The proposed federal and California regulations affecting emissions from on-highway diesel-powered vehicles have stimulated a critical examination of many forms of combustion chambers in search for a solution to the 5 g/bhp·h combined HC + NO2 requirement. Results of tests made on indirect and direct injection systems are presented. It has been shown that with a simple modification to the injection timing of a production engine equipped with a swirl chamber combustion system, combined emission on the order of 4 g/bhp·h can be achieved under both naturally aspirated and turbocharged conditions. These results are achieved without significant sacrifice of smoke-limited power. Data on heat loss to coolant with the engine tuned for low emissions are presented.The influence of injection and engine variables on the emission characteristics of direct injection systems has been investigated and it is shown that, in general, the measures required to generate low exhaust emissions result in a loss of smoke-limited power and a depreciation in specific fuel consumption. The superiority of the direct injection system over indirect injection chambers in regard to fuel economy is therefore reduced. The effects of exhaust gas recirculation and water injection on emissions are reviewed and it is concluded that neither is very attractive in regard to the reduction of emissions for the complication involved.The possible use of the high-speed diesel engine for use in passenger cars to meet the 1976 Federal Emission Requirements is also discussed.