Engine Systems 1 of 18
ENGINE FUEL SYSTEM Introduction The fuel system uses electronic, hydraulic and mechanical functions to regulate the power and adapt it to the requirements at any one time. Air pressure is also used in acceleration control and ventilation of certain components. is used as the hydraulic medium. The electronics circuitry will be dealt with in simple terms with a more detailed study in the electronics section. Electronic sensing of rotational speeds is obtained from three phonic wheels on the free turbine output shaft for Nf and a single phonic wheel in the engine accessory gearbox for Ng. Collective pitch variation is sensed by variable potentiometers (anticipators) fitted on the control run and T4 is sensed by the 4 thermocouples around the intermediate section of the engine. An Ng datum signal (PPNg.) is sensed by an internal sensor in the Control Unit (FCU.). The purpose of these signals will be discussed later. Mechanical sensing is by operation of the Flow Control lever (Speed Select Lever [SSL]) and a rotating centrifugal flyweight fuel governing system. ing is achieved by an independent starting device, again with electronic control. Two high-energy ignitors supply the initial 'light-up' with the fuel being supplied, in first instance, by two fuel injectors. The main fuel supply to the engine is via centrifugal injection wheel integral with the compressor shaft. This system has the advantages of not requiring high pressure for atomisation and less chance of clogging. Figure 19 gives a schematic outline of the components involved in the fuel system. Airframe Engine Boost Pumps Tank Control Unit Automatic ing System Heat Exchanger Electronic Control Unit Injector Electro Overspeed & Drain Main Injectors Injectors Figure 19 System Schematic 2 of 18
Supply The engine fuel supply is drawn from the tanks by the high-pressure pump in the FCU. The pump is a gear tooth type with a pressure relief valve set at 30 bar. It is capable of delivering more fuel than the engine requires in any speed range, the excess fuel being returned to the inlet side. See Figure 21. The high-pressure pump is assisted by the airframe boost pumps during start and in flight. The high-pressure fuel is passed on to the FCU and then via a filter and metering valves to the engine. The filter (20μ) has a by-pass valve and tell-tale visual clogging indicator mounted on top of the FCU. During normal running of the engine the fuel is passed through a pipe to a manifold in the rotating assembly connected to a centrifugal injection wheel. The rotation of this wheel atomises the fuel by centrifugal action and prevents clogging. See Figure 20. Metering valves are set by the FCU to give the required flow through to the injection wheel. During engine start and idle conditions, fuel is passed to the engine via the Injectors. This fuel is only admitted to the combustion chamber during the start cycle. There are two injectors per engine. Once the Injector Solenoid valve is closed P2 air pressure is passed through the injectors to prevent carbonisation. Injector Inlet Inlet Ignitor Ignitor Inlet Centrifugal Injection Wheel Figure 20 Injection System Inlet Supply From Tank Return Figure 21 Engine Pump Engine Pump PRV 30 bar Filter 20μ By-Pass ΔP = 2 bar Clogging Indicator Output 3 of 18
FUEL CONTROL UNIT - FCU The FCU receives numerous inputs and uses them together with internal components to establish the correct amount of fuel required by the engine during all stages of operation. Some of the main FCU inputs and functions are detailed below. Gas Generator Speed Ng The Ng signal to the FCU is taken from a phonic wheel driven in the engine accessory gearbox. The actual Ng is compared with a datum Ng, and utilising a centrifugal flyweight governor, the gas generator speed is maintained constant. The datum Ng is varied by changes in Nf, and actual Ng is increases or decreased accordingly to achieve a constant Nf and hence rotor RPM. Nf Servo The speed of the Power Turbine is detected by the NTL Harness. This sends an electronic signal to the Electronic Control Unit (ECU), which in turn sends an electronic signal to the Nf Servo in the FCU. The Nf Servo is used to effect change on the datum Ng and therefore actual Ng and control of engine speed. Caution The Nf Servo is spring-loaded to neutral. Therefore, should the electrical signal be lost, the fixed Ng datum signal will be 85% (28,200 RPM). This could cause a problem if the aircraft was on the ground with flat pitch applied, as the engines will run up rapidly to 28,200 RPM. This situation may be encountered if all electrical power is selected OFF. Minimum & Maximum Gas Generator Speed There are two adjustable stops in the FCU and they control minimum and maximum speed of the gas generator. The first setting is the idle stop. It is set to ensure that the engine does not idle any lower than 68% Ng (22,600 RPM). This idle speed is controlled by the starting servo valve. The second setting is the maximum Ng stop. This is set to ensure the maximum speed of the engine is not exceeded. This is set to 102.5% Ng (34,000 RPM). Acceleration Control In the FCU there is a device to control acceleration and deceleration. Its function is to ensure that the engine does not surge or flameout during rapid acceleration or deceleration. It utilises a capsule subjected to P2 (compressor discharge) and P0 (ambient air) pressure. As the engine is accelerated rapidly this capsule, through a linkage, will only increase the fuel flow as the compressor discharge pressure increases. This prevents too much fuel being added to the engine with insufficient air for combustion leading to the possibility of flameout. The same is true of engine deceleration. Engine Electronic Control Unit - ECU This has a significant input to the FCU during all stages of flight but particularly during engine start. They are located in the cabin of the aircraft below the electrical cabinet. Their functions will be looked at later in this chapter. Engine Anticipator This comes in the form of an electrical signal from the ECU and allows more fuel to the engines to cater for increased loading (increase in collective pitch). It is looked at in a little more detail later. 4 of 18
Over-speed & Drain Supply Pressurising 2.2 Bar A Injection Wheel Over-Speed Electronic Shut-Off Diaphragm Drain Figure 22 Over-speed & Drain Refer to Figure 22. This allows fuel to the Centrifugal Injection Wheel on start up and it is also used to shutdown the engine quickly following an over-speed situation. The electrical side of this is looked at in the engine electrical section later in this chapter. - During engine start, pressure builds in the line to the Over-speed & Drain valve. When it reaches 2.2 bar, the Pressurising opens and allows fuel into valve (A). At the same fuel pressure passes through a restrictor to the underside of the diaphragm raising it. The raising of the diaphragm allows fuel out of valve (A) and on to the Injection Wheel. This system prevents fuel getting to Injection Wheel at low pressure and causing combustion problems. Shut Down - During Shut Down, as the SSL is closed, pressure falls in the line to the Over-speed & Drain. As the pressure falls below 2.2 bar the Pressurising closes. This removes the fuel supply to the Injection Wheel, reduces the pressure under the diaphragm and allows valve (A) to close. The small quantity of fuel left in the system is then drained to a collector box under the combustion chamber and directed to the exhaust where it is burned off. Over-speed Should an engine over-speed be detected, the Over-speed Electronic Shut-off is energised. This has the immediate effect of reducing pressure under the diaphragm, which closes valve (A) thus shutting down the engine. 5 of 18
Servo Refer to Figure 23. Ng and T4 are sensed by the ECU and used to control the Servo electronically. In conjunction with the signal from the ECU its function is to control the engine start to ground idle. The electrical signal from the ECU controls the flapper valve in the Servo. In order to increase the rate of acceleration, the flapper valve moves to the right allowing more fuel to the start injectors. The Servo controls the engine at ground idle where it is fully open. Once the SSL is moved to the flight range the Servo is no longer doing any work and is closed. There is also the option to isolate the Servo. This allows the pilot to ability to carry out a manual start should this be required. This is discussed later in this chapter. From FCU Manual Open Servo Flapper Servo controlled by the ECU. Flapper moves to the right to increase fuel flow. T4 Limited 650 and controlled until engine reaches 68%. At 65% Ng acceleration is slowed by closing of the flapper valve. It is the Servo that governs engine speed at idle. Servo Injectors Injectors Electro P2 Air After Figure 23 Servo 6 of 18
3 Speed Signals For Governor Filter Filter Blockage Pressure Relief Nf Governor Indicator Pump (electronic stage) Injectors Nf Governor (hydromechanical stage) Anticipator Ball Injector Electro Ng Governor Cocks By-pass Speed Select lever Constant ΔP Ng Datum Transmitter PPNg Servo Acceleration Controller P0 P2 Injection Wheel Metering Needles Over-Speed Electro Pressurising Over-Speed & Drain Figure 24 Full Control Unit 7 of 18
SPEED SELECT LEVERS These have numerous positions and functions. The following five sections deal with the various positions and functions of the levers. Control Lever and Automatic ing In automatic starting mode, the fuel flow control lever intervenes only to determine the basic fuel flow rate required for starting and enables control of the starting circuit electric components. In Figure 25 the automatic start sequence is shown. The following narrative describes what is happening during the start. To start, the fuel flow control lever (10) is moved from position (A) to position (B). Micro switch (5) is released bringing on the ENG P DIFF NG and POWER warning lights. Micro switch (12) controlled by boss (13) trips, authorising the electric start sequences. From then on, pressing push-button (11) is all that is required to initiate the starting sequence. The basic fuel flow rate is determined by the position of the throttle lever and the main valve (2) starts to open. The governor metering unit is held fixed by the Acceleration Control Cam (1). The fuel flow it 10 litres per hr. The fuel flow rate is monitored by the Servo. This automatically monitors T4 and Ng and varies the fuel output accordingly. The start injectors solenoid valve is open and the starter runs. During the first stage (igniting phase), the Pressurising is closed and the two starting injectors diffuse the fuel, which is ignited by two ignitors. During the second stage (acceleration phase) the Pressurising opens (Pressure >2.2 bar) and fuel flows from the injection wheel. When the Ng reaches 45% (15,000 RPM) the starter cuts out, the injector solenoid valve closes and the engine becomes self-sustaining. The injectors are then purged continuously by P2 air. When the Ng reaches 65% (21,600 RPM) the starting phase is completed and the engine is at idle. This idle RPM is governed by the Servo as the Governor is held fixed by the acceleration control cam. The pilot now just has to move the fuel flow control lever forward into the flight detent. The start servo valve closes and the engine is taken over by its governing system as the cam is no longer holding the fuel metering unit fixed. 8 of 18
Acceleration Control Cam 1 Governor Metering Unit Injection Wheel Supply 2 Pressurising 2.2 bar THROT 3 Cocks Manual ing Ng T4 Automatic ing Mechanism Injector Solenoid Injectors Servo Throttle Auto- Position A Figure 25 SSL and Auto 9 of 18
Control Lever and Flight Idle Position Refer to Figure 26 below. From the automatic starting position (B) to the flight position (D), the main fuel flow valve (2) opens progressively. At (D) it is fully open The Acceleration Controller Cam (1) resists less and less to the opening of the metering unit. At (D) the cam no longer has any effect on the metering unit operation. The engine power rating increases and as it does so, the governor fuel metering unit opens taking over from the start servo valve which closes. When the rotor reaches its nominal speed the engine is taken over by its electronic governing system. The governor fuel metering unit increases or decreases the fuel flow, according to the power required (collective pitch) in order to maintain constant Nf and therefore Nr. Hence, if the pitch is increased, the metering unit increases the fuel flow and the Nr, which will tend to decrease, is re-established. If the pitch is decreased, the metering unit will reduces the fuel flow. Micro-switch (14) is made and once both SSLs are in the flight position, the THROT warning light will go OUT. NF Preset Speed NF True Speed Acceleration Control Cam 1 Governor Metering Unit Injection Wheel THROT Light goes out when BOTH SSLs are in the Flight Position Supply 2 Pressurising 2.2 bar THROT 3 Cocks Ng T4 Automatic ing Mechanism Injectors Servo Flight Position D Throttle Flight Position B Figure 26 SSL and Flight Position 10 of 18
Control Lever and Positive Manual Governing Range Refer to Figure 27 below. An active failure of the electronic governing system may open or close the fuel metering unit which then remains fixed in its new position. Having lost its governor the engine is either going to provide too much power (metering unit too far open) or not enough power (metering unit closed). In both these cases of failure the pilot may switch to manual governing mode to restore the power required for flight, thus reinstating the engine. Note that in the event of an electric power supply failure, the governor metering unit automatically adopts a position giving an Ng of 85% (28 220 rpm), which corresponds to the cruising power range. Note also that the metering unit minimum flow gives an Ng of 68 % (22,600 rpm). Hence, whatever the failure, the engine is not shut down. If the fuel metering unit is blocked (closed), the pilot, after clearing the over-rideable flight detent (9), may move the fuel flow control lever (10) into the manual +ve governing range, where it gradually opens the emergency valve (3) which by-passes the governor metering unit. Thus the pilot can set a SSL position giving the required fuel flow and therefore power setting. The emergency range retains the SSL in its new position by means of teeth on the quadrant. The trigger (9) must be operated to move the SSL forward or back in this range. Caution When operating in manual governing mode the T4, Ng and torque must be monitored closely as the normal protection of the FCU/ECU is not available. NF Preset Speed NF True Speed Acceleration Control Cam 1 Governor The THROT Light Comes on again in the Manual Governing Range Supply 2 Metering Unit Failed Closed Pressurising 2.2 bar Injection Wheel THROT 6 3 Cocks Automatic ing Mechanism Injectors Servo Manual Positive Governing Range E D Throttle Positive Governing Position Figure 27 SSL and Positive Manual Governing 11 of 18
Control Lever and Negative Manual Governing Range Refer to Figure 28 below. Should the governing system failure block the fuel metering unit fully open, the engine will supply too much power and there is a danger that the upper Nr limit will be exceeded (as well as the maximum Ng & T4 values). The pilot must therefore control the fuel flow (hence the power) by gradually closing the main fuel flow valve (2) by retarding the fuel flow control lever (10) aft of the flight detent into the ve range. NF Preset Speed NF True Speed Acceleration Control Cam 1 Governor The THROT Light Comes on again in the Manual Governing Range Supply 2 Metering Unit Failed Open Pressurising 2.2 bar Injection Wheel THROT 3 Cocks Automatic ing Mechanism Injectors Servo Manual Negative Governing Range D C Throttle Negative Governing Position Figure 28 SSL and Negative Governing 12 of 18
Control Lever and Manual ing Should the automatic starting mechanism fail, the Servo no longer controls the fuel flow required for starting (it may be open or closed). However starting remains possible controlling the fuel flow by displacing the fuel flow control lever (10) within the manual starting range. Refer to Figure 29. ing procedure Close the manual starting valve (on the side of the engine) which isolates the Servo. Move the fuel flow control lever slightly forward of the auto start gate. The electric control self-holding micro-switch (12) has moved beyond its boss and the auto start micro switch (12) is not made. Press and hold the start button (11) throughout the start phase. Moving the fuel control lever opens the main fuel valve (2) and by rotating cam (1), slightly opens the governor metering unit. It is by adjusting the metering unit opening that the pilot will control the fuel flow throughout the start phase. The lever movement should be neither too slow (Ng stagnation) or too fast (sudden rise in T4). The normal expected T4 is 650 C. The rest of the circuit operates in the same way as during the automatic start sequence. 1 st Stage Ignition The fuel sprayed by the injectors is ignited by the ignitors. 2 nd Stage Acceleration flows through the injection wheel as the Pressurising opens at 2.2 bar. When the Ng reaches 45% (15,000 RPM) the start button must be released. This cuts off power to the starter, the injector solenoid valve and the ignitors. The engine is now self-sustaining but as the start servo valve is inoperative, it is ungoverned at idle. The fuel control lever should be advanced to the flight detent for normal governing. Acceleration Control Cam 1 Governor Metering Unit Supply 2 Pressurising 2.2 bar THROT 3 Cocks Manual ing Closed Ng T4 Automatic ing Mechanism Injector Solenoid Injectors Servo D Throttle Manual Position Manual ing Range Figure 29 Manual ing 13 of 18
Engine Anticipators The roll of the Anticipator is to prevent the Rotor RPM from reducing as the rotor disc load is increased by application of collective pitch. See Figure 30. In earlier aircraft such as the SA330 Puma the Nr could droop from 272 RPM to 256 RPM with the application of collective lever. This loss of Nr has caused the pilot problems in the past. The anticipator fitted to the AS332L actually increases the Nr with increase in collective pitch. The Nominal Nr is 265 RPM. However at flat pitch on the ground the Nr is 255 RPM and it increases to 270 RPM at high pitch of 17. The anticipator works by taking an electrical signal directly from the movement of the flight controls. These anticipator potentiometers are located in the control cupboard behind the pilot and there is one per engine. This electrical signal is passed to the ECU and is compared with a preset fixed Nf reference signal and an actual Nf signal from the engine itself. The resultant of these three signals is converted in the ECU to a governor control signal and is passed to the FCU. The result is an increase or decrease in fuel flow to the engine and hence an increase or decrease in Nf/Nr. To Main Rotor Engine Control Unit Variable Reference Signal Actual Nf Speed Signal Collective Lever Anticipator Potentiometer Preset Nf Reference Signal Governor Control Signal Governor Metering Unit Injection Wheel Supply Figure 30 Engine Anticipator System 14 of 18
ing and Stopping Ng and T4 are sensed and they are used by the ECU to electronically control the start via the Servo completely independent of the FCU. Initial fuel supply for 'light up' is via two injectors in the combustion chamber fitted beside two high-energy ignitor plugs. Stopping is controlled by means of a combined over-peed and drain valve. ing Limitations ing Limits Time Ng T4 Maximum T4 Unlimited 750 C Max Transient T4 5 sec 800 C Absolute Maximum T4 2 sec 810 C er Cut Out 15,000 (45%) Idle Speed 22,600 (68%) 1. No more than 3 consecutive starts then wait 20 minutes 2. Maximum ventilation time is 15 seconds. 3. Minimum battery voltage during start is 14V. Operation During Refer to Figure 31. When the SSL is moved to the auto start gate the normal fuel cock is partially opened and a cam on the FCU holds the metering valves in a fixed position allowing approximately 10 liters per hour fuel flow through. This cam is coupled to the SSL and is inoperative in the normal flight position. All the additional fuel required for starting and acceleration to Ground Idle RPM is supplied via the electronically controlled Servo. When the start button is pressed the starter motor is powered, the injector electro valve is opened and the high-energy ignitors supplied. The engine starts rotating, driving the fuel pump, and the fuel passes through the Servo to the start injectors and main fuel injection wheel. The Pressurising, set at 2.2 bar, ensures that initial supply is to the start injectors. As the Ng increases and fuel pressure builds up, the Pressurising will be overcome and fuel will flow to below the diaphragm in the Over-Speed & Drain, lifting the diaphragm up & opening the supply valve to the main injection wheel. At the same time the drain valve will be closed. The flow through the Servo will be increased as the servo is opened under action of the electronics. It is fully open at approximately 65% Ng (21,600 RPM) with a fuel flow of approximately 80 litres per hour. At 45% Ng (15,000 RPM) an electronic signal cuts out supply to er, Injector Electro and the High-Energy Ignition. When the Injector valve closes, P2 air pressure will be allowed to 'purge' the injectors via the ball valve to prevent carbonisation. Movement of the SSL to the Fast Idle position rotates the cam controlling the metering valves and allows more fuel through the FCU. Ng will increase slightly (by approximately 1,000 RPM to 68%/22,600 Ng) and as the SSL is advanced to the Flight position the cam no longer controls the metering valve. The FCU takes over control, and the Servo is closed again (minimum flow of 10 litres per hour). Manual In the event of a hung start (i.e. no acceleration on starter cut-out) provision is made to enable a manually controlled start to be used. On the Servo is a Manually, which is normally in the open position. Rotating this valve through 180 closes off the Servo completely and the pilot can now start without the electronic control. 15 of 18
To do this, the SSL must be placed forward of the auto start gate, between auto-start and fast-idle. This increases the opening of the normal fuel cock and the metering valves via the cam. The start button is depressed and, because the micro switch is not made, must be held depressed. As the engine lights up, and begins to accelerate, control of T4 and Ng is carried out by slight movements of the SSL (not too fast but not too slow). The start button must be released at 15,000Ng and the SSL gradually moved forward to fast idle position. Having obtained a successful start the SSL can be advanced to flight in the normal manner. Caution As the Servo has been isolated for the manual start the engine is not properly governed at idle. It is important that on shutting down an engine after a start in manual, the SSL is retarded directly from flight to shut-off. In Speed Select Lever Servo Control Unit Manual Open Servo Flapper Injectors In Out Oil Cooler P2 Air After Pressurising 2.2 Bar A Injectors Electro Injection Wheel Over-Speed Electronic Shut-Off Diaphragm Drain Over-speed & Drain Figure 31 Over-speed and Drain 16 of 18
Engine Stopping Normal Moving the SSL to the shut-off position closes the normal fuel cock and the fuel supply is shut off. As the pressure falls under the diaphragm in the Over-Speed and Drain, the supply valve (A) closes and the drain valve opens. Any fuel remaining in the pipeline to the injection manifold is now drained to a collector box under the combustion chamber and directed to the exhaust where it is burnt off. Overspeed In the event of the free turbine over-speeding to 120% the over-speed electronic valve is energised. This has the effect of immediately reducing the pressure under the diaphragm in the Overspeed and Drain closing the supply valve (A) to the Centrifugal Wheel. There will be quite a lot of fuel passing to the exhaust, as the Pressurising is still open and will remain so until the fuel pressure falls below 2.2 bar. This does result in a rather alarming quantity of smoke from the exhaust. 17 of 18
INTENTIONALLY BLANK 18 of 18