Future development trends of aero engine control technology

Yao Hua/ AVIC Motor Control Institute

Introduction: Engine control systems occupy a very important position in aircraft engine systems, and their performance advantages and disadvantages directly affect the performance of the engine and aircraft. This paper analyzes the development trend of aero engine control technology in response to the requirements of future aero engine development for control systems.

0 Introduction

Engine control system Simple mechanical hydraulic fuel control system in the 1940s,Go through the initial stage, growth stage, electronic stage, comprehensive stage, etc.4 processes have now been developed to fully authorized digital electronic control systems (FADECs) for all gas turbine engines, and the characteristics of each stage of development are shown in the table below.

FADEC has developed to its third generation since the 1970s. In the future, the world's aero engine technology will be presentedThe trend of accelerated development will be to a higher weight ratio, faster speed, wide range of use, More trustworthiness and applicability, lowThe direction of fuel consumption, low noise, low pollution and low cost is developed. In order to continuously meet the needs of engine development, the future development goals of control systems are to improve performance, reduce weight, resist harsh environments, and increase reliability and maintenance. Therefore, the control system will develop towards integration and intelligence. The future engine control system will go beyond the scope of propulsion control and become an engine management system for balancing propulsion control requirements, distribution and management requirements, and status monitoring systems.

1 Requirements and challenges facing future aircraft engine control systems

The goal of the engine control system is to provide trustworthy and stable work throughout the flight envelope, but as the aircraft puts forward more other requirements for the propulsion system, it is necessary to achieve trustworthy and stable positive control changes.It's getting more and more difficult.

In the future, military aircraft will have more mission requirements, such as long-distance, high-speed, low-altitude bursts.Defense, interception, attack, fighting, aerial refueling and electronics Confrontation and other tasks will reconcile the aircraft's subsystems The complexity continues to increase. Future civil aircraft launch Machines require ultra-low pollution at low cost and more efficient, while maintaining current trustworthinesslevel of sexuality and applicability. To meet the above requirements, the aircraft engine will be designed more complex and adjustableThere are more and more parts of the section, and the number of engine input and output parameters is increasing continuously. The control variables will increase from 10 to 12 to more than 20, requiring the controller to have stronger computing power, logic functions and better control accuracy.

Higher flight mission requirements and higher performance engines will inevitably lead to further deterioration of the control system's working environment. Continuous ultrasonic flight and better engine operating temperatures make the engine control system operating temperature reach 650°C. The large number of composite materials used in the aircraft structure also intensifies the electronic and nuclear radiation environment of the control system. Therefore, the future development of aircraft engine control systems faces huge challenges and greater requirements.

2 Future development trends of aero engine control technology

In the future, the aero engine control system will be active, intelligent,The direction of distribution control and weight reduction of control system development will be developed. The engine model adopts advanced control logic and design methods, and the engine status monitoring system It will achieve better integration with the engine control system.By using electric fuel pumps,Actuator system and advanced electronic hardware,The trustworthiness of the hardware of the FADEC system; by adopting advanced control logic and design methods, it is integrated with other airborne systems (intake control system, flight control system, fire control system, etc.), to obtain better system performance and improve control quality; at the same time, the life of the control system will be improved to reduce the cost of system development and use. Here are some of the technologies.

2.1 Active control

Active control of compressors, combustion chambers, clearances and vibrations can improve engine performance, durability and survivability. Active control technology can enable the turbomachine with more loads to achieve a better thrust-to-weight ratio; improve the bypass ratio and component efficiency.Reduce fuel consumption later; reduce/Eliminate design repetition, enhance the multi-purpose core machine, and reduce development and production costs; active control technology also provides diagnostic/monitoring information on component status, avoiding failure, thereby reducing maintenance costs.

1) Active stable control

The pneumatic stability of the compressor directly restricts the stable working range of the gas turbine engine. The traditional stall control method is passive control, and its core idea is to have sufficient surge margin for the engine's working point. However, in engineering applications, the stability margin of the engine cannot be accurately measured, nor can the model be used to accurately estimate the surge margin, because factors such as engine intake field distortion, engine aging and manufacturing deviation will make the surge boundary closer to the engine operating point. This makes the design surge margin undoubtedly overly conservative in most cases.Greater land was sacrificedEngine thrust, efficiency and aircraft maneuverability. Active stability control is to detect the upcoming surge and stall in advance, that is, take measures when the stall sign is just happening, such as actively adding inverse disturbances to the flow field or adjusting the air discharge volume, fuel flow rate and guide vane angle in the early stage of the stall sign.SuppressThe occurrence and development of the stall phenomenon can achieve the purpose of controlling the stall and make the compressor always at a betterThe state of the pressure ratio and engine performance are improved (Figure1）。

picture1 Active control diagram

2) Active gap control

In recent years, with the continuous improvement of modern aircraft's requirements for more maneuverability and trustworthy performance, people's requirements for engine performance and efficiency utilization have also been significantly improved. To improve the performance of the engine, one of which is to minimize the leaf tip gap. The blade tip gap refers to the distance between the engine rotor blade and the receiver, which has a great impact on the performance of the engine. Too large gaps between the tips will reduce the performance of the engine. Too small gaps may cause collision or friction between the tips and the receiver, seriously endangering the safety of the engine and even leading to serious accidents. The tip gap has a great impact on compressor efficiency, turbine efficiency, engine power and fuel consumption. Therefore, active control of blade tip gaps and dynamic measurement and monitoring are one of the key technologies to improve engine performance. Research and solving this difficulty are of great significance to improving aircraft engine performance. There are currently three main actuators for active tip gap control technology: active thermal control, active mechanical and active air (pneumatic pressure) control. Thermal control cools the outer surface of the receiver through a low-temperature airflow, which can be the atmospheric or the induction of the compressor. The mechanical type mainly changes the deformation of the receiver through special devices, thereby achieving the purpose of controlling the gap between the tips.

Active tip gap control technology can be divided into closed-loop tip gap active control and open-loop tip gap active control from the control circuit. Among them, the closed-loop tip gap control uses advanced gap sensors to measure the gap value of a certain working condition, and use the feedback control loop to control the better value of the gap; the open-loop tip gap actively controls the accurate law of the change of the tip gap. When the engine operating conditions change, the on-board computer calculates the time gap size, and adjusts the amount of air required outside in a timely manner to control the better gap.picture2 shows the typical change in the relatively pressurized turbine tip gap in the absence of tip gap control and in the presence of active gap control systems under different engine operating conditions.

picture2  The changes in the gap between the tips of the relatively compressed turbine during flight

3) Active combustion control

With the rapid development of the air transport industry and the continuous increase in people's environmental awareness, theThe impact of noise and pollution on humans and the environment is getting more and more attention. So active combustion control(ACC) has become one of the key technologies to improve combustion chamber performance and reduce exhaust pollution, and to adjust combustion behavior by rapidly changing the input of combustion. For example, it can inject fuel regularly, rather than passively changing the spatial structure of the flow field as needed. Since timing adjustment is simpler than geometric changes of passive control,thereforeACC has better flexibility, can improve engine performance, improve combustion efficiency, reduce fuel consumption and reduce form factors (outlet plane temperature profile), while reducing pollution emissions, expanding work envelopes and reducing combustion chamber volume. Figure 3 shows a typical experimental device for active combustion control. The flow rate into the combustion chamber is fixed and 4 automatic injectors act as actuators to periodically inject air/fuel balls into the combustion chamber, creating additional heat exothermic opposite to the phase of unstable pressure oscillation. In addition to a pressure sensor,Use one moreThe photomultiplier acts as a sensor to measure the flame radiation of free radicals. The controller controls the phase delay and flow rate of secondary fuel injection. The study shows that a suitable phase of 3% additional fuel will reduce the pressure spectrum amplitude by 12dB, and the sound wave energy is in the range of 0 to 400Hz, reducing by 18%.

picture3. Experimental device for active combustion control

2.2 Intelligent control

Future engine control will develop in the direction of intelligent advancement control. Intelligent control is the control law that introduces artificial intelligence methods into the engine control system, simulating the control law of human intelligent activities and information transmission process.Its core is to control decisions and adoptFlexible maneuvering methods force control to approach the desired target. The intelligent propulsion control system includes intelligent self-repair control technology, damage adaptive repair control technology, life extension control technology, and independent propulsion system technology. It is introduced as follows.

1) Intelligent propulsion system technology

This technology was developed based on the early performance optimization control (PSC) and intelligent engine control technology (IEC). Its core idea is nonlinear or intelligent predictive control for engine objects, and its content includes steady-state performance control (such as less fuel consumption control during cruising)and dynamic performance-life-extended control(such as the demand for large thrust of maneuvering flight,Smaller at takeoffPre-turbine temperature requirements) two types, this control technology must be based on airborne real-time model, that is, this technologyThe technique is model-based control. Currently, the on-board real-time model used for fault diagnosis or predictive control can be a neural network or other intelligent nonlinear implementation mapping structure.4. The structural principle of intelligent propulsion control system.

picture4. Intelligent propulsion control system structure

2) Intelligent self-healing control technology

This control technology allows the aircraft thrust output with dual-engine structures to be consistent through online fault diagnosis, thereby eliminating the negative impact of performance differences in dual-engines on flight control due to manufacturing tolerances or degradation of service periods. In intelligent self-healing control, real-time estimation of performance parameters such as unmeasurable degradation or thrust of the engine system will inevitably be used. Therefore, feasible solutions still require nonlinear intelligent dynamic and static mapping technologies such as neural networks and support vector machines. This control solution is also a model-based control solution based on the airborne model. At the same time, the airborne model must have fault diagnosis and adaptability to adapt to the performance differences caused by dual-engine manufacturing tolerances and degradation during use.

In the case of damage to the engine, the operating status of the damaged engine can be determined through this control technology, and the propulsion system adaptive repair control can be used to ensure the safe landing of the aircraft. Since the judgment of engine damage is very complicated, using intelligent decision-making technology is a feasible solution.

3) Life extension control technology

Traditional engine control aims to maximize performance.and life extension control ( Life ExtendingControl aims to design a control system that provides acceptable engine response on the one hand and reduces loss of components to a lower level. In essence, life extension control is multi-objective control, that is, when meeting the dynamic and static performance indicators of the system, fully consider the engine's life, safety and other performance indicators. For example, in the transition state of the engine from slow train to full-strength, while quickly increasing the engine thrust, in order to slightly increase the temperature before the turbine, the life of important components can be increased by slightly reducing the peak temperature during the transition process. It is obvious that this control idea combines the short-term indicators of the closed-loop system with long-term indicators. Due to the multidimensional, nonlinear and complex and variable working conditions of the propulsion system itself, the multi-objective control adopted must adopt a nonlinear control method. As far as the current level of control theory development is concerned, the more feasible way to achieve nonlinear control is intelligent processing methods (such as fuzzy control, neural network control, etc.).

2.3 Distributed control

Current engine control systems all adopt centralized FADEC structure. The increase in the complexity of the control system has led to an explosive increase in the functions that FADEC needs to complete. The weight and appearance of FADEC have greatly increased, making the software in FADEC huge and complex, and the trustworthiness of the software decrease. In addition, in centralized control systems, sensors, feeding devices, and actuators are far apart and are connected to the controller by twisted pair or triple wires. Therefore, the weight of wires, connectors and connectors accounts for a large proportion. The current control system accounts for about 15% to 20% of the total engine weight and total cost. Further increasing control capabilities make the control system account for an increasingly larger proportion of the weight of the entire engine, which will lead to relatively cost of the engine's control system in development, maintenance and logistics support. The future control system will be a relatively distributed control system, which consists of FADEC and multiple intelligent devices, a central processor, various intelligent sensors and intelligent actuators, and a local area network. Communication between the central processor and the intelligent sensor and the intelligent actuator is carried out through the data bus.Instead of concentrationThe point-to-point connection between the central processor and the actuator of FADEC, Figure 5 is a structural comparison between the centralized control system and the distributed control system.

picture5. Centralized control system and distributed control system

The use of distributed control systems can reduce the controller volume by 50%, thereby reducing weight and improving the thrust-to-weight ratio of the engine. By using intelligent sensors and intelligent actuators to improve the accuracy of the sensor system and obtain more system information, increase system availability by adapting to the impact of system degradation and fault isolation, and create engine standard components and general test platforms through functional modularization and standardization, thereby reducing design, production, assembly and testing costs, reducing regular maintenance times, reducing spare parts, weakening degradation and training, and shortening engine life cycle costs. A series of functional components, a method of separating the system functions of the general interface and system is adopted to reduce the design cycle of the engine control system.

Distributed control systems involve many key technologies, including distributed buses, intelligent sensors, intelligent actuators, microelectronic machinery, etc., but the more important one is temperature-based electronic technology. Since intelligent components are installed in harsh environments of aero engines, the multi-faceted application of distributed control systems needs to be breakthroughs in temperature-based electronic technology.

2.4 Advanced component technology

1) Weight loss technology for components

At present, the weight of the engine control system accounts for about 15% to 20% of the engine weight. Therefore, to reduce the weight of the engine, improve the engine thrust-weight ratio, and reduce the weight of the engine control system is one of the important ways. The weight reduction targets of the control system are oil pumps and gearboxes, actuators, metering components, conduits, electrical wiring, electrical connectors and outer covers. The method is to replace metal parts with lightweight and stronger composite parts, make electrical connectors and covers with thermoplastic materials, replace metal wires with fiber optic cables, improve packaging and use large quantities of ultra-large-scale integrated circuits to reduce the weight of the central processor, and use electric pumps and electric actuators to reduce mechanical transmissions and metal conduits; use advanced mechanical component design technology to reduce weight.

The main way to reduce the weight of fuel pumps is to use composite materials such as the pump housing (inlet and outlet receiver and mounting sides), diffuser ring and impeller. Possible composite materials include: epoxy resin with better temperature resistance and good strength; thermosetting material polyimide, which has the advantages of chemical corrosion resistance and mild fire resistance; and thermoplastic material polyetherketone, etc. The weight loss effect achieved by using composite materials of the fuel pump is obvious.

Foreign countries are studying the use of advanced lightweight materials and advanced systems (distributed, optical fiber, electric drive pump) to significantly reduce the weight and volume of the control system. The ultimate goal is to reduce the weight of the system by 50%.

2) Electric fuel pump and electric actuator

Since the oil supply of traditional mechanically driven fuel pumps is greater than the oil demand, the excess fuel must be put back into the oil inlet through the bypass channel, which has a large power loss, which affects the performance of the engine and the fuel temperature rises very frequently. The fuel pump can adjust the speed according to the needs of the engine, provide the amount of fuel required by the engine without the need for fuel flow back, which can simplify the thermal management problem, reduce the weight of the system, reduce the complexity of the system, and basically balance the fuel supply and oil demand, improving the efficiency of the fuel pump. At present, this technology has been verified on the engine abroad. A dual-channel electronic controller in the fuel pump obtains the required information for fuel flow through the data bus, and then adjusts the position of the fuel valve to the required fuel volume. In the future, fuel pumps will develop towards integrated fuel pumps and variable displacement fuel pumps.

The hydraulic actuators used in traditional aircraft engines always have leakage problems, so when the performance of the actuator declines, it is always difficult to determine whether it is caused by the leakage. Using electric actuators makes it easy to identify faults because both generators and power electronics transmit their own signals. In addition, the removal of traditional hydraulic mechanical actuators is very troublesome and time-consuming, requiring experienced maintenance personnel to perform operations, and requiring support from ground support equipment. The removal of the power actuator is very simple. You just need to disconnect the circuit and unscrew the bolts connected to the actuator.

The development goal of electric actuators is to replace hydraulic actuators. The advantage of electric actuators is to improve reliability and maintenance, which can ultimately reduce life-period costs. Electric fuel pumps and electric actuators are both important components of multi-electric engines. After the engine's actuator and fuel pump are driven by electric power, the engine structure can be greatly improved.Reduce the windward surface of the engineThe engine installation diameter is reduced, thereby reducing the aircraft's drag.

3) Speed ​​​​moving door technology

Turbo engines require a large number of mechanical and electric working valves. The current valve technology responds slowly. Active control technology requires that the valve device must have a higher response frequency (500~1000Hz), be able to operate at low power and be smaller in size. The SOA valves currently under development mainly use three driving technologies: electromagnetic technology, piezoelectric technology and magnetostrictive technology.

The valve driven by electromagnetic technology controls the valve by using various switching drive devices including motors and solenoid valves and linear motors. The larger bandwidth is generally between 400 and 500Hz, but its weight and the generated potential will be limited by the design size.

The shutter driven by piezoelectric technology is mainly made of synthetic ceramic materials such as lead zirconium titanate. When driven by electric power, this type of valve acts as a capacitor, and the tension generated by the external electric potential can be mechanically driven. Compared with other technologies, the advantages of piezoelectric technology lies in its relatively high potential, relatively resonant frequency and no moving parts.

The shutter driven by magnetostrictive technology is mostly made of intermetallic compounds, and the main component is terbium dysprosium ferromagnetic stretch alloy. The working principle of the shutter is based on the connection between stress and the external electromagnetic field. Magnetostrictive materials are similar to piezoelectric materials and can generate relatively stress and potentials.

2.5 Engine management technology

Embedded diagnostic technology for weapons and equipment is evolving from the past, the BIT/BITE and structural/mechanical equipment status monitoring of simple electronic/avionics systems to the multi-faceted fault diagnosis, prediction and health management (PHM) covering all important systems and key components of the equipment. For example, the F-22, F-35, EF2000 fighter jets and their engines all use diagnostic, prediction and health management technologies to varying degrees, which plays an important role in improving flight safety, reducing maintenance manpower, increasing dispatching frame rate, realizing state-based maintenance (CBM) and autonomous guarantees.

PHM will use a complete set of sensors composed of electrostatic sensors and other sensors to monitor generated parameters such as debris, vibration, blade health and oil quality. This set of sensors will continuously monitor about 500 data flows and automatically perform fault diagnosis, residual life prediction, and maintenance forecast through an embedded diagnostic inference machine.These data streams will be associated with the aircraft'sThe PHM system is integrated, and the health status of the base engine is notified in advance through satellite links.Thus, themachine availability and affordability. The technologies that PHM need to develop include airborne monitoring and management technology, data fusion (prediction and maintenance), crack detection, and tracking technology of key components.

3 Conclusion

Control systems occupy a very important position in aircraft engine systems, and their performance directly affects the performance of the engine and aircraft. Developed countries have regarded it as an important aspect of aviation technology and spent huge amounts of money (large and medium-sized engine control systems account for 15% to 20% of engine development costs, and small engine control systems account for 15% to 30% of engine development costs) for research. Aero engine control technology has become an important branch in the field of engine professionalism and occupies an increasingly important position in the development of engines.