Pistons are the component that transfers the forces from expanding gas to the crankshaft in a reciprocating engine. It moves up and down inside the cylinder and is made gas-tight with the help of piston rings.
Automobiles using Internal combustion or IC engines run on the force generated by the expansion of air-fuel mixture inside the combustion chamber. The pistons then transfer this force to the connecting rod which then rotates the crankshaft. The piston has a reciprocating motion which is then converted to rotational with the help of connecting rod.
According to the purpose, working conditions and performance required the shape and proportion of the piston might vary. For example in older American muscle cars, the head of the piston will be in a semi-circular shape. This along with various other factors made the combustion very efficient and consequently better power. These engines were called “Hemi Engines” because of the hemispherical shape of pistons.
The piston must be able to withstand high temperature and pressure fluctuations as well as mechanical load. Piston’s material and durability contributes to the overall durability of the engine. Pistons are usually cast from Aluminium alloys and in case of some racing engines, it might be forged. Aluminium is used because of its excellent thermal conductivity and lightweight properties. The piston should be provided with proper tolerances for expansion. They expand under high temperature and if proper tolerance is not given, the piston might seize in the cylinder. Too much tolerance can also result in leakage of compression gases and piston noises.
The top part of the piston is known as crown while the bottom is called the skirt. The piston head is the area that faces tremendous amounts of heat and pressure during normal engine operation. Even though the piston looks round, it is actually a bit of an oval-shaped with a slight taper towards the skirt.
The piston and connecting rod are connected with the help of a pin known as piston pin or wrist pin or gudgeon pin. It is made of hardened steel and is fixed in the piston with the help of circlips but it can freely move in the connecting rod.
Piston rings prevent the gas sealing. They are narrow iron rings, fitted loosely on the grooves of the piston. They prevent the gases from going down the cylinder through the gap in between piston and cylinder walls and at the same time prevent oil from reaching the combustion chamber. The rings are split at some point so that it can press against the cylinder with light pressure. The upper rings have solid faces and provide gas sealing while the lower ones will have narrow edges and acts as oil scrapers. The upper rings are called compression rings and the lower ones are wiper rings and oil rings.
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Pistons account for at least 60% of the engine friction. Any change in piston or piston rings will have a direct impact on not only fuel consumption but the overall performance of the engine.