Octane Ratings Explained

by L.J. Martin

©Doug Wilson
(Some rights reserved)

Everybody has heard of octane, with the phrase "high octane" meaning exciting or extreme, but what is octane, and what does high octane actually mean? In this guide I will explain octane ratings and why they are important.

Firstly, why do we care about the octane rating of the fuel we put in our car? The answer is that the octane number of a fuel is a measure of its ability to prevent engine knock. So what is engine knock, and why do we want to prevent it?

The Problem: Engine Knock

Engine knock gets its name because of the sound it produces: a knocking or pinging sound coming from the engine. This is caused by the fuel detonating. But wait a minute, isn't the fuel supposed to detonate? Well, most people think a gasoline (petrol) engine works by mixing fuel and air, then making a spark that causes the mixture to explode. This is actually false, because in a properly functioning spark-ignition engine the fuel undergoes a fast, controlled burn rather than an explosion. If the fuel detonates rather than burning smoothly, the shockwave from the explosion will make the sound known as "knock".

So what's the problem with the fuel detonating rather than burning? The trouble with detonation is that it causes an explosive shockwave to rattle around inside the combustion chamber. This causes a sudden spike in pressure which can obviously cause great damage to the engine. I'm sure you agree that engine knock is a bad thing, as it can result in huge repair bills. But what actually causes the fuel to detonate instead of burning?

When the spark plug ignites the fuel, it creates a flame front that moves through the cylinder, burning the fuel/air mixture as it goes. We have all seen action movies where a fireball races towards our hero, only for him to leap to safety at the last millisecond. This is basically what happens inside the engine, with a fireball starting at the spark plug and moving through the cylinder. The fuel should not ignite until the fireball reaches it, but the unburned fuel is being subjected to increasing temperatures and pressures. (It is trapped in a little chamber right next to a fireball, after all.) When exposed to these high temperatures and pressures, the fuel molecules will tend to break down into smaller chunks. These broken chunks are highly reactive, and as they are mixed with an ideal quantity of air, they spontaneously explode before the fireball reaches them. This is the detonation that we were worried about. (Imagine that our action hero has a fireball racing towards him, but before he can heroically jump out of the way, a bomb next to him explodes due to the heat from the approaching fireball.)

What can be done to stop engine knock? There are two ways to approach the problem: changing the engine and changing the fuel.

There are ways to design an engine that will be highly resistant to knocking. For example, using a low compression ratio means that the engine doesn't squeeze the fuel/air mixture very much before igniting, which means the temperatures and pressures within the cylinder are reduced, so the fuel is less likely to explode before the flame front reaches it. The engine could also delay the spark timing, not igniting the fuel until the piston has started to move downwards. This means the fuel/air mixture will be less compressed when the flame front reaches it, again reducing the risk of detonation. The problem with these solutions is that they reduce the efficiency and power output of the engine. A high compression ratio makes an engine more efficient. Firing the spark very early while the piston is still moving upwards means that the cylinder pressure will be reaching its peak just as the piston starts to move downwards, applying the maximum force to the piston and producing more power.

So ideally, we would like to make an engine that is as powerful and efficient as possible, and change the fuel itself to reduce detonation. On the next page we will explain how this works, and where octane comes into it.