Basic Knowledge About Piston

1. What are the signs and causes of piston crown erosion? What management precautions should be taken?

Answer: Piston crown erosion manifests as the piston crown layer gradually thinning and peeling away, exhibiting pitting. The pitting varies in depth and size and is unevenly distributed. In severe cases, it can even burn through.

The main reason is that the piston crown surface is difficult to cool sufficiently, making it the hottest part of the combustion chamber and easily reaching the temperature that causes vanadium corrosion, resulting in high-temperature corrosion. Especially when there is carbon buildup of oil or scale on the inner surface of the piston crown, as well as carbon buildup on the piston crown due to poor combustion and prolonged overload operation, the piston crown will overheat, leading to severe erosion.

This erosion phenomenon is exacerbated by the increase in diesel engine cylinder diameter, boost pressure, and heat load. Especially when using heavy fuel oil, the corrosive effect of combustion products further accelerates the development of erosion.

Avoiding piston crown erosion inevitably involves fuel quality issues, thus involving economic problems, and cannot be solved simply. Therefore, only by ensuring efficient combustion in diesel engines through proper management and regularly removing carbon deposits on the piston crown and scale in the cooling space, can sufficient cooling of the piston crown be achieved, thus reducing the occurrence of the aforementioned accidents. In older engines, the shape of the erosion corresponds to the position of the fuel injection holes, indicating that atomization quality is the main cause of erosion. Severe erosion requires repair at a factory.

2. What are the causes of piston ring groove wear? How should it be repaired after wear?

Answer: After cylinder wear in a diesel engine, the cylinder diameter becomes unequal. In this case, in addition to the reciprocating motion with the piston, the piston rings also undergo opening and closing motion within the ring grooves, thus accelerating the wear of the upper and lower surfaces of the ring grooves (the bottom surface of the ring grooves in two-stroke engines suffers severe wear; both the upper and lower surfaces of the ring grooves in four-stroke engines show wear).

The wear of the piston ring grooves generally progresses from rectangular to trapezoidal. This is due to the wear caused by the radial movement of the piston rings within the grooves, the impact of the piston rings during piston reversal, and corrosion. Upon discovering this situation, the tapered surface should be machined flat on a lathe, and a thicker ring should be installed to prevent excessive increase in the clearance between the piston ring and the ring groove.

When the ring groove is severely worn, it can be repaired at a specialized factory. The ring groove is welded and then machined to restore its original dimensions. For large low-speed diesel engines, to increase the height of the piston ring groove, an annular spacer can be welded into the ring groove after machining.

3. What is the function of the friction-reducing ring on the piston skirt? Under what circumstances should it be replaced?

Answer: Some low-speed diesel engines often have lead-copper alloy friction-reducing rings embedded in the piston skirt. Their function is to facilitate the piston break-in process during the initial break-in stage. Experience shows that when friction-reducing rings are not used for break-in, severe cylinder scoring occurs after only a few hours of operation on low-speed engines. This demonstrates the good guiding effect of friction-reducing rings during the break-in stage. When the diameter of the friction-reducing ring is smaller than the diameter of the piston skirt, the piston skirt will directly bear the guiding function of the piston.

Therefore, unless severe wear or uneven wear occurs on the cylinder wall at the air port, the wear-resistant ring does not need to be replaced even if it wears down completely.

When replacing the cylinder liner, the wear-resistant ring on the piston of that cylinder should also be replaced to achieve a good break-in period. The replaced wear-resistant ring should have a clearance of approximately 0.0011–0.0012 mm between it and the cylinder wall.