A NUMERICAL STUDY ON COMBUSTION AND EMISSIONS CHARACTERISTICS OF DIESEL/BIODIESEL BLENDS IN A LOCOMOTIVE ENGINE

There is a pressing need for decarbonizing the hard-to-electrify transportation sectors, including rail, to achieve the net-zero-carbon goal by 2050. Due to the long lifetime of locomotive engines, adopting alternative drop- in fuels such as biofuels and their blends with conventional petroleum diesel in existing engine platforms is a promising solution to rapidly reduce the carbon footprint with relative low cost. It is therefore imperative to improve the understanding of the combustion and emissions characteristics of biofuels such as biodiesel, renewable diesels, and their blends with diesel to ensure a smooth transition away from conventional diesel in locomotive engines. In this study, a comprehensive numerical investigation is performed into the effect of substitution of diesel with a low-cloud-point (LCP) biodiesel on engine combustion performance and emissions characteristics in a four-stroke locomotive engine. A five-component surrogate for the LCP biodiesel is developed as part of this study using the Monte-Carlo sampling method targeting key physical and chemical properties. Results show that the newly developed biodiesel surrogate predicts the physical and chemical properties very well. A reduced chemical reaction mechanism is developed for the diesel/biodiesel blends and validated against literature data on ignition delay measurements. Three-dimensional (3-D) computational fluid dynamics (CFD) simulations are carried out using the new biodiesel surrogate, the V0a diesel surrogate, and the new reduced reaction mechanism. The engine CFD model is first validated against the measured pressure and heat release traces as well as the emissions data from experiments with the diesel fuel. The validated model is then applied to diesel/biodiesel blends with varying blending ratios. Results show that the combustion characteristics between different diesel/biodiesel blends are very similar. However, soot emissions decrease significantly by a factor of 1.55 while NOx emissions increase by a factor of 1.81 as the biodiesel blending ratio is increased.