Knock Damage Mechanisms in Spark-Ignition Engines

The objective of this preliminary investigation was to identify the mechanisms by which knocking combustion cause engine damage. The project was motivated by the need for a knock intensity measurement based on a damage related threshold and not the arbitrary octane-heptane scales. The scope of this work includes a detailed investigation to determine the modes of engine failure resulting from knocking combustion. Using the physical evidence from a collection of knock damaged pistons and other parts, the mechanisms of failure are reconstructed by deductive reasoning. The manner in which knock causes surface erosion, ring fracture, piston land cracking and fracture, piston blow-by and seizure are all addressed. The interrelationships between these various modes of failure are also considered. It is shown that there are two distinctly different aspects of knocking combustion that can initiate damage, namely global heat flux and local pressure-temperature pulses. It is thus proposed that either of these two primary failure mechanisms could form the basis of a damage-related knock-intensity criterion. This would result in two different values of damage potential for a given operating condition, each referring to its particular damage pathway.