A fuel enrichment valve within the FCU allows automatic or manual fuel enrichment during engine start. The primary function of this valve is to allow the engine to accelerate quickly through the critical RPM range of 18%-28% engine RPM.
Automatic fuel enrichment is a computer controlled function which occurs between 10%-60% engine RPM. This automatic function is designed to not only accelerate the engine during the start cycle but also to limit the maximum EGT to 695° C during the start cycle.
Manual fuel enrichment is available during manual mode operations and is pilot controlled through the associated START switch.
Pushing the START switch intermittently after 10% engine RPM allows fuel enrichment for engine acceleration, however, EGT must be monitored during manual enrichment to prevent exceeding EGT limitations (770°C for one second during ground operations).
Max. EGT during ground operations is 770°C for one second
The over speed governor in the FCU is a conventional (speeder spring flyweight) unit located in series with the fuel metering valve. The over speed governor is preset to between 104.5% and 105.5% engine RPM at which point it will restrict the flow of metered fuel to the engine therefore maintaining engine RPM.
A primary nozzles only solenoid valve is fitted to enhance the atomisation of fuel within the combustion chamber during the start cycle and is controlled by the 10% switch for opening and the 80% switch for closing during normal mode operation.
In manual mode the 10% switch will open the primary nozzles only valve with the 60% switch closing it as no automatic functions are available during manual mode operation.
A solid state electronic fuel computer is provided for each engine for operation in Normal mode. The computers control the engine throughout all phases of operation and are mounted under the cabin floor behind the co-pilot seat.
Fuel computers located under cabin floor behind co-pilot seat
Operation of the computers is based on sensing parameters and computer output signals which are transmitted to the fuel control unit and the propeller governor.
During normal mode operation the following input signals are supplied to the computer:
- Power lever position,
- Condition lever position,
- Engine torque pressure,
- Engine pressure ratio (Delta P/P),
- Engine inlet temperature (T2 probe),
- Exhaust Gas Temperature (EGT),
- Propeller governor RPM,
- Fuel control unit RPM, and
- 28 volts D.C. power
Diagrammatic view of Fuel computer inputs and outputs
The fuel computers also include an electronic under speed governor which is used primarily to control fuel flow to support propeller loads during ground operations and to prevent bog-down of engine RPM.
Three electronic speed switches within the computer perform various automatic functions and are described as the 10%, 60% and the 80% speed switches.
The 10% speed switch is used to:
- Automatically open the automatic shutoff valve with the FCU,
- Turn on the ignition,
- Open the fuel enrichment valve, and
- Open the primary nozzles only valve.
The 60% speed switch will:
- Terminate the starter operation,
- Turn off the ignition, and
- Close the oil vent valve.
The 80% speed switch will:
- Close the fuel enrichment valve,
- Close the primary nozzles only valve, and
- Engage the Single Red Line (SRL) system in the computer.
- Engage the flow limiting system on the bleed air line to the Air Cycle Machine (ACM) in normal mode.
During Manual mode operations the automatic electronic speed switches are inoperative and their functions are controlled by the pilot through the START MANUAL switches mounted on the left hand circuit breaker panel.
Start Manual switches on the left hand circuit breaker panel
During a Manual mode start the pilot moves the appropriate START MANUAL switch to the 10% position to perform the functions of the electronic 10% switch, and then to the 60% position to perform the functions of the 60% and 80% electronic switch functions. Fuel enrichment between 10% and 60% RPM is controlled by the intermittent pressing of the appropriate START switch.
A Fault monitoring system is incorporated into the fuel computers and constantly monitors the following four primary conditions:
- Power lever potentiometer,
- High FCU output,
- Low FCU output, and
- Low input voltage.
If the computer monitoring system senses any of the following conditions the fuel computer will trip to Manual mode operation requiring pilot input for all control inputs and monitoring:
- Conflicting power required and power output signals to the computer,
- Loss of power lever electrical input, and
- Electrical system voltage below 9 volts to the computers.
The fuel computers can also be manually switched off for manual mode operations by selection of the guarded fuel computer switches to the OFF position. These fuel computer switches are located next to the START MANUAL switches on the left hand circuit breaker panel in the cockpit. See the guarded fuel computer switches (located on the left hand circuit breaker panel) in the image above.
When the fuel computers are switched off either manually or automatically, annunciator lights will illuminate indicating to the pilot that the computer has been switched off and that the respective engine is now operating in Manual mode.
Fuel computer annunciator lights
The fuel computers in the Conquest include circuitry designed to automatically limit engine torque and temperature settings. The torque limiting system receives computer input signals from the torque meter which will in turn reduce fuel flow to the FCU once maximum torque settings are reached.
Maximum Torque limits are 1669 ft lbs @ 100% RPM
Maximum Torque limits are 1738 ft lbs @ 96% RPM
The temperature limiting system receives computer input signals from the EGT system, and like the torque limiting system, reduces the fuel flow through the FCU to maintain the maximum EGT.
The Single Red Line (SRL) limit set for the Conquest engine is 450° C.
Maximum EGT limits are 770° with the condition lever in START-TAXI and 450° at all other condition lever settings.
The Torque/Temp. limiting system is controlled by switches located on the left hand switch panel next to the pilot seat. With one switch per engine, these switches are labeled TORQUE-EGT-LIMIT with two positions either AUTO or MANUAL. The system is operational with the switch in AUTO and is inoperative with the switch in MANUAL.
Torque/Temp Limiting (TTL) switches located on the left switch panel