The Piston, Rings, and Wrist Pin

The piston converts the potential energy of the fuel, into the kinetic energy that turns the crankshaft. The piston is a cylindrical shaped hollow part that moves up and down inside the engine's cylinder. It has grooves around its perimeter near the top where rings are placed. The piston fits snugly in the cylinder. The piston rings are used to ensure a snug "air tight" fit. The piston requires four strokes (two up and two down) to do its job. The first is the intake stroke. This is a downward stroke to fill the cylinder with a fuel and air mixture. The second is an upward stroke to compress the mixture. Right before the piston reaches its maximum height in the cylinder, the spark plug fires and ignites the fuel. This action causes the piston to make its third stroke (downward). The third stroke is the power stroke; it is this stroke that powers the engine. On the fourth stroke, the burned gases are sent out through the exhaust system. The wrist pin connects the piston to the connecting rod. The connecting rod comes up through the bottom of the piston. The wrist pin is inserted into a hole (about half way up) that goes through the side of the piston, where it is attached to the connecting rod. Pistons are made of aluminum, because it is light and a good heat conductor. Pistons perform several functions. Pistons transmit the driving force of combustion to the crankshaft. This causes the crankshaft to rotate. The piston also acts as a moveable gas-tight plug that keeps the combustion in the cylinder. The piston acts as a bearing for the small end of the connecting-rod. Its toughest job is to get rid of some of the heat from combustion, and send it elsewhere. The piston head or "crown" is the top surface against which the explosive force is exerted. It may be flat, concave, convex or any one of a great variety of shapes to promote turbulence or help control combustion. In some, a narrow groove is cut into the piston above the top ring to serve as a "heat dam" to reduce the amount of heat reaching the top ring.