THE EFFECT OF OCTANE-ENHANCING ADDITIVES ON THE AUTOIGNITION CHARACTERISTICS OF HYDROCARBONS

An experimental and theoretical study of the low-temperature oxidation of model hydrocarbons in a jet stirred flow reactor has been performed in order to determine the fundamental chemical kinetic paths controlling the tendency of hydrocarbons to undergo autoignition phenomena. The low-temperature oxidation chemistry of n-pent ane (nC5) and methyl tert-butyl ether (MTBE) and their mixtures have been analyzed to illustrate the interplay between the different chemical reaction mechanisms of an octane-enhancing ether such as MTBE and a normal paraffin such as pentane. The overall activity of MTBE in reducing n-pentane reactivity is related to its capability to produce isobutene. In particular, isobutene has been identified as an especially effective retarding agent by providing a preferential fate for radicals and producing unreactive smaller species. The decomposition of MTBE is enhanced when it is in admixture with n-pentane since the latter, producing a large pool of radicals during its oxidation process, enhances the H-atom abstraction from MTBE molecules and hence its radical decomposition. As expected, as the conversion of nC5 is reduced for effect of temperature increase in the NTC region, also the MTBE decomposition is reduced for effect of the reduced radical pool. In this conditions MTBE decomposition approaches that of pure MTBE without significant influence of the nC5 oxidation. The kinetic model, previously developed and tested for the oxidation of individual pure fuels, shows to accurately reproduce the main features of the interaction between the two mixture components. Model predictions are able to reproduce quite well either the NTC and the reduced reactivity of the mixtures for effect of MTBE addition.