A research program was conducted to determine the effects of thermal barriers in the combustion chamber on emissions and combustion of a direct-injection diesel engine operated on selected alternate fuels. Steady-state exhaust emissions, cranking characteristics and combustion data of a baseline water-cooled engine were compared with those of a heat insulated uncooled engine with both engines operated on a reference fuel and two alternate fuels. A high compression ratio water-cooled engine with intake air heating was also included in the study.Operating the baseline water-cooled engine on alternate fuel generally raised the level of exhaust emissions. In addition, ignition delay and the rates and relative amounts of premixed and diffusion burning were significantly altered when the baseline water-cooled engine was tested with alternate fuels. Thermally insulating the combustion chamber decreased ignition delay which resulted in reduced particulate emissions but increased hydrocarbon emissions. Likewise, raising compression ratio and intake air temperature decreased ignition delay which resulted in a reduction in hydrocarbon and oxides of nitrogen emissions. However, particulate emissions increased under these conditions.Regarding starting, cranking speeds increased with decreasing fuel quality for the baseline water-cooled engine. Heat insulation of the combustion chamber reduced the adverse effects of the alternate fuels on starting, as did raising compression ratio and intake air temperature.