Effects of Exhaust Gas Recirculation under Fueling Rate or Air/Fuel Ratio–Controlled Strategies on Diesel Engine Performance and Emissions by Two-Zone Combustion Modeling

Abstract

The effects of exhaust gas recirculation (EGR) on combustion, performance, and exhaust emissions in direct injection (DI) diesel engines are assessed in a primarily numerical study under the influence of various EGR rates using two different strategies, that is, under constant engine fueling rate (FR) or air/fuel ratio (AFR). The analysis uses an in-house, comprehensive, two-zone diesel combustion model, which separates the in-cylinder working medium into an unburned zone (air, or air plus EGR gas) and a burned one into which fuel is injected from the injector-nozzle holes that are burned with the entrained gas mass from the unburned zone. The validity of the model computations is appraised positively compared to relevant experimental data, such as diagrams of in-cylinder pressures and temperatures, heat release rate (HRR), and nitric oxide (NO) and soot (smoke) emissions, with tests acquired in the laboratory on a standard, experimental, monocylinder, DI, naturally-aspirated (N/A) diesel engine at various operating conditions. A perplexity occurs in the literature of using either constant FR or AFR in the EGR strategy (sometimes overlooked), which can lead to an erroneous interpretation and comparison of the obtained results. This is delineated in this study by using its numerical results that supply insight into the local combustion and emissions formation mechanisms, identifying the important parameters in each case that affect the engine behavior under various operating conditions, in the light of the existing NO–smoke trade-off and the impact on engine power-output and efficiency. The results can be useful for optimizing the emissions and efficiency in each case, and assist with their implications for better designing of the related electronic control unit (ECU) in diesel engines bearing EGR systems.