Evaporable foam pattern for casting a cylinder block of a two-cycle engine

Abstract: An evaporable foam pattern for casting the engine block of a two-cycle engine is composed of a crankcase section (11) and a head section (12) which are joined along a parting line (13) disposed normal to the axis of the cylinders (4), with the parting line (13) intersecting the transfer passages adjacent the discharge end of the passages. A portion of the cylinder in the head section (12) located adjacent the parting line (13) is relatively larger in diameter and is connected to the cylinder by a plurality of ledges (15) that are disposed between the transfer passages. The pattern also includes a separate evaporable foam ring (16) which is disposed within the enlarged cylinder portion of the head section and one end of the ring has a plurality of ears which engage the ledges. Recessed edges of the ring extending between the ears bridge the transfer passages and partially define the transfer ports.

Description: The invention is directed to an evaporable foam pattern assembly for casting a metal engine block of a two-cycle engine. Cylinder blocks of two-cycle engines are normally fabricated by die casting. Recently there has been increased activity in casting the cylinder block by a lost foam process, in which the pattern is made from an evaporable polymeric material, such as polystyrene. In the typical lost foam process, the pattern is placed in a mold and an unbonded flowable material, such as sand, surrounds the pattern, as well as filling the cavities in the pattern. During casting, the heat of the molten metal will vaporize the pattern and the vapor will be entrapped within the interstices of the sand, while the metal fills the void created by evaporation of the foam material to provide a cast part identical in configuration to the pattern. The casting of a two-cycle engine block has proven difficult due to the complexity of the porting and particularly the transfer passages. A typical two-cycle engine includes one or more transfer passages which provide communication between the crankcase and the upper ends of the cylinders. With a loop charge system, a combustible charge is drawn upwardly from the crankcase through a group of three transfer passages to the head end of the cylinder. The charges converge below the spark plug and the confluent charge is compressed by the upward stroke of the piston. After ignition by the spark, waste gases of combustion are expelled through the cylinder exhaust port with the downward power stroke of the piston. To enable the fuel-air stream to flow in the desired course to the head and then flow to the exhaust port to effect removal of the spent gases, the discharge end of the transfer passage must be positioned in precise tolerance with respect to the cylinder head. It has been proposed to fabricate an evaporable foam pattern for a two-cycle engine block from a pair of sections, i.e. a crankcase section and a head section, which are joined along a parting line extending transversely through the axes of the cylinders. In this proposal, the end of the portion of each cylinder in the head pattern section adjacent the parting line is enlarged in diameter and the crankcase section is provided with a ring-like projection that extends longitudinally from the cylinder in the crankcase section and is received within the enlarged end portion in the head section. The ring-like extension is provided with a series of ears or lugs which engage ledges bordering the enlarged cylinder portion in the head pattern section. The undercut edges or recesses between the ears serve to define the lower edges of the transfer ports. The contiguous surfaces of the two pattern sections are joined together by an adhesive. However, it has been found difficult to properly apply the adhesive to the contiguous surfaces due to the lack of visibility when the two sections are assembled. If the layer of adhesive is inadequate, the molten metal during casting will fill the void where adhesive is lacking, while on the other hand, if excessive adhesive is applied, the adhesive can extrude from the abutting surfaces into the transfer passages and the extruded glued joint will result in a bead of metal in the cast block, which can adversely effect the flow of the charge through the transfer passages. The invention is directed to an evaporable foam pattern for use in casting a metal engine block of a two-cycle engine, such as an engine utilizing a cross-flow charge. The pattern, which is formed of an evaporable polymeric material, such as polystyrene, has a crankcase end and a head end, and a plurality of cylinders extend from the crankcase end toward the head end. A plurality of transfer passages are disposed longitudinally of each cylinder and the lower or inlet end of each transfer passage communicates with the crankcase end of the block, while the upper or discharge end of each transfer passage communicates through a transfer port with the cylinder. The pattern is formed of a pair of pattern sections, a crankcase pattern section and a head pattern section, which are joined along a parting line disposed normal to the axes of the cylinders. The parting line intersects the transfer passages adjacent the discharge ends of the passages. A portion of each cylinder in the head pattern section is enlarged in diameter adjacent the parting line, and a plurality of circumferentially spaced ledges interconnect the enlarged portions with the cylinder of the head section. The ledges are located between the transfer passages. The evaporable foam pattern also includes a separate ring which, in the assembled pattern, is located in the enlarged cylinder portion of the head pattern section. One end of the ring is provided with a plurality of circumferentially spaced projections or ears which are engaged with the ledges on the head pattern section, while the opposite end of the ring is flush with the parting line. The recessed edges between the ears on the end of the ring bridge the transfer passages and define the lower edge of each transfer passage. The contiguous surfaces of the three pattern sections are joined together by an adhesive of the type commonly used in evaporable foam casting procedures. To assemble the pattern, the ring is initially glued into the enlarged cylinder portion of the head pattern section, and the head and crankcase pattern are then secured together by application of the adhesive to the abutting surfaces of the parting line. In casting the engine block, the assembled pattern is placed in a mold and surrounded by an unbonded flowable material, such as sand, which also fills the cavities in the pattern. A molten metal, such as an aluminum alloy, is fed to the pattern via a sprue and the heat of the molten metal will vaporize the pattern, with the vapor being entrapped within the interstices of the sand, while the molten metal occupies the void created by vaporization of the foam to provide a cast engine block conforming in configuration to the pattern. The use of the ring pattern section in conjunction with the crankcase and head sections provides a more precise assembled pattern. The ring is initially assembled and glued to the head pattern section, and the interior of the cylinders and transfer passages are visible during this gluing operation, so that any excess glue or adhesive that may be extruded from the joint can be removed prior to assembling the crankcase and head sections together. As the curved discharge ends of the transfer passages are formed integrally with the head pattern section, the desired precise tolerance can be maintained between the transfer ports and the cylinder head to obtain optimum efficiency of combustion. In normal practice, the internal and external surfaces of the foam pattern are coated with a ceramic wash, and the pattern of the invention is designed so that the wash will freely enter and coat both the internal and external surfaces of the pattern and will fully drain therefrom. The use of the polymeric foam pattern also produces a smoother finish on the surfaces of the casting, as opposed to sand casting, and thereby provides a more attractive appearance for the cast part. Other objects and advantages will appear in the course of the following description. In the drawings: Fig. 1 is a perspective exploded view of the pattern of the invention; Fig. 2 is a top plan view taken along line 2-2 of Fig. 1 of the crankcase end pattern section; Fig. 3 is a section taken along line 3-3 of Fig. 2; and Fig. 4 is a section taken along line 4-4 of Fig. 2. The drawings illustrate an evaporable foam pattern 1 for casting a cylinder block of a two-cycle engine. The pattern is formed of an evaporable foam material, such as polystyrene or polymethylmethacrylate, and is used in a lost foam casting process. As the cast metal engine block is identical in configuration to the pattern, terminology of the components of the metal cylinder block will be used in describing the evaporable foam pattern. Pattern 1 includes a crankcase end 2 and a head end 3. In addition, the completely assembled pattern also includes an exhaust manifold cover pattern, an exhaust manifold water jacket cover pattern, and a cylinder head water jacket pattern, all of which are not shown in the drawings, but are adapted to be attached to block pattern 1 by glue or adhesive. Pattern 1 includes a pair of cylinders 4 and one end of each cylinder is enclosed by a dome-shaped head 5, while the opposite end of each cylinder is open to the crankcase end 2 of the pattern. An exhaust port 6 is formed in each cylinder 4 and communicates through an exhaust passage 7 to the exterior. While the cast engine block can be positioned in various orientations, for convenience of description, the term "upper", as used in the description and claims, is intended to mean a direction toward the head end of the block, while the term "lower" is intended to mean a direction toward the crankcase end of the block. As illustrated in Fig. 1, the cast engine block operates on a loop charge system, and thus the pattern is formed with a plurality of transfer passages 8a-8c, which extend longitudinally of each cylinder. The lower or inlet end of each transfer passage communicates with the crankcase end 2 of the pattern, while the upper or discharge end of each transfer passage is bordered by a generally curved wall 9 and communicates through a transfer port 10 with the respective cylinder adjacent the head 5 of the cylinder. Pattern 1 is formed of a crankcase section 11 and a head pattern section 12 which are joined together along a planar parting line 13, which extends normal to the axes of cylinders 4. Parting line 13 intersects transfer passages 8 adjacent the lower end of the curved wall section 9 of each passage. Parting line 13 divides each cylinder 4 into an upper cylinder portion 4a which is located within head section 12 and a lower cylinder portion 4b which is disposed within the crankcase section 11. The lower end of each cylinder portion 4a adjacent parting line 13 is enlarged in diameter, as indicated by 14, and the enlarged portion is connected to cylinder portion 4a by a series of circumferentially spaced ledges or shoulders 15a-15d. Ledges 15 extend generally normal to the axis of the respective cylinder. As best seen in Fig. 2, ledge 15a is located between exhaust port 6 and transfer passage 8a, ledge 15b is located between transfer passages 8a and 8c, ledge 15c is located between transfer passages 8b and 8c and ledge 15d is located between transfer passage 8b and exhaust port 6. Pattern 1 also includes a pair of separate rings 16 formed of evaporable polymeric material and which, in the assembled pattern, are disposed in the enlarged end portions 14 of the cylinders. The lower end 17 of each ring 16 is disposed at the parting line 13 and as illustrated in Fig. 3. The inner cylindrical surface 18 of ring 16 has the same diameter and forms an extension to the cylinder sections 4a and 4b. As shown in Fig. 1, the upper end of each ring 16 is formed with a plurality of ears or projections 20 which are adapted to engage the ledges 15a-d. Separating each ear is a recessed edge 21a-21d and the edges 21a-c define the lower edges of the transfer ports 10, while the recessed edge 21d is located substantially flush with the lower edge of the exhaust port 6. The three pattern pieces or sections 11, 12, and 16 are joined together along contiguous surfaces by an adhesive of the type commonly used in lost foam casting procedures. The adhesive is the type which will vaporize when exposed to the heat of the molten metal, so that there will be no residual adhesive remaining after the casting process. More particularly the adhesive is applied to the abutting surfaces 22 and 23 of the crankcase section 11 and head section 12, respectively, as well as between ears 20 and ledges 15, and between the outer peripheral surface of ring 16 and the contiguous cylindrical wall bordering enlarged portion 14 of the head section 12. To assemble the pattern 1, ring 16 is initially glued into the enlarged end portion 14 with ears 20 being bonded to ledges 15, and the crankcase and head sections 11 and 12 are then glued together by application of adhesive to the surfaces 17, 22 and 23 at the parting line 13. After the pattern has been assembled, it is normally subjected to a ceramic wash which serves to coat all the internal and external surfaces of the pattern with the ceramic material. The pattern is designed so that the wash will contact all internal and external surfaces and will readily drain therefrom. The ceramic coated pattern is then placed in a mold and surrounded by an unbonded, freely flowable material, such as sand, which also fills the cavities of the pattern. A sprue formed of evaporable polymeric material is connected to the pattern and the molten metal, such as an aluminum alloy, is fed via the sprue into contact with the pattern. The heat of the molten metal will vaporize the sprue and the pattern with the vapor being entrapped within the interstices of the sand, while the metal will occupy the void created by vaporization of the foam to provide a cast metal part identical in configuration to the pattern. The use of the separate ring 16 enables the pattern to be more precisely assembled. The ring is initially mounted in the enlarged end portion 14 of head section 12 and secured therein by the adhesive. As the ring is open, the interior of cylinder portion 4a of the head section, as well as the upper ends of the transfer passages 8, are visible and if any excess adhesive should be extruded from the joints into the transfer passages it can be readily removed. This ensures that there will be no beads of metal in the transfer passages of the final cast engine block resulting from an extruded bead of glue. As the transfer passages cannot be subsequently machined, it is critical that the cast walls bordering the transfer passages be smooth and free of obstructions. Any extruded beads of metal can adversely effect the flow of the fuel-air charge through the transfer passages and thus detract from the performance of the engine. As the discharge ends 9 of the transfer passages 8 are all located in the same pattern section 12 with the cylinder heads 5, the desired precise tolerance can be maintained between the transfer ports and the head to obtain optimum efficiency of combustion.

Claims: A pattern assembly (1) for casting a metal engine block for a two-cycle engine, the assembly comprising: a pattern formed of an evaporable polymeric material and having a crankcase end (2) and a head end (3) and having at least one cylinder (4) extending from the crankcase end (2) towards the head end (3); the pattern having at least one transfer passage (8) disposed longitudinally of the cylinder (4) with the transfer passage having an inlet end communicating with the crankcase end (2) of the pattern and having a discharge end communicating through a transfer port (10) with the cylinder (4) adjacent the head end; the pattern including a crankcase pattern section (11) and a head pattern section (12) joined together along a parting line (13) disposed normal to the axis of the cylinder, a first section (4b) of the cylinder being disposed in the crankcase section of the pattern and a second section (4a) of the cylinder being disposed in the head pattern section, a portion of the second cylinder section (4a) adjacent the parting line (13) being relatively larger in diameter, the head pattern section (12) having a plurality of circumferentially spaced ledges (15) adjacent the relatively larger portion of the second cylinder section, the ledges bordering the transfer passage; a separate ring (16) formed of evaporable polymeric material and disposed in the relatively larger portion of the second cylinder section with a first end disposed in engagement with the ledges, the portion of the first end of the ring disposed between the spaced ledges defining an edge of the transfer port (10); and, securing means for joining the respective contiguous surfaces of the head section, the crankcase section and the ring. An assembly according to claim 1, wherein said portion of the first end of the ring comprises a recessed edge (21a-c). An assembly according to claim 1 or claim 2, wherein the ring (16) has a second end disposed at the parting line. An assembly according to any of claims 1 to 3, comprising exhaust port means (6) disposed in the head pattern section and communicating with the second cylinder section. An assembly according to any of claims 1 to 4, wherein the ring has a plurality of said portions each disposed between adjacent ledges, one of the portions bordering exhaust passage means in the pattern assembly. An assembly according to any of claims 1 to 5, wherein the securing means is an adhesive means. An assembly according to claim 6, wherein the adhesive means is disposed between the first end of the ring and the ledges and is disposed between the outer surface of the ring and the surface of the head pattern section bordering the relatively larger portion of the second cylinder section and is disposed at the parting line. An assembly according to any of claims 1 to 7, comprising: a plurality of cylinders extending from the crankcase end towards the head end; a plurality of transfer passages disposed longitudinally of each cylinder, each transfer passage having an inlet end communicating with the crankcase end of the pattern and having a discharge end communicating through a transfer port with the respective cylinder adjacent the head end; the portion of each cylinder located in the head section and disposed adjacent the parting line being relatively larger in diameter to provide an enlarged cylindrical portion; a plurality of evaporable foam rings corresponding in number to the number of cylinders and disposed in a respective enlarged cylindrical portion, and a plurality of circumferentially spaced ears disposed on the first end of each ring and disposed in engagement with the ledges, each ring having a plurality of recessed edges between the ears and bridging the transfer passages and defining an edge of the respective transfer port. A method of forming an evaporable foam pattern for use in casting a metal engine block for a two cycle engine, comprising the steps of forming an evaporable polymeric foam crankcase pattern section (11) having a lower crankcase end and an upper end and having a lower cylinder section and a lower transfer passage section extending between the ends, forming an evaporable polymeric foam head pattern section (12) having a lower end and an upper end and having an upper cylinder section and an upper transfer passage section extending from the lower end and communicating with the upper cylinder section, forming a portion of the upper cylinder section adjacent the lower end with a relatively larger diameter with a ledge interconnecting the enlarged portion with the upper cylinder section, forming a ring (16) of evaporable polymeric foam material having an internal diameter substantially equal to the diameter of the cylinder sections, inserting the ring in the enlarged diameter portion and positioning an upper end of the ring against the ledge, securing the contiguous surfaces of the ring and the head pattern section together, disposing the upper end of the crankcase pattern section and the lower end of the head pattern section in abutting relation with the cylindrical sections and the transfer passage sections being in mating relation, and securing the abutting ends together. A method according to claim 9, wherein the step of securing the contiguous surfaces comprises applying an adhesive to the contiguous surfaces, and the step of securing the abutting ends comprises applying an adhesive to the abutting ends. A method according to claim 9 or claim 10, and including the step of positioning a second end of the ring in substantially flush relationship with the lower end of the head pattern section. A method according to any of claims 9 to 11, and including the step of forming a plurality of circumferentially spaced ledges between the enlarged portion and the upper cylinder section with the ledges bordering the upper transfer passage section, forming the first end of the ring with a plurality of circumferentially spaced ears with recessed edges between the ears, and positioning the ears in engagement with the ledges. A method of casting a metal engine block for a two-cycle engine, using a pattern assembly according to any of claims 1 to 8.

Inventors: CORBETT WILLIAM D, SHEAFFER BENJAMIN L, CORBETT, WILLIAM D., SHEAFFER, BENJAMIN L.
Applicants: BRUNSWICK CORP, BRUNSWICK CORPORATION
Application year: 1991
Main classification: B22C7
IPC codes: F02F1, B22C7, F02B75, B22C9