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The control of the engine air system is an essential part for meeting the emission levels of current and upcoming legislation. Up to now different strategies were presented in the literature and also applied on real systems. Starting from simple single-input-single-output structures in combination with feedforward parts leading to advanced multi-input-multi-output approaches. Nevertheless, independent of the used control approach for each of them suitable references are necessary. Although it seems adequate to directly use the emission target quantities in a closed loop air system control, a fast and accurate measurement is seldom available. An alternative is to use intermediate quantities as references, like fresh air mass flow or oxygen concentrations, which represent the state of the air system. However, for control purposes each of these quantities has to be determined, i.e., measured or calculated.