To understand the origins of why diesel did not become the chosen fuel for aviation turbine engines we need to go right back to the early days of flight. The initial development of fuel specifications focused on piston engines burning aviation gasoline (Avgas), and the arrival of turbine jet engines in the 1930s signalled a new era for designers. Jet engines burn fuel under steady state conditions and there was no longer the need for the high octane properties of Avgas. A safer choice was a fuel with a higher flash point that didn’t vaporize quite so easily.
Why the higher flash point? Flash point refers to the temperature at which the vapor above the liquid fuel catches fire when a flame or spark is present. For Avgas this temperature is very low, less than -30 °C, making it hazardous if spilt with an easily flammable vapor cloud. Jet and diesel fuels on the other hand have to be heated to greater than +38 and +55 °C respectively for the vapor to burn under ambient conditions.
Based on its higher flash point Frank Whittle and Power Jets Ltd decided, during the early jet engine testing days at Rugby, to run the new aviation turbine engine on diesel. At the time jet fuel hadn’t been developed. However, there were problems when refuelling with diesel. Carbon deposits blocked vaporizers, coated flame tubes, caused local overheating and power instability. A Gas Turbine Panel was formed to investigate. It found 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 standard ‘RDE/ F/ KER’ in 1944 which evolved into the specifications used today for Jet A and Jet A-1.
Developed by the aviation industry, jet fuel specifications are designed to improve the safety and reliability of flight. They also ensure that fuel reaches the aircraft in good condition and that additives are strictly regulated. For example, no blending of fatty acid methyl esters, as used in diesel, is permitted in jet fuel due to risks to low temperature properties, impact on aircraft range and fuel stability. Diesel specifications are set on a more local level and can vary considerably in content. Subsequently a diesel product may change dependent on refinery ability, market demand, government regulation, seasonal requirements and economics.
Another disadvantage with diesel is that at low temperatures it may freeze or form wax crystals blocking filters and stalling engines. This is a problem given aviators fly at altitude under bitterly cold conditions. Jet fuel is manufactured to keep flowing under these circumstances. It 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.
The bottom line comes down to flight safety and performance. With 80 years of industry jet fuel knowledge and oversight, Air bp will always advise fuelling your turbine engine with jet to ensure a more efficient, reliable and long-lasting performance from your aircraft.
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