This invention relates to a method of making a seal between a piston ring and a cylinder bore, for use particularly, but not exclusively, in a motor vehicle engine. The invention also relates to a piston ring for use in making such a seal, and to an engine incorporating such rings.

Piston rings are used to provide seals within the cylinder bores of an engine, for example a four stroke petrol engine, and to transfer heat from the piston to the cylinder walls.

Conventionally each ring is circular, with a gap in the circle to allow the ring to be opened up for fitting into an annular groove in the piston. The rings have a relaxed diameter greater than the cylinder bore internal diameter. When in place the rings are resiliently biased so that they are urged against the wall of the cylinder bore to form a seal. The resilience is provided by the natural resilience of the rings, and/or by the use of spring means or compressed gas.

In a four-stroke engine, the rings are free to rotate around the piston; however in a two-stroke engine the rings are generally pinned to prevent rotation. This is done to prevent the ends of the ring on either side of the gap from becoming caught in the inlet or outlet ports.

Two problems reduce the effectiveness of the seal between the piston ring and the cylinder bore. The first problem is that the rings are prone to change their shape at the operating temperature of the engine. For example, a piston ring which is circular at ambient temperatures may become oval in shape at high temperatures. This effect arises because of the presence of the free ends on either side of

the gap, which means that the ring is not uniform around its circumference.

This problem is described in British Patent number 2 155 828, and a solution is proposed which involves making piston rings which are non-circular at room temperature and which change their profiles to make a good seal at the operating temperature of the engine.

This solution overcomes the problems of piston ring distortion with temperature, but it does not address a second problem which is that the internal walls of the cylinder bores deviate from perfect circularity.

This deviation is caused in part by the mechanical load imposed by the cylinder head bolts surrounding the cylinder. In addition, the arrangement of cooling jackets around the cylinder may result in an uneven temperature distribution when the engine is running, causing thermal distortions of the cylinder bore. The pressure in the cylinder when the engine is running is a third factor contributing to the deviation of the cylinder bore from perfect circularity.

The factors influencing cylinder bore distortion have been analysed: see for example V V Dunaevsky, Tribology Transactions Vol 33 1, 33 (1990) , and D C Sun, J Appl Mech, 58, 141 (1991) ,

The deviation from circularity means that an increased tangential load needs to be applied to the piston ring to achieve an adequate seal. The increased load causes increased friction between the piston ring and the wall of the bore, thus reducing engine efficiency.

A further problem caused by the deviation of the bore from circularity is that the seal between the piston ring and the bore is less effective. This means that less pressure is generated during the combustion and compression strokes, and oil from the crank case may enter the cylinder, causing undesirable exhaust emissions.

The loss of pressure from the imperfect seal is particularly a problem for diesel engines, which have a high compression ratio, but the efficiency of petrol engines is also reduced.

Generally, mismatches between a cylinder bore and piston rings are greatest when an engine is new. A process of "bedding in" takes place as an engine is operated, during which the seal improves. This may be caused, for example, by changes in the shape of the piston, piston rings, or cylinder bore as a result of operation at the temperature of the engine, or by the abrading of small burrs or other artefacts from the manufacturing process. While the bedding in process is occurring oil consumption, and hence particulate emission, is relatively high.

However, even after bedding in is complete, the non-circular cylinder bore means that the seal is not complete.

Attempts have been made to improve the seal by trying to make the cylinder bores as perfectly circular in profile as possible. However to date it has not proved possible to achieve this with conventional designs. Designs which have given sufficiently circular bores are either very expensive or not feasible for large scale production.

According to the present invention there is provided a method of making a seal between a cylindrical piston and the

wall of an engine cylinder bore of non-circular internal profile, the method comprising the steps of:

1 determining the internal profile of the bore;

2 forming a piston ring that has an external edge with a profile that is matched to the internal profile of the bore so that the piston ring, when fitted to the piston, is a complementary fit for the bore;

3 fixing the piston ring around the piston so that rotation of the ring relative to the piston is prevented; and

4 locating the piston within the bore so that the complementary surfaces of the piston ring and of the bore are mated.

By matching the profile of the piston ring to the cylinder bore it is possible to provide an improved seal. A further benefit of this matching is that the "bedding in" time is reduced, thereby minimising oil consumption and particulate emission.

The invention may be used in any suitable engine; for example a two-stroke or four-stroke petrol or diesel engine.

The profile of the cylinder bore may be determined by measurement with appropriate instruments. Alternatively the profile may be determined by one or more calculations which allow for the effects of the cylinder head bolts, temperature, internal pressure within the cylinder, and any other relevant factors. The profile may be calculated by

means of finite element analysis techniques, and preferably by the use of these techniques in a computer program.

Preferably the internal profile of the bore is determined for the bore at the operating temperature of the engine.

Bores may each deviate from perfect circularity in different ways in a given engine. Each piston ring may be individually matched to the internal profile of the bore in which it is to be received.

A single piston ring may be used on a piston, but it is preferred that a plurality of rings are fitted, spaced axially apart on the piston. It is possible to use one piston ring which has an outer edge profile which is matched to the cylinder bore, and other rings which are conventional. Preferably all the rings are matched to the bore profile. For convenience hereafter, a single piston ring will be referred to.

The piston ring may be located in any suitable position around the piston. Preferably the piston is provided with an annular groove, and the piston ring is received in this groove.

Any suitable fixing means may be used for fixing the piston ring around the piston. Preferably the piston ring is fixed around the piston by means of one or more pins. Where the piston has an annular groove for receiving the piston ring, the pin or pins may be located in or adjacent to the groove.

Because the piston ring is fixed around the piston and unable to rotate, the preferred orientation of the piston ring to the cylinder bore wall is maintained.

It is preferred that an even pressure is exerted all around the piston ring to urge it against the cylinder bore to provide a uniform seal. This may be achieved by constructing the piston ring of a resilient material, preferably metal. Alternatively, or additionally, the piston ring may be provided with one or more independent springs and/or gas pressure to urge it against the cylinder bore.

According to a further aspect of the invention there is provided an internal combustion engine in which the cylinder bores have a non-circular internal profile, which engine has pistons operating in the cylinders, the pistons having piston rings which have an external edge with a profile which is matched to the internal profiles of the cylinder bores so that the piston rings, when fitted around the pistons, are a complementary fit for the bores.

The invention also extends to a piston ring which has an external edge with a profile that is matched to the non- circular internal profile of an engine bore in which the piston ring, after having been fitted around a piston, will operate so that the piston ring, when fitted around the piston, is a complementary fit for the bore.

The invention will now be further described, by way of example, with reference to the following drawings in which:

Figure 1 is a section through a engine block with a non-circular bore;

Figure 2 is a side view of a piston for use in the bore of Figure 1;

Figure 3 is a section through a cylinder bore in which is located the piston of Figure 2 fitted with two piston rings;

Figure 4 shows a non-circular piston ring in accordance with one aspect of the invention; and

Figure 5 is a section in the plane A-A through the cylinder bore of Figure 3.

Figure 1 shows part of an engine block 2 , which has a cylinder bore 8 for receiving a piston. Cylinder head bolts 4 cause the profile of the bore 8 to deviate from perfect circularity at indentations 6 around the bore. The deviation from circularity is very small, and not readily discernable to the naked eye. For the purpose of illustration of the invention the deviation is shown in this, and in the other diagrams, in a greatly exaggerated form. If a cylinder of perfectly circular profile, or a cylinder which is free to rotate, is inserted into the bore 8 it will not make a good seal because the innermost points 6 of the bore wall will define a rotational circumference for the piston which is smaller than that required to make a close seal with the remaining areas of the wall. If the cylinder is provided with one or more conventional piston rings, the rings will not have sufficient conformability to form a good seal. Conformability is the ability of a piston ring locally to deform to provide a seal. The greater the deviation of the bore from circularity, and the greater the frequency of deviation, the greater the conformity that is required of a piston ring to provide a seal.

Figure 2 shows a piston 14 for use in a cylinder bore. The piston 2 has annular grooves 16a, 16b in which are to be located piston rings.

Figure 3 is a section through a part of an engine block 2. The piston 14 is fitted with two piston rings 10a, 10b. The rings 10a, 10b are fixed around the piston 14 by means of pins 18. This pinning prevents rotation of the rings 10a, 10b around the piston 14. The rings 10a, 10b are urged outwardly from the piston 14 by springs 16 to make a seal with the wall of the cylinder bore 8.

Figure 4 shows a piston ring 10a which has been made with a non-circular outside edge. The ring 10a has been provided with indentations 12 which are complementary to the indentations of the bore wall of Figure 1. The ring 10a has a gap 20 for opening up the ring so that it may be fitted around a piston.

Figure 5 is a section in the plane A-A of Figure 3. The piston 14 has fitted to it the non-circular ring 10a of Figure 4. Each indentation 6 of the bore 8 is mated with a corresponding indentation 12 of the outer edge of the ring 10a, thereby forming a close seal. The pin 18 prevents rotation of the ring 10a relative to the piston 14, thereby ensuring that the close seal is maintained.

Although the invention has been described, for convenience, in terms of the use of one or two non-circular piston rings, it will be understood that the invention is not limited to this embodiment. A plurality of axially spaced non-circular piston rings may matched to the internal bore profile and used to provide a better seal. Alternatively, one or more non-circular piston rings may be used with one or more conventional circular piston rings.