The cetane number describes the ignition quality of diesel fuel. The more hydrocarbon molecules unbranched built are included in percentage of the fuel, the easier it ignites. This behavior is desirable in diesel engines.
The cetane number (CZ) of a fuel indicates that this behaves exactly like a mixture of n -hexadecane ( the old name for hexadecane is cetane ) and 1- methylnaphthalene with the specified volume fraction cetane. A mixture with 30 percent cetane has, for example, the cetane number 30
Instead of 1-methylnaphthalene and the synthetically accessible 2,2,4,4,6,8,8 - heptamethylnonane is used with a cetane number of 15 as of flammable fuel.
Due to a low cetane number of the ignition delay can be too high, so that by sudden, explosive fuel combustion a loud combustion noise is generated ( "knocking "). What is desired is no "explosion" but a controlled and directed burn.
The diesel fuel can tetranitromethane, amyl nitrate, acetone peroxide or 2 -ethyl- hexyl nitrate are added to increase the cetane number and thus improve the ignition performance, however, disadvantages such as toxicity, inferior storage stability and additional costs are involved.
Determination of cetane number
The determination of the cetane number is done in Germany according to DIN 51773rd this purpose, a special motor, the BASF engine or CFR engine used. Put simply, the ignition is determined by changing the intake air at constant ignition delay. The ignition delay is the time between the injection and the self-ignition of the fuel.
As a control fuels are known cetane number diesel fuels used. Their cetane number in turn is determined by comparative measurements with other motors before. The control fuel is only used to check the motor status of the test engine.
The test engine has the following features to determine the cetane number:
- Adjustment for fuel injection start and quantity
- Display means for determining the start of injection
- Means for determining the intake air quantity at a constant speed. By regulating the amount of air the compressor discharge pressure is reduced.
- Measurement device for the fuel quantity
- Measuring device for the ignition delay
After complete implementation of the fuel shall be designated:
- DIN51773 -CZ -50, 3 BASF - the number 50.3 in this example is the cetane number.
Especially for fuels and fuel mixtures with high Zündunwilligkeit ( diesel -alcohol mixtures ) or fuels with heavily modified viscosity ( rapeseed oil) to determine the cetane number according to DIN EN ISO 5165 leads ( test method with the CFR engine ) to inconsistencies. Newer testing methods are proposed (Fuel Injection Analyzer Company Fueltech AS in Trondheim, Norway, in the case of rapeseed oil).
Old diesel engines can be operated with cetane numbers from 40. However, modern high-speed diesel engines require cetane numbers from 50 The cetane number of fuels on the market today is already between 51 and 56 engine manufacturers call for a general increase to 58 Summer diesel tends to have a higher cetane number, as the winter fuel due to the necessary temperature properties of higher-boiling components are partially must be dispensed with. Aral offers an "Ultimate Diesel" with a minimum cetane rating of 60.