Patent Publication Number: US-8528515-B2

Title: Crankcase of internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2009-146382 filed on Jun. 19, 2009 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a crankcase of an internal combustion engine. 
     2. Description of Background Art 
     An internal combustion engine is known that includes a crankcase composed of an upper crankcase half and a lower crankcase half. In this kind of crankcase, the upper crankcase half and the lower crankcase half respectively have an upper journal support wall and a lower journal support wall for cooperatively supporting a crank journal of a crankshaft. The upper journal wall and the lower journal wall are fastened together by a journal bolt mounted from the lower surface of the crankcase. See, for example, Japanese Patent Laid-open No. 2004-308519. 
     In the above-mentioned internal combustion engine, lubricating oil (engine oil) is supplied from an oil pump to a cylinder portion, for example, and returned from one lateral side of the crankcase to an oil pan located below the crankcase. 
     However, in the case that the lubricating oil from the one lateral side of the crankcase is passed through a portion of the crankcase laterally outside of the journal support wall and the journal bolt and returned to the oil pan, the width of the oil pan must be increased to cause an increase in size of the oil pan. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is accordingly an object of an embodiment of the present invention to provide a crankcase of an internal combustion engine which can reduce the size of an oil pan. 
     In accordance with an embodiment of the present invention, there is provided a crankcase of an internal combustion engine that includes an upper crankcase half and a lower crankcase half, the upper crankcase half and the lower crankcase half respectively having an upper journal support wall and a lower journal support wall for cooperatively supporting a crank journal of a crankshaft with the upper journal support wall and the lower journal support wall being fastened together by a journal bolt mounted from the lower side of the crankcase. A wall portion for defining a space as a flow passage for a lubricating oil is formed below a journal bolt mounting surface of the lower crankcase half for mounting the journal bolt. A tool insertion hole for insertion of a tool for tightening the journal bolt is formed through the wall portion. 
     According to an embodiment of the present invention, the wall portion for defining the space as a lubricating oil passage is formed below the journal bolt mounting surface of the lower crankcase half, and the tool insertion hole for insertion of the tool for tightening the journal bolt is formed through the wall portion. Accordingly, the journal bolt can be mounted from the lower side of the crankcase, and the lubricating oil passage can be formed below the journal bolt mounting surface. This lubricating oil passage is located laterally inside of one side surface of the crankcase, so that the width of an oil pan to which the oil from the lubricating oil passage is returned can be reduced to thereby reduce the size of the oil pan. As a result, an increase in weight due to an increase in size of the oil pan can be prevented. 
     Further, since the lubricating oil passage is formed below the journal bolt mounting surface, it is possible to prevent an increase in friction due to the case that the lubricating oil passage bypasses the journal support wall and the journal bolt. 
     The wall portion may be located laterally outside of an oil pan of the internal combustion engine and the wall portion is formed with an exit oil passage for discharging the lubricating oil from the space toward the oil pan. With this configuration, the lubricating oil from the space defined by the wall portion can be smoothly returned through the exit oil passage toward the oil pan. 
     In addition, the space defined by the wall portion may open to one lateral side of the lower crankcase half, a side cover may be connected to the one lateral side of the lower crankcase half and the lubricating oil may be returned from the side cover flows into the space through its side opening. With this configuration, the size of the oil pan can be sufficiently reduced in relation to the configuration for returning the lubricating oil from the side cover to the oil pan. 
     Further, the wall portion may include a detachable oil filter. With this configuration, the journal bolt can be mounted from the lower side of the crankcase, and the lubricating oil passage may be formed below the journal bolt mounting surface. Furthermore, the tool insertion hole below the journal bolt mounting surface can be closed by the oil filter. Accordingly, the lubricating oil passage passing through the oil filter can be formed without bypassing the journal support wall and the journal bolt. As a result, a friction can be reduced and it is unnecessary to avoid the mounting position of the journal bolt in mounting the oil filter. 
     According to an embodiment of the present invention, the wall portion for defining the space as a lubricating oil passage is formed below the journal bolt mounting surface of the lower crankcase half, and the tool insertion hole for insertion of the tool for tightening the journal bolt is formed through the wall portion. Accordingly, the oil pan to which the oil from the space is returned can be reduced in size. 
     Further, the wall portion is located laterally outside of the oil pan, and the wall portion is formed with the exit oil passage for discharging the lubricating oil from the space toward the oil pan. Accordingly, the lubricating oil from the space can be smoothly returned through the exit oil passage toward the oil pan. 
     Further, the space defined by the wall portion opens to one lateral side of the lower crankcase half, and the side cover is connected to the one lateral side of the lower crankcase half, wherein the lubricating oil returned from the side cover flows into the space through its side opening. Accordingly, the size of the oil pan can be sufficiently reduced in relation to the configuration for returning the lubricating oil from the side cover to the oil pan. 
     Further, the wall portion includes the detachable oil filter. Accordingly, the lubricating oil passage passing through the oil filter can be formed without bypassing the journal support wall and the journal bolt. As a result, friction can be reduced and it is unnecessary to avoid the mounting position of the journal bolt in mounting the oil filter. 
     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 right side view of an engine to which a preferred embodiment of the present invention is applied; 
         FIG. 2  is a sectional view showing the internal configuration of the engine; 
         FIG. 3  is a sectional side view showing a supporting structure for a crankshaft and its peripheral configuration; 
         FIG. 4  is a bottom plan view of a lower crankcase half; 
         FIG. 5  is a left side view of the lower crankcase half; 
         FIG. 6A  is a sectional side view showing a bolt insertion hole and its periphery in the lower crankcase half; and 
         FIG. 6B  is a front elevation of an oil filter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described with reference to the attached drawings. 
     In the following description, the terms in relation to directions, such as front, rear, right, left, upper, and lower are the same as those with respect to a vehicle to which the present invention is applied unless otherwise specified. Further, in the drawings, the arrow Fr denotes the front side of the vehicle, the arrow R denotes the right side of the vehicle, and the arrow Up denotes the upper side of the vehicle. 
       FIG. 1  is a right side view of an engine  1  to which a preferred embodiment of the present invention is applied. 
     The engine (which is referred to also as an internal combustion engine)  1  is a water-cooled, four-stroke, V-type, four-cylinder engine. For example, the engine  1  is mounted on a vehicle body of a motorcycle as a prime mover thereof in such a manner that the rotation center C 1  of a crankshaft  2  extends in a direction (lateral direction) perpendicular to the traveling direction of the vehicle. 
     The engine  1  has a crankcase  3 . A front bank  4  is provided at a front upper portion of the crankcase  3  so as to be inclined to the front side of the vehicle, and a rear bank  5  is provided just behind the front bank  4  so as to be inclined to the rear side of the vehicle. The crankshaft  2  is provided in a front portion of the crankcase  3 , and a transmission (not shown) is accommodated in a rear portion of the crankcase  3 . 
     A plurality of intake pipe mounting portions  8  connected to a throttle body (not shown) are provided on the rear surface of the front bank  4  and on the front surface of the rear bank  5 . A plurality of exhaust pipe mounting portions  9  connected to an exhaust pipe (not shown) are provided on the front surface of the front bank  4  and on the rear surface of the rear bank  5 . An oil pan  10  is mounted on the lower side of the crankcase  3 . 
     Each of the front and rear banks  4  and  5  includes a cylinder body  12  formed integrally with the front upper portion of the crankcase  3 , a cylinder head  13  mounted on the upper end of the cylinder body  12 , and a head cover  14  mounted on the upper end of the cylinder head  13 . Further, a piston  16  is reciprocatably fitted in a cylinder bore (not shown) formed in each cylinder body  12 . 
     In  FIG. 1 , a front engine mount  1 A, a rear upper engine mount  1 B, and a rear lower engine mount  1 C are integral with the crankcase  3 . 
       FIG. 2  is a sectional view showing the internal configuration of the engine  1 . 
     A connecting rod  17  is pivotably connected at its small end to each piston  16  (see  FIG. 1 ). The big end of each connecting rod  17  is rotatably connected to a pair of left and right crankpins  18  and  19  of the crankshaft  2 . In  FIG. 2 , reference symbol CL denotes the lateral center of the engine  1 . 
     The crankshaft  2  has the pair of left and right crankpins  18  and  19 . The two connecting rods  17  in the left two cylinders of the front and rear banks  4  and  5  are connected to the left crankpin  18 , and the two connecting rods  17  in the right two cylinders of the front and rear banks  4  and  5  are connected to the right crankpin  19 . Further, a generator  20  is coaxially provided at the left end of the crankshaft  2 . The generator  20  is covered with a crankcase side cover (which is referred to also as a generator cover)  21 L connected to the left side of the crankcase  3 . 
     Each cylinder head  13  (see  FIG. 1 ) closes the upper opening of each cylinder bore to form a combustion chamber in cooperation with each piston  16 . In this preferred embodiment, the engine  1  is of a DOHC four-valve type such that the ceiling portion of each combustion chamber is formed with two intake openings of an intake port and two exhaust openings of an exhaust port, wherein the two intake openings are operatively closed by two intake valves  24  for each cylinder and the two exhaust openings are operatively closed by two exhaust valves  25  for each cylinder. 
     The stem (shaft portion) of each intake valve  24  and the stem of each exhaust valve  25  extend toward the upper end of each cylinder head  13  so as to form a V-shape as viewed in side elevation. The upper end of the stem of each intake valve  24  is in sliding contact with each cam of an intake camshaft  26  through a valve lifter (not shown). Similarly, the upper end of the stem of each exhaust valve  25  is in sliding contact with each cam of an exhaust camshaft  27  through a valve lifter (not shown). These camshafts  26  and  27  extend parallel to the crankshaft  2  (in the lateral direction of the vehicle). The rotation of the camshafts  26  and  27  causes the reciprocation of the valves  24  and  25  along their stems, thereby operatively closing the openings of the intake and exhaust ports exposed to the combustion chambers. A valve operating mechanism including these valves  24  and  25  and the camshafts  26  and  27  as main components is accommodated in a valve operating chamber formed by the cylinder head  13  and the head cover  14  in each of the front and rear banks  4  and  5 . 
     A cam gear train mechanism (which is referred to also as a gear type timing mechanism)  29  is provided on the right side of the front and rear banks  4  and  5 . The camshafts  26  and  27  cooperate with the crankshaft  2  through the cam gear train mechanism  29  to thereby operate the intake and exhaust valves  24  and  25 . The cam gear train mechanism  29  is accommodated in a gear train chamber (timing chamber)  32  formed in each right portion of the front and rear banks  4  and  5 . Each gear train chamber  32  functions also as a return passage for engine oil (lubricating oil) supplied to each valve operating chamber. 
     As shown in  FIG. 2 , the crankshaft  2  has three crank journals  33 ,  34 , and  35  at left, central, and right portions, respectively. Three pairs of upper and lower journal support walls  110  and  120  are formed in the crankcase  3 , so as to support the three crank journals  33 ,  34 , and  35 . Thus, the three crank journals  33 ,  34 , and  35  are rotatably supported through three metal bearings  36   a ,  37   a , and  38   a  to the three pairs of upper and lower journal support walls  110  and  120 , respectively. 
     The left crankpin  18  is supported through a pair of left crank webs (crank arms)  39  between the left and central crank journals  33  and  34 . Similarly, the right crankpin  19  is supported through a pair of right crank webs  40  between the central and right crank journals  34  and  35 . 
     A primary drive gear  41  is coaxially provided on a right end portion of the crankshaft  2  adjacent to the right side of the right journal  35 , and a cam drive gear  44  is coaxially provided on the right end portion of the crankshaft  2  adjacent to the right side of the primary drive gear  41 . Further, a pulser rotor  42  is coaxially provided on the right end portion of the crankshaft  2  adjacent to the right side of the cam drive gear  44 . 
     The cam drive gear  44  constitutes a part of the cam gear train mechanism  29 . As shown in  FIG. 1 , a rotational drive force from the cam drive gear  44  is transmitted to a single second gear (idle gear)  45  located above the cam drive gear  44 . Thereafter, the rotational drive force from the second gear  45  is dividedly transmitted to a pair of front and rear third gears (idle gears)  46  located on the right side of the cylinder bodies  12  of the front and rear banks  4  and  5 . Thereafter, the rotational drive forces from the front and rear third gears  46  are respectively transmitted to a pair of front and rear fourth gears (idle gears)  47  located on the right side of the cylinder heads  13  of the front and rear banks  4  and  5 . Thereafter, the rotational drive forces from the front and rear fourth gears  47  are respectively transmitted to an intake cam drive gear  48  in the front bank  4  and an exhaust cam drive gear  49  in the rear bank  5 . In each of the front and rear banks  4  and  5 , the intake and exhaust cam drive gears  48  and  49  are coaxially fixed to the right end portions of the intake and exhaust camshafts  26  and  27 , respectively, in the condition where these gears  48  and  49  are in mesh with each other. 
     Thus, the second gear  45 , the third gears  46 , and the fourth gears  47  constitute idle gears for transmitting the rotational drive force from the cam drive gear  44  to the cam driven gears  48  and  49  in the front and rear banks  4  and  5 . Accordingly, the rotation of the cam drive gear  44  rotating with the crankshaft  2  is accurately transmitted through the plurality of idle gears  45 ,  46 , and  47  to the camshafts  26  and  27  in the front and rear banks  4  and  5 . 
     In the cam gear train mechanism  29 , the camshafts  26  and  27  are rotated 360° by 720° rotating the crankshaft  2  (i.e., the cam drive gear  44 ), and the ratio in number of gear teeth between the cam drive gear  44  and the second gear (idle gear)  45  is set to 1:2, thereby obtaining the above rotational speed ratio. 
     As shown in  FIG. 2 , an externally threaded portion  52  is formed at the right end portion of the crankshaft  2  adjacent to the right side of the pulser rotor  42 . The externally threaded portion  52  is adapted to threadly engage a nut for fixing the pulser rotor  42 , the cam drive gear  44 , and the primary drive gear  41 . Further, a cylindrical portion  53  smaller in diameter than the externally threaded portion  52  is formed at the right end portion of the crankshaft  2  adjacent to the right side of the externally threaded portion  52 . 
     The outer side (right side) of the cylindrical portion  53  in the lateral direction of the engine  1  is covered with a crankcase side cover  21 R connected to the right side of the crankcase  3 . 
     That is, the engine  1  has such a configuration that the left and right crankcase side covers  21 L and  21 R are respectively connected to the left and right sides of the crankcase  3 . 
     A left balancer drive gear  54  is coaxially provided on the left end portion of the crankshaft  2  adjacent to the left side of the left crank journal  33 . The left balancer drive gear  54  functions as a drive gear for driving a left balancer (not shown) provided in a front left portion of the crankcase  3 . 
     The left end portion of the crankshaft  2  adjacent to the left side of the left balancer drive gear  54  is formed as a tapered portion  55  tapered toward the left end of the crankshaft  2 . For example, an outer rotor of the generator  20  is fixedly engaged with the tapered portion  55 . An externally threaded portion  56  is formed at the left end portion of the crankshaft  2  adjacent to the left side of the tapered portion  55 . The externally threaded portion  56  is adapted to threadly engage a nut for fixing the generator  20 . Further, a cylindrical portion  57  smaller in diameter than the externally threaded portion  56  is formed at the left end portion of the crankshaft  2  adjacent to the externally threaded portion  56 . 
     As shown in  FIG. 1 , a main shaft  59 , a counter shaft  60 , and an output shaft  61  are rotatably supported to the rear portion of the crankshaft  3  so as to extend in the lateral direction of the vehicle. In  FIG. 1 , reference symbols C 4 , C 5 , and C 6  denote the rotation centers of the main shaft  59 , the counter shaft  60 , and the output shaft  61 , respectively. The main shaft  59  and the counter shaft  60  are shaft members for supporting shift gears in the transmission. 
     A primary driven gear  62  having a relatively large diameter is coaxially provided on a right end portion of the main shaft  59  so as to be rotatable relative to the main shaft  59 . The primary driven gear  62  is in mesh with the primary drive gear  41  (see  FIG. 2 ). Accordingly, the rotational drive force of the crankshaft  2  is transmitted through the primary drive gear  41 , the primary driven gear  62 , and a clutch mechanism (not shown) to the main shaft  59 . 
     The primary drive gear  41  functions also as a drive gear for driving a right balancer (not shown) provided in a front right portion of the crankcase  3 . 
     An oil pump  68  for feeding the engine oil to oil passages (lubricating oil passages) in the engine  1  is provided in a lower portion of the crankcase  3 . The oil pump  68  is operatively connected through a chain to a pump drive sprocket (not shown) coaxially fixed to the main shaft  59 , so that the oil pump  68  is operated by the rotation of the crankshaft  2 . An oil strainer  70  extends downwardly from the lower portion of the oil pump  68  toward the oil pan  10 . Further, a discharge passage  71  extends from the front portion of the oil pump  68  toward an oil filter  69  provided on the front side of the lower portion of the crankcase  3 . In the following description, the engine oil will be referred to simply as oil. 
     During operation of the engine  1 , the oil pump  68  is operated to suck the oil stored in the oil pan  10  through the oil strainer  70 . The oil is next discharged through the discharge passage  71  to the oil filter  69  and next fed to various portions to be lubricated in the engine  1 . The oil used for lubrication in the engine  1  is returned to the oil pan  10  and fed again by the oil pump  68 , thus circulating in the engine  1 . 
       FIG. 2  shows a part of the oil passages in the engine  1 . As shown in  FIG. 2 , an entrance oil passage  82  is provided in the lower portion of the crankcase  3  so as to span the length of the crankshaft  2 . An exit oil passage  83  is provided in the front upper portion of the crankcase  3  so as to span the length of the crankshaft  2 . 
     The oil passages  82  and  83  extend in the left and right side walls of the crankcase  3 , i.e., in the left and right crankcase side covers  21 L and  21 R to form tunnel-shaped passages, which are connected to left and right shaft end oil chambers  84  and  85  adjacent to the left and right ends of the crankshaft  2 . 
     The entrance oil passage  82  has a horizontal passage  86  extending below the crankshaft  2  over substantially the same length thereof in the lateral direction of the vehicle and a pair of vertical passages  87  extending upwardly from the opposite ends of the horizontal passage  86 . That is, the entrance oil passage  82  has a substantially U-shaped configuration opening to the upper side as viewed in  FIG. 2 . A guide passage  88  extending from the oil filter  69  is connected to a right portion of the horizontal passage  86 . In  FIG. 2 , the arrows denote the flow of the oil. 
     The exit oil passage  83  has a horizontal passage  89  extending above the crankshaft  2  (between the lower end portions of the front and rear banks  4  and  5 ) over substantially the same length thereof in the lateral direction of the vehicle and a pair of vertical passages  90  extending downwardly from the opposite ends of the horizontal passage  89 . That is, the exit oil passage  83  has a substantially inverted U-shaped configuration opening to the lower side as viewed in  FIG. 2 . The oil passages  82  and  83  communicate with each other through the shaft end oil chambers  84  and  85  to thereby form a loop oil passage surrounding the crankshaft  2  as viewed in  FIG. 2 . 
     Left, central, and right bearing oil passages  91 ,  92 , and  93  respectively extend from left, central, and right portions of the horizontal passage  89  of the exit oil passage  83  toward the left, central, and right upper journal support walls  110 . Further, a pair of front and rear bank oil passages  94  and  95  extend upwardly from the right portion of the horizontal passage  89  of the exit oil passage  83  along the front and rear banks  4  and  5 , respectively. 
     When the oil pump  68  is operated during running of the engine  1 , the oil is first fed into the entrance oil passage  82  and once supplied into the shaft end oil chambers  84  and  85 . Thereafter, the oil is supplied through left and right axial oil passages  101  and  105  to the left and right crankpins  18  and  19 , respectively, and also supplied through the exit oil passage  83  and the bearing oil passages  91 ,  92 , and  93  to the metal bearings  36   a ,  37   a , and  38   a , respectively. 
     Further, a part of the oil in the exit oil passage  83  is supplied through the front and rear bank oil passages  94  and  95  to the valve operating chambers in the front and rear banks  4  and  5 , respectively. 
     The left axial oil passage  101  is formed in the left portion of the crankshaft  2  so as to extend in the axial direction thereof. The left end of the left axial oil passage  101  is connected to the left shaft end oil chamber  84 , and the right end portion of the left axial oil chamber  101  is connected through oil passages  103  and  104  to the left crankpin  18 , thereby supplying the oil to the sliding surfaces of the left connecting rods  17 . 
     Similarly, the right axial oil passage  105  is formed in the right portion of the crankshaft  2  so as to extend in the axial direction thereof. The right end of the right axial oil passage  105  is connected to the right shaft end oil chamber  85 , and the left end portion of the right axial oil passage  105  is connected through oil passages  107  and  108  to the right crankpin  19 , thereby supplying the oil to the sliding surfaces of the right connecting rods  17 . 
     The crankcase  3  is composed of a pair of upper and lower cases that can be separated from each other. That is, the crankcase  3  is composed of an upper crankcase half  111  as the upper case and a lower crankcase half  121  as the lower case. The upper crankcase half  111  has the upper journal support walls  110  for supporting the upper halves of the crank journals  33 ,  34 , and  35  of the crankshaft  2 . Similarly, the lower crankcase half  121  has the lower journal support walls  120  for supporting the lower halves of the crank journals  33 ,  34 , and  35  of the crankshaft  2 . 
     In  FIG. 1 , a mating surface  3 A is provided between the upper crankcase half  111  and the lower crankcase half  121  (which is referred to also as a split surface of the crankcase  3 ). 
       FIG. 3  is a sectional side view showing a supporting structure for the crankshaft  2  and its peripheral configuration. As shown in  FIG. 3 , the left crank journal  33  of the crankshaft  2  is supported to the upper journal support wall  110  and the lower journal support wall  120  at the left portion of the crankcase  3 . More specifically, the upper journal support wall  110  is formed with a semicircular recess  110 A corresponding to the outer circumference of the upper half of the crank journal  33 , and the lower journal support wall  120  is formed with a semicircular recess  120 A corresponding to the outer circumference of the lower half of the crank journal  33 . The crank journal  33  is fitted in these recesses  110 A and  120 A of the upper and lower journal support walls  110  and  120 . Further, a pair of front and rear journal bolts  131  and  132  are engaged into the crankcase  3  from its lower side to fasten the upper and lower journal support walls  110  and  120 . 
     While the upper and lower journal support walls  110  and  120  corresponding to the left crank journal  33  are fastened together by using the journal bolts  131  and  132 , the other upper and lower journal support walls  110  and  120  corresponding to the other crank journals  34  and  35  are also fastened together by using similar journal bolts  131  and  132 . 
     That is, the three pairs of upper journal support walls  110  and lower journal support walls  120  for respectively supporting the three crank journals  33 ,  34 , and  35  of the crankshaft  2  are formed between the upper crankcase half  111  and the lower crankcase half  121  so as to be spaced in the lateral direction of the engine  1  (i.e., in the lateral direction of the vehicle). 
     Further, as shown in  FIG. 3 , the lower crankcase half  121  is formed with a bearing portion  120 B for supporting the counter shaft  60  (see  FIG. 1 ) on the rear side of the recess  120 A of the lower journal support wall  120 . 
       FIG. 4  is a bottom plan view of the lower crankcase half  121 . As shown in  FIG. 4 , a plurality of bolt insertion holes  122  and  123  for insertion of the journal bolts  131  and  132  are exposed to the bottom surface of the lower crankcase half  121 , so that the journal bolts  131  and  132  can be easily engaged and disengaged from the lower side of the crankcase  3 . 
     After the oil lubricates a required portion of the engine  1  (e.g., a cylinder portion (i.e., the front and rear banks  4  and  5  in this preferred embodiment)), the oil is passed through a return oil passage (not shown) formed in the left crankcase side cover  21 L (see  FIG. 2 ). 
     In this case, if the oil from the crankcase side cover  21 L is returned through the laterally outer side of the journal support walls  110  and  120  and the journal bolts  131  and  132  to the oil pan  10 , so as to avoid interference with the journal support walls  110  and  120  and the journal bolts  131  and  132 , the width of the oil pan  10  must be increased to cause an increase in size of the oil pan  10 . 
     To cope with this problem, as shown in  FIG. 3 , a wall portion  140  for defining a space X as a flow passage for the return oil from the crankcase side cover  21 L is formed below a journal bolt mounting surface  125  of the lower crankcase half  121 , and this wall portion  140  is formed with a tool insertion hole  145  for insertion of a tool for tightening the journal bolt  131 . 
     The wall portion  140  will now be described in more detail. 
       FIG. 5  is a left side view of the lower crankcase half  121 . 
     As shown in  FIG. 3 , the wall portion  140  projects downward from the front lower portion of the lower crankcase half  121  so as to define the space X below the journal bolt mounting surface  125  for mounting the front journal bolt  131  in the lower crankcase half  121 . Further, as shown in  FIG. 5 , the space X defined by the wall portion  140  opens on the left side of the lower crankcase half  121  where the left crankcase side cover  21 L is connected. 
     The wall portion  140  has a bottom wall  141  (see  FIGS. 3 and 5 ) forming the bottom surface of the space X and a vertical wall  142  (see  FIG. 4 ) forming a laterally inner surface (right surface) of the space X. The upper surface of the space X is formed by the journal bolt mounting surface  125 . 
     The wall portion  140  is formed integrally with the lower crankcase half  121  in forming the lower crankcase half  121  by casting. As viewed in side elevation of the engine  1 , the wall portion  140  is formed at a position above the oil filter  69  (see  FIG. 1 ) located on the front side of the lower portion of the lower crankcase half  121 . This position of the wall portion  140  is set behind the front engine mount  1 A (see  FIG. 1 ) and before the front surface of the oil pan  10 . That is, the space X defined by the wall portion  140  is located before the front surface of the oil pan  10 , above the oil filter  69 , and behind the front engine mount  1 A. 
     As viewed in left side elevation of the lower crankcase half  121  as shown in  FIG. 5 , the bottom wall  141  extends obliquely downwardly from the lower end of a front wall  121 A of the lower crankcase half  121  and further extends substantially horizontally toward the rear side. The crankcase side cover  21 L is connected to the end surface of the front wall  121 A and the bottom wall  141 . 
     In other words, the end surface of the front wall  121 A and the bottom wall  141  functions as a mating surface M 1  (see  FIG. 5 ) for the crankcase side cover  21 L. The exit of the return oil passage formed in the crankcase side cover  21 L is in communication with the space X, thereby introducing the oil from the return oil passage into the space X. 
     As shown in  FIG. 3 , the bottom wall  141  is inclined downwardly toward the rear side in the vicinity of the journal bolt  131 , and the rear portion of the bottom wall  141  is formed with a through hole (exit oil passage)  143  extending through the wall of the lower crankcase half  121  along the inclination of an upper surface  141 A of the bottom wall  141 . 
     The through hole  143  is located laterally inside of the mating surface M 1  for the crankcase side cover  21 L, and communicates with the space X and a space Y leading to the oil pan  10 . 
     Accordingly, the return oil from the crankcase side cover  21 L is introduced into the space X formed laterally inside of the mating surface M 1  for the crankcase side cover  21 L. Thereafter, the oil is returned from the space X through the through hole  143  formed laterally inside of the mating surface M 1  to the oil pan  10 . 
     In this case, the oil can be returned from the crankcase side cover  21 L to the oil pan  10  without passing a portion laterally outside of the journal support walls  110  and  120  and the journal bolts  131  and  132 . That is, the oil pan  10  can be located laterally inside of the space X. 
     Accordingly, the oil pan  10  can be located laterally inside of the mating surface M 1  for the crankcase side cover  21 L, so that the width of the oil pan  10  can be reduced. 
     In  FIG. 4 , a mating surface M 2  of the lower crankcase half  121  for the oil pan  10  is provided. As shown in  FIG. 4 , the position of the left side M 2 L of the mating surface M 2 , i.e., the position of the left side surface of the oil pan  10  is set just laterally inside of the mating surface M 1  for the crankcase side cover  21 L. However, the lateral position of the left side surface of the oil pan  10  may be further retracted from the mating surface M 1 . 
     As shown in  FIG. 3 , the bottom wall  141  is formed with a cylindrical portion  145 A extending downwardly at a position below the front journal bolt  131 . The tool insertion hole  145  for insertion of the tool for tightening the journal bolt  131  is formed in the cylindrical portion  145 A. More specifically, the tool insertion hole  145  is formed so as to allow the insertion of the journal bolt  131  and the insertion of the tool for tightening the journal bolt  131 . 
     Further, a plug (not shown) is normally fitted in the cylindrical portion  145 A (in any cases other than the case of disassembling the engine  1 ), thereby closing the tool insertion hole  145 . For example, by forming internal threads in the cylindrical portion  145 A, various general-purpose screw parts may be used as the above-mentioned plug. 
     As shown in  FIG. 4 , the vertical wall  142  functions as a partition wall for partitioning the inside space of the lower crankcase half  121  into the space X and the other space in the lateral direction of the engine  1 . 
     The vertical wall  142  extends in the longitudinal direction of the engine  1  at a position laterally inside of the cylindrical portion  145 A in the vicinity thereof, and the front end of the vertical wall  142  reaches the front wall  121 A of the lower crankcase half  121 . 
     Accordingly, the length of the space X in the longitudinal direction of the engine  1  can be made relatively large and the volume of the space X can therefore be enlarged. 
     Further, as shown in  FIGS. 3 and 4 , the bottom wall  141  extends in the longitudinal direction of the engine  1  so as to span the lower side of the cylindrical portion  145 A, and the front end of the bottom wall  141  reaches the front wall  121 A of the lower crankcase half  121 . In other words, the space X ranges from the front wall  121 A of the lower crankcase half  121  to the rear side of the cylindrical portion  145 A in the longitudinal direction of the engine  1 . 
     As described above, the space X ranges from the front wall  121 A of the lower crankcase half  121  to the rear side of the cylindrical portion  145 A in the longitudinal direction of the engine  1 . Accordingly, the space X elongated in the longitudinal direction of the engine  1  can be formed without changing the longitudinal size of the lower crankcase half  121 , thereby enlarging the volume of the space X. 
     Due to such a large volume of the space X, the momentum of the return oil flowing from the crankcase side cover  21 L can be suppressed in the space X. Accordingly, the space X functions as a buffer for the return oil and it is also expected to have an effect of eliminating bubbles produced in the return oil. 
     As described above, the wall portion  140  for defining the space X as a flow passage for the return oil is formed below the journal bolt mounting surface  125  of the lower crankcase half  121 , and the tool insertion hole  145  for insertion of a tool for tightening the journal bolt  131  is formed through the wall portion  140 . Accordingly, the journal bolt  131  can be tightened from the lower side of the crankcase  3 , and a return oil passage can be formed below the journal bolt mounting surface  125 . This return oil passage is located laterally inside of the mating surface M 1  for the crankcase side cover  21 L, so that the width of the oil pan  10  in the lateral direction of the engine  1  can be reduced. 
     Further, as shown in  FIGS. 3 to 5 , the space X and the oil pan  10  are shifted in position from each other in the longitudinal direction of the engine  1 , so that the length of the oil pan  10  in the longitudinal direction of the engine  1  can also be reduced. As a result, both the width and the length of the oil pan  10  can be reduced to thereby reduce the size of the oil pan  10 . 
     As described above, the return oil passage (return passage for the lubricating oil) is formed below the journal bolt mounting surface  125 . That is, the return oil passage can be formed without passing through the laterally outer side of the journal support walls  110  and  120  and the journal bolts  131  and  132 . Accordingly, an increase in friction due to such bypassing can be prevented. 
     Thus, it is possible to prevent an increase in friction due to formation of a bypass oil passage, and it is also possible to prevent an increase in weight due to an increase in size of the oil pan  10 . 
     Further, the return oil passage can be formed by utilizing a dead space below the journal bolt mounting surface  125 . That is, the dead space can be effectively utilized. Further, it is unnecessary to increase the width of the lower crankcase half  121  in forming the return oil passage, so that an increase in width of the crankcase  3  can be prevented. Accordingly, the wall portion  140  for defining the space X as the return oil passage can be easily added to an existing lower crankcase half without such a wall portion  140  by design change. 
     As described above, the tool insertion hole  145  for insertion of the tool for tightening the journal bolt  131  is formed through the wall portion  140 , and the tool insertion hole  145  is closed by a plug. Accordingly, a closing structure for the tool insertion hole  145  is simple, so that an increase in cost therefore can be minimized. 
     Further, the wall portion  140  is located laterally outside of the oil pan  10  of the engine (see  FIG. 4 ), and the wall portion  140  is formed with the through hole (exit oil passage)  143  for discharging the return oil from the space X toward the oil pan  10  (see  FIG. 3 ). Accordingly, the oil from the space X can be smoothly returned through the through hole  143  toward the oil pan  10 . 
     As described above, the wall portion  140  defines the space X opening to the left side of the lower crankcase half  121 , and the return oil from the crankcase side cover  21 L connected to the left side of the lower crankcase half  121  flows into the space X through its left side opening. Accordingly, the size of the oil pan  10  can be sufficiently reduced in relation to the configuration for returning the oil from the crankcase side cover  21 L to the oil pan  10 . 
     As shown in  FIG. 4 , the wall portion  140  for defining the space X as the return oil passage is formed below the journal bolt mounting surface  125  (the mounting surface for the journal bolt  131  to be inserted into the left bolt insertion hole  122 A) near the left crankcase side cover  21 L. In addition, a wall portion  150  for defining a space as an oil passage is also formed below another journal bolt mounting surface  125  (the mounting surface for the journal bolt  131  to be inserted into the right bolt insertion hole  122 B) near the right crankcase side cover  21 R. Further, the wall portion  150  is formed with a vertically extending tool insertion hole  155 . 
       FIG. 6A  is a sectional side view showing the bolt insertion hole  122 B and its periphery in the lower crankcase half  121 , and  FIG. 6B  is a front elevation of the oil filter  69 . 
     The wall portion  150  formed below the bolt insertion hole  122 B defines a discharge oil space Z through which the discharge oil fed from the oil pump  68  is adapted to flow. As shown in  FIG. 6A , the tool insertion hole  155  of the wall portion  150  is closed by the detachable oil filter  69 . That is, the oil filter  69  constitutes a part of the wall portion  150  for defining the discharge oil space Z below the journal bolt mounting surface  125 . 
     As shown in  FIG. 6B , an oil inset port  69 A is formed on one side surface (left side surface) of the oil filter  69 . As shown in  FIG. 6A , two filter cartridges  69 B are arranged in the form of two layers in the oil filter  69 . The oil from the oil pump  68  is supplied through the oil inlet port  69 A into the oil filter  69  and next passed through the filter cartridges  69 B. The oil cleaned by the filter cartridges  69 B is supplied to the discharge oil space Z defined by the wall portion  150  and next fed to the required portions to be lubricated in the engine  1 . 
     Thus, the discharge oil passage (discharge passage for the lubricating oil) is also formed by utilizing a dead space below the journal bolt mounting surface  125  on the right side of the engine  1 . That is, as described above, the return oil passage is formed by utilizing the dead space below the journal bolt mounting surface  125  on the left side of the engine  1 . Accordingly, the oil pan  10  can be shifted in position from the space X toward the rear side, thereby reducing the length of the oil pan  10  in the longitudinal direction of the engine  1 . Since the oil pan  10  is shifted in position toward the rear side, a dead space is also formed below the right journal bolt mounting surface  125  on the front side of the oil pan  10 , and this dead space is utilized to form the discharge oil passage. 
     Accordingly, the discharge oil passage passing through the oil filter  69  can be formed without bypassing the journal support walls  110  and  120  and the journal bolts  131  and  132 . As a result, a friction can be reduced and it is unnecessary to avoid the mounting position of the journal bolt  131  in mounting the oil filter  69 . Accordingly, the flexibility to layout and shape of the oil filter  69  can be improved. 
     As shown in  FIG. 6A , the tool insertion hole  155  has a substantially rectangular opening directed obliquely downward toward the front side of the vehicle. A rectangular frame like filter mounting portion  156  projects frontward from the periphery of the front opening of the tool insertion hole  155 , and the oil filter  69  is mounted on the filter mounting portion  156  by means of a plurality of bolts (not shown). Accordingly, the oil filter  69  is connected to the lower crankcase half  121  so as to close the tool insertion hole  155 . 
     As shown in  FIG. 6A , the lower end of the bolt insertion hole  122 B opens to the tool insertion hole  155 , so that the journal bolt  131  and a bolt tightening tool can be easily applied to the bolt insertion hole  122 B. Further, the oil filter  69  can be mounted on the front surface of the lower crankcase half  121  so as to be raised substantially vertically, thereby reducing the length of the lower portion of the engine  1  including the oil filter  69  in the longitudinal direction of the engine  1 . 
     In  FIG. 6B , a plurality of bolt inserting portions  69 C are spaced from each other along the periphery of the oil filter  69 . The oil filter  69  is connected to the lower crankcase half  121  by means of the bolts inserted into the bolt inserting portions  69 C. 
     Having thus described a specific preferred embodiment of the present invention, it should be noted that the present invention is not limited to this preferred embodiment. For example, while the present invention is applied to a water-cooled, four-stroke, V-type, four-cylinder engine in this preferred embodiment, the present invention may be applied to any other types of internal combustion engines. 
     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.