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ALL ú ú C01 Page 20 ú Mar 15/97
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
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1. Three engine parameters establish the fuel rate:
_ CDP or P3 (modified for bleed, CBP) - KP3; CIT; and N2 core engine speed. These 3 parameters are used to position the 3D cam and the CDP cam, and through connecting linkages establish fuel flow limits. The contours of the 2 cams are designed to represent the fuel flow limits. The computing (summing/adding) levers mix the output of the two cams and compare the result to the fuel flow. The resultant comparison linkage then controls the limit pilot valve which in turn controls the servo pressure supply to (and the drain from) the governor pilot valve. By stopping the supply or blocking the drain, the governor action of positioning the fuel metering valve is limited as a function of the output of this cam and linkage system.
Acceleration/Deceleration Fuel Rate Limits
1. The deceleration fuel flow rate changes are 55
_ percent of the acceleration fuel flow rate changes. This is done by the limit pilot valve control of PC or PB to the governor pilot valve for deceleration versus acceleration. The deceleration fuel flow schedule is established at a rate which would prevent the engine from incurring a flame-out due to too much reduction in fuel flow relative to the air supplied by the compressor.
Surge Limit
1. The jump rate limiter valve is incorporated between
_ the governor pilot valve and the governor servo piston to prevent unwanted fuel flow transients.
Fuel Metering
1. The MEC meters fuel by varying the area of the fuel
_ metering valve orifice while maintaining a constant pressure drop across the metering valve orifice. The bypass valve maintains the constant pressure drop by varying the quantity of excess fuel (bypass) returned to the fuel pump. The bypass pressure, PB, is used as a reference pressure in the control servo mechanisms.
Fuel Metering Valve Operation
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73-21-00
ALL ú ú C01 Page 21 ú Mar 15/97
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
1. The fuel metering valve is actuated by the governor
_ servo piston on command from the governor pilot valve or limit pilot valve during acceleration or deceleration. Changes in the metering valve orifice size change the differential pressure P1-P2 sensed across the orifice. This differential pressure change operates the bypass valve to bypass more or less fuel if it is necessary to keep a constant pressure differential across the metering valve.
Specific Gravity Adjustment
1. The differential pressure, P1-P2, is adjusted to
_ provide the correct fuel flow for different specific gravity fuels by turning the P1, P2 reference spring screw of the differential pressure pilot valve. A higher differential pressure is established for a lower gravity fuel which will cause less fuel to be bypassed. This is done to supply the engine with more volume of fuel to make sure the same heat value for a given power command.
NOTE: The specific gravity adjustment should be
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used to correct for variations in specific
gravity of different fuels.
Safety Trips
1. The differential pressure pilot valve can be
_ actuated to open the bypass valve and bypass more fuel by an overspeed trip coupled to the tachometer servo system to prevent accidental overspeed of the engine and by command from the pump unloading valve as was described in the servo control system operation.
System Pressurization
1. The MEC contains a pressurizing valve down stream
_ of the fuel metering valve to keep system pressure during low flow conditions. This makes sure there is sufficient fuel pressure for servo operation and fuel metering.
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