Biodiesel is an alternative fuel derived from vegetable oils, animal fats, or other sources, and it can be made into biodiesel blends by mixing with conventional diesel. To achieve optimal engine combustion as well as minimal emissions with biodiesel blends, on-board blend level estimation system is one of the prerequisites. The paper investigates a blend level estimation system by evaluating exhaust oxygen fraction differences between diesel and biodiesel. In the system, a wideband oxygen sensor is utilized to measure the oxygen concentration. Then, research on the oxygen-content based biodiesel blend level estimation system is extended by taking the exhaust gas recirculation (EGR) into account. Since estimation accuracy under the lean conditions for this kind of system has been especially problematic, the effect of EGR on system estimation uncertainty is discussed. The theoretical formulation shows that oxygen consumption factor is an intrinsic indicator of fuels, which is not affected by the EGR level. However, the “shifting effect” caused by introducing EGR can help moving the estimation points into a range where not only the oxygen sensor but also the whole estimation system exhibits lower measurement uncertainty. Validations are provided by both simulation results based on a high-fidelity GT-Power engine model and experimental results on a medium-duty turbo-charged diesel engine platform.

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