Know-How: Diesel Engine often has a piston scuffing problem, why?
Recently, the service station has repaired several vehicles with engine failures, and there is a common problem with piston scuffing. The most serious car was a three-cylinder strain, and the piston was punctured, causing serious exhaust and oil discharge under the engine. The cost of changing a set of cylinder liners is enough to replace an engine. So what causes the engine piston scuffing?
COOPAL team will summarize three aspects as follows:
One: Piston set reason
- The piston ring clearance is too small.If the piston ring-opening clearance, side clearance, or back clearance is too small, then the piston ring will be blocked by thermal expansion during engine running, and it will be tightly pressed against the cylinder wall, or broken piston ring, so easily cause piston scuffing.
- Piston pin blew out. While piston pin and circlip are not installed, falls off, or breaks, then piston pin bounces out during movement. Which easily scuff the cylinder wall, causing air leaking from cylinder to crankcase.
- The clearance between the piston and cylinder is too small or too large. If the material of piston is poor, or the manufacturing size error is too large, or the piston is deformed after piston pin is assembled, causing the matching gap between piston and cylinder is too small, then the piston is stuck after thermal expansion, at last, cause piston scuffing the cylinder.
- Severe carbon deposits on the piston ring. Excessive carbon deposits cause the piston ring to stick or seize in the ring groove. At the same time, the carbon deposits is a hard abrasive, which will grind into longitudinal grooves on the cylinder wall.
- The piston is seriously deflected from the cylinder. Due to the bending and distortion of the connecting rod, causing the connecting rod journal, main journal, and piston pin seat parallelism and coaxiality is too large and piston to be obviously biased, which will accelerate the wear of piston ring, piston, and cylinder wall, and destroy the oil film Formation.
Second: The cylinder liner
- 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 goes down, destroys the oil film between piston and cylinder wall, and causes piston scuffing.
- The cylinder liner is deformed during assembly. For example, the upper-end surface of the cylinder liner protrudes too much, the cylinder liner will be deformed after installing the cylinder head; the water blocking ring of the cylinder liner is too thick, which will be deformed after being pressed into the cylinder block, all those will easily cause piston scuffing.
Third: Improper use and other aspects
- The air filter is not well sealed, which makes the filtering effect worse. The dust, sand, and other impurities in the air are sucked into the cylinder and cause abrasive wear. Tests have shown that if a few grams of dust are sucked in every day, the wear of the cylinder liner will increase by more than 10 times.
- Poor running-in. There are many micro-uneven on the surface of the cylinder liner, piston, and piston ring of a new engine or overhauled engine, making it difficult to form the lubricant film. If it is put into high-load operation immediately without running-in, it is easy to cause accidents such as piston scuffing.
- Start at low temperatures often. When the engine is started at low temperatures, the lubricating oil has a high viscosity and poor fluidity, so it is difficult to form an effective oil film on the inner wall of the cylinder. According to the research, when a diesel engines running below 30°C cooling water, the wear of cylinder liners and other parts is 5 to 7 times than running under normal water temperature.
- The engine is overheated. When the cooling system is poorly maintained or overloaded, the excessively high machine temperature not only reduces the mechanical strength of the parts but also prevents the formation of lubricating oil film on the inner cylinder wall. After the piston and other parts are heated and expanded, they are easy to get stuck in the cylinder liner. The result mostly is the piston partially melted and the inner wall of the cylinder liner is damaged, forcing the engine to stop operation.
- Long-term use of engine oil. Engine oil needs to be replaced regularly. If it is not replaced for a long time, it will cause poor lubrication of the engine. Iron filings in the oil will circulate to the piston again, which will not only fail to provide lubrication but will increase the friction coefficient between the piston and the cylinder liner. Cause the cylinder to be strained.
In actual operation, truck engine piston scuffing results are mostly above several factors. As a maintenance engineer, you must ensure that every car you have repaired is in a normal state, and you must strictly follow the operating standards when installing engine internal components. As a car owner, you need not only know how to drive the vehicle but also how to maintain the vehicle with good status.
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