Air spring

Abstract

An air spring 10 for a vehicle suspension system. The air spring 10 comprises a first chamber 12 having a variable volume, e.g. with a flexible outer wall 16; and a second chamber 14, which may define

Description

TECHNICAL FIELD The present disclosure relates to an air spring. Aspects of the invention relate to an air spring, to a suspension system, to a vehicle and to a method of operating an air spring. BACKGROUND It is known to provide vehicles, such as automobiles, sports utility vehicles (SUVs) or off-road vehicles, with an air suspension system. Air suspension systems are used in vehicles for maintaining and controlling ride height, as well as to dampen movements of the vehicle, particularly to reduce user perception of uneven road surfaces and to improve user comfort. Vehicle air suspension systems are known to include one or more air springs, which when installed around a vehicle and filled with compressed air, serve to adjust a vehicle ride height and to mitigate unwanted movement of the vehicle cabin caused by travelling over uneven surfaces. The air springs may be filled with compressed air to varying pressures to change a vehicle ride height or to adjust the vehicle response to travel over an uneven surface. The height or pressure of air in the air springs may be adjusted to maintain bump and rebound travel for dynamic conditions. The spring rate of the one or more air springs can be varied by changing the effective volume of air in the air spring. A low volume gives a high spring rate or stiffness. For a high level of driving comfort, a lower spring rate and therefore a higher air volume are required. When lifting a vehicle to an off-road height, the static pressure within the air springs is increased and increased bump travel is available. This results in an increased peak dynamic pressure for bump events. Stiffer springs with smaller volumes see greater peak pressures. This results in a trade-off between the maximum spring stiffness which can be offered to deliver the desired ride and handling, and the amount of lift which can be delivered to provide the desired off-road capability. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide an air spring, a suspension system and a vehicle as claimed in the appended claims According to an aspect of the present invention there is provided an air spring, for example an air spring for a vehicle suspension system. The air spring may comprise a first chamber, for example a first chamber having a variable volume. The air spring may comprise a second chamber. The second chamber may be connected to the first chamber, for example by a conduit. A valve may be arranged within the conduit. The valve may control a flow of fluid between the first and second chambers. The valve may be configured to open when the first chamber exceeds a threshold pressure. When open, the vale may allow fluid to flow from the first chamber to the second chamber through the conduit. According to an aspect of the present invention there is provided an air spring for a vehicle suspension system, the air spring comprising: a first chamber having a variable volume; a second chamber fluidly connected to the first chamber by a conduit; and a valve arranged within the conduit for controlling a flow of fluid between the first and second chambers, wherein the valve is configured to open when the first chamber exceeds a threshold pressure to allow fluid flow from the first chamber to the second chamber through the conduit. Advantageously, the air spring is configured to deliver the desired ride and handling for on-road driving, without compromising on comfort during bump events, for example during off-road driving. The air spring may comprising an equalizing element. The equalizing element may be configured to allow fluid to flow from the first chamber and / or the second chamber, independent of the conduit. The equalizing element advantageously enables the static pressures in the first and second chambers to be balanced and maintains the spring rate of the air spring. The conduit may define a first fluid flow path within the air spring. The equalizing element may define a second fluid flow path within the air spring. The second fluid flow path may be more constricted than the first fluid flow path. The more constricted second fluid flow path is too small to allow flow for dynamic inputs and so enables the spring rate of the spring to be maintained. The valve may be arranged to close against a valve seat to thereby block, the first fluid flow path. The second fluid flow path may be defined between the valve and the valve seat when the valve is closed. Advantageously, this enables an arrangement with fewer components, which is relatively lightweight and less complex to manufacture. The equalizing element may take the form of a capillary. The capillary may have a diameter of less than 3 mm. The diameter of the capillary may, for example, be 1 mm or less. The capillary advantageously provides greater control of the flow rate between the first and second chambers, for balancing static pressures between the first and second chambers and maintaining the spring rate. The air spring may comprise a housing. The conduit and the equalizing element may be located within the housing. The provision of the conduit and equalizing element within the air spring, for example within a housing of the air spring advantageously ensures that the internal environment of the air spring is kept free of contaminants and thus reduces the need to rebalance the system for lost air. The valve may comprise a resilient biasing element configured to compress and open the valve when the first chamber exceeds the threshold pressure. The valve may be a passive valve. Advantageously, this enables an arrangement that is less complex and more economical to manufacture. The second chamber may define a fixed volume. The fixed volume of the second chamber may be less than a maximum volume of the first chamber. This provides the benefits of a second chamber, whilst limiting the additional air and space or packaging requirements of the improved air spring. According to an aspect of the present invention there is provided a suspension system for a vehicle comprising at least one air spring according to the preceding aspects of the invention. According to another aspect of the present invention there is provided a vehicle comprising at least one air spring or the suspension system according to the preceding aspects of the invention. According to a further aspect of the present invention there is provided a method of operating an air spring, for example an air spring according to the preceding aspects of the invention. The method may comprise the steps of: supplying a volume of fluid to the first chamber to define a maximum volume of the first chamber; compressing the first chamber so that a pressure of fluid in the first chamber exceeds a threshold pressure, thereby opening the valve to allow fluid to flow from the first chamber to the second chamber through the conduit. The air spring may comprise a first chamber having a variable volume; a second chamber fluidly connected to the first chamber by a conduit; and a valve arranged within the conduit for controlling a flow of fluid between the first and second chambers, wherein the valve is configured to open when the first chamber exceeds a threshold pressure to allow fluid flow from the first chamber to the second chamber through the conduit. The method may comprise the steps of: supplying a volume of fluid to the first chamber to define a maximum volume of the first chamber; compressing the first chamber so that a pressure of fluid in the first chamber exceeds a threshold pressure, thereby opening the valve to allow fluid to flow from the first chamber to the second chamber through the conduit. According to another aspect of the present invention there is provided a method of operating an air spring, the air spring comprising: a first chamber having a variable volume; a second chamber fluidly connected to the first chamber by a conduit; and a valve arranged within the conduit for controlling a flow of fluid between the first and second chambers, wherein the valve is configured to open when the first chamber exceeds a threshold pressure to allow fluid flow from the first chamber to the second chamber through the conduit, the method comprising the steps of: supplying a volume of fluid to the first chamber to define a maximum volume of the first chamber; compressing the first chamber so that a pressure of fluid in the first chamber exceeds a threshold pressure, thereby opening the valve to allow fluid to flow from the first chamber to the second chamber through the conduit. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or 3 file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows an air spring according to an embodiment of the present invention in a first condition; Figure 2 shows the air spring of Figure 1 in a second condition; Figure 3 is a flow chart showing the steps in a method of operating the air spring of Figure 1; Figure 4 shows a graph comparing the effect of air spring displacement on the dynamic pressure in an air spring for a vehicle at normal ride height with 2 passengers and for a fully laden vehicle at off-road height; Figure 5 shows a graph of the effect of increasing the volume of the air spring on the dynamic pressure in the air spring for a fully laden vehicle at off-road height; Figure 6 shows a graph of the effect of the present invention on the dynamic pressure in the air spring for a fully laden vehicle at off-road height; and Figure 7 shows a vehicle in accordance with an embodiment of the invention. DETAILED DESCRIPTION An air spring 10 in accordance with an embodiment of the present invention is described herein with reference to the Figures 1 and 2. A method 100 of operating the air spring 10 is described with reference to Figures 3 to 6. As shown in Figure 7, the air spring 10 may be installed in the suspension system of a vehicle 200. With reference to Figures 1 and 2, the air spring 10 has a first chamber 12 and a second chamber 14. The first chamber 12 has a flexible outer wall 16, for example made from an elastomeric material, that can be expanded and compressed between a compressed condition, in which the first chamber has a first volume, and an expanded condition, in which the first chamber 12 has a second volume. In other words, the first chamber 12 has a variable volume. In contrast, the second chamber 14 defines a fixed volume. The fixed volume of the second chamber 14 is less than a maximum volume of the first chamber 12 (e.g. the fixed volume of the second chamber 14 is less than the second volume of the first chamber 12 when the first chamber 12 is in the expanded condition). The air spring 10 also includes a conduit 18, by which the second chamber 14 is fluidly connected to the first chamber 12. In other words, the conduit 18 defines a first fluid flow path within the air spring 10. A valve 20 is arranged within the conduit 18. The valve 20 controls the flow of fluid, for example air, between the first chamber 12 and the second chamber 14. More specifically, the valve 20 is configured to open when the pressure in the first chamber 12 exceeds the pressure in the second chamber 14 by a threshold pressure, for example 10 Bar, as will be described further below. When the valve 20 is open, fluid can flow from the first chamber 12 to the second chamber 14 through the conduit 18. The air spring 10 includes a valve seat 22. The valve 20 is arranged to close against the valve seat 22, to thereby block the first fluid flow path. Providing the valve 20 in the conduit 18 advantageously prevents the introduction of contaminants into either the first chamber 12 or the second chamber 14 of the air spring 10. A further advantage of this arrangement is that, in the event the valve 20 is opened unintentionally, fluid will flow from the first chamber 12 to the second chamber 14, which would make the air spring 10 softer, without loss of ride height. The valve 20 comprises a resilient biasing element 21 that biases the valve 20 against the valve seat 22. In other words, the valve 20, and therefore the conduit 18, is closed until there is a dynamic event, for example a bump caused by an uneven surface. The resilient biasing element 21 is configured such that when the difference between the pressure in the first chamber 12 and the pressure in the second chamber 14 exceeds the threshold pressure, the valve 20 overcomes the resilient biasing element 21, the resilient biasing element 21 is compressed and the valve 20 moves away from the valve seat 22. In this way, the valve 20 is opened when the difference between the pressure in the first chamber 12 and the pressure in the second chamber 14 exceeds the threshold pressure. In other words, the valve 20 is a passive valve, for example a pressure relief valve. Advantageously, the increase in effective volume when the valve 20 is opened results in a decrease in the peak force pressure caused by bump events, thereby preventing monotonic (or burst) failure of the spring. This may also result in improved comfort for passengers of the vehicle. The air spring 10 further includes an equalizing element 24. The equalizing element 24 is configured to allow fluid to flow from the first chamber 12 and / or the second chamber 14. The equalizing element 24 defines a second fluid flow path within the air spring 10. The second fluid flow path is more constricted than the first fluid flow path. The first flow path has an inner diameter that is larger than the inner diameter of the second flow path. This means that the cross-sectional area of the first fluid flow path is greater than the cross sectional area of the second fluid flow path. The flow rate of fluid in the first fluid flow path will thus be slower than the flow rate of fluid in the second fluid flow path. Under normal driving conditions, without uneven surfaces that would cause a bump event, the second fluid flow path allows the passage of fluid between the first chamber 12 and the second chamber 14 to ensure that the static pressure in the first chamber 12 and the static pressure in the second chamber 14 are balanced. In an example of the invention, the first flow path has an internal diameter that is approximately 30 mm and the second flow path has an internal diameter of approximately 3 mm or less, for example approximately 1 mm or less. The second fluid flow path ensures that the static pressures within the first and second chambers 12, 14 are balanced, whilst maintaining the spring rate of the air spring 10. The second fluid flow path may be defined between the valve 20 and the valve seat 22 when the valve 20 is closed. In other words, in an embodiment of the invention, the second fluid flow path may result from a small opening or leak between the valve 20 and the valve seat 22. The small opening or leak may be caused by the 5 pressure in the first chamber 12 exceeding the pressure in the second chamber 14 by an initial pressure, for example 1 Bar. The initial pressure may be sufficient to compress the resilient biasing element 21 such that the valve 20 moves away from the valve seat 22 creating the second fluid flow path, for example having an inner diameter of 1 mm or less. Alternatively, the equalizing element 24 may take the form of a capillary, for example a capillary that extends between the first chamber 12 and the second chamber 14. In some examples of the invention the capillary has a diameter of less than 1 mm. In other examples of the invention, the equalizing element 24 may take the form of a capillary, for example a capillary that extends through the valve 20 to provide a second fluid flow path between the first chamber 12 and the second chamber 14. In some examples of the invention the capillary has a diameter of less than 1 mm. The air spring 10 includes a housing 26, for example an outer housing 26 that surrounds the components of the air spring 10. The conduit 18 and the equalizing element 24 are each located within the housing 26. At least one air spring 10 is included in the suspension system of a vehicle 200, as shown in Figure 7. Operation of the air spring 10 will now be described. A method 100 of operating the air spring 10 is summarised in Figure 3. In step 102, a volume of fluid is supplied to the first chamber 12. The flexible outer wall 16 of the first chamber 12 is expanded such that the volume of the first chamber 12 is increased to the second (or maximum) volume. In this way, the air spring 10 is set to provide the desired ride and handling for on-road driving. The equalizing element 24 ensures that the pressure of the first chamber 12 and the pressure of the second chamber 14 are balanced in quasi-static conditions (e.g. when lifting the vehicle to an off-road right height or during ride height corrections). In the event that the vehicle 200 hits a bump, the outer wall 16 of the first chamber 12 is compressed such that the volume of the first chamber 12 is reduced to the first volume and, consequently, the pressure is increased. In other words, the first chamber 12 is compressed (step 104). The compression of the first chamber 12 results in the pressure within the first chamber 12 being increased. In the event the pressure in the first chamber 12 exceeds the pressure in the second chamber 14 by a threshold pressure, the valve 20 is opened. More specifically, the fluid within the first chamber 12 acts against the resilient biasing element 21 of the valve 20, moving the valve 20 away from the valve seat 22. This has the effect of opening the conduit 18 between the first chamber 12 and the second chamber 14, increasing the overall volume of the air spring 10 and therefore reducing the peak force pressure of the bump. This prevents monotonic (or burst) failure of the spring 10, and may also result in a more smooth ride for the passenger(s) of the vehicle 200. The invention is particularly advantageous as it provides a cost-effective air spring that delivers the desired ride and handling for normal, on-road, driving without compromising the amount of lift which can be offered to deliver off-road capability, as will be described in more detail with reference to Figures 4 to 6. Referring now to Figures 4 to 6, there are graphs comparing the effect of air spring displacement on the dynamic pressure in an air spring for a vehicle at normal ride height with 2 passengers (line A) and for a fully laden vehicle at off-road height (lines B, D and E). By fully laden, we mean that the vehicle is at maximum passenger occupancy and is laden with luggage. The vertical axis represents the gauge pressure of the air spring (Bar) and the horizontal axis represents the spring displacement versus that of the levelled condition (mm). A negative displacement is indicative of the air spring being expanded and a positive displacement is indicative of the air spring being compressed. The air spring may be compressed, for example, when going over a bump. The air spring has a burst pressure (line C) above which monotonic (or burst) failure of the air spring may occur. The static pressure in the valve 20 when the vehicle is at normal ride height with 2 passengers (line A) is approximately 8 Bar. The static pressure in the valve 20 when the vehicle is lifted to an off-road ride height and is fully laden (line B) is approximately 12 Bar. As can be seen in Figure 4, when the vehicle is at normal ride height with two passengers (line A), the maximum travel if the vehicle goes over a bump is approximately 50 mm and the peak force pressure experienced is approximately 16 Bar. This is approximately 8 Bar higher than the gauge pressure when the vehicle is in a levelled condition. Lifting the vehicle to an off-road ride height increases the static pressure in the air spring and increases the bump travel that is available. For example, and as represented by line B of Figure 4, when the vehicle is at off-road height and fully laden, in the event of a bump, the maximum travel is approximately 80 mm and the peak force pressure experienced is approximately 27 Bar. This is approximately 15 Bar higher than the gauge pressure when the vehicle is in a levelled condition and higher than the burst pressure. Stiffer air springs (with smaller volumes) see greater peak pressures. The effect of increasing the volume of the air spring on the dynamic pressure in the air spring of a fully laden vehicle is represented by line D of Figure 5. In particular, increasing the volume of the air spring by 800 cm3 results in the peak force pressure experienced in the event of 80 mm travel being approximately 22 Bar. This is less than the burst pressure. The increased volume of the air spring affects the ride and handling at normal ride height. There is, therefore, a compromise between providing an air spring having an increased stiffness (smaller volume) that delivers the desired ride and handling and a decreased stiffness (higher volume) that delivers the desired off-road capability. The invention provides a cost effective and reliable way to address this compromise. Figure 6 shows the dynamic pressure in the air spring of the present invention for a fully laden vehicle at off-road height. When the pressure in the first chamber 12 exceed the pressure in the second chamber 14 by approximately 10 Bar, the valve 20 is opened and the volume of the air spring is increased to the combined volume of the first and second chambers 12,14. In Figure 6, this is illustrated by the change in the gradient of line E when the gauge pressure is approximately 22 Bar(i.e. 10 Bar higher than the 12 Bar static pressure for a fully laden vehicle at off-road height). In this way, the peak force pressure experienced by the air spring 10 is kept below the burst pressure. Advantageously, the volume of the air spring 10 can be increased when required (e.g. to ensure desired off-road capability is achieved) without compromising the ride and handling when the vehicle is at normal ride height. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 5 In the example described above, for example, the valve 20 is a pressure relief valve with a resilient biasing element. It will be understood that the valve 20 could be replaced by any suitable valve that will open in response to the pressure in the first chamber 12 exceeding the pressure in the second chamber 14 by a threshold pressure. The threshold pressure that is required for the valve 20 to open may be affected by other factors, for example 10 how the car is loaded and what the pressure of air in the second chamber 14 is. The conduit 18 may be provided by a passage through adjacent walls of the first chamber 12 and the second chamber 14. Alternatively, the conduit 18 may be a separate component, for example a tube or other connector which extends between the first chamber 12 and the second chamber 14. 15

Claims

1. An air spring for a vehicle suspension system, the air spring comprising:a first chamber having a variable volume;a second chamber fluidly connected to the first chamber by a conduit; anda valve arranged within the conduit for controlling a flow of fluid between the first chamber and the second chamber, wherein the valve is configured to open when the first chamber exceeds a threshold pressure to allow fluid flow from the first chamber to the second chamber through the conduit.

2. The air spring as claimed in claim 1, further comprising an equalizing element configured to allow fluid to flow from the first chamber and / or the second chamber, independent of the conduit.

3. The air spring as claimed in claim 2, wherein the conduit defines a first fluid flow path within the air spring and the equalizing element defines a second fluid flow path within the air spring, wherein the second fluid flow path is more constricted than the first fluid flow path.

4. The air spring as claimed in claim 3, wherein the valve is arranged to close against a valve seat to thereby block the first fluid flow path, wherein the second fluid flow path is defined between the valve and the valve seat when the valve is closed.

5. The air spring as claimed in claim 3, wherein the equalizing element takes the form of a capillary, optionally wherein the capillary has a diameter of less than 5mm.

6. The air spring as claimed in any of claims 2 to 5 comprising a housing, wherein the conduit and the equalizing element are located within the housing.

7. The air spring as claimed in any of claims 1 to 6, wherein the valve comprises a resilient biasing element configured to compress and open the valve when the first chamber exceeds the threshold pressure.

8. The air spring as claimed in any of claims 1 to 7, wherein the valve is a passive valve.

9. The air spring as claimed in any of claims 1 to 8, wherein the second chamber defines a fixed volume,optionally wherein the fixed volume of the second chamber is less than a maximum volume of the first chamber.

10. A suspension system for a vehicle comprising at least one air spring according to any of claims 1to 9.

11. A vehicle comprising at least one air spring according to any of claims 1 to 9 or the suspension system of claim 10.

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