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A piston is an element formed by two pieces: a piston that moves in an alternative rectilinear movement and a jacket or guide to which it moves. This guide is sometimes also called a cylinder. In principle, the piston has a cylindrical shape and is usually made of metal, although it could be of any material.

The guide is actually a cavity that can be integrated in a different piece. It is part of the piston-crank-crank system that serves to transform rectilinear movements into rotary and vice versa.

As a definition of a piston, we can say that a piston is a part of a pump or cylinder of an engine that is driven by a fluid or by the explosion of fuel: The piston of an engine connected to the connecting rod.

On machines where the piston has a cylindrical or oval shape, the piston is often coupled to the crankshaft with the connecting rod. Often there are piston rings around the piston. The function of the piston rings is to ensure that there is a good seal between the cylinder wall and the piston.

Piston Function in a Thermal Engine

The function of the piston is to absorb the heat energy caused by the explosion of the fuel and convert it into linear kinetic energy by moving down. The speed and power with which the piston travels depends on the resistance of the engine and the intensity of the explosion. According to the second law of thermodynamics, the converted energy will always be inferior to the thermal energy of the explosion.

In a car-blast thermal engine, the piston is traditionally called a cylinder. The cylinder, in a car, is a cylindrical piece of steel. The movement of the piston is due to the fact that fuel is periodically thrown at the end of the jacket. Although the most common fuel is gasoline in Otto engines or gasoil in diesel engines there are also engines that run on gas and other elements.

As a result of the movement, a liquid or gas is displaced, or compressed, with pumps and compressors. In engines, the work is done on the piston with gases, which can then be transferred to other components, such as the crankshaft.

Piston Operation in a Thermal Engine

In the case of Otto engines, a spark caused by a spark plug is required to operate. In this case, they are also called spark ignition engines. The combination of the three elements causes a controlled explosion that pushes the piston piston away, thus generating a movement. This movement will be transmitted to the crankshaft. The pistons are located alternated to the crankshaft, so that those that are pushed out push the others inwards, and then those that have been left further to the bOttom will be the ones that push the previous ones back inside.

In the case of diesel engines, the operation is the same. The main difference between them is that the fuel explosion is not caused by a spark but is done by increasing the pressure.

The crankshaft is the part of an engine that converts the reciprocating linear motion of the piston into circular motion.

This occurs in engines that are called alternative or, more formally, alternative movement. At the end of each piston, in addition to the air and fuel inlets, the mixed air outlet of the combustion product, and the spark plug, has slots where the compression and lubrication segments are assembled.

The lubricant serves to decrease the frictional force of the plunger inside the sleeve. At the opposite end, the piston has a hole where the bolt that links the piston to the connecting rod is housed.

A piston valve is a mechanism used to control the movement of a fluid along a pipe thanks to the linear movement of a piston within a cylinder. It is used, among others, in most current metal wind instruments.

Piston Shape 

The true cold shape of the piston is not cylindrical, since the mass distribution and operating temperatures are uneven, and this leads to uneven expansion. Its shape is more or less truncated-conical in section (in fact, the maximum diameter is usually about 10 mm from the lower edge of the mantle) and oval in plan, where the lower axis passes through the pin and its flow rates. The precise shape for each engine and application is defined with computer simulations and performance tests, so that at the operating temperature it assumes the correct form to ensure the lowest oscillations in the barrel, the correct lubrication, the lowest frictions and the most effective: seal against flue gas.

In addition to the classic forms, there are also pistons, usually for rotary engines, where the shape is completely different, as in the case of the Wankel engine, where the piston or rotor has a trocoidal shape, but there is also the case of a classic engine (with alternating pistons) where these, instead of the usual round section, have an oval profile.


Published: November 15, 2016
Last review: February 20, 2020