What Is Cetane in Diesel and Why Do Cetane Numbers Matter?

Your diesel engine might be powerful, but it’s only as good as the fuel you put in it. To better understand what fuel your engine needs, you should learn about cetane and how it affects your vehicle’s combustion process.

What Is Cetane in Diesel?

For years, manufacturers have evaluated diesel fuel’s combustion quality according to its cetane number, but what exactly is cetane?

Cetane is a highly flammable chemical compound naturally found in diesel fuel. It ignites under high pressure, causing the fuel to combust.

Unlike gas-powered engines, diesel-powered engines don’t come with spark plugs. Instead, fuel is combusted through compression alone, which is why cetane plays such a big role in powering vehicles that run on diesel.

Most big rig truck fuel stations use a cetane of 40. Here’s a list of examples: 

• BP (Amoco branded), 51
• Chevron, 49 (51 with Techron D)
• ConocoPhillips, 48 (California 48-53)
• PetroCanada, 47-51
• Shell, 46;
• Sinclair, 46
• Marathon 45
• Exxon/Mobile, 43-46
• BP, 40-42 (Powerblend 47)
• HESS, 40-45
• Husky, 40 (Max 41-45)
• Sunoco, 40 (Sunoco Gold 45)
• Holiday Stations, 40-43
• Love’s: 40
• Pilot: 40
• Valero: 40
• Sheetz: 40
• Flying J, 40

What Is Cetane Rating?

Also known as a cetane number, cetane rating is the measurement of how much cetane is present in diesel fuel. It’s also used to indicate the combustion speed of diesel fuel. The higher a diesel fuel’s cetane rating is, the more complete combustion it can achieve.

Cetane ratings range from 0 to 100, with a score of 100 for pure cetane. Meanwhile, the American Society for Testing Materials (ASTM) minimum standard for ultra-low-sulfur diesel (ULSD) is 40.

Keep in mind that the ratings you see at gas station pumps aren’t always accurate. They only represent the minimum cetane number of the fuel you’re pumping into your tank.

Benefits of Using High-Cetane Diesel

Most diesel engines run best on fuel with a cetane number between 45 and 55. Once you pump your vehicle with high-cetane diesel, you’re likely to notice the following benefits:

Quicker Starting

You’re less likely to run into trouble starting your vehicle when you use high-cetane diesel. A higher cetane number means your engine ignites faster, leading to quicker starting.

Quiet Engine

Engines that run on high-cetane diesel tend to have less buildup inside, so they’re a lot less likely to make noise.

Diesel with higher cetane ratings also tends to burn for longer, causing the pressure inside the combustion chamber to rise gradually. This results in less engine knock and reduces stress on the engine.

Improved Fuel Efficiency

At the start of the combustion process, the injector sprays droplets of fuel into the combustion chamber. With a higher cetane rating, every droplet is more likely to ignite and produce more power, increasing your vehicle’s fuel efficiency.

How Does Low-Cetane Fuel Affect Engine Performance?

No matter how impressive your engine is, you can’t go very far with low-cetane diesel. A low cetane rating can cause issues such as the following:

Poor Fuel Economy

If you’re on a budget, you likely don’t want to use more fuel than necessary. Unfortunately, engines that run on low-cetane fuel typically create more soot particulates, which means your engine has to use more fuel to clear them out.

Less Power

Poor fuel economy can also lead to less engine power for your vehicle. Low-cetane fuel can cause incomplete or improperly timed combustion, which typically means there’s leftover fuel that can’t power your engine.

Increase in Emissions

Any unburned fuel left in the combustion chamber will continue through the system until it exits as white smoke and other particles, causing your vehicle’s emission levels to increase.

How To Boost a Fuel’s Cetane Number

With all the attractive benefits of high-cetane fuel, it’s no surprise that many people want to boost their fuel’s cetane number. Thankfully, there are a few easy ways to do this.

Use Cetane Improvers

There are plenty of aftermarket additives called cetane improvers that can increase your diesel’s cetane number and make it burn much faster. However, you might want to proceed with caution when it comes to these products.

You won’t run out of options to choose from, but it can be hard to tell which cetane boosters are legit and which ones are harmful snake oil.

Use Premium Diesel With Built-In Cetane Improvers

Your safest bet is to use premium diesel with built-in cetane improvers. This way, you’re less likely to fall victim to fake additives.

Cetane vs. Octane: What’s the Difference?

Just like cetane numbers, octane numbers measure a fuel’s combustion quality. The main difference between the two is that octane numbers rate gasoline, while cetane numbers rate diesel fuel.

An octane rating measures how well gasoline can resist pre-ignition due to compression. Gasoline with a higher octane rating can withstand greater compression from the piston before igniting in the engine.