【作者】 于吉超；

【导师】 舒歌群；

【作者基本信息】 天津大学 ， 动力机械及工程， 2007， 博士

【摘要】 提高车用发动机经济性与降低排放是目前社会所面临的两个重要课题,以往研究表明,如果采用氢气等辅助发动机燃烧,能够进一步扩大发动机的稀燃极限,使发动机在超过汽油可燃极限的空燃比下仍然能够稳定工作,发动机NOx排放水平非常低,然而这种技术的推广却受到车载氢源的限制。针对这种情况,本文提出了一种采用低温等离子体,借助发动机排气对汽油进行重整的方法,以制得富氢的混合气体辅助发动机实现混氢的稀薄燃烧,提高发动机经济性,同时在不依靠后处理的情况下,使发动机原机NOx排放满足目前排放法规要求。通过对系统的热力分析表明,利用发动机排气进行重整,具有比部分氧化重整更高的重整效率,将该重整系统用于辅助发动机稀薄燃烧时,不会对发动机的充气效率和最大功率产生太大影响。针对传统催化方法易中毒、体积大、响应慢等缺点,本文设计了一种利用冷等离子体进行汽油重整的装置,并提出了一种能够产生旋转滑动弧低温等离子体的方法,设计了一套等离子体试验电源。通过试验研究表明,利用该系统对汽油进行重整,当重整气体为发动机排气时,重整效率达到80%以上。将重整系统应用到稀薄燃烧发动机中的研究表明,随着重整比例的增大,发动机稀燃极限也逐渐增大,当发动机工作在可接受的发动机循环变动范围内时,NOx排放随稀燃极限的增大而降低。当重整比例30%时,发动机稀燃极限扩大到30左右,此时发动机NOx排放低于70ppm,低负荷时甚至低于40ppm;与发动机当量比工作时相比,NOx排放降低幅度达97%以上。虽然重整效率小于1,然而重整比例对于发动机经济性影响较小,在考察的工况范围内,发动机经济性提高约12~20%。另外,本文还对利用重整系统放出的热量对进气进行加热,研究表明,进气加热也能够起到扩大稀燃极限的作用,而且改善了发动机的经济性,NOx排放也随着发动机稀燃极限的增大而降低。本文建立了发动机准维燃烧模型,对采集的缸内压力信号进行分析,得到了MBR、燃烧速度ST、缸内温度、NOx排放等参数。通过对MBR、ST的分析表明,随着重整比例的增大,发动机的燃烧速度逐渐增大,因此发动机的燃烧稳定性得到增强,发动机稀燃极限随之提高。而随着发动机稀燃极限的增大,缸内温度历程以及最高温度都降低,甚至降到了1900K以下,在该温度下NOx生成速率降低,发动机NOx排放得到了很大的改善。更多还原

【Abstract】 Development of low emission and low fuel consumption engine becomes more important because of running out of fossil fuels, air pollution and global warming up problems. Previous research has shown that using hydrogen to enhance combustion, the engine’s lean burn limits can be further expanded. Therefor engine can still work stably with higher air-fuel ratio than the flammability limits of gasoline, and very low NOx emission can be achieved. However this technique is restricted by the short of hydrogen facilities on vehicle. In light of this situation, this paper presents a solution to use the exhaust gas, by the aid of cold plasma, to reform part of the fuel to hydrogen-rich gas, to improve lean burn of gasoline engine, for that the engine’s efficiency can be increased and the plane engine’s NOx emission can meet current regulations without any after treatment.Thermodynamic analysis of the reforming system shows that the reforming efficiency with the engine exhaust is higher than partial oxidation reforming with pure air. For supporting engine’s lean burn combustion, reforming will not have a significant negative impact on volumetric efficiency and maximum power of the engine. Since the traditional catalytic reformer has many shortcomings as easy poisoning, bulky, and slow response, this paper designed a new type of plasma reformer, presented a method to produce rotating gliding arc plasma, and an experiment power system is also contrived. Experimental studies show that using this system to reform gasoline with exhaust gas, the achieved reforming efficiency is more than 80%.With applying this reforming system to one lean burn engine, experimental study shows that the lean limit of engine is gradually increased when the reforming proportion is increased, and the NOx emission is sharply lowered. When 30% fuel is reformed to hydrogen rich gas, NOx emissions is lower than 70ppm, and even can be below 40ppm at low load conditions. NOx reduction rate is higher than 97% when compared with stoichiometric working conditions. Although reforming efficiency is lower than one, reforming part of fuel will not have much influence on engine efficiency. In the examined working conditions, the engine efficiency is increased by about 12~20%. In addition, this paper discusses using of the heat of the reforming system for heating intake air. The results show that, gas heating can also play a role in expanding the lean burn limit, and the engine efficiency is also improved. With the lean limits is increased, NOx emissions is getting lower.This paper establishes a quasi-dimensional combustion engine model. By analyzing the cylinder pressure, the mass burn ratio, flame speed, cylinder temperature, and NOx emissions can be predicted. The result of MBR and flame speed indicates that when the proportion of reforming is increased, the flame speed is gradually increasing, so the stability of combustion is enhanced, and the lean limit is also increased. With the engine’s lean limit increasing, the temperature is dropped and the maximum cylinder temperature is decreased to a very low level. At some conditions temperature is even below 1900K, and NOx formation rate is greatly lowered.更多还原