| Due to an increase in energy crisis, environmental pollution and further stringent emissioncontrol regulations, the investigation and utilization of clean and alternative fuels haveattracted much attention. As an alternative diesel fuel, biodiesel fuel is a clean and renewableenergy source. The oxygen content in the biodiesel provides an effective way to enhancecombustion process and inhibit soot formation in diesel engines. There have been a numberof experimental studies on combustion and emission characteristics of diesel engines fueledwith biodiesel fuels. However, there are few studies on spray, combustion, sootcharacteristics and their relationship for biodiesels. Meanwhile, the particulate emissions ofdiesel engine fueled with biodiesels still needs to be investigated further.The systemaic study conducted in this dissertation includes the spray, ignition, combustionand soot characteristics of biodiesels and diesel under the ultra-high injection pressure, andthe performance, combustion and emission characteristics of a turbocharged high-pressurecommon rail diesel engine fueled with biodiesels, diesel and biodiesel-diesel blends.Moreover, the investigation on the effects of ultra-high injection pressure and micro-holenozzle on soot formation of impinging diesel spray was also carried out. The investigationon spray, ignition, combustion and soot characteristics was conducted with a high speedvideo camera and several optical methods in a constant volume combustion vessel. Anexperimental setup of a modern turbocharged high-pressure common rail diesel engine wasused for the engine study. This dissertation provides some important information andguidance for the utilization of biodiesels in diesel engine. The innovative results of thisdissertation are as follows,（1） The non-evaporating and evaporating sprays of diesel, biodiesel from palm oil （BDFp）and biodiesel from used cooking oil （BDFc） were studied. Injection delay of biodiesel islonger than that of diesel. For non-evaporating spray, spray penetration and spray angle forbiodiesel are longer and smaller respectively than those of diesel. The spray projected areaand spray volume for biodiesels are smaller than those of diesel. The estimation on spraydroplet size shows that biodiesels generate larger Sauter mean diameter （SMD） due to thehigher viscosity and surface tension. BDFp shows these characteristics more obviously. Thisstudy proves theoretically that the behavior of biodiesels and diesel sprays in this study is similar to that of gaseous turbulent jets. Calculation using the quasi-steady jet theory showsthat the air entrainment of biodiesels is worse than that of diesel. For evaporating spray,biodiesels show longer spray liquid length than diesel, especially for BDFp. The differencein spray liquid length between diesel and biodiesels becomes larger as the injection pressureis increased.（2） The ignition and combustion characteristics of diesel, biodiesel from palm oil （BDFp）and biodiesel from used cooking oil （BDFc） were investigated. The ignition position locatesa short distance downstream from the spray liquid tip, where the fuel is in vapor phase. Withthe increase of injection pressure, the ignition position moves further downstream. The flamekernel develops both upstream and downstream after ignition occurs. The ignition delays ofbiodiesels are shorter than that of diesel, and BDFp shows the shortest ignition delay.Ignition delay is shortened with the increase of injection pressure. The lift-off lengths ofbiodiesels are shorter than that of diesel, and BDFp shows the shortest lift-off length. Forsome experimental conditions and test fuels, the liquid length is longer than lift-off length,which indicates that the liquid phase tip may be surrounded by the diffusion flame sheath.BDFp shows an obvious tendency to the phenomenon. With the increase of injectionpressure, the phenomenon disappears.（3） The soot characteristics of diesel, biodiesel from palm oil （BDFp） and biodiesel fromused cooking oil （BDFc） were investigated. Soot level for BDFp and BDFc is appreciablylower than that of diesel fuel at low injection pressure, while soot reduction of BDFp andBDFc is significant at ultra-high injection pressure. BDFc is more effective on soot reductionthan BDFp. Biodiesels show lower flame temperatures than that of diesel. The oxygen frombiodiesel fuels plays a more significant role on soot reduction than that from the entrained air.At low injection pressure, atomization and mixing are not “good” enough and the effect ofoxygen from biodiesel fuel molecule on soot oxidation is inhibited. While at high injectionpressure, the improved spray atomization and mixing are realized and thus the effect fromoxygen in fuel molecules on soot oxidation is obvious. Due to the small spray angle andshort lift-off length, BDFp shows a relatively poor air entrainment upstream of the lift-offlength than BDFc, which explains the more obvious effect of BDFc on soot reduction thanthat of BDFp.（4） The effects of ultra-high injection pressure and micro-hole nozzle on soot formation ofimpinging diesel spray were investigated. In the case of conventional injector nozzle （d=0.16mm）, ultra-high injection pressure generates appreciably larger flame size and appreciablylower soot formation respectively. In the case of micro-hole nozzle （d=0.08mm）, impingingspray flame shows much smaller size and lower soot formation. For the micro-hole nozzle（d=0.08mm）, ultra-high injection pressure is much effective on soot reduction. For theconventional injector nozzle （d=0.16mm）, flat wall impingement deteriorates soot formationsignificantly. While soot formation characteristics of free spray flame with the micro-hole nozzle （d=0.08mm） are not altered obviously by flat wall. Liquid lengths for the nozzles of0.16and0.08mm are longer and shorter respectively than the impingement distance （30mm）. Liquid impingement upon the wall is responsible for the deteriorated soot level ofimpinging flame compared to that of free flame with the conventional nozzle.（5） Performance, combustion and particulate emissions of a turbocharged common-railinjection diesel engine fueled with diesel, biodiesel from soybean oil （BDFs）, biodiesel fromused cooking oil （BDFc）, BDFs-diesel blends were investigated. When the diesel engine isfueled with biodiesels or biodiesel-diesel blends, the brake specific fuel consumption andthermal efficiency are increased slightly, the first peak of heat release rate corresponding topilot injection is lower than that of diesel, while the second peak corresponding to maininjection is higher and advanced to the top dead center （TDC）. HC and CO emissions areinsensitive to the test fuels in this diesel engine. When biodiesels or biodiesel-diesel blendsare used, CO2emission is decreased. The difference in the above characteristics areignorable when the diesel engine is fueled with BDFs and BDFc. Biodiesels show slightlylower NOx concentrations than diesel under low engine load; while at high engine load,biodiesels show obviously higher NOx concentrations. When the diesel engine is fueled withbiodiesels or biodiesel-diesel blends, particle size distributions are almost not altered, but theparticulate number concentration and mass concentration are reduced significantly at thesame time. The investigation reveals that BDFc is more effective than BDFs on the reductionof both NOx and particulate emissions. |
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