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

An evaporable foam pattern for casting the engine block of a two-cycle engine. The pattern 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 one end of each transfer passage communicates with the crankcase end, while the second or discharge end of the transfer passage communicates through a transfer port with the cylinder adjacent the head end of the pattern. The pattern is composed of a crankcase section and a head section which are joined along a parting line disposed normal to the axis of the cylinder, with the parting line intersecting the transfer passages adjacent the discharge end of the passages. A portion of the cylinder in the head section located adjacent the parting line is enlarged in diameter and is connected to the cylinder by a plurality of ledges that are disposed between the transfer passages. The pattern also includes an evaporable foam ring 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. An adhesive joins the abutting surfaces of the crankcase and head pattern sections, as well as securing the ring in the enlarged cylinder portion of the head pattern section.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
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. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter which is regarded as the invention.