Detailed kinetic modeling of conventional gasoline at highly boosted conditions and the associated intermediate temperature heat release

The combustion behavior of conventional gasoline has been numerically investigated by means of detailed chemical-kinetic modeling simulations, with particular emphasis on analyzing the chemistry of the intermediate temperature heat release (ITHR). Previous experimental work on highly boosted (up to 325 kPa absolute) HCCI combustion of gasoline (SAE 2020-01-1086) showed a steady increase in the charge temperature up to the point of hot ignition, even for conditions where the ignition point was retarded well after top dead center (TDC). Thus, sufficient energy was being released by early pre-ignition reactions resulting in temperature rise during the early part of the expansion stroke This behavior is associated with a slow pre-ignition heat release (ITHR), which is critical to keep the engine from misfiring at the very late combustion phasings required to prevent knock at high-load boosted conditions. The experimental results also showed that the amount of ITHR increased with increasing boost and reduced intake temperature. As a result, this experimental work achieved high efficiency (up to 47%) and high power output (up to 16.34 bar IMEPg) without incurring engine knock, and while keeping ultra-low NOx emissions. The ITHR was demonstrated to be fuel dependent as ethanol does not show an increase in the ITHR with boost (2010-01-0338), constraining the combustion timing to be much closer to TDC. The chemistry underlying these phenomena is not yet completely understood. In this numerical study, a chemical-kinetic modeling study of gasoline-fueled boosted HCCI combustion is presented and discussed. A numerical procedure to formulate a gasoline surrogate on the basis of its aromatic, alkene and alkane content and octane indexes was used to match the major properties of the real gasoline. The combustion behavior of this surrogate is modeled using the LLNL mechanism, and the results are compared with the experimental work performed at Sandia. The model successfully reproduces the main aspects of the combustion process shown by the experiments, and it provides insights into the ITHR phenomenon. Finally, a numerical analysis was conducted to explore the chemistry of the ITHR, allowing a correlation between the chemical structure of the fuel and the presence of the ITHR.