Heat Engines: External Combustion Engine, Internal Combustion Engine, Four stroke petrol engine and Two stroke petrol engine ~ MECHTECH GURU

Heat Engines: External Combustion Engine, Internal Combustion Engine, Four stroke petrol engine and Two stroke petrol engine


Heat Engines

Any type of engine or machine which derives heat energy from the combustion of fuel or any other source and converts this energy into mechanical work is termed as a heat engine. Heat engines may be classified into two main classes as follows :

1. External Combustion Engine.

2. Internal Combustion Engine.

1. External Combustion Engines (E.C. Engines)

In this case, combustion of fuel takes place outside the cylinder as in 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. These engines are generally needed for driving locomotives, ships, generation of electric power etc.

2. Internal Combustion Engines (I.C. Engines)

In this case combustion of the 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.

Development of I.C. Engines

Many experimental engines were constructed around 1878. The first really successful engine did not appear, however until 1879, when a German engineer Dr. Otto built his famous Otto gas engine. The operating cycle of this engine was based upon principles first laid down in 1860 by a French engineer named Bea de Rochas. The majority of modern I.C. engines operate according to these principles.

The development of the well known Diesel engine began about 1883 by Rudoff Diesel. Although this differs in many important respects from the otto engine, the operating cycle of modern high speed Diesel engines is thermodynamically very similar to the Otto cycle.

Different parts of I.C. Engines

A cross-section of an air-cooled I.C. engines with principal parts is shown in Fig.

A. Parts common to both petrol and diesel engines

1. Cylinder 

2. Cylinder head 

3. Piston

4. Piston rings 

5. Gudgeon pin 

6. Connecting rod

7. Crankshaft 

8. Crank 

9. Engine bearing

10. Crank case 

11. Flywheel 

12. Governor

13. Valves and valve operating mechanism.

B. Parts for petrol engines only

1. Spark plugs 

2. Carburettor 

3. Fuel pump.

C. Parts for Diesel engine only

1. Fuel pump. 

2. Injector.

An air-cooled four-stroke petrol engine
An air-cooled four-stroke petrol engine

Spark Ignition (S.I.) Engines

These engines may work on either four stroke cycle or two stroke cycle, majority of them, of course, operate on four stroke cycle.

Four stroke petrol engine

Fig. illustrates the various strokes/series of operations which take place in a four stroke petrol (Otto cycle) engine.

Suction stroke

 During suction stroke a mixture of air and fuel (petrol) is sucked through the inlet valve (I.V.). The exhaust valve remains closed during this operation.

Compression stroke

 During compression stroke, both the valves remain closed, and the pressure and temperature of the mixture increase. Near the end of compression stroke, the fuel is ignited by means of an electric spark in the spark plug, causing combustion of fuel at the instant of ignition. Working stroke. Next is the working (also called power or expansion) stroke. During this stroke, both the valves remain closed. Near the end of the expansion stroke, only the exhaust valve opens and the pressure in the cylinder at this stage forces most of the gases to leave the cylinder.

Exhaust stroke

 Next follows the exhaust stroke, when all the remaining gases are driven away from the cylinder, while the inlet valve remains closed and the piston returns to the top dead centre. The cycle is then repeated.

Four stroke otto cycle engine.
Four stroke otto cycle engine.

Two stroke petrol engine

In 1878, Dugald-clerk, a British engineer introduced a cycle which could be completed in two strokes of piston rather than four strokes as is the case with the four stroke cycle engines. The engines using this cycle were called two stroke cycle engines. In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh change of fuel entering the cylinder nearly at the end of the working stroke.

Fig.( below) shows a two stroke petrol engine (used in scooters, motor cycles etc.). The cylinder L is connected to a closed crank chamber C.C. During the upward stroke of the piston M, the gases in L are compressed and at the same time fresh air and fuel (petrol) mixture enters the crank chamber through the valve V. When the piston moves downwards, V closes and the mixture in the crank chamber is compressed. Refer Fig (i) the piston is moving upwards and is compressing an explosive change which has previously been supplied to L. Ignition takes place at the end of the stroke. The piston then travels downwards due to expansion of the gases [Fig. (ii)] and near the end of this stroke the piston uncovers the exhaust port (E.P.) and the burnt exhaust gases escape through this port [Fig. (iii)]. The transfer port (T.P.) then is uncovered immediately, and the compressed charge from the crank chamber flows into the cylinder and is deflected upwards by the hump provided on the head of the piston. It may be noted that the incoming air petrol mixture helps the removal of gases from the engine-cylinder ; if, in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease. The piston then again starts moving from bottom dead centre (B.D.C.) to top dead centre (T.D.C.) and the charge gets compressed when E.P. (exhaust port) and T.P. are covered by the piston ; thus the cycle is repeated.

Two-stroke petrol engine.
Two-stroke petrol engine.

The power obtained from a two-stroke cycle engine is theoretically twice the power obtainable from a four-stroke cycle engine.

Compression Ignition (C.I.) Engines

The operation of C.I. engines (or diesel engines) is practically the same as those of S.I. engines. The cycle in both the types, consists of suction, compression, ignition, expansion and exhaust. However, the combustion process in a C.I. engine is different from that of a S.I. engine as given below: 

In C.I. engine, only air is sucked during the stroke and the fuel is injected in the cylinder near the end of the compression stroke. Since the compression ratio is very high (between 14 : 1 to 22 : 1), the temperature of the air after compression is quite high. So when fuel is injected in the form of a spray at this stage, it ignites and burns almost as soon as it is introduced. The burnt gases are expanded and exhausted in the same way as is done in a S.I. engine.


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