GB2164419A

GB2164419A - Improvements in or relating to pistons

Info

Publication number: GB2164419A
Application number: GB08522476A
Authority: GB
Inventors: Robert Munro; David Clement Speaight
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1984-09-12
Filing date: 1985-09-11
Publication date: 1986-03-19
Also published as: GB2164419B; DE3567244D1; JPS62500186A; EP0193591B2; KR950000916B1; JPH0713496B2; US4702151A; GB8423050D0; KR860700280A; EP0193591A1; WO1986001858A1; CA1252677A; EP0193591B1; GB8522476D0

Description

1 GB2164419A 1

SPECIFICATION

Improvements in or relating to pistons The invention relates to a piston for an inter nal combustion engine comprising a crown, a piston axis, a ring band extending around the crown, gudgeon pin bosses supporting the crown and defining a gudgeon pin bore having an axis, and a skirt.

The purpose of the skirt is to transmit lateral thrust forces from the piston to an as sociated cylinder or liner. In many pistons, the skirt is formed by a generally cylindrical sur face extending around the piston and depend ing from the ring band for lubricated contact with the associated cylinder or liner. Such a skirt has the disadvantage that the oil film between the skirt and the associated cylinder or liner is of substantial area and so the fric tional forces generated by such film are also substantial. In addition, such a skirt is compar atively rigid and this requires the skirt to be a comparatively loose fit in the associated cylin der or liner in order to accommodate changes in piston and cylinder shape due to tempera ture changes, since the rigidity of the skirt does not allow the skirt to flex to accommo date such changes. Such a loose fit increases the incidence of piston slap so increasing 95 noise.

There have been various proposals for re ducing the load transmitting skirt area, to re duce the frictional forces, but some of these proposals have not provided adequate lubrica- 100 tion of these reduced area. In addition, others of these proposals have not dealt with the problem of the rigidity of these skirt areas.

According to a first aspect of the invention, there is provided a piston for an internal com bustion engine comprising a crown, a piston axis, a ring band extending around the crown, gudgeon pin bosses supporting the crown and defining a gudgeon pin bore having an axis and a skirt formed by two pairs of arcuate skirt portions on opposite sides of the gud geon pin bore, the arcuate skirt portions of each pair being connected to the gudgeon pin bosses and including at least one upper arcu ate skirt portion adjacent to but spaced from the ring band by a gap, and at least one lower arcuate skirt portion spaced from said at least one upper arcuate skirt portion by an axially and circumferentially extending gap, each arcuate skirt portion being disposed about a plane including the piston axis and normal to the gudgeon pin bore axis and ex tending only partially around the piston.

The following is a more detailed description of two embodiments of the invention, by way of example, reference being made to the ac companying drawings in which:

Figure 1 is a section through a first form of piston for an internal combustion engine, Figure 2 is a section on the line 11-11 of Fig- ure 1, Figure 3 is a section of a second form of piston for an internal combustion engine, and Figure 4 is a section on the line IV-1V of Figure 3.

Both the piston of Figures 1 and 2 and the piston of Figures 3 and 4 are made by a casting process using either an iron or a steel material, aluminium or an aluminium alloy.

They are pistons for Diesel engines.

Referring now to the piston. of Figures 1 and 2, this piston comprises a crown 10 surrounded by a ring band and having two gudgeon pin bosses 12a, 12b. The crown 10 is provided with a depression 13 which receives a precision cast steel combustion bowl 14 having on its undersurface projections 15 which space the bowl 14 from the depression 13 and so form an insulating air gap 16 there- between. The combustion bowl 14 is a shrink-fit in the depression. The crown 10 also has an annular outer surface which is spray-coated with a suitable ceramic 17 to a depth of 2mm in order to provide a heat- resistant coating on the crown 10.

The ring band 11 depends from the crown 10 and is formed with a plurality of piston ring grooves 18. Webs 19 are provided between the crown 10 and the ring band 11 in order to provide a ring band with the required rigidity.

A support 20 extends upwardly from each gudgeon pin boss 12a, 12b and is connected to the undersurface of the depression 13 in order to support the crown 10 and the ring band 11 on the gudgeon pin bosses 12a, 12b. Each gudgeon pin boss 12a, 12b is also formed with four struts 21a, 21b, 21c, 21d. Two of these struts 21 a, 21b extend up- wardly and outwardly from the associated gudgeon pin boss 12a, 12b on opposite sides thereof (see Fig.2). The other two struts 21c, 21d extend downwardly and outwardly from the associated boss 12a, 12b on opposite sides thereof (see also Fig.2). Thus, on each side of the gudgeon pin bosses, there are, considering both bosses, a pair of upwardly and outwardly directed struts 21a or 21b and a pair of downwardly and outwardly directed struts Each pair of struts 21 carries an associated arcuate skirt portion 22. Each arcuate skirt portion 22 has circumferentially extending upper and lower edges 23, 24 which lie in re- spective planes normal to the piston axis 26. These circumferential edges 23, 24 are interconnected by side edges 26 which lie in respective planes including a piston axis so that each arcuate skirt portion 22 is the shape of a segment of a cylinder. Each arcuate skirt portion 22 is symmetrically disposed about a plane including the piston axis and normal to the gudgeon pin bore axis (the plane of the section of Fig.2) and is of limited arcuate ex- tent. This extent may be between 70' and 2 GB2164419A 2 1100 but is preferably 800.

There is thus an axially and circumferentially extending gap 25 both between the upper edges 23 of the upper arcuate skirt portions 22 and between the lower edges 24 of the upper arcuate skirt portions 22a and the upper edges of the lower arcuate skirt portions 22b.

The axial width of each gap 25 is constant.

As shown in Figure 2, the axis 28 of the depression 13 and the combustion bowl 14 are symmetrical about an axis 28 which is offset from a vertical plane including the gud geon pin bore axis 27. This is to provide a correct positioning of the combustion bowl 14 in relation to a fuel injector of the associated 80 Diesel engine.

In use, the piston of Figures 1 and 2 reci procates in a cylinder of an internal combus tion engine, mounted on a connecting rod by a gudgeon pin extending through the gudgeon pin bosses 12a, 12b. The limited arcuate ex tent of the arcuate skirt portions 22 and their spaced support on the associated struts 21 allow the arcuate skirt portions 22 readily to adapt to the cross-sectional shape of an asso ciated cylinder or liner. In addition, since the arcuate skirt portions 22 are connected to the gudgeon pin bosses 12a, 12b and are not connected directly to the crown 10, the tem perature of these parts is much reduced in comparison with pistons where the skirt is connected directly to the crown.

The piston is light in weight because the traditional full cylindrical skirt is dispensed with. Due to its conformability, the piston can be fitted more closely within an associated cylinder or liner than conventional full-skirted pistons and this tends to reduce the noisiness of the engine. Although the arcuate skirt por tions provide a reduced area of contact be tween the piston and the associated cylinder or liner, in comparison with a piston having a cylindrical skirt, it has been found that these arcuate skirt portions are adequately lubricated at all times and that there is no mixed or boundary lubrication. This is due partly to the provision of wide gaps 25 which allow oil, thrown-up from the sump of the associated engine, to pass onto the cylinder liner wall. In this regard, the circumferential edges of the arcuate skirt portions may be provided with hydrodynamic ramps having a ramp angle of no more than 2'. These, in co-operation with the associated cylinder or liner, provide a hy drodynamic wedge action which forces oil on the cylinder or liner over the bearing surfaces of the acruate skirt portions and thus ensures hydrodynamic lubrication under all conditions.

Referring next to Figures 3 and 4, parts common to Figures 1 and 2 and to Figures 3 and 4 will be given the same reference numer als and will not be described in detail. In this embodiment, the crown 10 is not insulated by any ceramic material and the combustion bowl 29 is formed directly in the crown surface. In use, this piston operates in substantially the same way as the piston of Figures 1 and 2.

Claims

1. A piston for an internal combustion en gine comprising a crown, a piston axis, a ring band extending around the crown, guddeon pin bosses supporting the crown and defining a gudgeon pin bore having an axis, and a skirt formed by two pairs of arcuate skirt portions on opposite sides of the gudgeon pin bore, the arcuate skirt portions of. each pair being connected to the gudgeon pin bosses and including at least one upper arcuate skirt portion adjacent to but spaced from the ring band by a gap and at least one lower arcuate skirt portion spaced from said at least one upper arcuate skirt portion by an axially and circumferentially extending gap, each arcuate skirt portion being disposed about a plane including the piston axis and normal to the gudgeon pin bore axis and extending only partially around the piston.

2. A piston according to claim 1, wherein each arcuate skirt portion has upper and lower edges each lying in a respective plane normal to the piston axis so that the gap between the ring band and each at least one upper arcuate skirt portion and the gap between each at least one upper arcuate skirt portion and the associated at least one lower arcuate skirt portion is of constant width in a direction parallel to the piston axis.

3. A piston according to claim 1 or claim 2, wherein each arcuate skirt portion has side edges interconnecting said upper and lower edges with said side edges lying in respective planes including the piston axis.

4. A piston according to any one of claims 1 to 3, wherein the arcuate skirt portions are connected to the gudgeon pin bosses by struts.

5. A piston according to claim 4, wherein each boss has four struts projecting there- from, two of said at least four struts extending upwardly and outwardly of, and from opposite sides of, the associated gudgeon pin bosses and being connected to respective at least one upper arcuate skirt portions, the two remaining struts extending downwardly and outwardly of and extending from opposite sides of the associated gudgeon pin bosses and being connected to respective at least one lower arcuate skirt portion.

6. A piston according to any one of claims 1 to 5, wherein each pair of arcuate skirt portions comprises a single upper arcuate skirt portion and a single lower arcuate skirt portion.

7. A piston according to claim 6, wherein each arcuate skirt portion has an angular extent of between 70' and 110', preferably 80'.

8. A piston according to any one of claims 1 to 7, wherein the crown and the ring band are supported on the gudgeon pin bosses by 3 GB2164419A 3 at least two supports extending upwardly from the gudgeon pin bosses.

9. A piston according to any one of claims 1 to 8, wherein the crown includes a combus- tion bowl which is symmetrical about an axis which is offset from a vertical plane including the gudgeon pin bore axis.

10. A piston for an internal combustion engine substantially as hereinbefore described with reference to Figures 1 and 2 or to Figures 3 and 4 of the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1986, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.