Piston Heads
The three main components in every reciprocating engine are the piston heads, connecting rods and camshafts. The pressure of the combustion gases pushes down on the piston heads and connecting rods, turning the camshafts in the process. The camshafts are connected to the wheels through the transmission and driveshaft. The piston heads also push the combustion gases out of the cylinders and draw the fuel and air mixture into them. In two stroke engines, they also act as valves by blocking the inlet and exhaust ports during the compression stroke.
The piston heads are located in the cylinders, deep inside the engine block. They move up and down inside the cylinders, which in most vehicles are arranged in rows either side of the crankshaft below them. The top ends of the cylinders are sealed by the cylinder heads, which are bolted to the top of the engine block. The piston heads, cylinder heads, piston rings and valves form a sealed combustion chamber where the fuel and air mixture is ignited and burnt. Energy from the previous combustion is stored in a rotating flywheel attached to the crankshaft. This energy forces the pistons back up the cylinder during the compression stroke.
Piston heads resembles a short metal cylinder, closed at one end and open at the other. The heads are hollow to save weight and reduce inertia, but also have internal ribs for strength. The closed top end that faces the combustion gases is known as the piston crown. The crown may have circular cutouts, called valve relief pockets, that allow for greater compression without the valves striking the piston heads. The crown may also have raised bumps that improve compression by decreasing the volume of the combustion chamber.
The thick wall of each head is called the skirt and holds the connecting rod pin and the piston rings. Most of the piston heads used in vehicles have three piston rings. The top two are called compressing rings and stop the combustion gases from escaping down the side of the pistons. This improves performance by transferring more energy to the pistons. The lowest one is called the oil ring and helps to control the oil flow between the heads and the cylinder walls. Each ring has a gap that allows it to be expanded enough to fit around the head.
The connecting rods turn the camshafts by converting the reciprocating motion of the piston heads into rotary motion. Each rod is connected by a large pin which is held in place by locking rings. The pin passes through a hole in the connecting rod and through two reinforced sections of the piston skirt, called the pin bosses. There is no need for bearings inside the pin bosses since they are machined to precisely fit the pin. Engine oil from the sump is sprayed over the entire piston when it is operating, so the pin bosses receive adequate lubrication.
Piston heads are subjected to extreme thermal and mechanical stress inside the engine. Like all metal components, they will eventually fail due to fatigue cracking. This is the slow growth of tiny preexisting cracks due to cyclic stress. Their lifespan is further decreased by adverse effects like excessive acceleration, engine knocking and pre-ignition. However, the most common cause of failure is when a connecting rod breaks and the piston skirt impacts on the broken end of the rod.
Most internal combustion engines are the reciprocating type, with several piston heads and connecting rods. However, there are a few other engine types that do not use pistons. Pistonless rotary engines have been used in several production vehicles over the years. These engines have a triangular rotor that drives a central camshaft. They are simpler than reciprocating engines and have a higher power-to-weight ratio, but are generally less effective at sealing the combustion chamber.
