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Alkanes: Sources and Relevance

Alkanes: Octane Number

The octane number is a measure of resistance of an engine fuel to autoignition in internal combustion (Otto) engines. Coming from the engine, knocking is a sharp, pinging sound which is caused by the combustion of the air-fuel mixture in the cylinder. Normally, the flame front moves from the point of ignition through the cylinder in an orderly fashion. This generates shock waves which, because of localized overheating, can lead to self-ignition of the air-fuel mixture outside the flame front. The generated shock wave moves opposite to the original one. The resulting knocking decreases engine efficiency and increases the risk of engine damage.

At high pressure and high temperature, branched and aromatic hydrocarbons are less susceptible to self-ignition than long-chain, linear alkanes. Therefore, the former are preferred as fuel for Otto engines.

The octane number is based on an arbitrary scale with zero assigned to n-heptane and 100 to 2,2,4-trimethylpentane (isooctane).

Fig.1
n-Heptane
Fig.2
Isooctane

Mixtures of both alkanes exhibit octane numbers between 0 and 100 depending on the ratio of the mixture. Accordingly, fuel which behaves during combustion in the engine like a mixture of 92% isooctane and 8% of n-heptane is assigned the octane number 92.

Two variants of the octane number are measured in special test engines.

MON: Motor Octane Number indicates the octane number measured at 900 rpm.

RON: Research Octane Number indicates the octane number measured at 600 rpm.

Tab.1
Examples
Compound/FuelRONMON
Butane9390
Isopentane9290
Benzene9991
Toluene124112
ortho-Xylene120103
para-Xylene146127
Methyl tert-butyl ether110.5
Super (unleaded)9585
Normal (unleaded)9182.5

Addition of antiknock additives greatly increases the octane ratings. Previously, the addition of tetraethyllead (leaded gasoline) was common.

Fig.3

Burning of tetraethyllead generates lead oxide which poisons catalytic converters. Therefore, by resolution of the EU starting in 2000 (2005 based on some rules for exceptions), the sale of leaded gasoline in member states was banned. The mode of action of tetraethyllead is based on the interception of free radicals which are formed as intermediates during combustion. As an alternative to using leaded gasoline, the quality of the fuel was raised, for example, by using catalytic reforming and by adding other additives, such as tert-butyl alcohol and methyl tert-butyl ether. Both of these additives do not negatively effect the efficiency of catalytic converters.

Fig.4

According to the ratings scale, methyl tert-butyl ether has an octane number of 110.5!!!

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