Development and testing of diesel engine CFD models

Abstract

The development and validation of Computational Fluid Dynamic (CFD) models for diesel engine combustion and emissions is described. The complexity of diesel combustion requires simulations with many complex, interacting submodels in order to be successful. The review focuses on the current status of work at the University of Wisconsin Engine Research Center. The research program, which has been ongoing for over five years, has now reached the point where significant predictive capability is in place. A modified version of the KIVA code is used for the computations, with improved submodels for liquid breakup, drop distortion and drag, spray-wall impingement with rebounding, sliding and breaking-up drops, wall heat transfer with unsteadiness and compressibility, multistep kinetics ignition and laminar-turbulent characteristic time combustion models, Zeldovich NO x formation, and soot formation with Nagle-Strickland-Constable oxidation. The code also considers piston-cylinder-liner crevice flows and allows computations of the intake flow process in the realistic engine geometry with two moving intake valves. A multicomponent fuel vaporization model and a flamelet combustion model have also been implemented. Significant progress has been made using a modified RNG k-ε turbulence model. This turbulence model is capable of predicting the large-scale structures that are produced by the squish flows and generated by the spray. These flow structures have an important impact on the prediction of NO x formation since it is very sensitive to the local temperatures in the combustion chamber. Model validation experiments have been performed using a single-cylinder version of a heavy duty truck engine that features state-of-the-art high-pressure electronic fuel injection and emissions instrumentation. In addition to cylinder pressure, heat release, and emissions measurements, combustion visualization experiments have been performed using an endoscope system that takes the place of one of the exhaust valves. In-cylinder gas velocity (PIV) and gas temperature measurements have also been made in the motored engine using optical techniques. Modifications to the engine geometry for optical access were minimal, thus ensuring that the results represent the actual engine. Experiments have also been conducted to study the effect of injection characteristics, including injection pressure and rate, nozzle inlet condition and multiple injections on engine performance and emissions. The results show that multiple pulsed injections can be used to significantly reduce both soot and NO x simultaneously in the engine. In addition, when combined with exhaust gas recirculation to further lower NO x, pulsed injections are found to be still very effective at reducing soot. The intake flow CFD modeling results show that the details of the intake flow process influence the engine performance. Comparisons with the measured engine cylinder pressure, heat release, soot and NO x emission data, and the combustion visualization flame images show that the CFD model results are generally in good agreement with the experiments. In particular, the model is able to correctly predict the soot—NO x trade-off trend as a function of injection timing. However, further work is needed to improve the accuracy of predictions of combustion with late injection, and to assess the effect of intake flows on emissions.