The effects of nitrogen-enriched air, supplied by an air separation membrane, on NOx emissions from a 1.9-L turbocharged direct-injection diesel engine were investigated. To enrich combustion air with more nitrogen, prototype air separation membranes were installed between the after-cooler and intake manifold without any additional controls. The effects of nitrogen-enriched combustion air on NOx emissions were compared with and without exhaust gas recirculation (EGR). At sufficient boost pressures (>50 kPag), nitrogen-enriched air from the membrane provided intake oxygen levels that were similar to those of EGR. Compared with EGR, nitrogen-enriched air provided 10-15% NOx reductions during medium to high engine loads and speeds. At part loads, when turbocharger boost pressure was low, the air separation membrane was not effective in enriching air with nitrogen. As a result, NOx reduction was lower, but it was 15-25% better than when EGR was not used. The flow and pressure losses from the membrane were low, and proper waste gate modulation can compensate for the losses. On the basis of present tests with prototype membranes, nitrogen-enriched combustion air provided a viable alternative to EGR for reducing NOx, at least during engine operations in which there was sufficient boost pressure. In practice, membranes need to be custom designed and matched for a particular engine, along with the turbocharger, to obtain the maximum NOx reductions in the engine operating range of interest.