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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.

The development of smoke measurement procedures and instruments has been one of the significant factors contributing to the reduction of smoke from diesel-powered trucks. There is a need, however, for better information on the relation between measurements and the appearance of the smoke plume, for improved smokemeters, and for common international test procedures. Studies of smoke plumes using jet theory, alumina tracer, and hydraulic analog techniques indicate that the plumes are sometimes unstable and subject to large-scale shedding in the region where free-plume smokemeters are located. This introduces a significant random variation into smoke measurements made in this location. A future test procedure based on smoke measurements made with a long path (500 mm) in-line smokemeter during a dynamic cycle is suggested for consideration. Smoke limits would be based on tests similar to those conducted by MIRA to relate smoke levels and engine size to human response.