Octane Number

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Historical View

there was a demand for more powerful engines.

So they added more and larger pistons to the engines :some were even built with 16 cylinders that means we will increase the size of the engines to increase their power and this is not a practical method.

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Practical Method

The obvious method to get more power from an engine of a given size was to increase its compression ratio (a measure of the extent to which the gasoline–air mixture is compressed in the cylinder of an engine) The more the mixture is compressed before ignition, the more power the engine can deliver.

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•but the increased performance was accompanied by an increased tendency for detonation or knocking.

• The auto ignition of fuels causes a “knocking”.

• it ignites twice; once due to the high pressure and again when the spark ignites the gasoline. This causes the car engine to be less efficient and it can also damage the engine.

• Gasoline causes ignition.By/A.Nagy&A.Mostafa

Conclusion• Gasoline performance and hence quality of an

automobile gasoline is determined by its resistance to knock ”detonation during service”.

• The antiknock quality of the fuel limits the power and economy that an engine using that fuel can produce: the higher the antiknock quality of the fuel, the more the power and efficiency of the engine.

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• in 1933 by the general use of a single-cylinder test engine, which allowed comparisons of the antiknock characteristics of gasoline to be made in terms of octane numbers.

• The octane numbers formed a scale ranging from 0 to 100: the higher the number, the greater the antiknock characteristics.

• In 1939, a second and less severe test procedure using the same test engine was developed, and results obtained by this test were also expressed in octane numbers.

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• Octane numbers are obtained by two test procedures; those obtained by the first method are called motor octane numbers (indicative of high-s peed performance).

• and those obtained by the second method are called research octane numbers (indicative of normal road performance).

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• In the test methods used to determine the antiknock properties of gasoline, comparisons are made with blends of two pure hydrocarbons, n-heptane and iso-octane (2,2,4-trimethyl-pentane).Iso-octane has an octane number of 100 and is high in its resistance to knocking ;n-heptane is quite low (with an octane number of 0) in its resistance to knocking.

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Definition

• The percentage of iso-octane in that blend is termed the octane number of the gasoline. For example, gasoline with a knocking ability that matches that of a blend of 90% iso-octane and 10% n-heptane has an octane number of 90

• This system has been done by Russell Marker.

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What affecet the number of octane?

1. Octane number decreases with an increase in the carbon chain length.

• normal paraffins have low octane numbers and these become lower as the molecular weight increases.

2. Octane number increases with carbon chain branching.

• Iso-paraffins have higher octane numbers than the corresponding normal

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3. Octane number increases in aromatics with same number of carbons.

Olefins have markedly higher octane numbers than the related paraffins.

naphthenes are usually better than the corresponding normal paraffins but rarely have very high octane numbers.

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How octane number of a fuel can be increased?

1. By putting special additives into the fuel which discourage auto ignition.

2. By blending high-octane fuels in with the ordinary petrol.

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Anti-knocking additives

• Anti-knocking additives are substances which reduce the tendency of a fuel to auto-ignite, and so increase the octane number.

• small amounts of lead compounds have been used as economical and effective anti-knock additives. tetraethyl lead

• But it damage the environment.

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Additives

TolueneXylenetertiary-butyl-ether (MTBE)Methanol or EthanolAlcohol and Tertiary Butyl Alcohol

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Properties Of fuel mixed with gasoline

The shorter the alkane chain, the higher the octane number. However, the volatility of the hydrocarbon increases with a decrease in the chain length.

The short alkane chains have to be mixed in petrol to lower their volatility and higher the octane number of the petrol.

The more branched the alkane is, the higher the octane number.

that cracked gasoline caused less knock than straight-run gasoline.

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IS THERE AN OCTANE NUMER MORE THAN 100?

YES• many pure hydrocarbons and even

commercial gasoline have antiknock quality above an octane number of 100. In this range it is common practice to extend the reference values by the use of varying amounts of tetraethyl lead other additives in pure iso-octane.

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THANK YOU FOR YOUR ATTENTION

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A. Nagy