Progress Towards Diesel Combustion Modeling 952429
Progress on the development and validation of a CFD model for diesel engine combustion and flow is described. 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 NOx 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. Significant progress has been made using a modified RNG k-ε turbulence model, and a multicomponent fuel vaporization model and a flamelet combustion model have been implemented.
Model validation experiments have been performed using a single-cylinder 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, new combustion visualization experiments have also been performed using an endoscope system that takes the place of one of the exhaust valves. Modifications to the engine geometry for optical access were minimal, thus ensuring that the results represent the actual engine.
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 NOx 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-NOx 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.