Modeling End-Gas Knock in a Rapid-Compression Machine

摘要

A rapid-compression machine is a laboratory apparatus to study aspects of the compression stroke, combustion event, and expansion stroke of an Otto cycle. As a simple model of such a machine, unsteady one-dimensional nonisobaric laminar flame propagation through a combustible premixture, enclosed in a variable volume, is examined in the asymptotic limit of Arrhenius activation temperature large relative to the conventional adiabatic flame temperature. In this limit, a thin propagating flame separates nondiffusive expanses of burned and unburned gas. The pressure through the enclosure is spatially homogeneous for smooth flame propagation. However, expansion of the hot burned gas results in compressional preheating of the remaining unburned gas, and in fact the spatially homogeneous gas may undergo autoconversion prior to arrival of the propagation flame. If such an explosion is too rapid for acoustic adjustment, large spatial differences in pressure arise and the resulting nonlinear waves produce audible knock. Here attention is concentrated on what fraction (if any) of the total charge may undergo autoconversion for a given operating condition, and what enhanced heat transfer from the end gas would preclude autoconversion--though too great heat transfer from the end gas could result in flame quenching (unburned residual fuel). (Author)