• Method adopted/last revised:
• Method reapproved: 2019
• REF/ISBN:
• Status: Current
• First printed in STM books: 2018

This International Standard establishes the rating of liquid spark-ignition engine fuel in terms of an arbitrary scale of octane numbers using
a standard single-cylinder, four-stroke cycle, variable-compression ratio, carburetted, CFR engine operated at constant speed. Motor octane number (MON) provides a measure of the knock characteristics of motor fuels in automotive engines under severe conditions of operation. The motor octane number provides a measure of the knock characteristics of aviation fuels in aviation piston engines, by using an equation to correlate to aviation-method octane number or performance number (lean-mixture aviation rating).

This International Standard is applicable for the entire scale range from 0 MON to 120 MON, but the working range is 40 MON to 120 MON. Typical motor fuel testing is in the range of 80 MON to 90 MON. Typical aviation fuel testing is in the range of 98 MON to 102 MON. This International Standard is applicable for oxygenate-containing fuels containing up to 4,0 % (m/m) oxygen and for gasoline containing up to 25 % (V/V) ethanol.

NOTE 1 Although 25 % (V/V) of ethanol corresponds to approximately 9 % (m/m) oxygen, full applicability of
this test method for that oxygen range has only been checked for gasoline type of fuels.

NOTE 2 Work is under way to check the possibility to use the method for gasoline containing up to 85 %
(V/V) ethanol.

NOTE 3 This International Standard specifies operating conditions in SI units but engine measurements may
be specified in inch-pound units because these were the units used in the manufacture of the equipment,
and thus some references in this International Standard include these units in parenthesis.

NOTE 4 For the purposes of this standard, the terms “% (m/m)” and “% (V/V)” are used to represent the
mass fraction, μ, and the volume fraction, φ, of a material respectively.

NOTE 5 When determining conformance with specified limits in this standard, an observed or calculated value shall be rounded to the nearest unit in the last right-hand digit used in expressing that limit in accordance with the rounding method of ASTM Practice E29