A statistical analysis of developing knock intensity in a mixture with temperature inhomogeneities

Abstract

Knock formation and its intensity for a stoichiometric ethanol/air mixture under a representative end-gas auto-ignition condition in IC engines with temperature inhomogeneities are investigated using multi-dimensional direct numerical simulations (DNS) with a 40-species skeletal mechanism of ethanol. Two- and three-dimensional simulations are performed by systematically varying temperature fluctuations and its most energetic length scale, lT. The volumetric fraction of the mixture regions that have the propensity to detonation development, FD, is proposed as a metric to predict the amplitude of knock intensity. It is found that with increasing lT, FD shows a good agreement with the heat release fraction of the mixture regions with pressure greater than equilibrium pressure, FH. The detonation peninsula is well captured by FD and FH when plotting them as a function of the volume-averaged ξ, ξ¯, (ξ=a/Ssp is the ratio of the acoustic speed, a to the ignition front speed, Ssp). Decreasing lT is found to significantly reduce the super-knock intensity. The results suggest that decreasing lT, as in engines with tumble designs resulting in a smaller turbulence scale, will be effective in mitigating the the super-knock development.