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Diesel oil

Recently, diesel oil has become the most important motor fuel, one whose consumption will continue to grow. The European diesel-oil market is focused not only on truck transport – the number of small utility vehicles and particularly passenger cars with diesel engines shows significant growth as well.

What actually is diesel oil? Diesel oil is one of the most important petroleum refinery products. As regards its production, we can classify diesel oil as a medium petroleum distillate. It is obtained via distillation of petroleum, followed by other technological processes such as hydrogenation refining, hydrocracking, catalytic cracking, etc. In general, we can characterise diesel oil as a complex mixture mainly of petroleum hydrocarbons with 12 to 22 carbon atoms and a boiling point within the range of approx. 180 to 370°C. In order for this mixture to be usable as a motor fuel, it needs to comply with a range of qualitative standards, which are subject to thorough output control by all producers.

Diesel oil came into use at the beginning of the 20th century. The discoverer of the compression-ignition engine fuelled by diesel oil was the German inventor Rudolf Diesel, who was awarded the Grand Prix for his discovery at the World Exhibition in Paris in 1900. This is why the compression-ignition engine and diesel oil are often simply called “diesel,” and we especially find this term in use abroad.

In the beginning, only medium petroleum distillate was used as diesel oil and its properties were determined by the characteristics of the processed petroleum. With the expansion of automobile use, improved combustion engine design, and increased consumption of diesel oil, other technological processes began to be used in its production and the properties of diesel oil became more standardised.

One of the most important characteristics of diesel oil is its low-temperature behaviour. Ever since it was first used, two types of diesel oil – summer and winter – have been available on the market, which differ in their freezing points. It turned out, however, that freezing point is completely insufficient for characterising the behaviour of diesel oil at low temperatures and thus other parameters were introduced that were better at conveying information about the fuel.

For instance, in the 1960s the cloud point (i.e. the temperature at which the first crystals of solid paraffin begin to form) appeared in addition to freezing point, but not even this combination could satisfactorily describe the behaviour of diesel oil at low temperatures and thus the cloud point was replaced by the cold filter plugging point (CFPP) in the mid 1970s – this is the lowest temperature at which diesel oil is still able to pass through a strainer with a precisely defined mesh size. The freezing point was completely abandoned in the mid-1990s and winter-type diesel fuel is labelled with both its cloud point and CFPP.

As regards environmental protection, the issue of sulphur content in diesel oil came to the forefront in both Western Europe and the Czech Republic in the late 1970s. Sulphur content in diesel oil produced in Czechoslovak refineries began to decrease rapidly from the original 0.5% to 0.25% down to 0.15% m/m in 1987. Here, we should emphasise that Czechoslovakia thus ranked among the leading European nations as regards sulphur content in diesel oil. Perhaps only West Germany and some Scandinavian nations were ahead of us and we left behind such economic powers as France and Italy. In 1995, the Czech Republic set the maximum level of sulphur content in diesel oil to 0.05%, more than a year ahead of the EU. The reduction in sulphur content continues to this day and diesel oil produced today contains at most 50, in respect 10 mg/kg.

To improve the utility value of diesel oil, various additives (admixtures) are widely used. Some of these are applied directly at the refinery (e.g., additives regulating low-temperature properties, lubricating and conductivity-enhancing additives), while others are often used in the form of multifunctional “packages” used in particular by large, individual distribution companies to distinguish themselves form their competition and to maintain their market position. These “packages” usually contain detergents, corrosion inhibitors, de-emulsification admixtures, antifoam additives, etc.

In wintertime we may encounter diesel oil that is not totally clear, instead finding it to be to a certain extent turbid. This phenomenon is not necessarily detrimental. The temperature at which the turbidity appears, i.e., at which paraffin begins to crystallise, is called the cloud point. However, crystals formed in this way are small and do not prevent the fuel from passing through the fuel duct of the engine. The fuel’s ability to pass through the fuel lines is only impaired if the temperature drops below the so-called cold filter plugging point, i.e., the temperature at which the resulting paraffin crystals are so large that they clog the injection pump filter. The filter may also become clogged by prolonged periods of inactivity at temperatures near the cold filter plugging point. Winter-type diesel fuel distributed on our market has a cloud point below -8°C and CFPP below -20°C. The actual operability temperature is around -17°C. To ensure failure-free operation of diesel engines at low temperatures, refineries produce the so-called arctic diesel oil with an operability temperature all the way down to -30°C. This fuel is offered primarily by fuelling stations in the foothills of the Krkonoše and Šumava mountains.