Evaluating Oxygenated Fuel's Influence on Combustion and Emissions in Diesel Engines Using a Two-Zone Combustion Model

This work examines the effects of diesel fuel oxygenation on the combustion and emissions in a direct injection (DI) diesel engine. This is a predominately computational study, where the influence of the fuel oxygen content at various injection timings and loads is addressed by using an in-house, comprehensive, two-zone model of DI diesel engine combustion, which divides the cylinder contents into a nonburning zone of air and another zone in which fuel is continuously supplied from the injector and burned with entrained air from the air zone. The validity of the model results is assessed favorably against pertinent experimental data, such as cylinder pressure and heat release rate (HRR) diagrams and nitric oxide (NO) and soot emissions generated in this laboratory by conducting tests on an experimental, single-cylinder, DI Hydra (Ricardo/Cussons, Manchester, United Kingdom) diesel engine operated at two different loads and injection timings. Various degrees of oxygenation of conventional diesel fuel and various injection timings were undertaken in the theoretical study at each load. The numerical simulation results provide insight into the local combustion and emissions formation conditions. Numerical modeling determined that cylinder pressures, temperatures, and NO emissions increase, whereas soot emissions decrease with the degree of fuel oxygenation at any load, and also that cylinder pressures, temperatures, and NO emissions decrease, whereas soot emissions increase by retarding the injection timing at any load. These results are used for discussing the implications they have on the behavior of biofuels or diesel fuel blends, where the fuel-bound oxygen and ignition delay (embodied here in the injection timing variable) are the main parameters dictating their behavior. At least for the case of some common biofuels in blends with diesel fuel, it is shown that lower injection timings and higher degrees of biofuel blend oxygenation (inside limiting values) can alleviate the notorious NO-smoke trade-off with respect to a point of lower oxygenation degree lying on a higher injection timing NO-smoke curve (at any constant load).