Progress in Diesel Engine Intake Flow and Combustion Modeling 932458

The three-dimensional computer code, KIVA, is being modified to include state-of-the-art submodels for diesel engine flow and combustion. Improved and/or new submodels which have already been implemented are: wall heat transfer with unsteadiness and compressibility, laminar-turbulent characteristic time combustion with unburned HC and Zeldo'vich NOx, and spray/wall impingement with rebounding and sliding drops. Progress on the implementation of improved spray drop drag and drop breakup models, the formulation and testing of a multistep kinetics ignition model and preliminary soot modeling results are described. In addition, the use of a block structured version of KIVA to model the intake flow process is described. A grid generation scheme has been developed for modeling realistic (complex) engine geometries, and initial computations have been made of intake flow in the manifold and combustion chamber of a two-intake-valve engine. The research also involves the use of the code to assess the effects of subprocesses on diesel engine performance. The accuracy of the predictions is being tested by comparisons with engine experiments. To date, comparisons have been made with measured engine cylinder pressure, temperature and heat flux data, and the model results are in good agreement with the experiments. Work is in progress that will allow validation of in-cylinder flow and soot formation predictions. An engine test facility is described that is being constructed to provide the needed validation data.

The diesel engine is the leading heavy-duty power plant because of its superior energy efficiency. However, because of environmental concerns, proposed federal emission standards require reductions in both nitric oxides (NOx) and particulates. A detailed understanding of combustion is required in order to work effectively at reducing these by-products of combustion within the engine cylinder, while still not compromising engine fuel economy.

The objective of this research program is to develop an analytic design tool for use by the industry to predict engine performance and emissions. It is expected that the use of advanced modeling tools will enable engine development times and costs to be reduced. The three-dimensional computer code, KIVA [1,2] is being used since it is the most developed of available codes. Part of the research involves implementing state-of-the-art submodels into KIVA for spray atomization, drop breakup/coalescence, multi-component fuel vaporization, spray/wall interaction, ignition and combustion, wall heat transfer, unburned HC and NOx formation, soot and radiation, and modeling the intake flow process.

Previous progress has been described by Reitz et al. [3,4] where it was shown: that adding the effects of unsteadiness and compressibility to the heat transfer submodel improves the accuracy of heat transfer predictions; that drop rebound should be included in spray/wall interaction models since rebound can occur from walls particularly at low impingement velocities (e.g., in cold-starting); that the influence of vaporization on the atomization process should be considered since larger spray drops are formed at the nozzle with vaporization; that a laminar-and-turbulent characteristic time combustion model has the flexibility to match measured engine combustion data over a wide range of operating conditions; and, finally, that the characteristic time combustion model can also be extended to allow predictions of ignition.

The implementation of improved models for the prediction of spray drop drag and breakup, and for modeling ignition and combustion, intake flow and soot formation is described in this paper.

The research also involves the use of the computer code to assess the effects of subprocesses on diesel engine performance, and the accuracy of the predictions is being tested by comparisons with engine experiments. To date, comparisons have been made with measured engine cylinder pressure, temperature and heat flux data, an the model results are in good agreement with the experiments [3, 4].

Work is also in progress that will allow validation of in-cylinder flow and soot formation predictions. An engine test facility is described that is being constructed to provide the validation data that is required in order to work effectively at improving performance and reducing emissions while not compromising the engine's fuel economy.