01/05/16
Overview
The engine fuel system is automatic. The engine fuel and control system has a fuel control unit that includes a metered-fuel bypassing technology. This technology improves fuel metering accuracy at low flow range that is necessary for high altitude starting and operation of the auxiliary power unit (APU). The APU electronic control unit (ECU) controls the operation of the fuel system. The ECU controls the quantity of fuel which flows to the APU. The function of the fuel control unit (FCU) is to supply fuel at the correct rate of flow and pressure to the fuel nozzles.
The system is fully automatic requiring only a stable supply of fuel from the aircraft at pressure between 5 and 40 psig and external source of D.C. power for starting.
Control for fuel scheduling, speed control, valve sequencing and Auto shutdown is provided by electrical signals from a remotely located Electronic Control Unit (ECU).
The functions of the engine fuel system are to:
- Filter fuel and supply the correct quantity of fuel flow
- Keep the correct pressure necessary for smooth engine acceleration
- Keep a constant 100% rpm during different load conditions
Fuel Control Unit (FCU)
The function of the fuel control unit (FCU) is to supply fuel at the correct rate of flow and pressure. The FCU attaches to the oil pump with a coupling clamp. It has a shaft with an external spline which engages an internal spline in the oil pump. The oil pump supplies the power to operate the FCU which has the components that follow:
- Filter bypass valve
- Fuel filter impending-bypass-indicator
- High pressure fuel pump
- Relief valve
- Fuel strainer
- Torque-motor metering valve
- Differential pressure regulator
- Bypass orifice
- Pressurizing valve.
The FCU causes fuel from the aircraft to go through the fuel filter. After the fuel is filtered it flows to the high pressure fuel pump. The oil-pump drive shaft operates the fuel pump. The drive shaft turns at 2200 rpm. An ultimate relief valve keeps the outlet pressure between 725 and 775 psi (5002.5 and 5347.5 kPa). After the pump, the fuel passes through a fuel strainer and then to the torque motor. At the torque motor the fuel is divided and goes to:
- The torque-motor metering valve
- The differential pressure regulator.
The torque-motor metering valve has interfaces with the ECU and the differential pressure regulator. The ECU sends signals which put the torque-motor metering valve into the applicable position and the differential pressure regulator adjusts the pressure across the metering valve.
The differential pressure regulator keeps the torque-motor metering-valve differential-pressure between 20 and 30 psi (138 and 207 kPa). If the differential pressure increases to more than 30 psi (207 kPa), the differential pressure regulator will open and let fuel bypass to the pump inlet.
After fuel goes through the torque motor, it divides and goes to:
- The bypass orifice
- The pressurizing valve.
The bypass orifice sends a small amount of fuel to the pump inlet. The pressurizing valve keeps sufficient pressure at the bypass orifice. It makes sure that the fuel will flow through the bypass orifice at all pressures.
Fuel Filter
The FCU filter is a paper cellulose disposable filter (25 micron) located on the FCU. The fuel filter prevents foreign particles from entering the APU fuel control.
The aircraft boost pump provides a positive supply of fuel to the pump inlet at 10 psig to 40 psig necessary to prevent pump cavitation and ensure proper cooling of the pump. As the fuel becomes contaminated, a pressure differential of approximately 5 psi causes the impending filter bypass visual indicator to extend. This indicates that the fuel filter is contaminated and should be replaced. An excessively contaminated filter that creates a pressure differential of 8 to 12 psi pressure across the filter, causes a bypass valve to open. This provides an alternate path for fuel flow to the pump
Fuel Solenoid Shutoff Valve
The fuel solenoid shutoff valve (FSOV) is mounted inline with FCU discharge line .The fuel solenoid shutoff valve stops the flow of fuel to the nozzles and shuts down the APU. The FSOV is supplied with 28 VDC from the ECU to open during an APU start. The FSOV is a normally closed, two position, one way valve which is energized open at 10% rpm. The FSOV will remain open until the ECU removes power due to a normal or protective shutdown. In all cases, the 24 VDC power is removed and spring pressure closes the valve.
The fuel solenoid valve is line replaceable unit (LRU).
Flow Divider Valve and Manifolds
The flow divider is installed in the left side of the fuel manifold near the exhaust section. Also, the primary manifold is near the exhaust section extending in a clockwise rotation from the flow divider. The secondary manifold extends in a counterclockwise rotation. The flow divider sequences and distributes fuel to the primary and secondary fuel manifolds during the start sequence.
The fuel manifolds evenly distribute fuel to the atomizers. On the flow divider one port connects to the fuel inlet and directly to the primary manifold (always open). The check valve outlet connects to the secondary manifold. As the APU rpm and fuel discharge pressure increases proportionally during acceleration, fuel flows unrestricted to the three primary fuel nozzles. When fuel discharge pressure reaches 30 ±5 psig, the flow divider check valve opens, adding additional fuel flow, routed to the three secondary nozzles. At 100% rpm, the primary and secondary fuel nozzles are both providing fuel
The fuel flow divider valve is a line replaceable unit (LRU).
The manifolds are rigid steel lines and are not interchangeable.
The primary and secondary fuel manifolds are LRUs.
Fuel Nozzles
There are six fuel nozzles, three on the primary manifold and three on the secondary manifold. The purpose of the nozzles is to supply an even distribution of fuel spray into the combustion chamber. The fuel nozzle assemblies are radially installed around the plenum chamber. The cooler compressor air ensures proper nozzle tip temperature and reduces coking.
Plenum Drain
The drain orifice is located at the lowest point in the plenum and ensures complete drainage of unburned fuel. The plenum drain removes unused fuel from the turbine plenum, following a failed start attempt. The drain orifice is large enough that it is not susceptible to blockage by ordinary debris in the plenum chamber, but still small enough to minimize air leakage. Discharge from the drain provides the actuating air for the gearbox vent ejector pump as air flows from the fitting, to the pump, to the exhaust.
01/05/16
Operation
A power source of 24 VDC and a supply of fuel from the aircraft tanks is necessary for the operation. The aircraft supplies a constant and stable quantity of pressurized fuel for stable operation. The aircraft supplied fuel pressure is between 10 and 40 psi (69 to 276 kPa). The aircraft batteries provide the initial electric power.
An electrical boost pump, energized by the APU master switch, supplies the aircraft fuel to the FCU high-pressure fuel pump through a 25-micron filter element. If contamination is present, a visual impending filter bypass indicator extends at approximately 5 psid. At 8 to 12 psid across the filter, the fuel filter is bypassed by a filter bypass valve.
The APU electronic control unit (ECU) controls the operation of the fuel system. The ECU energizes this system after it gets a signal of RUN from the APU control panel. It then transmits a signal to the fuel distribution control panel.
The fuel distribution control panel tells the engine fuel system to supply the fuel from the applicable aircraft tank. The ECU then sends a 28 VDC signal to open the APU fuel shutoff-valve relay. The APU fuel shutoff-valve lets fuel go through the fuel inlet hose and into the fuel control unit (FCU). The FCU pressurizes the fuel.
Fuel is discharged from the pump and passes through a 70-micron filter screen (strainer), where it branches off internally to the torque motor metering valve, ultimate relief valve, and the differential pressure regulator valve. The ultimate relief valve opens to bypass fuel, back to the inlet side of the pump, when the fuel system pressure exceeds 675 to 725 psig. The differential pressure regulator keeps a constant 30 psid across the metering valve.
The fuel shutoff valve is energized open at 10% rpm by the ECU, allowing fuel flow through the primary passage of the fuel flow divider valve, directly to the primary nozzles (3). A flow divider check valve opens up at 30 psid to permit fuel flow to the secondary nozzles (3). This allows the rpm to reach and maintain 100% rpm.
At the same fuel pressure, the fuel flow through the primary spray nozzles is only half the flow through the secondary spray nozzles. The ECU can increase, decrease or stop the flow of the secondary fuel as applicable. To do this, the ECU opens or closes the flow divider valve or causes the FCU to adjust the fuel pressure.
When the APU master switch is selected OFF, the ECU deenergizes the fuel solenoid valve and the fuel control torque motor metering valve, shutting down the APU.
The ECU causes the fuel flow to stop when one of many sources sends a signal. For a usual shutdown, the ECU sends a simulated overspeed signal to the primary operating system. The ECU then does a check to make sure the signal to the fuel solenoid valve is de-energized. The ECU does one more check to make sure there is a decrease in APU speed. This is an indication that the fuel solenoid valve has closed. After that the ECU sends a signal to close the FCU torque motor.
Two tubes drain fuel from the APU. The fuel control drain tube connects to the FCU at the seal witness opening and drains fuel or oil leakage. After shutdown, the turbine-housing drain tube drains the fuel which does not burn from the APU. It connects to an orifice fitting installed in the turbine housing.
The APU FAULT advisory message will show when the APU fuel solenoid valve has failed.
The EICAS message that follows is related to the engine fuel system:
| EICAS MESSAGE(S) | LEVEL (COLOR) |
|---|---|
| APU FAULT | ADVISORY (cyan) |
