DEVELOPMENT AND TESTING OF DIESEL-ENGINE CFD MODELS

The development and validation of Computational Fluid Dynamic (CFD) mo dels for diesel engine combustion and emissions is described. The comp lexity of diesel combustion requires simulations with many complex, in teracting submodels in order to be successful. The review focuses on t he current status of work at the University of Wisconsin Engine Resear ch Center. The research program, which has been ongoing for over five years, has now reached the point where significant predictive capabili ty is in place. A modified version of the KIVA code is used for the co mputations, with improved submodels for liquid breakup, drop distortio n and drag, spray-wall impingement with rebounding, sliding and breaki ng-up drops, wall heat transfer with unsteadiness and compressibility, multistep kinetics ignition and laminar-turbulent characteristic time combustion models, Zeldovich NOx formation, and soot formation with N agle-Strickland-Constable oxidation. The code also considers piston-cy linder-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 m odel have also been implemented. Significant progress has been made us ing a modified RNG k-epsilon turbulence model. This turbulence model i s capable of predicting the large-scale structures that are produced b y the squish flows and generated by the spray. These flow structures h ave an important impact on the prediction of NOx formation since it is very sensitive to the local temperatures in the combustion chamber. M odel validation experiments have been performed using a single-cylinde r 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 measure ments, 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. Modific ations to the engine geometry for optical access were minimal, thus en suring 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 mult iple injections on engine performance and emissions. The results show that multiple pulsed injections can be used to significantly reduce bo th soot and NOx simultaneously in the engine. In addition, when combin ed with exhaust gas recirculation to further lower NOx, pulsed injecti ons are found to be still very effective at reducing soot. The intake flow CFD modeling results show that the details of the intake flow pro cess influence the engine performance. Comparisons with the measured e ngine cylinder pressure, heat release, soot and NOx emission data, and the combustion visualization flame images show that the CFD model res ults are generally in good agreement with the experiments. In particul ar, the model is able to correctly predict the soot-NOx trade-off tren d as a function of injection timing. However, further work is needed t o improve the accuracy of predictions of combustion with late injectio n, and to assess the effect of intake flows on emissions.