The authors evaluated the attainment of low-temperature diesel combustion in a medium-duty diesel engine apparatus. Attainment of low temperature combustion is determined by the simultaneous and substantial decreases in nitric oxide (NO) and smoke concentrations. The extreme low-temperature combustion condition results in greater than 50% reductions in both NO and smoke concentrations relative to a baseline conventional combustion mode. Combustion development occurs at a low speed (1400 rev/min) and nominally light load (nominally 68 N-m torque, or 1.9 bar brake mean effective pressure). The authors conclude that determination of low-temperature combustion attainment is not universally possible using phenomenologically based ignition delay calculations. Evaluations of two methods to determine start of combustion, which render a definition for ignition delay and for engine ignition delay, reveal that neither provides a consistent metric to determine low-temperature combustion attainment.