Abstract:
A compressor includes a motor, a drive shaft driven by the motor and connected thereto, a crank mechanism connected to the drive shaft, at least one compressed-air generation apparatus that is driven by the crank mechanism and is designed to generate compressed air, a crankcase that has an inner chamber wall in the shape of a hollow body, which receives the drive shaft at least in portions, an outer chamber wall that is spaced apart from the inner chamber wall radially with respect to the drive shaft, and a dividing wall, and a compressed-air storage container that is designed to receive compressed air generated by the compressed-air generation apparatus. The compressed-air storage container is formed by the inner chamber wall, the outer chamber wall, the end wall and the dividing wall.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a compressor, in particular to a compressor having a reciprocating piston compressor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Mobile compressors are used for example on construction sites for manual work in which compressed air is required for connected compressed-air tools. One type of compressor that is often used is the piston compressor, in which air is sucked into one or more cylinders, compressed by a piston and discharged again as compressed air. The amount of air delivered from the piston compressors is usually adapted to the compressed air required in each case by adjusting the drive speed of the machine driving the compressor. DE 10 2004 007 882 B4 discloses for example a compressor having a compressed-air sensor, depending on the measured value of which the speed of a piston compressor is adjusted. 
         [0003]    Due to the clocked operation thereof, piston compressors do not discharge compressed air continuously but rather generate compressed air in pulses. Conventionally, a specific compressed-air buffer volume is therefore retained in order to damp the compressed-air pulses by means of the compressor. This buffer volume is conventionally retained in separate storage containers so that compressed air at equally high pressure can be provided to a compressed-air consumer connected to the storage containers. DE 10 2009 052 510 A1 for example relates to a speed-variable piston compressor that has a lightweight and compact compressed-air tank made of plastics material. 
         [0004]    Various other attachments are provided for the design of compressed-air tanks for piston compressors: U.S. Pat. No. 6,089,835 A for example discloses a piston compressor having a compressed-air tank that is formed by a cover housing placed on the outside of the motor housing. U.S. Pat. No. 5,370,504 A discloses a piston compressor in which the compressor cylinders are completely embedded in a storage tank for compressed air. 
         [0005]    However, there is a need for solutions for compressors that have a lower weight and smaller dimensions so that they better suit manual transport. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect of the invention, a compressor is therefore provided, comprising a motor, a drive shaft driven by the motor and connected thereto, a crank mechanism connected to the drive shaft, at least one compressed-air generation apparatus that is driven by the crank mechanism and is designed to generate compressed air, a crankcase that has an inner chamber wall in the shape of a hollow body, which receives the drive shaft at least in portions, an outer chamber wall that is spaced apart from the inner chamber wall radially with respect to the drive shaft, an end wall, and a dividing wall, and a compressed-air storage container that is designed to receive compressed air generated by the compressed-air generation apparatus, wherein the compressed-air storage container is formed by the inner chamber wall, the outer chamber wall, the end wall and the dividing wall. 
         [0007]    The basic concept of the invention is that of embedding the storage container for compressed air generated by the compressor in the crankcase of the compressor by using the space around the drive shaft. In this case, it is highly advantageous that a separate storage container can be omitted, which in turn contributes to a considerable saving in terms of weight and cost. The entire structure of the compressor is more compact, and therefore the compressor remains easy to handle and portable despite having a large storage volume. 
         [0008]    In addition, by integrating the compressed-air storage container in the crankcase, the amount of components required is reduced, which in turn simplifies assembly of the compressor. By supporting the drive shaft in an integral crankcase portion, there is also no need for the complex adjustment of the individual bearing points with respect to one another. Furthermore, components that are required for operating the compressor, for example a pressure sensor, pressure indicator, safety valve, non-return valve or drain valve can be connected to the integrated compressed-air storage container in a cost-effective manner and without additional pipes. 
         [0009]    According to one embodiment of the compressor according to the invention, the compressor may also comprise a motor mount that receives and retains the motor and is connected to the crankcase by forming the end wall between the crankcase and the motor. 
         [0010]    According to another embodiment of the compressor according to the invention, the compressor may also comprise at least one first bearing that supports the drive shaft and is arranged within the hollow body formed by the inner chamber wall. 
         [0011]    In this case, the compressor may comprise at least one second bearing that supports the drive shaft. According to one variant, the second bearing may be arranged between the motor and the first bearing within the hollow body formed by the inner chamber wall. According to another variant, the second bearing may be arranged in the motor outside the hollow body formed by the inner chamber wall. The first and/or second bearing may for example be grease-lubricated rolling bearings. 
         [0012]    According to another embodiment of the compressor according to the invention, the crankcase may be monolithically formed with the inner chamber wall, the outer chamber wall and the dividing wall. In this case, the monolithic crankcase may be designed as a light metal cast part. 
         [0013]    According to another embodiment of the compressor according to the invention, the compressor may also have at least one brace that extends axially with respect to the drive shaft between the inner chamber wall and the outer chamber wall and divides the compressed-air storage container into at least two storage portions. 
         [0014]    According to another embodiment of the compressor according to the invention, the at least two storage portions may be fluidically interconnected by compressed-air lines, valves and/or constrictions. 
         [0015]    According to another embodiment of the compressor according to the invention, the compressor may also have at least one longitudinal rib that is formed integrally with the crankcase on the outside of the compressed-air storage container. 
         [0016]    According to another embodiment of the compressor according to the invention, the compressor may also comprise a motor mount that receives and retains the motor, wherein the crankcase is formed around the motor so as to be spaced apart from the motor mount, and wherein the compressed-air storage container extends at least in part around the motor between the crankcase and the motor mount. 
         [0017]    According to another embodiment of the compressor according to the invention, the compressed-air storage container may enclose the drive shaft within an angular range of 360°. 
         [0018]    According to another embodiment of the compressor according to the invention, the ratio of the distance between the axis of rotation of the drive shaft and the point on the inner wall of the compressed-air storage container that is furthest perpendicularly from the drive shaft to the distance between the axis of rotation of the drive shaft and the upper dead centre of a piston of the compressed-air generation apparatus may be between 0.2 and 1. 
         [0019]    According to another embodiment of the compressor according to the invention, the ratio of the distance between the axis of rotation of the drive shaft and the point on the inner wall of the compressed-air storage container that is furthest perpendicularly from the drive shaft to the maximum axial extent of the compressed-air storage container  25  may be between 0.3 and 2.5. 
         [0020]    According to another embodiment of the compressor according to the invention, the compressed-air generation apparatus may have at least one compressor chamber and the volume ratio between the volume of the compressed-air storage container and the sum of the geometric working volumes of the compressor chambers of the compressed-air generation apparatus may be between 5 and 25. 
     
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
         [0021]    The invention will be described in more detail below with reference to the embodiments and the accompanying drawings. 
           [0022]    The accompanying drawings are used in order to better understand the present invention and show variants of the invention. They are used to explain principles, advantages, technical effects and possible variations. Of course, other embodiments and many of the intended advantages of the invention are likewise conceivable, in particular with reference to the detailed description of the invention set out below. The elements in the drawings are not necessarily shown to scale and are simplified in part or shown schematically for reasons of clarity. Like reference signs denote like or similar components or elements. 
           [0023]      FIG. 1  is a schematic sectional view of a compressor according to one embodiment of the invention. 
           [0024]      FIG. 2  is a schematic cross section through the compressor in  FIG. 1 . 
           [0025]      FIG. 3  is a detailed view of the compressor in  FIG. 1  according to another embodiment of the invention. 
           [0026]      FIG. 4  is a schematic sectional view of a compressor according to another embodiment of the invention. 
           [0027]      FIG. 5  is a detailed view of the compressor in  FIG. 4  according to another embodiment of the invention. 
           [0028]      FIG. 6  is a schematic sectional view of a compressor according to another embodiment of the invention. 
           [0029]      FIG. 7  is a schematic sectional view of a compressor according to another embodiment of the invention. 
           [0030]      FIG. 8  is a schematic sectional view of a compressor according to another embodiment of the invention. 
       
    
    
       [0031]    Although specific embodiments are described and shown herein, it is clear to a person skilled in the art that an abundance of other, alternative and/or equivalent implementations can be selected for the embodiments, essentially without departing from the basic concept of the present invention. In general, all of the variations, modifications and deviations of the embodiments described herein should likewise be considered to be covered by the invention. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0032]      FIG. 1  is a schematic sectional view of a compressor  100 . The compressor  100  generally has a motor  40  that can be retained in a motor mount  41 . The motor  40  may for example be an electric motor having speed control. In this case, it may possible to use the synchronous motors thereof such as brushless DC motors or asynchronous motors. The motor  40  drives a drive shaft  24  that extends from the motor  40  into a crankcase  20 . In this case, the drive shaft  24  may be arranged substantially concentrically with the cross section of the crankcase shape  20  in the centre thereof. The drive shaft  24  is used to drive a crank mechanism  6  that reciprocates a piston  4  in a cylinder  5 , i.e. the crank mechanism  6  converts the rotational movement of the drive shaft  24  into a linear movement in the direction of extension of the piston  4  in the cylinder  5 . For this purpose, the crank mechanism  6  may have a counterweight, a crank web, a connecting rod, a connecting rod bearing and/or a gudgeon pin. In this case, a compressor chamber  11  is formed at the head of the cylinder housing, in which chamber air can be compressed in accordance with the main function of the compressor  100 . A fanwheel  45  may then be arranged on the crank mechanism  6 . 
         [0033]    The compressed-air storage container  25 , which is formed as an integral component of the crankcase  20  in  FIG. 1 , is a key component of the crankcase  20 . The crankcase  20  also has an inner chamber wall  26   a  that may be cylindrical, for example, with a circular or polygonal cross section and receives and supports the motor-side part of the drive shaft  24  such that it can rotate. At least one bearing  28   b  is therefore arranged in a first bearing seat inside the chamber wall  26   a.  The bearing  28   b  in the first bearing seat may support a non-motor-side part of the drive shaft  24  between the motor  40  and crank mechanism  6 , i.e. the bearing  28   b  supports the crank mechanism  6  in a floating manner. 
         [0034]    In addition, an additional bearing  28   a  may be formed in a second bearing seat inside the chamber wall  26   a  and may support a motor-side part of the drive shaft  24  between the motor  40  and crank mechanism  6 , i.e. the bearing  28   a  supports the motor  40  in a floating manner. Because the two bearings  28   a  and  28   b  are in the portion of the crankcase  20  that forms the compressed-air storage container  25 , the bearing seats of the bearings  28   a  and  28   b  can be better aligned to one another. This enables improved concentricity of the bearing seats with respect to one another. It is in this case possible for the two bearing seats of the bearings  28   a  and  28   b  in the crankcase  20  to be accessed from one side, in particular if the radial extent of the bearing  28   a  is less than that of the bearing  28   b.    
         [0035]    In order to illustrate the geometry of the compressed-air storage container  25 ,  FIG. 2  is an example of a cross section through the compressor  100  along the cross-sectional line AA in  FIG. 1 . The compressed-air storage container is arranged in this case so as to be substantially annular around the drive shaft  24 . The compressed-air storage container  25  may enclose a minimum angle of 200°, preferably of at least 240°, around the drive shaft  24 . In the example in  FIG. 2 , the crankcase  20  and therefore the compressed-air storage container  25  are in principle a hollow-cylindrical shape. The compressed-air storage container  25  is in this case delimited by the inner chamber wall  26   a  on one side and an outer chamber wall  26   b  on the other side in the radial direction relative to the axis of rotation of the drive shaft  24 . 
         [0036]    The outer chamber wall  26   b  is an outer wall of the crankcase  20  that completely receives the inner chamber wall  26   a  in its interior. In other words, the topology of the case formed by the outer chamber wall  26   b  and the inner chamber wall  26   a  substantially resembles two cylinders mounted inside one another, for example circular cylinders, prismatic cylinders or cylinders having a polygonal cross-sectional area. The cover areas of the cylinder shell surfaces formed between the by the outer chamber wall  26   b  and the inner chamber wall  26   a  may be enclosed by one or more dividing walls  34  on the other side or one or more end walls  23  on the other side in order to form the volume of the compressed-air storage container  25 . 
         [0037]    The dividing wall  34  or the dividing walls  34  each have a main direction of extension that substantially extends perpendicularly to the axial direction of the drive shaft  24 . The end wall  23  likewise has a main direction of extension that substantially extends perpendicularly to the axial direction of the drive shaft  24  and is spaced apart from the dividing wall  34  or the dividing walls  34  by a length that substantially corresponds to the longitudinal extent of the compressed-air storage container  25 . 
         [0038]    In the lateral direction, the compressed-air storage container  25  may be divided by one or more braces  33 . In this way, the compressed-air storage container  25  can be stabilised on the one hand and can be divided into a plurality of partial storage volumes on the other hand. Said partial storage volumes may be interconnected via compressed-air lines or other connection lines such as constrictions. Advantageously, compressed-air coolers and/or valves may also be arranged in the connection lines. In the example in  FIG. 2 , three braces  33  are shown that divide the completely surrounding compressed-air storage container  25  into three equal partial storage volumes that each cover 120° of the crankcase  20 . Of course, other divisions with more or fewer partial storage volumes or an asymmetrical division are likewise possible. The braces  33  may for example be integrally formed with the crankcase  20 , for example in a common metal cast part. 
         [0039]      FIG. 3  is a detailed longitudinal section through the compressor  100  in  FIG. 1 . The compressor  100  is shown in the example in  FIG. 3  as a dry-compressing speed-variable piston compressor  100  that works in accordance with the principle of reciprocating piston compression. In this case, however, it is likewise possible to use an oil-lubricated compressor instead of a dry-compressing compressor. The compression can in this case, as shown by way of example in  FIG. 3 , take place in one stage; however, it may also be possible to carry out the compression in a plurality of stages. 
         [0040]    The compressor according to  FIG. 3 , in a compressor portion  1  on the right-hand side of the figure, has a cylinder  5  in which a piston  4  is arranged in order to compress air from the surroundings. Air from the surroundings can be sucked through an intake air filter  2  into the compression chamber  11  via an inlet opening  3  having an inlet valve. This takes place when the piston  4  moves downwards. 
         [0041]    The linear working movement for the piston  5  is produced by a crank mechanism  6  that is connected to the rotor  43  of the motor  40  by means of a drive shaft  24 . The drive shaft  24  may be mounted so as to rotate relative to the crankcase  20  by means of two bearings  28   a  and  28   b,  for example prelubricated rolling bearings having fixed/floating bearings. The crankcase  20  has a crank mechanism portion  21  that encloses the crank mechanism  6  at least in part and has a storage portion  22  that adjoins the crank mechanism portion  21  and is arranged axially between said portion and the motor  40 . 
         [0042]    It is preferably provided for the dividing wall  34  to separate the compressed-air storage container  25  from the crank mechanism  21  inside the crankcase  20 , i.e. the crank mechanism  6  itself is not located in the air storage volume of the compressed-air storage container  25 . The storage portion  22  is therefore disjointedly formed with the crank mechanism portion  21 . In particular, it is also provided for the cylinder  5  and the piston  4  not to be arranged inside the storage portion  22 , i.e. for the volume of the compressed-air storage container not to include the cylinder  5  and the piston  4 . 
         [0043]    The storage portion  22  has an inner chamber wall  26   a  that is hollow or tubular in order to be arranged around the drive shaft  24  and receives the region of the drive shaft  24  leading through the storage portion  22  and at least one of the two bearings  28   a  and  28   b.  The inner chamber wall  26   a  may have recesses for one or more bearing seats of the bearings  28   a  and  28   b.  Furthermore, more than two bearings  28   a  and  28   b  may be provided. 
         [0044]    Furthermore, the storage portion  22  has an outer chamber wall  26   b  that may be arranged so as to be concentric around the inner chamber wall  26   a  and spaced apart therefrom. Preferably, the inner chamber wall  26   a  and the outer chamber wall  26   b  are integrally formed with the crankcase  20 , i.e. formed as an integral portion of the crankcase  20 . 
         [0045]    The inner chamber wall  26   a  and the outer chamber wall  26   b  define, together with one or more dividing walls  34 , the extension plane of which extends substantially perpendicularly to the axis of rotation of the drive shaft  24 , a compressed-air storage container  25  of the compressor  100 . The compressed-air storage container  25  is arranged annularly around the inner chamber wall  26   a  at least in portions so as to be concentric with the drive shaft  24 . In other words, the compressed-air storage container  25  therefore surrounds the drive shaft  24  at least in a partial angular range. In the example in  FIG. 3 , the compressed-air storage container  25  is arranged completely, i.e. in an angular range of 360°, around the drive shaft  24 . However, it may also be possible to provide only partial angular ranges of less than 360° around the drive shaft  24  in which angular chambers are defined by the chamber walls  26   a  and  26   b  and the dividing walls  34  for the function of the compressed-air storage container  25 . On the motor side, the compressed-air storage container  25  is tightly sealed with respect to the motor region or the motor mount  41  by an end wall  23  of the crankcase  20 . The compressed-air storage container  25  thus defines a control volume that is used to receive and temporarily store compressed air generated by the piston compressor by means of the corresponding dimensions of the chamber walls  26   a  and  26   b  and the axial distance L 3  between the dividing walls  34  and the end wall  23  of the crankcase  20 . 
         [0046]    The motor mount  41  may assume the function of supporting the torque between the rotor and stator of the motor  40 . The motor mount  41  may be a component that completely or only partially surrounds the motor  40  and may have closed bordering walls having braces, columns or the like. In this case, the motor mount  41  may also act as a completely closed motor housing. 
         [0047]    The motor mount  41  may in addition form the end wall  23 , which is arranged between the motor  40  and the storage portion  22  in the example in  FIG. 3 . However, it may also be provided for the end wall  23  to be arranged on the outside of the motor  40  so that the motor  40  is contained at least in part by the storage portion  22 , i.e. that the volume of the compressed-air storage container  25  extends at least in part in the axial direction of the drive shaft  24 , completely or in a partial angular range around the motor  40 . 
         [0048]    After a suction cycle of the piston  4 , the sucked-in air is compressed in the compression chamber  11  in a compression cycle when the piston  4  moves upwards and is output via the outlet opening  7  and an outlet valve arranged therein. The compressed air that is discharged via the outlet opening  7  may be output into a compressed-air line  8  that may comprise a region having a cooling line  9  for cooling purposes. The compressed air passes via the cooling line  9  through the non-return valve  10  to reach a compressed-air storage container  25  of the compressor  100 . 
         [0049]    Sealing with respect to the surroundings may expediently take place by means of seals  29  and  30 , for example O-rings. Both the crankcase  20  and the motor mount  41  may be reinforced by ribs  32 . Said ribs  32 , which can be attached to the outside of the crankcase  20  and/or of the motor mount  41  in a similar manner, contribute to better heat dissipation from the compressed air. In addition, it is possible to optimise the mechanical stability of the compressor  100  in this way. 
         [0050]    A compressed-air discharge line, for example a compressed-air tube for a tool operated by compressed air through which the compressed air may be extracted as required from the compressed-air storage container  25 , may be connected via a compressed-air coupling  31 . 
         [0051]    When the compressor is in operation, a compressor controller  60  may retrieve the pressure of the compressed air that is measured by a pressure sensor  27  arranged on the compressed-air storage container  25  via a control line  61 . If the measured target pressure in the compressed-air storage container  25  deviates from the target pressure stored in the compressor controller  60 , a target speed signal for the motor  40  can be determined from the control deviation, which signal is sent by the compressor controller  60  as an actuation signal via a control line  62  to a motor controller, for example to the frequency converter  70  of an electric motor  40 . The frequency converter  70  controls the speed of the motor  40  depending on the sent actuation signal. 
         [0052]    When the speed of the motor  40  is adjusted and the amount of delivered air from the compressor  100  is adapted as a result, it is advantageous for the size of the compressed-air storage container  25  to be able to be reduced while the switching frequency remains the same. As an alternative, it is likewise possible to reduce the switching frequency while the size of the compressed-air storage container  25  remains the same. By adjusting the speed, it is moreover advantageously possible to reduce the minimum amount of delivered air from the compressor, which in turn can lead to a smaller size of the compressed-air storage container  25  or a lower switching frequency. Finally, it is also possible to fill the compressed-air storage container  25  more rapidly after an idle phase, in particular if the compressor  100  is operated in a speed-adjusted manner and can provide a greater amount of delivered air at a low pressure. 
         [0053]    In the example in  FIG. 3 , the motor  40  is an electronically commutated synchronous external rotor motor in which a frequency converter  70  is directly attached to the stator  44 . The stator  44  bears the stator winding  46  and may for example be connected to the motor mount  41  by screws. The torque required for the compression of the compressor  100  is generated by the alternating magnetic field generated in the stator winding  46  in a known manner by interaction with the permanent magnets  48  in the rotor  43  of the motor  40 . 
         [0054]      FIG. 4  is a longitudinal section through a compact speed-variable piston compressor  100  having an alternative motor construction. Said compressor differs from the compressor  100  in  FIG. 1  substantially in that the motor  40  is an internal rotor motor having an external frequency converter.  FIG. 5  shows a more detailed view of the compressor from  FIG. 4 . In this case, the motor  40  has an external frequency converter  70  that is connected to the motor  40  via a motor connection cable  47 . If, for assembly reasons, the motor  40  cannot be attached to the crankcase  20  by means of the motor mount  41 , a cover can additionally be provided as the end wall  23  in the case of the compressor from  FIG. 5 . The cover  23  may attach the motor  40  to the motor mount  41 , which can then assume a housing function for the motor  40 . The cover  23  can also fluidically seal the compressed-air storage container  25 , which is located in the crankcase  20 . 
         [0055]    Both for the compressor  100  in  FIGS. 1 to 3  and the compressor  100  in  FIGS. 4 and 5 , the maximum radial extent L 2  (distance between the axis of rotation of the drive shaft  24  and the point on the inner wall of the compressed-air storage container  25  that is furthest perpendicularly from the drive shaft  24 ) may be in a specific ratio to the compressor length L 1  (distance between the axis of rotation of the drive shaft  24  and the upper dead centre of the piston). In the simplest case, the extent L 2  may be smaller than or equal to the compressor length L 1 . A ratio of L 2 /L 1 ≦ 2 / 3  is advantageous. The ratio L 2 /L 1  may in this case be between 0.2 and 1, preferably between 0.4 and 0.66. In absolute terms, the extent L 2  may be smaller than 150 mm, in order to ensure the compactness and therefore the portability of the compressor  100  for example. 
         [0056]    The maximum radial extent L 2  may also be in a specific ratio to the maximum axial extent L 3  of the compressed-air storage container  25 . If the compressed-air storage container  25  is arranged between the crank mechanism  6  and the motor  40 , the ratio L 2 /L 3  may be between 0.3 and 2.5, preferably between 0.5 and 1.33. 
         [0057]    In addition, the volume ratio between the volume V R  of the compressed-air storage container  25  and the geometric working volume V H  of the compressor chamber  11  (or the sum V H  of all the working volumes V Hi  of all the compressor chambers  11  in the case of a plurality of cylinder  5 ) can be set in order to be able to eliminate the damping of the compressed-air pulses in an optimum manner. The ratio V R /V H  may in this case be between 5 and 25. 
         [0058]    The crankcase  20  including all the chamber walls  26   a,    26   b  and end walls  23  and dividing walls  34  may be entirely formed in one piece in  FIGS. 1 to 5 , for example by a dead-mould casting method or a rapid prototyping method such as selective laser melting, 3D printing, additive layer manufacturing, electron beam melting, laser deposition welding or similar methods. Alternatively, it may also be possible for the chamber walls  26   a,    26   b  to be composed of a plurality of parts that are sealed with respect to one another and interconnected, for example screwed together. The crankcase  20  and the relevant components thereof, such as walls, dividing walls and end walls, may for example be produced in a pressure die casting method, for example from a light metal such as aluminium or magnesium. 
         [0059]      FIGS. 6, 7 and 8  are schematic views of additional variants of a compressor  100 . The compressors  100  in  FIGS. 6 and 7  differ from the compressors  100  in  FIGS. 1 and 4  substantially in that the second bearing  28   a  is housed in the motor  40  whereas in  FIG. 6  is it on the non-crankcase-side of the motor  40  and in  FIG. 7  it is on the crankcase-side of the motor  40 . The compressor  100  in  FIG. 8  has a crankcase  20  that together with the motor mount  41  forms a compressed-air storage container  25  that is extended axially with respect to the drive shaft. The compressed-air storage container  25  extends around the motor  40  inside the crankcase  20 , which is correspondingly spaced apart from the motor mount  41 . In this case, the ratio L 2 /L 1  of the maximum radial extent L 2  to the maximum axial extent L 1  of the compressed-air storage container  25  is between 0.12 and 1, preferably between 0.2 and 0.5. 
         [0060]    The compressed-air storage container  25  may enclose the motor  40  in a partial angular range of less than 360° or completely, i.e. over a circumference of 360°. It may also be possible for the compressed-air storage container  25  to completely enclose the motor  40  relative to the angular range around the drive shaft  24 , but to only partially enclose the motor  40  in the axial direction of the axis of rotation of the motor, i.e. is not completely formed up to the non-crankcase-end of the motor mount  40 .