The Octane Number (sometimes
erroneously called Octane Content) is a
measure of the
antiknock quality of petrol - the ability of a petrol to
resist
knocking upon combustion in an engine. Knocking is a
description of the sound that an engine makes when it runs on a too low
octane fuel, and it causes a drop in fuel efficiency.
In 1927, Graham Edgar suggested a test method to quantify the
knocking behavior of a fuel. Edgar proposed the use of two fuels: n-
heptane and 2,2,4-trimethyl pentane (iso-octane); fuels with a low
and a high antiknocking value respectively. Any commercial fuel that was
available on the market at that time could be compared in knocking
behavior to blends of these two compounds. The introduction of
reference fuels lead to the design of various test engines and test
conditions. Nowadays, the most commonly used test methods are the
Research Octane Number (RON), and the Motor Octane Number (MON).
The fuel property the octane ratings measure is the ability of the
unburnt end gases to spontaneously ignite under the specified test
conditions. Ideally, a fuel should resist decomposition before arrival
of the flame-front in the piston. This is a dependent on the hydrocarbon composition
of the fuel, but also on the presence of additives, such as oxygenates.
The Research Octane Number settings represent typical mild
driving, without consistent heavy loads on the engine.
Test Engine conditions Research Octane
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Test Method ASTM D2699-92 105
Engine Cooperative Fuels Research (CFR)
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm )
The conditions of the Motor Octane Number method represent
severe, sustained high speed, high load driving. For most
hydrocarbon fuels, including those with either lead or oxygenates, the
Motor Octane Number will be lower than the Research Octane Number.
Test Engine conditions Motor Octane
====================== ==================================
Test Method ASTM D2700-92 104
Engine Cooperative Fuels Research (CFR)
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm
In general, a combination of the RON and MON is used; e.g.
in the US a combination called the Antiknock Index (AKI) is defined
as:
AKI = (RON + MON) / 2
This is usually specified on the pump; next time you pull up at the gas station, you will
see :-)
Another important factor to quantify fuels is Sensitivity, defined as:
Sensitivity = RON - MON
A fuel can have a high Antiknock
Index value, but behave poorly under varying operating conditions. Most
modern fuels have a Sensitivity around 10.
information & data from: http://www.faqs.org/faqs/autos/gasoline-faq/