Heat Transfer as a Determent of End-Gas Knock

Abstract

Compressive preheating of the residual charge in an Otto-cycle-type, internalcombustion-engine cylinder can result in autoignition and rapid homogeneous burning (explosion). That is, the final portions of the combustible premixture may convert exothermically to product so rapidly that the reaction time may be less than the acoustic-adjustment time, In such cycles, large spatial nonuniformity in the pressure field, and onset of knock, result-s-events potentially damaging to cylinder components. Since operation at high compression ratio is thermally efficient, since (equally important) knock-free performance at conventional compression ratio is desirable, and since use of knock-inhibiting fuel additives is environmentally constrained, the end-gas-cooling characteristics required to achieve smooth flame propagation are studied, Specifically, a threezone model of the nonisobaric combustion event in a variable-volume enclosure is developed. The three zones are: the burned gas, the unburned charge in the normally cooled region, and the unburned charge in the region with augmented cooling. The heat transfer in these zones is distinguished by locally applicable polytropic relations, which prove to be tractable means of parametrically introducing heat transfer into this highly simplified unsteady one-dimensional model. The results indicate that augmented heat transfer from the end gas can prevent those temperatures for which autoconversion is likely to arise in the residual charge. At moderate engine speed (-2000 rpm), if ignition and final burn-up occur roughly symetrically with respect to piston-crown displacement from top dead center, for the prevention of knock, the fraction of the combustible mass that must be regarded as end gas (and subjected to enhanced cooling by heat transfer) is approximately one-half.