The present invention relates to pistons and in particular to pistons
for use in internal combustion engines.
In recent years great emphasis has been placed on the reduction of
size and weight in internal combustion engines. Nowhere has this emphasis been
more apparent than in engines for automotive uses, especially passenger vehicles.
Reduction in weight, especially in reciprocating mass, of components such as pistons
allows other consequential weight savings to be made in components co-operating
with the piston. With regard to engine size, particularly engine height; reduction
thereof allows lower bonnet lines leading to fuel savings due to improved aerodynamics.
One factor allowing the physical height of an engine to be reduced
is a reduction in the compression height of the piston. The compression height
of a piston is the axial distance between the gudgeon-pin axis and the top of the
One method which has been used successfully to reduce compression
height is the use of a piston crown having only two piston rings instead of the
more usual three. One example of such a piston is described in published European
Patent Application No. 0192114 of common ownership herewith.
Another approach to reducing piston compression height has been to
attempt to move the gudgeon-pin axis closer to the top of the piston crown and
indeed to depart from the traditional concept of a gudgeon-pin which bears the
total engine firing load in the portion disposed between the piston gudgeon-pin
bosses. One such proposal is described in GB patent application No. 2165619 wherein
the loads are borne by both the gudgeon-pin and by the top of the connecting rod
small end eye which bears simultaneously on the underside of the piston crown either
directly or via an intermediate wear-resistant insert. The principal problem with
this arrangement is that virtually no machining tolerances are permissible if the
loads are to be borne simultaneously on both the gudgeon-pin and the top of the
connecting rod small end.
Another approach which has been tried is to support the whole length
of the gudgeon-pin on the underside of the crown and to attach the connecting
rod thereto by other than a normal small end eye. DE-OLS-3332300A1 shows an arrangement
where the gudgeon-pin is in the form of an I-beam with curved faces and the connecting
rod is clinched into the I-beam channels.
A serious problem which ensues when attempting to provide a bearing
surface for the gudgeon-pin or connecting rod small end directly, or via an insert,
on the underside of the crown is that caused by the greatly increased temperature
in this region. The closer one moves the actual bearing surfaces towards the crown
underside the greater is the temperature. If the bearing surface is placed directly
on the crown underside the temperature problem is exacerbated due to the direct
conduction of heat through the crown into the bearing surface. The result of this
is that lubricants are degraded and the mating bearing surfaces between the aluminium
alloy and the ferrous material can partially seize, a condition known as "scuffing".
It is an object of the present invention to move the axis of relative
movement between the piston and the connecting rod closer to the crown whilst
maintaining the bearing surfaces themselves in a relatively cooler region where
adequate lubrication may be supplied. It is a further object to provide bearing
surfaces for relative movement between the piston and connecting rod which do not
necessarily include the alloy of which the piston body is constructed. A yet further
object is to reduce piston assembly weight. For a given cylinder block height
it is an object to provide a piston having a reduced compression height and thus
a lengthened connecting rod. One effect of a lengthened connecting rod is to reduce
reciprocating and secondary out-of-balance forces. Side thrust loadings may also
be reduced which may in turn result in reduced piston noise levels and reduced
According to the present invention a piston comprises a crown portion
and a skirt portion, the crown portion having at least one boss depending from
the underside thereof, the at least one boss having secured thereto at least one
bearing member and which at least one bearing member co-operates with an associated
In one embodiment of the present invention the bearing members comprise
a ferrous material and the piston an aluminium alloy.
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 section of a piston according to the present invention through
a plane which includes both the piston axis and the axis of the piston and connecting
- Figure 2 shows a view of the piston of Figure 1 in the direction 'A';
- Figure 3 shows a modification of the piston of Figures 1 and 2;
- Figure 4 shows an alternative embodiment of a piston according to the present
- Figure 5 which shows a fragmentary view of a modification of the embodiment
shown in Figure 4.
Referring now to Figures 1 and 2 of the drawings and where the same
features are denoted by common reference numerals.
A piston is denoted generally at 10 assembled to a connecting rod
(part shown) denoted generally at 11. The piston 10 comprises a crown portion 12
having a piston ring belt in which there are two piston ring grooves 13 and 14.
The piston further comprises a skirt portion 15 for accepting thrust loads during
operation in an associated cylinder (not shown). Depending from the underside 16
of the crown portion 12 and formed integrally therewith is a boss 17. The boss
17 has a hole 18 formed therethrough. Two circular rebated portions 19 are formed,
one each side, on the boss 17. Co-operating with the two rebates 19 are two cylindrical
steel bearing members 20. The bearing members 20 are secured to the boss 17 by
a dowel bolt and nut 21,22 so that the members 20 are maintained in fixed relationship
to the piston 10 and are mutally coaxial. The connecting rod 11 has a bifurcated
small end 25, the small end 25 having two portions 26 which co-operate with those
portions of the bearing members 20 which extend beyond the rebates 19. The lower
face 27 of the small end bears against the lowermost faces 28 of the members 20.
In the central portion 29 of the bifurcated small end 25 is a shallow trough 30.
In operation the firing loads in the engine are borne directly on
the co-operating faces 27 and 28 of the connecting rod and bearing members 20 respectively
and in the rebates 19 in the boss 17. Because there are virtually no bending loads
as with a conventional gudgeon-pin and because the bearing members 20 combined
with the bolt 21 are effectively solid there is practically no distortion in the
members 20 and their diameters may be reduced in comparison with that required
in a conventional gudgeon-pin. The retaining bolt 21 is not subjected to any significant
bending loads and, therefore, may be of relatively small section. The portions
26 need be only of sufficient strength to withstand the inertia forces generated
by the piston. Such inertia forces will be lower because of the lower weight of
the piston. The trough 30 acts as an oil reservoir so that the faces 27 and 28
where all the relative motion between the piston and connecting rod occurs is always
well-lubricated. Because the area of relative motion is relatively remote from
the underside 16 of the piston crown 12 the temperatures are lower.
The connecting rod small end may be fitted with, for example, a bronze
bush to provide an improved bearing material rather than the connecting rod material
itself. Alternatively the bearing members 20 may comprise a material such as a
leaded or phosphor bronze, for example, and co-operate directly with the connecting
Figure 3 shows a modification to the skirt portion 15 of the piston
10 in that there is a panel 40 extending between the thrust portions 15 on each
side to provide enhanced stiffness in some applications. Access to the bolt and
nut 21,22 is gained via apertures 41 in the panels 40.
In the embodiment shown in Figure 4 a single steel bearing member
50 is used. Bosses 51 depend from the underside of the piston crown. A third boss
52 is disposed between the bosses 51 the bearing member 50 being in contact therewith.
Machining of the contacting surface 54 of the boss 52 is effected during boring
of the bosses 51 to provide bores 55. Cylindrical light alloy spacers 56 are positioned
in the bores 55 and abut the end faces 57 of the member 50. The member 50 and
spacers 56 are held together and in fixed relationship to the piston by means
of the dowel bolt 58, nut 59 and washers 60. A birfurcated connecting rod end 62
is again used as in Figures 1 to 3. The firing loads on the piston are borne largely
in compression through the boss 52 member 50 and connecting rod bearing surface
In the modification shown in Figure 5 a single bearing member 70
is used the member extending uninterrupted through the bores 55 of the bosses 51.
This arrangement is slightly heavier than that shown in Figure 4.
In some applications the bearing members 20 may comprise a light
alloy such as a wrought aluminium-silicon-copper alloy or a modern high-strength
reinforced plastics material, for example.
The portions 26,27 which co-operate with the members 20 need not
necessarily be of circular bore. They may, for example, be oval with the minor
axis vertical. Therefore, the curvature of the lowermost portions of the connecting
rod bearing surface 27 is of slightly greater curvature than the members 20. The
effect of this is to enhance the lubrication of the bearing surface.
The invention has been illustrated with reference to a piston having
two piston rings; there may, however, be any required number of rings appropriate
to the application intended.