【作者】 郑尊清；

【导师】 姚春德；

【作者基本信息】 天津大学 ， 动力机械及工程， 2009， 博士

【摘要】 日益严格的排放法规对内燃机技术不断提出新的挑战,如何在保持高热效率的同时降低有害排放是目前柴油机面临的主要问题。本文围绕降低柴油机有害排放,在高压共轨柴油机上系统进行了采用EGR结合DOC方案实现国IV排放的研究,并进行了国V排放的初步探讨和含氧添加剂影响的研究。建立了基于可变喷嘴环增压器(VNT)的冷却高压EGR系统,进行了EGR对性能和排放影响的研究。结果表明,通过调节VNT开度可以有效控制EGR率,并能保证较充足的新鲜空气量。提高喷射压力可以明显改善NOx和烟度的trade-off关系。通过EGR率和燃油喷射参数优化,在NOx排放达到国IV限值的情况下,soot比排放相对原机国III降低约60%,经济性也有较大改善。多次喷射的研究表明,预喷对燃烧的主要影响是缩短主喷滞燃期、减少主喷预混燃烧。早的预喷具有更好的NOx和烟度折衷关系,但预喷参数应根据工况特点和控制需要权衡选择。后喷主要通过燃烧后期来对性能和排放产生影响,在适当的主-后喷间隔下,后喷对烟度降低效果明显,此时适当增大后喷油量烟度可以进一步降低。在所研究的大负荷工况将多次喷射与EGR相结合,相对单次喷射无EGR方案,在NOx比排放降低50%的情况下,烟度仍可获得明显改善。对燃烧室进行了降低压缩比的结构设计,研究结果表明,降低压缩比使NOx和烟度trade-off显著改善。根据本研究排放控制方案特点,确定了用于排放优化的多目标函数。优化结果表明,在NOx达到国IV限值的情况下,相对优化前soot排放降低30%以上,CR16.0(2)燃烧室的soot排放和经济性更好,说明该多目标函数方法适用于本方案排放的优化。加装DOC的排放测试表明,通过VNT/EGR控制、燃油喷射策略和燃烧室参数的优化,发动机满足了ESC测试循环的国IV排放要求,证明了采用EGR结合DOC满足国IV排放的可行性。B75工况国V排放的初步研究表明,将多次喷射和EGR结合,在NOx比排放达到2.0 g/kW.h(国V限值)情况下,soot比排放可以控制在0.01 g/kW.h以内,表明该技术方案具有在简化后处理条件下满足国V排放的潜力。柴油中加入含氧添加剂(正丁醇)对单次喷射soot改善明显,但对有后喷策略的soot改善作用很小,说明加入正丁醇可以简化喷射策略或减轻对后处理器效率的依赖。更多还原

【Abstract】 The increasingly stringent emission legislations have put more serious technical challenges on I.C. engines. One of the major issues diesel engine facing is how to reduce pollutant emissions while still maintaining high thermal efficiency. To reduce the emissions, systematical research for meeting China stage IV emissions by EGR combined with diesel oxidation catalyst (DOC) was carried out on a common rail diesel engine. The preliminary exploring of the China stage V emissions as well as the effects of oxygenated additive also had been carried out in this paper.A high pressure cooled EGR system based on VNT was designed, and the effects of EGR on the performance and emissions of the engine were studied. Results show that the EGR rate can be effectively regulated by adjusting VNT vane position while still ensuring sufficient fresh intake air mass flow. The trade-off between NOx and smoke was improved by increasing the injection pressure. Through the optimization of EGR rate and injection parameters, soot can be reduced by as much as 60% compared to its original result of China stage III test, while keeping NOx meeting China IV regulations, and fuel economy also can be improved significantly.The results of multiple injections show that, the pilot injection reduced the ignition delay and premixed combustion of the main injection. Relative earlier pilot injection is better for the trade-off between NOx and smoke. However, pilot injection parameters should be carefully calibrated according to the engine load and control requirement. The influences of post injection on the performance and emissions are mainly due to its effects on the latter parts of combustion. Smoke can be effectively reduced by applying appropriate main-post injection interval. And in these cases, smoke emissions can be further reduced through increasing the mass of post injection reasonably. Compared to single injection without EGR, NOx can be reduced by 50%, and smoke emission can be effectively improved with the combination of EGR and multiple injections at the high load.The combustion chamber with reduced compression ratio was designed, and the results show that the trade-off between NOx and smoke can be effectively improved by reducing compression ratio of the combustion chamber. Based on the emissions control strategy of this research, a multi-objective function for the emission optimization was selected. The multi-objective function optimization results show that, under the promise of NOx meeting China stage IV regulations, the optimized soot emissions could get a reduction of 30% compared to the result without optimization. A better soot emission and fuel economy could be obtained with the CR16.0 (2) combustion chamber. This indicates that multi-objective function method is suitable for the current research. The emissions test with DOC shows China stage IV emission standards under ESC test cycle was met through the optimization of VNT/EGR, injection strategies, and combustion chamber geometry. This confirmed that it is feasible to meet China stage IV emission regulations by EGR combined with DOC.Test results at B75 of ESC test cycle show that, through the combination of EGR and multiple injections, controlling NOx emission at constant value of 2.0 g/kW.h (limited value of China stage V standards), specific soot emission can be reduced to lower than 0.01 g/kW.h. This means that current emission control method has the potential to meet China stage V emission standards with simplified PM aftertreatment devices. The investigation on oxygenate additive (n-butanol) indicates that under single injection strategy, soot can be significantly reduced by blending n-butanol into diesel fuel. However, the reduction effect of soot is reduced under post injection strategy when n-butanol is used. It indicates that it is possible to simplify injection strategy or reduce the dependence of the aftertreatment efficiency by blending n-butanol into diesel.更多还原