A phenomenological description, or "conceptual model," of how direct-injection (DI) diesel combustion occurs has been derived from laser-sheet imaging and other recent optical data. To provide background, the most relevant of the recent imaging data of the author and co-workers are presented and discussed, as are the relationships between the various imaging measurements. Where appropriate, other supporting data from the literature is also discussed. Then, this combined information is summarized in a series of idealized schematics that depict the combustion process for a typical, modern-diesel-engine condition. The schematics incorporate virtually all of the information provided by our recent imaging data including: liquid-and vapor-fuel zones, fuel/air mixing, autoignition, reaction zones, and soot distributions. By combining all these elements, the schematics show the evolution of a reacting diesel fuel jet from the start of fuel injection up through the first part of the mixing-controlled burn (i.e. until the end of fuel injection). In addition, for a "developed" reacting diesel fuel jet during the mixingcontrolled burn, the schematics explain the sequence of events that occurs as fuel moves from the injector downstream through the mixing, combustion, and emissions-formation processes. The conceptual model depicted in these schematics also gives insight into the most likely mechanisms for soot formation and destruction and NO formation during the portion of the DI diesel combustion event discussed.

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February 1999This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. PREPRINTThis paper was prepared for submittal to the DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. 1999-01- Charles K. WestbrookLawrence Livermore National Laboratory ABSTRACT This paper proposes a structure for the diesel combustion process based on a combination of previously published and new results. Processes are analyzed with proven chemical kinetic models and validated with data from production-like direct injection diesel engines. The analysis provides new insight into the ignition and particulate formation processes, which combined with laser diagnostics, delineates the twostage nature of combustion in diesel engines. Data are presented to quantify events occurring during the ignition and initial combustion processes that form soot precursors.A framework is also proposed for understanding the heat release and emission formation processes.

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