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compensate for the gravity head across the manifold
at low fuel pressures to ensure that all spray nozzles
pass equal quantities of fuel.
99. Some combustion systems vaporize the fuel
(Part 4) as it enters the combustion zone.
FUEL HEATING
100. On many engines, a fuel-cooled oil cooler
(Part 8) is located between the L.P. fuel pump and
the inlet to the fuel filter (fig. 10-13), and advantage
is taken of this to transfer the heat from the oil to the
fuel and thus prevent blockage of the filter element
by ice particles. When heat transference by this
means is insufficient, the fuel is passed through a
second heat exchanger where it absorbs heat from a
thermostatically controlled airflow taken from the
compressor.
EFFECT OF A CHANGE OF FUEL
101. The main effect on the engine of a change
from one grade of fuel to another arises from the
variation of specific gravity and the number of heat
units obtainable from a gallon of fuel. As the number
of heat units per pound is practically the same for all
fuels approved for gas turbine engines, a comparison
of heat values per gallon can be obtained by
comparing specific gravities.
102. Changes in specific gravity have a definite
effect on the centrifugal pressure type of engine
speed governor (para. 15), for with an increase in
specific gravity the centrifugal pressure acting on the
governor diaphragm is greater. Thus the speed at
which the governor controls is reduced, and in
consequence the governor must be reset.
103. With a decrease in specific gravity, the
centrifugal pressure on the diaphragm is less and the
speed at which the governor controls is increased; in
consequence, the pilot must control the maximum
r.p.m. by manual operation of the throttle to prevent
overspeeding the engine until the governor can be
reset. The hydro-mechanical governor (para. 23) is
less sensitive to changes of specific gravity than the
centrifugal governor and is therefore preferred on
many fuel systems.
104. The pressure drop governor in the combined
acceleration and speed control system (para. 51) is
density compensated, by using a buoyant material
Fuel system
116
Fig. 10-18 A Duple fuel spray nozzle and
pressurizing valve.
for the governor weights, resulting in fuel being
metered on mass flow rather than volume flow.
105. Changes to a lower grade of fuel can lead to
production of carbon, giving a greater flame
luminosity and temperature, leading to higher
combustor metal temperatures and reduced
combustor and turbine life.
GAS TURBINE FUELS
106. Fuels for aircraft gas turbine engines must
conform to strict requirements to give optimum
engine performance, economy, safety and overhaul
life. Fuels are classed under two headings, kerosinetype
fuel and wide-cut gasoline-type fuel.
Fuel system
117
Fig. 10-19 An airspray nozzle.
Fig. 10-20 Fuel flow distributor.
Fuel requirements
107. In general, a gas turbine fuel should have the
following qualities:
(1) Be ’pumpable’ and flow easily under all
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
its viscosity or thickness, which is related to fuel
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
fuel filter or the orifices in the fuel system. Fuel
heating and anti-icing additives are available to
alleviate this problem.
109. For easy starting, the gas turbine engine
depends upon the satisfactory ignition of the
atomized spray of fuel from the fuel spray nozzles,
assuming that the engine is being motored at the
required speed. Satisfactory ignition depends upon
the quality of fuel in two ways:
(1) The volatility of the fuel; that is, its ability to
vaporize easily, especially at low
temperatures.
(2) The degree of atomization, which depends
upon the viscosity of the fuel, the fuel
pressure applied, and the design of the
atomizer.
110. The calorific value (fig. 10-21) of a fuel is an
　
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