Octane Number

Octane number structure gained popularity in America during the mid-to-late 1960s when gasoline companies advertised “high octane” levels. 

The majority of hydrocarbons are found in petroleum, including Octane. Octane is composed of 18 structural isomers. Nature and the earth’s surface contain petroleum, an oil found in nature. Petroleum consists of natural gas mixed with crude oil. Carbon chain length varies depending on how much and where the chain branches. There is no colour, and its density is 0.703 g cm−3. Octane rating is also defined by using Octane. 

A fuel is given an octane rating based on its heat resistance to prevent knocking. Octane reduces the possibility of a fuel-air mixture igniting before it’s supposed to, rather than enhancing combustion. Octane determines the resistance of a fuel to knocking.

Determination of Octane Number

An octane number, also known as an antiknock rating, measures how well a fuel resists knocking within an internal combustion engine when ignited in a mixture with air.

The carbon atoms in a straight-chain alkane are eight. The chemical formula of Octane is C8H18, and its structural formula is CH3(CH2)6CH3. Octane has different structural isomers depending on how much and where the carbon chain branches. In the octane rating system, 2,2,4-trimethylpentane (also known as isooctane) is used as standard values. Like all hydrocarbons with low molecular weights, Octane is highly volatile and flammable.

To determine the octane number, the knock intensity of the fuel is compared to that of two different reference fuels:

• Iso-octane

• Heptane

This is compared to isooctane, which resists knocking, and heptane, which readily knocks. A fuel being tested for octane number matches the percentage of isooctane in the mix of isooctane and heptane in a standard test engine.

Octane consists of 18 isomers:

• Octane

• 2-Methylheptane

• 3-Methylheptane

• 4-Methylheptane

• 2,2-Dimethylhexane

• 2,3-Dimethylhexane

• 2,4-Dimethylhexane

• 2,5-Dimethylhexane

• 3,3-Dimethylhexane

• 3,4-Dimethylhexane

• 3-Ethylhexane

• 2,2,3-Trimethylpentane

• 2,2,4-Trimethylpentane

• 2,3,3-Trimethylpentane

• 2,3,4-Trimethylpentane

• 2-Methyl-3-ethyl pentane

• 3-Methyl-3-ethyl pentane

• Tetramethylbutane

Pre-ignition and Engine Knock

• When using high octane or low octane gasoline, one will experience minor knocking since the octane level will be higher.

• An air-fuel mixture is ignited inside the engine by igniting the spark plug. During this process, the piston moves upwards, compressing the air-fuel mixture and simultaneously warming it up. Due to this compression, lower octane fuels can also ignite the air-fuel mixture.

• An audible knock can be heard when these flame fronts collide with the flame front ignited by the spark plug. If the fuel combusts and burns before the spark plug ignites it, it burns incompletely when the engine is pre-ignited. 

• This occurs as the air-fuel mixture within an engine compresses rather than being ignited by the spark plug. The residue of this incomplete ignition sticks to the inside of the fuel chamber, eventually causing the engine to sound like it knocks.

What is Gasoline, and how is it related to Octane Numbers?

Petrol-derived gasoline is primarily used in internal combustion engines (ICE), especially spark ignition Otto engines. Hydrocarbons are found in Gasoline and specific contaminants, including sulphur, nitrogen, oxygen, and certain metals. 

Gasoline is primarily composed of four main constituents:

• Olefins

•  Aromatics

•  Paraffin

• Naphthenes. 

Gasoline has an octane number (ON) rating that indicates its ignition performance or flammability. Motor Octane Number (MON) and Research Octane Number (RON) are the octane numbers. The RON value is expressed about an isooctane/n-heptane mixture. An antiknock index measures how well a fuel resists engine knock or octane quality. RON and MON are averaged to form AKI when chain length in hydrocarbon molecules increases, ON decreases. And when the carbon chain branches, ON increases. 

As an alternative to fuel octane boosters, such as tetraethyl lead, methyl tertiary-butyl ether, and ferrocene, the ON can also be increased by adding additives. Methanol, ethanol, and aromatic alcohols also contribute to a higher ON in gasoline. By adding oxygenates to Gasoline, such as MTBE, methanol, and ethanol, you make it cleaner and less polluting when the fuel burns.

Octane number example: Gasoline which has an octane number 92, has the exact knock as the mixture of 8% heptane and 92% isooctane.

Conclusion

Octane numbers are displayed on the pumps at gasoline stations to show how hard it will knock. Spark-ignition engines can be damaged by low-octane fuel, which can cause pre-ignition and knocking. The compression of the air-fuel mixture may cause fuel to detonate before the spark plug’s flame can reach it. When a detonation occurs, the engine may be under more pressure than it can handle.