Reliable reference determination for diesel engine emission control

To meet the current and upcoming emission legislation, engine air system control is a crucial aspect. To this end different approaches from SISO control in combination with feedforward structures up to advanced MIMO strategies are proposed in the literature. However, for each control strategy a reference quantity is necessary. Although it seems sensible to directly use the exhaust emission concentrations in a closed loop control, unfortunately a fast and accurate online measurement is seldom available. A typical approach to circumvent this issue is to use auxiliary quantities like fresh air mass flow, whereas the according reference values for these quantities are obtained during engine development. This paper presents a different approach where not only one but a combination of different candidates is used to define an optimal reference for the EGR valve control. The target for the overall control is to maintain a constant nitric oxides (NO_x) level during steady state operation without explicit NO_x measurement. Furthermore, a scheduling method is proposed to take into account the expected errors on the sensor signals depending on the current operating region. Finally the control was applied on a mean value engine simulation model and showed satisfactory results.