What Causes Slower Flame Propagation in the Lean-Combustion Engine?

TLDR: In this paper, a quasi-dimensional engine-simulation program and a multidimensional engine-flow and combustion code were used to study the effect of flame propagation in lean-burn engines.

Abstract: Previous engine data suggest that slower flame propagation in lean-burn engines could be due to slower flame expansion velocity at lean conditions than at stoichiometric combustion. This paper reports on two classes of model, a quasi-dimensional engine-simulation program and a multidimensional engine-flow and combustion code, used to study this effect in detail and to assess the capabilities of the models to resolve combustion details. The computed flame-speed data from each program differed somewhat in magnitude, but the predicted trends at various equivalence ratios were quite similar. The trends include: (1) The peak in-cylinder burned-gas temperature decreases by about 300 K as the equivalence ratio is decreased from 0.98 to 0.70. (2) Both the laminar flame speed and the flame-propagation speed, the latter computed from the time derivative of flame radius, decrease with decreasing equivalence ratio.