is the expression of the
antiknock properties of a gasoline,
relative to that of a standard reference fuel. There are two distinct types
of octane number measured in the laboratory: Research Octane Number (RON)
and Motor Octane Number (MON), determined in accordance with
ASTM-D-2699 and
D-2700, respectively. Both the RON and MON
tests are conducted in the same laboratory engine, but RON is determined
under less severe conditions, and is therefore numerically greater than MON
for the same fuel. The average of the two numbers — (RON + MON)/2 — is
commonly used as the indicator of a gasoline’s road antiknock performance.
The gasoline being tested is run in a special single-cylinder engine, whose
compression ratio can be varied (the higher the compression ratio, the
higher the octane requirement). The knock intensity of the test fuel, as
measured by a knockmeter, is compared with the knock intensities of blends
of isooctane (assigned a knock rating of 100)
and heptane (with a knock rating of zero),
measured under the same conditions as the test fuel. The percentage, by
volume, of the isooctane in the blend that matches the characteristics of
the test fuel is designated as the octane number of the fuel. For example,
if the matching blend contained 90% isooctane, the octane number of the test
fuel would be 90. In addition to the laboratory tests for RON and MON, there
is a third method, Road Octane Number, which is conducted in a specially
equipped test car by individuals trained to hear trace levels of engine
knock
Also see iantiknock
compounds, knock.