Method of and mold for casting a combined engine block and cylinder head for a twin piston engine

The mold comprises an outer mold part (10) which defines a mold cavity (11). Walls of the outer mold part form the outer surface of the combined engine block and cylinder head. There also are first and second inner mold parts (12, 13) which extend into the mold cavity and are maintained a distance from each other. Each of the first and second inner mold parts have a cylindrical portion (12a, 13a) which, during the casting procedure, forms the cylinder bores in the engine block. The first and second inner mold parts (12, 13) also each have an upper portion (12b, 13b) situated above the cylindrical portion. During the casting procedure the upper portions form portions of a combustion chamber common to the cylinder bores. The mold further includes an third inner mold part (15) which extends through the outer mold part (10) and cooperates with the first and second inner mold parts (12, 13) to form the combustion chamber during the casting procedure.

BACKGROUND OF THE INVENTION 
This invention relates to a press casting method for manufacturing a 
combined engine block and cylinder head for a twin piston engine, i.e., a 
piston engine having two cylinders and a common combustion chamber for the 
cylinders. The invention also relates to a mold for manufacturing such a 
combined engine block and cylinder head. 
DESCRIPTION OF THE RELATED ART 
Traditionally, a combined engine block and cylinder head for a small 
internal combustion engine, i.e., for a single cylinder two-cycle engine, 
is manufactured of light metal by means of a press casting method. For 
this production, a steel mold is used. The mold has a mold cavity in which 
one or more mold parts forming the cylinder bores are inserted before the 
mold cavity is filled with molten metal. Since each cylinder has a 
separate combustion chamber, the mold part which forms the combined 
cylinder and combustion chamber can be shaped as a single detail which can 
easily be withdrawn from the cast engine block when the metal has 
solidified. 
During recent years, demands for a reduction of engine exhaust gases has 
resulted in an increased interest in other types of engines. Included in 
these engines is the so-called twin piston engine which is known, per se, 
and has a single combustion chamber and a pair of parallel communicating 
cylinders. See, for example, French Patent No. 557,617 and U.S. Pat. No. 
2,184,603, the disclosures of which are expressly incorporated herein in 
their entireties. This construction is, however, from a manufacturing 
point of view, considerably more difficult to produce since the mold parts 
cannot easily be shaped in such a manner that they, at the same time, form 
the two cylinder bores and the common combustion chamber if it should also 
be possible to remove all mold parts from the finished detail. This type 
of engine has, therefore, been manufactured by means of ordinary 
destructive core technique, by means of extensive machining after casting 
and/or with the engine block and the cylinder head formed as separate 
parts. 
SUMMARY OF THE INVENTION 
The present invention provides a simple manufacturing technique for a twin 
piston engine. 
According to the present invention, a press casting method for producing a 
combined engine block and cylinder head for a twin piston engine is 
provided. The engine defines a pair of cylinders having a common 
combustion chamber wherein the combustion chamber is formed by cooperation 
between at least three mold parts. Two of the mold parts also each form a 
cylinder bore while a third mold part abuts the other two mold parts 
during press casting of the engine. 
In further accordance with the present invention, a mold for manufacturing 
a twin piston engine includes an outer mold part that defines a mold 
cavity. Walls of the outer mold part form an outside of the combined 
engine block and cylinder head. First and second inner mold parts are 
located in the mold cavity and maintained a distance from one another. The 
first and second inner mold parts each have a cylindrical portion which, 
during a press casting procedure, forms a cylinder bore. The first and 
second inner mold parts each have an upper portion above the cylindrical 
portion. The upper portions of the first and second inner mold parts 
cooperate with a third inner mold part during the press casting procedure 
to form a combustion chamber common to the cylinders.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIGS. 1 and 2, a mold 8 for a twin piston engine with an 
inlet 9 for molten metal includes an outer mold part 10, which is 
preferably formed out of steel, and consists of upper and lower sections 
10a, 10b. The outer mold part 10 defines a mainly box-shaped mold cavity 
11. The interior surfaces of the walls of the outer mold part 10, which 
are shown as flat surfaces, form the outside of the combined engine block 
and cylinder head. The walls can, however, have any suitable shape. For 
instance, they can be provided with recesses forming fins on the finished 
detail which, however, means that the outer mold part has to be divided 
into several sections in order to make it possible to separate the detail 
from the mold. 
Contained within the mold cavity 11 are first and second inner mold parts, 
12 and 13, respectively, which preferably are fixed to or are integrated 
with the lower section 10b. The inner mold parts 12, 13 each have a 
cylindrical lower portion 12a, 13a, and an upper portion 12b, 13b. The 
cylindrical lower portions 12a, 13a form or define the cylinder bores of 
the combined engine block and cylinder head. The upper portions 12b, 13b 
each form or define a part of a combustion chamber of the combined engine 
block and cylinder head. The first and second inner mold parts 12, 13 are 
spaced a distance from each other so that an intermediate space 14 is 
created therebetween. The intermediate space 14 is a part of the mold 
cavity 11 and forms or defines a partition wall between the cylinders 
during the casting procedure. 
The intermediate space 14 is, at its upper part, enclosed by means of a 
third inner mold part 15 which has a circular upper portion 15a. A lower 
portion 15b of the third inner mold part 15 is shaped as a truncated cone, 
and has an end surface 15c which forms an upper limiting surface of the 
intermediate space 14. The third inner mold part 15 extends through a 
circular hole 16 in the upper section 10a of the outer mold part 10 and 
can be moved in its length direction in this hole 16. The hole 16 is 
located so that the third inner mold part 15 can be withdrawn from the 
finished detail parallel to, but in the opposite direction, with respect 
to the draw out direction of the first and second inner mold parts 12, 13. 
The lower conical portion 15b of the third inner mold part 15 is, during 
the casting procedure, abutting corresponding recesses or sloping surfaces 
12c, 13c provided in the upper portion 12b, 13b of the first and second 
inner mold parts 12, 13, as illustrated in FIG. 2. 
It should be apparent that the third inner mold part 15 need not be 
cylinder-shaped, but can be of any suitable shape. The third inner mold 
part 15 can also be inserted into the mold cavity 11 from a direction 
different than that illustrated and described above without departing from 
the spirit and scope of the present invention as embodied in the claims 
appended hereto. 
A second preferred embodiment of the present invention is illustrated in 
FIGS. 3 and 4 wherein common structural features are identified with the 
same reference numerals used in FIGS. 1 and 2. In this embodiment, the 
third inner mold part 15 is inserted into and withdrawn from the mold 
cavity 11 in a direction which is perpendicular to or transverse the 
length direction of the cylinders. In this case, a small lower portion 15d 
of the cylinder-shaped surface of the third inner mold part 15 serves as 
an upper limitation surface of the space 14 in the mold cavity 11, 
whereas, the end surface 15c rests against an abutment surface in the 
outer mold part 10. It would, of course, also be possible to divide the 
third inner mold part 15 into several sections which could be withdrawn in 
different directions. 
It should also be mentioned that the hole formed by the third inner mold 
part 15 in the cast detail can easily be provided with threads and be used 
for installation of a spark plug (not shown). 
The same press casting technique could also be used for creating a common 
combustion chamber for the cylinders in a twin piston engine where the 
cylinders are not parallel with respect to each other, in which case, the 
mold parts forming the cylinder bores have to be withdrawn in different 
directions. 
The molds 8 illustrated in FIGS. 1-4 and described above are used to form, 
using the press-cast technique, a combined engine block and cylinder head 
for a twin-piston engine. The molds 8 are assembled by inserting the first 
and second inner mold parts 12, 13 into the mold cavity 11 of the upper 
section 10a of the outer mold part 10 and inserting the third inner mold 
part 15 through the opening or hole 16 in the upper section 10a. The lower 
section 10b of the outer mold part 10 abuts the upper section 10a and 
molten metal is allowed to be injected into the mold cavity 11 through the 
inlet 9 and flows around the inner mold parts 12, 13, 15. As such, the 
molds 8 according to the respective embodiments of the present invention 
are generally configured as illustrated in FIGS. 2 and 4. 
After molten metal is injected or allowed to flow into the mold cavity 11 
and around the inner mold parts 12, 13, 15, the molten metal is allowed to 
cool and solidify to form the combined engine block and cylinder head. 
Thereafter, the third inner mold part 15 is removed from the upper section 
10a of the outer mold part 10, and the lower section 10b of the outer mold 
part 10, together with the first and second inner mold parts 12, 13, is 
moved away from the upper section 10a. The cast detail is then removed 
from the upper or lower section 10a, 10b of the outer mold part 10. 
Although the preferred embodiments of this invention have been shown and 
described, it should be understood that various modifications and 
rearrangements of the parts may be resorted to without departing from the 
scope of the invention as disclosed and claimed herein.