ARRANGEMENT COMPRISING A HYDRODYNAMICALLY JOURNALLED RECIPROCABLE AND ROTATABLE PISTON

Abstract: An arrangement, more particularly a freely reci­ procating engine, comprising a piston which can reciprocate and also rotate in a cylinder (1) and which is journalled in the cylinder by means of a hydrodynamic groove bearing, the piston having two end faces which are acted upon during operation by different fluid pressures (P SB1,P₂), and the piston being provided with at least one sealing part (2) having a circular-cylindrical smooth sealing surface and with at least one bearing part (3) which is provided in its peripheral surface with a hydrodynamically acting groove pattern.

Description: The invention relates to an arrangement, more particularly a freely reciprocating piston engine, comprising a piston which can reciprocate and also rotate in a cylinder and which is journalled in the cylinder by means of a hydrodynamic groove bearing, the piston having two end faces which are acted upon during operation by different fluid pressures. In a known arrangement of the kind mentioned in the opening paragraph (described in an articale entitled "Test Results of High Efficiency Stirling Machine Components" by R.J. Vincent, W.D. Rifkin and G.M. Benson, pages 1867 - 1874, presented on the 17th IECEC in 1982 in Los Angeles, United States of America) which is constructed as a freely reciprocating Stirling engine, the possibility is mentioned of supporting the piston hydrodynamically in the cylinder by causing the piston to rotate as well as reciprocate. It should be noted that at higher speeds of rotation, smooth hydrodynamic bearings become unstable due to the so-called "half frequency whirl". According to this article, this is counteracted by providing the piston with a pattern of spiral grooves. A disadvantage of the known arrangement is that the pressure difference applied across the piston leads to a decrease in pressure across the hydrodynamic bearing which intro­ duces a radial dynamic instability of the bearing which is of a nature different from that of the said "half frequency whirl" and occurs especially at low speeds of rotation. As a result, the piston has a tendency to occupy an eccentric position in the cylinder, so that the leakage between the piston and the cylinder becomes considerably larger compared with the situation in which the piston is centrically disposed in the cylinder. The eccentricity also results in increased friction and wear. A possibility of avoiding this kind of instability is to cause the piston to rotate at a high speed, but for various reasons this is often very unattractive. The invention has for its object to provide an arrangement of the kind described above comprising a hydrodynamically journalled piston, which is journalled in a stable manner even at lower speeds of rotation and with a small eccentricity. The arrangement according to the invention is characterized in that between the two end faces the piston is provided with at least one sealing part having a circular-cylindrical smooth sealing surface and with at least one bearing part having in its peripheral surface a hydrodynamically acting pattern of grooves. The invention is based on the recognition of the fact that a circular-cylindrical smooth sealing surface, when it is disposed centrically in the cylinder, can keep leakage along the piston within acceptable limits, and that with an acceptable pressure difference across the bearing the said dynamic instability does not occur even at lower acceptable speeds of rotation. It has been found that the so-called angle of inclination of a grooved hydrodynamical bearing becomes larger than 90° when an excessively large pressure difference is applied across the bearing. This angle of inclination is the angle between the line along which the bearing capacity is directed and the line along which the eccentricity exists. At an angle of inclination exceeding 90°, the bearing capacity acts in the same direction as a possible disturbance so that no resetting force is present. The bearing is then unstable. The angle of inclination of a smooth circular-­ cylindrical sealing surface is about 90° with a small eccentricity, irrespective of the presence of absence of a pressure difference across the sealing part. The effect obtained when the pressure difference across the grooved bearing part is entirely or partly eliminated is that the angle of inclination of the bearing part becomes smaller than 90° in such a manner that the positive bearing capa­ city of the bearing part has a component in the direction of the eccentricity which is larger than the component of the negative bearing capacity of the smooth sealing part. As a result, the piston as a whole has become stable. Due to the measure taken, the piston will occupy a contactless stable position in the cylinder, as a result of which the leakage and the frictional losses can be minimal. In one embodiment of the arrangement according to the invention, the axial length and the diameter of the or each sealing part are chosen so that the pressure difference across the or each bearing part is at most 20 % of the overall pressure dif­ ference across the piston. A favourable embodiment of the arrangement accord­ ing to the invention is characterized in that the two sides of the or each bearing part are in open communication with each other through a duct so that no pressure difference is applied across each of the bearing parts. In order to ensure a good stable bearing, in a further embodiment, the piston is provided with two axially spaced sealing parts which, viewed in the axial direction, are located symmetrically adjacent the two end faces and between which a bearing part is disposed. In a corresponding manner, in another embodiment, the piston is provided with two axially spaced bearing parts which, viewed in the axial direction, are located symmetrically between the two end faces and between which a sealing part is disposed. In a further embodiment of the invention, the or each bearing part is provided with a pair of groove systems which pump in opposite axial directions and which together form a herring-bone pattern of grooves. The invention will be described more fully with reference to the drawings. Figs. 1 to 9 show diagrammatically a number of piston-cylinder combinations with a reciprocating and rotating piston comprising at least one sealing part and at least one bear­ ing part, and Fig. 10 shows diagrammatically in sectional view a double-acting compressor with a reciprocating and rotating piston. In Fig. 1, reference numeral 1 denotes a cylinder. This cylinder 1 accommodates a piston, which through means not shown can perform reciprocatory and also rotary movements in the cylinder. The piston comprises a sealing part 2 and a bearing part 3. The part 3 is provided in its peripheral surface with a pattern of grooves which, upon rotation of the piston in the indicated direction, has a pumping effect on a fluid medium in the bearing gap 4. The pumping effect is directed to the left as viewed in Fig. 1. The peripheral surface of the sealing part 2 is smooth and the sealing gap 5 is determined by the choice of the axial length of the part 2 and of its diameter. On one side of the piston a pressure p₁ prevails in the space 6, while on the other side a pressure p₂ prevails in the space 7. The pressure difference p₁-p₂ across the piston results in medium tending to flow from the space 6 to the space 7, or in the opposite direction, through the gaps 5 and 4. The gap 5 is a gap with parallel walls so that the flow therein does not influence the stability of the sealing part 2 of the piston. However, a hydrodynamic smooth cylindrical bearing is for other reasons always unstable at any speed of rotation with a small eccentricity. The bearing part 3 is provided with a hydrodynamic groove pattern. Such a hydrodynamically journalled body of revolution is stable at any speed of rotation if no pressure difference is applied across it. It has now been found that when a pressure difference is applied a disturbance will occur in the medium in the gap 4, which rise to instability. This instability can be prevented by increasing the speed of rotation to a high value, but this is not a very attractive solution. Another pos­ sibility is to ensure that little flow occurs in the gap 4. In the embodiment of Fig. 1 this is achieved by forming the piston with a sealing part 2 and a bearing part 3. The dimensions of the sealing part are chosen so that the major part of the pressure difference across the piston is applied across this sealing part so that only a small pressure difference is applied across the bearing part 3 and consequently the flow in the gap 4 will be limited so that the bearing part 3 occupies a stable centric position in the cylinder. Since the sealing part and the bearing part are rigidly connected to each other, this consequently means that the sealing part also occupies a stable centric position in the cylinder. The leakage through the gap will thus be minimal for the given circumstances. This principle, according to which the bearing function and the sealing function of a freely reciprocable and rotatable piston are separated at least in part, may be effected in a large number of ways. A number of these embodiments is shown diagrammatically in Figs. 2 to 10. Fig. 2 shows an embodiment in which the piston comprises two axially spaced sealing parts 2 and 6 and an inter­ mediate bearing part 3. The pressure difference is applied in this case for the major part across the sealing parts 2 and 6, while only a small pressure difference is applied across the bearing part 3. Another embodiment is shown in Fig. 3. In this case the piston comprises two axially spaced bearing parts 3 and 8 and an intermediate sealing part 2. It will be appreciated that also in this case the major part of the pressure difference is applied across the sealing part 2, while only a small pressure difference is applied across the bearing parts. In the embodiments shown in Fig.s 1, 2 and 3, the bearing part 3 or each of the bearing parts 3 and 8, respectively, is provided with a groove pattern which has a pumping effect in only one direction. In the embodiments shown in Figs. 4, 5 and 6, the bearing part 3 or each of the bearing parts 3 and 8, respectively, is provided with a herring-bone pattern of grooves, that is to say, the or each bearing part is provided with a pair of systems of grooves which pump in opposite directions. These embodiments are otherwise identical to those shown in Figs. 1, 2 and 3, respectively. Figs. 7, 8 and 9 show three possible embodiments in which no pressure difference is applied across the bearing part 3 or either of the bearing parts 3 and 8, respectively. This is achieved by providing a circumferential groove 9 or 10 in the peripheral surface of the piston between the or each bearing part and the or each sealing part of the piston. In Fig. 7, the groove 9 is connected through a duct 11 to a space 17 on the side of the bearing part remote from the sealing part so that the two sides of the bearing part are in open communication with each other and no pressure difference is applied across the bearing part and the overall pressure difference across the piston is applied across the sealing part 2. In Fig. 8, circumferential grooves 9 and 10 are formed between the bearing part 3 and the sealing parts 6' and 2, respectively, these grooves being interconnected through a duct 12 so that also in this case the two sides of the bearing part are in open communication with each other and no pressure difference is applied across the bearing part 3. In the embodiment shown in Fig. 9 circumferential grooves 9 and 10 are formed between the bearing parts 3 and 8, respectively, and the sealing part 2, and groove 10 being connected through a duct 13 to a space 16 on the side of the bearing part 3 remote from the sealing part 2 and the bearing part 8 is limited on one side by a groove 9 being connected through a duct 15 to the space 17 on the side of the bearing part 8 remote from the sealing part 2. Thus, the two sides of each bearing part are in open com­ munication with each other and the pressure difference is applied only across the sealing part 2. Fig. 10 shows diagrammatically an example of a double-acting compressor with a free piston. This compressor com­ prises a cylinder 101 which accommodates a piston 102. The piston 102 is provided on either side with a sealing part 103 and 104, respectively, and with a bearing part 105 and 106, respectively, which is provided in its peripheral surface with a herring-bone pattern of shallow grooves. The piston limits on one side a com­ pression space 107 and on its other side a compression space 108. The compression spaces 107 and 108 are sealed by covers 109 and 110, respectively, provided with inlet chambers 111 and 112, respectively, and outlet chambers 113 and 114, respectively. The cylinder wall supports coils 115 and 116, which cooperate with annular permanent magnets 117 and 118 arranged on the piston. By energization of the coils 115 and 116, respect­ ively, the piston can be driven with a reciprocating movement. The cylinder wall also supports a coils 120 which cooperates with cores 121 on the piston to rotate the piston. If desired, the coils 115 and 116 may be integrated with the coil 120 so that the reciprocation and rotation of the piston can be obtained with a single electrical system. Between the bearing parts 105 and 106 and the sealing parts 103 and 104, respectively, the piston 102 is formed with circumferential grooves 125 and 126, respectively, which are connected through ducts 127 and 128, respectively, to a space 129, which is in open communication with the sides of the bearing parts 105 and 106 remote from the sealing parts 103 and 104, respectively. Thus, it is achieved again that no pressure difference is applied across the bearing parts 105 and 106.

Claims: 1. An arrangement, more particularly a freely reci­ procating piston engine, comprising a piston which can reciprocate and also rotate in a cylinder and which is journalled in the cylinder by means of a hydrodynamic groove bearing, the piston having two end faces which are acted upon during operation by different fluid pressures, characterized in that between the two end faces the piston is provided with at least one sealing part having a circular-cylindrical smooth sealing surface and with at least one bearing part having in its peripheral surface a hydro­ dynamically acting pattern of grooves. 2. An arrangement as claimed in Claim 1, characterized in that the axial length and the diameter of the or each sealing part are chosen so that the pressure difference across the or each bearing part is at most 20 % of the overall pressure dif­ ference across the piston. 3. An arrangement as claimed in Claim 1, characterized in that the two sides of the or each bearing part are in open communication with each other through a duct so that no pressure difference is applied across each of the bearing parts. 4. An arrangement as claimed in Claim 1, 2 or 3, characterized in that the piston is provided with two axially spaced sealing parts which, viewed in the axial direction, are located symmetrically adjacent the two end faces and between which the piston is provided with a bearing part having a hydrodynamically acting groove pattern in its peripheral surface. 5. An arrangement as claimed in Claim 1, 2 or 3, characterized in that the piston is provided with two axially spaced bearing parts which, viewed in the axial direction, are located symmetrically between the two end faces and each of which has a hydrodynamically acting groove pattern in its peripheral surface between which bearing parts a circular-cylindrical smooth sealing part is situated. 6. An arrangement as claimed in any one of the pre­ ceding Claims, characterized in that the or each bearing part is provided in its peripheral surface with a pair of groove systems which pump in opposite axial directions and which together form a herring-bone pattern of grooves.

Inventors: BREMER, FRIDTJOF, MUIJDERMAN, EVERHARDUS ALBERTUS
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: F01B11
IPC codes: F02G1, F16C33, F04B53, F04B35, F16J1, F04B31, F04B39, F01B11, F01B3