GB2585627A - A planar mechanism affording improved working fluid utilisation in the stirling engine - Google Patents

Abstract

A planar mechanism comprising a crankshaft 1 crank pin 2, which may comprise an eccentric sheave, ternary links or coupler rods 3, 4 connected to binary links 7, 8 via points 5 and 6. The binary links are rotatably grounded to the machine at points 9 and 10. Couper points 11 and 12 describe more complex paths of movement (Fig.2) and the crank pin 2 may comprise an eccentric sheave (3, Fig.3) capable of forming a two-throw crankshaft. The mechanism is capable of coupling the rotary motion of a crankshaft with the respective collinear motions of the displacer and work piston of a coaxial Stirling engine in such a way as to permit the working fluid charge to be largely excluded from the compression space during the expansion phase and to be largely excluded from the compression space during the expansion phase.

Description

DESCRIPTION

The invention belongs in the category of planar mechanisms. It applies to engines which convert heat to work via the Stirling thermodynamic cycle and in particular to those variants of the genre in which work-piston and displacer reciprocate along a common axis -the so-called coaxial configuration.

The Stirling engine is routinely credited with high thermal efficiency, quietness of operation and simplicity -and thus affordability. Conversely, a Stirling engine combining all of these attributes with competitive power output has yet to be demonstrated. The present invention is directed at removing one of the obstacles to achieving the unfulfilled expectation.

Regardless of geometric configuration, all Stirling engines are reciprocating heat engines. Comparison with other such engines is in terms of specific cycle work defined as work per cycle per swept volume per unit of charge-gas pressure. Numerical values for the Stirling engine are typically between one and two percent of those for internal combustion engines of comparable swept volume.

A reason for the unfavourable comparison is the poor working-fluid utilisation typical of the genre: only a small percentage of the working fluid charge -if any -is processed through a complete cycle between the temperature extremes. By contrast, the internal combustion engine processes the majority.

The unsatisfactory utilisation is a result of the means by which the Stirling engine achieves the cyclic heat addition and rejection phases essential to the functioning of any heat-engine. This is by the respective cyclic variations in two internal volumes defined by work piston and displacer. These variations bias the charge cyclically between a containing surface which is maintained at low temperature by external cooling, and a separate containing surface which is maintained at high temperature by external heat supply.

To date, the accepted way of improving power output from a Stirling engine of given swept volume has been to increase the pressure of the charge gas. If the design of the gas-path is appropriate, the measure is effective, but it adds complexity and cost while sacrificing the simplicity of the original inventor's concept.

The measure is resorted to because crank mechanisms disclosed to date fail to achieve the target function, which is to confine the working-fluid charge to the cooled space during the compression phase, and to the heated space for the expansion phase.

That target is not fully attainable, but the present Invention makes possible a closer approximation than has hitherto been demonstrated.

The invention will now be described by reference to Figure 1 and with the aid of the vernacular of the formal study of planar mechanisms.

Figure 1 is constructed on the assumption that the engine operates with cylinder axis vertical, in other words, with the heat-input end uppermost. The Invention applies without modification to any other orientation of the coaxial configuration.

The figure shows crankshaft 1 viewed along the journal axis. The motions of all elements lie in the plane of the drawings, or parallel to that plane. Ternary links, or coupler rods, 3 and 4 are both rotatably connected to common crank-pin 2 and adapted at respective points 5 and 6 for rotatable connection to binary links 7 and 8 respectively. The latter are rotatably grounded to the frame of the machine at points 9 and 10 respectively. Ternary link 3 embodies coupler point 11. Ternary link 4 embodies coupler point 12. The arrangement constrains points 5 and 6 -although not coupler points 11 and 12 -to arcuate motion. The motions of points 11 and 12 crucially describe more complex paths.

Figure 2 employs stylised tokens from the study of planar mechanisms to represent the four basic possibilities achievable with the same component inventory. Each achieves the basic angular phase displacement between components of vertical motion of respective coupler points 11 and 12 as required for functioning as an engine. However, each combination yields its own unique set of higher harmonics, each set being independently variable.

Further flexibility is available in terms of wholesale re-location of coupler points 11 and 12.

Returning to Figure 1, elements 1, 3 and 7 together with the frame of the machine form a kinematic chain, specifically a 4-bar chain. Elements 1, 4 and 8, together with the frame of the machine, form another. In the classic study of planar mechanisms a 4-bar kinematic chain is said to have a single degree of freedom (see, for example, the treatment by Johnson*). The term gives a misleading impression of the flexibilities available to the designer wishing to tailor the volume variations to the thermodynamic requirement of the engine.

The linkage affords yet further flexibility through modification of the crank-pin function as illustrated separately at Figure 3 to make for a two-throw crankshaft. The upper diagram shows a unit fabricated from two sub-components forming an interference fit, namely, web 1 integral with main journal, and compound crank-pin 2 embodying eccentric sheave 3. The lower diagram suggests an alternative method of achieving the same kinematic outcome: a crankshaft in which the crank-pin is machined integral with journal and web, eccentric sheave 4 being by any suitable means secured to the crank-pin. Both arrangements permit assembly of coupler link 3, or of coupler link 4 in parallel duplex form, in which case it is the work-piston which will most appropriately be driven via the duplex coupler, while the more lightly-loaded coupler 4 is driven from the sheave. The enhanced control over the thermo-Johnson R Mechanical Design Synthesis van Nostrand Reinhold 1971 dynamic processes is in terms of the supplementary angular phase displacement and amplitude-difference possible between actuation of couplers 3 and 4 at crank-pin 2.

With or without the embodiment of the sheave, rotation of crankshaft 1 causes points 5 and 6 to perform near-linear oscillations, respective peaks of travel being separated by an angular phase difference. The longer the length specified for links 7 and 8 the more closely do the vertical excursions approximate the purely linear. In the limit they are indistinguishable from the respective motions of the well-known Rhombic Drive having the same ratio of coupler-rod centre distances to crank throw and the same perpendicular distance between crankshaft axis and vertical axis of reciprocation. Those familiar with the technology of the Stirling engine will notice that the present Invention achieves this feature without the need for the synchronising gear wheels or the opposed, mirror-image components of the Rhombic Drive.

The means whereby the vertical component of the motion of pivot-point 11 is imparted to work piston 13 does not form part of this invention but may be either direct or indirect via a cross-head. In either case, the connection may be by any preferred means, for example by a rigid binary link, by a flexure member or by a slider operating in a horizontal slot substantially perpendicular to the cylinder axis.

Displacer drive rod 14 is by some means constrained to linear motion along its vertical axis. The means whereby motion is imparted by coupler point 12 likewise does not form part of the invention, but as in the case of the work piston, may involve use of a rigid binary link, of a flexure member or of a sliding element.

In a practical embodiment of the invention, links 7 and 8 are of finite length. This and the fact that the siting of pivot points 9 and 10 is open to a degree of choice are responsible for much of the kinematic flexibility inherent in the mechanism. That flexibility allows the designer to modify the basic phase angle between piston and displacer motions by the addition of higher harmonic components.

In order for the Invention to aid achievement of the full potential of a Stirling engine, the designer must be able to embody the linear and angular dimensions indicated by due experiment aided by computation, the latter probably in the form of computer simulation. However, some dimensions, may not be open to free choice. In particular, the location of pivot points Q and Q' may be constrained by the choice of crank-case -open or enclosed. On this basis, dimensions and proportions suggeSted by Figures 1, 2 and 3 should not be used to guide design of any specific embodiment.

Claims (8)

1. CLAIMS1 A planar mechanism for actuating the respective motions of displacer and work piston of a coaxial Stirling engine in such a way that the working gas inventory is biased towards the variable-volume low-temperature space in advance of the compression phase and towards the variable-volume high-temperature space in advance of the expansion phase and the intermediate displacement phases are executed with overall volume maintained substantially constant.
2. 2 A mechanism as described in foregoing Claim 1 in which a basic angular phase-difference between the motions of displacer and work piston is achieved by separating the axis of the cylinder from that of the crankshaft journal by a perpendicular offset comparable in magnitude to the offset of the axis of the crank-pin from the axis of the crank-journal.
3. 3 A mechanism as described in foregoing Claims 1 and 2 in which a ternary link or coupler is free to pivot about the crank-pin and offers two further pivot points at either one of which attaches a binary link rotatably grounded at its far end to the engine frame and in which a second coupler is free to pivot about the same crank pin and also offers two pivot points either one of which connects pivotally to a binary link grounded rotatably to the frame of the machine.
4. 4 A mechanism as described in foregoing Claims 1, 2 and 3 in which one of the uncommitted ternary coupler points is by any means adapted to control the cyclic linear motion of the work piston while the other is by any means adapted to control the cyclic linear motion of the displacer.
5. A mechanism as described in foregoing Claims 1, 2, 3, and 4 in which the crankshaft is a two-throw crankshaft achieved by embodying in the crank-pin two cylindrical features, each defined by its unique respective radial offset, unique angular offset, or unique combination of both offsets.
6. 6 A mechanism as described in foregoing Claims 1, 2, 3, 4 and 5 in which the proportions and dispositions of crankshaft and links can be chosen with the aid as necessary of computer simulation and/or prior experiment so as to yield cyclic variations in heated and cooled volumes such that working fluid is substantially excluded from the variable-volume heated space during overall volume decrease, and substantially excluded from the variable-volume compression space during overall volume increase.
7. 7 A mechanism as described in foregoing claims 1, 2, 3, 4, 5 and 6 in which one or more of the pivots indicated in Figure 1 as cylindrical pins rotating in cylindrical bushes is replaced by a rolling-element bearing.
8. 8 A mechanism as described in foregoing claims 1, 2, 3, 4, 5, 6 and 7 in which one or both of pivots 9, 10 is a spherical bearing.