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Cause Analysis And Troubleshooting Of Abnormal Diesel Combustion
Case Analysis Of Wd615 Engine Accident Of Steyr Heavy Truck

Failure Analysis Of Piston Assembly Caused By Improper Maintenance

On April 14, 2009, our company accepted an accident of early wear failure of WD615 Euro II engine of Shanghai customer after replacing e2-4a piston. Because the customer only brought some damaged parts, we mainly analyzed the damaged piston and piston ring of piston assembly:

Accident Phenomenon:

After the engine is disassembled, there are a large number of hard carbon deposits on the top of the piston between the two inlet and exhaust valves; According to the customer’s understanding, the damage forms of the six pistons of this engine are basically the same. The ring height and radial thickness of the first piston ring are seriously worn.

Accident Analysis:

The piston is called the heart of the engine. It is one of the most important parts in the engine. Because the piston is constantly doing high-speed reciprocating motion under the action of high-temperature and high-pressure gas, it bears the thermal load and mechanical load of high-temperature and high-pressure, and its working conditions are very bad. The piston is the only aluminum part in the main moving parts of the engine. The failure of any related part in the engine or the working state of the engine is likely to cause the piston to be damaged or broken. Therefore, the engine fault is often exposed to the piston, but the part that really causes the engine fault is not necessarily the piston, Specific analysis needs to be carried out according to specific conditions!

1. There are obvious carbon deposits on the top of the piston with insufficient combustion stroke, and the outer surface is mixed with reddish brown solid attachments and a hard carbon deposit layer with a thickness of about 1mm (distributed between the two valve avoidance pits on the top of the piston).

2. The first gas ring of the piston ring is seriously worn. The side gap between the piston ring and the nickel iron heat insulation ring of the piston can be clearly seen in the above figure. The cross section of the first gas ring indicates that the upper and lower end faces of the piston ring have been seriously worn and deformed.

Whether in compression or power stroke, the piston ring seals the gas from top to bottom, The first gas ring (as shown in the right figure) about 80-90% of the gas is sealed, and about 10-20% of the gas is closed by the second gas ring. If the engine oil is relatively diluted or there is a large amount of hard carbon deposit during combustion, the first gas ring and the nickel iron heat insulation ring embedded on the piston will be worn, and some gas will burn between the first and second gas rings!

3. The second piston ring was observed when it was not disassembled. It is an anti twist ring of Euro II, and its wedge angle has been basically worn; Slight strain marks on the working surface of piston ring.

During disassembly and inspection, it is found that the lower end face of the second gas ring is worn very seriously, because the second ring groove of the piston is of aluminum alloy structure, which is not made of cast iron material of nickel iron heat insulation ring like the first gas ring groove. This type of wear is only caused by high-pressure gas and poor lubrication after the sealing effect of the first gas ring fails.

4. The oil ring is seriously damaged due to abrasive wear, which is not illustrated here.

Accident Overview:

This early damage phenomenon of the engine may be due to insufficient combustion of the engine and incomplete combustion of diesel oil, resulting in poor lubrication and increased wear of the first gas ring; When the first gas ring is close to failure, the lower end face of the second gas ring and the piston ring groove are abraded under the action of high-pressure gas. Due to the failure of the second gas ring, the pump oil effect increases, and too much lubricating oil participates in combustion to form hard carbon deposit, which also aggravates the wear of parts. This accident is not a piston quality problem.

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