The emission control of small naturally aspirated diesels has posed severe problems and it has been generally thought that their acceptability in pollution sensitive areas would decline. The investigation described attempts to reverse this view. The exhaust quality of engines of this type can be improved by reducing the combustion temperatures and/or reducing the initial heat release rate: a range of methods of achieving this were examined and found commercially unacceptable.A qualitative review of the problem led to the evolution of a modified cycle characterized by high turbulence leading to higher air/fuel mixing rates and faster diffusion burning. This “Squish Lip” combustion system allowed CARB 1977 projected emission levels to be met on development engines without performance deterioration. Bench and field trials are in hand and a second generation system for truck applications is being evaluated.