7.8 What is the effect of engine deposits?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
7.8 What is the effect of engine deposits?
A new engine may only require a fuel of 6-9 octane numbers lower than the
same engine after 25,000 km. This Octane Requirement Increase (ORI) is due to
the formation of a mixture of organic and inorganic deposits resulting from
both the fuel and the lubricant. They reach an equilibrium amount because
of flaking, however dramatic changes in driving styles can also result in
dramatic changes of the equilibrium position. When the engine starts to burn
more oil, the octane requirement can increase again. ORIs up to 12 are not
uncommon, depending on driving style [27,28,32,111]. The deposits produce
the ORI by several mechanisms:-
- they reduce the combustion chamber volume, effectively increasing the
compression ratio.
- they also reduce thermal conductivity, thus increasing the combustion
chamber temperatures.
- they catalyse undesirable pre-flame reactions that produce end gases with
low autoignition temperatures.
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