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6.4 Why are two ratings used to obtain the pump rating?




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

6.4 Why are two ratings used to obtain the pump rating?

The correct name for the (RON+MON)/2 formula is the "antiknock index",
and it remains the most important quality criteria for motorists [39].

The initial knock measurement methods developed in the 1920s resulted in a
diverse range of engine test methods and conditions, many of which have been
summarised by Campbell and Boyd [103]. In 1928 the Co-operative Fuel Research
Committee formed a sub-committee to develop a uniform knock-testing
apparatus and procedure. They settled on a single-cylinder, valve-in-head,
water-cooled, variable compression engine of 3.5"bore and 4.5" stroke. The
knock indicator was the bouncing-pin type. They selected operating conditions
for evaluation that most closely match the current Research Method, however
correlation trials with road octanes in the early 1930s exhibited such large
discrepancies that conditions were changed ( higher engine speed, hot mixture
temperature, and defined spark advance profiles ), and a new tentative ASTM
Octane rating method was produced. This method is similar to the operating
conditions of the current Motor Octane procedure [12,103]. Over several
decades, a large number of alternative octane test methods appeared. These
were variations to either the engine design, or the specified operating
conditions [103]. During the 1950-1960s attempts were made to internationally
standardise and reduce the number of Octane Rating test procedures.

During the late 1940s - mid 1960s, the Research method became the important
rating because it more closely represented the octane requirements of the
motorist using the fuels/vehicles/roads then available. In the late 1960s
German automakers discovered their engines were destroying themselves on
long Autobahn runs, even though the Research Octane was within specification.
They discovered that either the MON or the Sensitivity ( the numerical
difference between the RON and MON numbers ) also had to be specified. Today
it is accepted that no one octane rating covers all use. In fact, during
1994, there have been increasing concerns in Europe about the high
Sensitivity of some commercially-available unleaded fuels.

The design of the engine and vehicle significantly affect the fuel octane
requirement for both RON and MON. In the 1930s, most vehicles would have
been sensitive to the Research Octane of the fuel, almost regardless of the
Motor Octane, whereas most 1990s engines have a 'severity" of one, which
means the engine is unlikely to knock if a changes of one RON is matched by
an equal and opposite change of MON [32]. I should note that the Research
method was only formally approved in 1947, but used unofficially from 1942.

 

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