Abstract:
A journal wall of an in-line four cylinder engine is provided with an oilway and bolt fastening sections. A center of a breathing hole punched in the journal wall avoids the oilway and the bolt fastening sections by being offset by a predetermined amount with respect to a center of a breathing hole formed in an adjacent journal wall. In this way, it is possible to make the diameter of the breathing hole sufficiently large to alleviate friction loss in the four cylinder engine without making the oilway complicated or shortening the bolt fastening sections.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a crankcase for a multiple cylinder engine, and more particularly, to a structure for providing breathing holes having a sufficient hole diameter in journal walls of the crankcase. 
     2. Description of Related Art 
     In a multiple cylinder engine, due to reciprocal movement of the pistons there are large variations in the internal pressure of the crank chamber during high-speed rotation of the crankshaft. As a result, friction loss becomes large. In order to solve this problem, breathing holes are formed which open into the journal walls. These breathing holes for each journal wall are formed by machine processing in a direction parallel to a central axis of the crankshaft with respect to a cast cylinder block; this results in each breathing hole being arranged coaxially (see, for example, Japanese patent laid-open No. Hei. 10-77905). 
     Oilways and bolt fastening sections are also provided in the journal walls. These oilways and bolt fastening sections are integrally formed when the cylinder block is casted. Therefore, the breathing hole diameter must be made small enough to avoid the oilways and bolt fastening sections. However, this makes it difficult to provide breathing hole diameters large enough to reduce friction loss. 
     SUMMARY OF THE INVENTION 
     In order to solve the above problem of the conventional art, it is an object of the present invention to provide a multiple cylinder engine having crank chambers for each cylinder of the multiple cylinder engine arranged along the axial direction of a crankshaft, with breathing holes communicating between the crank chambers and respective journal walls supporting the crankshaft and constituting wall sections of each crank chamber, wherein the breathing holes are lined up along the crankshaft, and at least one of the breathing holes has a center different from centers of the other breathing holes. 
     A further object is to provide a multiple cylinder engine where the breathing holes are provided for each journal wall, and the number of breathing holes is the same as the number of journal walls. Various combinations of breathing holes may be implemented, from a combination where only the center position of one breathing hole is different, to a combination where the center positions of all the breathing holes are different. 
     Another object of the present invention is to provide a multiple cylinder engine where the journal wall, in which the breathing hole having a different center is provided, has an oilway and the breathing hole is formed at a position avoiding this oilway. 
     Yet another object of the present invention is to provide a multiple cylinder engine, where the journal wall with the breathing hole having a different center has a bolt fastening section and the breathing hole is formed at a position avoiding this bolt fastening section. 
     These and other objects of the present invention are accomplished by way of a plurality of breathing holes provided in each journal wall, each of breathing holes having a center different from all the others. Therefore, it is possible to vary the position of the breathing holes according to the structure of the journal wall, with the result that the diameter of the breathing holes can be made sufficiently large to ensure an opening area sufficient to reduce friction loss. 
     In the case where an oilway is provided in the journal wall, since the breathing hole is formed avoiding the oilway, it is possible to form the breathing hole having a sufficiently large diameter without interfering with the oilway. Therefore, the oilway may be formed in an uncomplicated manner. 
     In the case where a bolt fastening section is provided in the journal wall, and the breathing hole is formed avoiding the bolt fastening sections, it is possible to form the breathing hole having a sufficiently large diameter; therefore, preventing having to shorten the length of the bolts. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a cross-sectional view of a cylinder section taken along line  1 — 1  shown in FIG. 2; 
     FIG. 2 is a cross-sectional view of a crank chamber section of a multiple cylinder engine; and 
     FIG. 3 is a cross-sectional view of cylinder sections illustrating formation of breathing holes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2, reference numeral  1  is a cylinder head,  2  is a crankcase,  3  is a cylinder section,  4  is a lower case and  5  is a transmission. The crankcase  2  is divided into two parts, namely an upper case, that is formed integrally with the cylinder section  3 , and the lower case  4 . 
     Four cylinders  6 - 9  are formed in the cylinder section  3  and lined up along the axial direction of a crankshaft  10 . The crankshaft  10  is connected to pistons  11  which slide within each of the cylinders  6 - 9  via connecting rods  12 , and is supported by journal walls  13 - 17 . 
     The journal walls  13 - 17  each define cylinders  6 - 9  inside the crankcase  2  and crank chambers  20 - 23  corresponding to these cylinders. Breathing holes  24 - 28  are also formed in each of the journal walls  13 - 17 . 
     The breathing holes  24 - 28  are respectively formed one each in the journal walls  13 - 17 , with the same diameter, and are aligned along the axial direction of the crankshaft  10  communicating with each of the crank chambers  20 - 23 . 
     In FIG. 2, reference numeral  30  is an ACG cover, reference numeral  31  is a cam shaft drive sprocket, reference numeral  32  is a cam chain, reference numeral  33  is a cam chain chamber, and reference numerals  34 - 38  are journal walls for the crankshaft  10  at the lower case  4  side. 
     Next, the detailed structure of the journal walls of the cylinder section  3  will be described. 
     FIG. 1 illustrates a journal wall  14 , and a semicircular cavity  41  for housing the crankshaft  10  via a metal bearing  19  which is formed in a lower surface  40  of the journal wall  14 . An oilway  42  is also formed in the circumference of this cavity  41 . 
     The oilway  42  extends upward and leads to an attachment hole  43  for a pressure switch (omitted from the drawing) formed in a side of the journal wall  14 , extends from there through a side hole to the journal wall  13  (FIG.  2 ), bends at a thickened part of the journal wall  13  and connects to a separate oilway  44  that extends upward. 
     A pair of bolt attachment screw holes  45  are formed leading upward from the lower surface  40  on either side of the cavity  41 . These bolt holes  45  and bolts (not shown) facilitate the joining of the lower case  4  to the cylinder section  3 . 
     A breathing hole  25  is formed at a position leading away from the oilway  42  and the screw holes  45 . The diameter of this breathing hole  25  is sufficiently large to avoid friction loss, and is offset from a center position of a breathing hole  24  formed in the adjacent journal wall  13 . 
     Specifically, the center C 1  of the breathing hole  24  is offset by Δc from the center C 2  of the breathing hole  25 , and is eccentric by Δc from the oilway  42 . The centers of the breathing holes  24  and  25  are also positioned a sufficient distance away from the tips of the screw holes  45 . 
     As shown in FIG. 3, the breathing holes  24 - 27  are formed by punching the cast cylinder section  3  from a direction parallel to the axial direction of the crankshaft  10 , using a suitable fabrication system, such as NC machine tool  51  using an implement  50 . At the time the cylinder section  3  is cast, the cavity  41 , oilway  42  and screw holes  45 , before being tapped, etc. are integrally formed. 
     The implement  50  comprises a blade section  52  having an outer diameter D and a shaft section  53  having an outer diameter d smaller than the outer diameter D. The diameter D of the blade section  52  is approximately equal to the inner diameter of each of the breathing holes  24 - 27 , while the diameter d of the shaft section  53  is determined from the maximum offset amount of the breathing holes. 
     First, the breathing hole  24  is punched on the center C 1  using the implement  50 . The blade section  52  passes through the journal wall  13 , the shaft section  53  moves from there to the center C 2 , and the breathing hole  25  is formed in the journal wall  14 . The amount of movement a of the centers at this time is equivalent to an offset amount between the breathing hole  24  and the breathing hole  25  in a vertical direction in the drawing. 
     After punching the breathing hole  25 , the shaft section  53  is again moved by a distance b, and then the breathing hole  26  for the journal wall  15  is drilled. Thereby, the breathing hole  26  is formed having the center C 3  offset by the offset amount b in a vertical direction from the center C 1  of the breathing hole  24 . Thereafter, the breathing hole  27  and subsequent holes are punched in the same way. 
     The offset amounts a and b in FIG. 3 are offset amounts in the vertical direction of the Figures. The breathing holes are also appropriately offset in the lateral direction of FIG. 1 equal to the above described Δc. This offset amount is appropriately determined taking into consideration avoidance of the oilway  42  and the screw holes  45 , etc. and the structure of each of the journal walls  13 - 17 . 
     The outer diameter d of the shaft section  53  is determined by referencing one of the breathing holes. For example, as described above, the breathing hole  24  is used as a reference so that a difference between the outer diameter D of the blade section  52  and the outer diameter d of the shaft section  53  is approximately at least double an offset amount by which a predetermined breathing hole is most offset from the reference breathing hole, i.e. a maximum offset amount. 
     Next, the operation of the present invention will be described. The oilway  42  and the screw holes  45  are provided in the journal wall  14 . The breathing hole  25  is offset with respect to a breathing hole of another journal wall, for example the breathing hole  24  of the adjacent journal wall  13 , so that the breathing hole  25  is formed away from the oilway  42  and the screw holes  45 . 
     As a result, it is possible to make the diameter of the breathing hole  25  sufficiently larger, similarly to the diameter of other breathing holes, and it is possible to ensure a sufficient opening area to alleviate friction loss. Furthermore, it is possible to avoid having to form a complicated oilway passage, and to avoid the situation where the bolts are shortened as a result of the screw holes being short. 
     The breathing holes  24 , and  26 - 28  for the other journal walls  13  and  15 - 17  are also the same and can be formed at the most convenient positions; therefore, if it is necessary for the oilway and screw holes to avoid other parts, they can be formed as such. 
     The present invention is not limited to the above described embodiment, and various modifications are possible, for example, the number of cylinders can be more of less than in the embodiment as long as there is more than one. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.