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It is a standard measure of the performance of an engine or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating (igniting). In broad terms, fuels with a higher octane rating are used in high performance gasoline engines that require higher compression ratios. In contrast, fuels with lower octane numbers (but higher cetane numbers) are ideal for diesel engines, because diesel engines (also referred to as compression-ignition engines) do not compress the fuel, but rather compress only air and then inject fuel into the air which was heated by compression. Gasoline engines rely on ignition of air and fuel compressed together as a mixture, which is ignited at the end of the compression stroke using spark plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines. Use of gasoline with lower octane numbers may lead to the problem of engine knocking.
Research
Octane Number (RON)
The most common type of octane rating worldwide is the Research
Octane Number (RON). RON is determined by running the fuel in a
test engine with a variable compression ratio under controlled
conditions, and comparing the results with those for mixtures of
iso-octane and n-heptane.
Motor Octane
Number (MON)
Another type of octane rating, called Motor Octane Number (MON), is
determined at **0 rpm engine speed instead of the **0 rpm for
RON.[1] MON testing uses a similar test engine to that used in RON
testing, but with a preheated fuel mixture, higher engine speed,
and variable ignition timing to further stress the fuels knock
resistance. Depending on the composition of the fuel, the MON of a
modern pump gasoline will be about 8 to *2 octane lower than the
RON, but there is no direct link between RON and MON. Pump gasoline
specifications typically require both a minimum RON and a minimum
MON.[citation needed]
Anti-Knock
Index (AKI) or (R+M)/2
In most countries, including Australia, New Zealand and all of
those in Europe,[citation needed] the headline octane rating shown
on the pump is the RON, but in Canada, the United States, Brazil,
and some other countries, the headline number is the average of the
RON and the MON, called the Anti-Knock Index (AKI), and often
written on pumps as (R+M)/2). It may also sometimes be called the
Posted Octane Number (PON).
Difference
between RON, MON, and AKI
Because of the 8 to *2 octane number difference between RON and MON
noted above, the AKI shown in Canada and the United States is 4 to
6 octane numbers lower than elsewhere in the world for the same
fuel. This difference between RON and MON is known as the fuels
Sensitivity, and is not typically published for those countries
that use the Anti-Knock Index labelling system.
Observed Road
Octane Number (RdON)
Another type of octane rating, called Observed Road Octane Number
(RdON), is derived from testing gasolines in real world
multi-cylinder engines, normally at wide open throttle. It was
developed in the ***0s and is still reliable today. The original
testing was done in cars on the road but as technology developed
the testing was moved to chassis dynamometers with environmental
controls to improve consistency.
Octane
Index
The evaluation of the octane number by the two laboratory methods
requires a standard engine, and the test procedure can be both
expensive and time-consuming. The standard engine required for the
test may not always be available, especially in out-of-the-way
places or in small or mobile laboratories. These and other
considerations led to the search for a rapid method for the
evaluation of the anti-knock quality of gasoline. Such methods
include FTIR, near infrared on-line analyzers (ASTM D****5) and
others. Deriving an equation that can be used for calculating the
octane quality would also serve the same purpose with added
advantages. The term Octane Index is often used to refer to the
calculated octane quality in contradistinction to the (measured)
research or motor octane numbers. The octane index can be of great
service in the blending of gasoline. Motor gasoline, as marketed,
is usually a blend of several types of refinery grades that are
derived from different processes such as straight-run gasoline,
reformate, cracked gasoline etc. These different grades are
considered as one group when blending to meet final product
specifications. Most refiners produce and market more than one
grade of motor gasoline, differing principally in their anti-knock
quality. The ability to predict the octane quality of the blends
prior to blending is essential, something for which the calculated
octane index is specially suited.
Aviation
gasoline octane ratings
Aviation gasoline used in piston aircraft common in general
aviation have slightly different methods of measuring the octane of
the fuel. Similar to AKI, it has two different ratings, although it
is referred to only by the lower of the two. One is referred to as
the aviation lean rating and is the same as the MON of the fuel up
to **0. The second is the aviation rich rating and corresponds to
the octane rating of a test engine under forced induction operation
common in high-performance and military piston aircraft. This
utilizes a supercharger, and uses a significantly richer fuel/air
ratio for improved detonation resistance.
The most commonly used current fuel, **0LL, has an aviation lean rating of **0 octane, and an aviation rich rating of **0.
