The challenge of reducing nitrogen oxides (NOx) emissions in diesel engine exhaust gas can be met by the use of nitrogen-enriched air (NEA) for combustion. In this work, the separation performance of membranes producing NEA is simulated and experimentally validated. Retentate nitrogen concentration and stage-cut depend on the membrane properties oxygen-nitrogen selectivity, oxygen permeance and membrane area as well as on the operating parameters, feed pressure, permeate pressure and feed flow rate. This complex dependence is presented using correlations of dimensionless numbers for retentate oxygen mole fractions between 16 and 21 mol%. It is herewith possible to calculate membrane area and compression power loss for any given membrane at any engine load point. The correlations also serve to identify the most suitable membrane and operating parameters when having to meet design criteria, e.g. limited construction space or maximum power loss. In a design example, both membrane area and power demand are calculated for three state-of-the-art membranes at a number of characteristic load points of a medium-duty diesel truck engine. (C) 2008 Elsevier B.V. All rights reserved.