A piston – is a component of engine. Purpose of piston is to transfer force from expanding gas in the cylinder to the crankshaft via a connecting rod. Piston has cylinder form. It is moving component, which contained by a cylinder and is made gas-tight by piston rings.

Piston consist of: 1 – crown; 2 – piston pin; 3 – ring belt; 4 – gudgeon pin holes; 5 – skirt.

Piston upper surface is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder. Then this force acts along the centre-line of the cylinder downwards through the connecting rod and onto the crankshaft. The connecting rod is attached to the piston by a swivelling wrist pin.

«The force due to this pressure is equal to the cross-sectional area of the cylinder multiplied by the gas pressure».

The crown forms the upper surface on which the gas pressure acts, and the force due to this pressure is equal to the cross-sectional area of the cylinder multiplied by the gas pressure.

Freely piston moving in the cylinder provides some clearance.

To allow the piston to move freely in the cylinder it must have some clearance. This in turn allows gas to leak the combustion chamber past the piston. Since the greatest leakage occurs when pressures are highest and the gas is hottest, much of the oil film lubricating the piston will be burnt away or carbonized. After combustion the gases contain water vapour, carbon dioxide and, probably, small amounts of sulphur dioxide which may contaminate the lubricating oil and lead to corrosion of the engine parts. To reduce the leakage as much as possible piston rings are fitted inti grooves formed on the piston just below the crown.

The crown of the piston is directly exposed to the full heat of the burning gases during combustion. These gases are still extremely hot during the power and exhaust strokes the piston absorbs a great deal of heat from these hot gases and will reach a very high temperature unless heat is removed from the piston quickly enough to keep its temperature within reasonable limits. The piston can pass this heat on to the cylinder walls through the piston rings and skirt, and it can do this better if the metal of which the piston is made is a good conductor of heat.

Most metals expand with a rise in temperature, and since the piston gets hotter than the cylinder (which can be cooled more effectively). Certain designs of combustion chamber require the piston crown to be made a particular shape, such as the domed crown.

Note that, whatever the shape of the crown, the effective force on the piston due to gas pressure is always given by the cross-sectional area of the cylinder bore.