ENGINE & WORKING PRINCIPLES

ENGINE & WORKING PRINCIPLES
A heat engine is a machine, which converts heat energy into mechanical energy. The
combustion of fuel such as coal, petrol, diesel generates heat. This heat is supplied to a
working substance at high temperature. By the expansion of this substance in suitable
machines, heat energy is converted into useful work. Heat engines can be further divided into
two types:
(i) External combustion and
(ii) Internal combustion.
In a steam engine the combustion of fuel takes place outside the engine and the steam
thus formed is used to run the engine. Thus, it is known as external combustion engine. In the
case of internal combustion engine, the combustion of fuel takes place inside the engine
cylinder itself.
The IC engine can be further classified as: (i) stationary or mobile, (ii) horizontal or vertical
and (iii) low, medium or high speed. The two distinct types of IC engines used for either
mobile or stationary operations are: (i) diesel and (ii) carburettor.
Heat Engine
External Combustion Internal Combustion
Steam Engine
Reciprocating Wankel Rotary Gas
Turbine
CI Engine SI Engine
Two Stroke Four Stroke Two Stroke Four Stroke
Spark Ignition (Carburettor Type) IC Engine
In this engine liquid fuel is atomised, vaporized and mixed with air in correct proportion
before being taken to the engine cylinder through the intake manifolds. The ignition of the
mixture is caused by an electric spark and is known as spark ignition.
Compression Ignition (Diesel Type) IC Engine
In this only the liquid fuel is injected in the cylinder under high pressure.
CONSTRUCTIONAL FEATURES OF IC ENGINE:
The cross section of IC engine is shown in Fig. 1. A brief description of these parts is given
below.
Cylinder:
The cylinder of an IC engine constitutes the basic and supporting portion of the engine power
unit. Its major function is to provide space in which the piston can operate to draw in the fuel
mixture or air (depending upon spark ignition or compression ignition), compress it, allow it
to expand and thus generate power. The cylinder is usually made of high-grade cast iron. In
some cases, to give greater strength and wear resistance with less weight, chromium, nickel
and molybdenum are added to the cast iron.
Chart 1. Types of Heat Engines
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Piston:
The piston of an engine is the first part to begin movement and to transmit power to the
crankshaft as a result of the pressure and energy generated by the combustion of the fuel. The
piston is closed at one end and open on the other end to permit direct attachment of the
connecting rod and its free action.
The materials used for pistons are grey cast iron, cast steel and aluminium alloy. However,
the modern trend is to use only aluminium alloy pistons in the tractor engine.
Piston Rings:
These are made of cast iron on account of their ability to retain bearing qualities and elasticity
indefinitely. The primary function of the piston rings is to retain compression and at the same
time reduce the cylinder wall and piston wall contact area to a minimum, thus reducing
friction losses and excessive wear. The other important functions of piston rings are the
control of the lubricating oil, cylinder lubrication, and transmission of heat away from the
piston and from the cylinder walls. Piston rings are classed as compression rings and oil rings
depending on their function and location on the piston.
Compression rings are usually plain one-piece rings and are always placed in the grooves
nearest the piston head. Oil rings are grooved or slotted and are located either in the lowest
groove above the piston pin or in a groove near the piston skirt. Their function is to control
the distribution of the lubricating oil to the cylinder and piston surface in order to prevent
unnecessary or excessive oil consumption ion.
Fig. 1 Cross-section of a diesel engine
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Piston Pin:
The connecting rod is connected to the piston through the piston pin. It is made of case
hardened alloy steel with precision finish. There are three different methods to connect the
piston to the connecting rod.
Connecting Rod:
This is the connection between the piston and crankshaft. The end connecting the piston is
known as small end and the other end is known as big end. The big end has two halves of a
bearing bolted together. The connecting rod is made of drop forged steel and the section is of
the I-beam type.
Crankshaft:
This is connected to the piston through the connecting rod and converts the linear motion of
the piston into the rotational motion of the flywheel. The journals of the crankshaft are
supported on main bearings, housed in the crankcase. Counter-weights and the flywheel
bolted to the crankshaft help in the smooth running of the engine.
Engine Bearings:
The crankshaft and camshaft are supported on anti-friction bearings. These bearings must be
capable of with standing high speed, heavy load and high temperatures. Normally, cadmium,
silver or copper lead is coated on a steel back to give the above characteristics. For single
cylinder vertical/horizontal engines, the present trend is to use ball bearings in place of main
bearings of the thin shell type.
Figure 2. Components of the diesel engine
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Valves:
To allow the air to enter into the cylinder or the exhaust, gases to escape from the cylinder,
valves are provided, known as inlet and exhaust valves respectively. The valves are mounted
either on the cylinder head or on the cylinder block.
Camshaft:
The valves are operated by the action of the camshaft, which has separate cams for the inlet,
and exhaust valves. The cam lifts the valve against the pressure of the spring and as soon as it
changes position the spring closes the valve. The cam gets drive through either the gear or
sprocket and chain system from the crankshaft. It rotates at half the speed of the camshaft.
Flywheel
This is usually made of cast iron and its primary function is to maintain uniform engine speed
by carrying the crankshaft through the intervals when it is not receiving power from a piston.
The size of the flywheel varies with the number of cylinders and the type and size of the
engine. It also helps in balancing rotating masses.
Materials used for engine parts:
S. No. Name of the Parts Materials of Construction
1. Cylinder head Cast iron, Cast Aluminium
2. Cylinder liner Cast steel, Cast iron
3. Engine block Cast iron, Cast aluminum, Welded steel
4. Piston Cast iron, Aluminium alloy
5. Piston pin Forged steel, Casehardened steel.
6. Connecting rod Forged steel. Aluminium alloy.
7. Piston rings Cast iron, Pressed steel alloy.
8. Connecting rod bearings Bronze, White metal.
9. Main bearings White metal, Steel backed Babbitt base.
10. Crankshaft Forged steel, Cast steel
11. Camshaft Forged steel, Cast iron, cast steel,
12. Timing gears Cast iron, Fiber, Steel forging.
13. Push rods Forged steel.
14. Engine valves Forged steel, Steel, alloy.
15. Valve springs Carbon spring steel.
16. Manifolds Cast iron, Cast aluminium.
17. Crankcase Cast iron, Welded steel
18. Flywheel Cast iron.
19. Studs and bolts Carbon steel.
20. Gaskets Cork, Copper, Asbestos.
PRINCIPLES OF OPERATION OF IC ENGINES:
FOUR-STROKE CYCLE DIESEL ENGINE
In four-stroke cycle engines there are four strokes completing two revolutions of the
crankshaft. These are respectively, the suction, compression, power and exhaust strokes. In
Fig. 3, the piston is shown descending on its suction stroke. Only pure air is drawn into the
cylinder during this stroke through the inlet valve, whereas, the exhaust valve is closed. These
valves can be operated by the cam, push rod and rocker arm. The next stroke is the
compression stroke in which the piston moves up with both the valves remaining closed. The
AG ENGG. 243 Lecture 3 5
air, which has been drawn into the cylinder during the suction stroke, is progressively compressed
as the piston ascends. The compression ratio usually varies from 14:1 to 22:1. The
pressure at the end of the compression stroke ranges from 30 to 45 kg/cm2. As the air is
progressively compressed in the cylinder, its temperature increases, until when near the end of
the compression stroke, it becomes sufficiently high (650-80O oC) to instantly ignite any fuel
that is injected into the cylinder. When the piston is near the top of its compression stroke, a
liquid hydrocarbon fuel, such as diesel oil, is sprayed into the combustion chamber under
high pressure (140-160 kg/cm2), higher than that existing in the cylinder itself. This fuel
then ignites, being burnt with the oxygen of the highly compressed air.
During the fuel injection period, the piston reaches the end of its compression stroke and
commences to return on its third consecutive stroke, viz., power stroke. During this stroke
the hot products of combustion consisting chiefly of carbon dioxide, together with the
nitrogen left from the compressed air expand, thus forcing the piston downward. This is only
the working stroke of the cylinder.
During the power stroke the pressure falls from its maximum combustion value (47-55
kg/cm2), which is usually higher than the greater value of the compression pressure (45
kg/cm2), to about 3.5-5 kg/cm2 near the end of the stroke. The exhaust valve then opens,
usually a little earlier than when the piston reaches its lowest point of travel. The exhaust
gases are swept out on the following upward stroke of the piston. The exhaust valve remains
open throughout the whole stroke and closes at the top of the stroke.
The reciprocating motion of the piston is converted into the rotary motion of the crankshaft
by means of a connecting rod and crankshaft. The crankshaft rotates in the main bearings,
which are set in the crankcase. The flywheel is fitted on the crankshaft in order to smoothen
out the uneven torque that is generated in the reciprocating engine.
TWO-STROKE CYCLE DIESEL ENGINE:
The cycle of the four-stroke of the piston (the suction, compression, power and exhaust
strokes) is completed only in two strokes in the case of a two-stroke engine. The air is drawn
into the crankcase due to the suction created by the upward stroke of the piston. On the down
stroke of the piston it is compressed in the crankcase, The compression pressure is usually
very low, being just sufficient to enable the air to flow into the cylinder through the transfer
port when the piston reaches near the bottom of its down stroke.
The air thus flows into the cylinder, where the piston compresses it as it ascends, till the
piston is nearly at the top of its stroke. The compression pressure is increased sufficiently
Fig. 3. Principle of four-stroke engine
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high to raise the temperature of the air above the self-ignition point of the fuel used. The fuel
is injected into the cylinder head just before the completion of the compression stroke and
only for a short period. The burnt gases expand during the next downward stroke of the
piston. These gases escape into the exhaust pipe to the atmosphere through the piston
uncovering the exhaust port.
Modern Two-Stroke Cycle Diesel Engine
The crankcase method of air compression is unsatisfactory, as the exhaust gases do not escape
the cylinder during port opening. Also there is a loss of air through the exhaust ports during
the cylinder charging process. To overcome these disadvantages blowers are used to precompress
the air. This pre-compressed air enters the cylinder through the port. An exhaust
valve is also provided which opens mechanically just before the opening of the inlet ports
(Fig. 4).
FOUR-STROKE SPARK IGNITION ENGINE
In this gasoline is mixed with air, broken up into a mist and partially vaporized in a
carburettor (Fig. 5). The mixture is then sucked into the cylinder. There it is compressed by
the upward movement of the piston and is ignited by an electric spark. When the mixture is
burned, the resulting heat causes the gases to expand. The expanding gases exert a pressure
on the piston (power stroke). The exhaust gases escape in the next upward movement of the
piston. The strokes are similar to those discussed under four-stroke diesel engines. The
various temperatures and pressures are shown in Fig. 6. The compression ratio varies from
4:1 to 8:1 and the air-fuel mixture from 10:1 to 20:1.
Fig. 5. Principle of operation of four-stroke petrol engine
Fig. 4 Principle of two-stroke cycle diesel engine
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TWO-STROKE CYCLE PETROL ENGINE
The two-cycle carburettor type engine makes use of an airtight crankcase for partially
compressing the air-fuel mixture (Fig. 6). As the piston travels down, the mixture previously
drawn into the crankcase is partially compressed. As the piston nears the bottom of the stroke,
it uncovers the exhaust and intake ports. The exhaust flows out, reducing the pressure in the
cylinder. When the pressure in the combustion chamber is lower than the pressure in the
crankcase through the port openings to the combustion chamber, the incoming mixture is
deflected upward by a baffle on the piston. As the piston moves up, it compresses the mixture
above and draws into the crankcase below a new air-fuel mixture.
The, two-stroke cycle engine can be easily identified by the air-fuel mixture valve attached
to the crankcase and the exhaust Port located at the bottom of the cylinder.
COMPARISON OF CI AND SI ENGINES
The CI engine has the following advantages over the SI engine.
1. Reliability of the CI engine is much higher than that of the SI engine. This is because in
case of the failure of the battery, ignition or carburettor system, the SI engine cannot
operate, whereas the CI engine, with a separate fuel injector for each cylinder, has less
risk of failure.
2. The distribution of fuel to each cylinder is uniform as each of them has a separate
injector, whereas in the SI engine the distribution of fuel mixture is not uniform, owing to
the design of the single carburettor and the intake manifold.
3. Since the servicing period of the fuel injection system of CI engine is longer, its
maintenance cost is less than that of the SI engine.
4. The expansion ratio of the CI engine is higher than that of the SI engine; therefore, the
heat loss to the cylinder walls is less in the CI engine than that of the SI engine.
Consequently, the cooling system of the CI engine can be of smaller dimensions.
5. The torque characteristics of the CI engine are more uniform which results in better top
gear performance.
6. The CI engine can be switched over from part load to full load soon after starting from
cold, whereas the SI engine requires warming up.
7. The fuel (diesel) for the CI engine is cheaper than the fuel (petrol) for SI engine.
8. The fire risk in the CI engine is minimised due to the absence of the ignition system.
9. On part load, the specific fuel consumption of the CI engine is low.
Fig. 6 Principle of operation of two stroke petrol enine
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ADVANTAGES AND DISADVANTAGES OF TWO-STROKE CYCLE OVER
FOUR-STROKE CYCLE ENGINES
Advantages:
1) The two-stroke cycle engine gives one working stroke for each revolution of the
crankshaft. Hence theoretically the power developed for the same engine speed and
cylinder volume is twice that of the four-stroke cycle engine, which gives only one
working stroke for every two revolutions of the crankshaft. However, in practice,
because of poor scavenging, only 50-60% extra power is developed.
2) Due to one working stroke for each revolution of the crankshaft, the turning moment on
the crankshaft is more uniform. Therefore, a two-stroke engine requires a lighter
flywheel.
3) The two-stroke engine is simpler in construction. The design of its ports is much
simpler and their maintenance easier than that of the valve mechanism.
4) The power required to overcome frictional resistance of the suction and exhaust strokes
is saved, resulting in some economy of fuel.
5) Owing to the absence of the cam, camshaft, rockers, etc. of the valve mechanism, the
mechanical efficiency is higher.
6) The two-stroke engine gives fewer oscillations.
7) For the same power, a two-stroke engine is more compact and requires less space than a
four-stroke cycle engine. This makes it more suitable for use in small machines and
motorcycles.
8) A two-stroke engine is lighter in weight for the same power and speed especially when
the crankcase compression is used.
9) Due to its simpler design, it requires fewer spare parts.
10) A two-stroke cycle engine can be easily reversed if it is of the valve less type.
Disadvantages:
1. The scavenging being not very efficient in a two-stroke engine, the dilution of the
charges takes place which results in poor thermal efficiency.
2. The two-stroke spark ignition engines do not have a separate lubrication system and
normally, lubricating oil is mixed with the fuel. This is not as efrective as the
lubrication of a four-stroke engine. Therefore, the parts of the two-stroke engine are
subjected to greater wear and tear.
3. In a spark ignition two-stroke engine, some of the fuel passes directly to the exhaust.
Hence, the fuel consumption per horsepower is comparatively higher.
4. With heavy loads a two-stroke engine gets heated up due to the excessive heat produced.
At the same time the running of the engine is riot very smooth at light loads.
5. It consumes more lubricating oil because of the greater amount of heat generated.
6. Since the ports remain open during the upward stroke, the actual compression starts
only after both the inlet and exhaust ports have been closed. Hence, the compression
ratio of this engine is lower than that of a four-stroke engine of the same dimensions.
As the efficiency of an engine is directly proportional to its compression ratio, the
efficiency of a two-stroke cycle engine is lower than that of a four-stroke cycle engine
of the same size.