Hole in the piston crown (petrol engines)
DESCRIPTION OF THE DAMAGE
- Piston crown has a hole that penetrates all the way through, covered by the molten material.
- Skirt area displays seizure marks. Reason: the high temperatures and the piston material that has been ground down.
DAMAGE ASSESSMENT
This type of damage is caused by glow ignition. Glowing components exceed the self-ignition temperature of the air-gas mixture in the combustion chamber. These are essentially the spark plug, the exhaust valve and any combustion residue present in the combustion chamber. As a result, the mixture ignites before it is due to be ignited by the spark plug. This means that the flame acts for longer on the piston crown than in the normal combustion process.
Within a short space of time the piston crown heats up so much as a result of glow ignition that the material there becomes soft. The softened material is then carried away as a result of the inertia force on the reciprocal movements of the piston and the fast-streaming combustion gases. As such the combustion pressure forces a hole in through the remaining thickness of the piston crown wall. In many cases there will not be any seizure marks.
NOTE
Such rapid heating of a localised area on the piston crown is only possible as a result of glow ignition.
Within a short space of time the piston crown heats up so much as a result of glow ignition that the material there becomes soft. The softened material is then carried away as a result of the inertia force on the reciprocal movements of the piston and the fast-streaming combustion gases. As such the combustion pressure forces a hole in through the remaining thickness of the piston crown wall. In many cases there will not be any seizure marks.
NOTE
Such rapid heating of a localised area on the piston crown is only possible as a result of glow ignition.
POSSIBLE CAUSES FOR THE DAMAGE
- Heat value of the spark plugs too low.
- Mixture too lean, resulting in higher combustion temperatures.
- Damaged or leaking valves, or insufficient valve clearance, causing the valves to not close correctly. The combustion gases streaming past significantly increase the temperature of the valves, and the valves start to glow. This primarily affects the exhaust valves, as the intake valves are cooled by the fresh gases.
- Glowing combustion residue and oil carbon deposits in the combustion chamber.
- Incorrect injector installation dimension (missing or duplicate sealing rings).
- Unsuitable fuel with an octane rating that is too low. The fuel quality must correspond to the compression ratio of the engine, i.e. the octane rating of the fuel must cover the octane requirements of the engine under all operating conditions.
- Petrol contaminated by diesel, which lowers the octane rating of the fuel.
- High engine or intake air temperature caused by inadequate ventilation of the engine compartment.
- General overheating of the engine.
Side info