Toward developing simplified models for diesel/biodiesel engine pollutant emissions

Biodiesel is considered as an attractive alternative to petrodiesel for transportation applications. Substituting petrodiesel with domestically produced biodiesel increases energy independence, reduces the carbon footprint, and offers a viable path toward biomass utilisation. It has been found that biodiesel-fuelled engines emit up to 70% lower particulate matter (PM) compared to petrodiesel-fuelled engines, although they can emit up to 20% more nitric oxides (NO_x). However, the differences in physical and chemical properties of biodiesel compared to petrodiesel require re-optimization of diesel engine control systems, e.g. for injection timing and exhaust gas regeneration, when petrodiesel is substituted with biodiesel. Although there has been much focus in this area, there is still a need for fundamental research to aid in the development of closed-loop control systems. In this study, detailed chemical kinetic simulations are employed to improve the fundamental understanding of biodiesel and petrodiesel combustion under pressure and temperature conditions in engines. The detailed simulations are carried out with the objective of developing simplified mechanisms which can then be employed in engine models employed for engine development and to develop effective control strategies. The computations are carried out using a strained laminar flamelet code. In addition to exploring the effect of strain rate (turbulence) on the ignition, extinction, and pollutant formation characteristics, the fundamental chemical pathways that lead to PM and NO_x formation are studied by considering the evolution of precursor species. In general, ignition and extinction of flames in biodiesel combustion are more sensitive to turbulence than in petrodiesel flames. Soot volume fraction is lower in biodiesel combustion compared to petrodiesel combustion. This is consistent with measurements reported in engines. NO concentration is, however, lower for biodiesel combust- on when considering kinetics alone, suggesting that the volume phasing and operating parameters of engines influence observed NO results in engine emissions where NO is often observed to be higher in biodiesel-fuelled engines. It is shown that while increasing mixing reduces soot formation for both fuels, the reduction is significantly greater for biodiesel suggesting that increased mixing has a greater effect on PM emissions in biodiesel-fuelled engines.