The influences of fuel characteristics and engine parameters on exhaust emissions were studied experimentally using four diesel engines. All engines utilized direct-injection combustion systems and included a 2-cycle, air-scavenged unit and three 4-cycle engines. Two of the 4-cycle engines were naturally aspirated, and one was turbocharged.Seven fuels were selected to cover the broad compositional ranges of diesel fuels marketed throughout the country. Aside from the effect of fuel density on carbon monoxide (CO) emission, no fuel property showed an influence on emissions that was consistent for all engines tested.The study of engine variables included modifications to the fuel-injection system, variations in intake air temperature and density, exhaust recirculation, and water injection. As diesel engines generally emit large quantities of nitrogen oxides (NOx) and little CO or unburned hydrocarbon (HC), this study was aimed primarily at reducing NOx emissions while maintaining engine performance and an otherwise favorable emission pattern. Levels of emissions other than NOx are near minimum when the engine is adjusted for peak performance. Exhaust gas recirculation and addition of water to the intake air were found quite effective in limiting emissions of NOx with little sacrifice in engine performance or increase in CO and HC levels.