SCORCHING ON PISTON HEAD

TYPICAL DAMAGE APPEARANCE
-- Material from the piston head is burned away behind the rings.

-- There is no primary scuffing of the piston skirt, but melted material from the scorched area is smeared onto the piston skirt.

ASSESSMENT
Scorching of the piston head in petrol engines is the result of pre-ignition in engines with largely flat piston crown and large squish surfaces. Lack of clearance can be excluded as cause of damage since, in spite of the high temperature due to the piston head scorching, no scuffing occurs in the initial stages. However, the piston head becomes so severely over-heated in the squish zone that material in the top land and behind the rings down to the oil ring groove becomes pasty and is removed by a combination of inertia forces and the ever increasing penetration of combustion gases into the growing scorch zone.

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

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.
Diesel fuel in petrol.
Oil in combustion chamber due to leakage past piston rings or valve guides.
Sparking plugs with incorrect heat range.
The ignition timing and advance/retard characteristics and the distributor condition.
Combustion chamber deposits (hard and soft carbon).
Inlet temperature too high. This can be considerably raised by a faulty or incorrect exhaust, particularly with uniflow scavanged cylinder heads.
High engine or inlet temperature due to inadequate under-bonnet ventilation.
General overheating.

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