Gasoline and Combustion: Relationship between Molecular Structure and Performance

The efficiency of spark ignition engine is usually limited by the appearance of knock, which is linked to fuel performance, and more precisely to fuel octane number (Research Octane Number - RON and Motor Octane Number - MON). Improving fuel performance often implies a higher octane rating. However, meeting this condition is insufficient to predict combustion behavior. This paper proposes a way to characterize parameters related to knock resistance of fuels (RON, sensitivity and ignition delay time), using their compounds’ structure. This study uses a database consisting of 48 paraffins, with a carbon number ranging mainly from 4 to 9. The different hydrocarbons have different performance properties that can be characterized thanks to the number and length of their branches. Furthermore, ignition delay times of octane isomers have been measured and are closely correlated with the RON, confirming the different fuel properties between isomers. A kinetic study of paraffins’ low-temperature combustion helps in the understanding of those differences. Knock is caused by the exponential formation of radicals in the air/fuel mixture. The position of branches can facilitate or inhibit the formation of radicals because of the involved elementary reaction pathways. Quaternary carbons, steric hindrance or isolated radicals on a paraffin are factors making it possible to limit the radical propagation and thus possible to limit knock and improve fuel performance. This work finally aims at identifying high potential compounds that could be used in highly efficient fuel formulations. It shows that multi-branched paraffins with short main-chains have the best properties concerning knock resistance, with a specific consideration of the structural position of the branches.