Abstract:
A cooling structure for a cylinder head, wherein the cylinder head has formed therein an exhaust manifold having branch sections and also having a collecting section to which the branch sections connect; a first water jacket and a second water jacket, which cover the exhaust manifold; connection passages which supply cooling water in the second water jacket to the first water jacket; and a cooling water outlet which connects the first water jacket to the outside. The connection passages include the first connection passage which is closest to the collecting section, and the second connection passage which is located at the downstream end of each of the water jackets. The flow passage cross-sectional area of the second connection passage is set to be less than the flow passage cross-sectional area of the first connection passage.

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to a cooling structure for cooling a cylinder head that accommodates an exhaust manifold. 
     BACKGROUND OF THE INVENTION 
     There is a recent cylinder head that accommodates an exhaust manifold. Japanese Laid-Open Patent Publication No. 2010-275915 describes a cylinder head in which upper and lower water jackets are respectively arranged above and below an exhaust manifold to cover the exhaust manifold. The exhaust manifold is cooled by a coolant flowing through each water jacket. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-275915 
     SUMMARY OF THE INVENTION 
     However, the temperature of an exhaust manifold is uneven. When such an exhaust manifold is uniformly cooled using a water jacket, a coolant is excessively supplied to a portion having a low temperature. This may result in insufficient cooling of a portion having a high temperature. 
     It is an object of the present disclosure to provide a structure for cooling a cylinder head accommodating an exhaust manifold that effectively cools a portion of the exhaust manifold having a high temperature. 
     One aspect of the present disclosure is a cooling structure for a cylinder head. The cylinder head accommodates an exhaust manifold, a first water jacket, a second water jacket, a plurality of communication passages, and a coolant outlet. The exhaust manifold includes a plurality of branches and a collector. The branches are respectively connected to combustion chambers of cylinders. The branches converge at the collector. The first water jacket is located above the exhaust manifold and covers the exhaust manifold including at least the collector. The second water jacket is located below the exhaust manifold and covers the exhaust manifold including at least the collector. The communication passages supply coolant from the second water jacket to the first water jacket. The coolant outlet connects the first water jacket to the exterior. The cooling structure for the cylinder head is configured so that the coolant flows from each water jacket in a direction in which the cylinders are arranged in order to be discharged out of the coolant outlet. The communication passages include a first communication passage and a second communication passage. The first communication passage is located at an upstream side of the collector in a flow direction of the coolant and arranged at a position that is the most proximate to the collector. The second communication passage is located in a downstream side end of each water jacket. A cross-sectional passage area of the second communication passage is set to be smaller than a cross-sectional passage area of the first communication passage. 
     Exhaust constantly flows to the collector of the exhaust manifold from one of the branches. Thus, the collector of the exhaust manifold tends to have a high temperature due to heat of the exhaust. In general, the exhaust manifold is curved so that a portion at a downstream side is located below a portion at an upstream side. Thus, the exhaust flowing to the exhaust manifold from the combustion chamber tends to strike an upper portion of an inner wall of the exhaust manifold. This causes the upper portion of the exhaust manifold to have a high temperature compared to a lower portion. More specifically, in the exhaust manifold, the collector, particularly, an upper portion of the collector, tends to have a high temperature. 
     In this regard, the configuration described above effectively cools the exhaust manifold, particularly, the upper portion of the collector, which tends to have a high temperature. More specifically, in the above configuration, a coolant supplied to a lower water jacket, which serves as the second water jacket, flows in the cylinder arrangement direction. Some of the coolant flows to the second communication passage arranged in the downstream side end of the second water jacket. Then, the coolant flows to an upper water jacket, which serves as the first water jacket, through the second communication passage and is discharged from the coolant outlet arranged in the first water jacket. In this case, the cross-sectional passage area of the second communication passage is set to be smaller than the cross-sectional passage area of the first communication passage. Thus, more coolant is supplied to the first water jacket from the second water jacket through the first communication passage than when a different setting is used. This increases a flow rate of the coolant flowing to a portion of the first water jacket that covers the upper portion of the collector. Consequently, the upper portion of the collector may be effectively cooled. 
     As described above, the upper portion of the exhaust manifold tends to have a high temperature compared to the lower portion. In this regard, in one mode, the first water jacket is set so that an area of the exhaust manifold that is covered by the first water jacket is larger than an area of the exhaust manifold that is covered by the second water jacket. This mode effectively cools the upper portion of the exhaust manifold, which tends to have a high temperature while limiting excessive cooling of the lower portion of the exhaust manifold. 
     In another mode, the communication passages include a third communication passage located at a downstream side of the collector in the flow direction of the coolant. The collector is located between the third communication passage and the first communication passage. 
     In this mode, the coolant is supplied to a portion that covers the two opposite sides of the collector through the first communication passage and the third communication passage. This effectively cools not only the upper side of the collector but also the sides of the collector. 
     In still another mode, the second communication passage includes a downstream side opening that opens to the first water jacket. The second communication is configured so that a flow passage direction of the downstream side opening extends toward the coolant outlet. 
     In this mode, the coolant, which is supplied to the first water jacket from the second communication passage, flows toward the coolant outlet. This generates flow of the coolant toward the coolant outlet in the first water jacket. Consequently, more coolant may be discharged to the exterior from the coolant outlet. This increases the amount of the coolant flowing through each of the water jackets, thereby effectively cooling the exhaust manifold. 
     Other aspects and advantages of the present disclosure will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Novel features of the present disclosure will become apparent from the accompanying claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional view showing the structure of one embodiment of a cooling structure for a cylinder head; 
         FIG. 2  is a cross-sectional view showing the structure of a lower water jacket of the embodiment of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view showing the structure of an upper water jacket of the embodiment of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 3 ; and 
         FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of a cooling structure for a cylinder head will now be described with reference to  FIGS. 1 to 6 . 
     As shown in  FIG. 1 , a cylinder head  20  is arranged above a cylinder block  10  in an internal combustion engine. The cylinder head  20  includes an exhaust manifold  21  that is in communication with a combustion chamber  30 . The exhaust manifold  21  is curved so that a portion at a downstream side of the exhaust is located below a portion at an upstream side of the exhaust, which is at the side of the combustion chamber  30 . The cylinder head  20  includes an upper water jacket  22 , which is arranged above the exhaust manifold  21  and serves as a first water jacket, and a lower water jacket  23 , which is arranged below the exhaust manifold  21  and serves as a second water jacket. The lower water jacket  23  is in communication with a coolant passage  11  formed in the cylinder block  10 . 
     The structure of each of the upper water jacket  22  and the lower water jacket  23  will now be described with reference to  FIGS. 2 to 6 . 
     As shown in  FIG. 2 , the exhaust manifold  21  includes a plurality of branches  211 , which are connected to the combustion chambers  30 , and a collector  212 . The branches  211  converge into the collector  212 . The lower water jacket  23  extends in a direction in which cylinders are arranged (left-right direction in the drawing) and covers the collector  212  of the exhaust manifold  21  from a lower side. An area in which the lower water jacket  23  covers the exhaust manifold  21  is set to be 40% or less of the surface area of a lower portion of the exhaust manifold  21 . The exhaust manifold  21 , which is indicated by broken lines, includes a plurality of ends that are located below the lower water jacket  23  (located toward the cylinder block  10 ). 
     The lower water jacket  23  includes a first end and a second end in the cylinder arrangement direction (left-right direction in  FIG. 2 ). The first end includes a first inlet  24  to which a coolant is supplied from the cylinder block  10 . The second end includes a second communication passage  25  that is in communication with the upper water jacket  22 . Thus, the coolant, which is supplied to the lower water jacket  23  from the first inlet  24 , flows in the cylinder arrangement direction and is supplied to the upper water jacket  22  through the second communication passage  25 . 
     The upper water jacket  22  will now be described with reference to  FIG. 3 . 
     As shown in  FIG. 3 , the upper water jacket  22  extends in the cylinder arrangement direction (left-right direction in  FIG. 3 ) and covers the substantially entire exhaust manifold  21  including the collector  212  from an upper side. The area in which the upper water jacket  22  covers the exhaust manifold  21  is set to be 70% or more of the surface area of an upper portion of the exhaust manifold  21 . Thus, the area of the exhaust manifold  21  covered by the upper water jacket  22  is set to be larger than that covered by the lower water jacket  23 . 
     The upper water jacket  22  includes a first end and a second end in the cylinder arrangement direction. The first end includes a second inlet  26  to which the coolant is supplied from the coolant passage  11  of the cylinder block  10 . The second end is connected to the second communication passage  25  through which the coolant is supplied from the lower water jacket  23 . The second end also includes a coolant outlet  27  that connects the upper water jacket  22  to the exterior. Thus, the coolant, which is supplied to the upper water jacket  22  from the second inlet  26  and the second communication passage  25 , flows toward the coolant outlet  27  and is discharged from the coolant outlet  27  to, for example, a radiator arranged at an outer side. 
     As described above, in each of the water jackets  22 ,  23 , the first end, which includes the first inlet  24  or the second inlet  26 , corresponds to an upstream side end in a flow direction of the coolant. The second end, which includes the second communication passage  25 , corresponds to a downstream side end in the flow direction of the coolant. 
       FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 3 . 
     As shown in  FIG. 4 , the second inlet  26  of the upper water jacket  22  is in communication with the first inlet  24  of the lower water jacket  23 . The first inlet  24  is in communication with the coolant passage  11  of the cylinder block  10 . Thus, the coolant is supplied from the coolant passage  11  to each of the water jackets  22 ,  23  through each of the inlets  24 ,  26 . 
     Further, as shown in  FIGS. 2 and 3 , the water jackets  22 ,  23  include a first communication passage  28  and a third communication passage  29 . The coolant is supplied from the lower water jacket  23  to the upper water jacket  22  through the first communication passage  28  and the third communication passage  29  in addition to the second communication passage  25 . The first communication passage  28  is located at an upstream side of the collector  212  in the flow direction of the coolant and arranged at a position that is the most proximate to the collector  212 . The third communication passage  29  is located at a downstream side of the collector  212  in the flow direction of the coolant and arranged at a position where the distance from the third communication passage  29  to the collector  212  is substantially the same as the distance from the first communication passage  28  to the collector  212 . 
       FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 3 . 
     As shown in  FIG. 5 , the first communication passage  28  connects the two water jackets  22 ,  23  to each other at a location toward the upstream side from the collector  212 . The third communication passage  29  connects the two water jackets  22 ,  23  to each other at a location toward the downstream side from the collector  212 . That is, the collector  212  is located between the first communication passage  28  and the third communication passage  29 . Thus, the upper water jacket  22 , the first communication passage  28 , and the third communication passage  29  are arranged to surround an upper side and two opposite sides of the collector  212 . 
     The second communication passage  25  will now be described with reference to  FIG. 6 .  FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 3 . 
     As shown in  FIG. 6 , the second communication passage  25  includes an upstream side opening  251  that opens to the lower water jacket  23  and a downstream side opening  252  that opens to the upper water jacket  22 . The entire second communication passage  25  is inclined relative to the vertical direction (up-down direction in the drawing) so that the downstream side opening  252  is located closer to a side of the coolant outlet  27  than the upstream side opening  251  as viewed from the vertical direction (up-down direction in the drawing). More specifically, a coolant flow passage direction of the downstream side opening  252  extends toward the coolant outlet  27 . The cross-sectional passage area S 2  of the second communication passage  25  is set to be smaller than the cross-sectional passage area S 1  ( FIG. 5 ) of the first communication passage  28 . 
     The operation of the cooling structure for the cylinder head  20 , which is configured in the above manner, will now be described. 
     Exhaust constantly flows to the collector  212  of the exhaust manifold  21  from one of the branches  211 . Thus, the collector  212  tends to have a high temperature due to heat of the exhaust. The exhaust manifold  21  is curved so that the downstream side portion is located below the upstream side portion. Thus, the exhaust flowing to the exhaust manifold  21  from the combustion chamber  30  tends to strike an upper portion of an inner wall of the exhaust manifold  21 . This causes the upper portion of the exhaust manifold  21  to have a high temperature compared to a lower portion. More specifically, in the exhaust manifold  21 , the collector  212 , particularly, an upper portion of the collector  212 , tends to have a high temperature. 
     As indicated by arrows of  FIG. 2 , in the present embodiment, when a coolant is supplied from the cylinder block  10  to the lower water jacket  23  through the first inlet  24  and flows toward the second communication passage  25 , some of the coolant is supplied to the upper water jacket  22  from the first communication passage  28  and the third communication passage  29 . In this case, the cross-sectional passage area S 2  of the second communication passage  25  is set to be smaller than the cross-sectional passage area S 1  of the first communication passage  28 . Thus, more coolant is supplied to the upper water jacket  22  through the first communication passage  28  than when a different setting is used. This increases a flow rate of the coolant flowing to a portion of the upper water jacket  22  that covers the upper portion of the collector  212 . 
     As described above, the upper portion of the exhaust manifold  21  tends to have a high temperature compared to the lower portion. In this regard, in the present embodiment, the area of the exhaust manifold  21  covered by the upper water jacket  22  is larger than the area of the exhaust manifold  21  covered by the lower water jacket  23 . This increases the amount of the coolant flowing above the exhaust manifold  21  compared to the amount of the coolant flowing to the lower water jacket  23 . 
     The cylinder head  20  is configured so that the collector  212  of the exhaust manifold  21  is located between the first communication passage  28  and the third communication passage  29 . Thus, the coolant is supplied through the first communication passage  28  and the third communication passage  29  to a portion that covers the two opposite sides of the collector  212 . 
     In the second communication passage  25 , the flow passage direction of the downstream side opening  252 , which opens to the upper water jacket  22 , extends toward the coolant outlet  27 . Thus, when flowing to the upper water jacket  22  from the second communication passage  25 , the coolant flows toward the coolant outlet  27 . This generates flow of the coolant toward the coolant outlet  27  in the upper water jacket  22 . Consequently, more coolant is discharged to the exterior from the coolant outlet  27 . This increases the amount of the coolant flowing through each of the water jackets  22 ,  23 . 
     The embodiment, which has been described above, has the advantages described below. 
     (1) The present embodiment increases the flow rate of the coolant flowing to the portion of the upper water jacket  22  that covers the upper portion of the collector  212 . This effectively cools the upper portion of the collector  212 . 
     (2) The present embodiment effectively cools the upper portion of the exhaust manifold  21 , which tends to have a high temperature, while limiting excessive cooling of the lower portion of the exhaust manifold  21 . 
     (3) In the present embodiment, the coolant is supplied through the first communication passage  28  and the third communication passage  29  to a portion that covers the two opposite sides of the collector  212 . This effectively cools not only the upper portion of the collector  212  but also the sides of the collector  212 . 
     (4) The present embodiment increases the amount of the coolant flowing through each of the water jackets  22 ,  23 . This effectively cools the exhaust manifold  21 . 
     The embodiment may be modified as follows. 
     In the embodiment, the area of the exhaust manifold  21  covered by the lower water jacket  23  is set to be 40% or less of the surface area of the lower portion of the exhaust manifold  21 . The area of the exhaust manifold  21  covered by the upper water jacket  22  is set to be 70% or more of the surface area of the upper portion of the exhaust manifold  21 . However, the condition for the setting may be modified in accordance with various conditions, such as the degree of overheating of the exhaust manifold  21 . 
     In each embodiment, the third communication passage  29  is located at the downstream side of the collector  212  and arranged at the position where the distance from the third communication passage  29  to the collector  212  is substantially the same as the distance from the first communication passage  28  to the collector  212 . However, the distance from the third communication passage  29  to the collector  212  may be modified as long as the sides of the collector  212  can be cooled. 
     In each embodiment, the entire second communication passage  25  is inclined relative to the vertical direction so that the downstream side opening  252  is located closer to the side of the coolant outlet  27  than the upstream side opening  251  as viewed from the vertical direction. However, the flow passage direction may extend toward the coolant outlet  27  by inclining only the downstream side opening  252  or attaching another member to the interior of the downstream side opening  252 . The flow passage direction of the downstream side opening  252 , which opens to the upper water jacket  22 , only needs to extend toward the coolant outlet  27 . 
     In each embodiment, in at least one of the first communication passage  28 , the third communication passage  29 , and the second inlet  26 , the flow passage direction of a portion that opens to the upper water jacket  22  may extend toward the coolant outlet  27 . 
     In each embodiment, in the second communication passage  25 , the flow passage direction of the downstream side opening  252 , which opens to the upper water jacket  22 , is set to extend toward the coolant outlet  27 . However, this configuration may be omitted. Advantages (1) to (3) described above may be obtained even when this configuration is omitted. 
     The third communication passage  29  may be omitted from each embodiment. Advantages (1), (2), and (4) described above may be obtained even in a configuration in which the third communication passage  29  is omitted. 
     In each embodiment, when the temperature of the upper portion of the exhaust manifold  21  is not that high compared to the temperature of the lower portion, the area of the exhaust manifold  21  covered by the upper water jacket  22  may be set to be substantially the same as the area of the exhaust manifold  21  covered by the lower water jacket  23 . Advantages (1), (3), and (4) described above may be obtained even when the above setting is used. 
     DESCRIPTION OF REFERENCE SYMBOLS 
       10  cylinder block 
       11  coolant passage 
       20  cylinder head 
       21  exhaust manifold 
       22  upper water jacket 
       23  lower water jacket 
       24  first inlet 
       25  second communication passage 
       26  second inlet 
       27  coolant outlet 
       28  first communication passage 
       29  third communication passage 
       30  combustion chamber 
       211  branch 
       212  collector 
       251  upstream side opening 
       252  downstream side opening