The present invention relates to a piston ring for a piston in an internal combustion engine, particularly a two-stroke, uniflow-scavenged crosshead engine, in which the piston in its peripheral surface has several axially separated annular grooves, each having an upper and a lower groove face and a groove bottom, and in which the piston ring is inserted in the annular groove to seal the working chamber above the upper piston ring downwards during the compression and combustion strokes of the piston, the piston ring having an axial height smaller than the axial distance between the upper and the lower groove faces in the pertaining annular groove so that the piston ring can move axially in the annular groove, and having a substantially gastight ring partition allowing for an extension of the ring diameter at the mounting in the annular groove, and having several leakage tracks distributed over the periphery of the ring.

Such a piston ring is known from W094/12815, where the leakage tracks are grooves in the contact surface of the ring against the cylinder wall, i.e., the grooves extend from the upper surface to the lower surface of the ring.

Piston rings are also known with a radial cut at the ring partition where the pressure drop across the ring is controlled by means of the width of the obliquely extending radial cut, called the ring gap. In modern two-stroke crosshead engines the combustion pressure is high, resulting in high gas velocities down through the ring gap with consequent strong local heating of the underlying piston ring and of the inner surface of the cylinder liner. As the piston rings

rotate in the annular grooves, the underlying piston ring may be hit in so many places by the local overheat¬ ing that it looses its initial stress, whereupon it will not maintain the required sealing contact with the inner surface of the liner. After such a collapse of ring No. 2, the gas flow through the ring gap in the top ring is increased, because the cavity down to ring No. 3 has to be pressurized to achieve a smaller differential pressure across the top ring. This increases the thermal load on the top ring in the area around the ring gap and gives a risk of its collapse.

The piston top ring taught in W094/12815 solves this problem by means of a controlled gas leak distrib¬ uted on several small leakage grooves. However, these leakage grooves also involve disadvantages, especially with piston rings under a heavy load with regard to both heat and stress, partly because the material around the leakage grooves becomes very hot, partly because the grooves give rise to stress concentrations in an area of the ring which is under a heavy stress load.

US patent No. 1 988 727 describes a gastight piston ring, which completely fills out the pertaining annular groove in the direction of height. As this piston ring is not moved axially in relation to the annular groove during each engine cycle, it is unsuitable for use in modern high-output engines where the ring lifting in the annular groove is required to avoid coking and locking of the piston ring. The piston ring of the US patent is formed with a large number of recesses and notches in all ring surfaces in an attempt to avoid locking of the ring. In the lower ring surface, the major part of the contact area against the lower groove face has been removed, and there are two radially facing leakage openings of a square cross-section. The piston ring is very complex to manufacture.

US patent No. 1 365 348 describes another piston ring, which also completely fills out the pertaining annular groove in the direction of height. To avoid gas and oil accumulation in the annular groove behind the ring, the ring is formed with plane upper and lower surfaces to seal more tightly against the upper and lower surfaces of the annular groove and with radially extending tracks in its lower surface to lead away any small amounts of gas and oil seeping in from the back of the ring. This avoids a pressure build-up behind the ring.

As mentioned, it is essential in modern engines that the piston ring can move axially in the annular groove so that a ring lifting occurs in the annular groove in each engine cycle. Such a ring abuts the lower groove face during the combustion or working stroke, when the pressure in the combustion chamber is high. The annular space behind the piston ring is thus open towards the cylinder liner, for which reason there is no need to use the radially extending tracks known from said US patent to avoid a pressure accumulation behind the piston ring.

German patent No. 39 24 982 teaches a piston ring with a ring gap at the ring partition and with radially extending grooves in the upper ring surface. It is stated that such a ring design is not advantageous in heavily loaded piston rings owing to the stress concen¬ trations around the grooves, and the grooves are therefore preferably formed in the upper groove face of the annular groove. The differential pressure across the piston ring is reduced by gas flow through the ring gap. The radially extending grooves serve for rapid pressure equalisation across the piston rings when the sealing contact of the lower ring surface with the lower groove

face is broken at the lifting of the ring in the annular groove.

Japanese patent publication No. 2-48737 describes a piston ring having a plane upper and lower surface and a piston with annular grooves, where gas flow passages have been formed in the piston material between the annular grooves to control the pressure drop across the piston rings. These passages are difficult to manufac¬ ture. It is stated that the rings are dimensioned with leakages providing a uniform pressure drop across all the rings. This means that either the leakage amounts used are so small that the differential pressure across each ring only becomes slightly smaller so that no significantly wear-reducing effect is obtained, or that the volume flow of leak gas is very large in order to build up pressure in the spaces between the bottom piston rings, which increases the specific fuel consump¬ tion of the engine because a noticeable part of the gas in the combustion chamber escapes. The object of the present invention is to provide a piston ring with improved operating properties for the engine as well as for the ring itself and for the cylinder liner.

In view of this object, the piston ring according to the invention is characterized in that the leakage tracks are formed in the lower ring surface intended for contact with the lower groove face and are slit-shaped having a slit width facing in the peripheral direction of the ring and being at least several times larger than the slit height extending in the axial direction.

The location of the leakage tracks in the lower ring surface involves several advantages. Above all, the leak gas is prevented from hitting straight down on top of the underlying ring, and the stress concentrations become substantially smaller in the material at the

outer periphery of the ring, which is heavily loaded by tensile stresses and thus sensitive to fatigue frac¬ tures. The radially outflowing leak gas hits the inner surface of the cylinder liner, but as the movement of the piston ring in relation to this inner surface is far greater than in relation to the underlying piston ring, the risk of damage to the material is substantially smaller than at the above mentioned piston top ring with axially extending leakage tracks. Another advantage is that the leakage area is unaffected by the ring wear on the outer peripheral surface of the ring.

The slit shape of the leakage tracks is essential for achieving good operating properties. At the flow through the slit-shaped tracks, the gas flow is braked by the friction between the viscous gas and the wide, relatively closely positioned wall surfaces on the ring and the lower groove face of the annular groove, respectively, whereby the gas delivers a controlled amount of energy to the material around the track, which is heated by the energy delivered. The gas typically has a temperature in the interval of 800-1800°C, and the slit shape of the tracks can reduce the highest gas velocity in the track suitably so that the temperature of the track walls is brought close to or above 500°C, also in the cases where the volume of leak gas is modest. At these high temperatures, combustion residues cannot be deposited on the track walls and coke up. It is not necessary that the high temperatures in the material are obtained for a sustained period, so long as they are obtained once in a while, as any coke fixed in the tracks will be loosened when the temperature is brought up to about 500°C.

Preferably the slit height is smaller than 1 mm, as the cross-sectional area of each leakage track becomes too high at greater heights. Especially advan-

tageous gas flow and temperature conditions can be obtained with slit heights in the interval between 0.35 and 0.55 mm, because the closely positioned wide track walls also receive energy from the gas flow at the middle of the track and reduce the velocity thereof. At the same time largely uniform flow conditions can be achieved over the full track width. The advantageous small slit height also necessitates the use of several tracks to achieve the same cross-sectional area of the leakage tracks, and thus the total heat transmission to the ring during an engine cycle is distributed more evenly.

In a preferred embodiment, the ring height is at least eight times larger than the slit height, which gives the ring a large material thickness above the leakage track, so that the ring material distributes the energy received from the gas over a larger area. This ensures that the temperature at the upper surface of the ring is suitably lower than the high temperature on the surface of the leakage track.

The temperature in the ring material above the annular groove can also be limited by the width of the slit being no more than 20 times larger than the greatest height of the slit, as part of the heat will spread in the peripheral direction to the material located beside the annular groove.

Preferably, the ring area located diametrically opposite the ring partition is kept free of leakage tracks, as the mechanical stresses are largest in this area. By keeping the area free of leakage tracks a further load owing to stress concentrations and thermal loads is avoided.

A certain distribution of the thermal load on the ring is obtained, when it has at least four leakage tracks. However, a more suitable heat distribution is

obtained when the piston ring has at least six leakage tracks in the lower ring surface. In addition to the thermal distribution, other circumstances may also be taken into account when the most suitable number of tracks is determined, such as the risk of coke deposits in the annular space between the back of the ring and the groove bottom. The leak gas will flow down past several of the piston rings on the piston, and for this it needs to move in the peripheral direction of the piston to flow from the leakage tracks in a piston ring to the leakage tracks in the underlying piston ring. The gas may draw oil residues and residual products from the combustion with it, and the risk of deposition of residual material that may coke becomes greater, the longer the gas has to flow. So as to counteract coke deposits in the annular space behind the piston ring, the piston ring preferably has from eight to sixteen leakage tracks in its lower surface. The longest flow path in the peripheral direction between two piston rings corresponds to about half of the peripheral length of the ring divided by the number of leakage tracks, if the gas flows to the closest adjacent leakage track in the following ring. It appears from this that a larger number of tracks reduces the flow path, but at the same time the manufacturing costs are increased because more tracks have to be incorporated in the ring.

The leakage tracks may advantageously be evenly distributed over the periphery of the ring, perhaps excepting the area opposite to the ring partition, to obtain uniform operating conditions.

The total cross-sectional area A. . of the leakage tracks may suitably be in the interval from D /68000 to

D /5000, where D is the cylinder diameter in mm, and A. . is the area in mm . If the area becomes smaller th ^to a ^t n D2/68000, the reduction of the differential

pressure across the piston ring becomes too modest for a substantial reduction of the ring wear. The upper area limit of D /5000 indicates a suitable compromise between a substantial reduction of the ring wear and a not too strong deterioration of the combustion conditions and thus the specific fuel consumption. With the interval stated an advantageous long ring life can be achieved combined with good operating conditions for the engine as a whole. The area can be selected to be larger than the indicated upper area limit, but it must not be chosen to be so large that the pressure under the piston rings escapes so quickly immediately after the opening of the exhaust valve that the piston rings cannot be lifted up in the annular groove. In especially heavily loaded piston rings, the stress concentrations around the leakage tracks can be reduced by a suitable design of the tracks. In a very favourable embodiment as to stress, the cross-section of the upper surface of the leakage track is composed of at least three circular arc sections, of which the two outermost ones have their centres positioned above the lower ring surface, and an intermediate one has its centre positioned below the lower ring surface.

An example of an embodiment of the invention will now be described in further detail below with reference to the very schematic drawing, 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 perspective view on a larger scale of the piston ring in the area around the ring partition, Fig. 3 is a plane top view of a piston ring according to the invention,

Fig. 4 is a section on a larger scale of the piston ring of Fig. 3 in the area around a leakage track,

Fig. 5 is a side view of the ring section of Fig. 4,

Fig. 6 is a cross-section on a larger scale of a piston ring inserted in a ring groove, and Fig. 7 is a cross-section through a leakage track having an advantageous shape as concerns stress.

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. 6, having an upper groove face 4, a lower groove face 5 and a groove bottom 6. Piston rings 7 are inserted in the annular grooves to prevent the gas pressure in the working chamber 8 above the piston from spreading to the space below the piston. The piston ring has a largely gastight ring partition 9 rendering it possible to expand the ring diameter at the mounting in the annular groove 3 and permitting the two ends of the ring at the partition to pull away from each other as the ring gets worn.

An example of the design of an annular partition 9 is seen in Fig. 2, where gas flows through the parti¬ tion are largely prevented. The outer rim of the annular groove is indicated by the dot-and-dash lines 10. The outer surface 11 of the ring abuts the inner surface of the cylinder liner 2. At the ring partition, one end of the ring has a projection 12 passing into a corre¬ sponding recess 13 at the other end of the ring. In the radial direction, the projection 12 is narrower than the ring 7, and at the inner surface of the ring the recess 13 is defined by an axially extending wall abutting the inner surface of the projection. The height of the projection 12 is also smaller than the ring height, and the recess 13 is defined upwards by a radially project¬ ing wall abutting the upper surface of the projection. As the cross-section of the projection 12 fills out the cross-section of the recess 13, a substantially gastight

closure of the ring partition is achieved. It is well- known to design the ring partition in other gastight manners.

The piston ring 7 has several leakage tracks 14 in its lower surface 15. In the example shown in Fig. 3, the ring has ten leakage tracks 14, evenly distributed over the periphery of the ring in such a manner that the two leakage tracks arranged diametrically opposite the ring partition 9 are located symmetrically about the diameter 16 passing through the ring partition so that the most heavily loaded ring area 17 at this diameter is located between the two leakage tracks. If the piston ring is alternatively manufactured with an uneven number of tracks, it may be necessary for the distance between the two leakage tracks at the area 17 to be longer than between the other leakage tracks.

As shown most clearly in Fig. 4, the longitudinal axis of the leakage tracks 14 has an angle in relation to the radial direction of the ring so that the outflow- ing leak gas will act on the piston ring with a force in the peripheral direction, which makes the ring rotate in the annular groove. The angle a may, for example, be in the interval from 20° to 70° and in the example shown is 30°. Fig. 5 shows the piston ring seen from the outside with the outward end of the leakage track 14 fully drawn up, and the inward end shown by a dashed line. The piston ring has a height of 6.5 mm, and the leakage track has a height of h = 0.5 mm and a width of 5 mm. In cross-section, the bottom 18 of the leakage track is substantially rectilinear, and each of the sides of the track is S-shaped and composed of two circular arc sections, of which the outermost has a radius R of 2 mm with its centre above the lower ring surface 15 and a tangential course into the latter, while the innermost

has a radius of 2 mm with its centre located below the lower ring surface 15 and a tangential course into the bottom 18 of the leakage track to one side and into the outermost circular arc section to the other side. It is, of course, possible to form the leakage track with a quadrangular cross-section, but this results in larger stress concentrations at the corners of the track. In the alternative embodiment shown in Fig. 7, also the bottom 18 of the leakage track has a circular-arc-shaped cross-section with a relatively large radius of curva¬ ture and the centre thereof located below the lower ring surface 15.

In the cross-section shown in Fig. 6, the lower surface 15 of the piston ring abuts the lower groove face 5, and the leakage track 14 is seen to create gas flow communication between an annular space 19 between the inner surface 20 of the piston ring and the groove bottom 6 and an annular space 21 located below the piston ring between the inner surface of the cylinder liner 2 and the outer surface of the piston 1. As the upper surface 22 of the piston ring is some distance below the upper groove face 4, the gas may flow from an annular space located above the piston ring between the inner surface of the cylinder liner 2 and the outer surface of the piston via the space above the piston ring, the annular space 19, the leakage tracks 14 and into the annular space 21 to pressurize the latter and reduce the differential pressure across the piston ring.

The leakage areas in the piston rings can vary from ring to ring, and the bottom piston ring naturally has no leakage area, as it forms the bottom seal against the space below the piston.

When the piston ring is arranged in the annular groove 3 and the piston 1 is inserted in the cylinder liner 2, the ring diameter is smaller than in the

unstressed condition of the ring, and there are tensile stresses in the ring material near the outer surface 11 and compressive stresses in the material near the inner surface 20. The ring stresses therefore keep the outer surface 11 of the ring in contact with the inner surface of the cylinder liner.

The piston ring according to the invention is applicable for internal combustion engines of both the two-stroke and the four-stroke type, but it is especial- ly 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 10 to 19 bar, which results in large loads on the piston rings. In these engines, the piston ring according to the inven- tion renders it possible to achieve long life for piston rings and cylinder liner despite the high load on the rings.