Piston for internal combustion engine

A piston for an internal combustion engine includes a crown part, a land part extending from the crown part, a skirt part adjoining the land part, and a linking boss protruding from a reverse surface of the crown part and forming a spherical joint with a small end of a connecting rod. The piston is divided into an upper piston part having the land part, and a lower piston part having the skirt part and a plurality of molded plate members formed by press-molding plates.

FIELD OF THE INVENTION

The present invention relates to a piston for an internal combustion engine, which is partitioned into an upper piston part and a lower piston part.

BACKGROUND OF THE INVENTION

An example of a piston for an internal combustion engine, wherein ribs are formed radially on the reverse surface of a crown part of the piston, stresses concentrated on the crown part are alleviated, and the thickness and weight of the crown part is reduced, is proposed in Japanese Patent Laid-Open Publication No. 2005-133613. This piston for an internal combustion engine will be described based onFIG. 10hereof.

As shown inFIG. 10, a piston200is molded as an integrated member having: a crown part201; a cylindrical land part202that extends downward from the edge of the crown part201; cylindrical skirt parts203,203that extend further downward from the land part202; a cup-shaped support208that is formed on a reverse surface207of the crown part201in order to slidably link to a small end206of a con rod204; and a plurality of ribs211that extend downward from the reverse surface207of the crown part201.

The aforementioned cup-shaped support208has a narrow fixed base on the side of the crown part201and is shaped so that the diameter gradually increases from the fixed base. Therefore, when the piston200is to be formed by casting, a die with a plurality of partitions is used. Productivity decreases because the partitioned die must be dismantled when removing the piston element from the partitioned die. A method of manufacture that offers better productivity than casting has been needed.

Additionally, the piston200needs to be made lighter in order for the internal combustion engine to achieve higher rotation speeds.

Accordingly, a need has existed for a piston for an internal combustion engine offering improved productivity and lighter weight.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a piston for an internal combustion engine, which comprises: a crown part; a land part extending in a cylindrical shape from an edge of the crown part; a skirt part adjoining the land part; and a linking boss protruding from a reverse surface of the crown part and forming a spherical joint with a small end of a connecting rod, the piston being partitioned into an upper piston part having the land part, and a lower piston part having the skirt part, the lower piston part comprising a plurality of molded plate members formed by press-molding plates.

The piston for an internal combustion engine of the present invention can thereby improve the productivity of the lower piston part using a press mold. The shape of the upper piston part can be simplified, the cast can be simplified, and the piston element can be easily removed from the casting die. The productivity of the piston can also be improved thereby, and the cost of the piston can be reduced. The lower piston part, which has molded plate members, can also be made lighter.

Preferably, the piston comprises a pair of the molded plate members. Each of the molded plate members has an outer cylinder forming the skirt part; an inner cylinder positioned inside the outer cylinder so as to be attached to a cylindrical member that constitutes the spherical joint; and a round linking plate for integrally linking both the outer cylinder and inner cylinder. The pair of molded plate members is brought together and the round linking plates are joined; and the lower piston part has a box structure.

The rigidity of the lower piston part can thereby be increased and the weight can be reduced.

Desirably, the molded plate members are identical in shape.

One type of press mold is used. The tooling cost can be reduced, and the cost of the piston can be reduced.

Preferably, the round linking plate has a lightening hole with a rim which is burred.

The lightening hole enables the piston to be made lighter, and burring enables the round linking plate to be prevented from losing rigidity because of the presence of the lightening hole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown inFIG. 1, an internal combustion engine10has a cylinder block11, a piston13that is movably inserted into a cylinder core12provided to the cylinder block11, a connecting rod16linked to the piston13by a spherical joint14, and a sectional crankshaft18that is rotatably attached to a lower part of the cylinder block11and swingably supports the connecting rod16with a hollow crank pin17.

The cylinder block11is composed of: a cylinder21provided to the upper part; a cylindrical sleeve22fit within the cylinder21and forming the cylinder core12; and an upper crankcase23is attached to a lower part of the cylinder21.

The connecting rod16is molded as an integrated member having: a spherical small end24linked to the piston13; a large end25linked to the crank pin17; and a rod26linking the small end24and the large end25. The large end25is linked to the crank pin17by a sliding bearing31.

Reference numeral32designates a counterweight provided to the crankshaft18; reference numeral33designates a cylinder head attached to the upper part of the cylinder block11by a head gasket (not shown); reference numeral34designates an intake valve; reference numeral36designates an exhaust valve; reference numeral37is a combustion chamber; reference numeral38designates a lower crankcase attached to the lower part of the upper crankcase23by a plurality of bolts41in order to form a crankcase39with the upper crankcase23; and reference numeral42designates an oil pan attached to the lower part of the lower crankcase38by a plurality of bolts44.

As shown inFIG. 2, the piston13comprises an upper piston part51that is provided with a crown part50that forms the combustion chamber37(seeFIG. 1); an upper holding member56that slidably holds an upper hemisphere24aof the small end24of the connecting rod16by being positioned within a protrusion54that protrudes from a reverse surface53of the crown part50of the upper piston part51; a lower holding member57that slidably holds a lower hemisphere24bof the small end24of the connecting rod16; and a lower piston part58that is screwed to the upper piston part51in order to hold the lower holding member57.

The upper piston part51is molded as an integrated member having the circular plate-shaped crown part50, a thickened cylindrical land part61that extends downward from the edge of the crown part50, and the aforedescribed protrusion54.

The crown part50has a crown surface64that faces the combustion chamber37(seeFIG. 1).

The land part61is a component having a top land66, a top ring channel67, a second land68, a second ring channel71, a third land72, and an oil ring channel73, in order from the side of the crown surface64. A top ring (not shown) is fit into the top ring channel67, a second ring (not shown) is fit into the second ring channel71, and an oil ring (not shown) is fit into the oil ring channel73.

The protrusion54has a concavity75for housing the upper holding member56in a movable manner in the radial direction of the piston13; and a screw78that is formed on an outer perimeter surface77on the side of an opening76of the concavity75. The concavity75is a component that forms a small concavity82on a bottom part81.

The upper holding member56is made of a silicon nitride-type ceramic having excellent resistance to heat and wear, and is provided with a first spherical surface85that is a concave spherical surface fitted to the upper hemisphere24aof the small end24of the connecting rod16in a slidable manner; and a small convexity86positioned within the small concavity82of the upper piston part51. An inner perimeter surface87of the concavity75and an outer perimeter surface88of the upper holding member56both have a gap C on one side, and a gap C (not shown) is also present between the small concavity82and the small convexity86, each on one side. The gap C is also present between the inner perimeter surface87of the concavity75and the small end24of the connecting rod16, each on one side.

The lower holding member57is a member made of a silicon nitride-type ceramic (the materials given above are suitable), is partitioned into four parts, and is provided with a second spherical surface91that is a concave spherical surface fitted to the lower hemisphere24bof the small end24of the connecting rod16in a slidable manner; an outer perimeter surface92and a tapered part93that are fitted to the lower piston part58; and a contact surface94that comes into contact with an end surface54aof the protrusion54of the upper piston part51. The lower holding member57does not rotate relative to the protrusion54of the upper piston part51due to rotation-checking pins (not shown) and keeps the piston13from rotating relative to the connecting rod16through the provision of a guided surface that contacts the rod26of the connecting rod16.57athrough57d(57c,57dare not shown) are the four partitions that constitute the lower holding member57.

The lower piston part58is composed of: a cylinder95that is positioned to enclose the lower holding member57and the lower part of the protrusion54of the upper piston part51; and a plate assembly96that is attached to the cylinder95. An upper end surface97aof a skirt part97that is provided to the entire perimeter of the lower piston part58is a member that contacts a lower end surface61aof the land part61of the upper piston part51.

In a hole98of the cylinder95are provided a female tapered part101that is tightly attached to the male tapered part93of the lower holding member57; an inner perimeter surface102that is fitted to the outer perimeter surface92of the lower holding member57with a prescribed gap; and a female screw103that is formed on the upper part of the inner perimeter surface102so as to be threaded with the male screw78of the upper piston part51.105is a step part that holds the lower internal part of the plate assembly96in position.

The plate assembly96joins together: an upper molded plate member111that is formed from a plate by press molding; and a lower molded plate member112that is formed from a plate by press molding and is attached to the lower part of the upper molded plate member111.

The upper molded plate member111has the same shape as the lower molded plate member112and is molded as an integrated member having: an outer cylinder114that constitutes a part of the skirt part97; an inner cylinder115that is fitted to the cylinder95and welded to the cylinder95; and a tapered, round linking plate116that links both the outer cylinder114and the inner cylinder115.111a,111aare joining parts that are formed on both the upper molded plate member111and the lower molded plate member112in order to join together the upper molded plate member111and the lower molded plate member112.

Reference numeral121is a lightening hole formed in the round linking plate116, reference numeral122is a flange formed by burring the rim of the lightening hole121.

The outer cylinder114of the upper molded plate member111and the outer cylinder114of the lower molded plate member112are the components that constitute the skirt part97.

The lower piston part58thus has the configuration of with a box structure due to the formation of an annular, substantially closed space having a triangular cross-section from the cylinder95, the upper molded plate member111, and the lower molded plate member112. The lower piston part58can be lightened by decreasing the thickness of the upper molded plate member111and the lower molded plate member112and has high rigidity capable of resisting inertial force, thrust, and other forces.

The male tapered part93of the lower holding member57and the female tapered part101of the cylinder95are components capable of aligning the axis of the lower holding member57with the axis of the cylinder95by being fit together.

The aforementioned male screw78of the upper piston part51and the female screw103of the lower piston part58are the components that constitute a screw coupling124.

The connecting rod16is a member that has hollow lightening sections126,127,128in the interior of the rod part26, and has a hollow lightening section130in the interior of the small end24. The connecting rod16has oil holes134,135for feeding oil from the large end25(seeFIG. 1) to the sliding surface of the spherical joint14.

The aforementioned protrusion54, upper holding member56, lower holding member57, cylinder95, and small end24are the components that constitute the aforementioned spherical joint14that links the upper piston part51and the lower piston part58.

Reference numeral137is a center point that designates the center of the spherical small end24and coincides with the center of gravity of the piston13, but the center point137may also be made to coincide substantially with the center of gravity of the piston13.

Reference numeral138is an axis of the piston13. Reference numeral139is a straight line that passes through the center point137and is included on a plane that is orthogonal to the axis138.

The press-molded upper molded plate member111and the lower molded plate member112, which was also press molded, are joined together as shown inFIG. 3. The cylinder95is attached to the inner cylinders115of the upper molded plate member111and the lower molded plate member112, forming the lower piston part58into a box shape.

The upper molded plate member111and the lower molded plate member112, which were press-molded from plates, are thus included in the lower piston part58, and the lower piston part58is formed into a box shape. Therefore, the upper molded plate member111and the lower molded plate member112are easily manufactured, the productivity of the lower piston part58can be improved and the weight can be reduced. Furthermore, the rigidity of the lower piston part58can be increased.

As shown inFIG. 4, welding points142(on the reverse surface) at which the two round linking plates116are welded together are positioned near the outer cylinder114of the round linking plates116,116(designated by the forward116mark) of the lower piston part58and between each of the lightening holes121,121.

A method for manufacturing the lower piston part58will be described below.

A rectangular blank151made of an aluminum alloy (the material is, e.g., A7N01-O, where O is a physical property classification code that designates softness) is prepared, as shown inFIG. 5A.

The blank151shown inFIG. 5Ais punched out with a press, making a round plate152as shown inFIG. 5B.

A deep-drawing process is performed on the round plate152shown inFIG. 5B, resulting in a deep-drawn molded material154with a flange153attached to the rim, as shown inFIG. 5C.

A plate-shaped part157is formed by press molding on a bottom156of the deep-drawn molded material154shown inFIG. 5C, and a center hole158is punched out. Additionally, a plurality of lightening holes121is punched out and the flanges122are formed on the rims of the lightening holes121by burring, resulting in a partially molded material161, as shown inFIG. 5D.

The rim of the center hole156is raised by press molding, resulting in a standing rim163of the center hole and a preliminary molded plate member164, as shown inFIG. 5E.

Part of the flange153and the standing rim163of the center hole of the preliminary molded plate member164shown inFIG. 5Eare trimmed, resulting in the upper molded plate member111, as shown inFIG. 5F. The lower molded plate member112(seeFIG. 2) is fashioned in the same manner.

The lower molded plate member112is placed on the upper molded plate member111and held in position, and the joining parts111a,111aare welded and joined together by, e.g., electron-beam welding, as shown inFIG. 5G. The plate assembly96is thereby completed.

A plurality of holes112bis opened only in the lower molded plate member112in order to facilitate welding, as shown inFIG. 5H. The upper molded plate member111and the internal surfaces of the holes112bare then fused and joined together by electron-beam welding. Welding is performed from the side of the lower molded plate member112in order to minimize the effect of heat by keeping the welding points farther away from the crown part50(seeFIG. 2), which reaches the highest temperature when the piston13(seeFIG. 2) is in operation.

Electron-beam welding accelerates and focuses electrons using a high voltage in a vacuum, whereby the kinetic energy of the electrons is converted into thermal energy, and the materials to be welded are heated, fused, and joined together. Since the irradiated material is therefore rapidly and partially melted, little heat diffuses to the base metal and heat deformation or other effects caused by heat seldom occur.

The cylinder95is inserted into the inner cylinders115,115of the plate assembly96, and the inner cylinders115,115and the cylinder95are welded and joined together, as shown inFIG. 5I. The lower piston part58is thereby completed.

An upright lower piston part58is shown inFIG. 5J.

A upper molded plate member111and a lower molded plate member112having identical shapes can be rapidly manufactured by a series of steps using a press, as shown inFIGS. 5A through 5J. Productivity can be rapidly improved as compared with conventional lower piston parts manufactured by casting.

As shown inFIG. 6, a lower piston part170is composed of the cylinder95and a plate assembly171attached to the cylinder95. The lower piston part170has a skirt part (an outer cylinder178described hereinafter) that is provided to the entire perimeter and is separated from the land part61of the upper piston part51.

The plate assembly171joins an upper molded plate member175, which was formed from a plate by press molding, and a lower molded plate member176, which was formed from a plate by press molding.

The upper molded plate member175is molded as an integrated member having: the outer cylinder178that is the skirt part; an inner cylinder181that is fitted to the cylinder95and welded to the cylinder95; and a tapered, round linking plate182that links both the outer cylinder178and the inner cylinder181.

The lower molded plate member176is molded as an integrated member having: a joining part176athat is joined to a joining part175aprovided to the upper molded plate member175; an inner cylinder184fitted to the cylinder95and welded to the cylinder95; and a tapered, round linking plate185that links both the joining part176aand the inner cylinder184.

As shown inFIG. 7, a lower piston part190is composed of the cylinder95and a plate assembly191attached to the cylinder95.

The plate assembly191joins an upper molded plate member195, which was formed from a plate by press molding, and a lower molded plate member196, which was formed from a plate by press molding.

The upper molded plate member195is molded as an integrated member having: an outer cylinder201constituting a part of a skirt part198; an inner cylinder202fitted to the cylinder95and welded to the cylinder95; and a round linking plate203linking both the outer cylinder201and the inner cylinder202.

The outer cylinder201is composed of a cylindrical outer-cylinder base205and a small-diameter part206of the outer cylinder that has a smaller diameter than the outer-cylinder base205and is positioned concentrically with the outer-cylinder base205.

The lower molded plate member196is molded as an integrated member having: an outer cylinder211constituting a part of the skirt part198and welded to the outside of the small-diameter part206of the outer cylinder of the upper molded plate member195; an inner cylinder212fitted to the cylinder95and welded to the cylinder95; and a round linking plate213linking both the outer cylinder211and the inner cylinder212.

As shown inFIG. 8, a lower piston part220is composed of the cylinder95and a plate assembly221attached to the cylinder95. The plate assembly221brings together and joins an upper molded plate member223, which was formed from a plate by press molding, and a lower molded plate member224, which was also formed from a plate by press molding.

The upper molded plate member223is molded as an integrated member having: an outer cylinder227constituting a part of a skirt part226; an inner cylinder228fitted to the cylinder95and welded to the cylinder95; and a tapered, round linking plate231linking both the outer cylinder227and the inner cylinder228.

The lower molded plate member224is molded as an integrated member having: an outer cylinder233constituting a part of the skirt part226; an inner cylinder234fitted and welded to the cylinder95; and a round linking plate236linking both the outer cylinder233and the inner cylinder234.

Reference numerals223a,224ainFIG. 8are joining parts that are formed on both the upper molded plate member223and the lower molded plate member224in order to join together the upper molded plate member223and the lower molded plate member224.

The outer cylinder227of the upper molded plate member223and the outer cylinder233of the lower molded plate member224are the components that constitute the skirt part226.

As shown inFIG. 9, lightening holes244,244are formed on each round linking plate243of an upper molded plate member241and a lower molded plate member242(not shown) of a lower piston part240. Flanges245are formed on the edges of each of these lightening holes244.

The lightening holes244,244are formed on the side perpendicular to the direction of thrust/counter-thrust of the lower piston part240.

Having the lightening holes244,244formed on the side perpendicular to the direction of thrust/counter-thrust allows the rigidity of the round linking plates243to be kept from decreasing along the thrust/counter-thrust direction upon which thrust acts, and the lower piston part240can be made lighter.

The land part61is provided to the upper piston part51in the present embodiments, as shown inFIG. 2, but a skirt part may also be provided to the lower part of the land part61, and the amount of rigidity of this skirt part and the skirt part97of the lower piston part58may be made to be substantially continuous.