Important Terms from A to Z
The cetane number (CN) is an indicator of the ignitibility of diesel fuels. It provides information about the ignition delay, i.e. the speed of self-ignition of diesel fuel when injected into hot air through the fuel injector. In an engine that works on the diesel principle, the cetane number is authoritative for describing the quality of the combustion process. Higher cetane numbers mean shorter ignition delay and at the same time a better performance of the diesel engine: The injected fuel burns more evenly and completely, which usually results in higher-quality exhaust air, especially with regard to soot, particulate matter and unburned hydrocarbons. The ignition properties of a fuel not only affect exhaust properties, but also the combustion noise. If the ignition delay is high, for example due to a low cetane number, most of the injected fuel burns explosively. This leads to the typical loud diesel combustion noise (“knocking”).
The DIN 51773 standard describes how the cetane number is determined: The test fuel is burned in a specific test engine (BASF engine or CFR engine). A mixture of n-hexadecane (formerly cetane) and 1-methylnaphthalene is sought that has the same ignition quality in the same test engine as the fuel in question. Pure cetane is assigned the highest cetane number, i.e. 100 (most ignitable) and pure 1-methylnaphthalene the lowest cetane number 0 (least ignitable). The cetane number describes the percentage of cetane found in the mixture that corresponds to the ignition quality of the test fuel. So a fuel with a cetane number of 50 is exactly as ignitable as a mixture of 50% cetane and 50% 1-methylnaphthalene. Instead of 1-methylnaphthalene, heptamethylnonane (with a cetane number of 15) can also be used as the non-ignitable reference fuel.
The DIN EN 590 standard prescribes that a diesel fuel must have a cetane number of at least 51. But fuels can have higher values of 60 or in exceptional cases even 80. Such (usually more expensive) fuels can burn completely, even under unfavorable engine conditions (e.g. at high engine speeds). Because this means that no unburned or partially burned fuel components enter the exhaust, a higher exhaust gas quality is achieved. The cetane number a fuel needs to have for a given engine depends on the latter’s construction type (compression ratio, geometry of the combustion chamber, arrangement of the injectors):
- Large stationary diesel engines or engines that run slowly need diesel fuels with cetane numbers between 20 and 40,
- Older diesel engines and older car engines require a cetane number above 40,
- High-speed, modern diesel engines, especially vehicle engines, need cetane numbers above 52.
The cetane number of diesel fuel can be increased by mixing in certain additives such as tetranitromethane, amyl nitrate, acetone peroxide and 2-ethylhexyl nitrate. However, these additives increase the toxicity of the fuel. In winter, their addition may be necessary to provide sufficient fuel flowability even in heavy frost.
Biodiesel as a biofuel has a cetane number of 56-58, making it an excellent alternative to conventional diesel fuel. Unprocessed vegetable oils usually have low cetane numbers, burn incompletely, and are therefore a much poorer choice for diesel engines. “Gas-to-liquid” diesel, which is produced as a synthetic fuel from gases – such as natural gas or methane gas from biomass gasification – can have extremely high cetane numbers of 75 to 80. One reason for the high cetane numbers achieved by gas-to-liquid diesel is that such synthetic fuels contain no aromatics (cyclic compounds). Use of synthetic diesel could replace the sometimes toxic mix of additives that gives conventional diesel fuel higher cetane numbers.