Improvements in 3-D Modeling of Diesel Engine Intake Flow and Combustion 921627

A three-dimensional computer code (KIVA) is being modified to include state-of-the-art submodels for diesel engine flow and combustion: 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 the intake flow process. Improved and/or new submodels which have been completed 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. Results to date show that: adding the effects of unsteadiness and compressibility improves the accuracy of heat transfer predictions; spray drop rebound can occur from walls at low impingement velocities (e.g., in cold-starting); larger spray drops are formed at the nozzle due to the influence of vaporization on the atomization process; 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, the characteristic time combustion model can also be extended to allow predictions of ignition. The accuracy of the predictions is being assessed by comparisons with available measurements. Additional supporting experiments are also described briefly. To date, comparisons have been made with measured engine cylinder pressure and heat flux data for homogeneous charge, spark-ignited and compression-ignited engines, and also initial comparisons for diesel engines. The model results are in good agreement with the experiments.

A detailed understanding of diesel engine combustion is required in order to work effectively at improving performance and reducing emissions while not compromising fuel economy. The objective of this research is to apply advanced modeling techniques to study the influence of in-cylinder processes on efficiency and pollutant emissions. The program includes the development of a comprehensive analytical model of diesel engine combustion and flow. This model will be available as an analytic design tool for use by the industry to predict engine performance and emissions. The use of advanced modeling tools will enable engine development times and costs to be reduced.

The three-dimensional code, KIVA [1, 2], has been selected for use since it is the most developed of available codes. State-of-the-art submodels for the important physical processes in diesel combustion are being included in KIVA as part of the research effort. This paper summarizes progress to date on models for wall heat transfer, fuel drop vaporization, spray vaporization, atomization, ignition, combustion and the intake flow process. The intake flow process is being computed because the flow through the intake valves sets up the flow field within the combustion chamber, and these flow details have an important effect on engine combustion characteristics and performance.

The emphasis of the program is on the application of a comprehensive engine combustion code to assess the effect of the interacting subprocesses on diesel engine performance, rather than on the development of new models for the subprocesses themselves. The elements of the code will be assembled from existing state-of-the-art submodels. This use of multidimensional modeling as an engine development tool is timely and justifiable due to recent advances in submodel formulations. The accuracy of the predictions is being assessed by comparison with available experimental engine measurements.