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Initially used as a lighting fuel, kerosene is obtained from natural oil through refining and distillation. Its main function as reaction engine fuel is to be burned in the combustion chambers in order to produce a gas flow to power the aeroplane.  

Along with the characteristics described above, other important properties which make it ideal for aviation are its suitable thermal stability whilst it is stored; lubricity, fluidity, volatility rate, non-corrosiveness and cleanliness (absence of solid particles –which can obstruct filters- and of water, which may freeze). Other physical properties such as a suitable viscosity and freezing point for this use are characteristics of kerosene. 

In aeroplanes, as well as generating energy, the fuel is also used as hydraulic fluid in the engine control systems and as a refrigerant for certain components. Thermal stability is one of the most important properties of jet fuel because it is also used as a heat exchanger between the engine and other aircraft systems. For these and other reasons it is subjected to tests under extremely severe conditions. 

A little history 

Since the middle of the last century, kerosene has mainly been used in mixtures to power turbine engines. The turbine generates power by converting the calorific energy stored in the fuel into a combination of mechanical energy and heat. 

Two organisations established the specifications for commercial jet fuel and are responsible for enforcing them: ASTM (American Society for Testing and Materials) and MOD (United Kingdom Ministry of Defence). The specifications issued by both bodies are very similar. They define properties and proportions and limit potentially corrosive compounds such as mercaptans and organic acids, among others. 

If we refer to kerosene used for military purposes, the following types are produced:

The JP4, also known as 'wide cut', is essentially a combination of hydrocarbons including petrol and kerosene. However, it has a series of disadvantages, such as its high volatility, which causes losses through evaporation at high altitudes as well as the risk of catching fire when it is handled. Consequently, in the 1970s, 'wide cut' or Jet B was replaced –except in parts of Canada and Alaska- by JP8, a fuel composed almost entirely by kerosene. At the same time, there are others, such as JP5, a kerosene with a high combustion point.  

In the civilian market the following types of kerosene are produced: 

Jet A which has a freezing point of -40°C. In the rest of the world, Jet A-1 is more widely used, which has a freezing point of -47°C, a characteristic that makes it suitable for winter flights or polar routes. 

Some countries have their own jet fuel specifications, but they are practically identical to those issued by the ASTM and MOD. In the Commonwealth states and Eastern Europe the jet fuel specifications adopt the GOST standard. 

Importance of combustion 

The combustion heat is the heat released when a fraction of fuel is burned under certain conditions. Its value depends on the type of hydrocarbons making up the fuel and on its density. This value may be expressed by a volume (energy per unit of volume) or gravimetrically (energy per unit of weight). Normally, a less dense jet fuel has a lower gravimetric energy content and a denser jet fuel has a higher volumetric energy content. A high volumetric energy content increases the energy that can be stored in an aeroplane tank, allowing it to fly further. 

In an aeroplane turbine, during the combustion processes small carbonaceous particles are formed. These particles continue to burn as they pass through the flame and are totally burned. If they are not totally burned they can cause material erosion. At the same time, the carbonaceous particles are responsible for smoke. As such, a fuel’s composition also affects its emissions. 

Volatility, or the jet fuel’s tendency to vaporise, is also very important, as it must vaporise in order to be burned. However, if the volatility is very high it can lead to losses due to evaporation or what is known as 'vapour lock' in the fuel system which causes gaseous seals in the fuel pipe. 

The high combustion point, which indicates the risk of creating a flammable mixture with the air, and the high electric conductivity of static electricity charges in circuits and tanks (due to the anti-static additive content) are other aspects in favour of the use of kerosene as a jet fuel base.

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