GB2117868A - Improved piston ring seal - Google Patents

Abstract

A piston for an internal combustion engine comprises an annular ring groove which receives a compression ring 3. A seal assembly 4, 5, 6, 7, is arranged around and between the ring groove and the compression ring for reducing or preventing fluid flow around the radially inner surface of the compression ring. The seal assembly may be formed by inter-engaging projections and channels. Several different embodiments are shown. <IMAGE>

Description

SPECIFICATION Pistons This invention relates to pistons for petrol and diesel internal combustion engines.

A piston for an internal combustion engine is commonly provided with three or more piston rings which are received in corresponding piston ring grooves which extend around the piston. Each piston ring is intended to seal both against the associated cylinder or liner wall and against the side walls of the groove. These seals prevent the passage of gas from the combustion chamber above the piston to the lubricating oil beneath (commonly referred to as "blow-by") and the passage of oil to the combustion chamber.

Engine tests have shown that axial movement of a piston ring in the associated groove on reciprocation of the piston does not conform to a constant pattern and the ring, for a significant time, has its side faces not sealed against the sides of the piston ring groove but, in effect, flutters between the sides of the groove. When the ring is not sealed against the sides of the groove, gas under pressure flows round the back of the ring between the radially innermost edge of the ring and the base of the groove. This reduces the force which is applied by the gas on each side surface of the ring and which urges the opposite side surface of the ring into engagement with the adjacent groove surface to form a seal between the two surfaces.Since, to move the ring into sealing engagement with the groove side surface, this force has to overcome inertia forces and the friction forces created by engagement of the ring with the cylinder wall, this reduction in the force applied by the gas pressure will reduce the likelihood of a seal being made and so will increase leakage.

In addition, it has been found that compression rings invariably seal only on the radially outer edges of the groove sides with the downward force acting to form a seal between the ring and the groove only over a small area around this radially outer groove edge. This is due to the fact that the ring tilts so that its sides no longer lie in planes normal to the axis of the piston. This reduces the axial force caused by the gas pressure and thus reduces the seal against this small area so giving rise to fluttering.

According to the invention, there is provided a piston for an internal combustion engine comprising an annular ring groove, a compression ring received in the groove for sealing engagement with an associated cylinder or liner and a seal assembly within the ring groove and between the ring groove and the compression ring for reducing or preventing fluid flow around a radially inner end of the compression ring.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a cross-section of part of a piston showing an auxiliary ring sealing between a ring groove and a compression ring, Figure 2 shows a two part auxiliary ring, Figure 3 is a side elevation of a part of the piston of Figure 1 showing the compression ring modified by the addition of spacer lugs, Figure 4 is a similar view to Figure 1 but showing the compression ring formed with a rebate, Figure 5 is a similar view to Figure 1 but showing an auxiliary ring-receiving groove formed in an insert held in the piston, Figure 6 shows a similar view of a piston to that of Figure 1 but with a fixed projection extending.

from a base of the ring groove into a recess in a radially inner end surface of the compression ring, and Figure 7 is a similar view to Figure 6 but showing the fixed projection formed on an insert held in the piston.

Referring first to Figure 1, the piston 1 has a ring groove 2 receiving a compression ring 3, preferably an iron compression ring. A radially inner end surface of the compression ring 3 is formed with a recess, in the form of a channel 4, and a base of the ring groove 2 is also provided with a recess in the form of a channel 5. The channels 4 and 5 are aligned and receive spaced portions of an annular auxiliary ring, in the form of a steel rail 6. The auxiliary ring 6 may be formed in two or more segments (see Fig. 2) to facilitate its assembly into the piston. The rail 6 may be a tight fit in the channels 4, 5 or may be free to move into and out of the channels 4, 5.

The rail 6 thus forms a seal against the passage of both gas and oil around the radially inner end of the compression ring 3. The clearances between the sides of the groove 2 and the sides of the ring 3, and between the auxiliary ring 6 and the channels 4, 5 are sufficiently small to tend to increase the pressure differential across the compression ring and produce an axial force tending to move the sides of the compression ring 3 into full engagement with one or other of the side walls of the groove 2 so providing more effective ring-to-piston sealing.

Thus, in a sense, there is a form of labyrinth seal within the ring groove 2 at the back of the compression ring 3.

It is believed that the separate auxiliary ring 6 will have the advantage of providing a more effective seal both against a gas flow around the back of the compression ring 3 as well as against the escape of oil which, if allowed to escape, forms burnt deposits above the ring. In addition, the pressure differential across the ring will cause the compression ring 3 to react quickly and move against the appropriate piston groove wall over substantially the whole upper or lower surface area of the compression ring. The auxiliary ring 6 will seal well at its radially inner and outer edges and could thus achieve efficient sealing even if the compression ring 3 was not sealing along its lower side.

The auxiliary ring 6 can operate as the sole piston-to-ring seal, leaving the compression ring 3 to provide only piston-to-cylinder wall sealing.

This may be effected by the provision of means on the sides of the compression ring preventing it sealing against the ring groove walls. These means may take the form of lugs 1 3 (see Figure 3) provided on the sides of the compression ring 3.

As shown in Figure 1 above, the compression ring 3 is provided with a channel 4. As seen in Figure 4, this may be replaced by a rebate 14 extending around the radially inner end of the compression ring 3. As also shown in Figure 1, the channel 5 of the ring groove 2 is formed direct in the body of the piston 10. As seen in Figure 5, however, this channel 5 may be formed in an insert 10 held in the piston body 10 and forming also the base of the ring groove 2. The insert 10 may be encast during casting of the piston 10. The remaining parts shown in Figures 4 and 5 are as described above with reference to Figure 1 and bear the same reference numerals.

Referring next to Figure 6, parts common to Figures 1 to 5 and to Figure 6 will be given the same reference numerals and will not be described in detail. In the piston of Figure 6 the auxiliary ring 6 is replaced by an annular flange 7 projecting radially outwardly from the base of the ring groove 8. The flange 7 extends into a recess, in the form of a channel 9 in the radially inner end surface of the compression ring 3, which is substantially the same as that shown in Figure 1.

Of course, the projection 7 could be formed on the ring 8 and the channel 9 in the piston groove or insert. As with the embodiment of Figures 1 to 5, the flange 7 and the channel 9 could be a tight fit or a loose fit.

The fixed flange 7 does not form a complete seal, to allow the maximum build-up of a differential pressure across the compression ring to produce a sealing action against substantially the whole of the relevant ring surface by axial displacement. However, the flange 7 and channel 9 will provide what is effectively a labyrinth seal so allowing somre pressure build-up to produce more rapid sealing of the compression ring against the groove walls.

The channels 4, 5 and 9 may not be straightsided recesses and may be curved, convergent or divergent, or conical, in order to improve the effectiveness of the labyrinth seal and to reduce compression ring flutter and increase the axial force to quickly seal over the full extent of the compression ring.

The embodiments described above with reference to the drawings can improve the performance of a three or more ring piston.

Reduction in blow-by, by more efficient sealing, makes it possible to extend intervals between oil changes and to reduce oil consumption by avoiding the carbon build-up on the piston above the uppermost oil control ring.

For years, three, four or five ring pistons have been used in engines to ensure low blow-by and fuel consumption. The benefits of the embodiments described above with reference to the drawings are through to be substantial enough to make a two-ring system achievable, that is one compression ring and one oil control ring, as opposed to two or more compression rings and one oil control ring. This reduces the space occupied by the second compression ring on the ring belt, and thus the piston can be reduced in height and therefore weight, or alternatively, the skirt of the piston may be made longer for improved guidance.

Moreover, with more efficient sealing there is a possibility of obtaining a decrease in friction since, in an effort to get more efficient sealing, increased outward pressure of the ring against the cylinder walls is applied thus increasing wear and friction to achieve this better sealing, whereas a light, outward pressure, while still providing an effective and quick-acting seal, is preferable.

The embodiments described above with reference to the drawings have a further advantage, particularly in pistons for large diesel engines. Because of the pressures produced in such engines on the firing stroke, the compression ring is repeatedly tilted downwardly and forced against the lower sides of the associated groove and this can cause significant wear, both on the compression ring and the groove. In addition, the pressure can cause downward bowing or bending of the ring in a radial direction. The provision in the embodiments of Figures 1, 5, 6 and 7 of a projection engaging with the compression ring reduces these problems by making the compression ring less liable to tilting and by providing a resistance to the bowing or bending of the ring.

Claims (14)

1. A piston for an internal combustion engine comprising an annular ring groove, a compression ring received in the groove for sealing engagement with an associated cylinder or liner and a seal assembly within the ring groove and between the ring groove and the compression ring for reducing or preventing fluid flow around a radially inner end of the compression ring.

2. A piston according to claim 1, wherein the ring groove and the compression ring are provided with respective aligned recesses extending therearound, the seal assembly comprising an annular auxiliary ring having two spaced annular portions received in the ring groove recess and the compression recess respectively.

3. A piston according to claim 2, wherein the auxiliary ring is formed in two or more arcuate segments to facilitate assembly.

4. A piston according to claim 2 or claim 3, wherein the ring groove has spaced radially extending side walls and an annular base, the recess being formed by a channel in the base and wherein the compression ring has spaced radially extending sides and a radially inner end surface between said sides, the recess being formed in said radially inner end surfaces.

5. A piston according to claim 4, wherein the base of the ring groove is formed by an insert held in the piston, the channel being formed in said insert.

6. A piston according to any one of claims 2 to 5, wherein the compression ring recess is provided by a channel or a rebate in the compression ring.

7. A piston according to any one of claims 2 to 6, wherein the annular portions of the auxiliary ring and the recesses are so relatively dimensioned that the auxiliary ring portions are free to move into and out of the associated recesses.

8. A piston according to any one of claims 2 to 6, wherein the annular portions of the auxiliary ring and the recesses are so relatively dimensioned that at least one auxiliary ring portion is a tight fit in the associated recess.

9. A piston according to claim 1 , wherein the seal assembly comprises a projecting member extending into a co-operating channel member, one member being provided in the ring groove and the other member in the compression ring.

10. A piston according to claim 9, wherein the projecting member is provided in the ring groove and the channel member on the compression ring.

1 A piston according to claim 10, wherein the projecting member comprises an annular flange extending around a base of the ring groove lying in a plane normal to the piston axis and wherein the channel extends around a radially inner end surface of the compression ring and is generally co-planar with the flange.

12. A piston according to claim 10 or claim 11, wherein the base of the ring groove is formed by an insert held in the piston and wherein the insert provides the projecting member.

13. A piston according to any one of claims 1 to 12, wherein the compression ring has radially extending sides and wherein at least one of the sides is provided with means for preventing that side sealing therearound against the ring groove.

14. A piston substantially as hereinbefore described with reference to Figure 1 or to Figure 1 as modified by one or more of Figures 2 to 5 or to Figure 6 or to Figure 7 of the accompanying drawings.