DEPARTMENT OF MECHANICAL ENGINEERING
SEMINAR REPORT
ON
FUEL INJECTION SYSTEMS IN DIESEL ENGINES
Submitted To:
Submitted By:
Mr. Parveen Kumar
Mohit Bhola
Professor
Roll No. 2309634
Department of Mechanical Engineering
Final Year
Ambala College of Engineering and Applied Research, Devsthali Near Mithapur, Ambala Cantt. (Affiliated to Kurukshetra University, Kurukshetra) (2012-13)
ABSTRACT
For the compression ignition engine, it is very important to promote a means of injecting fuel into the cylinder at the proper time in the cycle. This is so because the injection system starts and controls the combustion process. Fuel injection equipment is to supply the engine with fuel in qualities exactly metered in proportion to the power required and timed with utmost accuracy, so that the engine will deliver that power within the limits prescribed for fuel consumption, exhaust smoke, noise and exhaust emissions. Fuel must be injected through suitable nozzles at pressures high enough to cause the required degree of atomization in the combustion chamber and to ensure that it mixes with sufficient air for complete combustion in the cycle time available. Compared with petrol, diesel is the lower quality product of petroleum family. Diesel particles are larger and heavier than petrol, thus more difficult to pulverize. Imperfect pulverization leads to more unburnt particles, hence more pollutant, lower fuel efficiency and less power. Common-rail technology is intended to improve the pulverization process. Conventional direct injection diesel engines must repeatedly generate fuel pressure for each injection. But in the CRDI engines the pressure is built up independently of the injection sequence and remains permanently available in the fuel line. CRDI system uses an ion sensor to provide real-time combustion data for each cylinder. The common rail upstream of the cylinders acts as an accumulator, distributing the fuel to the injectors at a constant pressure of up to 1600 bar. Here high-speed solenoid valves, regulated by the electronic engine management, separately control the injection timing and the amount of fuel injected for each cylinder as a function of the cylinder's actual need. In other words, pressure generation and fuel injection are independent of each other. This is an important advantage of common-rail injection over conventional fuel injection systems as CRDI increases the controllability of the individual injection processes and further refines fuel atomization, saving fuel and reducing emissions. Fuel economy of 25 to 35 % is obtained over a standard diesel engine and a substantial noise reduction is achieved due to a more synchronized timing operation.
ORIGIN OF FUEL INJECTION SYSTEMS
When Rudolph Diesel contracted with Augsburg and Krupp of Germany in 1893 to develop a more efficient internal combustion engine, one of his objectives was to use as fuel the mountainous piles of powdered coal which had been accumulating throughout the countryside. The first experimental coal dust burning engine was built that year using air to blast the fuel into the combustion chamber. His method is shown schematically in Fig. 1, which was reproduced from U.S. patent No. 542846 granted in 1895. The powdered coal was contained in hopper B provided with rotary valve D, and the compressed air was stored in tank A. When the injection valve E was lifted, the high pressure air flowed into the combustion chamber C through orifice F carrying with it the coal discharged through the rotating valve F. In attempting to start the engine it exploded, and all subsequent efforts to operate the engine on coal dust failed, so that oil was finally adopted as the fuel.
Figure 1 Coal dust injection system of Rudolph Diesel.
In the first experiments with oil, it was mechanically injected into the engine. The results were unsatisfactory, probably because of the crude injection equipment with large dead fuel volume, so that Dr. Diesel resorted to using the compressed air equipment available from his coal dust experiments. His first tests with air injection proved so successful that this became the accepted method of injection for many years. Thus, early in the development of this new engine the importance of the fuel injection process on engine combustion was emphasized, and subsequent progress in diesel engine development has been largely dependent upon improvements in fuel injection.
FUNCTIONAL REQUIREMENTS OF INJECTION SYSTEMS
For proper running and good performance from the engine, the following requirements must be met by the injection system:
Accurate metering of the fuel injected per cycle. This is very critical due to the fact of very small quantities of fuel being handled. Metering errors may cause drastic variation from the desired output. Timing the injection of fuel correctly in the cycle so that maximum power is obtained ensuring fuel economy and clean burning. Proper atomization of fuel into very fine droplets. Proper spray pattern to ensure rapid mixing of fuel and air. Uniform distribution of fuel droplets throughout the combustion chamber. To supply equal quantities of metered fuel to all cylinders in case of multi cylinder engines. No lag during beginning and end of injection i.e., to eliminate dribbling of fuel droplets into the cylinder.
CLASSIFICATION OF INJECTION SYSTEMS
In a constant pressure cycle or diesel engine, only air is compressed in the cylinder and then fuel is injected into the cylinder by means of a fuel injection system. The injection systems can be classified as:
FUEL INJECTION SYSTEM
MECHANICAL INJECTION SYSTEMS
AIR INJECTION
SOLID INJECTION
ELECTRONIC INJECTION SYSTEM
CRDi
MECHANICAL INJECTION SYSTEMS
AIR INJECTION SYSTEM In this system, fuel is forced into the cylinder by means of compressed air. This system is little used nowadays, because it requires a bulky multi-stage air compressor. This causes an increase in engine weight and reduces the brake power output further. One advantage that is claimed for the air injection system is good mixing of fuel with the air with resultant higher mean effective pressure. Another is the ability to utilize fuels of high viscosity which are less expensive than those used by the engines with solid injection systems. These advantages are off-set by the requirement of a multistage compressor thereby making the air-injection system obsolete.
SOLID INJECTION SYSTEMS In this system the liquid fuel is injected directly into the combustion chamber without the aid of compressed air. Hence, it is also called airless mechanical injection or solid injection system. Solid injection systems can be classified into four types.
Individual pump and nozzle system Unit injector system Common rail system Distributor system
All the above systems comprise mainly of the following components.
Fuel tank Fuel feed pump to supply fuel from the main fuel tank to the injection system. Injection pump to meter and pressurize the fuel for injection. Governor to ensure that the amount of fuel injected is in accordance with variation in load. Injector to take the fuel from the pump and distribute it in the combustion chamber by atomizing it into fine droplets. Fuel filters to prevent dust and abrasive particles from entering the pump and injectors thereby minimizing the wear and tear of the components.
A typical arrangement of various components for the solid injection system used in a CI engine is shown in Fig.2.
Figure 2 Typical Fuel Feed System of a CI Engine
Fuel from the fuel tank first enters the coarse filter from which is drawn into the plunger feed pump where the pressure is raised very slightly. Then the fuel enters the fine filter where all the dust and dirt particles are removed. From the fine filter the fuel enters the fuel pump where it is pressurized to about 200 bar and injected into the engine cylinder by means of the injector. Any spill over in the injector is returned to the fine filter. A pressure relief valve is also provided for the safety of the system. The above functions are achieved with the components listed above. The types of solid injection system described in the following section differ only in the manner of operation and control of the components mentioned above.
INDIVIDUAL PUMP AND NOZZLE SYSTEM
Figure 3: Individual Pump and Nozzle with separated pumps
The details of the individual pump and nozzle are shown in Fig 3. In this system, each cylinder is provided with one pump and one injector. In this arrangement a separate
metering and compression pump is provided for each cylinder. The pump may be placed close to the cylinder as shown in Fig (a) or they may be arranged in a cluster as shown in Fig (b). The high pressure pump plunger is actuated by a cam, and produces the fuel pressure necessary to open the injector valve at the correct time. The amount of fuel injected depends on the effective stroke of the plunger.
UNIT INJECTOR SYSTEM
Figure 4: Unit Injector System
The injector system, Fig 4, is one in which the pump and the injector nozzle are combined in one housing. Each cylinder is provided with one of these units injectors. Fuel is brought up to the injector by a low pressure pump, where at the proper time, a rocker arm actuates the plunger and thus injects the fuel into the cylinder. The amount of the fuel injected is regulated by the effective stroke of the plunger . The pump and the injector can be integrated in one unit as shown in Fig 9.4.
COMMON RAIL SYSTEM
Figure 5: Common Rail System
In the common rail system, Fig 5, a HP pump supplies fuel, under high pressure, to a fuel header. High pressure in the header forces the fuel to each of the nozzles located in the cylinders. At the proper time, a mechanically operated (by means of a push rod and rocker arm) valve allows the fuel to enter the proper cylinder through the nozzle. The pressure in the fuel header must be that, for which the injector system was designed, i.e., it must enable to penetrate and disperse the fuel in the combustion chamber. The amount of fuel entering the cylinder is regulated by varying the length of the push rod stroke. A high pressure pump is used for supplying fuel to the header, from where the fuel is metered by the injectors (assigned one per cylinder).
DISTRIBUTOR TYPE
Figure 6: Distributor System
Fig 6 shows a schematic diagram of a distributor type system. In this ayatem the pump which pressurizes the fuel also meters and times it. The fuel pump after metering the required amount of fuel supplies it to a rotating distributor at the correct time for supply to each cylinder. The number of injection strokes per cycle for the pump is equal to the no of cylinders. Since there is one metering element in each pump, a uniform distribution is automatically ensured. Not only that, the cost of the fuel-injection system also reduces to a value less than two-thirds of that for a individual pump system.
ELECTRONIC DIESEL INJECTION SYSTEM It may be noted that meeting future emission and other norms puts a large stress on the fuel injection system of diesel engine. All parameters related to the injection process like timing, rate of injection, end of injection, quality of injected fuel etc. have to be precisely controlled if the engine is to operate with a high efficiency and low emission levels. Such a control is difficult with conventional mechanical systems. Mechanical systems only sense a few parameters and meter the fuel quantity or adjust the injection timing. They seldom change the injection rate or the injection pressure.
Use of pilot injection systems can lead to significant advantages. Here a small quantity of fuel is first injected and allowed to undergo the ignition delay and burn. Subsequently the main injection takes place into gases, mixed or the uncontrolled combustion phase is minimized and this leads to a reduction in noise and NOx levels. Such a system will need an injection rate variation with time which is rather difficult to achieve precisely in mechanical systems. Hence, different types of injection systems with electronic controls have been developed. By means of EFI systems one can achieve the precise control of: a) Injection timing b) Fuel injection quantity c) Injection rate during various stages of injection. d) Injection pressure during injection e) Nozzle opening speed f) Pilot injection timing and its quantity The following are easy to obtain with such systems: a) Very high injection pressure b) Sharp start and stop of injection c) Cylinder Cut off d) Diagnostic capability e) Turbocharger control
CRDi CRDi stands for Common Rail Direct Injection meaning, direct injection of the fuel into the cylinders of a diesel engine via a single, common line, called the common rail which is connected to all the fuel injectors. Whereas ordinary diesel direct fuel-injection systems have to build up pressure anew for each and every injection cycle, the new common rail (line) engines maintain constant pressure regardless of the injection sequence. This pressure then remains permanently available throughout the fuel line. The engine's electronic timing regulates injection pressure according to engine speed and load. The electronic control unit (ECU) modifies injection pressure precisely and as needed, based on data obtained from sensors on the cam and crankshafts. In other words, compression and injection occur independently of each other. This technique allows fuel to be injected as needed, saving fuel and lowering emissions. More accurately measured and timed mixture spray in the combustion chamber significantly reducing unburned fuel gives CRDi the potential to meet future emission guidelines such as Euro V. CRDi engines are now being used in almost all MercedesBenz, Toyota, Hyundai, Ford and many other diesel automobiles.
DIRECT INJECTION SYSTEMS Direct injection means injecting the fuel directly into the cylinder instead of premixing it with air in separate intake ports. That allows for controlling combustion and emissions more precisely, but demands advanced engine management technologies.
Unlike petrol engines, diesel engines don’t need ignition system. Due to the inherent property of diesel, combustion will be automatically effective under a certain pressure and temperature combination during the compression phase of Otto cycle. Normally this requires a high compression ratio around 22 : 1 for normally aspirated engines. A strong
thus heavy block and head is required to cope with the pressure. Therefore diesel engines are always much heavier than petrol equivalent. The lack of ignition system simplifies repair and maintenance, the absence of throttle also help. The output of a diesel engine is controlled simply by the amount of fuel injected. This makes the injection system very decisive to fuel economy. Even without direct injection, diesel inherently delivers superior fuel economy because of leaner mixture of fuel and air. Unlike petrol, it can combust under very lean mixture. This inevitably reduces power output but under light load or partial load where power is not much an important consideration, its superior fuel economy shines. Another explanation for the inferior power output is the extra high compression ratio. On one hand the high pressure and the heavy pistons prevent it from revving as high as petrol engine (most diesel engine deliver peak power at lower than 4500 rpm.), on the other hand the long stroke dimension required by high compression ratio favors torque instead of power. This is why diesel engines always low on power but strong on torque.
To solve this problem, diesel makers prefer to add turbocharger. It is a device to input extra air into the cylinder while intake to boost up the power output of the engine. Turbocharger’s top end power suits the torque curve of diesel very much, unlike petrol. Therefore turbocharged diesel engines output, similar power to a petrol engine with similar capacity, while delivering superior low end torque and fuel economy.
CRDi LAYOUT
1 High Pressure Pump 4 Fuel Filter 7 High Pr. Accumulator 10 Turbocharger 13 Acc. Pedal Sensor 16 Intake Air Temp.
2 Element Shutoff Valve 5 Fuel Temp. Sensor 8 Rail Pressure Sensor 11 Coolant Temp. Sensor 14 Injector 17 Boost Pr. Sensor
3 Pressure Control Valve 6 Battery 9 ECU 12 Crankshaft Speed 15 Camshaft Speed Sensor 18 Air Mass Meter
BLOCK DIAGRAM
COMMON RAIL DIRECT INJECTION FEATURES
Simply explained, common rail refers to the single fuel injection line on the CRDi engines. Whereas conventional direct injection diesel engines must repeatedly generate fuel pressure for each injection, in CRDi engines the pressure is built up independently of the injection sequence and remains permanently available in the fuel line. In the CRDi system developed jointly by Mercedes-Benz and Bosch, the electronic engine management system continually adjusts the peak fuel pressure according to engine speed and throttle position. Sensor data from the camshaft and crankshaft provide the foundation for the electronic control unit to adapt the injection pressure precisely to demand. Common Rail Direct Injection is different from the conventional Diesel engines. Without being introduced to an antechamber the fuel is supplied directly to a common rail from where it is injected directly onto the pistons which ensures the onset of the combustion in the whole fuel mixture at the same time. There is no glow plug since the injection pressure is high. The fact that there is no glow plug lowers the maintenance costs and the fuel consumption. Compared with petrol, diesel is the lower quality fuel from petroleum family. Diesel particles are larger and heavier than petrol, thus more difficult to pulverize. Imperfect
pulverization leads to more unburned particles, hence more pollutant, lower fuel efficiency and less power. Common-rail technology is intended to improve the pulverization process. To improve pulverization, the fuel must be injected at a very high pressure, so high that normal fuel injectors cannot achieve it. In common-rail system, the fuel pressure is implemented by a very strong pump instead of fuel injectors. The highpressure fuel is fed to individual fuel injectors via a common rigid pipe (hence the name of "common-rail"). In the current first generation design, the pipe withstands pressures as high as 1,600 bar or 20,000 psi. Fuel always remains under such pressure even in stand-by state. Therefore whenever the injector (which acts as a valve rather than a pressure generator) opens, the high-pressure fuel can be injected into combustion chamber quickly. As a result, not only pulverization is improved by the higher fuel pressure, but the duration of fuel injection can be shortened and the timing can be more precisely controlled. Precise timing reduces the characteristic ―Diesel Knock‖ common to all diesel engines, direct injection or not. Benefited by the precise timing, common-rail injection system can introduce a "postcombustion", which injects small amount of fuel during the expansion phase thus creating small scale combustion after the normal combustion takes place. This further eliminates the unburned particles and also increases the exhaust flow temperature thus reducing the pre-heat time of the catalytic converter. In short, "post-combustion" cuts pollutants. The drive torque and pulsation inside the high-pressure lines are minimal, since the pump supplies only as much fuel as the engine actually requires. The high-pressure injectors are available with different nozzles for different spray configurations as Swirler nozzle to produce a cone-shaped spray and a slit nozzle for a fan-shaped spray. The new common-rail engine (in addition to other improvements) cuts fuel consumption by 20%, doubles torque at low engine speeds and increases power by 25%. It also brings a significant reduction in the noise and vibrations of conventional diesel engines. In emission, greenhouse gases (CO2) is reduced by 20%. At a constant level of NOx, carbon monoxide (CO) emissions are reduced by 40%, unburnt hydrocarbons (HC) by 50%, and particle emissions by 60%. CRDI principle not only lowers fuel consumption and emissions possible; it also offers improved comfort and is quieter than modern pre-combustion engines. Common-rail engines are thus clearly superior to ordinary motors using either direct or indirect fuelinjection systems. This division of labor necessitates a special chamber to maintain the high injection pressure of up to 1,600 bar. That is where the common fuel line (rail) comes in. It is connected to the injection nozzles (injectors) at the end of which are rapid solenoid valves to take care of the timing and amount of the injection.
The microcomputer regulates the amount of time the valves stay open and thus the amount of fuel injected, depending on operating conditions and how much output is needed. When the timing shuts the solenoid valves, fuel injection ends immediately. With the state-of-the-art common-rail direct fuel injection used an ideal compromise can be attained between economy, torque, ride comfort and long life.
THE INJECTOR A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel pump, and it is capable of opening and closing many times per second. When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel -- to make as fine a mist as possible so that it can burn easily. The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU. The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors. Each injector is complete and self-contained with nozzle, hydraulic intensifier, and electronic digital valve. At the end of each injector, a rapid-acting solenoid valve adjusts both the injection timing and the amount of fuel injected. A microcomputer controls each valve's opening and closing sequence.
SPIRAL-SHAPED INTAKE PORT FOR OPTIMUM SWIRL
The aluminum cylinder head for the CRDI engines is a new development. Among its distinguishing features are two spiral-shaped intake ports. One serves as a swirl port while the other serves as a charge port. Both ports are paired with the symmetrical combustion chamber, rapidly swirling the intake air before it enters the cylinders. The result is an optimum mixture, especially under partial throttle. The newly-designed injector nozzles (injectors) located in the middle of the cylinders provide for even distribution of fuel inside the combustion chambers valve.
INTEGRATED PORT FOR EXHAUST GAS RECYCLING Another novelty is the integrated port for exhaust gas recycling (EGR) in the cylinder head. Whereas older diesel engines lead exhaust gases outside around the engine the new CRDi engines are incorporated with a cast port for the direct injection motor which conducts the gases within the cylinder head itself. The exhaust gases recirculate directly from the exhaust side to the intake side. There are three advantages to this system. For one, it eliminates external pipes which are subject to vibration. Then, integrating EGR into the cylinder head means that part of the exhaust heat is transferred to the coolant, resulting in quicker engine warm-up. Finally, this new technique allows cooler exhaust gases and that means better combustion.
PRECISE TIMING COURTESY AIR FLOW METERING The hot-film mass air-flow meter is located in front of the turbocharger's compressor permitting an exact analysis of the air-mass that is being taken in. This mass will alter depending on temperature or atmospheric pressure. Due to this metering system, the microcomputer that controls engine timing receives precise data. It is thus able to regulate exhaust-gas recycling according to engine load and speed in the interest of lowering nitrous oxide and particle emissions. The compressed air from the turbocharger then flows through the intercooler which cools it down to 70 degrees centigrade. Since cool air has less volume than warm air, more air is taken inside the combustion chamber, thus amplifying the effect of the turbocharger. In the subordinate mixing chamber, fresh air and exhaust gas mingle in a computerdetermined ratio to match engine load at the moment. The mixing chamber is outfitted with a special exhaust-gas recycling valve and a butterfly valve controlled by a electropneumatic converter. The throttle increases the pressure gradient between the intake and outlet sides, thus increasing the recycled exhaust gases' effect on performance
SWIRL-CONTROL VALVES IN THE INTAKE MANIFOLDS Pneumatically guided swirl valves in the intake system help bring the fuel-air mixture to a high swirl rate at low rpm. This leads to efficient combustion and high torque. At high rpm the swirl is reduced and this in turn improves power output. On the way to the combustion chambers the compressed fresh air mixed with exhaust gases passes through swing manifolds. The intake area just before the cylinder head is single-channel, later becoming dual-channel. These two channels have different tasks. One acts as a spiral channel, swirling the mixture while the other serves as a charge channel which closes with the aid of electro-pneumatically activated valves under partialload operation. The advantage of this arrangement is that de-energizing increases the rate of swirl in the cylinders so that combustion produces less particle emissions than older direct-injection engines.
MULTIPLE PILOT INJECTION AND POST INJECTION The high combustion pressure of up to 145 bar (2130 psi) and the rate at which this pressure rises during the combustion process normally produce higher noise levels in direct injection engines than in their pre-chamber (indirect injection) counterparts. However, the CRDi system employs a piece of technical wizardry known as pilot injection' to overcome this problem: A few nanoseconds before the main fuel injection, a small amount of diesel is injected into the cylinder and ignites, thereby establishing the combustion process and setting the ideal conditions for the main combustion process.
Consequently, the fuel ignites faster with the result that the rise in pressure and temperature is less sudden. The system utilizes multiple pilot injections - small doses of fuel made prior to the main injection of fuel in each cylinder's firing, which help to smooth the sharp combustion character of the diesel engine to gasoline-like smoothness. The end effect, however, is not only a reduction in combustion noise but also a reduction in nitrogen oxide (NOx) emissions. Post injection is a similarly small dose of fuel injected after the main injection. Common rail technology's potential to lower particulate emissions is profound in this area. The small post injection is inserted with precise timing at the moment that is ideal for lower particulate discharge. Other methods to reduce noise are providing special cover for the cylinder head and the intercooler, and bracing on the oil pan, the timing-gear case and crankcase. The bottom line is that the noise produced by the new CRDI engines is lower than for comparable pre-combustion engines
POWERFUL MICROCOMPUTER The new direct-injection motors are regulated by a powerful microcomputer linked via CAN (Controller Area Network) data bus to other control devices on board. These devices exchange data. The engine's electrical controls are a central element of the common rail system because regulation of injection pressure and control of the solenoid valves for each cylinder - both indispensable for variable control of the motor - would be unthinkable without them. This electronic engine management network is a critical element of the common rail system because only the speed and spontaneity of electronics can ensure immediate pressure injection adjustment and cylinder-specific control of the injector solenoid valves.
NEWLY-DEVELOPED COATING
CATALYTIC
CONVERTERS
WITH
ZEOLITH
Besides electronically-controlled exhaust-gas recycling which contributes to lower nitrous oxide emissions, CRDi engines are equipped with catalytic converters near the motor and emission control devices on the underbody. These vouch for a high degree of efficiency. Emissions conform for the German "D3" norms which are 50 percent tighter than the maximum values prescribed in the EURO-2 guidelines. A new coating for the catalytic converters consisting of platinum, aluminum oxide and Zeolith crystals has been devised that besides oxidizing hydrocarbons and carbon monoxide, also converters diminish
nitrous oxide. The converter near the engine is equipped with a bypass channel via which a residual amount of hydrocarbons are passed on to the emission control devices on the underbody.
HIGH RIGIDITY CYLINDER BLOCK AND DUAL MASS FLYWHEEL To complement the new-generation common-rail system's unprecedented smoothness and low noise several enhancements have been added to its structure. Cylinder- block rigidity is increased by ribs in the water jacket and the crankshaft bearing cap is integrated into the lower block to greatly reduce engine vibration. A dual-mass flywheel is fitted to the engines to compensate for the harmonic effect of diesel engine on the powertrain elements, eliminating the characteristic rattle often associated with diesels.
UNIQUE INTAKE AND EXHAUST PORTS The CRDi engine uses an aluminium cylinder head with two spiral intake ports, one for swirling the fuel/air mixture and the other for filling the combustion chamber. Both ports are tuned to the symmetrically shaped combustion chambers and are designed to set the air into rapid swirling motion even before it reaches the cylinders. This ensures an optimal fuel/air mixture, especially in the part throttle range. Inside the combustion chambers, newly developed injectors are positioned in the middle of the cylinder to promote uniform fuel distribution. Another new feature of the CRDi engine is the integration of a port in the cylinder head for the exhaust gas recirculation (EGR) system. In most diesel engines this system is routed around the outside of the engine but in the CRDI system an EGR port has been cast into the cylinder head to channel gas from the exhaust side of the engine to the intake side. This design has three distinct benefits: It dispenses with external EGR lines, transfers exhaust heat to the coolant for quicker engine warm-up, and at the same time cools exhaust gases to further enhance combustion.
REDUCED NOISE LEVELS Diesel engines are known to be noisy. But the introduction of the CRDi engines has made many attributes of the old Diesel engines have become something of the past. One of these is noise. The noisy side of the old Diesel engines which was a cause of inconvenience has given way largely to a quietness in the CRDi technology, because
many functions executed by mechanical systems in the old Diesel engines are carried out electronically in the CRDi technology. This in turn enables the engine to run with much less noise. Moreover the carrying out of the injection via multiple injections instead of single is one of the causes which ensures the quietness of the engine. In the CRDi technology it is ensured that all the parts of the engine work in harmony, thereby minimizing the engine noise. Besides that, a high efficiency is achieved now even at low engine speeds. If the unequalled noise insulation is added to this it is almost impossible to hear any engine noise, especially inside the car.
CRDi – FUTURE TRENDS ULTRA-HIGH PRESSURE COMMON – RAIL INJECTION Newer CRDi engines feature maximum pressures of 1800 bar. This pressure is up to 33% higher than that of first-generation systems, many of which are in the 1600-bar range. This technology generates an ideal swirl in the combustion chamber which, coupled with the common-rail injectors’ superior fuel-spray pattern and optimized piston head design, allows the air/fuel mixture to form a perfect vertical vortex resulting in uniform combustion and greatly reduced NOx (nitrogen oxide) emissions. The system realizes high output and torque, superb fuel economy, emissions low enough to achieve Euro Stage IV designation and noise levels the same as a gasoline engines. In particular, exhaust emissions and Nox are reduced by some 50% over the current generation of diesel engines.
CRDI AND PARTICLE FILTER Particle emission is always the biggest problem of diesel engines. While diesel engines emit considerably less pollutant CO and Nox as well as green house gas CO2, the only shortcoming is excessive level of particles. These particles are mainly composed of carbon and hydrocarbons. They lead to dark smoke and smog which is very crucial to air quality of urban area, if not to the ecology system of our planet. Basically, particle filter is a porous silicon carbide unit; comprising passageways which has a property of easily trapping and retaining particles from the exhaust gas flow. Before the filter surface is fully occupied, these carbon / hydrocarbon particles should be burnt up, becoming CO2 and water and leave the filter accompany with exhaust gas flow. The process is called regeneration.
Normally regeneration takes place at 550° C. However, the main problem is: this temperature is not obtainable under normal conditions. Normally the temperature varies between 150° and 200°C when the driving in town, as the exhaust gas is not in full flow. The new common-rail injection technology helps solving this problem. By its highpressure, precise injection during a very short period, the common-rail system can introduce a "post-combustion" by injecting small amount of fuel during expansion phase. This increases the exhaust flow temperature to around 350°C.
Then, a specially designed oxidizing catalyst converter locating near the entrance of the particle filter unit will combust the remaining unburnt fuel come from the "postcombustion". This raises the temperature further to 450° C. The last 100°C required is fulfilled by adding an addictive called Eolys to the fuel. Eolys lowers the operating temperature of particle burning to 450° C, now regeneration occurs. The liquid-state additive is store in a small tank and added to the fuel by pump. The PF unit needs to be cleaned up every 80,000 km by high-pressure water, to get rid of the deposits resulting from the additive.
CRDI AND CLOSED-LOOP CONTROL INJECTION One feature of diesel-engine management had been holding back diesel's technical advance: the lack of true, closed-loop control of the injection system. This is significant because an open-loop system cannot accurately compensate for factors such as wear, manufacturing tolerances in the fuel injectors, or for variations in temperature and fuel quality. Gasoline-injection systems have been closed loop for years, and many of the advances in power, refinement, economy, and emissions seen today have been possible because of the real-time feedback that this provides. Its solution to this problem is an all-new common-rail, direct-injection system that uses an ion sensor to provide real-time combustion data for each cylinder. It is said to provide closed-loop control at a cost that will be roughly equivalent to today's best production systems. High-speed, common-rail direct-injection diesel engines are theoretically capable of excellent performance, economy, and emissions, but to achieve this they will require a much higher level of control than is possible with today's technology. With closed-loop systems and ion-sensing technology, the potential of diesel engines for automotive applications can be unlocked. The ion-sensing system creates an electrical field in the region where combustion starts by introducing a positive dc voltage at the tip of the glow plug. The field attracts the negatively charged particles created during combustion, producing a small current from the sensor to the piston and cylinder walls, which provide a ground. The current is measured by the engine control module (ECM) and processed to provide a signal that is proportional to the applied sensor voltage and to the level of ionization in the vicinity of the sensor. The difference in ionization before and after the start of combustion is quite pronounced, allowing the ion-sensing system to provide precise start-of-combustion (SOC) data that can be compared with a table of required SOC timings held by the ECM. The fuel control strategy can therefore be changed from open loop to closed loop, allowing the desired SOC to be maintained for all engine speeds, loads, temperatures, and fuel qualities; and to accommodate production tolerances and wear in each injector. Because the sensing function is combined with the glow plug, no engine modifications are required, and the sensor is in a near ideal location. One significant feature of the location is that soot build-up, which can reduce the resistance between the sensor and ground, can be easily detected and burnt off through a simple, automated routine. To reduce audible noise and NOx, a current production high-pressure common-rail system will typically inject a pilot pulse of around 3-5 mm3 of fuel before the main injection event. Pilot injection can reduce noise by 3-5 dB, but too large a pulse will compromise fuel consumption and emissions. Existing technology can reduce the pilot injection volume to around 1-2 mm3 but only at low injection pressures. Most engine designers would prefer higher pressures because this allows cylinders to be fueled more quickly and for the spray pattern to be improved, leading to increased torque and less smoke.
Closed-loop system allows a pilot volume of around 0.5-1.0 mm3 under high pressures using standard injectors, and is said to reduce particulates by around 10-20%. The precise volume of the pilot injection can be balanced between cylinders, leading to a further reduction in noise. The adaptively learned injector calibrations can also be applied to post-injection pulses, which provide a more complete combustion. 2-3% improvement in fuel consumption can be achieved compared with today's high-pressure systems by incorporating closed loop control.
CONCLUSION
The seminar that I had taken is Fuel Injection Systems in Diesel Engines from which we reached to the conclusion Fuel injection systems have evolved over the years, FISs of today have precise control over injection processes like timing, rate of injection, end of injection, quality of injected fuel to operate the engine with a high efficiency and low emission levels. Introduction of CRDi technology has revolutionized diesel engines. By using CRDi technology a lot of advantages are obtained, some of them are:
More power is developed. Increased fuel efficiency. Reduced noise. More stability. Pollutants are reduced. Particulates of exhaust are reduced. Exhaust gas recirculation is enhanced. Precise injection timing is obtained. Pilot and post injection increase the combustion quality. More pulverization of fuel is obtained. A very high injection pressure can be achieved. The powerful microcomputer makes the whole system more perfect. It doubles the torque at lower engine speeds.
The main disadvantage is that this technology increases the cost of the engine.
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