EFG
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CTC-201-263-01
EFFECTIVITY
73-21-18 Page 21
ALL CFM56-5B ENGINES FOR A319-A320-A321
ENGINE SYSTEMS Jan 03
CFMI PROPRIETARY INFORMATION
CFM56-5B TRAINING MANUAL
EFG
HYDROMECHANICAL UNIT
Delta P and bypass regulating system.
The Delta P valve and by-pass valves regulate a constant pressure drop across the FMV to ensure that the metered fuel flow is proportional to the FMV position.
The system consists of :
- A delta pressure detector.
-A by-pass valve.
The delta pressure detector senses the differential
pressure across the FMV.
Ps pressure (P1) acts on one side of the piston.
FMV output pressure (P2), plus force from a pre-loaded
spring, act on the other side of the piston. P1 - (P2 + spring load) = the stable position. The pre-loaded spring is calibrated to 36 psi.
At the stable position, the by-pass valve control signal is a constant value.
When changes occur in the Delta P across the FMV, the piston becomes unbalanced and moves axially.
This will either increase, or decrease, the by-pass valve control pressure. The by-pass valve piston will move to a more open, or closed position, thus increasing, or decreasing the by-passed fuel pressure.
This, in turn, will decrease or, increase the Ps flow (P1), and restore the Delta P to 36 psid.
Buffer.
There is a buffer, located between the Delta P valve and the by-pass valve.
This provides the necessary compensation to prevent speed overshoot and instability that could occur during overspeed limiter conditions.
EFG
..
CTC-201-264-01
EFFECTIVITY
73-21-18 Page 23
ALL CFM56-5B ENGINES FOR A319-A320-A321
ENGINE SYSTEMS Jan 03
CFMI PROPRIETARY INFORMATION
CFM56-5B TRAINING MANUAL
EFG
HYDROMECHANICAL UNIT
Overspeed governor system.
The overspeed governor system limits the core engine speed (N2) to a maximum of 106%, in the event of a malfunction that could drive the engine into an overspeed condition.
The OSG, which is hydro-mechanical and independent of the ECU, is installed on-line with the Delta P valve and consists of :
-A flyweight device, which is an N2 detector consisting of flyweights, pivot pins, bearings, and a ball head gear. It produces a force on a governor pilot valve proportional to the square of the flyweight speed.
- A governor pilot valve, which ports either Psf, or Pb, to the Delta P valve in order to modulate the by-pass valve. The pilot valve translation depends on the flyweight force, versus a pre-loaded spring force.
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