An experimental and kinetic modeling study of ethyl tert‑butyl ether. Part II: Intermediate and low temperature oxidation chemistry

Ethyl tert‑butyl ether (ETBE) has captured significant research attention due to its potential to reduce harmful emissions and consequently it is used as an oxygenate additive in gasoline. A comprehensive low- to high-temperature chemistry sub-model for ETBE has been developed for the first time and is validated against experimental data including ignition delay times (IDTs), species profiles, and laminar flame speeds. This paper focuses on the low- to intermediate-temperature kinetics of ETBE oxidation. IDTs of ETBE mixtures are measured in both a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) at pressures of 15 and 30 bar in the temperature range 615–1376 K at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’. The observed negative temperature coefficient behavior in ETBE oxidation can be explained by the competition between the reactions involving the formation of cyclic ethers and tert‑butyl vinyl ether (TBVE), and the reactions associated with the formation and consumption of carbonyl hydroperoxide species. Moreover, IDTs of 2,2-dimethylbutane (22DMB) and 2,2-dimethylpentane (22DMP) mixtures were also measured at 15 and 30 bar in the temperature range 666–1300 K at stoichiometric conditions in ‘air’ in order to compare the reactivities of these alkanes with their corresponding ethers, methyl tert‑butyl ether (MTBE) and ETBE. The oxygen lone pair in both MTBE and ETBE reduces the adjacent α C–H bond dissociation energy, making hydrogen atom abstraction at that site more facile which results in higher ether fuel reactivity at temperatures above 1000 K. At temperatures below 1000 K, the substitution of the corresponding secondary carbon atom in alkanes with an oxygen atom in ethers results in a much lower flux of fuel forming Q˙OOH radicals via a six-membered ring transition state which is the key species leading to low-temperature chain-branching reactions. This is why the reactivities of MTBE and ETBE are almost two orders of magnitude lower than their alkane counterparts 22DMB and 22DMP in the negative temperature coefficient region. Conversely, dimethyl ether displays nearly two orders of magnitude higher reactivity compared to propane at lower temperatures, because of the much higher fuel flux of RȮ₂ radicals proceeding to chain branching pathways through a six-membered ring transition state isomerization reaction compared to propane. This comparative analysis provides fundamental insights into structure-reactivity relationships in oxygenated fuel combustion chemistry.