DESCRIPTION

The present invention relates generally to pistons that are used in cylinders for gas compression and expansion processes, and particularly but not exclusively to pistons for use in applications such as heat engines, heat pumps, gas compressors and gas expanders.

In a standard piston gas compressor or piston engine, the piston is normally driven by a crankshaft and drive rod (or "con-rod"), which moves the piston in an oscillating manner within a cylinder. The velocity of the piston is normally sinusoidal, where the velocity is zero at Top Dead Centre (TDC) or Bottom Dead Centre (BDC) and the peak velocity occurs when the piston is at the mid point of the stroke. In this application the piston principally experiences two major sources of loads. These are gas loads and inertial loads, of which the inertial loads can often be the most significant and consequently the limiting factor for maximum operating speeds. These loads are generally diffuse loads as they are often spread over the whole piston and are reacted by the drive rod.

For large pistons the mass of the piston can increase as a cubic function, while the area of the piston only increases as a square function. Consequently very large pistons are much more prone to be limited in speed by their inertial loads over the gas loads. These large pistons are normally very expensive to machine and/or manufacture. It is also often important to minimise the amount of deadspace or clearance volume that is left in the cylinder. Deadspace or clearance Volume is the volume remaining between piston and cylinder when the piston is at TDC, for a normal engine, or either TDC or BDC, for double acting pistons, depending upon which face of the piston is chosen. This includes the space caused by the clearance of the piston and the space in any ports and passageways that are open to the working space.

Additionally in some reciprocating machinery it is often desirable for improved performance for the machinery to reach operating conditions quickly. This means that the thermal inertia of the components in the system should be kept low, so that they can achieve operating temperature very quickly. In addition if the operating conditions change they are able to respond to the new operating conditions and reach any new operating temperatures very quickly. Finally in pistons that use through piston valving it is desirable to provide a structure that allows the use and control of these valves and also gas paths for the gas to pass through.

The present applicant has identified the need for a novel piston assembly, which overcomes, or at least alleviates, some of the above-mentioned problems associated with conventional pistons.

In accordance with the present invention, there is provided a piston assembly comprising: a piston; a drive rod coupled to the piston for in use driving the piston; and a load transfer part (e.g. load transfer collar) comprising a stiffening structure, the load transfer part enclosing at least a portion of the drive rod and being coupled to the drive rod at a plurality of longitudinally spaced locations along the drive rod.

In this way an improved piston assembly is provided in which diffuse loads on the piston are transmitted to the drive rod over an elongate section of the drive rod by means of a lightweight load transfer structure offering a high degree of stiffness to take bending loads. Advantageously, this allows the piston to be used at higher pressures than would otherwise be possible without the load transfer part.

The piston assembly may comprise a cylinder for receiving the piston.

The drive rod may be coupled to the piston direct or via the load transfer part.

The drive rod may be a central drive rod (e.g. having a longitudinal axis extending through a central part of the piston). The load transfer part may partially or substantially enclose the portion of the drive rod. In the case of a central drive rod, the stiffening structure may substantially enclose the drive rod (e.g. with the stiffening structure being distributed substantially evenly around the longitudinal axis of the drive rod).

The stiffening structure may extend from substantially one lateral side of the piston to substantially an opposed lateral side of the piston. In one embodiment, the stiffening structure has a cross-sectional area substantially corresponding to that of the piston. The stiffening structure may have a thickness (e.g. mean thickness) of at least five times the thickness of the piston. For example, the stiffening structure may have a thickness which is at least ten times the thickness of the piston.

This piston and stiffening structure may have substantially similar cross-sectional profiles. In the case of a piston having a circular cross-sectional profile, the stiffening structure may have a substantially cylindrical outer profile concentric with the circular cross-sectional profile of the piston. The piston and stiffening structure cross-sectional profiles may be of substantially equal sizes. For example, the substantially cylindrical outer profile may have a diameter substantially equal to that of the piston.

The load transfer part may be coupled to the drive rod at a first end location nearest the piston and at a second end location furthest from the piston. For example, the load transfer part may be clamped to the drive rod with clamping contact occurring at the first and second end locations. In this way, the load transfer part may be constrained to move axially with the drive rod. In one embodiment, the load transfer part is further coupled to the drive rod at a plurality of longitudinally spaced locations between the first and second end locations. For example, the load transfer part may be bonded to the drive rod between the first and second end locations.

In one embodiment, the load transfer part is coupled to the drive rod at a plurality of discrete locations along the drive rod. In another embodiment, the load transfer part is coupled to the drive rod along a continuous portion of the drive rod extending along the drive rod.

The drive rod may have a cylindrical profile or any other suitable profile. In one embodiment, the drive rod defines a surface for abutting an underside surface of the load transfer part. In this way, the load transfer part may be clamped to the drive rod by locking part (e.g. a locking nut or suitable locking element) for engaging an upper surface of the load transfer part.

The load transfer part may be spaced from the piston (e.g. to define a space or chamber therebetween) and coupled to the piston by at least one member (e.g. a plurality of struts) laterally spaced from the drive rod and extending between the piston and the load transfer part. Advantageously, such an arrangement allows the passage of gas through a space between the piston and the load transfer part and allowing components such as valves to be positioned on the piston. Furthermore, the provision of the at least one member allows the thickness of the piston to be kept to a minimum. When used with through piston valves, this helps keep deadspace resulting from ports through the piston to a minimum.

A further advantage of spacing the load transfer part from the piston is that in high temperature applications a working face of the piston can be cooled by a gas flow passing across a side of the piston opposed to the working face. Advantageously, such an arrangement allows a piston assembly to be provided in which a structural part of the piston is maintained at a substantially lower temperature than the working face of the piston. In the case of a ceramic piston, a piston assembly may be provided which is capable of operating at very high temperatures.

A yet further advantage of a reducing piston thickness is an increase in thermal responsiveness which may allow a correct working temperature to be achieved more quickly than with a thicker piston, which in certain applications may be of significant benefit.

Another feature of this piston structure is that as the diameter of the piston increases the mass per unit piston area of the connecting rods and piston face is approximately constant and so the mass of these components in total increases in direct proportion to area, if the spacing of the rods and the pressure remain constant. The mass of the transfer structure increases at a faster rate than this as the depth of the transfer structure and/or the thickness of the faces must increase. The advantage of this is that the mass of the structure does not runaway as the piston area increases and allows the potential construction of large diameter pistons, e.g. pistons with diameters of 2 metres up to around 10 metres.

In one embodiment, the at least one member extends through the load transfer part.

In one embodiment, the drive rod is spaced from the piston.

The load transfer part and piston may define a pressurised space therebetween (e.g. pressurised chamber formed by the load transfer part, piston and walls of the cylinder). In one embodiment, the working gas pressure on the piston may substantially equal to or less than the pressure in the pressurised space. In this way, the drive rod can be of slender, lightweight construction. In another embodiment, the working gas pressure on the piston is higher than the pressure in the pressurised space. In this way, the drive rod will only see a predominantly compressive load. In this configuration, the drive rod must be designed to resist buckling (e.g. by means of a tubular construction). In one embodiment, the load transfer part decreases in stiffness with increased radial distance from the drive rod. For example, the structural dimensions, e.g. thickness and/or depth of the stiffening structure may decrease with increased radial distance from the drive rod. In this way, an efficient lightweight structure may be achieved for handling shear and bending loads across the load transfer part. The stiffening structure may define an outer profile which decreases in thickness

(e.g. along the longitudinal axis of the drive rod) with increased radial distance from the drive rod.

In another embodiment, the load transfer part comprises a localised strengthening element (e.g. "doubler") for stiffening a radially innermost portion of the stiffening structure. The localised strengthening element may be bonded to the structure. Alternatively the localised strengthening element can be machined or chemically etched or removed from a layer (e.g. outer face) of the load transfer part so as to minimise the weight of the layer.

The stiffening structure may comprise a plurality of spaced support elements forming a cellular stiffening structure (e.g. framework). The cellular stiffening structure may have a homogonous cell structure (i.e. a single repeating cell unit) or non-homogonous cell structure (e.g. with a plurality of different cell units).

The cellular stiffening structure may have a closed or open cell structure. Space between the plurality of spaced support elements forming the cellular stiffening structure may be filled by air or any other material which has a low density relative to the support elements.

The cellular stiffening structure may be formed from a different material to the piston. For example, the cellular stiffening structure may be formed from a material with a lower density than the piston. The cellular stiffening structure may form a core of a sandwich structure comprising first and second outer layers (e.g. first and second outer skins or plates). The cellular stiffening structure may be bonded (e.g. epoxy bonded or diffusion bonded in dependence upon the materials selected) to the first and second outer layers. The cellular stiffening structure may have a thickness which is substantially greater (e.g. at least ten times greater) than the combined thicknesses of the first and second outer layers. In one embodiment, the cellular stiffening structure may be formed from a material having low strength relative to the first and second outer layers.

In one embodiment, the first outer layer is provided by the piston.

In one embodiment, at least one of the first and second outer layers of the sandwich structure comprises a sheet of aluminium or stainless steel.

In another embodiment, at least one of the first and second outer layers of the sandwich structure may comprise a composite material. For example, in one embodiment the first and second outer layers may comprise carbon fibre or glass fibre.

At least one of the first and second outer layers of the sandwich structure may decrease in thickness (e.g. along the longitudinal axis of the drive rod) with increased radial distance from the drive rod. The cellular stiffening structure may comprise a honeycomb core (e.g. hexagonal honeycomb core). For example, the honeycomb core may be an aluminium, Nomex or stainless steel honeycomb core. In one embodiment the honeycomb core is an aluminium or Nomex honeycomb core and at least one of the first and second outer layers is an aluminium skin. In another embodiment, the honeycomb core is a stainless steel honeycomb core and at least one of the first and second outer layers is a stainless steel sheet.

In another embodiment, the cellular stiffening structure comprises a foamed core (e.g. with an irregular non-homogenous cell structure) .

In yet another embodiment, the cellular stiffening structure comprises a lattice structure. The lattice structure may comprise a truss structure (e.g. simple truss structure or space frame) comprising a plurality of struts connected together to form a plurality of nodes. The truss structure may be coupled to the drive rod at a plurality of nodes. The truss structure may be coupled to the piston or the at least one member extending between the piston and the load transfer part at a further plurality of nodes. Depending upon the strength of the lattice structure, the lattice structure may or may not need to be provided as part of a sandwich structure.

The piston may comprise at least one piston aperture located on a working face of the piston and a valve for allowing gas to selectively pass through the piston.

The valve may be located on either an inside surface or outside surface of the piston depending upon the pressure and gas flow path.

In one embodiment, the valve is located on a side of the piston nearest the load transfer part (i.e. between the piston and the load transfer part). This arrangement is particularly suitable where there is a pressurised space between the load transfer and the piston and the working gas pressure on the piston is substantially equal to or less than the pressure in the pressurised space.

In another embodiment, the valve is located on a side of the piston furthest from the load transfer part (i.e. between the piston and a cylinder head of the piston assembly). This arrangement is particularly suitable where the working gas pressure is substantially equal to or greater than the pressure in the pressurised space.

In one embodiment, the valve may comprise a reed valve.

In another embodiment, the valve comprises a rotary slide valve and a linear slide valve. The piston assembly may further comprise a retaining element for restricting movement of a slide valve (e.g. to ensure substantially planar movement of the slide valve relative to the piston).

In one embodiment, activation of the valve is achieved by means of a mechanism that is located within the drive rod. In another embodiment, activation of the valve is achieved by means of a mechanism that is located wholly outside of the piston assembly.

The piston assembly may further comprise high and low pressure gas reservoirs configured to drive opening and closure of the valve located on the piston.

In one embodiment, the piston comprises a substantially flat sheet of material. The substantially flat sheet of material may have at least one aperture for coupling the piston to the load transfer part. In the case of a piston comprising at least one piston aperture for through piston valving, the substantially flat sheet of material may comprise at least one further aperture for allowing gas flow through the piston.

The piston may comprise a composite construction including a structural layer for carrying loads experienced by the piston and at least one further layer. In one embodiment, the at least one further layer is a wearing layer for protecting the structural layer from mechanical wear. For example, the wearing layer may be configured to provide a rubbing surface for a valve located on the piston to move against. In another embodiment, the at least one further layer is a thermal layer for protecting the structural layer from adverse temperatures

In yet another embodiment, the at least one further layer is a filler layer for reducing deadspace when the piston is fully extended in a corresponding piston cylinder. The piston may (non-exclusively) comprise metallic, ceramic or plastics material.

In one embodiment, the piston assembly further comprising a seal associated with the load transfer part for sealing a space on a first side of the load transfer part from a space on a second side of the load transfer part.

The piston assembly may be a single-acting piston assembly. In another embodiment, the piston assembly is a double-acting piston assembly comprising a further piston as previously defined coupled (e.g. directly or indirectly) to the drive rod at a location longitudinally spaced from the first-mentioned piston. For example, the further piston may be coupled to the drive rod at an opposed end of the drive rod to the first mentioned piston. The double-acting piston assembly may comprise a further load transfer part as previously defined for coupling the further piston to the drive rod.

The first-mentioned piston and further piston may define a pressurised inter-piston space (e.g. inter-piston chamber) therebetween.

In one embodiment, the pressure on the first-mentioned piston and the pressure on the further piston are each substantially equal to or less than the pressure in the inter-piston space. In this way, the drive rod will only see a predominantly zero or tensile load and can therefore be of slender, lightweight construction.

In another embodiment, the pressure on the first-mentioned piston is substantially equal to or less than the pressure in the inter-piston space and the pressure on the further piston is substantially equal to or greater than the pressure in the inter-piston space. In this way, the portion of the drive rod connected to the further piston only experiences a compressive load.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic cross-sectional view of a piston assembly in accordance with a first embodiment of the present invention;

Figure 2 is a schematic cross-sectional view of a piston assembly in accordance with a second embodiment of the present invention;

Figure 3 is a schematic cross-sectional view of a piston assembly in accordance with a third embodiment of the present invention;

Figure 4 is a schematic perspective view of a piston assembly in accordance with a fourth embodiment of the present invention;

Figure 5 is a schematic cross-sectional view of a piston assembly in accordance with a fifth embodiment of the present invention;

Figure 6 is a schematic cross-sectional view of a piston assembly in accordance with a sixth embodiment of the present invention; Figure 7 is a schematic cross-sectional view of a double-acting piston assembly in accordance with a seventh embodiment of the present invention; and

Figure 8 is a schematic cross-sectional view of a double-acting piston assembly in accordance with an eighth embodiment of the present invention.

Figure 1 shows a piston assembly 10 comprising: a cylindrical drive rod 20 including an end flange 20a; a retaining nut 21 for engaging a screw-threaded portion of drive rod 20; an annular piston seal housing 30; a plurality of connecting rods 40; a piston

50 having a circular cross-sectional profile; and a load transfer part (or collar) 60 comprising a substantially cylindrical body 60a enclosing drive rod 20 and including a cellular core 61 having a substantially cylindrical outer profile extending between opposed first and second outer layers 62. Spaced connecting rods 40 extend through and connect load transfer part 60 to piston 50. In turn, load transfer part 60 is clamped to drive rod 20 between end flange 201 and retaining nut 21.

Figure 2 shows another piston assembly 10' comprising: a cylindrical drive rod 20' including an end flange 20a'; a retaining nut 21 ' for engaging a screw-threaded portion of drive rod 20'; an annular piston seal housing 30'; a plurality of connecting rods 40'; a piston 50' having a circular cross-sectional profile; and a load transfer part 60' comprising a substantially cylindrical body 60a' enclosing drive rod 20' and including a cellular core 61' having a substantially cylindrical outer profile extending between opposed first and second outer layers 62'. Each of the first and second outer layers 62' reduces in thickness as the radial distance from the drive rod 20' increases in order to minimise the mass of the piston assembly 10'.

Figure 3 shows yet another piston assembly 10" comprising: a cylindrical drive rod 20" including an end flange 20a"; a retaining nut 21 " for engaging a screw-threaded portion of drive rod 20"; an annular piston seal housing 30", a plurality of connecting rods 40", a piston 50" having a circular cross-sectional profile; and a load transfer part 60" comprising a substantially cylindrical body 60a" enclosing drive rod 20" and including a cellular core 61 " having a substantially cylindrical outer profile extending between opposed first and second outer layers 62". Cellular Core 61 " reduces in thickness as the radial distance from the drive rod 20" increases in order to minimise the mass of the piston assembly 10".

Figure 4 shows a piston assembly 110 comprising: a cylindrical drive rod 120; a retaining nut (not shown) for engaging a screw-threaded portion of drive rod 120; an annular piston seal housing 130; a plurality of connecting rods 140; a piston 150 having a circular cross-sectional profile; a load transfer part 160 comprising a substantially cylindrical body 160a enclosing drive rod 120 and including a cellular core (not shown)having a substantially cylindrical outer profile extending between opposed first and second outer layers 162 and a localised strengthening element 163 for stiffening a radially innermost portion of the load transfer part 160;an internal piston seal housing 164; and valves 170.

The piston 150 has multiple ports (not shown) within the surface that allow gas to pass through the piston 150 when the valve 170 is in the open position. The first and second outer layers 162 are locally reinforced by the localised strengthening element 163 to cope with the high concentration of shear loads around the drive rod 120.

Figure 5 shows another piston assembly 210 comprising: a cylindrical drive rod 220 including an end flange 210a; a retaining nut 221 for engaging a screw-threaded part of drive rod 220; a load transfer part 260 comprising a substantially cylindrical body 260a enclosing drive rod 220 and including a cellular core 261 having a substantially cylindrical outer profile extending between opposed first and second outer layers 262; an annular piston seal housing 263; and a lightweight filler layer 264a surrounding end flange 210a forming a piston face 264b having a circular cross-sectional profile. The lightweight filler layer 264 is designed to minimise deadspace by being shaped in appropriate manner to match as much of the cylinder space as possible at the minimum volume point of each stroke, allowing for suitable clearances and practical issues. In addition the lightweight filler layer 264 can also act as a thermal barrier to protect the piston from high or low temperatures.

Figure 6 shows yet another piston assembly 210' comprising: a cylindrical drive rod 220' including an end flange 220a'; a retaining nut 221 ' for engaging a screw- threaded part of drive rod 220'; a load transfer part 260' comprising a substantially cylindrical body 260a' enclosing drive rod 220' and including a cellular core 261' having a substantially cylindrical outer profile extending between opposed first and second outer layers 262'; an annular piston seal housing 263'; and a lightweight filler layer 264a' surrounding end flange 220a' forming a piston face 264b' having a circular cross-sectional profile.. Cellular core 261 " and first and second outer layers 262' both reduce in thickness as the radial distance from drive rod 220' increases in order to minimise the mass of the piston 210'.

Figure 7 shows a double-acting piston assembly 310 comprising: a cylindrical drive rod 320 including an end flange 320a; a retaining nut 321 for engaging a screw-threaded part of drive rod 320; and a load transfer part 360 comprising a substantially cylindrical body 360a enclosing drive rod 320 and including a cellular core 361 having a substantially cylindrical outer profile extending between first and second outer layers 362; annular piston seal housing 363; and lightweight filler layers 364a surrounding end flange 320a to form a pair of piston faces 364b. Figure 8 shows a further double-acting piston assembly 310' comprising: a cylindrical drive rod 320' including an end flange 320a'; a retaining nut 321' for engaging a screw-threaded part of drive rod 320'; a load transfer part 360' comprising a substantially cylindrical body 360a' enclosing drive rod 320' and including a cellular core 361 ' having a substantially cylindrical outer profile extending between opposed first and second outer layers 362'; annular piston seal housing 363'; and lightweight filler layers 364a' surrounding end flange 320a' to foπn a pair of piston faces 364b'.

In each of the embodiments of Figures 1-8, the cellular core may comprise a honeycomb core, a foamed core or a lattice structure (e.g. a truss structure or space frame comprising a plurality of struts connected together to form a plurality of nodes). The innermost and outermost layers may comprise sheets or skins of relatively stiff material (e.g. relative to the material of the cellular core).