GB2575495A

GB2575495A - A coolant system

Info

Publication number: GB2575495A
Application number: GB1811472.8A
Authority: GB
Inventors: David Flower Paul
Original assignee: Safran Electrical and Power SAS
Current assignee: Safran Electrical and Power SAS
Priority date: 2018-07-12
Filing date: 2018-07-12
Publication date: 2020-01-15
Also published as: GB201811472D0

Abstract

A coolant system 200 of an electrical generator 208 arranged to be driven by an aircraft 10 engine, the coolant system comprising: a coolant fluid circuit 202 containing a coolant fluid; and a pump 220 arranged to pump the fluid around the circuit to cool at least one cooled component of the generator, the pump being arranged to receive a drive from a rotor shaft of the generator such that the rotational speed of the pump varies with the rotational speed of the rotor shaft, wherein the pump is a variable displacement pump, and wherein the coolant system further comprises a displacement control 250 means configured to vary a displacement of the pump based on a coolant fluid pressure level at a predetermined location 260 n the coolant fluid circuit. The location may be remote from the pump. The system may comprise a cooler 210 located in the coolant circuit between the pump and the cooled component, wherein the location is disposed between the cooler and the cooled component, immediately upstream of the component. The pump may comprise a displacement control component such as a hydraulic actuator. The system may further comprise a biasing spring in opposition to the actuation force. The pump may be a variable displacement vane pump.

Description

A Coolant System

The present invention relates to coolant systems. In particular, the invention relates to coolant systems for use in cooling electrical generators connected to aircraft engines.

Background to the Invention

Electrical generators have both an operating temperature range (within which they can operate) and an optimum temperature range (within which they operate most efficiently). In use, electrical generators create heat due to inefficiencies in generation. Electrical generators are typically cooled by a circulating fluid to ensure that they are kept within their operating temperature range, and preferably kept within their optimum temperature range.

Aircraft propulsion systems typically comprise an engine, such as a turbine or jet engine, which may be connected to an electrical generator. The electrical generator is typically formed of an assembly of magnetic circuit components, comprising a rotor and a stator. Generally, aircraft engine electrical generators are cooled using a fluid - typically oil for large aircraft generators - by circulating the fluid using a mechanical pump. The pump itself is typically driven from the rotor shaft of the electrical generator. In other implementations, such as smaller generators, air cooling can be implemented using a fan. The oil flows along a cooling path which passes through, or adjacent to, the cooled components of the generator so that heat is drawn away from the cooled components.

When the generator is operating at lower speeds, for a given electrical load, currents in the windings of the generator rotor will be higher, which in turn generates more heat due to resistance. Therefore, more cooling is required at these lower speeds. Similarly, if the generator is operating at lower speeds, the rotational speed of the pump will also be lower, and thus the rate at which the oil flows around the circuit will be lower. Conversely, when the generator is operating at higher speeds, currents are lower, and thus less heat is generated. However, the drive speed delivered at the pump is higher, increasing the rate at which the oil flows around the circuit. In order to provide sufficient cooling at the lowflow-rate, high current operating conditions, the coolant pump will typically provide well over the flow rate required for cooling at high speeds.

To regulate the coolant flow rate through the cooling path, in typical known systems, oil flow is regulated using a pressure relief valve at a point where the oil is first used to cool the cooled components of generator. This ensures that oil flow through the cooling path is kept within a selected design flow rate range across the entire speed range, by venting any excess oil back to an oil reservoir or sump. Typically, this pressure relief valve is set to a fixed pressure, for example around 60psi (410kPa). However, since the hydraulic power provided by the pump is determined from the flow rate multiplied by the outlet pressure, the increased flow rate at high speeds combined with pump inefficiencies and the venting of oil back to the sump may result in a poor ratio of power consumption to useful cooling work achieved by the coolant system at cruise speeds. This may be exacerbated by the pressure drop in the coolant system, the majority of which occurs in the remotely mounted cooler and associated pipework of the cooling system and which can be in excess of 120psi (830kPa) when operating the engines at cruise speeds and higher. For example, in some systems, only around a third of hydraulic power provided by the pump is useful at cruise speed and in some circumstances, as little as 10 percent of pump power consumption might be converted into useful cooling work.

Therefore, an improved way of regulating coolant flow rate in coolant system is required.

Summary of the Invention

A first aspect of the present invention provides a coolant system of an electrical generator arranged to be driven by an aircraft engine, the coolant system comprising: a coolant fluid circuit containing a coolant fluid; and a pump arranged to pump the coolant fluid around the coolant fluid circuit to cool at least one cooled component of the electrical generator, the pump being arranged to receive a drive from a rotor shaft of the electrical generator such that the rotational speed of the pump varies with the rotational speed of the rotor shaft, wherein the pump is a variable displacement pump, and wherein the coolant system further comprises displacement control means configured to vary a displacement of the pump based on a coolant fluid pressure level at a predetermined location in the coolant fluid circuit.

By varying the displacement of the pump based on the coolant fluid pressure, the flow rate of coolant through the cooling path can be regulated at the pump according to the pressure level in the coolant fluid. This allows the coolant fluid flow rate through the cooling path to be kept within a selected design flow rate range across the entire speed range of the generator without the need for venting of coolant into a sump. The claimed arrangement may also facilitate a reduction in the pressure drop across a cooler in the coolant circuit and the associated pipework of the coolant circuit at higher speeds in comparison to known systems and may avoid the problem of excess power consumption at cruise speeds from pushing excess coolant around the coolant fluid circuit only to be diverted to the reservoir by the pressure relief valve. This results in a significant decrease in the power consumption of the pump at cruise speeds, leading to an increase in the overall efficiency of the coolant system.

The electrical generator is preferably a variable speed electrical generator.

The at least one cooled component may be any heat generating component of the electrical generator. For example, the at least one cooled component may include a magnetic heat generating component, an electric heat generating component, an electro-magnetic heat generating component, a mechanical heat generating component or any active generating component of the electrical generator, or any combination thereof.

Any suitable means may be used to take rotation from the rotor shaft of the electrical generator and transfer it to the pump. The pump is preferably driven by the rotor shaft of the electrical generator via a mechanical connection. For example, direct drive from the rotor shaft, via gears, a chain drive, or a pulley drive.

The predetermined location may be any suitable location in the coolant fluid circuit. The predetermined location may be within the pump. Preferably, the predetermined location is remote from the pump. The predetermined location may be downstream of the pump. In some embodiments, the coolant system further comprises a cooler located in the fluid coolant circuit between the pump and the at least one cooled component. In such embodiments, the predetermined location is preferably disposed between the cooler and the at least one cooled component. The predetermined location may be within the electrical generator. The predetermined location may be upstream of the at least one cooled component of the electrical generator. The predetermined location may be immediately upstream of the at least one cooled component. Where the coolant system includes a pressure relief valve configured to allow at least a proportion of the flow provided by the pump to bypass the electrical generator, the predetermined location is preferably upstream of the pressure relief valve, for example immediately upstream of the pressure relief valve.

The displacement of the pump may be varied by any suitable means. Preferably, the pump comprises at least one moveable displacement control component which is moveable between minimum and maximum displacement positions by the displacement control means in order to vary the displacement of the pump between minimum and maximum displacement configurations. The at least one moveable displacement control component may be switchable between minimum and maximum displacement positions so that the displacement of the pump may be switched between the minimum and maximum displacement configurations. In this manner, the pump has only two potential displacement configurations. This can result in less complex displacement control. In other examples, the at least one moveable displacement control component may be moveable to one or more intermediate positions between the minimum and maximum displacement positions so that the displacement of the pump may be varied to one or more intermediate displacement configurations between the minimum and maximum displacement configurations.

The coolant system may comprise any suitable displacement control means. The displacement control means may comprise one or more valves at an outlet of the pump which are configured to divert a portion of the flow from the pump back to a sump of the coolant system in order to vary the displacement of the pump. Preferably, the displacement control means comprises an actuator configured to apply an actuation force to the at least one moveable displacement control component to move the at least one moveable displacement control component between the minimum and maximum displacement positions.

The actuator may be any suitable actuator device for varying the displacement of the pump by applying an actuation force to the at least one moveable displacement control component. Preferably, the actuator is a hydraulic actuator. The hydraulic actuator may be positioned at the predetermined location. The coolant system preferably further comprises a pressure feed line in fluid communication with the predetermined location and configured to direct a proportion of the coolant fluid from the predetermined location to the hydraulic actuator to generate the actuation force. In this manner, the hydraulic actuator is fed by pressure in the coolant fluid at the predetermined location. The actuation force generated by the hydraulic actuator may be proportional to the coolant fluid pressure at the predetermined location. With this arrangement, the pump flow rate may be automatically compensated based directly on the coolant fluid pressure at the predetermined location.

Preferably, the actuator is configured to apply the actuation force to the at least one moveable displacement control component in a first direction towards the minimum displacement position. The actuator may be configured to apply the actuation force to the at least one moveable displacement control component in a second direction towards the maximum displacement position.

The actuator may be configured to move the at least one moveable displacement control component between the maximum and minimum positions and back again based on the coolant fluid pressure at the predetermined location. Preferably, the coolant system further comprises biasing means configured to apply a biasing force to the at least one moveable displacement control component in opposition to the actuation force. Thus, where the actuator is configured to apply the actuation force to the at least one moveable displacement control component in a first direction towards the minimum displacement position, the biasing means is configured to apply the biasing force to the at least one moveable displacement control component in a second direction towards the maximum displacement position. The biasing force is opposed to the actuation force. The displacement of the pump may depend on the relative magnitudes of the actuation force and the biasing force.

The biasing means may comprise a spring. The spring may be any suitable type of spring. For example, the spring may be a tension spring, a compression spring, or a torsion spring. The biasing means is preferably positioned within the pump. Preferably, the spring is a compression spring positioned within the pump.

Where the actuator is configured to apply the actuation force to at least one moveable displacement control component of the pump in a first direction towards the minimum displacement position, the biasing means is preferably configured such that the biasing force increases as the at least one moveable displacement control component moves towards the minimum displacement position. In other words, the closer the at least one moveable displacement control component is to the minimum displacement position, the greater the biasing force. This means that, where the pressure exceeds the target pressure level and the actuation force initially exceeds the biasing force, the at least one moveable displacement control component is moved in the first direction by the actuator towards the minimum displacement position either until the biasing force equals the actuation force, or until the at least one moveable displacement control component reaches the minimum displacement position.

The biasing means and the actuator may be configured such that the biasing force exceeds the actuation force when the coolant fluid pressure level at the predetermined location is below or equal to a target pressure level. This may be achieved through selection of the actuator, or the biasing means, or both. For example, where the actuator is a hydraulic actuator, the size of the hydraulic actuator may be selected based on the target pressure level desired. Alternatively, or in addition, where the biasing means is a spring, the spring rate of the spring may be selected based on the target pressure level desired. With this arrangement, the at least one moveable displacement control component may be held in the maximum displacement position by the biasing means until the coolant fluid pressure at the predetermined location exceeds the target pressure value. This means that the pump flow rate is at its maximum at the target pressure value and below. Once the coolant fluid pressure at the predetermined location exceeds the target pressure value, the actuator moves the at least one moveable displacement control component in the first direction against the biasing force to decrease the displacement of the pump. When this occurs, the flow rate from the pump is reduced.

The biasing means and the actuator may be configured such that when the coolant fluid pressure level at the predetermined location exceeds a target pressure level by a predetermined amount, the at least one moveable displacement control component is moved to the minimum displacement position. With this arrangement, the pump is in the minimum displacement configuration when the coolant fluid pressure level at the predetermined location exceeds the target pressure level by the predetermined amount. This may be achieved by selecting a hydraulic actuator piston size, or the spring rate of the biasing means, for example. The predetermined amount is selected based on the requirements of the coolant system. For example, the predetermined amount may be from about 5 psi to about 20 psi, preferably about 10 psi. The biasing means and the actuator may be configured such that when the coolant fluid pressure level at the predetermined location exceeds a target pressure level by less than the predetermined amount, the at least one moveable displacement control component is moved to an intermediate position between the minimum and maximum displacement positions.

The target pressure level may be any desired pressure level. For example, the target pressure level may be any value from about 40 psi (280kPa) to about 100 psi (690kPa), preferably from about 50 psi (340kPa) to about 70 psi (480kPa). In one example, the target pressure level is about 60 psi (410kPa).

The pump may be any suitable variable displacement pump. For example, the pump may be an axial piston pump. As will be understood, such a pump may comprise a number of pistons arranged in cylindrical voids around a cylinder block. The axial movement of the pistons is constrained at their lower ends by a swash plate which is set at an angle. Due to the angle of the swash plate, relative rotation between the cylinder block and the swash plate causes the pistons to reciprocate within the cylinders to alternately intake and discharge fluid from the pump. The displacement of such a pump may be varied by an actuator by changing the angle of the swash plate. Thus, in such embodiments, the at least one moveable displacement control component comprises the swash plate. Relative rotation may be achieved with a rotating swash plate, or a rotating cylinder block, or both. One of the swash plate and the cylinder block may be non-rotating.

Alternatively, the pump may be a variable displacement gerotor.

The pump may comprise a plurality of gerotor pumps. The displacement control means may comprise at least one valve located at an outlet of at least one of the plurality of gerotor pumps and configured to selectively divert at least a portion of the coolant flow to a sump of the coolant system in order to reduce the displacement of the pump. In such examples, even where the displacement of the individual pumps is fixed, the overall pump can be considered as a variable displacement pump since its displacement can be varied using the displacement control means. The plurality of gerotor pumps may be mounted on the same shaft. This can result in a compact yet effective arrangement. The plurality of gerotor pumps may all be mounted on a single rotor shaft of the pump. The displacement control means may comprise one or more valves located at an outlet of one or more of the plurality of gerotor pumps and configured to divert the coolant from the outlet to a sump of the coolant system in order to reduce the displacement of the pump. With this arrangement, the gerotor pump associated with the valve, which may be considered as an on/off pump, can have a very low pressure drop to pump against and, therefore, can have minimal power consumption. The plurality of gerotor pumps may comprise a first gerotor pump which is sized to provide a desired amount of cooling at cruise speed, and a second gerotor pump which is sized to provide additional cooling. The amount of additional cooling may correspond to the difference between the cooling requirements at cruise speed and the cooling requirements at minimum speed. The first gerotor pump may comprise a single pump, or multiple pumps. The second gerotor pump may comprise a single pump, or multiple pumps.

The pump is preferably a variable displacement vane pump. The variable displacement vane pump may have a rotor and a stator ring which is eccentric with the rotor. Preferably, the rotor is rotatable about a fixed axis and the at least one moveable displacement control component comprises the stator ring. The stator ring is moveable relative to the rotor to vary the displacement of the pump. The displacement of the pump may be varied by varying the degree of eccentricity between the stator ring and the rotor.

The displacement of the pump is preferably more than zero when the at least one moveable displacement control component is at the minimum displacement position. This allows the pump to continue to provide a minimum effective amount of cooling at high generator speeds, even in the event of failure elsewhere in the system. The pump preferably comprises a stop which is configured to prevent movement of the at least one moveable displacement control component beyond the minimum displacement position.

The coolant system preferably comprises a cooler located in the coolant fluid circuit.

The coolant system preferably includes a pressure relief valve configured to allow at least a proportion of the flow provided by the pump to bypass the at least one cooled component of the electrical generator, when coolant fluid pressure at the pressure relief valve reaches a threshold value. The pressure relief valve is preferably immediately upstream of the electrical generator. The pressure relief valve is preferably immediately upstream of the at least one cooled component. The pressure relief valve is preferably downstream of the predetermined location. The pressure relief valve may be immediately downstream of the predetermined location.

The coolant system may further comprise a fluid reservoir for supplying coolant fluid to the pump and receiving fluid from the coolant fluid circuit. Where the coolant system includes a pressure relief valve configured to allow at least a proportion of the flow provided by the pump to bypass the at least one cooled component of the electrical generator, the pressure relief valve is preferably configured to divert at least a proportion of the flow to the fluid reservoir.

The system may further comprise a filter located in the coolant fluid circuit.

A second aspect of the present invention provides an electrical generator arranged to be driven by an aircraft engine, the electrical generator comprising a rotor shaft and the coolant system according to any of the embodiments discussed above. The coolant system is configured to cool at least one component of the electrical generator. The pump of the coolant system is connected to a rotor shaft of the electrical generator such that the rotational speed of the pump varies with the rotational speed of the rotor shaft.

A third aspect of the invention provides an aircraft propulsion system comprising an aircraft engine and an electrical generator according to the second aspect which is driven by the aircraft engine.

A fourth aspect of the present invention provides an aircraft comprising an aircraft propulsion system according to the third aspect.

Features described in relation to one aspect of the invention may equally be applied to other aspects of the invention. In particular, features described in relation to the coolant system of the first aspect of the invention may equally be applied to the second, third and fourth aspects of the invention.

Brief Description of the Drawings

Further features and advantages of the present invention will become apparent from the following description of embodiments thereof, presented by way of example only, and by reference to the drawings, wherein:

Figure 1 is a schematic illustration of a coolant system in accordance with the prior art;

Figure 2 is a schematic illustration of an aircraft comprising a coolant system in accordance with an embodiment of the invention;

Figure 3 is a cross-sectional view of a pump for the coolant system of Figure 2, showing the pump in its maximum displacement configuration; and

Figure 4 is a cross-sectional view of the pump of Figure 3, showing the pump in its minimum displacement configuration.

Detailed Description of Preferred Embodiments

Figure 1 illustrates a coolant system 100 which is known in the prior art for use in aircraft propulsion systems. The coolant system 100 comprises a fluid coolant circuit 102. The coolant circuit 102 contains a coolant fluid (not shown) which can be circulated around the coolant circuit 102. The coolant is typically a liquid. In this illustrated example, the coolant can be oil, although any suitable coolant can be used. The coolant can be circulated to and from a coolant reservoir 104, commonly known as a sump.

The coolant system 100 also comprises a pump 106 arranged within the coolant circuit 102 and configured to circulate a flow of the coolant around the coolant circuit 102.

The coolant system 100 is associated with an electrical generator 108, which is cooled by the coolant circuit 102. In this respect, the electrical generator 108 is arranged in thermal communication with the coolant circuit 102 such that excess heat can be transferred from the electrical generator 108 to the coolant fluid. Typically, to achieve this effect, the coolant runs through one or more components of the generator, and more typically through one or more heat generating components of the generator. Heat generating components of the generator are typically those which generate heat due to electrical resistance during operation of the generator. The electrical generator 108 comprises a rotating component, known as a rotor, and/or a stationary component, known as a stator (not shown). It is these components in particular that the coolant circuit 102 is used to cool, though other components of the generator may be cooled in addition to or in place of those components of the generator. The rotor rotates on a rotor shaft (not shown) which is connected to the pump 106 to drive the rotation of the pump 106 and thereby pump coolant around the coolant circuit 102.

The coolant system 100 also comprises a cooler 110 located in the coolant circuit 102. This is typically located between the pump 106 and the generator 108. The pump 106 is configured to pump the coolant fluid towards the generator 108 through the cooler 110. This allows the cooled fluid from the cooler to flow on to the generator to perform its cooling function. To ensure that the fluid flow rate through the generator 108 remains broadly within a design range across the speed range of the generator 108, a pressure relief valve 112 is provided on a bypass line 113 extending from a bypass location 114 immediately upstream of the generator 108 to the reservoir 104. The pressure relief valve 112 is configured to open only when the pressure of the coolant fluid at the bypass location 114 exceeds a pressure threshold, for example 60psi (410kPa). This allows excess coolant fluid to return to the reservoir 104 when the pressure threshold is exceeded. The pressure relief valve 112 limits the pressure drop across the generator 108 and, therefore, limits the flow rate of the coolant through the generator 108. This gives a more predictable rate of removal of heat from the generator.

A filter 116 is also provided in the coolant circuit 102 to remove unwanted particulates from the coolant fluid. A cold start pressure relief valve 118 may also be provided to direct some of the coolant fluid such that it bypasses the cooler 110. As the oil is more viscous when it is cold, which requires more power from the pump 106, the cold pressure relief valve 118 helps to prevent overloading the system 100, for example, by directing some of the coolant fluid away from the cooler 110 and allowing it to bypass the cooler 110.

The prior art coolant system 100 described above is typical of a coolant system in which the fluid pressure is regulated by a standard pressure relief valve. In this instance the pressure relief valve is located substantially at the point at which the coolant circuit enters the cooled components of the generator and begins to cool the cooled components of the generator 108. However, it has been identified that the majority of the pressure drop in a typical coolant system 100 occurs in the cooler 110 in high speed modes of operation, i.e. at the cruising speed of an aircraft in which the generator may be mounted. At higher speeds, the flow rate increases, which in turn causes the pressure at the generator inlet to increase. Consequently, the pressure relief valve opens further to compensate for this increase and thereby keep the flow rate through the generator constant. This excess of flow rate at the generator, where the pressure relief valve is located, results in coolant being returned to the sump while bypassing the generator. This can result in a large proportion of the power consumed by the pump 106 being wasted.

Figure 2 shows an aircraft 10 comprising a coolant system 200 in accordance with an embodiment of the invention and an aircraft engine 300.

The coolant system 200 is similar to the prior art coolant system 100 and similar reference numerals are used to denote similar components. As with the coolant system 100, the coolant system 200 includes a coolant circuit 202 containing a coolant fluid (not shown) which can be circulated around the coolant circuit 202. The coolant is typically a liquid. In this illustrated example, the coolant can be oil, although any suitable coolant can be used. The coolant is circulated to and from a coolant reservoir 204.

The coolant system 200 also comprises a pump 220 arranged within the coolant circuit 202 and configured to circulate a flow of the coolant around the coolant circuit 202 to at least one cooled component of an electrical generator 208. The at least one cooled component is arranged in or near the coolant circuit 202, such that excess heat can be transferred from the at least one cooled component to the coolant fluid in order to cool the at least one cooled component. Thus, the coolant flows along a cooling path through, or adjacent to the at least one cooled component of the electrical generator 208. The cooling path forms part of the coolant circuit. The electrical generator 208 comprises a rotor 280 and a stator 290. The rotor 280 is driven by an aircraft engine 300, for example by an output shaft 302 extending from the aircraft engine 300. The rotor rotates on a rotor shaft which is mechanically connected to the pump 220 to drive the rotation of the pump 220 and thereby pump coolant around the coolant circuit 202 to cool the generator 208. The mechanical connection between the rotor shaft and the pump 220 may be may be achieved by any conventional means, for example via a geared connection.

The coolant circuit 202 is preferably configured such that coolant fluid flows first through the rotor 280 then through the stator 290, ensuring that the rotor 280 is fed the coldest coolant fluid coming from the cooler. However, it will be appreciated that the coolant circuit 202 may be configured such that coolant fluid flows first through the stator 290 then through the rotor 280, as indicated by the dashed fluid feed lines on Figure 2. In both examples, the rotor 280 and the stator 290 are cooled components of the electrical generator 208.

The coolant system 200 may also comprise a cooler 210 located in the coolant circuit 202 between the pump 220 and the generator 208. The pump 220 is configured to pump the coolant fluid towards the at least one cooled component of the generator 208, passing first through the cooler 210.

A pressure relief valve 212 is preferably provided on a bypass line 213 extending between a bypass location 214 in the coolant circuit 202 immediately upstream of the generator 208 and the reservoir 204. The pressure relief valve 212 is configured to open only when the pressure of the coolant fluid at the bypass location 214 exceeds a pressure threshold, for example 60psi (410kPa). This allows excess coolant fluid to return to the reservoir 204 when the pressure threshold is exceeded. The pressure relief valve 212 thereby puts an upper limit on the pressure drop across the cooling path and the flow rate of the coolant through the cooling path. This may act as a secondary mechanism by which coolant flow rate through the cooling path is regulated.

A filter 216 is preferably provided to remove unwanted particulates from the coolant fluid. A cold start pressure relief valve 218 can also be provided to direct some of the coolant fluid such that it bypasses the cooler 210 in situations where high viscosity of the coolant, or any blockage in the cooler, causes excessive pressure in the cooler 210.

To ensure that the fluid flow rate through the cooling path remains broadly within a design range across the speed range of the generator 208, the pump 220 used in the coolant system 200 is a variable displacement pump. The coolant system 200 further includes a displacement control means in the form of an actuator 250 connected to the pump 220 and configured to vary the displacement of the pump 220. The actuator 250 may be any suitable actuator. In this example, the actuator is a hydraulic actuator 250 which is fed by coolant fluid pressure at a predetermined location 260 downstream of the pump 220 via a pressure feed line 262 extending from the predetermined location 260 to the hydraulic actuator 250.

Preferably the predetermined location 260 is downstream of the cooler 210 and upstream of the optional pressure relief valve 212. In this example, the predetermined location 260 is immediately upstream of the pressure relief valve 212. This allows the displacement of the pump 220 to be varied based on the pressure of the coolant after the pressure drop across the cooler 210 and the majority of the pipework of the coolant circuit 202 has been overcome and before any flow is diverted to the reservoir 204 by the pressure relief valve 212. In other examples, the pressure relief valve may be located upstream of the predetermined location, provided the operating pressure at which the pressure relief valve is designed to open is greater than the pressure at which the pump is in its minimum displacement configuration. The pump 220 and the actuator 250 are discussed in more detail below with reference to Figures 3 and 4.

As shown in Figures 3 and 4, the pump 220 includes a pump housing 222 defining a pump cavity 224, a pump inlet 226 and a pump outlet (not shown). A rotor 228 is rotatably supported within the housing 222 by a drive shaft 230 which is in coaxial alignment with the central axis of the pump cavity 224. A stator ring 232 is positioned between the rotor 228 and the pump housing 222 in an eccentric relationship with the pump cavity 224 and the rotor 228. The rotor 228 has radially slidable vanes 234 positioned at regular intervals around its circumference. These vanes 234 are pressed into contact with the inner surface of the stator ring 232 by centrifugal force. Side plates (not shown) are provided on both sides of the vanes 234 so that pump chambers 236 are defined between adjacent vanes 234. Since the stator ring 232 and the rotor 228 are in an eccentric relationship, the volume of each pump chamber 236 varies as the rotor 228 rotates. An arc-shaped inlet port 235 is provided in one of the side plates adjacent to the portion of the rotor 228 in which the volume of the pump chambers 236 increases as the rotor 228 rotates, while an arc-shaped outlet port 237 is provided in one of the side plates adjacent to the portion of the rotor 228 in which the volume of the pump chambers 236 decreases as the rotor 228 rotates. The inlet port 235 is in fluid communication with the pump inlet 226 and the outlet port 237 is in fluid communication with the pump outlet.

In use, coolant fluid is drawn into the pump 220 through the pump inlet 226 and enters one or more of the expanding pump chambers 236 via the inlet port 235 in the side plate. As the rotor rotates, the pump chambers 236 decrease in volume and the coolant fluid is forced out of the pump 220 via the outlet port 237 in the side plate and through the pump outlet to drive coolant around the coolant circuit.

The stator ring 232 is held in position within the pump cavity 224 by the actuator 250 and a biasing means 238 acting on opposite sides of the stator ring 232. In this example, the biasing means comprises a compression spring 240 located in a spring cavity 242 in the pump housing 224 between the outer surface of the stator ring 232 and the inner surface of the spring cavity 242. The actuator 250 applies an actuation force F_A to the stator ring 232 to push the stator ring 232 against the spring 240 in a first direction towards a minimum displacement position. Conversely, the spring 240 applies a biasing force F_B to the stator ring 232 to push the stator ring 232 in a second direction towards a maximum displacement position. The relative magnitudes of the actuation force F_A and the biasing force F_b determine the lateral position of the stator ring 232 within the pump cavity 224 and, therefore, the eccentricity of the stator ring 232 and the displacement of the pump.

In this example, the actuator 250 comprises a hydraulic piston 252 which is moveable within a bore 254. The hydraulic piston 252 acts against the outer surface of the stator ring 232, while the bore 254 is in fluid communication with the predetermined location 260 via the pressure feed line 262. The fluid pressure in the pressure feed line 262 enters the bore 254 and acts on the head of the piston 252 to generate the actuation force. As will be understood, the greater the coolant fluid pressure at the predetermined location 260, the greater the actuation force, F_A. The actuator 250 may act directly on the stator ring 232, as shown in Figs. 3 and 4, or indirectly via an intermediate component which is slidably received in the pump housing 222 and which transfers the actuation force from the actuator 250 to the stator ring 232.

The spring 240 and the piston head are configured so that, when the coolant fluid pressure at the predetermined location is at or below the target pressure, the biasing force F_B is greater than the actuation force F_A and the spring 240 is fully extended. This is shown in Figure 3, in which the eccentricity of the stator ring 232 and the flow rate of the pump 220 are at their maximum values. In this state, the pump 220 is in its maximum displacement condition. When the coolant fluid pressure at the predetermined location exceeds the target pressure, for example when the rotational speed of the rotor 228 is increased, the actuation force F_A moves the stator ring 232 in the first direction to compress the spring 240 until the point at which the biasing force F_B is equal to the actuation force F_A. At this point, the displacement of the pump is somewhere between the maximum and minimum displacement configurations.

If the coolant fluid pressure at the predetermined location exceeds the target pressure by a predetermined amount, for example where the target pressure is exceeded by more than lOpsi (70 kPa), the spring 240 is fully compressed and further movement of the stator ring 232 in the first direction is prevented by a stop 244. In this state, the eccentricity of the stator ring 232 and the flow rate of the pump 220 are at their minimum values and the pump 220 is in its minimum displacement configuration. This configuration is shown in Figure 4. The presence ofthe stop 244 ensures that the pump 220 still provides a minimum safe oil flow rate to provide sufficient cooling and lubrication, for example in the event of a failure elsewhere in the system.

The pump 220 may further include a height adjustment device in the form of a height adjustment screw 244 by which the height ofthe stator ring 232 within the pump cavity 224 may be adjusted.

Systems according to the invention have been developed to address the problem of excess power being consumed at cruise speeds by excessive coolant flow through the cooler 110 as described above in relation to the prior art coolant system. As described above, at high engine speeds, typically, around half of the coolant reaching the pressure relief valve of the prior art system is returned directly to the oil reservoir without passing through the cooled generator components. This means that a majority of the pressure drop in the cooler is only used to push excess oil through the cooler and on round the circuit to the pressure relief valve, without cooling the generator. This can mean that in certain operational circumstances as little as 10% of the pump power is used to produce useful cooling work, with much ofthe rest being used to drive excess flow through the cooler.

By automatically compensating the flow rate of the pump by direct feedback from the coolant pressure, the coolant flow rate through the cooling path can be regulated within a design range across the speed range of the electrical generator without the need for excess oil to bypass the generator to the reservoir and without any significant increase in the pressure losses across the cooler at higher speeds. This results in a significant reduction in power consumption of the coolant system and a significant increase in the efficiency of the coolant system.

Various modifications, whether by way of addition, deletion and/or substitution, may be made to all of the above described embodiments to provide further embodiments, any and/or all of which are intended to be encompassed by the appended claims.

Claims

1. A coolant system of an electrical generator arranged to be driven by an aircraft engine, the coolant system comprising:

a coolant fluid circuit containing a coolant fluid; and a pump arranged to pump the coolant fluid around the coolant fluid circuit to cool at least one cooled component of the electrical generator, the pump being arranged to receive a drive from a rotor shaft of the electrical generator such that the rotational speed of the pump varies with the rotational speed of the rotor shaft, wherein the pump is a variable displacement pump, and wherein the coolant system further comprises a displacement control means configured to vary a displacement of the pump based on a coolant fluid pressure level at a predetermined location in the coolant fluid circuit.

2. A coolant system according to claim 1, wherein the predetermined location is remote from the pump.

3. A coolant system according to claim 2, further comprising a cooler located in the fluid coolant circuit between the pump and the at least one cooled component, wherein the predetermined location is disposed between the cooler and the at least one cooled component.

4. A coolant system according to claim 2 or claim 3, wherein the predetermined location is immediately upstream of the at least one cooled component.

5. A coolant system according to any of claims 1 to 4, wherein the pump comprises at least one moveable displacement control component which is moveable between minimum and maximum displacement positions by the displacement control means in order to vary the displacement of the pump between minimum and maximum displacement configurations.

6. A coolant system according to claim 5, wherein the displacement control means comprises an actuator configured to apply an actuation force to the at least one moveable displacement control component to move the at least one moveable displacement control component between the minimum and maximum displacement positions.

7. A coolant system according to claim 6, wherein the actuator is a hydraulic actuator and wherein the coolant system further comprises a pressure feed line in fluid communication with the predetermined location and configured to direct a proportion of the coolant fluid from the predetermined location to the hydraulic actuator to generate the actuation force.

8. A coolant system according to claim 6 or claim 7, wherein the actuator is configured to apply the actuation force to the at least one moveable displacement control component in a first direction towards the minimum displacement position.

9. A coolant system according to any of claims 6 to 8, further comprising a biasing means configured to apply a biasing force to the at least one moveable displacement control component in opposition to the actuation force.

10. A coolant system according to claim 9, wherein the biasing means and the actuator are configured such that the biasing force exceeds the actuation force when the coolant fluid pressure level at the predetermined location is below or equal to a target pressure level.

11. A coolant system according to claim 9 or claim 10, wherein the biasing means and the actuator are configured such that when the coolant fluid pressure level at the predetermined location exceeds a target pressure level by a predetermined amount, the at least one moveable displacement control component is moved to the minimum displacement position.

12. A coolant system according to any one of claims 9 to 11, wherein the biasing means comprises a spring.

13. A coolant system according to any one of claims 5 to 12, wherein the pump is a variable displacement vane pump comprising a rotor and a stator ring around the rotor, wherein the at least one moveable displacement control component comprises the stator ring and is moveable relative to the rotor to vary the displacement of the pump.

14. A coolant system according to any one of claims 5 to 13, wherein the displacement of the pump is greater than zero when the at least one moveable displacement control component is in the minimum displacement position, and wherein the pump further comprises a stop configured to prevent movement of the at least one moveable displacement control component beyond the minimum displacement position.

15. An electrical generator arranged to be driven by an aircraft engine, the electrical generator comprising a rotor shaft and a coolant system according to any one of claims 1 to 14, wherein the pump of the coolant system is connected to the rotor shaft such that the rotational speed of the pump varies with the rotational speed of the rotor shaft.

16. An aircraft propulsion system comprising an aircraft engine and an electrical generator according to claim 15, the electrical generator being driven the aircraft engine.

17. An aircraft comprising an aircraft propulsion system according to claim 16.