稀燃及废气再循环提高增压汽油机热效率的对比研究

在一台直列4缸增压直喷汽油机上针对万有特性最低油耗工况点,进行了稀薄燃烧与废气再循环(exhaust gas recirculation,EGR)提高发动机热效率的对比试验研究。试验结果表明:稀薄燃烧及EGR均能有效降低发动机燃油消耗率,稀释率分别为33%和19%时,采用稀燃和EGR时的最高有效热效率绝对值分别增加2.8%和1.7%,其中稀燃的有效热效率达到了39.9%,稀燃实现更高热效率主要归因于较低的传热损失和未燃损失。从燃烧角度来看,稀燃及EGR稀释都延长了燃烧滞燃期及持续期,但同样稀释率下稀燃的滞燃期更短,稀燃更高的稀释极限实现了更低的传热损失;但EGR抑制爆震,提前燃烧相位,使采用EGR时的排气能量损失低于稀燃。从排放角度来看,稀燃及EGR在高稀释率下均显著降低NO_x排放,而受益于高氧气浓度,相同稀释率下稀燃的HC及CO排放均低于采用EGR时,从而使稀燃的未燃损失更低。

A Comparison of Using Lean Burn and Exhaust Gas Recirculation to Improve Thermal Efficiency of a Turbocharged Gasoline Engine

Experiments on a 4-cylinders miller cycle gasoline engine were carried out to compare lean burn and exhaust gas recirculation (EGR) in terms of brake thermal efficiency (BTE) improvement. The operation point of 2500 r?min-1 and 1.1MPa brake mean effective pres-sure was selected for the study. The experimental results show that both lean burn and EGR are effective in reducing brake specific fuel consumption (BSFC), the maximum BTE increased by 2.8% and 1.7% respectively. The higher BTE achieved by lean burn is mainly a result of lower heat transfer loss and unburned fuel loss. Both lean burn and EGR extend the ignition delay and combustion duration, but the dilution limit of lean burn is higher than EGR because of shorter ignition delay. The higher dilution limit results in lower heat transfer loss of lean burn. EGR has earlier ignition timing and combustion phase than lean burn because of lower knock tendency, which leads to lower exhaust heat loss of EGR. As to emissions, both lean burn and EGR are good at reducing NOx emis-sions at high dilution rate, but lean bun has much lower CO and THC emission because of rich oxygen combustion, thus lean burn has much lower unburned combustion loss.