GB1588076A - Piston for an internal combustion engine - Google Patents

Description

(54) PISTON FOR AN INTERNAL COMBUSTION ENGINE (71) We, CUMMINS ENGINE COMPANY, INC., of 1000 Fifth Street, Columbus, Indiana 47201, United States of America, a body corporate organised and existing under the laws of the State of Indiana, United States of America, 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:- A type of piston presently in common use in reciprocating piston internal combustion engines is made of aluminium or an aluminium alloy, these materials having been chosen because of their relatively low weight for a given size. Such pistons, however, have an excessive rate of failure consisting mainly of cracks around the rim.This type of failure is the result of high piston temperatures and stresses, and these problems are expected to increase in the future because of the trend toward higher rated engines.

Higher strength materials such as iron have been tried and tested but, with conventional piston designs, such materials have also proven to be unsatisfactory. An iron piston of a conventional design is unacceptable because it weighs too much and because of failure, due to thermal stresses, of the part of the piston which supports the piston crown.

In another U.K. Patent Specification of ours (Serial No. 1,545,929) there is described and claimed a piston for an internal combustion engine, which piston, as viewed along an axis herein referred to as the longitudinal axis of the piston, has a circular peripheral shape centred on said axis, the piston comprising a crown and an outer wall extending from the periphery of the piston crown in a first direction parallel with said longitudinal axis, said outer wall being annular in crosssection perpendicular to said longitudinal axis and being formed with annular grooves therearound to accommodate piston rings, a support member being provided which is in the form of an inner wall encircling the longitudinal axis of the piston and which inner wall is spaced radially inwardly from said outer wall with respect to said longitudinal axis, said support member extending from said crown, in said first direction, to two bosses which are spaced apart on opposite sides of the longitudinal axis of the piston and have bores therethrough aligned along an axis extending in a direction perpendicular to that of the longitudinal axis of the piston, the last mentioned bores being provided to receive, in use, a gudgeon pin, the piston further including a plurality of arcuate thrust pads spaced apart angularly about the longitudinal axis of the piston and spaced angularly about said longitudinal axis fom said bosses, said thrust pads being spaced in said first direction from said outer wall, the thrust pads being connected with said bosses via struts, said support member, said crown, said bosses and said struts being formed integrally with one another.

It is a general object of the present invention to provide an improved piston design made of a high strength material, which will withstand the mechanical and thermal loads and which, when assembled with piston rings and a piston pin, weighs no more than a comparable conventional assembly including an aluminium or aluminium alloy piston.

In accordance with the present invention there is provided a piston for an internal combustion engine, comprising a circular crown, means connected to the outer periphery of said crown for supporting piston rings, gudgeon pin bosses having laterally extending bores formed therethrough for fastening said piston to a connecting rod of the engine, a plurality of angularly spaced arcuate thrust pads in the spaces between said bosses, an upwardly flaring conical support member located coaxially with said crown, said support member connecting said crown with said bosses for supporting said crown on said bosses, and a plurality of laterally extending struts connecting said pads and said bosses for supporting said pads on said bosses.

An embodiment of the invention is described below with reference to the accompanying drawings in which: Figure 1 is a plan view from above of a piston design embodying the invention; Figures 2 and 3 are sectional views taken on the lines 2-2 and 3-3 of Figure 1; and Figures 4 and 5 are sectional views taken on the lines 4-4 and 5 5 of Figure 3.

Referring to the drawings, a piston 101 comprises a crown 102, two gudgeon pin bosses 103 and 104, a support part 106 connecting the crown 102 and the bosses, two arcuate skirts or pads 107 and 108, struts 109 (Figure 4) connecting the bosses and the skirts, and an outer wall 111 formed at the outer periphery of the crown 102. The piston 101 is generally similar to that described in U.K. Patent Specification No. 1,545,929, and therefore only the differences in construction are described in detail here.

The upper surface of the crown 102 has spaced protrusions 112 and 113 formed thereon, and the recesses form valve pockets.

The configuration of the upper surface of the dome has a conventional design and does not form part of the present invention.

The outer wall 111 contains annular grooves 114 for three piston rings (not shown), and it is generally similar to the outer walls described in U.K. Patent Specification No. 1,545,929.

The struts 109 extend outwardly from the pin bosses 103 and 104 and are radially outside the support part 106. Cutouts 116 are machined at the lower corners of the skirts 107 and 108. The piston 101 is preferably cast in one piece and then the cutouts 116 are formed and the cutouts serve as chuckengaging surfaces during further machining operations on the piston.

The support part 106 has a generally conical shape and it extends from the upper side of the bosses 103 and 104 to the backside of the crown 102. As shown in Figures 3 and 4, the lower or smaller end 121 of the conical support part 106 merges with the upper sides of the bosses 103 and 104. In the two spaces between the two bosses 103 and 104, the part 106 includes a bridge portion 122 (Figures 2 and 3) which arches upwardly a short distance above the upper surface of the bosses 103 and 104. A radially inwardly extending shoe 123 is formed at the lower edge of each bridge portion 122 in order to strengthen the bridge portions. The upper or larger end 124 of the conical support part 106 merges with the back side of the crown 102.The circular line or band 126 (Figures 1 and 3) where the part 106 and the crown 102 merge, is approximately seven-tenths the distance from the vertical centre line or axis 127 of the piston 101 to the outer periphery. When viewed from above, as in Figure 1, one-half the area of the crown is within the line 126 and one-half the area is outside the line.

Consequently, the part 106 supports the crown 102 at the mid-point of its area.

The piston 101 has a number of advantageous features as compared with the piston disclosed in Patent Specification No.

1,545,929. First, the conical shape of the support part 106 enables the bosses 103 and 104 to be located close together because of the relatively small diameter of the lower end 121. The bending stress on the gudgeon pin is thereby reduced. Second, the relatively large diameter of the upper end 124 of the support part 106 enables the line 126 to be at the midpoint of the area of the crown. Consequently, the compression load on the upper surface of the crown 102, during operation of the engine, will be evenly divided between the surface areas inside and outside of the line 126. By balancing the loads on both sides of the line 126, the support for the crown 102 is improved and it is stiffened and able to withstand higher cylinder pressures. Third, the cone shape results in added material at its upper end and more rigidity near the crown.

Fourth, the arch-shape of the bridges 122 and the shoes 123 provides adequate support and rigidity in this area while also providing openings for ventilation and coolant flow through the central volume of the piston.

It will be apparent from the foregoing detailed description and from the drawings that an improved piston construction has been provided. The piston has the least amount of material required to withstand the mechanical and thermal loads encountered during use of the piston, and the piston assembly weight is comparable to an aluminium piston assembly even though the piston is made of a high strength metal. While the form of the piston described herein is made of a high strength ductile iron and is cast in one piece, it should be understood that the piston could instead be fabricated of higher strength rolled steel stock or from forgings, or a combination of cast parts and other fabricated parts could be used. Further, the crown could be made of a high temperature strength metal, and the other parts could be made of a less expensive metal.

The entire upper part of the piston is axisymmetrical, and consequently the stresses due both to mechanical loads and to thermal loads are nearly symmetrical about the axis of the piston. This produces a more uniform stress distribution and reduces the likelihood of piston failure. The conical support part provides uniform support for the crown, and it provides a short, direct mechanical load transmission path from the crown to the pin bosses and to the pin. The pin is relatively short, and the support part minimizes the bending movement on the pin by directing the mechanical loads from the crown as closely as possible to the connecting rod.

The outer wall includes only the amount of material necessary to support the piston rings and its own mass. The outer wall is maintained as distortion free as possible because of the axisymmetric upper part of the piston and because of the relatively thin connecting part between the crown and the outer wall. The connecting part serves as a thermal dam because of its relatively thin and long dimensions. This thinness also makes the connecting part relatively flexible which enables it to withstand deflections of the crown which arise from differences between the crown and outer wall temperatures.

Each of the main functioning parts of the piston is constructed to function as independently as possible of the others. The main functioning parts are, of course, the crown, the outer wall, the support part, the bosses, and the thrust pads.

While sufficient structural thickness is provided to withstand the stresses, no unnecessary material is used. This is important first because it keeps the weight down and also because the lack of extra or unnecessary material minimizes piston stresses caused by thermal gradients.

Still other advantages over conventional aluminium pistons are that, with the present design, clearances between the piston and the cylinder liner may be reduced in order to lessen piston noise, and the piston ring temperatures are reduced in order to increase ring life and lower oil consumption.

Regarding the diameter of the upper end of the crown support part relative to the crown, the diameter of the upper end of the conical support part is preferably such that the amount of crown area it encloses is substantially equal to the amount of crown area outside its diameter.

WHAT WE CLAIM IS: 1. A piston for an internal combustion engine, comprising a circular crown, means connected to the outer periphery of said crown for supporting piston rings, gudgeon pin bosses having laterally extending bores formed therethrough for fastening said piston to a connecting rod of the engine, a plurality of angularly spaced arcuate thrust pads in the spaces between said bosses, an upwardly flaring conical support member located coaxially with said crown, said support member connecting said crown with said bosses for supporting said crown on said bosses, and a plurality of laterally extending struts connecting said pads and said bosses for supporting said pads on said bosses.

2. A piston as in claim 1, wherein said piston parts are a one-piece cast construction.

3. A piston as in any of claims 1 or 2, wherein said crown extends to the outer periphery of the piston, and a heat dam part connects the outer periphery of said crown to said means for supporting piston rings, said heat dam part being thinner than said ring supporting means and said crown and therefore being relatively flexible, and the large diameter end of said support member being adjacent said heat dam part.

4. A piston as in claims 1 or 2 wherein the large diameter end of said support member is such that the area of said crown enclosed by said large diameter end is substantially equal to the area of said crown outside of said large diameter end.

5. A piston as in any one of the preceding claims wherein said conical support member is axisymmetrical.

6. A piston for an internal combustion engine, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. the piston and because of the relatively thin connecting part between the crown and the outer wall. The connecting part serves as a thermal dam because of its relatively thin and long dimensions. This thinness also makes the connecting part relatively flexible which enables it to withstand deflections of the crown which arise from differences between the crown and outer wall temperatures. Each of the main functioning parts of the piston is constructed to function as independently as possible of the others. The main functioning parts are, of course, the crown, the outer wall, the support part, the bosses, and the thrust pads. While sufficient structural thickness is provided to withstand the stresses, no unnecessary material is used. This is important first because it keeps the weight down and also because the lack of extra or unnecessary material minimizes piston stresses caused by thermal gradients. Still other advantages over conventional aluminium pistons are that, with the present design, clearances between the piston and the cylinder liner may be reduced in order to lessen piston noise, and the piston ring temperatures are reduced in order to increase ring life and lower oil consumption. Regarding the diameter of the upper end of the crown support part relative to the crown, the diameter of the upper end of the conical support part is preferably such that the amount of crown area it encloses is substantially equal to the amount of crown area outside its diameter. WHAT WE CLAIM IS:

1. A piston for an internal combustion engine, comprising a circular crown, means connected to the outer periphery of said crown for supporting piston rings, gudgeon pin bosses having laterally extending bores formed therethrough for fastening said piston to a connecting rod of the engine, a plurality of angularly spaced arcuate thrust pads in the spaces between said bosses, an upwardly flaring conical support member located coaxially with said crown, said support member connecting said crown with said bosses for supporting said crown on said bosses, and a plurality of laterally extending struts connecting said pads and said bosses for supporting said pads on said bosses.

2. A piston as in claim 1, wherein said piston parts are a one-piece cast construction.

3. A piston as in any of claims 1 or 2, wherein said crown extends to the outer periphery of the piston, and a heat dam part connects the outer periphery of said crown to said means for supporting piston rings, said heat dam part being thinner than said ring supporting means and said crown and therefore being relatively flexible, and the large diameter end of said support member being adjacent said heat dam part.

4. A piston as in claims 1 or 2 wherein the large diameter end of said support member is such that the area of said crown enclosed by said large diameter end is substantially equal to the area of said crown outside of said large diameter end.

5. A piston as in any one of the preceding claims wherein said conical support member is axisymmetrical.

6. A piston for an internal combustion engine, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.