The inevitability of engine-out NOx emissions from spark-ignited and diesel engines

Spark-ignition and diesel engine combustion experiments were carried out for fuels including natural gas. propane, and indolene, showing that flame extinction and high unburned hydrocarbon emissions result when in-cylinder combustion temperatures fall below a critical threshold value of approximately 1920 K. Laminar dame calculations were then carried out for methane and propane to relate these experimental limiting temperatures to those associated with lean-limit conditions for flame propagation. Model laminar flame results for pressures above 50 atm produced a lean limit at an equivalence ratio of 0.6, where the adiabatic flame temperature was about 1900 K. These results show that at the elevated pressures of engine combustion, the lean-limit product temperature for these hydrocarbon fuels is considerably higher than that obtained previously for lean-limit flames at atmospheric pressure and that these much higher temperatures will produce significant amounts of NOx. These coupled studies indicate that under conditions of pressure and temperature in both spark-ignition and diesel internal combustion engines, high product gas temperatures are required and relatively high NOx production rates are the inevitable result. Thus, it may; be impossible to reduce NOx production to very low levels if flame propagation at high pressures is involved, and it will he necessary to provide postcombustion aftertreatment to reduce NOx emissions to legislated levels.