Effect of Simulated-EGR (N2) on the distribution characteristics of equivalence ratio and the formation of exhaust emissions in a CI engine under early injection conditions

The objective of this study is to numerically investigate the effect of simulated-EGR by changing of N-2 gas mass fraction on the fuel mass fraction distributions and the exhaust gas formation in a cylinder of a CI engine under early injection timing conditions. This work provides the optimal operating conditions for a CI engine affected by simulated-EGR (N-2). In order to obtain experimental results to validate the numerical analysis, an experiment was conducted using a single-cylinder diesel engine which consisted of an intake system, a test engine control system, and exhaust emissions analyzers. The results of the numerical analysis were compared and validated with the obtained experimental results. The ECFM-3Z model and Mundo Tropea Sommerfeld model were applied as sub-models for the combustion and wall interaction analyses, respectively. The Mundo Tropea Sommerfeld model has two regimes as deposition and splash, which is determined by the splashing parameter (K). In order to simulate the EGR effect, the O-2 and N-2 mass fractions, which change the overall equivalence ratio from 0.54 to 1.01 in the cylinder, were varied under the same fuel injection mass. The start of energizing timing was swept from BTDC 03deg to BTDC 29deg. It was revealed that the peak cylinder pressure and IMEP value decreased when the overall equivalence ratio increased, because the combustion performance deteriorated by the low O-2 mass fraction in the cylinder. When the start of energizing timing was advanced, the peak cylinder pressure increased, because the enough air and fuel mixing time were secured. This allowed a homogeneous mixture to form in the cylinder, which tends to increase the proportion of premixed combustion. Specially, when the start of energizing timing was near BTDC 20deg, the ISNO and ISSoot simultaneously decreased. The reason is that a lot of fuel was injected to the upper end of the piston bowl, which formed a liquid wall film at the center area of the cylinder. The liquid wall film evaporated and suppressed the increase in cylinder temperature due to the effect of the latent heat of vaporization. Furthermore, it created a widely distributed high fuel mass fraction. Finally, the optimal operating conditions to improve engine performance was determined to be phi = 0.68 and t(eng) = BTDC 14deg because this condition had both low ISNO and ISSoot values, and high IMEP value. From these results, the found optimal operating conditions were expected to improve both exhaust emissions and combustion performance for the CI engine. (C) 2019 Elsevier Ltd. All rights reserved.