NOₓ and soot emissions trends for RME, SME and PME fuels using engine and spray experiments in combination with simulations

This study includes engine tests with neat FAME fuels and blends of 7% and 30% of RME in European Diesel, EN590 (non FAME added). The study was also completed with spray studies and numerical simulations for a deeper understanding of the engine results. The engine was operated at four operational points (A25, B50, B75 and C100) taken from the European Stationary Cycle, ESC cycle. The A25 case was also extended to achieve information how the soot emissions varies with the operational parameters; start of injection (SOI), NOₓ level (regulated by EGR) and needle opening pressure (NOP). The spray studies were performed in a high temperature/high pressure spray chamber at relevant conditions. For the numerical simulations the KIVA 3-V code was used with detailed chemical kinetics. The results shows that the neat FAME fuels lower the soot emissions by up to 90% compared with Diesel fuel. Moreover, even low blend ratios lowers the soot emissions significantly. Among the FAME fuels, combustion with PME had the lowest amount of soot emissions due to its lowest amount of di- and tri-unsaturated fatty acids (i.e. less double bonds) in comparison with SME and RME. The NOₓ emissions were increased for the neat FAME fuels in relation the Diesel fuel. The increase in NOₓ emissions for the neat FAME fuels is due to the higher flame temperature for the FAME fuels which could be a result of the oxygen content which causes a lower equivalence ratio (ϕ), i.e. leaner local reacting mixture. The A25 operational case showed that the most important factor to decrease soot emissions for Diesel fuel was NOP, while for the FAME fuels SOI had the greatest impact. Further, at constant level of NOₓ emissions for the included fuels it was observed that the FAME fuels still reduced the soot emissions significantly. The study shows that FAME fuels and Diesel blends with FAME fuels, can be a tool to meet future emission legislation, since low soot emissions levels can be reached for low NOₓ emissions and the fact that FAME fuels are renewable, the global warming impact is lower than for Diesel.