Cetane number
The cetane number is a value that describes the ignitability of diesel fuel - the higher, the more ignitable. The more unbranched hydrocarbon molecules there are in the fuel, the easier it is for it to ignite . This behavior is desirable in diesel engines and one of the functional principles.
definition
The cetane number (CZ) of a fuel indicates that it behaves exactly like a mixture of n- hexadecane (the old name for hexadecane is cetane) and 1-methylnaphthalene with the specified volume fraction of cetane. A mixture of 30% ignitable hexadecane (cetane) and 70% ignitable methylnaphthalene has a cetane number of 30.
Instead of 1-methylnaphthalene, the synthetically accessible 2,2,4,4,6,8,8-heptamethylnonane with a cetane number of 15 is used as an ignitable fuel .
Effects
A low cetane number can cause the ignition delay (the time from injection into the cylinder to ignition) to be too high, so that the sudden, explosive combustion creates a loud combustion noise (“nailing”). However, it is not an explosion that is desired , but controlled, directed combustion. In addition to the restriction of driving comfort and the noise pollution of the environment, the engine is also unnecessarily high stressed by nailing, and the exhaust gas values also deteriorate.
The so-called "cold running nailing", which initially occurs after a cold start of a diesel engine, is harmless.
Increase in cetane number
Tetranitromethane (highly carcinogenic), amyl nitrate , acetone peroxide (highly explosive, no longer in use today), di-tert-butyl peroxide or 2-ethylhexyl nitrate can be added to diesel fuel in order to increase the cetane number and thus improve ignitability. However, there are disadvantages such as increased toxicity , poorer storage stability and additional costs.
Determination of the cetane number
In Germany, the cetane number is determined in accordance with DIN 51773. A special motor, the BASF motor or CFR motor , is used for this purpose. The ignitability is determined with a defined constant ignition delay. The ignition delay is the time between injection and self-ignition of the fuel. In the test engine, this is done by changing the amount of intake air, which in turn determines the compression and combustion pressure. Diesel fuels with a known cetane number are used as control fuels. Their cetane number has been determined beforehand using comparative measurements with other engines. The control fuel is only used to check the engine status of the test engine.
The test engine has the following special features for determining the cetane number:
- Adjustment device for start of fuel injection and quantity
- Display device for determining the start of injection
- Device for determining the amount of air drawn in at constant speed . The discharge pressure is reduced by throttling the air volume.
- Measuring device for fuel quantity
- Measuring device for the ignition delay
After it has been completed, the fuel is given a designation:
- DIN51773-CZ-50.3 BASF - where the number 50.3 in this example represents the cetane number.
The determination of the cetane number in accordance with DIN EN ISO 5165 (test method with the CFR engine) leads to inconsistencies, especially in the case of fuels and fuel mixtures with a high degree of unwillingness to ignite (diesel-alcohol mixtures) or in the case of fuels with greatly changed viscosity ( rapeseed oil ). Newer test methods are proposed (Fuel Injection Analyzer from Fueltech AS in Trondheim, Norway in the case of rapeseed oil).
Applications
Old diesel engines can be operated with cetane numbers from 40, but modern high-speed diesel engines require cetane numbers from 50. The cetane number of fuels on the market is already between 51 and 56. Engine manufacturers are demanding a general increase to 58.
Winter diesel tends to have a lower cetane number, as winter diesel partly has to do without higher-boiling components due to the necessary low-temperature properties.
Various mineral oil companies offer premium fuels with increased cetane numbers.
See also
literature
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering . 3. Edition. Westermann Schulbuchverlag, Braunschweig 2000, ISBN 3-14-221500-X .
- Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology . 27th edition. Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .
- Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise . 5th edition. Holland + Josenhans, Stuttgart 1997, ISBN 3-7782-3520-6 .
- Josef Rath: Influence of structure and carbon number on the cetane number of kerosene . Vienna 1997 (diploma thesis).
Web links
Individual evidence
- ^ Van Basshuysen , Schäfer: Handbook internal combustion engine; P. 806; Vieweg-Verlag, Wiesbaden 2007. ISBN 978-3-8348-0227-9 .