The piston engine is known as the an internal-combustion heat-engine. The concept of the piston engine is that a supply of air-and-fuel mixture is fed to the inside of the cylinder where it is compressed and then burnt. This
internal combustion releases heat energy which is then converted into useful mechanical work as the high gas pressures generated force the piston to move along its stroke in the cylinder. It can be said, therefore, that
a heat-engine is merely an energy transformer.

To enable the piston movement to be harnessed, the driving thrust on the piston is transmitted by means of a connecting-rod to a crankshaft whose function is to convert the linear piston motion in the cylinder to a rotary crankshaft movement . The piston can thus be made to repeat its movement to and fro, due to the constraints of the crankshaft crankpin’s circular path and the guiding cylinder.

The backward-and-forward displacement of the piston is generally referred to as the reciprocating motion of the piston, so these power units are also known as reciprocating engines which show the internal combustion engine

The main problem in understanding the construction of the reciprocating piston engine is being able to identify and name the various parts making up the power unit.