环境湿度对柴油机NO_x及Soot排放特性的影响

Effect of Ambient Humidity on NO_x and Soot Emissions in a Diesel Engine

采用发动机台架试验和数值模拟相结合的方法,进行了某重型柴油机在不同进气湿度下的排放特性研究,探索了进气湿度对柴油机燃烧和排放的内在机制,并确定了进气湿度对柴油机排放特性影响的范围.结果表明:进气湿度对燃烧和排放的影响主要体现在进气湿度变化导致缸内环境氧浓度、工质比热容、水煤气反应以及高温离解自由基能力等情况发生变化.进气温度为30,℃时,氮氧化物(NO_x)排放随着进气湿度的增加迅速降低,直至接近0;进气湿度的含湿量达到27.20,g/kg(D30)前,碳烟(soot)排放缓慢增加;当进气湿度超过D30后,soot排放出现一个拐点,随着进气湿度的增加而大幅上升.在饱和湿空气下,当进气温度较低时,进气温度对燃烧的影响比进气湿度明显,进气温度越高,放热率峰值越大,NO_x排放增大,soot排放减小;当进气温度达到15,℃后,进气湿度对燃烧和排放的影响开始显现;随着进气温度的提高,饱和湿空气含湿量迅速增大,当进气温度达到25,℃后,进气湿度对燃烧的影响显著增强,使缸内最高燃烧温度和最高爆压都逐步降低,NO_x排放逐渐减为0,soot排放逐步增大.

Experimental and simulation studies were adopted to research emission characteristics and their influences by the ambient humidity in a heavy-duty diesel engine. Results show that the effect of ambient humidity on combus- tion and emissions of the diesel engine is mainly acted by the variation of the in-cylinder conditions, such as oxygen content, specific heat capacity, water-gas reaction and dissociating radical under high temperature. At the ambient temperature of 30 ℃, as the ambient humidity increases, NOx emission decreases to zero rapidly, while soot emis- sion increases tardily till the humidity reaches 27.20 g/kg dry air（D30）. Subsequently, soot emission swells up quickly. In saturated wet air environment, when the ambient temperature is lower, combustion is dominated by am- bient temperature rather than the ambient humidity. Higher temperature leads to greater peak heat release, higher NOxand less soot emissions. When the ambient temperature reaches 15 ℃, the influences of ambient humidity on combus- tion and emissions are apparent. When ambient temperature is greater than 25℃, the effects of ambient humidity on the combustion and emissions of the diesel engine are significantly enhanced. Higher ambient temperature leads to lower peak heat release and lower in-cylinder combustion temperature. NOx emissions gradually decrease to nearly zero, but soot emission increases all the way.