The development of a leanburn engine is described, in which optimized engine design, innovative engine management and exhaust gas aftertreatment using a special NOx-storage catalyst were combined to yield a significant improvement in fuel economy with reduced NOx emissions. To achieve stable combustion near the lean limit a swirl system was used and the appropriate parameters of the 2.2 I 4-cyIinder 4-valve SI engine were optimized. As a result, the mixture formation was improved and the lean limit was extended to higher air-fuel ratios. An adaptive lambda controller which was based on the evaluation of engine-smoothness calculated from the RPM-sensor was implemented to control each cylinder individually close to the lean limit. A model-based control system was developed to achieve extremely accurate air-fuel ratio control during transients. This system also enabled optimal regeneration of the NOx-storage catalyst, by optimizing the lean to rich air-fuel ratio transitions and by holding the engine torque constant so that the regeneration phase was imperceptible to the driver.Experimental results from the New European Driving Cycle (NEDC) show a significant improvement in fuel economy with exhaust emissions which are comparable to those of a stoichiometric concept. The whole system: optimized leanburn engine, advanced engine control system and exhaust gas aftertreatment which is introduced here is a promising way to improve fuel economy and to fulfill future emissions legislations.