Fuel Systems PDF

Fuel System Components I Engine Fuel Systems Gas turbine engines convert the latent energy of fuel into heat to provide energy for engine operation and thrust for the aircraft. The function of the fuel system is to provide the engine with fuel in a form suitable for combustion and to...

Fuel System Components I Engine Fuel Systems Gas turbine engines convert the latent energy of fuel into heat to provide energy for engine operation and thrust for the aircraft. The function of the fuel system is to provide the engine with fuel in a form suitable for combustion and to control its flow to the required rates necessary for easy starting, acceleration and stable running in all engine operating conditions. For a gas turbine engine to deliver the power required, it needs a system that supplies fuel in sufficient quantities to allow for varying conditions, ambient temperature, altitudes and power settings. 2022-08-08 B1-15b Gas Turbine Engine Page 86 of 291 CASA Part 66 - Training Materials Only  Fuel System Layout The layouts of aircraft and engine fuel systems vary with the type and size of aircraft. However, most systems include the following components: Fuel tank Boost pump Low-pressure (LP) shut-off valve Low-pressure transmitter Fuel heater High-pressure (HP) fuel pump Fuel filters Fuel pressure differential switch Fuel control unit High-pressure shut-off valve Fuel flow transmitter Pressurising and dump valve Fuel burners. Simplified fuel system for a gas turbine engine (73-00) 2022-08-08 B1-15b Gas Turbine Engine Page 87 of 291 CASA Part 66 - Training Materials Only  Low-Pressure Shut-Off Valves Low-pressure shut-off valves (LPSOVs) on modern aircraft (normally mounted behind the engine firewall) are used to isolate the engine fuel system from the airframe in case of fire or system maintenance. The two common types of shut-off valves are: Motor-driven gate valve Solenoid-operated valve. Motor-Driven Gate Valve The motor-driven gate valve shown below uses a reversible electric motor linked to a sliding valve assembly. The motor moves the valve gate in and out of the passage through which the fuel flows, thus shutting off or turning on the fuel flow. Motor-driven gate valve 2022-08-08 B1-15b Gas Turbine Engine Page 88 of 291 CASA Part 66 - Training Materials Only  Solenoid-Operated Valve A solenoid valve has an advantage over a motor-driven valve: it is much quicker to open or close. The valve shown below is a solenoid-operated, poppet type valve. When electrical current momentarily flows through the opening solenoid coil, a magnetic pull is exerted on the valve stem that opens the valve. When the stem rises high enough, the spring-loaded locking plunger is forced into the notch in the valve stem. This holds the valve open until current is momentarily directed to the closing solenoid coil. The magnetic pull of this coil pulls the locking plunger out of the notch in the valve stem, the spring closes the valve and shuts off the flow of fuel. Solenoid-operated poppet valve 2022-08-08 B1-15b Gas Turbine Engine Page 89 of 291 CASA Part 66 - Training Materials Only  Low-Pressure Transmitters For aircraft fitted with more than one fuel tank, it is desirable to have a means of warning the pilot that fuel in the supplying tank is exhausted (or the boost pump is not operating) and that the fuel selector must be set to draw fuel from another tank. The low fuel pressure switch is held open by normal fuel pressures, but it closes when the pressure falls. This turns on the warning light in the cockpit. Fuel pressure warning lights 2022-08-08 B1-15b Gas Turbine Engine Page 90 of 291 CASA Part 66 - Training Materials Only  Fuel Heaters Turbine-powered aircraft that operate at high altitudes and low temperatures for extended periods have the problem of water condensing out of the fuel and freezing on the fuel filters. To prevent this, these aircraft have a fuel temperature gauge and/or a filter differential pressure warning light that illuminates when ice obstructs the filter. The purpose of the fuel heater is to protect the fuel system from ice formation and to thaw ice that forms on the fuel filter screen. This is achieved by using hot air that has been heated by the compressor section of the engine. A fuel heater is depicted in the illustration below. Fuel heater 2022-08-08 B1-15b Gas Turbine Engine Page 91 of 291 CASA Part 66 - Training Materials Only  High-Pressure Fuel Pumps Engine-mounted fuel pumps are required to deliver a continuous supply of fuel at the proper pressure at all times during operation of the aircraft engine. They must be capable of delivering maximum needed flow at high pressure to obtain satisfactory nozzle atomisation and accurate fuel regulation. The two common types of engine-driven fuel pumps are: Spur gear type Piston type. Spur Gear Type Pumps Gear type pumps have approximately straight-line flow characteristics, whereas fuel requirements fluctuate with flight or ambient air conditions. Hence a pump of adequate capacity at all engine operating conditions has excess capacity over most of the range of operation. This characteristic requires the use of a pressure relief valve for disposing of excess fuel. A typical constant displacement gear pump is illustrated below. The fuel enters the pump at the impeller, which gives an initial pressure increase and discharges fuel to the two HP gear elements. Each of these elements discharges fuel through a check valve to a common discharge port. Shear sections are incorporated in the drive system of each element. Thus, if one element fails, the other continues to operate. The check valves prevent circulation through the inoperative unit. One element is capable of supplying sufficient fuel for moderate aircraft speeds. A relief valve is incorporated in the discharge port of the pump to allow fuel in excess of that required by the engine to be recirculated to the inlet side of the HP elements. Gear type pump 2022-08-08 B1-15b Gas Turbine Engine Page 92 of 291 CASA Part 66 - Training Materials Only  Piston Type Pumps The variable displacement pump system differs from the constant displacement pump system. Pump displacement is changed to meet the varying fuel flow requirements, that is, the amount of fuel discharged from the pump can be made to vary at any one speed. This is due to the inclination of the cam-plate; movement of the rotor imparts a reciprocating motion to the pistons, thus producing a pumping action. The stroke of the pistons is determined by the angle of inclination on the cam-plate. The degree of inclination is varied by the movement of a servo piston that is mechanically linked to the cam-plate and is biased by springs to give the full stroke position of the plungers. The piston is subject to servo pressure on the spring side and on the other side to pump delivery pressure, thus, variations in the pressure difference across the servo piston cause it to move with corresponding variations of the cam-plate angle and therefore pump stroke. With a variable flow pump, the fuel control unit can automatically and accurately regulate the pump pressure and delivery to the engine. Variable displacement piston type pump Relevant Youtube link: Pressure Compensated Pump (Video) 2022-08-08 B1-15b Gas Turbine Engine Page 93 of 291 CASA Part 66 - Training Materials Only  Fuel Filter Because the HP fuel pump, fuel control unit, pressurisation valve, dump valve and burners are manufactured to very fine tolerances and fitted with many small orifices, a filter is installed to protect the fuel control components from contaminants. The filter must be capable of removing particles measuring as small as 10 microns. Fuel filter internal diagram The three most common types of filters in use are the micron filter, the wafer screen filter and the plain screen mesh filter. The individual use of each of these filters is dictated by the filtering treatment required at a particular location. 2022-08-08 B1-15b Gas Turbine Engine Page 94 of 291 CASA Part 66 - Training Materials Only  Fuel filters physical example 2022-08-08 B1-15b Gas Turbine Engine Page 95 of 291 CASA Part 66 - Training Materials Only  Fuel Pressure Differential Switch The differential pressure switch is used in the fuel system to detect the presence of icing on the fuel filter. It illuminates a cockpit warning light when the pressure differential reaches a set amount. A fuel pressure differential switch takes a pressure reading from near the fuel flow transmitter and from between the fuel filter and HP pump. It indicates whether the fuel filter is becoming blocked by ice or foreign material in the fuel, enabling the pilot to select fuel heating to remove ice from the filter. Filter differential pressure sensor 2022-08-08 B1-15b Gas Turbine Engine Page 96 of 291 CASA Part 66 - Training Materials Only  Fuel Control Systems Fuel Control Units The control of power (or thrust) in a gas turbine engine is affected by regulating the quantity of fuel injected into the combustion chamber. If too much fuel is supplied to the combustion chamber, the turbine section may be damaged by excess heat, the compressor may stall or surge because of back- pressure from the combustion chambers, or a rich blowout may occur. A rich blowout occurs when the mixture is too rich to burn. Too little fuel entering the combustion chambers causes a lean blowout, which occurs when the mixture is too lean to burn. The usual method of varying the fuel flow to the combustion chamber is via a fuel control unit. Fuel control units operate under either hydro-pneumatic, hydromechanical or electro‑hydromechanical control. Fuel controls 2022-08-08 B1-15b Gas Turbine Engine Page 97 of 291 CASA Part 66 - Training Materials Only  Hydromechanical For many years the majority of fuel control units have been hydromechanical in operation. This means their operation is controlled both by hydraulic (fuel) and mechanical means to control the fuel flow to the engine. Hydro-pneumatic A hydro-pneumatic fuel control unit uses engine air pressures and mechanical forces to operate its fuel scheduling mechanisms. Electro‑hydromechanical Later model gas turbine engines are controlled by electronic fuel control systems known as electro‑hydromechanical fuel control units. These systems use computers, Electronic Engine Control (EEC), that sense inputs to set the hydromechanical section of the fuel control unit that limits the fuel flow to the engine. Fuel Flow Control The fuel control unit (FCU) is an engine-driven fuel metering device. It controls the fuel flow to the required quantity necessary for: Easy starting Acceleration Deceleration Stable running at all engine operating conditions. The FCU maintains a correct combustion zone air-to-fuel mixture ratio of 15:1 by weight automatically (engine power selection is achieved by a manual throttle). Automatic controls must prevent the engine from exceeding maximum limitations. To do this, one or more fuel pumps are used to deliver the fuel to the fuel spray nozzles, which inject it into the combustion system in the form of an atomised spray. Because the flow rate must vary according to the amount of air passing through the engine to maintain a constant selected engine speed or pressure ratio, the controlling devices are fully automatic with the exception of engine power selection, which is achieved by a manual throttle or power lever. A fuel shut-off valve control lever is also used to stop the engine, although in some instances these two manual controls are combined for single-lever operation. It is also necessary to have automatic safety controls that prevent the engine gas temperature, compressor delivery pressure and rotating assembly speed from exceeding their maximum limitations. 2022-08-08 B1-15b Gas Turbine Engine Page 98 of 291 CASA Part 66 - Training Materials Only  The fuel system often provides for ancillary functions, such as oil cooling and the hydraulic control of various engine control system actuators, for example, compressor airflow control such as bleed bands, variable stater vanes (VSVs) and variable guide vanes (VGVs). Control of the gas turbine engine’s power or thrust is affected by regulating the quantity of fuel injected into the combustion system. When a higher thrust is required, the throttle is opened and the pressure to the fuel spray nozzles increases due to the greater fuel flow. This increases the gas temperature, which in turn increases the acceleration of the gases through the turbine to give a higher engine speed and a correspondingly greater airflow, consequently producing an increase in engine thrust. This relationship between the airflow induced through the engine and the fuel supplied is, however, complicated by changes in the following: Aircraft altitude Air temperature Aircraft speed. These variables change the density of the air at the engine intake and consequently the mass of air induced through the engine. A typical change of airflow with altitude is shown in the graph below. To meet this change in airflow, a similar change in fuel flow must occur; otherwise, the ratio of airflow to fuel flow changes and increases or decreases the engine speed from that originally selected by the throttle lever position. Fuel flow changing with altitude 2022-08-08 B1-15b Gas Turbine Engine Page 99 of 291 CASA Part 66 - Training Materials Only  The method of varying the fuel flow to the spray nozzles is by adjusting the output of the HP fuel pump. The FCU varies the HP fuel pump output by: Varying piston stroke (piston type pump) Recirculating excess fuel delivery back to pump inlet (gear type pump). Pump output is varied through a servo system in response to the following: Throttle movement Air temperature and pressure Rapid acceleration and deceleration Signals of engine speed, gas temperature and compressor delivery pressure. A typical FCU takes inputs from sensors on the engine and airframe to adjust and compensate for changes in operating conditions: Compressor inlet pressure (Pt2) Compressor inlet temperature (T2) Compressor speed (N1, N2 and, in some engines, N3) Engine burner (combustor) pressure (Pb) Throttle position Fuel density (SG – specific gravity). In modern FADEC engines, fuel density is compensated for automatically. In older engines, fuel type and therefore density are adjustable on the FCU. Hydromechanical Fuel Control System Starting Prior to being activated, the FCU is in the following conditions: Fuel shut-off valve closed Power lever at idle Governor speeder spring in an expanded condition Governor flyweights in an underspeed condition Burner and inlet pressure bellows sensing barometric pressure and the multiplying linkage in the decrease position Differential pressure regulating valve closed Metering valve held off the minimum flow stop by the balanced spring pressures of the governor and main metering valve. 2022-08-08 B1-15b Gas Turbine Engine Page 100 of 291 CASA Part 66 - Training Materials Only  After the start button is pressed, the engine begins to rotate and the flyweights in the governor begin to open, overcoming initial speeder spring tension and moving the roller cage upwards, thus reducing the metering valve opening. The fuel pump pressurises the fuel system until the relief valve pressure in the pump is reached. When the engine has been accelerated by the starter to a set rpm, or after a certain period, the fuel shut-off valve is opened. The following are the results of that action: Fuel flows to the burners, causing a differential pressure across the metering valve. Therefore the differential pressure regulator senses the difference and begins to regulate the fuel pressure. Once combustion commences, the engine begins to accelerate and the burner pressure increases, causing the burner pressure bellows to move the multiplying linkage to begin opening the main metering valve through the roller cage. As the engine accelerates towards idle rpm, the speed governor and pressure bellows begin to regulate metering valve opening, commencing governed operation at idle speed. Hydromechanical fuel control starting 2022-08-08 B1-15b Gas Turbine Engine Page 101 of 291 CASA Part 66 - Training Materials Only  Governed or Steady Operation During governed or steady operation, consider that the power lever is set at a certain position and not changed. After the engine speed is set, the engine is subject to certain operating variables, such as aircraft speed and altitude, to which it must react. If an aircraft is increasing speed or descending, it increases inlet ram air pressure and mass airflow. Alternatively, an aircraft that is climbing and/or slowing decreases inlet air pressure and mass airflow. The inlet and burner pressure bellows sense these changes and move the multiplying lever in an appropriate direction to maintain the fuel mixture ratio. At the same time, the engine speed governor reacts to any speed variations, moving the pilot servo rod valve to return the engine to a steady governed state. Hydromechanical fuel control governed or steady state operation 2022-08-08 B1-15b Gas Turbine Engine Page 102 of 291 CASA Part 66 - Training Materials Only  Acceleration Movement of the power lever in an increase direction causes the spring cap to slide down the pilot servo valve rod and compress the flyweight speeder spring. In doing so, the spring base pushes down and forces the flyweights in at the top to an underspeed condition, moving the pilot valve rod downwards. The pilot servo valve slows the movement of the pilot servo control rod, preventing sudden fuel ratio changes by using its fluid displaced top to bottom as a restrictor. When the pilot valve rod moves down, the roller moves down the inclined plane and to the left. As it moves left, the roller forces the metering valve to the left against its spring, allowing increased fuel flow to the engine. As fuel flow increases, the differential pressure valve senses a decreased differential and closes to maintain the differential. With increased fuel flow, the engine speeds up and drives the fuel control shaft faster. As the engine speed increases, the burner pressure increases, which expands the burner pressure bellows that moves the multiplying linkage to the left, further increasing the fuel flow. The new flyweight force comes to equilibrium with the speeder spring force as the flyweights return towards an upright position. They are now in position to act at the next speed change. Hydromechanical fuel control acceleration 2022-08-08 B1-15b Gas Turbine Engine Page 103 of 291 CASA Part 66 - Training Materials Only  Deceleration Movement of the power lever in a decrease direction causes the spring cap to slide up the pilot servo valve rod and release pressure on the flyweight speeder spring. In doing so, the spring base moves up and the flyweights move to an overspeed condition, moving the pilot valve rod upwards. The pilot servo valve slows the movement of the pilot servo control rod, preventing sudden fuel ratio changes by using its fluid displaced top to bottom as a restrictor. When the pilot valve rod moves up, the metering valve spring forces the metering valve and the roller to the right as it moves up the inclined plane, allowing less fuel flow to the engine. With decreased fuel flow, the differential pressure valve senses the increased differential across the metering valve and opens to maintain the differential. The engine slows and drives the fuel control shaft slower. This slowing of the engine decreases the burner pressure, which through the bellows moves the multiplying linkage to the right, further decreasing the fuel flow. As the new flyweight force comes into equilibrium with the speeder spring force, the flyweights return towards an upright position. They are now in position to act at the next speed change. Hydromechanical fuel control deceleration 2022-08-08 B1-15b Gas Turbine Engine Page 104 of 291 CASA Part 66 - Training Materials Only  Shutdown Prior to shutdown, the engine must be allowed to stabilise at idle for a period to ensure a gradual cooling of the turbine and scavenging of propeller control oil in turbo propeller engines. On a simplified fuel control unit, shutdown occurs according to the following procedure: With the engine at idle governed speed, the fuel shut-off valve is closed. When the shut-off valve is closed, there is no fuel flow to give a differential fuel pressure, thus closing the differential pressure regulating valve, causing the fuel pump pressure relief valve to control maximum fuel pressure. Once combustion ceases, the engine speed begins to decrease, sending the governor into an underspeed condition. At the same time, the burner pressure decreases, moving the multiplying linkage to close the metering valve. Hydromechanical fuel control shut down 2022-08-08 B1-15b Gas Turbine Engine Page 105 of 291 CASA Part 66 - Training Materials Only  Hydro-Pneumatic Fuel Control Hydro-pneumatic fuel control units rely heavily on compressor discharge pressures to maintain the correct air-fuel ratio. A basic system is shown in the illustration below and provides: Rpm control Acceleration and deceleration control Minimum and maximum flow control. It has inputs of: Rpm command (power lever) Actual rpm Inlet temperature Compressor outlet pressure. Operation of Control Unmetered fuel pressure is supplied to the FCU by the fuel pump. The pressure regulator maintains a constant pressure drop across the metering head, ensuring constant flow. Excess fuel is bypassed back to the pump inlet. The air section is operated by compressor discharge air (Pc). When modified, this air becomes Px and Py, which act to position the metering valve. Tt2 Sensor The Tt2 sensors (bimetallic discs) are a typical means of sensing inlet duct temperature. They control a metering device, which affects pressures Px and Py. This reduces the acceleration rate under hot conditions, preventing excessive turbine temperature and the risk of compressor stall or surge. Power Lever Position 2022-08-08 B1-15b Gas Turbine Engine Page 106 of 291 CASA Part 66 - Training Materials Only  When the Power Lever Is Advanced: The flyweights droop in, the speeder spring force being greater than the flyweight force. The governor valve closes off the Py bleed. Px and Py pressures equalise on the surface of the governor. Px air contracts the acceleration bellows and the governor bellows rod is forced downwards. The diaphragm allows this movement. The main metering valve moves to open. The flyweights move outwards as engine speed increases and the governor valve opens to bleed Py air. Reduced Py value allows the governor bellows and rod to move up to a new stabilised position. The metering valve resumes a new position through the action of the torque rod assembly. When the Power Lever Is Retarded: The flyweights move outwards, speeder spring force being less than flyweight force due to high engine rpm. The governor valve opens, dumping Py air Px air expands the governor and deceleration bellows to its stop. The governor rod also moves up and the main metering valve moves towards close. Px air decreases with engine speed decrease, but the acceleration bellows holds the governor rod up. As engine speed slows, the flyweights move back in, closing the Py bleed at the governor valve and the backup valve. The deceleration bellows moves downwards. The metering valve moves slightly open to produce a stabilised fuel flow. The fuel pump supplies more fuel than is required and the bypass valve returns excess back to the pump inlet. The bypass valve incorporates a pressure regulator to ensure the pressure differential across the metering valve is unaffected by movement of the metering valve. Therefore, fuel flow is controlled only by metering valve position. The rpm is the primary control parameter, and compressor discharge pressure and inlet air temperature are secondary parameters. Together they control the metering valve via a servo bellows assembly. "On-speed" rpm is maintained by the governor in conjunction with the governor bellows pressure Py. The flyweights of the governor respond to an rpm change by increasing or decreasing the opening of the governor valve, which in turn alters Py and thus the extension of the governor bellows. The bellows assembly opens the metering valve slightly when there is a fall in rpm and closes it slightly when there is a rise in rpm. 2022-08-08 B1-15b Gas Turbine Engine Page 107 of 291 CASA Part 66 - Training Materials Only  Hydro-pneumatic fuel control system Unlike a hydromechanical FCU, which has a mechanical linkage between the power lever and the fuel metering valve, the hydro-pneumatic FCU has none and utilises a pneumatic coupling between the two. A hydro-pneumatic fuel control differs from a hydromechanical fuel control in that the former uses a pneumatic computing section that determines fuel flow rates based on the position of the power lever, N1 rpm, compressor discharge air (Px and Py), and outside air pressure and temperature. In the simplified schematic above, the N1 governor is connected to the pneumatic computing section, and it controls compressor turbine speed and senses compressor discharge air. Therefore, functions of the governors and pneumatic computing section are interdependent. 2022-08-08 B1-15b Gas Turbine Engine Page 108 of 291 CASA Part 66 - Training Materials Only  Governor Section For acceleration, an input force from the power lever increases compression on the speeder spring. This moves the flyweights of the governor inwards and closes the governor valve. With the governor valve closed, acceleration control pressure (Px) and governor pressure (Py) both increase with compressor pressure (Pc), causing the bellows assembly to gradually open the metering valve. System design ensures that increasing fuel flow matches increasing airflow through the engine and that acceleration takes place without risk of stall or surge. When the desired rpm is reached, the governor again maintains on-speed rpm. During deceleration, the reverse sequence occurs. The rate of deceleration is controlled by the deceleration bellows, which ensures smooth deceleration without the risk of flameout. The computing section automatically compensates for altitude by utilising an evacuated bellows to reset the metering valve position. Changes in ambient temperature are compensated by a sensor that allows compressor discharge pressure to be bled to ambient. Acceleration is controlled by the evacuated bellows, which repositions the fuel metering valve as Px changes. Deceleration is controlled by the deceleration bellows, which repositions the fuel metering valve as Py changes. Flow Control Flow control units regulate the fuel system by bypassing excess unwanted fuel back to the inlet side of the fuel pump. Electro‑Hydromechanical Control System Electro‑hydromechanical fuel control systems are sometimes referred to as electronic fuel controls because the majority of the system is made up of electronic circuits. Because of the need to precisely control many functions in the operation of modern high-bypass turbo fan engines, electronic engine control systems have been developed. These systems prolong engine life, save fuel, improve reliability, and reduce crew workload and maintenance costs. Two types of EEC systems in use are: Supervisory Electronic Engine Control Full Authority Digital Engine Control (FADEC). 2022-08-08 B1-15b Gas Turbine Engine Page 109 of 291 CASA Part 66 - Training Materials Only  Supervisory Electronic Engine Control System The supervisory EEC is a control that works with a proven hydromechanical fuel control. It includes a computer that receives information regarding various engine operating parameters and adjusts a standard hydromechanical FCU to obtain the most effective engine operation for the selected power setting. As can be seen in the diagram, the control amplifier (Supervisory EEC) receives a signal from turbine gas temperature (EGT) and two compressor speed signals (N1 and N2). The hydromechanical unit responds to EEC commands and actually performs the functions necessary for engine operation and protection. The hydromechanical element controls the basic operation of the engine, including starting, acceleration, deceleration and shutdown. The EEC, acting in a supervisory capacity, modulates the engine fuel flow to maintain the designated thrust. The pilot simply moves the throttle lever to a desired thrust setting position, such as full take-off thrust or maximum climb. The control adjusts engine pressure ratio (EPR) as required to maintain the thrust rating, compensating for changes in flight and environmental conditions. As the engine nears full power, the electronic circuit starts to function as a fuel-limiting device to control maximum EGT and N1 and N2 compressor speeds. If a problem develops, controls automatically revert to the hydromechanical system with no discontinuity in thrust. A warning signal is displayed in the cockpit, but no immediate action is required by the pilot. The pilot can also revert to the hydromechanical control at any time. Although the EEC uses aircraft electrical power for some of its functions, electric power for its basic operation is supplied by a separate engine-driven permanent magnet alternator (PMA). 2022-08-08 B1-15b Gas Turbine Engine Page 110 of 291 CASA Part 66 - Training Materials Only  © Aviation Australia Supervisory electronic engine control systems Full Authority Digital Engine Control Modern large high-bypass turbofan engines use the Full Authority Digital Engine Control (FADEC) system. The EEC is the primary component of the FADEC engine fuel control system. The EEC is a dual-channel computer that controls engine operation. It is normally mounted on the fan casing and cooling is either by natural fan case cooling air or directly by a dedicated fan air duct. A true FADEC system has no hydromechanical fuel control backup system. The system uses electronic sensors that feed engine parameter information into the EEC. The EEC gathers the needed information to determine the amount of fuel flow and transmits it to the fuel metering valve. The fuel metering valve simply reacts to the commands from the EEC. The EEC is the computing section of the fuel delivery system, and the metering valve meters the fuel flow. Power for the EEC comes from the aircraft electrical system or the PMA. When the engine is running, the PMA supplies power directly to the EEC. The PMA has a dual coil, so each channel is supplied power independently. 2022-08-08 B1-15b Gas Turbine Engine Page 111 of 291 CASA Part 66 - Training Materials Only  The electronic computer may have many inputs and outputs: EPR Engine pressure ratio N1 Fan speed N2 Intermediate-pressure compressor speed N3 High-pressure compressor speed Tt2 Inlet total temperature Tt8 High-pressure turbine inlet temperature Pt2 Inlet total pressure 28V DC inlet power PMA Permanent magnet alternator PLA Power lever angle VIGV An inlet guide vane angle Ps6 High-pressure compressor discharge static pressure Wf Fuel flow ACC Active clearance control (compressor and turbine blade cooling air supplied by fan air). The FADEC system performs all functions necessary to operate a turbofan engine efficiently and safely during all operating conditions from start-up to shutdown. Benefits of using electronic engine control are reduced crew workload, increased reliability and reduced fuel consumption. Flight crew workload is decreased because the pilot need only set target thrust and monitor the EPR and/or N1 indication. The EEC automatically accelerates or decelerates the engine to the required setting without the pilot having to monitor the engine gauges. Reduced fuel consumption is attained because the EEC controls the engine operating parameters to attain maximum thrust for the amount of fuel consumed. Also, optimising airflow control allows the engine to work nearer the surge line, thus increasing thrust while reducing the chance of surge or stall. To provide a high degree of reliability, FADEC systems are designed with several redundant and dedicated subsystems. An EEC consists of two redundant channels (A and B), housed in the same unit, that send and receive data. Each channel consists of its own processor, power supply (PMA), memory, sensors and actuators. In addition, any one channel can take information from the other channel. This way, the EEC can still operate even if several faults exist. If both channels are serviceable, the active channel alternates with each engine start. The other channel is in standby mode. Should both channels fail, there is a second backup and systems vary depending on engine manufacturer. For example, in one system the actuators are spring-loaded to a failsafe position so the fuel flow goes to maintain engine flight idle. In another system, the EEC goes into a Soft Reversionary mode using the last known ambient pressure signal, or into a Hard Reversionary mode which uses a fixed corner-point ambient temperature for the engine. 2022-08-08 B1-15b Gas Turbine Engine Page 112 of 291 CASA Part 66 - Training Materials Only  The EEC also controls several other subsystems of the engine through torque motors and solenoids, such as fuel and air oil coolers, bleed valves, variable stator vanes, turbine cooling air valves and the turbine case cooling system. The EEC is provided with feedback via valves and actuators fitted with dual sensors. The hydromechanical unit provides an interface between the EEC signal and fuel flow. It is achieved by an electrical hydraulic servo valve (EHSV) or torque motors actuating a fuel metering valve (FMV), thus controlling fuel supply to the nozzles. The Engine Rating Plug selects the proper software in the EEC for the thrust rating of the engine. The plug attaches to the engine fan case with a lanyard. When removing the EEC, the plug remains with the engine; it is not interchangeable. Engine rating plug Engine trimming is eliminated by the use of full authority EEC, as the engine fuel control system has fault sensing, self-testing and correcting features designed into it, greatly increasing its reliability and maintainability. As shown in the diagram below, the full authority EEC receives data from various areas. It then analyses the data and sends commands to position the actuators and solenoids and schedule fuel flow through the hydromechanical section of the FCU. 2022-08-08 B1-15b Gas Turbine Engine Page 113 of 291 CASA Part 66 - Training Materials Only  Electronic engine control interface Factors Controlling FCU Performance The FCU must sense the various operating and environmental parameters to supply engine fuel in the correct quantities. Parameters that directly affect the FCU are: Power lever angle Rpm Air temperature Air pressure Burner pressure Fuel density. Power Lever Angle The power lever angle is the pilot’s main control over the engine. The power lever angle schedules the fuel required to the engine without taking into account the other operating parameters. 2022-08-08 B1-15b Gas Turbine Engine Page 114 of 291 CASA Part 66 - Training Materials Only  Rpm To be able to produce the varying powers required, the engine must be able to operate at different speeds. Equipment is included to sense rpm so that the FCU can provide the appropriate fuel flow for the rpm at which the engine is operating. Air Pressure For a turbine engine, with an increase in altitude, air pressure decreases and the weight of the total air mass that flows through the engine at a given rpm reduces. Therefore an increase in altitude reduces thrust for a given throttle setting. Air Temperature Air temperature has a direct relationship with air density, i.e. an increase in temperature leads to a decrease in density. Therefore, for a turbine engine, an increase in air temperature reduces the weight of the total air mass that flows through the engine at a given rpm, requiring the FCU to reduce the fuel flow to maintain the combustion process. As can be seen in the graph below, temperature affects engine performance. Air temperature has an effect on the engine performance 2022-08-08 B1-15b Gas Turbine Engine Page 115 of 291 CASA Part 66 - Training Materials Only  Burner Pressure Static pressure in the combustion chamber is a useful measure of mass airflow. If the mass airflow is known, the air-fuel ratio can be more carefully controlled. As aircraft use bleed air from the engine compressor to provide various services, it is imperative that the burner pressure is known to accurately regulate fuel when these services are being used. Combustion chamber pressure and inlet pressure acting through bellows and lever assemblies can accurately control the fuel being introduced into the engine to control the air-fuel mixture. Fuel Density As the different types of fuels that may be used in gas turbine engines have different densities or specific gravities, the fuel control unit needs a method of adjusting for the various flows that occur if different fuels are used. Variation of the fuel differential pressure valve spring tension can be used to change the fuel flow to accommodate for different fuels’ specific gravities. Specific gravity adjustment, shown below, is a means of resetting the tension on the differential pressure regulator valve spring within the fuel control when an alternate fuel is used. Modern aircraft automatically measure the density of the fuel and input this information into the EEC. Fuel control fuel specific gravity adjustment 2022-08-08 B1-15b Gas Turbine Engine Page 116 of 291 CASA Part 66 - Training Materials Only  Fuel System Components II High-Pressure Shut-Off Valve The HP shut-off cock is a valve mounted in the FCU/hydromechanical unit and is used to give a definite shut-off of the fuel line from the FCU/hydromechanical unit to the fuel burner nozzles. The HP cock may be connected directly to the engine power lever or to an independent lever/switch. However, on turbo-propeller aircraft, it is normally connected in conjunction with the propeller feather control lever to give a movement through gates of engine run (HP cock open) to engine stop (HP cock closed), then propeller feather (HP cock closed). Fuel Flow Transmitter Fuel flow meters are used in fuel systems to show the amount of fuel consumed per hour by the engine, thus allowing the pilot or a Fuel Management Computer to accurately calculate the available flight time remaining. As fuel flows through the meter, it spins a small turbine wheel, and a digital circuit reads the number of revolutions in a specified period and converts this to a fuel flow rate. Fuel flow transmitter 2022-08-08 B1-15b Gas Turbine Engine Page 117 of 291 CASA Part 66 - Training Materials Only  Pressurising and Dump Valve A fuel pressurising and dump valve is normally required on engines using duplex type fuel nozzles. It divides the fuel flow into primary and main manifolds and drains fuel from these manifolds on shutdown. Fuel pressurising and dump valve Pressurising Valve The fuel pressurising valve controls the fuel flows required for all engine operation. For starting and idle, all fuel passes through the primary manifold. As the throttle is advanced above idle, fuel pressure increases, and at a preset pressure, the pressurising valve begins to open the main manifold. Fuel is continuing to flow through the primary manifold. Dump Valve The dump valve gives the capability to ‘dump’ or drain fuel from the fuel manifolds after shutdown. Manifold dumping is a procedure which sharply cuts off combustion and prevents fuel boiling, or after-burning, as a result of residual engine heat. This boiling tends to leave solid deposits which could clog finely calibrated passageways. 2022-08-08 B1-15b Gas Turbine Engine Page 118 of 291 CASA Part 66 - Training Materials Only  Operation The construction and operation of pressurisation and dump valves varies with different manufacturers. However, the following is a description of the operation of a typical pressurisation and dump valve as shown in the illustration below. When the power lever is opened, a pressure signal from the FCU moves the dump valve against the spring pressure, closing the dump port and opening the passageway to the manifolds. Metered fuel pressure builds at the inlet check valve until the spring tension is overcome and fuel is allowed to flow through the filter to the primary manifold. At a speed slightly above idle, the fuel pressure is sufficient to overcome the pressurising valve spring force, and fuel also flows to the main manifold. On shutdown, when the fuel lever is moved to OFF, the pressure signal holding the dump port closed and the fuel passage open is lost. Spring pressure closes the fuel passage and opens the manifolds to the fuel dump, or return, line. Pressuring and dump valve operation 2022-08-08 B1-15b Gas Turbine Engine Page 119 of 291 CASA Part 66 - Training Materials Only  Drain Valves The combustor drain valve shown below is a mechanical device located in the low point of a combustion case. It is closed by gas pressure within the combustor during engine operation and is opened by spring pressure when the engine is not in operation. This valve prevents fuel accumulation in the combustor. Fuel accumulates in the bottom of the lower combustion chamber following shutdown or a false start. When the air pressure in the combustion chamber reduces to near atmospheric, the valve opens and allows the accumulated fuel to drain away. It is imperative that this valve is in good working order; otherwise, a hot start during the next start attempt or an after-fire on shutdown is likely to occur. Combustor drain valve 2022-08-08 B1-15b Gas Turbine Engine Page 120 of 291 CASA Part 66 - Training Materials Only  Drain Tank Dump fuel, in years past, was allowed to spill onto the ground or be siphoned from a drain tank in flight. Current international regulations, however, prohibit this form of environmental pollution, and now the drain tank fuel must be captured, perhaps by hand-draining. To prevent hand-draining, several types of recycling systems have recently been developed. One such system returns fuel to the aircraft fuel supply. Another pushes fuel, which formerly would have been dumped, out of the fuel nozzles by introducing bleed air into the dump port. This prolongs combustion slightly until fuel starvation occurs. In the system shown below, a full tank causes a float valve to actuate and drain the tank via an educator type flow system. Drain tank 2022-08-08 B1-15b Gas Turbine Engine Page 121 of 291 CASA Part 66 - Training Materials Only  Fuel Nozzles Fuel Nozzles Fuel nozzles are the terminating points of the fuel system. They are located in the inlet of the combustion liner. Some mix air with fuel in correct proportions to atomise or vaporise the fuel. Air shrouds direct cooling air over the spray nozzle to aid atomisation. The highly polished nozzle delivers a precisely patterned, highly atomised spray. Fuel cannot be burned in a liquid state. It must first be mixed with air in correct proportions by atomisation or vaporisation. The pressure-atomising type of nozzle receives fuel under high pressure from a manifold and delivers it to the combustor in a highly atomised, precisely patterned spray. The cone-shaped, atomised spray pattern provides a large fuel surface area of very fine fuel droplets. This optimises fuel-air mixing and ensures the highest heat release from the fuel. The most desirable flame pattern occurs at higher compressor pressure ratios. Consequently, during starting and other off-design speeds, the lack of compression allows the flame length to increase. Various stages of fuel atomisation 2022-08-08 B1-15b Gas Turbine Engine Page 122 of 291 CASA Part 66 - Training Materials Only  If the spray pattern is also slightly distorted, the flame, rather than being held centred in the liner, can touch the liner surface and cause a hot spot or even burn through. Another problem that distorts the spray pattern is contaminant particles within the nozzle or carbon build-up outside the nozzle orifice. This can cause hot streaking, which is an un-atomised stream of fuel which forms and tends to cut through the cooling air blanket and impinge on the liner or on downstream components such as the turbine nozzle. Simplex Fuel Nozzles The simplex design is basically a small, round orifice which provides a single spray pattern. It incorporates an internally fluted spin chamber to impart a swirling motion and reduce axial velocity of the fuel to provide atomisation as it exits the orifice. The internal check valve in the simplex nozzle is there to prevent dribbling of fuel from the fuel manifold into the combustor after shutdown. Simplex fuel nozzle The simplex fuel nozzle was used in early jet engines and is still used in modern small engines, such as those used as APUs. This fuel nozzle gives good atomisation at the higher fuel flows, that is, at the higher burner pressures, but was unsatisfactory at the low pressures required at low engine speeds and especially at high altitudes. Because simplex nozzles are effective only at high pressure/high rpm, they are commonly used in constant speed applications, such as in APUs. 2022-08-08 B1-15b Gas Turbine Engine Page 123 of 291 CASA Part 66 - Training Materials Only  Duplex Fuel Nozzles Duplex fuel nozzles have two separate fuel flows: Primary flow Secondary flow (also known as main flow). There are two categories of duplex fuel nozzles: Single line Dual line. The duplex burner requires a primary and secondary fuel manifold and has two independent orifices, one much smaller than the other. Initial fuel flow is through the smaller orifice (primary); the larger orifice (secondary) opens to discharge fuel at higher fuel flows. Primary flow has a wide spray pattern used for engine start-up to idle. Secondary fuel flows with primary above idle. Combined primary and secondary flow has a narrower pattern. The narrow pattern ensures that the flame will not impinge on the combustion liner. Duplex nozzle fuel spray patterns A duplex nozzle is able to give effective atomisation over a wider flow range than the simplex spray nozzle. 2022-08-08 B1-15b Gas Turbine Engine Page 124 of 291 CASA Part 66 - Training Materials Only  Single-Line Duplex Nozzles The single-line duplex type receives its fuel at one inlet port and requires a flow divider to distribute fuel through two spray orifices. The round centre orifice, called the primary fuel, sprays at a wide angle during engine start and acceleration to idle. The annular outer orifice, referred to as secondary fuel, opens at a preset fuel pressure to flow along with the primary fuel. Fuel of much higher volume and pressure flowing from this outer orifice causes the spray pattern to narrow so that the fuel will not impinge on the combustion liner at higher power settings. Secondary fuel flows with primary from idle to full power. Single line duplex fuel nozzle 2022-08-08 B1-15b Gas Turbine Engine Page 125 of 291 CASA Part 66 - Training Materials Only  Dual-Line Duplex Nozzles A second type of duplex nozzle, called a dual-line duplex type, is quite similar to the single-line except that it contains no flow divider check valve to separate primary and secondary fuel. The check valve in this system is located in the Pressurising and Dump Valve and is labelled ‘Pressurising Valve’. The pressurising valve acts as a single, main flow divider for all of the fuel nozzles, whereas in the single-line duplex nozzle, each has its own flow divider in the form of its check valve. Only primary fuel is supplied for starting and low flow conditions. This fuel comes from a flow divider. Fuel sprays out from the centre primary orifice in a wide spray. The air shroud directs cooling air over the spray nozzle. Cooling air from compressor discharge also aids atomisation of the fuel. Fuel is discharged from both primary and secondary orifices for high speed conditions. Dual-line duplex fuel nozzles 2022-08-08 B1-15b Gas Turbine Engine Page 126 of 291 CASA Part 66 - Training Materials Only  Air Blast Fuel Nozzles The air blast fuel nozzle is a newer design and is being more widely used in various-sized engines because it enhances the atomisation process and produces finer fuel droplets. This nozzle is said to be more effective during starting, when low fuel pressure causes atomisation problems. By using high- velocity airflow, air blast nozzles more completely atomise the fuel than can be accomplished with fuel under pressure alone. Some nozzle designs completely vaporise the fuel by the high-velocity air as it enters the combustion chamber. This nozzle also has an advantage in that it utilises a lower system working pressure than the basic atomising types of nozzles. Air blast fuel nozzles 2022-08-08 B1-15b Gas Turbine Engine Page 127 of 291 CASA Part 66 - Training Materials Only  Vaporising Fuel Nozzles The vaporising fuel nozzle, shown below, connects to a fuel manifold in an arrangement similar to the atomising type. Instead of delivering the fuel directly into the primary air of the combustor, as the atomising type does, the vaporising tube premixes the primary air and fuel. Combustor heat surrounding the nozzle causes the mixture to vaporise before exiting into the combustor flame zone. There are two types of vaporiser nozzle: Tee-shaped – dual outlets Cane-shaped – single outlet. Vaporising fuel nozzle Whereas the atomiser nozzle discharges in the downstream direction, the vaporiser discharges in the upstream direction and the mixture then makes a 180° turn to move downstream. This arrangement provides a slow-moving, fine spray over a wide range of fuel flows and is said to produce more stable combustion in some engines than can be achieved by atomising nozzles, especially at low revolutions per minute. Some vaporisers have only one outlet and are referred to as cane-shaped vaporisers. Because vaporising nozzles do not provide an effective spray pattern for starting, there can be an additional set of small atomising type spray nozzles which spray into the combustor during starting. After light-off, start fuel is terminated on spool-up to idle. This system is generally referred to as a primer or starting fuel system. 2022-08-08 B1-15b Gas Turbine Engine Page 128 of 291 CASA Part 66 - Training Materials Only  Vaporising fuel nozzle fuel vapour flow Relevant Youtube link: Fuel Systems 1 (Video) Relevant Youtube link: Fuel Systems 2 (Video) 2022-08-08 B1-15b Gas Turbine Engine Page 129 of 291 CASA Part 66 - Training Materials Only