and Pre-ignition
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A pre-igniter was designed in which the temperature of the pre-igniting surface could be measured. It consisted of art electrically heated hot-spot consisting of 31 turns of Nimonic 75 wire, the unit being mounted in a sparking-plug body and screwed into the combustion chamber diametrically opposite the sparking plug.
The experimental investigation, said the authors, did not yield much information on the destructive effects of pre-ignition, because it was necessary to limit the severity of the conditions in order to obtain repeatable •observations. Pre-ignition of sufficient severity to stop the engine was frequently encountered, but because of the sturdy nature of the unit, no serious conseqttences resulted.
Pistons Burned in a multi-cylindered engine of small thermal capacity, the destructive effects would be more obvious. Pre-ignition failures, said the authors, often took the form of burning of the piston on one side, and there had been instances where piston crowns had been burnt through.
The most common source of preignition was the sparking plug. Ceramic plugs could withstand more arduous conditions than mica plugs.