Impact of biodiesel on NOx emissions in a common rail direct injection diesel engine

The impact of biodiesel on NOx emissions was investigated in a 2.5 L common rail direct injection diesel engine under both low load and high load conditions with different fuel injection strategies. Three fuels were used in this study, an ultralow sulfur diesel fuel (BP15), a blend of 20 vol % biodiesel in BP15 (B20), and a blend of 40 vol % biodiesel in BP15 (B40). Fuel injection timings were held the same for the biodiesel blends and the baseline diesel fuel to eliminate the potential injection timing differences due to the different fuel heating values. According to the needle lift profiles obtained from the needle lift sensor, no measurable injection timing differences were observed between the biodiesel blends and the baseline diesel fuel. Biodiesel blends were found to generally produce slightly lower NOx emissions than the baseline diesel fuel at the low load condition. Under the high load condition, evidently higher NOx emissions for biodiesel blends were observed under both single and double injection conditions. Overall, biodiesel diesel blends and the baseline diesel fuel had very similar heat release rate profiles. Injection system parameters were adjusted to reduce the increased NOx emissions for B20. Retarding injection timing under single injection conditions was found to be the more effective approach to reduce the NOx emissions than using pilot injection with retarded main injection in terms of NOx and fuel consumption tradeoff. Under the low load condition, the pilot injection strategy led to substantially reduced NOx emissions. When the performance of the 40 vol % biodiesel blend under various exhaust gas recirculation (EGR) rate conditions was investigated, the biodiesel blend showed no further combustion deterioration compared to the baseline diesel fuel. A low NOx and fuel-efficient engine operating condition was achieved with the 40 vol % biodiesel blend.