Amplified Pressure Waves During Autoignition: Relevance to CAI Engines 2002-01-2868

Controlled autoignition (CAI) engines ideally operate at very lean stoichiometries to achieve low NO_x emissions. But at high loads, when combustion approaches stoichiometric, they become noisy and severe engine knock develops. A possible cause is the development of amplifying pressure waves near the hot spots that inevitably occur in the autoigniting gas. This paper presents the results from numerical solutions at realistic engine conditions of the detailed chemical kinetic equations with acoustic wave propagation. Those calculations that involve hot spots must include a spatial dimension. Because of this, they are much more time-consuming than for the homogeneous case. A model system of mixtures of 0.5 H₂-0.5 CO with air for equivalence ratios, ϕ, between 0.45 and 1.0 has been used at engine-like temperatures and pressures. These calculations investigate the behaviour for various values of ϕ, hot spot size and temperature elevation. They demonstrate that strong pressure spikes and developing detonations occur when the spatial gradient of temperature is such that the autoignition moves into the unburnt mixture at approximately the speed of sound, so that the chemical and acoustic waves reinforce each other. It is also shown that this resonance, in itself, is not sufficient for a developing detonation to occur and it is also necessary for the mixture to be sufficiently reactive. The criterion for this in terms of the homogeneous chemical kinetics is developed. It is postulated that the conditions for the resonance will occur somewhere in the cylinder for most mixtures but only for ϕ close to stoichiometric do strong pressure waves develop, leading to noisy and possibly damaging combustion.