In addition to designing and developing hardware to control the exhaust gas recirculation (EGR) rate, it is equally important to tailor the other engine variables. Optimization of air-fuel ratio (A/F) and spark timing is required to achieve the best combination of oxides of nitrogen (NOx), hydrocarbon (HC), and carbon monoxide (CO) control, while minimizing the losses in fuel economy and drivability.This paper describes an engine dynamometer and vehicle study which defines the relationship between the above para meters. In general, increasing the spark advance with increasing EGR rates will minimize the losses in fuel economy and vehicle drivability while achieving significant reductions in NOx emissions. This approach is limited by a deterioration in HC emission control. The loss in HC control can be minimized by spark retard, but with a loss in fuel economy. The A/F must be tailored to maintain good combustion for HC emission control without excessive richness and loss of CO emission control.