This paper consists of two parts. Part I concerns the effects of injection timing, injection rate, and air swirl on emission of smoke and gaseous pollutants from direct-injection diesel engines. Studies show that fuel-injection equipment and variables such as nozzle configuration affect pollutant production and emission because they affect fuel-air mixing. An increased rate of injection or air swirl increases the rate of fuel-air mixing and reduces the amount of exhaust smoke and its dependence on injection timing. An increase in rate or swirl ratio increases nitric oxide emission at a given injection timing, but the increase is relatively small compared with reduction obtained by retarding injection timing. Substantial retard, in conjunction with increased rate of fuel-air mixing, limits loss in engine efficiency.Part II reports development of a model for calculating soot and nitric oxide formation. The model predicts effects of injection timing, injection rate, fueling, air swirl ratio, and engine speed on exhaust concentrations of these pollutants. The efficacy of the model and its contribution toward understanding and control of pollutants is discussed.