GB1593245A

GB1593245A - Pistons

Info

Publication number: GB1593245A
Application number: GB1017778A
Authority: GB
Original assignee: Associated Engineering Italy SpA
Current assignee: Associated Engineering Italy SpA
Priority date: 1978-03-15
Filing date: 1978-03-15
Publication date: 1981-07-15

Description

(54) PISTONS (71) We, ASSOCIATED ENGINEER ING ITALY, S.p.A., an Italian Company of Strada Valdellatore, 10091 Alpignano, Turin, Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to pistons, and particularly but not exclusively to pistons for four-stroke highly-rated internal combustion engines e.g. engines characterised by compression ratios exceeding 8: 1 and maximum rotational speeds over 5,000 r.p.m.

According to this invention there is provided a piston having a crown, an annular portion integral with the crown and in which ring grooves are formed, a pair of parallel projections extending directly from the underside of the crown and integral with it to form gudgeon pin bosses, an annular band integral with said annular portion, a pair of inserts of a material having a lower coefficient of thermal expansion than that of the material of the remainder of the piston, the central part of each insert being anchored in the respective projection, a pair of diametrically-opposed segmental main skirt parts, each main skirt part being carried by the corresponding ends of said inserts, the main skirt parts extending circumferentially over only a segment of the periphery, each segmental main skirt part extending on both sides of the plane of the thrust axis of the piston and being separated from said annular band by a transverse slot, wherein the diameter of the annular band is substantially less, when the piston is cold, than the diameter of the skirt segments both diameters being measured on the thrust axis, so that, in use, on starting the engine the piston is guided in the cylinder solely by the segmental main skirt parts.

An embodiment of this invention will now be described by way of example only with reference to the accompanying drawings in which: Figure lisa plan view on the piston crown; Figure 2 is an elevation showing one of the two inserts to be incorporated in the piston; Figure 3 is a partial longitudinal section of the piston on line 111-111 of Figure 1; Figure 4 is a cross section on the line IV-IV of Figure 3; Figure 5 is another partial longitudinal sec tion on the line V-V of Figure 1; and Figure 6 is another cross section on the line V1-V1 of Figure 5.

With reference to the attached drawing, the piston is mainly of aluminium alloy and has a crown 10, which is integral with an annular portion 11 in which ring grooves 12 are formed (the piston rings are not shown) and which has an annular band 13 forming the upper part of the skirt.

Projections 14 extend directly from the underside of the crown 10, are integral with the crown, are generally symmetrically located in relation to the thrust axis T-T (Figure 4), and are placed well inwardly from the piston periphery.

Two steel inserts 15 are cast-in, one in each projection 14, and extend chordwise parallel with the thrust axis T-T. The inserts have their ends let into the aluminium alloy of, and anchored to, the two skirt segments 16 which form the middle and lower portions of the piston sliding surfaces.

The top portion of the sliding surfaces is formed by the annular band 13 which is integral with the annular portion 11 of the piston crown and which is separated from skirt segments 16 by transverse slots 17 (Figures 3 and 5). Thus, skirt segments 16 are connected to the remaining part of the piston only by inserts 15 which are connected both to the skirt segments 16 to projections 14 which extend from the crown.

In order to provide good anchorage of inserts 15 in both skirt segments 16 and pro jections 14, the inserts 15 are provided with an aperture 19 (Figures 2, 3 and 6) having a circular portion allowing the formation of gudgeon pin bores 18, and a pair of slots 20 (Figures 2 and 3) opening into the circular portion of aperture 19. The slots 20 have a narrow part 20a toward the circular portion of aperture 19 and a wider part 20b toward the ends 15a of insert 15.

Some parts 15g of the steel inserts 15, between projections 14 and skirt segments 16 (see Figures 3, 4 and 6) are entirely free of a covering of aluminium alloy, on both faces, whilst the slots 20 (as seen in Figure 3) are so positioned as to be partly embedded in the aluminium alloy of projections 14 and partly uncovered. The alloy of projections 14 enters and occupies slots 20 at their narrow parts 20a, leaving the outer ends of the slots free. The slots 20, whose dimensions -and shape can be different from those shown in the drawing, not only assist in reducing the weight of each insert 15 but, by means of the reduction in width from the wider parts 20b to the narrow parts 20a, from wedge-shape seats into which the alloy of projections 14 enters, securely anchoring each insert 15 in its corresponding projection 14.

A similar mechanical locking action between each insert 15 and its corresponding projection 14 is ensured by features 15b, 15c, whilst other anchoring features are provided by formations 15d and 15e and slots 15f keying together the opposite ends 15a of each insert 15 and the respective skirt segment 16.

As the connection between inserts 15 and the aluminium alloy parts concerned is achieved by casting molten aluminium alloy into a die containing the inserts 15 properly positioned, the difference between the thermal expansion coefficients of the aluminium alloy forming the piston and the steel forming the inserts gives rise to a compressive action of the aluminium against the parts of the insert covered by it, as the aluminium becomes cooler and shrinks during manufacture. The result is that the insert is subjected to a number of compressive forces exerted by the aluminium alloy. The direction of the main compressive forces exerted by the aluminium alloy on each insert is shown in Figure 2 by arrows.

Through the particular shape of each insert 15, the shape of the slots 20 and of the slots 15f, the compressive forces exerted by the aluminium alloy on each insert assist in improving adhesion between the aluminium alloy and the steel.

The inserts 15 may be of a different shape; for example the ends 15a may be corrugated in cross section (as viewed looking on the end of the piston) rather than flat; further formations like 15d, 15e may be provided to interlock with the aluminium alloy; or the shape and number of slots 1 Sfmay be varied.

The piston in accordance with the invention is dimensionally designed so that, at ambient temperature, the minimum diametral clearance between the cylinder liner and skirt segments 16 is 0.025 mm approx., whereas the clearance between the annular band 13, forming the upper part of the skirt, and the cylinder liner is as high as 0.15 mm. Thus the piston diameter when cold, measured on the thrust axis T-T, is smaller at the annular band 13 than at the skirt segments 16. The result is that when the engine is started and temperatures have not reached their normal running levels, the bearing function of the piston is performed by the skirt segments 16.

As there is a very small clearance between the diameter of the skirt segments 16 and the diameter of the cylinder liner in operation, the engine when it is still cold will be satisfactorily noiseless.

Then, when the temperature of the crown 10 attains the normal running level, the skirt annular band 13, integral with the annular portion 11, will have expanded by thermal effect, without affecting the skirt segments 16 from which it is separated by means of slots 17. Under these conditions, the annular band 13 forming the upper part of the skirt also has a bearing function, thus contributing to satisfactory and noiseless engine operation.

The skirt segments 16 do not reach temperature levels as high as those of the annular band 13; also the thermal expansion in the diametral direction is controlled by the inserts 15, which, being in steel, have a thermal expansion coefficient lower than that of aluminium. The diametral expansion of skirt segments 16, which form the middle and lower portions of the sliding surface, is therefore smaller than that of the annular band 13 which forms the upper part of the skirt.

It has been found by experiment that the piston in accordance with the present invention may be so designed that, with the engine under load, the diametral dimension of the annular band 13 is equal to the diametral dimension of the skirt segments 16, whereby the bearing function of the skirt, as a whole, is spread between the annular band 13 and the skirt segments 16.

Also with engine running at normal speeds, the reduced clearance between the annular ring band portion 11 and the annular band 13 on the one hand and the cylinder liner on the other results in a better heat flow from the crown 10 through the annular band 13 to the cylinder liner and a consequent temperature reduction of the portion 11 and generally of the piston crown 10 and of the skirt segments 16.

The groove 21 for the oil control piston ring (not shown) is formed with oil holes 22.

The current trend is to limit the presence of oil holes 22 only to the areas near the thrust faces of the skirt because their presence all round the oil groove would weaken the gudgeon pin bosses which are normally carried by the piston skirt.

In the piston in accordance with the invention, since the gudgeon pin bosses are formed by projections 14 connected directly to the crown 10, they are not affected by the number and arrangement of radial oil holes 22, and thus these can be positioned all round the groove 21 to allow the most effective distribution of oil flow.

In a modification of this piston design, the projections 14 can be made wider at their base where they are attached to the crown 10, even to the extent of being connected to the annular band 13 in the regions which subtend an angle of about 90" at the piston axis and extend for 45" on either side of the longitudinal plane of the piston containing the gudgeon pin axis.

The invention is not restricted to the embodiment described and illustrated as an example, but includes any similar or equivalent construction.

WHAT WE CLAIM IS: 1. A piston having a crown, an annular portion integral with the crown and in which ring grooves are formed, a pair of parallel projections extending directly from the underside of the crown and integral with it to form gudgeon pin bosses, an annular band integral with said annular portion, a pair of inserts of a material having a lower coefficient of thermal expansion than that of the material of the remainder of the piston, the central part of each insert being anchored in the respective projection, a pair of diametrically-opposed segmental main skirt parts, each main skirt part being carried by the corresponding ends of said inserts, the main skirt parts extending circumferentially over only a segment of the periphery, each segmental main skirt part extending on both sides of the plane of the thrust axis of the piston and being separated from said annular band by a transverse slot, wherein the diameter of the annular band is substantially less, when the piston is cold, than the diameter of the skirt segments, both diameters being measured on the thrust axis, so that, in use, on starting the engine the piston is guided in the cylinder solely by the segmental main skirt parts.

2. A piston as claimed in claim 1, in which the diameter of the annular band is substantially 0.125mm (0.005 ins.) less than the diameter of the skirt segments when the piston is cold.

3. A piston as claimed in claim 1 or claim 2, in which the diameter of the annular band is substantially the same, when the piston is at its operating temperature, as the diameter of the skirt segments, both diameters being measured on the thrust axis, so that, in use, under normal running conditions, the annular band assists in guiding the piston in the cylinder.

5. A piston as claimed in any preceding claim in which the body of the piston is of aluminium alloy and the inserts are of steel.

6. A piston as claimed in any preceding claim in which the projections taper in width, being less wide at their free ends.

7. A piston as claimed in any preceding claim in which the projections are spaced at a substantial distance from the inner surface of the annular band which forms the upper part of the skirt, measured parallel to the gudgeon pin axis.

8. A piston as claimed in any of Claims 1 to 6 in which the projections are connected to the internal periphery of the annular band on the longitudinal plane of the piston containing the gudgeon pin axis.

9. A piston as claimed in any preceding claim, in which the inserts are so shaped as to provide a mechanical locking action with the material of the body of the piston.

10. A piston as claimed in Claim 9, in which each insert has an aperture having a circular portion through which the gudgeon pin bore passes, and a pair of slots opening into the circular portion, the slots having a narrow part toward the circular portion which contains material of the body part of the piston.

11. A piston as claimed in Claim 9 or Claim 10, in which each insert is provided with formations on its external periphery which provide a mechanical locking action between the ends of the inserts and the skirt segments.

12. A piston as claimed in any of Claims 9 to 11, in which each insert is provided with at least one slot at each end to provide a mechanical locking action between the ends of the inserts and the skirt segments.

13. A piston as claimed in any of Claims 1 to 7, and of Claims 9 to 12 when dependent on any of Claims 1 to 7, in which oil holes are provided between the lowermost ring groove and the interior of the piston, at spaced positions all round the periphery.

14. A piston substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The current trend is to limit the presence of oil holes 22 only to the areas near the thrust faces of the skirt because their presence all round the oil groove would weaken the gudgeon pin bosses which are normally carried by the piston skirt.

In the piston in accordance with the invention, since the gudgeon pin bosses are formed by projections 14 connected directly to the crown 10, they are not affected by the number and arrangement of radial oil holes 22, and thus these can be positioned all round the groove 21 to allow the most effective distribution of oil flow.

In a modification of this piston design, the projections 14 can be made wider at their base where they are attached to the crown 10, even to the extent of being connected to the annular band 13 in the regions which subtend an angle of about 90" at the piston axis and extend for 45" on either side of the longitudinal plane of the piston containing the gudgeon pin axis.

The invention is not restricted to the embodiment described and illustrated as an example, but includes any similar or equivalent construction.

WHAT WE CLAIM IS:

1. A piston having a crown, an annular portion integral with the crown and in which ring grooves are formed, a pair of parallel projections extending directly from the underside of the crown and integral with it to form gudgeon pin bosses, an annular band integral with said annular portion, a pair of inserts of a material having a lower coefficient of thermal expansion than that of the material of the remainder of the piston, the central part of each insert being anchored in the respective projection, a pair of diametrically-opposed segmental main skirt parts, each main skirt part being carried by the corresponding ends of said inserts, the main skirt parts extending circumferentially over only a segment of the periphery, each segmental main skirt part extending on both sides of the plane of the thrust axis of the piston and being separated from said annular band by a transverse slot, wherein the diameter of the annular band is substantially less, when the piston is cold, than the diameter of the skirt segments, both diameters being measured on the thrust axis, so that, in use, on starting the engine the piston is guided in the cylinder solely by the segmental main skirt parts.

2. A piston as claimed in claim 1, in which the diameter of the annular band is substantially 0.125mm (0.005 ins.) less than the diameter of the skirt segments when the piston is cold.

3. A piston as claimed in claim 1 or claim 2, in which the diameter of the annular band is substantially the same, when the piston is at its operating temperature, as the diameter of the skirt segments, both diameters being measured on the thrust axis, so that, in use, under normal running conditions, the annular band assists in guiding the piston in the cylinder.

5. A piston as claimed in any preceding claim in which the body of the piston is of aluminium alloy and the inserts are of steel.

6. A piston as claimed in any preceding claim in which the projections taper in width, being less wide at their free ends.

7. A piston as claimed in any preceding claim in which the projections are spaced at a substantial distance from the inner surface of the annular band which forms the upper part of the skirt, measured parallel to the gudgeon pin axis.

8. A piston as claimed in any of Claims 1 to 6 in which the projections are connected to the internal periphery of the annular band on the longitudinal plane of the piston containing the gudgeon pin axis.

9. A piston as claimed in any preceding claim, in which the inserts are so shaped as to provide a mechanical locking action with the material of the body of the piston.

10. A piston as claimed in Claim 9, in which each insert has an aperture having a circular portion through which the gudgeon pin bore passes, and a pair of slots opening into the circular portion, the slots having a narrow part toward the circular portion which contains material of the body part of the piston.

11. A piston as claimed in Claim 9 or Claim 10, in which each insert is provided with formations on its external periphery which provide a mechanical locking action between the ends of the inserts and the skirt segments.

12. A piston as claimed in any of Claims 9 to 11, in which each insert is provided with at least one slot at each end to provide a mechanical locking action between the ends of the inserts and the skirt segments.

13. A piston as claimed in any of Claims 1 to 7, and of Claims 9 to 12 when dependent on any of Claims 1 to 7, in which oil holes are provided between the lowermost ring groove and the interior of the piston, at spaced positions all round the periphery.

14. A piston substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.