Posted by Gomoto Media

The subject of using petrol of what octane usually elicits "passionate" debates that would sometimes degenerate into qualifications being thrown around in addition to name calling (usually referencing mental wellbeing).

But how did we get here? Why is higher octane petrol still being regarded as "providing the engine with more power?" The idea could have originated to the time when internal combustion engine technology was at its infancy and the push for higher engine performance meant that fuel technology had yet caught up. This was especially true during the Second World War when fighter aircraft engine broke new grounds in performance.

Anyhoo... It is best to talk about octane by understanding some aspects about the engine.

Compression ratio

The most important determining factor for using the correct octane rating is the engine's compression ratio.

This ratio is derived by dividing the largest volume to the smallest in a cylinder, as the piston moves up the cylinder and squeezes the fuel/air mixture into a smaller and smaller area. In other words, the cylinder's volume when the piston is at bottom dead centre (BDC) divided by when the piston is at top dead centre (TDC). Let us take a 250cc cylinder as an example: 250cc/20cc = 12.5:1

The mixture heats up more and more as it is squeezed, to more than 100-deg Celsius. This, even before the spark plug fires!

So, the higher the compression ratio, the higher temperature the fuel/air mixture will accumulate. Higher compression is great because more pressure is built up in the cylinder and when it translates to higher thermodynamic efficiency when the mixture is burned. This is why high performance engines such as on superbikes have much higher compression ratios compared to a cruiser or tourer.

Unfortunately, engineers discovered long ago that raising the compression ratio can cause the mixture to self-ignite before the spark plug fires. This is called "pre-ignition."

Self-ignition usually begins at the edges of the piston where new and cooler incoming mixture cannot reach. Starting from one hot spot, the self ignition triggers self-ignition in other spots around the piston and cylinder. The mixture that self-ignites does so at the speed of sound, sending shockwaves throughout the cylinder and in the end, you will end up with a piston looking like this.

However, this is actually how diesel engines work, by having between 14:1 to 25:1 compression ratios. The air in the cylinder is compressed to raise its temperature before diesel is sprayed into it, causing self-ignition.

Combating self-ignition

We have already mentioned that higher compression ratio yields better engine performance. So how to have a higher compression ratio without causing the fuel to self-ignite?

The answer is, you guessed it: Octane.

Now, octane is not some kind of chemical formula, for it is just a rating. The easy was to remember is: The higher the octane rating, the more resistant the fuel is to self-ignite.

"In other words, higher octane fuel resists pre-ignition better. And that is it. That is all that octane means."

Higher octane fuel say, RON 100 does not mean it stores more chemical power, waiting to be unleashed by the electrical arc over the spark plug's electrodes.

But why does my engine feel more powerful when I use higher RON petrol?

It could be psychological, but we do attest that the (electronically fuel injected) engine does feel more responsive with higher octane fuel.

That is because modern engines monitor the combustion process. More specifically, when regarding fuel, by using a knock sensor or two.

While the engine cannot raise or lower its compression ratio dynamically, it can alter the ignition timing. Ignition timing is when the spark plug fires depending on the engine load (RPM).

The flame, hence pressure resulting from ignition travels at a fixed speed. However, the time the piston flies up and down as speed picks up may not benefit from it as it may be out of phase. Imagine the fuel/air mixture on starting to burn when the piston is already approaching TDC... the resulting combustion pressure is wasted because it is at the end of the stroke.

So, the spark plug has to fire earlier to allow the flame to start propagating as the piston travels before TDC (BTDC). This is called advancing the ignition.

Conversely, ignition should happen nearer to TDC when the engine is at low RPMs because otherwise, there will be combustion pressure pushing down on the piston as it is still rising!

Back to pre-ignition, advancing the ignition timing too much can cause pre-ignition. Should this happen, the knock sensors pick up the sound waves and tell the ECU to retard the ignition timing. Result: Loss of response and power.

Since a higher octane fuel resists pre-ignition, the ECU allows the engine to operate closer or at its maximum potential.

But to repeat, it is NOT because higher octane fuel provides more power!

One more thing: Higher octane fuel does not mean it has better quality than the lower octane one. Neither does a higher RON fuel clean the engine better than a lower RON fuel.

Yes, we can hear your question: Then why does racing fuel have higher octane and cost a bomb?

Answer is above: Racing engines may be modified for even higher compression ratios and they are run at full throttle more than they do on public roads. They also have better quality (generally) than pump gas because they have less impurities from the manufacturing, transport and storage stages.

Do I choose higher or lower octane then?

The best is to choose the minimum octane your engine is rated for. It is entirely your choice to choose which one but using the minimum required octane does save lots of money.