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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.