Overview
The function of the fuel controlling system is to control the fuel for the operation of the engine in all ground/flight conditions.
The controlling system must control the fuel to be correct for the altitude, pressure and temperature. The controlling system receives inputs from the electronic control unit (ECU) of the full authority digital engine control (FADEC) system. From those inputs, the controlling system supplies sufficient fuel to operate the engine. This control is necessary because of different thrust command inputs from the aircraft. Fuel pressure is also supplied to operate the compressor guide vane assemblies of the engine.
01/07/16
Hydromechanical Unit
The hydromechanical unit (HMU) is attached on the aft side of the AGB with a V-band quick attach detach (QAD) clamp and weighs 28 lb (12.70 kg). The HMU has a fuel pump section and a fuel control unit (FCU). The HMU has a fuel filter module that is connected by fuel lines to the flow divider eco-valve (FDEV). The fuel pump supplies the high-pressure fuel flow into the flow metering part of the FCU. A metering valve (with a linear variable differential transformer (LVDT)), proportional bypass valve, and pressurizing valve are the components of the FCU and are used to control the engine necessary fuel flow. The FCU controls the output of the fuel pump. The ECU controls the torque motors of the three different electrohydraulic servo valves (EHSVs) in the FCU.
These EHSVs are as follows:
- Compressor variable geometry actuator (CVGA) EHSV (with two CVG torque motors)
- Metering valve (MV) EHSV (with two MV torque motors)
- Overspeed and cockpit shutoff EHSV (with two overspeed/shutdown torque motors and one run torque motor)
The fuel filter module is attached to the AGB. The fuel filter module contains a fuel filter, impending filter bypass switch, temperature sensor, and a fuel filter mechanical differential pressure indicator (DPI). The fuel filter cleans the fuel supply to the FCU high-pressure pump and keeps the contamination to a minimum. The fuel heater/oil cooler (FHOC) is upstream from the fuel filter module. The FHOC is of a tube configuration that supplies heat for the fuel system.
01/07/16
Operation
The fuel control system uses the HMU to control engine fuel flow through an electromechanical interface with the FADEC system. The FADEC system calculates engine fuel flow in reply to the throttle command inputs from the aircraft. The ECU in the FADEC system compares the metering valve position. The ECU transmits fuel requirements to the MV EHSV in the HMU. The MV EHSV changes the electrical signals into fluid pressure/flow which changes the metering valve.
The ECU sends command signals to the HMU which controls:
- Fuel pressurization
- Metered flow
- Control flows
- Ecology valve operation on shutdown
- Metered flow shutoff
The HMU performs the following functions:
- Supplies initiation of fuel flow on ECU command
- Supplies metered fuel flow to the engine
- Supplies regulated, filtered motive fuel flow to the aircraft
- Supplies electrical indication of metering valve position
- Supplies regulated, filtered fuel flow to the engine CVGA
- Supplies fuel shutoff on ECU overspeed command
- Supplies fuel shutoff on aircraft command
The HMU receives aircraft fuel at its fuel inlet port and sends it internally to the boost pump. From the boost pump, the fuel is sent externally through the fuel filter module (FFM) to the fuel heater/oil cooler. After the heat exchange, the fuel is sent back in the FFM to be filtered. From the FMM, the fuel goes into the HMU and is sent internally to the high-pressure pump.
High-pressure pump pressure flows to the metering valve inlet, proportional bypass valve, servo pressure regulator, and motive flow pressure regulator. The proportional bypass valve sends unwanted high-pressure pump discharge flow to the high-pressure pump inlet. The pump relief valve protects the fuel pump and FCU from very high-pressure surges.
From the high-pressure pump, the fuel also goes to the motive flow lockout valve. During startup, when the high-pressure pump discharge is low, the lockout valve sends all high-pressure flow into the control circuit of the metering valve. This allows starts at low engine speeds. When the engine speed increases, the lockout valve sends a motive flow to the aircraft fuel system for fuel feed functions.
The MV is set by the ECU through the dual-channel MV EHSV. The MV EHSV controls the metering valve open area and sends the metering valve position to the ECU through the LVDT. The metering valve sends metered fuel pressure to the pressurizing valve.
The pressurizing valve controls the quantity of metered fuel that goes out of the HMU. It keeps metered fuel in the HMU until the internal pressures are at a level that allows the HMU components to operate. This ensures the proportional bypass valve supplies a balanced pressure for the HMU components. The pressurizing valve also sends ecology valve control pressure and metered fuel pressure to the ecology valve of the FDEV. The pressurizing valve also sends fuel inlet pressure back to the boost pump.
The servo pressure regulator controls the servo pressure. It sends servo pressure to the metering valve, MV EHSV, and CVG EHSV. The motive flow pressure regulator controls the high-pressure pump discharge and supplies a low fuel pressure back to the aircraft fuel system for motive flow.
The HMU supplies pressurized fuel to the CVGA. The servo pressure relief valve protects the CVGA from too much pressure if there is a malfunction of the servo pressure regulator. The ECU controls the CVGA through a two-channel electrical circuit (ECU channels A and B). The CVGA has a LVDT which sends CVGA piston position feedback to the ECU.
The HMU reacts to one of the two ECU commands for an engine shutdown: an engine overspeed command (automatic) or an engine shutdown command (manual). An engine shutdown occurs through the overspeed and cockpit shutoff EHSV which sends the fuel flow of the proportional bypass valve back to the low-pressure return. This change in pressure causes the proportional bypass valve not to be in balance which then causes the pressurizing valve to close. This stops the fuel flow out of the HMU to the FDEV. The bias relief valve supplies protection for the overspeed and cockpit shutoff EHSV from too much pressure if there is a malfunction of the servo pressure regulator.
To stop the evaporation of fuel caught in the fuel manifold on an engine shutdown, the FDEV causes fuel to flow in the opposite direction. The FDEV holds the fuel which is drained from the fuel manifolds in a reservoir until the subsequent engine start. This occurs immediately after the overspeed and cockpit shutoff EHSV is energized. There is a fuel flow meter between the FCU section outlet and the FDEV. This fuel flow meter monitors the fuel flow from the HMU. The two ECUs receive a signal from this fuel flow meter.
The HMU supplies pressurized fuel to the FDEV which divides the flow internally. Then the FDEV sends this fuel to the upper and lower manifold to supply the air blast nozzles. The FDEV also sends fuel to the start fuel nozzles during startup. The nozzles supply a sufficient quantity of fuel to the combustion chamber as necessary for ground and flight engine operation.
The HMU receives the following inputs:
- Mechanical power from the engine AGB to turn the primary fuel pump
- Fuel pressure and flow from the aircraft fuel system
- Fuel pressure and flow from the fuel filter module
- Electrical signals from the ECU to the run torque motor
- Electrical power to energize the LVDT of the fuel metering valve
- Electrical signals to close the overspeed and cockpit shutoff EHSV. The signals can be fuel metering signals or overspeed signals from the ECU
The HMU supplies the following outputs:
- Fuel flow to the fuel filter module (from the boost pump section)
- High-pressure pump discharge fuel flow to operate the ecology valve in the FDEV
- Fuel flow to the aircraft fuel system (for the motive flow)
- Fuel flow to the CVGA
- Metered fuel flow to operate the ecology valve in the FDEV
- Electrical signal to the ECU related to the position of the LVDT in the fuel metering valve
System Interface
The fuel controlling system interfaces with the following systems/components:
- Accessory Gearbox (AGB)
- Fuel Distribution System
- Fuel Indicating System
- Compressor Variable-Geometry Actuator
- Full-Authority Digital Engine Control (FADEC) System
- Electronic Control Unit (ECU)