Strokes of Engine Operation Cycle

The following sequences of strokes make up the operation of engine:

1. Suction stroke

In this stroke, a mixture of fuel-vapour in correct proportion, is supplied to the engine cylinder.

2. Compression stroke

In this stroke, the fuel-vapour is first compressed in the engine cylinder.

3. Expansion or working stroke

In this stroke, the fuel-vapour is fired just before the compression is complete. It results in the sudden rise of pressure, due to expansion of the combustion products in the engine cylinder. The sudden rise of the pressure pushes the piston with a great force, and rotates the crankshaft. The crankshaft in turn drives the gearbox connected to it.

4. Exhaust stroke

In this stroke, the burnt gases (or combustion products) are exhausted from the engine cylinder, so as to make space available for the fresh fuel-vapour.

Note: The above-mentioned strokes are meant for gas and petrol engines. But in case of diesel engines, pure air is sucked in suction stroke which is compressed during the compression stroke. The diesel fuel is admitted into the engine cylinder (just before the beginning of the expansion stroke) and it is ignited by the hot air present in the cylinder. The expansion and exhaust strokes are similar to the gas and the petrol engines.

Two Stroke and Four Stroke Cycle Engines

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