Page 126 - The Jet Engine

Fuel system

pressures required for atomization of the fuel permits
the use of the comparatively lighter gear-type pump.

98. A flow distributor (fig. 10-20) is often required to
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
Fig. 10-18 A Duple fuel spray nozzle and which the governor controls is reduced, and in
pressurizing valve. consequence the governor must be reset.

spill and also for controlling the amount of spill flow 103. With a decrease in specific gravity, the
at various engine operating conditions. A centrifugal pressure on the diaphragm is less and the
disadvatage of this system is that excess heat may speed at which the governor controls is increased; in
be generated when a large volume of fuel is being consequence, the pilot must control the maximum
recirculated to inlet. Such heat may eventually lead r.p.m. by manual operation of the throttle to prevent
to a deterioration of the fuel. overspeeding the engine until the governor can be
reset. The hydro-mechanical governor (para. 23) is
97. The airspray nozzle (fig. 10-19), carries a less sensitive to changes of specific gravity than the
proportion of the primary combustion air (Part 4) with centrifugal governor and is therefore preferred on
the injected fuel. By aerating the spray, the local fuel- many fuel systems.
rich concentrations produced by other types of spray
nozzle are avoided, thus giving a reduction in both 104. The pressure drop governor in the combined
carbon formation and exhaust smoke. An additional acceleration and speed control system (para. 51) is
advantage of the airspray nozzle is that the low density compensated, by using a buoyant material

116