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�� INTERNAL COMBUSTION ENGINE

PREPARED BY ER. GOSTA PANDA

DEPARTMENT OF MECHANICAL ENGINEERING

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Introduction of I.C. Engine

Heat Engines - A machine or device which derives heat from the combustion of fuel and converts part of this energy into mechanical work is called a heat engine. Heat engines may be classified into two main classes as follows:

1. External combustion engines

2. Internal combustion engines.

1. External Combustion Engines - In this case, combustion of fuel takes place outside the cylinder as in the case of steam engines where the heat of combustion is employed to generate steam which is used to move a piston in a cylinder. Other examples of external combustion engines are hot air engines, steam turbine and closed cycle gas turbine.

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Introduction of I.C. Engine (contd..)

2. Internal Combustion Engines - In this case, combustion of fuel with oxygen of the air occurs within the cylinder of the engine. The internal combustion engines group includes engines employing mixtures of combustible gases and air, known as gas engines, those using lighter liquid fuel or spirit known as petrol engines and those using heavier liquid fuels, known as oil, compression ignition or diesel engines.

The important applications of I.C. engines are: (i) Road vehicles, locomotives, ships and aircraft, (ii) Portable standby units for power generation in case of scarcity of electric power, (iii) Extensively used in farm tractors, lawn movers, concrete mixing devices and motor boats.

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Classification of I.C. Engines

The internal combustion engines may be classified in the following ways:

1. According to the type of fuel used

a) Petrol engines, b) Diesel engines, and c) Gas engines.

2. According to the method of igniting the fuel

a) Spark ignition engines, and b) Compression ignition

engines.

3. According to the number of strokes per cycle

a) Four stroke cycle engines, and b) Two stroke cycle

engines.

4. According to the cycle of operation

a) Otto cycle engines, b) Diesel cycle engines, and c) Dual

cycle engines.

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Classification of I.C. Engines (contd..)

5. According to the speed of the engine

a) Slow speed engines, b) Medium speed engines, and

c) High speed engines.

6. According to the cooling system

a) Air-cooled engines, and b) Water-cooled engines.

7. According to the method of fuel injection

a) Carburettor engines, and b) Air injection engines.

8. According to the number of cylinders

a) Single cylinder engines, and b) Multi-cylinder engines.

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Classification of I.C. Engines (contd..)

9. According to the arrangement of cylinders

a) Vertical engines, b) Horizontal engines, c) Radial engines,

d) In-line multi-cylinder engines, e) V-type multi-cylinder

engines,

f) Opposite-cylinder engines, and g) Opposite-piston engines.

10. According to the valve mechanism

a) Overhead valve engines, and b) Side valve engines.

11. According to the method of governing

a) Hit and miss governed engines, b) Quantitatively

governed engines, and Qualitatively governed engines.

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Fig. Air-cooled I.C. engine

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Fig. Terms relating I.C. engines

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Terms relating to I.C. Engines (contd..)

5. Clearance volume – The volume contained in the cylinder above the top of the piston, when the piston is at top dead centre, is called the clearance volume.

6. Swept volume – The volume swept through by the piston in moving between top dead centre and bottom dead centre, is called swept volume or piston displacement. Thus, when piston is at bottom dead centre,

Total volume = swept volume + clearance volume.

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Two-stroke and Four-stroke Cycle

In a two-stroke engine, the working cycle is completed in two strokes of the piston or one revolution of the crankshaft. This is achieved by carrying out the suction and compression processes in one stroke (or in inward stroke), expansion and exhaust process in thr second stroke (or in outward stroke).

In a four-stroke engine, the working cycle is completed in four strokes of the piston or two revolutions of the crankshaft. This is achieved by carrying out suction, compression, expansion and exhaust processes in each stroke.

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Fig. Two-stroke cycle petrol engine

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Fig. p-V diagram for a two-stroke cycle engine

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Four-stroke Cycle Petrol Engine

It requires four strokes of the piston to complete one cycle of operation in the engine cylinder. The four strokes of a petrol engine sucking fuel-air mixture (petrol mixed with proportionate quantity of air in the carburettor known as charge) are described below:

1. Suction stroke – In this stroke, the inlet valve opens and charge is sucked into the cylinder as the piston moves downward from TDC. It continues till the piston reaches its BDC as shown in Fig.

2. Compression stroke – In this stroke, both the inlet and exhaust valves are closed and the charge is compressed as the piston moves upwards from BDC to TDC. As a result of compression, the pressure and temperature of the charge increases considerably. This completes one revolution of the crank shaft. The compression stroke is shown in Fig.

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Fig. Four-stroke Otto cycle engine

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Fig. Theoretical p-V diagram of a four-stroke Otto cycle engine

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Four-stroke Cycle Petrol Engine (contd..)

3. Expansion stroke – Shortly before the piston reaches TDC, the charge is ignited with the help of a spark plug. It suddenly increases the pressure and temperature of the products of combustion but the volume, practically remains constant. Due to the rise in pressure, the piston is pushed down with a great force. The hot burnt gases expand due to high speed of the piston. During this expansion, some of the heat energy produced is transformed into mechanical work. During this working stroke, as shown in Fig., both the valves are closed and piston moves from TDC to BDC.

4. Exhaust stroke – In this stroke, the exhaust valve is open as piston moves from BDC to TDC. This movement of the piston pushes out the product of combustion, from the engine cylinder and exhausted through the exhaust valve into the atmosphere, as shown in Fig. This completes the cycle, and the engine cylinder is ready to suck the charge again.

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Four-stroke Cycle Diesel Engine

It is also known as compression ignition engine because the ignition takes place due to the heat produced in the engine cylinder at the end of compression stroke. The four strokes of a diesel engine sucking pure air are described below:

1. Suction stroke – In this stroke, the inlet valve opens and pure air is sucked into the cylinder as the piston moves downwards from the TDC. It continues till the piston reaches its BDC as shown in Fig.

2. Compression stroke – In this stroke, both the valves are closed and the air is compressed as the piston move upwards from BDC to TDC. As a result of compression, pressure and temperature of the air increases considerably. This completes one revolution of the crank shaft. The compression stroke is shown in Fig.