Patent Publication Number: US-7712684-B2

Title: Fuel injection valve

Description:
The entire disclosure of Japanese Patent Application No. 2004-368558 including specification, claims, drawings and abstract is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fuel injection valve which injects a fuel from a nozzle hole, and more particularly to a fuel injection valve which generates a cavitation bubble in fuel flowing inside an injection valve. 
   2. Description of the Related Art 
   Fuel injection valves which generate cavitation bubbles in fuel flowing inside an injection valve in order to enhance atomization in a fuel spray have been proposed. Japanese Patent Laid-Open Publication No. 2003-83205, Japanese Patent Laid-Open Publication No. 2004-19481, and N. Tamaki et al., “Atomization Enhancement of the Spray and Improvement of the Spray Characteristics by Cavitation and Pin Inserted in the Nozzle Hole”, ICLASS, 2003 describe related art of such a structure. The fuel injection valve of Japanese Patent Laid-Open Publication No. 2003-83205 has a cavitation generator which generates cavitation bubbles in the fuel and a cavitation eliminator which eliminates the cavitation bubbles generated by the cavitation generator. In the fuel injection valve of this reference, disturbance is caused in the fuel flow within the nozzle hole by an impact pressure which is generated during disappearance of the cavitation bubbles to enhance atomization of the fuel spray. 
   In a fuel injection valve according to Japanese Patent Laid-Open Publication No. 2004-19481, the nozzle hole is separated into a first nozzle hole portion on the upstream side and a second nozzle hole portion on the downstream side. By setting the cross sectional area of the second nozzle hole portion to be larger than the cross sectional area of the first nozzle hole portion, a storage portion which stores the fuel is formed between the inner wall of the second nozzle hole portion and the fuel jet flowing from the first nozzle hole portion. Cavitation bubbles are generated within a shearing layer which is created by a velocity difference between the fuel stored in the storage portion and the fuel jet flowing from the first nozzle hole portion. In this manner, cavitation bubbles are formed near an outer peripheral surface of the fuel jet and the energy when the cavitation bubbles collapse is used for atomization of the fuel spray. 
   In the fuel injection valve of N. Tamaki et al., the nozzle hole is configured so that a gap portion is provided between an upstream nozzle hole and a downstream nozzle hole. Cavitation bubbles generated by the upstream nozzle hole collapse in the gap portion due to attenuation of the fuel flow and recovery of pressure. In addition, because a projecting pin is provided inside the nozzle hole, the cavitation bubbles also collapse in the downstream nozzle hole. A disturbance is caused in the fuel flow within the nozzle hole by the collapse of the cavitation bubbles so that the atomization in the fuel spray is enhanced. 
   In order to effectively achieve the atomization enhancement effect of the fuel spray by cavitation collapse over the entire region of the fuel jet after injection, it is desirable to inject fuel in which the cavitation bubbles are uniformly mixed (or mixed in an approximate uniform manner) from the nozzle hole. 
   In the fuel injection valves of Japanese Patent Laid-Open Publication No. 2003-83205 and N. Tamaki et al., because the cavitation bubbles disappear within the injection valve, only the fuel in liquid from is present in the fuel jet downstream of the exit of the nozzle hole. Therefore, the atomization enhancement effect of the fuel spray by the cavitation collapse cannot be obtained in the fuel jet after injection. 
   In the fuel injection valve of Japanese Patent Laid-Open Publication No. 2004-19481, although fuel in which the cavitation is mixed can be injected from the nozzle hole, the formation region of the cavitation bubbles is limited to a region near an outer peripheral surface of the fuel jet and a core of liquid phase remains around the center of the fuel jet. Because of this, the atomization enhancement effect of the fuel spray by the cavitation collapse cannot be obtained in a wide area in the fuel jet after injection. 
   SUMMARY OF THE INVENTION 
   The present invention advantageously provides a fuel injection valve which can further enhance atomization in a fuel spray which is injected. 
   According to one aspect of the present invention, there is provided a fuel injection valve which injects fuel from a nozzle hole, comprising a cavitation generation flow path in which a cavitation bubble is generated in fuel flowing inside the injection valve, and a bubble storage flow path which is connected to the cavitation generation flow path and the nozzle hole and which stores the cavitation bubble generated in the cavitation generation flow path. The fuel injection valve according to the present invention injects, from the nozzle hole, fuel containing the cavitation bubble stored in the bubble storage flow path. 
   In the present invention, cavitation bubbles are generated in the cavitation generation flow path, stored in the bubble storage flow path, and mixed with fuel of liquid phase and the fuel is introduced to the nozzle hole. As a result, fuel in a mixture state of gas and liquid can be injected from the nozzle hole. Therefore, according to the present invention, the atomization enhancement effect in the fuel spray due to collapse of cavitation can be effectively obtained over the entire region of the fuel jet after the injection, and consequently, the atomization of the injected fuel spray can be further enhanced. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, an exit which connects the bubble storage flow path and the nozzle hole is provided in the bubble storage flow path, and the bubble storage flow path is connected to the cavitation generation flow path and the nozzle hole while the exit to the nozzle hole is offset with respect to a direction of a fuel jet flowing in the cavitation generation flow path. With this structure, it is possible to inhibit flowing of the fuel jet from the cavitation generation flow path to the nozzle hole without any process in the bubble storage flow path. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, the fuel is injected from the nozzle hole while a cross sectional area of the bubble storage flow path is larger than a cross sectional area of the cavitation generation flow path. With this structure, a vertical vortex can be formed within the bubble storage flow path so that the cavitation bubble can be stored around the center of the vortex having a lower pressure than the surroundings. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, the fuel is injected from the nozzle hole while a cross sectional area of the nozzle hole is larger than a cross sectional area of the cavitation generation flow path. With this structure, the cavitation bubbles can effectively be generated in the fuel jet flowing into the bubble storage flow path. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, a cross sectional area of an entrance portion of the cavitation generation flow path is gradually reduced toward the bubble storage flow path. With this structure, because a discharge coefficient of the cavitation generation flow path can be increased, a number of generated cavitation bubbles can be increased in the fuel jet entering the bubble storage flow path. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, a cross sectional area of the cavitation generation flow path is reduced toward the bubble storage flow path. With this structure, because the discharge coefficient in the cavitation generation flow path can be increased, the number of generated cavitation bubbles can be increased in the fuel jet entering the bubble storage flow path. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, a wall which forms a part of the bubble storage flow path is provided, an exit which connects the bubble storage flow path and the nozzle hole is provided in the bubble storage flow path, and the wall has a curved surface which curves from a region which approximately opposes a fuel jet flowing in the cavitation generation flow path to the exit to the nozzle hole. With this structure, disappearance of the cavitation bubbles in the bubble storage flow path can be inhibited. 
   According to another aspect of the present invention, it is preferable that, in the fuel injection valve, a swirl generation flow path in which a swirl flow is generated in fuel flowing inside the injection valve is formed upstream of the cavitation generation flow path or in the cavitation generation flow path. With this structure, it is possible to alleviate non-uniformity in the flow rate distribution along a circumferential direction of the injection valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will be described in detail by reference to the drawings, wherein: 
       FIG. 1  is a diagram schematically showing an internal structure of a fuel injection valve according to a preferred embodiment of the present invention; 
       FIG. 2  is a diagram schematically showing a structure of a tip of a nozzle body of a fuel injection valve according to a preferred embodiment of the present invention; 
       FIG. 3  is a diagram for explaining an operation of a fuel injection valve according to a preferred embodiment of the present invention; 
       FIG. 4A  is a diagram of an experimental result of an experiment with the flow visualized, showing a generation state of cavitation bubbles; 
       FIG. 4B  is a diagram of an experimental result of an experiment with the flow visualized, showing a generation state of cavitation bubbles; 
       FIG. 4C  is a diagram of an experimental result of an experiment with the flow visualized, showing a generation state of cavitation bubbles; 
       FIG. 5  is a diagram showing a result of a numerical analysis of a void fraction (volume fraction) of bubbles; 
       FIG. 6  is a diagram showing a result of a numerical analysis of a void fraction (volume fraction) of bubbles; 
       FIG. 7  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 8  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 9  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 10  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 11  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 12  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 13  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 14  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 15  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 16  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 17  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 18  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 19A  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 19B  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 19C  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 20A  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 20B  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 20C  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 21A  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 21B  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 21C  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 22  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 23A  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 23B  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 23C  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 24A  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 24B  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 24C  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 24D  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 25  is a diagram schematically showing another structure of a tip of a nozzle body; 
       FIG. 26  is a diagram schematically showing another structure of a tip of a nozzle body; and 
       FIG. 27  is a diagram schematically showing another structure of a tip of a nozzle body. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will now be described.  FIGS. 1 and 2  are diagrams schematically showing a structure of a fuel injection valve according to a preferred embodiment of the present invention.  FIG. 1  shows an internal structure of the fuel injection valve and  FIG. 2  schematically shows a structure of a tip of a nozzle body. A fuel injection valve of the present embodiment is a fuel injection valve of an externally open valve type and used in, for example, internal combustion engines. 
   A nozzle body  7  is provided inside a valve housing  31 . A nozzle hole  3  is formed on a tip of the nozzle body  7 . A pintle  5  is inserted in a hollow portion formed in the central portion of the nozzle body  7  and is supported in a slidable state along an inner peripheral surface of the nozzle body  7 . One end of the pintle  5  is connected to a plunger  8  and a poppet valve  6  is provided on the other end of the pintle  5 . A biasing force toward the plunger  8  (toward the top of  FIG. 1 ) due to a spring  4  acts on the pintle  5 . 
   Fuel supplied in a pressurized manner by a fuel pump (not shown) flows into a fuel storage portion through a fuel supply flow path  32  formed in the nozzle body  7 . When a piezo-actuator (not shown) is not driven, because the pintle  5  is biased toward the plunger  8  (toward the top of  FIG. 1 ) by the spring  4 , the poppet valve  6  is in contact with a seat portion of the nozzle body  7  and the nozzle hole  3  is closed (valve closure state). In this configuration, the fuel is not injected from the nozzle hole  3 . When, on the other hand, the piezo-actuator is driven, a driving force toward the nozzle hole  3  (toward the bottom of  FIG. 1 ) acts on the plunger  8  and the pintle  5  is driven toward the nozzle hole  3  (toward the bottom of  FIG. 1 ). Because of this, the poppet valve  6  is separated from the seat portion of the nozzle body  7  and the nozzle hole  3  is opened (valve open state). In this case, fuel stored in the fuel storage portion  33  is injected from the nozzle hole  3 . 
   In the present embodiment, cavitation bubbles are generated in fuel flowing inside the nozzle body  7  when the fuel is injected. For this purpose, a projection  34  which projects toward the pintle  5  is formed on an inner peripheral surface of the nozzle body  7  at a position downstream of a fuel storage portion  33  so that a cavitation generation flow path  1  for generating cavitation bubbles is formed downstream of the fuel storage portion  33 . Because of this projection  34 , a cross sectional area of the flow path is stepwise reduced (rapid reduction) in transition from the fuel storage portion  33  to the cavitation generation flow path  1  and the cross sectional area of the flow path is stepwise increased (rapid expansion) in transition from the cavitation generation flow path  1  to a downstream flow path. 
   In addition, in the present embodiment, a bubble storage flow path  2  which stores the cavitation bubbles is formed downstream of the cavitation generation flow path  1  in order to inhibit disappearance of the cavitation bubbles generated in the cavitation generation flow path  1 . The bubble storage flow path  2  is connected to the cavitation generation flow path  1  at its entrance and to the nozzle hole  3  at its exit. A cross sectional area of the bubble storage flow path  2  (minimum cross sectional area) is larger than a cross sectional area of the cavitation generation flow path  1  (minimum cross sectional area). In addition, the exit of the bubble storage flow path  2  is provided at a position offset toward the external side along a radial direction of the injection valve with respect to a direction of the fuel jet flowing in the cavitation generation flow path  1 . In other words, at a position opposing the fuel jet flowing in the cavitation generation flow path  1 , the exit of the bubble storage flow path  2  is not provided and a wall of the bubble storage flow path  2  (an outer peripheral surface of the poppet valve  6 ) is formed. 
   In the structure of  FIG. 2 , the cross sectional area of the bubble storage flow path  2  is rapidly enlarged toward the external side along the radial direction of the injection valve, from the cross sectional area of the cavitation generation flow path  1 . In the bubble storage flow path  2 , while an entrance from the cavitation generation flow path  1  is placed at an inner peripheral side (a side near the pintle  5 ), the exit to the nozzle hole  3  is provided at an outer peripheral side (a side near the nozzle body  7 ). An angle of the nozzle hole  3  is set so that the direction of the fuel jet flowing in the nozzle hole  3  is tilted with respect to the direction of fuel jet flowing in the cavitation generation flow path  1 . 
   An operation of the fuel injection valve according to the present embodiment will now be described. 
   In a valve closure state in which the nozzle hole  3  is closed, the inside of the injection valve is filled with fuel in a liquid phase. When the piezo-actuator is driven from this state to open the nozzle hole  3 , a pressure of the bubble storage flow path  2  is gradually reduced due to formation of a flow field of the fuel and a pressure differential is created between the entrance and exit of the cavitation generation flow path  1 . Because of the creation of the pressure differential, cavitation bubbles start to be generated from a vertex portion of the entrance of the cavitation generation flow path  1  (vertex portion of the projection  34 ), as shown in  FIG. 3 . When the pressure differential is further increased, cavitation bubbles also start to be generated in a shearing layer between the fuel jet flowing out of the exit of the cavitation generation flow path  1  and the fuel surrounding the fuel jet, as shown in  FIG. 3 .  FIG. 3  shows a cavitation  41  generated at the entrance of the cavitation generation flow path  1  and a cavitation  42  generated in the shearing layer downstream of the exit of the cavitation generation flow path  1 . 
   In the present embodiment, the cross sectional area of the bubble storage flow path  2  (minimum cross sectional area) is set to be larger than the cross sectional area of the cavitation generation flow path  1  (minimum cross sectional area), to achieve a rapid-expansion flow at the exit of the cavitation generation flow path  1 , so that a vertical vortex  43  is formed in the bubble storage flow path  2  and cavitation bubbles are stored around the center of the vortex which has a lower pressure than the surroundings. The cavitation bubbles are also generated at the central portion of the vertical vortex  43 . In addition, in order to form a strong vertical vortex  43  in the bubble storage flow path  2 , the exit of the bubble storage flow path  2  to the nozzle hole  3  is offset with respect to the direction of the flow jet flowing in the cavitation generation flow path  1  so that flow of the fuel jet from the cavitation generation flow path  1  to the nozzle hole  3  without any processing is inhibited. 
   The fuel liquid of the bubble storage flow path  2  and the cavitation bubbles are mixed and introduced to the nozzle hole  3  and fuel in a state of mixture of gas and liquid is injected from the nozzle hole  3  as shown by an arrow  44  in  FIG. 3 . In the structure of  FIG. 2 , because the cross sectional area of the flow path is rapidly enlarged toward an external side along the radial direction of the injection valve in the transition from the cavitation generation flow path  1  to the bubble storage flow path  2 , a vortex is formed at an outer peripheral side of the bubble storage flow path  2 . Because of this, in the bubble storage flow path  2 , the void fraction (volume fraction) of the cavitation bubbles becomes relatively larger at the outer peripheral side than an inner peripheral side. By placing the exit to the nozzle hole  3  at the outer peripheral side of the bubble storage flow path  2 , it is possible to introduce a fuel flow containing a large amount of cavitation bubbles to the nozzle hole  3 . 
   Next, an experimental result and a result of a numerical analysis performed by the present inventors will be described. 
   Cavitation number Cn in the cavitation generation flow path  1  is defined by the following formula (1). In formula (1), P 1  represents a pressure of the fuel storage portion  33  (injection setting pressure), P 2  represents a pressure of the bubble storage flow path  2  (average pressure), and Pv represents a saturated vapor pressure of the fuel (saturated vapor pressure at a usage temperature).
 
 Cn =( P 1 −P 2)/( P 2 −Pv )  (1)
 
   Regarding liquids flowing through a nozzle in which the cross sectional area of the flow path is rapidly reduced at the entrance and rapidly enlarged at the exit, generation of cavitation bubbles were examined through a visualization experiment of the flow.  FIGS. 4A ,  4 B, and  4 C show the experimental results. When the pressure differential between the entrance and exit of the nozzle is increased to gradually increase the cavitation number Cn, cavitation bubbles start to be generated from the vertex portion of the entrance of the nozzle around cavitation number Cn of approximately 1, as shown in  FIG. 4A . When the pressure differential is further increased to further increase the cavitation number Cn, the cavitation bubbles also start to be generated in a shearing layer between a jet from the exit of the nozzle and liquid surrounding the jet around cavitation number Cn of greater than or equal to approximately 1.7 as shown in  FIGS. 4B and 4C . In order to effectively generate the cavitation bubbles by the cavitation generation flow path  1 , it is preferable that the cavitation number Cn defined by the formula (1) be 1.7 or greater. 
   When the discharge coefficient of the cavitation generation flow path  1  is C 1  (determined based on the shape of the cavitation generation flow path  1 ), the discharge coefficient of the nozzle hole  3  is C 2  (determined based on the shape of the nozzle hole  3 ), the cross sectional area (minimum cross sectional area) of the cavitation generation flow path  1  is A 1 , the cross sectional area (minimum cross sectional area) of the nozzle hole  3  during fuel is injected is A 2 , the environmental pressure in which the fuel is injected is Pa, and the density of the fuel is p, the following formula (2) can be derived from an equation of continuity. 
   
     
       
         
           
             
               
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   The formula (3) shows that the pressure (average pressure) P 2  of the bubble storage flow path  2  can be adjusted by a ratio A 1 /A 2  of the cross sectional areas of the flow paths between the cavitation generation flow path  1  and the nozzle hole  3 . Therefore, the cavitation number Cn can be adjusted by the ratio A 1 /A 2  of cross sectional areas of flow paths. Thus, it is preferable that the pressure (injection setting pressure) P 1  of the fuel storage portion  33  and the ratio A 1 /A 2  of the cross sectional areas of flow paths between the cavitation generation flow path  1  and the nozzle hole  3  be set so that the cavitation number Cn calculated using the formula (1) is 1.7 or greater. For this purpose, at least the full-lift amount of the poppet valve  6  must be set so that fuel is injected from the nozzle hole  3  while the cross sectional area (minimum cross sectional area) A 2  of the nozzle hole  3  is greater than the cross sectional area (minimum cross sectional area) A 1  of the cavitation generation flow path  1 . 
     FIG. 5  shows a result of a numerical analysis of void fraction (volume fraction) of the bubbles under the conditions of the injection setting pressure P 1  of 12 MPa, the environmental pressure Pa of 1.2 MPa, the ratio A 1 /A 2  of cross sectional areas of flow paths during injection of 0.5, an exit width x of the cavitation generation flow path  1  of 25 μm, and a length y of the bubble storage flow path  2  of 250 μm. As shown in  FIG. 5 , fuel in a state of mixture of gas and liquid can be injected from the nozzle hole  3  within 0.1 ms after the injection is started. In addition, the void fraction of the bubbles at the exit of the nozzle hole  3  is approximately 0.5 in the steady state of the full-lift of the poppet valve  6 , and it is thus confirmed by the numerical analysis that the fuel in which gas and liquid are almost uniformly mixed is injected from the nozzle hole  3 . 
     FIG. 6  shows a result of a numerical analysis of the void fraction (volume fraction) of the bubbles when the length y of the bubble storage flow path  2  is changed to 1000 μm. As shown in  FIG. 6 , when the length y of the bubble storage flow path  2  becomes extremely long, although cavitation bubbles are generated in the cavitation generation flow path  1  (refer to an arrow  45 ), the bubbles collapse inside the bubble storage flow path  2  (refer to an arrow  46 ), and the fuel is introduced to the nozzle hole  3  in a state of liquid phase (refer to an arrow  47 ). Therefore, the length y of the bubble storage flow path  2  is preferably set to a value small enough so that the bubbles do not collapse inside the bubble storage flow path  2 . 
   As described, in the present embodiment, the cavitation bubbles generated in the cavitation generation flow path  1  are not collapsed inside the injection valve, are temporarily stored in the bubble storage flow path  2 , and are mixed with a fuel of liquid phase, and the mixture is introduced to the nozzle hole  3 . In this manner, the fuel can be injected from the nozzle hole  3  in a state of mixture of gas and liquid, and thus, the atomization enhancement effect of the fuel spray by collapse of cavitation can effectively be obtained over the entire region of the fuel jet after injection. Therefore, the atomization of the injected fuel spray can be further enhanced. In addition, a spray penetration force can be reduced without losing the atomization enhancement effect due to the collapse of cavitation, and consequently, occurrence of adhesion of the fuel to a wall can be reduced. 
   In the present embodiment, by setting the flow in the transition from the cavitation generation flow path  1  to the bubble storage flow path  2  to the rapid expansion flow, a vertical vortex can be formed in the bubble storage flow path  2  and the cavitation bubbles can be stored around the center of the vortex which has a lower pressure than the surrounding. In addition, by offsetting the exit of the bubble storage flow path  2  to the nozzle hole  3  with respect to the direction of the fuel jet flowing in the cavitation generation flow path  1 , a strong vertical vortex can be formed in the bubble storage flow path  2  and exiting of the fuel jet from the cavitation generation flow path  1  to the nozzle hole  3  without any processing in the bubble storage flow path  2  can be inhibited. 
   In the present embodiment, by injecting the fuel from the nozzle hole  3  while the cross sectional area A 2  of the nozzle hole  3  is larger than the cross sectional area A 1  of the flow path of the cavitation generation flow path  1 , the cavitation bubbles can be effectively generated. Moreover, by setting the pressure (injection setting pressure) P 1  of the fuel storage portion  33  and the ratio A 1 /A 2  of the cross sectional areas of the flow paths so that the cavitation number Cn calculated by the formula (1) is 1.7 or greater, the cavitation bubbles can be more effectively generated. 
   Another configuration of the present embodiment will now be described. 
   In a configuration of the tip of the nozzle body as shown in  FIG. 7 , a curved surface having a predetermined radius is formed at an upstream vertex portion of the projection  34  so that the cross sectional area of the flow path at the entrance portion of the cavitation generation flow path  1  is gradually reduced toward the bubble storage flow path  2 . With this structure, because the discharge coefficient of the cavitation generation flow path  1  can be increased, the velocity of the fuel jet flowing out from the exit of the cavitation generation flow path  1  can be increased. Therefore, because generation of the cavitation bubbles in the shearing layer between the fuel jet flowing out from the exit of the cavitation generation flow path  1  and the fuel surrounding the fuel jet can be promoted, the atomization in the fuel spray by the collapse of the cavitation in the injected fuel jet can be further enhanced. 
   In the configuration of the tip of the nozzle body as shown in  FIG. 8 , a tapered shape is employed as the shape of the side wall of the outer periphery of the cavitation generation flow path  1  so that the cross sectional area of the cavitation generation flow path  1  becomes smaller toward the bubble storage flow path  2 . With this configuration also, because the discharge coefficient of the cavitation generation flow path  1  can be increased, generation of the cavitation bubbles in the shearing layer between the fuel jet flowing out of the exit of the cavitation generation flow path  1  and the fuel surrounding the fuel jet can be promoted.  FIG. 9  shows a configuration of the tip of the nozzle body in which the structures shown in  FIGS. 7 and 8  are combined. 
   In the configuration of the tip of the nozzle body as shown in  FIG. 10 , a slope having a curved shape is formed at a boundary between the pintle  5  and the poppet valve  6  so that the shape of the wall forming a part of the bubble storage flow path  2  includes a curved surface shape which is curved from a region which approximately opposes the fuel jet flowing in the cavitation generation flow path  1  to the exit to the nozzle hole  3 . When the fuel jet to which the cavitation bubbles are mixed collides with the wall of the bubble storage flow path  2 , the gas phase is again liquefied due to an increase in the pressure. Thus, a curved surface shape which is bowed toward the outside of the bubble storage flow path  2  is employed as the shape of the wall of the bubble storage flow path  2  from the region to which the fuel jet collides to the exit to the nozzle hole  3 , so that the direction of flow of the fuel is smoothly changed and the liquefaction of the gas phase can be inhibited. Therefore, the atomization of the fuel spray by collapse of cavitation in the injected fuel jet can be further enhanced. A result of a numerical analysis under the above-described conditions showed that the void fraction of the bubbles at the exit of the nozzle hole  3  in the structure shown in  FIG. 10  is greater than that of the structure shown in  FIG. 2  by approximately 0.1, indicating that a superior mixture state of gas and liquid has been achieved. 
   In a configuration of the tip of the nozzle body as shown in  FIG. 11 , a swirler (swirl generation flow path)  35  which generates a swirl flow along a circumferential direction of the injection valve in the fuel flowing inside the injection valve is formed upstream of the cavitation generation flow path  1 . In a configuration of the tip of the nozzle body as shown in  FIG. 12 , the swirler  35  is formed in the cavitation generation flow path  1 . According to the structures shown in  FIGS. 11 and 12 , because a swirl flow along a circumferential direction of the injection valve can be generated in the fuel flow, non-uniformity in the flow rate distribution along the circumferential direction of the injection valve can be alleviated. 
   In a configuration of the tip of the nozzle body as shown in  FIG. 13 , the radius of the pintle  5  is stepwise increased so that the cross sectional area of the flow path is rapidly reduced at the entrance of the cavitation generation flow path  1 . In addition, the radius of a hollow portion of the nozzle body  7  is stepwise increased so that the cross sectional area of the flow path is rapidly expanded at the exit of the cavitation generation flow path  1 . In a configuration of the tip of the nozzle body as shown in  FIG. 14 , in comparison to the configuration of  FIG. 13 , the radius of the hollow portion of the nozzle body  7  is stepwise increased and the radius of the pintle  5  is stepwise reduced so that the cross sectional area of the flow path is rapidly expanded at the exit of the cavitation generation flow path  1 . 
   In a configuration of the tip of the nozzle body as shown in  FIG. 15 , a projection  36  projecting toward the poppet valve  6  is formed at a position on the inner peripheral surface of the nozzle body  7  opposing the poppet valve  6  so that the cavitation generation flow path  1  is formed with the cross sectional area of the flow path rapidly reducing at the entrance and rapidly expanding at the exit. A step is formed in the poppet valve  6  to stepwise increase the radius of the poppet valve  6  so that the exit of the bubble storage flow path  2  is offset, with respect to the direction of the fuel jet flowing in the cavitation generation flow path  1 , toward the external side along the radial direction of the injection valve. In a configuration of the tip of the nozzle body as shown in  FIG. 16 , in comparison to the configuration of  FIG. 15 , a slope having a curved surface shape is formed in the poppet valve  6  instead of formation of the step. With this structure, the shape of the wall of the bubble storage flow path  2  from a region approximately opposing the fuel jet flowing in the cavitation generation flow path  1  to the exit to the nozzle hole  3  can be set to a curved surface shape which is bowed toward the outside of the bubble storage flow path  2 . According to the structure of  FIG. 16 , the direction of the fuel flow in the bubble storage flow path  2  can be smoothly changed so that liquefaction of the gas phase can be inhibited. In the structures of  FIGS. 15 and 16 , the direction of the fuel jet flowing in the nozzle hole  3  is approximately parallel to the direction of the fuel jet flowing in the cavitation generation flow path  1 . 
   In a configuration of the tip of the nozzle body as shown in  FIG. 17 , the radius of the poppet valve  6  is stepwise reduced so that the cross sectional area of the flow path at the exit of the cavitation generation flow path  1  is rapidly expanded toward the inside along the radial direction of the injection valve. In addition, by setting the inner peripheral surface of the nozzle body  7  to protrude toward the poppet valve  6 , the exit of the bubble storage flow path  2  is offset, with respect to the direction of the fuel jet flowing in the cavitation generation flow path  1 , toward the inside along the radial direction of the injection valve. In a configuration of the tip of the nozzle body as shown in  FIG. 18 , in comparison to the structure of  FIG. 17 , when the exit of the bubble storage flow path  2  is offset with respect to the direction of the fuel jet by setting the inner peripheral surface of the nozzle body  7  to protrude toward the poppet valve  6 , a slope having a curved surface shape is formed at the inner peripheral surface of the nozzle body  7 . Because of this structure, the shape of the wall which forms a part of the bubble storage flow path  2  contains a curved surface shape which is curved from a region approximately opposing the fuel jet flowing in the cavitation generation flow path  1  to the exit to the nozzle hole  3 . With this structure of  FIG. 18 , the direction of the fuel flow in the bubble storage flow path  2  can be smoothly changed so that liquefaction of the gas phase can be inhibited. Regarding the bubble storage flow path  2  in the structures of  FIGS. 17 and 18 , while the entrance from the cavitation generation flow path  1  is placed at an outer peripheral side (side near the nozzle body  7 ), the exit to the nozzle hole  3  is placed at an inner peripheral side (side near the poppet valve  6 ). 
   In the above description, configurations have been described in which the present invention is applied to fuel injection valves of externally open valve type. The present invention, however, is not limited to such a configuration and can be applied to fuel injection valves of internally open valve type, as will be described below. 
     FIGS. 19A ,  19 B, and  19 C schematically show configurations when the present invention is applied to fuel injection valves of an internally open valve type.  FIG. 19A  is a diagram schematically showing an internal structure and  FIGS. 19B and 19C  are cross sectional diagrams schematically showing cross sections (A-A cross section of  FIG. 19A ) of a plate  109  in which a nozzle hole  105  is formed. A needle  101  is inserted in a hollow portion of a nozzle body  110  and is supported in a slidable manner along an internal peripheral surface of the nozzle body  110 . The needle  101  can be driven by an electromagnetic actuator (not shown) and is in contact with a seat portion  106  of the nozzle body  110  when the fuel is not injected (when the electromagnetic actuator is not driven). 
   In the configuration shown in  FIGS. 19A ,  19 B, and  19 C, a cylindrical projection  102  is provided at a tip of the needle  101  and a ring-shaped gap is formed between the projection  102  and the nozzle body  110  so that a cavitation generation flow path  103  which generates cavitation bubbles in the fuel is formed downstream of the seat portion  106 . In addition, a plate  108  in which a cylindrical hollow section is formed is attached to the tip of the nozzle body  110 . The projection  102  at the tip of the needle  101  extends through the hollow section of the plate  108  and a ring-shaped gap is formed between the projection  102  and the plate  108  so that a bubble storage flow path  104  which stores the cavitation bubbles is formed downstream of the cavitation generation flow path  103 . A cross sectional area of the bubble storage flow path  104  is rapidly expanded toward the outside along the radial direction of the injection valve from the cross sectional area of the cavitation generation flow path  103 . 
   The plate  109  in which a nozzle hole  105  is formed is attached to the plate  108 . The nozzle hole  105  is formed at a position connected to the bubble storage flow path  104  and an entrance of the nozzle hole  105  (exit of the bubble storage flow path  104 ) is formed at a position which is offset, with respect to the direction of the fuel jet flowing in the cavitation generation flow path  103 , toward the outside along a radial direction of the injection valve. That is, regarding the bubble storage flow path  104 , while the entrance from the cavitation generation flow path  103  is placed at an inner peripheral side (side near the needle  101 ), the exit to the nozzle hole  105  is placed at an outer peripheral side (side near the plate  108 ). Alternatively, the plates  108  and  109  may be integrated. 
   A shape of the nozzle hole  105  may be a shape of a plurality of slits arranged along the circumferential direction of the plate  109  as shown in  FIG. 19B , or may be a shape of a plurality of circles arranged along the circumferential direction of the plate  109  as shown in  FIG. 19C . As described, the shape of the nozzle hole  105  is not limited to a particular shape. A total value A 2  of the cross sectional areas of the flow paths (minimum cross sectional area) of the plurality of nozzle holes  105  is set to be larger than the cross sectional area A 1  (minimum cross sectional area) of the cavitation generation flow path  103 . In addition, an angle of the nozzle hole  105  is set so that a direction of the fuel jet flowing in the nozzle hole  105  is tilted with respect to a direction of the fuel jet flowing in the cavitation generation flow path  103 . 
   With the above-described structure, the cavitation generation flow path  103 , bubble storage flow path  104 , and nozzle hole  105  are formed downstream of the seat portion  106  of the nozzle body  110 . Regarding other elements, the elements are substantially identical to the elements in the device of the externally open valve type, and therefore will not be described again. 
   An operation of the fuel injection valve having the structure of  FIGS. 19A ,  19 B, and  19 C will now be described. 
   When the fuel is injected, the needle  101  is driven by an electromagnetic actuator (not shown) so that the needle  101  is separated from the seat portion  106  of the nozzle body  110  and the fuel stored in the fuel storage portion  133  flows through the cavitation generation flow path  103  into the bubble storage flow path  104 . When the injection starts, a pressure P 2  of the bubble storage flow path  104  is equal to an environmental pressure Pa and a difference between a pressure P 1  at the entrance of the cavitation generation flow path (pressure of fuel storage portion  133 ) and a pressure P 2  at the exit of the cavitation generation flow path  103  (pressure of the bubble storage flow path  104 ) is maximum. Because of this, the cavitation number Cn defined by the formula (1) is at the maximum during start of the injection and cavitation bubbles can be easily generated in the cavitation generation flow path  103 . 
   When the bubble storage flow path  104  is filled with fuel, cavitation bubbles are further generated at a shearing layer between the fuel jet to which cavitation bubbles are mixed and which exits from the exit of the cavitation generation flow path  103  and the fuel surrounding the fuel jet, similar to the case of the device of an externally open valve type as described above. A vertical vortex is formed within the bubble storage flow path  104  and cavitation bubbles are stored around a center of the vortex having a lower pressure than the surroundings. In addition, cavitation bubbles are also generated in the central part of the vertical vortex. The fuel liquid in the bubble storage flow path  104  and the cavitation bubbles are mixed and introduced to the nozzle hole  105 , and fuel in a mixture state of gas and liquid is injected from the nozzle hole  105 . 
   When, on the other hand, the bubble storage flow path  104  is not filled with fuel, the fuel to which cavitation bubbles exiting from the cavitation generation flow path  103  are mixed is injected from the nozzle hole  105  while the bubble storage flow path  104  is filled with the fuel. Then, when the bubble storage flow path  104  is approximately filled with the fuel, the device operates in a manner similar to the above-described operation when the bubble storage flow path  104  is filled with the fuel. 
   In the above-described operation, similar to the device of the externally open valve type, the pressure P 1  of the fuel storage portion  133  (injection setting pressure) and the ratio A 1 /A 2  of the cross sectional areas between the cavitation generation flow path  103  and the nozzle hole  105  are preferably set so that the cavitation number Cn calculated by the formula (1) is 1.7 or greater. 
   Similar to the above, in the fuel injection valve of the internally open valve type also, because the fuel can be injected from the nozzle hole  105  in a mixture state of gas and liquid, atomization enhancement effect of the fuel spray by collapse of cavitation can be effectively obtained over the entire region of the fuel jet after injection. In addition, the spray penetration force can be reduced without losing the atomization enhancement effect by the collapse of the cavitations, and consequently, occurrence of adhesion of the fuel on the wall can be reduced. Moreover, a strong vertical vortex can be formed in the bubble storage flow path  104 , and it is possible to inhibit flow of the fuel jet from the cavitation generation flow path  103  to the nozzle hole  105  without any process. 
   Next, alternative configurations of the fuel injection valve of the internally open valve type will be described. 
   In a configuration of the tip of the nozzle body shown in  FIGS. 20A ,  20 B, and  20 C, in comparison to the configuration of  FIGS. 19A ,  19 B, and  19 C, the projection  102  of the needle  101  is omitted.  FIG. 20A  is a diagram schematically showing an internal structure,  FIG. 20B  is a cross sectional diagram schematically showing a structure of the plate  108  in which the bubble storage flow path  104  is formed, and  FIG. 20C  is a cross sectional diagram schematically showing a structure of the plate  109  in which the nozzle hole  105  is formed. In the configuration shown in  FIGS. 20A ,  20 B, and  20 C, a flow path formed between the needle  101  and the seat portion  106  when the needle  101  is separated from the seat portion  106  functions as the cavitation generation flow path  103 . In this case, the full-lift amount of the needle  101  is set so that a cross sectional area A 1  (minimum cross sectional area) of the cavitation generation flow path  103  (flow path between the needle  101  and the seat portion  106 ) is smaller than a total value A 2  of the cross sectional areas of the flow paths (minimum cross sectional area) of a plurality of nozzle holes  105  when the needle  101  is in the full-lift state. In addition, the full-lift amount of the needle  101  is preferably set so that the cavitation number Cn calculated using the formula (1) is 1.7 or greater. 
   In a configuration of the tip of the nozzle body shown in  FIGS. 21A ,  21 B, and  21 C, the plate  107  in which the cavitation generation flow path  103  is formed is attached to a tip of the nozzle body  110  and the plate  109  in which the bubble storage flow path  104  and the nozzle hole  105  are formed is attached to the plate  107 .  FIG. 21A  is a diagram schematically showing an internal structure,  FIG. 21B  is a cross sectional diagram schematically showing a structure of the plate  107 , and  FIG. 21C  is a cross sectional diagram (A-A cross section of  FIG. 21A ) schematically showing a structure of the plate  109 . A plurality of cavitation generation flow paths  103  are placed along the circumferential direction of the plate  107 .  FIGS. 21A-21C  show a configuration in which the cross sectional shape of the cavitation generation flow path  103  is circular, but the shape of the cavitation generation flow path  103  is not limited to this configuration. The bubble storage flow path  104  is formed by a ring-shaped channel formed in the plate  109  and a plurality of nozzle holes  105  which are connected to the bubble storage flow path  104  are placed along the circumferential direction of the plate  109 . In this configuration, the entrance of each nozzle hole  105  (exit of the bubble storage flow path  104 ) is offset, with respect to the direction of the fuel jet flowing in each cavitation generation flow path  103 , along the radial directions of the plates  107  and  109  (toward outside). Alternatively, it is also possible to offset, with respect to the direction of the fuel jet, the entrance of each nozzle hole  105  along the circumferential directions of the plates  107  and  109 . A total value A 2  of the cross sectional areas of the flow paths (minimum cross sectional area) of the plurality of nozzle holes  105  is set to be larger than a total value A 1  of the cross sectional areas of the flow paths (minimum cross sectional area) of the plurality of cavitation generation flow paths  103 . In addition, a pressure P 1  of the fuel storage portion  133  (injection setting pressure) and the ratio A 1 /A 2  of the cross sectional areas of the flow paths are preferably set so that the cavitation number Cn calculated by the formula (1) is 1.7 or greater. With the configuration of  FIGS. 21A ,  21 B, and  21 C, by forming the cavitation generation flow path  103  in the plate  107  which is a separate component from the nozzle body  110 , the machining precision of the cavitation generation flow path  103  can be improved, and the machining cost can be reduced. 
   In a configuration of the tip of the nozzle body as shown in  FIG. 22 , a ring-shaped bubble storage flow path  104  is formed by combining a concave plate  109  in which the central portion is depressed in a circular shape and a convex plate  107  on which a cylindrical projection is formed. In a configuration of the tip of the nozzle body as shown in  FIGS. 23A ,  23 B, and  23 C, a plurality of cavitation generation flow paths  103  are formed in the plate  107  in a linear manner and a plurality of nozzle holes  105  are formed on the plate  109  in a linear manner.  FIG. 23A  is a diagram schematically showing an internal structure,  FIG. 23B  is a cross sectional diagram schematically showing the structure of the plate  107 , and  FIG. 23C  is a cross sectional diagram (A-A cross section of  FIG. 23A ) schematically showing the structure of the plate  109 . 
   In a configuration of the tip of the nozzle body as shown in  FIGS. 24A ,  24 B,  24 C, and  24 D, in comparison to the configuration of  FIGS. 20A ,  20 B, and  20 C, the plate  107  in which the cavitation generation flow path  103  is formed is further provided between the nozzle body  110  and the plate  108 .  FIG. 24A  is a diagram schematically showing an internal structure,  FIG. 24B  is a cross sectional diagram schematically showing the structure of the plate  107  in which the cavitation generation flow path  103  is formed,  FIG. 24C  is a cross sectional diagram schematically showing the structure of the plate  108  in which the bubble storage flow path  104  is formed, and  FIG. 24D  is a cross sectional diagram schematically showing the structure of the plate  109  in which the nozzle hole  105  is formed.  FIGS. 24A and 24B  show a case in which the structure of the plate  107  is similar to that of  FIGS. 21A and 21B . However, the shape of the cavitation generation flow path  103  is not limited. With the structure of  FIGS. 24A-24D , by employing a cylindrical shape as the shape of the bubble storage flow path  104 , the machining cost of the bubble storage flow path  104  can be reduced. 
   In a configuration of the tip of the nozzle body as shown in  FIG. 25 , the cavitation generation flow path  103  and the bubble storage flow path  104  are formed upstream of the seat portion  106  of the nozzle body  110 . In  FIG. 25 , a projection  134  projecting toward the nozzle body  110  is formed on the needle  101  so that a cavitation generation flow path  103  is formed in which the cross sectional area of the flow path is rapidly reduced at the entrance and is rapidly expanded at the exit. In the bubble storage flow path  104 , while the entrance from the cavitation generation flow path  103  is placed at an outer periphery side (side near the nozzle body  110 ), the exit to the nozzle hole  105  is placed at an inner periphery side (side near the needle  101 ). The nozzle hole  105  is formed at the central portion of the tip of the nozzle body  110 . In addition, the full-lift amount of the needle  101  is set so that the minimum cross sectional area A 2  of the flow path from the seat portion  106  to the nozzle hole  105  is larger than the cross sectional area A 1  (minimum cross sectional area) of the cavitation generation flow path  103  when the needle  101  is at the full-lift state. In addition, the full-lift amount of the needle  101  is preferably set so that the cavitation number Cn calculated by the formula (1) is 1.7 or greater. 
   In a configuration of the tip of the nozzle body as shown in  FIG. 26 , in comparison to the structure of  FIG. 25 , a cone-shaped projection  111  is provided at the tip (downstream of the exit of the nozzle hole  105 ) of the needle  101 . With the structure of  FIG. 26 , the fuel injected from the nozzle hole  105  (in a mixed state of gas and liquid) expands in a conical shape along the conically shaped projection  111 , and thus, a widely spread spay having an approximate hollow conical shape can be formed. 
   In a configuration of the tip of the nozzle body shown in  FIG. 27 , in comparison with the structure of  FIG. 25 , a plurality of channels which are tilted with respect to an axis of the needle  101  are formed in the projection  134  so that a swirler (swirl generation flow path) which generates a swirl flow along a circumferential direction of the injection valve in the fuel flowing inside the injection valve is formed in the cavitation generation flow path  103 . With the structure of  FIG. 27 , a swirl flow is formed in the bubble storage flow path  104  and fuel having a swirl velocity component is injected from the nozzle hole  105 . The injected spray expands along the radial direction due to the centrifugal force and a widely spread spray having an approximate hollow conical shape can be formed. 
   In the configurations of  FIGS. 19A-27 , similar to the device of an externally open valve type as described above, the discharge coefficient of the cavitation generation flow path  103  can be increased by gradually reducing the cross sectional area of the flow path at the entrance of the cavitation generation flow path  103  toward the bubble storage flow path  104 . Alternatively, the discharge coefficient of the cavitation generation flow path  103  can be increased by employing a tapered shape for the cavitation generation flow path  103  to set the cross sectional area of the flow path of the cavitation generation flow path  103  to become smaller toward the bubble storage flow path  104 . Moreover, the shape of the wall of the bubble storage flow path  104  may be set to contain a curved shape which is curved from a region which approximately opposes the fuel jet flowing in the cavitation generation flow path  103  to the exit to the nozzle hole  105  so that the direction of the fuel flow in the bubble storage flow path  104  changes smoothly and the liquefaction of the gas phase is inhibited. 
   A preferred embodiment of the present invention has been described. The description of the preferred embodiment, however, should not be construed as limiting the present invention and various modifications can be made within the scope of the present invention.