Multi Piston Machine with at least Three Switchable Displacement Volumes

Abstract

A multi piston machine includes a rotor, pistons, a first control valve, and a second control valve. The pistons abut against a cam surface with multiple lobes. The machine is switchable between at least three non-zero displacement volumes using the first and the second control valves. The first control valve is connected to a first and a third fluid chamber. The second control valve is connected to a second and a fourth fluid chamber. The first and the second control valve are connected to first and the second working port respectively. A third number of second control openings is twice a second number of the lobes. There is a first and a second group of the second control openings. Adjacent second control openings belong to a different first or second group. The second control openings of the first group are either connected to the first or the second fluid chamber.

Claims

1. A multi piston machine comprising: a casing having a first working port and a second working port, the casing defining a cam surface having a second number of lobes; a rotor located within the casing and configured for rotation about an axis of rotation, the rotor defining a first control surface; a plurality of pistons; a first control valve; and a second control valve, wherein a first number of pistons of the plurality of pistons are received in the rotor in a linearly movable manner, wherein the pistons are configured to contact the cam surface of the casing, wherein a number of strokes of a single piston during one revolution of the rotor equals the second number, wherein each piston has one associated first control opening at the first control surface of the rotor, wherein the first control openings are located along a first circle having a center defined by the axis of rotation, wherein the casing defines a second control surface configured to abut against the first control surface, wherein the second control surface has a third number of second control openings located along a second circle having a center defined by the axis of rotation, wherein each of the first control openings overlap each of the second control openings in at least one rotational position of the rotor respectively, wherein there is a first, a second, a third, and fourth fluid chamber, wherein each of the second control openings is permanently connected to a single one of the first to fourth fluid chambers, wherein the multi piston machine is switchable between at least three non-zero displacement volumes using the first control valve and the second control valve, wherein the first control valve is connected to the first and the third fluid chambers, wherein the second control valve is connected to the second and the fourth fluid chambers, wherein the first and the second control valves are connected to the first and the second working ports respectively, wherein the third number is twice the second number, wherein there is a first and a second group of the second control openings, wherein adjacent second control openings belong to a different first or second group, wherein the second control openings of the first group are either connected to the first or the second fluid chamber, and wherein the second control openings of the second group are either connected to the third or the fourth fluid chamber.

2. The multi piston machine according to claim 1, wherein: the first control valve has a first and a second position, in the first position of the first control valve, the first fluid chamber is connected to the first working port, and the third fluid chamber is connected to the second working port, and in the second position of the first control valve, the first and the third fluid chambers are directly connected to each other, and the connection to the first and the second working ports is blocked.

3. The multi piston machine according to claim 2, wherein: the first control valve has a first control port, a pressure at the first control port urges the first control valve into the second position of the first control valve, in the second position of the first control valve, the first control port is connected to the first and the third fluid chambers, in the first position of the first control valve, the connection of the first control port to the first and the third fluid chambers is blocked.

4. The multi piston machine according to claim 3, wherein: the second control valve has a third and a fourth position, in the third position of the second control valve, the second fluid chamber is connected to the first working port, and the fourth fluid chamber is connected to the second working port, and in the fourth position of the second control valve, the second and the fourth fluid chambers are directly connected to each other, and the connection to the first and the second working ports is blocked.

5. The multi piston machine according to claim 4, wherein: the second control valve has a second control port, a pressure at the second control port urges the second control valve into the fourth position of the second control valve, in the fourth position of the second control valve, the second control port is connected to the second and the fourth fluid chambers, and in the third position of the second control valve, the connection of the second control port to the second and the fourth fluid chambers is blocked.

6. The multi piston machine according to claim 4, wherein when the first control valve is in the second position of the first control valve and the second control valve is in the fourth position of the second control valve, the first and the second working ports are directly connected to each other via the first and the second control valves.

7. The multi piston machine according to claim 1, wherein: the first fluid chamber is connected to a fourth number of the second control openings, the third fluid chamber is connected to the fourth number of the second control openings, the second fluid chamber is connected to a fifth number of the second control openings, the fourth fluid chamber is connected to the fifth number of the second control openings, and the fourth number and the fifth number differ by at least one.

8. The multi piston machine according to claim 1, wherein first number is larger than the third number by at least three.

9. The multi piston machine according to claim 7, wherein: the first and the second number have no common prime factor besides one, the cam surface has at least two dwell sections, in which a distance between the axis of rotation and the cam surface is constant, such that one of the pistons contacting the dwell section does not move while the rotor rotates, and an angular extent of at least two dwell sections is different from each other.

10. The multi piston machine according to claim 9, wherein: the first number is eighteen, the second number is seven, the third number is fourteen, the fourth number is three, and the fifth number is four.

11. The multi piston machine according to claim 1, wherein within the same first or second group at maximum two adjacent second control openings are connected to the same fluid chamber from the first to the fourth fluid chambers.

12. The multi piston machine according to claim 1, wherein: a moving direction of the pistons is radially with respect to the axis of rotation, and a first and a second row of pistons are offset relative to each other in a direction of the axis of rotation.

13. The multi piston machine according to claim 1, wherein: the lobes of the cam surface define multiple dead centers, a distance from the cam surface to the axis of rotation is a minimum or a maximum at the dead center, and each second control opening is located distant to named dead centers in a circumferential direction with respect to the axis of rotation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The disclosure is explained in more detail below with reference to the accompanying drawings.

[0027] FIG. 1 is a perspective view of an inventive multiple piston machine;

[0028] FIG. 2 is a perspective view of a rotor of the machine of FIG. 1;

[0029] FIG. 3 is a perspective view of a distributor of the machine of FIG. 1;

[0030] FIG. 4 is a schematic diagram illustrating pistons, first and second control openings, and first to fourth fluid chambers of the machine of FIG. 1;

[0031] FIG. 5 is a schematic illustrating first and the second control valves, the first to the fifth fluid chambers, and first and second working ports of the machine of FIG. 1;

[0032] FIG. 6 is a schematic diagram based on FIG. 3 illustrating dwell sections of the machine of FIG. 1;

[0033] FIG. 7 is a perspective view of another inventive multiple piston machine;

[0034] FIG. 8 is a perspective view of a rotor of the machine of FIG. 7;

[0035] FIG. 9 is a perspective view of a distributor of the machine of FIG. 7;

[0036] FIG. 10 is a schematic diagram illustrating pistons, first and second control openings, and first to fourth fluid chambers of the machine of FIG. 7;

[0037] FIG. 11 is a schematic illustrating first and the second control valves, the first to fifth fluid chambers, and first and the second working ports of the machine of FIG. 7; and

[0038] FIG. 12 is a schematic diagram based on FIG. 9 dwell sections of the machine of FIG. 1.

DETAILED DESCRIPTION

[0039] FIG. 1 shows a perspective view of an inventive multiple piston machine. The multiple piston machine 10 has a casing 60 comprising a first, a second, a third and a fourth casing part 64, which together enclose all components of the multiple piston machine 10 in a fluid tight manner. The first, the second and the third casing part 61; 62; 63 are fixed to each other. Preferably, the third casing part 63 has a first flange 65, which can be connected to a frame of a vehicle for example. The fourth casing part 64 is rotatable about an axis of rotation 13 with respect to the remaining casing 61; 62; 63. It has a second flange 66, which can be connected to a wheel of the named vehicle for example.

[0040] The cup shaped first casing part 61 holds the distributor (no. 30 in FIG. 3) and the first and the second control valve 41; 42, wherein no. 41; 42 actually point to a lid, which covers a spool of the first or second control valve 41; 42 respectively. The first and the second working port 11; 12 and the first and the second control port 43; 44 are located at the first casing part 61.

[0041] The second casing part 62 surrounds the rotor (no. 70 in FIG. 2) in a ring shaped manner. The inner circumferential surface of the second casing part 62 extents along the axis of rotation 13 with a constant cross section, wherein it forms the cam surface (no. 21 in FIG. 4).

[0042] The third casing part 63 surrounds a disc brake, which is known from EP 2 841 763 B1 for example. The fourth casing part 64 is fixed to the rotor (no. 70 in FIG. 2), via a splined shaft which is formed by the fourth casing part 64. The named shaft is supported by the third casing part 63 via roller bearings.

[0043] FIG. 2 shows a perspective view of the rotor 70. The rotor 70 at hand has eighteen pistons 1.1-1.18. The reference numerals 1.1-1.18 are assigned in numerical order around the rotor 70. There are two rows of pistons, wherein each row comprises one half of the total number of pistons. The two rows are shifted to each other in the direction of the axis of rotation 13 to allow for a larger diameter of each individual piston 1.1-1.18. The pistons 1.1-1.18 are movable radially with respect to axis of rotation 13, such that the axis of movement intersects axis of rotation 13 at 90°. It should be noted that the disclosure is not restricted to this angle. The axis of movement of the pistons could also be parallel to the axis of rotation 13 for example.

[0044] All pistons 1.1-1.18 are shown in their most inward position. During operation the piston 1.1-1.18 stick out of the rotor 70 such that they contact the cam surface (no. 21 in FIG. 4) with a roller 74. The roller 74 is held rotatably in the remaining piston via a hydrostatic bearing such that it can rotate with low friction despite the high forces acting on the pistons 1.1-1.18. All pistons are configured identically.

[0045] The rotor 70 has an even first control surface 71, which is perpendicular to the axis of rotation 13. On the first control surface 71 there is a first control opening 2.1-2.18 for each piston. The numbering (number after the point) of first openings 2.1-2.18 is identical to the numbering of the pistons 1.1-1.18. This means first control open 2.1 is connected to the cylinder (no. 73 in FIG. 4) of piston 1.1. The first control openings are equally distributed along a first circle 72 whose center is defined by the axis of rotation 13. All first control openings 2.1-2.18 are identical to each other, wherein they are circular.

[0046] The rotor 50 has a splined bore 75 via which it is connected to the fourth casing part (no. 64 in FIG. 1) in a rotationally fixed manner.

[0047] FIG. 3 shows a perspective view of the distributor 30. The distributor 30 is a one-piece part providing the second control surface 36 and the first to fifth fluid chamber 31-35. The even second control surface 36 is perpendicular to the axis of rotation 13. It has fourteen second control openings 3.1-3.14 which are nearly equally distributed along a second circle 37 in numerical order. The center of the second circle is defined by the axis of rotation 13, wherein its diameter is equal to the diameter of the first circle (no. 72 in FIG. 2). The second control openings 3.1-3.14 are nearly identical to each other, wherein they are formed like an oblong hole which extends in radial direction.

[0048] The distributer 30 has an outer surface which rotationally symmetric with respect to the axis of rotation 13 and which is adapted to the first casing part (no. 61 in FIG. 1) in a fluid tight manner. The hydraulic pressure in the first to fifth fluid chamber 31-35 urges the distributer 30 in the direction of the axis of rotation 13, such that the second control surface 36 abuts against the first control surface (no. 71 in FIG. 1) in a fluid tight manner. During one revolution of the rotor each first control opening overlaps each second control opening in at least in one rotational position of the rotor.

[0049] The first to fifth fluid chamber 31-35 are for formed by grooves on the circumferential surface of the distributor 30, which are arranged along the axis rotation 13 in numerical order. The connection between the first to fourth fluid chambers 31-34 with the second control openings will be explained with reference to FIG. 4 below. These permanent connections are formed by channels inside the distributor 30, which were made during the casting of the blank distributor. The fifth fluid chamber provides a fluid connection between the first and the second control valve, which is marked in FIG. 5 with no. 35.

[0050] The notch 38 prevents a rotation of the distributor 30 with respect to the axis of rotation 13. A pin, which is held by the second casing part, extends into the notch 38. By definition the notch is located between the second control openings 3.1 and 3.2.

[0051] FIG. 4 shows a schematic diagram comprising the pistons 1.1-1.18, the first and second control openings 2.1-2-18; 3.1-3.14 and the first to fourth fluid chamber 31; 32; 33; 34. For sake of clarity, some of the reference numerals 1.1-1.18; 2.1-2.18; 3.1-3.14 are not shown. In all three cases, there is a consecutive numbering, which ascends from left to right in FIG. 4.

[0052] The piston 1.1-1.18 are equally distributed around the axis of rotation (no. 13 in FIG. 2), wherein they are shown in an unfolded way in FIG. 4. The cam surface 21, the rotor 70 and the distributor 30 are shown correspondingly. The two dash-dot lines 15 refer to the same circumferential position with respect to the axis of rotation (no. 13 in FIG. 2, namely the center of piston 1.18.

[0053] The cam surface 21 on the inner circumference of the second casing part (no. 62 in FIG. 1) has seven lobes, wherein it is basically sinus shaped. Further details are explained with reference to FIG. 6. The fluid pressures in the cylinders 74 urge the moveable pistons 1.1-1.18 against the cam surface 21 such that they follow the cam surface 21 when the rotor 70 rotates. In consequence during one rotation of the rotor 70 each piston 1.1-1.18 executes seven strokes.

[0054] The distributor 30 has fourteen second control openings 3.1-3.14, i.e., two for each lobe of the cam surface 21. The rotational position of the distributor 30 relative to the cam surface 21 is fixed by a notch 38 (see FIG. 3 too) which engages with a cylindrical pin fixed in the second casing part (no. 62 in FIG. 1), such that each dead center (maximum or minimum) of the cam surface 21 is located between two neighboring second control openings 3.1-3.14.

[0055] The rotor 70 has eighteen pistons 1.1-1.18 which are accommodated in a respective cylinder 73 of the rotor 70 so that the can move linearly. Each piston 1.1-1.18 contacts the cam surface 21 via a roller (no. 74 in FIG. 2) which is not shown in FIG. 4. Each cylinder 73 has as a respective first control opening 2.1-2.18, wherein each first control opening 2.1-2.18 overlaps each second control opening 3.1-3.14 during one rotation of the rotor 70.

[0056] Below no. 30 in FIG. 4 the internal connections of the distributor are shown, which are selected to provide three switchable non-zero displacement volumes according to the disclosure. Since the first number of pistons 1.1-1.18 and the third number of second control openings 3.1-3.14 have number two as a common prime factor there are pairs of pistons which have a 180° phase relation. For instance piston 1.2 and 1.11 show a phase relation of 180°. In theory it is possible to short circuit such a pair of pistons so that it does no contribute to the overall displacement volume of the multi piston machine, wherein no pressure peaks are produced. This is the basic working principle of US 6,050,173 A. But exactly this option is not used with the present disclosure. There is no position of the first and second control valve (no. 41; 42 in FIG. 5) which results in such a short circuit of the named pair of pistons 1.2/1.11, i.e. in a short circuit between the second and third fluid chamber 32; 33 in the rotor position shown in FIG. 4. Instead the disclosed connections are used.

[0057] There is a first and second group of second control openings A; B, wherein neighboring second control openings 2.1-2.14 belong to a different first or second group A; B. The first fluid chamber 3.1 is permanently connected to three second control openings 3.6; 3.8; 3.14 belonging to the first group A. The second fluid chamber 32 is permanently connected to four second control openings 3.2; 3.4; 3.10; 3.12 belonging to the first group A. The third fluid chamber 33 is permanently connected to three second control openings 3.1; 3.7; 3.9 belonging to the second group B. The fourth fluid chamber 34 is permanently connected to four second control openings 3.3; 3.5; 3.11; 3.13 belonging to second group B.

[0058] FIG. 5 shows a schematic comprising the first and the second control valve 41; 42, the first to fifth fluid chamber 31-35 and the first and the second working port 11; 12. The first and the second control valve 41; 42 and the first and the second auxiliary valve 45; 56 are preferably configured as spool valves respectively. The first control valve 41; has a first and a second position 51; 52, wherein the second control valve 42 has a third and a fourth position 53; 54. In FIG. 5, the first and third positions 51; 53 are active such that all second control openings belonging to group A are connected to the first port 11, wherein all second control openings belonging to group B are connected to the second working port 12. Namely the first control valve 41 connects the first working port 11 with the first fluid chamber 31 and the second working port 12 with the third fluid chamber 33. The second control valve 42 connects the first working port 11 with the second fluid chamber 32 and the second working port 12 with the fourth fluid chamber 34. The fifth fluid chamber 35 is not used in this switching position, in which the multi piston machine works with the maximum displacement volume.

[0059] The second to maximum displacement volume is active, when the first control valve 41 is in the second position 52, wherein the second control valve 42 is in the third position 53. Then the first working port 11 is only connected to the second fluid chamber 32, wherein the second working port 12 is only connected to the fourth fluid chamber 34 wherein both connections are provided by the second control valve 42. The first control valve 41 provides a direct connection between the first and the third fluid chamber 31; 33 via its first short circuit connection 55. Consequently the second control openings 3.1; 3.6; 3.7; 3.8; 3.9; 3.14 are connected to each other. The two pairs 3.1/3.8 and 3.8/3.14 provide an exact 180° phase shift. The remaining pair 3.6/3.9 does not exactly provide a 180° phase shift, but nearly. To minimize pressure peaks due to this small miss-match the first auxiliary valve 45 provides a connection between the first short connection 55 and the first control port 43, when it is switched into its open position by a pressure in the first control port 43, which urges the first control valve 41 into is second position 52. In this state eight of the fourteen second control openings contribute to the net displacement volume so that the net displacement volume is 8/14 of the maximum displacement volume.

[0060] The third to maximum displacement volume is active, when the first control valve 41 is in the first position 51, wherein the second control valve 42 is in the fourth position 54. Then the first working port 11 is only connected to the first fluid chamber 31, wherein the second working port 12 is only connected to the third fluid chamber 33 wherein both connections are provided by the first control valve 41. The second control valve 42 provides a direct connection between the second and the fourth fluid chamber 32; 34 via its second short circuit connection 56. Consequently the second control openings 3.2; 3.3; 3.4; 3.5; 3.10; 3.11; 3.12; 3.13 are connected to each other. The three pairs 3.3/3.10; 3.4/3.11; and 3.5/3.12 provide an exact 180° phase shift. The remaining pair 3.2/3.13 does not exactly provide a 180° phase shift, but nearly. To minimize pressure peaks due to this small miss-match the second auxiliary valve 46 provides a connection between the second short connection 46 and the second control port 44, when it is switched into its open position by a pressure in the second control port 44, which urges the second control valve 42 into its fourth position 54. In this state six of the fourteen second control openings contribute to the net displacement volume so that the net displacement volume is 6/14 of the maximum displacement volume.

[0061] When first control valve 41 is switch into its second position 52 and the second control valve 42 is switched into its fourth position 54 the multi piston machine is in a free-wheeling state. There is a direct connection between the first and the second working 11; 12 ports via the fifth fluid chamber 35. Furthermore, the first to fourth fluid chamber 31-34 are short circuited to each other. When the multi piston machine drives an associated wheel of a vehicle, the wheel can be turned with low resistance, wherein fluid pressure at the first or second working port 11; 12 does not drive the vehicle.

[0062] FIG. 6 shows a schematic diagram based on FIG. 3 comprising the dwell sections 25. As described above in the reduced displacement modes some deviation from a perfect 180° phase relationship is accepted. Most of the time the resulting pressure peaks are satisfactorily minimized by the first and second auxiliary valves (no. 45; 46 in FIG. 5). The pressure peaks may be further reduced by introducing dwell sections 25 at critical positions of the cam surface 21. A dwell section 25 extends to about 0.5° to 2° of rotor rotation. The dwell section 25 is formed in a way that a piston contacting the dwell section 25 does not move when the rotor is turned, as long as it contacts the dwell section 25.

[0063] Based on experiment it can easily be figured out at which rotational positions the pressure peaks occur. Based on the description above the pistons producing the pressure peaks are known. The dwell sections 25 are introduced correspondingly. Their length is selected as low as possible but long enough to reduce the offending pressure peaks. Experiments of the applicant have shown that it is desirable the choose dwell sections 25 with different length at different positions. Preferably, the dwell sections 25 are located at a position where the offending piston e.g. 1.1 is at the center between two neighboring second control openings e.g. 3.10; 3.11.

[0064] FIG. 7 shows a perspective view of another inventive multiple piston machine 10. The multiple piston machine 10 has a casing 60 comprising a first, a second, a third and a fourth casing part 61; 62; 63; 64, which together enclose all components of the multiple piston machine 10 in a fluid tight manner. The first, the second and the third casing part 61; 62; 63 are fixed to each other. Preferably, the third casing part 63 has a first flange 65, which can be connected to a frame of a vehicle for example. The fourth casing part 64 is rotatable about an axis of rotation 13 with respect to the remaining casing 61; 62; 63. It has a second flange 66, which can be connected to a wheel of the named vehicle for example.

[0065] The cup shaped first casing part 61 holds the distributor (no. 30 in FIG. 9) and the first and the second control valve 41; 42, wherein no. 41; 42 actually point to the section of the casing 60, which covers a spool of the first or second control valve 41; 42 respectively. The first and the second working port 11; 12 and the first and the second control port 43; 44 are located at the first casing part 61.

[0066] The second casing part 62 surrounds the rotor (no. 70 in FIG. 8) in a ring shaped manner. The inner circumferential surface of the second casing part 62 extents along the axis of rotation 13 with a constant cross section, wherein it forms the cam surface (no. 21 in FIG. 10).

[0067] The third casing part 63 surrounds a disc brake, which is known from EP 2,841,763 B1 for example. The fourth casing part 64 is fixed to the rotor (no. 70 in FIG. 8), via a splined shaft which is formed by the fourth casing part 64. The named shaft is supported by the third casing part 63 via roller bearings.

[0068] FIG. 8 shows a perspective view of the rotor 70. The rotor 70 at hand has eighteen pistons 1.1-1.18. The reference numerals 1.1-1.18 are assigned in numerical order around the rotor 70. The pistons 1.1-1.18 are movable radially with respect to axis of rotation 13, such that the axis of movement intersects axis of rotation 13 at 90°. It should be noted that the disclosure is not restricted to this angle. The axis of movement of the pistons could also be parallel to the axis of rotation 13 for example.

[0069] All pistons 1.1-1.18 are shown in their most inward position. During operation the piston 1.1-1.18 stick out of the rotor 70 such that they contact the cam surface (no. 21 in FIG. 10) with a roller 74. The roller 74 is held rotatably in the remaining piston via a hydrostatic bearing such that it can rotate with low friction despite the high forces acting on the pistons 1.1-1.18. All pistons are configured identically.

[0070] The rotor 70 has an even first control surface 71, which is perpendicular to the axis of rotation 13. On the first control surface 71 there is a first control opening 2.1-2.18 for each piston. The numbering (number after the point) of first openings 2.1-2.18 is identical to the numbering of the pistons 1.1-1.18. This means first control open 2.1 is connected to the cylinder (no. 73 in FIG. 10) of piston 1.1. The first control openings are equally distributed along a first circle 72 whose center is defined by the axis of rotation 13. All first control openings 2.1-2.18 are identical to each other, wherein they are circular.

[0071] The rotor 50 has a splined bore 75 via which it is connected to the fourth casing part (no. 64 in FIG. 7) in a rotationally fixed manner.

[0072] FIG. 9 shows a perspective view of the distributor 30. The distributor 30 is a one-piece part providing the second control surface 36 and the first to fifth fluid chamber 31-35. The even second control surface 36 is perpendicular to the axis of rotation 13. It has fourteen second control openings 3.1-3.14 which are nearly equally distributed along a second circle 37 in numerical order. The center of the second circle 37 is defined by the axis of rotation 13, wherein its diameter is equal to the diameter of the first circle (no. 72 in FIG. 2). The second control openings 3.1-3.14 are nearly identical to each other, wherein they are formed like an oblong hole which extends in radial direction.

[0073] The distributer 30 has an outer surface which is rotationally symmetric with respect to the axis of rotation 13 and which is adapted to the first casing part (no. 61 in FIG. 7) in a fluid tight manner. The hydraulic pressure in the first to fifth fluid chamber 31-35 urges the distributer 30 in the direction of the axis of rotation 13, such that the second control surface 36 abuts against the first control surface (no. 71 in FIG. 7) in a fluid tight manner. During one revolution of the rotor each first control opening overlaps each second control opening in at least in one rotational position of the rotor.

[0074] The first to fifth fluid chamber 31-35 are for formed by grooves on the circumferential surface of the distributor 30, which are arranged along the axis rotation 13 in numerical order. The connection between the first to fourth fluid chambers 31-34 with the second control openings will be explained with reference to FIG. 10 below. These permanent connections are formed by channels inside the distributor 30, which were made during the casting of the blank distributor. The fifth fluid chamber provides a fluid connection between the first and the second control valve, which is marked in FIG. 11 with no. 35.

[0075] The notch 38 prevents a rotation of the distributor 30 with respect to the axis of rotation 13. A pin, which is held by the second casing part, extends into the notch 38. By definition the notch is located between the second control openings 3.1 and 3.2.

[0076] FIG. 10 shows a schematic diagram comprising the pistons 1.1-1.18, the first and second control openings 2.1-2-18; 3.1-3.14 and the first to fourth fluid chamber 31; 32; 33; 34. For sake of clarity some of the reference numerals 1.1-1.18; 2.1-2.18; 3.1-3.14 were missed out. In all three cases there is a consecutive numbering, which ascends from left to right in FIG. 10.

[0077] The piston 1.1-1.18 are equally distributed around the axis of rotation (no. 13 in FIG. 8), wherein they are shown in an unfolded way in FIG. 10. The cam surface 21, the rotor 70 and the distributor 30 are shown correspondingly. The two dash-dot lines 15 refer to the same circumferential position with respect to the axis of rotation (no. 13 in FIG. 8, namely the center of piston 1.18.

[0078] The cam surface 21 on the inner circumference of the second casing part (no. 62 in FIG. 7) has seven lobes, wherein it is basically sinus shaped. Further details are explained with reference to FIG. 12. The fluid pressures in the cylinders 74 urge the moveable pistons 1.1-1.18 against the cam surface 21 such that they follow the cam surface 21 when the rotor 70 rotates. In consequence during one rotation of the rotor 70 each piston 1.1-1.18 executes seven strokes.

[0079] The distributor 30 has fourteen second control openings 3.1-3.14, i.e., two for each lobe of the cam surface 21. The rotational position of the distributor 30 relative to the cam surface 21 is fixed by a notch 38 (see FIG. 9 too) which engages with a cylindrical pin fixed in the second casing part (no. 62 in FIG. 7), such that each dead center (maximum or minimum) of the cam surface 21 is located between two neighboring second control openings 3.1-3.14.

[0080] The rotor 70 has eighteen pistons 1.1-1.18 which are accommodated in a respective cylinder 73 of the rotor 70 so that the can move linearly. Each piston 1.1-1.18 contacts the cam surface 21 via a roller (no. 74 in FIG. 8) which is not shown in FIG. 10. Each cylinder 73 has as a respective first control opening 2.1-2.18, wherein each first control opening 2.1-2.18 overlaps each second control opening 3.1-3.14 during one rotation of the rotor 70.

[0081] Below no. 30 in FIG. 10 the internal connections of the distributor are shown, which are selected to provide three switchable non-zero displacement volumes according to the disclosure. Since the first number of pistons 1.1-1.18 and the third number of second control openings 3.1-3.14 have number two as a common prime factor there are pairs of pistons which have a 180° phase relation. For instance piston 1.2 and 1.11 show a phase relation of 180°. In theory it is possible to short circuit such a pair of pistons so that it does no contribute to the overall displacement volume of the multi piston machine, wherein no pressure peaks are produced. This is the basic working principle of U.S. Pat. No. 6,050,173 A. But exactly this option is not used with the present disclosure. There is no position of the first and second control valve (no. 41; 42 in FIG. 11) which results in such a short circuit of the named pair of pistons 1.2/1.11, i.e., in a short circuit between the second and third fluid chamber 32; 33 in the rotor position shown in FIG. 10. Instead the disclosed connections are used.

[0082] There is a first and second group of second control openings A; B, wherein neighboring second control openings 2.1-2.14 belong to a different first or second group A; B. The first fluid chamber 3.1 is permanently connected to three second control openings 3.6; 3.8; 3.14 belonging to the first group A. The second fluid chamber 32 is permanently connected to four second control openings 3.2; 3.4; 3.10; 3.12 belonging to the first group A. The third fluid chamber 33 is permanently connected to three second control openings 3.1; 3.7; 3.9 belonging to the second group B. The fourth fluid chamber 34 is permanently connected to four second control openings 3.3; 3.5; 3.11; 3.13 belonging to second group B.

[0083] FIG. 11 shows a schematic comprising the first and the second control valve 41; 42, the first to fifth fluid chamber 31-35 and the first and the second working port 11; 12. The first and the second control valve 41; 42 and the first and the second auxiliary valve 45; 56 are preferably configured as spool valves respectively. The first control valve 41; has a first and a second position 51; 52, wherein the second control valve 42 has a third and a fourth position 53; 54. In FIG. 11, the first and third positions 51; 53 are active such that all second control openings belonging to group A are connected to the first port 11, wherein all second control openings belonging to group B are connected to the second working port 12. Namely the first control valve 41 connects the first working port 11 with the first fluid chamber 31 and the second working port 12 with the third fluid chamber 33. The second control valve 42 connects the first working port 11 with the second fluid chamber 32 and the second working port 12 with the fourth fluid chamber 34. The fifth fluid chamber 35 is not used in this switching position, in which the multi piston machine works with the maximum displacement volume.

[0084] The second to maximum displacement volume is an active, when the first control valve 41 is in the second position 52, wherein the second control valve 42 is in the third position 53. Then the first working port 11 is only connected to the second fluid chamber 32, wherein the second working port 12 is only connected to the fourth fluid chamber 34 wherein both connections are provided by the second control valve 42. The first control valve 41 provides a direct connection between the first and the third fluid chamber 31; 33 via its first short circuit connection 55. Consequently the second control openings 3.1; 3.6; 3.7; 3.8; 3.9; 3.14 are connected to each other. The two pairs 3.1/3.8 and 3.8/3.14 provide an exact 180° phase shift. The remaining pair 3.6/3.9 does not exactly provide a 180° phase shift, but nearly. To minimize pressure peaks due to this small miss-match the first auxiliary valve 45 provides a connection between the first short connection 55 and the first control port 43, when it is switched into its open position by a pressure in the first control port 43, which urges the first control valve 41 into is second position 52. In this state eight of the fourteen second control openings contribute to the net displacement volume so that the net displacement volume is 8/14 of the maximum displacement volume.

[0085] The third to maximum displacement volume is an active, when the first control valve 41 is in the first position 51, wherein the second control valve 42 is in the fourth position 54. Then the first working port 11 is only connected to the first fluid chamber 31, wherein the second working port 12 is only connected to the third fluid chamber 33 wherein both connections are provided by the first control valve 41. The second control valve 42 provides a direct connection between the second and the fourth fluid chamber 32; 34 via its second short circuit connection 56. Consequently the second control openings 3.2; 3.3; 3.4; 3.5; 3.10; 3.11; 3.12; 3.13 are connected to each other. The three pairs 3.3/3.10; 3.4/3.11; and 3.5/3.12 provide an exact 180° phase shift. The remaining pair 3.2/3.13 does not exactly provide a 180° phase shift, but nearly. To minimize pressure peaks due to this small miss-match the second auxiliary valve 46 provides a connection between the second short connection 46 and the second control port 44, when it is switched into its open position by a pressure in the second control port 44, which urges the second control valve 42 into its fourth position 54. In this state six of the fourteen second control openings contribute to the net displacement volume so that the net displacement volume is 6/14 of the maximum displacement volume.

[0086] When first control valve 41 is switch into its second position 52 and the second control valve 42 is switched into its fourth position 54 the multi piston machine is in a free-wheeling state. There is a direct connection between the first and the second working 11; 12 ports via the fifth fluid chamber 35. Furthermore the first to fourth fluid chamber 31-34 are short circuited to each other. When the multi piston machine drives an associated wheel of a vehicle, the wheel can be turned with low resistance, wherein fluid pressure at the first or second working port 11; 12 does not drive the vehicle.

[0087] FIG. 12 shows a schematic diagram based on FIG. 9 comprising the dwell sections 25. As described above in the reduced displacement modes some deviation from a perfect 180° phase relationship is accepted. Most of the time the resulting pressure peaks are satisfactorily minimized by the first and second auxiliary valves (no. 45; 46 in FIG. 11). The pressure peaks may be further reduced by introducing dwell sections 25 at critical positions of the cam surface 21. A dwell section 25 extends to about 0.5° to 2° of rotor rotation. The dwell section 25 is formed in a way that a piston contacting the dwell section 25 does not move when the rotor is turned, as long as it contacts the dwell section 25.

[0088] Based on experiment it can easily be figured out at which rotational positions the pressure peaks occur. Based on the description above the pistons producing the pressure peaks are known. The dwell sections 25 are introduced correspondingly. Their length is selected as low as possible but long enough to reduce the offending pressure peaks. Experiments of the applicant have shown that it is desirable the choose dwell sections 25 with different length at different positions. Preferably, the dwell sections 25 are located at a position where the offending piston e.g. 1.1 is at the center between two neighboring second control openings e.g. 3.10; 3.11.

REFERENCE NUMERALS

[0089] A first group of second control openings [0090] B second group of second control openings [0091] 1.1 . . . 1.18 piston [0092] 2.1 . . . 2.18 first control opening [0093] 3.1 . . . 3.14 second control opening [0094] 10 multi piston machine [0095] 11 first working port [0096] 12 second working port [0097] 13 axis of rotation [0098] 14 angle of rotation [0099] 15 dash-dot line [0100] 21 cam surface [0101] 22 lobe [0102] 23 cam ring [0103] 24 dead center [0104] 25 dwell section [0105] 30 distributor [0106] 31 first fluid chamber [0107] 32 second fluid chamber [0108] 33 third fluid chamber [0109] 34 fourth fluid chamber [0110] 35 fifth fluid chamber [0111] 36 second control surface [0112] 37 second circle [0113] 38 notch (alignment cam surface) [0114] 41 first control valve [0115] 42 second control valve [0116] 43 first control port [0117] 44 second control port [0118] 45 first auxiliary valve [0119] 46 second auxiliary valve [0120] 47 spring [0121] 51 first position [0122] 52 second position [0123] 53 third position [0124] 54 fourth position [0125] 55 first short circuit connection [0126] 56 second short circuit connection [0127] 60 casing [0128] 61 first casing part [0129] 62 second casing part (cam ring) [0130] 63 third casing part [0131] 64 fourth casing part [0132] 65 first flange [0133] 66 second flange [0134] 70 rotor [0135] 71 first control surface [0136] 72 first circle [0137] 73 cylinder [0138] 74 roller [0139] 75 splined bore