US3644069A

US3644069A - Rotary engine construction

Info

Publication number: US3644069A
Application number: US848989A
Authority: US
Inventors: George R Stewart
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-08-11
Filing date: 1969-08-11
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22

Abstract

A rotary internal combustion engine including a toroidal housing having an annular cylinder formed therein, provided with spaced inlet and exhaust ports; a plurality of drive pistons equally angularly spaced in the cylinder and fixed through a drive disk to an output shaft; compression pistons in the cylinder between successive drive pistons, the compression pistons being fixed through a compression disk to a tubular countershaft coaxial with the output shaft; means for oscillatorily moving the countershaft and compression pistons during rotation of the output shaft and countershaft, including a raceway and rollers cammingly rollable in the raceway, the rollers being linkingly connected to the output shaft and to countershaft; sealing ring assemblies between the disks and between each disk and the housing, each ring assembly comprising two interengaged relatively rotatable rings, each ring having an outer side lying in a plane normal to the ring axis, an inner side describing a single turn helix about said axis, and a juncture portion having an axial length equal to the pitch of the helix, the rings being assembled with their inner helical sides in slidable juxtaposition, the ring assembly including resilient means for biasing apart the two juncture portions whereby to tend to increase the effective overall width of a ring assembly; means to minimize leakage and blowby of compressed gases past the pistons during operation, including turbulence-inducing elements carried by the pistons adjacent the edges of the piston faces, and turbulence-inducing configurations such as indentations on the walls of pistons, cylinder, or both, and on adjacent surfaces of other relatively movable members between which sealing is necessary, including radial surfaces of the drive and compression disks and adjacent radial surfaces of the housing; liners or coatings on piston and cylinder walls of material such as asbestos, fused silica and pyrolitic boron nitride having low-thermal conductivity, low coefficient of thermal expansion, and high-temperature strength to improve combustion efficiency.

Description

United States Patent Stewart Feb. 22, 1972 [54] ROTARY ENGINE CONSTRUCTION George R. Stewart, 10049 Sideview Drive, Downey, Calif. 90240 [22] Filed: Aug. 11, 1969 [21] Appl. No.: 848,989

[72] Inventor:

[52] U.S.Cl ..418/33,418/141,418/143,

Y 418/179, 123/18A [51] Int. Cl. ..F0lc 19/00, F040 27/00, F02b 53/00 [58] Field ofSearch ..4l8/3338, 141,

418/104,l12,l40,142,143,144, 179; 103/129, 126 S; 230/144; 123/11, 11 A, 11 B, 8 SS, 18 A, 91/60; 277/53, 55, 135 J, 151, 193

Primary Examiner-William L. Freeh Assistant Examiner-John J. Vrablik Att0meyMiketta, Glenny, Poms and Smith [57] ABSTRACT A rotary internal combustion engine including a toroidal housing having an annular cylinder formed therein, provided with spaced inlet and exhaust ports; a plurality of drive pistons equally angularly spaced in the cylinder and fixed through a drive disk to an output shaft; compression pistons in the cylinder between successive drive pistons, the compression pistons being fixed through a compression disk to a tubular countershaft coaxial with the output shaft; means for oscillatorily moving the countershaft and compression pistons during rotation of the output shaft and countershaft, including a raceway and rollers cammingly rollable in the raceway, the rollers being linkingly connected to the output shaft and to countershaft; sealing ring assemblies between the disks and between each disk and the housing, each ring assembly comprising two interengaged relatively rotatable rings, each ring having an outer side lying in a plane normal to the ring axis, an inner side describing a single turn helix about said axis, and a juncture portion having an axial length equal to the pitch of the helix, the rings being assembled with their inner helical sides in slidable juxtaposition, the ring assembly including resilient means for biasing apart the two juncture portions whereby to tend to increase the efi'ective overall width of a ring assembly; means to minimize leakage and blowby of compressed gases past the pistons during operation, including turbulence-inducing elements carried by the pistons adjacent the edges of the piston faces, and turbulence-inducing configurations such as indentations on the walls of pistons, cylinder, or both, and on adjacent surfaces of other relatively movable members between which sealing is necessary, including radial surfaces of the drive and compression disks and adjacent radial surfaces of the housing; liners or coatings on piston and cylinder walls of material such as asbestos, fused silica and pyrolitic boron nitride having low-thermal conductivity, low coefficient of thermal expansion, and high-temperature strength to improve combustion efficiency.

5 Claims, 11 Drawing Figures III I. ll 1 a0 III/Ill PATENTEDFEBZZ 1972 3,644,069

SHEET 3 or 4 f/vvE/vroe. 050865 B 5 7514/9 12 7' firrale/vew'.

PATENTEDFEB22 m2 SHEET b 0F 4 M a Z in 75 1 N 2 m Mn N. 7 IR E e w. G

ROTARY ENGINE CONSTRUCTION BACKGROUND AND SUMMARY OF THE INVEN- TION Rotary internal combustion engines have a number of important advantages in terms of efficiency, compactness, lightness in weight and simplicity of construction, but such engines have not been widely used in the past. Certain problems exist in these engines and, despite many attempts in the past, the problems have not been satisfactorily solved.

For example, adequate sealing means have been lacking in these engines, in order to minimize blowby of the gases past the piston, from a chamber which is instantaneously undergoing the power stroke to adjacent chambers. Another aspect of the sealing problem in rotary engines relates to the sealing between the disks on which the pistons are mounted, and between each of such disks and the housing.

In accordance with a preferred form of the present invention hereinafter illustrated and described in detail, there are provided means and configurations by which to form improved sealing between the pistons and the annular cylinder in which they are housed, such sealing being particularly effective at high speeds of relative movement between the pistons and the cylinder walls. Such sealing means may take the form of turbulence producing elements carried by the piston faces adjacent the cylinder walls; and the walls of the pistons, as well as the inner surface of the cylinder walls, may be provided with a roughened configuration characterized by a multiplicity of surface irregularities such as indentations, concavities or depressions creating turbulence by which to minimize the flow of gases past the piston, from one face of the piston to the opposite face. Moreover, liners or coatings of a roughened material such as asbestos, fused silica, and pyrolitic boron nitride may be permanently affixed to the piston and cylinder walls, not only to minimize heat loss and thereby improve efficiency, but also to create turbulence in gas flow past the pistons and thereby enhance sealing.

In a rotary engine having pistons carried rigidly on one or more supporting disks, no contact between the pistons and the cylinder wall is needed or is even desirable. In this respect, the rotary engine of the present type differs basically from the typical internal combustion engine having a plurality of pistons connected through connecting rods to eccentric bearings of a crankshaft. In this latter situation, the forces acting on the pistons include nonaxial vectorial components urging the pistons laterally into contact with the cylinder walls. Under these conditions, it is obvious that the piston and cylinder walls must be very smooth in order to minimize friction during operation, and this requirement of smoothness virtually precludes use of the present invention in such conventional engines, insofar as the roughened configuration is concerned.

Accordingly, the principal object of the present invention is to disclose and provide novel improvements in rotary engines. More specifically, it is an object of the present invention to provide, in the type of rotary engine including a plurality of drive pistons and compression pistons in alternating relation in an annular cylinder, improved sealing means to minimize gas leakage or blowby; to provide liners or coatings on the piston or cylinder walls or both in a rotary engine of the type referred to, by which to substantially decrease heat loss from the engine and thereby increase its efficiency; to provide improved sealing ring construction, particularly applicable to sealing between mutually movable disks on which pistons are mounted, and between such disks and the engine housing; and for other and additional related objects and purposes as will be understood from a study of the following description of a preferred embodiment of the invention taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS HO. 1 is a vertical sectional view showing a preferred embodiment of the invention.

FIG. 2 is a sectional view on line II--II of FIG. 1.

FIG. 3 is a fragmentary sectional view, on an enlarged scale, taken on the broken line III-III of FIG. 2.

FIG. 4 is a fragmentary sectional view, on an enlarged scale, of the area encircled at IV in FIG. 2.

FIG. 5 is a view similar to FIG. 4, but showing a modified construction of the rings carried by the piston for generating turbulence.

FIG. 6 is a fragmentary sectional view, on an enlarged scale, showing a modified form of turbulence-inducing configuration on the surface of a piston, including adjacent portions of the annular cylinder.

FIG. 7 is a perspective view of a sealing ring assembly in accordance with the invention;

FIG. 8 is a fragmentary sectional view, on an enlarged scale, taken on line VII-VIII of FIG. 7.

FIG. 9 is a fragmentary sectional view of a portion of one of the pistons and the adjacent portion of the cylinder wall, showing an alternative means for providing turbulence and heat sealing, including a thin layer on the piston and on the cylinder wall of a material such as asbestos or the like.

FIG. 10 is a fragmentary vertical sectional view of the upper portion of an engine, wherein the sealing between the disks or flanges supporting the pistons and the adjacent portions of the housing shells is achieved by indentations formed in such surfaces.

FIG. 11 is a fragmentary sectional view taken on line XI- XI of FIG. 10.

DETAILED DESCRIPTION The principal features and characteristics of the present invention may be incorporated in rotary power-producing machines having chambers of cyclically varying volumes, including engines such as internal combustion, steam and compressed air engines, both air cooled and liquid cooled, as well as in comparable power-operated machines such as blowers, pumps and compressors. The specific exemplary embodiment of the invention selected for illustration and description herein is a liquid-cooled intemal'combustion engine, and the general construction of an engine in accordance with the present invention will be understood by reference to FIGS. 1 and 2.

As there shown, and with reference first to FIG. 1, an engine indicated generally at 10 includes a generally annular housing made up of mating shell members indicated generally at 12 and 14, each provided with a

peripheral flange

13 and 15 respectively, the shells being retained in assembled relation by suitable fastening members as shown at 16, extending through a number of arcuately spaced openings formed in the flanges I3 and 15. It may be desirable to provide cooling means of conventional arrangement in the present engine, and such means are here shown as including, in each of the housing shells l2 and 14, a generally annular jacket for coolant, such as water, the jackets being indicated generally at 18 and 19 respectively. Coolant circulation means are provided in communication with the jacket, including coolant means 20 and outlet means 21. A series of angularly spaced

openings

22 and 23 are formed in the inner walls of the

respective shells

12 and 14, the opening being in registration with one another, in order to provide fluid communication between the jackets l8 and 19. One of the shells, as 12, may be provided with an opening for receiving therein a conventional spark plug indicated generally at 25.

Within the housing formed by mating shells l2 and 14 there is provided an annular cylinder indicated generally at 30, and formed by annular recesses in each of the two housing shells, each of the recesses being semicircular as seen in section and disposed so that, when the shells l2 and 14 are assembled, the recesses are in registration to form a cylinder having a circular section.

Each of the shells l2 and 14 is centrally apertured, and is provided with antifriction bearing means indicated generally at 31 and 32 respectively, for supporting a centrally disposed shaft indicated generally at 33. Rotatably mounted on shaft 33, within the housing 10, is a countershaft indicated generally at 35, there being a sleeve bearing 36 between shaft 33 and countershaft 35. It will be noted that countershaft 35 is relatively short, and at its inner end, or left end as seen in FIG. 1, it is provided with an upwardly extending annular disk or flange 38. As will be best understood by reference to FIG. 3, shaft 33 is also provided with an outwardly extending disk or flange 40, each of the flanges 38 and 40 being fixed to its respective shaft 33 and countershaft 35.

Within annular cylinder 30 are mounted two pairs of pistons, including a pair of pistons herein denominated as d4ive pistons, and a pair of compression" pistons. With reference to FIG. 2, the drive pistons are indicated generally at 42 and 44, and the compression pistons are indicated generally at 46 and 48. It will be noted that the pistons are arranged in alternating relation, that is, each compression piston is between two drive pistons. The pistons of each pair are diametrically opposite one another.

Conventional means are provided in the present engine for imparting oscillatory rotation to countershaft 35 and

compression pistons

46 and 48 fixed thereto, during rotation of shaft 33 and

drive pistons

42 and 44 fixed to that shaft. In the present embodiment of the invention, such means are indicated generally at 50 and are well known in the art, as illustrated for example by the Gardner U.S. Pat. No. 2,147,290, as well as by the Stewart U.S. Pat. No. 3,292,602.

During operation of the rotary engine of the present invention, and with further reference to FIG. 2, an explosive mixture of suitable gas and air is introduced into the cylinder through an inlet port 52. The general motion of the pistons in FIG. 2 is assumed to be counterclockwise. The mixture is thus impressed during the compression phase of the cycle, and the power stroke commences with the firing of the mixture by the spark plug 25, imparting power to the pistons by forcing them apart, and the burned gases are exhausted through port 54.

The elements, mechanisms and sequence of operation thus far described are conventional and well known in the rotary engine art. In accordance with the present invention, the operative surfaces of the pistons and the cylinder wall are configured to induce turbulence and thereby fluid friction to the gases seeking to pass from one face of a piston to the other. Otherwise stated, the present invention does not contemplate the use of sealing rings to maintain pressure between adjacent pistons in the cylinder, but rather relies upon the friction induced by the turbulence resulting from the configurations of the piston and cylinder walls.

One form of arrangement for inducing such turbulence will be understood by reference to the fragmentary view of FIG. 4, showing on a greatly enlarged scale the adjacent portions of piston 46 and the cylinder wall.

The construction as shown in FIG. 4 includes the provision of an annular member surrounding the face of piston 46 and spaced slightly therefrom. Thus, the periphery of the face of piston 46 may be recessed, as at 58, and a ring 60 is disposed in the recessed portion and is fixedly mounted to piston 46 by means permitting gas flow around the ring 60. For example, as seen in FIG. 4, such support means for the ring may include a plurality of angularly spaced bars or rods, one of which is indicated at 62, each such rod having its innermost portion received in and fixed to the piston itself, with the outermost portion fixed to the ring 60. The number of rods 62 for a particular apparatus will be determined by strength and related requirements, but typically approximately eight or of such rods, equally angularly spaced about the periphery of the face of piston 46, will be sufficient to provide mounting support for the ring 60 and maintain it in desired spaced relationship with the piston proper. Ring 60 is here shown as being substantially circular in cross section, although it will be understood that other curved shapes such as elliptical, as well as polygonal sections may be used.

The present invention contemplates additional means for creating turbulence in gases which attempt to flow between piston and cylinder, from one face of the piston to the opposite face of the piston. As appears in FIG. 4, the inner surface defining the cylinder 30 may be configured to form a roughened surface, such as by the provision of a series of grooves or indentations 66 formed in the cylinder wall. In the illustration of FIG. 4, that portion of the cylinder wall is provided by the outermost surface of the flange 38, and it will be understood that grooves similar to grooves 66 are provided throughout the surfaces defining cylinder 30.

Additionally, the arcuate sidewall of piston 46 itself may be provided with configurations to cause further turbulence and consequent friction and resistance to gas flow along the annular space around the piston within the cylinder. Thus a number of grooves 68 are formed in the outer arcuate surface of piston 46, extending completely around the circumference of the piston.

Additional turbulence and consequent resistance to gas flow may be provided by one or more additional rings of the type illustrated by ring 60 in FIG. 4. Thus, as seen in FIG. 5, a piston indicated generally at 146 is provided with a recessed portion 158 substantially larger than the recessed portion 58 previously mentioned and described in connection with FIG. 4. Within the recessed portion 158 there are disposed two concentric rings, 160 and 161, each carried by the piston by means of a set of mounting bars or rods, as in the case of ring 60 of FIG. 4. Thus, in FIG, 5, ring 160 is supported by such mounting bars 162, one of which is shown in FIG. 5, while ring 161 is supported by a number of angularly spaced bars, one of which is shown at 163. The inner portions of the

bars

162 and 163 are fixed to the piston 146, while the outer portions are fixed by suitable means, as by welding or the like, to the

respective rings

160 and 161. The construction illustrated in FIG. 5 is otherwise substantially similar to that of FIG. 4, including grooves 66 formed in the flange 38, and grooves, one of which is illustrated at 168, formed in piston 146.

The construction of FIG. 4 will be more completely understood by reference to the showing of FIG. 6, illustrating in section a complete piston within the cylinder, the adjacent walls of piston and cylinder being configured in the manner described in connection with FIG. 4, and each of faces of the piston being provided with rings of the type illustrated at 60 in FIG. 4. Thus, in FIG. 6, on the face of piston 46 opposite to that seen in FIG. 4, a ring 63 is disposed in an annular recess and is supported by a series of angularly spaced mounting bars or rods being indicated at 64 and 65. Also shown in FIG. 6 is a further clarification of the grooves 66 formed in the flange 38, and corresponding grooves 67 formed in the concave wall defining cylinder 30. It may be noted that the grooves 67, as well as the grooves 68 formed in the outer surface of the piston, are here shown as tapering in their width, from a maximum at the outer radius from the engine axis to a minimum at the smallest radius from the engine axis. Alternatively, the

width of such grooves may be virtually uniform throughout their length.

An alternative construction for providing the indentations or irregularities in the adjacent surfaces of piston and cylinder is illustrated in FIG. 9, showing a fragmentary portion of a piston indicated generally at 246 within a cylinder defined partially by a portion 248 of one of the mating shells constituting the housing of the present engine. In this form of the invention, the outer surface of piston 46 is covered with a layer or coating, and the concave inner surface of the member 248 forming a portion of the annular cylinder is similarly provided with a lining or coating 262. Each of the

coatings

260, 262 is desirably made of a composition such as asbestos, fluxed silica, pyrolitic boron nitride or combinations thereof, having low thermal conductivity and low coefficient of thermal expansion and high-temperature strength. The advantages of layers such as 260 and 262 of such material are not only to provide a surface or surfaces having indentations which will induce turbulence and thereby perform the sealing function heretofore referred to in connection with FIGS. 4 and 5; but also to minimize loss of heat by heat transfer from the annular cylinder, thereby employing the heat generated during combustion more efficiently in producing output power in shaft 33.

It will accordingly be seen that the turbulence inducing configurations of the adjacent faces of piston and cylinder, in accordance with the preceding description and accompanying drawings, provide effective sealing without actual contact between the piston and cylinder, as in the case of conventional arrangements employing piston rings and the like. It is especially to be noted, that the construction here disclosed is eminently suited for rotary engines, as distinguished from the type in which a piston moves rectilinearly within a longitudinal cylinder, with a connecting rod or similar linkage connecting the piston to a crankshaft. In this more conventional arrangement just referred to, it is obvious that the piston is subjected to vectors of force normal to its axis of movement, causing the piston necessarily to contact the cylinder wall. Such lateral vectors are not present in a rotary engine in the type under present consideration.

In order to further enhance the sealing characteristics of the present engine, there may also be provided in accordance with the invention a composite sealing ring, or sealing ring assembly, made up of two component annular members. Such a device is seen in FIGS. 7 and 8, and its use in the present invention will be understood by reference to FIGS. 1 and 3, particularly the latter. Thus, with specific reference to FIG. 7, a ring assembly indicated generally at 100 includes a pair of

mating ring elements

102 and 104. Each of the two component rings includes an inner face which describes effectively a helical surface about the axis of the ring, and an outer surface lying in a plane normal to that axis. The two helical surfaces are arranged to be in mutually slidable relation when the component rings are assembled into the ring assembly as seen in FIG. 7, and each of the helical surfaces terminates in a juncture portion lying in a plane containing the axis of the ring. Thus ring component 102 includes a helical face 106 and a juncture portion 108. Ring 104 includes an inner helical face 110 and an outer planar face 112, the helical face'110 terminating at each of its ends in a juncture portion 114.

The enlarged fragmentary sectional view in FIG. 8 will clarify the relationship between the component rings of the assembly in the vicinity of the juncture portions. As will be seen in FIG. 8, means are provided for resiliently biasing apart the two juncture portions 108 and 1 14, in order to effectively tend to increase the overall width of the ring assembly, as measured parallel to the axis of the ring. In FIG. 8 such resilient means are shown as including a compression spring 116, the upper end of which is received in a bore 117 formed in ring 104, and the lower portion of spring 116 is received in a mating bore 118, formed in ring 102. As will be seen by reference to FIG. 7, two of such biasing springs may be used.

In FIG. 3 there are shown three ring assemblies of the type just described. One such assembly is indicated generally at 120, received in an annular recess formed in housing shell member 12 adjacent to disk or flange 40, which is fixed to shaft 33. A second such ring assembly is shown in FIG. 3 and is indicated generally at 130, being received in a recess formed in flange 40, and serving to seal between that flange and disk or flange 38 immediately adjacent. A third such ring assembly, indicated generally at 140, is received in an annular recess formed in housing shell member 14, and serves to seal between that shell member and flange 38.

In the use of ring assemblies, such as 120, 130 and 140, it may be desirable to provide on their outermost annular faces indentations or similar irregularities to impart turbulence to gases seeking to pass between such outer surfaces of the ring and an adjacent piston.

In practicing the present invention, the exact dimensions of clearance between piston and cylinder, and average or effective depth of indentations such as grooves and the like, may vary widely depending upon environmental circumstances. For example, with reference to FIGS. 4 and 5, the minimum radial clearance between the relatively moving parts may be of the order of 0.025 of an inch, and the diameter of the ring 60, as well as the diameter of each of the rings I60 and 161 in FIG. 5, may also be of approximately this value. Similarly, the

radial depths of the

grooves

66, 68 and 168 may be from about 0.010 to 0.025 of an inch.

In the form of the invention shown in FIG. 9, it is contemplated that the facing surfaces of the liners or

coatings

260 and 262 will be generally of irregularly indented contour, with a net radial clearance at a minimum of about 0.005 of an inch.

The roughened configuration to provide turbulence and consequent sealing between relatively moving surfaces may also be incorporated in the radially extending surfaces of the disks or flanges on which the pistons are mounted, and the adjacent portions of the walls of the housing shells. With particular reference to FIG. 10, there is shown an embodiment of the invention incorporating such construction, the showing of FIG. 10 corresponding to the upper portion of FIG. 1. Thus the annular cylinder is indicated generally at 330, and mounted therein is a piston indicated generally at 346, it being assumed for purposes of illustration that piston 346 is one of the compression pistons heretofore mentioned. This piston is fixed to a radial disk or flange 338, which in turn is fixed to countershaft indicated generally at 335. The countershaft is rotatably mounted on the principal drive shaft 333, desirably including bearing means 336.

Adjacent flange 338 is flange 340, to which is attached the drive pistons in the manner previously described, the drive pistons being omitted from the showing of FIG. 10.

On the radial faces of flanges or

disks

338 and 340, and on inwardly directed faces 370 and 372 of the housing shells there are provided indentations or other roughened configurations, corresponding to those previously described in connection with FIG. 6. Thus, as best appears in FIG. 11, such indentations may take the form of irregularities similar to knurling 350, disposed annularly immediately radially inwardly of the pistons to minimize gas flow between the disks. The radially inner annular portions of the disks and of the adjacent surfaces of the housing shells are preferably not configured with indentations or otherwise roughened, but are smooth, in order to provide bearing surfaces to maintain the disks in their proper position axially of the engine, In this connection it may be noted also that the countershaft 335 is prevented from moving rightwardly as seen in FIG. 10 by suitable means, here shown as including a retaining plate 360 attached to the housing shell by suitable means 362, and including an inner annular shoulder 364 and an annular element of bearing material 366, abutting a shoulder 368 formed in the countershaft.

It will be understood that toroidal as used herein in describing the cylinder is not limited to a cross section which is circular as illustrated, but includes also a rectangular or other polygonal cross section, a closed conic section such as an ellipse, or other closed figure. Modifications and changes from the forms of the invention hereinabove shown and described, and from the dimensions mentioned as illustrated, are contemplated.

I claim:

1. In a rotary engine having a plurality of pairs of pistons disposed in a toroidal cylinder, a corresponding piston of each pair being connected to a central output shaft and means for moving the other piston in oscillatory relation with the output shaft pistons, said pistons being defined arcuately by faces disposed in planes generally radially of the output shaft, the improvement comprising the provision of:

means mounting the pistons in the toroidal cylinder to provide a radial clearance between the adjacent walls of the pistons and the cylinder;

turbulence producing means in the path of the gas flow between adjacent piston and cylinder walls comprising a multiplicity of indentations in said walls,

wherein the depth of indentations is in the order of magnitude of the radial clearance between adjacent piston and cylinder walls, and wherein each of piston faces is provided with a circumferential recess extending therearound;

and a ring disposed in each of said recesses, fixedly mounted on the piston, and spaced therefrom.

piston.

4. The invention as defined in claim 1 wherein the ring is tangent to the plane of the surface face.

6. The invention as defined in claim 4 wherein the ring is 5 circular in cross section and is spaced from the piston by a distance approximately equal to its diameter.

Claims

1. In a rotary engine having a plurality of pairs of pistons disposed in a toroidal cylinder, a corresponding piston of each pair being connected to a central output shaft and means for moving the other piston in oscillatory relation with the output shaft pistons, said pistons being defined arcuately by faces disposed in planes generally radially of the output shaft, the improvement comprising the provision of: means mounting the pistons in the toroidal cylinder to provide a radial clearance between the adjacent walls of the pistons and the cylinder; turbulence producing means in the path of the gas flow between adjacent piston and cylinder walls comprising a multiplicity of indentations in said Walls, wherein the depth of indentations is in the order of magnitude of the radial clearance between adjacent piston and cylinder walls, and wherein each of piston faces is provided with a circumferential recess extending therearound; and a ring disposed in each of said recesses, fixedly mounted on the piston, and spaced therefrom.

2. The invention as defined in claim 1 wherein the depth of the circumferential recess is in the order of magnitude of that of the indentations.

3. The invention as defined in claim 1 wherein the circumferential recess is substantially larger than the indentations, and wherein a plurality of rings are located in the recesses, said rings being spaced from each other as well as from the piston.

6. The invention as defined in claim 4 wherein the ring is circular in cross section and is spaced from the piston by a distance approximately equal to its diameter.