The present invention relates to a method of manufacturing a cylinder liner for a piston engine, such as a large two-stroke crosshead engine, in which the running surface for the piston rings on the inner surface of the liner is established first by cutting a wave pattern having a difference in levels between the wave crests and troughs of at least 0.005 mm into the inner surface with at least one cutting tool having a curved cutting edge and then by removing the wave crests from the pattern, at least in the running surface closest to the piston top dead centre position, so that in longitudinal section the inner surface of the finished liner has a partially wave-shaped surface in which the wave troughs are separated by substantially plane areas.

German patent No. 683262 describes a cylinder liner manufactured by a method of this kind where the wave crests in the wave pattern is removed by honing the inner surface of the liner. This method requires a shift from one machining device cutting the wave pattern into the inner surface, to a new set-up in a honing machine. Besides, honing in itself is a costly and time- consuming machining, in which a head with several rotating honing stones are passed along through the liner while it rotates so that the honing stones grind away the material in the wave crests. Particularly in case of larger cylinder liners, the honing equipment is costly to acquire.

Swiss patent No. 342409 describes a cylinder liner in which the running surface for the piston rings is established on the inner surface of the liner by cutting a wave-shaped pattern into the inner surface. Such a

liner is called wave cut, and the pattern is usually helical, the cutting tool being fed in the longitudinal direction of the liner at a certain rate while the liner is rotated. An advantage mentioned by the Swiss patent is that the grooves collect lubricating oil so that oil pockets appear which promote lubrication between the piston rings and the inner surface of the liner.

This wave cutting of the inner surface of the liner in a wave pattern which is coherent in the longitudinal direction of the liner provides the manufacturing advantage that honing of the inner surface is avoided, because the wave cutting machines the liner to the desired internal diameter dimension. When the liner is put into operation, the piston rings will abrade the wave crests so that plane areas appear between the wave troughs, but the piston rings are at the same time worn down.

The development of large two-stroke crosshead engines moves towards ever increasing cylinder outputs and thus also increasing effective mean pressure. The most recent engines can be manufactured with cylinder outputs of up to 5,700 kW at an effective mean pressure of 18.2 bar. This makes very large demands on piston rings and cylinder liner, because the pressure drop across the piston rings and thus also the force of their contact with the inner surface of the liner become large. It is therefore possible to envisage problems at the running-in of pistons and liner if the inner surface of the liner is cut in a pure wave pattern, the protrud- ing, sharp wave crests being capable of provoking seizure of the piston rings.

Danish patent No. 139111 describes a cylinder liner having in its inner surface a helically cut groove in which the pitch of the helical shape is so large that the wave troughs are separated by plane areas having a

length L of, for example, 4 mm in the longitudinal direction of the cylinder. Before the groove is cut, this liner has to be honed, which renders the liner expensive to manufacture, because it first has to be machined to its approximate final internal dimension in one set-up, and then it has to be set up in a honing machine and be honed, and then again relocated to the first set-up for cutting of the groove. Cylinder liners for large engines are heavy components which it is time- consuming to relocate and set up in machining equipment.

JP-A 5-65849 describes a cylinder block for a piston engine, in which the cylinder after boring is subjected to a honing operation creating grinding marks or grooves in a waffle pattern. These grinding marks include small sharp projections which can cause damage to the piston rings. In order to prevent this the inner side of the cylinder is burnished by several rolling tools. Such a burnishing operation smoothening out small projections on a cylindrical surface is a well known process. The cylinder block described in this Japanese document must also be relocated between several set-ups in different machines.

The object of the invention is to provide a method of manufacturing cylinder liners with the advantageously broken wave pattern in such a way that the costly honing equipment can be avoided and that the handling of the liner is facilitated, and the time consumed by its manufacture is reduced.

In view of this, the method according to the invention is characterized in that the cylinder liner has an internal diameter in the interval from 25 cm to 100 cm and a length in the interval from 100 cm to 400 cm, that the wave crests are removed, without using honing, by plastic compression of at least 0.004 mm of their heigth into said substantially plane areas, and

that the bottom of the wave troughs after the compres¬ sion is at a level at least 0.001 mm lower than these areas .

The plastic compression can be made by a technical- ly uncomplicated process to be performed with relatively simple and cheap equipment and the very large liners can be held in one and the same set-up while the wave pattern is cut into the inner side of the liner and the wave crests are compressed into the substantially plane areas. In addition the investments in honing equipment is saved, said equipment being very expensive for liners of this large size. Further, in the approximately plane areas between the wave troughs the inner surface of the liner achieves a surface character which is very favourable to the running-in of liner and piston rings. The rolled surface is free from sharp projections, but on the other hand is not completely smooth or mirror- bright, which might cause problems to the lubrication between liner and piston rings. The plastic compression of the wave crests may be performed, for example, by means of rolling with a small rolling tool which is preferred because the equipment for this is the most simple. Alternatively, rolling may be performed by means of a single roller extending along the full length of the liner. The mentioned limits on the heights of the wave-shaped surface are particularly advantageous to a wave-shaped pattern which, before rolling, has a difference in levels between wave troughs and crests of 0.01 - 0.02 mm. With a plastic deformation of the wave crests within the above interval limits, the inner surface of the liner obtains a surface providing a gentle running-in of the piston rings. If the depth of the wave troughs becomes smaller than 0.001 mm, the lubricating conditions achieved will not be satisfac- tory.

Preferably, the wave pattern is cut into the inner surface of the liner by said at least one cutting tool which is advanced in the longitudinal direction of the liner by a boring bar at a feed rate while the liner 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 which is moved forward by the same boring bar as the cutting tool. This avoids time-consuming relocation of the cylinder liner from one set-up in one machine to a set-up in another machine. When the spiral cut has been cut in the inner surface of the liner, the boring bar may be run out of the liner and the rolling tool be mounted, whereupon the boring bar is reinserted into the liner and the rolling is performed. The cutting and rolling tools may also be mounted in respective cross slides or in respective holders so that a proper change of tools may be replaced by running the tools back or forth in relation to the inner surface of the liner according to need. The boring bar with the cutting tool is adapted to adjust the cutting depth of the cutting tool by radial displacement of the tool, and therefore the rolling pressure may suitably be adjusted by displacement of the rolling tool in the radial direction of the liner so that the existing adjustment options of the boring bar are exploited.

The rolling may also be performed with a rolling tool having several rollers mounted in a tool head, which is known from the rolling of the inner surface of pipes, but such a tool is most suited for relatively small pipe diameters where the pipe diameter is con¬ stant. Preferably, the plastic compression is performed by rolling with a rolling tool having a single roller, the radial position of which in relation to the inner surface of the liner can be adjusted, and which can be

moved forward in the longitudinal direction of the liner while the liner is being rotated. This enables the same tool to be used for rolling of liners with different internal diameters. The use of a single roller also allows the rolling pressure to be adjusted very accu¬ rately by radial displacement of the roller so that excessive rolling down of the wave pattern is avoided. If several rollers are used, a simultaneous control of the rollers must be performed within narrow limits, which may be difficult, particularly because varying forces on one roller can be transmitted to the other roller (s) .

It is desirable that the rolling tool is associated with an indicator for the current rolling pressure so that the rolling pressure can be monitored and possibly be finely adjusted during the rolling of the inner surface. Cylinder liners are often manufactured in series for a single engine or for several engines of the same size, and in such a serial manufacture, the indicator can also be used to re-apply experience on a suitable rolling pressure for a specific size of cylinder liner and to adjust the rolling tool at the initial rolling of a liner.

To facilitate manufacture of the liner, the cutting of the wave-shaped pattern may be performed at a tolerance of the cut internal liner diameter of, for example, ± 0.1 - 0.2 mm when the liner diameter is in the interval between 25 and 100 cm. Despite this tolerance, the wave height in the pattern is cut at a much finer tolerance of, for example, ± 0.003 mm or less, because the arc-shaped cutting edge of the tool bit in the cutting tool has a very large radius of, for example, from 100 mm to 800 mm, depending on the internal diameter of the liner and the desired wave height in the pattern, and because the variations in the

diameter take place so slowly that neighbouring waves are cut with substantially identical diameters. Owing to the tolerance of the liner diameter, the rolling tool may suitably maintain the desired rolling pressure at movement of the tool in the longitudinal direction of the liner, although the internal diameter of the liner varies over the length of the liner.

As the diameter variations are small, the rolling pressure in a very simple tool design may be maintained by the rolling tool being supported by an arm which is bent inwards within its elasticity limit in the radial direction of the liner when the rolling pressure is applied, whereby the arm compensates for the diameter variations by resilience in the radial direction. Alternatively, the rolling tool may be mounted on a cross slide which is continuously adjustable in the radial direction by means of an adjustment drive on the basis of signals from the above indicator for the current rolling pressure. When the piston engine is running, the pressure in the chamber above the piston drops at the latter's movement away from the upper dead centre position, and the reducing pressure leads to smaller forces between the piston rings and the liner. In certain cases it may be possible to manufacture the liner in such a manner that the rolling is only carried out in an upper liner section comprising the area on which the uppermost piston ring slides when the piston is moved from its upper dead centre and part of the piston stroke down- wards towards the bottom dead centre position. The rolling of the inner surface takes place so quickly that no substantial time is gained by limiting the rolling to the upper liner section, but a saving in the rolling equipment may be achieved, particularly in case of very

large liners of lengths up to 400 cm, because the boring bar need not be so long.

The wave crests may be deformed so by the rolling that the area of the substantially plane areas between the wave troughs constitute from 25 per cent to 75 per cent of the total area of the liner in the rolled area. If the plane areas constitute less than 25 per cent, the contact area to the piston rings becomes too small, which may cause damage to the material of the rings owing to excessive heating, because the heat is not conducted sufficiently away to the liner. An insuffi¬ cient contact area may also destroy the pressure-sealing effect of the piston rings. If the plane areas consti¬ tute more than 75%, the lubricating conditions (the tribological conditions) deteriorate because the oil pockets become too small. Preferably, the wave crests are deformed so by the rolling that the area of the substantially plane areas between the wave troughs constitute from 40 per cent to 60 per cent of the total liner area in the rolled area. This is a compromise between the contradictory considerations to lubricating conditions and to thermal load and pressure sealing, at the same time providing a suitable distance to the above threshold limits so that a certain manufacturing inaccuracy will not be of vital importance to the operating conditions of the liner.

The ability of the piston rings to seal against very high pressures in the combustion chamber can be ensured by deforming the wave crests so by the rolling that the substantially plane area between consecutive wave troughs has an extent in the longitudinal direction of the liner which, within an interval of ±1 mm, corresponds to a quarter of the ring height of the piston ring having the smallest ring height. When the piston in a cylinder liner so manufactured is moved

longitudinally along the area with the wave-shaped pattern, each of the piston rings is surrounded by at least two consecutive plane areas, which prevents the pressurized gas above the piston from blowing through the helical groove or trough and down below the piston ring.

Most suitably, the wave crests may 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 crests is compressed into the substantially plane areas, and that the bottom of the wave troughs is at a level at least 0.002 mm lower than these areas. If these narrower limits are exceeded locally, it is still possible for the inner surface of the liner to have an acceptable surface.

In a preferred method according to the invention, the wave-cut pattern is deformed so that the average radial difference in levels between the provided substantially plane areas and the wave troughs consti- tutes between 7 per cent and 66 per cent of the average difference in levels between the wave crests and the wave troughs in the pattern before the compression, and preferably between 16 per cent and 36 per cent thereof. The invention also relates to a cylinder liner for a piston engine, such as a large two-stroke crosshead engine, having a running surface for the piston rings on the inner surface of the liner, which running surface, at least in the area closest to the piston top dead centre position, has a partially wave-shaped pattern in which the wave troughs are separated by substantially plane areas. This cylinder liner according to the invention is characterized in that it has an internal diameter in the interval from 25 cm to 100 cm and a length in the interval from 100 cm to 400 cm, that the substantially plane areas are rolled surfaces free

from sharp projections, that the bottom of the wave troughs is at a level at least 0.001 mm lower than these areas, and that the substantially plane area between consecutive wave troughs has an extent in the longitudi- nal direction of the liner which, within an interval of ± 1 mm, corresponds to a quarter of the ring height of the piston ring having the smallest ring height. The cylinder liner exhibits the above mentioned advantageous properties of the running surface. Examples of the invention will now be explained in further detail below with reference to the very sche¬ matic drawing, in which

Fig. 1 shows partially a side view, partially a longitudinal section of a cylinder liner, Fig. 2 shows a perspective view of a cylinder liner set up in a machining apparatus, partially shown,

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

Fig. 4 is a side view of another rolling tool,

Fig. 5, greatly enlarged, shows a longitudinal section through an inner surface of a cylinder liner, rolled according to the invention,

Fig. 6, five times enlarged, shows a photo of the inner surface of a wave-cut and partially honed cylinder liner, Fig. 7 is a similar photo of a cylinder liner which has been wave-cut and rolled according to the invention,

Fig. 8 is a copy of a roughness measurement made on the liner inner surface shown in Fig. 6, and

Fig. 9 is a copy of a roughness measurement made on the liner inner surface 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 may be manufactured in various sizes with internal diameters typically in the interval from 25 cm to 100 cm, and corresponding typical lengths in

the interval from 100 cm to 400 cm. The liner is normally manufactured from cast iron, and it may be cast integrally or be divided in two parts joined together end to end. In the Figure, the liner half shown to the right of the longitudinal axis 2 is shown in a longi¬ tudinal section. In a well-known manner, the liner may be mounted in the engine, not shown, by positioning an annular, downward-facing surface 3 on the top plate of the engine frame box or cylinder block, whereupon a piston 4 with piston rings 5 is mounted in the cylinder, and a cylinder cover is arranged on top of the liner on its annular upward-facing surface 6 and is 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 which ensures good lubrication between the rings and the inner surface so as to avoid scuffing or seizure between outer sides of the rings and the inner surfaces of the liner. During the running-in of the piston and the liner in a new engine, the structure of the surface is of particularly great importance. As mentioned above, it is therefore desired to manufacture the cylinder liner with a wave- shaped pattern in its inner surface, where the wave crests are removed. It is possible to manufacture the liner with the pattern in question along the whole inner surface. The pattern can also be machined in an upper section only of the liner, such as the section being swept by the piston rings 5 in the first 40 per cent of the downward piston stroke. The section may also have other relative sizes, such as 20 per cent, 25 per cent, 30 per cent or 35 per cent or intermediate values.

Before machining scavenging air slots 8 in the lower section of the liner, the machining of the inner surface 7 of the liner is finished. This takes place in

a very large boring machine designed as a kind of lathe of heavy dimensions, only partially shown in Fig. 2. In the following the machine is referred to as a lathe. By means of a crane, the liner with horizontal longitudinal axis is lifted and centred in relation to the rotational axis of the lathe, whereupon one end of the liner is clamped to the driving spindle of the lathe by means of four chucks 9, while the other end of the liner is supported in a centred position by a holder 10 having several supporting rollers 11 which run on the outer surface of the liner. The holder 10 is displaceable on the lathe bed 12.

At the end opposite to the spindle, the lathe has a saddle, not shown, which supports a very heavy and rigid boring bar 13, which is moved by the displacement of the saddle on the lathe bed into or out of the cylinder liner coaxially with its longitudinal axis. At the end nearest the spindle, the boring bar has a tool holder 14 in the form of a cross slide capable of adjusting a tool 15 in the radial direction of the liner.

When the liner is set up, the spindle with the liner is made to rotate, and the inner surface 7 is coarse-turned at an accuracy of, for example, 5 mm to the diameter. Then fine turning is performed with a tool bit having a curved cutting edge so that the cut produces the desired shape of the wave troughs in the wave-cut pattern in the inner surface of the liner produced by the fine turning. The distance S (Fig. 5) between two consecutive wave crests is adjusted as desired by means of the forward feed in the longitudinal displacement of the boring bar, the distance being of the same length as the feed rate. In a cylinder liner with an internal diameter of 98 cm, a feed rate of 8 mm per revolution of the cylinder liner may be suitable,

while a feed rate of 4 mm may be selected for a cylinder liner of an internal diameter of 50 cm or less. The pitch may be selected to correspond to half the ring height of the ring with the smallest ring height among those of the piston.

The radial difference in levels h (Fig. 5) between the wave crests and troughs is determined by the curva¬ ture of the edge of the tool bit, as a stronger curva¬ ture provides a larger difference in levels. The difference in levels may be as large as 0.06 mm, but normally from 0.01 to 0.02 mm is preferred.

After cutting the wave pattern, the boring bar is run out of the liner, and a rolling tool is positioned radially in relation to the inner surface 7, whereupon the inner surface is rolled so that the material in the wave crests is deformed plastically, i.e., pressed radially outwards so that the finished inner surface obtains the shape shown in Fig. 5 with a helical groove or wave trough 17. The longitudinal section in the inner surface of the liner, shown in Fig. 5, is distorted for the sake of clarity so that the dimensions in the radial direction are enlarged many times. In the longitudinal direction, the wave troughs are separated by plane areas 18, together constituting 25-75 per cent, and typically 40-60 per cent, of the liner length with the wave-shaped pattern.

In a simple design shown in Fig. 3, the rolling tool may comprise a roller 19, which is rotatably mounted in a forked head 20 at the end of a cross arm 21 fixed in a recess in a tool holder 22 which is supported by the boring bar 13. The tool holder or the tool itself may have a certain limited flexibility in the radial direction of the liner so that variations of few tenths of millimetres in the diameter of the liner 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 liner.

Another example of the design of the rolling tool is seen in Fig. 4, where a roller 23 is embedded unilat- erally in a head 24, and at its back, the roller contacts a supporting roller 25. The head is mounted on an obliquely extending angular portion divided into two parts, 26a and 26b, which are mutually resilient, but maintain the set rolling pressure. An indicator 27 shows the magnitude of the current rolling pressure. Instead of a visual indicator, the tool may be equipped with an inductive system for measuring the rolling pressure and for making electrical signals which can be used for adjustment purposes or for remote reading. Via an intermediate piece 28, the angular portion is mounted in the tool holder 14 of the boring bar so that the rolling pressure can be adjusted by radial displacement of this tool holder. A tool of this type is commercially available from the German company, W. Hegenscheidt GmbH, Celle, under the type designation EG 14.

The rolling pressure indicator may be in-built into the cross slide of the boring bar, the cross slide being influenced by substantially the same radial pressure as the rolling tool. The lathe may also have a display with, for example, digital display of the displacement of the cross slide in the radial and axial directions, respectively. Such a display may be reset when the rolling tool is arranged in force-less contact with the inner surface of the liner, whereupon the outward displacement of the cross slide will be representative of the rolling pressure.

The longitudinal axis of the roller may form a free angle with the inner surface of the liner, where the apex of the angle faces forwards in the feed direction shown by the arrow A.

Now follows a description of examples made with a cylinder liner having an internal diameter of 35 cm.

Example 1 The liner was made of the liner material usual to large engines, cast iron, and after the coarse turning, the inner surface of the liner was finely turned over its full length in a helical, wave-cut pattern with a distance S = 4 mm between wave crests and a wave height of approximately h = 0.015 mm. Then the cutting tool of the boring bar was replaced by the rolling tool shown in Fig. 3. The rolling pressure was adjusted by first bringing the roller into force-less contact with the inner surface of the liner, whereupon the cross slide of the boring bar was set at an outward displacement of 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 corre¬ sponding radial displacement of the rolling tool, as a substantial part of the displacement is used for pressure-loading the cross slide, the tool holder and the tool, i.e. to build up the rolling pressure. This is a substantial difference from the setting of the cutting tools normally used in a lathe. The liner was made to rotate at 90 rpm, which yielded a relative velocity between the rolling tool and the inner surface of the liner of V = 100 m/min, and the boring bar was displaced into the liner at a feed rate of s = 0.5 mm/rev.

Visual inspection showed that a larger rolling pressure was desirable, and that the feed rate could be substantially higher.

Example 2

Apart from the rolling parameters, the cylinder liner was manufactured in the same manner as in example 1. The rolling was performed 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 suitable, and that the areas between wave troughs had a well-defined extent and were substantially plane.

Example 3 Apart from the rolling parameters, the cylinder liner was manufactured in the same manner as in example 1. The rolling was performed 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 suitable, and that the areas between the wave troughs had obtained a larger extent and constituted about 30 per cent of the inner surface of the liner.

Example 4 Apart from the rolling parameters, the cylinder liner was manufactured in the same manner as in example 1. The rolling was performed with the parameters V = 100 m/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 suitable, and that the areas between the wave troughs had obtained a larger extent and constituted about 40 per cent of the inner surface of the liner.

A comparative test was made in which a cylinder liner was manufactured in the same manner as in example

1, but where the rolling was replaced by a partial honing which removed the wave crests.

The surfaces of the liners manufactured in example 4 and by the partial honing were photographed at an enlargement of five times, vide Figs. 6 and 7, and the surface roughness was measured with a Perthen roughness testing instrument, vide Figs. 8 and 9, where the amplification in the radial direction was adjusted to be very strong. On the strips recorded, 10 mm in the direction of the y axis indicates a distance of 0.025 mm, while 10 mm in the direction of the x axis indicates a distance of 1 mm.

Fig. 6 shows clear annular grinding marks or grooves from the honing, and the roughness test in Fig.

8 shows a large number of small points in the approxi¬ mately plane areas where the wave crests have been removed.

The rolled surface shown in Fig. 7 has a consider¬ able nicer appearance, and the roughness test in Fig.

9 shows plane areas between the wave troughs with far fewer sharp protruding points, but the surface still has a number of small rounded-off differences in levels in the plane areas, which contributes to achieving good oil adhesion to the surface.

In the dimensional indications above for the wave- cut pattern and the rolled pattern it should be under- stood that the values mentioned are average values. As shown on the strips of the roughness tests, the surface locally has large depressions not included in the dimensions, as these are typically graphite deposits in the surface or similar variations determined by the alloy. These depressions are also present in the substantially plane areas which may also be called plateau areas.