Fuel system comprising two-stage gear pump for aero-engine

Abstract

A fuel system for an aero-engine including a two-stage gear pump includes a low pressure pump; the first gear pump and the second gear pump are respectively communicated with the low-pressure pump; the metering valve is provided with a core part, a first inlet window and a second inlet window, and the first gear pump and the second gear pump are directly communicated with the metering valve; the first gear pump and the second gear pump are in parallel connection, the metering valve is provided with a first outlet window, a second outlet window and a third outlet window, the metering valve is communicated with the fuel grading valve through the first outlet window and communicated with the downstream of the first gear pump through the second outlet window, and the metering valve is communicated with the downstream of the second gear pump through the third outlet window. And a core configured to move between a first position in which the core permits fuel to flow through the third outlet window and a second position in which the core blocks the third outlet window and permits fuel to flow through the second outlet window, the core being configured to move between a first position in which the core permits fuel to flow through the third outlet window and a second position in which the core blocks the third outlet window and permits fuel to flow through the second outlet window.

Description

Technical Field

[0001] The present invention relates to the technical field of aero-engine fuel control systems, and particularly to a fuel system for an aero-engine including a two-stage gear pump. Background Art

[0002] As shown in the traditional aero-engine fuel system Figure 1 it mainly includes a low-pressure pump 1, a high-pressure pump 2, a constant-pressure valve 3, an actuating component 4, a high-pressure shut-off valve 5, a fuel grading valve 6, a first group of fuel nozzles 7, a second group of fuel nozzles 8, an over-speed valve 9, a metering valve 10, a differential-pressure valve 11, a return valve 12, etc. The fuel supplied by the aircraft passes through the low-pressure pump 1 and the high-pressure pump 2 for pressurization, and then enters the fuel manifold and the fuel nozzles through the metering valve 10 and the high-pressure shut-off valve 5 and is supplied to the engine combustion chamber. The metering valve 10 is used to meter the fuel quantity to the engine combustion chamber; the high-pressure shut-off valve 5 is used to maintain a sufficient minimum servo pressure in the system and cut off the fuel to the engine combustion chamber after the engine stops; the differential-pressure valve 11 is used to ensure a constant differential pressure before and after the metering valve 10, so that controlling the position of the metering valve 10 can control the fuel quantity to the combustion chamber; the return valve 12 is used to return the fuel provided by the high-pressure pump 2 that is more than the engine demand back to the low-pressure pump 1; the over-speed valve 9 can introduce high-pressure oil into the control chamber of the high-pressure shut-off valve 5, thereby closing the high-pressure shut-off valve 5 to cut off the fuel, and at the same time controlling the opening of the return valve 12 to ensure a certain system pressure in the fuel system. The fuel grading valve 6 is used to improve the atomization effect of the fuel nozzles. When the flow rate in the combustion chamber is small, the fuel is supplied to some of the nozzles in the combustion chamber, and when the flow rate in the combustion chamber is large, the fuel is supplied to all the nozzles in the combustion chamber.

[0003] Generally speaking, a gear pump has the advantages of simple structure, strong pressurization ability, high reliability, etc. Therefore, the gear pump is more suitable for the high-pressure pump of an aero-engine fuel system that requires high reliability and easy maintenance.

[0004] The input shaft of the gear pump is driven by the high-pressure rotor of the aero-engine through a gearbox, with the same percentage speed, supplying a fuel flow rate proportional to the percentage. Then, the return valve of the fuel system returns the fuel that the combustion chamber does not need. This mode will cause a large amount of fuel to be returned in the fuel system in the scenario of high engine speed and small flow rate. After the high-pressure fuel is returned, the pressure energy is converted into heat energy, resulting in too high a temperature of the fuel system and being unable to provide refrigerant for the fuel system, leading to overheating of the fuel system and further endangering the safety of the engine.

[0005] Therefore, it is necessary to design a fuel system with new pump technology to fundamentally change the current situation of overheating of the fuel system. Summary of the Invention

[0006] To solve the problem that the fuel system is prone to overheating under low flow rates, the present invention designs a fuel system for an aeroengine that includes a two-stage gear pump. This fuel system uses two parallel gear pumps to respectively match the fuel flow requirements of the fuel system under large operating conditions (e.g., takeoff) and small operating conditions (e.g., idle). A metering valve is used to distribute and switch the output flow of the two pumps, so that the fuel system has a reduced displacement and reduced return oil under small operating conditions, thereby reducing the fuel temperature rise and completely solving the problem of fuel system overheating.

[0007] Specifically, this fuel system for an aeroengine that includes a two-stage gear pump includes: a low-pressure pump; a first gear pump and a second gear pump, which are respectively connected to the low-pressure pump; a metering valve that has a core, a first inlet window, and a second inlet window, and the first gear pump and the second gear pump are respectively connected to the first inlet window and the second inlet window of the metering valve, thus being directly connected to the metering valve; and an electro-hydraulic servo valve configured to control the movement of the core of the metering valve. Among them, the first gear pump and the second gear pump form a parallel relationship. The metering valve has a first outlet window, a second outlet window, and a third outlet window. The metering valve is connected to a fuel staging valve through a first oil passage extending from the first outlet window, is connected to the downstream of the first gear pump through a second oil passage extending from the second outlet window, and is connected to the downstream of the low-pressure pump through a third oil passage extending from the third outlet window. And the core is configured to move between a first position and a second position. In the first position, the core allows fuel to flow through the third outlet window, and in the second position, the core partially or completely blocks the third outlet window and allows fuel to flow through the second outlet window.

[0008] Preferably, the low-pressure pump, the first gear pump, and the second gear pump are driven by the same drive shaft.

[0009] One or more of the first inlet window, the second inlet window, the first outlet window, the second outlet window, and the third outlet window of the metering valve are formed into a thin-wall small-hole structure, so that the flow pressure difference passing through conforms to the formula Wf = μA√2ΔPρ, where Wf is the metering flow rate, μ is the flow coefficient, A is the area of any window of the metering valve, ΔP is the pressure difference between the upstream and downstream of the metering valve, and ρ is the fuel density. After ΔP is constant, Wf is proportional to A. The metering flow rate can be only related to the window area of the metering valve, so the linearity and accuracy of the metering flow rate can be guaranteed.

[0010] Advantageously, the first gear pump is smaller than the second gear pump. The first gear pump can be set for the fuel flow rate in the small operating condition, and the second gear pump can be set for the flow rate supplement in the large operating condition.

[0011] Advantageously, the second outlet window is larger than the third outlet window to offset the different pressures in the fuel supply line and the fuel return line, enabling smooth distribution.

[0012] In this embodiment, the fuel system further includes a fuel return valve configured to send the fuel provided by the second gear pump and more than the engine demand back to the downstream of the low-pressure pump through the second oil path. The fuel return valve consists of two nested valves, and the two valves are in the form of proportional-integral valves to improve the dynamic performance of constant differential pressure regulation.

[0013] In this embodiment, the fuel system further includes a check valve disposed in the second oil path to prevent the fuel from the first gear pump from leaking to the upstream of the second gear pump.

[0014] The additional features and advantages of the described fuel system for an aeroengine including a two-stage gear pump will be set forth in the detailed description below and will be apparent to those skilled in the art from the following or will be recognized by those skilled in the art from practicing the embodiments described herein. These descriptions include the following detailed description and the drawings. Description of the Drawings

[0015] For the above purposes, the technical features of the present invention are clearly described in the following embodiments, and its advantages are apparent from the following detailed description with reference to the drawings. The drawings illustrate by way of example the preferred embodiments of the present invention without limiting the scope of the inventive concept.

[0016] Figure 1 is a schematic diagram of a typical fuel system for an aeroengine;

[0017] Figure 2 is a schematic diagram of a fuel system for an aeroengine including a two-stage gear pump according to an embodiment of the present invention;

[0018] Figure 3 is a structural diagram of a fuel system for an aeroengine including a two-stage gear pump according to an embodiment of the present invention; and

[0019] Figure 4 is a simplified schematic diagram of the gear pump distribution of a fuel system for an aeroengine including a two-stage gear pump according to an embodiment of the present invention, where the engine is in the first state; and

[0020] Figure 5 is a simplified schematic diagram of the gear pump distribution of a fuel system for an aeroengine including a two-stage gear pump according to an embodiment of the present invention, where the engine is in the second state.

[0021] Reference Signs:

[0022] 1 - Low-pressure pump; 2 - High-pressure pump; 3 - Constant pressure valve; 4 - Actuating component, 5 - High-pressure shut-off valve; 6 - Fuel staging valve; 7 - First group of fuel nozzles; 8 - Second group of fuel nozzles; 9 - Overspeed valve; 10 - Metering valve; 11 - Differential pressure valve; 12 - Return oil valve;

[0023] 101 - Low-pressure pump; 102 - First gear pump; 103 - Second gear pump; 104 - Constant pressure valve; 105 - Actuating component; 106 - High-pressure shut-off valve; 107 - Fuel staging valve; 108 - First group of fuel nozzles, 109 - Second group of fuel nozzles; 110 - Overspeed valve; 111 - Metering valve; 112 - Differential pressure valve; 113 - Return oil valve;

[0024] 201 - Low-pressure pump; 202 - First gear pump; 203 - Second gear pump; 204 - Metering valve; 205 - Core; 206 - Electro-hydraulic servo valve; 207 - First inlet window; 208 - Second inlet window; 209 - First outlet window; 210 - Second outlet window; 211 - Third outlet window; 212 - First oil circuit; 213 - Second oil circuit; 214 - Third oil circuit; 215 - Differential pressure valve; 216 - Return oil valve; 217 - Throttle nozzle; 218 - Throttle nozzle; 219 - High-pressure shut-off valve; 220 - Check valve Detailed implementation mode

[0025] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, so as to more clearly understand the purpose, features and advantages of the present invention.

[0026] The term "directly connected" used herein refers to the connection relationship between two components. Specifically, it means that fuel can reach the second component from the first component through the oil circuit without passing through any other components on the way, or vice versa.

[0027] The term "first state" used herein refers to the state where the engine requires a relatively small fuel flow rate. For example, the state of the engine during idle speed and cruise can be referred to as the first state. In this article, the "first state" can be used interchangeably with the "small state", "small flow rate", etc.

[0028] The term "second state" used herein refers to the state where the engine requires a relatively large fuel flow rate. For example, the state of the engine during takeoff can be referred to as the second state. In this article, the "second state" can be used interchangeably with the "large state", "large flow rate", etc.

[0029] Note that the fuel flow rate required by the engine during idle speed is usually about 4% of the fuel flow rate required during takeoff, and the fuel flow rate required during cruise is usually about 20% of the fuel flow rate required during takeoff (assuming the fuel flow rate required during takeoff is 100%).

[0030] As used herein, the term "upstream" refers to the position before the fuel of a component (such as a pump, valve, etc.) passes through the component. Thus, the "upstream pressure" refers to the pressure of the fuel before it passes through the component. Similarly, the term "downstream" as used herein refers to the position after the fuel of a component (such as a pump, valve, etc.) passes through the component. Thus, the "downstream pressure" refers to the pressure of the fuel after it passes through the component.

[0031] Figure 2 The schematic diagram of the fuel system of the aeroengine two-stage gear pump according to an embodiment of the present invention is shown, which mainly includes a low-pressure pump 101, a first gear pump 102, a second gear pump 103, a constant-pressure valve 104, an actuating component 105, a high-pressure shutoff valve 106, a fuel grading valve 107, a first group of fuel nozzles 108, a second group of fuel nozzles 109, an over-speed valve 110, a metering valve 111, a differential-pressure valve 112, a return valve 113, etc.

[0032] The flow path of the fuel in the fuel system of this embodiment is described below.

[0033] Refer to Figure 2 , the fuel passes through the low-pressure pump 101 in the direction of arrow A. The first gear pump 102 and the second gear pump 103 are respectively connected to the low-pressure pump 101, so that the fuel can pass through the first gear pump 102 and the second gear pump 103 respectively to form two oil paths.

[0034] On the one hand, a part of the fuel passing through the first gear pump 102 is supplied to the constant-pressure valve 4 of the servo system to modulate the fuel to a constant pressure for the actuating component 5, while another part of the fuel is supplied to the metering valve 111. The metering valve 111 measures the fuel flow rate to the engine combustion chamber (also known as "metering flow rate"). The differential-pressure valve 112 connected to the metering valve 111 senses the pressure before and after the metering valve, and then maintains a constant differential pressure before and after the metering valve by controlling the return valve 113, so that the metering flow rate of the metering valve 111 is only related to the opening of the metering valve 111. The fuel passes through the metering valve 111 and enters the high-pressure shutoff valve 106. When the metering flow rate is small, the high-pressure shutoff valve 106 closes to ensure that the fuel system has enough high-pressure oil. When the metering flow rate increases, the high-pressure shutoff valve 106 opens. The metered fuel enters the fuel grading valve 107 after passing through the high-pressure shutoff valve 106, and then is supplied to the first group of fuel nozzles 108 and / or the second group of fuel nozzles 109 under the control of the fuel grading valve 107.

[0035] On the other hand, the fuel passing through the second gear pump 103 is directly supplied to the metering valve 111. In different states of the engine (the first state and the second state, and any state therebetween), according to the movement of the metering valve 111 to the corresponding position, the metering valve 111 distributes the fuel passing through the second gear pump 103 to the upstream of the second gear pump 103 (arrows B and C) or the downstream of the first gear pump 102 (arrow D), so that the upstream and downstream of the second gear pump 103 are interconnected to form an idling state, without extracting excessive hydraulic power. In the second state or the large state, at least most of the fuel passing through the second gear pump 103 is distributed to the downstream of the first gear pump 102, so that more fuel passes through the metering valve 111 along the oil path from the first gear pump 102 to the metering valve 111 and enters the subsequent high-pressure shutoff valve 106, fuel staging valve 107, and the first set of fuel nozzles 108 and / or the second set of fuel nozzles 109. Thus, the switching of the pump working mode is achieved by using the metering valve 111.

[0036] The over-speed valve 110 can be used as an electrically controlled valve. After the valve is turned on, it will introduce high-pressure oil into the control chamber of the high-pressure shutoff valve 106, thereby closing the high-pressure shutoff valve to cut off the fuel. At the same time, it controls the opening of the oil return valve 113 to ensure a certain system pressure in the fuel system.

[0037] Figure 3 The structural diagram of a fuel system including a two-stage gear pump for an aeroengine according to an embodiment of the present invention is shown.

[0038] The working modes of the various components of the fuel system in this embodiment are described below.

[0039] Refer to Figure 3 , a low-pressure pump 201 such as a centrifugal pump is used to boost the fuel passing through it to form fuel with a low-pressure Pb. Then, the fuel is further boosted by the first gear pump 202 under the action of a load to form fuel with a high-pressure Ps (hereinafter referred to as "high-pressure fuel"). One path of the high-pressure fuel is supplied to the constant-pressure valve of the servo system (not shown here) as described above for modulating fuel with a constant-pressure Pc, where the various pressures in the oil path leading to the servo system satisfy Ps > Pc > Pb. Another path of the high-pressure fuel is supplied to the metering valve 204. The upstream pressure of the metering valve 204 is P1, and the downstream pressure of the metering valve 204 is P2, where the various pressures in the oil path passing through the metering valve 204 satisfy Ps > P1 > P2.

[0040] The second gear pump 203 is in a parallel relationship with the first gear pump 202, and these two gear pumps are preferably driven by the same transmission shaft together with the low-pressure pump 201. The fuel before passing through the first gear pump 202 and the second gear pump 203 has a low-pressure Pb. The fuel passing through the gear pump 203 first enters the metering valve 204 and is distributed by the metering valve 204 to downstream of the first gear pump 202 or downstream of the low-pressure pump 201 (i.e., upstream of the second gear pump 203).

[0041] In this embodiment, the metering valve 204 is used to meter the fuel quantity into the engine combustion chamber and is also used for the working mode switching of the gear pump.

[0042] Specifically, the metering valve 204 has a core 205. The electro-hydraulic servo valve 206 for controlling the metering valve 204 switches the output of the electro-hydraulic servo valve 206 between a constant-pressure Pc and a low-pressure Pb (return oil pressure) at both ends of the metering valve 204 according to the electrical signal sent by the electronic controller to control the movement of the core 205 of the metering valve 204, where the value of Pc - Pb is a constant. In Figure 3 when the electro-hydraulic servo valve 206 outputs the low-pressure Pb, the core 205 moves downward to the first position; when the electro-hydraulic servo valve 206 outputs the constant-pressure Pc, the core 205 moves upward to the second position. The electro-hydraulic servo valve 206 is a conventional component in the art, so its structure will not be described here.

[0043] The metering valve 204 also has a first inlet window 207 and a second inlet window 208. The first gear pump 202 and the second gear pump 203 are respectively communicated with the first inlet window 207 and the second inlet window 208 of the metering valve 204, so as to be directly communicated with the metering valve 204, enabling the fuel passing through the first gear pump 202 and the second gear pump 203 to directly enter the metering valve 204.

[0044] The metering valve 204 also has a first outlet window 209, a second outlet window 210, and a third outlet window 211. The metering valve 204 is communicated with the fuel staging valve through a first oil passage 212 extending from the first outlet window 209, is communicated with the downstream of the first gear pump 202 through a second oil passage 213 extending from the second outlet window 210, and is communicated with the upstream of the second gear pump 203 through a third oil passage 214 extending from the third outlet window 211. The core 205 can be in the first position (see also Figure 4 ) and the second position (see also Figure 5) Move between them. In the first position, the core 205 allows fuel to flow through the third outlet window 211 and return to the upstream of the second gear pump 203 through the third oil passage 214, so that the upstream and downstream of the second gear pump 103 are interconnected to form an idling state, without extracting excessive hydraulic power, thereby reducing the temperature of the fuel system and improving safety. At the same time, the core 205 can partially block the first outlet window 209 to reduce the fuel flow through the metering valve 204. In the second position, the core 205 partially or completely blocks the third outlet window 211, so that the third oil passage 214 is substantially blocked, and thus the fuel turns to flow through the second outlet window 210 and reaches the downstream of the first gear pump 202 through the second oil passage 213. These fuels can enter the first oil passage 212 through the metering valve 204 to meet a greater fuel demand.

[0045] Preferably, one or more of the first inlet window 207, the second inlet window 208, the first outlet window 209, the second outlet window 210, and the third outlet window 211 of the metering valve 204 can be a thin-wall small hole structure, so that the flow pressure difference passing through the window conforms to the thin-wall small hole throttling formula

[0046] Wf = μA√2ΔPρ

[0047] Where, Wf is the metering flow rate, μ is the flow coefficient, A is the window area of the metering valve 204, ΔP is the upstream and downstream pressure difference of the metering valve 204, ΔP = P1 - P2, ρ is the fuel density. After ΔP is constant, Wf is proportional to A, that is, the metering flow rate can be only related to the window area of the metering valve 204. Therefore, the linearity and accuracy of the metering flow rate can be guaranteed, and the proportional relationship between Wf and A can be calibrated in advance to perform feedforward control on the engine.

[0048] In addition, the second outlet window 210 can be designed to be larger than the third outlet window 211 to facilitate offsetting the different pressures of the fuel supply passage and the fuel return passage, so that the distribution can proceed smoothly.

[0049] Continue to refer to Figure 3, the upper chamber of the differential pressure valve 215 senses the downstream pressure P2 of the metering valve 204, and the lower chamber senses the upstream pressure P1 of the metering valve 204, where P2 + spring force = P1 (this spring force is provided by the spring in the differential pressure valve 215). The fuel with the high-pressure Ps in the oil circuit leading to the servo system passes through the throttle nozzle 217 and then simultaneously leads to the lower chamber of the return valve 216 and the side window of the differential pressure valve 215, forming a voltage divider system. If the movement of the metering valve 204 or other reasons cause the downstream pressure P2 + spring force of the metering valve 204 ≠ the pressure P1 in front of the metering valve 204, the differential pressure valve 215 will move up and down, and the area of the side window of the differential pressure valve 215 will also change accordingly, further changing the pressure in the lower chamber of the return valve 216. The return valve 216 moves up and down to perform negative feedback compensation on the P1 pressure, and re-restores the relationship of the downstream pressure P2 + spring force of the metering valve 204 = the upstream pressure P1 of the metering valve 204, ensuring that the differential pressure ΔP between the upstream and downstream of the metering valve 204 is equal. By controlling the return valve 216 to maintain the constant differential pressure ΔP before and after the metering valve 204, the metering flow rate is only related to the window opening of the metering valve 204.

[0050] The return valve 216 is composed of two nested valves, and these two valves are in the form of proportional integral valves to improve the dynamic performance of the constant differential pressure regulation. The upper and lower sides of the inner valve directly sense the upper and lower pressures of the metering valve 204 and are used to increase the speed of differential pressure control.

[0051] The throttle nozzle 218 is used to adjust the movement speed of the return valve 216, add damping to the system, and improve the system stability.

[0052] The fuel passes through the metering valve 204 and enters the high-pressure shut-off valve 219, and the high-pressure shut-off valve 219 can move up and down. When the metering flow rate is small, the high-pressure shut-off valve 219 closes to ensure that the fuel system has enough fuel with the high-pressure Ps, and when the metering flow rate increases, the high-pressure shut-off valve 219 opens. The downstream pressure of the high-pressure shut-off valve is P22, where P2 > P22. The metered oil enters the fuel grading valve through the high-pressure shut-off valve 219 and is supplied to one or more groups of fuel nozzles under the control of the fuel grading valve.

[0053] The function of setting the high-pressure shut-off valve 219 is to increase the back pressure of the constant differential pressure control system to improve the system pressure and maintain the system stability. At the same time, it increases the servo pressure of the system under small flow rates. After the system fuel supply is closed, it reduces the fuel leakage to the downstream. To reduce the driving force required for the high-pressure shut-off valve 219 to act, a small hole structure is provided in the oil distribution structure to balance the fuel pressure received by the oil distribution structure. The high-pressure shut-off valve 219 has an end face sealing feature to reduce the oil leakage to the downstream after the high-pressure shut-off valve 219 is closed.

[0054] Advantageously, a one-way valve 220 is provided in the second oil passage to prevent fuel from the first gear pump 202 from leaking to the upstream of the second gear pump 203.

[0055] The following will combine Figures 4 - 5 to describe the switching of the working mode of the pump by the metering valve 204 of the present invention. Note that for ease of explanation, some components are omitted in Figures 4 - 5 the figure.

[0056] Refer to Figure 4 , the centrifugal pump 201 outputs fuel flow rate and supplies it to both the first gear pump 202 and the second gear pump 203. When the engine state is relatively small, the opening of the first outlet window 210 of the metering valve 204 is relatively small, and the core 206 is in a lower position. The gear pump distribution window of the metering valve 204 allows a large amount of fuel to be distributed to the upstream of the second gear pump 203, enabling the front and rear of the second gear pump 203 to communicate with each other, forming an idling state and not extracting excessive hydraulic power.

[0057] Refer to Figure 5 , as the engine state rises, the required fuel flow rate increases significantly. The core 206 of the metering valve 204 moves upward, closing or reducing the third oil passage 214 that supplies fuel from the second gear pump 203 to the upstream of the gear pump 203, and simultaneously opening the second outlet window 211 that supplies fuel from the second gear pump 203 to the downstream of the first gear pump 203, so that more fuel enters the metering valve 204 through the first inlet window 207 to participate in engine combustion, thereby meeting the fuel demand in the large state. Generally speaking, assuming that the fuel flow rate is 100% during engine takeoff, about 4% during idle speed, and about 20% during cruise, in this case, the first gear pump 202 can be set to provide fuel flow rate for small states such as ignition, startup, and cruise, and the second gear pump 203 can be set to provide flow rate supplement for high states such as takeoff and climb. Therefore, the first gear pump 202 can be made smaller than the second gear pump 203 and meet the requirements of most flight state requirements, further avoiding a large amount of temperature rise caused by high-pressure fuel return.

[0058] A dual-stage gear pump fuel system for an aeroengine of the present invention has the following beneficial effects:

[0059] 1. It can reduce the fuel temperature rise in the idle speed state and reduce the risk of fuel system overheating;

[0060] 2. It can improve the cooling capacity for the lubricating oil system and reduce the risk of lubricating oil system overheating;

[0061] 3. It can reduce the demand of the engine for an air-oil radiator and reduce the weight of the engine;

[0062] 4. Except for adding gear pumps and modifying the metering valve, no additional components are added, and the impact on the fuel system modification is small.

[0063] Although the structure of the present invention has been described in conjunction with the preferred embodiments above, those of ordinary skill in the art should recognize that the above examples are for illustrative purposes only and should not be construed as limiting the present invention. Therefore, the present invention can be modified and varied, and these modifications and variations will fall within the scope of this application.

Claims

1. A fuel system for an aeroengine including a two-stage gear pump, comprising: a low-pressure pump; a first gear pump and a second gear pump, the first gear pump and the second gear pump being respectively in communication with the low-pressure pump; a metering valve having a core, a first inlet window and a second inlet window, the first gear pump and the second gear pump being respectively in communication with the first inlet window and the second inlet window of the metering valve, so as to be in direct communication with the metering valve; and an electro-hydraulic servo valve configured to control the movement of the core of the metering valve, wherein, the first gear pump and the second gear pump form a parallel relationship, wherein, the metering valve has a first outlet window, a second outlet window and a third outlet window, the metering valve is in communication with a fuel grading valve through a first oil passage extending from the first outlet window, is in communication with the downstream of the first gear pump through a second oil passage extending from the second outlet window, and is in communication with the downstream of the low-pressure pump through a third oil passage extending from the third outlet window, and wherein, the core is configured to move between a first position and a second position, in the first position, the core allows fuel to flow through the third outlet window, and in the second position, the core partially or completely blocks the third outlet window and allows fuel to flow through the second outlet window.

2. The fuel system for an aeroengine including a two-stage gear pump according to claim 1, characterized in that the low-pressure pump, the first gear pump and the second gear pump are driven by the same transmission shaft.

3. The fuel system for an aeroengine including a two-stage gear pump according to claim 1, characterized in that one or more of the first inlet window, the second inlet window, the first outlet window, the second outlet window and the third outlet window of the metering valve are formed into a thin-wall orifice structure such that the flow differential pressure passing therethrough conforms to the formula Wf = μA√2ΔPρ wherein, Wf is the metering flow rate, μ is the flow coefficient, A is the area of any window of the metering valve, ΔP is the pressure differential between the upstream and downstream of the metering valve, ρ is the fuel density, wherein, after ΔP is constant, Wf is proportional to A.

4. The fuel system for an aeroengine including a two-stage gear pump according to claim 1, characterized in that the first gear pump is smaller than the second gear pump.

5. The fuel system for an aeroengine including a two-stage gear pump according to claim 1, characterized in that the second outlet window is larger than the third outlet window.

6. The fuel system for an aeroengine including a two-stage gear pump according to claim 1, characterized in that it further includes a return oil valve configured to send the fuel provided by the second gear pump and more than the engine demand back to the downstream of the low-pressure pump through the second oil passage, wherein, the return oil valve is composed of two nested valves, and the two valves are in the form of proportional integral valves.

7. The fuel system for an aeroengine including a two-stage gear pump according to claim 1, Characterized in that, It further includes a check valve, and the check valve is arranged in the second oil passage to prevent the fuel from the first gear pump from leaking to the upstream of the second gear pump.

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