Most modern fuel control units meter the flow of fuel by
keeping the pressure drop or difference across the metering valve a
constant value, while varying the orifice of the metering valve.
Another way to control fuel is to keep the valve orifice a constant
size and vary the pressure acting upon the fluid. The operation of a
gas turbine requires that a number of variable conditions be given
careful thought to provide for safe, efficient operation. Among
these are engine rpm, acceleration, exhaust gas temperature (egt),
compressor inlet temperature, compressor discharge pressure, and
throttle or power control setting. All these conditions affect or
are affected by fuel flow, which is increased only to the point where
the limiting temperature is reached. As the engine accelerates and
airflow through the engine increases, more fuel is added. If turbine
inlet temperature were the only engine limitation, a temperature
pickup sensing this temperature could be used. However, it is also
necessary to avoid the operating range that would cause a compressor
surge and stall. Because more than one factor limits engine
operation, it is necessary to schedule the accelerating fuel in
accordance with a combination of these factors. Because turbine
engine compressors are susceptible to surges and stalls, a control
with a longer acceleration time is used than is needed for a
reciprocating engine. This acceleration time is known as a "lag,"
and the pilot must be aware of the time it takes the engine to
accelerate and give him the power change he requires. Compressor
discharge pressure or burner pressure is commonly used as the
variable for these controls, since they vary both with engine speed
and inlet air temperature. By evaluating these variable conditions,
a fair indication of the amount of fuel which can be burned without
exceeding engine limitations is obtained.
Two fuel control systems are discussed in the following
subparagraphs.
a. Automatic control system. The amount of fuel required to
run the engine at rated rpm varies with the inlet air temperature and
pressure. For example, it requires less fuel to run the engine on a
hot day than on a cold day. To relieve the pilot of the necessity of
resetting the power lever to compensate for changes in outside air
temperature and pressure, a speed governor is used. A simple speed
governor consists of flyweights balanced by a spring. When the
engine is running unloaded, at rated speed, the metering valve is
open only far enough to supply the small amount of fuel required. If
a load is applied to the engine, the speed decreases. This decrease
in rpm causes the flyweights to move in under the force of the spring
tension and the fuel valve to open wider and admit more fuel. With
the additional fuel, the engine picks up speed again, and, as the
rated
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