GB2448544A

GB2448544A - Piston skirt design

Info

Publication number: GB2448544A
Application number: GB0707663A
Authority: GB
Inventors: Ian Graham Pegg
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2007-04-20
Filing date: 2007-04-20
Publication date: 2008-10-22
Anticipated expiration: 2027-04-20
Also published as: GB0707663D0; GB2448544B

Abstract

A piston for use in a cylinder of an internal combustion engine, the piston having a barrelled skirt surface (24) for stabilising the motion of the piston (10) when reciprocating in the cylinder of the engine, characterised in that the skirt surface (24) is textured so as to create areas of localised higher pressure within a converging oil film existing between the skirt surface (24) and a wall of the cylinder, such that the motion of the piston (10), acting on the oil, generates an increased pressure forcing the piston away from the cylinder wall (28).

Description

Piston Skirt Design

Field of the invention

This invention relates to piston skirt design and more specifically to reducing the friction between the piston skirt and the cylinder wall of an internal combustion engine.

Background of the invention

Internal combustion engines have many areas of parasitic losses. The piston assembly is one of the largest and the piston skirt, according to some research, is the largest contributor. These losses reduce the overall engine efficiency. This is especially so when the engine operates at light loads such those typically seen in passenger cars or light commercial vehicles. These losses are however important for all engines at all operating conditions.

The piston skirt is an extended area of the piston projecting downwards from the piston crown and guiding the piston as it reciprocates within the cylinder bore. The piston is connected to the crankshaft by way of a connecting rod. This is pivoted to the piston about a gudgeon or wrist pin. By virtue of the way in which the motion of the piston is imparted through the connecting to the crank shaft, the piston rocks within the bore about the gudgeon pin axis. The skirt is used to decrease the extent of this rocking and reduce the chances of the edges of the piston "picking up" in the cylinder wall and damaging both the piston and the bore.

The skirt is therefore in direct contact with the wall of the cylinder for much of the time and as a result much has been done to reduce the wear that the piston skirt must endure. Typically this is achieved through coatings such as PTFE and relying on oil splash from the crank shaft bearings as well as dedicated spray jets aimed at cooling the piston and/or lubricating the bore. Oil from these jets also splashes onto the bore due the oil returning to the sump being picked up and thrown off by the crankshaft and con rod.

The oil is kept from reaching the combustion chamber by the piston rings. The bore finish is designed to ensure residual oil remains on the bore for lubrication between the piston and the bore.

Some designs of piston skirt have attempted to reduce friction by providing a number of raised pads. These are located proximate the upper and lower edges of the piston skirt, but have a tendency to scrape oil away. The raised also cause high contact pressures, causing a lowering of the load support capability.

Summary of the invention

With a view to reducing the friction between the cylinder wall and the piston skirt, there is provided a piston for use in a cylinder of an internal combustion engine, the piston having a barrelled skirt surface for stabilising the motion of the piston when reciprocating in the cylinder of the engine, characterised in that the skirt surface is textured so as to create areas of localised higher pressure within a converging oil film existing between the skirt surface and a wall of the cylinder, such that the motion of the piston, acting on the oil, generates an increased pressure forcing the piston away from the cylinder wall. This reduces the oil film shear rate and therefore the frictional losses.

Preferably the textured skirt includes elongate pockets Alternatively the elongates pockets are arranged substantially perpendicular to the direction of relative motion of the piston.

Advantageously the elongate pockets form discontinuous circumferential bands extending across the width of the piston skirt.

The piston may be provided with two or more bands of pockets spaced axially from one another. The number of bands is preferably greater than two, five being recommended.

The pockets of each band may be out of alignment with the pockets on adjacent bands.

Preferably texturing of the skirt surface is provided in two distinct regions, one proximate a lower edge of the piston skirt and the other proximate an upper edge of the piston skirt, such that there is a pocket-free region (26c) coinciding approximately with the fullest part of the barrelled skirt surface.

In an alternative embodiment a lower textured region (26a) is provided proximate a lower edge of the skirt on one side of the piston, and an upper textured region is provided proximate an upper edge of the skirt on the opposite side of the piston.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:-Figure 1. is a side view of a conventional piston showing the piston skirt, Figure 2 is a cross section along the section line A-A of Figure 1, Figure 3 is a side view of a piston according to a preferred embodiment of the present invention, and Figure 4 is a section through the skirt of a piston according to the present invention and a corresponding graph showing the effect on oil pressure created by the pockets within the piston skirt.

Detailed description of the preferred embodiment(s) Turning to Figure 1 which shows a side view of a conventional piston 10. The upper area of the circumference of the piston is occupied by the ring assembly 12, usually consisting of an upper compression ring, a central ring and a lower oil control or scraper ring.

Immediately below this, central to the piston is the skirt, the surface of which is indicated 24 and is bisected by the section line A-A.

The line A-A serves to show the plane through which the cross section shown in Figure 2 is cut. The hatched area of this diagram indicates the body of the piston 10. The piston also shows a piston crown 18 and a bowl 16 in which the combustion takes place. Pistons may of course varying in shape depending on the type of engine in which they are to be used. For example, the piston shown also includes a cooling gallery 30 which is supplied with oil by a focused oil jet (not shown) sprayed from below.

Figure 2 also shows the gudgeon or wrist pin journal 20, which receives the gudgeon pin and pivotally attaches it the connecting rod (now shown). The gudgeon pin is typically retained within the gudgeon pin journal by circlips, but may also be secured by a press fit with the connecting rod pivoting about the pin rather than the pin being held in a floating arrangement.

Figure 2 also shows the barrelled shape of the surface 24 of skirt 14. The barrelled area protrudes beyond the width of the ring assembly 12. The barrelling 22 is approximately centred on the axis of the gudgeon pin since it is this axis about which the piston rocks when reciprocating in the cylinder wall 28, when in use. By barrelling the surface 24 of the skirt 14, the area of the contact patch with the cylinder wall 28 is reduced, whereas in the absence of any barrelling, wear would alternate between the upper most ring land and the bottom most area of the skirt 14 depending on the direction of piston stroke.

The barrelling also results in the skirt and the cylinder wall no longer being parallel to one another.

During reciprocation, the piston 10 rocks about the gudgeon pin, of which the axis is represented by a dotted line in Figure 3. The rocking motion combined with the barrelled shape of the skirt 14 causes the trapped volume between the cylinder wall 28 and the skirt 14 to converge along the leading skirt edge towards the fullest part of the barrelling, and then diverge along the trailing edge of the skirt (see Figure 4 in which the arrow indicates the direction of piston motion). The relative movement of oil within this converging volume, generates a hydrodynamic oil bearing.

WO 2004/063533 A2 teaches that by texturing thrust bearing surfaces, specifically with pockets created by either machining or laser etching, the pressure within the converging oil film is increased. This has the effect of forcing the two surfaces further apart, and by increasing the separation of the surfaces, friction is reduced.

By texturing the surface of the piston skirt in this way, similar advantages can be achieved which provide a significant reduction in engine parasitic losses.

The entire surface 24 of piston skirt 14 may be etched or machined with such pockets, however, this has been shown to be detrimental to load support capability. Therefore, in a preferred embodiment shown in Figures 3 and 4, pockets are contained within two separate regions, a lower region 26a and an upper region 26b (Figure 3).

This ensures that the pocket-free central region 26c is always proximate the cylinder wall 28 at the point where the oil film is diverging.

During a downward stroke, as shown in Figure 4, the non-pocketed region 26c is closest to the cylinder wall 28.

The corresponding graph of Figure 4 shows the effect on the pressure of the oil disposed between the surface 24 and the cylinder wall 28 as a direct result of the convergence of the oil and the band of pockets. The result is an increased net force pushing the skirt 14 and piston 10 further away from the cylinder wall 28 and reducing the friction between them.

Since in upward strokes and downward strokes the piston may rock in opposite directions, it is often found, that downward strokes produce wear on a lower region of the skirt on one side of the piston, and upward strokes cause wear on an upper region of the skirt on the other side of the piston. It is therefore preferable, in order to reduce manufacturing costs, to provide only a lower skirt pocket region on one side of the piston and only an upper skirt pocket region on the opposite side of the piston. Of course, with such pistons, the orientation of the piston within the cylinder, during assembly of the engine is critical.

The pocketed regions 26a, 26b are formed from discontinuous bands of elongate pockets having a length preferably ranging from 0.5 to 2mm, and a width ranging from microns to 1mm. The pockets are separated horizontally and vertically from one another by approximately 0.5mm to 2mm and each is preferably 5-30 microns deep.

It is foreseeable that each pocketed region consists of one band of pockets, but it is preferable for the regions to consist of between two and five bands as shown in Figure 3.

It is further preferable that the pockets of adjacent bands are circumferentially off set from one another.

It is also preferable that the long axis of the pockets is substantially perpendicular to the axis of the piston.

Claims

1. A piston for use in a cylinder of an internal combustion engine, the piston having a barrelled skirt surface (24) for stabilising the motion of the piston (10) when reciprocating in the cylinder of the engine, characterised in that the skirt surface (24) is textured so as to create areas of localised higher pressure within a converging oil film existing between the skirt surface (24) and a wall of the cylinder, such that the motion of the piston (10), acting on the oil, generates an increased pressure forcing the piston away from the cylinder wall (28).

2. A piston as claimed in claim 1, wherein the textured skirt includes elongate pockets.

3. A piston as claimed in claim 2, wherein the elongate pockets are arranged substantially perpendicular to the direction of relative motion of the piston.

4. A piston as claimed in claim 2 or 3, wherein the elongate pockets form discontinuous circumferential bands extending across the width of the piston skirt.

5. A piston as claimed in claim 4, wherein the piston is provided with two or more bands of pockets spaced axially from one another.

6. A piston as claimed in claim 5, wherein the piston is provided with five bands of pockets.

7. A piston as claimed in claim 5 or 6, wherein the pockets of each band are circumferentially offset from the pockets of adjacent bands.

8. A piston as claimed in any preceding claim, wherein texturing of the skirt surface is provided in two distinct regions, one proximate (26a) a lower edge of the piston skirt (14) and the other (26b) proximate an upper edge of the piston skirt, such that there is a pocket-free region (26c) coinciding approximately with the fullest part of the barrelled skirt surface (24)

9. A piston as claimed in claim 8, wherein a lower textured region (26a) is provided proximate a lower edge of the skirt (14) on one side of the piston (10), and an upper textured region (26b) is provided proximate an upper edge of the skirt (14) on the opposite side of the piston.

10. A piston substantially as herein described with reference to and as illustrated in the accompanying drawings.