Abstract
Advancing the spark and increasing the compression ratio can improve the thermal efficiency of spark-ignited internal combustion engines, but only within limits set by knock occurrence. Further critical factors are strongly inter-related flame parameters, including burning velocity, turbulent flame speed and flame surface perturbations. The purpose of this study was to determine characteristics of the most suitable fuels for sparkignition engines. Therefore we have evaluated the relationships, for various fuels, among burning velocity, ignition delay time and flame instability (defined in terms of Lewis number, i.e. the diffusion coefficient to thermal diffusivity ratio). Measurements obtained from images of combustion in a combustion chamber show that, of the tested fuels, burning velocities were highest for ethanol followed by n-heptane, toluene and isooctane. Parallel sensitivity analyses of elementary reactions involved in the combustion indicate that the methyl radical degradation reaction, which is normally stable, is highly temperature sensitive in ethanol combustion, explaining ethanol’s high burning velocity. Furthermore, zero dimensional chemical kinetic calculations show that ignition delay time is prolonged by the addition of ethanol. Hence, ethanol combustion should have low knock tendencies due to the combination of high burning velocity and prolonged ignition delay time. Further numerical analyses of flame instability indicate that perturbations of the surface in cellular flames are most strongly affected by the interactions between mass and thermal diffusion. Thermal diffusion effect becomes unstable when the Lewis number is lower than unity. We have compared the Lewis numbers of various fuels combusting under lean conditions. Experimental results show that ethanol is most stable, so ethanol has a stabilizing effect on flames under lean conditions. Overall, the results indicate that a fuel, such as ethanol, which has fast burning velocity, long ignition delay time and stable propagation characteristics is the most suitable for spark-ignition internal combustion engines.
Keywords: Spark ignition, Flame propagation, Combustion, Knock, Flame instability, Lewis number