Piston is a reciprocating part in the cylinder body of automobile engine. The basic structure of the piston can be divided into top, head and skirt. The top of the piston is the main part of the combustor. The flat - top piston is often used in gasoline engines. There are often various pits on the top of the piston of a diesel engine, whose specific shape, position and size must be adapted to the requirements of mixture formation and combustion of diesel engine. The whole piston can be divided into three parts: piston top, piston head and piston skirt. The main function of the piston is to bear the combustion pressure in the cylinder and transmit this force through the piston pin and connecting rod to the crankshaft. In addition, the piston and cylinder cover, cylinder wall combustor. Piston top is the combustor component, so often made into different shapes, gasoline engine piston top adopts flat or concave top, in order to make the combustor structure compact, small heat dissipation area, simple manufacturing process. Convex piston is often used in two - stroke gasoline engines. The piston top of diesel engines is often made into various pits. Piston head is the part above the piston pin seat. Piston ring is installed in the piston head to prevent high temperature and high pressure gas from entering the crankcase and engine oil from entering the combustion chamber. Most of the heat absorbed by the top of the piston also passes through the head of the piston to the cylinder, which in turn passes through the cooling medium. Piston head machining several rings installed ring groove, the number of piston rings depends on the sealing requirements, it is related to the engine speed and cylinder pressure. High - speed engines have fewer cycles than low - speed engines and gasoline engines have fewer cycles than diesel engines. General gasoline engine USES 2 gas rings and 1 oil ring; The diesel engine has three air rings and one oil ring. Low speed diesel engines use 3 ~ 4 air rings. In order to reduce the friction loss, the height of the ring should be reduced as far as possible. All the parts below the piston ring groove are called piston skirts. Its function is to guide the piston to reciprocate in the cylinder and to bear the side pressure. When the engine works, due to the action of the air pressure in the cylinder, the piston will produce bending deformation. After the piston is heated, because the metal in the piston pin is much, its expansion is greater than that in other places. In addition, the piston will also produce extrusion deformation under the action of side pressure. As a result of the above deformation, the piston skirt section becomes an ellipse of the long axis in the piston pin direction. In addition, due to the uneven distribution of temperature and mass along the axis of the piston, the thermal expansion of each section is large on the top and small on the bottom.
