Ignition Studies in a Motored Engine for Kinetic Mechanism Development and Validation

Abstract

An accurate combustion model for real-world fuels is a key part of internal combustion engine simulation and design to reduce pollutants and greenhouse gas emissions. However, existing chemical kinetic mechanisms are not adequately accurate, especially for low-temperature oxidation (LTO). In this study, the autoignition of a jet fuel surrogate and pure components were investigated in a motored engine via coupled experiment and simulation to validate and improve the oxidation kinetic mechanism. A multizone model was developed to simulate homogeneous charge compression ignition (HCCI) combustion in a modified CRF octane rating engine. In a simulation study for three pentane isomers, the multizone model was accurate for autoignition simulation and effective for kinetic mechanism validation. An existing mechanism for pentane isomers was accurate in predicting reactivity, heat release, and oxidation intermediate species for all three pentane isomers. However, the low-temperature reactivity for iso-pentane was slightly underpredicted, with a 32.2% underprediction for the first-stage fuel consumption. Among the oxidation intermediate species, cyclic ethers were overpredicted by a minimum of 58.9%, while chain-branching products acetaldehyde or acetone were underpredicted by more than 57.5%. 2-Pentene and 2-methyl-2-butene production was overpredicted by 104% and 126%, showing that concerted elimination reaction rates and the effect of H-atom availability need to be improved. In addition, acetone production should be negligible but was significantly overpredicted during iso-pentane oxidation. Chain-branching pathways following the first and second O2 addition to the tertiary carbon were overestimated and needed to be eliminated in the kinetic mechanism. The ignition properties of the jet fuel surrogate and its pure components were investigated through coupled engine experiments and simulation. The UM-3 Jet-A surrogate showed strong low-temperature reactivity, which the kinetic mechanism, SKE360, successfully captured. However, the global reactivity was underpredicted due to underestimated n-dodecane and decalin reactivity, and underestimated toluene oxidation. In the surrogate mixture, iso-cetane, decalin, and toluene oxidation were significantly enhanced by n-dodecane low-temperature oxidation. The radical pool from n-dodecane low-temperature oxidation enhances H-atom abstraction, fuel radical formation, and production of small intermediate species via consecutive beta-scission reactions. This oxidation enhancement was successfully predicted for iso-cetane and decalin, but underpredicted by 79.1% for toluene. For the pure components of the jet fuel surrogate, existing mechanisms underestimated n-dodecane reactivity, and their different reaction rates led to significant differences in reactivity prediction. Low-temperature oxidation was insignificant during iso-cetane oxidation at the test conditions, but was overpredicted by the jet fuel surrogate mechanism SKE360. Reaction rates for R + O2 <=> RO2 reactions need to be improved to eliminate O2 addition to the iso-cetane fuel radical in alkene formation. Decalin reactivity was underpredicted by SKE360. The production of benzene, cyclohexadiene, and cyclohexene was overpredicted by more than 10 times during decalin oxidation, showing the opening of one ring in this bicyclic alkane molecule was overestimated. The main oxidation pathways following the C-C bond breaking between the two tertiary carbons were missing and need to be added. In this work, motored engine experimental measurements were for the first time used for quantitative evaluations of kinetic mechanisms. The method developed in this study and the ignition data generated improved our fundamental understanding of combustion chemistry. Our discussions provided directions for future mechanism development.