Abstract:
To simplify the connection of a cylinder barrel and a cylinder head to, a crankcase and to reduce the weight of an engine. A liquid-cooled engine is provided with the crankcase wherein a cylinder barrel having a water jacket on the cylinder side for cooling is connected to the crankcase. A cylinder head is connected to the cylinder barrel. A plurality of mounting bosses extend from fitting planes to a crankcase of the cylinder barrels and to cylinder heads that are integrated with cylinder blocks. The cylinder barrels and the cylinder heads are integrated in a state in which the mounting bosses encircle the cylinder bores and the cylinder blocks are fastened to the crankcase by bolts inserted into each mounting boss.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2003-279247 filed on Jul. 24, 2003 the entire contents thereof is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a liquid-cooled engine provided with a crankcase that supports a crankshaft so that the crankshaft can be rotated, a cylinder barrel having a cylinder bore and a water jacket on the cylinder side for cooling. The water jacket encircles the cylinder bore and is connected to the crankcase. A cylinder head includes a water jacket on the head side communicating with the water jacket on the cylinder side and is connected to the cylinder barrel. 
   2. Description of Background Art 
   A liquid-cooled engine is disclosed in JP-A-2002-213302. In the conventional type liquid-cooled engine, the cylinder barrel and the cylinder head are separately formed. The work necessary for connecting the cylinder barrel and the cylinder head to the crankcase is troublesome. In addition, to maintain the sealing performance of a gasket inserted between the cylinder barrel and the cylinder head, a bolt for fastening the cylinder barrel and the cylinder head is required, the number of parts increases, and the weight of the engine increases. 
   The present invention is made in view of situation discussed above. It is an object of the present invention to provide a liquid-cooled engine in which the connection of a cylinder barrel and a cylinder head to a crankcase is simplified and which can contribute to a reduction in weight. 
   To achieve the object, the present invention provides a liquid-cooled engine with a crankcase that supports a crankshaft so that the crankshaft can be rotated. A cylinder barrel includes a cylinder bore and a water jacket on the cylinder side for providing a cooling fluid that encircles the cylinder bore and is connected to the crankcase. A cylinder head includes a water jacket on the head side that communicates with the water jacket on the cylinder side and is connected to the cylinder barrel. A plurality of mounting bosses extend from a fitting plane to the crankcase of the cylinder barrel to the cylinder head and are integrated with a cylinder block in which the water jacket on the cylinder side and the water jacket on the head side mutually communicate. The cylinder barrel and the cylinder head are integrated in a state in which the mounting bosses encircle the cylinder bore and the cylinder block is fastened to the crankcase by bolts inserted into each mounting boss. 
   The present invention provides a coupling wall for coupling at least one set of the mounting bosses out of the mutually adjacent two sets of a pair of mounting bosses between the cylinder bores mutually adjacent to each other in an axial direction of the crankshaft and being integrated with the cylinder block having the plurality of cylinder bores arranged in the axial direction of the crankshaft. 
   Further, the present invention provides rod guide pipes wherein each rod to which power from the crankshaft is transmitted and which configures a part of a valve system is inserted so that the rod can be axially moved and the coupling wall are arranged on sides of both cylinder blocks. 
   According to the present invention, as the cylinder block is composed of the cylinder barrel and the cylinder head which are integrated and the cylinder block is fastened to the crankcase, the connection of the cylinder barrel and the cylinder head to the crankcase can be simplified. No gasket is required to be positioned between the cylinder barrel and the cylinder head. No bolt for is required for maintaining the sealing performance of the gasket. The number of parts may be reduced. Thus, the weight of the engine can be reduced. In addition, the circumferences of the cylinder bore can be sufficiently reinforced by the plurality of mounting bosses encircling the cylinder bore so that a fastening load of the bolts inserted into the mounting bosses can be born. 
   According to the present invention, no bolt is required to be arranged between each cylinder bore. Thus, the strength of the cylinder barrel between the cylinder bores can be increased by the coupling wall, distance between the cylinder bores in the axial direction of the crankshaft can be reduced, and the engine can be miniaturized. 
   Further, according to the present invention, the balance in rigidity between the cylinder blocks is enhanced and the occurrence of distortion relative to the inside diameter of the cylinder bore with which the cylinder block is provided can be inhibited. 
   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 side view showing an engine equivalent to a first embodiment; 
       FIG. 2  is a plan view wherein a part of which is cut off showing the engine; 
       FIG. 3  is an enlarged front view viewed from a direction shown by an arrow  3  in  FIG. 1 ; 
       FIG. 4  is a plan showing the body of the engine; 
       FIG. 5  is a bottom view showing the body of the engine; 
       FIG. 6  is a sectional view viewed along a line  6 — 6  in  FIG. 3 ; 
       FIG. 7  is a sectional view viewed along a line  7 — 7  in  FIG. 4 ; 
       FIG. 8  is an enlarged sectional view viewed along a line  8 — 8  in  FIG. 4 ; 
       FIG. 9  is a schematic drawing in which the engine is viewed from the rear side to show a circulating system of cooling water; 
       FIG. 10  is a sectional view viewed along a line  10 — 10  in  FIG. 9 ; and 
       FIG. 11  shows a second embodiment corresponding to  FIG. 9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIGS. 1 through 10  show a first embodiment wherein the present invention is applied to a four-cycle horizontally opposite four-cylinder engine. 
   In  FIGS. 1 through 3 , the four-cycle horizontally opposite four-cylinder engine is mounted on an airplane, for example. The four-cycle horizontally opposite four-cylinder engine is housed in a front cowl of the airframe of the airplane with the axis of a crankshaft  11  extending longitudinally of the airplane, and a spinner having a plurality of propellers is coaxially coupled to the crankshaft  11 . 
   As also shown in  FIG. 4 , the engine has an engine body  12  including a left engine block  13 L disposed on the left side of the engine as viewed from behind and a right engine block  13 R disposed on the right side of the engine as viewed from behind. 
   The left engine block  13 L includes a left crankcase  14 L and a left cylinder block  15 L coupled to the left crankcase  14 L. The right engine block  13 R includes a right crankcase  14 R coupled to the left crankcase  14 L and a right cylinder block  15 R coupled to the right crankcase  14 R remotely from the left crankcase  14 L. 
   The left cylinder block  15 L includes a left cylinder barrel  16 L coupled to the left crankcase  14 L and a left cylinder head  17 L integrally formed with the left cylinder barrel  16 L remotely from the left crankcase  14 L. The right cylinder block  15 R includes a right cylinder barrel  16 R coupled to the right crankcase  14 R and a right cylinder head  17 R integrally formed with the right cylinder barrel  16 R remotely from the right crankcase  14 R. 
   As also shown in  FIGS. 5 and 6 , the cylinder barrels  16 L,  16 R of the cylinder blocks  15 L,  15 R have respective pairs of cylinder bores  18 L,  18 L;  18 R,  18 R disposed on both sides of the crankshaft  11  and confronting each other. The cylinder bores  18 L,  18 L;  18 R,  18 R are arrayed in the axial direction of the crankshaft  11  and offset with respect to each other in the axial direction of the crankshaft  11 . Pistons  20 L . . . ,  20 R . . . , which define combustion chambers  19 L . . . ,  19 R . . . between the pistons and the cylinder heads  17 L,  17 R, are slidably fitted in the respective cylinder bores  18 L . . . ,  18 R . . . . 
   The engine blocks  13 L,  13 R are arranged in opposed relation to each other with the axes of the cylinder bores  18 L . . . ,  18 R . . . being disposed substantially horizontally. The left and right crankcases  14 L,  14 R are fastened to each other to jointly make up a crankcase  21 . The crankshaft  11 , connected to the pistons  20 L . . . ,  20 R . . . by connecting rods  22 L . . . ,  22 R . . . , is rotatably supported between the left and right crankcases  14 L,  14 R. 
   The left crankcase  14 L has a front journal support wall  23 L, a first intermediate journal support wall  24 L, a second intermediate journal support wall  25 L, a third intermediate journal support wall  26 L and a rear journal support wall  27 L. The walls support a left half of the crankshaft  11  on both the front and rear sides of the connecting rods  22 L . . . and are longitudinally spaced from each other. The right crankcase  14 R has a front journal support wall  23 R, a first intermediate journal support wall  24 R, a second intermediate journal support wall  25 R, a third intermediate journal support wall  26 R and a rear journal support wall  27 R. The walls support a right half of the crankshaft  11  on both the front and rear sides of the connecting rods  22 R . . . and are longitudinally spaced from each other. The crankshaft  11  is rotatably supported by the journal support walls  23 L through  27 L of the left crankcase  14 L and the journal support walls  23 R through  27 R of the right crankcase  14 R. 
   The journal support walls  23 L through  27 L and  23 R through  27 R of the left and right crankcases  14 L,  14 R are fastened by pairs of stud bolts  28  . . . and nuts  29  . . . , which extend vertically across the crankshaft  11 . 
   The stud bolts  28  . . . for fastening the front journal support walls  23 L,  233 R and the rear journal support walls  27 L,  24 R are longer than the stud bolts  28  . . . for fastening the first, second, and third intermediate journal support walls  24 L through  26 L;  24 R through  26 R. 
   The nuts  29  . . . engage an outer surface of the right crankcase  14 R and are threaded over the stud bolts  28  . . . which are mounted on the front journal support wall  23 L of the left crankcase  14 L and inserted through the front journal support wall  23 R of the right crankcase  14 R. The nuts  29  . . . engage an outer surface of the left crankcase  14 L and are threaded over the stud bolts  28  . . . which are mounted on the rear journal support wall  27 R of the right crankcase  14 R and inserted through the rear journal support wall  27 L of the left crankcase  14 L. 
   The nuts  29  . . . are threaded over the stud bolts  28  . . . that are mounted on the second and third intermediate journal support walls  25 L,  26 L of the left crankcase  14 L and are inserted through the second and third intermediate journal support walls  25 R,  26 R of the right crankcase  14 R. The nuts  29  . . . are held in engagement with the second and third intermediate journal support walls  25 R,  26 R. The nuts  29  . . . are threaded over the stud bolts  28  . . . that are mounted on the first intermediate journal support wall  24 R of the right crankcase  14 R and inserted through the first intermediate journal support wall  24 L of the left crankcase  14 L. The nuts  29  . . . are held in engagement with the first intermediate journal support walls  24 L. 
   The left and right engine blocks  13 L,  13 R are coupled to each other by pairs of through bolts  30  . . . and pairs of two sets of stud bolts  32  . . . that are disposed in portions corresponding to the first, second, and third intermediate journal support walls  24 L through  26 L and  24 R through  26 R of the crankcases  14 L,  14 R. 
   The through bolts  30  . . . extend through the left and right engine blocks  13 L,  13 R in such a manner to sandwich, between themselves and the crankshaft  11 , the pairs of stud bolts  28  . . . . The stud bolts  28  are disposed on the first, second, and third intermediate journal support walls  24 L through  26 L and  24 R through  26 R in order to fasten the support walls  24 L through  26 L and  24 R through  26 R to each other. Nuts  31  . . . are threaded over the opposite ends of the through bolts  30  . . . , which project from the cylinder heads  17 L,  17 R of the left and right engine blocks  13 L,  13 R. In order to prevent the through bolts  30  . . . from rotating when the nuts  31  . . . are tightened, hexagonal tool engaging portions  30   a  for engagement with a tool (not shown) are coaxially disposed on the opposite ends of the respective through bolts  30  . . . so as to project from the nuts  31  . . . . 
   Of the two sets of stud bolts  32  . . . , one set of stud bolts  32  . . . is mounted on the front journal support wall  23 L of the left crankcase  13 L and extends through the right engine block  13 R and nuts  33  . . . are threaded over the stud bolts  32  . . . , which project from the cylinder head  17 R of the right engine block  13 R. Of the two sets of stud bolts  32  . . . , the other set of stud bolts  32  . . . is mounted on the rear journal support wall  27 R of the right crankcase  13 R and extends through the left engine block  13 L, and nuts  33  . . . are threaded over the stud bolts  32  . . . which project from the cylinder head  17 L of the left engine block  13 L. 
   The stud bolts  32  . . . are disposed in positions for sandwiching, between themselves and the crankshaft  11 , the pair of stud bolts  28  . . . fastening the front journal support walls  23 L,  23 R of the left and right crankcases  13 L,  13 R and the pair of stud bolts  28  . . . fastening the rear journal support walls  27 L,  27 R of the left and right crankcases  13 L,  13 R. 
   As shown in  FIG. 7 , the through bolts  30  . . . and the stud bolts  32  . . . are disposed in a surrounding relation to the cylinder bores  18 L . . . ,  18 R . . . at 90°-spaced intervals. The cylinder blocks  13 L,  13 R have a plurality of integral mounting bosses  34  . . . for the through bolts  30  . . . and the stud bolts  32  . . . to extend therethrough. The mounting bosses  34  . . . extend from the surfaces of the cylinder barrels  16 L,  16 R, which are attached to the crankcase  21 , to the cylinder heads  17 L,  17 R and surround the cylinder bores  18 L . . . ,  18 R . . . . 
   Joint walls  35  . . . are integrally mounted on the cylinder blocks  13 L,  13 R as shown in  FIG. 8 . The joint walls  35  . . . join at least one of the two adjacent sets of the mounting bosses  34 ,  34 , which are disposed on corresponding portions between the mutually adjacent cylinder bores  18 L,  18 L;  18 R,  18 R arrayed in the axial direction of the crankshaft  11 . The pair of mutually adjacent mounting bosses  34 ,  34  are on upper walls of the cylinder blocks  13 L,  13 R in the first embodiment. 
   A support tube  38 , which is jointly made up of the left and right crankcases  14 L,  14 R, is formed so as to project forwardly on a front portion of the crankcase  21 . The crankshaft  11  has a front portion extending coaxially through the support tube  38  and projecting from the front end of the support tube  38 . A ring gear  39  is fixed to the portion of the crankshaft  11 , which projects from the front end of the support tube  38 . The spinner (not shown) is coaxially mounted on the ring gear  39 . A slide bearing  40  is interposed between the front portion of the support tube  38  and the crankshaft  11 , and an annular seal member (not shown) is interposed between the support tube  38  and the crankshaft  11  forwardly of the slide bearing  40 . 
   For starting the engine, a starter  41  applies a rotational drive force to the crankshaft  11 . The starter  41  includes a starter motor  42  and a pinion  43 . The motor  42  is supported on a lower portion of the left crankcase  14  of the crankcase  21 . The pinion  43  projects into mesh with the ring gear  39  when the rotational speed of the starter motor  42  becomes a predetermined value or higher. After the engine has started to operate, the pinion  43  is released out of mesh with the ring gear  39  back into its original position. 
   The crankshaft  11  has a plurality of circumferentially spaced teeth  44  within the support tube  38 . A pair of crankshaft angle sensors  45 ,  45 , for detecting a crankshaft angle, is mounted on the support tube  38  by the projections  44  . . . in 180°-spaced relation to each other. 
   As also shown in  FIGS. 9 and 10 , a water pump  46 , which can be rotated by the crankshaft  11 , is mounted on an end of the crankcase  21  along the axis of the crankshaft  11 , i.e., a rear end of the crankcase  21  in the first embodiment. 
   A drive gear  47  is coaxially mounted on a rear end of the crankshaft  11 , which projects from the rear journal support walls  27 L,  27 R. A rotor  49  of a generator  48 , which is mounted in a rear portion of the crankcase  21 , is coaxially and relatively immovably connected to the drive gear  47 . A cover  51  is mounted on the rear end of the crankcase  21 . The generator  48  has a stator  50  mounted on the cover  51 . 
   The water pump  46  has a pump housing  54  including a case  52 , which integrally has a cylindrical shaft support  52   a  that is fitted in the cover  51  in a light-tight manner and a pump cover  53  sandwiching the case  52  between itself and the cover  51 . The case  52  and the pump cover  53  are fastened together to the cover  51 . 
   A pump shaft  55 , which extends through the shaft support  52   a  in a light-tight manner, is rotatably supported by the shaft support  52   a  . An end of the pump shaft  55 , which projects from the shaft support  52   a , is rotatably supported by the crankcase  21 . Rotary vanes  57  are fixed to the other end of the pump shaft  55  within a pump chamber  56  that is defined in the pump housing  54 . A driven gear  58 , which is fixed to the pump shaft  55  between the shaft support  52   a  and the crankcase  21 , is held in mesh with an idle gear  59  that is rotatably supported between the crankcase  21  and the cover  51 . The idle gear  59  is in mesh with the drive gear  47 . 
   Cooling cylinder water jackets  60 L,  60 R are disposed in the respective cylinder barrels  16 L,  16 R. Head water jackets  61 L,  61 R communicating respectively with the cylinder water jackets  60 L,  60 R are disposed in the respective cylinder heads  17 L,  17 R, which are integrally formed with the cylinder barrels  16 L,  16 R. The water pump  46  serves to circulate a coolant between the cylinder and head water jackets  60 L,  60 R;  61 L,  61 R. The crankcase  21  has coolant supply passages  62 L,  62 R for guiding the coolant from the water pump  46  and coolant return passages  63 L,  63 R for guiding the coolant that is delivered out of the cylinder water jackets  60 L,  60 R. The coolant supply passages  62 L,  62 R and the coolant return passages  63 L,  63 R are disposed parallel to the axis of the crankshaft  11  on both sides of the axes of the cylinder bores  18 L . . . ,  18 R . . . . 
   The cylinder and head water jackets  60 L,  60 R;  61 L,  61 R are formed such that the coolant supplied from the coolant supply passages  62 L,  62 R returns from the cylinder water jackets  60 L,  60 R via the head water jackets  61 L,  61 R to the cylinder water jackets  60 L,  60 R. The cylinder water jackets  60 L,  60 R are divided into supply jacket portions  64  . . . and return jacket portions  65  . . . . The supply jacket portions  64  . . . communicate with the coolant supply passages  62 L,  62 R and also with the head water jackets  61 L,  61 R. The return jacket portions  65  . . . communicate with the head water jackets  61 L,  61 R at positions spaced from the supply jacket portions  64  . . . and are defined in the cylinder barrels  16 L,  16 R. 
   The cover  51  joined to the crankcase  21  has a passage  66  for guiding the coolant discharged from the pump chamber  56  of the water pump  46  to the coolant supply passages  62 L,  62 R. 
   The supply jacket portions  64  and the return jacket portions  65  . . . , with superposed portions surrounding substantial half of the cylinder bores  18 L . . . ,  18 R . . . , are formed in the cylinder barrels  16 L,  16 R. A plurality of supply and return branch passages  67 L,  67 R;  68 L,  68 R are disposed in the crankcase  21  and the cylinder barrels  16 L,  16 R. The supply and return branch passages  67 L,  67 R;  68 L,  68 R are connect the portions of the supply jacket portions  64  . . . and the return jacket portions  65  . . . , which correspond to the cylinder bores  18 L . . . ,  18 R . . . , to the coolant supply passages  62 L,  62 R and the coolant return passages  63 L,  63 R. 
   The coolant supply passages  62 L,  62 R are reduced in diameter stepwise in a direction away from the water pump  46 . The inside diameter of the supply passages  62 L,  62 R in the cylinder bores  18 L,  18 R that is most remote from the water pump  46  is smaller than the inside diameter thereof in the cylinder bores  18 L,  18 R closer to the water pump  46 . 
   When the cylinder blocks  13 L,  13 R are cast, the cylinder water jackets  60 L,  60 R are formed within the cylinder barrels  16 L,  16 R as ring-shaped cavities surrounding the respective cylinder bores  18 L . . . ,  18 R . . . . Rod members  69  . . . are fitted into the cylinder barrels  16 L,  16 R from the cylinder heads  17 L,  17 R so as to lie on a straight line interconnecting the axes of the cylinder bores  18 L . . . ,  18 R . . . . The rod members  69  . . . divide the cavities halfway into the supply jacket portions  64  . . . and the return jacket portions  65  . . . . 
   The rod members  69  . . . have intermediate portions smaller in diameter for forming an annular passage  70  for removing air from the supply jacket portions  64  . . . into the return jacket portions  65  . . . , between themselves and the cylinderbarrels  16 L,  16 R. 
   Plug members  105  . . . ,  106  . . . , for preventing the rod members  69  . . . from being released from.the cylinder heads  17 L,  17 R, are threaded in the cylinder heads  17 L,  17 R in abutment against the rod members  69  . . . . 
   Referring to  FIG. 9  in particular, the coolant return passages  63 L,  63 R communicate with each other through a joint pipe  71 . A first return conduit  72  communicates with the coolant return passage  63 L and is connected to the crankcase  21  for guiding the coolant to a radiator (not shown). The coolant, which returns from the radiator, is guided to a second return conduit  74 . A thermostat  73  for guiding the coolant from the first return conduit  72  to the second return conduit  74  by bypassing the radiator when the temperature of the coolant is low is disposed between the first and second return conduits  72 ,  74 . The second return conduit  74  is connected to a return joint pipe  75  that is joined to the pump cover  53  of the water pump  46 . 
   A steam conduit  76 , for guiding a steam evaporated by heating into an expansion tank  77 , is connected to an upper portion of the first return conduit  72 . A third return pipe  78  for guiding the coolant that is condensed in the expansion tank  77  is connected to a return joint pipe  79  that is joined to the pump cover  53  of the water pump  46 . An oil filter  81  is disposed laterally of an oil pan  80  mounted on a lower portion of the crankcase  21 . The oil filter  81  houses therein an oil cooler that is supplied with the coolant from the water pump  46  via a supply conduit  82 . The coolant from the oil cooler is returned to the water pump  46  via a return conduit  83 . 
   Intake ports  85 L . . . ,  85 R . . . corresponding individually to the combustion chambers  9 L . . . ,  19 R . . . are defined in upper portions of the left and right cylinder heads  17 L,  17 R. The intake ports  85 L . . . ,  85 R . . . are bifurcated and communicate with the combustion chambers  19 L . . . ,  19 R . . . . 
   Arcuately curved intake pipes  86 L . . . ,  86 R . . . are connected respectively to the intake ports  85 L . . . ,  85 R . . . . Electromagnetic fuel injector valves  87 L . . . ,  87 R . . . for injecting a fuel into the intake ports  85 L . . . ,  85 R . . . are mounted respectively in intermediate portions of the intake pipes  86 L . . . ,  86 R . . . . The electromagnetic fuel injector valves  87 L . . . in the left engine block  13 L are connected to a common fuel rail  88 L, and the electromagnetic fuel injector valves  87 R . . . in the right engine block  13 R are connected to a common fuel rail  88 R. 
   An intake chamber  89  is disposed above the crankcase  21  of the engine body  12  and supported by the engine body  12 . The intake pipes  86 L . . . ,  86 R . . . have upstream ends connected to downstream ends of joint pipes  90 L . . . ,  90 R . . . , which have upstream ends projecting into the intake chamber  89  from both sides thereof. In the intake chamber  89 , the upstream ends of the joint pipes  90 L  90 R . . . are spread into a flaring shape and open rearwardly. 
   Throttle bodies  92 ,  92  each having a throttle valve  91  angularly movably supported therein with downstream ends juxtaposed and connected to a rear portion of the intake chamber  89 . Air cleaners  93 ,  93  are connected respectively to upstream ends of the throttle bodies  92 ,  92 . The air cleaners  93 ,  93  are supported on support stays  94 ,  94 , which are mounted on the intake chamber  89  and extend rearwardly. 
   Exhaust ports  95 L . . . ,  95 R . . . , which correspond individually to the combustion chambers  19 L . . . ,  19 R . . . , are defined in lower portions of the left and right cylinder heads  17 L,  17 R. Exhaust pipes  96 L . . . ,  96 R . . . extending below the engine body  12  and rearwardly are connected respectively to the exhaust ports  95 L . . . ,  95 R . . . . 
   Substantially H-shaped head covers  97 L,  97 R are joined respectively to the left and right cylinder heads  17 L,  17 R. Valve operating devices (not shown) for actuating intake valves and exhaust valves to control the introduction of intake air into the combustion chambers  19 L . . . ,  19 R . . . and the discharge of exhaust gases from the combustion chambers  19 L . . . ,  19 R . . . are disposed between the head covers  97 L,  97 R and the cylinder heads  17 L,  17 R. Covers  98 L,  98 R are fastened to upper portions of the head covers  97 L . . . . The covers  98 L,  98 R cover intake valve operating portions of the valve operating devices. Covers  99 L,  99 R, which cover exhaust valve operating portions of the valve operating devices, are fastened to lower portions of the head covers  97 L . . . . 
   The intake valve operating portions of the valve operating devices, which are disposed between the head covers  97 L,  97 R and the cylinder heads  17 L,  17 R, produce valve opening drive forces with push rods that are pushed upwardly in the intake stroke by the power transmitted from the drive gear  47  of the crankshaft  11 . The push rods associated with the respective combustion chambers  19 L . . . ,  19 R . . . are axially movably inserted in rod guide tubes  100 L,  100 R. The tubes  100 L,  100 R are disposed below the cylinder blocks  15 L,  15 R on the left and right sides of the crankcase  21  and interconnecting longitudinally central portions of the lower portions of the left and right crankcases  14 L,  14 R and the head covers  97 L,  97 R. 
   The exhaust valve operating portions of the valve operating devices, which are disposed between the head covers  97 L,  97 R and the cylinder heads  17 L,  17 R, produce valve opening drive forces with pull rods that are pulled downwardly in the exhaust stroke by the power transmitted from the drive gear  47  of the crankshaft  11 . The pull rods associated with the respective combustion chambers  19 L . . . ,  19 R . . . are axially movably inserted in rod guide tubes  101 L,  101 R. The tubes  101 L,  101 R are disposed below the rod guide tubes  100 L,  100 R and for interconnecting the longitudinally central portions of the lower portions of the left and right crankcases  14 L,  14 R and the head covers  97 L,  97 R. 
   Thus, the rod guide tubes  100 L,  100 R,  101 L . . . ,  101 R . . . are disposed to interconnect the longitudinally central portions of the lower portions of the left and right crankcases  14 L,  14 R and the head covers  97 L,  97 R. The pair of mutually adjacent mounting bosses  34 ,  34  on the upper wall of the cylinder blocks  13 L,  13 R are connected by the joint walls  35  . . . and are integral with the cylinder blocks  13 L,  13 R. The rod guide tubes  100 L,  100 R,  101 L . . . ,  101 R . . . and the joint walls  35  are disposed on upper and lower sides of the cylinder blocks  13 L,  13 R. 
   Pairs of ignition plugs  102 L,  102 L . . . ,  102 R,  102 R . . . , which are associated with the respective combustion chambers  19 L . . . ,  19 R . . . , are mounted in the cylinder heads  17 L,  17 R. Ignition coils  103 L . . . ,  103 R . . . as electric accessories are mounted on upper side surfaces of the cylinder. heads  17 L,  17 R between the intake pipes  86 L,  86 L;  86 R,  86 R. The ignition coils  103 L  103 R . . . are disposed by a pair on each side of the intake chamber  89 . Pairs of high-tension cords  104  . . . connected to the ignition coils  103 L . . . ,  103 R . . . are connected to the ignition plugs  102 L,  102 L . . . ,  102 R,  102 R . . . . 
   To allow the fuel to be reliably ignited in the combustion chambers  19 L . . . ,  19 R . . . even in the event that one of the ignition coils  103 L . . . ,  103 R . . . malfunctions, a pair of high-tension cords  104 ,  104  connected to the same ignition coils  103 L . . . ,  103 R . . . is connected to the ignition plugs  102 L . . . ,  102 R . . . of the different combustion chambers  19 L . . . ,  9 R . . . . 
   An electronic control unit  105 ′ for controlling the operation of the engine is mounted on the outer surface of a front side wall of the intake chamber  89 . An intake pressure sensor  106 ′ and an intake temperature sensor  107  are inserted from the electronic control unit  105 ′ into the intake chamber  89  through the front side wall of the intake chamber  89 . The intake pressure sensor  106 ′ and an intake temperature sensor  107  is for detecting the intake pressure and temperature, respectively, in the intake chamber  89 . 
   The electromagnetic fuel injector valves  87 L . . . ,  87 R . . . , the ignition coils  103 L . . . ,  103 R . . . , and the electronic control unit  105 ′ are disposed around the intake chamber  89 . The electromagnetic fuel injector valves  87 L . . . ,  87 R . . . , the ignition coils  103 L . . .  103 R . . . and the electronic control unit  105 ′ are covered with a shield cover  108 , which is mounted on the engine body  12  in a covering relationship to at least a portion of the intake chamber  89 . 
   In the first embodiment, the shield cover  108  is made of a steel sheet, for example, in a covering relationship to a substantial portion of the intake chamber  89  except a rear portion thereof and an upper portion of the engine body  12 . The shield cover  108  has an opening edge formed in contact with the engine body  12 . Portions of the high-tension cords  104  extending from the ignition coils  103 L . . .  103 R . . . are also covered with the shield cover  108 . 
   Since the electromagnetic fuel injector valves  87 L . . . ,  87 R . . . , the ignition coils  103 L . . . ,  103 R . . . , and the electronic control unit  105 ′ are covered with the single shield cover  108 , the electric accessories can be shielded. The number of parts used is reduced and the overall engine is made more compact than if the electric accessories are individually shielded. As the portions of the high-tension cords  104  . . . are covered with the shield cover  108 , those portions of the shield cover  108  may have their individual shields removed. Therefore, a secondary voltage drop across the high-tension cords  104  . . . may be improved by removing the individual shields. 
   Thus, the electronic control unit  105 ′ is mounted on the outer surface of the front side wall of the intake chamber  89 . Further, the intake pressure sensor  106 ′ and the intake temperature sensor  107  for detecting the intake pressure and temperature, respectively, in the intake chamber  89  are inserted from the electronic control unit  105 ′ into the intake chamber  89  through the front side wall of the intake chamber  89 . The electronic control unit  105 ′ can be shielded, and also the intake pressure sensor  106 ′ and the intake temperature sensor  107  can be directly connected to the electronic control unit  105 ′. As a result, the labor of connecting lead wires can be eliminated. 
   The operation of the first embodiment will be described below. The water pump  46  is mounted on an end of the crankcase  21  along the axis of the crankshaft  11 . The coolant supply passages  62 L,  62 R guide the coolant from the water pump  46 , and the coolant return passages  63 L,  63 R guide the coolant that is delivered out of the cylinder water jackets  60 L,  60 R of the cylinder barrels  16 L,  16 R. The supply passages  62 L,  62 R and the return passages  63 L,  63 R are formed parallel to the axis of the crankshaft  11  on both sides of the axes of the cylinder bores  18 L . . . ,  18 R . . . . The cylinder water jackets  60 L,  60 R and the head water jackets  61 L,  61 R are formed such that the coolant supplied from the supply passages  62 L,  62 R returns from the cylinder water jackets  60 L,  60 R via the head water jackets  61 L,  61 R to the cylinder water jackets  60 L,  60 R. 
   Therefore, no piping is required outside of the engine for guiding the coolant from the water pump  46  to the cylinder barrels  16 L,  16 R, and no piping is required outside of the engine for delivering out the coolant from the cylinder heads  17 L,  17 R. Therefore, the coolant piping around the engine is simplified. 
   The cylinder water jackets  60 L,  60 R are divided into supply jacket portions  64  . . . and return jacket portions  65  . . . and are defined in the cylinder barrels  16 L,  16 R. The supply jacket portions  64  . . . communicate with the coolant supply passages  62 L,  62 R and also with the head water jackets  61 L,  61 R. The return jacket portions  65  . . . communicate with the coolant return passages  63 L,  63 R and also with the head water jackets  61 L,  61 R at positions spaced from the supply jacket portions  64  . . . . Consequently, the coolant supply passages  62 L,  62 R and the coolant return passages  63 L,  63 R can be cast or drilled in one direction along the axis of the crankshaft  11 . Therefore, the machinability for forming the passages is increased. 
   The supply jacket portions  64  and the return jacket portions  65  . . . , with superposed portions surrounding substantial half of the cylinder bores  18 L . . . ,  18 R. . . , are formed in the cylinder barrels  16 L,  16 R, which have the cylinder bores  18 L . . . ,  18 R . . . arrayed in the axial direction of the crankshaft  11 . A plurality of supply and return branch passages  67 L,  67 R . . . ;  68 L,  68 R . . . are disposed between the crankcase  21  and the cylinder barrels  16 L,  16 R. The plurality of supply and return branch passages  67 L,  67 R . . . ;  68 L,  68 R . . . connect the portions of the supply jacket portions  64  . . . and the return jacket portions  65  . . . , which correspond to the cylinder bores  18 L . . . ,  18 R . . . , to the coolant supply passages  62 L,  62 R and the coolant return passages  63 L,  63 R. Thus, a passage structure for uniformly cooling portions corresponding to the respective cylinder bores  18 L . . . ,  18 R . . . in a multicylinder engine can easily be constructed. 
   By changing stepwise the diameters of the coolant supply passages  62 L,  62 R and the coolant return passages  63 L,  63 R, which extend linearly, the amount of the coolant flowing through the cylinder water jackets  60 L,  60 R and the head water jackets  61 L,  61 R, which correspond to the cylinder bores  18 L . . . ,  18 R . . . , can be made more uniform. 
   Since the cylinder barrels  16 L,  16 R and the cylinder heads  17 L,  17 R are integrally formed by mutually joining the cylinder water jackets.  60 L,  60 R and the head water jackets  61 L,  61 R, a sand core in the shape of a succession of the cylinder and head water jackets  60 L,  60 R;  61 L,  61 R is integrally formed for increased productivity for the cylinder barrels  16 L,  16 R and the cylinder heads  17 L,  17 R. 
   The rod members  69  . . . divide the ring-shaped cavities, which is defined in the cylinder barrels  16 L,  16 R in surrounding relation to the cylinder bores  18 L . . . ,  18 R . . . in a casting process, halfway into the supply and return jacket portions  64  . . . ,  65  . . . . The rod members  69  are fitted into the cylinder barrels  16 L,  16 R from the cylinder heads  17 L,  17 R. Therefore, the portion of the sand core, which corresponds to the cylinder bores  18 L . . . ,  18 R . . . of the cylinder water jackets  60 L,  60 R, may be ring-shaped for easy sand removal to increase productivity and castability. In addition, the cylinder water jackets  60 L,  60 R can easily be divided into the supply jacket portions  64  . . . and the return jacket portions  65  . . . . 
   The cylinder blocks  13 L,  13 R, which have the cylinder barrels  16 L,  16 R and the cylinder heads  17 L,  17 R integrally formed to provide communication between the cylinder water jackets  60 L,  60 R and the head water jackets  61 L,  61 R have a plurality of mounting bosses  34  . . . , The bosses  34  . . . extend from the surfaces of the cylinder barrels  16 L,  16 R, which are attached to the crankcase  21 , to the cylinder heads  17 L,  17 R. The bosses  34  . . . surround the cylinder bores  18 L . . . ,  18 R . . . in the cylinder heads  17 L,  17 R. The cylinder blocks  13 L,  13 R are fastened to the crankcase  21  by the through bolts  30  . . . and the stud bolts  30  . . . extending through the mounting bosses  34  . . . . Therefore, the cylinder barrels  16 L,  16 R and the cylinder heads  17 L,  17 R can simply be joined to the crankcase  21 . A gasket is not required between the cylinder barrels  16 L,  16 R and the cylinder heads  17 L,  17 R, bolts, which would otherwise be needed to keep the sealing ability of gaskets. As a result, the number of parts used is reduced, and the weight of the engine can be reduced. Furthermore, the surrounding areas of the cylinder bores  18 L . . . ,  18 R . . . can sufficiently be stiffened by the mounting bosses  34  . . . surrounding the cylinder bores  18 L . . . ,  18 R . . . . The surrounding areas can withstand the tightening loads on the through bolts  30  . . . and the stud bolts  32  . . . that are inserted through the mounting bosses  34  . . . . 
   The joint walls  35  . . . are integrally mounted on the cylinder blocks  13 L,  13 R. The joint walls  35  . . . join at least one of the two adjacent sets of the mounting bosses  34 ,  34 , which are disposed on corresponding portions between the mutually adjacent cylinder bores  18 L,  18 L;  18 R,  18 R arrayed in the axial direction of the crankshaft  11 . The joint walls  35  . . . join the pair of mutually adjacent mounting bosses  34 ,  34  on the upper walls of the cylinder blocks  13 L,  13 R in the first embodiment. Therefore, no bolts need to be disposed between the cylinder bores  18 L . . . ,  18 R . . . , and the mechanical strength of the cylinder barrels  16 L,  16 R between the cylinder bores  18 L . . . ,  18 R . . . can be increased by the joint walls  35  . . . . The distance between the cylinder bores  18 L . . . ,  18 R . . . in the direction along the axis of the crankshaft  11  can be shortened for making the engine smaller in size. 
   The rod guide tubes  100 L,  100 R,  101 L . . . ,  101 R . . . axially movably insert the push rods and the pull rods of the valve operating device for transmitting the power of the crankshaft  11 . The rod guide tubes  100 L,  100 R,  101 L . . . ,  101 R . . . and the joint walls  35  are disposed on the upper and lower sides of the cylinder blocks  13 L,  13 R. Therefore, the rigidity of the cylinder blocks  13 L,  13 R is of an improved balance, preventing the inside diameter of the cylinder bores  18 L . . . ,  18 R . . . in the cylinder blocks  13 L,  13 R from varying. 
     FIG. 11  shows a second embodiment of the present invention. The parts of the second embodiment corresponding to those of the first embodiment are denoted by identical reference characters. 
   A pair of water pumps  46  rotatable by the crankshaft  11  is mounted on the crankcase  21  on one end of the crankshaft  11 , i.e., a rear end of the crankshaft  11  in the second embodiment. 
   The crankcase  21  has a coolant supply passage  62 L, a coolant supply passage  62 R, a coolant return passage  63 L and a coolant return passage  63 R. The coolant supply passage  62 L guides the coolant from one of the water pumps  46  into the supply jacket portion  64  in the cylinder water jacket  60 L in the left cylinder barrel  16 L. The coolant supply passage  62 R guides the coolant from the other of the water pumps  46  into the supply jacket portion  64  in the cylinder water jacket  60 R in the right cylinder barrel  16 R. The coolant return passage  63 L guides the coolant that is delivered out of the return jacket portion  65  in the cylinder water jacket  60 L in the left cylinder barrel  16 L. The coolant return passage  63 R guides the coolant that is delivered out of the return jacket portion  65  in the cylinder water jacket  60 R in the right cylinder barrel  16 R. These passages are defined parallel to the axis of the crankshaft  11  on both sides of the cylinder bores  18 L,  18 R as viewed in a figure projecting onto a plane perpendicular to the axes of the cylinder bores  18 L,  18 R. 
   Individual return conduits  110 L,  110 R are individually connected to the coolant return passages  63 L,  63 R. The return conduits  110 L,  110 R are connected in common to the first return conduit  72 . The coolant returning from the non-illustrated radiator is guided into the second return conduit  74 , which is connected in common to the water pumps  46 . A thermostat  73  is disposed between the first and second return conduits  72 ,  74 . The thermostat  73  guides the coolant from the first return conduit  72  to the second return conduit  74  by bypassing the radiator when the temperature of the coolant is low. 
   A steam conduit  76  for guiding steam evaporated by heating into an expansion tank  77  is connected to an upper portion of the cylinder block  13 L in communication with the coolant return passage  63 L. A third return pipe  76  for guiding the coolant that is condensed in the expansion tank  77  is connected to the second return conduit  74 . An oil cooler disposed in an oil filter  81  is supplied with the coolant from the water pumps  46 , . 46  via the supply conduit  82 , and the coolant from the oil cooler is returned to the thermostat  73  via a return conduit  111 . 
   The second embodiment offers the same advantages as those of the first embodiment. 
   While the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, but various design changes may be made without departing from the invention as defined in the scope of claims for patent. 
   For example, the present invention has been described as being applied to a horizontally opposed multicylinder engine in the first and second embodiments described above. However, the present invention is also applicable to a V-shaped multicylinder engine or a single-cylinder engine. 
   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.