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Ever wondered what the difference is between jet fuel and diesel?

Release date:
March 2020
A technical comparison between Jet and Diesel
Alisdair Clark, Air BP’s aviation fuels research and development manager, explains there’s much more to this than both products simply being different to gasoline

Let’s start right at the beginning, in the early days of turbine engine design. Here was an aviation engine which burnt the fuel under steady-state conditions, a little bit like a blow-torch. No longer was there the worry of ‘waiting for the spark’ and risk of detonation in the world of the Aviation Gasoline (Avgas) piston engine. Any fuel would do, or that’s what Sir Frank Whittle and Power Jets Ltd originally thought in the 1930s when test work started in Rugby. 

A good choice looked to be a product which was easy to manufacture with a higher flash point than Avgas for safety. Why the higher flash point? Flash point refers to the temperature at which the vapour above the liquid fuel catches fire when a flame or spark is present. For a pool of Avgas this temperature is very low, less than -30°C, making it hazardous if spilt and easily flammable. Jet and diesel fuels usually have to be heated for the vapour to burn under ambient conditions, to greater than +38 and +55 °C respectively.

So, Whittle decided to run the new aviation turbine engine on diesel based on availability and its higher flash point, jet fuel not having been invented. But all was not well - carbon deposits blocked the vaporisers, coated flame tubes, caused local overheating and power instability. A Gas Turbine Panel was formed to investigate. It concluded that while aviation turbine engines could burn many fuel types, for reliable and efficient operation a cleaner burning kerosene with a low freeze point offered the best solution. This resulted in the development of the first jet fuel specification ‘RDE/F/KER’ which has evolved into the standards used today for Jet A and Jet A-1. 

While both jet and diesel fuel have a better flash point than Avgas, how else do they differ? Aviators fly at high altitude under bitterly cold conditions – jet fuel  keeps flowing under these conditions while diesel would freeze or form wax crystals blocking filters and stalling engines. Jet can also act as a coolant for the engine oil and maintain clean injector nozzles, avoiding carbon deposits in the severe temperature regime of modern turbine engine design, a lesson learnt in the early testing days at Rugby. Specifications are also very strict regarding the approval and use of additives or unusual components. For example, no blending of fatty acid methyl esters, as used in diesel, is permitted due to risks to low temperature properties, impact on aircraft range and fuel stability.  

Could an aviation turbine engine run on other fuels? In rare cases such approvals do exist for particular engines / airframes, usually for emergency use or where there is major difficulty in fuel supply. However, at Air BP, we will always advise fuelling your turbine engine with jet to ensure more efficient, reliable and long-lasting performance from your aircraft – you are purchasing 80 years of industry jet fuel knowledge and oversight.

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