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This paper reports on research conducted at AVL on swirl supported and quiescent combustion systems on heavy-duty and medium-duty direct injection diesel engines towards defining the most appropriate combustion system technologies which will enable the EPA 1991 HD diesel emissions standards to be met without the use of exhaust after-treatment devices and with good prospects for meeting the 1994 standards. Based on both theoretical work and development efforts on a variety of engines, an analysis is conducted of the various aspects of swirl and fuel injection with regard to their effect on the formation of combustible mixtures and on particulate formation. Particular reference is given to the lube oil particulate fractions and to the dry soot fraction, recognized as the main contributors to total particulates. Based on this analysis, the main features of a low emission combustion system are derived, and a strategy to satisfy the 1991 and 1994 standards is presented.