Diesel engines often have piston wear problems, why?

Recently, we discovered that our customers’ engines had a common problem of piston scratches. The most serious car suffered a three-cylinder strain, and the piston was punctured, resulting in severe exhaust and oil discharge under the engine. The cost of replacing a set of cylinder liners is enough to replace an entire engine. So what causes engine piston scratches?

The FJT team summarized the problems in three aspects as follows:

1). Reasons for the piston group

  1. Piston ring clearance is too small. If the piston ring opening clearance, side clearance or rear clearance is too small, the piston ring will be clogged due to thermal expansion during engine operation, and will be tightly pressed against the cylinder wall, or the piston ring will be broken, thus easily causing piston scratching. hurt.
  2. Piston pin burst. When the piston pin and circlip are not installed, fall off, or break, the piston pin will pop out during movement. It is easy to wear the cylinder wall, causing the cylinder to leak into the crankcase.
  3. The gap between the piston and the cylinder is too small or too large. If the material of the piston is not good, or the manufacturing size error is too large, or the piston is deformed after the piston pin is assembled, causing the matching gap between the piston and the cylinder to be too small, the piston will get stuck after thermal expansion, and finally, the piston will scratch the cylinder. hurt.
  4. There is severe carbon deposits on the piston rings. Excessive carbon deposits can cause piston rings to stick or become stuck in the ring grooves. At the same time, carbon deposit is a hard abrasive that will grind into longitudinal grooves on the cylinder wall.
  5. The piston is severely offset from the cylinder. Due to the bending and deformation of the connecting rod, the parallelism and coaxiality of the connecting rod journal, main journal and piston pin seat are too large and the piston cannot be significantly offset. This will accelerate the wear of the piston ring, piston and cylinder wall, and damage the Formation of oil film.

2). Cylinder liner

  1. The roundness and cylindricity tolerances of the cylinder liner exceed the allowable range, which greatly reduces the sealing performance of the piston and cylinder liner. The high-temperature gas in the cylinder drops, destroying the oil film between the piston and the cylinder wall, causing the piston to be scratched.
  2. The cylinder liner was deformed during assembly. For example, if the upper surface of the cylinder liner protrudes too much, the cylinder liner will deform after the cylinder head is installed; the water retaining ring of the cylinder liner is too thick and will deform after being pressed into the cylinder block. These can easily cause the piston to be scratched.

3). Improper use, etc.

  1. The air filter is not tightly sealed, which makes the filtration effect worse. Dust, sand and other impurities in the air are drawn into the cylinder and cause abrasive wear. Tests have shown that if a few grams of dust are inhaled every day, the wear of the cylinder liner will increase by more than 10 times.
  2. Poor running-in. There are many tiny uneven surfaces on the cylinder liners, pistons, and piston rings of new or overhauled engines, making it difficult to form a lubricating film. If it is immediately put into high-load operation without running in, it is easy to cause accidents such as piston scratches.
  3. Often started at low temperatures. When the engine is started at low temperatures, the lubricating oil has high viscosity and poor fluidity, making it difficult to form an effective oil film on the inner wall of the cylinder.
  4. According to research, when a diesel engine operates in cooling water below 30°C, the wear of the cylinder liner and other components is 5 to 7 times that of normal water temperature.
  5. The engine is overheating. When the cooling system is poorly maintained or overloaded, excessive machine temperatures will not only reduce the mechanical strength of the parts, but also prevent the formation of a lubricating oil film on the inner cylinder wall. After parts such as pistons expand due to heat, they can easily get stuck in the cylinder liner. The main result is that the piston is partially melted and the inner wall of the cylinder liner is damaged, forcing the engine to stop running.
  6. Use engine oil for a long time. Engine oil needs to be changed regularly. If it is not replaced for a long time, it will cause poor engine lubrication. The iron filings in the oil will circulate to the piston again, which not only fails to provide lubrication, but will increase the friction coefficient between the piston and the cylinder liner. causing the cylinder to tighten.

In actual operation, the results of truck engine piston scratches are mostly caused by the above factors. As a maintenance engineer, you must ensure that every vehicle you repair is in proper condition and strict operating standards must be followed when installing internal engine components. As a car owner, you not only need to know how to drive your vehicle, but you also need to know how to keep it in good condition.

Welcome to contact FJT for diesel engine or Cummins parts supply! hazel@fjtco.cn

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