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Detailed chemical kinetic models for cleaner internal combustion engines

Final Report Summary - CLEAN-ICE (Detailed chemical kinetic models for cleaner internal combustion engines)

The key objective of the Clean-ICE project was to promote cleaner and more efficient combustion technologies through the development of theoretically grounded and more accurate chemical models. The developed models for the combustion of constituents of gasoline, kerosene, and diesel fuels do a reasonable job in predicting auto-ignition and flame propagation parameters, and the formation of the main regulated pollutants. When coupled with CFD simulations, they contribute to actual engine research in order to achieve a significant improvement in efficiency. However the success rate of these models deteriorates sharply in the prediction of the formation of minor pollutants (alkenes, dienes, aromatics, aldehydes) and soot nano-particles, which have a deleterious impact on both the environment and on human health. At the same time, despite an increasing emphasis in adding biofuels into fossil fuels, there was a great lack of chemical models for the combustion of oxygenated reactants especially those formed from non edible biomass.
The main scientific focus was then to enlarge and deepen the understanding of the reaction mechanisms associated with the combustion of an increased range of fuels and to elucidate the formation of a large number of minor pollutants. Experimental data have been obtained in well defined laboratory systems (jet stirred reactor (JSR), premixed laminar flames (PLF)) by using analytical techniques able to detect pollutants formed at ppm level. Theoretical methods based on quantum mechanics have been used to study important individual reactions, that in connection with automatic generation of mechanisms. The most significant results have already been obtained in 5 directions:
• Probing low temperature hydrocarbon oxidation
A particularly detailed speciation of the products formed during the low-temperature oxidation of hydrocarbons has been achieved for the first time, thanks a new system coupling a JSR to a molecular-beam mass spectrometer combined with tunable synchrotron vacuum ultraviolet photoionization. Hydroperoxides, including ketohydroperoxides, the species responsible for the start of hydrocarbon auto-ignition, the key of the design of cleaner engines, have been quantified. Acids, such as acetic acid which contribute to the acidity of the atmosphere have also been analyzed.
• Probing intermediate temperature hydrocarbon oxidation
A JSR has been coupled to two spectroscopic methods, cw CRDS (continuous wave cavity ring-down spectroscopy) in the near infrared allowing the quantification of hydrogen peroxide (H2O2), and FAGE (Fluorescence Assay by Gas Expansion), a technique based on based on Laser Induced Fluorescence, allowing the detection of hydroxyl (OH) and hydroperoxy (HO2) radicals. These are the first reliable quantifications of H2O2, OH and HO2, the most important species triggering the full development of auto-ignition.
• Revisiting the kinetics of the first steps of hydrocarbon oxidation using quantum chemistry
The kinetic data of the major primary reactions, involved in the oxidation of alkanes and alkenes (the isomerizations of alkylperoxy radicals giving QOOH radicals and the subsequent decompositions to give cyclic ethers), have been calculated using highly accurate methods. The related reactions have been up-dated in automatically generated models for the combustion of C3-C6 alkanes. These models have been then used to successfully reproduce minor product (e.g. cyclic ethers, aldehydes) formation.
• Minor pollutants from furan derivatives
Thank to a large use of quantum chemistry calculations, the first models for the combustion of furan derivatives, promising biofuel additives in gasoline, have been proposed. These models predict a surprising high tendency of dimethylfuran to form species responsible of the formation of ultrafine soot particles influencing human health and earth climate. This has been confirmed by PLF studies using gas chromatography and mass spectrometry analyses after probe sampling.
• Experimental and modeling study of minor pollutant formation during the oxidation of methyl ester
JSR experimental studies of the numerous minor pollutants formed during the oxidation of large methyl esters, representative of those present in biodiesel (palmitate, oleate) in mixture with a large hydrocarbon, and for a proposed surrogate, decanoate, have been performed. Automatically generated models have been proposed for saturated methyl esters up to C19, reproducing well the pollutant formation.