EGR对减少UHC和CO排放的影响与分析

分析了二冲程均质发动机UHC和CO的来源,介绍了一种能够大幅度降低UHC和CO的方法.试验是在二冲程发动机怠速废气再循环量较大情况下进行的,怠速时,通过在SI二冲程发动机上增加EGR,发现UHC从7 800×10-6降低到3 000×10-6,CO从3%降低到0.2%,循环变动也减小了.EGR中的活性物质与新鲜充量充分反应改善了预燃过程.由此证明,可以通过增加EGR实现发动机稳定运行,并在当量比和稀燃条件下得到了同样结果.试验表明,UHC和CO主要来自余隙中的残余燃料,说明燃烧温度不是影响UHC和CO的主要因素,决定因素是第一阶段燃烧前缸内发生的预燃反应程度.活性物质的重要性已被试验证实,试验中用氮气和空气代替EGR稀释混合气,发现火花点火在EGR中是可行的,在空气和氮气中却不行.

Effect and Analysis of EGR on Reducing UHC and CO Emission in Homogeneous Charge Two-Stroke Engines

This paper analyzes the sources of unburned hydrocarbons(UHC) and CO from two-stroke homogeneous charge spark ignition (SI) engines, describing an approach with the potential to substantially reduce UHC and CO in this system based on the experiments carried out on a two-stroke engine in idle condition with high levels of internal exhaust gas recirculation (EGR). By increasing internal EGR in a spark ignited two-stroke engine at idle, UHC was reduced from 7 800 x 10-6 to 3 000 x 10-6, CO was reduced from 3% to 0.2%, and cyclic variability was reduced as well. Increased EGR allowed active species in the burned gases to react with the fresh charge well in advance of the first stage ignition. The results that stable engine operation can be obtained with additional internal EGR. Similar results were obtained in stoichiometric and lean conditions. Further experimental and model results indicate that the main source of UHC and CO emissions is the fuel trapped in crevices,which suggests that the combustion temperature is not the main factor affecting UHC and CO. The critical factor is the pre-ignition reaction that occurs in the cylinder prior to the first stage of auto-ignition. The importance of active species is confirmed by experiments where air or N2 with similar quantities to real EGR are used to dilute the charge.It was found that spark ignition was possible with real EGR but not possible with either air or N2.