Octane Number (RON, MON) & Knock Resistance
The octane number (ON) is a measure of the knock resistance of gasoline. It defines a numerical
value from 0 to 100, and describes the behavior of the fuel in the engine during combustion. In
determining the octane number, a distinction is primarily made between the research octane
number (RON) and the motor octane number (MON). Reference is also sometimes made to the
front octane number (FON), also called RON 100, and the observed road octane number (RdON).
Research octane number (RON)
The research octane number (RON) describes the behavior of the fuel in the engine at lower
temperatures and speeds, and is an attempt to simulate acceleration behavior. This octane
number is posted on pumps in Germany. According to DIN EN 228, the RON of a Super gasoline
must be at least 95.0 and at least 98.0 for a Super Plus gasoline. The RON is usually higher than
the specified required minimum.
Motor octane number (MON)
The motor octane number (MON) describes the behavior of the fuel in the engine at high
temperatures and speeds – a full-throttle range, comparable to driving fast on a highway. This
octane number is not generally known to the public, as it is not specified at service stations.
According to DIN EN 228, the MON for a Super gasoline must be at least 85.0 and at least 88.0 for
a Super Plus gasoline. Due to the nature of the product, the MON is usually at the limit of the
specification.
Knock resistance
Knock resistance is a fuel’s ability not to self-ignite and burn in an uncontrolled way while the fuel
is being compressed. This means that the air-fuel mixture in the engine is not ignited only by the
ignition spark, but also by compression. An octane number describes this phenomenon under
defined conditions.
A high octane number can help increase the efficiency and thus performance of an engine.
However, the octane number is not a measure of energy content or better combustion. More
performance can only be achieved by adjusting the engine parameters to the fuel, not simply by
fuelling with higher-octane gasoline. Diesel fuel in the tank due to misfuelling reduces the octane
number and the continuous mixing of the gas – which is why just a few drops of diesel fuel in the
gasoline may cause problems of the drive mode.
Determining the octane numbers
The octane numbers (RON, MON) can be determined using one or more comparison fuels that are
blended in the test lab. The check indicator of a test engine is used to show whether the
comparison fuel and the sample have the same knock resistance. A “CFR test engine” can be used
for this test. A CFR test engine is a 4-stroke gasoline engine with an adjustable compression
cylinder. It takes its name from the Cooperative Fuel Research Committee, which developed the
method and the engine. A CFR test engine has up to four tanks, which can be individually switched
with the engine running.
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From Wikipedia, the Free Encyclopedia
An octane rating, or octane number, 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.
The problem: pre-ignition and knocking
In a normal spark-ignition engine, the air-fuel mixture is heated because of being compressed and
is then triggered to burn rapidly by the spark plug. During the combustion process, if the unburnt
portion of the fuel in the combustion chamber is heated (or compressed) too much, pockets of
unburnt fuel may self-ignite (detonate) before the main flame front reaches them. Shockwaves
produced by detonation can cause much higher pressures than engine components are designed
for, and can cause a "knocking" or "pinging" sound. Knocking can cause major engine damage if
severe.
The most typically used engine management systems found in automobiles today have a knock
sensor that monitors if knock is being produced by the fuel being used. In modern computer-
controlled engines, the ignition timing will be automatically altered by the engine management
system to reduce the knock to an acceptable level.
Isooctane as a reference standard
Octanes are a family of hydrocarbons that are
typical components of gasoline. They are
colorless liquids that boil around 125 °C (260
°F). One member of the octane family,
isooctane, is used as a reference standard to
benchmark the tendency of gasoline or LPG
fuels to resist self-ignition.
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-
Figure 1 octane) and heptane that would have the same
2,2,4-Trimethylpentane (iso-octane) (upper) has an anti-knocking capacity as the fuel under test:
octane rating of 100 whereas n-heptane has an octane
rating of 0. the percentage, by volume, of 2,2,4-
trimethylpentane in that mixture is the octane
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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. A rating of 90 does not mean
that the gasoline contains just iso-octane and heptane in these proportions but that it has the
same detonation resistance properties (generally, gasoline sold for common use never consists
solely of iso-octane and heptane; it is a mixture of many hydrocarbons and often other additives).
Because some fuels are more knock-resistant than pure iso-octane, the definition has been
extended to allow for octane numbers greater than 100.
Octane ratings are not indicators of the energy content of fuels. (See Effects below and Heat of
combustion). They are 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. Ethanol BTUs can be compared with gasoline
BTUs in heat of combustion tables.
It is possible for a fuel to have a Research Octane Number (RON) more than 100, because iso-
octane is not the most knock-resistant substance available. Racing fuels, avgas, 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
tetraethyllead was once a common additive, but its use for fuels for road vehicles has been
progressively phased-out worldwide, beginning in the 1970s.
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)
Another type of octane rating, called Motor Octane Number (MON), is determined at 900 rpm
engine speed instead of the 600 rpm for RON. 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 pump gasoline will be about 8 to 12 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.
Anti-Knock Index (AKI) or (R+M)/2
In most countries in Europe (also in Australia, Pakistan and New Zealand) 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 simple mean or 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).
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Difference between RON, MON, and AKI
Because of the 8 to 12 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 fuel's 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 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.
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 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 especially suited.
Aviation gasoline octane ratings
Aviation gasolines used in piston aircraft engines common in general aviation have a slightly
different method of measuring the octane of the fuel. Similar to an 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 100. 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, 100LL, has an aviation lean rating of 100 octane, and an
aviation rich rating of 130.
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The RON/MON values of n-heptane and iso-octane are exactly 0 and 100, respectively, by the definition
of octane rating. The following table lists octane ratings for various other fuels.
Fuel RON MON AKI or (R+M)/2
hexadecane < −30
hydrogen > 130
diesel fuel 15–25
1-pentene 34
1-heptene 60
"Regular gasoline" in Japan (Japanese Industrial Standards) 90
benzene 101
ethane 108
n-octane −20 −17 -18.5
n-heptane (RON and MON 0 by definition) 0 0 0
2-methylheptane 23 23.8 23
n-hexane 25 26.0 26
2-methylhexane 44 46.4 45.2
3-methylhexane 55.0
n-pentane 62 61.9 62
n-butanol 92 71 83
Neopentane (dimethylpropane) 80.2
"Aral Super 95" in Germany, "Aral Super 95 E10" (10%
95 85 90
Ethanol) in Germany
"EuroSuper" or "EuroPremium" or "Regular unleaded" in
95 85–86 90–91
Europe, "SP95" in France, "Super 95" in Belgium
E85 gasoline 102-105 85-87 94-96
"Premium" or "Super unleaded" gasoline in US (10% ethanol
97 87-88 92-93
blend)
"Shell V-Power Nitro+ 99" "Tesco Momentum 99" In the
99 87 93
United Kingdom
"Eni (or Agip) Blu Super +(or Tech)" in Italy 100 87 94
"SuperPlus" in Germany 98 88 93
"Shell V-Power" in Italy and Germany 100 88 94
Petro-Canada "Ultra 94" in Canada 101.5 88 94
Aral Ultimate 102 in Germany 102 88 95
Petrobras Podium in Brazil 102 88 97
2,5-Dimethylfuran 101.3 88.1 94.7
methanol 108.7 88.6 98.65
"Shell V-Power 98", "Caltex Platinum 98 with Techron", "Esso
Mobil Synergy 8000" and "SPC LEVO 98" in Singapore, "BP
Ultimate 98/Mobil Synergy 8000" in New Zealand, "SP98" in 98 89–90 93–94
France, "Super 98" in Belgium, Great Britain, Slovenia and
Spain
ethanol 108.6 89.7 99.15
n-butane 94 90.1 92
Isopentane (methylbutane) 90.3
t-butanol 103 91 97
"BP Ultimate 102 - now discontinued" 102 93–94 97–98
2,2-dimethylbutane 93.4
2,3-dimethylbutane 94.4
propane 112 97 105
i-butane 102 97.6 100
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1-propanol 118 98 108
isopropanol 118 98 108
ethylbenzene 112 99 106
2,3,3-trimethylpentane 106.1 99.4 103
ExxonMobil Avgas 100 99.5 (min)
2,2,3-trimethylpentane 109.6 99.9 105
"isooctane" (RON and MON 100 by definition) 100 100 100
2,2,3-trimethylbutane 112.1 101.3 106
isopropylbenzene (cumene) 112 102 107
toluene 121 107 114
xylene 118 115 116.5
VP C16 Race Fuel 117 118 117.5
Effects
Higher octane ratings correlate to higher activation energies: the amount of applied energy
required to initiate combustion. Since higher octane fuels have higher activation energy
requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise
known as auto ignition or detonation.
Because octane is a measured and/or calculated rating of the fuel's ability to resist auto ignition,
the higher the octane of the fuel, the harder that fuel is to ignite and the more heat is required to
ignite it. The result is that a hotter ignition spark is required for ignition. Creating a hotter spark
requires more energy from the ignition system, which in turn increases the parasitic electrical load
on the engine. The spark also must begin earlier in order to generate sufficient heat at the proper
time for precise ignition. As octane, ignition spark energy, and the need for precise timing
increase, the engine becomes more difficult to "tune" and keep "in tune". The resulting sub-
optimal spark energy and timing can cause major engine problems, from a simple "miss" to
uncontrolled detonation and catastrophic engine failure.
The other rarely-discussed reality with high-octane fuels associated with "high performance" is
that as octane increases, the specific gravity and energy content of the fuel per unit of weight are
reduced. The net result is that to make a given amount of power, more high-octane fuel must be
burned in the engine. Lighter and "thinner" fuel also has a lower specific heat, so the practice of
running an engine "rich" to use excess fuel to aid in cooling requires richer and richer mixtures as
octane increases.
Higher-octane, lower-energy-dense "thinner" fuels often contain alcohol compounds incompatible
with the stock fuel system components, which also makes them hygroscopic. They also evaporate
away much more easily than heavier, lower-octane fuel which leads to more accumulated
contaminants in the fuel system. Its typically the hydrochloric acids that form due to that water
and the compounds in the fuel that have the most detrimental effects on the engine fuel system
components, as such acids corrode many metals used in gasoline fuel systems.
During the compression stroke of an internal combustion engine, the temperature of the air-fuel
mix rises as it is compressed, in accordance with the ideal gas law. Higher compression ratios
necessarily add parasitic load to the engine, and are only necessary if the engine is being
specifically designed to run on high-octane fuel. Aircraft engines run at relatively low speeds and
are "undersquare". They run best on lower-octane, slower-burning fuels that require less heat and
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a lower compression ratio for optimum vaporization and uniform fuel-air mixing, with the ignition
spark coming as late as possible in order to extend the production of cylinder pressure and torque
as far down the power stroke as possible. The main reason for using high-octane fuel in air-cooled
engines is that it is more easily vaporized in a cold carburetor and engine and absorbs less intake
air heat which greatly reduces the tendency for carburetor icing to occur.
With their reduced densities and weight per volume of fuel, the other obvious benefit is that an
aircraft with any given volume of fuel in the tanks is automatically lighter. And since many
airplanes are flown only occasionally and may sit unused for weeks or months, the lighter fuels
tend to evaporate away and leave behind fewer deposits such as "varnish". Aircraft also typically
have dual "redundant" ignition systems which are nearly impossible to tune and time to produce
identical ignition timing so using a lighter fuel that's less prone to auto ignition is a wise "insurance
policy". For the same reasons, those lighter fuels which are better solvents are much less likely to
cause any "varnish" or other fouling on the "backup" spark plugs.
Because of the high cost of unleaded, high-octane avgas, and the tendency of piston-engine
aircraft to be recreational vehicles and the potential for fuel loss via evaporation/theft/leaks, most
general aviation pilots attempt to save money by tuning their fuel-air mixtures and ignition timing
to run "lean of peak".
That practice can be fine for "cruising" straight and level and in smooth air, but in case of the need
to rapidly add power (sudden climb, headwind, etc.), the additional "insurance policy" of excess
octane in avgas helps prevent dangerously lean fuel-air mixtures from rapidly melting and
physically "burning" the aluminum pistons in an air-cooled aircraft engine. With only fuel,
lubricating oil and airflow to cool the engine and a "lean of peak" mixture with no "excess" fuel for
cooling, the engine is only a few seconds of dangerously lean mixtures, autoignition and
detonation away from catastrophic failure.
