Diesel is a fuel for diesel engines with internal combustion. Diesel engines have better efficiency and higher mensural volumetric power compared to spark ignition engines, so they are used in freight transport, agricultural machinery, etc. Diesel is a liquid fuel formed by a mixture of hardly evaporable hydrocarbons and other additives. The usual number of carbon atoms in the individual hydrocarbon molecules forming the basis of the mixture is between 9 and 22.
Diesel is most often produced by hydrogenation of fractions from crude oil distillation, usually at temperatures of 160 – 370 ° C. Diesel must meet the requirements specified in standard EN STN 590, it is a sulfur-free fuel with a sulfur content of up to 10 ppm, while its essential component must be also biofuel.
The quality of diesel is expressed by the cetane number, which expresses its ignition quality. The cetane number thus represents a parallel to the octane number for petrol. The higher the cetane number of diesel, the better its combustion characteristics. In this case, the direct injection engine starts better, has better performance, quieter and smoother operation, and also the exhaust gases contain less unwanted exhaust fumes. Thanks to better performance, fuel consumption also decreases. All this leads to a reduction in the burden on the environment.
Even in diesel, similarly to petrol, are added small amounts of additives that improve its properties. Additives are usually added, in particular:
- to ensure low temperature properties; this additive is added seasonally during the winter and transition periods
- to increase the cetane number
- antifoam additive.
Especially in winter, the emphasis should be placed on the quality of diesel used in cars, as diesel can freeze, unlike petrol. Petrol contains light hydrocarbons, which have very good low-temperature properties, so there is no risk of freezing. Diesel is heavier, it contains longer chains of unbranched hydrocarbons (paraffins) to ensure good combustion properties, but at low temperatures these form crystals which cause “freezing”.
Therefore, from mid-November to the end of February, diesel producers produce so-called winter diesel. As the refineries are prepared for this situation and produce winter-quality diesel on time, it is up to the seller to buy such diesel on time, thus avoiding the problem of diesel “freezing” and to avoid problems with starting and operation of the engine at low temperatures. At the same time, it is recommended that the petrol station operator drains the diesel tank before the winter season. As the diesel at petrol stations contains FAME biofuel, which is hygroscopic, it is recommended to store such diesel for a maximum of 3 months, because prolonged storage causes the fuel to age, increase the water content and thus degrade the diesel.
Types of diesel according to biofuel content and diesel labeling
Under current European legislation, all Member States are required to label dispensers at petrol stations with a single symbol. This is related to meeting the environmental requirements for reducing CO2 emissions from fuels. SAPPO member companies meet these requirements, in particular through the blending of biofuels into fossil fuels. In the conditions of the Slovak Republic, the biodiesel is currently used as biofuel in diesel. Biodiesel is an ester made from vegetable oil or animal fat, the so-called FAME. Another biofuel that is used is hydrotreated oil of vegetable or animal origin, so-called HVO. In the conditions of the Slovak Republic, only diesel with a biofuel content with a maximum FAME volume of 7% is currently sold at petrol stations. The volume of biofuel in diesel is regulated by the relevant legislation of the Slovak Republic as well as the quality standard STN EN 590.
The label for diesel consists of the letter B, which means FAME biofuel and a number expressing its maximum percentage volume.
Symbol B7 indicates diesel that contains maximum 7% of biofuel volume.