6.12 How do other fuel properties affect octane?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
6.12 How do other fuel properties affect octane?
Several other properties affect knock. The most significant determinant of
octane is the chemical structure of the hydrocarbons and their response to
the addition of octane enhancing additives. Other factors include:-
Front End Volatility - Paraffins are the major component in gasoline, and
the octane number decreases with increasing chain length or ring size, but
increases with chain branching. Overall, the effect is a significant
reduction in octane if front end volatility is lost, as can happen with
improper or long term storage. Fuel economy on short trips can be improved
by using a more volatile fuel, at the risk of carburettor icing and
increased evaporative emissions.
Final Boiling Point.- Decreases in the final boiling point increase fuel
octane. Aviation gasolines have much lower final boiling points than
automotive gasolines. Note that final boiling points are being reduced
because the higher boiling fractions are responsible for disproportionate
quantities of pollutants and toxins.
Preignition tendency - both knock and preignition can induce each other.
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