The heavy-duty engines covered in this report include eight gasoline and four diesel engines used in farm, construction, and industrial applications. The report includes descriptions and photographs of the test engines, instrumentation systems used, explanations of test sequences and calculation methods employed. They were monitored for CO, CO2, NO, NOx, O2, hydrocarbons, aldehydes, HCHO, particulates, and smoke. The engines were tested using well-accepted steady-state procedures for gaseous emissions measurement, and in addition, the Federal procedure for smoke certification was used for testing the diesel engines. The emissions results obtained in this study, as well as data obtained from other sources, were used in conjunction with information on engine population and usage to estimate emission factors. National impact was estimated separately for each of three engine applications.