A piston is a mobile component contained by a cylinder and is made gas tight. The racing piston moves up and down in a linear motion. Its primary purpose in an engine is to transfer force from expanding gas in the cylinder to the top of the crank through a connecting rod. The connecting rod is attached to the piston by wrist pin that is mounted to the piston. This pin is made of hardened steel and it is free to move into the connecting rod as opposed to the piston itself.
Piston manufacturers ensure that there are gas sealing. This gas sealing in the cylinder is achieved by use of piston rings. They are iron rings fitted loosely into grooves in the piston just below the crown. The rings are split at a point in the rim enabling them to press against the cylinder with a light spring pressure. There are upper rings which have solid faces and serves to seal gas. The lower rings on the other side act as oil scrapers; they have a narrow edge and u-shaped profile to facilitate their function.
Pistons are made from aluminum alloys. Early pistons were made of iron. The arising need to have lighter engines made it necessary to use lighter alloys. The need to have pistons that could survive engine combustion temperatures led to the development of alloys such as Y alloy and Hiduminium. Purposely these alloys were created to be used in making pistons.
The piston serves to transform the energy of expanding gases into mechanical energy. This mechanical energy is that which enables the device say a car to move or a machine to function as required. This is what makes the pistons the main component of an engine without which the engine cannot function. A piston is basically a performance valve as it controls the motion of a fluid along a tube which is the cylinder. They control admission of steam or fluids into the tube, its exhaustion from the cylinders after a while enables a locomotive to move under its own power. This power is what is referred to as mechanical force.
Piston designs have changed overtime. Pistons replaced slide valves which were used earlier in the 19th century. The major reason for this replacement was that it was complicated to lubricate the slide valves in the presence of superheated steam. Pistons made it easier to reduce the steam passage making it shorter. This served to reduce resistance to the flow of steam and hence improve efficiency in the operation of the engine.
When an engine is functioning, steam is required to enter the piston at a controlled rate in order for the machine to function efficiently. This involves controlling the admission and exhaustion of steam to and from the cylinders. Steam enters and exits the valve through a steam port, located in the middle position of the piston. In some engines the piston acts as a valve by covering and uncovering ports in the cylinder wall. This is essentially its function in an engine.