Fuel Injection And Combustion Of Diesel Engines Fueled With Dme-biodiesel Blends

Because of the challenge of increased vehicle population and environmental pollution, clean alternative fuels are becoming increasingly important. Both dimethyl ether (DME) and biodiesel are clean alternative fuels. Their problems to separate application can be solved by blending them. The injection process, spray, combustion, engine performance and performance prediction of DME-biodiesel blends are investigated in this dissertation.The injection process of DME-biodiesel blends is experimentally studied on a pump test bench. In common rail injection system, compared to biodiesel, the injection of DME starts later, and the injection of other fuel blends start almost at the same time. By increasing DME proportion, fuel injection ends later and injection duration is prolonged significantly, but peak injection rate changes little. With the increase of rail pressure, the early change rate of injection rate with time and peak injection rate increase; the injection start and injection end among blended fuels vary less, but injection rate curve fluctuates stronger. With the increased injection pulse width, the injection start and peak injection rate remain unchanged, but injection duration increases significantly. As nozzle opening pressure increases from 17MPa to 20MPa, the injection starts later and peak injection rate increases. For in-line-pump injection system, with the increase of DME proportion, the injection start later, and the early change rate of injection rate with time and the peak of injection rate decrease with retarded peak phase; furthermore, the injection duration extends and the sound velocity decreases. Compared to common rail injection system, with in-line-pump injection system, DME proportion has more effect on injection timing, peak injection rate and injection rate pattern.The spray characteristics of DME-biodiesel blends are studied using high-speed photography. The results show that spray penetration increases rapidly first and then slowly with time. With increased DME proportion, spray penetration decrease, but spray angle increases. As ambient pressure increases, spray penetration decrease, but spray angle increases; spray penetration differs less distinctly among various fuels. With the increase of rail pressure, spray velocity, spray penetration and spray angle increase. With the increase of injection pulse width, spray penetration increase while spray angle changes little.An experimental study is conducted on a naturally aspirated diesel engine fueled with DME-biodiesel blends. The results show that, with the increase of DME proportion, the peak in-cylinder pressure decreases with retarded peak pressure phase; the peak premixed combustion decreases, but the peak diffusion combustion increases, and ignition delays; the peak in-cylinder temperature and peak pressure rise rate decrease, and their phases retard. As DME proportion increases, the brake specific fuel consumption (BSFC), exhaust temperature, NOx and smoke emissions decrease, especially with greatly reduced smoke emissions at high loads; CO emissions are very low at low to middle loads, and decreases at high loads. The heat release analysis is associated with in-cylinder pressure four-layer wavelet analysis of blended fuels. The results show that just as the peak heat release rate, pressure rise rate and pressure rise acceleration, so the peaks of subsignals at four layers are located within premixed combustion phase. The peaks of subsignals represent pressure oscillation of premixed combustion. The peak subsignal and wavelet relative energy at fourth level are the largest. With the increase of DME proportion, wavelet relative energy at fourth level increases, but wavelet relative energy at other levels decreases.Another experimental study is conducted on a turbocharged diesel engine fueled with DME-biodiesel blends with 0, 30%, 50%, 70% and 100% DME proportion individually. The effects of nozzle parameter and DME proportion on combustion are investigated. The results show that, with the increase of DME proportion, inlet pressure increases, but the ignition delays, the peak in-cylinder pressure and heat release rate, peak in-cylinder temperature and peak pressure rise rate decrease, and their phases retard. By increasing DME proportion from 30% to 100%, BSFC, NOx emissions and smoke emissions decrease, while HC emissions and CO emissions decrease firstly and then change little. Compared to the nozzle of 6×0.40mm, the peak cylinder pressure and peak heat release rate increase with nozzle of 6×0.35mm, and their phases are advanced. As DME proportion increases, their differences become greater. To get low BSFC and NOx emissions, with low DME proportion blended fuels,nozzle of 6×0.35mm should be adopted; and with high DME proportion blended fuels,nozzle of 6×0.40mm adopted. An artificial neural network trained by Levenberg-Marquardt algorithm is developed to predict the performance of the blended fuels. Its accurate prediction is verified through experiment.The effects of EGR on emissions and performance are also investigated. The results show that with EGR, inlet pressure increases;for all five blended fuels, NOx emissions decrease drastically, but CO emissions and HC emissions increase; smoke emissions increase for blended fuels containing no more than 50% of DME, but change little for blended fuels containing 70% DME or more due to low baseline levels. With the increased load, the effect of EGR on emissions becomes obvious.