Shock tube experiments and kinetic modeling of ignition of unsaturated C5 methyl esters

Ignition delay times (IDTs) of methyl crotonate (MC), methyl 3-butanoate (MB3D), and methyl methacrylate (MMA), were systematically measured to explore the influences of the position of unsaturated CC double bond and the structure of alkyl chain on ignition characteristics of unsaturated C5 methyl esters. Experiments were performed at 1108–1783 K and 4–16 atm with the equivalance ratio ranging from 0.25 to 2 in a shock tube. Reactivities of three unsaturated esters are similar: their IDTs increase with decreasing temperature and increasing equivalence ratio but are less sensitive to pressure. To gain the insight into differences in kinetics during fuel ignition, the literature MC, MB3D and MMA models were evaluated and the MMA model was updated. Sensitivity and reaction pathway analyses show that the hydrogen-abstraction, unimolecular decomposition, and radical addition reactions are important in controlling MC ignition. However, only hydrogen-abstraction and radical addition reactions show the significance during MB3D and MMA ignitions. Furthermore, the impact of saturation was also explored. The saturated methyl esters have slightly longer IDTs than MC, MB3D, and MMA under fuel-lean conditions with T> 1350 K, because the unsaturated methyl esters are the key intermediates and the hydrogen-abstraction reactions dominate the consumption of saturated esters.