Difference between octane and cetane number

Technician blog

In this article we are going to talk about the difference between octane number and cetane number. Both are used as fuel rating scales. Our goal is to let you know the difference between these two numbers.
Octane numbers are used as a rating scale for petrol. And the cetane numbers are used as a rating scale for diesel fuels. But first I will tell you about the basic definition. We're going to go by octane number and then we're going to talk about what cetane number is.


The octane number is a measure of the impact resistance of a fuel. And what that means is a high octane fuel. That can be compressed more without this fuel mixture igniting. It's a measure of how much you can compress that air-fuel mixture before it ignites. The higher the number, the more you can compress it. The lower the number, the more likely the fuel will ignite on its own. Of course, on gasoline engines you want to use a spark plug to start the ignition. You don't want to use compression unlike diesel engines, and high octane ratings are a good thing. This changes the rating scale from 0 to 100.


If you're wondering, yes you can go over 100. There are fuels with octane numbers greater than 100. But in general we are talking about this range with heptane, zero and octane is 100. Both of these are hydrocarbons.
Octane has a rating of 100 heptane and a rating of 0. This means that heptane is very likely to be ignited by compression. Octane isn't very likely to ignite from compression so why would you want that higher number?


Well, because of that greater knock resistance. Of course, knocking is a bad thing. Because you don't want to scan it in time with the spark plug by compression. But using a higher octane rating can have higher compression ratios. And higher compression ratios lead to more torque and more efficiency.
They also allow you to move the timing forward. You can ignite the spark plug earlier. And get maximum torque when the piston is closer to top dead center. This process will produce more torque and have better efficiency. They are also useful for forced induction, turbocharger and compressor applications.

If you have higher pressures in that cylinder. And you want to make sure you don't have any knocking problems. You can lower the fuels with anti-knock. And that lower octane rating, this will create less resistance to knocking. You will see this more with naturally aspirated engines. Because when you know that you will not face these higher burdens. Then you don't necessarily have to worry about getting knocked.

What is a fuel that has a ninety octane rating?


Well what that says is a kind of equivalence. If it was made of heptane and octane. That said, a ninety would be equivalent to a fuel that was 90 percent octane. And ten percent heptane. However, that does not mean that the fuel is made of it. But it would behave like a fuel made up of octane and heptane.


One of the things that always comes up when you talk about octane rating is. That everyone in Europe says that; USA fuel quality is terrible and our octane rating is super low. This is due to different scales that are used. The u.s. use AKI and Europe use RON. And RON tends to be a higher number even if the fuels are exactly the same.

But what about the cetane number?


Now let's talk about the cetane number. Which is a measure of the ignition delay of a diesel fuel. What does that mean? Well, it's a measure of the time from when fuel is injected into the cylinder. And when does this fuel start to burn. It is a measure of the time and you are looking at a high cetane number.


This means that once the fuel is injected with a low cetane number, it will ignite very quickly. Which means it will take a while for the fuel to ignite. After it is injected into the cylinder. And so you scale your rating again. We use two different hydrocarbons for a rating scale of 0 and 100. Cetane, just like octane, is a 100 cetane number.


For example, cetane 100 is C16 H34 and 0 is 1-methylnaphthalene. That is definitely C11 H10. And in fact, they have since switched to isocetane, which has the same chemical nature of composition.

C 16 H 34 as a cetane has a cetane number of 15. The reason for this is more likely than methylnaphthalene. Because it's a bit cheaper and easier to work with. And so they can look exactly the same C16 H34. But the structure of them is very different. Because how the atoms are actually connected to each other is very different.


That gives the lower cetane number. These numbers take a long time to ignite, you will have a longer burn. Because these numbers don't take very long to ignite. The burn is shorter and the benefit is faster burn. That you can generate more torque.
You can also tune your engines to run at a higher speed, as the combustion does not take as long. This is because you will have less time to burn when you get into that higher RPM. And it is important that the burn rate and ignition be retarded for a very short time.


It's also great for timing when maximum torque is occurring. With faster combustion, you can change the injection timing. But ultimately because of this shorter ignition delay. You can better synchronize this with the point in time at which this peak torque should occur within the cylinder. You have the maximum amount of work. That makes it great for timing because it burns very quickly and it ignites very quickly. This will give you fewer hydrocarbon emissions.


To achieve better performance and fuel efficiency. Chiptuning is a good thing for your car. It's very simple and is done in 4 easy steps.

 

Some facts about octanes and cetanes


There have been some studies that have shown. That you can get a more complete burn with a higher cetane number. But because it burns faster it tends to burn a little hotter. As a result, you tend to get more particulate matter emissions.


Some studies show that the lower combustion temperatures result with these lower cetane numbers. This can lead to lower particulate matter emissions. But overall you have longer combustion and less torque.
Hopefully that will clear up the difference what each of those two numbers mean when you sit on the pump and try to fill up your car. I would still recommend going with what your car manufacturer recommends.