Octane rating
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A US gas station pump offering five different (R+M)/2 octane ratings
Definition: A value used to indicate the resistance of a motor fuel to knock.
Octane numbers are based on a scale on which isooctane is 100 (minimal knock)
and heptane is 0 (bad knock).
Octane is a gas 2,2,4-trimethylpentane (iso-octane) and heptane which is used as a
reference standard to describe the tendency of Gasoline, Petrol, or Benzin fuels to self
ignite during compression prior to the desired position of the piston in the cylinder as
appropriate for valve and ignition timing. The problem of premature ignition is
referred to as pre-ignition and also as engine knock, which is a sound that is made
when the fuel ignites too early in the compression stroke.
Severe knock causes severe engine damage, such as broken connecting rods, melted
pistons, melted or broken valves and other components. The Octane rating is a
measure of how likely a gasoline or liquid petroleum fuel is to self ignite. The higher
the number, the less likely an engine is to pre-ignite and suffer damage. A higher
Octane rating is better for engine life.
The most typically used engine management systems found in automobiles today
monitor the level of knock that is being produced by the fuel being used. Many high
performance engines require a rating of their fuel at 93 Octane. They will however
run on lower octane fuels as necessary. The timing of the ignition will be
automatically altered by the fuel management system to reduce the pre-ignition to an
acceptable level.
The octane rating of Gasoline is measured in a test engine, and is defined by
comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptane
which would have the same anti-knocking capacity as the fuel under test: the
percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number
of the fuel. For example, petrol with the same knocking characteristics as a mixture of
90% iso-octane and 10% heptane would have an octane rating of 90.[1] This does not
mean that the petrol contains just iso-octane and heptane in these proportions, but that
it has the same detonation resistance properties. Because some fuels are more knockresistant 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. Where the octane number is raised by blending
in ethanol, energy content per volume is reduced.
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 may have
octane ratings of 110 or significantly higher. Typical "octane booster" gasoline
additives include MTBE, ETBE, isooctane and toluene. Lead in the form of tetraethyl 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.
Contents
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1 Measurement methods
o 1.1 Research Octane Number (RON)
o 1.2 Motor Octane Number (MON)
o 1.3 Anti-Knock Index (AKI)
o 1.4 Difference between RON and AKI
o 1.5 Observed Road Octane Number (RdON)
2 Examples of octane ratings
3 Effects of octane rating
4 Regional variations
5 See also
6 References
7 Further reading
8 External links
[edit] Measurement methods
[edit] 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.
[edit] 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 determined at 900 rpm engine speed, instead of the 600 rpm for
RON.[2][3] 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 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,
however there is no direct link between RON and MON. Normally, fuel specifications
require both a minimum RON and a minimum MON.[citation needed]
[edit] Anti-Knock Index (AKI)
In most countries, including Australia and all of those in Europe, the "headline"
octane rating shown on the pump is the RON, but in Canada, the United States and
some other countries, like Brazil, 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 Pump Octane Number (PON).
[edit] 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.
[edit] Observed Road Octane Number (RdON)
The final 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 1920s 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.[4]
[edit] Examples of octane ratings
The RON and MON of n-heptane and iso-octane are exactly 0 and 100, by definition.
The following table lists octane ratings for various other fuels.[5][6]
Fuel
hexadecane
n-octane
n-heptane (RON and MON 0 by definition)
diesel fuel
2-methylheptane
n-hexane
1-pentene
2-methylhexane
3-methylhexane
1-heptene
n-pentane
requirement for a typical two-stroke outboard
engine[7]
Pertamina "Premium" gasoline in Indonesia
n-butanol
2,2-dimethylpropane
"regular" gasoline in Australia, New Zealand,
Canada and the US
Pertamina "Pertamax" gasoline in Indonesia
Shell "Super" in Indonesia
RON
< -30
-10
0
15–25
23
25
34
44
MON
AKI
0
0
23.8
26.0
26
46.4
55.0
60
62
61.9
69
65
67
71
80.2
83
82–83
87
88
92
91–92
92
92
n-butane
2-methylbutane
Pertamina "Pertamax Plus" gasoline in Indonesia
Shell "Super Extra" in Indonesia
Shell "FuelSave " in Malaysia
"EuroSuper" or "EuroPremium" or "Regular
unleaded" in Europe
Shell "V-Power 97" in Malaysia
"SuperPlus" in Germany, Great Britain, Slovenia
and Spain, "SP98" in France
2,2-dimethylbutane
2,3-dimethylbutane
ExxonMobil Avgas 100[9]
Shell "V-Power Racing" in Australia discontinued July 2008 [10]
"isooctane" (RON and MON 100 by definition)
benzene
i-butane
"BP Ultimate 102 - now discontinued"[12]
t-butanol
2,3,3-trimethylpentane
ethane
2,2,3-trimethylpentane
toluene
94[8]
90.1
90.3
95
95
95
95
85–86
90–91
89–90
93–94
97
98
93.4
94.4
99.5 (min)
100
100
101
102[11]
102
103
106.1[13]
108
109.6[13]
111
100
100
97.6
93–94
91
99.4[13]
97–98
97
103
99.9[13]
95
E85 gasoline
propane
2,2,3-trimethylbutane
xylene
isopropanol
methanol
ethanol
2,5-Dimethylfuran
methane
hydrogen*
*Hydrogen
112
112.1[13]
117
118
108.7[15]
108.6[15]
119
120
> 130
105
103
9496[14]
97
101.3[13]
106
98
88.6[15]
89.7[15]
108
98.65
99.15
120
very
low[16]
120
does not fit well into the normal definitions of octane number. It has a
very high RON and a low MON,[16] so that it has low knock resistance in practice,[17]
due to its low ignition energy