Overview
The compressor control system controls airflow through the high-pressure compressor. It operates to give the most satisfactory engine operation and to prevent surge or engine stall conditions.
The compressor control system has a compressor variable geometry (CVG) vane part and compressor air bleed part. These two parts operate together to control the airflow through the high-pressure compressor in all conditions of engine operation.
The high-pressure (HP) compressor-airflow control system controls the quantity of the air flow through the HP compressor. The system prevents surge and stall of the compressor and supplies better engine performance.
The HP compressor-airflow control system includes a compressor variable geometry (CVG) subsystem and a compressor bleed subsystem. The compressor bleed subsystem controls the pressure in the combustion chamber when the CVG subsystem cannot control the compressor air flow.
The full-authority digital engine-control (FADEC) system supplies the necessary controls to operate the HP compressor-airflow control-system.
Each engine has one HP compressor-airflow control-system. The HP compressor-airflow control-system includes:
- The compressor bleed subsystem that includes:
- Two large main-surge bleed valves (the two valves are the same; one is on the left side of the engine, the other is on the right side of the engine)
- One small trim-surge bleed valve.
- Two large main-surge bleed valves (the two valves are the same; one is on the left side of the engine, the other is on the right side of the engine)
- The CVG subsystem that includes a compressor variable-geometry actuator with its mechanical link components.
Main Surge Bleed Valves
The two main-surge bleed valves are installed on the aft subsystem of the compressor case. The valves are installed with four captive bolts. The valves are spring-loaded to the OPEN position, and are connected to the combustion chamber (P3 pressure) through openings in the combustion chamber case.
The valves each have a solenoid to control the P3 pressure (servo pressure) for the main-surge bleed valve. Also, each valve has a receptacle for the solenoid control signal.
Trim Surge Bleed Valves
The trim-surge bleed valve is installed on the aft subsystem of the compressor case. The valve is installed with four captive bolts. The valve is spring-loaded to the OPEN position, and is connected to the combustion chamber (P3 pressure) through an opening in the combustion chamber case.
The valve has a solenoid to control the P3 pressure (servo pressure) for the trim-surge bleed valve. Also, the valve has a receptacle for the solenoid control signal.
Compressor Variable Geometry Actuator
The CVG actuator is installed on the left lower side of the HP compressor case. The body of the actuator is attached to the compressor case through an actuator mount bracket.
The rod end of the actuator is attached to a CVG connecting rod that supplies a mechanical link to the variable vanes in the compressor. The connecting rod is connected to two unison rings by the levers and connecting links. One of the unison rings is connected to the inlet guide vanes and the second unison ring is connected to the compressor variable vanes.
The inlet guide vanes and the compressor variable vanes can turn to change their angle as necessary. The CVG actuator has two ports to receive pressure control through two fuel pressure hoses. It has also a third port that connects to a drain hose.
The CVG actuator has two receptacles that supply the electrical interface for the position feedback signals.
Operation
In normal operation, the CVG vanes control the air flow through the compressor. The CVG vanes are operated by the compressor variable-geometry actuator. The actuator operation is controlled with the fuel pressure that it gets from the HMU. The HMU supplies the actuator with fuel pressure through the fuel pressure hoses. The electronic control units (ECUs) control the supply of fuel pressure from the HMU to the actuator.
The actuator extends or retracts the connecting rod in relation to the electronic control unit (ECU) commands. The levers and the connecting links transmit the movement to turn the unison rings. The movement of the unison rings causes the inlet guide vanes and the compressor variable vanes to change the angle. The angle of the vanes control the flow of air through the compressor. When the vanes move to close, less air flows into the compressor. When the vanes move to open, more air flows into the compressor.
When the CVG vanes cannot control the compressor air flow, the three surge bleed valves (SBVs) control the pressure in the combustion chamber. This condition is usually only present during the engine start and shutdown. The FADEC system schedules the control of the SBVs.
The reason there are three SBVs is because if surge margin is lost, it is not efficient to open one very large valve all at once to get back surge margin. This would be a large step change in engine operating condition, which would cause the temperature to increase, the speed to change, and the performance to decrease.
It is more efficient to incrementally open one valve at a time, and wait until necessary to open each. At the end of the process, all three valves can be open, but the effect is more gradual, and the performance level is kept stable as long as possible. Also, the engine control can adjust speeds better because of the smaller step changes.
The smaller valve (trim-surge bleed valve) is used when the aircraft is in a stable condition where fuel burn must not be excessive. This valve supplies a small increase in surge margin which gives the stability protection necessary to prevent excessive fuel burn.
All of the SBVs operate in the same manner. The valves are spring-loaded open at the engine start. When the valves are in the open condition, they let the air bleed from the aft end of the compressor to the bypass duct. The SBVs are sequentially set close by the ECU as the engine speed increases during acceleration. The SBVs are sequentially set open by the ECU during deceleration. The ECU calculates and gives the commands when a given surge bleed valve must open. When the SBVs are in the OPEN position, and there is a supply of air, the torque that the air turbine starter (ATS) must supply to turn the compressor, is decreased.
The compressor supplies pressure to the combustion chamber, and the ECUs use this pressure to control the positions of each of the bleed valves. The ECUs operate the solenoid of each valve to control the supply of P3 pressure to the related bleed valve. The bleed valves open and close to control the quantity of air that flows from the compressor to the combustion chamber.
System Interface
The HP compressor-airflow control system interfaces with the following systems/components:
- Hydromechanical Unit (HMU)
- Full-Authority Digital Engine Control (FADEC)