A Comparison and Model of NOx Formation for Diesel Fuel and Diethyl Ether

Exhaust NO_x and particulate measurements were obtained at equivalent operating conditions in a direct-injection compression ignition engine for diesel and diethyl ether fuel. Particulate levels for diethyl ether were very low relative to the diesel fuel and did not increase significantly until the fuel to oxygen equivalence ratio was above 0.8. The log of fuel specific NO_x for both the diesel and diethyl ether fuel were found to correlate well with the inverse of adiabatic flame temperature as has been observed by others in previous research. A newly defined heat release averaged, adiabatic flame temperature improved the correlation of both fuels. Differences in the correlations of the two fuels with this newly defined temperature were smaller than the scatter of the data. A simple model based on the assumptions of a NO_x formation zone proportional in volume to the rate of heat release was successful at predicting fuel specific NO_x over a range of simulated EGR conditions, speeds, and loads to within approximately ± 30%. The results suggest that the differences in NO_x obtained with different fuels are a result of the fuels effect on flame temperature and not a fundamental change in kinetic reaction or mixing rates.