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The Underlying Physics and Chemistry behind Fuel Sensitivity
ISSN: 1946-3952, e-ISSN: 1946-3960
Published April 12, 2010 by SAE International in United States
Citation: Mittal, V., Heywood, J., and Green, W., "The Underlying Physics and Chemistry behind Fuel Sensitivity," SAE Int. J. Fuels Lubr. 3(1):256-265, 2010, https://doi.org/10.4271/2010-01-0617.
Recent studies have shown that for a given RON, fuels with a higher sensitivity (RON-MON) tend to have better antiknock performance at most knock-limited conditions in modern engines. The underlying chemistry behind fuel sensitivity was therefore investigated to understand why this trend occurs. Chemical kinetic models were used to study fuels of varying sensitivities; in particular their autoignition delay times and chemical intermediates were compared.
As is well known, non-sensitive fuels tend to be paraffins, while the higher sensitivity fuels tend to be olefins, aromatics, diolefins, napthenes, and alcohols. A more exact relationship between sensitivity and the fuel's chemical structure was not found to be apparent. High sensitivity fuels can have vastly different chemical structures.
The results showed that the autoignition delay time (τ) behaved differently at different temperatures. At temperatures below 775 K and above 900 K, τ has a strong temperature dependence. However, between 775 K and 900 K, τ has a decreased temperature dependence. The change in temperature dependence in this region was found to correlate with fuel sensitivity. The autoignition of fuels with a higher sensitivity have a higher temperature dependence in that region. A stronger temperature dependence on τ in this region results in slower low temperature chemistry and faster high temperature chemistry.
As a consequence, fuels behave differently depending on the temperature regime of the end-gas. If two fuels have the same RON, the autoignition integral for the two fuels approaches 1 at the same time in the RON test. Lower end-gas temperatures would allow sensitive fuels, which have slower low temperature chemistry, to have better antiknock performance. However, higher end-gas temperatures, such as those in the MON test, would allow non-sensitive fuels to have better antiknock performance.
The fuels with larger sensitivities studied here were predicted by kinetic models to produce large amounts of aldehydes, which are relatively stable at low temperatures, but react rapidly at high temperatures. These aldehydes appear to be an important cause of the octane sensitivity.