A Criterion for Knock in Petrol Engines

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Abstract
The problem was to correlate the tendency of a fuel to knock in a petrol engine, with the physical conditions in the cylinder which cause the knock, and with its ignition qualities in a bomb. The method of approach was to find a function of the physical conditions, the attainment of a critical value of which, during the stroke, is a criterion of knock. Theories of the mechanism of knock are reviewed, and suggest a rational form for this criterion. Engine tests of fuels were made at incipient and heavier knock under a variety of engine conditions, and indicator diagrams were obtained by an oscillograph indicator. Assumptions, based on the rational form, were made for the form of the criterion; and by analysis of the diagrams, etc., a reasonably satisfactory form was selected. Substantially the same critical value was then obtained for incipient knock under all running conditions, including change of compression ratio. The possible application to fuel rating is discussed. The ignition experiments indicated that the same critical value of the same function is a criterion of auto-ignition in a bomb, and they suggested a similarity between the mechanism of knock in an engine and auto-ignition in a bomb.

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