Patent Publication Number: US-7913660-B2

Title: Cylinder head

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 2007-252145 filed in Japan on Sep. 27, 2007, 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 cylinder head of an engine. 
     2. Description of the Related Art 
     In general, a cylinder head of an engine is built on an upper side of a cylinder block with head bolts, and intake valves for opening and closing intake ports of combustion chambers, exhaust valves for opening and closing exhaust ports of the combustion chambers, a camshaft and rocker arms which actuate the intake and exhaust valves to open and close the intake and exhaust ports, a rocker arm shaft which supports the rocker arms and the like are provided on the cylinder head. The rotation of a crankshaft is transmitted to the camshaft via timing gears or a timing belt. 
     A plurality of camshaft support portions for supporting the camshaft rotatably and a plurality of rocker arm shaft support portions for supporting the rocker arm shaft rotatably are provided on the cylinder head. Specifically, these camshaft support portions and rocker arm support portions are each formed into a concave shape (a semi-circular shape) for accommodating the shafts and their journal portions for support. These shaft support portions are disposed along axial directions of the shafts. 
     On the other hand, as has been described above, the cylinder head is fixed to the cylinder block with the head bolts. A plurality of head bolt boss portions are formed on an upper surface of the bottom wall portion of the cylinder head in positions where the head bolts are passed. 
     Incidentally, a space is necessary on the periphery of the head bolt boss portion for a head bolt tightening tool to be inserted. Because of this, a relatively wide space is secured on the periphery of the head bolt boss portion. 
     The cylinder head tends to be enlarged in size as the camshaft support portions, the rocker arm shaft support portions and the head bolt boss portions are formed in the interior thereof and the spaces need to be secured on the peripheries of the head bolt boss portions as described above. However, it is required from the viewpoint of reducing the overall size of the engine to arrange the shaft support portions and the head bolt boss portions in such an appropriate way as to suppress the enlargement of the cylinder head. 
     For example, Japanese Patent No. 3263118 can be raised as disclosing a construction which can realize the reduction in size of the cylinder head. In Japanese Patent No. 3263118, there is proposed a devised positional relationship between supporting structures and head bolts. 
     In Japanese Patent No. 3263118, lower bearing members are installed on and between left and right support walls which are provided on an upper surface of a cylinder head in such a manner as to be erected therefrom. Cam caps are installed on the lower bearing members, and a camshaft is supported between the lower bearing members and the cam caps. Head bolts are disposed between the support walls, and spaces are secured between the head bolts and the support walls so that the head bolts are inserted and removed therethrough. The cylinder head is fastened to a cylinder block using the head bolts, and thereafter, the lower bearing members are built on and between the support walls. 
     In the construction disclosed in Japanese Patent No. 3263118, the lower bearing members and the head bolts are overlapped each other in a height direction, whereby the reduction in size of the cylinder head is realized. 
     On the other hand, in addition to the reduction in its size, it is required for the cylinder head to ensure a sufficient rigidity, and to make this happen, reinforcement portions such as ribs are provided. However, in the event that such high rigidity is attempted to be realized in the construction disclosed in Japanese Patent No. 3263118, separately from the support walls which are disposed in consideration of ensuring the head bolt insertion and removal spaces, reinforcement portions such as ribs need to be disposed within the cylinder head. It is considered that the disposition of such ribs in the cylinder head makes it difficult to reduce the size of the engine to a sufficiently small level. 
     In addition, since the structures for supporting the camshaft on the cylinder head are provided on a bottom wall of the cylinder head in such manner as to be erected integrally therefrom, it is considered that the supporting structures themselves function to reinforce the cylinder head to make it highly rigid. However, in the camshaft supporting structures disclosed in Japanese Patent No. 3263118, since the construction is adopted in which the support wall and the lower bearing member are made up of the separate parts, the strength is reduced compared to an integral construction, and the function as the reinforcement portion of the cylinder head is reduced. 
     Furthermore, in the event that the support wall and the lower bearing member are made up of the separate parts as with the construction disclosed in Japanese Patent No. 3263118, an increase in production costs is called for due to an increase in the number of parts involved. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a cylinder head which can be made compact while increasing its rigidity. 
     In order to achieve the object, according to the invention, there is provided a cylinder head, comprising: 
     a bottom wall portion, disposed on a side which faces a cylinder block; 
     an outer circumferential wall portion, erected from a circumference of the bottom wall portion, and defining inside thereof a disposition space where
         opening and closing valves which open and close combustion chambers,   rocker arms which are brought into abutment with the opening and closing valves,   a camshaft which actuates the rocker arms, and   a rocker arm shaft which supports the rocker arms are disposed;       

     a longitudinal wall portion, extending along the camshaft within the disposition space, and both ends of which are integrally formed with the outer circumferential wall portion; 
     a lateral wall portion, one end of which is integrally formed with the longitudinal wall portion, and the other end of which is integrally formed with a portion of the outer circumferential wall portion which confronts the longitudinal wall portion; 
     a camshaft support portion, formed on the lateral wall portion for supporting the camshaft; 
     a rocker arm shaft support portion, formed on the lateral wall portion for supporting the rocker arm shaft in such a manner that a height position of the rocker arm shaft from the bottom wall portion is positioned to be shifted from a height position of the camshaft from the bottom wall portion; and 
     a head bolt boss portion, formed on the bottom wall portion in such a manner as to overlap the lateral wall portion in a height direction from the bottom wall portion, and an interior of which a head bolt is passed through for fixing the bottom wall portion to the cylinder block in such a state that the bottom wall portion confronts the cylinder block, wherein 
     the lateral wall portion includes a passage portion through which the head bolt is passed into the head bolt boss portion and which overlaps the head bolt boss portion in the height direction. 
     The rocker arm shaft support portion may be lower than the camshaft support portion in the height direction. 
     The longitudinal wall portion may include an injector holding portion for holding an injector for supplying fuel to interiors of the combustion chambers. 
     The head bolt boss portion may overlap the camshaft support portion in the height direction. A portion of the lateral wall portion on which the camshaft support portion may be formed is thicker than the other portion of the lateral wall portion. The passage portion may be disposed between both edge portions of the portion of the lateral wall portion in a thickness direction. 
     The rocker arm shaft support portion may be disposed between the camshaft support portion and the bottom wall portion in the height direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       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 perspective view schematically showing an engine which includes a cylinder head according to an embodiment of the invention; 
         FIG. 2  is a sectional view of the engine taken along the line F 2 -F 2  shown in  FIG. 1 ; 
         FIG. 3  is a perspective view showing the engine sectioned along the line F 3 -F 3  shown in  FIG. 1 ; 
         FIG. 4  is a perspective view showing the engine sectioned along the line F 4 -F 4  shown in  FIG. 1 ; 
         FIG. 5  is a sectional view of the engine showing a state in which an intake camshaft and a rocker arm mechanism are built on the cylinder head shown in  FIG. 4 ; and 
         FIG. 6  is a plan view showing the cylinder head shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A cylinder head according to an embodiment of the invention will be described using  FIGS. 1 to 6 .  FIG. 1  is a perspective view showing schematically an engine  10  which includes a cylinder head  30  of the embodiment. 
     As is shown in  FIG. 1 , the engine is, for example, an in-line four-cylinder diesel engine. The engine  10  includes a cylinder block  20  and a cylinder head  30 .  FIG. 5  is a sectional view showing schematically an interior of the cylinder block  20 . As is shown in  FIG. 5 , cylinders  22  are formed within the cylinder block  20 . Pistons  23  are accommodated in the cylinders  22 . Connecting rods, not shown, are connected to the pistons  23 , respectively, and these connecting rods are connected to a crankshaft, not shown. 
     Combustion chambers  24  are formed between the respective pistons  23  and the cylinder head  30 , which will be described later. The pistons  23  are put in motion by energy produced within the combustion chambers  24  in the power stroke, and such motions of the pistons  23  are transmitted to the crankshaft by the connecting rods, as a result of which the crankshaft rotates. 
     As is shown in  FIG. 1 , the cylinder head  30  is disposed on a deck surface  21  of the cylinder block  20  to which the cylinders  22  are made to open. A seal member such as a gasket, not shown, is interposed between the cylinder block  20  and the cylinder head  30 . 
     The cylinder head  30  is fixed to the cylinder block  20  with head bolts such as a head bolt  90  shown in a right-hand edge portion of the figure. The fixing construction of the cylinder head  30  to the cylinder block  20  using the head bolds  90  will be described in detail later. 
       FIG. 2  is a sectional view of the engine taken along the line F 2 -F 2  shown in  FIG. 1 .  FIG. 2  shows a section of the engine which results when the engine is sectioned in a direction which extends across the crankshaft. Note that in  FIG. 2 , only the outline of the cylinder block  20  is shown by a chain double-dashed line. In addition, the illustration of intake and exhaust camshafts  39 ,  40  and rocker arm mechanisms  50 , which will be described later, is omitted. 
     As is shown in  FIGS. 1 ,  2 , the cylinder head  30  includes an opening which is opened to a side opposite to a side which faces the cylinder block  20 . For example, a head cover, not shown, is placed on the opening in the cylinder head  30 . As is shown in  FIGS. 1 ,  2 , the cylinder head  30  has a bottom wall portion  31 , an outer circumferential wall portion  32  and a longitudinal wall portion  33 . 
     As is shown in  FIG. 2 , the bottom wall portion  31  is disposed on the side of the cylinder head  30  which faces the deck surface  21  of the cylinder block  20  and constitutes a base portion of the cylinder head  30 . 
     As is indicated by arrows in  FIG. 1 , a direction in which the respective combustion chambers  24  are aligned, that is, a direction in which the crankshaft extends is made to be a longitudinal direction A of the engine  10 , and a direction which extends across the direction in which the respective combustion chambers  24  are aligned is made to be a width or transverse direction B of the engine  10 . A direction in which the cylinder block  20  and the cylinder head  30  are aligned (a direction in which the cylinder block  20  and the bottom wall portion  31  are aligned) is made to be a vertical direction C of the engine  10 , and the cylinder head  30  is positioned to lie above the cylinder block  20 . Note that in this embodiment, the vertical direction C is parallel to a height direction from the bottom wall portion  31 . 
     The outer circumferential wall portion  32  is erected, for example, substantially in the vertical direction from a circumferential edge portion of the bottom wall portion  31  and continues in a circumferential direction. The outer circumferential wall portion  32  defines an inside and an outside of the cylinder head  30  and has outer longitudinal wall portions  35  which extend substantially along the longitudinal direction A and external lateral wall portions  36  which extend substantially along the transverse direction B. 
     The longitudinal wall portion  33  is disposed substantially centrally of the cylinder head  30  in the transverse direction B and extends along the longitudinal direction A. In addition, the longitudinal wall portion  33  is formed integrally with the bottom wall portion  31  in such a manner as to be erected substantially in the vertical direction C and is formed integrally with the outer lateral wall portions  36  at both ends thereof in the longitudinal direction A. 
     As is shown in  FIG. 1 , a plurality of injector holding boss portions  38 , adapted to hold injectors  37  (shown in  FIG. 5 ) for injecting fuel into the combustion chambers, are formed on the longitudinal wall portion  33 . The injector holding boss portions  38  are formed to correspond individually to the combustion chambers. The bottom wall portion  31 , the circumferential wall portion  32  and the longitudinal wall portion  33  are formed integrally. 
     Intake valves  34  (partially shown in  FIG. 5 ), exhaust valves, an intake camshaft  39  and an exhaust camshaft  40  which are actuated by the rotation of the crankshaft being transmitted thereto, rocker arm mechanisms  50  (partially shown in  FIG. 4 ) and the like are built in the cylinder head  30 . Note that these intake valves  34 , exhaust valves, intake and exhaust camshafts  39 ,  40  and rocker arm mechanisms  50 , as well as the injectors  37  described above are examples of parts that are built in the cylinder head  30 . 
     The intake valve  34  is provided in an intake port which communicates with the combustion chamber  24 . The exhaust valve (not shown) is provided in an exhaust port which communicates with the combustion chamber  24 . 
     As is shown in  FIG. 1 , the intake camshaft  39  and the exhaust camshaft  40  rotate to actuate the intake valves  34  and the exhaust valves by the rotation of the crankshaft being transmitted thereto. The intake camshaft  39  is disposed on an intake side (a right-hand side in the figure) of the engine  10 , so as to actuate the rocker arm mechanisms  50 . The exhaust camshaft  40  is disposed on an exhaust side (a left-hand side in the figure) of the engine  10 , so as to actuate the exhaust valves. Note that both the intake and exhaust camshafts  39 ,  40  are schematically shown in  FIG. 1 . 
     Supporting structures for supporting the intake and exhaust camshafts  39 ,  40  and the rocker arm mechanisms  50  are provided on the cylinder head  30 . The supporting structures will be described specifically. 
     Firstly, a construction for supporting the exhaust camshaft  40  will be described. A plurality of exhaust side lateral wall portions  41  for supporting the exhaust camshaft  40  are formed on an exhaust side of the cylinder head  30 . The exhaust side lateral wall portions  41  are formed in such a manner as to extend in the transverse direction B between the longitudinal wall portion  33  and the outer longitudinal wall portion  35  which is disposed on the exhaust side of the cylinder head  30  (the outer longitudinal wall portion  35  which is disposed on the left-hand side of the cylinder head  30  as viewed in  FIG. 1 ). The exhaust side lateral wall portions  41  are disposed in positions where they lie adjacent to the injector holding boss portions  38  and are formed integrally with the longitudinal wall portion  33  at one ends while formed integrally with the outer longitudinal wall portion  35  at the other ends thereof. 
     In the sectional view shown in  FIG. 2 , the exhaust side lateral wall portion  41  is not sectioned but is shown as viewed from the front. In addition,  FIG. 3  is a perspective view showing the engine  10  which is sectioned along the line F 3 -F 3  shown in  FIG. 1 . Note that in these figures, only the outline of the cylinder block  20  is shown by a chain double-dashed line. In addition, the illustration of the intake and exhaust camshafts  39 ,  40  and rocker arm mechanisms  50  is omitted. 
     As is shown in  FIGS. 2 ,  3 , the exhaust side lateral wall portion  41  is formed integrally with the bottom wall portion  31  near end portions thereof. In the exhaust side lateral wall portion  41 , a space is formed between a portion other than the end portions and the upper surface of the bottom wall portion  31 . 
     As is shown in  FIG. 1 , exhaust camshaft support portions  42  are formed on the exhaust side lateral wall portions  41  for supporting journals  40   a  of the exhaust camshaft  40 . The exhaust camshaft support portion  42  is formed in cross section into a concave shape which is cut out semi-circularly and accommodates therein the journal  40   a  of the exhaust camshaft  40 . Note that in  FIG. 1 , the journals  40   a  of the exhaust camshaft  40  are portions shown by chain double-dashed lines. 
     The exhaust camshaft  40  is supported on the cylinder head  30  by the journals  40   a  being supported on the exhaust camshaft support portions  42 . The plurality (four in this embodiment) of exhaust side lateral wall portions  41  is formed to be aligned in the longitudinal direction in such a manner that one exhaust side lateral wall portion  41  supports one of the journals  40   a  of the exhaust camshaft  40 . In addition, exhaust camshaft support portions  42  are also formed on the outer lateral wall portions  36  for supporting end portions of the exhaust camshaft  40 . 
     Next, supporting structures of the intake camshaft  39  and the rocker arm mechanisms  50  will be described. As is shown in  FIG. 1 , first intake side lateral wall portions  43  are formed on an intake side of the cylinder head  30  in such a manner as to extend in a straight line in the transverse direction B. 
       FIG. 3  is the perspective view in which none of the first intake side lateral wall portion  43  is sectioned. As is shown in  FIG. 3 , the first intake side lateral wall portions  43  are formed in such a manner as to extend between the longitudinal wall portion  33  and the outer longitudinal wall portion  35  which is disposed on the intake side of the cylinder head  30  (the outer longitudinal wall portion  35  which is disposed on the right-hand side of the cylinder head  30  as viewed in  FIG. 1 ). 
     The plurality of first intake side lateral wall portions  43  is formed in such a manner that one first intake side lateral wall portion  43  is disposed in a substantially central position between the injector holding boss portions  38  which lie adjacent to each other in the longitudinal direction A. In this embodiment, four first intake side lateral wall portions  43  are formed. The first intake side lateral wall portions  43  are formed integrally with the longitudinal wall portion  33  at one ends while formed integrally with the outer longitudinal wall portion  35  at the other ends thereof. 
     The first intake side lateral wall portion  43  has an intake camshaft support portion  44  for supporting a journal  39   a  of the intake camshaft  39 . The intake camshaft support portion  44  is formed on a side of the first intake side lateral wall portion  43  which faces the longitudinal wall portion  33 . The intake camshaft support portion  43  is formed in cross section into a concave shape which is cut out semi-circularly and accommodates therein the journal  39   a  of the intake camshaft  39 . 
     Thus, as has been described above, the intake camshaft  39  is supported on the cylinder head  30  by being supported within the intake camshaft support portions  44 . Note that in  FIG. 1 , the journals  39   a  of the intake camshaft  39  are portions indicated by chain double-dashed lines. The plurality of first intake side lateral wall portions  43  is disposed to be aligned in the longitudinal direction A in such a manner that one first intake side lateral wall portion  43  supports one of the journals  39   a  of the intake camshaft  39 . In addition, intake camshaft support portions  44  are also formed on the outer lateral wall portions  36  in a similar fashion for supporting both ends of the intake camshaft  39 . 
       FIG. 4  is a perspective view showing the engine  10  which is sectioned along the line F 4 -F 4  shown in  FIG. 1 . In  FIG. 4 , the exhaust side lateral wall portion  41  is sectioned. Note that in the figure, the illustration of the intake and exhaust camshafts  39 ,  40  and the rocker arm mechanisms  50  is emitted. In addition, only the outline of the cylinder block  20  is indicated by a chain double-dashed line. 
     As is shown in  FIG. 4 , the rocker arm mechanisms  50  are disposed between the longitudinal wall portion  33  and the intake-side outer longitudinal wall portion  35 . As is indicated by chain double-dashed lines in  FIG. 4 , the rocker arm mechanisms  50  are disposed between the first intake side lateral wall portions  43 . 
     The rocker arm mechanism  50  is of variable type in which the intake valve  34  is driven variably in association with the running state of the engine  10 . In addition, two intake valves  34  are used for one combustion chamber  24 . The intake valves  34  are aligned in, for example, in the longitudinal direction A. The intake valve  34  is an example of an opening and closing valve. 
     In  FIG. 4 , one of the rocker arm mechanisms  50  which corresponds to one cylinder is shown in an enlarged fashion in an area D surrounded by a chain double-dashed line. As is shown in the area D, a low-speed cam  45  and a high-speed cam  46  are formed on the intake camshaft  39 . 
     The low-speed cam  45  actuates the intake valve  34  at a valve opening and closing timing which is suitable for the low-speed running of the engine  10  and has a cam profile which can provide a valve lift amount which is suitable for such a low-speed running. The high-speed cam  46  actuates the intake valve  34  at a valve opening and closing timing which is suitable for the high-speed running of the engine  10  and has a cam profile which can provide a valve lift amount which is suitable for such a high-speed running. 
     The rocker arm mechanism  50  includes a rocker arm shaft  51 , a low-speed rocker arm  52  and a high-speed rocker arm  53 . As is shown in  FIG. 1 , the rocker arm shaft  51  is disposed on the intake side of the cylinder head  30 . As is shown in  FIGS. 1 to 4 , rocker arm shaft support portions  54  are formed on the first intake side lateral wall portions  43  for supporting the rocker arm shaft  51 . 
     The rocker arm shaft support portion  54  is disposed on the first intake side lateral wall portion  43  in a position which lies adjacent to the intake camshaft support portion  44  and in the proximity to its end which faces the outer longitudinal wall portion  35 . Consequently, the rocker arm shaft support portion  54  is disposed in the position which lies adjacent to the intake camshaft support portion  44  in the transverse direction B. 
     The rocker arm shaft support portion  54  is formed in cross section into a concave shape which is cut out semi-circularly for accommodating therein a journal  51   a  of the rocker arm shaft  51 . In  FIG. 1 , the journal  51   a  is a portion indicated by a chain double-dashed line. The rocker arm shaft support portions  54  accommodate the rocker arm shaft  51  inside thereof. The rocker arm shaft  51  is supported on the cylinder head  30  by the journals  51   a  thereof being supported by the rocker arm shaft support portions  54 . One rocker arm shaft support portion  54  supports one of the journals  51   a  of the rocker arm shaft  51 . 
     In addition, a receiving surface  100  of the rocker arm shaft support portion  54  which supports the rocker arm shaft  51  is positioned further downwards than a receiving surface  101  of the intake camshaft support portion  44  which supports the intake camshaft  39 . Namely, the rocker shaft support portion  54  is disposed further downwards (towards the cylinder block  20 ) than the intake camshaft support portion  44 . 
     This will be described specifically.  FIG. 5  is a sectional view of the engine  10  which shows a state in which the intake camshaft  39  and the rocker arm mechanisms  50  are built on the cylinder head  30  shown in  FIG. 4 . In addition,  FIG. 5  is the sectional view taken along the line F 4 -F 4  shown in  FIG. 1 . 
     As is shown in  FIG. 5 , the locker arm shaft support portion  54  is formed such that in such a state that the locker arm shaft support portion  54  supports the rocker arm shaft  51 , the rocker arm shaft  51  is disposed further downwards than the intake camshaft  39  (in a position where the low-speed cams  45  and the high-speed cams  46  do not interfere with the rocker arm shaft  51 ). 
     By being disposed in the position which is shifted downwards relative to the intake camshaft  39 , the rocker arm shaft  51  is allowed to be disposed inwards (towards the longitudinal wall portion  33 ) of the cylinder head  30  while preventing the interference thereof with the low-speed cams  45  and the high-speed cams  46 . 
     In addition, the arrangement in which the intake camshaft  39  and the rocker arm mechanisms  50  are disposed in such a manner as to overlap each other in the vertical direction can be realized by the locker arm shaft  51  being disposed below the intake camshaft  39 . 
     As is shown in  FIG. 3 , below the first intake side lateral wall portion  43 , a space S is formed between the intake camshaft support portion  44  and the bottom wall portion  31 . 
     In  FIG. 4 , a state in which the rocker arm mechanism  50  is exploded is shown in an area E indicated by a chain double-dashed line. As is shown in  FIG. 4 , the low-speed rocker arm  52  has a low-speed boss portion  55  and a low-speed arm portion  56 . 
     The low-speed boss portion  55  is formed into a cylindrical tube shape, and the rocker arm shaft  51  is passed through an interior thereof. The low-speed boss portion  55  is made to rotate relative to the rocker arm shaft  51 . The low-speed arm portion  56  is formed on the low-speed boss portion  55  in such a manner as to extend from the low-speed boss portion  55  towards the intake valve  34 . 
     A distal end of the low-speed arm portion  56  is bifurcated substantially into a Y-shape and each of the bifurcated portions actuates one intake valve  34 . In addition, as is shown in FIG.  5 , the intake valve  34  is constructed to open the intake port when it is depressed and is normally urged in a direction in which it closes the intake port (in a direction in which it is depressed) by, for example, a coil spring  57 . 
     As is shown in  FIG. 4 , a low-speed roller member  58  is provided on the low-speed arm portion  56 . The low-speed roller member  58  is disposed below the low-speed cam  45 , and the low-speed cam  45  is made to be brought into abutment with the low-speed roller member  58 . 
     The low-speed roller member  58  is urged to be brought into abutment with the low-speed cam  45  at all times by the low-speed arm portion  56  being urged (depressed) by the coil spring  57  of the intake valve  34 . Because of this, the low-speed roller member  58  is displaced while following the cam profile of the low-speed cam  45  when the intake camshaft  39  rotates, whereby the low-speed rocker arm  52  is actuated to rotate on the rocker arm shaft  51  as a fulcrum. 
     The high-speed rocker arm  53  is supported on the rocker arm shaft  51  and is disposed adjacent to the low-speed rocker arm  52  along the rocker arm shaft  51 . The high-speed rocker arm  53  includes a high-speed boss portion  59  and a high-speed roller member  60 . 
     The high-speed boss portion  59  is disposed adjacent to the low-speed boss portion  55 . The high-speed boss portion  59  is formed into a cylindrical tube shape, and the rocker arm shaft  51  is passed through an interior thereof. The high-speed boss portion  59  is made to rotate freely relative the rocker arm shaft  51 . 
     The high-speed roller member  60  is provided on the high-speed boss portion  59  and is disposed below the high-speed cam  46 . The high-speed cam  46  is in abutment with the high-speed roller member  60 . An abutment portion  61  is formed at a lower end portion of the high-speed rocker arm  53 . The abutment portion  61  is formed into a downwardly projecting shape. 
     As is shown in  FIG. 5 , a push-up member  62  is brought into abutment with the abutment portion  61  from the side of the cylinder block  20 . The push-up member  62  is made to extend and contract over its overall length and is constructed to incorporate therein a spring member. An upper end face  63  of the push-up member  62  urges the abutment portion  61  upwardly by the upper end face  63  being urged by the spring member. 
     Because of this, since the high-speed roller member  60  is made to be in abutment with the high-speed cam  46  at all times, the high-speed roller member  60  is displaced while following the cam profile of the high-speed cam  46  when the intake camshaft  39  rotates. As a result, the high-speed rocker arm  53  is actuated to rotate on the rocker arm shaft  51  as a fulcrum. 
     The push-up member  62  is provided on a second intake side lateral wall portion  70 . As is shown in  FIGS. 1 ,  4 , the second intake side lateral wall portion  70  is formed to extend in the transverse direction B between the longitudinal wall portion  33  and the outer longitudinal wall portion  35 . The periphery of the second intake side lateral wall portion  70  is formed integrally with the longitudinal wall portion  33 , the bottom wall portion  31  and the outer longitudinal wall portion  35 . 
     An upper end face  71  of the second intake side lateral wall portion  70  is positioned below the abutment portion  61 . An accommodation hole  72  is bored in the upper end face  71  of the second intake side lateral wall portion  70  for accommodating therein the push-up member  62 . 
     A switching mechanism  73  is provided between the high-speed rocker arm  53  and the low-speed rocker arm  52  which switches between the transmission of displacement of the high-speed rocker arm  53  to the intake valve  34  and the transmission of displacement of the low-speed rocker arm  52  to the intake valve  34 . The switching mechanism  73  includes a storage tubular portion  74 , a piston  75 , a coil spring  76 , a transmission arm  77  and a hydraulic mechanism, not shown. 
     The storage tubular portion  74  is provided on the low-speed boss portion  55 . The storage tubular portion  74  is formed into a tubular shape. A window portion  78  is formed in part of a back side portion of the storage tubular portion  74  which is opposite to a side thereof which confronts the intake camshaft  39  by cutting out the portion in question. The storage tubular portion  74  communicates with the outside thereof through the window portion  78 . 
     The piston  75  is stored in the storage tubular portion  74 . As is shown in the figure, a cut-out portion  79  is formed in an upper end portion of the piston  75  by cutting partially the upper end portion. The window portion  78  is positioned on a low-speed boss portion  55  side (downwards) of the storage tubular portion  74 . 
     When the piston  75  is positioned on the low-speed boss portion  55  side, the cut-out portion  79  is exposed to the outside through the window portion  78 . In addition, when the piston  75  moves to an upper end side (a side opposite to the low-speed boss portion  55 ) of the storage tubular portion  74 , the cut-out portion  79  is covered any other portion (a wall portion) than the window portion  78  on a back side portion thereof. 
     The coil spring  76  is stored within the storage tubular portion  74  and is disposed between the piston  75  and the upper end of the storage tubular portion  74 . Because of this, the piston  75  is urged downwards (towards the low-speed boss portion  55 ) by the coil spring  76 . Consequently, the cut-out portion  79  is normally made to be exposed to the outside through the window portion  78 . 
     The transmission arm  77  is formed on the high-speed boss portion  59 . A distal end  80  of the transmission arm  77  is formed in such a manner as to enter the interior of the storage tubular portion  74  through the window portion  78  in association with the rotation of the high-speed rocker arm  53 . 
     Because of this, when the piston  75  is positioned such that the cut-out portion  79  confronts the window portion  78 , the distal end  80  of the transmission arm  77  is in no case brought into abutment with the piston  75  through the window portion  78 , whereby the distal end  80  of the transmission arm  77  can enter the interior of the storage tubular portion  74 . Consequently, since the high-speed rocker arm  53  is put in a state in which it oscillates idly, the rotational displacement of the high-speed rocker arm  53  is not transmitted to the low-speed rocker arm  52 . 
     When the piston  75  is positioned such that the window portion  78  is covered by the piston  75  (the portion thereof other than the cut-out portion  79 ), the transmission arm  77  is allowed to be brought into abutment with the piston  75 . As a result of this, the rotational displacement of the high-speed rocker arm  53  is transmitted to the low-speed rocker arm  52  via the piston  75 . The lift amount of the intake valve  34  by the high-speed cam  46  is larger than the lift amount thereof by the low-speed cam  45 . Because of this, the rotational displacement of the high-speed rocker arm  53  is made to be transmitted to the intake valve  34  via the low-speed rocker arm  52 . 
     The hydraulic mechanism has a function to push up the piston  75  against the elastic force of the coil spring  76 . The hydraulic mechanism switches positions of the piston  75  in accordance with the running state of the engine  10 . 
     The hydraulic mechanism does not urge the piston  75  when the engine  10  is in the low-speed running state. Because of this, since the cut-out portion  79  is made to confront the window portion  78 , the rotational displacement of the high-speed rocker arm  53  is not transmitted to the low-speed rocker arm  52 , whereby the intake valve  34  is actuated by the low-speed rocker arm  52 . 
     When the engine  10  is in the high-speed running state, the hydraulic mechanism urges the piston  75 , whereby since the cut-out portion  79  is dislocated from the window portion  78 , the window portion  78  being covered by the portion of the piston  75  other than the cut-out portion  79 , the rotational displacement of the high-speed rocker arm  53  is transmitted to the low-speed rocker arm  52  via the transmission arm  77  and the piston  75 . As a result of this, the intake valve  34  is actuated by the high-speed rocker arm  53 . 
     Next, a fixing construction of the cylinder head  30  to the cylinder block  20  will be described specifically. As is shown in  FIG. 1 , the cylinder head  30  is fixed to the cylinder block  20  with the head bolts  90 . A plurality of head bolt boss portions  91  is formed on the bottom wall portion  31 . 
       FIG. 6  is a plan view showing the cylinder head  30 . As is shown in  FIG. 6 , head bolt boss portions  91  which are disposed on the exhaust side of the cylinder head  30  are disposed in such a manner that one head bolt boss portion  91  is disposed between the adjacent exhaust side lateral wall portions  41  in the longitudinal direction A. Because of this, a sufficient working space for inserting the head bolt  90  into the head bolt boss portion  91  can be secured on the periphery of the head bolt boss portion  91  so disposed. 
     Head bolt boss portions  91  are also disposed below the exhaust camshaft support portions  42  which are formed on the outer lateral wall portions  36 . In the exhaust camshaft support portion  42  formed on the outer lateral wall portion  36 , a passage hole through which the head bolt  90  and a tool used for head bolt tightening work are passed is formed in a portion which overlaps the head bolt boss portion  91  in the vertical direction C. Consequently, the head bolt  90  and the tool used for the head bolt tightening work can be inserted from above through the exhaust camshaft support portion  42 . 
       FIG. 2  is the sectional view taken along the plane which passes through the first intake side lateral wall portion  43 . As is shown in  FIGS. 2 ,  6 , head bolt boss portions  91  which are disposed on the intake side of the cylinder head  30  are disposed below the camshaft support portions  44  formed on the first intake side lateral wall portions  43 . 
     As is shown in  FIGS. 3 ,  4 ,  6 , when viewed from the top, the intake camshaft support portion  44  which is formed on the first intake side lateral wall portion  43  is formed thicker in the longitudinal direction A than the other portions of the first intake side lateral wall portion  43  than the intake camshaft support portion  44 . In addition, a passage hole (a passage portion)  44   a  is formed between both edge portions  44   b  of the intake camshaft support portion  44  in the longitudinal direction A in such a manner as to reach the head bolt boss portion  91 . The intake-side head bolt boss portion  91  is exposed in such a manner as to be accessed from above through the passage hole  44   a.    
     The passage hole  44   a  is sized to allow the passage of the head bolt  90  and the tool used when the head bolt  90  is built into the head bolt boss portion  91 . Thus, a space is secured on the periphery of the intake-side head bolt boss portion  91  which is necessary to build the head bolt  90  into the head bolt boss portion  91 . 
     In addition, the intake camshaft support portion  44  which is formed on one of the other outer lateral wall portions  36  is also constructed such that a passage hole  44   a  is formed therein in a similar manner to that in which the passage hole  44   a  is formed in the intake camshaft support portion  44  formed on the first intake side lateral wall portion  43 , and the head bolt  90  and the tool can also be passed therethrough. 
     Note that the exhaust side lateral wall portions  41  and the first and second intake side lateral wall portions  43 ,  70  are formed integrally when the cylinder head  30  is formed through casting, and therefore, the bottom wall portion  31 , the longitudinal wall portion  33  and the circumferential wall portion  32  are made integral with each other. 
     In the cylinder head  30  that is configured as has been described heretofore, since the longitudinal wall portion  33  which supports the exhaust side lateral wall portions  41  and the first and second intake side lateral wall portions  43 ,  70  extends in the longitudinal direction A to be connected to the outer circumferential wall portion  32  at the ends thereof, the longitudinal wall portion  33  functions as a reinforcement rib of the cylinder head  30 . 
     Because of this, the rigidity of the cylinder head  30  is increased. Furthermore, by the longitudinal wall portion  33  functioning as the reinforcement rib, the necessity is obviated of providing a separate reinforcement rib, whereby the cylinder head  30  is made compact. 
     Furthermore, the rigidity of the cylinder head  30  is increased by the exhaust side lateral wall portions  41  and the first and second intake side lateral wall portions  43 ,  70  which extend in the transverse direction B functioning as reinforcement ribs. 
     Moreover, the cylinder head  30  can be made compact in the transverse direction B by the rocker arm shaft  51  being positioned below the intake camshaft  39  and being disposed inwards of the cylinder head  30 . 
     In addition, since the arrangement can be realized in which the intake camshaft  39  and the rocker arm mechanisms  50  are disposed in such a manner as to overlap each other in the vertical direction by the rocker arm shaft  51  being disposed below the intake camshaft  39 , the cylinder head  30  can be made compact in the transverse direction B. 
     Furthermore, by disposing the head bolt boss portions  91  in the (downward) positions where they overlap the intake camshaft support portions  44  and enabling the access to the head bolt boss portions  91  through the passage holes  44   a , not only can the cylinder head  30  be made compact but also the reduction in rigidity can be suppressed which would otherwise be caused by the intake camshaft support portions  44  being made up of separate parts. 
     The cylinder head  30  is made compact while increasing its rigidity by the advantages described above. 
     In addition, since the injector holding boss portions  38  are formed on the longitudinal wall portion  33 , the necessity is obviated of providing additional injector holding boss portions  38  within the cylinder head  30 . Because of this, the cylinder head  30  can be made compact. 
     Additionally, the intake camshaft support portions  44  support the intake camshaft  39  at their edge portions  44   b . Because of this, the intake camshaft  39  is held while well balanced. 
     In addition, the head bolt boss portions  91  are disposed below the intake camshaft support portions  44  and the rocker arm shaft support portions  54  are disposed below the intake camshaft support portion  44 . As a result of this, since the disposition of the rocker arm mechanisms  50  in a higher position relative to the bottom wall portion  31  is suppressed, the increase in overall height of the engine  10  is suppressed. Namely, the engine  10  is made compact. 
     Note that in the embodiment, the rocker arm mechanisms  50  are provided on the intake side of the cylinder head  30 , and no rocker arm mechanism is provided on the exhaust side thereof. However, the rocker arm mechanisms may be provided on the exhaust side of the cylinder head  30 . In this case, the positional relationship between the camshaft and the rocker arm shaft and the position of the head bolt boss portions  91  may be similar to those on the intake side. By this configuration, the same advantage can be obtained even in the event that the construction on the intake side is adopted on the exhaust side. 
     In addition, while the embodiment of the invention is described as being applied to the diesel engine, the invention is not limited thereto. The same advantage can be obtained even in the event that the invention is applied to a gasoline engine. 
     According to an aspect of the invention, the longitudinal wall portion not only supports the lateral walls but also functions as reinforcement portions of the cylinder head. By the camshaft support portions being formed on the lateral wall portions, a sufficient strength can be secured while suppressing the increase in production costs, compared with the configuration in which the shaft support portions are formed as the separate elements from the cylinder head. In addition, by the rocker arm shaft being made to be shifted from the camshaft in the height direction, for example, as to the arrangement of the rocker arms, the parts which are involved in the valve train can be disposed to overlap the camshaft in the height direction. Additionally, the lateral wall portions and the head bolt boss portions are prevented from being situated on the same plane, whereby the reduction in size of the cylinder head can be realized. 
     According to an aspect of the invention, the parts such as the rocker arms which are built on the rocker arm shaft can be disposed in the low position by disposing the camshaft in the position which is shifted further downwards than the rocker arm shaft in the height direction. 
     According an aspect of the invention, since the injector holding portions are formed by making use of the longitudinal wall portion, the necessity is obviated of providing additionally injector holding spaces within the cylinder head. 
     According to an aspect of the invention, since the camshaft support portions are made to support the camshaft at both the edges of the thick portions (both side portions of the passage portions), the stable support of the camshaft is realized. In addition, since the head bolts can be inserted through the passage portions from above the camshaft support portions and can be tightened by operating a tool above the camshaft support portions, the necessity is obviated of securing tool spaces for tightening and removing the head bolts within the disposition spaces inside the cylinder head, this also contributing to the reduction in the overall size of the cylinder head. 
     The invention thus described, it will be obvious that the same say be varied in many ways. Such variations are not to be regarded as a departure from the sprit 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.