NO317176B1 - Method of Preparing a Cylinder Lining for a Piston Engine, and a Cylinder Lining - Google Patents

Description

The present invention relates to a method for producing a cylinder liner for a piston engine, such as a cylinder liner with an inner diameter in the range from 25 to 100 cm and a length in the range from 100 cm to 40 cm, where the running surface for the piston rings on the inner surface of the liner is formed by first cutting a wave pattern with a level difference (h) between the wave peaks and valleys of at least 0.005 mm in the inner surface with at least one cutting tool with a curved cutting edge, and then removing the wave peaks from the pattern at least in the running surface closest to the top dead center position of the piston, so that the inner surface of the finished liner in longitudinal section has a partially wavy surface where the wave valleys are separated by plateau sections which are mainly flat sections.

German patent no. 683 262 describes a cylinder liner which is produced by a method of this type, where the wave peaks in the wave pattern are removed by honing the inner surface of the liner. This method requires a change from one machining device that cuts the wave pattern in the inner surface, to a new tension in a honing machine. Moreover, honing in itself is an expensive and time-consuming machining operation, where a head with several rotating hone stones is guided lengthwise through the groove as it rotates, so that the hone stones grind away the material in the wave crests. Especially in the case of large cylinder liners, the honing equipment is expensive to acquire.

Swiss patent no. 342409 describes a cylinder liner where the running surface for the piston rings is formed on the inner surface of the liner by cutting a wave-shaped pattern in the inner surface. Such a lining is called wave cut, and the pattern

is usually helical, as the cutting tool is fed in the liner's longitudinal direction at a certain speed, while the liner is rotated. An advantage mentioned in the Swiss patent is that the grooves collect lubricating oil, so that oil pockets are created that promote lubrication between the piston rings and the groove

one's inner surface.

This wave cutting in the liner's inner surface, in a wave pattern which is continuous in the liner's longitudinal direction, entails the advantage in production that honing of the inner surface is avoided because the wave cutting machines the chamfering to the desired dimension of the inner diameter. When the chamfer is put into use, the piston rings will grind away the wave crests so that plateau sections occur between the wave valleys, but the piston rings are worn down at the same time.

The development of large two-stroke crosshead engines moves towards ever-increasing cylinder performances and thus also increasing effective mean pressures. The latest engines can be produced with cylinder outputs of up to 5,700 kW at an effective mean pressure of 18.2 bar. This places very high demands on piston rings and cylinder liners, because the pressure drop across the piston rings and thus the force of their contact with the liner becomes large. It is therefore possible to encounter problems during the run-in of pistons and grooves if the inner surface of the grooves is cut in a clean wave pattern, as the protruding, sharp wave crests will be able to cause the piston rings to stick.

Danish patent no. 139111 describes a cylinder liner whose inner surface is provided with a helical cut groove in which the pitch of the helical shape is so great that the wave valleys are separated by plateau sections with a length L of e.g. 4 mm in the longitudinal direction of the cylinder. Before the groove is cut, the liner must be honed, which makes the liner expensive to manufacture because it must first be machined to its approximate final internal dimension in one clamping, then clamped in a honing machine for honing, and then again moved to the first clamping for cutting the track. Cylinder liners for large engines are heavy components that are time-consuming to move and tension in machining equipment.

JP-A 5-65849 describes a cylinder block for a reciprocating engine, where the cylinder, after boring, is subjected to a honing operation which forms grinding marks or grooves in a waffle pattern. These grinding marks include small sharp protrusions that can cause damage to the piston rings. To prevent this, the cylinder is polished bright using several rolling tools. Such a smooth polishing operation to smooth out small protrusions on a cylindrical surface is a well-known process. The cylinder block described in this Japanese document must also be moved between several tensioners in different machines.

The purpose of the invention is to provide a method for the production of cylinder liners with the advantageous broken wave pattern, so that the expensive honing equipment can be omitted, that the handling of the liner is facilitated, and that the time consumption during production is reduced.

In light of this, the method according to the invention is characterized by the fact that at least 0.004 mm of the wave crests are plastically compressed into the plateau areas without the use of honing, whereby the bottom of the wave valleys after compression is at a level that is at least 0.001 mm lower than the plateau areas.

The plastic compression can be carried out by a technically uncomplicated process that can be carried out with relatively simple and cheap equipment, and the very large linings can be held in one and the same tension while the wave pattern is cut inside the ring and the wave crests are pressed together into the plateau sections, which are mainly flat parties. In addition, the investment in honing equipment is saved, which is very expensive for liners of this large size. Furthermore, the inner surface of the groove, in the plateau sections between the wave valleys, gets a surface character that is very favorable for the run-in of the liner and piston rings. The rolled surface is free of sharp protrusions, but on the other hand, it is not completely smooth or mirror-gloss, which would have caused problems for the lubrication between the liner and the piston rings. The plastic compression of the wave crests can be carried out e.g. by means of rolling with a small rolling tool, which is preferred because the equipment for this is the simplest. Alternatively, rolling can be carried out using a single roll that extends along the entire length of the furrow. The aforementioned limits for the heights of the corrugated surface are particularly advantageous for a corrugated pattern which, before rolling, has level differences between the troughs and peaks of 0.01 to 0.02 mm. By a plastic deformation of the wave crests within the above-mentioned limitations, the inner surface of the liner achieves a surface which provides a gentle run-in of the piston rings. If the depth of the wave valleys becomes less than 0.001 mm, the lubrication conditions achieved will not be satisfactory.

Preferably, the wave pattern is cut into the inner surface of the groove by at least one cutting tool which is advanced in the longitudinal direction of the liner by means of a drill rod at a feed rate, while the groove is rotated so that the wave pattern is formed as at least one spiral cut, and the plastic compression is carried out by rolling the inner surface with a rolling tool that is moved forward with the same drill rod as the cutting tool. This prevents time-consuming relocation of the cylinder liner from a tension in one machine to a tension in another machine. When the spiral cut has been cut in the inner surface of the groove, the drill rod can be moved out of the liner and the rolling tool mounted, after which the drill rod is again introduced into the liner and the rolling is carried out. The cutting and rolling tools will also be able to be mounted so that a correct change of tools can be replaced by running the tools backwards or forwards in relation to the liner's inner surface, as required. The drill rod with the cutting tool is designed to adjust the cutting depth of the cutting tool by radial displacement of the tool, and therefore the roller pressure will be suitably adjusted by displacement of the roller tool in the radial direction of the groove, so that the existing adjustment options for the drill rod are utilized.

The rolling will also be able to be carried out with a rolling tool with several rolls mounted in a tool head, which is known from rolling the inner surface of pipes, but such a tool is most suitable for relatively small pipe diameters where the diameter is constant. Preferably, the plastic compression is carried out by rolling with a rolling tool with a single roller whose radial position in relation to the liner's inner surface can be adjusted, and which can be moved forward in the liner's longitudinal direction while the liner is rotated. This makes it possible to use the same tool for rolling liners with different inner diameters. The use of a single roller also enables very precise adjustment of the roller pressure by radial displacement of the roller, so that redundant rolling down of the wave pattern is avoided. If several rollers are used, simultaneous control of the rollers must be carried out within narrow limits, which can be difficult, especially because varying forces on one roller can be transferred to the other roller(s).

It is desirable that the rolling tool is associated with an indicator for the relevant rolling pressure, so that the rolling pressure can be monitored and possibly fine-tuned during the rolling of the inner surface. Cylinder liners are often produced in series for a single engine or for several engines of the same size, and in such series production the indicator can also be used to reapply experience with a suitable rolling pressure for a specific cylinder liner size and to adjust the rolling tool at the beginning of rolling of a liner.

In order to simplify the production of the lining, the cutting of the wavy pattern can be carried out with a tolerance of the cut inner lining diameter of e.g. ± 0.1 - 0.2 mm when the diameter of the liner is in the range between 25 cm and 100 cm. Despite this tolerance, the wave height in the pattern is cut with a much finer tolerance of e.g. ± 0.003 mm or less, because the arc-shaped cutting edge of the tool insert in the cutting tool has a very large radius of e.g. from 100 mm to 800 mm, depending on the inner diameter of the ring and the desired wave height in the pattern, and because the variations in diameter take place so slowly that neighboring waves are cut with essentially identical diameters. Due to the groove diameter tolerance, the rolling tool will suitably be able to maintain the desired roll pressure when moving the tool in the longitudinal direction of the groove, although the inner diameter of the groove varies along the length of the groove.

As the diameter variations are small, the rolling pressure can be maintained with a very simple tool design by the rolling tool being supported by an arm that bends inwards within its elastic limit in the radial direction of the liner when the rolling pressure is applied, whereby the arm compensates for the diameter variations by springing in the radial direction. Alternatively, the rolling tool could be mounted on a transverse slide which is continuously adjustable in the radial direction by means of an adjustment displacement on the basis of signals from the above-mentioned indicator for the relevant roll pressure.

When the piston engine runs, the pressure in the chamber above the piston drops as it moves away from the top dead center position, and the decreasing pressure leads to less force between the piston rings and the liner. In certain cases, it will be possible to manufacture the liner in such a way that the rolling is only carried out in an upper liner section comprising the area where the top piston ring slides when the piston is moved from its top dead center and part of the piston stroke downwards towards the bottom dead center position. The rolling of the inner surface takes place so quickly that no significant time is gained by limiting the rolling to the upper groove part, but a saving on rolling equipment can be achieved, especially in the case of very large liners with a length of up to 400 cm, because the drill rod does not need to be that long.

The wave crests will be able to be deformed in such a way during the rolling that the area of the plateau sections between the wave valleys constitutes from 25% to 75% of the total area of the bearing in the rolled section. If the plateau portions are less than 25%, the contact area with the piston rings is too small, which could cause damage to the material in the rings due to overheating, because not enough heat is transferred to the ring. An insufficient contact area could also destroy the piston rings' pressure-sealing effect. If the plateau sections make up more than 75%, the lubrication conditions (the tribological conditions) deteriorate because the oil pockets become too small. Preferably, the wave peaks are deformed in such a way during the rolling that the area of the mainly flat parts between the wave valleys constitutes from 40% to 60% of the total area of the liner in the rolled part. This is a compromise between conflicting considerations in terms of lubrication conditions and thermal stress and pressure sealing, and at the same time maintaining a suitable distance to the above threshold limitations so that a certain inaccuracy in production will not be of vital importance to the liner's operating conditions.

The ability of the piston rings to seal against very high pressures in the combustion chamber can be ensured by deforming the wave peaks during rolling so that the plateau area between successive wave valleys has an extent in the longitudinal direction of the groove which, within a range of ± 1 mm, corresponds to a quarter of the ring height of the piston ring with the smallest ring height. When the piston in a cylinder liner produced in this way is moved longitudinally along the part with the wave-shaped pattern, each piston ring is surrounded by at least two consecutive planar parts which prevent the pressurized gas above the piston from blowing through the helical groove or wave valley and down under the piston ring.

Most expediently, the wave crests will be deformed so that at least 0.006 mm and at most 0.018 mm, preferably at most 0.015 mm of the height of the wave crest is pressed together into the plateau sections, and that the bottom of the wave valleys is at a level that is at least 0.002 mm lower than these parties. If these narrow limits are exceeded locally, it is still possible for the inner surface of the furrow to have an acceptable surface.

In a preferred method according to the invention, the wave-cut pattern is deformed so that the average radial level difference between the provided plateau sections and the wave valleys is between 7% and 66% of the average level difference between the wave peaks and wave valleys in the pattern before the compression, and preferably between 16% and 36% of this one.

The invention relates to a cylinder liner and piston assembly for a piston engine, such as a large two-stroke cross-head engine with a piston ring running surface on the inner surface of the liner, which running surface, at least in the region nearest the top dead center position of the piston, in longitudinal section has a partially wavy pattern where the wave valleys are separated by plateau sections. This cylinder liner according to the invention is characterized in that it has an inner diameter in the range from 25 cm to 100 cm and a length in the range from 100 cm to 400 cm, that the plateau sections are rolled surfaces, that the bottom of the wave valleys is at a level that is at least lies 0.001 mm lower than these parts, and that the plateau part between successive wave valleys has an extent in the liner's longitudinal direction that lies within ± 1 mm of a quarter of the ring height of the piston ring that has the smallest ring height. The cylinder liner exhibits the above-mentioned advantageous properties

creator of the running surface.

Examples of the invention will now be explained in more detail below with reference to the attached very schematic drawings, where

fig. 1 shows a partial side view and partial longitudinal section of a cylinder liner,

fig. 2 shows a perspective view of a cylinder liner which is clamped in a partially shown machining device,

fig. 3 is a perspective view of a rolling tool,

fig. 4 is a side elevation of another rolling tool,

fig. 5 shows a greatly enlarged longitudinal section of the inner surface of a cylinder liner which has been rolled according to the invention,

fig. 6 shows a five times enlarged photograph of the inner surface of a wave cut and partially honed cylinder bearing,

fig. 7 is a similar photograph of a cylinder liner that is wave cut and rolled according to the invention,

fig. 8 is a copy of a roughness measurement made of the inner surface of the bearing, shown in fig, 6, and

fig. 9 is a copy of a roughness measurement made of the inner surface of the groove, shown in fig. 7.

Fig. 1 shows a cylinder liner 1 for a large two-stroke crosshead engine. Depending on the engine size, the cylinder liner can be manufactured in different sizes with inner diameters that typically range from 25 cm to

100 cm, and corresponding typical lengths in the range from 100 cm to 400 cm. The lining is usually made of cast iron,

and it will be able to be cast in one piece or split into two parts which are joined end-to-end. In the figure, the liner, which is half shown to the right of the longitudinal axis 2, is shown in longitudinal section. In a well-known way, the furrowing will be able to be mounted in one

engine, not shown, by positioning an annular, downward-facing surface 3 on the top plate of the engine rack case or cylinder block, after which a piston 4 with piston rings 5 is mounted in the cylinder and a cylinder cover is arranged on top of the groove, on its annular upward-facing surface 6, and clamped to the top plate.

The piston rings 5 slide along the inner surface of the liner 7 and it is therefore important that the inner surface has a structure that ensures good lubrication between the rings and the inner surface, to avoid subbing or sticking of the outer side of the rings to the inner surface of the groove. During the running-in of the piston and the groove in a new engine, the structure of the surface is of particular importance. As mentioned above, it is therefore desirable to produce the cylinder liner with a wave-shaped pattern on the inner surface, where the wave crests have been removed. It is possible to produce the lining with the relevant pattern along the entire inner surface. The pattern can also be machined in only an upper part of the liner, such as the part swept by the piston rings 5 in the first 40% of the downward piston stroke. The lot will also be able to have other relative sizes, such as 20%, 25%, 30% or 35%, or intermediate values.

Before machining the flushing air slots 8 in the lower part of the groove, the machining of the inner surface of the liner 7 is finished. This takes place in a very large drilling machine designed as a type of lathe with large dimensions, which is only partially shown in Fig. 2. In the following, the machine is called a lathe. Using a crane, the liner is lifted with a horizontal longitudinal axis and centered in relation to the lathe's axis of rotation, after which one end of the liner is clamped to the lathe's drive spindle by means of four jaws 9, while the other end of the groove is supported in a centered position by of a holder 10 which is provided with several support rollers 11 which run on the outer surface of the furrow. The holder 10 is displaceable on the lathe bracket 12.

At the opposite end of the spindle, the lathe has a carriage, not shown, which supports a very heavy and rigid drill rod 13 which is moved by displacement of the carriage on the lathe frame into and out of the cylinder liner, coaxially with its longitudinal axis. At the end closest to the spindle, the drill rod has a tool holder 14 in the form of a transverse slide which can adjust a tool 15 in the radial direction of the liner.

When the liner is clamped in, the spindle with the liner is set in rotation and the inner surface 7 is roughly turned with an accuracy of the diameter of e.g. 5 mm. Fine turning is then carried out with a tool insert with a curved cutting edge, so that the cutting provides the desired shape of the wave valleys in the wave-cut pattern in the inner surface of the groove produced by the fine turning. The distance S (fig. 5) between two successive wave crests is adjusted as desired by means of the forward feed by the longitudinal displacement of the drill rod, the length of the distance being the same as the feed rate. In the case of a cylinder liner with an inner diameter of 98 cm, a feed rate of 8 mm per revolution of the cylinder liner would be appropriate, while a feed rate of 4 mm would be selectable for a cylinder liner with an inner diameter of 50 cm or less. The pitch can be chosen so that it corresponds to half the height of the one of the piston rings that has the smallest ring height.

The radial difference in levels h {fig. 5) between wave peaks and troughs is determined by the curvature of the tool cutting edge, as a stronger curvature produces a greater difference in the levels. The difference in levels could be as large as 0.06 mm, but normally from 0.01 mm to 0.02 mm is preferred.

After cutting the wave pattern, the drill rod is moved out of the liner and a rolling tool is placed radially in relation to the inner surface 7, after which the inner surface is rolled so that the material in the wave crests is plastically deformed, i.e. pressed radially outwards, so that the finished inner surface takes the shape shown in fig . 5, with a helical groove or wave valley 17. The longitudinal section in the inner surface of the groove, shown in fig. 5, is distorted for the sake of clarity, so that the dimensions in the radial direction are many times enlarged.

In the longitudinal direction, the wave valleys are separated by planar portions 18, which together constitute 25% to 75%, and typically 40% to 60% of the length of the corrugated pattern of the furrow.

In a simple design, as shown in fig. 3, the rolling tool can comprise a roller 19 which is rotatably mounted in a fork head 20 at the end of a cross arm 21 which is fixed in a recess in a tool holder 22 which is supported by the drill rod 13. The tool holder or the tool itself will be able to have a certain limited flexibility in the radial direction of the groove, so that variations of a few tenths of a millimeter in the diameter of the groove are absorbed as an elastic bending of the holder. The cross arm is adjustable in its longitudinal direction, i.e. in the radial direction of the groove.

Another example of the design of the rolling tool is shown

on fig. 4, where a roller 23 is encased on one side in a head 24, and at its rear end the roller is in contact with a support roller 25. The head is mounted on an obliquely extending angle-shaped t part which is divided into two parts, 26a and 26b which is

mutually elastic, but maintains the set roller pressure. An indicator 27 shows the magnitude of the roller pressure in question. Instead of a visual indicator, the tool could be equipped with an inductive system to measure the roller pressure and to emit electrical signals that can be used for adjustment purposes or for remote reading. Via an intermediate piece 28, the angular part is mounted in the drill rod's tool holder 14, so that the roller pressure can

adjusted by radial displacement of this tool holder. A tool of this type is commercially available from the German firm, W. Hegenscheidt GmbH, Celle, under the type designation EG 14.

The roll pressure indicator could be built into the cross slide of the drill rod, which is affected by essentially the same radial pressure as the roll tool. The lathe can also be equipped with a projector, e.g. a digital display of the displacement of the cross slide in the respective radial and axial direction. Such a display will be able to be reset when the rolling tool is arranged in pressureless contact with the liner's inner surface, after which the outward displacement of the transverse slide will be representative of the rolling pressure.

The longitudinal axis of the roller will be able to form a free angle a with the inner surface of the ring, where the tip of the angle faces forwards in the feed direction, shown by arrow A.

Now follows a description of samples made with a cylindrical ring with an inner diameter of 35 cm.

Example 1

The casing was made from the lining material common to large engines, cast iron, and after rough-turning, the inner surface of the casing was fine-turned along its full length in a helical, wave-cut pattern with a distance S = 4 mm between wave crests and a wave height h of about. 0.015 mm. Then the drill rod cutting tool was replaced with the rolling tool shown in fig. 3. The roller pressure was adjusted by first bringing the roller into pressure-free contact with the inner surface of the ring, after which the cross slide of the drill rod was set to an outward displacement F = 0.03 mm, measured on the diameter, i.e. a radial displacement of 0.015 mm. It should be noted that the adjustment of the cross slide does not entail

a corresponding radial displacement of the rolling tool, as a significant part of the displacement is used for pressure load-

ning of the cross slide, the tool holder and the tool, i.e. building up the roller pressure. This is a significant difference from the setting of the cutting tools normally used in a lathe. The liner rotation speed was set at 90 rpm, resulting in a relative velocity between the rolling tool and liner inner surface of V = 100 m/min, and the drill spindle was advanced into the groove at a feed rate of s = 0.5 mm/rev.

Visual inspection showed that a greater roll pressure was desirable, and that the feed rate could have been significantly higher.

Example 2

Apart from the rolling parameters, the cylinder liner was produced in the same way as in example 1. The rolling was carried out with the parameters V = 100 m/min, F = 0.10 mm on the diameter and s = 4.0 mm/rev.

Visual inspection and roughness measurement showed that the feed rate was appropriate and that the sections between the wave valleys had a well-defined extent and were mainly flat.

Example 3

Apart from the rolling parameters, the cylinder liner was manufactured in the same way as in example 1. The rolling was carried out with the parameters V = 100 m/min, F = 0.15 mm on the diameter and s = 4.0 mm/rev.

Visual inspection and roughness measurement showed that the feed rate was still appropriate and that the parts between the wave valleys had reached a greater extent and amounted to approx. 30% of the furring's inner surface.

Example 4

Apart from the rolling parameters, the cylinder liner was produced in the same way as in example 1. The rolling was carried out with the parameters V = 100 mm/min, F = 0.20 mm on

the diameter and s = 4.0 mm/rev.

Visual inspection and roughness measurement showed that the feed rate was still appropriate and that the parts between the wave valleys had reached a greater extent and amounted to approx. 40% of the lining's inner surface.

A comparative test was carried out with a cylinder liner produced in the same way as in example 1, but where the rolling was replaced by a partial honing which removed the wave crests.

The surfaces of the linings produced in example 4 and by partial honing were photographed five times enlarged, see fig. 6 and 7, and the surface roughness was measured with a Perthen roughness tester, see fig. 8 and 9, where the amplification in the radial direction was adjusted so that it was very strong.

On the strips with the registration, 10 mm in the direction of the y-axis indicates a stretch of 0.025 mm, while 10 mm in the direction of the x-axis indicates a stretch of 1 mm.

Fig. 6 clearly shows ring-shaped grinding marks or traces from the honing, and the roughness test in fig. 8 shows a large number of small points in the approximately flat sections where the wave crests have been removed.

The rolled surface shown in fig. 7 has a considerably nicer appearance, and the roughness test in fig. 9 shows flat sections between the wave valleys with far fewer sharp protruding points, but the surface still has a number of small rounded level differences in the flat sections, which contribute to achieving good oil adhesion to the surface.

In the above dimension indications for the wave-cut pattern and the rolled pattern, it must be understood that the mentioned values are mean values. As shown on the roughness sample strips, the surface has locally large depressions which are not included in the dimensions, as these are typical graphite deposits in the surface, or similar variations determined by the alloy. These depressions are also found in the mainly flat parts, which can also be called plateau parts.

Claims (10)

Method for manufacturing a cylinder liner (1) for a piston engine, such as a cylinder liner with an inner diameter in the range from 25 cm to 100 cm and a length in the range from 100 cm to 400 cm, where the running surface for the piston rings on the inner surface (7) of the liner is formed by first cutting a wave pattern with a level difference (h) between the wave crests and valleys of at least 0.005 mm in the inner surface with at least one cutting tool with a curved cutting edge, and then removing the wave crests from the pattern, at least in the running surface closest to the top dead center position of the piston, so that the inner surface (7) of the finished groove (l) in longitudinal section has a partially wavy surface where the wave valleys (17) are separated by plateau sections (18) which are mainly flat sections, characterized by at least 0.004 mm of the wave peaks are compressed plastically into the plateau parts (18) without the use of honing, whereby the bottom of the wave valleys (17) after the compression is left at a level that is at least e r 0.001 mm lower than the plateau areas. 2. Method according to claim 1, characterized in that the wave pattern is cut into the inner surface of the groove using said at least one cutting tool, which is advanced in the longitudinal direction of the groove by a drill rod at a feed speed (s) while the groove is rotated so that the wave pattern is formed as in at least one spiral cut, and that the plastic compression is carried out by rolling the inner surface with a rolling tool that is moved forward using the same drill rod as the cutting tool. 3. Method according to claim 1 or 2, characterized in that the plastic compression during rolling is carried out with a rolling tool with a single roll (19; 23) whose radial position in relation to the liner's inner surface can be adjusted, and which can be moved forward in the liner's longitudinal direction while the ring (l) is rotated. 4. Method according to claim 3, characterized in that the rolling tool is connected to an indicator (27) for the current rolling pressure. 5. Method according to claim 3 or 4, characterized in that the rolling tool maintains the desired rolling pressure during movement of the tool in the longitudinal direction of the liner, although the inner diameter of the ring (1) varies along the length of the liner. 6. Method according to one of claims 2-5, characterized in that the rolling is only carried out in an upper fringing part, comprising the part on which the top piston ring slides when the piston (4) is moved from its top dead center position and part of the piston stroke downwards towards it bottom dead center position. 7. Method according to one of claims 2-6, characterized in that the wave crests are deformed by the roller in such a way that the area of the mainly flat parts (18) between the wave valleys (17) constitutes from 25% to 75%, preferably from 40% to 60% of the ffiring's (1) total area in the rolled part. 8. Method according to claim 7, characterized in that the wave crests are deformed by rolling such that the mainly flat part (18) between successive wave valleys (17) has an extent in the longitudinal direction of the lining which, within a range of ± 1 mm, corresponds to a quarter of the ring height of the piston ring that has the smallest ring height. 9. Method according to one of the preceding claims, characterized in that the wave peaks are deformed so that at least 0.006 mm and at most 0.018 mm, preferably at most 0.015 mm, of the height of the wave peaks is pressed into the mainly flat parts (18), and that the bottom of the wave valleys (17) is at a level that is at least 0.002 mm lower than these parts. 10. Method according to one of the preceding claims, characterized in that the wave-cut pattern is deformed so that the average radial difference in levels between the provided mainly flat parts (18) and the wave valleys amounts to between 7% and 66% of the average difference in levels ( h) between the crests and troughs of the pattern before the compression, and preferably between 16% and 36% thereof. 11]/<*> Cylinder liner (1) for a piston engine, such as a large two-stroke crosshead engine, having a running surface for the piston rings on the inner surface (7) of the liner, which running surface, at least in the portion nearest the top dead center position of the piston, has a partially wave-shaped pattern where the wave valleys (17) are separated by mainly flat parts (18), characterized in that the cylindrical ring has an inner diameter in the range from 25 cm to 100 cm and a length in the range from 100 cm to 400 cm, that they are mainly flat parts (18) are rolled surfaces, that the bottom of the wave valleys (17) is at a level that is at least 0.001 mm lower than these parts, and that the mainly flat part (18) between successive wave valleys (17) has an extent of the longitudinal direction of the ring which, within a range of ± 1 mm, corresponds to a quarter of the ring height of the piston ring which has the smallest ring height.