Exhaust emission characteristics of a production automotive diesel engine were studied. The particular engine was chosen because it is being marketed with a prechamber as well as with a direct-injection combustion system and, in both configurations, either naturally aspirated or turbocharged. In addition, an aftercooler was fitted to the turbocharged direct-injection engine.Methods for reducing exhaust emissions are discussed. A brief survey is given of potential emission control by catalytic converter, exhaust recirculation, and LPG dual-fuel operation. Basic investigations, conducted on single-cylinder direct-injection (DI) engines are reported.It is concluded that direct injection is the most promising combustion system for low-emission commercial automotive diesel engines, particularly when turbocharged and aftercooled, and that this type of engine can satisfy the 1975 California standards.