TITLE OF THE INVENTION: VALVE APPARATUS FOR FUEL INJECTION VALVE
TECHNICAL FIELD
[0001]
The present invention relates to a valve apparatus for a fuel injection valve that is used to supply fuel to an internal combustion engine of an automobile, etc., and particularly relates to a valve apparatus that can promote atomization in spraying characteristics.
BACKGROUND ART
[0002]
In recent years, the change to fuel injection (Fl) in internal combustion engines is advancing even in motorcycles that have a small engine displacement, adoption of fuel injection valves is increasing, and there is demand for atomization of fuel sprays that are sprayed from fuel injection valves.
[0003]
In consideration of such conditions, various kinds of investigation have been made into the atomization of fuel spray.
In Patent Literature 1, for example, a conventional valve apparatus has been disclosed in which a disk-shaped fuel cavity that communicates between a fuel passage and nozzle holes is disposed between a nozzle hole plate and a valve seat, the cavity having a tapered shape in which a height of the cavity reduces toward a radially outer side. In this conventional valve apparatus, fuel that has flowed into the cavity from the fuel seat portion changes its direction of flow and flows through the cavity from a

vicinity of an axial center toward the radially outer side, and collides with the nozzle holes. Since the height of the cavity decreases toward the radially outer side in a tapered shape, a radially outer height at the nozzle holes is lower than a height near an axis. Thus, because a cross-sectional area of the flow that flows into the nozzle holes from radially outside (backflow) is reduced, and the flow speed of the backflow is increased, turbulence is increased in an upper portion of the nozzle holes due to collision between the main flow, which is fuel flow that flows through the nozzle holes from a vicinity of the axial center, and the backflow. In conventional valve apparatuses, this turbulence due to the collision between the main flow and the backflow has been used to attempt to atomize the fuel spray.
CITATION LIST PATENT LITERATURE
[0004]
Patent Literature V- Japanese Patent Laid-Open No. 2003-155965 (Gazette^ Figure 4)
SUMMARY OF THE INVENTION
PROBLEM TO BE SOLVED BY THE INVENTION
[0005]
However, in the conventional valve apparatus according to Patent Literature 1, fluid loss due to intense collision between the main flow and the backflow in the upper portions of the nozzle holes is large, and there has been a limit to the atomization of the fuel spray.
[0006]
Specifically, in conventional valve apparatuses, because an outer circumferential wall surface of the cavity is formed into a cylindrical surface

that has a central axis of the cavity as an axial center, a comparatively large-volume portion that is surrounded by an upper surface (tapered surface), a lower surface (an upper surface of the nozzle hole plate), and the outer circumferential wall surface of the cavity exists radially outside the nozzle holes from the central axis of the cavity. Thus, the flow rate of fuel that flows between the nozzle holes and turns into backflow is increased. In addition, because fuel that has passed between the nozzle holes and collided with the outer circumferential wall surface rises up perpendicular to the upper surface of the nozzle hole plate, and arrives above the nozzle holes along the upper surface of the cavity, the flow of this backflow does not attenuate easily. The backflow thereby forms a large-mass, high-speed flow, and collides intensely with the main flow. Because kinetic energy in the main flow is reduced by this collision, when the fuel is sprayed into atmospheric air from the nozzle holes, its speed relative to the air is reduced. Thus, one problem has been that the shearing force by which the air breaks up the fuel droplets is reduced, making atomization of the fuel spray deficient.
[0007]
In addition, the above-mentioned comparatively large-volume portion leads to increases in the overall volume of the cavity. Changes in flow rate that accompany changes in ambient pressure (dynamic flow rate) are increased thereby. Specifically, because the volume of the cavity is increased, and the amount of fuel that remains inside he cavity after completion of valve closing is increased, when spraying into a negative-pressure atmosphere, the amount of fuel that is sucked out of the nozzle holes into the engine air intake pipe due to negative pressure after completion of valve closing is increased, and problems arise such as changes in flow rate being greater. In addition, because flow speed of the fuel that is sucked out from inside the cavity is reduced, another problem has been

that fuel that has inconsistent particle size is sprayed immediately after valve closing.
[0008]
The present invention aims to solve the above problems and an object of the present invention is to provide a valve apparatus for a fuel injection valve that can promote atomization of a fuel spray and that can also suppress changes in flow rate that accompany changes in ambient pressure by enabling reductions in volume in a volume portion that exists radially outside nozzle apertures inside a cavity.
MEANS FOR SOLVING THE PROBLEM
[0009]
A valve apparatus for a fuel injection valve according to the present invention includes: a valve seat including: a truncated conical seat surface in which a diameter reduces toward a downstream end; and a cylindrical opening portion that is formed downstream from the seat surface so as to be coaxial to the seat surface, the valve seat having an axial center of the seat surface and the opening portion as a central axis! a valve member that stops outflow of fuel from the opening portion by sitting on the seat surface, and that allows outflow of fuel from the opening portion by separating from the seat surface! a nozzle aperture plate that is fixed to a downstream end surface of the valve seat such that a flat upper surface faces upstream, the nozzle aperture plate including a plurality of nozzle apertures that are each formed so as to have a cylindrical shape, that are positioned in a plane that includes the central axis such that an axial center of the cylindrical shape approaches the central axis toward an upstream end, and that are disposed so as to be distributed around the central axis on a circumference of a common circle! and a disk-shaped cavity that is disposed between the valve seat and the nozzle aperture plate, and that

communicates between the opening portion and the nozzle apertures. An axial height of the cavity is constant at least in a radial region from the opening portion to a point of intersection between the upper surface of the nozzle aperture plate and the axial centers of the nozzle apertures, an outer circumferential wall surface of the cavity includes^ a truncated conical tapered surface that has the central axis as an axial center, and in which a diameter reduces toward an upstream end; and a rounded joint that links an upstream surface of the cavity and the tapered surface, and portions of openings of the nozzle apertures on the upper surface of the nozzle aperture plate overlap with the outer circumferential wall surface when viewed from an axial direction of the central axis.
EFFECTS OF THE INVENTION
[0010]
In this invention, because the outer circumferential wall surface of the cavity has: a truncated conical tapered surface that has a central axis as an axis, and in which the diameter reduces toward an upstream end; and a rounded joint that links an upstream surface and the tapered surface of the cavity, and portions of the openings of the nozzle apertures on the lower surface of the cavity overlap with the outer circumferential wall surface when viewed from the axial direction of the central axis, a volume of the volume portion that exists radially outside the nozzle apertures inside the cavity can be reduced.
[0011]
Thus, the flow rate of fuel that flows between the nozzle holes and turns into backflow is reduced. In addition, because fuel that has passed between the nozzle holes and collided with the outer circumferential wall surface is inverted by the outer circumferential wall surface, which is inclined at an acute angle relative to the upper surface of the nozzle hole

plate, and arrives above the nozzle holes along the upstream surface of the cavity, the flow of backflow is attenuated. Because the backflow forms a lowspeed, small-mass flow and collides with the main flow, reductions in kinetic energy in the main flow are suppressed. Thus when the fuel is sprayed into atmospheric air from the nozzle holes, because reductions in its speed relative to the air are suppressed, the shearing force by which the air breaks up the fuel droplets is increased, making atomization of the fuel spray sufficient.
[0012]
Due to the above-mentioned reduction in the volume portion, increases in the overall volume of the cavity are suppressed. Thus, the volume of the cavity is smaller, reducing the amount of fuel that remains inside the cavity after completion of valve closing. The amount of fuel that is sucked out into the engine air intake pipe from the nozzle apertures after completion of valve closing due to the negative pressure when spraying into a pressure atmosphere is thereby reduced, enabling increases in changes in flow rate to be suppressed. In addition, the amount of fuel spray that has poor particle size that is sprayed at valve closing completion is reduced.
[0013]
Because the axial height of the cavity is constant at least in a radial region from the opening portion to a point of intersection between the upper surface of the nozzle aperture plate and the axial centers of the nozzle apertures, the volume of the cavity can be reduced compared to conventional techniques, in which the axial height of the cavity reduces gradually toward a radially outer side, and flow in a direction that is perpendicular to the central axis, which is the flow of fuel from the opening portion toward the nozzle apertures, is also strengthened. The flow of fuel that has flowed into the nozzle apertures thereby detaches at the inlet portion of the nozzle apertures, facilitating generation of liquid films inside

the nozzle apertures, in other words, facilitating atomization of the fuel spray.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
Figure 1 is a longitudinal cross section that explains a configuration of a fuel injection valve according to Embodiment 1 of the present invention!
Figure 2 is a longitudinal cross section that shows a vicinity of a valve seat of the fuel injection valve according to Embodiment 1 of the present invention!
Figure 3 is a cross section that is taken along Line III - III in Figure 2 so as to be viewed in the direction of the arrows!
Figure 4 is a cross section that shows part of a valve apparatus of the fuel injection valve according to Embodiment 1 of the present invention!
Figure 5 is a cross section that shows part of the valve apparatus of the fuel injection valve according to Embodiment 1 of the present invention!
Figure 6 is a graph that shows a relationship between average particle size of fuel spray and M/D in the valve apparatus of the fuel injection valve according to Embodiment 1 of the present invention!
Figure 7 is a graph that shows a relationship between average particle size of fuel spray and H/D in the valve apparatus of the fuel injection valve according to Embodiment 1 of the present invention!
Figure 8 is a longitudinal cross section that shows a vicinity of a valve apparatus of a fuel injection valve according to Embodiment 2 of the present invention!
Figure 9 is a schematic diagram that explains fuel flow inside a cavity in the valve apparatus of the fuel injection valve according to Embodiment 2 of the present invention! and

Figure 10 is a schematic diagram that shows a vicinity of a nozzle aperture in the valve apparatus of the fuel injection valve according to Embodiment 2 of the present invention.
DESCRIPTION OF EMBODIMENTS
[0015] Embodiment 1
Figure 1 is a longitudinal cross section that explains a configuration of a fuel injection valve according to Embodiment 1 of the present invention, Figure 2 is a longitudinal cross section that shows a vicinity of a valve seat of the fuel injection valve according to Embodiment 1 of the present invention, and Figure 3 is a cross section that is taken along Line III - III in Figure 2 so as to be viewed in the direction of the arrows. Moreover, a "longitudinal cross section" is a cross section in a plane that includes a central axis A0 of the fuel injection valve.
[0016]
In Figure 1, a fuel injection valve 100 includes: a valve apparatus! a solenoid apparatus that generates an electromagnetic force that opens the valve apparatus; and a spring 8 that generates a spring force that closes the valve apparatus.
[0017]
The solenoid apparatus includes: a core 1 that is produced using a magnetic metal material so as to have a cylindrical shape! a coil 2 that is disposed so as to be embedded in a bobbin 3 that is made of an insulating resin so as to surround a first end portion of the core V, a yoke 4 that is produced using a magnetic metal material so as to have a two-tiered cylindrical shape, and that is disposed so as to accommodate the coil 2 inside a larger diameter portion; and a cap 5 that is produced using a magnetic metal material so as to have a ring shape that has a notch portion

for leading out terminals 6, that is disposed so as to cover an opening at an end of the larger diameter portion of the yoke 4, that is fixed to the yoke 4 by welding, and that is also disposed so as to contact the core 1, the solenoid apparatus being formed integrally in a housing 7 that is made of an insulating resin.
[0018]
The spring 8 is disposed in an internal portion of the core 1, and is fixed to the internal portion of the core 1 such that a rod 9 can adjust the spring force from the spring 8.
[0019]
The valve apparatus includes: a holder 10 that is produced using a magnetic metal material so as to have a cylindrical shape, and that is disposed coaxially at a first end of the core 1 so as to be spaced apart from the core 1 magnetically by means of a nonmagnetic sleeve 11; an armature 12 that is disposed inside the holder 10 so as to face a first end surface of the core 1 so as to be movable in a direction that is parallel to the central axis A0 of the fuel injection valve 100; a pipe 13 that is fixed in a vicinity of a first end of the armature 12; a ball 14 that functions as a valve member that is fixed to a first end of the pipe 13, and that is disposed inside a vicinity of a first end of the holder 10; a valve seat 15 that is fixed inside a first end portion of the holder 10; and a flat nozzle aperture plate 16 that is fixed to a first end surface of the valve seat 15 such that a flat upper surface faces toward the valve seat 15.
[0020]
A second end of the pipe 13 contacts the spring 8, and bears the spring force from the spring 8. As shown in Figure 2, the valve seat 15 includes: a truncated cone-shaped seat surface 15a that has a tapered shape in a vicinity of a first end; a cylindrical opening portion 15b that is formed in a vicinity of a first end of the seat surface 15a! and a cylindrical

sliding surface 15c that is formed in a vicinity of a second end of the seat surface 15a. Moreover, central axes of the seat surface 15a, the opening portion 15b, and the sliding surface 15c are aligned with the central axis AO. As shown in Figures 2 and 3, an outer circumferential portion of the ball 14 is machined into a pentagonal shape, and the ball 14 is able to sit on and separate from the seat surface 15a by moving in a direction that is parallel to the central axis AO such that corner portions of the pentagonal shape thereof are guided by the sliding surface 15c.
[0021]
A plurality of nozzle apertures 17 are formed on the nozzle aperture plate 16 so as to be spaced apart from each other on a circumference of a common circle that is centered around the central axis AO. These nozzle apertures 17 are each produced so as to have a cylindrical aperture shape, and are positioned outside the opening portion 15b relative to the central axis AO. Furthermore, an axial center Al of each of the spraying apertures 17 is positioned on a plane that includes the central axis AO, and is inclined so as to approach the central axis AO toward a second end.
A cavity 18 is formed between the valve seat 15 and the nozzle aperture plate 16 by indenting the first end surface of the valve seat 15 in a vicinity of the opening portion 15b. This cavity 18 has a disk shape, and communicates between the opening portion 15b and the nozzle apertures 17.
[0022]
Next, operation of the fuel injection valve 100 that is configured in this manner will be explained.
[0023]
In an initial state, there is no passage of electric current to the coil 2, the pipe 13 is pressed toward the valve seat 15 by the spring force from the spring 8, and the ball 14 is placed in contact with the seat surface 15a of the

valve seat 15, forming a valve closing state. The armature 12 is spaced apart from the core 1. Fuel is supplied to the fuel injection valve 100 from a vicinity of a second end of the central axis A0.
[0024]
When current is passed through the coil 2 from outside by means of the terminals 6, to generate magnetic flux in a magnetic passage that is constituted by the core 1, the cap 5, the yoke 4, the holder 10, and the armature 12, magnetic attraction that attracts the armature 12 toward the core 1 is generated. The armature 12 thereby moves toward the core 1 in opposition to the spring force from the spring 8, and the ball 14, which is linked to the armature 12 by means of the pipe 13, enters an open position that is separated from the seat surface 15a of the valve seat 15. Thus, fuel that has been supplied to the fuel injection valve 100 flows toward the ball 14 through the internal portion of the core 1. The fuel then passes through the flat passages between the ball 14 and the sliding surface 15c, passes through the flat passages between the ball 14 and the seat surface 15a, and flows out of the opening portion 15b into the cavity 18, as indicated by arrows in Figure 2. The fuel that has flowed into the cavity 18 flows radially through the cavity 18, and is sprayed from the nozzle apertures 17 of the nozzle aperture plate 16.
[0025]
When the passage of electric current to the coil 2 is stopped, the magnetic attraction that attracts the armature 12 toward the core 1 disappears. The armature 12 thereby moves toward the valve seat 15 due to the spring force from the spring 8, placing the ball 14 in a valve closing position that is in contact with the seat surface 15a, and spraying of the fuel is stopped.
[0026]

Construction of the nozzle apertures 17 and the cavity 18 of the valve apparatus will now be explained with reference to Figure 4. Figure 4 is a cross section that shows part of a valve apparatus of the fuel injection valve according to Embodiment 1 of the present invention. Here, a surface of the cavity 18 that faces the nozzle aperture plate 16 is an upper surface 20, a surface of the nozzle aperture plate 16 that is on a side near the valve seat 15 is a lower surface 21 of the cavity 18, and a surface that links an outer circumferential edge portion of the upper surface 20 and an outer circumferential edge portion of the lower surface 21 is an outer circumferential wall surface 22. Furthermore, a radial direction is direction that is perpendicular to the central axis A0 in a cross section that includes the central axis A0. Furthermore, for simplicity, hatching has been omitted.
[0027]
The outer circumferential wall surface 22 of the cavity 18 has: a tapered surface 23 that has the central axis A0 as its axial center, and that is formed so as to have a truncated conical surface that tapers toward a second end of the central axis A0, which is an upstream end; and a rounded joint 24 that links the upper surface 20 and the tapered surface 23. An axial height H of the cavity 18 is constant except for the outer circumferential wall surface 22. In addition, portions of the openings of the nozzle apertures 17 on the lower surface 21 of the cavity 18 overlap with the outer circumferential wall surface 22 when viewed from the axial direction of the central axis A0.
[0028]
Here, it is not necessary for the height H of the cavity 18 to be constant from the opening portion 15b to the joint 24, provided that it is constant at least from the opening portion 15b to directly above central positions of the nozzle apertures 17 on the lower surface 21 of the cavity 18.

Moreover, "central positions" of the nozzle apertures 17 on the lower surface
21 of the cavity 18 are points of intersection between the lower surface 21 of
the cavity 18 and the central axes Al of the nozzle apertures 17.
[0029]
In the cavity 18 that is configured in this manner, because the outer circumferential wall surface 22 has: a truncated conical tapered surface 23 that has a central axis AO as an axis, and in which the diameter reduces toward an upstream end; and a rounded joint 24 that links the upper surface 20 and the tapered surface 23 of the cavity 18, and portions of the openings of the nozzle apertures 17 on the lower surface 21 of the cavity 18 overlap with the outer circumferential wall surface 22 when viewed from the axial direction of the central axis AO, a volume of the volume portion that exists radially outside the nozzle apertures 17 from the central axis AO can be reduced compared to conventional techniques in which the outer circumferential wall surface of the cavity is formed into a cylindrical surface that is centered around the central axis AO. Thus, the flow rate of fuel that flows radially outward between the nozzle apertures 17 and turns into backflow is reduced compared to conventional techniques.
[0030]
An angle of inclination 6 of the outer circumferential wall surface 22, which is an angle that is formed between the tapered surface 23 of the outer circumferential wall surface 22 and the lower surface 21, is an acute angle. Thus, as indicated by arrows in Figure 4, fuel that has flowed into the cavity 18 from a opening portion 15b flows radially outward through the cavity 18, is inverted after reaching the outer circumferential wall surface 22, and flows along the outer circumferential wall surface 22. A change in angle from the radially outward flow that is perpendicular to the central axis AO to turn into backflow that flows along the outer circumferential wall surface
22 in which the angle of inclination 6 is an acute angle is greater than a

change in angle in conventional techniques from the radially outward flow to turn into backflow that flows along an outer circumferential wall surface in which the angle of inclination is perpendicular. Thus, the flow of backflow according to the present application is attenuated significantly compared to conventional techniques.
[0031]
Because the flow rate of fuel that turns into backflow is reduced, and the flow of the backflow is attenuated in this manner, the backflow becomes a lowmass low-velocity flow before colliding with the main flow. Thus, because reductions in kinetic energy of the main flow due to collision with the backflow are suppressed, reductions in relative speed of the fuel that is sprayed into atmospheric air from the nozzle apertures 17 relative to the air are suppressed, increasing shearing force by which the air breaks up the fuel droplets. The fuel droplets that are sprayed are thereby broken up by the air, facilitating atomization of the fuel spray.
[0032]
Because the volume of the volume portion that exists radially outside the nozzle apertures 17 from the central axis AO can be reduced, the volume of the cavity 18 is reduced, reducing the amount of fuel that remains inside the cavity after completion of valve closing. Thus, during spraying into a negative-pressure atmosphere, because the amount of fuel that is sucked out through the nozzle apertures 17 into the engine air intake pipe due to negative pressure after completion of valve closing is reduced, changes in flow rate can be reduced. The amount of fuel spray that has poor particle size that is sprayed at valve closing completion is also reduced, suppressing deterioration in spray characteristics.
[0033]
Because the height Hoi the cavity 18 is also constant at least from the opening portion 15b to directly above central positions of the nozzle

apertures 17, the volume of the cavity can be reduced compared to conventional techniques, in which the axial height of the cavity reduces gradually toward a radially outer side, and flow in a direction that is perpendicular to the central axis AO in a cross section that includes the central axis AO of the main flow through the cavity 18 from the opening portion 15b toward the nozzle apertures 17 is also strengthened. The flow of fuel that has flowed into the nozzle apertures 17 thereby detaches at the inlet portion of the nozzle apertures 17, facilitating formation of liquid films inside the nozzle apertures 17, in other words, facilitating atomization of the fuel spray.
[0034]
Next, reductions in the volume portion that is present radially outside the nozzle apertures 17 from the central axis AO will be investigated.
In order to reduce the volume of the volume portion in question, it is desirable for a starting point of the outer circumferential wall surface 22, which is a point of intersection between the lower surface 21 and the tapered surface 23, and an end point of the outer circumferential wall surface 22, which is a point of intersection between the upper surface 20 and the joint 24, to be positioned closer to the central axis AO.
Because the height H oi the cavity 18 is constant at least from the opening portion 15b to directly above central positions of the nozzle apertures 17, a position that is directly above the central positions of the nozzle apertures 17 is a position that is as close as possible to the central axis AO for the end point of the outer circumferential wall surface 22.
A position that has clearance that takes manufacturing irregularities such as processing and welding into consideration is a position that is as close as possible to the central axis AO for the starting point of the outer circumferential wall surface 22.

[0035]
If the angle of inclination 6 of the outer circumferential wall surface 22 is increased, the end point of the outer circumferential wall surface 22 shifts radially outward from the positions directly above the central positions of the nozzle apertures 17. Thus, the main flow toward the nozzle apertures 17 will collide head-on with the backflow that has formed a flow that is parallel to the upper surface 20 from the outer circumferential wall surface 22, reducing the speed of the main flow, and giving rise to deterioration in the atomization of the fuel spray. However, because the velocity component of the backflow that collides with the main flow is reduced in a range in which influence of the flow of the backflow that is parallel to the upper surface 20 is less than influence of the flow of the backflow that is parallel to the outer circumferential wall surface 22, reductions in the velocity of the main flow are suppressed, reducing the influence on the atomization of the fuel spray. Furthermore, if the angle of inclination 6 of the outer circumferential wall surface 22 is reduced, then the volume of the volume portion that exists radially outside the nozzle apertures 17 increases due to restrictions on the position of the end point of the outer circumferential wall surface 22, increasing the influence of the backflow.
[0036]
An upper limit of the angle of inclination 6 of the outer circumferential wall surface 22 will now be investigated.
[0037]
A distance Li between the starting point and the end point of the outer circumferential wall surface 22 is expressed by Expression (l).
Li - H/cosO... Expression (l)
A distance L2 between the central positions of the nozzle apertures 17 and the radially outermost positions of the openings of the nozzle

apertures 17 on the lower surface 21 in a direction that is perpendicular to the central axis AO in a cross section that includes the central axis AO is expressed by Expression (2).
L2- DA2'cosd) ... Expression (2)
Moreover, D is a diameter of the nozzle apertures 17, and a is a nozzle aperture angle of the nozzle apertures 17, which is an angle that is formed between the axial centers Al and the central axis AO.
[0038]
If the position of collision between the main flow and the backflow is considered to be a position directly above a central position 0 of a nozzle aperture 17, then the distance Xthat the backflow flows along the upper surface 20 in a direction that is perpendicular to the central axis AO in the cross section that includes the central axis AO is in a range of
0 DA2'cosd) ... Expression (4)
[0040]

Next, a lower limit of the angle of inclination 6 of the outer circumferential wall surface 22 will be investigated. Figure 5 is a cross section that shows part of the valve apparatus of the fuel injection valve according to Embodiment 1 of the present invention. Here, 7 is a radius of the opening portion 15b, ri is a radius of the upper surface 20 of the cavity 18, which is a distance from the central axis A0 to the end point of the outer circumferential wall surface 22, r^is a radius of the lower surface 21 of the cavity 18, which is a distance from the central axis A0 to the starting point of the outer circumferential wall surface 22, L3 is a distance in a direction that is perpendicular to the central axis A0 from the starting point to the end point of the outer circumferential wall surface 22 in a cross section that includes the central axis A0, and p is a distance from the central axis A0 to the central position 0 of the nozzle apertures 17 in the cross section that includes the central axis A0, in other words, a nozzle aperture pitch radius. Furthermore, for simplicity, hatching has been omitted.
[0041]
If the central position 0 of the nozzle apertures 17 is considered a boundary between the main flow and the backflow, then as indicated by broken lines in Figure 5, fuel volume V2 of the backflow on the outer circumferential side of the central position 0 of the nozzle apertures 17 increases as the angle of inclination 6 of the outer circumferential wall surface 22 is reduced. If the fuel volume V2 of the backflow on the outer circumferential side of the central position 0 of the nozzle apertures 17 becomes greater than fuel volume Vi of the main flow from the opening portion 15b to the central position 0 of the nozzle apertures 17, then the flow rate of the backflow that collides with the main flow is greater, leading to reductions in the velocity of the main flow, which gives rise to deterioration in the atomization of the fuel spray. In other words,

reductions in the velocity of the main flow are suppressed by making Vi greater than V2, enabling atomization of the fuel spray to be promoted.
[0042]
Now, n, r2, and L3 are expressed by Expressions (5) through (7).
ri - p + X... Expression (5)
r2 - ri + L3... Expression (6)
L3- HAa.nO... Expression (7)
Vi and 1/^are expressed by Expressions (8) and (9).
Vi=(j^- P)nH... Expression (8)
V2- i{(ri2 + riT2+ r£)M) " p*\nH... Expression (9)
[0043]
Thus, atomization of the fuel spray can be promoted by setting the angle of inclination 6 of the outer circumferential wall surface 22 so as to satisfy Expression (10). In that case, in a range of 0 < 6 < n/Z, ta.nO is reduced as #is reduced. Because T^is proportional to lAan20, Expression (10) is an expression that defines a lower limit of 6.
(p2 - P)nH> [{(n2 + nr2+ r^)/S} ■ pfiiiH ... Expression (10)
[0044]
Next, a relationship between the shortest distance M from the opening portion 15b to the central position 0 of the nozzle apertures 17 in the cross section that includes the central axis AO and a diameter D of the nozzle apertures 17 will be investigated.
[0045]
Flow channel cross-sectional area of a flow channel that is formed by a cylindrical surface that has the central axis AO of the cavity 18 as an axial center increases radially outward. Because the flow channel cross-sectional area in question is increased excessively if M/D becomes

greater than five, the velocity of the main flow is reduced, making atomization of the fuel spray poor.
Because rectification of the main flow is insufficient if M/D is less than or equal to four, the velocity of the main flow will not rise to the desired velocity, also making atomization of the fuel spray poor.
Consequently, because rectification of the main flow can be achieved, and reductions in the velocity of the main flow are also suppressed, by configuring the cavity 18 such that M/D satisfies Expression (ll), atomization of the fuel spray can be promoted.
4