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Empirical and analytical data on the minimum possible flame temperatures for combustion processes rapid enough to be effective for engine operation are presented. The fundamental basis for these minimum temperatures is explored with chemical kinetic analysis. The combination of these minimum temperatures and the time scales associated with engine processes yield minimum possible levels of in-cylinder NOx production for both diesel and spark-ignition engines. These minimum NOx levels are identified and validated empirically. Legislated NOx levels lower than those indicated will require exhaust aftertreatment in addition to in-cylinder combustion control.

This paper proposes a structure for the diesel combustion process based on a combination of previously published and new results. Processes are analyzed with proven chemical kinetic models and validated with data from production-like direct injection diesel engines. The analysis provides new insight into the ignition and particulate formation processes, which combined with laser diagnostics, delineates the two-stage nature of combustion in diesel engines. Data are presented to quantify events occurring during the ignition and initial combustion processes that form soot precursors. A framework is also proposed for understanding the heat release and emission formation processes.