A PISTON RING FOR AN INTERNAL COMBUSTION ENGINE The present invention relates to a piston ring for use in an annular groove of a piston of a large two-stroke internal combustion engine, particularly a uniflow-scavenged crosshead engine. The piston ring has an upper ring face and a lower ring face and a ring partition allowing expansion and contraction of the piston ring for adjustment of the ring diameter during engine operation and at the mounting in the annular groove. Further, the invention relates to a large two- stroke internal combustion engine, particularly a uniflow- scavenged crosshead engine, comprising a plurality of pistons having in their peripheral surface several axially separated annular grooves with a piston ring mounted therein. The invention also relates to a method for treating a piston ring for use in an annular groove of a piston of a large two-stroke internal combustion engine.

BACKGROUND OF THE INVENTION WO-A-97/11295 discloses a piston ring for a large two-stroke internal combustion engine, such as a marine diesel engine.

These types of piston rings are generally made of cast iron and have a partition that enables the ring to expand and retract, e. g. upon mounting in the annular groove of the piston. The partition can be formed by a simple radial cut or be formed by mutually overlapping tongues which bar a major part of the gap so as to form a substantially gas tight partition.

The cast iron ring is machined and subjected to hardening treatments before it is placed in the annular groove in the

piston. The piston ring has an axial height smaller than the axial distance between the upper and the lower groove face in the pertaining annular groove, so that the piston ring has a loose fit in the annular groove. It is known that the piston ring is displaced in the circumferential direction of the ring groove with each stroke of the piston concerned. This causes wear of the lower ring face and the lower groove face. The lower groove face is provided with a wear resistant coating, usually a chrome plating, in order to improve its wear resistance. The high thermal load and abrasive content of the gas in the combustion chamber can cause the chrome plating on the lower groove face to wear down below acceptable limits before an overhaul of the piston would be due. Wear on the lower ring face has never been an issue, since the term for replacement is normally dictated by the wear on the periphery of the piston ring. Replacing a piston ring is a relatively uncomplicated procedure. Renewing the relatively thick chrome plating of the lower groove face is however a relatively complicated and expensive operation. The piston needs to be removed from the engine. The lower groove face has to be machined before and after the chrome plating-by spraying several layers to build up the lower groove face-is applied.

There is therefore a need to increase the durability of the lower groove face.

DISCLOSURE OF THE INVENTION On this background, it is an object of the present invention to provide a piston ring of the kind referred to initially, which reduces wear of the lower groove face. This object is achieved in accordance with claim 1 by providing a piston ring of said kind in which a part of the lower ring face proximate

to the ring partition is provided with a wear resistant coating.

It has been realized now, that the movement of the piston ring relative to the annular groove is largely caused by expansion and contraction of the piston ring with each reciprocal movement of the piston. During the movements up and down in the cylinder liner the piston ring adapts its outer diameter namely to the inner diameter of the cylinder liner. The inner wall of the cylinder liner is slightly converging towards the top due to uneven distribution of the thermal expansion along the height of the cylinder liner caused by the relatively high temperature at the top and relatively low temperature at the bottom of the latter.

It has now been recognized that the wear is mainly 3-part abrasive which means that the rate of wear is directly dependent on the amount of relative movement and the surface pressure between the parts. It has also been realized that the expanding and retracting movement of the piston ring causes practically no movement between the annular groove and piston ring in the part of the ring opposite to the ring partition, since the part opposite to the ring partition acts as a flexible hinge, and is thus practically not moving when the piston ring expands and retracts. Most of the relative movement takes therefore place at the part of the ring closest to the ring partition. Thus, most of the wear to the chrome plating on the lower groove face takes place in the area around the ring partition, wherever it happens to be.

Although the ring partition changes place during engine operation, it has been found out that the wear rate of the chrome plating on the lower groove face can be reduced by

changing the properties of the lower ring face in the area of the ring partition. By providing a part of the lower ring face proximate to the partition with a wear resistant coating, such as a chrome plating, a hard face to hard face combination is obtained. This combination is particularly suitable for resisting 3-part abrasive wear. The wear to the lower groove face can thus be significantly reduced.

The best result is obtained with coatings with a high hardness. Typically, electrolytically applied coatings are suitable for the purpose, such as chrome or nickel based coatings, e. g. a nickel-cobalt alloy.

The wear resistant coating may also comprise hard particles dispersed in a metal matrix. The metal matrix can be formed by a chrome or nickel based material. The hard particles are typically different metal carbides, oxides or silicates, such as titanium carbides, aluminum oxides, wolfram carbides, chrome oxides, and vanadium carbides.

In order to achieve a satisfactory effect, the wear resistant coating needs to extend tangentially from both sides of the ring partition for approximately 30° to 60°, though preferably at least approximately 45°.

The ring partition can be of the type that is substantially gastight and provided with several leakage tracks formed in the lower ring face or in the periphery of the piston ring.

Alternatively, the ring partition can be a simple radial slit.

It is another object of the present invention to provide a large two-stroke internal combustion engine, with improved wear resistance of the lower groove face of the annular

grooves in the pistons. This object is achieved in accordance with claim 6 by providing a part of the lower ring face proximate to the ring partition with a wear resistant coating, preferably a hard plating.

It is yet another object of the present invention to provide a method for treating a piston ring of said kind which reduces wear of the lower groove face. This object is achieved in accordance with claim 8 by providing a method comprising the steps of suspending the piston ring from a suspension member with the upper and lower ring faces arranged substantially vertically and the ring partition arranged down, lowering the piston ring into an electrolytic coating bath until a part of lower ring face proximate to the ring partition is submerged, electrolytically depositing a wear resistant coating on the submerged part of the piston ring, and raising the piston ring up from the electrolytic coating bath.

When only a part of the lower ring face needs to be provided with a coating only a part the ring needs to be submerged.

When applying a wear resistant coating to the complete lower ring face, the tooling holding the piston ring needs to be at least partially submerged, and therefore masked before each treatment to avoid contamination of the catalytic bath. This creates a large increase in the amount of resources required to produce the piston ring. Thus, the realization that it is sufficient to provide only the part of the lower ring face proximate to the ring partition with a wear resistance coating allows for considerable savings in production costs.

The electrolytically deposited wear resistant coating preferably has a thickness between 0,05 mm and 0,5 mm.

Further objects, features, advantages and properties of the piston ring, combustion engine and method for producing a piston ring according to the invention will become apparent from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawings, in which: Fig. 1 shows part of a cross-section through piston rings according to the invention arranged in annular grooves in a piston arranged in a cylinder, Fig. 2 is a plane top view of a piston ring according a first preferred embodiment of the invention, Fig. 3 is a plane side view of a piston ring according to a first preferred embodiment of the invention, Fig. 4 is a cross-section on a larger scale of a piston ring inserted in an annular groove, Fig. 5 illustrates diagrammatically the movements of the different parts of the piston ring according to the invention.

Fig. 6 shows a bath for coating a piston ring, and Fig. 7 is a plane top view of a piston ring according to a second embodiment of the invention.

DETAILED DESCRIPTION Fig. 1 shows a piston 1 mounted in a cylinder liner 2. The piston has several axially separated annular grooves 3, each as shown in Fig. 4, having an upper groove face 4, a lower groove face 5 and a groove bottom 6.

The lower groove face 5 is coated with a layer of chrome 10 to form a hard surface that is resistant to 3-part abrasive wear.

If the up to approximately 1 mm thick layer of chrome should however get worn the piston 1 needs to be removed from the engine to apply a new layer of chrome. The lower groove face 5 has to be machined before and after applying the new chrome layer.

The piston rings 7 are inserted in the annular grooves to prevent the gas pressure in the working chamber 11 above the piston 1 from spreading to the space below the piston. The piston ring 7 has a ring partition 9 formed by a simple radial cut rendering it possible to expand the ring diameter when mounting in the annular groove 3 and permitting the piston ring to expand and contract to adapt the ring outer diameter to the inner diameter of the cylinder liner 2 as the piston 1 moves up and down. The ring partition 9 also allows the two ends of the ring at the partition to pull away from each other as the ring gets worn.

Figs. 2 and 3 show the piston ring 7 according to a first preferred embodiment in greater detail. The ring has a lower ring face 15 and an upper ring face 16. The lines 18 and 19 shown in Fig. 2 demarcate the part of the lower ring face that is coated with a layer of chrome 17 to improve the hardness of the face material and to create a face material combination that is particularly durable. The thickness of the coating is typically between 0,05 and 0,5 mm. The coated area extends in both directions for about 45° from the ring partition. It is understood though that is possible to cover a smaller area or a larger area with chrome plating, depending on circumstances such as load conditions and the amount of abrasive particles to which the piston ring and the annular groove are exposed.

It is also possible to use other coating materials, such as nickel based alloys, i. e. a nickel-cobalt alloy.

Electrolytically applied coatings that give a high surface hardness are however preferred.

The wear resistant coating can alternatively be formed by hard particles dispersed in a metal matrix (not shown). The metal matrix can be formed by a chrome or nickel based material. The hard particles are typically different metal carbides, oxides or silicates, such as titanium carbides, aluminum oxides, wolfram carbides, chrome oxides and vanadium carbides.

In the cross-section shown in Fig. 4, the lower ring face 15 of the piston ring 7 abuts the lower groove face 5, and the chrome plating 10 on the lower groove face 5 and the partial chrome plating 17 on the lower ring face 15 are diagrammatically illustrated. Thus, in the area where the lower ring face 15 is coated with chrome there is a hard face to hard face contact. This type of surface combination is particularly suitable for resisting abrasive 3-part wear, i. e. wear that is dependent on the amount of movement between the surfaces involved.

Fig. 5 shows illustrates the movements of the different parts of the piston ring 7 due to expansion and retraction to adapt the ring diameter to the slightly converging inner diameter of the cylinder liner 2. The vectors in the figure represent the moved distance (enlarged for visibility reasons) and direction. The parts close to the ring partition make the largest movement, in a practically tangential direction.

Towards the area of the ring opposite to the ring partition the movements are very small in a practically radial direction. Wear to the chrome layer 10 is caused by sliding

contact with the lower ring face 15. Most of the movement between the lower groove face 5 and the lower ring face 15 takes place in the part of lower ring face that is proximate to the ring partition and most of the wear of the chrome layer 10 happens in the area where the ring partition is located, whichever part of the annular groove this happens to be. In this area there is however a hard face to hard face material combination that reduces wear and thus the chrome layer 10 lasts.

Fig. 6 shows a bath 20 for catalytic coating of the piston rings. The piston ring 7 (or a plurality of piston rings) is suspended from a tool 21 that can lower and raise the piston ring 7 in and out of the catalytic bath. The rings are hung on the tool 21 with the ring partition down. The piston ring is partially lowered into the catalytic bath (cf. Fig. 6) to apply the chrome coating (or other suitable coating). At least about one fifth of the piston ring 7 needs to be submerged, depending though on requirements to wear reduction it is preferred to submerge at least one quarter of the piston ring during the coating procedure. When the coating is applied the piston ring is lifted from the bath and eventually subjected to other treatments before mounting it in the annular groove of a piston.

Fig. 7 shows an another preferred embodiment of the piston ring according to the invention. The ring partition 9 in this embodiment formed by mutually overlapping tongues which bar a major part of the gap to form a largely gastight partition.

The piston ring 7 has several leakage grooves 14 in its lower surface 15. In the example shown in Fig. 7, the ring has ten leakage tracks 14, evenly distributed over the periphery of the ring. Alternatively, the leakage tracks can be formed in

the periphery of the piston ring (not shown). The lines 18 and 19 shown in Fig. 7 demarcate the part of the lower ring face that is coated with a layer of chrome 17 to improve the hardness of the face material. The coated area extends in both directions for about 45° from the ring partition, but the extend of the coated area can be adjusted in accordance with circumstances.

The piston rings according to the invention are applicable for internal combustion engines of both the two-stroke and the four-stroke type, but they are especially advantageous in large two-stroke crosshead engines with uniflow scavenging, viz. with scavenging air ports in the lower section of the cylinder liner and an exhaust valve in the cylinder cover.

Such engines may, for example, have cylinder diameters in the interval from 250 to 1000 mm, and stroke lengths in the interval from 900 to 2400 mm, and they can be operated at very high mean pressures in the interval from 16 to 20 bar, which results in large loads on the piston rings. In these engines, the piston ring according to the invention renders it possible to achieve long life for piston rings and the ring groove despite the high load on the rings.

Although the present invention has been described in detail for purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the scope of the invention. Thus, while the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the scope of the appended claims.