The present invention relates to pistons and particularly to pistons
of the articulated type.
Pistons generally comprise three distinct portions; the crown portion
which includes the piston ring groove belt, the gudgeon-pin boss portion and the
skirt portion. Pistons, heretofore, have mainly been produced from aluminium alloys.
With ever increasing demands for improved power output, fuel economy and emission
reductions aluminium alloys are showing strength limitations in the crown and
pin boss regions especially in diesel engines. Much attention is now being paid
to pistons in steel and cast-iron.
Steel or cast-iron is better able to withstand the thermal and mechanical
loads imposed on the crown in the highest rated engines. The piston ring grooves
are more wear-resistant and may be positioned nearer to the crown top surface
because of lower thermal conductivity. Gudgeon-pin bosses in ferrous material
are also better able to withstand the firing loads at higher temperatures without
cracking than are aluminium alloys.
One disadvantage with a single piece ferrous alloy piston may be
in increased weight although with modern designs and production techniques for
thin wall casting the weight may be no more than a comparable aluminium alloy
piston. The use of non-ferrous or plastics materials in the skirt portion, however,
has certain intrinsic advantages. These advantages generally relate to the bearing
properties of such materials which are generally superior to ferrous alloys, especially
in lubricated contact with a cast-iron cylinder bore. This does not, however, preclude
the use of ferrous alloy skirt portions which may be employed in some engine applications
or may have surface treatments or coatings to improve the compatibility of the
piston skirt with the cylinder bore.
Articulated pistons have several advantages. Crown and hence piston
ring stability is greatly enhanced since side loads imposed on the crown portion
are small and attributable mainly to gudgeon pin friction. Contact between crown
and cylinder bore is usually also greatly reduced due to the removal of significant
side thrusts and hence sideways motion and piston lands are therefore able to run
with smaller clearances. This latter feature is beneficial with respect to improved
fuel efficiency and lower emissions. Because the skirt portion of the piston is
effectively isolated from the crown portion except for conduction of heat via
the gudgeon pin the skirt portion of an articulated piston may run significantly
cooler than in a one-piece piston. Because of the lower running temperature of
the skirt portion there is considerable freedom of choice of materials. Thus the
materials of the crown portion and the skirt portion may be more effectively optimised
in respect of the duties required of each. Skirt design freedom may also be enhanced
by relative independence from the crown portion.
According to the present invention a piston comprises a crown portion
which also includes a piston ring belt and gudgeon-pin bosses and which crown
portion comprises a ferrous alloy and a separate articulated skirt portion which
also includes gudgeon-pin bores the crown portion and the skirt portion having
a common gudgeon-pin wherein at least part of the skirt portion surfaces which
co-operate with an associated cylinder or cylinder liner comprise a material selected
from the group comprising aluminium alloys, plastics material and coated steel
or cast iron.
In preferred embodiments of the present invention the rubbing surface
area of the skirt portion with the associated cylinder or cylinder liner may be
provided in the form of axially spaced-apart circumferentially extending bearing
Such lands in cast monolithic skirt constructions may be provided
in the form of separate distinct lands supported by struts depending from the associated
gudgeon-pin boss and the lands not being connected to each other in any way other
than via the struts and gudgeon-pin bosses. It has been found that such a construction
permits a closer fit between the skirt and associated cylinder and thus assists
in lowering the noise generated by the piston. The closer fit is permitted due
to the enhanced conformability of the skirt lands with the cylinder surface which
tends to distort due to local temperature variations. This type of skirt construction
may be applicable to cast aluminium alloys and to ferrous castings having surface
treatments or coatings on the bearing lands.
Separated bearing lands may alternatively be provided by removal
of intervening skirt portions.
Spaced-apart bearing lands may also be provided by relieving of the
intervening material between the lands without total removal thereof. This may
be appropriate to fabricated skirt constructions which will be described in greater
Spaced-apart bearing lands provide lower friction due to lower rubbing
areas than conventional skirts where high-area oil films increase the viscous drag
between piston and cylinder. Spaced-apart lands also tend to be inherently better
lubricated due to unimpeded access of oil to the contact surfaces.
Fabrication of the skirt portion from sheet or strip may afford a
greater choice of materials. Many plastics materials, for example, are available
which are amenable to being bonded to steel, aluminium or copper alloy strip and
sheet. Indeed many such materials are available and which are produced as journal
bearing materials. Examples of such plastics material bonded to steel via a porous
bronze sintered interlayer include various polytetrafluoroethylenebased plastics,
alloys of polyphenylenesulphone and polyetherketones, polyetheretherketones and
amideimide resins having fillers of other dry-bearing or self-lubricating materials.
The choice of materials for the fabrication of skirt portions from
sheet or strip is not only limited to plastics-lined materials as described above.
Many metallic bearing materials which are either not amenable to casting or do
not have the requisite mechanical properties in the cast form are available in
bimetal form bonded to a steel backing. Such bearing materials include, for example,
alloys based on aluminium-tin-copper, aluminium-silicon-copper, aluminium-lead-tin-copper,
aluminium-silicon-tin-copper, copper-lead-tin, etc. These alloys have superior
wear and seizure-resistance.
In order that the present invention may be more fully understood
examples will now be described by way of illustration only with reference to the
accompanying drawings of which:
- Figure 1 shows a view of a piston according to the present invention sectioned
in two mutually perpendicular planes, one plane being that which includes both
the gudgeon pin axis and the piston axis; the half-section on the left being viewed
in the direction of the gudgeon-pin axis and the half-section on the right being
viewed normal to the gudgeon-pin in the direction of arrows 'A'.
- Figure 2 shows a first alternative piston sectioned as in Figure 1;
- Figure 3 shows a second alternative piston sectioned as in Figure 2. The right-hand
half-section is viewed in direction 'B' with reference to the left section; and
- Figure 4 which shows a sectioned view of a piston in two mutually perpendicular
planes, the section on the left being through a plane including both the gudgeon-pin
and piston axes and that on the right is viewed in the direction of arrow 'C' parallel
to the gudgeon-pin axis.
Referring now to Figure 1 and where a piston is denoted generally
at 10. The piston comprises two separate components; a crown portion 11 and a
skirt portion 12. The crown portion 11 consists of a stainless steel casting having
piston ring grooves 13, 14 and 15, a combustion bowl 16, pillars 17 depending from
the bowl 16, the pillars 17 further including gudgeon pin bosses 18 having bores
19. The skirt portion 12 consists of an aluminium alloy casting 21 of basically
cylindrical form of which portions 22 have been machined away to leave bearing
lands 24 and 25 to support the thrust side loads. The skirt portion 12 further
includes gudgeon-pin bores 28 having circlip grooves 29 for location of a common
gudgeon pin 30 (not shown). The bores 19 and 28 are both of substantially equal
The piston of Figure 2 again shows a steel crown portion casting
11 similar to that of Figure 1. The skirt portion 12 however comprises a complex
aluminium alloy die-casting which comprises pin-boss pieces 30 having bores 31,
lands 32 and 33 which are supported by struts 34 and 35 depending from the boss
pieces 30, the bores 19 and 31 again being of substantially equal diameter for
support by a common gudgeon-pin 30 (not shown).
Figure 3 shows a piston having a steel crown portion 11 and a fabricated
skirt portion 12. The skirt portion comprises two boss pieces 40 having bores
41 to which has been brazed or welded at joints 42 a wrapped, generally cylindrical
member 43. The member 43 has been wrapped from a flat bimetal sheet material having
a steel backing 44 to which has been cold pressure welded by roll-bonding an aluminium-silicon
11%-copper 1% alloy 45. The alloy has been machined away in regions 46, 47 and
48 to leave two lands 49 of the alloy 45 extending part-way around the finally
wrapped member 43.
Figure 4 shows a piston comprising a stainless steel crown portion
11 and a steel skirt portion 12. The crown portion is essentially as described
with reference to the preceding three figures. The skirt portion 12 comprises
an investment casting having pin boss pieces 50, thin wall, arcuate load bearing
skirt lands 51 and 52 supported on the boss pieces 50 by struts 53 and 54 depending
therefrom. The boss pieces have bores 55 of substantially the same bore 19 as
those of the crown portion bosses 18. The boss pieces 50 further include circlip
grooves 56 for axial location of the gudgeon-pin 30 (not shown).
In further alternative embodiments of the invention the bearing lands
24 and 25 of Figure 1 or lands 32 and 33 of Figure 2 may be profiled so as to generate
oil films for hydrodynamic lubrication of the piston in operation in accordance
with known practice. Alternatively the portions 22 may not be machined away but
the periphery of the skirt casting 21 may be suitably profiled as disclosed in
US 4,535,682 or PCT/GB85/0055. Any other known profiles may be employed to generate
favourable distribution of the lubricant.
The skirt portion 12 of Figure 1 may be manufactured from aluminium
alloy tube the bores 28 being formed, for example, in upset portions of the tube.
The wrapped member 43 of Figure 3 may not necessarily be formed from
a bimetal material but may be formed from a metal-backed material having thereon
a lining comprising a plastics material. One such plastics material suitable for
such applications is based on polyetherketone (PEK) or polyetheretherketone (PEEK).
PEEK with additions of polytetrafluoroethylene (PTFE), bronze and graphite is particularly
suitable. Furthermore the member 43 may not have the lining material 45 be it
metallic or plastics based completely removed to expose the backing metal but
may be profiled away only sufficiently to produce bearing lands, pads or other
profiles to generate hydrodynamic oil lubrication films in accordance with known
practice as disclosed in US patent 4,535,682 of common ownership herewith.
Fabricated crown portion structures may also be employed in place
of casting. It will be evident to the person skilled in the art that where features
such as, for example, circlips have been described for the axial location of gudgeon-pins
that any other known method or device such as buttons in the pin ends may be used.
Although the invention has been described with reference to pistons
having steel or cast-iron crown portions the scope of the invention is intended
to encompass articulated pistons having aluminium alloy crown portions. Crown
portions having air gaps incorporated into the combustion bowls are also envisaged.
Pistons having crowns and combustion bowls of the type envisaged are disclosed
in GB 2,125,517 of common ownership herewith.