Combustion and the thermodynamic performance of spark ignition engines

Abstract

Some overall considerations for the optimization of the energy chain for the spark ignition engine are briefly discussed and figures given for the relative inhaled energies with different fuels. There follows a description of the nature of turbulent flame propagation in engines, showing the computed characteristics of the burning rate for stoichiometric and lean mixtures at different engine speeds. These reveal the factors influencing partial quenching and misfire. Flame instabilities are shown theoretically to become more important with increasing pressure. The associated flame wrinkling and cellularity increase the burning rate of both laminar and turbulent gaseous flames. Fundamental aspects of the present trend towards the direct injection of gasoline are discussed and the unique instabilities in aerosol combustion are shown to increase the burning velocity above that of the corresponding gaseous mixture. The degree to which evaporative cooling of droplets can enhance the volumetric efficiency is discussed. The onset of autoignition in an engine is calculated with recent shock tube data, which are also used to show that evaporative cooling can increase the knock resistance by about four octane numbers.