Effect of coolant temperature on air-fuel mixture formation and combustion in an optical direct injection spark ignition engine fueled with gasoline and butanol

Diversification of the energy mix and the drive for increasing security of supply have extended the use of alternative fuels in internal combustion engines. Butanol is a viable energy source for spark ignition (SI) power units featuring higher energy density and compatibility with existing systems. The present work investigated the use of n-butanol in an optically accessible wall guided direct injection SI engine, operated at low load, as well as wide open throttle. Engine speed and injection pressure were kept constant, while coolant temperature was alternated between two values, so as to simulate cold-start and fully warmed-up conditions. In-cylinder pressure and exhaust gas emission measurements were coupled with optical results obtained through UV-visible flame visualization and 2D chemiluminescence. This allowed a more detailed insight into the occurrence of diffusive flames near the piston surface and an analysis of flame front propagation, as well as its morphology. The correlation of thermodynamic data and flame imaging with band-pass filters for recording the spatial distribution of the OH radical, soot precursors and carbonaceous structures gave information on the evolution of chemical species during combustion. While at low load the alcohol performed slightly better compared to gasoline, at wide open throttle the opposite was recorded. The effect of coolant temperature was more evident for butanol. These observations were correlated to the presence of liquid fuel film on the piston crown, which resulted in slower flame propagation and higher related emissions for the alcohol.