A Theoretical and Experimental Study of the Modes of End Gas Autoignition Leading to Knock in S. I. Engines 942060
A 2-D simulation of fluid dynamic and chemistry interaction following end gas autoignition has demonstrated three distinct modes of reaction, dependent upon the temperature gradient about an exothermic centre. All three modes (deflagration, developing detonation and thermal explosion) can contribute to knock; the developing detonation case, associated with intermediate temperature gradient, has been identified as the more damaging. The simulation code (LUMAD) has been used in a systematic parametric study designed to separate the complex interacting events which can lead to mixed modes in real engines. A most significant finding related to the sequential autoignition of multiple exothermic centres. For two exothermic centres characterised by large temperature gradients, which would normally yield the relatively benign deflagrative mode, it was found that pressure waves emanating from the first could modify the temperature gradient and so promote violent developing detonation mode at the adjacent centre. The LUMAD code was modified to allow arbitrary flame front definition and autoignition centre location. It was also adapted to include a particle tracking routine. It was then applied to data collected in an associated series of engine experiments to allow comparison of velocity fields and pressure records. Generally good agreement was obtained, allowing inference of data for events too fast to monitor with conventional instrumentation.