充量密度和氧体积分数对柴油机高负荷下热效率和排放的影响

Effects of Charge Density and Oxygen Volume Fraction on Thermal Efficiency and Emissions in a High Load Operation Diesel Engine

采用试验和数值模拟相结合的方法研究了上止点时的充量密度、温度和氧体积分数对柴油机高负荷条件下的燃烧路径、排放和热效率的影响.结果表明:高的充量密度导致低的全局燃氧当量比,增强了燃氧混合率和化学反应速率,有利于提高热效率.提高充量密度对NOx的形成有两个相反的作用:增加充量的总热容,抑制燃烧温度;增加空气的卷吸率,提高混合气的温度.低的氧体积分数降低了燃油/气体混合气的化学反应速率,推迟放热率.高负荷柴油发动机在燃烧过程中有大量不完全燃烧产物CO形成和存在,这被称为"CO的冷储藏",其推迟了放热率相位,降低了已燃气体的温度,致使NOx排放降低.研究还发现发动机的碳烟排放与"CO的冷储藏"量相关.

Experimental and simulation studies on the effects of charge density, temperature at top dead center and oxygen volume fraction on combustion paths, emissions and thermal efficiency of a high load operation diesel engine were conducted. Results reveal that higher charge density leads to lower overall fuel/oxygen equivalence ratio, enhances the rate of mixing and chemical reaction and improves thermal efficiency. Increasing the charge density plays two opposite roles in NOx formation： inhibiting combustion temperature rise because of increasing total heat capacity of charge and increasing mixture temperature because of increasing air entrainment rate. Lower oxygen volume frac- tion essentially decreases the chemical reaction rate of the fuel/gas mixture and retards the phase of heat release. For- mation and existence of large amount of incomplete combustion products, CO, are found in high load operation en- gine during the combustion process which is named as ＂cold storage of carbon-monoxide＂, and this retards the heat release phase and decreases the burning gas temperature and NOx emissions. It was found that soot emission was corre- lated with the amount of the ＂cold storage of CO＂.