Numerical analysis of the autoignition of methanol, ethanol, n-heptane and n-octane sprays with detailed chemistry

The autoignition process of fuel sprays has relevance to compression-ignition engines. The present study focusses on a detailed description of the processes involved. In particular, variable liquid and gas properties, detailed transport in the gas phase, and detailed reaction mechanisms are considered. The model uses a simplified geometry and assumes a uniform spray. Results of numerical calculations of the autoignition of methanol, ethanol, n-heptane, and n-octane are presented, where special attention is focussed on differences in liquid properties. It appears that the vaporization rate is dominated by the ratio M(F)/phi, the molecular weight of the fuel over the molar ratio of oxygen to fuel, and by the latent heat of vaporization, L(V). For the fuel methanol, both M(F)/phi and L(V) are largest, so that in spite of the high volatility of methanol, both spray lifetime and ignition delay are largest for a methanol spray compared to ethanol, n-heptane and n-octane. Ignition delay increases with initial droplet size if pure air is used as the oxidizer. There is a minimum ignition delay time when the air is premixed with fuel vapor.