The increasing importance of environmental protection requires a drastic reduction of NOx, particulates and CO2-emissions. This has motivated the development of an exhaust gas recirculation system with closed loop control on a direct injected (DI) Diesel engine. As input for the control system the air/fuel ratio of the exhaust gas is used, measured using an oxygen sensor. It is shown in this paper that by proper choice of the oxygen sensor, control oriented modelling and robust control design based on the linear approximation of the model very good improvements in NOx can be obtained, while keeping particulates emissions low.