Heat engine engine is a device which transforms chemical energy of fuel and thermal energy to produce mechanical work engine normally convert thermal energy into mechanical energy and therefore they are called heat engines.

Heat engine

1.external combustion engine e.g- boiler

2.internal combustion engine

e.g -petrol engine/diesel engine/gas turbine

Internal combustion engine

A) continuous internal combustion engine (steady energy flow )
E.g -gas turbine,rocket

B) Intermittent internal combustion engine (non steady energy flow )

Intermittent internal combustion engine

A)S.I Engine (premixed charge )
a) four stroke
b) two stroke

B)C.I Engine (Non – premix charge )
a)four stroke
b)two stroke

Internal combustion engine :-

In an internal combustion engine, the power provided by a combustion fuel is transformed directly into mechanical energy through controlled combustion of fuel in an enclosed space.

The explosive fuel -air mixture may be ignited by an electric Spark or by the resulting compression temperature .
In reciprocating engine, explosion causes the rotation of some engine parts (crankshaft ) by driving the piston in the cylinder. The motion is transmitted to the crankshaft by means of the connecting rod.

we can classify internal combustion engine according to number of strokes of the Piston in one complete working cycle named as two stroke and four stroke engine.

Engine cycles

The term cycles refers to one complete sequence of operation required to produce power in an engine. This cycle of operations is continuously repeated while the engine is running.For a diesel engine it consists of four operation within the cylinder.

1.Compression of charge of air.

2.Injection of fuel which then ignite

3.Expansion of the hot gases formed during combustion.

4.Expulsion of the used gas to exhaust

The complete cycle of event in two stroke engine that means suction, compression,expansion and exhaust is accomplished in a single revolution of crankshaft

Or, 2 stroke Piston as compression and expansion of the charge takes place during one stroke while the admission of the fresh charge occurred during the other stroke, simultaneously with escape of burning gases.

In four stroke engine, the complete engine cycle is accomplished within 2 revolution of crankshaft.


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