Diesel Engine Model Development and Experiments 951200

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, combustion visualization experiments have also 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 NOx simultaneously in the engine. In addition, when combined with exhaust gas recirculation to further lower NOx, 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 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 in order to improve the accuracy of predictions of combustion with late injection, and to assess the effect of intake flows on emissions.

AUTOMOBILES AND TRUCKS afford such a convenient means of transportation that they will continue to be demanded by our mobile society. As a result, the requirement to meet the challenge of producing cleaner and more efficient power-plants will intensify further in the next years. This challenge requires an increased commitment to research by the transportation industry. The industry has already improved engine performance significantly through the use of new technologies such as ultra-high injection pressure fuel sprays (e.g., to reduce pollutant emission levels) and the use of advanced materials (e.g., ceramics to influence engine heat transfer losses). More recently, advanced computer models are finding increased use in the industry as a tool to accelerate the pace of change. Progress on the development and validation of an advanced CFD model for diesel engine combustion is described in this study.

The internal combustion engine represents one of the more challenging fluid mechanics problems to model because the flow is compressible, low Mach number, turbulent, unsteady, cyclic, and non-stationary, both spatially and temporally. The combustion characteristics are greatly influenced by the details of the fuel preparation process and the distribution of fuel in the engine which is, in turn, controlled by the in-cylinder fluid mechanics. Fuel injection introduces the complexity of describing the physics of dense, vaporizing two-phase flows. Pollutant emissions are controlled by the details of the turbulent fuel-air mixing and combustion processes, and a detailed understanding of these processes is required in order to improve performance and reduce emissions while not compromising fuel economy.

Much progress has been made in CFD model development for engines in recent years. For example, recent modeling studies have been conducted for direct-injection diesel engines [1]*, stratified-charge rotary engines [2], and homogeneous-charge reciprocating engines [3]. However, considerable work is needed in the development of physical submodels [4]. In spite of the detailed nature of even the most comprehensive of engine codes, they will not be entirely predictive for the foreseeable future due to the wide range of length and time scales needed to describe engine fluid mechanics. For example, to resolve the flow-field around 10 μm diameter drops (typical of the Sauter mean diameter of diesel spray drops) in al 10 cm diameter combustion chamber requires about 10¹² grid points. Practical super-computer storage and run times limit calculations to about 10⁵ grid points. The missing 7 orders of magnitude will not be realized soon, even with the most optimistic projections about computer power increases.

Thus, it is necessary to introduce submodels for processes that occur on time and length scales that are too short to be resolved such as atomization, drop drag and vaporization, drop breakup and coalescence, drop turbulence dispersion and turbulence modulation effects, spray/wall interaction and turbulent combustion. The use of submodels to describe unresolved physical processes necessarily introduces empiricism into computations. However, the compromise between accuracy and feasibility of computation is justified by the insight which model calculations offer. Confidence in the model predictions and knowledge of their limitations is gained by comparison with experiments.

The KIVA code [5, 6, 7] has been selected for use in the present work since it is the most developed of available codes. This code has the ability to calculate three-dimensional flows in engine cylinders with arbitrary shaped piston geometries, including the effects of turbulence, sprays and wall heat transfer. The code has been modified as described below and in Refs. [1, 4, 8, 9 and 10] by including state-of-the-art submodels for the important physical processes that occur in diesel combustion.

The model validation experiments have been performed using a single-cylinder research engine that features state-of-the-art high pressure electronic fuel injection and emissions instrumentation. Details of comparisons between measured and predicted engine performance, soot and NOx emissions and combustion visualizations are presented. The model results are generally in good agreement with experiments, indicating that reliable computer models are now available to the industry to help reduce engine development times and costs.