Cetane number

The cetane number describes the ignition quality of diesel fuels and indicates the ignition delay when injected into hot air. Higher values promote more even combustion and better exhaust gas quality.

Definition
The cetane number is a measure of the ignition quality of diesel fuels, which is decisive for the combustion process in diesel or compression ignition engines. It is determined by the percentage of n-hexadecane (cetane) in a mixture with 1-methylnaphthalene or heptamethylnonane in a test engine.

Background / Origin
The cetane number is determined in accordance with the DIN 51773 standard, in which a diesel fuel to be tested is burned in a special test engine to determine its ignitability. The comparison is made with a defined mixture of n-hexadecane and 1-methylnaphthalene or heptamethylnonane. Pure cetane has the highest cetane number at 100 and 1-methylnaphthalene the lowest at 0.

The DIN EN 590 standard stipulates that diesel fuels must have a minimum cetane number of 51, although higher values of 60 to 80 can be achieved. High-quality fuels enable complete combustion, especially under demanding engine conditions, which improves exhaust gas quality.

Additives are used to improve the cetane number. However, these additives, such as tetranitromethane, increase the toxicity of the fuel and may be necessary, especially in winter, to ensure sufficient fluidity.

Properties
The cetane number of diesel fuels reflects their ignitability and is associated with several physical and chemical characteristics:

- Composition: the cetane number is determined by the proportion of cetane in a blend with compounds that are unwilling to ignite.
- Physical characteristics: Fuels with high cetane numbers burn more evenly, resulting in lower noise and soot emissions.
- Quality differences: Biodiesel typically has a cetane number of 56 to 58, while unprocessed vegetable oils are significantly lower and cannot be used optimally for diesel engines.
- Chemical characteristics: Gas-to-liquid diesel achieves extremely high cetane numbers of 75 to 80, as these synthetic fuels do not contain aromatics.

Application and importance in the market
The cetane number has a direct influence on exhaust gas behavior and engine performance. For diesel engines, a cetane number of at least 51 is required to ensure ignition. Higher-grade fuels (60-80) offer advantages in terms of engine performance, especially at high engine speeds.

The use of additives can increase the cetane number, but this is often accompanied by increased toxicity. In comparison, biodiesel can have a higher cetane number than conventional diesel, depending on the fuel used. The market is trending towards the use of fuels with higher cetane numbers to reduce emissions and increase efficiency.

Differentiation from similar terms
- Cetane number vs. octane number: While the cetane number describes the ignitability of diesel fuels, the octane number refers to gasoline fuels and their resistance to spontaneous combustion.
- Biodiesel vs. conventional diesel fuel: Biodiesel has a higher cetane number (56-58) but is a suitable alternative, while unprocessed vegetable oils have lower cetane numbers and are more inefficient.

Summary
The cetane number is an important measure of the ignition quality of diesel fuels, which influences engine performance and exhaust gas quality. It is determined by comparison in test engines and should be at least 51. Higher cetane numbers enable better combustion and lower emissions. Additives can increase the cetane number, but have a negative effect on the toxicity of the fuel. Biodiesel and gas-to-liquid diesel are promising alternatives to conventional diesel.