Multidimensional Modeling of Fuel Composition Effects on Combustion and Cold-starting in Diesel Engines

A computer model developed for describing multicomponent fuel vaporization, and ignition in diesel engines has been applied in this study to understand cold-starting and the parameters that are of significant influence on this phenomena. This research utilizes recent improvements in spray vaporization and combustion models that have been implemented in the KIVA-II CFD code. Typical engine fuels are blends of various fuels species, i.e., multicomponent. Thus, the original single component fuel vaporization model in KIVA-II was replaced by a multicomponent fuel vaporization model (based on the model suggested by Jin and Borman). The model has been extended to model diesel sprays under typical diesel conditions, including the effect of fuel cetane number variation. Necessary modifications were carried out in the atomization and collision sub-models. The ignition model was also modified to account for fuel composition effects by modifying the Shell ignition model. The improved model was applied to simulate diesel engine cold-starting. The effect of fuel residual from previous cycles was studied and was found to be important. Other injection parameters, such as injection timing and duration were also studied. Another factor that was investigated was engine geometry and how it can be modified to improve on cold-starting in diesel engines. Cold-starting was found to be enhanced by the presence of a small fuel vapor residual and by a shorter injection duration, while engine geometry modifications were found to be helpful in selecting an optimum location on the cylinder head for an ignition aid. THE SIGNIFICANT NUh4BER of diesel engines which provide automotive power, together with recent concerns for the environment, resulted in the introduction of more legislation to limit their pollutant emissions. Of particular interest are the Nitrogen Oxides (NOx) and soot emissions. Both of these pollutants depend significantly on both the fuel and the injection system. The trend to lower emissions motivates new research efforts with the objective of improving engine performance. A significant fraction of engine emissions, particularly unburned hydrocarbons, is produced during the cold-starting phase of the engine operation. Furthermore, starting the diesel engine under cold ambient conditions represents a difficulty in itself Thus, more research is needed to identify mechanisms that would improve the cold-startability of the engine. Cold-starting is characterized as a situation where the engine does not fire at all for several cycles, or fires for one cycle and skips firing for the several following cycles, as seen by Henein et al. [l]*. There are several other issues of importance in cold-starting of diesel engines as discussed in Gonzalez et al. [2]. Fuel carryover from cycle-to-cycle is important as liquid fuel remains on the piston surface from misfiring and borderline cycles. Also, another factor in cold-starting in diesels is the excessive wear that results from high peak pressures reached after combustion after misfiring. Blowby gases also tend to increase at the slower cranking speed and that further reduces the compression temperature. Finally, the issue of unburned hydrocarbons and white smoke contributes significantly to the engine emissions. The objective of this work is to understand the process of cold-starting and the various parameters that can help improve the cold-startability of the engine using the most updated and improved computational models in KIVA-II. The controlling processes in diesel engine combustion are very complicated. To have a better understanding of these processes, they can be broken down to several distinct sub-processes such as injection, atomization, vaporization, ignition, combustion, etc. Once each process is investigated, a comprehensive understanding of diesel combustion can be achieved. + Numbers in brackets designate references listed at the end of the paper. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.