MODULE III

INTERNAL COMBUSTION (IC) ENGINES

2.1. I. C. Engines: An Internal combustion engine more probably called as IC Engine, is a heat
engine which converts heat energy released by the combustion of fuel taking place inside the
engine cylinder into mechanical work. It has advantages such as high efficiency, light weight,
compactness, easy starting, adaptability, suitability for mobile applications, comparatively lower
initial cost has made its use as a prime mover.

2.1.1. Classification of IC Engines:

i. Nature of Thermodynamic cycle as:

1. Otto Cycle engine. 2. Diesel engine. 3. Dual combustion cycle engine.
ii. Type of Fuel used as:
1. Petrol Engine 2. Diesel engine. 3. Gas engine. 4. Bi-fuel engine.
iii. Number of strokes as:
1. Four stroke engine. 2. Two stroke engine.
iv. Method of ignition as:
1. Spark ignition engine, known as S.I engine.
2. Compression ignition engine, known as C.I. Engine.
v. Number of cylinders as:
1. Single cylinder engine. 2. Multi cylinder engine.
vi. Position of Cylinder as:
1. Horizontal engine. 2. Vertical engine. 3. V- engine. 4. Opposed cylinder engine.
5. Radial engine.
vii. Method of cooling as:
1. Air cooled engine. 2. Water cooled engine.

2.2. Parts of I.C. Engines:

1. Cylinder: The heart of the engine is the cylinder in which the fuel is burnt and the power is
developed. The inside diameter is called bore. To prevent the wearing of cylinder block, a sleeve
will be fitted tightly in the cylinder. The piston reciprocates inside the cylinder.

2. Piston: The piston is a close fitting hollow cylindrical plunger moving to-and-fro in the cylinder.
The power developed by the combustion of the fuel is transmitted by the piston to the crankshaft
through the connecting rod.

3. Piston rings: The piston rings are the metallic rings inserted into the circumferential grooves
provided at the top end of the piston. These rings maintain a gas-tight joint between the piston and
the cylinder while the piston is reciprocating in the cylinder. They also help in conducting the heat
from the piston to the cylinder.

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 Fig 2.1 Parts of I. C. Engine

4. Connecting rod: It is a link that connects the piston and the crankshaft by means of pin joints. It
converts the rectilinear motion of the piston into rotary motion of the crankshaft.

5. Crank and crankshaft: The crank is lever that is connected to the end of the connecting rod by a
pin joint with its other end rigidly connected to a shaft called crankshaft. It rotates about the axis
of the crankshaft and causes the connecting rod to oscillate.

6. Crank case: It is the lower part of the engine serving as an enclosure for the crankshaft and also
sump for the lubricating oil.

7. Valves: The valves are the devices which controls the flow of the intake and the exhaust gas to
and from the cylinder. They are also called poppet valves. These valves are operated by means of
cams driven by crankshaft through a timing gear and chain.

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8. Fly wheel: It is a heavy wheel mounted on the crankshaft of the engine to maintain uniform
rotation of the crankshaft.

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4- STROKE PETROL ENGINE: (S. I. Engine)

Petrol engines works on the principle of theoretical Otto cycle, also known as constant volume
cycle. It consists of cylinder, piston, connecting rod, crank, crankshaft, inlet valve, exhaust valve
and spark plug. The spark plug fitted at the top of the cylinder initiates the ignition of the petrol,
hence the name spark ignition engine.

Fig .2.6. Four Stroke Petrol Engine.

1. SUCTION STROKE:

• During this stroke the piston moves from TDC to BDC. The inlet valve is open and exhaust valve
is closed. The crankshaft rotates by half a rotation. As the piston moves downwards, suction is
created in the cylinder, as a result, fresh air-petrol mixture is drawn into the cylinder through the
inlet valve. At the end of this stroke, the piston is in BDC, the cylinder is filled with air-petrol
mixture and inlet valve closes. Horizontal line AB on the P-V diagram.

Fig .2.7. Theoretical Otto Cycle

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2. COMPRESSION STROKE:

• During this stroke the piston moves from BDC to TDC. Both the inlet valve and exhaust valves
are closed. The crankshaft rotates by half a rotation. As the piston moves upwards, the fuel
mixture in the cylinder will be compressed. The ratio of compression ratio in petrol engines ranges
from 7:1 to 11:1, represented by the BC curve in the P-V diagram. When the piston reaches TDC,
the spark plug ignites the fuel mixture. Since the spark plug ignites the fuel (air-petrol), this type
of engine is also called as spark ignition or S.I Engine. The combustion of fuel takes place
increasing the pressure at constant volume, represented by the line CD in the P-V diagram.

3. WORKING OR POWER STROKE:

• During this stroke the piston moves from TDC to BDC. Both the inlet valve and exhaust valves
are closed. The crankshaft rotates by half a rotation. The high pressure of the burnt gases forces
the piston downwards performing power stroke. The linear motion of the piston is converted to
rotary motion of the crankshaft by connecting rod and crank. It is represented by curve on DE on
PV diagram. At the end of the stroke, the piston is in BDC, the exhaust valve opens which release
the burnt gases to the atmosphere. This will bring pressure in the cylinder to atmospheric at
constant volume, represented by the line EB in the P-V diagram.

4. EXHAUST STROKE:

• During this stroke the piston moves from BDC to TDC. The inlet valve is closed and exhaust
valve is open. The crankshaft rotates by half a rotation. As the piston moves towards the TDC, the
burnt gases will be expelled out through the exhaust valve. Line BA on the P-V diagram. When
the piston reaches the TDC, the exhaust valve closes and this completes the cycle.

4 STROKE DIESEL ENGINE: (C. I. Engine)

Diesel engines works on the principle of theoretical Diesel cycle, also known as constant pressure
cycle. It consists of cylinder, piston, connecting rod, crank, crankshaft, inlet valve, and exhaust
valve and fuel injector. The fuel injector fitted at the top of the cylinder supplies the measured
quantity of diesel at high pressure.

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 Fig 2.8. Four Stroke Diesel Engine.

1. SUCTION STROKE:

• During this stroke the piston moves from TDC to BDC. The inlet valve is open and exhaust valve
is closed. The crankshaft rotates by half a rotation. As the piston moves downwards, suction is
created in the cylinder, as a result, fresh air is drawn into the cylinder through the inlet valve. At
the end of this stroke, the piston is in BDC, the cylinder is filled with air and inlet valve closes.
Horizontal line AB on the P-V diagram.

Fig 2 .9. Theoretical Diesel Engine

2. COMPRESSION STROKE:

• During this stroke the piston moves from BDC to TDC. Both the inlet valve and exhaust valves
are closed. The crankshaft rotates by half a rotation. As the piston moves upwards, the air in the
cylinder will be compressed. The ratio of compression ratio in diesel engines ranges from 16:1 to
22:1, represented the BC curve in the P-V diagram. As the air gets compressed its pressure and
temperature increases and attains a temperature greater than the ignition temperature of diesel.
Diesel is sprayed into the cylinder through the fuel injector. The high temperature of the air

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 ignites the diesel as soon as it is sprayed and undergoes combustion at constant pressure. Line CD
on the P-V diagram. Since the compresses air ignites the diesel, this type of engine is also called
as compression ignition or C.I Engine.

3. WORKING OR POWER STROKE:

• During this stroke the piston moves from TDC to BDC. Both the inlet valve and exhaust valves
are closed. The crankshaft rotates by half a rotation. The high pressure of the burnt gases forces
the piston downwards performing power stroke. The linear motion of the piston is converted to
rotary motion of the crankshaft by connecting rod and crank. It is represented by curve DE on PV
diagram. At the end of the stroke, the piston is in BDC, the exhaust valve opens which release the
burnt gases to the atmosphere. This will bring pressure in the cylinder to atmospheric at constant
volume, represented by the line EB in the P-V diagram.

4. EXHAUST STROKE:

• During this stroke the piston moves from BDC to TDC. The inlet valve is closed and exhaust
valve is open. The crankshaft rotates by half a rotation. As the piston moves towards the TDC, the
burnt gases will be expelled out through the exhaust valve. Line BA on the P-V diagram. When
the piston reaches the TDC, the exhaust valve closes and this completes the cycle.

In 4 stroke engine, the 4 strokes constitute one cycle, hence the name 4 stroke cycle engine. The
crankshaft makes two revolutions to complete one cycle. The power is developed in every
alternate revolution of the crankshaft. 4 Stroke diesel engines produce higher power than 4 Stroke
petrol engines.

2.4. Two - Stroke Engine:

A 2 stroke engine performs only TWO strokes to complete one cycle. Crankshaft makes only one
revolution to complete the cycle. The power is developed in every revolution of the crankshaft.
Based on the type of fuel used they are classified as

1) 2-Stroke Petrol engine

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 Fig.2.3. Two Stroke engine parts

2.4.1. Two-Stroke Petrol Engine:

The 2-stroke engine cylinder has inlet, exhaust and transfer ports on its circumference, as shown
in fig.3.

Inlet port – admits fresh air-fuel mixture (charge) into the crankcase.

Transfer port – transfers the charge from the crankcase into the cylinder.

Exhaust port – discharges the burnt gases from the cylinder.

These ports are opened and closed by the reciprocating piston. The connecting rod and the crank
convert the reciprocating motion of the piston into the rotary motion of the crankshaft.

FIRST STROKE:

• Piston moves from TDC to BDC. The spark plug ignites the compressed petrol and air mixture
(charge). The hot gases are released during combustion increasing the pressure in the cylinder
which forces the piston downwards. The piston moves downwards performing the power stroke
until the top of the piston uncovers the exhaust port. The burnt gases escape through the exhaust
port. As the piston descends it covers the inlet port and uncovers the transfer port and charge
flows from crankcase into the cylinder.

• This charge entering the cylinder drives out the remaining burnt gases through the exhaust port
and the process is called scavenging. This process continues till the piston covers both exhaust &
transfer port during the next ascending stroke. The crankshaft rotates by half rotation.

SECOND STROKE:

• Piston moves from BDC to TDC. As the piston ascends, it covers the transfer port and the supply
of charge to the cylinder is cut-off. Further upward movement covers exhaust port and
compression of the charge begins. In the mean time, inlet port opens and fresh charge enters the
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crankcase. Further ascend of piston will compress the charge in the cylinder. The compression
ratio ranges from 7:1 to 11:1. After piston reaches TDC first stroke repeats again. The crank
rotates by half rotation.

Fig.4. Two Stroke Petrol Engine.

COMPARISON OF 4 STROKE AND 2 STROKE ENGINE:

PRINCIPLE 4 STROKE 2 STROKE

1. Number of strokes per Four Two

cycle

12. Uses Cars, trucks, tractors, jeeps, buses, Mopeds, scooters, motor
etc., cycles, etc.,

3. Power Developed In every alternate revolution of the In every revolution of the

crankshaft crankshaft

4. Flywheel Heavy Light

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 5. Admission of charge Directly to the engine cylinder First to the crankcase & then
transferred to the engine
cylinder

6. Exhaust gases Driven through the outlet during Driven out by scavenging
exhaust stroke operation

7. Valves Opened & closed by mechanical Opened & closed by piston

valves

8. Noise Less High

9.Lubricating oil Less More

consumption

10. Fuel consumption Less More

11. Mechanical efficiency Low High

COMPARISON OF PETROL AND DIESEL ENGINE:

1. Cycle of operation Otto cycle (constant volume) Diesel cycle (constant

pressure)

2. Fuel used Petrol Diesel

3. Admission of fuel During suction stroke At the end of compression

stroke.

4. Charge drawn during Air and petrol mixture Only air

suction

5. Compression ratio 7:1 to 12:1 16:1 to 22:1

6. Type of ignition Spark ignition Compression or auto

ignition

7. Uses Scooter, motor cycle, car, etc., Trucks, tractors, buses, etc.,

8. Engine speed High about 7000rpm Low from 500 to 3000rpm

9. Power output capacity Less More

10. Thermal efficiency Less High

11. Noise & vibration Almost nil High

12. Weight of the engine Less High

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13. Initial cost Less More

14. Operating cost High Less

15. Maintenance cost Less Slightly higher

16. Starting of the engine Easily started Difficult to start in cold

weather

17. Exhaust gas pollution Less More

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 REFRIGERATION AND AIRCONDITIONING

Refrigeration: It is defined as a method of reducing temperature of a system below

that of the surroundings and maintaining it at the lower temperature by continuously
abstracting the heat from it.

Refrigerant: The medium or working substance that continuously extracts heat from
the space within the refrigerator which is to be kept cool at temperature less than
atmospheric by rejecting heat to atmosphere is called refrigerant.
Refrigeration concepts:
1. Heat flows from a system at higher temperature to a system at lower
temperature.
2. Fluids absorb heat, change from liquid phase to vapor phase and condenses
back to liquid while by giving off heat.
3. The boiling and freezing temperatures of fluid depends on its pressure.
4. Heat can flow from a system at lower temperature to a system at higher
temperature only with the aid of external work.
Refrigerating effect: The rate at which the heat is absorbed in a cycle from the
interior space to be cooled is called refrigerating effect.

Unit of refrigeration: The capacity of refrigeration system is expressed in tons of

refrigeration.

A ton of refrigeration is defined as the quantity of heat absorbed in order to form one
ton of ice in 24hours from water at 00C.

In S.I System

1 ton of refrigeration = 210 kJ/min

= 3.5 Kw

Coefficient of performance: The coefficient of performance (COP) of a refrigeration

system is defined as the ratio of the refrigerating effect (heat absorbed or removed) to
the work supplied.

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If Q = Heat absorbed or Removed, kW

W = Work supplied, kW

Q
Then, COP = ⁄W

Ice making capacity

The capacity of a Refrigerating system to make ice beginning from water (at water
temperature) to solid ice. It is usually specified by kg/hr.

Relative COP

It is defined as the ratio of Actual COP to the Theoretical COP of a refrigerator.

Relative COP = Actual COP/Theoretical COP

Refrigerants commonly used:

1. Ammonia – in vapor absorption refrigerator.

2. Carbon dioxide – in marine refrigerators.

3. Sulphur dioxide – in household refrigerators.

4. Methyl chloride – in small scale & domestic refrigerators.

5. Freon – 12 (Dichlorodifluoromethane) – in domestic vapor

compression refrigerators.

6. Freon – 22 (difluoromonochloromethane) – in air conditioners.

Properties of a good refrigerant:

• Must have low boiling point.

• Must have low freezing point.

• Evaporator & condenser pressure should be slightly above the atmospheric

pressure.

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 • Latent heat of evaporation must be very high.

• Specific volume must be very low.

• Toxicity - should be non-toxic.

• Flammability - should not be flammable.

• Corrosiveness - should be non-corrosive.

• COP must be high.

• Odour - must be odourless.

• Leakage should be easily detectable.

VAPOUR COMPRESSION REFRIGERATOR

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• It mainly consists of a compressor, a throttle valve, a condenser and an
evaporator made of coiled tubes installed in the freezing compartment of the
refrigerator.
• The refrigerant at low pressure and temperature passing in the evaporator
coiled tubes absorbs heat from the contents in the freezing compartment and
evaporates.
• This lowers the temperature of freezing compartment.
• The vapour refrigerant at low pressure from evaporator is drawn by the
compressor which compresses it to high pressure.
• This increase in pressure increases the saturation temperature of the refrigerant
higher than the temperature of the cooling medium (atmospheric air) in the
condenser so that vapour can reject heat in the condenser.
• In the condenser it gives off its latent heat to the atmosphere air and condenses
to liquid.
• The high pressure liquid refrigerant now flows to the throttle valve in which it
expands to a low pressure.
• Temperature reduces to -10°C and vapour will be wet.
• This wet vapour now passes to the evaporator coils where it absorbs heat from
the surrounding and the cycle repeats.
• Thus heat is continuously removed from the contents of the refrigerator in the
evaporator and rejected in the condenser to the atmospheric air.
• This will keep the contents of the refrigerator at lower temperature.
• The most commonly used refrigerant in vapour compression refrigerator is
dichlorodifluoromethane popularly known as Freon 12.

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VAPOR ABSORPTION REFRIGERATION SYSTEM

• This refrigerator mainly consists of an absorber, a circulating pump, heat

exchanger, generator, condenser, expansion valve and evaporating coiled
tubes.

• Low pressure ammonia vapor is dissolved in the cold water contained in the
absorber, which will produce a strong ammonia solution.

• The strong ammonia solution from absorber is pumped to heat exchanger

where it is warmed by the warm weak ammonia solution flowing back from
the heat separator.

• The warm high pressure ammonia solution now passes to the heat separator
where it is heated by heating coils.

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 • The heating will drive out the ammonia vapor from it. Now the solution in
heat separator becomes weak and flows back to the heat exchanger where it
warms up the strong ammonia solution passing through it.

• The high pressure ammonia vapor from heat separator now passes to a
condenser, where it rejects heat and is condensed. (liquid)

• The high pressure ammonia liquid is now expanded to low pressure and low
temperature in the throttle valve.

• The low pressure condensed ammonia liquid at low temperature is passed

onto the evaporator coils provided in the freezing compartment, where it
absorbs the heat and evaporates.

• The low pressure ammonia vapor from freezing compartment is passed again
to the absorber and the cycle repeats.

Comparison between Vapor Compression and Vapor Absorption Systems:

Principle Vapor compression Vapor absorption

Working method Refrigerant vapor is compressed Refrigerant vapor is absorbed and heated
Type of energy Mechanical Heat
supplied

Work or energy To compress refrigerant To run the pump

supplied

COP Higher Relatively low & remains same

Capacity Up to 1000 tons Above 1000 tons
Noise More Almost quiet
Refrigerant Freon-12 Ammonia
Leakage problem Chances are more No leakage
Maintenance High Less
Operating cost High Less

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AIR CONDITIONING:

Providing a cool congenial indoor atmosphere by cooling, humidifying, or

dehumidifying, cleaning and recirculating the surrounding air is called air
conditioning. The artificial cooling of air and conditioning it to provide maximum
comfort to human beings is called comfort air conditioning.

The artificial cooling of air and conditioning it to provide a controlled

atmosphere required in some engineering, manufacturing and processing is called
industrial air conditioning.

ROOM AIR CONDITIONER AND PRINCIPLES OF AIR CONDITIONING:

An air conditioner continuously draws air from an indoor space to be cooled,

cools it by the refrigeration principles and discharges back into the same indoor space
that needs to be cooled.

It mainly consists of an evaporator, condenser, compressor, two fans one each

for evaporator and condenser units usually driven by the single motor, capillary, etc. It
is generally mounted on a window sill such that the evaporator unit is inside the room
and the condenser part projecting outside the building.

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• The high pressure, high temperature liquid refrigerant from the condenser is
passed to the evaporator coils through the capillary tube where it undergoes
expansion.

• The refrigerant in evaporator coils absorbs heat from the air passing over it
from the interior and evaporates.

• The high temperature evaporated refrigerant is compressed to high pressure

by a compressor and delivered to the condenser, where it is cooled or
condensed to liquid by giving off the heat to the atmospheric air passing over
it.

• The cooled high pressure refrigerant now passes through the capillary tube
where it undergoes expansion and again re-circulated to repeat the cycle
continuously.

ABSOLUTE HUMIDITY: It is defined as ratio of water vapour contained in a

given volume of air.

SPECIFIC HUMIDITY: It is defined as the ratio of weight of water vapour to

the total weight of air.

RELATIVE HUMIDITY: It is defined as the ratio of the actual vapour content

of the air to the vapour content of the air at the same temperature when saturated
with water vapour.

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