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Specific Methods
Lean-Mixture Aviation Rating
ASTM D 2700 / IP 236 – Standard Test Method for Motor Octane Number
of Spark-Ignition Engine Fuel
2
This method uses a standardized, single-cylinder, four-stroke-cycle, variable-compression-ratio,
carbureted engine to rate a fuel’s ability to resist auto-ignition. The engine is operated under
specific conditions of speed, fuel-air mixture temperature, and spark timing. The fuel-air ratio
and compression ratio of the engine are adjusted to produce a knock of standardized intensity for
the test fuel, as measured by an electronic knock meter. Reference fuels are run to identify those
with knock intensities that bracket the knock intensity of the test fuel. The result for the test fuel
is then determined by interpolating between the octane numbers of the bracketing reference fuels
and reported as a Motor octane number.
The method employs two sets of primary reference fuels:
• Primary reference fuels with octane numbers of 100 and less are blends of normal heptane
(n-heptane), whose octane number is defined to be zero, and isooctane (2,2,4-trimethylpentane),
whose octane number is defined to be 100. The octane number of each blend is equal to the
volume percent of isooctane it contains.
• Primary reference fuels with octane numbers greater than 100 are blends of isooctane and
tetraethyl lead. The method provides an equation to convert the concentration of tetraethyl
lead in units of milliliters per U.S. gallon to octane number.
D 2700 includes a table to convert a Motor octane number to an Aviation rating equivalent to
one that would be obtained using the now-discontinued D 614 method (see sidebar). For Motor
octane numbers between 75 and 100, the conversion generally reduces the value by a fraction of a
number and the result is reported as an octane number. For Motor octane numbers between 100
and 110, the conversion increases the value by up to 20 numbers and the result is reported as a
performance number.
Rich-mixture Supercharge Rating
ASTM D 909 / IP 119 – Standard Test Method for Knock Characteristics of Aviation
Gasolines by the Supercharge Method
This method uses a supercharged, single-cylinder engine operated at constant speed, compression
ratio, air temperature, and spark timing. The inlet manifold air pressure and the fuel-air ratio are
varied. The engine is connected to a dynamometer and power output at light knocking intensity
is measured for a series of fuel-air ratios running from lean to rich.
The dynamometer measures the work available at the crankshaft, in units of brake mean effective
pressure (BMEP). However, another unit – indicated mean effective pressure (IMEP) – is more
History of Octane Testing*
In the late 1910s and early 1920s, the initial
systematic studies of the relationship
between engine knocking and fuel quality
were conducted by A.H. Gibson and Harry
Ricardo in England and Thomas Midgely and
Thomas Boyd in the United States. In 1927,
Graham Edgar, following an extensive investi-
gation of the detonation characteristics of
pure hydrocarbons, proposed using normal
heptane and isooctane to define a 100-point
knock testing rating scale. At about the same
time, the Cooperative Fuel Research Commit-
tee (CFR), sponsored jointly by automotive
and oil organizations, specified a single-
cylinder, variable-compression engine for
knock testing. By 1929, oil companies were
using the reference fuels and test engines to
obtain octane ratings.
The CFR engine, when operated at specified
speed and temperature conditions, became
the basis for the CFR Motor Method for anti-
knock testing. The method used a knock
meter to determine knocking. The CFR Motor
method evolved into ASTM Method D 357,
Standard Method for Knock Characteristics of
Motor Fuels of 100 Octane Number and
Below by the Motor Method. In 1968, D 357
was replaced by ASTM D 2700, Standard Test
Method for Motor Octane Number of Spark-
Ignition Engine Fuel. In the 1930s, the CFR
Motor Method was the specification method
used by the U.S. Navy and commercial
airlines to determine the (lean-mixture) anti-
knock rating of avgas. The U.S. Army Air
(continued next page)
* Alexander R. Ogston, A Short History of Aviation
Gasoline Development, 1903-1980, Society of
Automotive Engineers Paper No. 810848, 1981.
2 This is the test used to determine the “M” in the (R
M)/2 antiknock index posted on motor gasoline pumps
at service stations.
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