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 flame 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 NO x . 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 NO x production rates are the inevitable result. Thus, it may be impossible to reduce NO x production to very low levels if flame propagation at high pressures is involved, and it will be necessary to provide postcombustion aftertreatment to reduce NO x emissions to legislated levels.