Rating Diesel: Understanding Cetane Numbers

Rating Diesel; Understanding Cetane Numbers

Hydrocarbons, a compound comprised of carbon and hydrogen, typically make up the bulk of various petroleums. Cetane is one such hydrocarbon that appears in a colorless fluid form. When it comes under extreme compression, the pressure causes it to ignite quite quickly. Cetane is assigned the base rating of a hundred. Accordingly, it is then used as a measurement to indicate how well other fuels, like diesels or even biodiesels, perform.

What Does Cetane Number Indicate?

Standard gasoline uses a rating system of octane numbers to measure its efficiency. In a similar manner, diesel uses cetane numbers as a measurement of how well it combusts. However, there is a significant difference in what exactly these two ratings measure. For gasoline, the octane number measures how well the substance will resist spontaneously combusting at average temperatures without a helper ignition source. This is known as auto-ignition. Comparatively, the cetane number (or CN for short) tests how long the diesel delays its ignition time after the fuel enters the combustion chamber.

Since diesel works with compression instead of a flame or spark, it is most effective when it ignites as quickly as possible. If it has a high cetane number, this means that the delay period is relatively short. In this case, the short delay period allows the diesel to burn more completely. In turn, it helps the vehicle’s engine to run more smoothly and powerfully while producing less emissions. Diesel that takes a longer time to ignite does not work as efficiently. Keep in mind that the cetane number is mostly used for lighter diesel fuels. The efficiency of heaver diesel is measured using a couple of other ratings called the CII and CCAI.

How is the Cetane Number Test Calculated?

There are a number of tests and special procedures that are used to measure cetane ratings. This typically involves engines used specifically for testing purposes, as well as running an analysis of the diesel under varying conditions. However, these methods are not only costly, but they also take a great deal of time and effort. As an alternative, there is another process called the calculated method for finding the cetane number of a fuel. Within the calculated method are many different tests that can be run. One such test that is most commonly used is known as the ASTM D976. Another popular one is called the ASTM 4737. Both of these processes derive cetane numbers by measuring not only the fuel density, but also its boiling and evaporation levels.

What Effect Does the Cetane Number Have on Engine Performance?

With standard cars, it generally does not benefit the engine if the driver fills it with a gasoline that features an octane rating more than the amount recommended by the manufacturer. Similarly, vehicles that run on diesel do not perform any better by using fuel with a cetane number higher than the recommended amount for that specific engine. Despite this, there is sometimes a misconception that any higher cetane number will result in better engine performance and power.

There are many different factors that are involved in determining the best cetane number for a certain type of engine. This normally includes the physical size of the engine, the way it is designed, how fast it operates, as well as its load variations. A less factor, but one that is counted all the same, is external weather or climate conditions. On the other hand, if an engine is operated with a fuel that features a cetane number lower than the recommended amount, there can be several drawbacks. The vehicle will not operate as smoothly, and the poor operation can result in vibrations as well as extra noises. Additionally, it could create a larger amount of emissions and wear on the engine. In some cases, the driver may even have difficulty starting the engine.

Types of Diesel Fuels and Corresponding Cetane Numbers

Most diesels for standard vehicles and general highway usage normally require a cetane rating that falls between 45 to around 55. The table below outlines the different grades of cetane numbers that correspond with various diesel fuels that are compression ignited.

Types of Diesel Cetane Numbers
Regular Diesel 48
Premium Diesel 55
Biodiesel (B100) 55
Biodiesel Blend (B20) 50
Synthetic Diesel 55

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