A passive recirculation of hydrogen using ejectors is effective and efficient only in a limited operating window, which kept ejectors until recently from being integrated into automotive fuel cell systems, where a more dynamic operation is expected. Strategies like parallel setup or a PWM-drive employed to expand the operating window demand reliable control algorithms. Such algorithms can greatly benefit from the predictive power a mathematical model. In a fuel cell system, the anode and cathode side cannot be separated, and a mathematical description should encompass all components to a reasonable degree, to allow the model to run on a low power automotive-grade platform in real time. This contribution demonstrates an approach to reduce the computation expense of the mathematical model.