NOx emissions can be controlled through engine operation with lean homogeneous air/fuel mixtures. This emission control approach precludes the need for exhaust gas recirculation (EGR) and secondary air injection systems. The Lean Mixture concept results in similar emissions, fuel economy, and driveability when compared to EGR systems tailored to similar emission levels with similar aftertreatment systems. The Lean Mixture approach does offer the potential for less engine emission control hardware.The minimum NOx level achieved experimentally at the lean driveability limit was about 1.2 g/mi but with significantly higher HC emissions. Lean Mixture systems are sensitive to variations in engine air/fuel ratio which produce a significant effect on their emissions and fuel economy. Due to this sensitivity, it appears that the Lean Mixture concept is limited to the current NOx emission standards (3.1 g/mi) unless technological advances in fuel/air metering occur that reduce engine and carburetor air/fuel ratio variations.Vehicle programs have demonstrated the effect of lean engine air/fuel ratios on base emissions and fuel economy as well as in conjunction with various aftertreatment systems; oxidizing catalytic converters, and lean manifold reactors. Lean Mixture systems are not compatible with reducing catalysts.A Lean Mixture-Manifold Reactor emission control system was used to evaluate the effect of compression ratio and leaded fuel on vehicle emissions and fuel economy. With the constraint of equal HC emission levels, the classical relationship between compression ratio and fuel economy appears to have been altered. The use of higher compression ratio did not result in improved fuel economy. In addition, the use of leaded fuels resulted in both a direct and a long term increase in HC emissions.System durability is shown for a Lean Mixture-Catalytic Converter system with lead-free fuel and for a Lean Mixture system without aftertreatment with leaded fuel. An engine dynamometer program demonstrated the incentive for improved mixture distribution.