transverse girder and they pass through the casting through tubes constructed in the engine frames and through the entablature or cylinder jackets where locking nuts are fitted. Tie-bolts are prestressed during assembly and carry the firing forces from the cylinder cover to the transverse, beam and thence the ship's hull. Tie-bolts should be as close to the
54 crankshaft axis as possible to minimise bending stress on the transverse girders of the bedplate and to prevent unbalanced loads being transmitted to the welds. The further the tie-bolts are the greater will be the bending stress. Hence any method of bringing the tie bolts close together will decrease the stress. Therefore ‘Sulzer’ and ‘Fiat’ engines have used jacking bolts between A-frames and main bearing up per half keeps. This ensures that the tie-bolts are as close as together as possible. Great care must be taken that the tie-bolts are correctly tensioned before tensioning jacking bolts otherwise if the tie-bolts were tensioned after jacking bolts, the latter and main bearing keeps could be over-stressed.
‘Sulzer’ Engine Main Bearing Jack bolt arrangement (See page 40 drawing for full details). CONSEQUENCES OF RUNNING AN ENGINE WITH SLACK TIE BOLTS : Cylinder beam would flex and lift at the location of the slack bolt landing faces of the tie bolt upper and lower nuts, landing faces of the cylinder beam on the frame would fret and machined faces would eventually get destroyed. The fitted bracing bolts between the cylinder jackets will also slacken and the fit of the bolts would be lost. If fretting has occurred in an uneven pattern where the cylinder beam lands, and the tie bolts are tightened, the alignment of cylinder to the piston stroke will be destroyed. The fitted bracing bolts between the cylinder jackets will also slacken an d fit of the bolts will be lost. Fretting may make the nut landing face out of square and if tie bolts are tightened on the damaged face, a bending moment will be induced in the tie bolt, this may cause an uneven stress pattern in the tie bolt which could lead to early fatigue failure. Damage may take place in the bedplate in way of cross girder. Rigidity of the whole structure will be destroyed. Guide force will have to be absorbed by the frame bolts and dowels, which may stretch and slacken allowing the structure to ‘work’. This may destroy the piston alignment. Guide faces and bars may get slackened (these are bolted to the supporting structure).
TENSIONING OF TIE RODS AND CHECKING THE PRETENSION (‘SULZER RLA’. ENGINE). Bedplate, columns, cylinder jackets are greatly relieved of the gas forces set up and freed from tensile stresses when tie rods are properly tensioned. In order to av oid vibration all tie rods are held in position by special guide bushes located on the lower end of the cylinder jackets. These bushes are of two parts and clamped on to the rods. Clamping bolts jam the tie rods in the bores. Tie rods are pretensioned by
55 hydraulic tensioning device. Tightening is carried out in two steps to avoid to reduce additional stresses on the jackets. Note: For new engines it is recommended that all tie rods be checked for correct pretension after the first year of service and if necessary pretensioned to the valve specified. After that it is sufficient to make random cheeks during major overhaul. The bolts should be checked approximately 4000 to 6000 running hours.
1. Cylinder jacket. 2. Tie rod lower washer. 3. Tie rod main nut. 4,5,6. Hydraulic tool lower half with cylinder. 7. Hydraulic tool stretching piston screwed to tie rod. 94933. Hydraulic oil connections. 11. Oil vent screw. 12.Tommy to tighten the nut after stretching. K. Washer cylinder to keep the tie rod nut clear. Tie rod tensioning hydraulic tool as fitted to the tie rod top. Procedure for checking the pretension of the tie rods. 1. Remove the thread protecting hoods from all tie rods and clean the contact face of the intermediate ring. 2. Screw pretensioning jacks on the tie rods (two on opposite sides) until the lower part of the cylinder rests on the intermediate ring. Slightly slacken vent screws. 3. Connect both pretensioning jacks with hoses to the high-pressure oil pump and operate pump until air has escaped. Retighten vent screws. 4. Operate pump until 600 bar pressure is reached and maintain this pressure. 5. Check with a feeler gauge through measuring point ‘S’ for any clearance. 6. If any clearance does exist tighten the tie rod not by a tommy bar, until it rests firmly on the intermediate ring. (Check with feeler gauge). If no clearance, pressure is to be released immediately. All the tie rods are to be checked in this manner. 7. After checking has been completed the threads to be protected with anticorrosive grease. Procedure for Loosening or tightening of tie rods. 1. Before loosening or tensioning tie rods, the thrust bolts of the main bearings must be loosened. 2. Clamping bolts must be removed. 3. Loosening and tensioning has to be carried out in stages (three stages).
56 The order is shown in the diagram given below:
Half full value.◄----------------------1 st Strech.--------------------------►Half full value. Full Value. ◄----------------------------------------2 nd Strech. --------------------------------►Full Value. Full Value (Second time).◄----------------------- 3 rd Strech (Final) ---------------------► Full Value. Tie rod loosening: Apply pressure slightly over the engine pressure prescribed slowly. Then nut should be slackened by say 1 to 1½ turns. (If pressure is set for 600bar, then apply say 620bar). Tensioning: Preparation. 1. Clamping bolts to be tightened. 2. The lower tie rod nut is screwed on and secured with looking screw. 3. The contact surface of intermediate ring and upper tie rod nut are to be cleaned and coated with ‘molykote’ paste.
57 4. Screw eye bolt into the tie rod and carefully lift until the lower tie rod nut rests tightly against bearing girder. (It is best to use block and tackle between crane hook and tie rod. The tie rod can thus be lifted by hand until the lower nut lands on the girder. With a crane, it may not be possible to feel when the lower nut lands). 5. In this Position, the upper tie rod nut is tightened with a tommy bar until it rests firmly on the intermediate ring and the tie rod is removed from the lifting device. (There must be no clearance between the bearing girder and the lower nut). Procedure of Tensioning. 1. Once all the preparations mentioned above have been madethe engine manufacturers’ instruction for tensioning has to be followed. 2. Mount pre-tensioning jacks on the two tie rods 1/1 in the middle as shown in the sketch and the lower part of the cylinder has to rest on the intermediate ring. 3. Connect pre-tensioning jacks to high-pressure oil pump and vent the system. 4. Operate pump until a pressure of about 350 bar. (1st stage, Half value) is reached. Maintain this pressure while the two upper nuts are tightened by tommy bar and a snug fit is obtained. 5. After the operation and tensioning of tie rods 1/1, go over to 2/2, 3/3, 4/4 till out ward end of the engine. 6. Repeat the same for full pressure from 1/1 to out ward end. 7. For a third time do the same for full pressure once again to eliminate any residual stresses on the cylinder jackets. 8. After completion of pre-tensioning procedure, coat the protruding thread with non-acidic grease and fit the protective hoods. Important points to note . 1. Individual tie bolt must not be loosened completely without slackening the rest at least partially. 2. Thrust bolts of bearing covers must be slackened before loosening or tightening the tie bolts. 3. All components and contact surfaces involved in the process of tensioning the bolts must be checked for cleanliness, levelness, perpendicularity and parallelism. 4. Every care should be taken to protect the threads. 5. Tightening or loosening should be always done in stages. 6. Before checking the bolt tightening, check that the supporting chocks are firmly fixed. 7. Increase the pressure of the hydraulic tightening tool very slowly, while checking bolts. 8. Always ensure that the hydraulic tool is kept in good condition along with the flexible hoses and the pressure gauge fully calibrated. The following faults can be found in the structure: 1. Cracks. Tremendous improvement has taken place in welded structures, but still cracks can be found at junction welds. Cracks can also occur around boltholes, or where the stress pattern is complex. So, the most likely places for inspection to detect cracks are: a) Behind the guides. b) Around main bearing pockets. c) All junction welds. d) At any weld. e) Securing bolts and dowels between bedplate and frame, frame and cylinder. f) Around guide securing bolts. 2. Loose Bolts. Tie bolts keep the engine structure under compression throughout the cycle and the structure is designed accordingly. Tie bolts may get slackened and if the slackening is considerable, the structure will not remain under compression during combustion. The guide force will have to be taken up by the frame bolts and dowels. This could be high enough to stretch and slacken the bolts and this would allow the structure to "work". Bolts holding the guide faces and bars to the supporting structure may also g et
58 slackened. This may seriously affect the piston alignment. Fretting would take p lace at the landing faces of all the parts held together. 3. Misalignment. Alignment of the whole structure (assemble) is of extreme importance. The initial alignment may be carried out by a plumb line from crossed laths on the top of the liner, frame is now adjusted until the plumb line lies evenly in a hole through lath, which is mounted between frame positions. Fitted bolts or clear bolts and dowels now secure the frame. Now the liner and guide alignment is carried out by piano wire, calipers/micrometers. Misalignment may occur due to: a) Settling of the structure. b) Fire. c) Grounding, collision. d) Cracking of frames. e) Distortion of bedplate. Indications of misalignment. a) Overheated bearings. b) Overheated guide slippers. c) Uneven wear of liner. d) Piston slapping. e) Excessive vibration. f) Wear of stuffing box, piston rod. ********************************** Kv**************************************** Reference: Running & Maintenance of Marine Diesel Engine By Mr. John Lamb. B&W, Sulzer, MAN, Doxford Engine manufacturers’ Manual. End of engine frames, cylinder blocks and tie rods/BIT/AMET/BE/KV/May 2003.
59
Cylinder Cover: This, in combination with the cylinder walls and piston crown provides the perimeter of the combustion chamber. It is therefore exposed to high mechanical and thermal loads. Sufficient penetrations must be made in the cover to house: 1. Inlet & exhaust valves. 2. Fuel valve or valves. 3. Air Starting Valve. 4. Relief valve. 5. Indicator cock. This makes the cover complicated and it is therefore usually cast. Stresses in a Cylinder cover.
Valve housing - holding down studs of valve cause tensile stress in cover, which increases as the valve expands if the valve sealing face Is at the bottom of the pocket. Thermal load can also cause tensile stresses & distortion of inner face of cover. 4-Stroke Engine. These are usually made of cast iron because of the number of valve penetrations and the need for large inlet air and exhaust gas passages. To accommodate the passages and give adequate strength due to the use of cast iron a very deep casting is needed. In order to avoid high thermal loads good cooling is needed and this in turn demands thin metal sections. To achieve optimum strength and reduce temperature stresses is very difficult and therefore cylinder covers are prone to failure. The biggest problem area is between the valve and fuel injector pockets, and this is the most likely area for cracks to occur. To overcome the problem the fuel valve may be offset from the center of the head, which adversely affects combustion, but permits larger cooling water passages and therefore improves cooling. Further improvements can be made if: a) Water-cooled valve cages are used, particularly for the exhaust valve. b) Separate sleeves for fuel valves are screwed & rolled into the cover. The first gives more direct cooling as the full flow of jacket-cover coolant is directed through spaces adjacent to the seat and stem and the ease of removal allows more frequent overhauling. The latter are
60 an advantage as the sleeve can expand and contract within the cover reducing the total stresses and being of thin section occupy little space so that an adequate water flow can be arranged between the valve pocket and the exhaust valve. A further improvement is the use of four valves (2 inlet valves and 2 exhaust valves). These allow more room for the central fuel valve, provide longer areas for gas flow and reduce valve inertia. They increase the complexity of the cover however. The cover shown below uses a water-cooled cage for the exhaust valve but a directly mounted air inlet valve with a renewable seat. In order to reduce the material thickness a strong back type of construction is used with the lower face (called the flame plate) made thin and supported through heavy vertical ribs from an integrally cast strong back plate. The cover strength is further improved by using a deep casting.
Cylinder cover for 4 – Stroke Engine. With the advancement of metallurgy and machining modern cylinder covers are integral cast with inserts or bore cooled. A modern cylinder cover for a 4 – stroke engine is shown below along with service temperatures of the combustion chamber. To remove heat and keep all components in the combustion zone with in the designed thermal loading the cooling system is modernised. Major advantages of the new bore cooling system cover design are: 1. Extremely low valve seat temperatures and excellent temperature distribution. 2. Extremely large stiffness resulting in valve seats with considerably improved sealing properties and low dynamic stresses. 3. With the above properties, the valve overhaul intervals even with low quality fuel service will certainly be greatly increased.
61
Sulzer Z 40/48 Engine Measured temperatures in *C in combustion chamber area . 500 r.p.m. b.m.e.p. 20.50 kp/cm. 687 BHP/Cyl.
AS 25/30 Sulzer engine Cylinder head temperatures between original design and new bore cooled design for the same engine of same power. 1000 r.p.m. – b.m.e.p. 16 .29 bar.
62
2 – Stroke Engine: In a 2-s cycle more heat is liberated in the cylinder in a given time than with the 4-s engine, consequently cooling is more important. However fewer penetrations occu r in the cover because no air inlet valves are fitted and in loop-scavenged engines no exhaust valves either. Because the heat stresses are greater but in a simpler cover cast alloy steel can be used. To further improve the arrangement a 2-part cover can be used. Sulzer Engine Cylinder cover : Water-cooled cast steel outer cover forming the majority of the combustion space wall with a watercooled S. G. cast iron central insert carrying all valves. This cover holds only one fuel valve, one air start valve, one relief valve and an indicator cook. Latest RND-M engine uses a single, forged steel cover employing bore cooling.
Sulzer RND – Type Engine Combustion Chamber: Cylinder Cover normal Cast Type.
Sulzer RN 90 M type Engine Combustion Space: Cylinder cover Bore Cool Type.
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Sulzer Engine Progressive Advancement in Cylinder Cover design and cooling. B&W Engine Cylinder Cover: Water-cooled cast steel cover into which piston crown penetrates at top dead center. This carries the fuel valves (2 or 3), air start valve, relief valve etc. A large cast iron central insert carries the exhaust valve & gas passage. The cover/liner joint is below the combustion zone, which relieves the liner of the firing load and protects the joint. Latest B & W engine again uses a forged steel cover with bore cooling.
B&W :VT2BF: Engine Cylinder Cover (Old conventional Type).
64
B&W Engine early Bore Cooling Arrangement of Cylinder Cover.
B&W Engine: K90MC: Old Type Cylinder Cover. Cast In Cooling pipes.
B&W Engine: K90MC: New Type Cylinder Cover. Bore Cooled Type.
65 MAN Engine Cylinder Cover: Upper & lower parts with water-cooling to lower only. The lower accepts the thermal stresses while the upper acts as a strong back to absorb the mechanical load. The last engines built by them also used bore cooling for their cylinder covers.
MAN Engine: K/Z Type: Three piece Cylinder Cover.
MAN Engine: KSZ-B Old Conventional Type .
MAN Engine: KSZ-BL-C. Bore cooled Piston only .
66
MAN: KSZ: C/CL Engine Piston, Liner and Cylinder Cover.
MAN: KSZ 52/105 C/CL Engine Section through Piston, Liner and Cylinder Cover showing the temperature gradient due using Bore Cooling.
67
Sulzer Engine: Bore Cooled Piston, Liner and Cylinder Cover showing the temperature gradient.
Defects in cylinder covers: 1. Cracking: Due to the same process as cracking in piston crowns. Generally occur around the fuel valve pocket or between the fuel and exhaust valve pocket. They are caused by overheating, casting strains or notch effects (particularly in 2 – stroke cast covers). Cracks can be repaired by chain studding for temporary repair, ‘Metalock’ for a semi-permanent repair or by welding if the material is suitable. 2. Burning: Due to flame impingement. Repair is by welding if the material is suitable. 3. Distortion: Due to uneven tightening down of the cylinder cover over the liner face, overheating of cylinder cover (particularly if scale is present) or unrelieved casting strains. It causes liner joint leakage and or liner flange cracking. 4. Deposits: Scale & silt due to poor quality water or contaminated water. Not usually found when distilled water is used. 5. Corrosion: Due to inadequate or nonexistent water treatment. ************************************ Kv********************************************* End of Cylinder Covers/BIT/AMET/BE/Kv/May 2003.
68
5. Engine Valve Gear and Valves: Valve Gear: It designates the combination of all parts, including the various valves, which control the admission of air charge and the discharge of exhaust gases in four stroke engines, the discharge of exhaust gases in some two stroke engines (uniflow scavenging type), the admission of fuel in air- injection and some mechanical-injection engines, and the admission of compressed air for starting most of the larger engines. Valve Actuating Gear: It designates the combination of those parts only which operate or actuate the various intake, exhaust, fuel and air-starter valves, open and close them at the proper moment in respect to the position of the piston and crankpin, and hold them open during the required time. Valve Timing Gear: It designates the combination of those parts only which affect and control the moment of opening and closing of the valves with respect to crank and piston position. These parts include cams, camshaft and camshaft drive. The valve gears of diesel engines vary considerably in their construction, depending o n type, speed, and size of the engines. The action of the various parts of a valve gear may be best explained using the figure on following pages. The crankshaft drives the camshaft by chain or gearing. A cam on camshaft lifts the push rod, which operates the rocker arm, which in turn, changes the upward motion of the valve, thus opening it. As soon as the closing side of the cam moves under the push rod, the valve spring starts to return the valve to its seat and eventually closes it.
Arrangement of Valve Gear in a Four - Cycle Diesel Engine.
69 Valve Actuating Gear: In most engines, this gear consists of rocker arms, which actuate the valves, push rods which connect the rocker arms and the cams on the camshaft, and a drive connecting the camshaft to the crankshaft Rocker Arms: The rocker arm has one end on valve stem and the other end, through a hardened steel roller, with the cam profile; if the camshaft is located near the cylinder-head, as shown in the figure.
Cam and Rocker-arm. If the camshaft is located much lower, the other end of the rocker arm is in contact with the upper end of the push rod and lower , with the rotating cam through a cam follower. The rocker arm is pivoted at or near the centre and the pivot pin rest is held in brackets rigidly bolted to cylinder head. Secured to the cylinder head, pivot pin rests in a bronze bushing or needle bearing. Rocker arm's contact to valve stem is by means of roller or more often by means of a setscrew, which is used to adjust ‘Tappet clearance’ (or lash).
Figure below shows the valve operating gear large marine Diesel Engine (B&W Engine K-EF Type).
The cams shrunk onto the camshaft 4 operates the exhaust valves through rocker arm 12 mounted on pivot pin 13, push rod 10 guided by guide bush 11 and roller 2 which runs in needle bearing 3.
70 The roller guide 1 is prevented from turning in the bores in the housing ho using by the key and keyway 6. The housing is closed at the top by a cover 7, which is provided with a scraper ring 8 to prevent oil leakage. The housing around the cam discs serves as a lubricating oil bath. Automatic Valve – Lash Adjusters:
Automatic valve - lash adjusters are used on some engines to avoid the necessity of a clearance otherwise needed in the valve gear to allow for expansion due to temperature changes. They also eliminate the need for manual adjustment in order to take care of wear at various points of the valve gear. Automatic adjusters may be either mechanical o r hydraulic. The mechanical type uses a cam (generally located at the end of o f the rocker arm over the valve stem) and a spring, which turns the cam so as to take up the clearance when the valve is on its seat.
Hydraulic Lash Adjuster.
The above figure shows a hydraulic lash adjuster built into the end of the rocker arm above the push rod. It consists of a small cylinder (called ‘lifter cylinder’) containing a plunger, spring, and ball check valve. The plunger rests against the upper part pa rt of the rocker arm, while the spring pushes pu shes the cylinder downwards toward the push rod. In operation, oil under pressure from the lubricating oil system enters the lifter cylinder, past the ball check valve and is trapped under the plunger, which has previously taken up the clearance. When the rocker arm moves downward to open the valve, the trapped oil transmits its force through the cylinder to the push rod. If the valve stem expands, there is sufficient leakage of oil past the plunger to permit the lifter cylinder to rise slowly so that there is no danger of holding the valve open. In some engines such as B&W: K-EF type, automatic valve adjusters are incorporated at the bottom of the push rod. Push Rods:
These are generally hollow, to obtain stiffness without unnecessary weight. Usually (in small high speed engines), lower end of push rod carries a head or ‘follower’ of flat or mushroom shape, which rides on the cam; a rounded head at the upper end fits into a cup on one end of the valve rocker arm. In many engines, side thrust on the push rod is avoided by using a hinged h inged follower, which rests on the cam and transmits the cam action to the push rod; the follower carries usually a roller, which runs on the cam and thus reduces friction. A cam and follower arrangement is shown in the figure (Page 59).
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Hinged Cam – Follower. Mechanical valve actuated Exhaust valve for large uniflow early B&W Engine . The mechanical valve actuating gear (see page 57 for operating gear) has got shortcomings. Due to inertia of parts, there is inherent delay in opening and closing of valves. The buckling of push rods, valve bouncing and inherent problems of mechanical linkages make hydraulic actuation of valves an attractive option.
B&W. K type – Engine: Exhaust Valve. Mechanically Operated.
72 Hydraulic Valve Actuating Gear: Hydraulic Valve Actuating Gear is now more common. For Example B&W Engine K-CIF Type and later versions use Hydraulic Valve Actuating gear as described below. The figure on the following pages shows the construction and operation.
Hydraulically Operated Exhaust Valve showing the arrangement of connections. Description.
A cam on the camshaft actuates the Exhaust valve. Though a roller, the movement is transmitted through a push rod to the plunger in a hydraulic oil cylinder, which through a high-pressure pipe, is connected with the hydraulic cylinder on the exhaust valve.
73
B&W: L – GFCA Type Engine Exhaust Valve Shown in Detail.
The roller guide is pressed downwards onto the cam by the action of a helical spring. The push rod rests on a retaining washer. Hydraulic oil cylinder is attached to the push rod housing byeight studs. The cylinder is provided with an exchangeable liner and has on the outside a skirt, which forms the outer wall of a cooling duct round the cylinder. The skirt is sealed in an oil tight manner against the cylinder by means of two rubber rings. The pressure oil from the camshaft lubricating system is supplied through an elbow union at the bottom of the cylinder. Part of oil passes through cooling d uct and is drained to the oil pan of the roller guide housing. The remainder of oil is led through two holes, one at bottom and one on top with a non-return valve, to the pressure space of the cylinder. The lower bore in the cylinder of exhaust valve is provided with a throttle valve used for adjusting flow by which fine adjustment of closing and opening of exhaust valve can be made. Leaking oil from the hydraulic cylinder on the exhaust valve is drained through bleed pipe to top of hydraulic cylinder of the
74 roller guide, from where it is drained along w ith any oil from safety valve and the protective hose to the roller guide housing. Air Spring Type Exhaust Valves: Modern engine exhaust valves now a days have eliminated the spring actuation of the valves. Springs that held the valves to the valve seat and allowed the valve to open and close are totally eliminated due to bouncing effects and the damage it caused to valve seats. The springs are replaced with “Air Spring or Air piston” type of valve operation, where the air pressure holds the valve on its seat. The figures below will illustrate such modern valves, which are in use these days.
Combustion Space and Air Spring Operated Exhaust Valve: Sulzer: RTA 58 – 84 Engine.
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Air Spring Exhaust Valve With Hydraulic Actuating System:
Position Of Exhaust Valve With Out Air Pressure.
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Air Spring Exhaust Valve In Closed Position.
Air Spring Exhaust Valve In Open Position. ************************************* Kv********************************************
77
Inlet and Exhaust Valves: Air inlet and exhaust valves of the mushroom type are always used in four stroke engines and sometimes in two stroke engines. To handle large flow rates, they are of large size. Both open into the cylinder, so that the greater the gas pressure in the cylinder, the more firmly are the lids pressed against their seats. Therefore the springs employed to close the valves require being strong enough to keep the lids on its seat during the low pressure period of the cycle of the engine. It is not unusual to find exhaust valves having smaller diameter than inlet valves. The reasons are: a) Exhaust valves open against higher pressures within cylinder. b) Exhaust gases assist in expelling the gases through open exhaust valve, unlike the inlet valve. c) Being smaller assists in keeping them cool which is important as exhaust valves operate at higher temperatures. However, large engines can have them of same size. As inlet and exhaust valves withstand different thermal loads, they might differ in material also. Also, exhaust valves require cooling. In large engines it is better to duplicate inlet and exhaust valve. It gives better gas and airflow resulting in reasonable sized valves, better volumetric efficiency, better scavenging, cooler piston and liner and better performance. The figure on the following pages illustrate typical valves and shows common terminology. Typically the valve seat is angled at 45* for diesel engines, although some valves use a narrower 30* angle from the horizontal. The 30* angle allows less restriction across the seat and flow can start sooner and end later. Valve guides, typically make of Cast iron, guide the valve stems, which tend to wear against the valve guides due to angularity of up and down motion of the valve contribution by rocker arm action. It is important that the clearance between the valve and the valve guide i.e. valve guide clearance, be within the manufacturer's specification. The valve seat must be smooth, not only to prevent leakage, but also to allow for good heat transfer. As valve seats are prone to damage, burning and distortion, Exhaust valves have replaceable valve seat inserts, as shown in solid black in the figure on following page 66. As the inserts are ground away and exceed dimensional limitations desired, they are replaced with new inserts. If a valve had to be repaired or replaced the entire head might have to be removed because the valves are installed solidly in the head. Heads can be removed on small engines but on large engines head removal is certainly difficult. Valve cages are used to overcome this difficulty. An example is illustrated in the figure on next page. A copper gasket is inserted between the combustion chamber and the cylinder and the head in the valve cage. This copper gasket prevents both leakages from the combustion chamber and carbon build up. Valve Cages. To help prevent welding or freezing up valves due to deposition of carbon particles around and to avoid uneven wear, exhaust valves of modern large engines are given rotation by providing vanes or ‘rotocaps’. Each valve cage for exhaust valves may have its own water jacket. Also, to have effective cooling, Bore cooling is incorporated in the modern exhaust valves.
78
Valve Construction.
Caged Exhaust Valve.
Valve Seat Insert.
79 Large Slow Speed Engine Exhaust Valves: B&W: KGF Type:
The figure on the next page shows an exhaust valve used in K-GF type B&W Engines. The exhaust valve housing is made of pearlitic cast iron and provided with a chamber for the cooling water. The valve housings are provided with loose valve seating 33 made of steel with stellite valve seating surfaces. The loose seating is fixed with the screws 32 and can easily be replaced when worn or burned through. The exhaust valves are mounted in the centre bores in the cylinder covers and tightened against seating at the bottom by means of studs, nuts and the sleeves 28. Seating at the assembly surfaces is achieved by grinding. A valve guide 8 is pressed into the valve housing are provided with bronze linings 21. The exhaust valve spindles 18 are produced from one-piece stainless steel forgings having high strength and corrosion Properties. The seating surface is stellite. The shield 7 is shrunk on to the valve spindle and serves to prevent gas leakage and oil-coke deposits on the valve spindles and guides. The ends of the spindles, at the point where they are activated by the rocker arms, are hard-faced with a material of great wear-resistance. The steel split ring 14 is mounted in a groove around the spindle and prevents the spindle from dropping down into the cylinder during possible replacement of a broken valve spring, which can be carried out without removing the exhaust valve. To prevent the escape of gas, the spindles are provided with a sealing ring 11. This ring is mounted in a recess in the bottom of the upper spindle guide. Exhaust valve closing is effected by means of two sets of coil springs 12 and 13 for each valve. The one set of springs is arranged con-centrically inside the other, and each set consists of two springs mounted end-to-end. In the middle of this spring assembly is mounted a spring guide 20 which is connected to the rocker arm by means of studs. The lower springs rest in the spring guide 9, while the top of the upper springs presses against the spring guide 16, which transfers the spring force to the valve spindles through the tapered and two-part locking rings 17. The two part tapered ring is forced into a recess in the valve spindle by the spring force and rests against two small conical surfaces at the top and bottom. As any play at the contact surfaces would very quickly give rise to wear, the fitting of the ring in both the spindle recess and in the spring guide 16 is carried out very accurately. The two halves in each valve are matched together and must not be exchanged with ring halves from other valves. The supply of cooling water to the exhaust valve-cooling chamber takes place through short elbow joints 3 which transfer the cooling water from the cooling water chamber in the cylinder covers. The elbow joints are fitted into holes in the top of the cylinder covers and sealing is provided by means of the rubber rings 2. The discharge of cooling water from each valve takes place through an opening 10 from where the water passes through a pipe provided with a thermometer and a vent cock. The cooling water is led from these pipes to a common discharge pipe. The exhaust valve housings are fitted with cleaning covers 31. Sketches: Page 68 for details: Page 57 for Operating System: Page 59 for B&M-K Type Valve :
80
Main Engine Exhaust Valve: B&W: K-GF Type.
81 Recent Design Exhaust Valves:
Modern exhaust valves with latest technology have been explained in pages 62, 63, and 64 under the heading “Air spring types of exhaust valves”. The exhaust valve is centrally located in the cylinder. It is forged from a Nimonic heat resistant alloy and is mounted in a cage with a bore cooled valve seat. It could rotate on its seat by the force acting on the vanes provided on the valve stem. This valve is hydraulically actuated from camshaft and has an air spring. Use of air spring contributes to a very smooth dynamic behavior of the whole valve system. Thus the valve gear failure by vibrations happening due to use of helical springs is minimised. Also, valve lift can be increased. Exhaust Valve Casing. Exhaust Valve. Cooling Water outlet.
Cooling Water in. Cooling Water In.
Sulzer: RTA: Engine Exhaust Valve Seat Arrangement with Cooling system.
Exhaust Valve and Valve Seat Temperature Gradient Curve for RTA: 58 Engine. Note: At R1 rating show an average of 360*C measured at the valve seating face with a fully symmetrical temperature distribution well below the critical 450*C for good service independent of fuel quality.
82 Hydraulic valve actuating system is well suited to long-stroke engines with mid h eight camshafts valve rotation is essential for reliable heavy fuel operation. The valve impeller on the stem is a simple and very effective means of rotating the valve. It helps to ensure a uniform seat temperature distribution and to keep the seat clean as well as dent and impression free as much as possible. Valve Material: Inlet Valves: Any good quality steel that can be heat-treated e.g. 3% Ni-steel. Exhaust Valves: Material Requirements. 1. The material should retain its greatest strength at high temperatures. 2. No tendencies to air harden. 3. Critical temperature above 800*C. 4. No tendency of high temperature scaling. 5. Hot and cold corrosion resistant. 6. Able to be forged and machined easily. 7. Capable of consistent and reliable heat treatment. Most diesel engines use an Austenitic heat-resisting alloy steel. The seating surface c an be stellited. Typical heat treatment: Heat up to 950*C and cool in air to give a Brinnel Hardness of 269. Surface Treatment: Surface treatment is frequently used to improve or modify valve steel characteristics. Chrome-cobalttungsten alloy available in various grades of h ardness is widely used. The hardness when deposited is in the order of 375 to 425 Brinnel. The valve head is treated to more than 430*C to reduce contraction stresses. The value face is now sweated by an oxyacetylene flame and the alloy deposited continually by welding (1.02 mm to 1.52 mm). Valve Seat Inserts: Alloy Irons, with high percentage of molybdenum and Chromium with a Brinnel number of Approx. 500 are best. Alloy steel with stellited seating surface are also in common use. The methods employed for fitting the inserts include screwing and shrinking. Valve Guides: Valve guides are mostly made of Cast Iron. To avoid scaling etc at high temperatures alloy Irons are preferred. Phosphor Bronze and Gun metal have also been successfully used. Alloy Iron guides with Bronze linings also are in common use. Valve Housing: Mostly made of pearlitic cast iron and provided w ith a chamber for cooling water. Valve problems and methods of increasing valve life, reducing overhaul frequency :
Most of the problems are the consequence of operating at increased output imposing greater mechanical and thermal stresses. The larger diesel engines often use heavy residual fuels, which contain relatively high ash and sulphur content together with traces of metal salts capable of causing exhaust valve corrosion or forming a brittle glass like deposit on the h ot valve seat of four stroke engines. Two-stroke engine runs much cooler, at the same time much longer service life requiring between overhauls. Modifications done on valves to over come all these problems: a) Exhaust valve material have improved strength and hardness at high temperatures. Material used is Austenitic steel. This material avoids cupping, cracking and seat deformation. b) Valve rotors (Rotocaps) will help minimize local over heating of valve seat and valve body.
83 c) Valve temperature can be reduced by: i) Uniform cooling of cylinder head around exhaust valve seating. ii) Using as wide valve seat as possible. iii) Radially thin valve seat insert. iv) Using as close a valve stem to guide clearance as possible. v) Well-cooled guide should be as close to the valve as possible. vi) Using bore-cooling system. vii) In some highly rated engines some extra cooling is always obtained by scavenge air and over lap valve timing. Modern highly rated engines using poor quality residual fuel oil have the following problem. Vanadium in the fuel forms vanadium pentoxide (V2O5) with a dew point of 690*C. Above and around this temperature vanadium pentoxide is a corrosive liquid and apart from corrosive effects, a number of complex sodium, vanadium salts can be formed by the combustion of residual fuel oil. These salts can adhere to the valve seat if the temperature is high. The acidic effect corrodes the valve seat. The deposit hinders the heat transfer from valve to the valve seat leading to high temperature condition of the valve body. Eventually, the deposits breakaway locally leading to local blow-by and valve burning. This is some times called “Hot corrosion” by engineers. Depositing a layer of ‘Stellite’ or ‘Dolite’ or similar metal combats seat “tramping” on the valve head seat and the valve seat inserts. The coating must be as thin as possible. Being brittle material thick coating will hinder heat transfer in this area leading to valve over heating and failure. There is a tendency in turbocharged engines, to find that the inlet valve and its seat wear excessively at a greater rate than the exhaust valve. This because of fretting caused by little oil in the valve/seat surface. The above problem can be solved by: a) Improved lubricating conditions. b) Decrease valve seat loading by reducing the seat angle from 45* to30*. c) Oil additives specially barium or calcium. d) Valve head rigidity. Valve bouncing can be reduced by the following modifications: 1. By increasing the number of springs instead of using one or two heavy springs of bigger diameter. In B&W: K-GF type engines six helical springs are used. 2. By means of hydraulic valve actuating gear as described in earlier part of this notes. 3. By improving the spring material. Typical spring material value may be: UTS 150 Kg/mm2 Yield point. 120 Kg/mm2 Elongation. 5% BHN 400 . Carbon. 45 to .55 . Mn. 30 to .60 . Si. 15 to .30 . Cr. 75 to 1.1 . V. 15 to .25 S. Not over .03 . P. Not over 035
84 Opposed piston engines are called ‘Cover less’ (Doxford Engine) or Semi-covered’ (Harland & Wolff Engine) if the top piston is smaller in diameter than the main piston. These engines have the following disadvantages: a) Complex running gear. b) Increased maintenance and survey. c) Piston cooling is necessary and more complicated than exhaust valve cooling. d) Long bearing span to provide room for side crank or eccentrics for driving the top piston. In view of above these engines are no more manufactured and have become obsolete. Shrouding of Valves: If the seat material is softer than the valve, shroud builds up in the seat and vice-versa, due to operation of the valve. Small amount of shroud can seriously affect the air or gas flow across the valve because of reduction in effective opening area and less streamlined flow. Shroud should be removed by seat cutter, machining or special grinding machine. Ceramic Coated Valves and Piston crowns : Some engine-builders feel that the peak is being reached in diesel engine development. To develop further, new materials may have to be developed. In this context, ceramics may well prove to be a main link in the chain to develop the next generation ultra-efficient diesel. Work on ceramics has been progressing in the USA by a company well known as an authorised repairer of many engine makes. AMT in Miami has experimented with bonding a type of ceramic to steel, aluminium and cast-iron. In particular, work has concen trated on coating piston beads and Valves. Results from operators using the company’s ‘Cerro-Plasmic HB’ coatings, have shown a number of significant improvements in engine performance.
For instance, results with a coated piston crown showed the metal temperature to be reduced by 194*F and crown underside temperature reduced by 135*F. On a six-cylinder engine, the effect on two cylinders with the pistons, cylinder heads and valves coated, was that fuel pump rack position could be reduced by 10.5%. In another example, an engine with coated piston and valves increased its output at 1800 rev/min from 1880 to 1940 bhp. In all cases, the company reports that bonding method proved successful and there wear no instances of coating failure. Currently, ship owners are conducting trials with coated valves in large B&W slow-speed diesels. Also, tests are being undertaken with coated hot gas inlets and outlets on BBC-VTR750 turbo-chargers. Results are extremely good and found that great reductions in corrosion and heat loss in the components. ********************************* Kv************************************** End of Engine Valves/BIT/AMET/BE/KV/May 2003.
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Cams: A cam is a device for transforming uniform rotary motion to intermittent reciprocating motion. An eccentric differs in providing a continuous reciprocating motion similar to that of a crank. The cam drive has been universally adopted because cams can be made in shapes that will give the desired rapid opening and closing of the valves not possible with the eccentric. Another advantage of cam drive is that it simplifies the reversing mechanism, by allowing endwise movement of camshaft. Cams are precisely positioned on this shaft, being shrunk on, integral or keyed for positive positioning. Some cams have provision for a limited range of adjustment, but in all cases the cam must be tightly secured before the engine can be operated. At present, even some of large engines have cams forged or cast integral with the camshaft and then machined, usually ground to the required exact shape. The advantage of such an integral camshaft is that if one valve of one cylinder is timed correctly, all the valves in all cylinders will be timed co rrectly. On the other hand, any change in timing will affect all valves and cylinders. In operation, cams are subjected to impact and are hardened in order to reduce wear. The shape of cam determines the points of opening and closing of the valve, the velocity of opening and closing, and the amount of the valve lifts from its seat. The desired cam shape or profile is obtained by accurate grinding. The grinding stone repeats the shape of a master cam and thus ensures accuracy of all cams. Following figures show some profile of cams:
Profiles of intake and exhaust cams. The difference in profiles of various cams can be noticed. In two stroke engines there is no cam for the intake, but if exhaust valves are used, at least two exhaust valves per cylinder are present which may be operated either by a common or two separate cams, so that the number of cams is about the same as in 4-stroke engine. When twin cams are fitted, in reversing the camshaft is made to slide laterally so that the ahead cam slips out of gear and the astern cam into gear on the same roller. Cam profile regulates opening and closing of valve. Timing of fuel valve opening and closing can be advanced or retarded by adjustable toe piece. Roller clearance also affects on fuel admission side, cam profile should show a gradual rise or slope, on closing side sharp drop. Thus usually the same cam is not equally suitable for both ahead and astern running.
Fuel-injection cams.
86 In case of Sulzer two cycle engines, same cam is used and the fuel timing for both ahead and astern running is unaltered.
The figures show the cams for SULZER and M.A.N. engines, the arrangements for cam ad justments can be noticed easily. Some Air-starting valve cams are shown below:
Camshafts: In most of the modern engines the cams and shaft are forged or cast in one piece. In some engines the camshaft is a straight round shaft and the cams are separate pieces, machined and keyed to the shaft. In some larger engines, the camshafts are made up of two or more sections bolted together by flanges with fitted reamed holes to assure accurate timing. Most camshafts are made of forged steel, usually of nickel-chromium alloy steel, and the larger camshafts are often bored hollow. They are heat-treated and cams are usually surface hardened. The camshafts are carried in plain bearings.
Camshaft bearings. To insure good support, the camshaft is usually carried by a series of camshaft bearings. One bearing being located between each pair of cylinders. Bearings may be either plain bushings or split sleeves. If plain bushings are used, their bores are larger than the cams, so that the camshaft may be withdrawn endwise. If split bushings are used, the camshaft may be removed sidewise from the engine.
87 Sulzer Engine Camshaft.
Slow Speed Engines. Camshaft bearings are usually of the Journal type, operating in white-metal lined or bronze bearing shells or bushes, and lubricated by oil supplied from and returning to the engine lubricating oil System. If there are chances of fuel oil contamination in the camshaft bearing or ca m lubrication then the lubricating oil system for camshaft will be an independent one.. Medium Speed Engines. Similar to slow speed or roller or needle-roller bearings. High Speed Engines. Needle-roller bearings. Camshaft Drive: In four-stroke engine, the camshaft speed must be exactly one-half the crankshaft speed, so that the camshaft makes one complete revolution while the crankshaft makes two. In two-stroke engine, camshaft speed is exactly same as the crankshaft speed. Because these speed relations must be exact, the connecting drive must be positive.
The drive arrangement used for particular engine-depends largely on where the camshaft is located and on whether an auxiliary camshaft (for fuel pumps, etc.) or a ‘power take’ off shaft is included. The camshaft may be located on the cylinder block, using short push rods, or at the cylinder head level, without push rods. For the sake of good appearance and cleanliness, the camshaft and push rods are often enclosed completely. This requires use of gears or chains. Many d rive arrangements are used.
88 Figures below show six typical layouts for camshaft drive .
Typical Camshaft Drives. Camshaft Gear Drive. Gears for camshaft drive must be accurately cut and heat-treated to resist wear. Helical teeth are preferred to spur teeth for greater quietness and more even transmission of power. A fiber or other nonmetallic gear is sometimes introduced in to the train of gears for the same reason.
MAN Engine: Cam Shaft Gear Train.
89 Depending on the number of cylinders of the engine, the camshaft ca mshaft drive is located either in the middle or at the after end of the engine (flywheel-end). The rotation of the crankshaft is transferred to the camshaft by way of four spur gears. In order to facilitate erection, the spur wheel on the crankshaft is of two-part design. The two intermediate gear wheels are supported on bearing pins, which are flanged on one and on to the column and on the other end held in position with clamp flanges. The axial clearances of the intermediate gear wheels are adjusted by machining the locating ring to the required size. The bearings of the intermediate wheels and the gear teeth are lubricated by b y the low-pressure oil system. The lubricating oil nozzles spray direct onto the gear teeth. These nozzles can be dismantled for cleaning. The screw and pipe connections in the gear wheel space must be secured by wire respective locking plates. Marking of the gear wheels: All gear wheels for camshaft drive are stamped ‘Flywheel end’ on after side. In case the drive has to be overhauled the gear wheels must therefore be refitted with the marks facing flywheel side. The two intermediate gear wheels are stamped ‘Upper wheel’ and ‘lower wheel’ markings are to be observed, when refitting these gear wheels. Further the intermediate gear wheel bearing pins and flanges are marked together with the column, which again must be observed in case of an overhaul. One should adhere to the engine manufacturer’s instructions for that engine while adjusting, overhauling and testing. Chain Drive: Multiple link chain drives are used on many engine designs. Following figures shows a multiple-strand silent chain drive used on a 4-stroke cycle, engine. Drive is from the crankshaft sprocket (12), passing the idler sprocket (10) and the chain ch ain takes up adjustment (9). The camshaft sprocket (7) and camshaft drive gear (8) engage the camshaft gear (4) and provide a 2 to 1 reduction in speed. Over speed Governor (1) is driven through pinion (2) from governor drive gear (3), which is turning at camshaft c amshaft speed, as is the fuel transfer pump (5). The drive chain (16) goes to the pump drive sprocket (15) and the pump drive gear (14) from which the water pump is driven through gear (13). Chain Drives must be kept tight. Hence, period check up on the idler adjustment is necessary. The chain is kept tant from the camshaft drive (7) across the pump drive (15) an d on to the crankshaft sprocket. Travel is clockwise, as viewed, with the crankshaft sprocket pulling the chain. If slack developed, timing changes would occur, and slapping of the loose chain ch ain result in serious fluctuation in timing.
Chain drive system of camshaft and auxiliary drives.
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MHI: UEC85LSII: Engine. Bore: 850mm; Stroke: 3,150; Power output: 5250PS/Cyl; RPM: 76. Camshaft gear train, Turning gear, Crank shaft, Fuel pump with cam and all components could be seen.
91 Chain Drive: B&W Engine: K-EF Type: The figure on the following page shows a chain drive for B&W Engine (K-EF Type). The camshaft (2) is driven from the chain wheel on crankshaft (6) by means of a chain drive d rive consisting of two identical roller chains (3) guided by means of two guide rails. The Tension of the chains can be adjusted by means of a tensioning arm (4). A separate chain transmission runs from the intermediate wheel (8) to the chain wheel (10), which drives the cylinder lubricators, generator, and starting air distributor. A further chain transmission (9) from the intermediate wheel drives the engine g overnor. Chains and guide rails are lubricated through spray nozzles mounted between the guide rails and the chain wheels. Cams and couplings are shrunk on to camshaft and can be adjusted or removed by means of hydraulic tools. After adjustment on the test bed, the engine is provided with a series of marks and corresponding. Pin gauges enabling checks to be made for correct adjustment.
Chain Drive of B&W Engine (K-EF Type).
92 Roller Chains: Design: Chains are manufactured to comply with BS 228 or ISO R606 standards. This ensures interchangeability between chain makes and provides for minimum breaking load. The factor of safety employed is high and for camshaft chains this is about 50. For chains up to 64mm pitch, standard chains are used. Above this however, greater accuracy is paid to: a) Link pitch: By grinding and horning the bores of bushes in assembled links. b) Surface finish: On links, pins and rollers, to improve resistance to fatigue.
Details of Roller Chain.
Provided the chain is adjusted correctly and dampers provided with a generous supply of lubricant, the chain drive efficiency may be as high as 99% and this efficiency varies little during its operating life. The Chain Pitch : This is determined by the crankshaft speed and the shaft diameter. The Sprocket Size: This is governed by the space available. Normal and Peak Loads : The maximum allowable bearing pressure between pin and bush determines these loads, after accounting for Factor of Safety (governs the diameter and length of bearing). The anticipated fatigue life for a two-stroke diesel engine chain may be about 80,000 hours, during which time the chain will have worn up to 0.7% of its normal length (most of the increase in length is due to wear). This is about 30% of the chain rated wear capacity. 2% elongation is considered the end of useful life, but to ensure reliability replacement is required at 1.5%. Adjustment: Excessive tension results in overloading of the bearing surfaces, both on the chain and shaft, causing unnecessarily rapid wear. Insufficient tension results in excessive vibration, slap and backlash, all of which can cause higher than normal peak loads. This may also cause damage to the Neoprene faced dampers. Incorrect tension will result in incorrect fuel injection timing. Too tight: Causes advancement of injection. Too Slack : Causes retardation of injection.
93 Chain adjustment is measured on the longest length between two wheels, preferably the strand containing the adjusting jockey wheel. Turn the engine on the turning gear, so that the strand on which the adjustment has to be measured, is on the slack side of the drive. Using the adjusting jockey, tighten the chain until the total movement at mid point in the strand, when moved by hand, is equal to ONE PITCH when measured normal to the strand.
Adjusting jockey adjusting system used in B&W Engine Chain Drive : Adjustment of chain tension: 1. Turn engine so as to slacken the longest free length of the chain. 2. At middle of the longest free end pull the chain away from the guide bar. In this manner it shall normally be possible to move the chain a distance corresponding to approximately one half of the chain link (pitch) away from the guide bar. If the chain is too slack or too tight, adjust the chain tension. 3. Loosen nut locking. Loosen nuts A –B – C – D. 4. Tighten nut C until load set length of the spring reduced to ‘x’ mm (as per instruction manual of the engine). 5. Turn nut B to sit on lock bar.
94 6. Turn nut A and lock. 7. Now the chain is tensioned by the spring tension and held in position. 8. Tighten nut C till the thrust washer of the spring bears against the distance pipe of the chain tightening bolt. 9. Tighten nut D and lock. 10. Now the chain is tensioned to the correct value and the spring is gagged. 11. Repeat item 2, and check the tension is correct. Inspect the Neoprene facing on the dampers and replace if required. After the chain is adjusted then adjust the dampers. On the taught side of the chain, measure accurately over the longest practical length between bearing pin centers, and calculate percentage elongation from: Percentage Elongation = 100 (M—XP) XP M = Length measured in cm. X = Number of link pitches in the length M. P = Pitch of the chain in mm. Faults that can be found in chain drive: a) Crack or breakage of roller or side plates. b) Stiff joints. c) Seized rollers. d) Wear of ends of bearing pins. e) Scoring on inner surface of links, due to misalignment. f) Damage to ends of the link plates. Some rollers are heat treated to provide durability, rather than extreme hardness, so that some superficial marking may be present but will not affect performance.
Valve Gear Maintenance: To keep the valve gear in good operating condition, all components of the valve gear are maintained according to a maintenance schedule, which depends on the engine, operating conditions and fuels used. Though the checking, dismantling and overhauling is to be done whenever necessary, yet following schedule that is typical of a B&W KGF type engine. Exhaust Valve: i) Cheek and adjustment ----------------- 500 - 1000 hours. ii) Overhauling ------------------------------ 8000 hours. iii) Overhaul of hydraulic oil cylinder ----- 8000 hours. iv) Inspection of roller guide ---------------- 500 - 1000 hours. Chain Drive: 1. Inspection of chain drive, chain wheels, Rubber guide bars, bolt connections, L.O. System, 500 1000 hours for new engine, and for normal engine 8000 hours. 2. Check of Chain Tension, 500 - 1000 hours for new engine and normal 4000hours. 3. Inspection of running Surface of cams around 8000 hours. 4. Check tightening of camshaft bearing and coupling bolts 500 - 1000 hours for new engine and normal 4000 hours. 5. Inspect ion of camshaft bearings done at 4 yearly surveys. 6. Replacement of cams on camshaft only when necessary. 7. Adjustments of camshaft due to chain wear only when necessary. 8. Check control gear timings 500 to 1000 hours for new engine and when necessary.
95 The abovementioned are figures for a typical slow speed large Marine Diesel engine. However, for medium speed engines, the figures might vary, as given below. Exhaust Valve For heavy oil operation -------------------------- 500 - 1000 hours. For Light Fuel operation ------------------------- 2000 - 8000 hours. Inspection Procedure and Criteria. 1. Exhaust valves should be examined for carbon deposits, cracks, scores and pitting, burning or abrasions of valve and valve seats. 2. Valve and valve seats are to be examined for ‘shrouding’ or ‘pocketing’ which can take place as a result of excessive wear or frequent regrinding. 3. Valve stem diameter and bore of valve guide should be measured. 4. Valve stem should be tested for straightness by rolling on a surface table or on a centre lathe. 5. Valve springs should be examined to see that they are free from pitting, cracking, scoring and corrosion. The free length of spring should be checked. 6. Cams, cam followers, tappets and all the valve operating gear parts should be inspected to see that there are no cracks, flaking, scores or burrs, and to see that wear has not reached a stage which would prevent the accurate setting of clearances. 7. Push rods should be checked for straightness. 8. In camshaft gear drive, the teeth should be checked for pitting, abrasion, deformation and backlash. 9. Camshaft chain drives: a) Rollers and links should be inspected for dents, pitting, abrasion and corrosion marks. b) Bearing pins and links should be inspected for cracks and fracture. c) Chain should be inspected for 'stretch'. d) Chain tension should be checked. Allowable Limits . a) Chain ‘Stretch’ 2% max. Usually 1% for 10 links. b) Chain Tension (sag), ½ link min. 1 link max. c) Gear Back Lash, 0.25 mm (normal) 150% (max.). d) Tappet Clearance (Typical): i) Exhaust Valve -------------- 0 .6 mm, 1.0 mm. max. ii) Inlet Valve ------------------ 0 .5 mm, 1.0 mm. max. Valve Bouncing.
The valves are opened by the rocker arm pushing on the valve stem tip, but they are closed by two or sometimes three coil springs. Multiple springs are used, not only because of the additional force they
96 provide but because of a characteristic of the spring known as resonance. Valve opening is done with a series of impulses, and at a certain engine RPM, these impulses will occur at the resonant frequency o f the spring. When this happens, the spring loses its effectiveness and allows the valve to float. To prevent this floating condition, two or more springs having different pitch, diameter, and wire size are used; and because of their different configuration, they have different resonant frequencies, so the engine can operate throughout its full range of RPM without valve float problems. Valve bouncing is a phenomenon, which happens due to sudden release of compression energy of the spring at the moment of valve closure due to tappet clearance allowance. The valve would jump up and down on seat before closing shut finally. Spring surge (resonant vibration) at this moment also tends to help generate valve bouncing. Valve bouncing can be reduced by: (i) Increasing number of springs. (ii) Improving spring material. (iii) Using inner and outer springs, right and left handed. (iv) Using hydraulic valve actuating gear. (v) Using air spring. Tappet Clearance. The tappet clearance, which is the clearance between push rod roller and cam base circle at valve closure in cold engine, is usually measured between rocker arm and valve spindle top. Tappet clearances are necessary to allow for thermal expansion of valve spindles. Clearances should normally be set while the engine is cold and the cam follower is off the cam peak. Wear of valve gear tends to increase clearances. Excessive tappet clearance will cause the valve to open late and close early and will reduce valve lift. It will also cause noise and damage working surfaces. Insufficient clearance will cause the valve to open early and close late with increased valve lift. It may prevent the valve from closing completely causing burning of valve, low compression etc. Vibration in chain drive. Following reasons can cause vibrations: (i) Excessive stretch. (ii) Misalignment of wheels. (iii) Error of pitch of chain or chain wheels. (iv) Unmatched chains. (v) Stiff links. (vi) Twined or bent chain. (vii) Improper balancing of engine power. The various reasons must be dealt with at earliest to avoid failure of chain drives. *********************************** Kv**************************************** References: 1. Diesel Engine Operation and maintenance by Mr. V. L. Maleev. 2. Diesel and High Compression Gas Engines by Mr. Edger J Kates. 3. Instructions for K-GF Type Engines by B&W Engineering. 4. Diesel Motor Ships Engines by Christen Khak. 5. Instruction Manual for Sulzer RND-M Engines and B&W K-EF and K-GF Engines. ***********************Kv**************************** End of Engine Valve Gear and Valves/BIT/AMET/BE/KV/May 2003.
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6. Cylinder Liners. The liner is regarded as a thick cylinder under the action of a fluid pressure. The material is to be strong to with stand the tensile loop stress. The interior surface forms the wall of the combustion chamber. There is a considerable temperature stress on the material of the body. The two surfaces tend to expand at differential rates for being at different temperatures. But the body prevents their free expansion this causes a stress to be set up. The liner is secured at the top flange by cover studs. A compressive stress is set up on this part of the liner. Besides, the surface of the liner needs to be resistant to wear and corrosion. The choice of material must also consider such factor as its amenability to various metalworking and forming processes such as casting, machining, surface treatment etc. Cylinder Liner. Wet Type. Slow and Medium Speed Engines. Cooled by Cooling Water.
Dry Type. Small Engines. Air Cooled (Fined liners used).
Liner should withstand high mechanical load (Gas pressure) and thermal load caused by heat flow. To achieve above olden days they used one, two or three piece liner construction. Liners are cylindrical construction fixed at one point on the top and expand at the other point downwards. Mechanical Stress. Maximum pressure: Firing pressure: 45 - 50bar for non-supercharged engine and around 85 - 100bar in supercharged engines.
This pressure produces circumferential stress (Hoop stress) and longitudinal stress. Hoop Stress = 2 x Longitudinal stress. So Hoop stress is only considered. Hoop Stress = P x D Where P: Cylinder pressure. D: Cylinder diameter. T: Cylinder thickness. 2T Hoop stress will increase if bore size and firing pressure increases. Thermal Stress. Resistance to heat flow through the metal of the liner produces a temperature gradient across the metal. Due above expansion will be different at the inner and outer wall of the liner.
∆T = Temperature gradient. Stress δt ∞ ∆T. Temperature gradient will increase as metal thickness increases.
98 Increase in metal thickness is good for withstanding mechanical stress but bad for thermal stress.
So liner design was a complex issue for long time, but with the advent of bore cooling, availability of new-alloyed material and new machining technology the above problem to great extent has been solved.
Comparison of stresses in a liner.
99 Material. Cast iron is generally regarded as a suitable material for construction of diesel engine cylinder liner. In order to improve strength and induce specific desirable properties such as strength and surface properties, cast iron is alloyed with the inclusion of small quantities of nickel, chromium, molybdenum, vanadium, copper etc. Such inclusions refine the grain structure of the material. The total percentages of alloying inclusions should not exceed beyond 5%. Good quality ‘Pearlitic Grey Cast Iron’ consist of the following alloying material: Carbon: 3 to 3.4%. Its graphite flakes assist lubrication. Silicon: 1 to 2.0%. Improves fluidity and graphite formation. Manganese: 0.6 - 0.8% Phosphorous: 0.5% maximum. Reduces porosity Vanadium: 0.15%. Refines grain structure Titanium: 0.05%. Improves strength Specification Ultimate tensile strength: 200 Mn/mm2. Ultimate bending strength: 520 Mn/mm2. Ultimate compressive strength: 900 Mn/mm2. Brinell Hardness: 180 - 220 HB. Ductility: 1 to 5% Elongation. Reasons for using Cast Iron: 1. Can be cast in to intricate shapes. 2. Has good wear resistance: a) Due to large surface of irregular shaped graphite flakes. b) Due to semi-porous surface holding oil pockets. 3. Possesses good thermal conductivity. 4. Damps out vibrations due to rapid combustion. 5. Cheap material.
One of the advantages of cast iron is its excellent performance as a lubricated sliding surface. The presence of graphite in its microstructure is mainly responsible for this. During the running in period, fresh surface containing graphite is exposed, leaving minute pores on the working surface. These cavities store oil. The graphite also acts, as a lubricant in the dry state. The porous structure of cast iron is of particular advantage in the liner operation as a remedy against intensive galling action. When scuffing and scoring takes place, the interaction between two rubbing surfaces tears the metal at the surface, exposing fresh graphite, which acts as lubricant. This minimises the use of extreme condition i.e. seizure. The porous surface of cast iron helps to retain lubricating oil in the minute cavities, which will readily ooze out to keep the surface wetted in molecular layer of lubricating oil. As the liners are put to more severe conditions of working as regards temperature, pressure, with oil ash and in corrosive environment the need for a harder wear and corrosive resistant surface was realised. Some other methods and materials were tried. Harder surfaces of cast iron or steel were not successful as it failed to meet the demand of the service. Austenite cast iron, although provided adequate protection against abrasion and corrosion, failed to produce a mated gas sealed surface with the piston ring. Hardened steel liners did not protect the surface, as surface was quickly dried up due to lack of retentivity of oil. Low alloyed cast iron, therefore, remains the choicest material for liner construction. The surface is lapped with a coarse hone and this together with a surface, which is naturally porous keeps lubricating oil adhered to it.
100 Some liner sketches with reference to type of scavenging.
Simple loop-flow scavenging.
Loop-flow opposed-port scavenging.
Uniflow valve scavenging.
Sulzer: RD Liner .
Full loop-flow port scavenging.
Uniflow port scavenging.
Sulzer: RND Liner.
101
B&W: EF Type Engine Liner (See bore cooling arrangement).
Sulzer: RND Engine Liner (See bore cooling arrangement above the liner sketch).
102 Sulzer: RN-M Engine Liner Port Openings.
For the loop-scavenging each scavenge port and exhaust port has specific design to enhance scavenging process. 1: Exhaust ports. 2: Scavenge ports. 3: Lubricating oil quill fitting space.
103
Doxford Opposed Piston Engine: 3- Piece Liner top two sketch: Below is bore cooled liner.
104 Sulzer: RND 90: Bore cooled Liner: Below RN 90 & RN90M Liner Stress shown.
105
Latest Sulzer RTA Engine Liner with water guide ring . 1: ‘0’ Ring. 2: Cylinder liner. 3 & 3a: ‘0’ Ring. 4: Water guide ring. 5: SIPWA pick-up or plug. 6: ‘0’ Ring. 7: Water guide jacket. D, D1: Leakage drain. L: Water transfer hole. KB, KB1: Check holes for leakage water. KW: Cooling water space. LR: Empty space. TB: Cooling drillings. SS: Scavenge Ports. ********************** Kv**************************
106
Improvements for Sulzer RND-M engines due to Bore-cooled cylinder cover:
Its introduction was prepared by: 1. Extensive Finite Element analysis of mechanical and thermal loading (above figure shows the appropriate FE- network. The cover was analysed in combination with the cylinder liner and part of the jacket, in order to assure proper boundary conditions). 2. Testing of prototype cover on test engine with experimental measurements of strains and temperatures and investigations regarding bore holes clogging. 3. In-service testing of a prototype cover on a marine engine with regular checking. 4. Strain and temperature measurements on the first series covers. Liner Cooling: With reference to all the liner figures shown in previous pages: The cylinder liner is of special close grain cast iron. It is fresh water-cooled and the division bars between the exhaust parts are hollow and also water-cooled. In order to reduce the thermal stresses of the cylinder liners, particularly for the really large bore engines, the so-called bore cooling system has been applied in the upper part of the cylinder liner. This is a system of holes drilled tangentially at an oblique angle into the cylinder wall so that the cooling is led as close as possible to the hot liner wall of the liner, thus the temperature of the running surface is reduced. The cold part of the cylinder outside the bores is relatively thick and embraces the inner portion like a shrunk-on ring and so reduces the pulsating stresses caused by the gas pressure. The cooling water is collected in the water guide ring and left out of the cylinder cover. Causes of cylinder liner wear. The causes and prevention of cylinder liner, piston ring and ring groove wear, are one of the most controversial subjects connected with diesel engines. Cylinder wear is also one of the most important factors governing maintenance costs and repair work on engines. The combination of very highly rated two stroke engines and the use of residual fuels have aggravated this problem.
107 The main causes of cylinder liner wear may be broadly classified as follows: 1. Normal frictional wear: Caused by metal-to-metal contact under boundary lubrication conditions. This may be aggravated by oil with inadequate load carrying properties, too low a viscosity or an inadequate oil supply. 2. Abrasive wear: Caused by hard foreign matter introduced with the induction air and by hard particles of carbon, asphalt, wear debris and ash forming constituents present either in the fuel or lubricating oil. Frictional and abrasive wear are often linked together under the general description of abrasive wear. Sources of hard particles are air borne dust, ash in the fuel and carbon from combustion. To minimize this type of wear one have to keep engine air filters clean, keep scavenge space clean, effective centrifuging fuel oil, fuel pump and injector in good condition, fuel timing correct and keeping the fuel temperature correct thus achieving good combustion. 3. Corrosive wear : Caused by acidic products of combustion, especially condensed sulphur oxides. This is especially troublesome when burning high sulphur content residual fuels. Sulphur burns Combines with oxygen – Produce heat – Sulphur dioxide. Hydrogen + Oxygen gives water H2O – Gives out heat. Sulphur dioxide + Water = Sulphurus acid. Sulphur dioxide + Oxygen (Catalyst - vanadium pentoxide) SO3. SO 3 + H 2O → H2SO4. This H2SO4 in dilute condition causes all the damage. The dew point of this acid is around 110*C to 180*C depending on concentration. This temperature condition does exist in the liner face so results in corrosion. To minmise this corrosive wear one should use alkaline based cylinder oil. Fuel with 4 to 5% sulphur should use alkaline oil with Total Basicity Number 70. Fuel with 1% sulphur should use alkaline oil with 20 to 30 TBN cylinder oil. Alkalinity in the oil neutralizes the acid thus prevents corrosion. This is only effective when the correct amount oil is fed in to the liner and the correct number TBN oil is used for that particular fuel oil sulphur content. If either the feed rate or the TBN number is not correct for that amount of sulphur content in the fuel, particular wear pattern will occur in the liner, generally termed as ‘Clover Leafing’. Lubricating oil supply via quill.
Lubricating oil supply. Sketch showing the ‘clover Leafing’ pattern.
108 Reasons for the above type of wear: a) Insufficient oil supply. b) Inadequate TBN number oil. c) Failure of oil distribution in liner face. d) Jacket cooling water temperature too low. e) Turbo-charger air cooler not properly controlled. Supplying cold air to scavenge space. f) Jacket cooling water outlet temperature low, keeping the liner surface temperature around dew point of the acid. Keep the cooling water temperature as high as possible with out forming vapour pockets and spoil rubber seal rings on liner. g) Keep quantity of starting air to the engine while maneuvering as possible. h) To protect ‘Waste Heat System’ always bypassed when the engine on light load. 4. Scuffing wear: Scuffing wear occurs when lubricant oil film fails to separate the ring face on the liner face and subsequent contact while operating, friction heat is generated to such an extent that “Micro-welding’ or ‘Micro-seizer’ takes place. The reasons for this is as follows: 1. Liner surface too smooth therefore retains too little oil on its face. 2. Water on surface of the liner repelling the oil film formation. 3. Blow by of combustion gases across the ring sealing face breaking the oil film formation. 4. Poor oil distribution on the liner surface by the quills and/or the gutter groves. 5. Deposit on piston top landing absorbing the lubricating oil. Liner Wear Pattern. Maximum normal wear occurs at the top of the liner in a Port-Starboard direction and around scavenge ports. The reasons are: 1. High temperature prevalent at the top dead center reduces oil viscosity and therefore oil thickness. 2. High gas pressure increases ring loading causing penetration of oil film. 3. Slow movement of piston allows oil wedge to break down (Reversal of piston movement). 4. Movement of ship is maximum in Port – Starboard side than Forward -Aft side causing excess wear. 5. High temperature makes oil film less resistance to acid penetration. Wear Rate. For Two Strake engine a wear rate of 0.1 mm per 1000 hour is normal. Maximum acceptable rate is 0.25 mm per 1000 hr. Maximum total wear is acceptable is 0 .75% of bore. Useful life span: 70,000 - 80,000 hours For Four Stroke engine the wear rate is 0.02 mm per 1000 hours. Faults (See sketch page 104). Crack across liner flange due to uneven and excessive tightening of cylinder cover studs. Hoop stress crack due to poor liner support. Circumferential crack along wear ridge due to stress concentration or more likely new rings hitting ridges. 4. ‘Star’ or ‘Craze’ cracks caused by flame impingement. 5. Star cracks around lubricating quill due to water leaks. 6. Cracks across port bars due to over loading, poor cooling, scavenge fire, poor fitting of liner in its position and usage of wrong ‘O’ rings on the liner.
Liner 1. 2. 3.
109 Liner Faults.
Causes of Excessive Wear.
When excessive wear of piston rings and cylinder liner occurs, the cause is usually one o r more of the following factors: 1. Improper running in. 2. Misalignment of the pistons, or distortion of cylinders, preventing bedding-in of pistons and cylinders. 3. Inadequate oil supply, or unsatisfactory arrangements for lubricant type and quality. 4. Lubricating oil too low in viscosity, or too low in alkalinity (Total Base Number -TBN). 5. Piston ring clearances incorrect. 6. Unsuitable cylinder liner material or unsuitable piston ring material or hardness factor between ring material and liner material not compatible. 7. Contamination of lubricating oil, by extraneous abrasive material. 8. Cylinder wall temperatures too high or too low. 9. Overloading the engine. 10. Scavenge air temperature too low, especially in humid climates, resulting in excessive quantities of condensed water entering the cylinder thus aiding acidic formation. 11. Inefficient combustion, promoting deposit formation degradation of the lubricating oil. 12. Use of a low-sulphur fuel (containing less than say 1% sulphur) in conjunction with highly alkaline cylinder oil - this particular fuel/lubricant combination is not necessarily harmful but cylinders wear due acidic wear and/or scuffing. Low Sulphur fuel oil with high T.B.N lubricating oil will leave balance alkaline salts which due to heat will form in to abrasive material and mixing with lubricating oil will score the liner leading to abrasive wear. Cylinder Liner Wear: (See figure above). Liner shown is a single acting type. a) Travel of the top ring b) Travel of the bottom ring.
110
Vertical Wear Profile of Liner.
111
Specimen Engine Cylinder Liner Gauging Chart (Chart given is for Opposed Piston Engine).
112 Liner Lubrication: The inner surface of the cylinder liner is lubricated through quills, which are equipped with non-return valves. In order to prevent any leakages into the fresh water-cooling system, outside sleeves have been arra1nged around the quills, which are sealed by rubber joints. Any oil leakages from the quill as well as water leakage from the cooling spaces will pass to the outside. The quills may be inspected with out draining off the jacket cooling water. Sulzer RD and RND Engines development of cylinder lubricating oil stud or quill.
Requirements of a cylinder liner/rings lubricant. It may be stated that the essential properties of a good cylinder lubricant are: 1. It must reduce sliding friction between rings and liner to a minimum, thereby minimising metalto-metal contact and frictional wear. 2. It must possess adequate viscosity at high working temperatures and still be sufficiently fluid to spread rapidly ever the entire working surface to form a good absorbed oil film. 3. It must form an effective seal in conjunction with the piston rings, preventing as ‘blow-by’ burning away of the oil film and lack of compression. 4. It must burn cleanly, leaving as little and as soft a deposit possible. This is especially true of high additive content oils as unsuitable types can form objectionable ash deposits. 5. It must effectively prevent the built-up of deposits in the ring zone and in the parts of port exhausted 2-stroke engines. 6. It must effectively neutralize the corrosive effects of numerous acids formed during combustion of the fuel. Lubrication is difficult to achieve because: a) Piston direction changes every stroke. b) In two stroke engine no non-working stroke. c) In diaphragm engine no oil is returned. Therefore supply is limited and consumption is controlled. So no cooling effect for the liner.
113 d) Piston and rings distort due to gas pressure and temperature. e) All fuels contain abrasive contaminants. f) Liner temperature (working surface) varies causing change in viscosity. For two stroke engine one quill fitted per 300 mm circumference or less. Total per cylinder is abo ut 6 to 10 numbers. Best position for the quill in line with 1st and 2nd ring position with the piston at the top dead center.
Sight Glass Fluids: a) Water: b) Calcium Nitrate solution: c) 75% Glycerin and water.
Early Typical Cylinder Liner Lubricator.
The lubricator shown above is still in use in older engines. Modern engines are fitted with multi plunger lubricator pumps as shown in the next page. These pumps are required to control the feed rate accurately to ensure proper lubrication of the liner and also to keep the alkalinity value correct such that all acid is nutralised and there is no alkaline salt left in the oil after it has lubricated.
114
1. Dust cover. 2. set screw. 3. Housing cover. 4. Upper cover. 5. Operating twisted disc. 6. Control twisted disc. 7. Main plunger. 8. Control plunger. 9. Pump element. 10. Base plate. 11. Lubricator housing. 12. Worm gear. 13. Pump shaft. 15. Drive shaft. 16. Pipe connection to quill. A: Suction pipe. B: Delivery pipe.
I.V.O. Cylinder Lubricator.
Accumulator Cylinder Lubrication System. This system could control cylinder oil feed with reference to engine load change and also could control quantity of oil feed with respect to fuel sulphur content/ alkalinity value.
115 Matching of Cylinder oil Alkalinity to Fuel Sulphur Content for Two Stroke Diesel Engine. B&W Engine Manufacturer’s Recommendation.
The above chart helps operating engineers to select the correct TBN cylinder oil along with the correct quantity of oil to be fed to the liner depending on the sulphur content of the fuel oil burnt for that voyage. The quantity of oil has to be controlled for change in engine load and while manuvering. Running in of new liners or running in of new piston rings should be done with great care. The engine manufacturers use various methods, which are given in the next page.
116 Maximum Liner wear rate mm/1000 hours.
↑
Maximum Cylinder Liner Wear Rate/ Engine Running Hours.
Maximum wear rate mm/1000 hours. ↑
→
Cylinder oil feed rate: g/ bhp /hr.
→
Effect of Cylinder oil feed on liner wear rate in Turbo-charged Uniflow Engine.
117 Feed rate increase above Normal Maximum
↑
→ Running Hours.→ Cylinder Liner Running in Method No: 1: Cylinder oil feed rate adjustment during running in.
Cylinder Liner running method No: 2: Progressive Increase of Engine Speed and Cylinder Lubricating Oil Alkalinity during running in Period. ******************************* Kv******************************** End of Cylinder Liners/BIT/AMET/BE/KV/June 2003.
End of Part One of Marine Diesel Engines.
118
Question Bank for Marine Internal Combustion Engineering: I & II. For BE (Marine Engineering) Cadets.
1. 2. 3. 4. 5. 6.
What is the function of main engine bedplate and frames? What are the advantages of supercharging an internal combustion engine? Sketch and describe the cycle of operation of a four-stroke engine with a timing diagram. How are the main engine tie rods tightened? Explain why tie rods are required? Which internal combustion engines do not require tie rods? Write short notes on: (a) Jackets and liners of a large powered engine. (b) Pistons of four-stroke medium speed engine. (c) Cross heads. (d) Bedplates of large main engine. 7. Sketch and describe the different types of scavenging and indicate their advantages and disadvantages. 8. Sketch and describe a turbo charger suitable for large slow speed engine. 9. Sketch and describe the operation of crankcase explosion relief valve. 10. What are the differences between two and four stroke engines? 11. Explain in detail three types of cylinder liner wear? 12. Name all the parts fitted to a four stroke engine cylinder head and explain their function. 13. What are the types of scavenging used in large two stroke engines? 14. What are the types of supercharging used in diesel engines? 15. Sketch and describe the working cycle of a two-stroke engine with timing diagram. 16. Describe in detail the differences between slow speed, medium speed and high-speed diesel engines. 17. a) Explain why the air coolers are fitted between the turbo-charger and the scavenge manifold of the engine. b) Sketch a water separator fitted after the cooler. c) Describe the problems that would occur if the tubes of the cooler became fouled. d) Describe the problems the unit will face if the scavenge air is cooled to a very low value. 18. Sketch and describe pulse and constant pressure types of supercharging indicating advantages and disadvantages. 19. Sketch and describe in detail the jacket cooling water system suitable for a large marine diesel engine. 20. Discuss in detail the causes of scavenger fires. How are they detected and prevented? 21. What method is adopted for tensioning the tie rod in Diesel engines and why? 22. What are the reasons for an overheated crankshaft main bearing? 23. What are the reasons for an overheated main engine crosshead bearing? 24. Which method of scavenging gives maximum scavenging efficiency and why? 25. What is the need for grouping the exhausts in a multi-cylinder diesel engine? 26. What type of compressor is used for turbo supercharging? 27. What do you understand by ‘solid’ injection system? 28. Why does increasing compression ratio reduce delay period in a diesel engine? 29. What is the first step taken when oil moist detector indicates a warning signal? 30. What amount of heat (in percentage) is lost through exhaust gas and how? In modern ship power plant explain how this heat lost is recovered to enhance efficiency of the plant? Kv/ICE/BE/QB/03.
*** 2.
119 Page: 2.
31. Draw the timing diagram of a two stroke supercharged engine and explain the significance of a) Exhaust port or valve opening early or late. b) Scavenge port opening early or late. c) Fuel injection early or late. 32. Discuss the development of a modern piston of a marine diesel engine with reference to a) Material of construction. b) Thermal deformation. c) Cooling medium. 33. Compare the uniflow scavenging with reversed flow or loop scavenging pertaining to diesel engines. 34. Explain the need and utility of after cooler in a turbo-charging system of a marine diesel engine. 35. Discuss supercharging of a Marine Diesel engine by constant pressure and pressure pulse systems. Comment on their relative advantages and disadvantages. 36. Enumerate the troubles that might arise due to improper cooling of engines. 37. a) Discuss the cause and origins of scavenge fires. b) How are they detected? c) What precautions are taken to prevent damage? 38. Why an engine is called an Internal Combustion Engine? 39. Describe a Trunk Piston Diesel Engine? 40. Name and describe two principal constructional differences between a uniflow type large slow speed engine and a similar loop scavenging Marine Diesel Engines. 41. What are bedplate and frames pertaining to an internal combustion engine? 42. Draw a valve Timing Diagram of a 4 stroke Diesel Engine. 43. What purposes does Scavenging process serve in a 2-Stroke Diesel Engine? 44. What is a Supercharged Engine? 45. What is the purpose of Supercharging? 46. Sketch and describe with the help of a line diagram a Turbo Charger suitable for a large diesel engine. 47. What is the significance of turbulence during combustion process? 48. Indicate two main requirements of a fuel Injector. 49. What are the four phases of combustion and explain each one. 50. Why does a piston ring break? 51. Explain in detail why are A-frames required for Marine Diesel engines? 52. What are the advantages of 4-stroke cycle engine over 2-stroke cycle engine with reference to scavenging? 53. Draw the pressure-crank angle diagram during scavenging process of a 2-stroke cycle engine. 54. How does a scavenge fire manifest itself and how would you avoid it? 55. Why is oil mist detection important in a diesel engine? 56. Discuss the valve timings of a 4-stroke cycle engine for Marine applications and the significance of various events. 57. Discuss the valve timings of a large 2-stroke engine operating on uniflow scavenging and loop scavenging system. 58. Discuss the constructional details of a cross head as fitted to a Marine Diesel engine.
Kv/ICE/BE/QB/03.
120 Page: 3.
59. Explain the requirements and the special features of a Marine Diesel Engine with long stroke fitted for main propulsion. 60. Discuss the manufacturing details of connecting rods employed for Marine Engines. 61. Discuss the types of crankshafts manufactured for use on a large slow speed diesel engine. 62. Discuss the advantages of constant pressure type turbo charging over pulse method. 63. Why is uniflow scavenging preferred over other methods of scavenging? Compare their relative performance in terms of scavenging efficiency. 64. With the help of a neat sketch, explain the principle of working of turbo compressor. 65. What are the requirements to be satisfied by the fuel injection system when residual fuel is burned in Marine Diesel engines? 66. Discuss the causes and prevention of crank case explosions in Marine Engines. 67. Discuss in detail combustion process in a Diesel engine. 68. Clearly explain with the help of neat diagrams the theoretical working cycles of 4-stroke and 2stroke Diesel Engines. 69. Explain in detail the jacket water-cooling system for a Marine Diesel Engine, which uses telescopic pipes out side the engine casing. 70. Explain with the help of a neat sketch the operation of a Rotary Scavenging Pump. 71. Explain the Uniflow Scavenging in an Opposed Piston Engine. 72. Clearly explain the different types of combustion chambers used for fuel mixing with air in a Diesel Engine. 73. Explain in detail how a fuel is prepared for efficient combustion. 74. What is the nature of the stressing of the ‘big end bearing bolts’ of a 2 stroke cycle engine and how does this influence the material selection for the bolts? 75. What is the purpose of cross head in a large marine diesel engine? State two important reasons why this is incorporated in such engines. 76. Name and describe two essential safety devices fitted to the crankcase of a large marine diesel engine. 77. What are the mountings fitted on a cylinder head of a diesel engine and explain their purpose. 78. What is the safety feature and device employed in a diesel engine cylinder head and what does it protect the cylinders head from? 79. A large marine diesel engine (usually of the 2 cycle type) is constructed as an assembly and not as a mono block. What is the single most important component in such an engine that holds all these together, also relieving these of the combustion loads? 80. Sketch and describe a fuel cam, exhaust valve cam and an air-starting cam. 81. Where leaf springs are used in a centrifugal clutch what is the preferred material and strength? 82. Describe a centrifugal coupling employing slippers and its role in power transmission. 83. What will happen if a solid non-ferrous bush bearing in a rigid housing begins to run dry and why? 84. Why is the surface finish of a journal in a bearing very important? 85. What conditions predetermine the type of bearing such as angular contact and spherical roller bearings to be employed? 86. What is type of damage that can occur in the rolling contact bearings of a large Turbo charger? How is it obviated?
Kv/ICE/BE/QB/03
