Effects of exhaust gas recirculation (EGR) on combustion and emissions during cold start of direct injection (DI) diesel engine

Through experiments conducted on a single cylinder direct injection (DI) diesel engine, effects of exhaust gas recirculatoin (EGR) on combustion and emission during cold start were investigated. Combustion of first firing cycle can be promoted significantly by introducing EGR. In experiments, when partially closed choking valve and partially or fully opened EGR valve, peak cylinder pressure of first firing cycle was about 45% higher than that under normal condition without EGR, and the start of combustion (SOC) was also much earlier. EGR also had effects on combustion stability. In the case, which kept 50% or 100% opening of EGR valve (OEV) and kept 100% opening of choking valve (OCV), more stable combustion process was achieved when common rail pressure decreased during cold start. However, excessive amount of EGR led to extreme unstable combustion and even misfiring. Opacity and NO emissions were also analyzed in detail. In the case with maximum EGR, the lowest average opacity, which was less than 4%, was achieved during initial several firing cycles of cold start. But in the later phase, excessive amount of EGR led to a great deal of white smoke emission. NO emission during initial phase of cold start is mainly affected by increase in fuel amount of injection. When combustion became stable gradually, EGR showed significant effect on NO reduction.