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7.9 What is the Road Octane Number of a Fuel?




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This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.

7.9 What is the Road Octane Number of a Fuel?

The CFR octane rating engines do not reflect actual conditions in a vehicle,
consequently there are standard procedures for evaluating the performance
of the gasoline in an engine. The most common are:-
1. The Modified Uniontown Procedure. Full throttle accelerations are made
from low speed using primary reference fuels. The ignition timing is
adjusted until trace knock is detected at some stage. Several reference
fuels are used, and a Road Octane Number v Basic Ignition timing graph is
obtained. The fuel sample is tested, and the trace knock ignition timing
setting is read from the graph to provide the Road Octane Number. This is
a rapid procedure but provides minimal information, and cars with engine
management systems require sophisticated electronic equipment to adjust
the ignition timing [28].

2. The Modified Borderline Knock Procedure. The automatic spark advance is
disabled, and a manual adjustment facility added. Accelerations are
performed as in the Modified Uniontown Procedure, however trace knock is
maintained throughout the run by adjustment of the spark advance. A map
of ignition advance v engine speed is made for several reference fuels
and the sample fuels. This procedure can show the variation of road octane
with engine speed, however the technique is almost impossible to perform
on vehicles with modern management systems [28].

The Road Octane Number lies between the MON and RON, and the difference
between the RON and the Road Octane number is called 'depreciation" [111].
Because nominally-identical new vehicle models display octane requirements
that can range over seven numbers, a large number of vehicles have to be
tested [28,111].

 

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