If you own a diesel vehicle, chances are that you’ll have come across the term "cetane". Whether you're a truck enthusiast, a professional in the automotive industry, or simply a curious driver, gaining an understanding of cetane can be incredibly useful. So, what is cetane? In this guide, we will explain everything you need to know about cetane, from cetane numbers to cetane boosters, and the role they play in diesel engines.

What is cetane?

Cetane is a colorless gas that is liquid at room temperature. It’s made of hydrocarbon molecules, specifically n-hexadecane, and has the chemical formula C16H34, meaning each molecule has 16 carbon atoms and 34 hydrogen atoms. It’s particularly important in the context of diesel engines and fuel performance as it is used as a way of measuring the ignition quality of diesel fuel. 

What is a cetane number or rating?

The cetane number (CN) measures the ignition quality of diesel fuel, similar to how octane ratings measure gasoline quality. The CN is determined by comparing the ignition delay of the diesel fuel to a blend of cetane and isocetane. Pure cetane has a CN of 100, while isocetane has a cetane number of 15. 

The CN of a particular diesel fuel is found by creating a blend of these two substances that match the fuel's ignition delay and then calculating a volume-weighted average. A higher CN means shorter ignition delays, leading to quicker and more efficient combustion.

In simple terms, the CN will tell you how well diesel fuel will work in your engine. It works as a score for how fast and smoothly the fuel will ignite.

High CN: The fuel ignites quickly and makes the engine run smoothly.

Low CN: The fuel takes longer to ignite and can make the engine run rough or knock.

A good metaphor is to imagine it like rating how fast a match lights when you strike it. A higher CN means the diesel fuel lights up faster and burns more efficiently, which helps your engine run better and produce fewer emissions.

The significance of CN in diesel fuel

The CN of diesel fuel is a critical measure of its quality and performance, specifically indicating the fuel's ignition properties. It’s an incredibly useful metric that can be used to determine a number of things about diesel fuel. Understanding the CN of your vehicle can have a number of benefits:

Ignition quality

The CN measures how quickly and efficiently diesel fuel ignites under compression in a diesel engine. 

Engine performance

The CN directly affects engine performance. Higher cetane numbers lead to smoother and more efficient combustion, resulting in improved power output and fuel efficiency. 

Cold Starts

A higher CN improves the engine's ability to start in cold weather by reducing the ignition delay.

Smooth operation

Fuels with higher CN tend to cause less knocking and vibration, resulting in quieter and smoother engine operation.

Fuel economy

Optimal cetane levels can improve fuel efficiency as the fuel burns more completely and cleanly.

Reduced emissions

Cleaner combustion from higher cetane fuels means less unburned hydrocarbons and carbon monoxide, contributing to a reduction in the overall environmental footprint of diesel engines.

Factors affecting CNs

So we’ve looked at what cetane is, what a CN is, and how it can help assess diesel fuel. But what makes one type of diesel better than another, and what effect does this have on your vehicle? Well, there are several factors that can influence the CN of a particular diesel fuel:

Fuel composition

The specific chemical makeup of a particular diesel fuel can have a significant impact on its CN. Fuels with a higher proportion of straight-chain hydrocarbons tend to have higher CNs, while those with more branched-chain hydrocarbons have lower cetane numbers.

Refining process

The refining process used to produce diesel fuel can also affect its cetane number. Hydrocracking, a refining process that breaks down larger molecules into smaller ones, typically results in fuel with a higher cetane number compared to traditional distillation methods.

Diesel fuel additives

Various additives can be used to enhance the CN of diesel fuel. These additives, known as cetane boosters, help improve fuel combustion efficiency and overall engine performance. Archoil’s AR6500 Diesel Treatment is one such additive. Simply add it to your tank every time you fill up and boost your CN by up to eight points.

CN standards and regulations

There are a wide range of standards and regulations that govern the CN of diesel fuel in different countries around the world. These regulations vary depending on a range of political and environmental factors. For comparison, we will look at the American and European standards.

The US standard: ASTM D975 is the standard used for diesel fuel in the United States. The specification ensures that diesel fuels meet key parameters such as CN, sulfur content, viscosity, flash point, distillation characteristics, and lubricity. In relation to CN, the regulations require a minimum number of 40 for all No 1-D and 2-D grades of diesel fuel.

The EU standard: EN 590 is the European standard for diesel fuel and covers parameters like CN and sulfur content. The main difference when compared to the US is that it imposes stricter sulfur limits. In relation to CN, the regulations require a minimum cetane number of 51, much higher than in the US.

In the US, diesel fuel has a lower CN than in the European Union. This difference in cetane standards means that vehicles designed and manufactured in the EU, won’t perform optimally when using US diesel. These vehicles often have engines with tighter tolerances that rely on higher CNs to run efficiently. Using the lower-cetane fuel that is available in the US can lead to issues such as reduced engine performance, increased emissions, and higher maintenance costs.

What is cetane booster?

The cetane number of a particular fuel can be improved by using an off-the-shelf cetane booster, such as Archoil’s AR6500 Fuel Treatment. Simply add this additive to your tank every time you refuel your vehicle and boost the CN of your fuel by up to 8 points. Using a cetane booster can provide a number of benefits:

Improved engine performance

Cetane boosters enhance the ignition quality of your diesel fuel, leading to better engine performance and fuel efficiency.

Reduced emissions

Higher cetane numbers result in cleaner combustion, helping to reduce the amount of emissions coming from your vehicle.

Enhanced fuel economy

Better combustion efficiency means more energy is extracted from every bit of fuel, leading to improved fuel economy.

Reduced engine noise

Cetane boosters also help to minimize engine knocking and rough idling, resulting in a quieter and smoother-running engine.

Commercially available diesel fuel

Having read about all the benefits of using diesel fuel with a high cetane number, you might be wondering why most diesel fuels in the US only have a CN of around 40. The simple answer is that it comes down to economics and the law. Firstly, the Environmental Protection Agency (EPA) requires a minimum CN of 40 for all diesel fuel, meaning that is the lowest CN diesel that you will find. But most engines run at optimum efficiency with a cetane number of 47-50.

So why the discrepancy? Well, the low limit was set so that all fuels could reasonably achieve the minimum CN of 40 without having to add cetane boosters, which would make production more expensive. The particular value of 40 was determined to be an acceptable one, even if it’s not ideal for almost all modern vehicles.

That’s another reason it’s recommended to use a cetane booster, such as Archoil’s AR6500 Fuel Treatment. By using this diesel fuel additive every time you fill up, you will not only help to protect your engine from low-quality diesel, but you will also save money in the long run through increased combustion efficiency.

Common cetane myths

Understanding cetane can be difficult, and as with many technical topics, there are a few misconceptions about how it works and its effect in diesel engines. We will address some of the most common myths that we hear below:

Myth 1: Higher cetane always means better performance

As we have explained above, higher cetane numbers generally indicate better ignition quality. But it’s not always the case that higher cetane fuels will result in better performance for all engines. The optimal cetane number can vary depending on the engine design and operating conditions. It's essential to use fuel that meets the specific requirements of your particular engine.

Myth 2: Cetane boosters are harmful to engines

Cetane boosters, when used correctly and according to manufacturer guidelines, are not harmful to engines. In fact, they can improve engine performance and longevity by enhancing combustion efficiency. The issues that people have with cetane boosters usually come from overuse or improper use, so it's vital to always follow the recommended dosages.

Myth 3: Cetane number is the only factor in diesel fuel quality

While the CN is an important factor in determining diesel fuel quality, it's not the only one. Other factors, such as sulfur content, lubricity, and stability, also play key roles in determining the overall quality of your diesel fuel. Cetane number works great as a quick indicator, but there’s more that goes into determining a fuel’s quality.

Conclusion

Low standards of cetane in diesel fuel in the US lead to many issues for diesel vehicles. That’s why it’s so important to use a cetane booster like Archoil’s AR6500. By understanding the importance of cetane and taking steps to improve the cetane number of your fuel, you can ensure you are optimizing your engine’s performance, promoting cleaner combustion, and reducing emissions. 

Want to understand more about what you put into your car or truck? Or maybe you’re interested in learning more about other aspects of vehicle care and maintenance. Check out our Resources section for a range of in-depth guides from our auto experts.