Piston head seizure due to the use of incorrect pistons (diesel engines)
DESCRIPTION OF THE DAMAGE
- Localised score marks can be seen on the piston head, going all around the circumference of the piston.
- Score marks start at the piston crown and end at the second compression ring.
- Score marks are centred around the piston top land.
DAMAGE ASSESSMENT
This damage has been caused by abnormal combustion. However, the fault lies in the use of an incorrect piston, not with the fuel injection system. Engines are designed in accordance with the statutory prescribed emission standards. The pistons for the relevant emission standard are often barely any different to look at.
In this example, pistons with different bowl diameters were used on the same engine type to meet different emission standards. The piston for the Euro 1 emission standard (bowl diameter of 77 mm) was replaced during engine repair with a piston for the Euro 2 emission standard (bowl diameter of 75 mm).
Because of the smaller bowl diameter, the injection nozzle was also spraying onto the edge of the bowl rather than just into the bowl. The edge of the bowl and the piston material heated up at the points of contact and their thermal expansion therefore also increased. This led to the localised seizure marks.
If the pistons used are not intended for the engine type and emission standard, this can result in serious abnormal combustion with unforeseeable consequences. Other less severe effects would be failure to comply with exhaust gas values, lack of engine performance and increased fuel consumption.
In this example, pistons with different bowl diameters were used on the same engine type to meet different emission standards. The piston for the Euro 1 emission standard (bowl diameter of 77 mm) was replaced during engine repair with a piston for the Euro 2 emission standard (bowl diameter of 75 mm).
Because of the smaller bowl diameter, the injection nozzle was also spraying onto the edge of the bowl rather than just into the bowl. The edge of the bowl and the piston material heated up at the points of contact and their thermal expansion therefore also increased. This led to the localised seizure marks.
If the pistons used are not intended for the engine type and emission standard, this can result in serious abnormal combustion with unforeseeable consequences. Other less severe effects would be failure to comply with exhaust gas values, lack of engine performance and increased fuel consumption.
POSSIBLE CAUSES FOR THE DAMAGE
- Pistons with an incorrectly shaped bowl or an incorrect bowl depth or diameter.
- Deviating piston dimensions (e.g. compression height).
- Incorrect piston design. For example, a piston without an oil cooling gallery must not be used if the engine manufacturer specifies an oil cooling gallery for the particular application.
- Use of incorrect components or components that are unsuitable for the intended purpose (injection nozzles, fuel injection pumps, cylinder head gaskets or other components that affect the mixture formation or combustion process).