GB2384541A

GB2384541A - Piston with gap between crown and body portion to allow for heat expansion

Info

Publication number: GB2384541A
Application number: GB0201820A
Authority: GB
Inventors: William John Frederick Morgan; Kenneth John Street
Original assignee: Federal Mogul Bradford Ltd
Current assignee: Federal Mogul Bradford Ltd
Priority date: 2002-01-25
Filing date: 2002-01-25
Publication date: 2003-07-30
Also published as: GB0201820D0

Abstract

A piston (10) comprises a crown portion (12), and a body portion (32) and bolts (50) clamping the portions together. The crown portion and the body portion each have an abutment surface (20, 34) which abuts the abutment surface of the other of said piston portions. The bolts (50) act to apply clamping force which urges said abutment surfaces together. One or both of said abutment surfaces is so shaped that, with no clamping force, the abutment surfaces (20; 34) contact one another at radially inward areas (58) thereof and a gap (60) is formed between other areas of said abutment surfaces with the width of the gap, over at least a part of the radial plane of the piston, increasing in radially outward directions relative to the axis (14) of the piston. One or both of said abutment surfaces are further shaped so that the width of said gap, over at least a part of the radial plane of the piston, varies in the circumferential direction relative to the axis of the piston.

Description

PISTON This invention is concerned with a piston for an internal combustion engine, the piston being of the so-called"composite"type. Such a piston is formed from an assembly of piston portions. The piston portions comprise a crown portion which borders a combustion chamber of the engine and therefore has to be made of a material which can withstand the high temperatures and forces generated and also the effects of corrosive gases. The piston portions also comprise a body portion which acts to guide the piston in its movement and to provide a connection to a connecting rod. The body portion can be made of a different material from the crown portion having properties more appropriate to its function. Both the crown portion and the body portion are generally circular or slightly oval in outline when viewed in a plane normal to an axis of the piston along which it reciprocates in the operation of the engine. The possibility that these portions are not truly circular is ignored herein when using geometric terms such as "radial" and "circumferential". Both the crown portion and the body portion extend around the axis of the piston which is generally central thereof.

In the composite type of piston, the crown portion and the body portion each have at least one abutment surface which, in the assembled condition of the piston, abuts an abutment surface of the other of said piston portions, the portions being axially-aligned and adjacent to one another. The piston also comprises at least two bolts extending generally axially of the piston and acting to apply clamping force which urges said abutment surfaces together. The clamping force has to ensure that the two piston portions remain attached to one another and do not move relative to one another in a way which would cause wear. However, in service, the crown portion becomes hotter than the body portion and expands to a greater extent. This causes the crown portion to bulge into a slightly convex form which, if the abutment surfaces were planar and normal to the axis of the piston, would cause stress at the radially outer edges of

the abutment surfaces, with a consequent risk of cracking of the piston portions or of the bolts, and would also cause a gap to form at the radially Inner edges of the abutment surface, with a consequent risk of damage through fretting This problem is made more acute in some cases where the coefficient of thermal expansion of the material of the crown portion is greater than that of the body portion. In order to address this problem, German patent specification number 12 69 414 proposes that a gap should be provided between the abutment surfaces which gap has a width which increases in the radially outward direction, thereby relieving stress at the outer edges of the abutment surfaces. Such a gap can be provided by providing that one of said abutment surfaces is shaped so that it is frusto-conical. This means that said gap increases in a uniform manner.

British patent specification number 2020782 discloses an improvement to address the problem that the angle of the cone can only be optimised for one specific working condition. The improvement proposed is that the abutment surface is shaped so that it is convexly curved in the radial direction with the gap therefore increasing in the radially outward direction in a non-uniform manner.

It is an object of the present invention to improve upon the solutions proposed in the aforementioned British and German specifications. In particular, it is an object to provide that the loading on the bolts can be distributed as desired over the area of the abutment surfaces, for example being uniformly distributed instead of being concentrated in the vicinities of the bolts.

The invention provides a piston formed from an assembly of piston portions, said piston portions comprising a crown portion extending around an axis of the piston, and a body portion also extending around the axis of the piston, the crown portion and the body portion each having at least one abutment surface which, in the assembled condition of the piston, abuts an abutment surface of the other of said piston portions, the piston also comprising at least two bolts extending generally axially of the piston and acting to apply clamping force which urges said abutment surfaces together, one or both of said abutment surfaces being so shaped that, with no clamping force, the abutment surfaces contact one another at radially inward areas thereof and a gap is formed between other areas of said

abutment surfaces with the width of the gap, over at least part of the radial plane of the piston, increasing in radially outward directions relative to the axis of the piston, wherein one or both of said abutment surfaces are further shaped so that, over at least part of the radial plane of the piston, the width of said gap varies in the circumferential direction relative to the axis of the piston.

In a piston according to the invention, the advantages given by a gap which increases in the radially outward direction are retained, including the possibility that the increase may be non-uniform. The increase in gap width in the radially outward directions may be applied over the whole (360 degree) radial plane of the piston or may be dispensed with regions remote from the bolts so that the increase is applied only in two or more segments of said radial plane, said segments being located in the vicinities of the bolts. Furthermore, the variation in the gap width in the circumferential direction allows distribution of the clamping force circumferentially. Preferably, the shape is applied to only one of the abutment surfaces with the other being planar and normal to the axis of the piston.

In a piston according to the invention, the width of said gap between said abutment surfaces, in the circumferential direction, is at its maximum in axial planes of the piston which include one of the bolts. For example, where there are two bolts or groups of bolts, said gap may reach its maximum width at 180 degree intervals about the axis of the piston, these positions corresponding to the positions of the bolts or the centres of the groups of bolts.

The variation of the width of said gap in the circumferential direction will be different in different piston designs. In some cases, the variation is up to 100 microns. The increase of the width of said gap in radially outward directions will be also different in different piston designs, for example the increase may be up to 40 microns.

In a piston according to the invention, the or each bolt may extend through a passage which has an opening in each of said abutment surfaces. The

abutment surfaces may be generally annular about the axis of the piston.

A piston according to the invention will usually comprise a plurality of bolts distributed substantially uniformly about the axis of the piston or arranged in groups distributed substantially uniformly about the axis of the piston. In this case, said abutment surfaces are preferably so shaped that, with no clamping force, said abutment surfaces contact one another at areas thereof which are substantially half way, in the circumferential direction, between adjacent bolts or the centre lines of adjacent groups of bolts. This arrangement assists in causing a uniform distribution of force across the abutment surfaces.

In a piston according to the invention, the piston portions may each have a further abutment surface spaced radially from the aforementioned abutment surface thereof, said further abutment surfaces, in the assembled condition of the piston with clamping force applied, abutting one another, said further abutment surfaces being so shaped that, with no clamping force, they contact one another or make their closest approach to one another at predetermined areas thereof and a gap is formed between other areas of said further abutment surfaces, one or both of said further abutment surfaces being shaped so that, over at least part of the radial plane of the piston, the width of said gap varies in the circumferential direction relative to the axis of the piston. If desired, the shape may be such that, over at least part of the radial plane of the piston, the width of the gap between said further abutment surfaces increases in radially outward directions relative to the axis of the piston. Said further abutment surfaces may be spaced axially relative to the aforementioned abutment surfaces.

There now follows a detailed description, to be read with reference to the accompanying drawings, of a piston which is illustrative of the invention.

In the drawings: Figure 1 is a vertical cross-sectional view taken through one half of the illustrative piston; Figure 2 is a plan view of the illustrative piston;

Figure 3 is a diagrammatic representation of the width of a gap between abutment surfaces of the illustrative piston taken in the circumferential direction; and Figure 4 is a view similar to Figure 3 but representing the width of a gap between further abutment surfaces of the illustrative piston.

The illustrative piston 10 is of the composite type being formed from an assembly of piston portions. Said piston portions comprise a crown portion 12 which extends around a longitudinal axis 14 of the piston. The crown portion 12 is generally cylindrical in form having an outer peripheral surface 13 which defines three ring grooves 16 to receive piston rings (not shown). The crown portion 12 has an upper surface 17 which, in the operation of the piston, borders the combustion chamber of the engine. The surface 17 defines a combustion bowl 18. The crown portion 12 has a lower surface formed by an annular abutment surface 20 which encircles an opening to a central recess 22 in the crown portion and is itself encircled by an opening to an annular outer recess 24 in the crown portion. The lower surface of the crown portion 12 is also formed by a further annular abutment surface 30 which encircles the opening to the recess 24 and extends to the outer periphery of the crown portion 12. The surface 30 is axially spaced downwardly (viewing Figure 1) from the surface 20.

The piston portions of the piston 10 also comprise a body portion 32 which also extends around the axis 14 of the piston 10. The body portion 32 has an upper surface which contacts the lower surface of the crown portion 12. This upper surface is formed by an abutment surface 34 which engages the abutment surface 20 of the crown portion 12 and a further abutment surface 36 which engages the further abutment surface 30 of the crown portion 12. The abutment surface 34 is generally annular and surrounds an opening to a central recess 37 defined by the body portion 32. The recess 37 co-operates with the recess 22 of the crown portion 12 in forming a cooling chamber of the piston 10. The abutment surface 34 is itself encircled by the opening to a recess 38. The opening to the recess 38 is encircled by an annular portion 39 of the body portion 32 which has a step therein. The further abutment surface 36 is formed by a

lower surface of this step. The step is formed so that it can receive the portion of the crown portion 12, on which the further abutment surface 30 is formed.

Thus, the crown portion 12 and the body portion 32 each have an abutment surface (20, respectively 34) which, in the assembled condition of the piston 10, abuts the abutment surface of the other of said piston portions. The crown portion 12 and the body portion 32 also each have a further abutment surface (30, respectively 36) which, in the assembled condition of the piston 10 with clamping force applied, abuts the further abutment surface of the other of said piston portions. When there is no clamping force applied, the surfaces 30 and 36 may not contact one another, being prevented from doing so by contact between the surfaces 20 and 34, but the application of clamping force brings them into contact.

The piston 10 also comprises four bolts 50 each extending generally axially of the piston. The bolts 50 act to apply clamping force which urges said abutment surfaces 20 and 34 together and also urges said further abutment surfaces 30 and 36 together. Each bolt 50 extends through one of four passages 52 defined partially by the crown portion 12 and partially by the body portion 32. Each passage 52 has aligned openings in the abutment surfaces 20 and 34. Specifically, each bolt 50 has a threaded end portion which is received in a thread cut into the crown portion 12 and a head which bears on a washer 54 which in turn bears on a lower surface of the body portion 32. As can be seen in Figure 2, the four bolts 50 are arranged in pairs, ie in two groups of two, at diametrically-opposed positions relative to the axis 14.

The abutment surface 20 is machined to be planar and transverse to the axis 14 but the abutment surface 34 of the body portion 32 is machined into a complex shape which will be defined on the assumption that no clamping force is applied to distort the shape, ie the bolts 50 are not tightened to an extent which applies an appreciable force across the surfaces 20 and 34. Specifically, the surface 34 is so shaped that the abutment surfaces 20 and 34 contact one another at radially inward areas 58 thereof and a gap 60 is formed between other areas of

said abutment surfaces. The areas 58 are located half way, measuring circumferentially relative to the axis 14, between the two groups of bolts 50 (see Figure 2). Over some parts of the radial plane of the piston, specifically segments which are spaced centrally between the pairs of bolts 50, the width of the gap 60 does not increase in radially outward directions relative to the axis 14 (see Figure 3). Elsewhere, the amount of the increase in the gap 60 in radially outward directions varies across the segments of the radial plane of the piston where the increase occurs. In the vicinity of the bolts 50, the gap 60 is at its maximum and does increase in radially outward directions. Furthermore, the surface 34 is so shaped that the width of said gap 60 varies in the circumferental direction relative to the axis of the piston. In this case, the circumferential variation in width of the gap 60 occurs over the entire radial plane of the piston 10 but, in some cases, the variation may be only over part of said radial plane.

The further abutment surface 30 is machined to be planar and transverse to the axis 14 but the further abutment surface 36 of the body portion 32 is shaped.

This shape is also described on the assumption that there is no significant clamping force. Specifically, the surface 36 is so shaped that the further abutment surfaces 30 and 36 make their closest approach to one another or contact one another at areas 68 thereof which are located approximately half way between the pairs of bolts 50. Note that, in this case, the areas 68 extend radially across the surfaces 30 and 36 rather than being at the inward radial edges of the surfaces. A gap 70 is formed between other areas of said further abutment surfaces. The width of the gap 70 is constant in radially outward directions relative to the axis 14 (see Figure 4) but, in modifications, this width may increase in radially outward directions in similar fashion to the gap 60.

However, the surface 36 is so shaped that the width of said gap 70 varies in the circumferental direction relative to the axis 14 of the piston (see Figure 4). In this case, the variation is only over two diametrically-opposed segments of approximately 90 degrees of the radial plane of the piston, with the width remaining constant over the remaining segments.

Figure 3 shows, in graphical form and on a greatly increased scale, the width of

the gap 60 along the line A to A (plot 60a) and along the line B to B (plot 60b) in Figure 2. The line A to A extends along the inner peripheral edge of the surface 34 and the line B to B extends along the outer peripheral edge of the surface 34.

In Figure 3, the Y axis represents the width of the gap 60 which is shown below a line 62 representing the plane of the surface 20. The X axis represents the angle subtended at the axis 14, the plots 60a and 60b both extending over 90 degrees or one quadrant of the radial plane of the piston. The left (viewing Figure 3) ends of the plots 60a and 60b are positioned on a line bisecting one of the pairs of bolts 50. The radial position of one of the bolts 50 and of one of the areas 58 is indicated.

It can be seen from the plot 60a in Figure 3 that, around the inner peripheral edge of the surface 34, the width of the gap 60 is greatest in the vicinity of the bolts 50 and reduces to zero at the area 58 at the right-hand end of the plot. It can be seen from the plot 60b that the width of the gap 60 around the outer peripheral edge of the surface 34 follows a similar pattern to the width of the inner peripheral edge shown by the plot 60a. However, a comparison of the plots 60a and 60b shows that, along the outer peripheral edge of the surface 34, the width of the gap 60 is greater than its width along the inner peripheral edge of the surface 34 except close to the axial plane of the areas 58 where the width is the same.

Figure 3 shows the widths of the gap 60 over a first quadrant of the radial plane of the piston. In the second quadrant, the widths are a mirror image of those in the first quadrant and, in the third and fourth quadrants, the width are, respectively, the same as in the first and second quadrants.

Figure 4 is similar to Figure 3 but illustrates (plot 70c) the width of the gap 70 between the surfaces 30 and 36. In Figure 4, the Y axis represents the width of the gap 70 which is shown as a distance from a line 72 representing the plane of the surface 30. The X axis represents the angle subtended at the axis 14, the plot 70c extends over 90 degrees or one quadrant of the radial plane of the piston. The left (viewing Figure 4) end of the plot 70c is positioned on a line

bisecting one of the pairs of bolts 50. The plot 70c is taken along the line C to C which extends along the inner peripheral edge of the surface 36 but, as the gap 70 is constant in width in radial directions, the plot 70c also represents the width of the gap 70 at the outer peripheral edge of the surface 36 or at any point in between.

It can be seen from plot 70c that the width of the gap 70 varies circumferentially over the quadrant represented. Specifically, between the left edge of Figure 4 and the centre thereof (which represents a 45 degree position from the centre line of a pair of the bolts 50), the width of the gap is constant and then decreases gradually to its minimum which occurs at the right edge of Figure 3 which represents a position half way between the pair of bolts 50. At the minimum width of the gap 70 there is still a gap (the distance of closest approach) between the surfaces 30 and 36; this is because the surfaces 20 and 34 contact one another before the surfaces 30 and 36.

Figure 4 shows the widths of the gap 70 over a first quadrant of the radial plane of the piston. In the second quadrant, the widths are a mirror image of those in the first quadrant and, in the third and fourth quadrants, the width are, respectively, the same as in the first and second quadrants.

It should be understood that the variation in the widths of the gap 60 and 70 described herein and illustrated by Figures 3 and 4 represent examples only and many other possibilities exist within the scope of the invention.

The effect of the shaping of the abutment surfaces 34 and 36 is that, when the piston portions 12 and 32 are assembled and before clamping forces are applied, as aforementioned, contact occurs only in the area 58. In the areas 68 the surfaces 30 and 36 make their closest approach but do not contact one another. As the bolts 50 are tightened, thereby applying increasing clamping force, the areas of contact 58 expand towards the bolts 50 and the areas 68 come into contact and expand towards the bolts 50. This arrangement ensures a more uniform force distribution across the surfaces 20 and 34, and 30 and 36.

The increase in the gap 60 in the radial directions retains the advantage mentioned above in relation to German patent specification number 12 69 414 and British patent specification number 2020782.

Claims

CLAIMS 1 A piston formed from an assembly of piston portions, said piston portions comprising a crown portion extending around an axis of the piston, and a body portion also extending around the axis of the piston, the crown portion and the body portion each having at least one abutment surface which, in the assembled condition of the piston, abuts an abutment surface of the other of said piston portions, the piston also comprising at least two bolts extending generally axially of the piston and acting to apply clamping force which urges said abutment surfaces together, one or both of said abutment surfaces being so shaped that, with no clamping force, the abutment surfaces contact one another at radially inward areas thereof and a gap is formed between other areas of said abutment surfaces with the width of the gap, over at least part of the radial plane of the piston, increasing in radially outward directions relative to the axis of the piston, wherein one or both of said abutment surfaces are further shaped so that, over at least part of the radial plane of the piston, the width of said gap varies in the circumferential direction relative to the axis of the piston
2 A piston according to claim 1, wherein the width of said gap between said abutment surfaces, in the circumferential direction, is at its maximum in axial planes of the piston which include the bolts.
3 A piston according to either one of claims 1 or 2, wherein the width of said gap varies in the circumferential direction by up to 100 microns.
4 A piston according to any one of claims 1 to 3, wherein, over said part of the radial plane of the piston, the width of said gap increases in radially outward directions by up to 40 microns.

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5 A piston according to any one of claims 1 to 4, wherein the or each bolt extends through a passage which has an opening in each of said abutment surfaces.
6 A piston according to any one of claims 1 to 5, wherein said abutment surfaces are generally annular about the axis of the piston.
7 A piston according to any one of claims 1 to 6, wherein the piston comprises a plurality of bolts distributed substantially uniformly about the axis of the piston or arranged in groups distributed substantially uniformly about the axis of the piston and said abutment surfaces are so shaped that, with no clamping force, said abutment surfaces contact one another at areas thereof which are substantially half way, in the circumferential direction, between adjacent bolts or the centre lines of adjacent groups of bolts.
8 A piston according to any one of claims 1 to 7, wherein said piston portions each have a further abutment surface spaced radially from the aforementioned abutment surface thereof, said further abutment surfaces, in the assembled condition of the piston with clamping force applied, abutting one another, said further abutment surfaces being so shaped that, with no clamping force, they make their closest approach or contact one another at predetermined areas thereof and a gap is formed between other areas of said further abutment surfaces, wherein one or both of said further abutment surfaces are shaped so that, over at least a part of the radial plane of the piston, the width of said gap varies in the circumferential direction relative to the axis of the piston.
9 A piston according to claim 8, wherein the shape is such that, over at least part of the radial plane of the piston, the width of the gap between said further abutment surfaces increases in radially outward directions relative to the axis of the piston

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10 A piston according to either one of claims 8 or 9, wherein said further abutment surfaces are spaced axially relative to the aforementioned abutment surfaces.
11 A piston substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.