In general, a piston is a sliding plug that slides inside the bore of a cylinder. Its purpose is either to change the volume enclosed by the cylinder, or to exert a force on a gas or fluid inside the cylinder.
Most pistons fitted in a cylinder have piston rings. Usually there are two spring-compression rings that act as a seal between the piston and the cylinder wall, and one or more oil control rings below the compression rings. The head of the piston can be flat, bulged or otherwise shaped. Pistons can be forged or cast. A special type of cast piston is the Hypereutectic piston. The piston is an important component of a piston engine and of hydraulic pneumatic systems.
As the piston moves inside the cylinder, it transforms the energy from the expansion of a burning gas (usually a mixture of petrol or diesel and air) into mechanical power (in the form of a reciprocating linear motion). From there the power is conveyed through a connecting rod to a crankshaft, which transforms it into a rotary motion, which usually drives a gearbox through a clutch.
Pistons by Brand
Piston engines
Internal combustion engines
There are two ways that an internal combustion piston engine can transform combustion into motive power: the two-stroke cycle and the four-stroke cycle. A single-cylinder two-stroke engine produces power every crankshaft revolution, while a single-cylinder four-stroke engine produces power once every two revolutions. Older designs of small two-stroke engines produced more pollution than four-stroke engines. However, modern two-stroke designs, like the Vespa ET2 Injection utilise fuel-injection and are as clean as four-strokes. Large diesel two-stroke engines, as used in ships and locomotives, have always used fuel-injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-storey houses, has pistons nearly 1 metre in diameter and is one of the most efficient mobile engines in existence. In theory, a four-stroke engine has to be larger than a two-stroke engine to produce an equivalent amount of power. Two-stroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two-stroke engines were reputed to need more maintenance (despite exceptions like the Ricardo Dolphin engine, and the Twingle engines of the Trojan car and the Puch 250 motorcycle). Even though the simplest two-stroke engines have fewer moving parts, they could wear out faster than four-stroke engines. However fuel-injected two-strokes achieve better engine lubrication, and cooling and reliability should improve considerably.
Steam engines
Steam engines are usually double-acting (i.e. steam pressure acts alternately on each side of the piston) and the admission and release of steam is controlled by slide valves, piston valves or poppet valves.
In an Otto or Diesel engine, the head of the piston forms one wall of an expansion chamber inside the cylinder. The opposite wall, called the cylinder head, contains inlet and exhaust valves for gases.
Drawbacks
Since it’s a main reciprocating part of engine and hence it create the problems of unbalancing due to its inertia. Due to friction between wall of the cylinder and piston rings its life becomes short and it generates the unpleasant sound due reciprocating mechanism. To transmit the energy of reciprocating piston, it is connected to a connecting rod and crank mechanism and due to these part there occurs mechanical loss. The motion of the crank shaft is not smooth, since energy supplied by the piston is not continuous and it is impulsive in nature. To supply the fuel into the cylinder there is need of the valves and valve mechanism and during opening and closing & the valve mechanical noise occurs also mechanical vibration.
Additional information and source: Piston