The Significance of Laboratory Octane Numbers in Relation to Road Anti-knock Performance

Abstract

Fuel composition plays a prominent part in deciding the compression ratio to be adopted for an engine, since composition directly affects the knocking tendencies of motor fuels, and knock in turn imposes a limitation on engine performance. The engine conditions which affect knock are discussed and the fact that all fuels do not react equally to these conditions is dealt with in some detail. Laboratory and road knock rating methods are described, and results of tests on a number of fuels and cars, both in Great Britain and in the United States of America are used to demonstrate a relationship between laboratory and road antiknock ratings. It is shown that the road anti-knock performance of fuels is directly affected by their “sensitivity”, to a greater or lesser degree depending on engine “severity”. Severe engines are at a disadvantage in not being able to make full use of the high Research Method Ratings of sensitive fuels, and the factors in engine design and operation which cause high severity are discussed. It is emphasized how necessary it is to produce designs of engines of lower “severity”.