The present invention relates to a Stirling machine of the type comprising at least one group of four uniformly oriented cylinders positioned in a quadrangular arrangement, a double- acting piston displaceably arranged within each cylinder and dividing the cylinder into a warm and a cold cylinder cham¬ ber, respectively, the warm chamber of each cylinder being interconnected with the cold chamber of the succeeding cylin¬ der in a succession of cylinders along the perimeter of the quadrangular arrangement, and a crank mechanism common to all of the pistons and connected thereto such that the cyclical movement of each piston in the group is approximately 90° in advance of the movement of the preceding piston in said succession of cylinders.

A Stirling engine of the above type is disclosed in US patent No. 4,138,897. The known Stirling engine comprises a pair of parallel, co-rotating crankshafts, and a crosshead for re¬ ceiving transverse forces or side forces is associated with each cylinder. Furthermore, because of the two crankshafts it is necessary to provide a transmission mechanism to transmit the power from each of the crankshafts to a common power output shaft.

The present invention provides a Stirling machine as that described above having an improved crank mechanism which in a simple and efficient manner achieves one of the objects of the invention of allowing all the pistons to transmit their motive power to a single crankshaft and the cyclic movement of each piston in the group to be approximately 90° in advance of the preceding piston in said succession of pis- tons, while substantially eliminating transverse forces or side forces between the pistons and the associated cylinder surfaces and at the same time eliminating the need for a power transmission mechanism which in itself is a complicat¬ ing factor and, particularly in large machines, not very practical.

The Stirling machine according to the invention is characte¬ rized in that the crank mechanism comprises a single crank¬ shaft having a pair of longitudinally spaced, radially oppositely directed cranks and being journalled in a housing, a pair of yokes each pivotally- connected to one of the cranks at a first pivot and pivotally connected to a pair of piston rods of a pair of said pistons at second and third pivots which are substantially equally spaced from the first pivot, the first, second, and third pivots defining an essentially right-angled triangle having its right angle at the first pivot, the right-angled vertices defined at the first pivots of the two yokes being oppositely directed, and a laterally extending rocking lever connecting each yoke to said housing, the opposite ends of each rocking lever being pivotally connected to the yoke and to the housing, respectively. Thus, the Stirling machine according to the invention comprises at least one group of four cylinders operating with a phase difference of essentially 90° and a simple crank mechanism which does not apply any substantial transverse forces or side forces to the piston and which has a single crankshaft common to all of the cylinders. The extremely low piston side forces which may be obtained permit the use of unlubricated pistons.

So as to minimise the transverse movement of the piston rods each rocking lever is preferably connected to the yoke sub¬ stantially at the midpoint of the hypotenuse of the triangle.

The Stirling machine according to the invention may comprise four, eight, twelve or any other multiple of four cylinders which are aligned in two parallel, transversely spaced rows so as to define one or more quadrangular arrangement of cylinders. A combustion chamber common to a group of four cylinders may then be arranged within, above, below or adja¬ cent to each quadrangular arrangement of cylinders, and each combustion chamber may have walls defining therein closed gas spaces, the warm cylinder chamber of each cylinder of the

group communicating with one end region of at least one of said gas spaces, which at the opposite end region communi¬ cates with the cold cylinder chamber of the succeeding cylin¬ der through at least one heat regenerator and at least one cold space. The combustion chamber may, for example, be arranged within, above, below or adjacent the quadrangular arrangement of cylinders, whereby heat from the combustion chamber may effectively be transferred to the warm chambers of the cylinders.

The gas spaces communicating with the warm and cold cylinder chambers may be of any suitable shape and may, for example, be defined within hollow panels. In the preferred embodiment, however, each of the gas spaces is at least partly defined within a plurality of upright tubes. These tubes may communi- cate directly with the chambers of the associated cylinders. Preferably, however, the upright tubes communicate indirectly with the cylinder chambers. Thus, the end regions of said plurality of upright tubes may be interconnected by manifold tubes communicating with the warm and cold chambers of the cylinders.

The cylinders of the Stirling machine could in fact be arranged along a pair of non-parallel straight or curved lines. In such case the yokes associated with adjacent pairs of opposite cylinders would be different. In the preferred embodiment, however, the quadrangular arrangement is a rec¬ tangular arrangement so that the cylinders are arranged along a pair of mutually parallel lines and the yokes are ident¬ ically shaped.

The axes of the cylinders in each group of cylinders may extend in different directions. In the preferred embodiment, however, the axes of all of the cylinders in each group of cylinders are substantially parallel. The axes of the cylin¬ ders are preferably substantially vertical.

So as to further reduce the transverse or side forces on the pistons, the connection between the piston rods and the second and third pivots of the yokes may comprise a crosshead for each piston arranged in a cylinder coaxial with the piston cylinder and rigidly connected to the piston rod at a distance from the piston, the crossheads being pivotally connected to the yokes at the second and third pivots by crosshead rods pivotally attached to the crossheads. Hereby the movements of the second and third pivots in a transverse direction in relation to the axes of the piston cylinders do not cause transverse forces on the piston rods as these movements are taken up by the crosshead rods, the pivotal connection of which to the crossheads and the yokes being at a distance from the warm chambers of the piston cylinders and therefore not subject to detrimental thermal influence.

Preferably, the crosshead cylinders corresponding to the yoke having its second and third pivots arranged closer to the pistons than the first pivot are arranged farther away from the pistons than the second and third pivots, while the crosshead cylinders corresponding to the yoke having its second and third pivots arranged farther away from the pis¬ tons than the first pivot are arranged closer to the pistons than the second and third pivots.

In Stirling machines of the type in question the working medium in the chambers of the piston cylinder leaks past the sealing means between the piston rod and the aperture between the cylinder and the crank mechanism housing into said hous¬ ing giving rise to waste of working medium if it can escape to the surrounding environment wherefore said sealing means must be as efficient as possible. Therefore it is an advan¬ tage that the said housing is hermetically sealed. The pres¬ sure in the housing will build up to substantially the mean working pressure in the piston cylinder chambers because of said leakage and therefore the piston rod sealing means may be much simpler, as they will be subjected to a lower pres¬ sure differential and to dynamic pressure variations. The

housing can be hermetically sealed in a much simpler manner when the crank mechanism only comprises a single crankshaft than when two crankshafts with interconnecting means are involved. The mean pressure in the housing can be reduced to the minimum pressure in the cold cylinder chambers by intro¬ ducing at least one check valve between the cold cylinder chambers and the housing.

The working gas leakage building up a high pressure in the hermetically sealed housing will, however, particularly in large machines, entail expensive structural design of said housing. Therefore, preferably at least one of the crossheads and its corresponding cylinder constitute a compressor cylin¬ der for compressing the working gas of the Stirling machine leaking into the housing, the crosshead being provided with piston rings and the crosshead cylinder having a transverse closure plate defining a compression chamber in the cylinder together with the crosshead, the compression chamber communi¬ cating with the interior of the housing through an inlet valve and communicating with an exhaust conduit through an outlet valve and the exhaust conduit communicating with the exterior of the housing.

The operation of the crossheads can be replaced by grease lubricated linear needle or ball bearings reducing the trans¬ verse or side forces in the same manner as the crossheads. One of the piston rods being guided in such a linear guide bearing may be attached to a small compressor piston co¬ operating with a separate compressor cylinder to obtain the same function as the crosshead and crosshead cylinder consti¬ tuting the compressor.

Preferably the compressor cylinder is constituted by one of the crosshead cylinders arranged farther away from the pis¬ tons than the first and second pivots of its corresponding yoke, its transverse closure plate being arranged farther away from the pistons than its crosshead.

When the power output of the Stirling machine is controlled by the working gas mean pressure it is advantageous, when more power is needed, to increase the mean pressure from a lower level to a higher by communicating the exhaust conduit with the piston cylinder chambers. The- exhaust conduit may communicate selectively with the piston cylinder chambers or an intermediate buffer working gas container for storing the working gas exhausted from the housing when it is not needed to increase the mean pressure in piston cylinder chambers.

According to a further aspect the present invention also relates to a crank mechanism for a Stirling machine as that described above, and the crank mechanism according to the invention is characterized in comprising a single crankshaft having a pair of longitudinally spaced, radially oppositely directed cranks and adapted for being journalled in a hous¬ ing, a pair of yokes each pivotally connected to one of the cranks at a first pivot and adapted to be pivotally connected to a pair of piston rods at the second and third pivots which are substantially equally spaced from the first pivot, the first, second, and third pivots defining an essentially right-angled triangle having its right angle at the first pivot, the right-angled vertices defined at the first pivots of the two yokes being oppositely directed, and a laterally extending rocking lever for connecting each yoke to a hous¬ ing, the opposite ends of each rocking lever being adapted to be pivotally connected to the yoke and to the housing, res¬ pectively.

Preferably each rocking lever is connected to the yoke sub- stantially at the midpoint of the hypotenuse of the triangle.

Preferably the connection between the piston rods and the second and third pivots of the yokes comprises a crosshead for each piston arranged in a cylinder coaxial with the piston cylinder and rigidly connected to the piston rod at a distance from the piston, the crossheads being pivotally

connected to the yokes at the second and third pivots by crosshead rods pivotally attached to the crossheads.

Furthermore, preferably the crosshead cylinders corresponding to the yoke having its second and third pivots arranged closer to the pistons than the first pivot are arranged farther away from the pistons than the second and third pivots, while the crosshead cylinders corresponding to the yoke having its second and third pivots arranged farther away from the pistons than the first pivot are arranged closer to the pistons than the second and third pivots.

It is advantageous that said housing is hermetically sealed and that at least one of the crossheads and its corresponding cylinder constitute a compressor cylinder for compressing the working gas of the Stirling machine leaking into the housing past the piston rods, the crosshead being provided with piston seals and the crosshead cylinder having a transverse closure plate defining a compression chamber in the cylinder together with the crosshead, the compression chamber communi¬ cating with the interior of the housing through an inlet valve and communicating with an exhaust conduit through an outlet valve and the exhaust conduit communicating with the exterior of the housing.

The operation of the crossheads can be replaced by grease lubricated linear needle or ball bearings reducing the trans- verse or side forces in the same manner as the crossheads. One of the piston rods being guided in such a linear guide bearing may be attached to a small compressor piston co¬ operating with a separate compressor cylinder to obtain the same function as the crosshead and crosshead cylinder consti- tuting the compressor.

Preferably, the compressor cylinder is constituted by one of the crosshead cylinders arranged farther away from the pis¬ tons than the first and second pivots of its corresponding

yoke, its transverse closure plate being arranged farther away from the pistons than its crosshead.

It is advantageous that the exhaust conduit communicates with the piston cylinder chambers. The exhaust conduit may com- municate selectively with the piston cylinder chambers or an intermediate buffer working gas container for storing the working gas exhausted from the housing.

The invention will now, by way of example, be further described with reference to the drawings, wherein

Fig. 1 is a diagrammatic top plan view of the cylinder arrangement and the heating arrangement of a double-action Stirling engine according to the invention,

Fig. 2 is a diagrammatic perspective view showing the prin¬ ciple of the Stirling engine according to the invention,

Fig. 3 is a series of four diagrammatic sectional views along lines A-A and B-B, respectively, in Fig. 1 of cylinder row 1 and cylinder row 2 of a Stirling engine according to the invention, where the crank mechanism and pistons are shown in four different positions corresponding to successive 90° angular displacements of the crankshaft,

Fig. 4 is a diagrammatic perspective view of the inter¬ communication between the cylinders of the Stirling engine according to the invention,

Fig. 5 is a partially sectional view of an embodiment of a Stirling engine according to the invention, showing cylinder row l and its associated yoke,

Fig. 6 is a view corresponding to the view in Fig. 5, showing cylinder row 2 and its associated yoke,

Fig. 7 is a partially sectional view of the crank mechanism housing of cylinder row 1 of another embodiment of a Stirling engine according to the invention very similar to the one shown in Figs. 5 and 6, and

Fig. 8 is an enlarged view of the portion of Fig. 7 encompassed by the circle.

The double-action Stirling engine shown in Figs. 1-4 com¬ prises four cylinders 10 arranged in two pairs or rows (cyl¬ inder row 1 being to the left and cylinder row 2 being to the right in Figs. 2 and 3) and having substantially vertically extending parallel axes and being placed in a rectangular arrangement as best shown in Figs 1, 2 and 4. A combustion chamber 11 is defined by heater elements 12 forming combus¬ tion chamber walls. Each heater element comprises a row of vertically extending tubes 13 and a substantially hori¬ zontally extending manifold conduit 14 interconnecting the inner spaces of the vertical tubes 13 in each row and com¬ municating with the warm cylinder chamber 15 of the res¬ pective cylinders 10. The cold cylinder chambers 16 (Fig. 4) of the respective cylinders 10 communicate with manifold conduits 17 communicating with cooling elements 18 in turn communicating with the heater elements 12 through heat re¬ generators 19. The overall orientation of the Stirling engine may be as shown or upside down or at any angle suitable for the application of the engine.

Referring now to Figs. 2 and 3, each cylinder 10 contains a piston 20 having a piston rod 21 pivotally attached to and extending from the piston 20 into a housing 22 for a crank mechanism. Each piston rod 21 is arranged within a sealing tube or sleeve 23 fixedly attached to the piston 20 and extending through and sealingly slidable in an aperture in the partition wall between the cylinder 10 and the crank mechanism housing 22. The crank mechanism comprises a single crank shaft 24 common to all four piston rods 21 and having two radially oppositely directed cranks 25 and 26. The crank

shaft 24 is journalled in the housing 22 as indicated at 27 and 28.

The cranks 25 and 26 are pivotally attached to yokes 29 and 30, respectively, by means of pivots 31 and 32, respectively. The yokes 29 and 30 are pivotally attached to the respective piston rods 21 by means of pivots 33, 34 and 35, 36, respect¬ ively. The set of pivots 31, 33 and 34 and the set of pivots 32, 35 and 36 each define a right-angled, equilateral tri¬ angle having oppositely directed apexes defined by the pivots 31 and 32, respectively. The yokes 29 and 30 may have any suitable shape allowing such an arrangement of said sets of pivots 31, 33, 34 and 32, 35, 36, respectively.

The yokes 29 and 30 are pivotally attached to rocking levers 37 and 38, respectively, by means of pivots 39 and 40, re- spectively. The pivots 39 and 40 are each positioned substan¬ tially midway on a straight line connecting the pivots 33 and 34 and the pivots 35 and 36, respectively. The rocking levers 37 and 38 are pivotally attached to the wall of the housing 22 by means of pivots 41 and 42, respectively.

During operation of the Stirling engine shown in Figs. 1-4, the heater element tubes 13 are heated by combustion of for instance wood chips in the combustion chamber and the Stir¬ ling process takes place in the Stirling engine in a well known manner. The quadrangular configuration with a single crankshaft allows short manifold conduits 14 and 17 which reduces thermal stress.

The stroke of each piston 20 is cyclically phased 90° with respect to the strokes of the succeeding and preceding pis¬ tons 20 of the adjacent cylinders 10 in the quadrangle. This is illustrated in Fig. 3 where the pistons 20 of cylinder row 1 and cylinder row 2 with associated piston rods, yokes, rocker levers and the crankshaft are shown in four consecu¬ tive positions from top to bottom corresponding to quarter revolutions starting from 0 revolution and ending with 3/4

revolution. In the 0 revolution position illustrated at the top of Fig. 3 the leftmost piston is in its top position, the middle left piston is in its ascending middle position, the rightmost piston is in its lowest position and the middle right piston is in its descending middle position, each of said pistons 20 thus being cyclically phased by 90° with respect to the succeeding and preceding piston in this sequence. This is achieved in a simple manner by means of the arrangement of the attachment points of the piston rods 21 to the yokes 29 and 30 and to the attachment points of said yokes to the cranks 25 and 26, respectively, of the crank¬ shaft 24.

It will be appreciated that two or more such quadrangles of cylinders may be arranged beside each other having a common crank shaft and crank mechanism housing, the phases of the cylinders of the various quadrangles being such that an optimal balancing of the common crankshaft is obtained.

Referring now to Figs. 5 - 7, there is shown a Stirling engine having four cylinders 50 arranged in two cylinder rows (cylinder row 1 shown in Figs. 5 and 7 and cylinder row 2 shown in Fig. 6) forming a quadrangle, each cylinder 50 having a piston (not shown) fixedly attached to a piston rod 51. The warm chambers of the cylinders 50 communicate by means of manifold conduits 52 and 53 with heater elements comprising tubes or risers 54 heated by radiation and fin elements 55 heated by convection, the heat originating from combustion of for example wood chips in the quadrangular combustion space defined between the heater elements. A cooler element 56 is arranged around the four cylinders 50 and communicates with the cold chambers of the cylinders 50 by means of manifold conduits (not shown) , the cooling water for the cooler elements being circulated through conduits 57.

A hermetically sealed crank mechanism housing 58 common to the four cylinders 50 rotatably accommodates a single, common crankshaft 59 having two oppositely directed cranks 60 and

61, corresponding to cylinder row 1 and 2, respectively. The cranks 60 and 61 constitute pivots for pivotally attaching yokes 62 and 63, respectively, to the crankshaft 59, which yokes 62 and 63 are pivotally attached to crosshead rods 64, 65 and 66, 67, respectively, by means of pivots 68, 69 and 70, 71, respectively. The crosshead rod 64 is shown in its entirety in Fig. 7. The crosshead rods 64-67 are pivotally attached to respective crossheads (not shown, except for the crosshead 72 in Fig. 7 attached to the crosshead rod 64 by means of a pivot 73) each slidably arranged in crosshead cylinders 74, 75, 76 and 77, respectively, fixedly attached to the housing 58. The crossheads are fixedly attached to the piston rods 51 either directly as in cylinder row 2 in Fig. 6 or via a fork shaped, rigid connector 51a as in cylinder row 2 in Figs. 5 and 7.

The set of pivots 60, 68 and 69 and the set of pivots 61, 70 and 71 each define a right-angled, equilateral triangle having oppositely directed apexes defined by the pivots or cranks 60 and 61, respectively.

The yokes 62 and 63 are pivotally attached to rocking levers 78 and 79, respectively, by means of pivots 80 and 81, res¬ pectively. The rocking levers 78 and 79 are pivotally attached to the housing 58 by means of pivots 82 and 83, respectively.

The operation of the Stirling engine of Figs. 5 - 7 is sub¬ stantially the same as described with reference to Figs. 1 - 3 with the exception of the influence of the movement of the yokes on the movement of the piston rods. The transverse movement of the piston rods caused by the non linear movement of the pivots connecting the yokes to the piston rods is eliminated by said non linear movement being taken up by the crosshead rods 64 - 67.

Referring now to Figs. 7 - 8, the crosshead 72 in the crosshead cylinder 74 is provided with a piston ring 85, and

the bottom of the cylinder 74 is closed by means of a trans¬ verse plate 86 provided with two apertures 87 and 88 serving as exhaust and inlet openings, respectively. A valve body 89 is seated in the aperture 87 biased in the opposite direction of -arrow D by a pressure spring 91. A valve body 90 is seated in the aperture 88 biased in the opposite direction of arrow E by a traction spring 92. The aperture 87 communicates the space between the crosshead 73 and the transverse plate 86 with an exhaust conduit 93 extending through the wall of the housing 58. The aperture 88 communicates the space between the crosshead 73 and the transverse plate 86 with the interior of the housing 58.

In operation, the working gas (e.g. helium) leaks from the working chambers of the cylinders 50 past the sealing means between the piston rods 51 and the corresponding apertures in the partition wall between the cylinders 50 and the housing 58 and leaks into the housing 50 thereby building up the pressure therein. The crosshead 73 acts as a piston in a compressor and sucks working gas through the aperture 88, past the valve body 90, against the bias of the spring 92 and into the cylinder 74 during its ascending stroke. During the descending stroke, the crosshead expels working gas through the aperture 87, past the valve body 89, against the bias of the spring 91 and through the exhaust conduit 93 to the exterior of the housing 58.

The pressure of the working gas in the housing 58 is thus maintained lower than the mean working gas pressure in the working chambers of the cylinders 50 thereby reducing the structural stresses on the housing and allowing a lighter and simpler construction thereof and a simpler construction of the sealing means between the housing 58 and an extension of the crankshaft 59 connected to generator means outside the housing in case the generator means are not located inside the housing. The exhaust conduit 93 may communicate with an intermediate working gas buffer tank for replenishing the working gas charge of the cylinders 50 or it may communicate

directly with the working chambers of the cylinders 50. The compression ratio of the compressor is chosen such that a pressure above the maximum working pressure of the cylinders 50 is achieved in the exhaust conduit 93 while maintaining a pressure as low as possible inside the housing 58. Thus the working gas exhausted from the housing can be used to regu¬ late the charge of working gas in the cylinders 50 and there¬ by the power output of the Stirling engine without needing any supplementary compressor. For example, when working with helium as working gas and Stirling engines with power outputs of 40 kW and 150 kW, the compressor comprising the crosshead 73 and the cylinder 74 increases the helium pressure from 5 - 10 bar to approx. 40 bar.

The principles of the invention may be applied to all types of Stirling machines of the type in question and they may be applied to other piston machines utilizing four cylinders and where a 90° phase difference is desirable, for instance four or eight cylinder two-stroke internal combustion engines.

In the above mentioned 40 kW and 150 kW Stirling engines the engines are arranged upside down with the combustion chamber for burning wood chips arranged below the cylinders and the engines being part of a combined district heating and elec¬ tricity generation installation. The sizes of the Stirling machines in which the invention can be embodied is not limited to the above range as the principles can be applied to all sizes of such machines.