A new ignition time model applied to super knock

Abstract

Abstract An ignition time model is developed to model super knock in a compression engine. The model assumes that thermoacoustic interaction is the primary mechanism for the onset of super knock. By ignoring diffusive effects, a simple transport equation for the time to ignition of a fluid particle is derived. The significantly reduced cost of the chemistry model allows for complex hydrocarbon fuels to be simulated. Additionally, a zonal model for the secondary ignition of a charge due to the action of an expanding flame is developed. The flame compresses the unburned gas, causing the temperature and pressure to rise, which yields a pre-ignition in the unburned gas before the charge is engulfed by the flame. It is shown that the ignition time model compares well to the detailed chemical model with less than 1% difference in the prediction of ignition delay. Using this ignition time model, a multi-dimensional simulation of super knock in a rapid compression machine corresponding to the configuration of Wang et\xa0al. [1] is performed. It is found that interaction of the shock with the flame and the side wall of the cylinder significantly enhances the strength of the shock, and the in-cylinder pressure exceeds 300\xa0bar. From the pressure rise predicted by the simulation, it is concluded that simulated ignition is a super knock event. Since the ignition time model excludes diffusive effects on the chemistry, it is proposed that acoustic resonance of the cylinder is the primary driver in the development of super knock for the configuration under examination and that inhomogeneous ignition due to transient flame compression could be a key mechanism for super knock.