CFD Modelling of the Effects of Exhaust Gas Recirculation (EGR) and Injection Timing on Diesel Combustion and Emissions

摘要

Emissions from Diesel engines have been a major concern for many years, particularly with regards to the impact of NO_x and particulate matter on human health. Exhaust gas re-circulation (EGR) is a widely used method in diesel engines for controlling NO_x production. While EGR rates can be varied to ensure engine performance and reduce NO_x emissions, EGR also influences the ignition delay, reduces the peak combustion temperature and increases particulate emissions. Moreover, the injection timing directly affects NO_x and particulate emissions under the broad and highly variable operating conditions. An effective CFD-based design tool for diesel engines must therefore include robust and accurate predictive capabilities for combustion and pollutant formation, to address the complex design tradeoffs. The objective of the present study is to evaluate CFD modeling of diesel engine combustion and emissions for various combinations of EGR rates and injection timings. In particular, we examine the impact of using detailed fuel-chemistry and multi-component spray models. Model sensitivities were analyzed to provide insight about the prediction accuracy and dependencies. The engine operating conditions are varied with EGR levels between 8% and 40% and with different injection timing in a validated CFD setup. Numerical predictions of cylinder pressure trace, heat release rate, ignition delay, NO_x and particulate emissions were compared with a large set of experimental results. From these comparisons conclusions were drawn regarding predictive capabilities for varying injection timing and EGR rates.