【作者】 侯军兴；

【导师】 乔信起；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2011， 博士

【摘要】 随着汽车保有量的不断增加和排放法规的日益严厉,发展清洁能源成为当前能源与环境领域的一个重要课题。二甲醚和生物柴油均为柴油机清洁代用燃料,两种燃料混合后可弥补各自不足。本文针对非增压、增压发动机燃用二甲醚-生物柴油混合燃料开展了系统的研究,涉及喷油、喷雾、燃烧、性能和性能预测等。在喷油泵试验台上试验研究了二甲醚-生物柴油混合燃料的喷射过程。研究表明,对共轨燃料系统,与生物柴油相比,纯二甲醚的喷油始点较晚,其他燃料的喷油始点差别不明显。随着燃料中二甲醚比例的增加,喷油终点延后;喷油持续期明显延长,最大喷油速率变化不大。轨压升高,所有燃料的喷油速率曲线上升段变陡,喷油速率峰值增加;各混合燃料的喷油始点、喷油终点差别均变小,喷油速率在喷油持续期的波动变大。同一燃料,喷射脉宽增大,则喷油始点一致,最大喷油速率基本不变,喷油持续期明显增加。启喷压力由17MPa升高为20MPa,喷油始点变晚,最大喷油速率略增加。对泵-管-嘴燃料系统,随着二甲醚比例的增加,喷油始点延后,喷油速率曲线上升段变平缓,最大喷油速率降低,其相位依次延后;喷油持续期增加;高压油管内声速和长管内声速降低。共轨系统与泵-管-嘴系统比,燃料中二甲醚比例对前者喷油始点、最大喷油速率、喷油速率曲线形状的影响要远远小于后者。采用高速摄影研究了二甲醚-生物柴油混合燃料的喷雾特性。研究表明,各燃料喷雾贯穿距离随时间增加的规律是先快后慢。随着燃料中二甲醚比例的增加,喷雾贯穿距离减小;喷雾锥角变大。背压升高,各燃料的喷雾贯穿距离减小,喷雾锥角变大;各燃料喷雾贯穿距离的差别变小。轨压升高,各燃料的喷雾贯穿速度、贯穿距离、喷雾锥角变大。脉宽增加,各燃料的喷雾贯穿距离增大,喷雾锥角差别不大。非增压发动机燃用二甲醚-生物柴油混合燃料研究表明,随着燃料中二甲醚比例的增加,缸内压力峰值降低且相位延后;预混合燃烧减弱,其放热峰值降低;扩散燃烧放热峰值升高;混合燃料的燃烧始点延后。缸内最高温度和最大压力升高率降低,其相位延后。掺混二甲醚后,混合燃料的燃油消耗率、排气温度、NOx排放和碳烟排放降低,高负荷时碳烟排放降低更显著。中低负荷时各燃料的CO排放均很低,高负荷时燃料中掺混二甲醚,CO排放显著降低。对缸内压力进行四层小波分解,提取压力时频信息,并将之与放热率、缸内压力升高率和压力升高加速度关联。结果表明,放热率峰值、压力升高率峰值、压力升高加速度峰值都在预混合燃烧阶段;缸内压力在各层的子带信号峰值也在此阶段,各层子带信号峰值反应了预混合燃烧在各频域的冲击。各混合燃料燃烧时均为第四层子带信号峰值和小波相对能量最大,不同负荷都如此。随着燃料中二甲醚比例的增加,第四层小波相对能量增加,其他层小波相对能量减少。针对二甲醚-生物柴油混合燃料的理化特性,试验研究了喷嘴参数、混合比例等对增压发动机燃烧和性能的影响,试验的混合燃料中二甲醚占质量比例分别为0%、30%、50%、70%和100%。研究发现,采用同一喷嘴时,随着燃料中二甲醚比例的增加,发动机进气压力增加,着火延迟,缸内压力峰值、放热率峰值、最高缸内温度、压力升高率峰值降低,其相位延后。二甲醚掺混比例从30%到100%时,燃油消耗率、NOx排放和碳烟排放下降,HC排放和CO排放先降低,之后变化平缓。6×0.35mm喷嘴与6×0.40mm喷嘴比较,前者的缸内压力峰值、压力升高率峰值和放热率峰值均高于后者,峰值相位提前;随着燃料中二甲醚比例的增加,差别更明显。综合考虑燃油消耗率和NOx排放,燃用低比例二甲醚混合燃料时,宜选用6×0.35mm喷嘴;燃用高比例二甲醚混合燃料时,宜选用6×0.40mm喷嘴。建立了预测混合燃料发动机性能的神经网络,该网络采用Levenberg-Marquardt算法,收敛快速,预测精度高,并试验验证了其泛化能力。研究了废气再循环EGR对二甲醚-生物柴油混合燃料增压发动机性能的影响。研究表明,引入冷EGR后,发动机进气压力增加;各燃料的NOx排放均大幅下降;CO排放和HC排放均升高。二甲醚比例不大于50%的混合燃料的碳烟排放随EGR的引入明显升高,二甲醚比例不小于70%的混合燃料的碳烟排放很低,变化不明显。负荷增大,EGR对排放的影响更明显。更多还原

【Abstract】 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.更多还原