EXPANSION CONTROL IN PISTON
During working of engine, the piston runs hotter than the cylinder, because the cylinder is surrounded by the cooling water. Hence the piston expands more than the cylinder. This expansion must be controlled to avoid loss of piston clearance. The loss of piston clearance could lead to serious trouble like excessive friction, excessive wear & at the end of possible seizer of piston in the cylinder leading to complete engine failure. Pistons which are made up of aluminum expands more rapidly than the iron with the rise of temperature. The expansion of piston skirt can be controlled by several ways.
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| Controlling Piston Expansion |
By keeping the heat possibly away from the lower part of the cylinder - This can be done in full-skirt piston by cutting horizontal slots just below the lower part of the piston's oil control ring groove. These slots minimize the path for heat travelling from the cylinder head to the skirt. Thus, skirt doesn't become so hot & doesn't expand so much.
By making the heat dam - Heat dam consists of a groove cut near the top of the piston. This minimizes the size of path the heat can travel from piston to the skirt. The skirt, therefore, runs cooler & does not expand so much.
By cam grinding the piston - The pistons are manufactured so the they are given slightly elliptical or oval shape when cold. These are called cam-ground pistons. when these pistons warms up. It assumes a round shape. Its area of contacting to the cylinder wall, therefore, increases. This minor axis of the ellipse lies in the direction of the piston pin axis. The more expansion along minor axis being 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 the struts, bands or belts cast into the piston. This cause the outward thrust of the expanding piston head to 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.
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