基于可控燃烧路径的直喷柴油机预混燃烧和高密度低温燃烧及排放控制机理研究

In order to break through the limits of the thermal efficiency and emissions of a direct-injection diesel engine, it is a hot research topic to control the combustion process path finely, using the φ-T map to avoid entering soot and NOx generation areas. Based on controllable combustion paths in the φ-T map, different combustion control strategies are adopted under different working conditions: premixed combustion strategy for low and medium loads, high density-low temperature combustion for the high load. Taking the two kinds of high efficiency and clean combustion modes in a diesel engine as the research objects, this project is based on the basic science problems such as effects of control parameters on the mixing process, the combustion reaction and the emission generation mechanisms and influencing factors, innovatively applies the new concept of the mixing time, in combination with the numerical simulation and the test method to explore the influencing factors and laws of realizing the high efficiency and clean combustion paths. The effects of optimal injection strategies (pulse number, ratio, interval, timing, etc) on the mixing time, the mixing space, the mixture equivalence ratio, temperature distribution and stratification of each injection pulse before and after changing of the engine speed are studied. Combined with other technical means, such as intake boost, the injection strategy for further reducing the emissions is summarized. The mechanism of the influence of premixed combustion and high density-low temperature combustion on the formation of soot, CO and UHC emissions is revealed. The innovative results of the optimal control strategies for controllable combustion paths are proposed to enrich and develop the low-temperature combustion theory.

为了突破直喷柴油机热效率和排放极限，围绕φ-T图避免进入碳烟和NOx生成区，精细化控制燃烧过程路径成为研究热点。基于φ-T图可控燃烧路径分析，不同工况采用不同的燃烧控制技术：中小负荷采用预混燃烧策略，高负荷采用高密度低温燃烧策略。本项目以该两种高效清洁柴油机燃烧方式为研究对象，围绕各控制参数对其混合过程、燃烧反应和排放物生成机理及影响因素等基础科学问题，创新性地应用混合时间的新概念结合数值模拟及试验的方法，探究其对实现高效清洁燃烧路径的影响因素及规律。重点研究转速改变前后优化喷油策略（脉冲次数、比例、间隔、定时等）对各喷油脉冲的混合时间、混合空间、混合气当量比、温度分布和分层的影响及规律。耦合进气增压等其它技术手段，总结出进一步降低各排放的喷油策略。揭示预混燃烧和高密度低温燃烧方式对其碳烟、CO和UHC排放物生成影响的机理，提出实现可控燃烧路径优化控制策略创新成果，丰富和发展低温燃烧理论。

为了突破直喷柴油机热效率和排放极限，重型柴油机中小负荷采用预混燃烧策略，高负荷采用高密度低温燃烧策略，探究转速改变前后优化喷油策略（脉冲次数、比例、间隔、定时等）的影响。主要研究结论有：（1）低负荷预混燃烧转速拓展时，喷油定时应与燃烧室形状配合以最大程度地利用燃烧室的形状优势；转速提高后采用多次喷油策略能够克服喷油持续期延长带来的喷油速率降低、油气混合程度降低等缺陷。单次喷油优化喷油策略后，低转速工况NOx降低了38%，碳烟降低了一个数量级，指示热效率提高了8.66%；中转速工况指示热效率保持不变，NOx降低了59.3%，碳烟减少了70%；高转速工况指示热效率略提高，NOx略增加，碳烟显著减少。（2）进气压力、转速、多脉冲喷油定时对中负荷预混燃烧和排放的影响。进气增压使得高温着火时刻提前，稀混合气质量分数提高，指示热效率提高，燃烧损失减小。转速提高使得高温着火时刻推迟，过浓混合气质量分数降低；NOx、碳烟、CO和UHC均降低；指示热效率提高，燃烧损失降低，排气损失提高。多脉冲喷油定时推迟，碳烟、CO和UHC均降低，NOx变化不大；指示热效率提高，燃烧损失降低，排气损失提高。（3）主喷模式（次数）、后喷比例和后喷定时对高负荷低转速工况高密度低温燃烧和排放的影响。多次喷油策略的NOx均低于单次喷油策略，并且主喷次数越多，NOx越低。需要足够的主后喷喷油间隔以确保主后喷相对独立，卷吸更多的新鲜空气，降低碳烟排放。过迟的后喷定时由于较低的缸内温度，有增加碳烟排放的倾向。形成和氧化综合作用决定最终碳烟排放。为了获得NOx和碳烟排放的最佳折中，与单次喷油相比，1主+1后、2主+1后和3主+1后模式分别在后喷比例为15%、后喷定时为25、30和35oCA ATDC时，碳烟分别降低了26.7%、 -34.5%和-112.8%，NOx分别减少了5.88%、21.2%和40.3%。