PRE-IGNITION DAMAGE -- HOLE IN HEAD OF PISTON





TYPICAL DAMAGE APPEARANCE
  1. A hole has formed in the piston crown after a relatively short running time.
  2. The upper third of the hole has formed by melting, the remaining two thirds, where the hole opens out in the downward direction, by breakage.
  3. The piston skirt and undamaged part of the piston head often show no sign of thermal overload, and the hardness relaxation is within normal limits. In many cases scuffing has not even occurred.
ASSESSMENT

Holes in the piston crown form mostly in high compression engines with predominantly hemispherical combustion chambers. Specific combustion defects cause such rapid local overheating that material in the affected area becomes pasty. The inertia forces due to the axial piston movement and the fast flowing combustion gases cause removal of the soft material. As a result of the loss of strength in this area, the combustion pressure then causes mechanical failure of the remaining two-thirds of the piston crown thickness. This lower part of the hole opens out downwards. Such a rapid local overheating of the piston crown to a pasty condition can only be caused by pre-ignition. Combustion is initiated some time prior to the actual spark enabling the flame to operate on the piston crown much longer than usual. Tests have shown that the piston crown heats up locally to the melting point within a few seconds under continuous pre-ignition.

With pre-ignition, combustion is initiated by a glowing area in the combustion chamber when the mixture self-ignition temperature is exceeded. Probable sources are the sparking plug, the exhaust valve and deposits adhering to the combustion chamber walls.

POSSIBLE CAUSES
  1. The fuel quality must match the compression ratio of the engine, i.e. the octane rating of the fuel must cover the engine octane requirement at all operating conditions
  2. Diesel fuel in petrol.
  3. Oil in combustion chamber due to leakage past piston rings or valve guides.
  4. Sparking plugs with incorrect heat range.
  5. The ignition timing and advance/retard characteristics and the distributor condition.
  6. Leaking exhaust valves.
  7. Combustion chamber deposits (hard and soft carbon).
  8. Inlet temperature too high. This can be considerably raised by a faulty or incorrect exhaust, particularly with uniflow scavanged cylinder heads.
  9. High engine or inlet temperature due to inadequate under-bonnet ventilation.
  10. General overheating.






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