Fuels are chemical compounds compounds whose energy content is release through combustion and is used for the creation of a propulsion.

Fuels are chemical compounds whose energy content is release through combustion and is used for the creation of a propulsion. The largest area of application includes transportation means such as automobiles, motorcycles, aircraft and ships. Petrol, diesel, LPG and natural gas are widespread.


Petrol is fuel for gasoline engines. Although most people just say petrol, "motor fuel" would actually be the correct term. The petrol available to drivers at service stations is distilled from crude oil. However, the basic fuel obtained in this process has an octane number of 50 – 70 RON. Modern engines require an octane number of 95 to 98 RON. Therefore, the basic fuel made from crude oil is “enhanced” by adding other chemical compounds with a high anti-knock rating.

Diesel fuel

Diesel fuel, also known as diesel or diesel oil, is a mixture of various hydrocarbons and is manufactured from gas oil obtained by distilling crude oil. The name comes from the inventor of the diesel engine, Rudolf Diesel. Diesel and extra-light heating oil (EL) have a similar distillation range, which is why they are interchangeable in technically unsophisticated devices. Until the 1990s the two were even identical, however, heating oil was coloured red, because its use in diesel engines was not permitted for taxation purposes. For modern engines this is no longer applicable, because heating oil contains up to 2000 mg/kg sulphur, whereas diesel contains around 10 mg/kg. Today's engines and the exhaust cleaning systems would be severely damaged with heating oil.

As is the case with the petrol engine, ignition accelerators are added to diesel as well. They increase the combustion quality of the fuel, which is indicated by the cetane number. Diesel with a higher cetane number decreases the time between injection and auto-ignition of the fuel.

Basically, diesel has more heavier components than petrol. Therefore it tends to produce more soot, resulting in higher concentrations of fine particulate matter in urban areas. This has led to the introduction of diesel particulate filters and environmental zones. This, of course, also places special requirements on engines and glow plugs.

LPG, Liquified Petroleum Gas

LPG is a gas mixture (butane/propane) and is also known as liquid gas or auto gas. It is gaining in popularity as an alternative to petrol. LPG impacts the environment less than petrol and therefore the tax is lower. The emission of nitrogen oxide is only one-fifth of that in petrol combustion. The CO2 emissions are reduced by 15 per cent and unburned hydrocarbons by half.

Many million vehicles in Europe are already equipped for operation with petrol and liquid gas. However, this places special demands on the engine and ignition. Since LPG burns hotter, especially resistant and high-quality spark plugs are needed. In addition, it has a higher octane rating than petrol - in other words it is more difficult to ignite. This also means that the use of precious metal spark plugs is recommended, because they need little ignition voltage.

Natural gas (compressed natural gas, CNG)

CNG is a combustible natural gas which occurs in underground reservoirs and is obtained as a “by-product” in oil mining. Its most important constituent is methane and it can often contain large amounts of higher hydrocarbons.

Since it contains fewer impurities than fossil fuels, natural gas burns more cleanly. Therefore, the tax on natural gas is lower. Since the mid-1990s the automobile industry has been mass producing cars which run on natural gas; however, the availability of natural gas at service stations in Germany is still relatively low.

Octane rating

The octane rating indicates the anti-knock properties of petrol. The knock resistance, in turn, is the quality of a fuel to not combust out of control through self-ignition ("to knock"), rather precisely controlled through ignition sparks, injection or compression.

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