This is an optimization study on the use of filtered exhaust gas recirculation (EGR) to reduce the NO emissions of diesel engines. Control of the particulate emissions and provisions for filtered EGR were achieved by an Aerodynamically Regenerated Trap (ART) with collection efficiencies in the order of 99%. The amount of EGR was regulated to provide for substantial NO reduction, without unacceptably decreasing the thermal efficiency of the engine or increasing the CO emissions. EGR regulation was accomplished by monitoring the injection pump setting which was correlated to the fuel flow rate, the speed of the engine, the amount of EGR flow, and the ambient air temperature. Through these parameters, the mixture strength expressed as the equivalence ratio, ϕ, was calculated and related to the power output of the engine. Thus, a map of engine performance parameters was generated and related to measured NO and CO emissions. A series of road tests showed that EGR most effectively reduces NO emissions at high ϕ's (by a factor of two at 20% EGR) which, however, is accompanied by an increase in CO emissions by a factor of two, and a penalty in fuel economy by 8%. Benefits and losses can be optimized by automatically varying the level of EGR, using feedback from the aforementioned engine parameters. An algorithm was developed to govern the electrically controlled EGR valve and tests showed that the NO levels decreased by 30%, while the CO increased by 30%, showing no penalty in fuel economy. The resulting specific NO and CO emissions were well within the current US EPA standards.