This application is a National Stage of International Application No. PCT/GB2013/050015, filed Jan. 7, 2013, which claims the benefit of GB Patent Application No. 1200506.2, filed Jan. 12, 2012, the contents of which are herein incorporated by reference.
The present invention relates to Stirling cycle machines, for example Stirling cycle engines (also referred to as Stirling engines) and Stirling cycle coolers (also referred to as Stirling coolers).
Stirling engines have the potential for generating power efficiently from diverse heat sources that include solar, biomass and radio-nuclides. There has been considerable development of Stirling engines for more than twenty years but the resulting configurations have still not attained significant exploitation.
Large Stirling engines have tended to use “kinematic” configurations that have oil lubricated crank mechanisms. These have demonstrated high efficiency but are relatively expensive to operate, particularly as they generally require frequent servicing—typically at intervals of ˜8000 hrs.
Oil free engines have been developed that have demonstrated long maintenance free life e.g. engines made by Sunpower and Infinia. Such configurations use linear technologies that avoid the requirement for crank mechanisms etc. They are capable of high efficiency but so far they have been limited to powers of ˜1 kW. This is too small for many potential applications e.g. renewable power using solar and biomass heat sources. There are a number of issues that inhibit scaling to larger sizes. For example, these linear engines do not have any means for controlling the power generated; the beta geometries used require displacer components that become more difficult to resonate; and the annular heater geometry used does not scale well to larger sizes.
Although there are many different configurations of Stirling cycle machine, they all basically consist of a gas filled assembly of two variable volumes Vc, Ve connected by a number of heat exchangers—i.e. a cooler 2, a regenerator 4 and a heater 6, as illustrated in
The varying volumes Vc, Ve, generated by the piston Pc, Pe and cylinder 5 assemblies, operate at different temperatures with a phase between them that is typically between 60 to 120 deg. The volume with the retarded phase is termed the compression volume Vc and in it work is done on the gas by the piston Pc. The other volume is termed the expansion volume Ve and in this case the gas does work on the piston Pe. The net work of the machine is the difference between the work output of the expansion volume Ve and the work input of the compression volume Vc. For work output to be positive, i.e. for the machine to operate as an engine, the expansion volume temperature Te must be higher than the compression volume temperature Tc. For efficient operation the ratio Te/Tc is made as high as possible. For a practical Stirling engine Te and Tc are typically 1000 K and 300 K respectively.
A key aspect of the configuration of a Stirling engine is the means used to transfer power from the expansion volume Ve to the compression volume Vc so as to maintain engine operation.
In “alpha” type engines the compression and expansion volumes Vc, Ve are quite separate and they are generally mechanically connected via a common crank mechanism 8 as in
In “beta” and “gamma” engines (the general arrangement of a gamma engine is illustrated in
Beta engines are similar in operation to gamma engines but are arranged so that the piston and displacer share the same cylinder with the heat exchangers forming an annulus around the cylinder. They have the advantage of a more compact arrangement.
There also exist multi-cylinder engine configurations that use double acting pistons to transfer power. In a Rinia multi-cylinder configuration there are effectively four engines integrated together in a loop so that adjacent engines are 90 degrees out of phase. This arrangement allows each piston to act as an expansion piston for one engine and a compression piston for the engine adjacent to it. The compression power for each engine is therefore supplied directly by the expansion power of an adjacent engine.
All four configurations have been exploited in various Kinematic engines. For high power, high efficiency engines the alpha single cylinder and Rinia multi-cylinder configurations have been the preferred configurations.
Nearly all linear configurations have used a beta configuration although more recently multi-cylinder configurations have being developed. Single cylinder alpha configurations have not generally been used in linear machines because of the lack of a suitable power transfer mechanism. An exception to this is a configuration disclosed in U.S. Pat. No. 5,146,750 (Moscrip). This describes a particular electrical power transfer mechanism.
It is an object of the present invention to provide a configuration for a linear Stirling cycle machine that is geometrically well suited to larger sizes and which can readily incorporate power control mechanisms.
According to an aspect, there is provided a Stirling cycle engine, comprising: an expansion volume structure defining an expansion volume; a compression volume structure defining a compression volume; a gas spring coupling volume structure defining a gas spring coupling volume; a first reciprocating assembly comprising an expansion piston configured to reciprocate within the expansion volume and an expander gas spring piston rigidly connected to the expansion piston and configured to reciprocate within the gas spring coupling volume; and a second reciprocating assembly comprising a compression piston configured to reciprocate within the compression volume and a compressor gas spring piston rigidly connected to the compression piston and configured to reciprocate within the gas spring coupling volume, wherein: the gas spring coupling volume structure and the first and second reciprocating assemblies are configured such that power is transferred in use from the expansion piston to the compression piston via the gas spring coupling volume.
This arrangement incorporates a novel arrangement for transferring power from the expansion volume to the compression volume. The expansion and compression volumes may be part of the same engine unit or different engine units. The arrangement is especially suited for linear, alpha configuration machines. The arrangement can be scaled up easily without losing efficiency and is therefore geometrically well suited to larger sizes. The arrangement can readily incorporate power control mechanisms. In an embodiment, the power control mechanisms comprise one or more transducers that interact with the first and/or second reciprocating assemblies.
In an embodiment, a controller is provided that controls one or more of the following: the power output of the engine, the amount of power transferred from the first reciprocating assembly to the second reciprocating assembly, the phase difference between the movements within the first and second reciprocating assemblies, the frequency of the movement of the first and second reciprocating assemblies. In an embodiment, the controller controls a transducer in the first and/or second reciprocating assemblies.
In an embodiment, pairs of linear suspension springs are provided for guiding movement of components within one or both of the first and second reciprocating assemblies. The pairs of linear suspension springs provide the basis for highly accurate linear guiding of components. In an embodiment, the expansion piston, expander gas spring piston, compression piston and/or compressor gas spring piston can be guided to move within corresponding close-fitting bores without the need for lubricant and/or direct contact between the piston(s) and bore(s). Lubricant free, long-life operation is therefore facilitated.
In an embodiment, balanced engine operation is achieved by providing two sets of said first reciprocating assembly, said second reciprocating assembly, and said gas spring coupling volume structure, each set being arranged so that, in use, the position of the center of mass of the engine remains constant.
In an embodiment, balanced engine operation is achieved by providing a third reciprocating assembly comprising a further compression piston configured to reciprocate within a further compression volume and a further compressor gas spring piston rigidly connected to the further compression piston and configured to reciprocate within the gas spring coupling volume. In an embodiment, the second and third reciprocating assemblies are positioned on opposite sides of the first reciprocating assemblies and configured such that a resultant inertial force arising from movement within the second and third reciprocating assemblies acts along the axis of reciprocating movement within the first reciprocating assembly. In an embodiment, a balancer mass is provided that is configured to act along the axis of reciprocating movement within the first reciprocating assembly.
According to an aspect, there is provided a Stirling cycle cooler, comprising: an expansion volume structure defining an expansion volume; a compression volume structure defining a compression volume; a gas spring coupling volume structure defining a gas spring coupling volume; a first reciprocating assembly comprising an expansion piston configured to reciprocate within the expansion volume and an expander gas spring piston rigidly connected to the expansion piston and configured to reciprocate within the gas spring coupling volume; and a second reciprocating assembly comprising a compression piston configured to reciprocate within the compression volume and a compressor gas spring piston rigidly connected to the compression piston and configured to reciprocate within the gas spring coupling volume, wherein: the gas spring coupling volume structure and the first and second reciprocating assemblies are configured such that power is transferred in use from the expansion piston to the compression piston via the gas spring coupling volume.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:
As mentioned above, typical prior art alpha type Stirling cycle engines (as illustrated in
An embodiment of the type illustrated in
In an embodiment, the expansion piston Pe engages within the expansion volume structure 20 and is configured to be movable in a reciprocating manner therein. The expansion piston Pe is part of a first reciprocating assembly. In the embodiment shown, the expansion piston Pe is mechanically (e.g. rigidly) connected to the armature 22 of a linear generator 23 via an expansion coupling member 26. In such an embodiment, the expansion coupling member 26 is also part of the first reciprocating assembly. In an embodiment, the expansion coupling member 26 is provided in the form of a shaft or rod. In an embodiment, movement of the armature 22 relative to a stator 24 of the linear generator 23 generates electricity. In an embodiment, the piston Pe is also coupled to a gas spring coupling 14, optionally via the expansion coupling member 26. In an embodiment, the piston Pe is coupled (e.g. rigidly) to an expander gas spring piston 28, which in this embodiment is part of the first reciprocating assembly and is configured to reciprocate within a gas spring coupling volume 34. The gas spring coupling volume 34 is defined by a gas spring coupling volume structure 44. The expander gas spring piston 28 is part of the gas spring coupling 14.
In an embodiment, the compression piston Pc engages within the compression volume structure 18 and is configured to be movable in a reciprocating manner therein. The compression piston Pc is part of a second reciprocating assembly. In the embodiment shown, the compression piston Pc is mechanically (e.g. rigidly) connected to a compressor gas spring piston 30, which in this embodiment is part of the second reciprocating assembly and is configured to reciprocate within the gas spring coupling volume 34, optionally via a compression coupling member 32 (which in this embodiment is also part of the second reciprocating assembly). In an embodiment, the compression coupling member 32 is provided in the form of a shaft or rod. The compressor gas spring piston 30 is also part of the gas spring coupling 14.
In the embodiment shown in
In describing the operation of the engine it is helpful to refer to different faces of a piston. A north/south direction is shown in
The north faces of the compression and expansion pistons Pc, Pe compress and expand the gas in the Stirling engine components (the compression and expansion volumes Vc, Ve). As described above, the expansion displacement is typically 60 to 120 degrees in advance of the compression displacement. There is a power input from the compression piston Pc into the gas and a power output from the gas into the expansion piston Pe. For an engine the expansion power is larger than the compression power so there is net power generation. The gas spring coupling 14, which is a coupling based on the principle of a gas spring, provides a power transfer between the first reciprocating assembly (which may also be referred to as the expansion assembly) and the second reciprocating assembly (which may also be referred to as the compression assembly). In this way the compression power (required by the compression piston Pc) is provided by the expansion piston Pe and the linear generator 23 is used to transform the remaining power into an electrical power output.
The operation of a gas spring will now be described in more detail.
For a phase difference between the displacements other than 0 and 180 degrees it is found that although there is still no overall power consumption, there is a net transfer of power from one piston to the other. This can be seen by considering two pistons with equal displacements. When the pistons are in phase the gas pressure variations are in anti-phase. If one piston is advanced 60 degrees with respect to the other then consideration of the point of minimum volume determines that the pressure variation will advance 30 degrees with respect to one piston and be retarded by 30 degrees with respect to the other piston. There is therefore an equal and opposite work done by each piston. Overall the piston that is advanced gains power from the other piston.
More generally a gas spring coupling can have two or more pistons (i.e. displacement mechanisms) that are undergoing some cyclic variation—e.g. as determined by sinusoidal motion. The displacements will combine to produce a pressure variation. The pistons whose minimum volume is in advance of the peak pressure will absorb energy. The pistons whose minimum volume is retarded with respect to the peak pressure will lose energy. In this way power is transferred between pistons. The phase relationship determines the polarity of the power transfer. The magnitude is determined by swept volume, i.e. piston diameter and stroke, and phase angle.
Returning to the embodiment shown in
For the Stirling engine to operate it has already been stated that the expansion piston Pe must be in advance of the compression piston Pc and the phase difference is typically in the range 60 to 120 degrees. If the south faces of the two gas spring pistons 28, 30 are considered for the gas spring coupling then it is found that the phase difference is incorrect—the gas spring coupling would transfer power from the compression piston Pc to the expansion piston Pe. A way round this is to introduce a 180 degree phase shift by combining a north face for one gas spring piston 28, 30 with a south face for the other 30,28. For example, in
In the embodiment shown in
In the description given above, possible losses in the gas spring coupling 14 are not discussed. In practice these losses can be significant and for efficient operation of an engine it is desirable that they be kept to a minimum. There are two loss mechanisms to be considered:
The piston seal loss is due to gas leakage past one or more of the pistons 28,30 in the gas spring coupling 14, driven by the pressure variations. This is a common engineering problem and can be controlled by a variety of means; small piston cylinder clearance, contacting seals (e.g. pistons rings), lubricants etc.
The gas spring loss due to heat transfer is more complex and has only been analyzed in detail for a few specific geometries; nonetheless the general mechanisms are well understood. The main requirement for the gas spring is that the compression and expansion processes should be reversible. In principle there is a choice; either the processes are isothermal—they are reversible because the temperature variations are very small, or the processes are adiabatic—they are reversible because there is no heat exchange. In between these limits the processes exchange heat with significant temperature drops and the inherent irreversibilities lead to significant losses. The factor deciding the scale of the loss is the Peclet number. This is a dimensionless parameter that gauges where a process lies between the isothermal and adiabatic extremes. A high Peclet number denotes an adiabatic process; a low one denotes an isothermal process.
It is found that for machines operating at 50 Hz with dimensions consistent with power outputs of 1 kW, reversibility is more easily attained by pursuing adiabatic processes. In practice this demands that heat transfer should be minimized as far as possible by minimizing the surface area and also keeping flow velocities down.
Accurate values for adiabatic gas springs are not readily calculated for arbitrary geometries. However losses for cylindrical geometries have been subject to both theoretical and experimental investigations that resulted in a fairly reliable loss correlation (see Kornhauser A. A, Smith J. L, “The Effects of heat Transfer on Gas Spring Performance”, Transactions of the ASME, Vol 115, March 1993 pages 70 to 75). Estimates of losses using this correlation suggest that very high efficiency can be obtained using suitable gas spring geometries.
It is noted there are changes in volume wherever there are displacements and that every piston has two faces. There may therefore be unintended pressure variations in other parts of the engine, e.g. around the armature 22. The magnitude of these variations can be reduced by ensuring there is sufficient volume. Nonetheless such volumes may have extended heat transfer surfaces and so may introduce significant losses. This aspect is considered again below in the context of a more detailed example.
The embodiments described above have focused on the use of the gas spring coupling 14 to provide efficient power transfer (i.e. feedback) from the expansion piston Pe (and/or first reciprocated assembly) to the compression piston Pc (and/or second reciprocating assembly). In this basic form there is no provision for controlling power or modifying operating characteristics. The feedback is mainly fixed by the geometry and the dynamics and these are not readily changed by external intervention.
In an embodiment, features for implementing synchronization, controlling the power output of the engine, the amount of power transferred from the first reciprocating assembly to the second reciprocating assembly, the amplitude (position/stroke) of the movement within the first reciprocating assembly and/or the second reciprocating assembly, the phase difference between the movements within the first and second reciprocating assemblies and/or frequency of the movement of the first and second reciprocating assemblies are provided. In an embodiment, a controller is provided. In an embodiment, the controller controls operation of a transducer in the first and/or second reciprocating assemblies. In an embodiment a measurement device is provided for measuring one or more operating characteristics of the engine. In an embodiment, the measurement device measures one or more of the following: the power output of the engine, the amount of power transferred from the first reciprocating assembly to the second reciprocating assembly, the amplitude (position/stroke) of the movement within the first reciprocating assembly and/or the second reciprocating assembly, the phase difference between the movements within the first and second reciprocating assemblies and/or the frequency of the movement of the first and second reciprocating assemblies. In an embodiment, the measurement device is configured to provide input to the controller. Such features are particularly useful if multiple engine units are to be integrated together to give a common output.
In an embodiment, a valve 46 is provided for controllably venting the gas spring coupling volume 34. The valve 46 provides a simple but effective way of exercising power control. With the valve 46 shut the power transfer will be at its most efficient and the engine will run at its maximum design power. If the valve 46 is opened sufficiently then this will ruin the feedback and the engine will stop. In between there is the possibility of throttling the flow so that some power control is possible. The throttling process will dissipate energy so this will not necessarily be the most efficient method. Various valve geometries can be used as well as different mechanisms for their operation.
In an embodiment, an electromagnetic transducer 48 is integrated into the compressor assembly (the second reciprocating assembly). An example of such a configuration is shown in
The gas spring coupling power transfer mechanism can be designed to provide either too much or too little power. In both cases, embodiments may be provided in which the electromagnetic transducer 48 is configured to modify engine operation by adding or subtracting power.
In an example embodiment, the transducer 48 has an external power input or is connected to a load so it provides damping that will reduce the power in the compressor assembly (the second reciprocating assembly).
In an embodiment, a direct electrical feedback circuit 52 is provided. The direct electrical feedback circuit 52 operates in a manner that is analogous to the gas spring coupling 14. In an embodiment, different reactive components are used and/or the polarity of the transducer 48 with respect to the generator 23 is changed, to arrange for the electrical feedback to reinforce the mechanical power transfer or to oppose it, as desired.
In an embodiment, the engine is configured so that most of the power transfer is provided by the gas spring coupling 14. An electrical feedback is then used to fine tune the engine balance so that the feedback to the compressor assembly (the second reciprocating assembly) is slightly insufficient. A small external input is then used to control the engine power and/or determine its operating frequency and/or phase so that it can be readily integrated with other power sources. In an embodiment, the valve 46 is configured to act as an emergency “on/off valve” in the event of a loss of generator load.
In a Stirling engine that uses linear drive mechanisms, the position of the pistons is not geometrically determined by crank mechanisms. Instead it is determined by the dynamics of the two moving assemblies (the first and second reciprocating assemblies). In practice this dictates that mechanical resonance for both the first and second assemblies need to be equal or close to the operating frequency depending on the engine phase angle required. The mechanical resonances are determined by the moving masses and the spring stiffnesses. In an embodiment, it is desirable to minimize the sizes of the moving masses, subject to providing the necessary strength and rigidity. In such an embodiment, adjustment of the mechanical resonances is carried out predominantly by adjusting the spring stiffnesses. In an embodiment, the mass is also adjusted.
There are four possible sources of spring stiffness:
The spring stiffness contributed by mechanical springs is significant for small engines e.g. <100 W power, but for engines in the 1 kW+ range it is small enough to be neglected.
In an alpha configuration engine it is found that the compression piston Pc has significant spring stiffness. The expansion piston Pe however generally has an effective value ˜0—it is quite possible for the spring stiffness to be slightly negative.
Significant spring stiffness can be generated by the gas spring coupling 14 for both compressor and expander assemblies (first and second reciprocating assemblies), depending on the piston diameters and phases etc.
Additional gas springs can be added to both compressor and expander assemblies (first and second reciprocating assemblies) to further increase spring stiffness.
There is therefore considerable scope for adjusting the dynamics to that required. The main proviso that needs consideration is that as the engine size is increased the stroke is also increased to retain workable dimensions for the linear motors etc. For a given displacement and pressure excursion the spring stiffness reduces rapidly with increasing stroke. It is therefore inevitable that as size increases the maximum operating frequency is reduced. It is found that for a ˜10 kW engine 50 Hz operation is possible but above this size the frequency may need to be reduced.
The description given above has referred generally to linear technologies that do not require lubrication. A specific technology that is well suited to this engine configuration is one which has been developed for coolers used in space. This uses sets of flexures to provide accurate linear suspension systems—equivalent to a linear bearings. Each flexure may be referred to as a linear suspension spring. In an embodiment, pairs of linear suspension springs are provided that guide reciprocating movement of a piston within a bore. Contacting seals are not used. Instead, a small clearance is maintained between the piston and the bore (such that the piston and corresponding bore are “close-fitting”) that maintains a leakage loss at an acceptable level. In an embodiment, the clearance is about 10 microns.
In other embodiments, linear gas bearings are used, as an alternative oil free mechanism, to guide movement of one or more pistons of the Stirling cycle engine.
In the example shown, linear suspension springs 54 are provided on each side of the generator 23 to guide linear, reciprocating movement of the expansion piston Pe and the expander gas spring piston 28 within corresponding respective bores 56. In the example shown, linear suspension springs 54 are also provided on each side of the motor 48 to guide linear, reciprocating movement of the compression piston Pc and the compressor gas spring piston 30 within corresponding respective bores 58.
The embodiments described in detail above (with reference to figures prior to
There are a number of ways of producing a balanced engine. One method is to use two separate engines and arrange them so that the two sets of piston assemblies are horizontally opposed either with heat exchangers on the inside or outside (i.e. NSSN or SNNS). Each piston is then equally balanced by a mirrored companion.
Another method that will give even better balance is to have a single engine but adopt balanced piston pairs for both compression and expansion volumes. With matching pistons and an engine pressure variation that is common to both sets, symmetry should ensure that very good balance is achieved. An example of such an arrangement is illustrated in
In the example shown in
In an alternative embodiment, the cooler-regenerator-heater assembly is arranged so that each half has its own cooler 2 and regenerator 4 but share a common heater 6, as is shown in
In the embodiment shown in
Referring again to the embodiment of
In the embodiment shown in
In an embodiment, the cross-sectional area of the supporting shaft 74 of the expander gas spring piston 28 is equal to the cross-sectional area of the expansion piston Pe. This helps to reduce variations in the size of dead volumes within the first reciprocating assembly, for example in the region of the transducer 23. Losses associated with pressure variations caused by reciprocating movement within the first reciprocating assembly can thereby be reduced. In an embodiment, the cross-sectional area of the supporting shaft 74 of the compressor gas spring piston 30 is equal to the cross-sectional area of the compression piston Pc. This helps to reduce variations in the size of dead volumes within the second reciprocating assembly, for example in the region of the transducer 48. Losses associated with pressure variations caused by reciprocating movement within the second reciprocating assembly can thereby be reduced.
Embodiments have so far been described with particular reference to Stirling engines—i.e. Stirling cycle machines that generate power. Any one of the described embodiments can also be applied singly or in combination to Stirling cycle machines that are used to pump heat e.g. coolers and heat pumps.
The detailed description given above with reference to
The embodiments described above comprise a gas spring coupling to transfer power between the compression and expansion volumes of the same engine. Further embodiments are possible where a gas spring coupling is used to transfer power from the expansion volume of one engine to the compression volume of another engine. Such an arrangement is illustrated in
In
Arrangements of the type shown in
In an embodiment in which there is a phase difference of 180 degrees between the two engine units 101,102 there is no longer the need to introduce an extra phase difference by using different faces of the gas spring pistons 28,30 as was shown for single engine embodiments. For example, in the arrangement of
The two engines units 101,102 shown in
This can be done by either having two engine units opposed in a “boxer” formation as described above or alternatively by having two engine units side by side.
In a range of embodiments, a gas spring coupling is provided that transfers some power between one or more expansion and compression assemblies belonging to one or more alpha configuration Stirling cycle machines. The power transferred by the gas spring coupling can constitute the entire power transfer. Alternatively it can be part of the transfer with the rest being transferred by other means e.g. by electrical means to give some control of engine operation.
The power transferred by the gas spring coupling can be between expansion volumes and compression volumes that belong to the same engine unit or alternatively it can be between separate engine units. The power transfers can be contained within loops. Alternatively, the power transfer can be part of an open sequence of engine units.
In an embodiment, in addition to displacements associated with the expansion and compression assemblies (first and second reciprocating assemblies), the gas spring coupling is configured to accommodate additional displacements that modulate the operation of the gas spring and hence the engine. An example of such an arrangement is depicted in
In an embodiment, a single gas spring coupling has inputs/outputs for single expansion and compression volumes. In other embodiments, a single gas spring coupling has multiple inputs/outputs for a plurality of expansion and/or compression volumes. In each case, the phases are configured to give the desired power flows. It is also possible to have multiple gas spring couplings operating in parallel.
In an embodiment, additional gas forces are used to input or output power from the assemblies. An example of such an embodiment was described above with reference to
Number | Date | Country | Kind |
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1200506.2 | Jan 2012 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2013/050015 | 1/7/2013 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2013/104894 | 7/18/2013 | WO | A |
Number | Name | Date | Kind |
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4367625 | Vitale | Jan 1983 | A |
4702903 | Keefer | Oct 1987 | A |
4717405 | Budliger | Jan 1988 | A |
5146750 | Moscrip | Sep 1992 | A |
8074457 | Bin-Nun | Dec 2011 | B2 |
8820068 | Dadd | Sep 2014 | B2 |
20070295201 | Dadd | Dec 2007 | A1 |
Number | Date | Country |
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102004018782 | Nov 2005 | DE |
102004056156 | May 2006 | DE |
WO2011020988 | Feb 2011 | WO |
Entry |
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Kornhauser, et al., The Effects of Heat Transfer on Gas Spring Performance, Transactions of the ASME, vol. 115, Mar. 1993, pp. 70-75. |
International Search Report and Written Opinion issued in PCT/GB2013/050015, Jun. 24, 2013. |
GB Search Report issued in GB1200506.2, May 8, 2012. |
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Number | Date | Country | |
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20150052887 A1 | Feb 2015 | US |