Heat release and instantaneous injection rate data was obtained from a series of experiments on a 121mm bore, single-cylinder, deep-bowl, non-swirling D.I. diesel research engine, using a variety of fuel injection pump builds. Results from tests at constant air-fuel ratio and constant start of combustion angle show that increasing the mean fuel injection kinetic energy (M.I.K.E.) at a given engine speed reduces the heat release time and increases the fuel-air mixing rate. Also, at constant fuel injection kinetic energy, increasing the engine speed increases the fuel-air mixing rate. These experimental trends have been interpreted with the aid of a novel but mathematically very simple analytical approach, based on the hypothesis that all fuel-air mixing in a DI diesel combustion system is promoted by kinetic energy inputs.A “Combined Mixing Efficiency” has been identified which appears to be a characteristic constant of a DI diesel combustion system geometric configuration. This is designated ηcomb75 which is the ratio of (A) an “ideal minimum mixing energy” and (B) the sum of (1) the fuel injection kinetic energy and (2) the air turbulence kinetic energy at TDC multiplied by an arbitrary factor. This efficiency parameter is shown to be independent of engine speed and only weakly dependent on the magnitude of fuel injection kinetic energy supplied.This work suggests that fuel-air mixing in a DI diesel can be enhanced by engine speed-related air turbulence effects in a manner more significant than has been previously thought, even in so-called ‘quiescent’ combustion systems.