6.10 What is the Octane Distribution of the fuel?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
6.10 What is the Octane Distribution of the fuel?
The combination of vehicle and engine can result in specific requirements
for octane that depend on the fuel. If the octane is distributed differently
throughout the boiling range of a fuel, then engines can knock on one brand
of 87 (RON+MON)/2, but not on another brand. This "octane distribution" is
especially important when sudden changes in load occur, such as high load,
full throttle, acceleration. The fuel can segregate in the manifold, with
the very volatile fraction reaching the combustion chamber first and, if
that fraction is deficient in octane, then knock will occur until the less
volatile, higher octane fractions arrive [27,28].
Some fuel specifications include delta RONs, to ensure octane distribution
throughout the fuel boiling range was consistent. Octane distribution was
seldom a problem with the alkyl lead compounds, as the tetra methyl lead
and tetra ethyl lead octane volatility profiles were well characterised, but
it can be a major problem for the new, reformulated, low aromatic gasolines,
as MTBE boils at 55C, whereas ethanol boils at 78C. Drivers have discovered
that an 87 (RON+MON)/2 from one brand has to be substituted with an 89
(RON+MON)/2 of another, and that is because of the combination of their
driving style, engine design, vehicle mass, fuel octane distribution, fuel
volatility, and the octane-enhancers used.
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