Influence of Fuel Injection Pressure and Biodiesel upon NO _x Emissions

Biodiesel is a potential alternative to Ultra Low Sulfur Diesel (ULSD); however, it often suffers from increased fuel consumption in comparison to ULSD when injection timings and/or pressures are similar. To decrease fuel consumption, increasing biodiesel injection pressure has been found to mitigate the issues associated with its relatively high viscosity and lower energy content. When doing so, the literature indicates decreased emissions, albeit with potentially greater nitrogen oxide (NO_x) emissions in contrast to ULSD. In order to better understand the trade-off between fuel consumption and NO_x emissions, this study explores the influence of fuel injection pressure on ULSD, Waste Cooking Oil (WCO) biodiesel, and their blends in a single-cylinder compression ignition (CI) engine. In particular, fuel injection pressures and timings for WCO biodiesel and blended fuels are adjusted to attempt to mimic the in-cylinder pressure profile of operation using ULSD. Furthermore, this effort analyzes fuel physical properties, heat release, fuel consumption, and engine-out emissions, to draw thorough comparisons between various fuel blends and ULSD.

Operation with low-percentage blends of biodiesel was found to be similar in behavior in comparison to ULSD. However, fuel blends with a larger composition of biodiesel exhibit significant reductions in overall performance, thus requiring greater injection pressures to make up the deficit. Emissions species exhibited expected behavior with respect to engine load, while also decreasing with greater biodiesel content, thanks to improved combustion efficiency. However, NO_x emissions for all fuel blends displayed complex behavior in comparison to ULSD by decreasing at lower loads while increasing at higher loads, with all changes becoming more prominent with growing biodiesel composition. Overall, the onset of noticeable deviation in measured parameters was found to occur around 20% biodiesel content, suggesting that significant engine calibration may only be required for fuel blends at or beyond this point. Moreover, it is possible to decrease biodiesel fuel consumption without exceeding ULSD NO_x emissions.