What’s That: Cetane

Chances are you have heard of cetane, but you may be lacking a complete understanding of the term. While cetane levels might not be a primary concern, overlooking them can lead to expensive consequences. Using diesel fuel with an inadequate cetane number can cause reduced efficiency, engine wear, and many other issues. Understanding what it is, how it’s calculated, and why it matters can help you make better choices for your diesel engine’s performance and longevity. So, let’s break it down!

What is it?

Cetane is a highly combustible chemical compound that occurs naturally in diesel fuel. It ignites under high pressure, leading to the combustion of the fuel. More specifically, cetane number (CN), is a measure of the combustion quality of diesel fuel during compression ignition. It’s similar to the octane rating for gasoline but operates inversely. While high-octane gasoline resists early ignition, high-cetane diesel fuel encourages quick ignition of the fuel when compressed.

Cetane ratings influence several aspects of your diesel engine’s performance, such as emissions, engine health, and performance. Higher rates typically burn more completely, reducing harmful emissions. Fuels with appropriate numbers can reduce engine noise, wear and tear, and maintenance needs. Higher cetane helps in the cold starting of engines by ensuring quick ignition at lower temperatures. Overall, a suitable rating can lead to smoother engine performance and improved fuel efficiency.

How is it Calculated?

The rating of a particular diesel blend is determined by the amount of cetane present in the fuel, which is a clear, colorless hydrocarbon that ignites under high pressure. Pure cetane, representing the utmost purity achievable in diesel fuel, has a rating of 100.

This number is calculated based on the ignition delay—the time between the start of injection and the start of combustion (ignition) of the fuel. In practical terms, cetane numbers are measured in a controlled engine test, following specific standards (ASTM D613 for those interested in the technical details). Essentially, the number measures how short the delay is between the injection of fuel into the combustion chamber and the start of combustion. A higher number indicates that this delay is reduced, allowing the fuel to ignite more quickly and easily under compression. This quicker ignition leads to a more complete combustion of the fuel.

Diesel fuels typically have cetane ratings ranging from 40 to 55. For most engines, a rating of 50 or above is considered excellent, providing a smoother, quieter engine operation, quicker start-up, reduced emissions, and generally better performance, especially in cold weather.

Where are Cetane Ratings found?

Cetane ratings are not always as prominently displayed as octane ratings at gas stations. However, premium diesel fuels often advertise their levels. You might need to check the fuel specifications from the supplier or manufacturer for the exact number. It’s also a good practice to ask at the pump or consult your vehicle’s manual, as some manufacturers recommend specific cetane ratings.

How does low-cetane fuel impact engine performance?

You have probably heard the phrase “diesel is diesel.” Diesel may be diesel, but your engine is only as efficient as the quality of the fuel you use. Regardless of your engine’s capabilities, using diesel with a low cetane rating can hinder its performance. One of the primary issues is poor fuel economy. Operating on low-cetane fuel often results in higher production of soot particulates. This forces your engine to consume more fuel to manage and expel these particulates, which is especially concerning if you’re budget-conscious and aiming to minimize fuel consumption. Additionally, low cetane fuel typically leads to less power output from your engine. This happens because of incomplete or improperly timed combustion, often leaving residual fuel that fails to contribute to engine power. This unburned fuel affects performance and increases emissions. It passes through the system and exits as white smoke and other pollutants, elevating your vehicle’s emission levels.