GB2136088A

GB2136088A - Lubricating connecting rod small end

Info

Publication number: GB2136088A
Application number: GB08300508A
Authority: GB
Inventors: Roger Gojon
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1983-01-10
Filing date: 1983-01-10
Publication date: 1984-09-12
Also published as: FR2539191A1; DE3400486A1; IT8447522A0; FR2539191B1; GB8300508D0; GB2136088B; IT1178062B

Abstract

In a connecting rod assembly for the piston 11 of an internal combustion engine, particularly the arrangement of the small end bearing 15 in a two-stroke diesel engine, a gudgeon pin 13 is journalled in a small end bush 14 which is off-set laterally by a distance X in the small end 15 of the connecting rod 12. This urges the piston 11 away from the small end 15 at a predetermined position of the connecting rod 12 during its cycle of oscillation so that the bearing can be lubricated. <IMAGE>

Description

SPECIFICATION Connecting rod assembly The present invention relates to a connecting rod assembly for the piston of an internal combustion engine, and is particularly concerned with the arrangement of the small end bearing in a two stroke engine. In the present specification engines will be described as having pistons moving vertically with the piston crown uppermost, though naturally, the engines may have any orientation.

The small end of a piston rod generally oscillates with respect to the piston through about 1 50 to either side of the vertical during one cycle.

This motion is not conducive to developing effective hydrodynamic lubrication conditions between the gudgeon pin and the small end bush.

In four-stroke engines there is at least during the descending induction stroke, a reversal in the direction of the force exerted by the gudgeon pin on the small end bush. This stroke enables oil to be introduced into the zone which will be put under load in a later part of the cycle, when the direction of the force is reversed. Thus, the system works at least partially hydrodynamically and undue compression of the oil film is avoided. This permits four stroke engine small end bearings to function effectively regardless of load and speed provided there is sufficient lubrication available.

In the small end bearings of two-stroke engines, however, no reversal of the direction of force exerted by the gudgeon pin on the small end bush occurs, because, in general, the gas pressure in the cylinder exceeds the inertia forces which would otherwise tend to move the gudgeon pin upwards relative to the small end bush. As a result, the gudgeon pin is always forced downwards on to the small end bush and the small end bearing may operate as a nonhydrodynamic system with a far higher coefficient of friction resulting in problems with increasing temperatures. This may impose limits on the power output of the engine.

It is an object of the present invention to provide a system in which the small end bearing may be properly lubricated regardless of whether or not the direction of the force between the gudgeon pin and the small end bush reverses.

It has been noted by the applicants that although the resultant force of the gas pressure and inertia forces is always in a direction away from the piston crown in a two-stroke engine, this force is relatively weak. For that reason, it is proposed to tend to move the piston independently of the gudgeon pin.

According to the invention, a piston connecting rod assembly comprises a piston, a gudgeon pin connected to the piston, and a connecting rod connected to the gudgeon pin at a small end, the small end having an eccentric portion relative to the axis of the gudgeon pin adapted to urge the piston away from the small end at a predetermined position of the connecting rod during its cycle of oscillation. Thus, the gudgeon pin may be raised relative to the small end sufficiently to provide lubrication beneath the gudgeon pin.

Preferably the opening defined by the small end bush is slightly oval, with the major axis vertical, to allow for the required slight relative vertical movement between the gudgeon pin and the bush.

The predetermined position of the connecting rod is preferably when it is generally at the limit of its oscillation from the vertical on the up stroke, i.e. at the 2700 position of its cycle. The eccentric portion may contact the under surface of the piston crown (or an extension thereof) directly or may contact an intermediate member which is in contact with the piston.

In a preferred construction, the gudgeon pin is mounted eccentrically in the piston by means of pin holes which are slightly displaced laterally and the opening for the gudgeon pin in the small end is aiso displaced laterally by a corresponding distance thus defining the eccentric portion of the small end bush.

The eccentric portion may comprise a portion of the connecting rod small end or may comprise a portion of the small end bush itself.

The invention may advantageously be applied to two stroke engines, in particular small diesel engines.

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 accompanying drawings in which: Figure 1 is a vertical section through a piston and small end showing one embodiment of the invention; Figure 2 is a view similar to Figure 1 but omitting the connecting rod showing a second embodiment; Figure 3 is a vertical section at right angles to Figure 2; Figure 4 is a vertical section of the connecting rod omitted from Figure 2; Figures 5 and 6 are views similar to Figure 5 to an exaggerated scale showing the connecting rod at top-dead-centre and at 270 respectively; Figure 7 is a view similar to Figure 1 showing a third embodiment; Figure 8 is a diagrammatic representation of the connecting rod cycle; and Figure 9 is a graph showing the downward pressure exerted by the gudgeon pin over the connecting rod cycle.

Figure 1 shows a piston 11 and a connecting rod 12 connected to the piston by means of a gudgeon pin 13 located in two gudgeon pin holes in the piston skirt. The gudgeon pin 13 is journalled in a small end bush 14 which is offset laterally by a distance x in the small end 1 5 of the connecting rod 1 2. A yoke 1 6 is located between the small end 1 5 and the piston inside surface.

Thus, it will be understood that as the connecting rod 12 oscillates towards the left in Figure 1, the eccentric portion 1 7 lifts the yoke 1 6 until it contacts the piston 11. Further movement of the rod 12 then tends to lift the piston 11 and so the gudgeon pin 13, thus releasing the downward pressure of the pin 13 on the bush 14, allowing oil to enter the small clearance between the pin 13 and the bush 14.

In Figures 2 and 3 the gudgeon pin holes 21 are offset laterally by a distance y. In addition, the small end bush 24 has a slightly elliptical shape to allow a slight relative movement between the bush 24 and the gudgeon pin 13. Furthermore, the bush 24 is offset laterally by a distance y equal to the offset of the gudgeon pin holes 21.

An arch 22 extends downwards from the piston interior towards the region to be occupied by the small end of the connecting rod 12, shown in Figure 4. As a result, the yoke 1 6 of Figure 1 can be omitted, since as the connecting rod 12 oscillates, the eccentric portion 1 7 engages the arch 22. This is shown most clearly in Figures 5 and 6 in which the degree of offset etc. is exaggerated for the sake of clarity.

When the connecting rod is in the top-deadcentre position of Figure 5, the small end 1 5 is spaced from the arch 22 and the gudgeon pin 13 is located at the bottom of the bush 24. As the connecting rod 12 oscillates to the left to the position of Figure 6, the eccentric portion 1 7 engages the arch 22 and lifts the piston 11 and so the gudgeon pin 13. This leaves a slight clearance 25 between the gudgeon pin 13 and the bush 24.

In the embodiment of Figure 7, the eccentric portion is designated 37 and is constituted by the small end bush itself, designated 34. In this case, the bore of the bush 34 is offset laterally by a distance z. The piston 11 has an arch 22 which is engaged by the eccentric portion 37 in a similar manner to the embodiment of Figures 2 to 6.

In order to ensure smooth operation in all the embodiments, a bearing layer can be placed between the eccentric portions 17, 27, 37 and either the yoke 1 6 or the arch 22 as appropriate.

Additionally, this region of contact would be lubricated.

Figure 8 shows diagrammatically the oscillation of the connecting rod 1 2 relative to the piston 11.

As the rod 12 approaches the 2700 position, the gudgeon pin 13 begins to be lifted. This proceeds until the rod 12 has passed the 2700 position by about the same degree. The extent of this lifting period is shown in Figure 9 and is designated 41.

Claims

1. A piston connecting rod assembly comprising a piston, a gudgeon pin connected to the piston, and a connectil-,g rod connected to the gudgeon pin at a small end, the small end having an eccentric portion relative to the axis of the gudgeon pin adapted to urge the piston away from the small end at a predetermined position of the connecting rod during its cycle of oscillation.

2. An assembly as claimed in Claim 1 in which the opening defined by the small end bush is slightly oval, with the major axis vertical.

3. An asssembly as claimed in Claim 1 or Claim 2 in which the said predetermined position of the connecting rod is generally at the limit of its oscillation from the vertical on the upstroke.

4. An assembly as claimed in any preceding claim in which the eccentric portion contacts the under surface of the piston crown, or an extension thereof, directly.

5. An assembly as claimed in any of Claims 1 to 3 in which the eccentric portion contacts an intermediate member which is in contact with the piston.

6. An assembly as claimed in any preceding claim in which the gudgeon pin is mounted eccentrically in the piston by means of pin holes which are slightly displaced laterally and the opening for the gudgeon pin in the small end is also displaced laterally by a corresponding distance thus defining the eccentric portion of the small end bush.

7. An assembly as claimed in'any preceding claim in which the eccentric portion comprises a portion of the connecting rod small end.

8. An assembly as claimed in any of Claims 1 to 7 in which the eccentric portion comprises a portion of the small end bush itself.

9. A piston connecting rod assembly constructed and arranged substantially as herein specifically described with reference to and as shown in Figure 1, Figures 2 to 6, or Figure 7 of the accompanying drawings.