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the engine with fuel in a form suitable for combustion
and to control the flow to the required quantity
necessary for easy starting, acceleration and stable
running, at all engine operating conditions. To do
this, one or more fuel pumps are used to deliver the
fuel to the fuel spray nozzles, which inject it into the
combustion system (Part 4) in the form of an
atomized spray. Because the flow rate must vary
according to the amount of air passing through the
engine to maintain a constant selected engine speed
or pressure ratio, the controlling devices are fully
automatic with the exception of engine power
selection, which is achieved by a manual throttle or
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power lever. A fuel shut-off valve (cock) control lever
is also used to stop the engine, although in some
instances these two manual controls are combined
for single-lever operation.
2. It is also necessary to have automatic safety
controls that prevent the engine gas temperature,
compressor delivery pressure, and the rotating
assembly speed, from exceeding their maximum
limitations.
3. With the turbo-propeller engine, changes in
propeller speed and pitch have to be taken into
account due to their effect on the power output of the
engine. Thus, it is usual to interconnect the throttle
lever and propeller controller unit, for by so doing the
correct relationship between fuel flow and airflow is
maintained at all engine speeds and the pilot is given
single-lever control of the engine. Although the
maximum speed of the engine is normally
determined by the propeller speed controller, overspeeding
is ultimately prevented by a governor in the
fuel system.
4. The fuel system often provides for ancillary
functions, such as oil cooling (Part 8) and the
hydraulic control of various engine control systems;
for example, compressor airflow control (Part 3).
MANUAL AND AUTOMATIC CONTROL
5. The control of power or thrust of the gas turbine
engine is effected by regulating the quantity of fuel
injected into the combustion system. When a higher
thrust is required, the throttle is opened and the
pressure to the fuel spray nozzles increases due to
the greater fuel flow. This has the effect of increasing
the gas temperature, which in turn increases the
acceleration of the gases through the turbine to give
a higher engine speed and a correspondingly greater
airflow, consequently producing an increase in
engine thrust.
6. This relationship between the airflow induced
through the engine and the fuel supplied is, however,
complicated by changes in altitude, air temperature
and aircraft speed. These variables change the
density of the air at the engine intake and consequently
the mass of air induced through the engine.
A typical change of airflow with altitude is shown in
fig. 10-1. To meet this change in airflow a similar
change in fuel flow (fig. 10-2) must occur, otherwise
the ratio of airflow to fuel flow will change and will
increase or decrease the engine speed from that
originally selected by the throttle lever position.
7. Described in this Part are five representative
systems of automatic fuel control; these are the
pressure control and flow control systems, which are
Fuel system
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Fig. 10-1 Airflow changing with altitude.
Fig. 10-2 Fuel flow changing with altitude.
Fuel system
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Fig. 10-3 Simplified fuel systems for turbo-propeller and turbo-jet engines.
Fuel system
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Fig. 10-4 A pressure control system (turbo-propeller engine).
hydro-mechanical, and the acceleration and speed
control and pressure ratio control systems, which are
mechanical. With the exception of the pressure ratio
control system, which uses a gear-type pump, all the
systems use a variable-stroke, multi-plunger type
fuel pump to supply the fuel to the spray nozzles.
8. Some engines are fitted with an electronic
system of control and this generally involves the use
of electronic circuits to measure and translate
changing engine conditions to automatically adjust
the fuel pump output. On helicopters powered by gas
turbine engines using the free-power turbine
principle (Part 5), additional manual and automatic
controls on the engine govern the free-power turbine
and, consequently, aircraft rotor speed.
FUEL CONTROL SYSTEMS
9. Typical high pressure (H.P.) fuel control systems
for a turbo-propeller engine and a turbo-jet engine
are shown in simplified form in fig. 10-3, each
basically consisting of an H.P. pump, a throttle
control and a number of fuel spray nozzles. In
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