Air can be enriched with oxygen and/or nitrogen by selective permeation through a nonporous polymer membrane; this concept offers numerous potential benefits for piston engines. The use of oxygen-enriched intake air can significantly reduce exhaust emissions (except NOx), improve power density, lessen ignition delay, and allow the use of lower-grade fuels. The use of nitrogen-enriched air as a diluent can lessen NOx emissions and may be considered an alternative to exhaust gas recirculation (EGR). Nitrogen-enriched air can also be used to generate a monatomic-nitrogen stream, with nonthermal plasma, to treat exhaust NOx. With such synergistic use of variable air composition from an on-board polymer membrane, many emissions problems can be solved effectively. This paper presents an overview of different applications of air separation membranes for diesel and spark-ignition engines. Membrane characteristics and operating requirements are examined for use in automotive engines. Several prototype membranes have been developed and tested, and the impli-cations of their various characteristics are evaluated in terms of achieving lower parasitic power requirements and smaller module sizes. Prototype test results show that current polymer membranes can meet the needs of automotive engines and have the potential for a wide variety of applications. Based on our studies, a new variable air composition concept is proposed to achieve low emissions goals for both diesel and spark-ignition engines.