Page 128 - The Jet Engine

Fuel system

Fuel requirements Other factors which affect the choice of heat per unit
107. In general, a gas turbine fuel should have the of volume or weight, must also be taken into consid-
following qualities: eration; these include the type of aircraft, the
duration of flight, and the required balance between
(1) Be 'pumpable' and flow easily under all fuel weight and payload.
operating conditions.
(2) Permit engine starting at all ground
conditions and give satisfactory flight
relighting characteristics.
(3) Give efficient combustion at all conditions.
(4) Have as high a calorific value as possible.
(5) Produce minimal harmful effects on the
combustion system or the turbine blades.
(6) Produce minimal corrosive effects on the
fuel system components.
(7) Provide adequate lubrication for the moving
parts of the fuel system.
(8) Reduce fire hazards to a minimum.

108. The pumping qualities of the fuel depend upon Fig. 10-21 Relationship between calorific
its viscosity or thickness, which is related to fuel value and specific gravity.
temperature, Fuel must be satisfactory down to
approximately -50 deg. C. As the fuel temperature
falls, ice crystals may form to cause blockage of the 111. Turbine fuels tend to corrode the components
fuel filter or the orifices in the fuel system. Fuel of the fuel and combustion systems mainly as a result
heating and anti-icing additives are available to of the sulphur and water content of the fuel. Sulphur,
alleviate this problem. when burnt in air, forms sulphur dioxide; when mixed
with water this forms sulphurous acid and is very
109. For easy starting, the gas turbine engine corrosive, particularly on copper and lead. Because it
depends upon the satisfactory ignition of the
atomized spray of fuel from the fuel spray nozzles, is impracticable to completely remove the sulphur
assuming that the engine is being motored at the content, it is essential that the sulphur be kept to a
required speed. Satisfactory ignition depends upon controlled minimum. Although free water is removed
the quality of fuel in two ways: prior to use, dissolved water, i.e. water in solution,
cannot be effectively removed, as the fuel would re-
(1) The volatility of the fuel; that is, its ability to absorb moisture from the atmosphere when stored in
vaporize easily, especially at low a vented aircraft or storage tank (para. 118).
temperatures.
(2) The degree of atomization, which depends 112. All gas turbine fuels are potentially dangerous
upon the viscosity of the fuel, the fuel and therefore handling and storage precautions
pressure applied, and the design of the should be strictly observed.
atomizer.
Vapour locking and boiling
110. The calorific value (fig. 10-21) of a fuel is an 113. The main physical difference between kerosine
expression of the heat or energy content per pound and wide-cut fuels is their degree of volatility, the latter
or gallon that is released during combustion. This type of fuel having a higher volatility, thus increasing
value, which is usually expressed in British thermal the problem of vapour locking and boiling. With
units, influences the range of an aircraft. Where the kerosine-type fuels, the volatility is controlled by distil-
limiting factor is the capacity of the aircraft tanks, the lation and flash point, but with the wide-cut fuels it is
calorific value per unit volume should be as high as controlled by distillation and the Reid Vapour
possible, thus enabling more energy, and hence Pressure (R.V.P.) test. In this test, the absolute
more aircraft range, to be obtained from a given pressure of the fuel is recorded by special apparatus
volume of fuel. When the useful payload is the with the fuel temperature at 37.8 deg. C. (100 deg. F.).
limiting factor, the calorific value per unit of weight
should be as high as possible, because more energy 114. Kerosine has a low vapour pressure and will
can then be obtained from a minimum weight of fuel. boil only at extremely high altitudes or high tempera-

118