Patent Publication Number: US-10309294-B2

Title: Engine

Description:
TECHNICAL FIELD 
     The present invention relates to an engine, and particularly to an engine configured to inject fuel directly to a combustion chamber in a cylinder in a predetermined operation range in a period from a second half of a compression stroke until a first half of an expansion stroke and perform ignition after a compression top dead center. 
     BACKGROUND ART 
     Typically, engines using gasoline or fuel containing gasoline as a major component widely adopt a spark ignition method of performing ignition by a spark plug. To improve fuel efficiency and the like, a technology has been developed in recent years, in which: a high compression ratio (for example, 14 or more) is applied as a geometrical compression ratio of the engine; gasoline or fuel containing gasoline as a major component is used; and in a predetermined operation range, compression self ignition (specifically, homogeneous-charge compression ignition (HCCI)) is performed. 
     A combustion chamber structure of the engine configured to perform the compression self ignition is disclosed in, for example, PTL 1. Regarding a combustion chamber structure applied to a high compression ratio engine, PTL 1 discloses a technology of improving filling efficiency by configuring the combustion chamber structure such that an inside of a cavity formed on a middle portion of a piston upper surface is adequately scavenged. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Laid-Open Patent Application Publication No. 2014-43782 
     SUMMARY OF INVENTION 
     Technical Problem 
     to the above-described high compression ratio engine, in a predetermined operation range (for example, a low-rotation high-load range), to suppress so-called preignition, it is necessary to: inject fuel from a plurality of injection openings of a fuel injection valve in a period from a second half of a compression stroke until a first half of an expansion stroke; perform forced ignition by a spark plug after a compression top dead center; and complete combustion in a short period of time. 
     However, for example, when a combustion chamber ceiling of an engine is formed in a gable roof shape (pent roof shape), shapes of paths along which the fuel injected from the injection openings flows in the combustion chamber are not the same as one another, so that arrival positions of the fuel at the time of the spark ignition are different from one another. In addition, a time from when the fuel is injected until when the fuel is ignited is short. Therefore, a thick part and thin part of the fuel-air mixture tend to be generated in the combustion chamber at the time of spark ignition, i.e., homogeneity of the fuel-air mixture in the combustion chamber tends not to be secured. When the homogeneity of the fuel-air mixture is not secured as above, the fuel-air mixture containing the fuel is discharged without being combusted, or combustion (after-burning) occurs after a combustion timing. Thus, the fuel efficiency deteriorates. In addition, smoke is generated, and emission also deteriorates. 
     The present invention was made to solve the above problems, and an object of the present invention is to provide an engine configured to inject fuel in a period from a second half of a compression stroke until a first half of an expansion stroke and perform ignition after a compression top dead center, the engine being capable of appropriately securing homogeneity of a fuel-air mixture in a combustion chamber at an ignition timing. 
     Solution to Problem 
     To achieve the above object, an engine according to the present invention is an engine configured to inject fuel directly to a combustion chamber in a cylinder in a predetermined operation range in a period from a second half of a compression stroke until a first half of an expansion stroke and perform ignition after a compression top dead center, the engine including: a piston including a cavity that is concave downward at a middle portion of an upper surface of the piston; a cylinder head configured so as to form a combustion chamber having a pent roof shape; a fuel injection valve arranged at the cylinder head so as to be located at a position corresponding to a middle portion of the piston, the fuel injection valve being configured to inject the fuel into the cavity of the piston in the period from the second half of the compression stroke until the first half of the expansion stroke; and a spark plug arranged at the cylinder head so as to be provided at a position located at a radially outer side of the middle portion of the piston and corresponding to an upper side of the cavity of the piston, the middle portion corresponding to a position where the fuel injection valve is provided, wherein: the fuel injection valve includes a plurality of injection openings which are arranged in a circumferential direction surrounding a longitudinal axis of the fuel injection valve and through each of which the fuel is injected in a direction inclined relative to the longitudinal axis by a predetermined injection angle; and each of the injection openings is formed such that when a height of a ceiling of the combustion chamber at a position corresponding to an edge end portion of the cavity in an injection direction of the injection opening is large, the injection angle of the injection opening is large. 
     According to the present invention configured as above, each of the plurality of injection openings which are arranged in the circumferential direction surrounding the longitudinal axis of the fuel injection valve and through each of which the fuel is injected in a direction inclined relative to the longitudinal axis by a predetermined injection angle is formed such that when the height of the ceiling of the combustion chamber at a position corresponding to the edge end portion of the cavity in the injection direction of the injection opening is large, the injection angle of the injection opening is large. Therefore, the increase in the injected fuel flow path length by the large height of the ceiling of the combustion chamber at a position corresponding to the edge end portion of the cavity in the injection direction of the injection opening can be suppressed by the increase in the injection angle of the injection opening. With this, timings at which the fuel injected from the injection openings reaches the combustion chamber ceiling can be set to be equal to one another. Thus, the homogeneity of the fuel-air mixture in the combustion chamber at the ignition timing can be surely secured. 
     Further, in the present invention, preferably, the injection angles of the injection openings are set such that each of injected fuel flow path lengths each of which is a length of a path along which the fuel injected from the injection opening flows through the cavity to reach the ceiling of the combustion chamber becomes equal to the injected fuel flow path length of the injection opening located closest to the spark plug. 
     According to the present invention configured as above, the timing at which the fuel injected from each injection opening reaches the combustion chamber ceiling can be set to be equal to the timing at which the fuel-air mixture containing the fuel injected from the injection opening located closest to the spark plug reaches the vicinity of the spark plug. Thus, while securing the homogeneity of the fuel-air mixture in the combustion chamber at the ignition timing, the ignitability of the spark plug can be appropriately secured. 
     Advantageous Effects of Invention 
     The engine according to the present invention is an engine configured to inject fuel in a period from a second half of a compression stroke until a first half of an expansion stroke and perform ignition after a compression top dead center, and homogeneity of a fuel-air mixture in a combustion chamber of the engine can be appropriately secured. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic plan view showing one cylinder of an engine according to an embodiment of the present invention when viewed from a lower side in a cylinder axial direction. 
         FIG. 2  is a plan view showing a piston according to the embodiment of the present invention when viewed from an upper side in the cylinder axial direction. 
         FIG. 3  is a partial sectional view taken along line of  FIG. 1  and showing the piston, a cylinder head, and the like according to the embodiment of the present invention. 
         FIG. 4  is a plan view showing a tip end portion of a fuel injection valve according to the embodiment of the present invention when viewed from a lower side in a direction along a longitudinal axis of the fuel injection valve. 
         FIG. 5  is a partial sectional view taken along line V-V of  FIG. 4  and showing the tip end portion of the fuel injection valve according to the embodiment of the present invention. 
         FIG. 6  is a schematic plan view showing one cylinder of the engine according to the embodiment of the present invention when viewed from the lower side in the cylinder axial direction and is a diagram showing a plurality of fuel injection regions located in respective injection directions of injection openings of the fuel injection valve according to the embodiment of the present invention. 
         FIG. 7  is a partial sectional view taken along line VII-VII of  FIG. 6  and showing the piston, the cylinder head, and the like according to the embodiment of the present invention and is a diagram showing injected fuel flow paths of the fuel injected from the injection openings of the fuel injection valve according to the embodiment of the present invention. 
         FIG. 8  is a partial sectional view taken along line VIII-VIII of  FIG. 6  and showing the piston, the cylinder head, and the like according to the embodiment of the present invention and is a diagram showing the injected fuel flow paths of the fuel injected from the injection openings of the fuel injection valve according to the embodiment of the present invention. 
         FIG. 9  is a table showing injection opening diameters and injection angles of the injection openings of the fuel injection valve according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an engine according to an embodiment of the present invention will be explained in reference to the drawings. 
     First, before explaining details of the embodiment of the present invention, a premise configuration of the engine according to the embodiment of the present invention will be briefly explained. The engine according to the embodiment of the present invention drives at a high compression ratio such as a geometrical compression ratio of 14 or more (preferably 18 to 20) and also performs homogeneous-charge compression ignition called HCCI in a predetermined low-load range. Further, in a predetermined operation range (for example, a low-rotation high-load range), to suppress preignition and the like, the engine according to the embodiment of the present invention injects fuel (performs retarded injection) in a period from a second half of a compression stroke until a first half of an expansion stroke and performs ignition after a compression top dead center. Such premise configuration of the engine is realized by control of an ECU (Electronic Control Unit) in a vehicle. 
     Next, a combustion chamber structure of the engine according to the embodiment of the present invention will be specifically explained in reference to  FIGS. 1 to 3 . 
       FIG. 1  is a schematic plan view showing one cylinder of the engine according to the embodiment of the present invention when viewed from a lower side in a cylinder axial direction.  FIG. 2  is a plan view showing a piston according to the embodiment of the present invention when viewed from an upper side in the cylinder axial direction.  FIG. 3  is a partial sectional view taken along line of  FIG. 1  and showing the piston, a cylinder head, and the like according to the embodiment of the present invention. It should be noted that  FIG. 3  is a diagram when the piston is located at the compression top dead center. 
     In  FIG. 1 , a reference sign Z denotes a cylinder axis extending in a direction perpendicular to a paper surface, and a reference sign Y denotes a line segment extending in a paper surface upward/downward direction and corresponding to a crank axis. The engine according to the present embodiment adopts a combustion chamber including a combustion chamber ceiling located at the cylinder head and having a gable roof shape (pent roof shape) (also see  FIG. 3 ). The line segment shown by the reference sign Y corresponds to a pent roof-shaped ridgeline (hereinafter suitably referred to as “pent roof ridgeline”) constituting the combustion chamber. Further, a reference sign X denotes a line segment extending through a center of the combustion chamber, i.e., a center axis of the cylinder and perpendicular to the pent roof ridgeline Y. It should be noted that  FIG. 3  is a sectional view taken along a surface spreading along the line segment X perpendicular to the pent roof ridgeline Y and showing a part of the piston, the cylinder head, and the like. 
     As shown in  FIG. 1 , one cylinder includes two intake valves  1  provided at one of regions (i.e., a left region in  FIG. 1 ) sandwiching the pent roof ridgeline Y. These two intake valves  1  are provided so as to be lined up in a direction along the pent roof ridgeline Y. Reference signs  5  in  FIG. 1  denote intake ports that are opened and closed by the respective intake valves  1 . Further, the cylinder includes two exhaust valves  2  provided at the other of the regions (i.e., a right region in  FIG. 1 ) sandwiching the pent roof ridgeline Y. The two exhaust valves  2  are provided so as to be lined up in the direction along the pent roof ridgeline Y. Reference signs  6  in  FIG. 1  denote exhaust ports that are opened and closed by the respective exhaust valves  2 . Furthermore, one fuel injection valve  3  is provided on the cylinder axis Z. In addition, one of two spark plugs  4  is provided between the two intake valves  1 , and the other of the two spark plugs  4  is provided between the two exhaust valves  2 . 
     Next, as shown in  FIG. 2 , a cavity  11  that is concave downward is formed at a middle portion of an upper surface of a piston  10  (also see  FIG. 3 ). Specifically, the cavity  11  is formed to have a substantially circular shape when viewed from a direction along the cylinder axis Z, and a mountain-shaped protruding portion  11   a  is formed at a middle portion of the cavity  11 . A concave portion  11   b  having a lower height than the protruding portion  11   a  is formed at a radially outer side of the protruding portion  11   a  so as to surround the protruding portion  11   a . The fuel injection valve  3  is arranged right above the protruding portion  11   a  of the cavity  11 , and the two spark plugs  4  are arranged in the concave portion  11   b  of the cavity  11  (also see  FIGS. 1 and 3 ). 
     Further, an annular portion  13  extending from an outer edge of the cavity  11  to an outer edge of the upper surface of the piston  10  and surrounding a radially outer side of the cavity  11  is provided at an upper portion of the piston  10 . The annular portion  13  includes four valve recesses  15  each of which is concave downward by, for example, about 1 mm. These four valve recesses  15  are provided at positions corresponding to the two intake valves  1  and positions corresponding to the two exhaust valves  2 . Further, portions  17  each located between the adjacent valve recesses  15  are not concave (i.e., are higher than the valve recesses  15 ) and are substantially flat. Hereinafter, the portion  17  between the valve recesses  15  is suitably referred to as a “piston upper surface portion  17 .” 
     Next, as shown in  FIG. 3 , the fuel injection valve  3  is provided at a portion of a cylinder head  40 , the portion corresponding to the middle portion of the piston  10 . Specifically, the fuel injection valve  3  is provided such that a longitudinal axis of the fuel injection valve  3  coincides with the cylinder axis Z. The fuel injection valve  3  injects the fuel directly to the combustion chamber  30 . The fuel injection valve  3  includes a plurality of injection openings  27 , and the fuel is sprayed from the injection openings  27  so as to form a conical shape that is symmetrical about the cylinder axis Z. In this case, an injection angle θ of the fuel injected by the fuel injection valve  3  through the injection openings  27  is set such that the fuel injected based on the control of the ECU in the period from the second half of the compression stroke until the first half of the expansion stroke (for example, 60° before top dead center) gets into the cavity  11  of the piston  10  (see arrows of  FIG. 3 ), in other words, the fuel does not collide with the annular portion  13  of the piston  10  or a cylinder side wall (for example, a cylinder liner). Further, the injection angle θ of the fuel injection valve  3  is set such that a spray collision distance from a fuel injection position to a position of the cavity  11  with which the fuel collides is larger than a length (division length) from the fuel injection position to a position where an initial division of the fuel occurs. 
     It should be noted that the injection angle θ corresponds to an inclination angle of the injection direction of the fuel injected from each injection opening  27 , the inclination angle being defined based on the longitudinal axis (i.e., the cylinder axis Z) of the fuel injection valve  3 . Further, the fuel is supplied to the fuel injection valve  3  at relatively high fuel pressure (for example, 40 to 120 MPa). 
     Further, the two spark plugs  4  are provided at portions of the cylinder head  40 , the portions being located at a radially outer side of the middle portion of the piston  10  and corresponding to an upper side of the cavity  11  of the piston  10 . To be specific, each of the spark plugs  4  is provided at such a position that an electrode  4   a  of a tip end portion of the spark plug  4  is located within the cavity  11  in a radial direction. Further, each of the spark plugs  4  is arranged such that the electrode  4   a  is located along a combustion chamber ceiling  30   a  (in other words, along a lower surface of the cylinder head  40 ; The same is true in the following explanations). Specifically, each of the spark plugs  4  is provided at the cylinder head  40  such that an inclination direction of the electrode  4   a  is set along an inclination of the combustion chamber ceiling  30   a  while suppressing projection of the electrode  4   a  toward the combustion chamber  30  as much as possible. 
     It should be noted that in  FIG. 3 , an area shown by a reference sign SA denotes a squish area that is a space formed at a gap between the piston upper surface portion  17  and the combustion chamber ceiling  30   a . The squish area SA is formed not only at the gap between the piston upper surface portion  17  and the combustion chamber ceiling  30   a  but also at a gap between the combustion chamber ceiling  30   a  and each of upper surfaces of the valve recesses  15  (see  FIG. 2 ) provided at positions corresponding to the intake valves  1  and the exhaust valves  2 . 
     Next, the fuel injection valve  3  according to the embodiment of the present invention will be explained in detail in reference to  FIGS. 4 and 5 .  FIG. 4  is a plan view showing a tip end portion of the fuel injection valve  3  according to the embodiment of the present invention when viewed from a lower side in a direction along the longitudinal axis of the fuel injection valve  3 .  FIG. 5  is a partial sectional view taken along line V-V of  FIG. 4  and showing the tip end portion of the fuel injection valve  3  according to the embodiment of the present invention. 
     As shown in  FIGS. 4 and 5 , the fuel injection valve  3  includes a bottomed cylindrical valve body extending in the direction along the longitudinal axis of the fuel injection valve  3 . A columnar needle  21  extending in the direction along the longitudinal axis of the fuel injection valve  3  is provided in the valve body  19 . The needle  21  is driven by a high-responsiveness solenoid (not shown) in an upward/downward direction along the longitudinal axis of the fuel injection valve  3 . A bottom surface of the valve body  19  is formed in a concave spherical shape that is concave downward. A seat portion  23  is formed at an outer peripheral portion of the bottom surface of the valve body  19 . A tip end portion of the needle  21  moved downward by the high-responsiveness solenoid is pressed against the seat portion  23 . Further, a space between an inner peripheral surface of the valve body  19  and an outer peripheral surface of the needle  21  is a fuel passage  25 . Furthermore, a plurality of injection openings  27  are formed on the bottom surface of the valve body so as to be located at a tip end side of the seat portion  23 . 
     The injection openings  27  are arranged in a circumferential direction surrounding the longitudinal axis of the fuel injection valve  3 . Each of the injection openings  27  is formed such that the fuel is injected in a direction inclined relative to the longitudinal axis of the fuel injection valve  3  by a predetermined injection angle θ. In the present embodiment, as shown in  FIG. 4 , ten injection openings  27  are arranged on the fuel injection valve  3  at equal angular intervals (i.e., at intervals of 36°) in the circumferential direction surrounding the longitudinal axis of the fuel injection valve  3 . Further, as shown in  FIG. 5 , each of the injection openings  27  are formed such that an angle formed by a center axis of the injection opening  27  and the longitudinal axis of the fuel injection valve  3  becomes θ. 
     Next, the sizes and injection angles of the injection openings  27  of the fuel injection valve  3  according to the embodiment of the present invention will be explained in reference to  FIGS. 6 to 9 . 
       FIG. 6  is a schematic plan view showing one cylinder of the engine according to the embodiment of the present invention when viewed from the lower side in the cylinder axial direction and is a diagram showing a plurality of fuel injection regions located in respective injection directions of the injection openings  27  of the fuel injection valve  3  according to the embodiment of the present invention.  FIG. 7  is a partial sectional view taken along line VII-VII of  FIG. 6  and showing the piston  10 , the cylinder head  40 , and the like according to the embodiment of the present invention and is a diagram showing the injected fuel flow paths of the fuel injected from the injection openings  27  of the fuel injection valve  3  according to the embodiment of the present invention.  FIG. 8  is a partial sectional view taken along line VIII-VIII of  FIG. 6  and showing the piston  10 , the cylinder head  40 , and the like according to the embodiment of the present invention and is a diagram showing the injected fuel flow paths of the fuel injected from the injection openings  27  of the fuel injection valve  3  according to the embodiment of the present invention.  FIG. 9  is a table showing injection opening diameters and injection angles of the injection openings  27  of the fuel injection valve  3  according to the embodiment of the present invention. 
     First, as shown in  FIG. 6 , to determine the sizes of the injection openings  27 , the combustion chamber  30  at the compression top dead center is divided into a plurality of fuel injection regions by virtual vertical surfaces each extending in a radial direction of the cylinder from the longitudinal axis of the fuel injection valve  3  through a middle between the adjacent injection openings  27 . 
     In the example of  FIG. 6 , ten injection openings  27  of the fuel injection valve  3  are shown by respective letters A-J, and directions in which the injection openings  27  are directed are shown by one-dot chain lines. To be specific, when the cylinder is viewed from the lower side in the cylinder axial direction, the injection opening  27  directed upward on the paper surface of  FIG. 6  in the direction along the pent roof ridgeline Y is shown by the letter A, and the other injection openings  27  are shown by the respectively letters B-J clockwise. 
     Then, a fuel injection region V A  located in the injection direction of the injection opening A is defined by: a virtual vertical surface P AB  extending in the radial direction of the cylinder from the longitudinal axis (i.e., the cylinder axis Z) of the fuel injection valve  3  through the middle between the adjacent injection openings A and B; and a virtual vertical surface P JA  extending in the radial direction of the cylinder from the longitudinal axis (i.e., the cylinder axis Z) of the fuel injection valve  3  through the middle between the adjacent injection openings J and A. Similarly, fuel injection regions V B  to V J  located at the respective injection directions of the injection openings B to J are defined. As shown in  FIG. 6 , each of the fuel injection regions is formed in a fan shape in plan view, and the fuel injection regions are arranged in the circumferential direction surrounding the longitudinal axis of the fuel injection valve  3 . 
     Each of the injection openings  27  is formed such that in a case where the combustion chamber  30  is divided into the plurality of fuel injection regions corresponding to the respective injection openings  27  as above, and when the volume of the fuel injection region located in the injection direction of the injection opening  27  is large, an opening area of the injection opening  27  is large. More preferably, the injection openings  27  are formed such that a ratio of the opening areas of the injection openings  27  and a ratio of the volumes of the fuel injection regions located in the respective injection directions of the injection openings  27  coincide with each other. 
     In the example of  FIG. 6 , the volumes of the fuel injection regions V A  and V F  each defined so as to extend in the direction along the pent roof ridgeline Y and each having a largest height (i.e., distance between the upper surface of the piston  10  and the combustion chamber ceiling  30   a ) are the largest, and the volumes of the fuel injection regions V C , V D , V H , and V I  each defined so as to extend in a direction closest to the line segment X perpendicular to the pent roof ridgeline Y are the smallest. As shown in  FIG. 9 , in the present embodiment, when each of the smallest volumes of the fuel injection regions V C , V D , V H , V I  is regarded as 1, the ratio of the volumes of the fuel injection regions is represented by “(V C , V D , V H , V I ):(V B , V E , V G , V J ):(V A , V F )=1:1.1:1.3.” 
     In the present embodiment, the injection opening diameters of the injection openings  27  are set such that: the ratio of the opening areas of the injection openings  27  coincides with the ratio of the volumes of the fuel injection regions located in the respective injection directions of the injection openings  27 , i.e., “(C, D, H, I):(B, E, J):(A, F)=1:1.1:1.3” is realized; and an average value of the injection opening diameters of the circular injection openings  27  is 0.100 mm. Specifically, as shown in  FIG. 9 , each of the injection opening diameters of the injection openings C, D, H, and I is set to 0.095 mm. Each of the injection opening diameters of the injection openings B, E, and J is set to 0.100 mm. Each of the injection opening diameters of the injection openings A and F is set to 1.3 mm. 
     Further, to determine the injection angles of the injection openings  27 , injected fuel flow path lengths are specified. Each of the injected fuel flow path lengths is a length of a path along which the fuel injected from the injection opening  27  flows through the cavity  11  to reach the combustion chamber ceiling  30   a.    
     The injected fuel flow path length is calculated as a sum of: a distance from the injection opening  27  of the fuel injection valve  3  to a position where the fuel injected at the injection angle θ collides with a surface of the cavity  11 ; and a travel distance from the position where the fuel collides with the surface of the cavity  11  to the combustion chamber ceiling  30   a  through the concave portion  11   b  of the cavity  11 . 
     In the example of  FIG. 7 , an injected fuel flow path length L C  of the injection opening C is a sum of: a distance L C1  from the injection opening C to a position where the fuel injected at an injection angle θ C  collides with the surface of the cavity  11 ; and a travel distance L C2  from the position where the fuel collides with the surface of the cavity  11  to the combustion chamber ceiling  30   a  through the concave portion  11   b  of the cavity  11 . An injected fuel flow path length L H  of the injection opening H is a sum of: a distance L H1  from the injection opening H to a position where the fuel injected at an injection angle θ H  collides with the surface of the cavity  11 ; and a travel distance L H2  from the position where the fuel collides with the surface of the cavity  11  to the combustion chamber ceiling  30   a  through the concave portion  11   b  of the cavity  11 . 
     In the example of  FIG. 8 , an injected fuel flow path length L A  of the injection opening A is a sum of: a distance L A1  from the injection opening A to a position where the fuel injected at an injection angle θ A  collides with the surface of the cavity  11 ; and a travel distance L A2  from the position where the fuel collides with the surface of the cavity  11  to the combustion chamber ceiling  30   a  through the concave portion  11   b  of the cavity  11 . An injected fuel flow path length L F  of the injection opening F is a sum of: a distance L F1  from the injection opening H to a position where the fuel injected at an injection angle θ F  collides with the surface of the cavity  11 ; and a travel distance L F2  from the position where the fuel collides with the surface of the cavity  11  to the combustion chamber ceiling  30   a  through the concave portion  11   b  of the cavity  11 . 
     In a case where the injection angles of the injection openings  27  are equal to one another, and when a height of the combustion chamber ceiling  30   a  at a position corresponding to an edge end portion  11   d  of the cavity  11  in the injection direction of each injection opening  27  is large (i.e., when a distance from the edge end portion  11   d  of the cavity  11  to the combustion chamber ceiling  30   a  is long), the corresponding injected fuel flow path length is long. Further, in a case where the heights of the combustion chamber ceiling  30   a  at positions corresponding to the edge end portion  11   d  of the cavity  11  in the injection directions of the injection openings  27  are equal to one another, and when the injection angle θ of each injection opening  27  is large, the corresponding injected fuel flow path length is short. 
     Therefore, in the present embodiment, each of the injection openings  27  is formed such that when the height of the combustion chamber ceiling  30   a  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening  27  is large, the injection angle of the injection opening  27  is large. Thus, the injected fuel flow path lengths of the injection openings  27  become equal to one another. 
     In the example of  FIG. 6 , the heights of the combustion chamber ceiling  30   a  at positions corresponding to the edge end portion  11   d  of the cavity  11  in the injection directions of the injection openings C, D, H, and I directed in the direction closest to the line segment X perpendicular to the pent roof ridgeline Y are the smallest. The heights of the combustion chamber ceiling  30   a  at positions corresponding to the edge end portion  11   d  of the cavity  11  in the injection directions of the injection openings A and F directed in the direction along the pent roof ridgeline Y are the largest. 
     For example, as shown in  FIGS. 7 and 8 , a height h C  of the combustion chamber ceiling  30   a  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening C and a height h H  of the combustion chamber ceiling  30   a  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening H are equal to each other. However, each of a height h A  of the combustion chamber ceiling  30   a  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening A and a height h F  of the combustion chamber ceiling  30   a  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening F is larger than the height h C . Therefore, the injection angle θ C  of the injection opening C and the injection angle θ H  of the injection opening H are set to be equal to each other, and each of the injection angle θ A  of the injection opening A and the injection angle θ F  of the injection opening H is set to be larger than the injection angle θ C  of the injection opening C. Thus, the injected fuel flow path lengths of the injection openings A, C, F, and H become equal to one another. 
     In the present embodiment, as shown in  FIG. 9 , when each of the heights h C , h D , h H , and h I  of the combustion chamber ceiling  30   a  at positions corresponding to the edge end portion  11   d  of the cavity  11  in the injection directions of the injection openings C, D, H, and I is set to 0 mm, each of the heights h B , h E , h G , and h J  of the combustion chamber ceiling  30   a  at positions corresponding to the edge end portion  11   d  of the cavity  11  in the injection directions of the injection openings B, E, and J becomes 0.90 mm, and each of the heights h A  and h F  of the combustion chamber ceiling  30   a  at positions corresponding to the edge end portion  11   d  of the cavity  11  in the injection directions of the injection openings A and F becomes 2.80 mm. 
     When each of the injection angles θ C , θ D , θ H , and θ I  of the injection openings C, D, H, and I each corresponding to the smallest height of the combustion chamber ceiling  30   a  is set to 50°, each of the injected fuel flow path lengths L C , L D , L H , and L I  is 40 mm. 
     In this case, when each of the injection angles θ B , θ E , θ G , and θ J  of the injection openings B, E, and J is set to 52°, and each of the injection angles θ A  and θ F  of the injection openings A and F is set to 55°, each of the injected fuel flow path lengths of the injection openings  27  becomes 40 mm. Thus, all of the injected fuel flow path lengths of the injection openings  27  become equal to one another. 
     Next, modified examples of the embodiment of the present invention will be explained. 
     The above embodiment has explained the engine including the combustion chamber  30  having the pent roof shape (see  FIG. 3 , for example). However, the present invention is also applicable to an engine including the combustion chamber  30  having a shape (such as a semispherical shape or a bathtub shape) other than the pent roof shape. 
     The above embodiment has explained the fuel injection valve  3  including the ten injection openings  27 . However, the present invention is also applicable to an engine including the fuel injection valve  3  having a plurality of injection openings  27  other than the ten injection openings  27 . 
     Next, operational advantages of the engines according to the embodiment of the present invention and the modified examples of the embodiment of the present invention will be explained. 
     First, each of the plurality of injection openings  27  arranged in the circumferential direction surrounding the longitudinal axis of the fuel injection valve  3  is formed such that in a case where the combustion chamber  30  at the compression top dead center is divided into the plurality of fuel injection regions, located in the respective injection directions of the injection openings  27 , by the vertical surfaces each extending in the radial direction of the cylinder from the longitudinal axis of the fuel injection valve  3  through the middle between the adjacent injection openings  27 , and when the volume of the fuel injection region located in the injection direction of the injection opening  27  is large, the opening area of the injection opening  27  is large. Therefore, even when the volumes of the fuel injection regions located in the respective injection directions of the injection openings  27  are different from one another since, for example, the combustion chamber  30  of the engine is formed in the pent roof shape, the fuel can be injected from the injection openings  27  at the amounts corresponding to the volumes of the fuel injection regions. With this, the homogeneity of the fuel-air mixture in the combustion chamber  30  at the ignition timing can be secured. 
     Especially, the injection openings  27  are formed such that the ratio of the opening areas of the injection openings  27  and the ratio of the volumes of the fuel injection regions located in the respective injection directions of the injection openings  27  coincide with each other. Therefore, the amount of fuel injected from the injection opening  27  can be set to be proportional to the volume of the fuel injection region located in the injection direction of the injection opening  27 . With this, even when the volumes of the fuel injection regions are different from one another, the concentrations of the fuel-air mixtures in the fuel injection regions can be set to be equal to one another. Thus, the homogeneity of the fuel-air mixture in the combustion chamber  30  at the ignition timing can be surely secured. 
     Each of the plurality of injection openings  27  which are arranged in the circumferential direction surrounding the longitudinal axis of the fuel injection valve  3  and through each of which the fuel is injected in a direction inclined relative to the longitudinal axis by a predetermined injection angle is formed such that when the height of the ceiling of the combustion chamber  30  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening  27  is large, the injection angle of the injection opening  27  is large. Therefore, the increase in the injected fuel flow path length by the large height of the ceiling of the combustion chamber  30  at a position corresponding to the edge end portion  11   d  of the cavity  11  in the injection direction of the injection opening  27  can be suppressed by the increase in the injection angle of the injection opening  27 . With this, timings at which the fuel injected from the injection openings  27  reaches the combustion chamber ceiling  30   a  can be set to be equal to one another. Thus, the homogeneity of the fuel-air mixture in the combustion chamber  30  at the ignition timing can be surely secured. 
     Especially, the injection angles of the injection openings  27  are set such that each injected fuel flow path length that is the length of the path along which the fuel injected from the injection opening  27  flows through the cavity  11  to reach the ceiling of the combustion chamber  30  becomes equal to the injected fuel flow path length of the injection opening  27  located closest to the spark plug  4 . Therefore, the timing at which the fuel injected from each injection opening  27  reaches the combustion chamber ceiling  30   a  can be set to be equal to the timing at which the fuel-air mixture containing the fuel injected from the injection opening  27  located closest to the spark plug  4  reaches the vicinity of the spark plug  4 . Thus, while securing the homogeneity of the fuel-air mixture in the combustion chamber  30  at the ignition timing, the ignitability of the spark plug  4  can be appropriately secured. 
     LIST OF REFERENCE CHARACTERS 
       1  intake valve 
       2  exhaust valve 
       3  fuel injection valve 
       4  spark plug 
       4   a  electrode 
       5  intake port 
       6  exhaust port 
       10  piston 
       11  cavity 
       11   a  protruding portion 
       11   b  concave portion 
       11   c  curved surface of cavity 
       11   d  edge end portion of cavity 
       13  annular portion 
       15  valve recess 
       17  piston upper surface portion 
       19  valve body 
       21  needle 
       23  seat portion 
       25  fuel passage 
       27  injection opening 
       30  combustion chamber 
       30   a  combustion chamber ceiling 
       40  cylinder head