The traditional philosophy that all knocking combustion is damaging has caused engine and fuel manufactures to operated with large “safety” margins, incurring both fuel production and consumption costs.
This paper describes an attempt to relate knocking conditions to erosion damage. One of the goals was to discover a measurable knock intensity parameter which describes the propensity of knock to cause erosion damage.
An experimental technique was developed in which test specimens were subjected to controlled knocking conditions in the combustion chamber of a modified engine. A photographic method was employed to measure the relative level of erosion on the test specimens. This methodology was used to determine the effect of fuel type and operating conditions on knock-induced erosion. It was confirmed that the geometry as well as the temperature played a considerable role in the damage mechanism.
The data from these tests showed no correlation between the classical knock intensity criteria (i.e.: peak-to-peak pressure amplitude) and the measured erosion damage. This result was understood as an indication that the classical knock intensities are a measure of the global aftermath of knocking combustion, which is not directly related to the local condition in the end-gas crevices during the auto-ignition-detonation process.
This was confirmed by a stress calculation applied to bent specimens, which revealed the existence of local pressure differences of greater than 350 bar. This is many times greater than that recorded by the pressure transducers.
The erosion test results showed that the cylinder pressure at the point of auto-ignition was correlated to the measured erosion damage. It was concluded that the strength of the developing-detonation process in the crevice region was strongly related to the initial conditions in the end-gas prior to auto-ignition.
Microscopy revealed that knock damage is accompanied by the development and propagation of micro-cracks within the erosion zones.
This work now provides a means of measuring the knock-erosion potential and not simply the knock intensity.