Diffusion Flame Temperature-Its Influence on Diesel Particulate and Hydrocarbon Emissions

The influence of flame temperature on particulate carbon, gas-phase hydrocarbon and particulate-bound hydrocarbon emissions for two divided-chamber diesel engines is described. The data base consisted of emissions from both a high-swirl and a low-swirl engine operating over a wide range of speeds, loads and intake-air compositions. At a given engine speed, load, timing and combustion chamber geometry, variations in the intake oxygen concentration were utilized to determine the dependence of these emissions on the characteristic diffusion flame temperature. This dependence was not significantly affected by the engine operating conditions, implying that the majority of the fuel burns in the diffusion mode at a temperature related to the stoichiometric adiabatic flame temperature. However, emissions from the low-swirl engine were found to be much more sensitive to flame temperature than emissions from the high-swirl engine. The dependence of the flame temperature effect on the combustion chamber geometry can be interpreted as a change in the competition between formation and oxidation processes. A phenomenological description of the competition between these two mechanisms is used to discuss the relative roles of flame temperature on emissions.