Effects Of Alternative Fuel Properties On Combustion And Emissions Characteristics Of Diesel Engine

The world faces two critical problems of energy shortage and air pollution. Developmentand utilization of clean alternative fuels is an effective solution. At present, biodiesel andgas-to-liquids (GTL) are most promising alternatives. They can be mono-fuel or blendedwith few diesel engine structure modification and parameters tuning. On the same dieselengine, this paper analyzes the physical and chemical characteristics of biodiesel andGTL blended fuel, their effects on combustion process, exhaust gas pollutants and PMsize distribution, and finding the key factors.The advanced test platform is established for high pressure common rail diesel enginewith advanced measurement facilities. Two partial flow dilution sampling systems aredesigned for PM mass and PM size measurement. Accroding to the requirement for theengine exhaust particle size spectrometer of TSI EEPS3090, the optimal dilution ratio isset in200~300.Seven physical and chemical properties of biodiesel and GTL blended fuel, aresummarized, as well as cetane number, density, aromatic content, distillationcharacteristic temperature T90/T95, sulfur content, oxygen content, viscosity, n-alkanesconcentration of GTL base on measurement.The results show that the peak pressure decreases, the maximum rate of heat releaseand the maximum pressure rising rate increases with more biodiesel. But, the case iscomparably different under low load. However ignition delay period increases slightlyand combustion duration decreases. Under high load, more biodiesel blended content,less CO, HC, PM and soot emission concentration, while more NOx. Under low load, PMand soot increases, NOxdecreases. CO and HC resulting from biodiesel B100arehigher than those of diesel. NOxand PM trade-off relationship are not obviouslyimproved.The particle size distribution curve of diesel is unimodal at steady operating conditions,while that of biodiesel is bimodal with nuclei mode and accumulation mode. Thenumber density of nuclei mode particle from biodiesel is higher than that of diesel,however, that of accumulation mode is less. Under higher load, the number densityrising rate of accumulation mode from diesel is bigger than that of nuclei mode, and thepeak shifts to the big size. The number density of nuclei mode from biodiesel decreasesand the peak shifts to the small size, while that of the accumulation mode rises and thepeak shifts to the big size. With more blended biodiesel under steady operatingconditions, particle distribution gradually changes from unimodal of diesel fuel tobimodal of biodiesel, but the transition trend is distinct under different operating condition. The particle size distribution from diesel dominates with50nm accumulationmode under transient operating conditions, and that of below50nm nuclei mode. Theaverage number density of biodiesel is higher than that of diesel, while that of nucleimode is higher than that of accumulation mode. With the transient rate decreases, theaverage number density of nuclei mode decreases, while that of accumulation is muchsmall. The average number density of nuclei mode gradually increases with moreblended biodiesel, while that of accumulation mode gradually decreases, the highestcan amount to97%.The biodiesel is of higher viscosity, cetane number, oxygen content and lower aromaticcontent, density. They have great effect on combustion，exhaust gases pollutants andPM size distribution.With more blended GTL, the shorter ignition delay period and longer combustionduration. At the same time pre-mixing combustion quantity decreases and diffusioncombustion ratio increases. The peak pressure, maximum pressure rise rate and themaximum rate of heat release are distinct under different operating condition. CO, HC,NOx, PM and soot emissions gradually decrease, but NOx increases slightly at10%lowload, GTL can significantly improve the NOxand PM trade-off relationship.Under steady operating conditions, PM number density distribution from diesel, GTL andblended fuel take on unimodal. At about25nm, the number density is the smallest. Thepeak area lies between50~100nm. That of accumulation mode from GTL is biggerthan that of nuclei mode. With more blended GTL，that of nuclei mode below25nmgradually decreases under different load, that of nuclei mode above25nm and that ofaccumulation mode above50nm decreases under low load decrease, however theyincrease at medium and higher load. Under transient operating conditions, the numberdensity of small particle size of nuclei mode increases under higher load at early stage,peak near interim and reduce in the later. That of accumulation mode change very smallunder higher load. With more blended GTL, the peak number density of nuclei modeadvances and that of large particle size of accumulation mode increases. With lessblended GTL, the peak number density of nuclei mode gradually decreases with moreblended GTL, especially for large transient rate condition better effect.The GTL is of higher cetane number, lower aromatic content, less density which hasgreat effects on combustion, exhaust pollutants and PM size distribution.