Third Paper: The Influence of Engine Design on Spark Knock at High Speeds

Abstract

Modern motor roads permit cars to be driven at high speeds for many miles. In these circumstances it is possible for engines to be operating in a condition of spark knock for a considerable time. Such knock might not be detected by the driver, but could initiate a chain of events leading to damage to the engine.

A realistic procedure has been devised for carrying out tests for spark knock at high speeds, employing cars mounted on vehicle dynamometers. Tests on 30 European car models, all having manual transmissions, led to the critical knock speed concept for making comparisons between the high speed knock susceptibilities of various designs of engine. These comparisons have shown that, in service, over-advanced spark timing is frequently the prime reason for the occurrence of high speed knock.

With spark timing set at the minimum advance required for the engine to develop its best torque, octane requirement is influenced by compression ratio, brake horsepower per litre, cylinder swept volume and disposition of the exhaust valves. Also to a small extent, by combustion chamber design and the amount of induction heating employed. Given attention to the above factors, it is felt that the high speed anti-knock requirements of the present generation of engines can be met by currently marketed gasolines of 98/99 Research octane number (ON).