PISTON FOR RECIPROCATING ENGINE

Abstract: A reciprocating machine component (23; 33; 38; 45; 55; 73; 85), such as a piston of an internal combustion engine or a compressor, is provided with bearing pads (22; 32; 42; 43; 60; 70; 84) for transmitting lateral thrust to an associated guide surface (26; 36; 46; 59; 75), such as a cylinder or liner. The bearing pads are mounted such that they tilt during reciprocation in order to provide a wedge-shaped lubricant film between the pad and the surface. The mounting is also such as to ensure that the direction of tilt of the pads changes with the direction of reciprocation.

Description: Ths invention relates to pistons for reciprocating engines, compressors and pumps. Figure 1 shows diagrammatically a typical known such piston where a rotating crank 1 and connecting rod 2 are linked by a pivot 3 to a piston 4 which reciprocates in a cylinder or guide 5. The force 6 along the line of action of the piston combines with the connecting rod force 7 to produce a resultant lateral force 8. The sliding speed of the piston 4 varies approximately sinusoidally from zero at each end of the stroke, where the direction reverses, to a maximum value near the mid point. The lateral force 8 also varies cyclically, its magnitude and direction depending upon the particular type of machine. For example, in most internal combustion engines now manufactured, the lateral force on the piston reverses at the end of the stroke causing the piston to cross over from one side of the cylinder to the opposite side. As a result of long practical experience the specific loading or guide surfaces on conventional pistons and crossheads is very low compared with that on the rotating bearings, commonly one-tenth or less. Specific loading is defined as the applied force divided by the area over which it is distributed, projected in the direction of the force - that is To obtain a low specific loading relatively large surfaces are required and these cause a concomitant frictional drag due to the shearing of the film of lubricating oil between the surfaces. Any reduction in this friction will increase the mechanical efficiency of the machine and, in the case of an engine, will reduce the fuel consumption for a given power output. The theory and practice of lubrication confirm that parallel sliding surfaces have a limited specific load capacity and tend to also have limitations to the sliding velocity which may safely be applied and this applies to the guide surfaces already described. Both specific loading and sliding speed capacities can be greatly increased if the surfaces are inclined very slightly so that a converging film of lubricant is formed. Figure 2 illustrates this convergent, or wedge-shaped film 9a essental to high performance, where a surface 9 slides over a guide surface 11 in the direction of arrow 10. For optimum performance the angle 12 between the surfaces will generally be between 0.0001 and 0.01 radian. Thus, if the sliding surface of component 4 were to be inclined relative to the non-operating surface, the lubrication would be improved, specific loading could be increased allowing a reduction in area and thereby reducing the frictional drag. An improvement in mechanical efficiency would ensue. Two inclinations would be required, one for each direction of sliding, and the no-operating surface would still contribute to frictional drag, but there could still be a net gain over conventional designs. However, a fixed inclination is only suitable for a narrow range of speed and specific load conditions whereas the piston or crosshead 4 can in many cases be subject to widely varying speed and load conditions, each cyclical in nature but not always in step. FR-A-1451882 discloses a bearing in which a sliding surface is supported on two elements which rock on domed heads. Lubricant is supplied between the sliding surface and the surfaces of the elements. US-A-4178899 discloses a piston for an internal combustion engine in which a film of oil is maintained around the whole of the skirt and the ring band. This produces, according to the theory given above, a low specific loading but has the disadvantage of high frictional drag. GB-A-1008510 discloses in the embodiment of Figures 19-22, a piston in which an annulus of glide blocks are carried in grooves on the surface of a piston body. The glide blocks adjust themselves during reciprocation so that wedge-shaped oil films are obtained between the glide blocks and the cylinder wall. In JP-A-57-92038 a piston is provided with a skirt carrying vibration insulation material in recesses in the skirt. An abrasion prevention material is carried on the vibration insulation material. According to the invention, there is provided a piston of the kind comprising a piston body for reciprocating sliding movement on a surface of a cylinder or liner of an internal combustion engine or compressor with a fluid lubricant therebetween, the piston body being provided with one or more bearing pads for guiding the piston during said reciprocation, the or each bearing pad being mounted for tilting during said reciprocation to allow the formation between the piston and an associated cylinder or liner, of a fluid pressure wedge of lubricant whose leading edge at any instant has the greater thickness, characterised in that the or each bearing pad is mounted on two flexural connections, in the form of two flexural columns connected between the or each pad and the piston body, the two flexural columns being spaced in a direction parallel to the direction of relative motion, the lenghts of said columns meeting at a point on the opposite side of the pad face from the columns, the columns deflecting during said reciprocation to allow the associated pad to tilt to form said lubricant pressure wedge. The invention may be carried into practice in various ways and some embodiments will now be described, by way of example, with reference to the Figures 3 to 5 of the accompanying drawings, in which:- Figure 3 is a diagrammatic section of a reciprocating machine comprising a bearing pad mounted for reciprocating sliding motion on a co-operating guide surface, Figure 4 is a schematic representation of the bearing pad of Figure 3 for use in demontrating how tilting of the pad occurs, Figure 5 shows a reciprocating machine in the form of a crosshead incorporating the second form of bearing pad shown in Figures 3 and 4. The reciprocating machine includes a bearing pad 43 and is shown in Figures 3 to 5. The pad 43 is supported on two inclined flexing columns 44 connected to the component 45 which has relative sliding motion along the surface 46. The columns 44 are spaced in the direction of relative motion of the component 45 and the surface 46 and are inclined so that their axes 47, when produced, meet at a point 48 on the opposite side of the pad face from the columns. The effect of this arrangement is illustrated diagrammatically in Figure 4. If the component 45 carrying the pad 43 is considered to be moving in the direction of arrow 49 over the stationary surface 46, then the frictional drag force on the pad will act in the direction of arrow 50. The pad 43 and the column axes 47 will deflect to the positions shown by the dotted lines, causing the pad to adopt a tilted position. At the same time the point of intersection 48 of the column axes produced will move to the position indicated at 51. Since the movement from 48 to 51 is, by geometry, greater than the deflection of the pad 43, to a degree dependent on the angle between the column axes 47, the effective centre of support of the pad moves towards the trailing edge 52. It is known from lubrication theory that this will improve the film conditions. Thus the frictional force and the hydrodynamic force act in unison to cause flexing of the support columns. The geometric proportions can be selected to optimise the film lubrication conditions to suit each individual application. As a matter of convenience more than two flexing columns may be used, providing that their axes when produced all meet at the same point, on the opposite side or the pad face from the columns. Referring next to Figure 5, the reciprocating machine shown therein comprises a crosshead 55 rigidly connected to a piston rod 56 and is pivotally attached to a connecting rod 57 by a pin 58. The crosshead 55 reciprocates in guides 59 which may be of either cylindrical or flat form as desired. The lateral force is carried by a pad 60 on each side, although more than one pad may be fitted if desired. The pads 60 are of the kind shown in Figure 3 and 4. By way of comparison the dotted lines 61 indicate the shape of a crosshead of conventional form, requiring the length of the guides to be increased as indicated by the dotted lines 62. Thus there is the benefit of a substantial reduction in the length of the crosshead guides, which may also permit a shorter connecting rod to be used so providing a useful saving in the overall height of the machine and a corresponding reduction in weight and cost. The embodiment described above with reference to the drawings has been applied to pistons and crossheads in engines compressors and like machines so that the examples illustrated all reciprocate in an axial direction in cylindrical bores. However, it is to be understood that they are equally applicable to crossheads operating on flat guide surfaces, and also to reciprocating motion in a curved path as in a cylindrical or annular sector in a plane normal or inclined to the axis. In the example quoted the pads are carried on the moving component sliding along a stationary guide surface, but they are equally applicable to an arrangement wherein the pads are carried by a stationary component supporting a surface sliding over them as, for example, a ram or a machine tool table. The bearing pads may be made of any suitable metallic or plastics material or any combination of these. A preferred material is polyetheretherketone reinforced with carbon or glass fibres. The sliding surface or face of each pad may be treated, or coated with another material to improve the bearing or wear characteristics. For example, pads made of aluminium alloy may have the surface treated with hard anodising or pads for crossheads may be faced with white metal. The guide surfaces may be similarly treated or coated as required. In all the drawings the thickness of the oil film and the angles of tilt are greatly exaggerated for purposes of clarity. In practice, both are extremely small. In all the embodiments of the invention described above with reference to the drawings, the use of tilting pads permits an increase in specific loading and therefore a reduction of sliding surface area of an order of 10, resulting in a significant improvement in the overall efficiency of the machine. In the case of an internal combustion engine, it has been estimated that this could provide a fuel saving of 4% for a given power output.

Claims: 1. A piston of the kind comprising a piston body (45; 55) for reciprocating sliding movement on a surface of a cylinder or liner of an internal combustion engine or compressor with a fluid lubricant therebetween, the piston body (45; 55) being provided with one or more bearing pads (43; 60) for guiding the piston during said reciprocation, the or each bearing pad being mounted for tilting during said reciprocation to allow the formation between the piston and an associated cylinder or liner, of a fluid pressure wedge of lubricant whose leading edge at any instant has the greater thickness, characterised in that the or each bearing pad (43; 60) is mounted on two flexural connections, in the form of two flexural columns (44) connected between the or each pad and the piston body, the two flexural columns being spaced in a direction parallel to the direction of relative motion, the lengths of said columns meeting at a point on the opposite side of the pad face from the columns, the columns deflecting during said reciprocation to allow the associated pad to tilt to form said lubricant pressure wedge. 2. A piston according to claim 1, characterised in that the or each bearing pad (42) has a sliding surface which is convexly curved in planes normal to the co-operating surface and parallel to the direction of relative motion.

Inventors: HILL, ALFRED
Applicants: AE PLC
Application year: 1985
Main classification: F16J1
IPC codes: F04B39, F16J1, F02F3, F04B53