Repsol YPF

Choosing JET as a fuel for aviation turbines

To increase efficiency of gas turbine engines, the internal temperature has been increased leading to a higher combustion pressure. This makes current engines extremely sensitive to the fuel´s quality.

Current turbojets with complex features compress air up to 3 or 4 times more than piston engines. Air compression, prior to its blending with fuel and combustion, is one of the most highly influential factors where fuel finances are concerned. However, obtaining  a compression ratio of 30:1 and 40:1 is far from easy. It also requires heavy and large rotary compressors with high speed rotation. 

As the engine compression ratio increases, so too does the amount of steps to be followed, the cost, the size and weight of the compressor, etc. For example: the Pratt & Whitney PW800 has: Fan + low pressure compressor,  3 steps + high pressure compressor, 5 steps, before the combustion chamber.

The whole process starts with the fan, the large wheel made up of blades that can be seen at the entrance to the engine. Specialists know that the fan is the initial compressor that stands out because of the size of its diameter.

The compressor rotates and compresses air. The function of the turbine is to provide the necessary power to support the compressor's huge efforts.The compressor and the turbine are, therefore, on the same axle. Just like the pump, located where the water flow narrows, the turbine transforms the energy of the flow of combustion gases into its own rotation movement. This rotation is then transmitted to the compressor.

The low pressure turbine pulls the fan and the compressor. These are called low-pressure because the air (in the compressor) or the combustion gases (in the turbine) have low or intermediate pressure compared to the pressure reaches inside the engine. As these engines compress air to an often very high degree, they need to keep compressing it in a different high pressure compressor that is connected to a high pressure turbine which provides the rotation boost.

The FBP (Final Boiling Point) is kept under control to prevent the temperature from rising above safety levels for the engine. This avoids thermal cracking of the heavy components which in turn can lead to the production of carbon particles.

These particles, normally associated with rubber, would reduce the transfer of heat and interfere with the fuel flow.

In modern engines, JET A-1 can be used as fuel, a lubricant and a heat exchanger for cooling:

• the engine oil
• the hydraulic circuit 
• the air conditioning equipment          

The rise in temperature that the fuel experiences when it is used for refrigeration purposes requires supervision of rubber and lacquer formations, not only through the rubber content test but also through the JFTOT that simulates the heating conditions and then checks the lacquer deposit levels. These deposits  would damage the heat exchange mechanism as well as measuring appliances, filters and injection nozzles.

This effect is of even greater importance when it comes to supersonic aircraft due to the additional heat in the fuselage as a result of friction with the air.

All of this requires strict control of the thermic stability.

Comparing the three most common fuels, petrol, gas-oil and kerosene:

• Petrol: It is too volatile for extreme altitude flights. Problem with the vapour lock.
• Gas-oil: Crystallization point is too high, FBP (< 380ºC), even higher than that of petrol, which means larger Carbon formations.
• Kerosene: Low vapour pressure. Low volatility. No vapour lock. Inflammation point safer than that of petrol. Has a good crystallization point for high altitude flights. FBP<300ºC, lower than that of gas-oil.