BIRLA INSTITUT
OF TECHNOLOGY AND SCIENCE, PILANI
Introduction to I ternal Combustion E gines
Dr. Ravi Ind Inder Sing ngh h E ail id: raviinder@pil ni.bits-pilani.ac.in
Introduction =
• Heat engine : It can be de ined as any engine that convert thermal energy to mechanic l work output. Examples of hea engines include: steam engine, diesel engine, and gasoline (petrol engine. •
n e as s o ow erma energy s e ng ng e vere o wo wor n fluid of the heat engine, heat e gine can be classified as an interna combust bustio ion n engin nginee and exter tern l combu ombust stio ion n engine gine..
2
• In an Internal combustion engine, combustion take place within working fluid of the engine, thus fluid get contaminated with combustion prod produc uctts. – Petrol engine is an e ample of internal combustio engine, where the w rking fluid is a mixture of ai and fuel . • In an External combusti n engine, working fluid get energy using boilers by bu ning fossil fuels or any othe fuel, thus the working fluid does not come in contact wit comb combus usti tion on prod produc ucts ts.. – Steam engine is an e ample of external combustio engine, where the wo king fluid is steam.
Classification of Heat Engines
Internal combustion engines ma be classified as : –
Spark Ignition engine .
–
Compression Ignition engines.
• Spark ignition engine (SI
ngine): An engine in which the
spark. • Compression ignition engin (CI engine): An engine in which the combustion process star s when the air-fuel mixture self ignites due to high temper ture in the combustion chamber caused by high compression. –
Spark ignition and C mpression Ignition engine operate on either a four stroke cycle or a two stroke cycle.
Spark Ignition Engine (SI engine):
Figure 1(a)
Compression Ignition Engine (CI engine):
Figure 1(b)
Figure 2: Engi e components
ternal combustion Engine Components:
. Engine components shown in fi ure1(a), 1(b) and figure 2 are defined as follows: Block : Body of the engine containing c linders, made of cast iron or aluminum. Cylinder : The circular cylinders in the ngine block in which the pistons reciprocate back and forth. Head : The piece which closes the end f the cylinders, usually containing part o the clearance volume of the combustion chamber. Combustion chamber: The end of the ylinder between the head and the piston face where combustion occurs.
–
The size of combustion chamber continuously changes from minimum volume when the piston is at TDC to a maximum volume when the piston BDC.
Crankshaft : Rotating shaft throu h which engine work output is supplied to external systems.
– The crankshaft is connected to the engine block with the main bearings. – It is rotated by the recipro ating pistons through the connecti rods connected to the cran shaft, offset from the axis of rotation. This offset is so etimes called crank throw or crank radius. Connecting rod : Rod connecting the piston with the rotating crankshaft, usually made of steel or alloy forging in most engines but may be aluminum in some small e gines. Piston rings: Metal rings that fit into circumferential grooves around the piston and form a sliding surfa e against the cylinder walls.
Camshaft : Rotating shaft use to push open valves at the proper time in the engine cycle, either directly or through mechanical or hydraulic linkage (push rods, r cker arms, tappets) . Push rods : The mechanical li kage between the camshaft and valves on overhead valve en i es with the camshaft in the crankcase. Crankcase : Part of the engine block surrounding the crankshaft.
– In many engines the oil p n makes up part of the crankcase housing. Exhaust manifold : Piping sy tem which carries exhaust gases away from the engine cylinder , usually made of cast iron .
Intake manifold :Piping syste cylinders, usually made of cast
which delivers incoming air to the etal, plastic, or composite material
–
In most SI engines, fuel is ad ed to the air in the intake manifold system either by fuel injector or with a carburetor.
–
The individual pipe to a single cylinder is called runner.
Carburetor : A device which eters the proper amount of fuel int the air flow by means of pressure differential.
–
For many decades it was the asic fuel metering system on all automobile (and other) engin s.
Spark plug : Electrical device sed to initiate combustion in an SI engine by creating high voltage ischarge across an electrode gap.
.C. Engine componen s apart from omponents shown in the figure: Exhaust System: Flow system or removing exhaust gases from th cylinders, treating them, and ex austing them to the surroundings.
– It consists of an exhaust manifold which carries the exhaust gases away from the engine, a thermal or catalytic converter t , , tailpipe to carry the exhau t gases away from the passenger compartment. Flywheel : Rotating mass with large moment of inertia connected to the crank shaft of the engine.
– The purpose of the flywhe l is to store energy and furnish large angular momentum t at keeps the engine rotating between power strokes an smooth's out engine operation.
•
Fuel injector : A pressuriz d nozzle that sprays fuel into th incoming air (SI engines )or nto the cylinder (CI engines).
•
Fuel pump : Electrically or mechanically driven pump t supply fuel from the fuel tan (reservoir) to the engine.
•
Glow plug : Small electric l resistance heater mounted insid the combustion chamber of many CI engines, used to prehea the chamber enough so that combustion will occur when firs starting a cold engine.
– •
The glow plug is turn off fter the engine is started.
Starter : Several methods a e used to start IC engines. Most ar started by use of an electric otor (starter) geared to the engin flywheel. Energy is supplied from an electric battery.
Figure 3 : Engin Terminology
ngine Terminology Figure 3, shows the pressure volu e diagram of ideal engine cycle along with engine terminology a follows: • Top Dead Center (TDC): Position of the piston when it stops at th furthest point away from the cra kshaft.
– Top because this positio is at the top of the engines (not , . Because in some engines TDC is not at the top of the engines(e.g: horizontally oppose engines, radial engines, etc,.) Some sources call this position ead End Dead Center (HEDC). – Some source call this point TOP Center (TC). – When the piston is at TDC, the volume in the cylinder is a minimum called the clea ance volume.
Bottom Dead Center (BDC): Positi n of the piston when it stops at the point closest to the crankshaft.
–
Some sources call this Crank nd Dead Center (CEDC) because it is not always at the bottom of the engine. Some source call this point Bottom Center (BC).
Stroke : Distance traveled by the piston from one extreme position to the othe . Bore :It is defined as cylinder diameter or piston face diameter; piston face diameter is same as cylinder diamete ( minus small clearance). Swept volume/Displacement volum : Volume displaced by the piston as it travels through one stroke.
–
Swept volume is defined as str ke times bore.
–
Displacement can be given for one cylinder or entire engine (one cylind times number of cylinders).
Clearance volume : It is the m nimum volume of the cylinder available for the charge (air or ir fuel mixture) when the piston reaches at its outermost point (t p dead center or outer dead center during compression stroke of t e cycle.
–
Minimum volume of c mbustion chamber with piston at .
Compression ratio : The ratio of total volume to clearance volum of the cylinder is the compressi n ratio of the engine.
–
Typically compression ratio for SI engines varies form 8 to 12 and for CI engine it varies from 12 to 24
lassification of IC en ines
- Internal Combustion Engines – two stroke 1. Intake / Compression
2. Power / Exhaust
a. b.
a. b.
c.
inlet port opens compressed fuel-air mixture rushes into the cylinder piston upward movement rovides further com ression
c.
ignition piston moves downward compressing fuel-air mixture in the crankcase exhaust port opens
Internal Combustion Engines
starting position
four stroke -
1. in ake
2. compression
a. piston st rts moving down b. intake valve opens c. air-fuel ixture gets in
a. piston moves up b. both valves closed c. air-fuel mixture gets compressed
Internal Combustion Engines – four stroke -
ignition
3. ower a. air- uel mixture explodes driving the piston down
4. exhaust a. piston moves up b. exhaust valve opens c. exhaust leaves the cylinder
II Cycle of operation
Otto Cycle
Diesel Cycle
III Type of fuel used 1. 2. 3. 4. 5.
Volatiles liquid fuels [Gasoli e, Alcohol, Kerosene] Gaseous fuels[Compressed atural Gas(CNG), LPG, Blast furnace gas and biogas] Solid fuels [Solid fuels converts into gaseous fuels] Viscous liquid like heavy a d light diesel oils [Heavy and Light Diesel Oils] Engines using two fuels [ ighly Volatile fuel is injected along with air and other fuel s injected in combustion space]
IV Method of Charging I Naturally Aspirated Engines II Supercharged Engines
V Type of Ignition I Battery Ignition
II Magneto Ignition system
VI Type of Cooling
Air Cooled Engine
Water Cooled Engine
Cylinder Arrangements
First Law Analysis of Engine ycle
Engine Terminology (Recap)
Compression ratio ( r) = VT/VC = VC + VS / VC
ngine Performance Parameter s Indicated Thermal Efficiency(η ) ith
Brake Thermal Efficiency( η bth )
Mechanical Efficiency (
Indicated power is the theoreti maximum output power of the engine. The indicated power is the total po available from the expanding of the gases the cylinders negating any friction, heat l or entropy within the system.
Brake power is the power output of th drive shaft of an engine without with o the power loss caused by gear transmission friction etc.
η m)
Mechanical efficiency is how much of the power developed by the expanding of the gases in cylinders is actually delivered as useful power.
ngine Performance Parameters (Cont…) Volumetric Efficiency(
η v)
Volumetric efficiency is defined as the ratio f actual volume flow rate into the intake system to th rate at which the volume is displaced by the system.
.
η v
=
m
a
ρ a V d i s p N
/ 2
Relative Efficiency( η ) rel
η rel
ean
=
Actual − thermal − efficiency Air − s tan dard − efficiency
ec ve pressure pm
Mean Effective pressure is the average pressure inside the cylinders of an internal combustion engine based on the calculated or easured power output . It increase as manifold pressure increases.
ip
=
pm LAnK 60 × 1000
pm =
60000×ip
LAnK
pb
=
60000 × bp
AnK
Area of the indicator diagra
pim Length of the indicator diagr
ngine Performance Parameters (Cont…) Mean Piston Speed ( S ) p
S p
=
2 LN
Specific Power Output (Ps) Specific Power Output (P s) of an engine is defined as t e power output per unit piston area.
s
=
= Constant × p bm ×
S
Specific Fuel Consumption (sfc) Sfc = Fuel consumption per unit time Power
p
ngine Performance Parameters (Cont…) Inlet – Valve Mach Index (Z)
A
u
=
C
i
p
Ai
V
Fuel-Air (F/A) or Air Fuel Ratio (A/F)
A mixture that contains just enough air for c mplete combustion of all the fuel in the mixture is called a chemically correct or stoichiometric fuel-air ratio. SI engines : 15:1 CI engines : 18:1 to 80: 1 from ful load to no load
φ
=
Actual fuel-air rati Stoichiometric fuel air ratio
