The oxidation of a gasoline fuel surrogate in the negative temperature coefficient region
Authors:   , Lenhert David Burton , Cernansky Nicholas Peter 
Published: July 2021

The oxidation of simple blends or chemical surrogates has allowed the detailed examination of the combustion processes for fuels that either have too many compounds for detailed study or have a variable composition. For gasoline fuels, mixtures of nheptane and 2,2,4-trimethyl-pentane (iso-octane) have been used as a surrogate to determine the octane rating of a fuel. Neat and binary mixtures of these primary reference fuels (PRFs) have been studied and modeled extensively to elucidate the mechanism of autoignition for real world fuels. However, modern gasoline fuels are complex mixtures of alkanes (~60%), alkenes (~10%), and aromatics (~30%). From the limited number of studies that have been conducted on mixtures of alkane/alkene and alkane/aromatic, we know that the addition of an alkene or aromatic compound had a significant influence on the oxidation process. The aim of this study was to examine the oxidation of a chemical surrogate that included alkane, alkene, and aromatic components and to elucidate the effects of the components on the oxidation process. The surrogate was a mixture of 4.6% 1-pentene, 31.8% toluene, 14.0% n-heptane, and 49.6% isooctane and had been developed to mimic the behavior of several industry standard fuels in the low and intermediate temperature regime. This experimental program studied the oxidation of each of the individual components and then the entire mixture in the low and intermediate temperature regime (600 - 800 K) at elevated pressures (8 atm) under dilute and lean or stoichiometric xv conditions. Samples were extracted and analyzed over a range of temperatures at a fixed residence time and over a range of residence times at a fixed temperature by a suite of online analyzers, namely non-dispersive infrared measurements of CO and CO₂ and total hydrocarbon measurements with a flame ionization detector. Samples were also extracted for subsequent offline analysis using gas chromatography and gas chromatography with mass spectrometry to identify the major intermediates species. The neat 1-pentene results suggested that it undergoes both alkane and alkene type decomposition pathways, but is dominated by alkene pathways, specifically hydroperoxy radical (HO[radical]₂) addition to the double bond. The neat n-heptane results were compared to the current detailed n-heptane model of Curran et al. (1998a). Generally, the model predictions and experimental data were in good agreement. The neat iso-octane results were compared to the current detailed iso-octane model of Curran et al. (2002). The model predictions and experimental data were generally in very poor agreement, as the decomposition of isooctane was significantly over predicted. The major pathway identified for the formation of aldehydes in n-heptane and iso-octane was the decomposition of the dihydroperoxy radical. The neat toluene results are not included, since neat toluene did not react at all conditions examined. The surrogate results showed that approximately 10% of the toluene reacted to form benzaldehyde, benzene, phenol, and ethyl-benzene. The addition of the toluene and 1-pentene also affected the relative rates of production of the intermediate species, namely, by increasing the formation of ethers and conjugate alkenes.