A US gas station pump offering five
different AKI octane ratings
The
octane rating is a measure of the resistance
of
gasoline and other
fuels to
Autoignition_temperature in
spark-ignition internal combustion
engines.
The octane number of a fuel is measured in a test engine, and is
defined by comparison with the mixture of
iso-octane and
heptane
which would have the same anti-knocking capacity as the fuel under
test: the percentage, by volume, of
iso-octane in that mixture is the octane number
of the fuel. For example, gasoline with the same
knocking characteristics as a mixture of 90%
iso-octane and 10%
heptane would have an octane rating of 90. This does
not mean that the gasoline contains just iso-octane and heptane in
these proportions, but that it has the same detonation resistance
properties. Because some fuels are more knock-resistant than
iso-octane, the definition has been extended to allow for octane
numbers higher than 100.
Octane rating does not relate to the energy content of the fuel
(see
heating value). It is only a
measure of the fuel's tendency to burn in a controlled manner,
rather than exploding in an uncontrolled manner.
It is possible for a fuel to have a Research Octane Number (RON)
greater than 100, because iso-octane is not the most
knock-resistant substance available. Racing fuels,
AvGas,
liquefied
petroleum gas (LPG), and
alcohol
fuels such as
methanol or
ethanol may have octane ratings of 110 or
significantly higher — ethanol's RON is 129 (102 MON,
116 AKI). Typical "octane booster"
gasoline additives include
MTBE,
ETBE,
isooctane and
toluene.
Lead in the form of
tetra-ethyl lead was once a common
additive, but since the 1970s, its use in most of the
industrialised world has been restricted, and its use is currently
limited mostly to
aviation
gasoline.
Measurement methods
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)
There is another type of octane rating, called
Motor Octane
Number (
MON), or the aviation lean octane
rating, which is a better measure of how the fuel behaves when
under load as it is done at 900 rpm instead of the 600 rpm of the
RON. MON testing uses a similar test engine to that used in RON
testing, but with a preheated fuel mixture, a higher engine speed,
and variable
ignition timing to
further stress the fuel's knock resistance. Depending on the
composition of the fuel, the MON of a modern gasoline will be about
8 to 10 points lower than the RON. Normally, fuel specifications
require both a minimum RON and a minimum MON.
Anti-Knock Index (AKI)
In most
countries, including all of those of Australia and Europe the
"headline" octane rating shown on the pump is the RON, but in
Canada
, the United States
and some other countries, the headline number is
the average of the RON and the MON, called the Anti-Knock
Index (AKI). It may also sometimes
be called the
Road Octane Number
(
RdON),
Pump Octane Number
(
PON), or
(R+M)/2.
Difference between RON and AKI
Because of the 8 to 10 point difference noted above, the octane
rating shown in the United States is 4 to 5 points lower than the
rating shown elsewhere in the world for the same fuel. See the
table in the following section for a comparison.
Examples of octane ratings
The MON of n-
heptane and iso-octane are
exactly 0 and 100, by definition. The following table lists octane
ratings for various other fuels.[10]
*Hydrogen does not fit well into the normal definitions of
octane number. It has a very high RON and a low MON, so that it has
low knock resistance in practice, due to its low ignition energy
(primarily due to its low dissociation energy) and extremely high
flame speed. These traits are highly desirable in rocket engines,
but undesirable in Otto-cycle engines. However, as a minor blending
component (e.g. in a
bi-fuel vehicle),
hydrogen raises overall knock resistance. Flame speed is limited by
the rest of the component species; hydrogen may reduce knock by
contributing its high thermal conductivity
Effects of octane rating
Higher octane ratings correlate to higher
activation energies. Activation energy is
the amount of energy necessary to start a chemical reaction. Since
higher octane fuels have higher activation energies, it is less
likely that a given compression will cause autoignition.
It might seem odd that fuels with higher octane ratings are used in
more powerful engines, since such fuels explode less easily.
However, an uncontrolled explosion is not desired in an internal
combustion engine. The fuel must be fired at a precise time. An
explosion too early will cause the resulting forces to try to turn
the crankshaft in the reverse direction. This will not cause the
engine to rotate in the reverse direction because of the
kinetic energy in the
rotating assemblies and the flywheel,
but will
strain the
crankshaft. This strain is the source of the
characteristic 'ping' noise heard during
detonation. This reduces power output, because
much of the energy is absorbed as
strain and
heat in
parts of the engine, rather than being converted to
torque at the
crankshaft.
A fuel with a higher octane rating can be run at a higher
compression ratio without causing detonation. Compression is
directly related to power (see
engine
tuning), so engines that require higher octane usually deliver
more
motive power. Engine power is a
function of the fuel, as well as the engine design, and is related
to octane rating of the fuel. Power is limited by the maximum
amount of fuel-air mixture that can be forced into the
combustion chamber. When the
throttle is partially open, only a small fraction
of the total available power is produced because the
manifold is operating at
pressures far below atmospheric. In this case, the octane
requirement is far lower than when the throttle is opened fully and
the
manifold pressure increases to
atmospheric pressure, or higher in the case of
supercharged or
turbocharged engines.
Many high-performance engines are designed to operate with a high
maximum compression, and thus demand high-octane premium gasoline.
A common misconception is that power output or fuel mileage can be
improved by burning higher octane fuel than a particular engine was
designed for. The power output of an engine depends in part on the
energy density of its fuel, but similar fuels with different octane
ratings have similar density. Since switching to a higher octane
fuel does not add any more hydrocarbon content or oxygen, the
engine cannot produce more power.
However, burning fuel with a lower octane rating than required by
the engine often reduces power output and efficiency one way or
another. If the engine begins to detonate (knock), that reduces
power and efficiency for the reasons stated above. Many modern car
engines feature a
knock sensor – a
small
piezoelectric microphone which
detects knock, and then sends a signal to the
engine control unit to retard the
ignition timing. Retarding the
ignition timing reduces the tendency to detonate, but also reduces
power output and fuel efficiency.
Most fuel stations have two storage tanks (even those offering 3 or
4 octane levels), and you are given a mixture of the higher and
lower octane fuel. Purchasing premium simply means more fuel from
the higher octane tank. The detergents in the fuel are the
same.
The octane rating was developed by chemist
Russell Marker at the
Ethyl Corporation c1926. The selection of
n-
heptane as the zero point of the
scale was due to the availability of very high purity
n-heptane, not mixed with other
isomers of heptane or
octane,
distilled from the
resin of the
Jeffrey Pine. Other sources of heptane produced
from crude oil contain a mixture of different isomers with greatly
differing ratings, which would not give a precise zero point.
Regional variations
The selection of octane ratings available at the pump can vary
greatly from region to region.
- Australia, "regular" unleaded fuel is
91 RON, "premium" unleaded with 95 RON is widely
available, and 98 RON fuel is also reasonably common. Shell
used to sell 100 RON petrol from a small number of service
stations, most of which are located in capital cities (stopped in
August 2008).
- Italy
, 95 RON
is the regular gasoline offered (verde), and most fuel stations
offer 98 RON as the premium type (Super/Blu Super), many Shell
stations close to the cities offer also V-Power Gasoline rated at
100 RON
- Malaysia
, the
"regular" unleaded fuel is 95 RON, "premium" fuel is rated at
97 RON, and Shell's V-Power at 98 RON.
- Netherlands, Shell V-Power is a 97 RON (labelled as 95 due
to the legalities of only using 95 or 98 labelling), whereas in
neighbouring Germany Shell V-Power consists of the regular
100 RON fuel.
- Ireland
, 95 RON is the only petrol type available
through stations.
- Russia
and CIS countries, 80 RON (76 MON) is the minimum
available, the standard is 92 RON, however, the most used type
is 95 RON.
- South Africa, "regular" unleaded
fuel is 95 RON in coastal areas with most fuel stations optionally
offering 97 RON. Inland (higher altitude) "regular" unleaded fuel
is 93 RON, once again most fuel stations optionally offer 95
RON.
- United Kingdom
, 'regular' petrol has an octane rating of
95 RON, with 97 RON fuel being widely available as the
Super Unleaded. Tesco
and Shell both offer 99 RON fuel.
BP is currently trialling the public selling of
the super-high octane petrol BP Ultimate Unleaded 102, which as
the name suggests, has an octane rating of 102 RON. Although
BP Ultimate Unleaded (with an octane rating of 97 RON) and BP
Ultimate Diesel are both widely available throughout the UK, BP
Ultimate Unleaded 102 is (as of October 2007) only available
throughout the UK in 10 filling stations, and is priced at about
two and half times more than their 97 RON fuel. Also offered Shell
V-Power, but in a 99 RON octane rating, and Tesco fuel
stations also supply the Greenergy
produced 99 RON "Tesco 99".
- United States, in the Rocky Mountain (high altitude) states,
85 AKI is the minimum octane, and 91 AKI is the maximum
octane available in fuel. The reason for this is that in
higher-altitude areas, a typical naturally-aspirated engine draws
in less air mass per cycle due to the reduced density of the
atmosphere. This directly translates to less fuel and reduced
absolute compression in the cylinder, therefore deterring knock. It
is safe to fill up a carbureted car that normally takes 87 AKI
fuel at sea level with 85 AKI fuel in the mountains, but at
sea level the fuel may cause damage to the engine. A disadvantage
to this strategy is that most turbocharged vehicles are unable to
produce full power, even when using the "premium" 91 AKI fuel.
In some east coast states, up to 94 AKI is available [22569]. In parts of the Midwest (primarily
Minnesota, Iowa, Illinois and Missouri) ethanol based E-85 fuel with
105 AKI is available [22570]. Often, filling stations near US racing
tracks will offer higher octane levels such as 100 AKI.
California fuel stations will offer 87, 89, and 91 AKI octane
fuels, and at some stations, 100 AKI or higher octane, sold as
racing fuel. Until summer 2001 before the phase-out of methyl
tert-butyl ether aka MTBE as an
octane enhancer additive, 92 AKI was
offered in lieu of 91.
Generally, octane ratings are higher in
Europe than they are in
North America and most other parts of the
world. This is especially true when comparing the lowest available
octane level in each country. In many parts of Europe, 95 RON
(90-91 AKI) is the minimum available standard, with
97/98 RON being higher specification (being called
Super
Unleaded). The higher rating seen in Europe is an artifact of
a different underlying measuring procedure. In most countries
(including all of Europe and Australia) the "headline" octane that
would be shown on the pump is the
RON, but in
Canada, the United States and some other countries the headline
number is the average of the
RON and the
MON, sometimes called the Anti-Knock Index (AKI),
Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.
Because of the 8 to 10 point difference noted above, this means
that the octane in the United States will be about 4 to 5 points
lower than the same fuel elsewhere: 87 octane fuel, the "regular"
gasoline in Canada and the US, would be 91-92 in Europe. However
most European pumps deliver 95 (RON) as "regular", equivalent
to 90–91 US AKI=(R+M)/2, and deliver 98, 99 or
100 (RON) (93-94 AKI) labeled as
Super Unleaded -
thus
regular petrol sold in much of Europe corresponds to
premium sold in the United States.
In other countries "regular" unleaded gasoline, when available, is
sometimes as low as 85 RON (still with the more regular fuel,
95, and premium, around 98, available).
See also
References
- http://www.texacoursa.com/glossary/r.html
- http://www.texacoursa.com/glossary/m.html
- Petroleum and Coal
-
http://www.iupac.org/publications/pac/1983/pdf/5502x0199.pdf
- Johnson Operation and Maintenance Manual, 1999
- BP Ultimate 102
- LIQUID HYDROGEN AS A PROPULSION FUEL,1945-1959
- SAE standard J 1297 Alternative Automotive Fuels, Sept
2002
- Khoo, Kenny K. Understanding Octane and its Related Components.
Yellowknife: Smithsonian Press, 2006.
External links
Octane ratings of some hydrocarbons
Information in general
, big
suppliers like Shell or Aral offer 100 RON gasoline (Shell V-Power, Aral Ultimate) at almost every
fuel station.
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