We report quantitative experimental investigations on the air/fuel distribution in the combustion chamber of a spark ignition engine prior to ignition and during the first stages of combustion. A four cylinder VW four-stroke engine was modified to give optical access to the combustion chamber via the piston. The fuel concentration was visualised by planar laser-induced fluorescence (PLIF).
The choice of an appropriate fuel dopant is very important. Several properties have to be considered simultaneously. The most crucial influence results from the sensitivity to quenching by oxygen. Since fuel distributions recorded at different engine operating conditions wore to be compared on a quantitative scale, this effect had to be taken into account most carefully.
The long fluorescence lifetime and the extraordinarily low quenching rate of vapour-phase fluoranthene in a high pressure environment as pertaining to engines led to its choice as dopant. The selection was corroborated by comprehensive investigations, the results of which are reported in this paper.
Highly resolved temporal and spatial images of the fuel distribution were obtained for different engine operating conditions after correcting the influence of quenching. The air/fuel-ratio, engine speed, time of injection and charge motion within the cylinder were varied. In this context we also analysed two different combustion chamber configurations. The results are presented as air/fuel-equivalence ratio (λ) mappings. Local differences in the charge homogeneity are discussed and interpreted. The influence of the fuel distribution on the evolution of the combustion process was quantified via cylinder pressure records.
We give recommendations for effective engine design in terms of the charge homogeneity and its effect on flame propagation, cyclic variations and pollutant formation.