【作者】 张俊军；

【导师】 黄震；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2009， 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,因此寻找新的发动机代用燃料就成了人们的当务之急。二甲醚(DME)是一种非常适合柴油机使用的优质代用燃料,其应用可以有效缓解我国对石油基燃料的依赖。本文对二甲醚发动机进行了燃烧排放特性及其性能优化的研究。本文对一台车用增压柴油机进行改造,根据二甲醚的理化特性,设计了适合二甲醚的燃油系统,进行了燃用二甲醚的试验研究。结果表明,二甲醚发动机在中低转速时,燃油消耗率比柴油低,在中高转速时比柴油机高。在发动机的所有工况范围内,二甲醚发动机的NOx排放比柴油机有明显下降,HC和CO排放处于很低水平,碳烟排放为零,能实现无烟燃烧。燃料供给温度升高会造成二甲醚发动机功率下降。结合燃料供给温度变化,对发动机燃料系统结构参数优化的试验结果表明,加大柱塞直径和柱塞有效行程可提高循环供油量,可确保不同燃料状态下发动机的功率输出。相同功率下,采用相同规格喷油泵,当喷油器喷孔孔径减小时,NOx排放升高,燃料经济性较好。综合考虑二甲醚发动机的燃油消耗率与排放,得到两种较为理想的燃料喷射系统技术方案:柱塞直径为13mm、升程为12mm的P7100喷油泵匹配6×0.40mm喷油器和柱塞直径为12mm、升程为14mm的P8500喷油泵匹配6×0.43mm喷油器。采用这两种技术方案,不需废气再循环和氧化后处理,二甲醚发动机的ESC试验循环测试结果均能满足欧Ⅲ排放标准限值。采用KIVA-3V对增压二甲醚发动机和柴油机额定功率点的缸内燃烧过程与NOx排放进行了数值模拟研究。耦合到KIVA-3V中的二甲醚化学反应机理包括78个化学组份和336步基元反应;选用正庚烷化学反应机理模拟柴油燃烧,包括65个化学组份,248步基元反应。结果表明,计算所得的缸内压力和放热率与实测值吻合较好。对缸内燃烧的温度计算表明,柴油滞燃期较二甲醚的长,柴油燃烧初期,其高温区分布于喷雾浓侧,且在缸内气流作用下沿垂直于喷雾方向扩散;二甲醚的着火点位于喷嘴附近,在燃烧初始时刻,其喷雾稀薄侧温度明显高于柴油,随喷雾的进行,其燃烧高温区从喷嘴附近延伸到燃烧室壁面,呈现狭长的高温带。与柴油相比,二甲醚发动机缸内燃烧最高温度显著降低,且其温度梯度较小。选用的9步NOx生成机理可较好地预测发动机实际运行中所产的NOx排放水平。为了降低二甲醚发动机的排放,本文建立了涡前压后高压引流废气再循环(EGR)系统,进行了EGR和氧化后处理(DOC)降低排放的试验研究。涡前压后高压引流EGR使得进、排气温度和燃油消耗率均升高,进气流量降低。EGR缩短了燃料滞燃期,延长了燃烧持续期,燃烧终点被延迟。EGR降低了NOx排放,但引起CO排放升高,对HC排放影响不明显。通过氧化后处理装置,HC和CO排放大幅度降低。燃油正时优化试验结果表明,在3°CA BTDC供油提前角下,二甲醚发动机ESC试验循环测试结果可满足欧Ⅳ排放标准限值,同时其燃料经济性较原柴油机有优势。为了进一步降低NOx排放,搭建了适合增压二甲醚发动机的涡前压前高压引流废气再循环系统,研究了EGR率对燃烧和性能的影响。试验结果表明,与涡前压后高压引流EGR相比,采用涡前压前方式组织废气再循环可获得较大的EGR率,发动机NOx排放下降的幅度更为显著。3°CA BTDC供油提前角下,采用涡前压前高压引流EGR和氧化后处理,二甲醚发动机ESC试验循环测试结果可满足欧Ⅴ排放标准限值。在5°CA BTDC下采用两级冷却EGR技术,与3°CA BTDC时相比,在降低二甲醚发动机NOx排放的同时改善了燃料经济性。结合柴油机燃用二甲醚HCCI燃烧与缸内直喷燃烧各自的优点,提出了柴油机燃用二甲醚气道-气缸喷射复合燃烧方式。在一台改造过的2-135直列泵柴油机上,考察了预混合率、直喷供油提前角、进气燃料设计以及进气添加CO2对复合燃烧的影响。结果表明,二甲醚气道-气缸喷射复合燃烧过程包括HCCI燃烧和缸内直喷燃料的预混及扩散燃烧多阶段放热,主要受预混合率和供油提前角的影响而呈现不同的放热特征。采用适当的预混合率和直喷供油提前角,与HCCI燃烧比,复合燃烧在保持NOx基本不变的条件下有效地拓宽了发动机工况范围,同时降低了HC和CO排放。更多还原

【Abstract】 The development of China’s automotive industry is facing major challenges of energy supply and environmental protection. It is of great importance to develop clean and alternative fuels for internal combustion engines and vehicles. Dimethyl ether (DME) is expected as a promising alternative fuel for its smoke-free combustion and rich resources. Promotion of DME utilization can not only relieve the current crisis of energy, but also effectively reduce air pollution. The study on combustion and performance of DME engine in this dissertation is summarized as follows:The DME fuel supply and injection system tailored for DME fuel was developed for a turbocharged diesel engine. An experimental study on performance and emission characteristics was conducted on the engine fueled with DME. The results show that, at full load, the brake specific fuel consumption (BSFC) of DME engine at low engine speed is lower than that of diesel engine, and NOx emissions of the DME engine decrease remarkably. HC emissions reduce while CO emission slightly increases. The combustion of DME engine is free of smoke at all test engine loads.Engine power output and torque decrease with increasing DME fuel supply temperature. Through the parameter optimization of DME fuel injection system, it is found that increasing the plunger diameter and effective stroke can ensure the constant engine power output at different fuel temperature. At the same engine load, using a nozzle orifice with a smaller diameter, NOx emission level is higher, and BSFC is lower. Under the required engine power output, a good compromise for DME fuel consumption and NOx emissions can be obtained using P7100 pump with the plunger diameter of 13mm and the stroke of 12mm matched with a injector of 6×0.40mm, or using P8500 pump with the plunger diameter of 12mm and the stroke of 14mm matched with a injector of 6×0.43mm. The results of ESC test show that, by means of optimization of injection system, emissions of DME engine can meet the requirements of EURO-Ⅲwithout employing EGR and aftertreatment. Using a CFD KIVA-3V model, combustion process and NOx formation of both diesel fuel and DME on a turbocharged diesel engine at the rated power were investigated. The chemical kinetic mechanism of DME consists of 78 species participating in 336 reactions, and n-heptane mechanism is used to present diesel fuel combustion including 65 species participating in 248 reactions. It is found that the calculated in-cylinder pressure and heat release rate are in good agreement with the measured results for both fuels. The ignition delay period of DME is shorter than that of diesel fuel. At initial period of combustion, the high temperature region locates at rich side of diesel fuel spray and propagates along the direction perpendicular to the spray development under the effect of in-cylinder gas flow. For DME, the ignition position occurs near the nozzle. At initial period of DME combustion, the temperature at lean side of DME spray is higher than that of diesel fuel. With the development of DME spray, the high temperature region appears as a narrow belt along the DME spray from the nozzle to the tip of the spray. In comparison to diesel fuel, the local maximum temperature of DME combustion is lower than that of diesel fuel, and in-cylinder temperature gradient of DME engine is small. The selected mechanism of NOx formation with 9 reactions can well predict NOx emission level of the engine.In order to reduce emissions of the DME engine,the high-pressure loop exhaust gas recirculation (HP-EGR) system was developed on a turbocharged DME engine,and the experimental study of emissions reduction was conducted by means of EGR and oxidation catalyst converter (DOC). In this HP-EGR system, exhaust gas before turbine flows through an EGR valve and an EGR cooler, and then enters the air intake pipe after the inter-cooler. The results indicate that,by means of EGR, inlet air temperature, exhaust gas and fuel consumption increase, and the mass air flow decreases. EGR shortens the ignition delay period, and increases the combustion duration. With EGR, a substantial reduction in NOx emissions without smoke is obtained while fuel consumption and CO emission tend towards deterioration, and HC emissions show no remarkable change. HC and CO emissions are significantly reduced by means of DOC. Through the parametric optimization, at the fuel delivery advance angle of 3°CA BTDC, ESC test results show that emissions of DME engine meet the requirements of EURO-IV. Meanwhile, there is a good fuel economy in comparison to the base engine.In order to further reduce the NOx emissions, second type of HP-EGR system was developed. In this HP-EGR system, exhaust gas before turbine flows through an EGR valve and EGR coolers, mixing wih fresh air, and then enters into the compressor of turbocharger. The effect of EGR rate on combustion and performance was investigated. The results show that, using this type of HP-EGR, a higher EGR rate can be obtained, and a remarkable reduction in NOx emissions can be observed. The results of ESC emission test show that, at the fuel delivery advance angle of 3°CA BTDC, emissions of DME engine can meet the requirements of EURO-V by means of EGR and DOC. In comparison to fuel delivery advance angle of 3°CA BTDC, a reduction in NOx emissions is obtained while the fuel economy is improved by using two-stage cooling EGR technology at the fuel delivery advance angle of 5°CA BTDC.Taking advantage of homogeneous charge compression ignition (HCCI) combustion and in-cylinder direct injection combustion, a new concept combustion system, namely compound charge compression ignition (CCCI) combustion system by port aspiration and direct injection of DME, is proposed. The effect of premixed fuel ratio and fuel delivery advance angle on CCCI combustion was investigated. In order to further reduce emissions and improve the fuel consumption in a CCCI engine, fuel design concept and port aspiration of CO2 were employed. The experimental results show that CCCI combustion exhibits a multi-stage combustion mode including HCCI combustion , pre-mixing combustion and diffusion combustion. The heat release pattern mainly depends on premixed fuel ratio and fuel delivery advance angle. Compared with HCCI combustion mode, CCCI combustion can extend the operating range with little change in NOx emissions and a considerable reduction in HC and CO emissions.更多还原