A piston is considered one of the most essential parts of a reciprocating engine as it helps convert the chemical energy obtained by fuel combustion into useful mechanical power. The purpose of the piston is to provide a means of conveying the expansion of the gases to the crankshaft, via connecting rod, without loss of gas from above and oil from below.
A piston is essentially a cylindrical plug that moves up and down in the cylinder. It is equipped with piston rings to provide a good seal between the cylinder wall and the piston. Although the piston appears to be a simple part, it is actually quite complex from a design standpoint.
The efficiency and economy of the engine primarily depend on the working of the piston. It must operate in the cylinder with minimum friction and should be able to withstand the high explosive force developed in the cylinder and also the very high temperature ranging from 2000 degrees Celsius to 2800 degrees Celcius during operation. The piston should be as strong as possible, however, its weight should be minimized as far as possible in order to reduce the inertia due to its reciprocating mass.
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The piston must possess the following qualities-
- Rigidly to withstand high pressure.
- Lightness reduces the weight of the reciprocating masses and so enables higher engine speeds.
- Good heat conductivity reduces the risk of donation so allowing a higher compression ratio.
- Silence in operation.
- Materials having low expansion and provision to allow for different expansion rates of cast iron cylinder block and an aluminum piston.
- Correctly formed skirt to give uniform bearing under working conditions.
The piston performs the following functions-
- To receive the thrust generated by the explosion of the gas in the cylinder and transmit it to the connecting rod.
- To reciprocate in the cylinder as a gas-tight plug causing suction, compression, expansion, and exhaust strokes.
- To form a guide and bearing to the small end of the connecting rod and to take the side thrust due to the obliquity of the rod.
The top of the piston is called the Head. Ring grooves are cut on the circumference of the upper portion of the piston. The parts below the ring grooves are called skirts. The portions of the piston that separate the grooves are the lands.
How Piston Made-
The material used for pistons is mainly aluminum alloy. Aluminum pistons can be either cast or forged. Cast iron is also used for pistons. In the early years, cast iron was an almost universal material for pistons because it possesses excellent wearing qualities, coefficient of expansion, and general stability in manufacture. But due to the reduction of weight in reciprocating parts, the use of aluminum for pistons was essential. To obtain equal strength a greater thickness of metal is necessary, the same advantage of the light metal is lost. Aluminum is inferior to cast iron in strength and wearing qualities; its greater coefficient of expansion necessitates greater clearance in the cylinder to avoid the risk of seizure. The heat conductivity of aluminum is about three times that of cast iron; and this combined with the greater thickness necessary for strength, enables, an aluminum alloy to run at much lower temperatures than a cast iron one. As a result, carbonized oil does not form on the underside of the piston; and the crankcase, therefore, keeps cleaner. This cool running property of aluminum is now recognized as being quite as valuable as its lightness indeed.