Piston assembly and method of making a piston

An improved piston for an opposed piston internal combustion engine is provided. The piston includes a piston body that extends along an axis from a crown portion to a skirt portion with a full piston skirt and to a pin boss portion. The piston body includes a plurality of ring grooves in the crown portion and at least one ring groove in the skirt portion. A wrist pin which has a length that is longer than a maximum diameter of the piston body is joined with the piston body at the pin boss portion and extends past the pin boss portion for receiving a pair of connecting rods on opposite sides of the piston body. The piston body is a monobloc piston body which is made of one integral piece or of multiple pieces that are welded or adhered together.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related generally to pistons for internal combustion engines and more particularly to pistons for opposed piston internal combustion engines.

2. Related Art

Typical opposed piston internal combustion engines include an inner piston which has a combustion surface that faces away from the crankshaft and an outer piston which has a combustion surface that faces towards the crankshaft. In operation, both the inner and outer pistons are connected to the same crankshaft and simultaneously transfer power to the crank shaft in response to a fuel and air combustion in a combustion cylinder between the combustion surfaces of the inner and outer pistons. The outer piston typically has a crown portion, a skirt portion and a pin boss portion. A wrist pin is received within the pin boss portion and extends radially outwardly on either side of the piston for receiving a pair of connecting rods to interconnect the outer piston with a crankshaft. Current designs of outer pistons for such opposed piston engines typically include a number of separate pieces, for example, up to eight, which are all joined together through pins, bolts and other removable/non-permanent connections.

SUMMARY OF THE INVENTION

One aspect of the present invention is related to an improved piston for an opposed piston internal combustion engine. The piston includes a piston body which extends along an axis from a crown portion to a skirt portion and to a pin boss portion with the skirt portion being located between the crown and the boss portion. The pin boss portion includes a pair of opposing pin bosses, and the skirt portion includes a full piston skirt. The pin boss body further includes a plurality of ring grooves which substantially circumferentially surround the crown portion and at least one ring groove which substantially circumferentially surrounds the skirt portion. A wrist pin which has a length that is longer than a maximum diameter of the piston body is joined with the pin bosses and extends past each of the pin bosses for receiving a pair of connecting rods. The piston body is a monobloc piston bod made of one integral piece or of multiple pieces which are welded or adhered together. This allows for simpler manufacturing of the piston body and for improved cost effectiveness. Additionally, the full piston skirt provides for improved performance of the piston body in the opposed piston internal combustion engine.

According to another aspect of the present invention, an inner wall of the pin boss portion includes at least one horizontal rib which extends at least partially in a direction that is perpendicular to said axis, and the skirt portion includes at least one vertical rib which extends in a direction that is parallel with the axis. The horizontal and vertical ribs may allow for the other portions of the piston body to be formed with a reduced wall thickness without compromising the structural integrity of the piston body. The reduced wall thicknesses in certain areas of the piston body may provide for material cost savings in addition to improved performance by reducing the total mass or weight of the piston body.

Another aspect of the present invention provides for an improved method of making a piston for an opposed piston internal combustion engine. The method includes the steps of preparing a first piece of a piston body and preparing a second piece of a piston body. The method continues with the step of joining the first and second pieces through one of welding or adhesives to form a monobloc piston body having a crown portion, a skirt portion including a full piston skirt and a pin boss portion including opposing pin bosses. The method proceeds with the step of receiving a wrist pin which has a length that is greater than a maximum diameter of the piston body into the pin bosses of the pin boss portion of the piston body.

According to a further aspect of the present invention, the first piece includes less than the entire crown portion and the second piece includes the remainder of the crown portion, the skirt portion and the pin boss portion.

DESCRIPTION OF THE ENABLING EMBODIMENT

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, an exemplary embodiment of a piston20constructed according to one aspect of the present invention is generally shown inFIG. 1. The exemplary piston20is configured for use as an outer piston of an opposed piston two-stroke internal combustion engine (not shown) having a single crankshaft. For example,FIG. 5shows the exemplary piston20in engagement with a pair of connecting rods22of such a two-stroke internal combustion engine.

Referring back toFIG. 1, the piston20of the exemplary embodiment includes a monobloc piston body24which extends along an axis A from a crown portion26to a skirt portion28and then to a pin boss portion30, i.e., the skirt portion28is disposed axially between the crown and pin boss portions26,30. The crown portion26, skirt portion28and pin boss portion30are all either made of one integral piece or are made of multiple pieces which are welded or adhered together to establish permanent connections between these components.

The crown portion26has an upper combustion surface with a generally flat top face34and a recessed combustion bowl36and an annular ring belt which is configured to receive a plurality of piston rings (not shown) for sealing the piston body24with a cylinder wall (not shown). In the exemplary embodiment, the ring belt38includes three axially spaced ring grooves40(hereinafter referred to as “upper ring grooves40”) which are configured to receive a pair of compression rings for sealing combustion gasses in a combustion chamber above the piston body24and an oil control ring for scraping oil off of the cylinder wall. It should be appreciated that the ring belt may include any suitable number of ring grooves for receiving any suitable number of piston rings.

Because the pin boss portion30is axially below, as opposed to axially aligned with, the skirt portion28, the skirt portion28of the piston body24is a full piston skirt in that it includes a generally flat outer surface which extends uninterruptedly substantially continuously around the piston body24, i.e. through 360 degrees around the axis A. This provides the skirt portion28of the piston body24with improved performance as compared to non-continuous skirts of other known pistons. Adjacent the flat, uninterrupted outer surface, the skirt portion28includes a pair of ring grooves42(hereinafter referred to as “lower ring grooves42”) which are spaced axially from the upper ring grooves40in the crown portion26and are located adjacent the pin boss portion30of the piston body24. In the exemplary embodiment, the skirt portion28has two lower ring grooves42. However, it should be appreciated that any suitable number of lower ring grooves42may be provided. The piston rings in between the upper ring grooves40on the crown portion26and the piston rings in the lower ring grooves42on the skirt portion28together restrict the passage of air and fuel from one or more intakes on the cylinder wall from leaking past the piston body24in either axial direction during operation of an opposed piston20two-stroke internal combustion engine. As best shown inFIG. 3, in the exemplary embodiment, the inner wall of the skirt portion28is recessed in the area of the lower ring grooves42to allow for a reduced wall thickness in the area of the generally flat outer wall of the skirt portion28.

Referring now toFIG. 4, the pin boss portion30of the piston body24includes a pair of pin bosses44which are aligned with one another and are spaced from one another by a distance which, in the exemplary embodiment, corresponds approximately with the maximum diameter of the piston body24. Each of the pin bosses44has an opening, and a wrist pin46(also known as a gudgeon pin) extends through the aligned openings and is joined with the pin bosses44. The wrist pin46has a length which is greater than the maximum diameter of the piston body24, and the ends of the wrist pin46project outwardly from the pin bosses44for engaging connecting rods22on opposite sides of the piston body24as is shown inFIG. 6. The wrist pin46may be connected with the pin bosses44of the piston body24through any suitable process or means including, for example, welding, brazing, mechanical fasteners, etc. As best shown inFIG. 3, in the exemplary embodiment, the wrist pin46is hollow for mass savings purposes.

As best shown inFIGS. 2 and 3, the inner walls of the pin bosses44have horizontally (or circumferentially) extending ribs48which provide structural reinforcement to the pin boss portion30of the piston body24. The skirt portion28also includes vertically (or axially) extending ribs50which depend from the crown portion26and extend from the crown portion26to the horizontal ribs48on the pin boss portion30. The horizontal and vertical ribs48,50may allow for the other portions of the piston body24to be formed with a reduced wall thickness without compromising the structural integrity of the piston body24. The reduced wall thicknesses in certain areas of the piston body24may provide for material cost savings in addition to improved performance by reducing the total mass or weight of the piston body24.

Referring back toFIG. 3, the monobloc piston body24of the exemplary embodiment is made of two separate pieces54,56(hereinafter referred to as an “upper piece54” and a “lower piece56” which are formed separately and are subsequently joined together. For example, in the exemplary embodiment, the upper and lower pieces54,56are joined together at a weld seam57. The upper and lower pieces54,56are preferably formed of the same material (such as steel or aluminum, for example) and are individually shaped, for example, through casting, forging and/or machining processes. The upper and lower pieces54,56are then preferably joined together through a friction welding operation. However, it should be appreciated that the upper and lower pieces54,56could alternately be formed of different materials and they could be joined together through any suitable process including, for example, other types of induction welding, resistance welding, brazing, soldering, laser welding, charge carrier rays, gluing, screwing, bolting, synching, mechanical deformation, etc. The piston body24may require additional machining after the upper and lower pieces54,65are joined together.

As best shown inFIG. 3, in the exemplary embodiment, the weld seam where the upper and lower pieces54,56are joined together is located within the crown portion26of the piston body24. As such, some of the crown portion26is presented by the upper piece54, and some of the crown portion26is presented by the lower piece56. The upper and lower pieces54,56are each shaped such that joining them together results in the formation of an oil gallery58below the combustion surface32of the crown portion26, i.e., the oil gallery58is presented at least partially by each of the upper and lower pieces54,56. The lower piece56is provided with ports60for allowing a cooling oil (not shown) to enter and exit the oil gallery58, thereby allowing the cooling oil to extract heat from the crown portion26of the piston body24during operation of the engine. The ports60may be formed into the lower piece56either during the initial formation of the lower piece56or after the upper and lower pieces54,56have been joined together. It should be appreciated that the upper and lower pieces54,56or the entire piston body24could also be made through an additive manufacturing technique, such as3dprinting or sintering.

The piston body24is either made of one integral piece of material or is made of multiple pieces (preferably two or fewer pieces) which are welded or adhered together. This allows the piston body24to be manufactured more efficiently and at a lower cost than other known piston bodies for opposed piston two-stroke internal combustion engines because it lacks the bolts, pins and other mechanical fasteners that join the various components of other piston bodies together. Additionally, the overall structure of the piston body24is more rigid and less likely to rock while oscillating during operation of the engine which reduces skirt scuffing. Even further, the horizontal and vertical ribs48,50and the recessed inner wall of the skirt portion28allow for material savings and weight reduction advantages.

Another aspect of the present invention provides for a method of making a piston20for an opposed piston internal combustion engine. The exemplary method includes the step of preparing a first (or upper) piece54of a piston body including less than an entire crown portion26. The method continues with the step of preparing a second (or lower) piece56of a piston body24including less than an entire crown portion26. The second piece54also includes a skirt portion which includes a generally flat outer surface that extends uninterruptedly substantially continuously around the second piece54and a pin boss portion30which includes a pair of opposing pin bosses44. The second piece56is preferably formed with a horizontal rib48in the pin boss portion30and a vertical rib50in the skirt portion28.

The first and second pieces54,56may be made of any suitable metal including, for example, aluminum or steel and may be shaped through any suitable process or combination of processes including, for example, casting, forging and/or machining. The exemplary method continues with the step of joining the first and second pieces54,56together through at least one of welding and adhesives to form a monobloc piston body24.

The exemplary method proceeds with the step of joining a wrist pin46which has a length that is greater than a maximum diameter of the first and second pieces54,56with the pin bosses44in the pin boss portion30of the piston body24. The wrist pin46is preferably joined with the pin boss portion30through a welding process.