Piston (Image credit: btechgurus.blogspot.com)

Piston is one of the most important part in a reciprocating engine in which it helps to convert the chemical energy obtained by combustion of fuel into useful mechanical power. The purpose of the engine is to provide a means of conveying the expansion of the gases to the crankshaft via the connecting rod, without loss of gas from above or oil from below. Piston is essentially a cylindrical plug that moves up and down in the cylinder. It is equipped with piston rings to provide a good seal between the cylinder wall and piston. The efficiency and economy of the engine primarily depends on the working of piston. It must operate in the cylinder with minimum friction and should be able to withstand a high explosive force developed in the cylinder and also the very high temperature ranging from 2000 C to over 2800 C during operation. The piston should be as strong as possible, however its weight should be minimized as far as possible in order to reduce the inertia due to its reciprocating mass. The top of the piston is called head. Ring grooves are cut on the circumference of the upper portion of the piston. The parts below the ring grooves is called skirt. The portion of the piston that separates the grooves are called the lands. Some pistons have a groove in the top land called a heat dam which reduces heat transfer to the rings. The piston bosses are the reinforced sections of the piston designed to hold the piston pin or wrist pin.

The material used for piston is mainly aluminium alloy. Aluminium pistons can be either cast or forged. Cast iron is also used for piston.

Pistons are usually small in diameter than the bore of the cylinder. The space between the piston and the cylinder wall is called the piston clearance. This clearance is essentially for the following reasons:

1. It provides a space for a film of lubricant between the piston and cylinder wall to reduce friction.
2. It prevents piston seizure due to very high operating temperature, piston and cylinder block expand the cylinder is cooled better than the piston, hence sufficient clearance should be provided for the piston to expand, fail in which the piston seizure will result.
3. If there is no clearance between the piston and cylinder, it will be impossible for the piston to reciprocate in the cylinder.

Piston head shape or piston crown:

Piston heads (Image credit: miata.net)