GB2104621A - Piston ring lubrication - Google Patents

Abstract

A piston assembly comprises a piston (11) and plurality of piston rings (17, 18, 19, 20). Drillings (29) extending through the piston (11) from the base of the top ring groove (13) to the circumferential surface of the piston between the top and second ring grooves (13, 14) enable gas pressure in the cylinder to be transferred from the top ring (17) to the second ring (18) which takes an increased proportion of the piston sealing load. This arrangement, used in conjunction with a top ring (17) design which facilitates the downward scraping of lubricant towards the sump eliminates, or substantially reduces in volume the standing ring of lubricant formed on the cylinder walls at top ring top dead centre when a conventional ring arrangement is used and results in less production of harmful deposits in the vicinity of the piston rings, and reduced evaporation and degradation of the oil in the engine as a whole. Alternatively, or in addition, the top ring (17) may have transverse slots 31 in at least a part of its outer face to facilitate the transfer of gas pressure (and excess lubricant) from the top ring to the second ring. <IMAGE>

Description

SPECIFICATION Piston assembly for use in reciprocating piston and cylinder machine The present invention relates to a piston assembly for use in a reciprocating piston and cylinder machine, such as an internal combustion engine or a compressor.

In reciprocating piston and cylinder machines, the piston assembly commonly comprises a piston with a plurality of parallel, spaced-apart piston rings disposed around its circumferential face. The piston rings each are designed for specific functions. For example, in spark-ignition and compression-ignition engines, the piston ring nearest to the piston crown (ring No. 1) is intended to take most of the burden of piston sealing and is more or less neutral with respect to the movement of lubricating oil along the cylinder wall either towards the combustion chamber or towards the lubricating oil sump. Riny No. 1 is commonly referred to as the compression ring. Its outer face is usually cylindrical.The piston rings which are the second and third nearest to the piston crown (rings 2 and 3) are primarily intended to direct excess lubricating oil along the cylinder wall in the direction of the oil sump, and to this end, they are often tapered so as to be narrower in the direction of the piston crown. In addition to their intended primary use, these rings have a secondary sealing function. The fourth piston ring in distance from the piston crown and any subsequent piston rings are intended to scrape excess lubricating oil from the cylinder wall and to direct it to the sump or other receptacle. The or each of these rings is often formed with a circumferential recess in its outer face, and the faces on each side of the recess are tapered so as to be narrower towards the piston crown to facilitate the oil scraping function.

The fourth and any subsequent rings have a plurality of radial slots for the passage of lubricant inward to the base of the groove, and in addition, there may be drillings in the fourth (and subsequent) grooves for the passage of lubricant to the underside of the piston.

During the operation of a conventional piston and cylinder machine, it has been found that the arrangement of the piston rings promotes the formation of a standing ring of lubricant on the cylinder walls just above the top dead centre position of ring No. 1, and in internal combustion engines this ring of oil is exposed to the combustion chamber flame, and to the effects of hot gases and high pressures. It is believed that this exposure of the lubricant is a principal factor in the chemical and thermal degradation of the lubricant which may cause evaporation losses leading to increased lubricant viscosity and increased additive concentrations.In some cases, the exposure of the lubricant to the extreme conditions in the combustion chamber may result, in due course, in the formation of deposits on the piston, piston rings and in the piston ring grooves, all of which adversely affect the performance and in-service life of the engine.

The present invention provides a piston assembly for use in a reciprocating piston and cylinder machine comprising a plurality of piston rings circumferentially received around the piston and a channel or channels for communicating gas pressure from the vicinity of the first piston ring, nearest to the piston crown, to another piston ring which is more remote from the piston crown.

During use of the piston assembly of the invention in a reciprocating piston and cylinder machine, the piston ring nearest to the crown is intended to have little or no sealing function, but to serve merely as a lubricant scraper which directs oil downwards along the cylinder wall in the direction of the oil sump (or equivalent receiving space), and it may have any form suited to this duty.

The said other piston ring, more remote from the piston crown serves to take a much greater proportion of the burden of piston sealing than in prior piston assemblies due to the fact that the pressure in the cylinder is communicated to the said outer piston ring via the aforesaid channel(s).

By virtue of this novel ring arrangement, the standing ring of lubricant at the top ring top dead centre, exposed directly to the flame, or temperature and pressures in the vicinity of the piston crown, is eliminated, or at least considerably reduced in volume, and lubricant degradation and loss tends to be mitigated, thereby improving the operation and in-service life of the machine, and also reducing lubricant consumption and increasing the service life of the lubricant.

The said other piston ring may be the piston ring which is the second nearest to the piston crown, but this is by no means essential. Thus, the said other piston ring may be the piston ring which is the third or fourth nearest piston ring to the piston crown or in any other suitable position to provide the benefits of the invention without any unacceptable drawbacks.

The invention is now further described with reference to some non-limitative embodiments thereof given by way of illustrative example only, and with reference to the accompanying diagrammatic drawings in which Figure 1 is a diagrammatic cross-sectional elevation showing part of a piston assembly of the invention and a cylinder wall; and Figure 2 is a diagrammatic cross-sectional elevation similar to Figure 1 but showing a different embodiment of the invention.

Referring first to Fig. 1 , the piston assembly 10 comprises a piston 11 having a piston crown 1 2 and circumferential grooves 13, 14, 15, 1 6 in which are received respective piston rings 17, 18, 19, 20. The piston assembly 10 is shown within a cylinder with the piston rings contiguous with the wall of the cylinder liner 21.

The piston ring 17 nearest the plane of the piston crown 12 (ring No. 1) has a circumferential recess defined between two radially extending portions whose outer faces, adjacent to the liner 21, are tapered in the direction of the plane of the piston crown 12. The taper has the effect, during reciprocation of the piston assembly 10 within the cylinder, of scraping oil in a (downwards) direction away from the space 22 above the piston crown.

When the piston assembly 10 is employed in an internal combustion engine, the space 22 will contain hot gases resulting from the combustion of fuel. Ring 17 has a plurality of radial drillings 23 to facilitate the passage of oil scraped from the cylinder liner to the base of the groove 13 inward of the ring 1 7, and thence via the channels 29 towards the second ring 1 8. The presence of a liberal supply of lubricant which will tend to flood the space between ring 1 8 and the cylinder wall 21 further assists the sealing function of ring 1 8.

Piston ring 18 (ring No. 2) is an annular ring having substantially plain cylindrical outer face, and piston ring 1 9 (ring No. 3) is similar except that its outer face is tapered in the direction of the plane of the piston crown 1 2 so that the radial faces thereof (particularly the lower radial face) serve to direct lubricating oil along the wall of the liner 21 away from space 22.Piston ring 20 (ring No. is similar to ring 1 7 in having a circumferential recess 24 defined between radially extending portions whose outer faces are tapered in the direction of the plane of piston crown whereby during reciprocating motion of the piston assembly 10 in the cylinder, the radial faces of the radially extending portions will tend to direct lubricant along the wall of the liner 21 away from the space 22. In addition, ring 20 has radial drillings 25 for the passage of lubricant radially inwardly to the base of the groove 1 6, and there may be a drilling 26 through the piston 11 from the base of groove 1 6 to the underside face 27 of the piston 11 for the passage of lubricant out of groove 1 6 to the space defined by the underside face 27 of the piston 11.

The piston assembly 10 is provided with a channel for communicating at least some of the gas pressure in space 22 from the vicinity of piston rings 1 7 to act on piston ring 1 8. As depicted in Fig. 1, the channel is in the form of a plurality of drillings or ducts 29 which extend from the base of groove 1 3 to the circumferential surface of the piston 11 between groove 1 3 and groove 14.The ducts 29 (of which only one is shown in Fig. 1) provide a channel by which at least part of the gas pressure forces which would normally act on ring 17 will act on ring 18, and moreover, at least some of the lubricant which normally tends to form a ring on the radial surface of ring 17 (more particularly the radial surface thereof nearest to the plane of the piston crown 12) is caused to pass, under the influence of gas pressure, through the duct 29 to the space between the ring 1 7 and the ring 1 8 where it is less exposed to conditions prevailing in the space 22.The oil thus transferred through duct 29 tends to flood the radial face of piston ring 1 8 so that piston ring 1 8 is further assisted in taking a substantial proportion of the burden of sealing the piston assembly 10 in the cylinder.

Fig. 2 depicts a piston 11 in which the piston ring 11 7 nearest to the plane of the piston crown is similar in form to the piston ring 17 of Fig. 1 except that its lower radially standing portion is provided with a series of transverse slots 31 for the transfer of gas pressure (and excess lubricant) via the plurality of radial drillings 32 to the next piston ring (not shown) which is more remote from the plane of the piston crown than ring 117.

A further embodiment of the invention is to use a top ring (nearest to the piston crown) tapered so that it is narrower in the direction of the piston crown, and having a series of transverse slots cut in its face to facilitate the transfer of gas pressure from the combustion chamber to another ring more remote from the plane of the piston crown.

It is understood that both of these embodiments of the invention may be used in conjunction with a piston 11 having drillings 29 as shown in Fig. 1.

The benefits and advantages of the piston assembly of the invention include the following: (1) reduced oil consumption; (2) reduced thickening of the oil film between the piston ring faces and the cylinder, resulting in reduced power requirements to reciprocate the piston assembly in the cylinder; (3) reduced formation of deposits on the piston rings, in the piston ring grooves and on the piston, thereby increasing the in-service life of the machine between maintenance and repair operations; (4) reduced degradation of the lubricant used in the machine, resulting in longer lubricant life.

Although the invention has been described with particular, but not exclusive, reference to internal combustion engines, the piston assembly of the invention may be useful in other piston and cylinder reciprocating machines such as (but not restricted to) positive displacement compressors.

Claims (12)

1. A piston assembly for use in a reciprocating piston and cylinder machine comprising a plurality of piston rings circumferentially received around the piston and at least one channel for communicating gas pressure from the vicinity of the first piston ring, nearest to the piston crown, to another piston ring which is more remote from the piston crown.

2. An assembly as in claim 1 in which the channel or channels extend from the first piston ring groove in which the first piston ring is received to a circumferential surface of the piston between the first piston ring groove and another piston ring groove in which the said other piston ring is received.

3. An assembly as in claim 1 or claim 2 in which the said other piston ring groove is the second nearest piston ring groove to the piston crown.

4. An assembly as in any one of claims 1 to 3 in which the channel or channels is or are in the form of at least one duct extending through the piston from within the first piston ring groove.

5. An assembly as in any one of claims 1 to 4 in which the first piston ring has at least one groove on its radially outer face, the said groove being transverse with respect to the radially outer face of the piston and constituting, at least in part, the said channel.

6. An assembly as in any one of claims 1 to 5 in which the first piston ring is tapered in the direction of the plane of the piston crown.

7. An assembly as in any one of claims 1 to 6 in which the first piston ring has at least one port which extends therethrough from an outer face thereof to an inner face thereof for communicating excess lubricating oil and gas from the first piston ring to the second.

8. An assembly as in claim 7 in which the said port extends through the piston ring from the radially innermost face of a circumferential depression in the outer face of the ring.

9. A piston assembly substantially as hereinbefore described.

1 0. A reciprocating piston and cylinder machine comprising a piston assembly as in any one of claims 1 to 9.

11. A machine as in claim 10 wherein the machine is an internal combustion engine.

12. A reciprocating piston and cylinder machine substantially as hereinbefore described.