A piston is an element made up of two parts: a piston that moves in a rectilinear reciprocating movement and a sleeve 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 made of any material.
The guide is actually a cavity that can be built into a different part. It is part of the piston-connecting-rod-crank system that is used to transform rectilinear movements into rotary ones and vice versa.
As a definition of piston, we can say that a piston is a part of a pump or cylinder of an engine that moves 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 attached to the crankshaft with the connecting rod. There are often 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.
What Is the Piston for in a Heat 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 moving downwards. The speed and power with which the piston moves depends on the resistance of the engine and the intensity of the explosion. According to the second law of thermodynamics, the energy converted will always be less than the thermal energy of the explosion.
In an automobile internal combustion engine, the piston is traditionally called the cylinder. The cylinder, in an automobile, 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 sleeve. Although the most common fuel is gasoline in Otto engines or diesel in diesel engines, there are also engines that work with gas and other elements.
As a result of movement, a liquid or gas is displaced, or compressed, with pumps and compressors. In engines, work is done on the piston with gases, which can then be transferred to other components, such as the crankshaft.
How Does the Piston of a Reciprocating Engine Work?
In the case of Otto engines, a spark caused by a spark plug is needed to work. In this case, they are also called spark ignition engines. The combination of the three elements causes a controlled explosion that pushes the plunger of the piston away, thus generating movement. This movement will be transmitted to the crankshaft. The pistons are located alternately to the crankshaft, so that those that are pushed out push the others in, and then those that have been further to the bottom will be the ones that push the previous ones back in.
In the case of diesel engines, the operation is the same. The main difference between them is that the explosion of the fuel is not caused by a spark but rather 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 reciprocating or, more formally, reciprocating. At the end of each piston, in addition to the air and fuel inlets, the outlet of mixed air resulting from combustion, and the spark plug, it has grooves where the compression and lubrication segments are assembled.
The lubricant serves to decrease the friction force of the piston inside the sleeve. At the opposite end, the piston has a hole where the pin that connects 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 inside a cylinder. It is used, among others, in most current brass instruments.
Characteristics of a Piston
The true cold shape of the piston is not cylindrical, as 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 reality, 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 caudals.
The precise shape for each engine and application is defined with computer simulations and performance tests, so that at operating temperature it assumes the correct shape to ensure the lowest oscillations in the barrel, the correct lubrication, the lowest frictions and the most effective: seal against combustion gases.
In addition to the classic shapes, 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 the rotor has a trochoidal 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.