Although diesels, as a group, are a relatively small source of air pollutants, emissions standards which limit emissions from diesels have been adopted by California and the federal government. Test procedures and instrumentation for measuring diesel emissions have been developed, and an understanding of how engine design parameters affect emissions is evolving.Smoke and carbon monoxide are primarily functions of fuel-air ratio. Smoke is also affected by injection timing, air motion, and fuel spray characteristics. Hydrocarbon emissions are most affected by details of injector design and matching of the spray geometry with the combustion chamber shape. Nitric oxide emissions are controlled by local oxygen availability in regions of high temperature and residence time at the high temperature. Retarded injection timing, rich local fuel-air ratios, lower mixing rates of fuel and air, lower compression ratio, and lower intake air temperature tend to reduce nitric oxide emissions but may increase smoke.Smoke suppressant additives, water induction, exhaust gas recirculation, and catalytic converters reduce one or more of the emissions but are less desirable than emissions control through engine modifications.