Abstract
Methyl heptanoate (MHP) is a potential surrogate component for fatty acid methyl esters found in biodiesel. The carbon chain length of MHP is long enough to enable low temperature (low-T) reactivity and negative temperature coefficient oxidation behavior during the combustion experiments, similar to the real biodiesel fuels. This paper investigated the low-T oxidation of MHP at 780 Torr and equivalence ratios of 0.5, 1.0 and 1.5 in a jet-stirred reactor. Detailed speciation profiles of fuel, intermediates and products were obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. A comprehensive kinetic model with 779 species and 3594 reactions was developed and validated against the new experimental data. Model analysis indicated that the dominant reaction pathways for MHP consumption were H-abstraction reactions by radicals of OH, HO2 to produce MHP radicals under all experimental conditions. In this low-T oxidation region, O2 addition reactions were responsible for the consumption of MHP radicals. The formation pathways of unsaturated methyl esters were strongly related to the decomposition of cyclic ethers. Furthermore, the formations of CH3OOH and C2H5OOH were closely linked to the reactions of CH3O2 and C2H5O2 as well as the radicals of CH2O and HO2. This work provides detailed information relevant to low-T biodiesel oxidation chemistry and guidance for the application of biodiesel in internal combustion engines.