Daniel Schwarz considers the mass fraction of oxygen as a qualitative descriptor of the intake air of a diesel engine. He highlights the advantages of this measure over the EGR rate as a descriptor during transient phases of the engine. The author presents physical modeling approaches for the gas system of the diesel engine that utilize the information from the measured mass fraction of oxygen. This includes a mean value model that determines the gas state in the cylinder when using internal exhaust gas recirculation through variable charge exchange control. Additionally, he explains a control-oriented model of the low-pressure exhaust gas recirculation path and examines the regulation of the mass fraction of oxygen in the intake path. The author applies nonlinear control algorithms and compares them with each other. He discusses the use of observers for estimating disturbances and implements them.