Auto-ignition and Octane Numbers
Auto-Ignition
Auto ignition is when fuel ignites itself without a spark. The tendency for fuel to auto-ignite is measured out of 100 called an octane number. The higher the octane number, the lower the tendency to ignite. If the fuel in a car, for
example petrol auto-ignites it can cause damage to the engine, reduced engine performance and it also makes a knocking or “pinking” sound.
Octane Numbers
Octane numbers are measured in a test engine and is defined by a comparison of 2,2,4-trimethylpentane and heptane. Therefore if a fuel has an octane number of 75, this means that it has the same tendency to auto-ignite as a mixture of 75% 2,2,4-timethylpentane and 25% heptane.
Auto ignition is when fuel ignites itself without a spark. The tendency for fuel to auto-ignite is measured out of 100 called an octane number. The higher the octane number, the lower the tendency to ignite. If the fuel in a car, for
example petrol auto-ignites it can cause damage to the engine, reduced engine performance and it also makes a knocking or “pinking” sound.
Octane Numbers
Octane numbers are measured in a test engine and is defined by a comparison of 2,2,4-trimethylpentane and heptane. Therefore if a fuel has an octane number of 75, this means that it has the same tendency to auto-ignite as a mixture of 75% 2,2,4-timethylpentane and 25% heptane.
Comparison of Octane Numbers
Different hydrocarbons have different octane numbers. The comparisons between different hydrocarbons can be found below.
- Shorter chained compounds have a higher octane number in comparison to longer chain compounds.
- Branches that come off chained compounds increase the octane number, and more branches increase the octane number
- Arenes have a higher octane number than cycloalkanes which also have a higher octane number than the corresponding straight chained alkanes
- Oxygenates have higher octane numbers than corresponding alkanes.
In fuel such as petrol, we are looking to minimise the chance of auto-ignition and therefore damage to the car and engine. For this reason the highest octane numbered petrols are used. The picture below shows a petrol pump and all of the petrol types available for purchase have a high octane number. As you can see the more expensive premium petrols can be found at the right and the cheaper regular petrol on the left. The premium petrol is more expensive because of its higher octane number and therefore it is less likely to auto-ignite or cause damage to the car's engine.
Different hydrocarbons have different octane numbers. The comparisons between different hydrocarbons can be found below.
- Shorter chained compounds have a higher octane number in comparison to longer chain compounds.
- Branches that come off chained compounds increase the octane number, and more branches increase the octane number
- Arenes have a higher octane number than cycloalkanes which also have a higher octane number than the corresponding straight chained alkanes
- Oxygenates have higher octane numbers than corresponding alkanes.
In fuel such as petrol, we are looking to minimise the chance of auto-ignition and therefore damage to the car and engine. For this reason the highest octane numbered petrols are used. The picture below shows a petrol pump and all of the petrol types available for purchase have a high octane number. As you can see the more expensive premium petrols can be found at the right and the cheaper regular petrol on the left. The premium petrol is more expensive because of its higher octane number and therefore it is less likely to auto-ignite or cause damage to the car's engine.