Piston for internal combustion engine

A piston for an internal combustion engine in which the skirt portion of the piston is slightly oval rather than cylindrical and the size of the oval is varied along the skirt to produce contact surfaces of a particular shape engaging the cylinder. Specifically, the smallest diameter of the oval continuously increases from the piston rings downwardly and the largest diameter increases for a portion thereof. The contact surface shape has sloping sides to enhance the flow of lubricating oil away from the rings.

The present invention relates to a piston for an internal combustion engine 
and, in particular, to the shape of the skirt part of the piston below the 
piston rings. 
In a conventional piston of the type having a slightly oval, rather than 
perfectly cylindrical, skirt portion, the largest diameter of the skirt is 
progressively changed in the axial direction of the piston, whereas the 
smallest diameter remains unchanged. As shown in FIG. 5, a conventional 
piston 1' contacts the cylinder inner surface through a contact surface 
20' of approximately the size and shape shown by the chain line. This 
contact surface 20' has a configuration which inhibits the downward flow 
of lubricating oil scraped from the cylinder wall by an oil scraping 
piston ring fitted in a lowermost piston ring fitting groove 4. As a 
result, the lubricating oil does not readily and smoothly return to the 
crankcase whereby some may squeeze past the piston rings and unfavorably 
enter the combustion chamber which in turn results in an undesirable 
increase in consumption of the lubricating oil. 
In view of such circumstances, it is a primary object of the present 
invention to provide a piston of a particular shape which allows the 
lubricating oil to readily and smoothly return to the crankcase of an 
internal combustion engine. A more specific object of the present 
invention is to provide a piston with a skirt portion which is gradually 
reduced in both its largest and smallest diameters in the direction toward 
the lowermost piston ring fitting groove.

Referring first to FIGS. 1, 2 and 3, a piston 1 is provided at its upper 
portion with three piston ring fitting grooves 2, 3 and 4 spaced from each 
other in the axial direction of the piston 1 and extending around the 
entire circumference thereof. Sealing piston rings 5 and 6 are 
respectively fitted in the upper two piston ring fitting grooves 2 and 3, 
while an oil scraping piston ring 7 is fitted in the lowermost piston ring 
fitting groove 4. 
The piston 1 is provided with a skirt portion 8 below the lowermost piston 
ring fitting groove 4. This skirt portion 8 is not perfectly cylindrical 
but rather it has the cross-sectional configuration of an oval which has 
its smallest diameter DS extending in axial direction of the piston pin 9 
and its largest diameter DL extending in the direction perpendicular to 
that pin axial direction. 
A Pin boss 10 for supporting the piston pin 9 is provided on the outer face 
of the piston on both sides and has a rectangular recess 11. A pair of 
guide grooves 13 and 14 are provided in each pin boss 10 such as to extend 
between the lower edge of the rectangular recess 11 and the lower end of 
the skirt portion 8 while interposing a land 12 therebetween. 
Referring now to FIG. 4, the skirt portion 8 is comprised of first and 
second oval portions 17 and 18. The upper or first oval portion 17 extends 
between approximately the axis of the piston pin 9 and the lowermost 
piston ring fitting groove 4, while the lower or second oval portion 18 
consists of the remaining portion of the skirt portion 8. 
In the first oval portion 17, both its largest and smallest diameters DL 
and DS are gradually reduced in the upward direction from the location of 
pin 9 toward the lowermost piston ring fitting groove 4. In the second 
oval portion 18, its smallest diameter DS is gradually reduced from the 
lower end toward the first oval portion 17, whereas its largest diameter 
DL remains unchanged. More specifically, as shown in FIG. 4, assuming now 
that the largest and smallest diameters of the first oval portion 17 at 
its upper end are respectively represented by DLc and DSc, while the 
largest and smallest diameters of the skirt portion 8 at the boundary 
between the first and second oval portions 17 and 18 (i.e. the level of 
pin 9) are respectively represented by DLb and DSb, and the largest and 
smallest diameters of the second oval portion 18 at its lower end are 
respectively represented by DLa and DSa, the above-described diameters 
assume relationships therebetween as those respectively stated by the 
following formulae (1) and (2): 
EQU DLa=DLb&gt;DLc (1) 
EQU DSa&gt;Dsb&gt;DSc (2) 
Such first and second oval portions 17 and 18 are integrated with each 
other in such a manner that the boundary portion therebetween has a smooth 
surface, whereby the skirt portion 8 is formed. It is to be noted that 
FIG. 4 shows the difference between the corresponding dimensions in an 
enlarged scale for the sake of convenience of illustration whereas the 
difference is actually very small. 
As a result of these relationships among the diameters DL and DS, the 
piston 1 has contact areas or surfaces 20 formed on the outer surface of 
the skirt portion 8 of the general shape shown within the chain lines in 
FIGS. 1-3 through which the skirt portion 8 contacts the inner surface of 
a cylinder 19. More specifically, by forming the skirt portion 8 in such a 
manner that both the above-described formulae (1) and (2) are satisfied, 
the contact surface 20 has a configuration whereby it is gradually reduced 
in its width in the direction toward the upper end thereof, with boundary 
lines 21 and 22 thereby being formed which slant in such a manner that 
they come closer to each other toward the upper end of the contact surface 
20. 
The following is a description of the operation of this embodiment. As the 
piston 1 reciprocates within the cylinder 19, the lubricating oil is 
supplied in a conventional manner from the large end portion of the 
connecting rod such as to effect lubrication between the piston 1 and the 
cylinder 19. This lubricating oil is scraped off by means of the lowermost 
piston ring 7. At this time, the scraped lubricating oil is guided toward 
the rectangular recess 11 along the boundary lines 21 and 22 of each of 
the contact surfaces 20 in the manner shown by the arrows in FIG. 3 and is 
then returned to the crankcase from the rectangular recess 11 through the 
guide grooves 13 and 14. Accordingly, it is possible to prevent intrusion 
of the lubricating oil into the combination chamber as much as possible. 
In contrast, with the conventional shaped piston shown in FIG. 5, the 
lubricating oil tends to become trapped between the nearly flat upper 
extremity of the contact surface 20' and the lowermost piston ring and to 
then bypass the rings. 
Further, the contact surface 20 of the piston of this invention has a 
relatively large width at its lower portion and therefore allows the skirt 
portion 8 to slidably contact the cylinder 19 over a relatively wide area 
at the lower portion of the skirt portion 8 for increased stability but 
which is subjected to a relatively low degree of heat load. For this 
reason, generation of piston slap noise is reduced as much as possible. 
As has been described above, according to the present invention, the piston 
skirt is provided with a portion which is gradually reduced in both its 
largest and smallest diameters toward the piston ring fitting groove. It 
is therefore possible to allow each of the contact surfaces of the skirt 
through which it contacts the cylinder to have a configuration that is 
gradually reduced in its width in the upward direction toward the piston 
ring fitting groove. Thus, it is possible for the lubricating oil which is 
being scraped off the cylinder wall to be guided toward the end portions 
of the piston pin and thereby smoothly returned to the crankcase. 
Accordingly, it is advantageously possible to reduce the consumption of 
lubricating oil.