Important terms used in IC engine:


Stroke is the distance covered by the piston between the top dead centre(TDC) and bottom dead centre(BDC) in an IC engine.

Fig: Stroke


Bore is the inner diameter of the cylinder.

Fig: Bore

Mean Piston Speed

It is the average speed of the piston during one revolution in an IC engine. 

= (Piston distance in one revolution)(Rate of the crankshaft rotation)


Where n=no of revolution per sec

Low mean piston speed – 4.5 to 7m/sec

Medium mean piston speed – 7 to 10m/sec

High mean piston speed – 10 to 15m/sec

Bottom Dead Centre(BDC)

It is the point at which the piston of an engine is lowest and  nearest to the axis of the crankshaft during a stroke.

Piston at BDC
Fig: Piston at BDC

Top Dead Centre(TDC)

It is the point at which the piston of an engine is at top during a stroke.

Piston at TDC
Fig: Piston at TDC

Compression ratio

It is the ratio of the volume of the cylinder when the piston is at BDC and at TDC. The higher the ratio, the higher is the power output.

Formula of compression ratio
Compression ratio
Fig: Compression ratio 10:1


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