Piston is considered to be one of the most important parts in a reciprocating engine in which it helps to convert the chemical energy obtained by the combustion of fuel 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 the connecting rod, without loss of gas from above or oil from below. 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 piston. The efficiency and economy of the engine primarily depends on the working of 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 degree Celsius to over 2800 degree Celsius 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. The material used for pistons is mainly aluminium alloy. Aluminium pistons can be either cast or forged. Cast-iron is also used for piston. In early years cast iron was almost universal material for pistons because it possesses excellent wearing qualities, coefficient of expansion and general suitability in manufacture. But due to the reduction of weight in reciprocating parts, the use of aluminium for piston was essential. To obtain equal strength a greater thickness of metal is necessary, the same of the advantage of the light metal is lost. Aluminium is inferior to cast iron in strength and wearing qualities, and its greater co-efficient of expansion necessitates greater clearance in the cylinder to avoid the risk of seizure. Pistons are usually small in diameter than the bore of the cylinder. The space between the cylinder and the cylinder wall is called the piston clearance.
Posted on: Fri, 21 Mar 2014 15:52:39 +0000
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