Shengrong GUO, Jinhua CHEN, Yueliang LU, Yan WANG,Hongkang DONG

a Nanjing Engineer Institute of Aircraft System Jincheng, AVIC, Nanjing 211100, China

b Aviation Science and Technology Key Laboratory of Aero Electromechanical System Integration, Nanjing 211100, China

c School of Transportation Science and Engineering, Beihang University, Beijing 100083, China

KEYWORDS

Abstract The piston pump is the key power component in the civil aircraft hydraulic system, and the most common pump used in the aviation field is the pressure compensated variable displacement type. In this review paper, a basic introduction to the civil aircraft piston pump is presented,including the classification,structure,working principle,design features,and achievements by some research groups.Then,the future directions of the aircraft pump are reported from various perspectives.Further,the critical technologies are analyzed and summarized in detail from six thrust areas:friction couples,noise reduction,inlet boost,thermal management,fault diagnosis and health management,and mechanical seal.Finally,the challenges and limitations of the research on the aircraft pump are discussed to provide valuable insight for future scholars.

1. Introduction

In the hydraulic system of civil aircraft,the piston pump is one of the most critical power components.1The pumps convert mechanical energy into hydraulic energy, supplying power to the actuators to fulfill the flight posture adjustment, and retraction and extension of the landing gear and braking.Due to their compact and simple design,swash plate type axial piston pumps are widely used in the aviation field,and they are capable of working at extremely high pressures and speed while maintaining high overall efficiency.

High pressure, high power, integration and intelligent control are the main trends of the fluid power systems in modern civil aircraft. The characteristics of civil aircraft, such as high reliability, safety and a long service life, also present technical requirements of the pump. Unlike industrial pumps, high speed aviation piston pumps may encounter many challenges:cavitation caused by low inlet pressure, large pulsation and noise, and tipping of rotating components. These undesirable phenomena could reduce the volumetric efficiency and aggravate the fatigue wear, eventually leading to a short life of the pump.In addition,the highly compact design of the hydraulic system weakens heat dissipation, which can easily cause an extremely rapid temperature increase and function failure.Although some of the challenges have drawn considerable attention, many problems remain unsolved.

The authors have read a great number of previous literatures and this review attempts to outline systematically the current state of the hydraulic pumps in civil aircrafts and major problems faced by designers and researchers. It is organized as follows: In Section 2, the working principle and design features of the typical aircraft piston pump are presented, followed by a brief introduction into the impressive achievements made by some research groups in Section 3. The future directions are described in Section 4. Section 5 summarizes the critical technologies and describes some research breakthroughs in detail. Finally, concluding remarks about this review paper are given in Section 6.

2. Working principle and design features

2.1. Classification and structure principle

The aircraft pump can be divided into fixed and variable displacement pumps.Fixed displacement pumps provide the flow that is directly proportional to the speed of rotation, with the ratio being the displacement of the pump and expressed as a volume per revolution.Variable displacement pumps can vary their displacements in response to some control mechanism.Due to their more compact and simple design, fixed displacement pumps can still be found in many applications, such as electro-hydrostatic actuator(EHA),and breaking and steering systems. Variable displacement pumps are typically used in central hydraulic systems characterized by multiple consumers where pump output is controlled by the means other than pump speed. Table 1 provides the classification of some commonly used pumps.

Currently, the flat cut-off pressure compensated type is the most commonly used for variable displacement control of aircraft pumps, which can provide nearly constant pressure at pump output at all flows lower than the maximum flow capacity of the pump.2Without too much wastage of power, this type of pump can meet the continuous demand for the pump throughout a flight with frequent variations in magnitude.3Fig.1 illustrates the cross-section view of a typical variable displacement pump in the aircraft hydraulic system. As the drive shaft rotates, the pistons reciprocate within the cylinder block bores. The piston shoes are held against a bearing surface by compression force during the discharge stroke and by the shoe hold-down plate and retainer during the intake stroke. The pump displacement can be adjusted by changing the angle of the yoke through an actuator piston, which is driven by the control pressure oil out from the compensator valve.The characteristic curve of the flow and pressure is shown in Fig.2.4The preload on the pressure compensator spring determines the system pressure at which the pump begins regulating.Compressing the spring (increasing preload) causes the pump to regulate at a higher pressure, and conversely, relaxing the spring (decreasing preload) provides regulation at a lower pressure.

Table 1 Types of some commonly used pumps.

Fig.1 Cross-section view of a typical civil aircraft pump.4

Fig.2 Flow-pressure characteristic (flat cut-off).4

2.2. Design features

The original concept of piston pumps can date back to the 16th century, when Ramelli developed a piston pump to draw water in mines.5However,it was not until 1905 that American engineer Reynold designed the first valve plate type hydraulic pump for the steering system in naval vessels and introduced mineral oil as the transmission medium,which was the prelude to modern hydraulics. During the following decades, the optimization and modification based on Reynold’s pump never stopped, and particularly in the 1950s, Denison and Lucas designed a novel swash plate pump which increased the pressure up to 35 MPa,marking a leap for the piston pump.After World War II, the piston pump began to be used in civil aircraft, and specific designs had to be developed in order to adapt to the harsh conditions in high altitude. As the world leaders in aerospace hydraulic systems and components,Eaton and Parker continually develop their products with extensive analysis and verification tests,and at present hydraulic pumps have incorporated numerous design features to enhance the efficiency, reliability and maintainability of the unit.4,6

2.2.1. Centrifugal boost impeller

For the suction performance at low inlet pressures,an impeller is used to ensure that the piston bores fill properly.The impeller adds the length and weight of the pump, but results in weight reduction and increased reliability of the overall system.

2.2.2. Attenuation

The attenuator minimizes outlet pressure pulsations in the aircraft hydraulic system. This reduces the wear on the components downstream of the pump, thus improving the overall reliability of the hydraulic system.

2.2.3. Electrical depressurization valve (EDV)

Energizing the EDV will port outlet pressure to a depressurizing position; therefore, the leakage is circulated at a low pressure instead of the rated pressure during idle periods, and the power loss is further reduced.Additionally,owing to the EDV,the outlet pressure can build up rather rapidly.

2.2.4. Blocking valve

Incorporated in conjunction with a depressurization circuit,the outlet blocking valve is hydraulically balanced for rapid response and ease of manufacture. Viscous dampening of the blocking valve piston retards the closure rate to allow sufficient decompression of the outlet fluid of the system prior to the valve closure.

2.2.5. Gerotor

A gerotor ensures case drain flow against the maximum back pressure of the system at the case drain port, decreasing the operating temperature of the rotating group and minimizing the pressure loads on the rotating group components.

2.2.6. Rotating mechanical seal

A mechanical seal is composed of a rotating ring and a static one, with the former sliding against the latter. The static one is generally made of high quality materials such as bearing grade bronze or carbon. The interface between these two elements ensures the sealing of the fluid in high speed cases.

3. Research focuses

In order to understand the physical phenomena of the piston pump,early researchers generally conducted theoretical analysis assisted with simplified test devices. In the 1960s, Yamaguchi derived the formula for calculating the pressure change in the cylinder to reveal the effects of trapping phenomena and the pressure ripple.7In the following decades, he and his colleagues continued researching the motion of pistons inside the cylinder bores and the lubricating characteristics of the friction interfaces.8–15In 1980, Rvachev and Slesarenko calculated the temperature distribution in the cylinder block of an axial piston pump, and the results were compared with the experiment.16Edge and others from the Fluid Power Centre in University of Bath first carried out the theoretical and experimental study of the pressure fluctuations generated by hydraulic pumps.17–20In the 1990s, this team remarkably introduced a creative method for measuring the source flow ripple and source impedance of hydraulic pumps.21This method was the ‘secondary source’ method, adopted as the ISO standard in 1996.22Although the early works were impressive, their deviation from actual results is significant due to the limitations of computing capabilities and test conditions.

In the 21st century, the rapid development of computer technology and numerical methods has witnessed the wide use of Computational Fluid Dynamics(CFD)and virtual prototyping technology in various research areas of the piston pump,enabling the multi-domain analysis in complex models.Focusing on the research of the axial piston pump, several active research teams have made numerous achievements in recent years.The Maha Fluid Power Research Center(Maha),led by Ivantysynova, continuously carries out the design and optimization of pumps and motors. The simulation program Calculation of Swash Plate Type Axial Piston Pump/Motor(CASPAR) was developed as a design tool for optimizing swash plate machines based on non-isothermal gap flow models of critical lubricating interfaces.23–25The program allows the calculation of real flow ripples at both ports, the calculation of the instantaneous cylinder pressure, the internal and external volumetric losses, viscous friction forces, gap heights,and oscillating forces and moments exerted on the swash plate.26–34Another research interest of Maha is noise generation and reduction. Kim and Ivantysynova investigated swash plate active vibration control (AVC) techniques with twoweight notch least mean square/filtered-x least mean square(LMS/FxLMS)filters.Utilizing the proposed controllers,their tests showed effective swash plate vibration reductions at the targeted frequency.35,36In addition, Vacca et al. investigated the cavitation in the hydraulic pumps by lumped parameter and CFD approaches.37–39The institute for fluid power drives and controls (IFAS) of RWTH Aachen University has a special group for the research and development of hydrostatic displacement units. Its work is oriented towards improving the tribological performance and using new materials and surface coatings.40–45Manring, a professor from the University of Missouri-Columbia, investigated the noise reduction through optimizing the valve-plate slot geometries46and altering the trajectory of the piston travel.47Recently, he focused on the improvement of power density48,49and operating efficiency.50,51Division Fluid and Mechatronic Systems(FLUMES) at Linko¨ping University has a long tradition in research into hydraulic pumps and motors. The main focus in many projects has been the methods of measurement and simulation of flow pulsations in the pumps, which has produced several innovations and methods to significantly reduce noise.52–54Additionally, the team also investigated and analyzes the displacement control of pumps via the comparison between pressure and flow control.55The researchers in Cardiff University, under the direction of Prof. Watton, carried out some investigations on flow ripple, pump leakage, barrel dynamics and lubricating interface via numerical and experimental methods.56–60Particularly, they measured the pressure ripple inside the pistons chambers for the first time using extremely small pressure transducers,and the technology might be a solution for fault diagnosis analysis.

The State Key Lab of Fluid Power&Mechatronic Systems(SKLoFP), Zhejiang University, is the most important research center in the field of fluid power in China, which has conducted much work on the high speed aircraft piston pump. Xu and Song optimized the structure of the precompression volume and the valve-plate slots to reduce flow ripple.61–63Zhang and Chao investigated the cylinder block tilt in a high-speed electro-hydrostatic actuator pump of aircraft,considering the effect of piston-slipper assembly mass difference and the geometric errors of the cylinder block.64–67Ouyang et al. conducted dynamic analyses of the swash plate vibration and pressure pulsation of an aircraft piston pump based on fluid-structure interactions (FSIs),68and presented the attenuating characteristics of the integrated buffer bottles.69,70

In addition, the researchers from Beihang University have carried out effective studies and achieved significantly in fault diagnosis, and prognostics and health management (PHM) of aviation mechatronic systems and components. Wang et al.analyzed the failure mechanism of high-speed aircraft pumps,and presented the fault diagnosis methods based on the layered clustering algorithm,71Dempster–Shafer evidence theory72and a nonlinear unknown input observer.1Ma et al. studied the typical failure modes of the aircraft hydraulic pump such as wear,fatigue, and thermal aging,and proposed the accelerated lifetime test methods including strengthening the load and worsening the operation conditions.73–77

The civil aircraft pump is a typical component which has the characteristics of multi-domains (mechanical, electronic and hydraulic)and multi-scales(from microns to the decimeter level).It is necessary to develop fully coupled simulation models in order to comprehend the physical phenomena.Additionally, the advanced data acquisition systems and test methods contribute to obtaining the information unavailable before,such as the high frequency pressure fluctuation,and film characteristics of lubricating interfaces.

4. Future directions

According to the analysis of current literatures, the future directions of civil aircraft pumps are likely to move toward the followings: (a) high pressure; (b) low pulsation; (c) high reliability and long service life; (d) intelligence and energysaving; (e) intelligence and energy-saving.

4.1. High pressure

Fig.3 Operational pressure used in different types of aircraft.

High pressure is conductive to reducing the size and weight of fluid power equipment, and improving the capability and maneuverability of aircraft.Fig.3 shows the operational pressure used in different types of aircraft. In the 1950s, Martin proposed to the Air Force that 3000 psi (21 MPa) pressure was a practical upper limit of aircraft hydraulic systems,which remained in effect for a long time.78It was in this period that aircraft hydraulic systems used a constant pressure variable pump as the power supply source. Rockwell performed some simulations and experiments to prove that the optimal fluid pressure of aircraft hydraulic systems is 8000 psi (56 MPa).79Studies have shown that the Airbus A380 was able to reduce its overall weight by 1 T after adopting a 5000 psi hydraulic system.80Comparative studies showed that by increasing the hydraulic pressure from 3000 psi (21 MPa) to 8000 psi(56 MPa), the weight of a hydraulic system can be reduced by 30%, whereas its volume can be reduced by 40%.81Therefore, super-high pressure is a future direction.

However, the increasing demand for high pressure leads to some problems.First,the power loss of pumps is proportional to the square of pressure,which means that increasing the fluid pressure will result in increased leakage in the hydraulic system. There is a similar law of the leakage for other hydraulic components.For example,as the pressure of a hydraulic pump increases from 1500 psi (10.5 MPa) to 3000 psi (21 MPa), the volume of power loss increases from 0.735 to 2.94 kW.82Next,high pressure has generated new technical requirements for hydraulic components, whose strength and seal performance need to be enhanced for reliability.

4.2. High rotational speed

The future development of general aircraft systems is directed towards highly integrated and power efficient systems,83which makes the electric motor pump(EMP)used more widely.Generally, the EMP, for example the pump used in the electrohydrostatic actuator (EHA)84system, has a higher rotational speed when compared with the general primary pump of aircraft, engine driven pump (EDP). Table 2 shows the rated speed and displacement of the pumps used in Boeing and Airbus civil aircrafts85.As it can be seen in the table,the speed of the EMP has increased from less than 5000 r/min in early aircrafts to approximately 8000 r/min in modern civil aircrafts,and even more than 12000 r/min in some more electric aircrafts. However, high-speed operating conditions will bring several practical problems imposed on aircraft pumps,such as pressure pulsation, cavitation, cylinder block tilt etc.,64–67,86which are challenges to future pumps design.

4.3. Low pulsation

Flow ripple that superimposes upon the mean flow rate is an inherent characteristic of a piston pump; its main frequency component varies with the rotation speed of the pump. The flow pulsations can be divided into two parts: kinematic flow pulsations, which are created by the geometrically defined motions of the limited number of pistons, and compressible flow pulsations due to the limited stiffness of oil. These flow pulsations interact with the connected pipeline system and transform into pressure pulsations, which then spread throughout the pipelines to other parts of the system.Pressure pulsations and the accompanying vibrations are often the sources of unreliability and fatigue of an aircraft hydraulic power system.Some pipe crack cases occurred in early aircraft due to the large pressure pulsation.Therefore,according to theSAE Aerospace standard,87the procurement specification provided by the purchaser shall state the maximum permitted amplitude of the discharge pressure pulsations. In general,the amplitude of pressure pulsations shall not exceed 5% of the rated pressure under any condition or a pressure band specified by pump specifications.The pump pressure pulsation can be as low as ±1% in some well-designed systems, for instance the Airbus A380 hydraulic system, and as high as±10% as allowed in older systems.2Thus, it can be seen that reducing the pressure pulsation is a permanent topic for the aircraft piston pump.

Table 2 Information of EDPs and EMPs used in Boeing and Airbus aircrafts.

4.4. High reliability and long service life

As an important part of the aircraft entire system,the hydraulic system should have high reliability to ensure flight safety,which also creates the demand for a long service life of the pump. In general, the service life of civil aircraft is more than 60,000 hours.88The engine driven pump (EDP), mounted directly in the engine transmission case, is required to have high reliability for the consideration of maintenance. The EDP used in the traditional civil aircraft could serve for more than 20,000 hours,89but the modern civil aircraft has higher requirements for the reliability of the pump. For instance,the average life of the EDP on Airbus A380 is assured to be 35000 hours by the supplier.80Thus, it can be concluded that a long life is a future direction of the aircraft piston pump.

There are many factors that affect the reliability of the pump,including wear of friction couples,oil cleanliness,operation conditions, poor suction capability, etc. For the pump itself, the key to a long life is the design of friction couples in high speed and high pressure conditions;the film characteristics of the lubricating interfaces have heavy influence on the reliability of the pump,and sometimes can even be decisive.In addition, the other critical technologies, such as anticavitation,pulsation reduction,and temperature management,also contribute to the long-life piston pump,which will be presented in detail in Section 5.

4.5. Intelligence and energy saving

Under normal conditions,the aircraft hydraulic system uses an EDP to supply the pressurized hydraulic fluid to the flight control and utility systems. As mentioned above, the typical EDP is a pressure compensated variable displacement axial piston pump capable of delivering a variable volume of fluid to maintain the pressure in a hydraulic system.However,the constant output pressure is set according to the maximum pressure during maneuver flight,which accounts for only less than 10%of the total flight time.81This causes significant power loss, further resulting in poor efficiency and temperature rise.

An intelligent hydraulic pump system (IHPS) is generally defined as a kind of pump source systems whose output can be easily controlled by virtue of an intelligent controller to meet the requirements of an actual aircraft hydraulic system.To realize feedback control of the output, necessary sensors are installed on an IHPS, including pressure sensors, displacement sensors, and temperature sensors. Based on the state parameters of an IHPS and its actual operating condition,the controller adjusts the displacement of the pump in accordance with the pressure signal, and then achieves optimal matching with the load. However, it will cause flow shortage and rapid pressure drop when the IHPS responds sluggishly and influences flight safety seriously.Moreover,the additional parts on the pump could significantly reduce the reliability of the whole system.As a result,the IHPS has never been applied on civil hydraulic systems.Currently,the dual pressure aircraft pump, equipped with an electrical depressurization valve, is adopted as an alternative solution, due to its advantages of high efficiency and energy saving.

5. Critical technologies

As discussed in Section 4,high pressure,high rotational speed,low pulsation, highest reliability and long service life, intelligence and energy saving are the main directions for future pump designs. These future directions raise new problems or make the pre-existing problems more outstanding. In order to meet the requirements of future pumps, it is necessary to improve the following critical technologies.

5.1. Design of friction couples in high speed and high pressure conditions

The lubricating gaps of the friction couples in piston machines represent the main source of power loss.A deep understanding of the complex physical phenomena characterizing the complex fluid-structure interaction is crucial for improving the existing designs and designing more efficient machines. The three main lubricating gaps (see Fig.4)27in these machines must fulfill the functions of sealing and bearing. Unlike other tribological contacts, the gaps of axial piston machines fulfill simultaneously bearing and sealing functions under extreme oscillating loads, making the optimization of gap geometry an extremely challenging task. Besides the main movement(for example, axial movement and piston spin motion for the piston), a micro movement is performed in the piston, slipper and cylinder block,changing the film thickness and generating an additional squeeze film effect.Recently,the problem of friction couples has become a hot topic due to its importance and complexity.

Fig.4 Lubricating interfaces in swash plate type axial piston machines.27

Pelosi,28Zecchi30and Schenk31investigated the fluidstructure interaction modeling of the three primary sliding interfaces in swash plate type axial piston machines. In their dissertations, each lubricating interface model can capture the complex fluid-structure interaction and thermal phenomena affecting the non-isothermal fluid film conditions. In particular, the models consider the change in fluid film thickness and squeeze film effect due to the component micro-motion as well as the elastic deformations of solid boundaries. The elastic deformation of the surfaces is related to the fluid film pressure and thermal stresses. The models couple iteratively different numerical domains and solution schemes,as depicted in the case of piston/cylinder in Fig.5. Different numerical methods, discretization schemes and solvers are necessary in order to solve different physical problems/domains. The communication and coupling are allowed through advanced interpolation methods based on nearest neighbor searching.

The numerical model can be used to investigate better lubricating interface designs, including novel material combinations90,91and micro-shaped surfaces.92The researchers in MAHA discovered that a micro metric sine waved piston shape could reduce power loss generated in the piston/cylinder assembly. Simulations have demonstrated the potential decrease of overall power loss up to 50% at full displacement and 65%at partial displacement at higher pressures,and even up to 20% and 60% at full and partial displacements, respectively, at lower pressures.93,94These results represent a major breakthrough in this research direction, suggesting that an even deeper study of the possible new technologies will lead to a new generation of pumps and motors.

Fig.5 Piston/cylinder model coupled numerical domains (see Ref. 27 for related variables).

Fig.6 PVD coated piston and coating structure.95

To avoid the two hard surfaces running against each other,RWTH Aachen University40,41developed new physical vapor deposition (PVD) coatings to coat the piston and slipper(Fig.6)95. For the pump mode, the pistons, which require contoured bores, are cylindrical without contours. The PVD coatings use a graded‘‘Zirconium Carbide”(ZrCg)which features increased carbon content over the coating thickness as indicated in the lower part of the figure. As a consequence,the hardness varies and decreases towards the top of the coating. The coating thickness is in the range of smaller double digit μm, allowing a rather limited run in while the soft outer layer is smoothened.Thus,it becomes possible to use tempered steel as a counter body.

5.2. Noise reduction

It is widely accepted that the noise emitted from the piston pump can be attributed to two main sources, namely, Fluid Borne Noise Sources (FBNS) and Structure Borne Noise Sources (SBNS). Both FBNS and SBNS are generated from the pressure changes in displacement chambers. Currently,the methods for pulsation reduction can be divided into two categories: optimizing the existing structure and installing the attenuators.

Pressure pulsations and the accompanying vibrations are often caused by the improper design of the valve plate; thus,optimizing the structure of the valve plate is a hot topic for reducing the pulsation. Manring46investigated the principal advantages of using various valve-plate slot geometries within an axial piston pump, as shown in Fig.7 (in the figure,rrepresents the piston pitch radius;drepresents the depth of the valve-plate slot; ω represents the shaft angular speed;wrepresents the bottom length of the triangle slot; and φf(shuō)represents the angular length of the valve-plate slot). The results of this study suggest that the use of quadratically varying slot geometry is not effective since it offers no obvious performance improvement.

Based on a comparison of the effectiveness of three valve plates (Fig.8), Seeniraj et al.96found that among the passive design methods, precompression grooves and precompression filter volume (PCFV) were most effective in reducing noise sources.The authors also explained the limitations of precompression grooves and PCFV, and further proposed a new design method which combines the precompression grooves,PCFV and decompression filter volume (DCFV).

Xu et al.97proposed a new design method for the transition region of valve plate based on the matching of the flow area and the reduction of transient reverse flow. The authors discussed the impact of the flow ripple in the discharge line of an axial piston pump and the impact of the pressure overshoot and undershoot in the piston chamber on the fluid borne noise.The results showed that the new method could reduce the flow ripple and eliminate the pressure overshoot and undershoot.In 2016, this research group continued the investigation of the potential of flow ripple reduction using a combination of cross-angle and pressure relief grooves.63As shown in Fig.9(in this figure, γ represents the cross angle of valve-plate; φ represents the timing; θ represents the rotational angle of the cylinder; and φs1, φe1, φs2, φe2represent the starting and ending locations of the valve plate inlet and outlet ports), the authors chose proper matches between the cross-angle and timing in order to guarantee that the precompression and decompression angles for different cross-angles were the same with those of the original design.The findings showed that the flow ripples from the optimal solutions could be reduced by using the cross-angle and pressure relief grooves, comparing with the pumps using the cross-angle and ordinary precompression and decompression angles.

Fig.7 Typical slot geometries that are used within axial-piston pumps.46

Fig.8 Three types of valve plates.95

Fig.9 Valve plate designs and the simulation results of outlet flow rate.63

Optimizing the structures other than valve plate could also reduce noise and vibration, which has been demonstrated by previous studies. In 2004, Manring and Dong98pioneered the analysis of the control and containment forces acting on the swash plate. In their research, the dynamic characteristics of the control and containment forces were calculated by deriving instantaneous and average equations of the motion for the swash plate.The results showed that using a secondary swash plate angle could limit the magnitude of the required control effort for the pump, and further reduced the flow ripple in a large range of operating conditions.Recently,Kim and Ivantysynova35,36proposed the concept of swash plate active vibration control with Two-Weight Notch Least Mean Square/Filtered-x Least Mean Square (LMS/FxLMS) Filters.The concept of AVC is the reduction of the swash plate vibration by means of creating a destructive interference force using the swash plate control. As a pump rotates, the oscillating swash plate moment (MX)is converted to the oscillating force(FSL) which is acting on the swash plate control actuator. The active vibration control adjusts the high response servo valve and generates a destructive interference force to the oscillating force (FSL) as shown in Fig.10 (in this figure,FSLrepresents the self-adjusting force;FControlrepresents the control force;SPAccrepresents the acceleration of the swash plate;usvrepresents the control voltage of servo valve).

To date it is difficult to improve the noise reduction effect only by optimizing the existing structures of the piston pump,and more studies therefore have focused on developing new pulsation attenuators for the aircraft piston pump. A novel hydraulic silencer99,100has been developed,employing an engineered compliant material lining (see Fig.11). The device has fewer parts than current products, so it will be less costly to manufacture, and its lining is maintenance-free. The device has been shown to have performance acceptable to industry in direct comparison to the available products. The material has been used beyond the silencer, and is being considered for compliance control in other devices. In 2013, Gao et al.69developed a novel pulsation attenuator for the pressurecontrolled aircraft piston pump. The attenuator, as shown in Fig.12, can adjust itself to keep the attenuation effect when the pump parameters change within a certain range, and it is also compact enough to be integrated into the pump.

At present,the EDP with integrated attenuator(see Fig.13)is wildly used in modern civil aircraft. For instance, Airbus A380 installs eight PV3-300-31 piston pumps as the EDP,which are supplied by Eaton Vickers.This type of piston pump has eleven pistons and an integrated spherical attenuator with the consideration of pressure pulsation reduction.It is verified that the amplitude of pressure fluctuation is only ±1%; thus,this pump is regarded as the quietest pump on the market.The AP series EDP,6manufactured by Parker Hannifin, is also available to equip the attenuator as the optional features,adopted by a number of civil and military aircrafts from Airbus, Boeing, Lockheed Martin, Dassault, etc.

Fig.10 Concept of swash plate AVC.

Fig.11 Compact hydraulic in-line silencer.

Fig.12 Schematic of a pressure attenuator.

5.3. Inlet booster impeller

Fig.13 EDPs with integrated attenuators.

Compared with common industrial pumps,the EDP and EMP in modern aircrafts are more likely to suffer from cavitation damage than those in other engineering applications since they work at extremely high rotation speed (eg., more than 10000 r/min).84–86,101Further, the requirements of size and weight for airborne components limit greatly the diameter of the inlet pipe,and the distance between the tank and the pump inevitably elongates the pipe.Therefore,it is necessary to boost the inlet pressure in order to feed the suction chamber with sufficient fluid flow.In practice,there are three types of solutions to deal with the problem: (1) pressurizing the tank,81,102(2)boosting the inlet pressure with volumetric pump,103and (3)boosting the inlet pressure with the centrifugal impeller.4,6Currently, the bootstrap type and pressurized gas type reservoirs are widely used in civil and military aircraft. However, the cavitation phenomenon still exists if pressurizing the tank is solely adopted, because the pressure loss before the pump significantly limits the pressurization effect especially in large flow conditions. In small flow conditions, most outlet flow of the volumetric type booster could enter the return circuit,so that the induced rise in the weight of the return circuit is a great burden for aviation equipment. Comparatively, a small size impeller can easily permit the pump to operate at inlet pressure well below atmospheric pressure, according to an experimental research carried out by Chen.104Unfortunately, until now, little literature has described how to design and develop the inlet boost impeller for the aircraft piston pump.

As mentioned in Section 2, the most common aircraft piston pump is the variable displacement pressure compensated type, so the output flow changes from zero to maximum. In order to avoid insufficient suction, the flow from the boost impeller must meet the requirements for the maximum flow.On the other hand, the impeller is directly mounted on the main shaft, meaning that it has the same rotation speed with the cylinder block. As a result, it is unable to match the flow requirement by changing the speed of the impeller. Furthermore, the water hammer caused by abrupt velocity change of fluid flow is inevitable and sometime harmful to the impeller blades. Therefore, the issues discussed above must be taken into account during the design of inlet boost impeller.

5.4. Thermal management

The tendency of the hydraulic system in civil aircraft is higher pressure and higher power.The higher pressure means greater power loss, and the aircraft pump is belong to the compact design aviation equipment, resulting in poor heat dissipation and rapid temperature increase. An increase in temperature reduces the service life of the fluid and seals. A predictable increase in aircraft hydraulic system pressure from 3000 psi(21 MPa) to 8000 psi (56 MPa) will cause a temperature rise from 110 to 180°C.81The statistic data indicates that the average stable life will reduce by 90% if the oil temperature increases by 15°C.81Additionally, an increase in oil temperature aggravates the sediment accumulation, reduces the lubrication property, and can result in the function loss of the whole fluid power system. Furthermore, future aircraft, especially the military one, will increasingly use composite materials that are characterized by poor heat-transfer capability considering the reduction of the radar cross section (RCS).During the supersonic flight, an increase in aircraft surface temperature will further increase the hydraulic system temperature. Hence, it is necessary to find solutions to keep the system temperature at the appropriate level.

At present, several methods have been attempted to solve the heating problem of the piston pump.Power loss originates from the lubricating gaps between the friction couples, which has great influence on the temperature rise of the piston pump.Therefore,the proper design of the friction couples is advantageous to effective management of the heating problem. A detailed description of this issue can be referred to in Section 5.1. Guo and Yin105comprehensively summarized the static and dynamic calculation methods of the temperature in aircraft hydraulic systems, and further proposed thermal management strategies. Parker Hannifin has developed a remarkable technique of forced heat dissipation to be applied on the EDPs.6A gerotor is used inside the pump to accelerate the case drain flow,decreases the operating temperature of the rotating group,and minimizes the pressure loads on the rotating group components.However,the rule of matching the suction pump with the piston pump needs to be further investigated to avoid cavitation or over pressure in the case.

In some rare cases, however, the temperature need to be maintained at a high level. The air driven pump (ADP), also named ram air turbine(RAT)pump,generates power for flight control in an emergency. The RAT pump can start rapidly in cold environment, attributing to an orifice for heating the hydraulic liquid.Wang et al.106proposed a calculation method for the size of the temperature control orifice based on the thermal equilibrium.

5.5. Fault diagnosis and health management

Efficient diagnosis of the aircraft fluid power system is one of the key techniques in the prognostic and health management(PHM) system. Generally, there are three or four redundant hydraulic power systems in aerial designs to ensure flight security.As a crucial component in the hydraulic system,the EDP must be sufficiently reliable during the flight. However, the EDP consists of many precision parts which work in highspeed and high-pressure conditions, so faults occur easily.Therefore, monitoring the status and predicting the faults of the EDP in real-time becomes particularly important. As shown in Fig.14,the PHM system should contain the following functions:state monitoring via signal acquisition,fault diagnosis and prediction,and evaluation mechanism of PHM.Due to the PHM system, the fault in the EDP can be located quickly and accurately; thus, the on-condition maintenance is realized in order to reduce the total maintenance cost and time.

Fig.14 Structure of PHM system.

To obtain the health state and fault features of the EDP,the essential sensors should be installed in the hydraulic system.PHM collects information from these sensors and extracts fault features from characteristic parameters. However, the information always contains many different interference signals, so it is necessary to adopt a method to extract the fault features effectively, eliminate the disturbance, and highlight the failure features. The common approaches used in the PHM system for feature extraction include wavelet analysis,107cepstrum envelope analysis,108empirical mode decomposition,109and the chaos-based weak signal extraction.110The most significant difference between a PHM and traditional fault diagnosis lies in the prognostic function. PHM can use the prior fault knowledge and the current state to predict the variation trend of parameters or performance, and obtain the residual life quickly and accurately.With the effective fault prediction, PHM can guide the repair decision effectively before faults occur, and stop the fault development. Hence the fault prediction can prevent the catastrophic failure after mastering the performance degradation. In PHM evaluation,the reliability, safety, maintainability, failure coverage rate,and alarm rate should be comprehensively considered.

5.6. High-parameter mechanical seal

The mechanical seal is widely used as the shaft seal in highspeed rotating machinery such as pumps, turbines and compressors. The main role of the mechanical face seal is to prevent leakage inside instruments because any problem in this element can cause significant economic loss and casualties. As for the aircraft piston pump, the mechanical seal does not belong to the frontier technologies, but it is significant for the reliability of the pump.The expected life of the mechanical seal should be up to tens of thousands of hours even though it works with high parameters, including the high rotation speed4(the recommended maximum speed is up to 22500 r/min) and wide temperature range (from -55°C to 200°C).Moreover, the seal often suffers from severe vibration impact,which is harmful to the performance of the seal.It is generally accepted that wear is the main form which disables the function of the mechanical seal. There are many causes leading to the wear such as load, geometry shape, processing technique, and materials, and these factors provide ways to improve the friction behavior of the mating elements. Eaton Vickers has developed a shaft seal configuration especially designed for use in aerospace pumps.4This face type shaft seal(Fig.15)is not a‘‘package”design,but a combination of simple elements made of high quality materials such as bearing grade bronze or carbon.The major difference from other seals is that the sealing element rotates with the shaft while the heavier mating ring is stationary in the housing. The seal assembly is driven by two tabs on the retainer that engages with the pump drive shaft.Static sealing around the circumference of the drive shaft is accomplished with the elastomeric grommet,as shown in Fig.15(b),which is held in contact with the shaft by a garter spring. Dynamic sealing is affected by forcing the rotating element against the stationary mating ring.

Fig.15 The shaft seal used in Eaton Vickers pumps.4

Fig.16 Mechanical seal with circular notches.111

Fig.17 Mechanical seal with spiral grooves.112

In addition, some scholars have improved the sealing performance by grooving on the lubricating surface. Djama?¨et al.111found that thermos hydrodynamic mechanical face seals, which are equipped with a notched rotating face, as shown in Fig.16 (in this figure, ω represents the rotational speed of the rotating ring), are efficient in reducing friction,thus used in heavy-duty applications.The notches are assumed to create periodic thermoelastic deformations of the rotating face,improving the hydrodynamic lift and preventing the contact between the faces. In 2012, Qiu and Khonsari112developed a three-dimensional thermohydrodynamic (THD) CFD model to study the characteristics of an inward pumping spiral groove mechanical seal, as shown in Fig.17. It is found that thermal behavior plays an important role in the overall performance of a seal.The spiral groove parameter can be optimized to achieve desired performance depending on the application requirements of the seal.

6. Conclusions

The research on civil aircraft piston pumps has lasted for more than 60 years. Particularly in recent years, advances in computer science, CFD, materials, processes technology, sensors and signal acquisition have significantly pushed forward the development of the aircraft piston pump. This paper presents an overview of the description, future directions and critical technologies of the piston pump used in modern civil aircraft.The design and development of the piston pump involve mechanical engineering, electronics, fluid mechanics, material technology,control theory,etc..This review attempts to cover all these areas by introducing the achievements from some active research institutes. Furthermore, the details of some critical technologies have been presented,providing significant insight into related investigations in the future.

High pressure,high rotational speed,low pulsation,highest reliability and long service life, intelligence and energy saving are the future directions of the aircraft piston pump, which lead to many challenges to the pump design.Some of the challenges,such as the design of the key friction couples and noise reduction, have attracted increasing attention and achieved some breakthroughs; however, little research has been conducted on the other challenges, such as the inlet boost technique, and forced heat dissipation. In addition, according to the concept of intelligent manufacturing presented in Industry 4.0, ‘being smart’ is a core element in future industry. Therefore, the integration of sensors, controls and embedded software is another challenge for future pump designs and a huge opportunity for fluid power technology.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51775013) and the Aeronautical Science Foundation of China (No. 2016ZC09007).