Higher octane petrol doesn’t have more energy; it has less!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

09 February 2025

Early this week, YouTuber by the name Kim JH visited several filling stations in Nairobi, bought petrol and tested its octane rating with a portable measuring device. His results showed that one of the high-priced brands that is normally marketed for “high performance” engines actually had lower octane number than the rest. This caused a big publicity storm.

Whether or not the test was credible, the lingering question is: does petrol with higher octane improve the performance of a car? It turns that the octane rating of petrol is NOT an indicator of the amount of energy it contains.

To understand what’s going on, let’s first understand the meaning of the octane rating. If pure petrol is mixed with air and then compressed, it will detonate and explode even if there is no spark to ignite it. This is not good for an engine because the detonation can happen before the compression stroke is complete.

That would not only reduce the energy output of the engine but can also cause permanent damage. To stop the spontaneous detonation, chemicals are added to the petrol to stabilise it – the most common today is ethanol (yes, the same stuff that people ingest to get drunk!).

To get the octane number, the behaviour of petrol is compared to that of a mixture of octane plus heptane. Octane is a chemical with a chain of 8 carbon atoms while heptane has a chain of 7 carbons.

Heptane detonates very easily under light compression while octane is a lot more resilient; it will only explode under extremely high pressure. In this scale, a petrol sample that behaves like heptane is assigned octane number zero since it is zero percent octane.

A sample that behaves like 100 per cent octane is assigned octane number 100. In the same way, if the petrol behaves like a 50:50 mixture of octane-heptane, then its octane number is 50. Ordinary premium petrol in Kenya is rated at octane number 91; this means that it has been treated with ethanol to make it behave like a mixture of 91pc octane and 9pc heptane.

Now the energy content of petrol is about 45 mega joules per kilogram (MJ/kg) while that of alcohol is 27MJ/kg. Cleary then, adding alcohol to petrol is bound to reduce the total energy. So why do so many people – including professional mechanics – believe that higher octane petrol has more energy?

Well, that is a story for another day.