Piston of internal combustion engine

Details Piston of internal combustion engine Piston of internal combustion engine

1999-04-19

2002-03-19

Nguyen, Hoang (Department: 3748)

Expansible chamber devices

Piston

Open-ended hollow skirt type

C092S239000, C123S193600

Reexamination Certificate

active

06357341

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the improvements of a reciprocating piston of an internal combustion engine suitable for automotive vehicles.
2. Description of the Prior Art
In reciprocating pistons used for automotive internal combustion engines, during reciprocating motion of the piston, the piston serves to transmit combustion pressure through a piston pin and a connecting rod to a crank pin and thus convert the combustion pressure into rotational force (torque) of an engine crankshaft. The piston operates with the piston crown or piston head exposed to extremely hot combustion gases, whereas the piston skirt contacts the comparatively cool cylinder wall. This results in a temperature gradient from the top of the piston to the bottom. Generally, the temperature of the piston top exposed to the combustion chamber is higher than that of the piston bottom. Thus, there is a difference of thermal expansion from the top to the bottom. Additionally, the piston moves up and down at high speeds, during engine operation. Of various engine parts, the piston is always subjected to very severe circumstances, namely thermal stresses and mechanical stresses. The piston must have satisfactory durability to live under these severe conditions, while performing its function and while smoothly sliding against the cylinder wall. During the operation of the engine, the resultant force, which is obtained as the product of the combustion pressure applied to the piston crown and the inertia force of the reciprocating engine parts, acts on the piston. With the connecting rod tilted, the resultant force is divided into a component force acting in a direction of the connecting rod, and a component force (called side thrust or major thrust) acting in a thrust direction perpendicular to the direction of action of the resultant force. In order to dispersedly transmit the side thrust acting on the cylinder wall or the cylinder bore, the piston is formed with a piston skirt at both sides of piston pin-boss portions. The greater the circumferential width of the piston skirt, the greater the contact-surface area (or the thrust face) of the piston skirt. With the greater thrust face of the skirt, the side thrust can be effectively dispersed, thus avoiding high localized stresses acting on the cylinder wall. However, the greater the skirt surface area or the circumferential width of the skirt, the greater friction loss during the reciprocating motion of the piston, thus increasing power loss of an internal combustion engine. To balance these two contradictory requirements, that is, increased friction loss and good dispersion of side thrust, there have been proposed and developed various unsymmetrical piston skirt structures where two sides (a major thrust side and a minor thrust side) of the piston skirt are unsymmetrical with respect to the piston pin-boss portions. Such a light-weight piston having an unsymmetrical skirt, has been disclosed in Japanese Utility-model Provisional Publication No. 64-3054 and in U.S. Pat. No. 4,274,372. In the conventional piston structures with an unsymmetrical skirt, as disclosed in the Japanese Utility-model Provisional Publication No. 64-3054 and in the U.S. Pat. No. 4,274,372, a skirt surface area of a major thrust side, on which a comparatively great side thrust acts when the piston descends during the power stroke, is dimensioned to be greater than a skirt surface area of a minor thrust side, on which a comparatively small side thrust acts when the piston moves upwards during the compression stroke, so as to effectively disperse the side thrust force, while, at the same time, reducing friction loss. As is generally known, the differences in thermal expansion between the cylinder and piston during operation, caused by variations in temperature, would change the fit between the cylinder wall and the piston skirt such that it would be either loose to tight. If the fit is too tight, high contact-surface pressure may occur between the cylinder wall and piston skirt owing to thermal expansion, thereby resulting in wear. To reduce undesired cylinder-wall wear or skirt wear and to satisfy various requirements, namely increased flexibility of the skirt in the thrust direction for thermal-expansion control, proper durability (to such an extent that permanent set never takes place under great side thrust), and less possibility of deformity by thermal or mechanical causes, the previously-noted piston structure with an unsymmetrical skirt is often utilized. Referring now to
FIG. 11
, there is shown a bottom view of a prior art piston
10
with an unsymmetrical skirt, disclosed in the Japanese Utility-model Provisional Publication No. 64-3054. As seen in
FIG. 11
, the major-thrust-side skirt
20
has a greater circumferential width than a minor-thrust-side skirt
22
. The piston
10
is formed integral with two diametrically opposing connecting wall portions (
24
,
24
), each interconnecting one side edge of the major-thrust-side skirt
20
and one side edge of the minor-thrust-side skirt
22
via the associated piston pin-boss portion
18
. Each of the connecting wall portions (
24
,
24
) is formed into a substantially circular-arc shape so that each connecting wall portion expands radially outwards. The radius-of-curvature R of each of the connecting wall portions (
24
,
24
) is dimensioned to be greater than the radius of the piston
10
. Additionally, the center-of-curvature C of each of the connecting wall portions (
24
,
24
) is offset somewhat to the major thrust side (see the eccentricity E shown in FIG.
11
). With the connecting wall portions
24
and
24
, each expanding radially outwards, as a whole, the rigidity of the piston (in the radial direction) can be lowered. Therefore, even when the piston experiences interference fit between its side wall and the cylinder wall, the side wall of the piston is able to effectively deflect by virtue of proper flexibility of each connecting wall portion (
24
,
24
), thereby avoiding the contact-surface pressure between the cylinder wall and the piston skirt surface from excessively rising. This reduces undesired cylinder-wall wear or skirt wear. In
FIG. 11
, reference sign
14
denotes a piston crown portion, whereas reference sign
26
denotes a stiffening rib portion. However, with the previously-noted piston skirt structure, the angle at intersection point between the minor-thrust-side skirt
22
and each of the connecting wall portions (
24
,
24
) is an obtuse angle. Furthermore, in comparison with the major-thrust-side skirt
20
, the circumferential width of the minor-thrust-side skirt
22
is short. Thus, the rigidity of the minor-thrust-side skirt
22
remains kept high. On the other hand, both the radially-outward expanded connecting wall portions
24
and
24
contribute to reduction in the radial durability of the piston. Actually, the piston pin-boss portions (
18
,
18
) are located in the middle portion of the respective connecting wall portions (
24
,
24
). Each of the pin-boss portions has a comparatively high rigidity. Probably, it will be impossible to induce adequate deflection of the connecting wall portions.
SUMMARY OF THE INVENTION
Frictional resistance imposed on the piston is broadly classified into (i) a frictional force created between the cylinder wall and the major-thrust-side skirt surface on expansion or power stroke, caused by a relatively great thrust force occurring owing to the combustion pressure, and (ii) a frictional force created between bearing surfaces of the cylinder wall and piston during the intake, compression, and exhaust stroke and caused by inertial force of the reciprocating parts and thermal expansion with less effect of combustion pressure or without providing the effect of combustion pressure. Practically, the engine operation is greatly effected by the frictional resistance applied to the piston at comparatively low engine speeds, and thus the magnitude of thrust force arising from inertia force based the reciprocating

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