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Content archived on 2024-04-15

DEVELOPMENT OF A MECHANISM OF THE COMBUSTION OF MOTOR FUELS IN THE PRESENCE OF BOOSTERS OF OCTANE OR CETANE.

Objective

THE AIM OF THIS PROJECT IS TO EXPLAIN THE ANTI-KNOCKING ROLE OF THE ASYMMETRIC ETHERS MTBE AND TAME THROUGH THE ELUCIDATION OF THEIR DETAILED OPERATING KINETICS AND MECHANISM AND AS A RESULT TO POSSIBLY SUGGEST NEW BOOSTER FORMULATIONS.
The results obtained on the pyrolysis and oxidation reactions of methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME) and ethyl tert-butyl ether (ETBE) have led to a better understanding of the oxidation mechanism and of the antiknock effect of these oxygenates.
The decomposition reaction of these oxygenates in the absence of oxygen is an intramolecular 4-centre elimination and not a chain radical mechanism.
The oxidation reaction at about 300 C looked more like a pyrolysis initiated by oxygen than like a genuine oxidation (the oxygen consumption is very low).
The major primary products of the oxidation (and pyrolysis) reactions of these ethers were products which also have good octane numbers (research and blending) and antiknock properties when used in an engine. This very important conclusion extends the conditions which have been derived from the investigation of MTBE oxidation and show that an organic substance should be able to inhibit the oxidation reaction of another substance.
There was a very strong similarity between the mechanistic oxidation schemes of these ethers. It was shown that their oxidation schemes (at about 450 C) were limited to elementary steps of the high temperature scheme. These elementary steps account for the formation of the major primary products (which all have high octane numbers) and rather unreactive water radicals. which all were consistent with the observed antiknock effects.

The present study shows that a chemical like an ether can have a positive effect on the octane number on condition that this substance is inert enough towards oxidation reaction on one hand and on the other hand that it yields, when reacting in the presence of oxygen, rather unreactive radicals. It should also yield molecular products (alcohols, olefins) which have high octane numbers, especially blending octane numbers.

Ethers like MTBE, TAME and ETBE are therefore a means of generating in situ substances which have high octane numbers. A lthough able to provide a logical explanation of the observed properties and phenomena, it still seems difficult, because of a lack of fundamental knowledge on alcohol and olefin oxidation reactions, to be able to predict the antiknock properties of oxygenates on the basis of the logical reaction schemes.
THERE ARE LIMITATIONS TO THE VIABLE PRODUCTION OF THE OLEFINIC PRECURSORS OF THE OCTANE BOOSTERS LIKE MTBE AND TAME FOR UNLEADED MOTOR FUELS. A DEEP UNDERSTANDING OF THEIR OPERATING MECHANISM MAY LEAD TO PROPOSALS OF ALTERNATIVE PRODUCTS OR OF A BETTER USE OF THE KNOWN ONES,E.G. VIA SYNERGISTIC MIXTURES.
KNOCKING IS RELATED TO THE SPONTANEOUS SELF-IGNITION PROPERTIES OF THE FUEL. VERY RELIABLE CORRELATIONS HAVE BEEN FOUND BETWEEN THE BEHAVIOUR OF A SUBSTANCE AS TO SELF-IGNITION PROPERTIES UNDER ENGINE CONDITIONS AND THE CHARACTERISTICS OF ITS NON-ISOTHERMAL OXIDATION BETWEEN 300 AND 450 CELSIUS DEGREES UNDER STATIC CONDITIONS AT SUB-ATMOSPHERIC PRESSURE IN A PYREX LABORATORY REACTOR. BY THIS METHOD IT IS MEANT TO DETERMINE THE SELF-IGNITION LIMITS OF EACH ETHER AS WELL AS OF THE BINARY MIXTURES OF THEM WITH A REFERENCE ALKANE.
OTHER ASYMMETRIC ETHERS AND POSSIBLY T. BUTYLALCOHOL WILL ALSO BE EXAMINED. IT IS ALSO INTENTED TO DETERMINE THE REACTION PRODUCTS AT LOW CONVERSION. THIS WILL ALLOW THE ELEMENTARY STEPS OF THE RADICAL MECHANISM TO BE QUALITATIVELY GUESSED. THE BENSON, GOLDEN, O'NEAL METHODS OF THERMOCHEMICAL KINETICS SHALL BE USED TO GET THE CORRESPONDING QUANTITATIVE PICTURE. FOR THEM, COMPUTER PROGRAMMES ARE BEING MADE READY. A MATHEMATICAL MODEL SHALL BE SET UP TO ACCOUNT FOR HEAT EXCHANGES BETWEEN THE SYSTEM AND THE SURROUNDINGS.
THE QUINN'S KINETIC MODEL OF SELF-IGNITION OF ALKANES WILL THEN BE MODIFIED BY INTRODUCING THE NEWLY FOUND ELEMENTARY PROCESSES BROUGHT ABOUT BY THE BOOSTERS AND THE RESULTS COMPARED WITH THE EXPERIMENTAL ONES. TRANSPOSITIONS WILL ALSO BE MADE TO KNOCKING MODELS LIKE NATARJAN-BRACCO'S OR SCHAPERTONS'S. SHOULD THE OPERATING MECHANISMS OF THE MTBE AND TAME BE VERY SIMILAR, THEN OTHER MOLECULES OF EASIER AVAILABILITY MIGHT BE ENVISAGED. IF NOT, THE DIFFERENCES WOULD STIMULATE PROPOSALS OF OTHER CLASSES OF BOOSTERS.
NOTE: KAISERSLAUTERN UNIVERSITY IS ALSO INVOLVED IN THIS PROJECT: SEE CONTRACT EN3C004400-D (B).

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Institut National Polytechnique de Lorraine
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