Who's Got Better Fuel? Octane Ratings Explained and Mythbusted

So what is the octane rating of fuel? The octane rating of fuel is actually an indicator of just one single thing and that is the fuel’s ability to resist detonation or knock. That’s it. The octane number has nothing to do with the energy content of fuel nor it’s impact on the power output of the engine. 
The octane rating is also only relevant for gasoline engines because gasoline engines compress air and fuel together and then ignite the compressed mixture using a spark plug. When we compress air we we bring the air molecules closer together causing them to bump against each other more which then increases the temperature of air. And if we compress it enough to raise the temperature enough this can then lead to the spontaneous ignition of the air fuel mixture which is independent of the action of the spark plug. This spontaneous ignition of the air fuel mixture is also known as detonation or knock. It’s an uncontrolled event which can damage the engine if it’s strong enough or persists long enough and hence must be avoided. 

So if your engine requires let’s say regular 87 octane fuel and you put in premium 94 octane in it simply means that you’ve increased the knock resistance of the fuel inside your engine. but this does nothing because your engine is designed for 87 octane and wasn’t knocking in the first place. In other words you’re increasing protection against a non-existant risk. The only time when a higher octane might have helped is if there was something actually wrong with your engine which was causing it to knock. Your ecu detects the knock through the knock sensor and then retards ignition timing to stop the knocking which reduces engine performance. You then pour in the premium fuel which prevents the knocking and your ECU restores normal timing. In this case the higher octane fuel didn’t give you any additional performance but only restored original performance by acting as a band-aid fix for the fault inside your engine. In fact on most modern engines this scenario is impossible because knocking would trigger a check engine light or even limp-mode and engine performance could not be restored without resetting the codes regardless of the fuel you put into the engine. The reality of things is that putting in high octane fuel in a normally operating engine that doesn’t need high octane fuel actually reduces the performance of that engine. And this is because a higher octane number doesn’t just increase knock resistance, it also usually leads to a lower flame speed of the combustion and this can lead to reduced performance. 

Now diesel engines don’t have spark plugs and they only compress air which means that the octane rating is irrelevant for diesels. Diesel engines introduce fuel into the combustion chamber only when the air has been compressed sufficiently to be hot enough to ignite the fuel. This is why the tendency to self-ignite under heat and pressure is actually desirable for diesel fuels and the reason why they use a completely different rating called the Cetane number which measures this. 

So how is the octane number of fuel even measured? Well it’s measured using a special machine which is essentially a little single cylinder four stroke engine, as you can see here’s the cylinder and here we have the valve springs, but what makes it different from real engines is that it has a variable compression ratio. 

The compression ratio of the engine is the ratio between the largest and the smallest volume of the cylinder, in order words it’s the ratio between the total cylinder volume when the piston is at bottom dead center and when it is at top dead center. The compression ratio of an engine is fixed and can’t be changed when the engine is running. To change it you have to take the engine apart and make mechanical changes to it’s internals to change the compression ratio. For example installing a piston with a large dome is going to decrease both the smallest and the largest cylinder volume leading to an increased compression ratio.

The higher the compression ratio the more we compress and heat up the air fuel mixture leading to higher chances of knock. This is why high compression ratio gasoline engines need higher octane fuels. Another important factor is the presence of forced induction. A turbo or supercharger also compresses the air and pushes it into the engine which means that forced induction also contributes to increased air temperatures and increased chances for knock.


So the compression ratio of a real engine is fixed but it’s not fixed for our octane rating test engine. Older versions of these machines featured a manual handle which raises or lowers the height of the cylinder head thus changing the compression ratio of the engine while it’s in operation. 

A special thank you to my patrons:
Daniel
Daniel Morgan
Pepe
Brian Alvarez
Jack H
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Nelson

#d4a #octane #gasoline