Piston.
Piston is a reciprocating movement in the cylinder body of an 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 combustion chamber, and its shape is related to the selected combustion chamber form. Gasoline engines mostly use flat top piston, which has the advantage of small heat absorption area. Diesel engine piston top often has a variety of pits, its specific shape, position and size must be with the diesel engine mixture formation and combustion requirements.
The piston top is a component of the combustion chamber, so it is often made of different shapes, and the gasoline engine piston at most uses a flat top or a concave top, so that the combustion chamber is compact, the heat dissipation area is small, and the manufacturing process is simple. Convex head pistons are commonly used in two stroke gasoline engines. The piston tops of diesel engines are often made of various pits.
The piston head is the part above the piston pin seat, and the piston head is installed with a piston ring to prevent high temperature and high pressure gas from entering the crankcase and prevent oil from entering the combustion chamber; Most of the heat absorbed by the top of the piston is also transmitted to the cylinder through the piston head, and then transferred through the cooling medium.
The piston head is processed with several ring grooves for mounting piston rings, and the number of piston rings depends on the requirements of the seal, which is related to the engine speed and cylinder pressure. High-speed engines have fewer rings than low-speed engines, and gasoline engines have fewer rings than diesel engines. General gasoline engines use 2 gas rings and 1 oil ring; The diesel engine has 3 gas rings and 1 oil ring; Low speed diesel engine uses 3 ~ 4 gas rings. In order to reduce friction loss, the height of the belt part should be reduced as far as possible, and the number of rings should be reduced under the condition of ensuring sealing.
All parts of the piston ring below the groove are called piston skirts. Its role is to guide the piston in the cylinder for reciprocating motion and withstand side pressure. When the engine is working, due to the effect of the gas pressure in the cylinder, the piston will bend and deform. After the piston is heated, the expansion amount is greater than that of other places due to the metal at the piston pin. In addition, the piston will produce extrusion deformation under the action of side pressure. As a result of the above deformation, the section of the piston skirt becomes an ellipse in the direction of the long axis perpendicular to the piston pin. 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.
The main failures of the piston assembly and their causes are as follows:
1. Ablation of top surface of piston. Piston ablation appears on the top of the piston, with loose pitting in light cases and local melting in heavy cases. The main reason for the ablation of the top of the piston is caused by abnormal combustion, so that the top accepts too much heat or runs under large load after the piston ring is stuck and broken.
2, the top surface of the piston cracks. The direction of the crack on the top surface of the piston is generally perpendicular to the axis of the pin hole of the piston, which is mainly caused by the fatigue crack caused by thermal stress. The reason: the overload operation of the engine leads to excessive deformation of the piston, resulting in fatigue cracking of the top surface of the piston;
3, piston ring groove side wall wear. When the piston moves up and down, the piston ring should be radial telescopic with the deformation of the cylinder, especially the temperature of the first ring groove is high, and it is affected by the "impact" of gas and the oil wedge, so the ring friction and vibration occur in the ring groove, causing wear;
4. The piston ring is coke stuck in the ring groove. Piston ring coking is the result of lubricating oil oxidation deposition or ring loss of freedom of movement in the tank, this failure is very harmful. The main reasons: diesel engine overheating or long-term overload work, so that the lubricating oil gum, piston ring, cylinder serious thermal deformation; Lubricating oil pollution is serious, lubricating oil quality is poor; The crankcase ventilation device works poorly, causing excessive negative pressure or poor air tightness of the cylinder, resulting in oil rushing. Therefore, it is necessary to ensure the use of qualified oil to prevent the diesel engine from overheating.
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