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
======================       ==================================
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.

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