Abstract:
An oscillating-piston machine comprises a housing, in which a first and at least a second piston are arranged, which pistons can together revolve in the housing about an axis of rotation that is fixed with respect to the housing, and which pistons, as they revolve about the axis of rotation, execute oppositely directed reciprocating pivoting movements about a pivot axis running perpendicular to the axis of rotation and through the centre of the housing, the first piston having a first end face, and the at least second piston having a second end face facing the first end face, the end faces delimiting a working chamber, characterized in that the pistons are arranged in such a way that the axis of rotation runs through the working chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application claims priority of German patent application No. 10 2005 010 775.3 filed on Feb. 25, 2005. 
   BACKGROUND OF THE INVENTION 
   The invention generally relates to oscillating-piston machines. 
   Oscillating-piston machines, and in particular an oscillating-piston machine in accordance with the present invention, can be used as internal combustion engines, as pumps or as compressors. An oscillating-piston machine in accordance with the present invention is preferably used as an internal combustion engine and is described in this form in the present description. 
   If an oscillating-piston machine is used as an internal combustion engine, the individual working strokes of intake, compression, ignition of the combustion mix and expansion and exhaust of the burnt combustion mix are produced by reciprocating pivoting movements of the individual pistons between two limit positions. 
   In the case of the oscillating-piston machine which is known from document WO 03/067033 A1, in the name of the same Applicant, four pistons are arranged in the housing, and these pistons revolve jointly about an axis of rotation which is arranged fixedly in the centre of the housing, and as they revolve they execute reciprocating pivoting movements about a pivot axis in the housing, with in each case two adjacent pistons pivoting in opposite directions. In this known oscillating-piston machine, in each case two pistons arranged diametrically opposite one another with respect to the centre of the housing are rigidly connected to one another to form a double piston, and two piston pairs of this type are in a crossed-over arrangement in the centre of the housing. In each case one working chamber is formed between in each case two end faces of the piston of the piston pairs facing one another, so that the known oscillating-piston machine has two working chambers. The size of the two working chambers which are arranged diametrically opposite with respect to the centre of the housing increases and decreases in the same direction with the reciprocating pivoting movement of the pistons. 
   The pistons of this known oscillating-piston machine are arranged in such a way in the housing that in their TDC position, in which the volumes of the two working chambers are at a minimum, they are positioned perpendicular to the axis of rotation. In this position, the centrifugal forces acting on the pistons during the revolution of the pistons about the axis of rotation are at a maximum. The result of this is that at high rotational speeds the expansion or movement of the pistons away from one another has to take place counter to the centrifugal forces, since the centrifugal forces counteract this movement of the pistons away from one another. In this oscillating-piston machine, the working chambers are always located outside and perpendicular to the axis of rotation. 
   The pistons of the known oscillating-piston machine are substantially in the form of a wedge of a sphere, and correspondingly so is the geometry of the working chambers. 
   Although the known oscillating-piston machine has very good operating properties, it is an object of the present invention to provide a new design of an oscillating-piston machine which differs from the design of the known oscillating-piston machine described above. 
   SUMMARY OF THE INVENTION 
   Therefore, the invention is based on the object of providing a new design of this type for an oscillating-piston machine of the type described in the introduction. 
   According to the invention, an oscillating-piston machine is provided, comprising a housing defining a centre, a first piston and at least a second piston, the first piston and the at least second piston being arranged in the housing and being able to revolve together in the housing about an axis of rotation which is stationary with respect to the housing, the first piston and the at least second piston executing oppositely directed reciprocating pivoting movements about a pivot axis, as the first and the at least second piston revolve about the axis of rotation, the pivot axis running perpendicular through said axis of rotation and through the centre of the housing, the first piston having a first end face, and the at least second piston having a second end face facing the first end face, the first end face and the second end face delimiting a first working chamber, and the first piston and the at least second piston being arranged in such a way that the axis of rotation runs through the first working chamber. 
   The novel design of the oscillating-piston machine according to the invention compared to the known oscillating-piston machine accordingly consists in the at least two pistons being arranged in such a way that the at least one working chamber is not located perpendicular to the axis of rotation, but rather on the axis of rotation or around the axis of rotation. The centrifugal forces which act on the two pistons delimiting the working chamber as they revolve about the axis of rotation are lower on account of the reduced distance between the pistons and the axis of rotation, and moreover they act in the direction in which the two pistons are moved away from one another, i.e. the centrifugal forces assist the working stroke of expansion. The centrifugal forces which occur perpendicular to the axis of rotation during revolution of the pistons about the axis of rotation therefore assist with the expansion of the at least one working chamber. 
   In a preferred configuration, it is provided that the first and second end faces of the first and at least second pistons are circular in form. 
   In this configuration, the first and at least second pistons are cylindrical at least in the region which adjoins their end faces, and are therefore very similar to conventional pistons of linear reciprocating-piston engines in this region. One advantage which results from this is that piston rings, if appropriate with a corresponding curvature, can be used as seals for the two pistons, so that in this respect it is possible to make use of long-standing experience in solving sealing problems in linear reciprocating-piston engines. In this configuration, the working chamber delimited by the two end faces of the first and at least second piston has the geometry of a cylinder or toroid section curved about the oscillation axis. 
   However, as an alternative to a circular configuration of the end faces of the first and at least second pistons, it is also possible to select a different geometry, for example an oval shape, which contributes to increasing the size of the at least one working chamber in particular if the interior of the housing is spherical-symmetrical. 
   In a further preferred configuration, the first and the at least second pistons are designed substantially in the form of an arc. 
   It will be understood that the arcuate configuration of the first and at least second pistons may be restricted to the region adjoining their end faces, i.e., as will be described in more detail below, outer sides of the pistons which are remote from the end faces may be used as functional elements for controlling the pistons in order to derive the pivoting movement from the revolving movement of the pistons, and for this purpose may be configured in different ways. 
   In a further preferred configuration, the first piston and/or the at least second piston has at least one running member which, when the first and/or at least second piston is revolving, is guided along a corresponding designed control cam, in order to generate the pivoting movements of the first and at least second piston, the control cam being arranged on the housing, at at least approximately a maximum distance from the axis of rotation. 
   In the known oscillating-piston machine, there is a comparable control mechanism for controlling the pivoting movements of the pistons, but in that case the control cam is at a shorter distance from the axis of rotation, in the vicinity of the end sides of the housing. The advantage of the greater distance between the control cam and the axis of rotation consists in improved lever ratios, in order to derive the pivoting movements of the at least two pistons from their revolving movement about the axis of rotation. 
   In this context, it is also preferable if the at least one running member is a ball which is mounted rotatably in a ball socket on an outer side, facing the housing, of the first and/or at least second piston, and if the control cam is designed as a groove with a cross section in the form of part of a circle in the housing, in which groove the ball partially engages. 
   A control mechanism of this type, which uses a ball as the at least one running member, has the advantage of optimum reduction in friction in the control mechanism, since the ball can rotate freely in the ball socket of the at least one piston, and also in the groove in the housing, so that the ball can follow the control cam with particularly little friction, on account of the fact that it can rotate in all directions. 
   The ball socket may be designed in such a way that it holds the ball captively, or the ball can be held in the ball socket by adhesion forces by means of a lubricating film which is provided by oil lubrication. 
   It is preferable for both the first and the at least second piston to have a running member in the form of a ball, which balls can run at a distance from one another in the same groove-like control cam in the housing. 
   In a further preferred configuration, the first and the at least second piston are mounted slideably in a piston cage, which is arranged in the housing, concentrically with respect to and rotatable about the axis of rotation, the piston cage being rotationally fixedly connected to the first and at least second piston with respect to the revolving movement about the axis of rotation. 
   The ball cage and the first and at least second piston therefore form the “inner machine” or “inner motor” of the oscillating-piston machine. The sliding mounting of the two pistons in the piston cage is used for the pivoting mobility of the two pistons about the pivot axis, while on account of being rotationally fixedly connected to the piston cage in terms of the revolving movement about the axis of rotation, the pistons revolve with the piston cage about the axis of rotation. The piston cage can now advantageously be used as a drive or output member and may accordingly be designed as a shaft extension protruding out of the housing. 
   In a further preferred configuration, the piston cage, approximately perpendicular to the axis of rotation, has a bore in which the first and at least second piston are partially received, such that they slide therein, and which delimits the working chamber in the circumferential direction. 
   The bore therefore defines, together with the two end faces, which face one another, of the first and at least second piston, the at least one working chamber of the oscillating-piston machine. The geometry of the bore in the piston cage is also selected according to the geometry of the end faces of the two pistons, i.e. for example to be circular or, as has already been mentioned above, oval or some other shape corresponding to the shape of the end faces of the pistons. If the end faces of the two pistons are circular in form, the result, in combination with the circular bore in the piston cage, is a working chamber which corresponds to a curved cylinder or a toroid section. The pistons are then preferably sealed against the wall of the bore of the piston cage by means of seals, the latter, in the case of a circular bore and circular end faces, advantageously being designed as piston rings matched to the shape of the working chamber. 
   In a further preferred configuration, a passage passes through the piston cage and on one side opens out in the bore, while on the other side it opens out toward the housing, in order to be in communication with an inlet opening or an outlet opening in the housing, depending on the rotational position of the piston cage. 
   The advantage of this measure is that the piston cage, by means of the abovementioned passage or opening, acts as a type of valve for the inlet and outlet openings in the housing. It is therefore not necessary for the inlet and outlet openings in the housing to be provided with separate valves, or to provide a complex control of the valve for the instant of opening or closing, as is the case in conventional linear reciprocating-piston engines. The opening and closing of the inlet and outlet openings to admit combustion air and/or fuel and to discharge burnt combustion mix take place automatically at the correct stroke as a result of the revolving movement of the piston cage about the axis of rotation. 
   In a further preferred configuration, the piston cage has at least one passage for a medium, in particular coolant/lubricant, which extends at least partially over the circumference and through the interior of the piston cage. 
   One advantage of this arrangement is that the piston cage advantageously performs a further function, namely that of supplying all the moving parts within the housing with a cooling and/or lubricating medium. A cooling/lubricating medium can be supplied via connections arranged at the housing, in which case the at least one passage preferably extends as an annular passage on the outer side of the piston cage, so that the at least one passage is always in communication with the supply connections. 
   In a further preferred configuration, a bore, which preferably widens out at its ends, passes through the piston cage at the level of and in the direction of the pivot axis. 
   This bore advantageously serves as a further coolant/lubricant passage, which makes a contribution to particularly intensive circulation of a cooling/lubricating medium of this type, since this bore extends perpendicular to the axis of rotation, and therefore the cooling/lubricating medium which is then located therein is subjected to centrifugal forces as the piston cage revolves about the axis of rotation, causing the cooling/lubricating medium to move towards the widening ends of the bore. As a result, a ventilation effect advantageously occurs during the circulation of the cooling/lubricating medium. 
   In a further preferred configuration, a third and a fourth piston are arranged in the housing, which third and fourth pistons are arranged diametrically with respect to the first and second pistons, based on the pivot axis, and can pivote about this axis, can revolve about the axis of rotation with the first and second pistons and define a second working chamber. 
   In this configuration, a system which is advantageously symmetrical, and therefore balanced in terms of mass, with respect to the axis of rotation and pivot axis is also created in the oscillating-piston machine according to the invention. 
   In this context, it is preferable if the four pistons are arranged in such a way that the first and second working chambers, as the pistons revolve about the axis of rotation, increase and decrease in size in the same directions. 
   This configuration makes a contribution to the four pistons forming a mass-balanced system in every position of revolution and pivoting. The four pistons are preferably in each case arranged in diametrically opposite pairs with respect to the pivot axis, but unlike in the known oscillating-piston machine it is preferably provided that the four pistons be arranged individually in the housing, i.e. are not rigidly connected to one another in pairs to form double pistons. 
   In a further preferred configuration, the piston cage extends on both sides of the oscillation axis and also accommodates the third and fourth pistons. 
   Overall, therefore, this creates a particularly simple structure, requiring only a small number of parts, in which the piston cage accommodates all four pistons. For the third and fourth piston, the piston cage, if this is provided for the first and second pistons as described above, likewise has a bore for the third and fourth pistons, in which bore the third and fourth pistons are mounted slideably and are rotationally fixedly connected to the piston cage with respect to the axis of rotation, this bore, together with the end faces of the third and fourth pistons, then delimiting the second working chamber. 
   In a further preferred configuration, a housing inner wall of the housing is substantially spherical in form. 
   This configuration advantageously creates a spherical-symmetrical oscillating-piston machine, which has already proven its worth in the known oscillating-piston machine. 
   As an alternative, however, it can also be provided that a housing inner wall of the housing, when seen in section along a plane which includes the axis of rotation and the pivot axis, is oblong in the direction of the axis of rotation. 
   In this context, the term “oblong” is to be understood as meaning that the housing of the oscillating-piston machine comprises two halves of a sphere, between which is inserted a portion which is elongate in the direction of the axis of rotation. The oblong shape of the housing inner wall of the housing advantageously opens up the possibility of providing the following preferred configurations. 
   For example, it is preferable if a hollow pin, which can rotate about the pivot axis, is arranged in the housing, and in the wall of the hollow pin there is an opening which is in communication with the first working chamber or if appropriate with the second working chamber as a function of the rotary position of the hollow pin. 
   This hollow pin can advantageously be used to feed fresh air, in particular pressurized fresh air, into the working chamber or, if two working chambers are provided, into the two working chambers alternately, via the circumferentially delimited opening provided in the hollow pin. As a result, combustion air can be passed into the working chambers at an admission pressure, making it possible to achieve greater compression of the fuel-air mix in the working chambers. In this way, the oscillating-piston machine is suitable in particular as a diesel engine. 
   In this context, it is preferable if the hollow pin is connected to a transmission mechanism which, when the pistons revolve about the axis of rotation, makes the hollow pin rotate about the pivot axis. 
   In this way, the rotational movement of the hollow pin to enable its opening to communicate with one working chamber or the other can be derived, in an advantageously simple way, directly from the revolving movement of the pistons about the axis of rotation, without the need for an external control mechanism. If the step-up ratio of the transmission mechanism is selected appropriately, the rotational speed of the hollow pin is synchronized in a simple way with the rotational speed of the oscillating-piston machine. 
   In this context, it is also preferable if the transmission mechanism includes worm toothing gear, which is connected to the hollow pin and meshes with at least one set of toothing which is arranged at the housing and extends around the axis of rotation. 
   A transmission mechanism of this type is of particularly simple design, can be accommodated in the housing without an increased need for space, and given a suitable configuration of the worm toothing the rotational speed of the hollow pin can then be adapted as a function of the rotational speed of the revolving movement of the pistons about the axis of rotation. 
   Further advantages and features will emerge from the following description and the appended drawing. 
   It will be understood that the features which have been listed above and are yet to be explained below can be used not only in the combination given in each instance, but also in other combinations or as stand-alone measures without departing from the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawing and are described in more detail below with reference to the drawing, in which: 
       FIG. 1  shows a perspective overall view of an oscillating-piston machine; 
       FIG. 2  shows a view of the oscillating-piston machine from  FIG. 1  in the direction of arrows II in  FIG. 1 ; 
       FIG. 3  shows a longitudinal section through the oscillating-piston machine on a plane parallel to the axis of rotation and perpendicular to the pivot axis, with the pistons of the oscillating-piston machine illustrated in a first operating position; 
       FIG. 4  illustrates the oscillating-piston machine in the same operating position of the pistons as in  FIG. 3 , in the form of a slightly perspective view, without the pistons being shown in section; 
       FIG. 5  shows an illustration of the oscillating-piston machine comparable to that shown in  FIG. 4 , with the pistons illustrated in a second operating position; 
       FIG. 6  shows a longitudinal section through the oscillating-piston machine from  FIGS. 1 to 5 , with the pistons illustrated in a third operating position; 
       FIG. 7  illustrates the oscillating-piston machine with the pistons in the same operating position as in  FIG. 6 , in the form of a slightly perspective view without the pistons being illustrated in section; 
       FIG. 8  shows a section through the oscillating-piston machine on line VIII-VIII from  FIG. 3 ; 
       FIG. 9  shows a section through the oscillating-piston machine on line IX-IX from  FIG. 3 ; 
       FIG. 10  shows a longitudinal section on line X-X from  FIG. 3  through the oscillating-piston machine as shown in  FIGS. 1 to 9 ; 
       FIG. 10A  shows an illustration comparable to  FIG. 10  of a modified exemplary embodiment of the oscillating-piston machine; 
       FIG. 11  shows a longitudinal section through the oscillating-piston machine similar to that shown in  FIG. 3  or  4 , but without the piston cage and the pistons being illustrated in section; 
       FIG. 12  shows a view of the oscillating-piston machine with one half of the housing removed; 
       FIG. 13  shows a perspective illustration of the arrangement of piston cage and pistons alone, in perspective; 
       FIG. 14  shows a perspective view of an inner side of a housing half of the oscillating-piston machine alone; 
       FIG. 15   a ) to  d ) show various perspective views and sections of a piston of the oscillating-piston machine including its running member in stand-alone form; 
       FIG. 16  shows a longitudinal section through an oscillating-piston machine in accordance with a further exemplary embodiment; and 
       FIG. 17  shows a longitudinal section through the oscillating-piston machine from  FIG. 16  in section along a plane which is rotated through 90° with respect to the section plane in  FIG. 16 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 to 10  and  FIGS. 11 and 12  show various illustrations of an oscillating-piston machine provided with the general reference numeral  10 . Further details of the oscillating-piston machine  10  are illustrated in  FIGS. 13 to 15 . 
   In the present exemplary embodiment, the oscillating-piston machine  10  is designed as an internal combustion engine. 
   The oscillating-piston machine  10  has a housing  12  which is assembled from two housing halves  14  and  16 . The housing halves  14  and  16  each have a flange  18   a  and  18   b , by means of which the housing halves  14  and  16  are releasably connected to one another. 
   Inlet connection pieces  20  and  24  for fresh air/fuel, which are arranged diametrically opposite with respect to the centre of the housing and the openings of which pass through the housing (cf.  FIG. 9 ), are arranged at the housing  12 . Outlet connection pieces  22  and  26  are likewise provided. The inlet connection pieces  20  and  24  are used to supply fresh air or combustion air, while the outlet connection pieces  22  and  26  are used to discharge burnt fuel-air mix. The inlet connection pieces  20  and  24  are each assigned a connection for a fuel injection nozzle, as illustrated by a connection  25  for the connection piece  24  (cf. also  FIG. 9 ).  FIG. 2  illustrates a corresponding connection  21  for the inlet connection piece  20 . 
   Furthermore, a plurality of connections  28  to  38  for supplying and discharging and/or circulating a cooling/lubricating medium through the interior of the oscillating-piston machine  10  are arranged at the housing. 
   In the exemplary embodiment of the oscillating-piston machine  10 , a housing inner wall  39  is substantially spherical in form or is spherical-symmetrical, as can be seen, for example, from  FIG. 3 . 
   Four pistons  40  to  46 , which in the housing  12  can jointly revolve about an axis of rotation  48  as indicated by an arrow  49  ( FIG. 3 ), are arranged in the interior of the housing  12 . During this revolving movement, the pistons  40  to  46  execute an pivoting movement, which is superimposed on the revolving movement, about a pivot axis  50  between two limit positions, one limit position being illustrated in  FIG. 3  (referred to as the BDC position), and the other limit position being illustrated in  FIG. 6  (referred to as the TDC position). 
   Both the axis of rotation  48  and the pivot axis  50 , which are to be understood as geometric axes, pass through the centre of the spherical housing  12 . Furthermore, the pivot axis  50  is always perpendicular to the axis of rotation  48 , but likewise revolves around the latter in accordance with the revolving movement of the pistons  40  to  46  likewise about the axis of rotation  48 . 
   Of the pistons  40  to  46 , in each case two pistons are positioned diametrically opposite one another with respect to the pivot axis  50 , specifically in every pivot position of the pistons  40  to  46 , and specifically pistons  40  and  44 , on the one hand, and pistons  42  and  46 , on the other hand, are arranged diametrically opposite one another. However, the pistons  40  to  46  are mounted individually in the housing  12 , i.e. are not rigidly connected to one another in pairs. 
   Each of the pistons  40  to  46  has an end face, i.e. the piston  40  has an end face  52 , the piston  42  has an end face  54 , the piston  44  has an end face  56  and the piston  46  has an end face  58 . 
   End faces which respectively face one another, i.e. in the present case the end faces  54  and  56  of the pistons  42  and  44  and the end faces  52  and  58  of the pistons  40  and  46 , in each case delimit a working chamber  60  and  62  serving as combustion chambers. The axis of rotation  48  passes through both working chambers  60 ,  62 , preferably centrally in each position of the pistons. 
   Since respectively adjacent ones of the pistons  40  to  46  execute pivoting movements in opposite directions to one another as they revolve about the axis of rotation  48 , the working chambers  60  and  62  always increase and decrease in size in the same direction as one another. 
   By way of example, starting from the state in which the working chambers  60  and  62  have their maximum volume, as shown in  FIG. 3 , the pistons  40  and  46  pivote towards one another ( FIG. 5 ), as do the pistons  42  and  44 . In the process, the volumes of the working chambers  60  and  62  are reduced until the limit position illustrated in  FIG. 6  is reached, in which the working chambers  60  and  62  adopt their minimum volume. 
   It will be understood that the pistons  40  and  46 , as they pivote about the pivot axis  50 , always remain to the left-hand side of line VIII-VIII in  FIG. 3 , and pistons  42  and  44  always remain to the right-hand side of the said line. 
   To derive the pivoting movements of the pistons  40  to  46  about the pivot axis  50  from the revolving movement of the pistons  40  to  46  about the axis of rotation  48 , each piston  40  to  46  has a running member  64  (piston  40 ),  66  (piston  42 ),  68  (piston  44 ) and  70  (piston  46 ). The running members  64  to  70  are balls which are in each case mounted in a ball socket  72 , as illustrated for piston  40  in  FIG. 15 , with the ball socket being arranged on an outer side of the respective piston  40  to  46 , facing the housing inner wall  39 . 
   As illustrated in  FIG. 3 , the balls  64  to  70  may be mounted loosely in the ball sockets  72  and held there by adhesion produced by a lubricating film, in which case the ball sockets  72  do not extend beyond the diameter of the balls  64  to  70 , or alternatively the ball sockets may, as illustrated in  FIG. 15   a ) and  b ), hold the balls  64  to  70  in a positively-locking manner and therefore captively by means of an extension  74  extending beyond the diameter of the balls. 
   In any case, the balls  64  to  70  can rotate freely in the ball socket  72  in all directions about their respective centres. 
   The running members or balls  64  to  70  are assigned two control cams in which the balls  64  to  70  run. More accurately, the balls  64  and  70  of the pistons  40  and  46  are assigned a first control cam  76 , which is designed as a groove with a cross section in the form of part of a circle in the housing inner wall  39 . A corresponding control cam  78  is assigned to the running members or balls  66  and  68  of the pistons  42  and  44 . 
   The balls  64  and  70  therefore run in the same control cam  76 , and the balls  66  and  68  run in the same control cam  78 . The balls  64  and  70 , on the one hand, and the balls  66  and  68 , on the other hand, are in each case offset by 180° from one another with respect to the axis of rotation  48 . 
   The control cams  76  and  78  are arranged at least approximately at the maximum distance from the axis of rotation  48 , as can be seen from  FIG. 3 , i.e. they are located approximately at the level of the pivot axis  50 . Overall, the control cams  76  and  78  run substantially orthogonally to the axis of rotation  48 . 
     FIG. 14 , which shows the housing half  14  alone, provides a perspective illustration of the control cams  76  and  78  in detail. 
   The pistons  40  to  46  are mounted in the housing  12 , in a piston cage  80  which revolves about the axis of rotation  48  together with the pistons  40  to  46  and is described in more detail below together with further details of the pistons  40  to  46 .  FIGS. 11 to 13  illustrate the piston cage  80  in the form of views which are not taken in section. 
   In the exemplary embodiment shown and preferably, the piston cage  80  is a single-piece component, although a multi-piece design is also conceivable instead of a single-piece design. 
   The piston cage  80  extends along the axis of rotation  48  over the entire length of the housing  12 , with shaft extensions  86  and  88  of the piston cage  80  projecting out of the housing. 
   The piston cage  80  in each case has a main bearing section  82  and  84  which adjoins the shaft extensions  86  and  88  and via which the piston cage  80  is mounted in the housing  12  such that it can rotate about the axis of rotation  48 . The bearing sections  82  and  84  are connected in the centre of the housing by way of a centre section  90 , which has a pin-like section  92  which extends along the pivot axis  50  and on which the pistons  40  and  46  are mounted with respect to the centre of the housing or the pivot axis  50 . 
   In accordance with  FIG. 10 , the piston cage  80  has two bores  94  and  96 , in which the pistons  40  to  46  are slideably mounted. More accurately, the pistons  40  and  46  are mounted slideably in bore  94 , and the pistons  42  and  44  are mounted slideably in bore  96 . The bores  94  and  96  are circular in form, and accordingly the end faces  52  to  58  of the pistons  40  to  46  are likewise of circular design. The pistons  40  to  46  are mounted in the bores  94  and  96  by means of piston rings for sealing the working chambers  60  and  62 , as illustrated by seals  98  (outside) and  100  (inside) for piston  40  in  FIG. 3 . In accordance with  FIG. 3 , pistons  42  to  46  have corresponding seals on their radially outer side and their radially inner side. 
   The bores  94  and  96 , together with the end faces  52  to  58 , delimit the working chambers  60  and  62 . 
   In the bores  94  and  96  in the piston cage  80 , the pistons  40  to  46  are rotationally fixedly connected to the piston cage  80 , so that the pistons  40  to  46 , together with the piston cage  80 , revolve about the axis of rotation  48 , while the pistons  40  to  46  can move slideably within the bores  94  and  96 , in accordance with their pivoting movements about the pivot axis  50 , in order to carry out the individual working strokes of intake, compression, expansion and exhaust. 
   The pistons  40  to  46  are designed substantially in the form of arcs, as illustrated in  FIG. 15 , and the working chambers  60  and  62  are also approximately in the form of a curved or arcuate cylinder, with the curvature being concentric with respect to the pivot axis  50 . 
   The arrangement made up of piston cage  80 , pistons  40  to  46  as well as the running members  64  to  70  forms the “inner motor” of the oscillating-piston machine  10 , i.e. this arrangement comprises all the moving parts of the oscillating-piston machine  10 . 
   As illustrated by way of example in  FIGS. 4 and 9 , a plurality of passages  102  and  104  are present in the bearing sections  82  and  84 , respectively, of the piston cage  80 , which passages extend circumferentially and through the interior of the bearing sections  82  and  84  of the piston cage  80  and are in communication with the connections  28 ,  30  and  36 ,  38  which have already been mentioned above, so that a cooling/lubricating medium for cooling and lubricating the piston cage  80  can be passed through the passages  102 ,  104 . The passages  102  and  104  serve primarily to cool the inner motor in the vicinity of the working chambers  60 ,  62 . 
   In accordance with  FIG. 4 , cooling/lubricating medium passages  106  and  108  are likewise formed in the housing  12 , with a bore  110 , which likewise serves as a cooling/lubricating medium passage, passing through the centre section  90  of the piston cage  80  in the direction of the pivot axis  50 . When the piston cage  80  rotates about the axis of rotation  48 , the cooling/lubricating medium which is present in the bore  110  is thrown towards the housing inner wall  39  as a result of centrifugal forces. In this way, the pistons  40  to  46  and the running members  64  to  70  in the centre of the inner motor are cooled and/or lubricated. At the running members  64  to  70 , the lubricating film which forms also serves to hold the running members  64  to  70  in the ball sockets  72  of the pistons  40  to  46  through adhesion, unless, as illustrated in  FIG. 15 , this is achieved by a positively locking action. 
   The bore  110  widens out in the shape of a trumpet at both its ends, in order to improve the distribution of the cooling/lubricating medium in the centre of the housing  12  still further. 
   In accordance with  FIGS. 9 and 10 , two further bores or passages  114  and  116  are also provided in the piston cage  80 ; these bores or passages on one side open out in the bores  94  and  96 , respectively, and on the other side open out towards the housing inner wall  39 , specifically at the level of the inlet or outlet connection pieces  20  and  22  or  24  and  26 , respectively. The passages  114  and  116  are used to admit a fuel-air mix to the working chambers  60 ,  62  through the inlet connection pieces  20  and  24 , respectively, in one rotational position of the piston cage  80  about the axis of rotation  48 , and to discharge burnt fuel-air mix through the outlet connection pieces  22  and  26  in a different rotary position. In the other rotary positions, the piston cage  80  closes off these connection pieces. The piston cage  80  therefore simultaneously performs the function of a valve for opening and closing the connection pieces  20  to  26 . 
   As can also be seen from  FIG. 10 , a spark plug  118  and  120  for each working chamber  60  and  62  is provided in the piston cage  80 , these spark plugs being arranged on the axis of rotation  48  and rotating about the latter together with the piston cage  80 . Electrical supply conductors (not shown) are correspondingly connected to the spark plugs  118  and  120  via slip rings, for example. If the oscillating-piston machine  10  is used as a diesel engine, the plugs  118  and  120  are correspondingly glow plugs. 
   The arrangement of the connection pieces  20  and  22  offset through 180° about the axis of rotation  48  with respect to the connection pieces  24  and  26  serves to ensure that an expansion operation always takes place in at least one of the working chambers  60  and  62  as the pistons  40  and  46  revolve through 360° about the axis of rotation  48 . Therefore, precisely when an expansion stroke is taking place in the working chamber  60 , an exhaust stroke for discharging burnt fuel-air mix is taking place in the working chamber  62 , and vice versa. 
   The way in which the oscillating-piston machine  10  functions is described below. 
   Starting from the operating position of the pistons  40  to  46  shown in  FIGS. 3 and 4 , the pistons  40  and  46  in that position are in what is known as their BDC (bottom dead centre) position. After rotation through 45° about the axis of rotation  48 , the pistons  40  and  46  or  42  and  44  have moved halfway towards one another, as illustrated in  FIG. 5 . The volume of the working chambers  60  and  62  has there been reduced by approximately half. The pivoting movement of the pistons  40  to  46  was in this case imparted by the running members  64  to  70  being guided in the control cams  76  and  78 . 
   After further rotation through 45° about the axis of rotation  48 , the pistons  40  to  46  then adopt the TDC (top dead centre) position illustrated in  FIGS. 6 and 7 , in which the volumes of the working chambers  60  and  62  are at a minimum. After further rotation through 45° about the axis of rotation  48 , progressing in the same direction, the pistons  40  to  46  then return to the position shown in  FIG. 5 , and after further rotation through 45° they once again adopt the position shown in  FIG. 3 . The working chambers  60  and  62  are once again at a maximum after rotation through 180° about the axis of rotation  48 . 
   Therefore, after a full revolution through 360°, the four strokes of intake, compression, expansion and exhaust have taken place once in each of the working chambers  60  and  62 . 
     FIG. 10A  illustrates a slightly modified configuration of an oscillating-piston machine  10 ′, which differs from the oscillating-piston machine  10  only by virtue of the fact that the bores  94 ′ and  96 ′ in the piston cage  80 ′, and accordingly the end faces  52 ′ and  54 ′ (and the same is also true of the end faces  56 ′ and  58 ′, which are not illustrated) are not circular, but rather, as illustrated by way of example in  FIG. 10A , are oval or elliptical in form. This allows the size of the working chambers  60 ′ and  62 ′ to be increased compared to the circular configuration. 
     FIGS. 16 and 17  illustrate yet another exemplary embodiment of an oscillating-piston machine  10 ″, which differs from the oscillating-piston machine  10  or oscillating-piston machine  10 ′ as follows. 
   Whereas the housing  12  of the oscillating-piston machine  10  and of the oscillating-piston machine  10 ′ is spherical-symmetrical, the housing  12 ″ of the oscillating-piston machine  10 ″ is of oblong design. More specifically, the housing  12 ″ comprises two hemispheres  13 ″ and  15 ″, between which there is inserted an elongate section  17 ″ extending in the direction of the axis of rotation  48 ″. This makes the housing  12 ″ longer in the direction of the axis of rotation  48 ″ compared to the design of the housing  12 , which allows the following measures. 
   A hollow pin  122 , which has an opening  124  in its wall, is arranged on the inner side of the central section  90 ″ of the piston cage  80 ″, which in accordance with  FIG. 17  is likewise designed to be oblong in cross section. The central section  90 ″ has two openings  126  and  128  on the axis of rotation  48 ″, with which the opening  124  in the hollow pin  122  is in communication depending on its rotational position, although the opening  124  can in each case only be in communication with one of the openings  126  and  128  at a time. The hollow pin  124  is mounted in the central section  90 ″ such that it can rotate about the pivot axis  50 ″. The rotational movement of the hollow pin  122  about the pivot axis  50 ″ is derived from the revolving movement of the piston cage  80 ″ about the axis of rotation  48 ″. For this purpose, at one end the central section  90 ″ has a transmission mechanism  130 , which includes worm toothing  123  fixedly connected to the hollow pin  122 . The worm toothing or worm gear  132  meshes with toothing  134  arranged concentrically around the axis of rotation  48 , so that when the central section  90 ″ including the hollow pin  122  revolves about the axis of rotation  48  the worm toothing  132  and therefore the hollow pin  122  are made to rotate about the pivot axis  50 ″. 
   Furthermore, an inlet  136  for fresh air, which can be opened and closed by, for example, a standard valve device  138 , is provided in the housing. Fresh air, in particular precompressed fresh air, can now be introduced into the interior of the hollow pin  122  through the inlet  136 , and then, depending on the rotational position of the hollow pin  122  relative to the openings  126 ,  128 , the fresh air is introduced into the working chambers  60 ″ or  62 ″, specifically in addition to the supply of fuel-air mix through the connection pieces  20 ″ and  24 ″. This makes the oscillating-piston machine  10 ″ what is known as a supercharged engine. 
   The worm toothing  132  and the toothing  134  are accordingly to be designed in such a way that the rotational movement of the hollow pin  122  about the pivot axis  50 ″ is suitably synchronized with the piston positions of the pistons  40 ″ to  46 ″. This means that the supply of fresh air through the hollow pin  122  into the working chamber  60 ″ or into the working chamber  62 ″ should preferably take place when, or the opening  124  should be in communication with the respective opening  126  and  128  when, the ignition of the fuel-air mix admitted through the inlet connection pieces  20 ″ and  24 ″ is just on the verge of igniting. Rotation of the hollow pin through 360° about the axis of rotation  48 ″ should cause it to rotate through 360° about the pivot axis  50 . 
   Otherwise, the oscillating-piston machine  10 ″ corresponds to the configurations of the oscillating-piston machine  10  or  10 ′, and consequently in this respect reference can be made to the description given of those oscillating-piston machines.