Abstract:
Provided is an engine hood comprising: an outer panel that constitutes an outer part of the engine hood; and an inner panel that constitutes an inner part of the engine hood and that is joined to the outer panel such that a space is formed between the inner panel and the outer panel. A porous section that comprises a plurality of through holes is provided to the inner panel. An external air intake hole that guides external air into the space is provided to the engine hood.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an engine hood. 
       BACKGROUND ART 
       [0002]    In automobiles such as passenger cars, trucks, and buses, noise is caused in an engine room while an engine is operating. Such noise is transmitted from the inside of the engine room to the outside and inside of a vehicle compartment. 
         [0003]    Patent Document 1 discloses a sound absorbing material attached to a lower surface of a hood panel to absorb engine noise of an automobile and made of a porous base material. The porous base material is made of organic fibers such as polyester fibers or polyethylene fibers, or is made of a plastic foaming body with an open-cell structure, such as a polyester foaming body or polyethylene foaming body. 
         [0004]    However, such a sound absorbing material is typically effective to noise outside a vehicle compartment, i.e., noise in a frequency band with a relatively-high frequency of equal to or higher than 1000 kHz. However, the sound absorbing material is less effective to noise inside the vehicle compartment, i.e., noise in a low frequency band of equal to or lower than 1000 Hz. 
         [0005]    Thus, Patent Document 2 discloses a soundproof engine hood configured to reduce noise inside a vehicle compartment by a sound absorbing effect of a resonance sound absorbing pipe provided in an engine hood. Moreover, Patent Document 3 discloses a muffling device of an internal combustion engine configured such that a resonance muffling chamber communicating with an air intake path of the engine is provided in a hollow reinforcing frame attached to a rear surface of an engine hood. 
       CITATION LIST 
     Patent Document 
       [0006]    Patent Document 1: JP 2004-334022 
         [0007]    Patent Document 2: JP 2001-247055 
         [0008]    Patent Document 3: JP 04-284154 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0009]    However, the Helmholtz resonators disclosed in Patent Documents 2 and 3 basically act only on a single frequency. For this reason, a wide variety of noise in a low frequency band of equal to or lower than 1000 Hz cannot be reduced. 
         [0010]    The present invention is intended to provide an engine hood capable of reducing a wide variety of noise in a low frequency band in an engine room. 
       Solution to Problem 
       [0011]    The engine hood of the present invention is an engine hood covering an engine room provided at a front portion of a vehicle. The engine hood includes an outer panel forming an outer portion of the engine hood, and an inner panel joined to the outer panel such that a space is formed between the inner panel and the outer panel and forming an inner portion of the engine hood. The inner panel includes a porous portion provided with a plurality of through-holes, and the engine hood is provided with an external air intake hole through which external air is introduced into the space. 
       Advantageous Effects of the Invention 
       [0012]    According to the present invention, since the space between the outer panel and the inner panel communicates with the engine room through the porous portion, the hollow engine hood acts as a Helmholtz resonator. This can reduce noise in a low frequency band in the engine room. Moreover, when sound waves from an engine pass through the through-holes, part of sound wave energy is converted into thermal energy by friction with an inner wall surface of each through-hole. After the sound waves have passed through the through-holes, eddies are caused at the outlet and inlet of each through-hole due to the alternating flow of the sound waves, leading to a pressure loss. This results in greater attenuation by the holes. Thus, a wide variety of noise in the low frequency band in the engine room can be reduced. In addition, the external air introduced into the space through the external air intake hole passes, as a steady flow (wind), through the through-holes. Due to the eddies caused when such a steady flow passes through the through-holes, the sound waves from the engine are simultaneously attenuated when passing through the through-holes. As a result, the sound wave energy lost in the holes increases. For this reason, sound wave attenuation becomes greater, and a sound absorption coefficient increases in a wide low frequency band. Consequently, a wide variety of noise in the low frequency band in the engine room can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of a front portion of a vehicle. 
           [0014]      FIG. 2  is a view of an engine hood from below. 
           [0015]      FIG. 3A  is an enlarged view of a main portion III of  FIG. 1 . 
           [0016]      FIG. 3B  is an enlarged view of the main portion III of  FIG. 1 . 
           [0017]      FIG. 4  is a conceptual diagram of an embodiment of the present invention. 
           [0018]      FIG. 5  is a schematic configuration diagram of a sound tube experimental device. 
           [0019]      FIG. 6  is a graph of measurement results of a sound absorption coefficient. 
           [0020]      FIG. 7  is a view of an engine hood from below. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Hereinafter, suitable embodiments of the present invention will be described with reference to drawings. 
       First Embodiment 
       [0022]    (Vehicle Configuration) 
         [0023]    An engine hood  1  of a first embodiment of the present invention is, as illustrated in  FIG. 1 , provided at a vehicle  10  such as an automobile. The vehicle  10  includes an engine room  11  at a front portion of the vehicle  10  in a forward direction indicated by an arrow in  FIG. 1 . A radiator  12 , a cooling fan  13 , and an engine  14  are arranged in this order from the front side toward the rear side in the engine room  11 . A front wall  15  defining a front end of the engine room  11  is provided with an external air intake port  16  through which external air is introduced into the engine room  11 . An upper portion of the engine room  11  is openably covered with the engine hood  1 . A vehicle compartment  17  in which a driver operates the vehicle  10  is provided on the rear side of the engine room  11 . 
         [0024]    (Engine Hood Configuration) 
         [0025]    The engine hood  1  of the present embodiment is made of metal, and includes an outer panel  2  and an inner panel  3 . The outer panel  2  forms an outer portion of the engine hood  1 . The inner panel  3  is, by, e.g., welding, joined to the outer panel  2  such that a space  4  is formed between the inner panel  3  and the outer panel  2 , and forms an inner portion of the engine hood  1 . 
         [0026]    The inner panel  3  includes a porous portion  3   a  provided with a plurality of through-holes. The porous portion  3   a  communicates with the space  4 . The diameter of each through-hole of the porous portion  3   a  is 0.3 mm to 5 mm, and preferably equal to or greater than 1 mm, for example. When the diameter of each through-hole is equal to or greater than 1 mm, a later-described steady flow can suitably pass through the through-holes. Moreover, the aperture ratio of the porous portion  3   a  is about 1% to about 5%, for example. Note that the diameter of the through-hole and the aperture ratio are not limited to the above-described values. 
         [0027]    A sound absorbing material  18  is attached to the surface of the inner panel  3  close to the engine room  11 . The sound absorbing material  18  is made of a porous material such as glass wool. The sound absorbing material  18  is disposed on the rear side of the porous portion  3   a  to face the engine  14 . Thus, the sound absorbing material  18  absorbs noise in the engine room  11 , particularly noise in a high frequency band. 
         [0028]    The engine hood  1  is provided with an external air intake hole  5  through which external air is introduced into the space  4 . The external air intake hole  5  is provided at the position on the front side of the porous portion  3   a,  and in the present embodiment, is provided at a front end of the engine hood  1 . Note that the external air intake hole  5  may be provided at the boundary between the outer panel  2  and the inner panel  3 , or may be provided at the outer panel  2  or the inner panel  3 . In the present embodiment, the external air intake hole  5  is provided at the inner panel  3 . 
         [0029]    With the external air intake hole  5  provided at the engine hood  1 , part of external air flowing from the front side toward the rear side of the vehicle  10  is introduced into the space  4  through the external air intake hole  5  while the vehicle  10  is running in the forward direction. Subsequently, the external air introduced into the space  4  passes, as a steady flow (wind), through the through-holes of the porous portion  3   a,  and then, is introduced into the engine room  11 . Such air is discharged to the outside of the engine room  11  through a plurality of not-shown vent holes communicating with the engine room  11 . 
         [0030]    The engine hood  1  includes a pair of guides  6  in the space  4 .  FIG. 2  is a view of the engine hood  1  from below. As illustrated in  FIG. 2 , the pair of guides  6  is configured to guide, to the porous portion  3   a,  the external air introduced into the space  4  through the external air intake hole  5 . 
         [0031]    The engine hood  1  includes a partitioning member  7  configured to divide the space  4  into a communication space  4   a  communicating with the porous portion  3   a  and the external air intake hole  5  and a closed space  4   b . The porous portion  3   a  and the communication space  4   a  form a Helmholtz resonator. The resonance frequency targeted for sound absorption by the resonator varies depending on the volume of the communication space  4   a . A greater volume of the communication space  4   a  results in a lower resonance frequency. Since the space  4  is divided by the partitioning member  7 , the volume of the communication space  4   a  is optimized so that the Helmholtz resonator formed of the porous portion  3   a  and the communication space  4   a  can absorb noise with the frequency targeted for sound absorption. 
         [0032]    A sealing member  20  is attached to the surface of the inner panel  3  close to the engine room  11 . As illustrated in  FIG. 1 , the sealing member  20  is configured to seal the clearance between the inner panel  3  and an upper wall  19  protruding from an upper end of the front wall  15 , which defines the front end of the engine room  11 , toward the rear side to face the inner panel  3  when the upper portion of the engine room  11  is covered with the engine hood  1 . With this configuration, when the vehicle  10  moves forward, external air is not introduced into the engine room  11  through the clearance between the upper wall  19  and the inner panel  3 , but is introduced into the space  4  through the external air intake hole  5 . 
         [0033]    As illustrated in  FIG. 3A  as an enlarged view of a main portion III of  FIG. 1 , the inner panel  3  is provided with an upstream guide  3   b  configured to guide external air toward the external air intake hole  5 . The upstream guide  3   b  is provided to extend from the lower side of the external air intake hole  5  toward the rear side in the forward direction (see  FIG. 1 ). The left end of the upstream guide  3   b  as viewed in  FIG. 3A  is separated from the external air intake hole  5 , and is positioned in the vicinity of the upper wall  19 . The right end of the upstream guide  3   b  as viewed in  FIG. 3A  is connected to the inner panel  3 . The external air entering the clearance between the upper wall  19  and the inner panel  3  is guided to the external air intake hole  5  by the upstream guide  3   b.  Then, such air is introduced into the space  4  through the external air intake hole  5 . As described above, since external air is guided to the external air intake hole  5  by the upstream guide  3   b , external air can be easily taken through the external air intake hole  5 . 
         [0034]    Note that as illustrated in  FIG. 3B  as an enlarged view of the main portion III of  FIG. 1 , an upstream guide  19   b  may be provided at the upper wall (a wall portion)  19 . At part of the upper wall  19  facing the external air intake hole  5 , the upstream guide  19   b  is provided in the shape raised toward the external air intake hole  5 . The upstream guide  19   b  has the surface extending from the lower side of the external air intake hole  5  toward the rear side in the forward direction (see  FIG. 1 ) to incline to the external air intake hole  5 . An external air intake path is formed between such an inclined surface and the external air intake hole  5 . Even with the upstream guide  19   b  described above, external air can be easily taken through the external air intake hole  5 . 
         [0035]    A conceptual diagram of the present embodiment is illustrated in FIG.  4 . A speaker  22  as a sound source is attached to one end of a sound tube  21 , and a porous plate  23  provided with a plurality of through-holes is placed in the sound tube  21 . An external air intake hole  24  is provided at the other end of the sound tube  21 . The speaker  22  corresponds to the engine  14 , the space A between the speaker  22  and the porous plate  23  corresponds to the inside of the engine room  11 , and the space B between the porous plate  23  and the other end of the sound tube  21  corresponds to the space  4  (the communication space  4   a ) between the outer panel  2  and the inner panel  3 . 
         [0036]    In such a configuration, noise is, as illustrated in  FIG. 1 , caused in the engine room  11  during operation of the engine  14 . Such noise is transmitted from the inside of the engine room  11  to the outside of the engine room  11 , i.e., the outside of the vehicle compartment  17 , and then, is transmitted to the inside of the vehicle compartment  17 . Since the space  4  between the outer panel  2  and the inner panel  3  communicates with the engine room  11  through the porous portion  3   a,  the hollow engine hood  1  acts as the Helmholtz resonator. This can reduce noise in a low frequency band in the engine room  11 . In  FIG. 4 , the porous plate  23  and the space B act as a Helmholtz resonator. 
         [0037]    However, the Helmholtz resonator basically acts only on a single frequency (a resonance frequency). For this reason, in the present embodiment, the plurality of through-holes of the porous portion  3   a  of the inner panel  3  absorb a wide variety of noise in the low frequency band in the engine room  11 . That is, when sound waves from the engine  14  pass through the through-holes, part of sound wave energy is converted into thermal energy due to friction with an inner wall surface of each through-hole. After the sound waves have passed through the through-holes, eddies are caused at the outlet and inlet of each through-hole due to the alternating flow of the sound waves, leading to a pressure loss. This results in greater attenuation by the holes. Thus, a wide variety of noise in the low frequency band in the engine room  11  can be reduced. For example, noise outside the vehicle compartment, i.e., noise in a low frequency band of equal to or lower than 1000 Hz can be reduced. In  FIG. 4 , the sound waves passing through the through-holes of the porous plate  23  are attenuated. 
         [0038]    In addition, in the present embodiment, the external air introduced into the space  4  through the external air intake hole  5  passes, as illustrated in  FIG. 1 , through the through-holes as a steady flow (wind). Due to the eddies caused when such a steady flow passes through the through-holes, the sound waves from the engine  14  are simultaneously attenuated when passing through the through-holes. As a result, the sound wave energy lost in the holes increases. For this reason, sound wave attenuation becomes greater, and a sound absorption coefficient increases in a wide low frequency band. Consequently, a wide variety of noise in the low frequency band in the engine room  11  can be reduced. In  FIG. 4 , the steady flow passing through the porous plate  23  by way of the external air intake hole  24  enhances sound wave attenuation. Thus, the sound waves output from the speaker  22  and reflected on the porous plate  23  are weaker than the sound waves output from the speaker  22 . 
         [0039]    A sound wave is a longitudinal wave (a compressional wave). For this reason, when passing through the through-holes, the sound waves are simultaneously attenuated due to the eddies caused when the steady flow passes through each through-hole even if the direction of the steady flow is the same as or opposite to the traveling direction of the sound wave. Note that in the case where the direction of the steady flow is the same as the traveling direction of the sound wave in  FIG. 1  (i.e., the case where the direction of the steady flow is opposite to the direction indicated by the arrow in  FIG. 1 ), the steady flow tends to direct toward a region with a lower pressure loss. For this reason, the steady flow does not tend to direct toward the through-holes, but tends to direct toward the not-shown vent holes communicating with the engine room  11 . As a result, a majority portion of the steady flow does not pass through the through-holes, leading to a smaller effect of the steady flow. On the other hand, in the case where the direction of the steady flow is opposite to the traveling direction of the sound wave as in the present embodiment, there is nowhere that the steady flow can pass through, except for the through-holes. Thus, the steady flow can suitably pass through the through-holes, leading to a greater effect of the steady flow. 
         [0040]    As illustrated in  FIG. 1 , since the engine hood  1  is provided with the external air intake hole  5  positioned on the front side of the porous portion  3   a,  external air can be constantly introduced into the communication space  4   a  through the external air intake hole  5  while the vehicle  10  is running. Thus, while the vehicle  10  is running, the steady flow constantly passes through the through-holes. Consequently, a sound wave attenuation effect can be suitably enhanced. 
         [0041]    As illustrated in  FIG. 2 , the volume of the communication space  4   a  can be freely changed by the partitioning member  7  dividing the space  4  into the communication space  4   a  and the closed space  4   b.  With this configuration, the volume of the communication space  4   a  can be optimized such that the Helmholtz resonator formed of the porous portion  3   a  and the communication space  4   a  absorbs noise with the frequency targeted for sound absorption. In  FIG. 4 , the volume of the space B is optimized depending on the frequency targeted for sound absorption. 
         [0042]    Moreover, as illustrated in  FIG. 2 , the external air introduced into the communication space  4   a  through the external air intake hole  5  is guided to the porous portion  3   a  by the guides  6 , and therefore, it is ensured that the air passes through the through-holes. Thus, the steady flow passing through the through-holes can be suitably generated. 
         [0043]    Further, as illustrated in  FIGS. 3A and 3B , the external air is guided to the external air intake hole  5  by the upstream guides  3   b,    19   b,  and therefore, can be easily taken through the external air intake hole  5 . As a result, more external air can be taken through the external air intake hole  5 . 
         [0044]    (Sound Absorption Coefficient Measurement) 
         [0045]    Next, results of measurement of a change in a sound absorption coefficient due to a steady flow passing through a porous plate will be described.  FIG. 5  is a schematic configuration diagram of a sound tube experimental device  30 . As illustrated in  FIG. 5 , the sound tube experimental device  30  is configured such that a porous plate  33  provided with a plurality of through-holes is placed in a sound tube  31  whose one end is attached to a speaker  32 . In both of the case where an air flow circulates in the sound tube  31  such that a steady flow passes through the porous plate  33  and the case where an air flow does not circulate in the sound tube  31  such that no steady flow passes through the porous plate  33 , the sound from the speaker  32  was measured by a microphone  34 . The sound absorption coefficient of the porous plate  33  was measured based on the obtained sound measurement results. Note that the direction of the steady flow passing through the porous plate  33  is opposite to a sound wave traveling direction. The results are shown in  FIG. 6 . In the case where the steady flow passes through the porous plate  33 , the results show that the sound absorption coefficient of the porous plate  33  increases in a wide low frequency band of 250 Hz to 1750 Hz. 
         [0046]    (Advantageous Effects) 
         [0047]    According to the engine hood  1  of the present embodiment, the space  4  between the outer panel  2  and the inner panel  3  communicates with the engine room  11  through the porous portion  3   a  as described above, and therefore, the hollow engine hood  1  acts as the Helmholtz resonator. Thus, noise in the low frequency band in the engine room  11  can be reduced. Moreover, when the sound waves from the engine  14  pass through the through-holes, part of the sound wave energy is converted into the thermal energy due to friction with the inner wall surface of each through-hole. After the sound waves have passed through the through-holes, the eddies are caused at the outlet and inlet of each through-hole due to the alternating flow of the sound waves, leading to the pressure loss. This results in greater sound wave attenuation by the holes. Thus, a wide variety of noise in the low frequency band in the engine room  11  can be reduced. In addition, the external air introduced into the space  4  through the external air intake hole  5  passes through the through-holes as the steady flow (wind). Due to the eddies caused when the steady flow passes through the through-holes, the sound waves from the engine  14  are simultaneously attenuated when passing through the through-holes. As a result, the sound wave energy lost in the holes increases. For this reason, sound wave attenuation becomes greater, and the sound absorption coefficient increases in a wide low frequency band. Consequently, a wide variety of noise in the low frequency band in the engine room  11  can be reduced. 
         [0048]    Since the engine hood  1  is provided with the external air intake hole  5  positioned on the front side of the porous portion  3   a,  external air can be constantly introduced into the space  4  through the external air intake hole  5  while the vehicle  10  is running. Thus, while the vehicle  10  is running, the steady flow constantly passes through the through-holes. Consequently, the sound wave attenuation effect can be suitably enhanced. 
         [0049]    The volume of the communication space  4   a  can be changed by the partitioning member  7  dividing the space  4  into the communication space  4   a  and the closed space  4   b.  With this configuration, the volume of the communication space  4   a  can be optimized such that the Helmholtz resonator formed of the porous portion  3   a  and the communication space  4   a  absorbs noise with the frequency targeted for sound absorption. 
         [0050]    The external air introduced into the space  4  through the external air intake hole  5  is guided to the porous portion  3   a  by the guides  6 , and therefore, it is ensured that the air passes through the through-holes. Thus, the steady flow passing through the through-holes can be suitably generated. 
         [0051]    External air is guided to the external air intake hole  5  by the upstream guides  3   b,    19   b,  and therefore, can be easily taken through the external air intake hole  5 . As a result, more external air can be taken through the external air intake hole  5 . 
       Second Embodiment 
       [0052]    (Engine Hood Configuration) 
         [0053]    Next, an engine hood  201  of a second embodiment of the present invention will be described. Note that the same reference numerals as those used for the above-described components are used to represent equivalent elements, and description thereof will not be repeated.  FIG. 7  is a view of the engine hood  201  of the present embodiment from below. The engine hood  201  of the present embodiment illustrated in  FIG. 7  is different from the engine hood  1  of the first embodiment in that a porous portion  3   a  is provided at an inner panel  3  at the most downstream position in the flow of external air introduced into a communication space  4   a  through an external air intake hole  5  and flowing in the communication space  4   a.    
         [0054]    The external air introduced into the communication space  4   a  through the external air intake hole  5  flows in the communication space  4   a . Such air passes, as a steady flow (wind), through through-holes of the porous portion  3   a  provided at the most downstream position in the flow of the air, and then, is introduced into an engine room  11 . Thus, it is ensured that the external air passes through the through-holes, and the steady flow passing through the through-holes can be suitably generated. 
         [0055]    The engine hood  201  includes a sound absorbing material  8  provided in the communication space  4   a.  The sound absorbing material  8  contains a porous material such as glass wool. The sound absorbing material  8  is configured to absorb noise in a high-frequency band, the noise entering the communication space  4   a  through the porous portion  3   a.  Thus, a wider band can be targeted for sound absorption. 
         [0056]    Note that the engine hood  201  may include a not-shown sound absorption mechanism instead of the sound absorbing material  8  or in addition to the sound absorbing material  8 . Examples of the sound absorption mechanism include the mechanism including a porous plate provided with a plurality of through-holes, a back plate disposed to face the porous plate with a predetermined distance from the porous plate, and a frame body surrounding the space between the porous plate and the back plate. According to such a sound absorption mechanism, the porous plate and the space form a Helmholtz resonator, and such a resonator provides an effect of reducing noise with a resonance frequency and absorbing sound waves passing through the through-holes. 
         [0057]    (Advantageous Effects) 
         [0058]    According to the engine hood  201  of the present embodiment, the porous portion  3   a  is, as described above, provided at the most downstream position in the flow of external air introduced into the communication space  4   a  through the external air intake hole  5  and flowing in the communication space  4   a.  Thus, it is ensured that the external air passes through the through-holes, and the steady flow passing through the through-holes can be suitably generated. 
         [0059]    The sound absorbing material  8  is provided in the communication space  4   a  to absorb the noise entering the communication space  4   a  from the engine room  11 . Thus, a wider band can be targeted for sound absorption. 
         [0060]    The sound absorbing mechanism including the Helmholtz resonator formed of the porous plate and the space is provided in the communication space  4   a,  and therefore, the noise entering the communication space  4   a  from the engine room  11  can be absorbed by a sound absorption effect by the through-holes and a sound absorption effect by a resonance principle. Thus, a wider band can be targeted for sound absorption. 
         [0061]    (Variations of the Embodiments) 
         [0062]    The embodiments of the present invention have been described above, but have been set forth merely as specific examples. These embodiments are not intended to limit the present invention, and design change can be optionally made to, e.g., a specific configuration. Moreover, the features and advantageous effects described in the embodiments of the present invention have been merely listed as most preferable features and advantageous effects of the present invention, and the features and advantages effects of the present invention are not limited to those described in the embodiments of the present invention. 
         [0063]    This application is based on the Japanese patent application (Japanese Patent Application No. 2014-25737) filed on Feb. 13, 2014, the entire contents of which are incorporated by reference herein. 
       EXPLANATION OF REFERENCE NUMERALS 
       [0000]    
       
           1 ,  201  engine hood 
           2  outer panel 
           3  inner panel 
           3   a  porous portion 
           3   b  upstream guide 
           4  space 
           4   a  communication space 
           4   b  closed space 
           5  external air intake hole 
           6  guide 
           7  partitioning member 
           8  sound absorbing material 
           10  vehicle 
           11  engine room 
           16  external air intake port 
           18  sound absorbing material 
           19  upper wall (wall portion) 
           19   b  upstream guide