Basics of Internal Combustion Engines by Indranil Mandal
1. INTERNAL COMBUSTION ENGINE
Module – I
A heat engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this
energy to produce mechanical work. It is classified into two types-
(a) External combustion engine
(b) Internal combustion engine
Classification of Heat Engines:
L: Large over 10000 KW
M: Medium between 1000- 10000 KW
S: Small under 1000 KW
2. External combustionengine:
In this engine, the products of combustion of air and fuel transfer heat to a second fluid which is the working
fluid of the cycle.
In the steam engine or a steam turbine plant, the heat of combustion is employed to generate steam
which is used in a piston engine (reciprocating type engine) or a turbine (rotary type engine) for
Figure 1: External Combustion Engine
In steam engine or steam turbine the heat generated due to combustion of fuel and it is employed to generate
high pressure steam, which is used as working fluid in a reciprocating engine or turbine (From Figure 1).
In a closed cycle gas turbine, the heat of combustion in an external furnace is transferred to gas,
usually air which the working fluid of the cycle.
Internal combustion engine:
In this engine, the combustion of air and fuels take place inside the cylinder and are used as the direct
motive force. It can be classified into the following types:
1. According to the basic engine design- (a) Reciprocating engine (Use of cylinder piston arrangement),
(b) Rotary engine (Use of turbine)
2. According to the type of fuel used- (a) Petrol engine, (b) diesel engine, (c) gas engine (CNG, LPG),
(d) Alcohol engine (ethanol, methanol etc)
3. According to the number of strokes per cycle- (a) Four stroke and (b) Two stroke engine
4. According to the method of igniting the fuel- (a) Spark ignition engine, (b) compression ignition
engine and (c) hot spot ignition engine
5. According to the working cycle- (a) Otto cycle (constant volume cycle) engine, (b) diesel cycle
(constant pressure cycle) engine, (c) dual combustion cycle (semi diesel cycle) engine.
6. According to the fuel supply and mixture preparation- (a) Carburetted type (fuel supplied through the
carburettor), (b) Injection type (fuel injected into inlet ports or inlet manifold, fuel injected into the
cylinder just before ignition).
7. According to the number of cylinder- (a) Single cylinder and (b) multi-cylinder engine
8. Method of cooling- water cooled or air cooled
9. Speed of the engine- Slow speed, medium speed and high speed engine
10. Cylinder arrangement-Vertical, horizontal, inline, V-type, radial, opposed cylinder or piston engines.
11. Valve or port design and location- Overhead (I head), side valve (L head); in two stroke engines: cross
scavenging, loop scavenging, uniflow scavenging.
3. 12. Method governing- Hit and miss governed engines, quantitatively governed engines and qualitatively
13. Application- Automotive engines for land transport, marine engines for propulsion of ships, aircraft
engines for aircraft propulsion, industrial engines, prime movers for electrical generators.
Comparison between external combustion engine and internal combustion engine:
ADVANTAGES OF INTERNAL COMBUSTION ENGINES:
1. Greater mechanical simplicity.
2. Higher power output per unit weight because of absence of auxiliary units like boiler, condenser and feed
3. Low initial cost
4. Higher brake thermal efficiency as only a small fraction of heat energy of the fuel is dissipated to cooling
5. These units are compact and requires less space
6. Easy starting from cold conditions
DISADVANTAGES OF INTERNAL COMBUSTION ENGINES:
1. I C engines cannot use solid fuels which are cheaper. Only liquid or gaseous fuel of given specification
can be efficiently used. These fuels are relatively more expensive.
2. I C engines have reciprocating parts and hence balancing of them is problem and they are also susceptible
to mechanical vibrations.
4. INTERNAL COMBUSTION ENGINE PARTS AND THEIR FUNCTION
Cylinder: It is the main part of the engine inside which piston reciprocates to and fro. It should have high
strength to withstand high pressure above 50 bar and temperature above 2000°C. The ordinary engine is
made of cast iron and heavy duty engines are made of steel alloys or aluminum alloys. In the multi-cylinder
engine, the cylinders are cast in one block known as cylinder block.
Cylinder head: The top end of the cylinder is covered by cylinder head over which inlet and exhaust valve,
spark plug or injectors are mounted. A copper or asbestos gasket is provided between the engine cylinder
and cylinder head to make an air tight joint.
Piston: Transmit the force exerted by the burning of charge to the connecting rod. Usually made of
aluminium alloy which has good heat conducting property and greater strength at higher temperature.
Figure 2 and 3 shows the different components of IC engine:
Figure.2: Different parts of IC engine
5. Figure.3: Different parts of IC engine
Piston rings: These are housed in the circumferential grooves provided on the outer surface of the piston
and made of steel alloys which retain elastic properties even at high temperature. 2 types of rings-
compression and oil rings. Compression ring is upper ring of the piston which provides air tight seal to
prevent leakage of the burnt gases into the lower portion. Oil ring is lower ring which provides effective seal
to prevent leakage of the oil into the engine cylinder.
Connecting rod: It converts reciprocating motion of the piston into circular motion of the crank shaft, in the
working stroke. The smaller end of the connecting rod is connected with the piston by gudgeon pin and
bigger end of the connecting rod is connected with the crank with crank pin. The special steel alloys or
aluminium alloys are used for the manufacture of connecting rod.
Crank: - It is a lever between connecting rod and crank shaft.
Crankshaft: It converts the reciprocating motion of the piston into the rotary motion with the help of
connecting rod. The special steel alloys are used for the manufacturing of the crankshaft. It consists of
eccentric portion called crank.
Crank case: It houses cylinder and crankshaft of the IC engine and also serves as sump for the lubricating
Flywheel: It is big wheel mounted on the crankshaft, whose function is to maintain its speed constant. It is
done by storing excess energy during the power stroke, which is returned during other stroke.
IC ENGINE – TERMINOLOGY
1. Cylinder bore (D): The nominal inner diameter of the working cylinder.
2. Piston area (A): The area of circle of diameter equal to the cylinder bore.
3. Stroke (L): The nominal distance through which a working piston moves between two successive
reversals of its direction of motion.
4. Dead centre: The position of the working piston and the moving parts which are mechanically connected
to it at the moment when the direction of the piston motion is reversed (at either end point of the stroke).
6. (a) Bottom dead centre (BDC): Dead centre when the piston is nearest to the crankshaft.
(b) Top dead centre (TDC): Dead centre when the position is farthest from the crankshaft.
5. Displacement volume or swept volume (Vs): The nominal volume generated by the working piston
when travelling from the one dead centre to next one and given as,
Vs=A × L
6. Clearance volume (Vc): the nominal volume of the space on the combustion side of the piston at the top
7. Cylinder volume (V): Total volume of the cylinder. V= Vs + Vc
8. Compression ratio (r): r= Vs/ Vc
FOUR-STROKE CYCLE S-I ENGINE - PRINCIPLE OF OPERATION:
Figure.3: cross section of a SI Engine
In Four-stroke cycle engine, the cycle of operation is completed in four-strokes of the piston or two
revolutions of the crankshaft. Each stroke consists of 180°, of crankshaft rotation and hence a cycle consists
of 720°of crankshaft rotation.
The series of operations of an ideal four-stroke SI engine are as follows (see Figure.4 & Figure.5)
1. Suction stroke
Suction stroke 0-1 starts when the piston is at top dead centre and about to move downwards. The inlet valve
is open at this time and the exhaust valve is closed. Due to the suction created by the motion of the piston
towards bottom dead centre, the charge consisting of fresh air mixed with the fuel is drawn into the cylinder.
At the end of the suction stroke the inlet valve closes.
7. 2. Compression stroke
The fresh charge taken into the cylinder during suction stroke is compressed by the return stroke of the
piston 1-2. During this stroke both inlet and exhaust valves remain closed. The air which occupied the whole
cylinder volume is now compressed into clearance volume. Just before the end of the compression stroke the
mixture is ignited with the help of an electric spark between the electrodes of the spark plug located in
combustion chamber wall. Burning takes place when the piston is almost at top dead centre. During the
burning process the chemical energy of the fuel is converted into sensible energy, producing a temperature
rise of about 2000°C, and the pressure is also considerably increased.
3. Expansion or power stroke
Due to high pressure the burnt gases force the piston towards bottom dead centre, stroke 3-4, and both the
inlet and exhaust valves remaining closed. Thus power is obtained during this stroke. Both pressure and
temperature decrease during expansion.
4. Exhaust stroke.
At the end of the expansion stroke the exhaust valve opens, the inlet valve remaining closed, and the piston
is moving from bottom dead centre to top dead centre sweeps out the burnt gases from the cylinder, stroke 4-
0. The exhaust valve closes at the end of the exhaust stroke and some 'residual' gases remain in the cylinder.
Each cylinder of a four-stroke engine completes the above four operations in two engine revolutions.
One revolution of the crankshaft occurs during the suction and compression strokes, and second revolution
during the power and exhaust strokes. Thus for one complete cycle, there is only one power stroke while the
crankshaft turns by two revolutions. Most of the spark-ignition internal combustion engines are of the four-
stroke type. They are most popular for passenger cars and small aircraft applications.
The construction and working of a four-stroke petrol engine is shown below:
Figure.4:The four-stroke spark-ignition (SI) engine cycle (Otto cycle or constant volume cycle)
8. Figure.5: Ideal and actual indicator diagrams for four-stroke SI engine
Figure.6: Four-stroke petrol engine valve timing diagram in relation to the pressure volume diagram
9. Valve Timing Diagram:
Valve timing is the regulation of the points in the cycle at which the valves are set to open & close. In the
ideal cycle inlet & exhaust valve open & close at dead centers.
Figure.7: Theoretical valve timing diagram
But in actual cycles, they open or close before or after dead centers as explained below: -
There are two factors, one mechanical & other dynamic, for the actual valve timings tot be different from the
theoretical valve timing.
a) Mechanical factor: - The puppet valves of the reciprocating engines are opened & closed by cam
mechanisms. The clearance between cam, tappet and valve must be slowly taken up & valve slowly
filled, at first, if noise and wear is to be avoided, for the same reasons the valve cannot be closed
abruptly, else it mill bounce on its seat. (Also the cam counters should be so designed as to produce
gradual and smooth changes in directional acceleration.) Thus, the valve opening & closing periods
are spread over a considerable number of crankshaft degrees. As a result, the opening of valve must
commence ahead of time at which it is fully opened ie before dead centers.) The same reasoning
close after the dead centers.
b) Dynamic factor: - Besides mechanical factors of opening & closing of valves, the actual valve
timing is set taking into considerations, the dynamic effects of gas flow.
Valve timing Diagram for 4 stroke S.I. Engine:-
1) Intake valve timing: - Intake valve timing has a bearing on the actual quantity of air sucked during
the suction stoke, i.e. if affects the volumetric efficiency. From the actual valve timing diagram for
both low and high speed S.I. engine it is seen that the intake valve opens 10° before the arrival of the
piston at TDC on the exhaust stock. This is to ensure that the valve will be fully open the fresh
charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves out in
the suction stroke, the fresh charge is drawn in through the intake valve. When the piston searches
BDC and starts to move in the compression stroke, the inertia of the entering fresh charge tends to
cause it to continue to move into cylinder. To take advantage of this, the intake valve is closed after
TDC so that maximum air is taken in. This is called ram effect. The time the intake valve should
remain open after TDC is decided by the speed. The charge speed is low and so the air inertia is low
and hence the intake valve should close relatively early, after BDC for a slow speed engine as
compared to a high speed engine.
10. Figure.8: Valve timing for low and high speed four-stroke SI engine
2) Exhaust valve timing: - The exhaust valve is set to open before BDC (say about 25° before BDC in
low speed engines & 55° before BDC is high speed engines). If the exhaust valve did not start to
open until BDC, the pressures in the cylinder would be considerably above the atmospheric pressure
during the first portion of the exhaust stroke, increasing the work required to except the exhaust
gases. But opening the exhaust valve earlier reduces the pressure near the end of the power stroke &
thus, causes some loss of useful work on this stoke. However, the overall effect of opening the valve
prior to the timing the piston reaches BDC results in overall gains in output.
The closing timing of exhaust valve effects the volumetric efficiency. By closing the exhaust valve a
few degrees after TDC (after 15° for low speed and 20° for high speed), the inertia of exhaust gases
tends to scavenge the cylinder by conveying out a greater mass of the gas left in the clearance
volume. This results in increased volumetric efficiency.
The period when both the intake & exhaust valves are open at the same time is called as valves over
11. Table 1–Typical valve timings for four-stroke SI engines
Valve timing is different for different makes of engines.
b-before, a-after TDC-Top dead centre BDC-Bottom dead centre.