Observer Based Oxygen Fraction Estimation for a Dual-Loop EGR Diesel Engine Fueled With Biodiesel Blends

This paper investigates the influence of biodiesel on the effectiveness of exhaust gas recirculation (EGR) in modern Diesel engines equipped with dual-loop EGR systems. Intake manifold oxygen fraction, which is an important factor for both combustion and emissions, is selected as a new reference for evaluating the equivalent EGR level instead of EGR ratio. A Luenberger-like observer for the oxygen fraction is designed based on the dynamic model of the air-path loop with consideration of the existence of oxygen content in the fuel. The convergence of the observer is proved with the assistance of some physical insight into the engine system. The performance of the observer is validated on a high-fidelity engine model built in GT-Power. The results show that when the same amount of fuel is injected, there is an increase in the exhaust oxygen concentration for biodiesel as oxygen content in fuel increases. Then the higher exhaust oxygen concentration leads to an increase in the intake manifold oxygen fraction, since the engine control unit (ECU) commanded EGR valve angles are constant across different fuels. This real-time oxygen fraction estimation approach is potentially useful for mitigating the biodiesel NO _x emission effect.