This work has demonstrated that the use of oxygen enriched air in the inlet to a direct injection diesel engine can result in a significant reduction in particulates emissions (in excess of 80% at full load), increased thermal efficiency if injection timing control is employed, substantial reductions in exhaust smoke under most conditions, ability to burn inferior quality fuels which is economically and environmentally attractive and achievement of turbo-charged levels of output. NO(x) emissions are generally increased due to the improved combustion efficiency, but can be reduced to normally aspirated engine levels at some efficiency penalty. The successful use of a self-contained oxygen generation system using a prototype flat sheet polymeric membrane module, compressor and turbocharger design based on the initial engine test work with oxygen from compressed gas cylinders has been demonstrated for the first time. This work has identified an important role for membrane gas separation in this area and represents a major step forward in improving diesel engine efficiency and emission control. Current work is aimed at optimising membrane/turbine/compressor combinations with emphasis on the use of improved hollow fibre membranes and demonstration on an operating diesel engine.