【作者】 吴君华；

【导师】 黄震；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2007， 博士

【摘要】 我国汽车工业的发展正面临着日益严峻的能源供应和环境保护双重压力。发展新的清洁能源,尤其是符合我国能源资源特点的汽车代用燃料已迫在眉睫。二甲醚由于其特殊物化特性,能够实现高效低污染燃烧,并改善我国的能源结构,近年来日益受到人们的重视。本文在国家科技部“十五”科技攻关专项“清洁汽车行动计划”的资助下,深入研究了增压二甲醚发动机的燃烧特性和排放特点,并重点研究了二甲醚发动机的排放控制策略。本文对D6114ZLQB柴油机进行改造,根据二甲醚的物性特点,专门设计了适合二甲醚的燃料供给和喷射系统。系统研究了燃油系统参数和运转参数对二甲醚发动机性能和排放的影响。试验发现二甲醚燃料温度对发动机动力性能影响显著,温度升高,功率下降明显,应用中必须控制二甲醚燃料的温度。比较了二甲醚发动机和柴油机的燃烧、性能和排放特性。和柴油相比,二甲醚的最高爆发压力、最大压力升高率、燃烧噪声均比柴油低。二甲醚扩散燃烧速度比柴油快,燃烧持续期比柴油短。和柴油机相比,二甲醚发动机低速扭矩显著提高, NOX排放显著下降,HC排放下降,CO排放略有升高。在各运转工况下,二甲醚发动机碳烟排放为零。二甲醚发动机排放全面达到欧Ⅲ排放标准,并有较大余量。研究了二甲醚中甲醇含量对发动机性能和排放的影响,并测试了非常规排放HCHO。研究表明:随着甲醇比例的增加,发动机油耗率、排温升高,NOX排放降低,HC和CO排放增加,HCHO排放基本呈依次上升的规律,但总体来说,HCHO排放处于很低水平,浓度排放低于15×10-6。试验发现,当甲醇含量高于1.5%时,油耗率增加明显,HC和CO排放显著增大。从性能、排放方面考虑车用二甲醚燃料成分,建议甲醇含量不高于1.5%。为今后车用二甲醚标准的制定奠定了基础。为了进一步降低二甲醚发动机的NOX排放,本文研发了适合增压二甲醚发动机的“低压回路”废气再循环系统,研究了EGR率对燃烧和排放性能的影响。研究表明:在所有运转工况下,随着EGR率增大,缸内燃烧压力峰值降低,压力峰值出现的位置延后,着火点延后,预混合燃烧峰值增大,扩散燃烧峰值降低,燃烧持续期增大。随着EGR率增大,发动机排温升高。EGR对油耗率影响比较复杂,低负荷下,适当的EGR量会降低发动机的油耗率。在中高负荷下,EGR的加入会使油耗率增大,经济性恶化。随着EGR率的增加,发动机的NOX排放大幅度下降,高负荷下降幅度更大。HC排放随EGR率增加而上升。EGR率在一定范围内时,EGR率对CO排放影响不大,但当EGR率超过一定范围时,EGR率的增大会引起CO排放急剧升高。EGR率对碳烟排放影响不大,二甲醚发动机碳烟排放保持为零。针对采用EGR后二甲醚发动机HC和CO排放升高的问题,本文采用氧化后处理技术,成功的解决了EGR引起HC和CO排放升高的问题。研究表明,经过氧化后处理器,发动机HC和CO排放显著下降,CO排放下降幅度达到90%。在此基础上,本文提出了二甲醚发动机实现超低排放的控制策略,即采用废气再循环技术降低NOX排放,氧化后处理技术降低HC和CO排放。研究了掺混LPG对二甲醚发动机燃烧和排放的影响。研究表明:随着LPG含量的增加,滞燃期延长,着火点延后,燃烧持续期缩短。油耗率随LPG含量增大有一个低谷,LPG含量过高或过低都会导致油耗率升高。试验表明,在二甲醚中掺混30%LPG时油耗率最低。LPG对NOX排放影响较复杂,低负荷时,LPG含量增大,NOX排放上升。中高负荷时,随LPG含量增大,NOX排放先增大后减小。LPG含量增大使HC排放略有升高,而导致CO排放迅速增加。提出了二甲醚发动机预混合燃烧的实现策略。利用二甲醚蒸汽压高、易于汽化的特点,采取增大柱塞直径以及添加高热值LPG燃料等措施缩短喷射持续期,通过大的废气再循环和推迟喷油技术以及添加低十六烷值的LPG延长滞燃期,使尽可能多的燃料在滞燃期内喷入气缸,着火前形成准均质混合气,扩大预混合燃烧的范围,实现发动机的高效和低排放。试验表明,采用预混合燃烧,二甲醚发动机的油耗率略有改善,NOX排放大幅度下降,800r/min时各负荷下NOX排放下降幅度接近90%。更多还原

【Abstract】 The development of China’s automotive industry is facing two major constraints: energy security and environment protection. It is an imperative task for China to develop alternative fuels for automobiles with due consideration to the country’s structure of energy supply. DME, with high thermal efficiency and low emission due to its special properties, has in recent years attracted increasingly wide attention and has been considered as an alternative fuel which will contribute to the rationalization of China’s energy structure and the improvement of the environment. Funded by the“Clean Auto Action”, a project sponsored by the Ministry of Science and Technology of China, this paper investigates the characteristics of combustion and emissions of a turbocharged DME engine with special focus on the emission control strategy.Based on the D6114ZLQB diesel engine, a DME fuel supply and injection system was developed to fit in with the special properties of DME. The effect of fuel supply system and operating parameters on the DME engine was investigated. It was found that DME temperature has a great effect on the power of the DME engine. The output power of the DME engine would be reduced with the increase of DME temperature. The fuel temperature should be controlled in order to keep steady power of the DME engine in practical use.Combustion, performance and emissions characteristics between the DME engine and a diesel engine were compared. Compared with the diesel engine, the injection delay of DME is longer than that of diesel while ignition delay is shorter than that of diesel. However, the maximum cylinder pressure, maximum pressure rise rate and combustion noises of the DME engine are lower than those of diesel. The combustion velocity of DME is faster than that of diesel, resulting in shorter combustion duration of DME. The maximum torque and power of the DME engine at full load are greater than those of the diesel engine, particularly at low engine speeds. The fuel consumption at low engine speeds is lower than diesel. NOX emissions of the DME engine is decreased remarkably, which is more obvious at high engine speeds. HC emission is reduced while CO emission is increased slightly. The DME engine is smoke free throughout all the operating points of the engine. The emissions of the DME engine are far below the limits of EURO-III.The effect of methanol content on performance and emissions characteristic of the DME engine was investigated. HCHO emission of the engine was tested. The experimental results show that fuel consumption and exhaust gas temperature are improved with the increase of methanol content. HC and CO emissions are increased while NOX emission is reduced. HCHO emission is increased with methanol, but it is lower than 15×10-6. It can be found that HC and CO emissions would be increased greatly as the methanol content is higher than 1.5%. Taking into consideration of the performance and emissions of the DME engine, it is recommended that the methanol content should be lower than 1.5%. This will be a scientific basis for setting the DME standard for vehicle in future.In order to further reduce the NOX emission of the DME engine, a“low pressure routine”EGR system was developed specifically. The effect of EGR rate on combustion and emissions was investigated. It is found that: At all engine operating points, with the increase of EGR rate, the cylinder pressure is reduced; the position of peak pressure is delayed. Ignition timing is prolonged; the peak of pre-mixed combustion ROHR is increased while the peak of scatter combustion ROHR is reduced. The combustion duration is enlarged and the exhaust gas temperature is increased with EGR rate. The effect of EGR rate on fuel consumption is more complex; the fuel consumption is decreased at low load with EGR while it is increased at medium and high loads with EGR. With the increase of EGR rate, NOX emission of DME engine is significantly reduced, especially at high load, while HC emission is increased. When the EGR rate is lower than a certain value, EGR has little effect on CO emission. When EGR rate is lager than a certain value, it would lead to a sharp increase of CO emission. EGR has little effect on smoke, the smoke of DME engine is still zero with EGR.The oxidation catalyst technology was used to solve the problem of HC and CO emissions caused by EGR. It is found that HC and CO emissions are reduced greatly by oxidation catalyst,CO emission is reduced by 90%. The HC and CO emissions of DME engine achieve a very low level. The strategy for ultra-low emission is proposed: EGR technology is taken to reduce NOX emission. Oxidation catalyst technology is taken to reduce HC and CO emissions.The effect of LPG content on DME engine was investigated. With the increase of LPG content, ignition delay is prolonged; the combustion duration of blend fuel is reduced especially at low load. LPG had two effects on fuel consumption: fuel consumption would be increased with LPG content for increase of ignition delay. On the other hand, fuel consumption would be decreased with LPG content for shorting combustion duration. The fuel consumption would be reduced firstly then increased for the two effects. It was found that the DME engine achieved better efficiency when the content of LPG in blend fuel was 30%. With the increase of LPG rate, NOX emission is increased at low load, while NOX emission is increased firstly and decreased secondly at medium and high load. HC emission is increased slightly while CO emission is increased greatly with LPG content.The strategy of pre-mixed combustion for DME was proposed. Taking advantage of the characteristic of high vapor pressure and good spraying of DME, Methods such as enlarging plunger diameter and blending a certain LPG in DME were taken to shorten combustion duration. Methods such as cooled-EGR technology, lagging fuel supply timing and blending LPG were taken to increase ignition delay. All those measures were taken to achieve premixed combustion. More DME was injected into cylinder in ignition delay, forming mixed gas before ignition, enlarging the scope of pre-mixed combustion. The concept of DME engine with high performance and ultra-low emission was accomplished. It is found that the DME engine kept high efficiency and NOX emission is significantly decreased. NOX emission is reduced by 90% at the speed of 800r/min.更多还原