An important goal for CFD simulation in engine design is to be able to predict the combustion behavior as operating conditions are varied and as hardware is modified. Such predictive capability allows virtual prototyping and optimization of design parameters. For low-temperature combustion conditions, such as with high rates of exhaust-gas recirculation, reliable and accurate predictions have been elusive. Soot has been particularly difficult to predict, due to the dependence of soot formation on the fuel composition and the kinetics detail of the fuel combustion. Soot evolution in diesel engines is impacted by fuel and chemistry effects, as well as by spray dynamics and turbulence. In this work, we present a systematic approach to accurately simulate combustion and emissions in a high-performance BMW diesel engine. This approach has been tested and validated against experimental data for a wide range of operating conditions. Nine operating conditions have been modeled that span engine loads of 3-21 bar MEP, engine speeds of 1000-4400 rpm and external EGR of 0-38%.
For the simulations, FORTÉ CFD software is used to simulate spray injection, spray breakup and vaporization, combustion phasing and emissions. The European diesel fuel from the experiments was modeled using a four-component surrogate fuel. The composition of the fuel surrogate was determined to represent the diesel properties of cetane number, lower heating value, H/C ratio, threshold sooting index and distillation curve. A discrete multi-component spray-vaporization model considers the vaporization properties of each surrogate component during the engine simulation. The fuel-combustion model is a 495-species detailed chemistry mechanism that was derived from Reaction Design's Model Fuel Library. The well-validated mechanism includes detailed reaction pathways for ignition and emissions chemistry for all four surrogate components as well as for NOx formation and soot formation from multiple soot precursors. The advanced chemistry solver in FORTÉ results in reasonable turnaround times of ∼18 hours on 16 cores.
The results demonstrate the ability of the detailed chemistry to accurately model the combustion phasing and the emissions of the high-performance BMW diesel engine. The impact of pilot ignition and injection timing on combustion phasing is reproduced well by the model. The trends of soot emissions as a function of EGR and fuel loading are also captured. Over the wide range of operating conditions, the simulations were performed without modification to model input parameters, using a consistent approach throughout. In this way, the results demonstrate the potential for predictive simulation in a wide range of design studies.