The present invention relates to pistons and particularly to pistons
for internal combustion engines.
Broadly speaking a piston may be categorised into two regions; the
crown and piston ring belt forming one region and the skirt including the gudgeon
pin bosses forming the second region.
When being machined to final shape the crown and ring belt region
is usually machined to a circular polar profile. There are, however, exceptions;
specific designs of some engines having deviations from this generalisation. The
skirt region, however, is generally neither circular in polar profile nor linear
in axial profile.
The skirt region is often machined on a cam turning machine to an
oval polar profile. One example of such an oval polar profile is an ellipse where
the minor diameter is parallel to the gudgeon pin axis and the major diameter
lies in a direction at right-angles to the gudgeon pin axis. The ovality of a piston
may be defined as the difference between the dimension of the major diameter and
that of the minor diameter. The resulting figure of ovality, however, only gives
the maximum deviation from a true circular polar profile and gives no indication
of the shape of the curve between the major and minor diameters. Any given ovality
figure could in theory be applied to an infinite number of different curves.
The reasons for machining piston skirts to shapes other than circular
in polar profile include some based on thermal considerations and others based
on dynamic considerations relating to the environment in which the piston operates.
Since for practical reasons pistons may only be economically produced
by machining at ambient temperatures the form to which it is machined is an attempt
to compensate for the thermal distortion which occurs during heat-up to and dynamic
operation at the normal running temperature of the piston. Such distortion may
be anything but uniform and will be greatly influenced by the temperature variations
which occur within the piston itself. Temperatures will be higher nearer to the
crown and lower at the lower skirt portions. Furthermore, the changes of section
thickness in the skirt region will also influence the manner and degree of the
distortion and will vary from the thin sections of the skirt per se to the thicker,
stiffer regions around the pin boss and ring belt.
The dynamic considerations to be made stem from the stress/strain
effects during operation of the engine. The forces acting on the piston are generally
at a maximum during the power stroke of the engine during combustion of the fuel.
The forces due to combustion are borne mainly on one side of the piston known as
the thrust face of the piston. The forces generated in the remainder of the cycle
are much lower and are borne both by the thrust face and the other side of the
piston which is known as the counter-thrust face. These forces are not necessarily,
however, spread evenly between the thrust and counter-thrust faces of the piston.
The curvature to which the piston is machined generally attempts
to produce in the running engine a "bedding" or contact area between the piston
and its associated cylinder wall which lies within an arc subtending between approximately
40° and 80° on the thrust and counter-thrust faces of the piston.
Heretofore piston skirts have been machined symmetrically about the
plane which includes both the piston and the minor axis. The curvature of the
piston skirt has generally been calculated to produce the desired bedding area
to accommodate the combustion generated forces on the thrust face of the piston.
It has now been discovered that significant improvements in, for example, noise
reduction, stability and scuffing between a piston and its associated cylinder
wall may be obtained by machining non-symmetrically about the plane which includes
both the piston axis and the minor axis.
According to the present invention there is provided a piston having
a crown and piston-ring belt region and a skirt region and is characterised by having
over at least a part of the skirt region asymmetrical curvatures of either the
polar profile or the axial profile or both the polar and axial profiles about a
plane which includes both the piston axis and the minor axis.
The plane which includes both the piston axis and the minor axis
may also include the gudgeon pin axis.
The degree of asymmetry to which the piston skirts are machined may
also vary along the axial length of the piston skirt. Furthermore, in addition
to the polar profile of the piston skirt being asymmetric the axial length of at
least part of the piston skirt portion may also be formed to a convex curvature.
Such axial curvature may be either symmetrical about the axial plane which includes
both the piston axis and the minor axis or may be asymmetrical, i.e. the curvature
either side of the axial plane may be different at any given polar plane.
Pistons according to the invention may be conveniently visualised
as having ovoid-shaped polar profiles. The concept of an ovoid-shaped piston is
of course greatly exaggerated but nevertheless the general idea of the piston
polar profile having a relatively high convexity (or low radius of curvature) on
one working face and a relatively low convexity (or high radius of curvature)
on the other working face is applicable.
Pistons according to the invention may be constructed wherein the
piston thrust side has the higher convexity and the counter-thrust side has the
lower convexity profile, or vice versa, for better performance in respect of noise
or scuffing, for example. Furthermore it is also envisaged that the pistons may
have skirts wherein the relative orientation of the ovoid shape with respect to
thrust and counter-thrust side changes within a single piston. For example, at
the lower end of the skirt the polar profile may have the lower convexity on the
thrust side whilst at the top of the skirt near to the ring belt the higher convexity
profile may be on the thrust side with the curvature gradually changing over along
the skirt length.
With respect to the axial profile in, for example, the piston skirt
near to the piston ring belt region the convexity may, for example, generally
be lower on the thrust side than on the counter-thrust side. The physical manifestation
of this is that the radial dimension which is measured normal to the plane which
includes both the piston axis and the gudgeon pin axis to the outer skirt wall
in the relevant polar plane is greater on the thrust side than on the non-thrust
side. In some engine designs, however, superior performance may be obtained with
these parameters reversed.
Preferably pistons according to the invention may have the intervening
skirt portions between the working faces relieved in some manner such as, for
example, by machining or utilising piston castings having cast-in skirt relief.
Generally speaking the polar profile of the intervening skirt between the working
faces is immaterial provided that contact with the cylinder bore does not occur
in places where contact is undesirable.
In order that the invention may be more fully understood examples
of pistons will now be given by way of illustration only with reference to the
accompanying drawings, of which:
Figure 1 shows an elevation of a schematic piston indicating the various regions;
Figure 2 shows a diagrammatic comparison, greatly exaggerated, of a polar profile
through a prior art piston and one polar profile through a piston according to
the present invention;
Figure 3 shows a diagrammatic comparison, greatly exaggerated, between a prior
art piston axial profile and alternative axial profiles of a piston according
to the present invention.
Referring now to the figures and where in Figure 1 a piston is indicated
generally at 10. The piston comprises a crown and ring belt region 11 and a skirt
region 12 also including the gudgeon pin boss region 13. For the purposes of illustration
the skirt 12 may be sub-divided into three levels; Level 1 lying just below the
ring belt 11, Level 3 lying near the lower end of the skirt and Level 2 lying between
Levels 1 and 3 approximately coplanar with the gudgeon pin axis for convenience
and for no other specific reason. The thrust face is denoted by T and the counter-thrust
face by T¹.
Figure 2 shows a greatly exaggerated representation of a skirt profile
through one polar plane. Profile 20 is typical of a conventional prior art piston,
the oval shape 20 being symmetrical about both the major axis TT¹ and the
minor axis PP¹. Profile 21 shows one plane through a piston according to
the invention. As may be seen the curvature of the thrust face T is of high convexity
and that of the counter-thrust face T¹ is of lower convexity.
Figure 3 shows a greatly exaggerated representation of the axial skirt
profile. The profile shown by the lines 30 represents a conventional symmetrical
piston having what is known as a "barrelled" profile. The degree of barrelling
varies from piston to piston and will depend upon the specific design thereof
for the engine application. A piston according to the invention may have an axial
profile shown by the dashed lines 31 or with a profile indicated by dashed lines
32 between Levels 1 and 2. The effect of this would be to offset the polar profile
21 of Figure 2, for example, either slightly to the thrust or non-thrust side depending
upon the specific piston. Such asymmetries may also be included at the lower skirt
regions around Level 3 and further schematic examples of such asymmetries are
given by dashed lines 33 and 34. Asymmetry may also be incorporated if appropriate
at any axial level including Level 2.
During operation of the piston in the engine the stresses imposed
by combustion are much higher on the thrust face T. Therefore, the convexity is
higher so that the piston skirt may deform against its associated cylinder wall
to provide an optimum bedding or contact area. The stresses on the counter-thrust
face T¹ are much lower and, therefore, if the convexity of face T were applied
to face T¹ the bedding or contact area between skirt and wall would be much
less leading, in some engines, to piston instability. A low convexity profile is,
therefore, adopted on face T¹ to increase bedding area and promote piston
stability and hence increase performance and reduce noise. The dotted portions
of the curve 21 represent portions of the piston skirt which are relieved or removed
either by machining or by having cast-in relieved panels for example. The bedding
area is approximately represented by the solid portions of the curve 21 and lies,
in a real piston, approximately within arcs subtending between 20° and 40° either
side of the major axis TT¹ measured from the piston axis 22.
The types of curve shown in Figure 2 may be denoted by 'A' for the
curvature of a typical symmetrical type oval piston, 'B' for the relatively higher
relatively convexity portion of curve 21 and 'C' for the lower convexity portion
of curve 21.
The complete symmetrical profile 20 may be regarded as a reference
profile applicable to a typical conventional piston. The actual shape of the curve
'A' may be combined in one polar plane with a shape of type B or C to form a piston
according to the present invention. For the purpose of clarification it should
be noted that the curved shapes A, B and C only denote relative curvatures and
not absolute curvatures. Thus a conventional piston may, for example, comprise
curves of type B in the same polar planes on both thrust and counter-thrust faces
along the axial length of the skirt.
Depending upon the requirements of a particular piston the various
types of curvature typified in Figure 2 may be incorporated into the skirt in
any desired combination as indicated in the table below.
The principles disclosed in the present invention may be applied
to almost any piston design to optimise the bedding areas on the piston working
1. A piston having a crown and piston-ring belt region (11) and a skirt region
(12) characterised by having at least a part of the skirt region asymmetrical curvatures
of either the polar profile (21) or the axial profile (31,32,33, 34) or both the
polar and axial profiles about a plane which includes both the piston axis (22)
and the minor axis.
2. A piston according to Claim 1 characterised by the gudgeon pin axis also
lying in the plane which includes both the piston axis and the minor axis.
3. A piston according to either Claim 1 or Claim 2 and characterised by the
thrust face having a curvature in a polar plane of greater convexity than the
curvature of the counter-thrust face in the same polar plane.
4. A piston according to either Claim 1 or Claim 2 and characterised by the
thrust face having a curvature in a polar plane of lower convexity than the curvature
of the thrust face in the same polar plane.
5. A method of optimising performance with respect to one or more of stability,
noise, scuffing and wear of a piston comprising a crown and ring belt region (11)
and a skirt region (12), the method being characterised by machining the thrust
and counter-thrust faces of the piston skirt to asymmetric polar profiles (21)
or axial profiles (31,32,33,34) or both polar and axial profiles about the plane
which includes both the piston axis (22) and the minor axis over at least a part
of the skirt length.