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.
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.