A review of the fuel consumption per capita and population trends is presented which suggests that any significant relaxation in diesel emission goals is undesirable.An analysis is then made of the chemical, thermodynamic and fluid dynamic factors which control the formation of nitric oxide, unburned hydrocarbon, carbon monoxide and exhaust smoke in diesel combustion. Based on this analysis it is hypothesized that a reduction in the quantity of fuel burned in diffusion flames and increased mixing rates of air and fuel vapor will significantly lower emission levels. It is suggested, therefore, that future trends in direct injection combustion development will need to incorporate increased fuel atomization and toroidal air motion to achieve the required favorable combustion conditions.A comparison is made of commercially acceptable, state-of-the-art diesel engines developed to meet low emission levels to allow an estimate of the potential improvements from combustion development. The comparison suggests that intense mixing and good fuel atomization, characteristic of the prechamber process, are the keys to lower emissions and can yield up to 40% reductions in HC + NO2 levels for the direct injection diesel combustion process.A forecast of future engine requirements is made and indicates that hybrid engines combining the diesel and Rankine cycles are feasible for heavy duty engine applications. The combination of the highly efficient diesel cycle and a reciprocating steam engine using diesel engine exhaust energy is a potentially efficient, low emission, socially acceptable prime mover.