What exactly is a diesel fuel?


Diesel fuel is the energy source for diesel engines and consists of hydrocarbons of crude oil. As an energy source for internal combustion engines, a distinction is made mainly between gasoline and diesel. Diesel is a liquid energy carrier with a density of about 900g per liter. This is about 10% higher than the density of gasoline. The color of diesel is colorless-yellowish. Its calorific value is 42.6MJ/kg, which is slightly higher than the calorific value of gasoline, which is about 41MJ/kg. Diesel fuel thus has a higher energy density, which can also be exploited by the engine, resulting in lower fuel consumption. Diesel consists of relatively longer molecular chains compared to gasoline. This leads to its more difficult flammability and is the main cause of the technical differences in the functioning of the two types of engines.

The production of diesel fuel takes place in refineries. One ton of petroleum is used to make various products, including diesel and gasoline fuel. To produce diesel, the extracted crude oil is heated and distilled. The steam that forms at low temperatures is used to make gasoline. The medium-weight components in the oil vapor subsequently condense and are collected. The collected liquid consists of longer molecular chains and forms the diesel fuel. Since middle distillation filters heavier components from crude oil, the density of diesel is higher than that of gasoline.

The ratio of gasoline to diesel is not fixed in principle in this production, but it is within a narrow range. Since both fuels are produced in coupled production, manufacturers must always consider and market both products in a networked fashion.

The minimum requirements for diesel fuels are specified in DIN EN 590. Diesel is characterized by the cetane number. This index describes the fuel's ignition propensity. The higher the number, the more ignitable is the fuel. Ignition willing behavior is needed by diesel engines to prevent the so-called nailing. If the fuel does not burn, or does not burn completely, in one or more working cycles in succession, fuel accumulates in the combustion chamber. This burns in one fell swoop in the following work cycle and there is a sharp rise in pressure in the engine block. This results in mechanical charges and can be heard as a regular banging, or nailing, sound.

Biodiesel and additives

Biodiesel (chemical name: fatty acid methyl ester) is not produced from crude oil, but by combining vegetable and animal fats or oils with alcohol. This can, technically, be mixed in any ratio with regular diesel. In Germany, a biofuel quota of 7% has been in force for service stations since 2009.

Other substances can also be added to the tank to improve runability or increase the cetane number. However, all the important additives are usually already added to the fuel by the manufacturer. At extremely low temperatures, diesel becomes thick. This problem is well known, which is why winter diesel with the appropriate additives is then sold at gas stations. If the vehicle is not moved frequently, it should be emptied and refueled before a drop in temperature to prevent any fuel residues from the summer from clumping in the tank.

Instead of legally approved diesel fuel from the service station, EL heating oil can in principle also be added to the diesel tank. However, since it is taxed differently, this is tax evasion and is punishable in Germany. In addition, the operating license of the vehicle expires. It is a misdemeanor and there are other penalties in addition to a fine.

The diesel engine

The diesel engine differs in its mode of operation from the gasoline engine and thus also in its design. The diesel engine is self-igniting and uses the ignition ability of the diesel fuel. This means that no spark plugs are installed in the diesel engine. The fuel is so highly compressed by the pistons that it burns by itself. The compression ratio is about 18:1, while for the gasoline engine it is only 8:1. During this process, higher temperatures are generated than with the gasoline engine. In addition, higher pressures are generated in the combustion chambers, which the diesel engine must be able to withstand. This is counteracted by a more solid construction method. The diesel engine therefore has a lower power-to-weight ratio (kg/Ps), as it has to be built heavier. Heavier pistons mean that moving masses increase and slightly lower engine speeds can be achieved. Its response behavior is correspondingly poorer. Due to the higher temperatures, in some cases injection nozzles are installed in the engine to cool down the pistons. Its advantages are that due to the higher energy density in the diesel fuel and the better efficiency of the diesel engine, the fuel consumption is lower.

Diesel vehicles in Germany and worldwide

In 2016, 46.9% of all newly registered vehicles were equipped with a diesel engine. Their sales have fallen by 2% compared with the previous year. A total of around 15 million diesel vehicles share the German roads with 30 million gasoline-powered vehicles. However, the high proportion of diesels is a purely European phenomenon: worldwide, the proportion of diesel vehicles is only 14. In Germany and Europe, diesel is favored by tax advantages. As a result, diesel is available at German gas stations at a lower price than gasoline. The mineral oil tax on diesel is, because of the importance for the economic traffic, clearly smaller, than on gasoline. This tax was included in the Renewable Energy Act in 2006. Thus per liter gasoline 65.96 cent, on Diesel however only 47.43 cent tax is raised. This price difference has a direct effect on the prices at the gas stations.

Diesel vehicles are subject to higher vehicle tax, more frequent maintenance intervals and higher insurance costs. Therefore, the purchase of a diesel vehicle is often only worthwhile for frequent drivers. If one compares for example the performance-similar Golf 1.0 TSI (gasoline) and the Golf 1.6 TSI (diesel) with each other, the purchase of the diesel vehicle, taking into account the total cost, is worthwhile from about 25.000 kilometers driven per year.

Future prospects for diesel

There are studies that say that diesel engines will be completely irrelevant by 2025. The diesel does emit slightly less CO₂ than the gasoline engine. For it however the emission of NOx (nitrogen oxides) is around a multiple higher. These substances are carcinogenic. In order to protect the environment and relieve inner cities, German municipalities are to be given the option of being able to impose explicit driving bans on diesel vehicles in inner cities. There are already city center bans for diesel in European cities, for example in Paris, Madrid and Athens-Central. The cost argument could also change. Hybrid vehicles are to become the most favorable drive systems.

Diesel vehicles are major consumers of diesel fuels, which are inevitably produced during the refining of crude oil. However, in order not to devalue them, diesel engines will continue to exist in the medium term and will be economical to run due to the tax benefits on the fuel.

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