Piston-
A Piston is a cylindrical drum used in an automobile engine to generate power inside the cylinder. The piston moves up and down into the cylinder, which is called reciprocating motion. A piston is considered one of the most crucial parts of the engine. When the chemical charge bursts on the top of the piston head, it takes all the heat and pressure and comes down to the bottom dead center.
The purpose of the piston is to provide a means of conveying the expansion of the gases to the crankshaft. To withstand heat and pressure the piston is made of aluminum alloy. The heat conductivity and pressure coefficient make the aluminum alloy the best choice for the piston. To withstand the pressure the heated piston has some integral cuts and holes. Today I will discuss this.
Expansion Control-
During operation, the piston runs many degrees hotter than the cylinder, because the cylinder is surrounded by cooling water. Hence the piston expands more than the cylinder. This expansion must be controlled in order to avoid loss of adequate piston clearance. Such a loss may cause serious engine trouble. The problem is more accurate with aluminum pistons because aluminum expands more rapidly than iron with the rise of temperature. The expansion of the piston skirt can be controlled by several methods as follows:
By keeping heat away-
The expansion can be controlled by keeping the heat away from the lower part of the piston as much as possible. This can be done in a full-skirt piston by cutting horizontal slots in the piston just below the lower oil-control ring groove. These slots reduce the path for the heat traveling from the cylinder head to the skirt. Thus, the skirt does not become so hot and does not expand so much. In some full-skirt pistons, vertical slots are also cut in the skirt, which would allow metal expansion in the skirt, with an appreciable increase in the piston diameter. The horizontal slot reduces the path for the heat travels. The vertical slot allows for expansion without an increase in piston diameter.
By making a heat dam-
The heat dam consists of a groove cut near the top of the piston. This reduces the size of the path the heat can travel from the piston head to the skirt. The skirt, therefore, runs cooler and does not expand so much.
By cam grinding the piston-
The pistons are finished so that they are slightly oval (elliptical) when cold. These pistons are called cam-ground pistons. When a cam ground piston warms up, it assumes a round shape. Its area of contact with the cylinder wall, therefore, increases. The minor axis of the ellipse lies in the direction of the piston pin axis. The more expansion along the minor axis is caused by the piston bosses. Thus, the piston after expansion at operating temperature becomes circular.
By using struts-
The piston expansion may also be controlled by using the struts, bands, or belts cast into the piston. These cause the outward thrust of the expanding piston head to be carried more toward the piston-pin bosses than toward the thrust faces so that the effect is similar to that of the cam-ground pistons.