Octane number

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2,2,4-trimethylpentane or isooctane with octane number 100 (top) and n-heptane with octane number 0 (bottom)

The octane number defines a measure of the negative Zündunwilligkeit and thus for the anti-knock properties of a gasoline fuel or motor gasoline . The numerical value of the octane number up to 100 indicates how much volume percentage of ignition-retardant isooctane C 8 H 18 (RON = 100) must be in a mixture with ignitable n- heptane C 7 H 16 (RON = 0) in order for it to be the same Knock resistance (in a test engine according to RON or MON) like the fuel to be tested. For example, an octane number of RON = 95 (colloquially: 95 octane) of a gasoline would mean that its knock resistance corresponds to a mixture of 95% by volume isooctane and 5% by volume n- heptane.


There are many substances, such as some aromatics , natural gas and liquid gas , that have an octane number greater than 100. However, these are difficult to measure in terms of measurement technology, since the reference system with isooctane is only defined up to an octane number of 100. Octane numbers greater than 100 must therefore be extrapolated . The octane number above 100 RON / MON corresponds to the octane number of a mixture of iso- octane and tetraethyl lead (TEL) ; the octane number of the mixture is assigned to a certain volume fraction of TEL in iso- octane. This assignment is based on the table specified in DIN 51756 Part 1. Unlike the octane number up to 100, it cannot be read directly from the mixture ratio of the reference fuel. In this context, the term blende count is also used; in aviation it is more the COP, the latter given as a two-part “fraction” such as 115/145, which means that the fuel used here has a COP of 115 with a lean mixture and a COP of 145 with a rich mixture.

Dependence of the octane number (RON) on the ignition temperature

Isooctane is knock- proof , n- heptane quickly causes what is known as knocking in the engine . The reason for this is that the n -heptane ignites in an uncontrolled manner during the compression process due to the compression heat in the cylinder . Isooctane can be highly compressed without spontaneous ignition occurring . In the Otto engine , the gasoline-air mixture should be ignited by an ignition spark and burn down with a defined flame front (in the further development of the Otto engine with homogeneous compression ignition , the ignition spark is partially omitted).

You can differentiate between different octane numbers:

  • RON - R esearch- O ktan z ahl
  • MOZ - M otor- O ktan for ahl
  • SCO - S train path O ktan for ahl
  • FOZ - F ront- O ktan for many ahl, with RON 100 ° C referred

Octane number and efficiency

The increase in octane number went hand in hand with the further development of combustion engines. In the past, raw gasoline / naphtha was used as fuel, just like it is obtained from primary distillation. The engines developed after the Second World War needed more knock-resistant fuel. Greater compression can increase the efficiency of the engine and thus the specific power.

In order to increase the octane rating was in 1924 in the US, and from 1936 to 1996 in Germany gasoline tetraethyl lead added. As a radical scavenger, the lead prevents uncontrolled self-ignition of the fuel-air mixture during compression. It also has a lubricating effect on the valve seats. Because lead and its compounds are poisonous, the lead content of petrol was limited by law in the Federal Republic of Germany from 1971 , initially to 0.4 g / l and later to 0.15 g / l. In the 1980s, unleaded gasoline was introduced together with the exhaust gas catalytic converters because the lead additives would have rendered the catalytic converters ineffective. Finally, on January 1, 2000, leaded petrol was banned in the European Union . There were hardly any vehicles whose valve seats were designed for lead in fuel.

The different octane numbers of the fuels available at filling stations are due to the different uses of the components produced in an oil refinery . Premium gasoline contains more high-quality components than regular gasoline. High-quality components are generally more expensive to manufacture, so high-octane gasoline is more expensive.

Methyl tertiary butyl ether (MTBE) is often added to increase the knock resistance, up to 15% vol. Is allowed. Because of its poor degradability in water, MTBE is classified as hazardous to water ( WGK  1 = slightly hazardous to water). In a number of states in the USA, MTBE has already been "banned" from gasoline again. Nowadays, ethyl tertiary butyl ether (ETBE) is used more and more . ETBE offers several advantages over MTBE due to its higher boiling point and, as it is obtained from bio-ethanol, among other things, is of interest for tax purposes as a fuel component. Like MTBE, ETBE also has the disadvantage that it is difficult to break down in the groundwater.


Irregular ignition has been observed in engines since around 1912. The noise was known as "knocking", which quickly destroyed the engine. Initially, the new battery-powered, electrical ignition systems were assumed to be the cause. On closer examination it turned out that the knocking was related to the compression rate, which the engine engineers increased in order to achieve more power (relationship between the ignition temperature of the fuel and the temperature increase of the fuel during the reduction of the volume during compression. See Boyle's law ). Different measurement methods were tried, but due to the many variables (fuel composition, ignition timing, compression, engine temperature, cylinder design ...) none of the measurement methods prevailed.

In 1927, Graham Edgar had the idea that pure substances could be used as reference systems. Two substances were required (one with a strong knocking effect with a low ignition temperature and one that was resistant to knocking with a high ignition temperature), which could be produced in great purity and in sufficient quantities. Furthermore, these two substances should have very similar properties (melting and boiling point, density and evaporation properties). n- Heptane could be obtained in great purity by distillation and had very poor knocking properties. 2,2,4-Trimethylpentane (“iso-octane”) could be synthesized by the addition of isobutene to isobutane and purified by distillation and had very good knocking properties.

The fuels available at the time had knocking behavior, which could be represented by mixtures of 40:60 to 60:40 i-octane: n-heptane. So they can be characterized well with this system. Thus, before 1930, normal car gasoline had octane numbers of 40 to 60, but could be made more “compression-resistant” by adding large amounts of “potato fuel” or benzene or adding “lead tetraethyl” or iron carbonyls .

Octane numbers

Dispenser in the USA with five types of gasoline (octane number indicated as AON)

In Europe, petrol stations usually indicate the RON (Researched Octane Number) as the "Pump Octane Number", whereas in the USA it is usually the AON ("Average Octane Number"). The AON must be calculated using the formula AON = ( RON + MON ): 2, which is relatively time-consuming . Since the RON values ​​are higher than those of MOZ or AON and are even easier to calculate, the RON number has established itself at European petrol stations.

Researched (Explored) -Oktanzahl (RON)

The RON is determined using the single-cylinder CFR test method.

Both the MOZ and the RON are determined in the CFR engine (variable compression ratio) by comparison with a reference fuel made from isooctane (OZ = 100) and normal heptane (OZ = 0). The volume fraction of isooctane of the reference fuel, which has the same knock intensity as the fuel to be tested, is its octane number. The MOZ is usually lower than the RON, as it is determined at a higher engine speed and mixture preheating to approx. 149 ° C (300 ° Fahrenheit).

The RON determined using the research method (DIN EN ISO 5164) is intended to describe the knocking behavior at low engine load and low engine speeds.

Engine octane number (MOZ)

The "engine octane number" determined using the engine method (DIN EN ISO 5163) is intended to describe the behavior at high engine loads and high thermal loads. Here, the standard engine applies tougher conditions, namely instead of 600 now 900 revolutions per minute, an automatically adjustable ignition setting and mixture preheating to at least 149 ° C. This means that the MON is always less than or equal to the RON.

Octane numbers are determined in the CFR engine or BASF engine by comparison with a reference fuel made from isooctane (OZ = 100) and normal heptane (OZ = 0). The isooctane volume fraction of the reference fuel, which has the same knock intensity as the fuel to be tested, is its octane number.

The difference between RON and MON is as "sensitivity" (sensitivity) referred to and brings the temperature dependence of the octane number expressed. A high sensitivity means that the fuel reacts sensitively to higher thermal loads.

Street octane number (SOZ)

The comparison values ​​are measured under realistic conditions on the road. This takes you to the fuel's performance limit: consistently high engine speed at full throttle . So that the SCO value is comparable, it is subject to a standard.

Front Octane Number (FOZ)

The FOZ describes the octane number of the fuel components that boil up to approx. 100 ° C. The research octane number of the fuel components that have evaporated up to 100 ° C is determined (hence the name RON 100 ° C ). The FOZ thus describes the behavior of the fuel at low engine temperatures shortly after the engine has been started (cold start behavior).

Octane numbers of some basic substances

RON MOZ annotation
Heptane n -heptane 0 0 by definition
Isooctane 2,2,4-trimethylpentane 100 100 by definition
butane n -Butane 93.4 90.1
Isobutane Methyl propane 102 98
Pentane n -pentane 61.8 63.2
Isopentane 2-methylbutane 92.3 90.3
Neopentane 2,2-dimethylpropane 86 80
Cyclopentane   103 86
Hexane n -hexane 24.8 26.0
Isohexane 2-methylpentane 73.4 73.5
  3-methylpentane 74 74
Neohexane 2,2-dimethylbutane 94 93
Diisopropyl 2,3-dimethylbutane 102 101
Cyclohexane   83 77
benzene benzene 99 91
toluene Methylbenzene 124 112
Xylene o -xylene 120 102
Xylene m -xylene 145 124
Xylene p -xylene 146 127
Ethylbenzene Ethylbenzene 124 107
Ethanol   130 96
MTBE tert-butyl methyl ether 118 100
ETBE tert-butyl ethyl ether 118 102
Dicyclopentadiene Tricyclo [ 2,6 ] deca-3,8-diene 229 167

Octane number requirement of a gasoline engine

The octane rating describes the need for knock resistance of the fuel in an engine so that it does not lead to unwanted auto-ignition . The octane number required depends on the operating conditions of the engine (speed, temperature, combustion chamber geometry, compression ratio, mixture composition, air pressure, humidity, ignition point, deposits, etc.). In order for the engine to work properly, the octane number of the fuel must be so high that the engine's octane number is still met even under the most unfavorable operating conditions - for example, the octane number of an engine at full throttle can be 10 octane numbers higher than when idling. The use of octane numbers above the engine specification usually has no advantages. Modern engines with electronic map ignition in combination with knock sensors can be driven with different octane numbers with reduced power.

The octane requirement can be found either in the tank cap or in the operating instructions. The research octane number (RON) describes the behavior of the fuel in the engine at lower temperature and speed, the engine octane number (MOZ) describes the behavior of the fuel in the engine at high speed and high temperature range. One can also speak of a full load range here, comparable to driving on the motorway. In contrast to ours, in the USA there are the values ​​AON and RON. This is calculated using AON = (RON + MON) / 2 and accordingly differs from the values ​​known to us with 91, 95 or 98 octane.

Octane numbers of petrol

variety RON MOZ AEON annotation
normal at least 91 82.5 86.75
Super 95 E5 at least 95 85 90
Super 95 E10 at least 95 85 90 Fuel with 10% ethanol
SuperPlus 98 at least 98 88 93
BP Ultimate Super at least 98 88 93
OMV MaxxMotion Super 100plus at least 100 88 94
Shell V-Power 100+ at least 100 88 94
Aral / BP Ultimate 102 at least 102 88 95
MoGas at least 98 88 93 Premium gasoline approved for flight operations
AvGas UL91 at least 96 91 93.5 Aviation fuel, unleaded
AvGas 100LL at least 130 100 115 Standard aviation fuel, leaded
LPG 103-111 97-99 100-105
natural gas
Ethanol E85 (fuel with 85% ethanol ) at least 107
Formula 1 gasoline maximum 102 earlier to 108 RON

In Austria , OMV AG introduced Super Plus with 100 RON in 2004, in Switzerland BP is also introducing Super Plus with 100 RON. This has already been changed in many cases in Germany , and Ultimate102 is now also listed there.


  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3 .
  • Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise. 5th edition, Holland + Josenhans Verlag, Stuttgart, 1997, ISBN 3-7782-3520-6 .
  • Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4 .
  • Kurt-Jürgen Berger, Michael Braunheim, Eckhard Brennecke: Technology automotive engineering. 1st edition, Verlag Gehlen, Bad Homburg vor der Höhe, 2000, ISBN 3-441-92250-6 .

See also

Web links

Wiktionary: Octane number  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. MTBE ban ( memento of July 26, 2009 in the Internet Archive ) (PDF; 226 kB).
  2. a b c d e f g h i j k l m n Entry for octane number. In: Römpp Online . Georg Thieme Verlag, accessed on December 26, 2012.
  3. a b Fuels for general aviation. Retrieved December 11, 2019 .