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Hydromechanical Fuel Control

The basic principle of turbine engine fuel metering is to vary the flow of fuel to

the discharge nozzle by maintaining a constant pressure drop across a metering orifice

whose area is varied by Pilots throttle position and by engine operating condition.

The Fuel Control Unit (FCU) is mounted on the rear flange of fuel pump.

A splinted coupling between pump and fuel control unit transmits a speed signal,

proportional to compressor rotor and turbine speed Ng; to the governing section of fuel

control unit. The fuel control unit determines the fuel schedule for the engine and power

required by controlling the speed of compressor.

The fuel control unit consists of following major components: condition lever

that selects the start, low idle and high idle functions, power lever that selects the gas

generator speed between high idle and maximum through which three dimensional

cam, cam follower lever, fuel valve, fly weight governor that controls the fuel flow to

maintain selected speed, the pneumatic bellows that controls the acceleration schedule.

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The basic fuel control senses the pilots throttle position. The engine RPM, and

the burner pressure to control the amount of fuel metered to the nozzles. A fly weight

governor is used to control the position of throttle valve. The flyweights are driven by the

engine speed proportional to engine RPM, and the compression of the speeder spring is

controlled by the pilot throttle lever.

A bellows actuated burner pressure sensor valve between the discharge side of

the throttle valve and the inlet side of the fuel pump senses the difference between the

burner pressure and ambient pressure so the fuel pressure at the nozzle can vary as

the burner or compressor discharge pressure changes.

The pilot moves the throttle forward to increase thrust from the engine. This

movement increases compression of the speeder spring. The fly weights move inward,

and the throttle valve moves down, sending more fuel to the engine. The engine speeds

up, and centrifugal force causes the flyweights to sling outward until they return to their

upright, or on speed condition. The throttle valve has increased the size of metering

orifice, and the differential relief valve maintains the constant pressure drop across the

throttle valve in its new position.

When the fuel flow is initially increased, it is possible that the fuel air mixture will

enrich enough to cause a rich blowout before the engine speeds up enough to move

sufficient air through the compressor and provide the correct fuel air mixture. But this is

prevented by burner-pressure sensor.

The bellows in the burner pressure sensor opens the bypass valve and some

of the fuel bypasses back to the inlet side of the pump instead of going to the nozzles.

As soon as the engine accelerates to the desired speed, the burner pressure increases

enough to shut off the fuel bypass, and all the metered fuel goes to the nozzles.