The main reasons for piston knock are as follows: the clearance between piston and cylinder wall is too large. The standard clearance between piston skirt and cylinder of WD615 Series engine is 0.143-0.182, and the large wear limit is 0.35-0.40. After the engine runs for a period of time, the piston of the cylinder is worn, and the lubrication is not good. The clearance between the piston and the cylinder is increased due to the wear, and a serious step appears at the lower part of an air ring, which makes the piston strike the cylinder and make an abnormal noise. After running for a period of time, the piston skirt and cylinder are worn seriously, resulting in serious out of round and knocking cylinder. Some connecting rods are deformed due to various reasons, resulting in eccentric wear of piston, larger clearance and cylinder knock. Piston knock will lead to high fuel consumption, engine oil channeling, high oil consumption and poor economy. When the piston knocks the cylinder seriously, it will also break the piston and damage the cylinder, resulting in the fracture of the connecting rod and the damage of the cylinder block.

自上料搅拌车活塞敲缸的判断方法：

Judgment method of piston knock of self feeding mixer

(1)逐缸断油：采取逐缸断油的方法来确定敲缸的位置，如果断油某缸时，声音明显的减小或者消失，而当恢复供油时能听到明显的“嗒嗒”声，说明该缸活塞敲缸。

(1) Cylinder by cylinder oil cut-off: the method of cylinder by cylinder oil cut-off is adopted to determine the knock position. If the oil cut-off reaches a certain cylinder, the sound obviously decreases or disappears, and when the oil supply is restored, the obvious "click" sound can be heard, indicating that the piston of the cylinder knocks.

(2)为了进一步证实该缸活塞敲缸，可以将该缸的喷油器卸下来，向气缸内加入少量的CD级中增压机油(起密封作用)，再装好喷油器，发动发动机，敲击声消失或者减弱，运行一会敲击声再度出现，则是该缸活塞敲缸无疑。

(2) In order to further confirm the knock of the cylinder piston, the fuel injector of the cylinder can be removed, a small amount of CD grade medium supercharging oil can be added into the cylinder (which plays a sealing role), and then the fuel injector can be installed, and the engine can be started. If the knock disappears or weakens, and the knock appears again after running for a while, the knock of the cylinder piston is undoubted.

自上料搅拌车 遇到活塞敲缸可以采用以下方法处理：发动机起动后，低温运行时有敲击声，温度正常后，声音消失，可以暂不处理，继续运行。发动机温度正常时，有明显的敲击声，应尽量避免高速运行，尽快拆卸维修。拆下气缸盖，抽出活塞以后，发现气缸严重失圆、拉伤，或者活塞和气缸的间隙太大，应进行更换，更换气缸套、活塞及活塞环。如果发现(混泥土搅拌车)连杆变形，应尽量更换。

The following methods can be used to deal with the piston knock of the feeding mixer: after the engine is started, there is a knock sound when the engine is running at low temperature, and the sound disappears when the temperature is normal, so it can not be dealt with temporarily and continue to run. When the engine temperature is normal, there is obvious knocking sound. Try to avoid high-speed operation and disassemble for maintenance as soon as possible. After the cylinder head is removed and the piston is pulled out, it is found that the cylinder is seriously out of round, strained, or the gap between the piston and the cylinder is too large, so it should be replaced. The cylinder liner, piston and piston ring should be replaced. If the connecting rod of concrete mixer is found to be deformed, it should be replaced as far as possible.