Piston, particularly for internal combustion engines

A piston, particularly for internal combustion engines, of which the bearing surfaces with which the piston comes into contact with the engine cylinder bore lie on a generated surface which runs asymmetrically to the axis of an engine cylinder accommodating the piston without clearance in the longitudinal direction of the piston and/or in the circumferential direction.

BACKGROUND TO THE INVENTION 
This invention relates to a piston, particularly for internal combustion 
engines, comprising a head and a skirt having annular grooves therein to 
receive piston rings and gudgeon pin bores the bearing surfaces of which 
skirt come into contact with the bore of the cylinder in which it 
reciprocates lie on a generated surface which has an essentially circular 
or elliptical outer contour in all cross-sections vertical to the 
longitudinal axis of the piston, and in the longitudinal axis of the 
piston straight or curved generating lines. 
It is known to make the outer contour of the generated surface in which the 
running surfaces of the piston lie deviate from a round or elliptical 
basic shape in cross-section. The generated surface also generally does 
not exhibit a straight course in the longitudinal direction of the piston. 
The pistons are instead generally barrel-shaped or convex in this 
direction. 
The cross-section particularly of light metal pistons generaly have a round 
basic shape with superimposed ovalities. The small axis of ovality is 
always in the direction of the gudgeon pin. The difference in length 
between the large and small axes of ovalilty is generally of the order of 
0.1-0.8 mm. 
Pistons with a basic shape which is already elliptical in cross-section may 
also contain corresponding superimposed ovalities. 
It is also known to shape the outer contour in the circumferential 
directiion on the thrust and anti-thrust sides of the piston differently, 
as shown for example in DE-AS No. 12 94 740. However, in the case of the 
different outer contours the two generatrices opposite each other in the 
connecting rod reciprocating plane run symmetrically to the longitudinal 
axis of the piston. Moreover, the cross-sectional contours are in mirror 
symmetry to the connecting rod reciprocating plane running through the 
longitudinal axis of the piston. 
In addition to the above reference the most relevant prior art known in the 
applicant is as follows: 
DE-PS No. 856 247 
DE-PS No. 755 235 
DE-OS No. 29 13 418 
DE-OS No. 14 75 846 
DE-AN No. A22663 XII/47 published Feb. 16, 1956 
U.S. Pat. No. 34,63,957 
Japanese Pat. No. 23 048-84 
OBJECT OF THE INVENTION 
An object of the invention is to adapt the skirt contour still further to 
the particular conditions of engine operation or deviations of shape in 
the contour of the running surfaces of the engine cylinder accommodating 
the piston. Quieter and more friction-free or in extreme cases 
seizure-resistant running should ultimately be achieved. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a piston, particularly for 
internal combustion engines, comprising a head and a skirt having annular 
grooves therein to receive piston rings and gudgeon pin bores the bearing 
surfaces of which skirt come into contact with the bore of the cylinder in 
which it reciprocates lie on a generated surface which has an essentially 
circular or elliptical outer contour in all cross-sections vertical to the 
longitudinal axis of the piston, and in the longitudinal axis of the 
piston straight or curved generating lines, the improvement being that two 
opposite generating lines, extending at least over a considerable common 
part of the axial length of the piston, do not run symmetrically in the 
connecting rod reciprocating plane passing through the longitudinal axis 
of an enclosing cylinder placed without clearance around the piston. 
With this solution in accordance with the invention it is possible to take 
the generatrix of a convex-shaped piston skirt of cylindrical basic shape 
in the upper shaft region closer to the generatrix of the engine cylinder 
on the anti-thrust side than on the thrust side. At the same time the 
course of the ovality on the thrust and anti-thrust sides can still be 
different. The following advantage can be achieved by means of a slight 
drawing in of the skirt in the upper region of the anti-thrust side in the 
case of a convex-shaped piston skirt in comparison with the shaping of the 
skirt contour on the thrust side. 
In the region of the top dead centre high normal forces must be adsorbed on 
the pressure side by the piston skirt after ignition because of the 
oblique position of the connecting rod. These lateral forces and the 
ignition force itself distort the piston in a way which necessitates 
considerable drawing in in the upper skirt region and substantial 
deviations from circular shape (ovalities) in the circumferential 
direction. 
The necessary deviations on the pressure side were also transmitted to the 
anti-thrust side in the case of the previous mutually symmetrical shank 
shape generating lines in the connecting rod reciprocating plane, although 
smaller distortions of the skirt exist there. During the course of the 
piston stroke the piston contact changes several times from the thrust to 
the anti-thrust side, so that a shape which is as cylindrical and circular 
as possible would be necessary for the piston to be well aligned; this 
also greatly influences the blow quantity and oil consumption. This need 
can be met by the envisaged asymmetrical shape in accordance with the 
invention. 
The formation of an oblique contact picture becoming established during 
engine operation can additionally be avoided on the thrust and anti-thrust 
sides of the skirt in that the region in which the piston skirt on the 
thrust and anit-thrust sides actually lies against the path of the 
cylinder and makes itself visible in the case of the run piston because of 
the friction which has taken place there is generally designated as the 
contact picture. In the case of a piston running perfectly, essentially 
symmetrical contact pictures form vertically to the connecting rod 
reciprocating plane in the longitudinal axis of the piston. However, as is 
known different operating temperatures may occur over the circumference of 
the cylinder, which cause cylinder distortions and oblique contact 
pictures. 
The outer surfaces of the piston running against the cylinder running 
surface of the engine may lie on completely differently formed generated 
surfaces in the region of the ring part of the piston and the piston 
shank, running particularly over the circumference of the piston 
completely asymmetrically to the axis of the engine cylinder accommodating 
the piston (if the piston lies against this with no clearance). 
The generated surface of the piston skirt on which the piston surfaces 
running against the path of the cylinder lie and which are formed 
asymmetrically to the extent described, should in accordance with the 
subclaims extend at least over a height of 1/4 to preferably 1/2 of the 
height of the piston skirt. 
The piston does not need to have a closed running surface. The important 
point is merely that all the running surface regions lie on the outer 
contour of a an assumed generated surface indicated in accordance with the 
invention, so that the piston bearing surface can also be divided up into 
several individual regions separated from each other provided these 
individual regions likewise lie in the generated surface in question.

DESCRIPTION OF PREFERRED EMBODIMENT 
The embodiment relates to a piston of light metal with a diameter of 
approximately 110 mm. In the sections through the piston according to 
FIGS. 3-5 X designates the direction of the gudgeon pin and Y the line 
intersecting the connecting rod reciprocating plane with the drawing 
plane. Point C lies on the anti-thrust side of the piston, whilst point D 
lies on the thrust side of the piston. Points A and B lie on the sides of 
the piston adjacent to the axis of the gudgeon pin. 
Circle K with centre M has a diameter which corresponds to the nominal 
diameter imposed by the engine cylinder, if the piston lies against the 
engine cylinder without clearance. In FIGS. 2a and 2b curves A to D 
indicate the profile of the piston skirt at the corresponding points A to 
D in the longitudinal direction of the piston. Curves A' to D' represent 
the profile of the generating lines in the ring part of the piston. 
In the cross-sectional plane presented in FIG. 3, the generating line 
contour determined by points A to D runs asymmetrically to the axis marked 
by point M throughout the circumference of the piston. The dimensions 
entered on FIGS. 2a and 2b indicate clearly the extent to which the 
generating line of the piston is distant from the contour of the circle in 
the individual intersecting planes, at the individual points A to D. 
The disymmetry of the contour of the circumference of the generating line 
in relation to mirror symmetry at the connecting rod reciprocating plane 
was chosen in the indicated manner for the embodiment in order to avoid an 
oblique contact picture in the piston caused by slightly distorted 
cylinder paths in engine operation. An engine in which point A is in the 
vicinity of the exhaust valve and point B is on the side of the intake 
valve was taken as the basis. A perfect symmetrical contact picture on the 
piston shank could be achieved as a result of the indicated disymmetry of 
the contour of the circumference of the generating line in relation to 
mirror symmetry at the piston canting plane. 
The disymmetry of the contour of the circumference of the generating line 
in relation to mirror symmetry on the plane passing through the 
longitudinal axis of the piston rod and the axis of the gudgeon pin 
indicated in the embodiment, in which the distance MD on the pressure side 
in the upper shank region is smaller than the distance MC, has brought the 
following advantage. 
Better alignment of the piston over the stroke length is achieved, i.e. its 
canting movements in the direction of the connecting rod reciprocating 
plane are reduced. As a result the blow quantity, the consumption of 
lubricating oil and the noise level caused by piston canting were reduced.