A truck in your fleet clatters at idle, produces white smoke during cold starts, and cranks longer than it should on cold mornings—yet the injectors check out and the filters are new. When mechanical issues have been ruled out, the ignition quality of the fuel itself may be part of the problem, and that quality is expressed as the cetane number. The confusing part is that cetane often gets compared to octane, even though the two measure very different properties, and the cetane quality of the fuel you're buying isn't always obvious. This post explains what cetane does inside a compression-ignition engine, what the fuel specifications require, and how to determine whether low cetane may be contributing to the problems you're seeing.
Quick Answer
Cetane number measures how readily diesel fuel ignites after it is injected into the combustion chamber. A higher cetane number means a shorter ignition delay between injection and the start of combustion. Higher cetane can improve cold starting, reduce combustion noise, decrease white smoke during cold starts, and promote smoother engine operation. In the United States, ASTM D975 requires a minimum cetane index of 40 or a minimum cetane number of 40. When the available pump diesel fuel has marginal cetane quality, a cetane improver such as Super-Tane can help raise the cetane number.
Table of Contents
Cetane number is a measure of a diesel fuel's ignition quality, expressed by the length of its ignition delay—the time between fuel injection into the combustion chamber and the start of combustion. A higher cetane number means a shorter ignition delay: the fuel begins burning more promptly, producing a smoother pressure rise instead of allowing excess fuel to accumulate before ignition.
That timing difference is what you feel and hear. Diesel engines are compression-ignition engines—there is no spark plug, so the fuel must auto-ignite reliably when the piston compresses the air charge and raises its temperature. When ignition delay is too long, more fuel accumulates in the combustion chamber before ignition begins, resulting in a rapid pressure rise once combustion starts. That rapid pressure rise produces the characteristic diesel knock and increases combustion harshness. When cetane quality is appropriate for the engine and operating conditions, combustion begins more predictably, improving cold-start performance, reducing combustion noise, promoting smoother engine operation, and helping minimize white smoke during cold starts.
Cetane and octane both describe a fuel's combustion characteristics, but they measure opposite properties. A gasoline fuel's octane rating measures its resistance to auto-ignition, helping prevent knock or detonation in spark-ignition engines. A diesel fuel's cetane number measures how readily it auto-ignites under compression. In a diesel engine, a higher cetane number generally means a shorter ignition delay and more predictable combustion.
The two ratings do share one important principle: using more than the engine requires typically provides little or no additional benefit. Filling a gasoline engine designed for regular fuel with premium gasoline usually won't improve performance, just as using diesel fuel with a cetane number well above an engine's requirements generally won't produce additional improvements. The cetane number an engine needs depends primarily on its design, fuel injection system, operating speed, and expected load, while factors such as cold weather and high altitude can also influence how important good ignition quality becomes.
In the United States, ASTM D975 requires on-road diesel fuel to have either a minimum cetane number of 40 or a minimum cetane index of 40. In practice, most pump diesel in North America falls in the low-to-mid 40s, although the exact cetane number varies by refinery, region, season, and fuel supplier. Diesel marketed as Premium Diesel under the National Conference on Weights and Measures (NCWM) definition adopted in NIST Handbook 130 must have a minimum cetane number of 47.
Europe sets a higher baseline. The EN 590 specification requires a minimum cetane number of 51 for automotive diesel fuel. Biodiesel also tends to have higher cetane values than conventional petroleum diesel, although the exact number depends on the feedstock. Biodiesel produced from vegetable oils typically falls in the upper 40s to low 50s, while biodiesel made from animal fats often has cetane numbers in the mid-to-upper 50s.
Although some retail stations advertise the cetane number of their diesel fuel, many do not, making it difficult for drivers to know the ignition quality of the fuel they're purchasing. For fleets and fuel distributors, the difference between the ASTM minimum and the cetane level that provides the best cold-starting and combustion characteristics under their operating conditions is where cetane management can provide the greatest value.
Cetane number influences how readily diesel fuel ignites during engine startup, particularly in cold weather. Because diesel engines rely on the heat of compression rather than a spark plug, the fuel must auto-ignite quickly once it is injected into the hot compressed air. A higher cetane number shortens the ignition delay, making combustion begin more readily when temperatures are low.
Modern diesel engines are far better at cold starting than their predecessors thanks to advances such as high-pressure common-rail fuel systems, electronically controlled injection timing, and glow plugs or intake air heaters. Even so, cetane quality can still make a noticeable difference, especially in very cold conditions or when an engine is already operating near the limits of its starting system.
For fleet operators, emergency generators, and equipment that must start reliably after long periods of inactivity, good cetane quality is one factor that can improve cold-weather starting and reduce rough running, white smoke, and combustion noise immediately after startup.
The symptoms of marginal cetane quality can include hard starting, rough idle immediately after startup, increased combustion noise, white smoke during cold starts, and sluggish engine operation until the engine reaches operating temperature. None of these symptoms, however, is unique to low cetane. Dirty or worn injectors, degraded fuel, water contamination, microbial growth, restricted fuel filters, and other mechanical or fuel-quality problems can produce many of the same complaints. You can't determine a fuel's cetane number simply by looking at it or by observing engine symptoms alone.
That's why it's important to diagnose the problem before choosing a treatment. Fuel testing can determine the cetane quality of the fuel while also evaluating other factors that commonly affect engine performance, including fuel stability, water contamination, and microbial growth. Bell's fuel testing services help identify the underlying cause of fuel-related performance issues so you can choose the appropriate corrective action instead of relying on guesswork. Treating a microbial contamination problem with a cetane improver—or trying to solve marginal cetane with a biocide—adds unnecessary cost while allowing the real problem to persist.
When testing confirms that marginal cetane quality is contributing to the problem, a dedicated cetane improver is the most direct solution. The most widely used cetane improver in the industry is 2-ethylhexyl nitrate (2EHN), which increases a fuel's cetane number by shortening ignition delay. Super-Tane is Bell Performance's dedicated cetane improver. It uses proven 2EHN chemistry to improve ignition quality, promoting easier cold starts, smoother engine operation, reduced combustion noise, and improved combustion consistency at a treat rate of 1:1000 with each addition of new fuel.
It's also important to recognize that not every diesel performance complaint is caused by low cetane. Injector deposits, degraded fuel, water contamination, and microbial growth can all produce symptoms that resemble poor ignition quality. In high-mileage engines, detergent fuel treatments such as Dee-Zol can restore injector cleanliness and help recover lost performance, but they do so by improving fuel delivery and combustion efficiency—not by increasing the fuel's cetane number. Many commercial operators use both products together when appropriate: a detergent to keep the fuel system clean and a cetane improver when additional ignition quality is needed.
| Application | Product | Treat Rate | When to Apply |
|---|---|---|---|
| Raise cetane in diesel, biodiesel, or heating oil | Super-Tane | 1:1000 (variable with desired cetane gain) | With each addition of new fuel |
ASTM D975 requires on-road diesel fuel in the United States to have either a minimum cetane number of 40 or a minimum cetane index of 40. In practice, most pump diesel falls in the low-to-mid 40s, while diesel marketed as Premium Diesel under the NCWM definition in NIST Handbook 130 must have a minimum cetane number of 47. The ideal cetane number depends on the engine and operating conditions, but engines operating in cold weather or demanding applications may benefit from fuel with higher cetane quality than the ASTM minimum.
Not directly. Higher cetane does not increase the energy content of diesel fuel or add horsepower to an engine that is already operating as designed. Instead, it shortens ignition delay, which can improve cold starting, reduce combustion noise, and promote smoother engine operation. If marginal cetane quality is causing poor combustion, raising the cetane number can help restore normal performance, but once the fuel meets the engine's ignition quality requirements, additional cetane generally provides little or no further benefit.
No. Although both are fuel quality ratings, they measure opposite combustion characteristics. Octane measures a gasoline fuel's resistance to auto-ignition, helping prevent knock in spark-ignition engines. Cetane measures how readily a diesel fuel auto-ignites under compression. In gasoline engines, higher octane resists ignition. In diesel engines, higher cetane promotes prompt ignition.
Cold weather makes it more difficult for diesel fuel to ignite because the compressed air in the combustion chamber starts at a lower temperature. Modern diesel engines use technologies such as high-pressure fuel injection, electronic engine controls, and glow plugs or intake air heaters to improve cold starting, but fuel quality still plays a role. If the fuel has marginal cetane quality, ignition delay can increase, resulting in longer cranking, rough running, white smoke, and increased combustion noise immediately after startup. It's also important to remember that fuel gelling, weak batteries, or mechanical problems can produce similar symptoms and require different solutions.
Most cetane improvers use 2-ethylhexyl nitrate (2EHN), the industry's standard cetane-improving chemistry. As the fuel is compressed inside the cylinder, 2EHN decomposes and promotes earlier ignition, shortening ignition delay and increasing the fuel's cetane number. Super-Tane uses this proven chemistry to improve ignition quality and is formulated to be added with each delivery of new diesel fuel at a 1:1000 treat rate.
Yes. Hard starting, rough idle, increased combustion noise, and poor cold-weather performance can result from marginal cetane quality, but they can also be caused by degraded fuel, water contamination, microbial growth, injector problems, or restricted fuel filters. Fuel testing helps determine whether cetane quality is contributing to the problem while also identifying other fuel-related issues. Bell Performance's fuel testing services evaluate ignition quality alongside fuel stability, water contamination, and microbial growth so you can select the appropriate treatment instead of relying on trial and error.
If your fleet or stored diesel is showing hard starts, black smoke, or rough operation, start by confirming the fuel is the problem—then treat it. Super-Tane is formulated to raise cetane number, reduce black smoke, and smooth engine operation at a 1:1000 treat rate with each fuel delivery. See Super-Tane details and order, or talk to Bell FTS about testing your stored fuel first.