Abstract:
A diesel engine in which a lubrication oil on the wall surface of the cylinder liner is prevented from being burned and diluted, and a smoke resulting from the local lack of air in the combustion chamber is also prevented from being generated. The diesel engine comprises a group injection hole nozzle  20  and a piston  110 . A step  115  having a height gradually increased toward the radial outer circumference of the piston is formed over the entire circumference of the top surface  111  of the piston. Furthermore, a tilted part  115   b  is formed as the step surface of the step  115.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a diesel engine. In more detail, the present invention relates to a structural art of a combustion chamber of a diesel engine having a group injection hole nozzle. 
         [0003]    2. Background Art 
         [0004]    A combustion chamber of a diesel engine is demarcated by a top surface of a piston moving upward or downward and a wall surface of a cylinder liner. A fuel injection nozzle is provided in the upper portion of the combustion chamber. The fuel injection nozzle has a nozzle body forming an injection hole and a nozzle opening and closing the injection hole. There is well known a group injection hole nozzle as a fuel injection nozzle that two or more injection holes adjacent to each other in series along the nozzle axis direction. 
         [0005]    Compared with a single injection hole nozzle (normal nozzle), the group injection hole nozzle can be designed so as to have smaller diameter of the injection hole. Namely, compared with the normal nozzle, the group injection hole nozzle has smaller spray particle diameter, wider compound spraying angle, and larger spray tip range. Accordingly, by using the group injection hole nozzle, the performance of mixture of air and fuel is improved so as to reduce combustion noise and smoke. 
         [0006]    Generally, with regard to the normal nozzle, passing-through ability of spray is reduced in a diesel engine with a large bore diameter. However, with regard to the group injection hole nozzle, passing-through ability of spray is maintained in a diesel engine with a large bore diameter by interaction of adjacent sprays. 
         [0007]    The Japanese Patent Laid Open Gazette 2006-070802 discloses a group injection hole nozzle comprising two injection holes provided in series and twisted against each other. 
         [0008]    However, with regard to the group injection hole nozzle disclosed in the Japanese Patent Laid Open Gazette 2006-070802, when fuel injection is performed at downward movement process of a piston, fuel injected from the group injection hole touches the piston top surface and then moves to a wall surface of a cylinder liner and collides with the wall surface. Then, it is disadvantageous that burn of lubricating oil and dilution of lubricating oil occur on the wall surface of the cylinder liner. It is also disadvantageous that localized insufficient of air occurs in a combustion chamber so as to generate smoke. 
       BRIEF SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0009]    The purpose of the present invention is to provide a diesel engine in which lubricating oil on a wall surface of a cylinder liner is prevented from being burned and diluted, and smoke resulting from localized lack of air in a combustion chamber is also prevented from being generated. 
       Means for Solving the Problems 
       [0010]    A diesel engine according to the present invention has a group injection hole nozzle, and a step is provided over all perimeter of a piston top surface so that height of the step is increased outward along a diameter of the piston. 
         [0011]    Accordingly, movement direction of spray of fuel injection touching a piston top surface is changed upward at the downward movement of the piston so that the spray is prevented from touching a cylinder liner wall surface. 
         [0012]    With regard to the diesel engine according to the present invention, preferably, a step surface of the step is perpendicular to the piston top surface. 
         [0013]    Accordingly, even if the group injection hole nozzle has injection hole angle which increases passing-through ability of spray along the direction of diameter, the spray is prevented from touching a cylinder liner wall surface. 
         [0014]    With regard to the diesel engine according to the present invention, preferably, a step surface of the step is tilted. 
         [0015]    Accordingly, even if the piston has large heat load, the spray is prevented from touching a cylinder liner wall surface. 
         [0016]    With regard to the diesel engine according to the present invention, preferably, a step surface of the step is curved. 
         [0017]    Accordingly, even if the shape of the combustion chamber has strong the squish flow, the wall surface forming the step is circular arc-like shaped so as to prevent back squish flow from being spoiled. 
         [0018]    With regard to the diesel engine according to the present invention, preferably, each of injection holes of the group injection hole has optional opening angle between the injection holes, and compound spraying angle of each of the group injection holes is different from each other. 
         [0019]    Accordingly, degree of freedom of design is improved, for example, compound spraying angle is increased by making each of opening angle different from each other. 
         [0020]    With regard to the diesel engine according to the present invention, preferably, distance between each of the group injection holes and the step is equal to each other. 
         [0021]    Accordingly, even if the center of the combustion chamber is different from the nozzle axis because of constraint of design, or even if the center of the combustion chamber is different from the nozzle axis and the opening angle of the group injection hole is changed corresponding to the distance between the group injection hole and the wall surface of the cylinder liner, the spray is prevented from touching a cylinder liner wall surface. 
         [0022]    With regard to the diesel engine according to the present invention, preferably, when the piston is at a top dead point, spray from each of the group injection holes collides with a wall surface of a combustion chamber. 
         [0023]    Accordingly, since the combustion in the combustion chamber is normally started or finished in the vicinity of the top dead point of the piston, the whole spray is supplied into the combustion chamber and is combusted except for at the downward movement process of the piston. 
       EFFECT OF THE INVENTION 
       [0024]    According to the diesel engine of the present invention, lubricating oil on the wall surface of the cylinder liner is prevented from being burned and diluted, and smoke resulting from localized lack of air in the combustion chamber is also prevented from being generated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0025]      FIG. 1  It is a sectional schematic drawing of comparison of a group injection hole nozzle with a single injection hole nozzle according to the present invention. 
           [0026]      FIG. 2  It is a sectional schematic drawing showing problems of the group injection hole nozzle. 
           [0027]      FIG. 3  It is a sectional schematic drawing of embodiment 1. 
           [0028]      FIG. 4  It is a sectional schematic drawing of embodiment 2. 
           [0029]      FIG. 5  It is a sectional schematic drawing of embodiment 3. 
           [0030]      FIG. 6  It is a sectional schematic drawing of embodiment 4. 
           [0031]      FIG. 7  It is a sectional schematic drawing of embodiment 5. 
           [0032]      FIG. 8  It is a sectional schematic drawing of embodiment 6. 
           [0033]      FIG. 9  It is a schematic side view showing relation between injection hole angle and opening angle. 
           [0034]      FIG. 10  It is a plan view and sectional schematic drawing showing distance between each of group injection holes to a step. 
           [0035]      FIG. 11  It is a plan view and schematic side view showing distance between each of group injection holes to a step. 
           [0036]      FIG. 12  It is a sectional schematic drawing at a top dead point. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Best Mode for Carrying Out the Invention 
       [0037]    Explanation will be given on a single injection hole nozzle (normal nozzle)  10  referring  FIG. 1(   a ). Each of cylinder heads of a direct injection type diesel engine is attached thereto with the single injection hole nozzle  10  which is a fuel injection nozzle injecting fuel pressurized by a fuel injection pump or fuel pressurized in a common rail. 
         [0038]    The single injection hole nozzle  10  has a nozzle holder  12  substantially cylindrical about a nozzle axis P and a needle (nor shown). A semi-globular round head portion  13  is formed at the tip of the nozzle holder  12 . The direction along the nozzle holder  12  toward the round head portion  13  is defined as the downward direction of the nozzle axis P. An injection hole  14  is bored in the round head portion  13  downward aslant about the nozzle axis P. 
         [0039]    Another injection hole  14  (not shown) is bored in the same surface in which the nozzle axis of the injection hole  14  passes through so as to be symmetrical to the injection hole  14  about the nozzle axis. The angle between the axes of the injection holes  14  (alternate long and short dash lines in  FIG. 1(   a )) downward about the nozzle axis P is defined as injection hole angle α. The angle formed by spraying by each of the injection holes  14  is defined as spraying angle θ. 
         [0040]    A group injection hole nozzle  20  is a fuel injection nozzle in which two or more injection holes close to each other on the same line are arranged. Group injection holes  24 ,  24  are bored in the group injection hole nozzle  20  so as to be close to each other on the same line on the same section in which the nozzle axis P passes through. A nozzle holder  22  and a round head portion  23  have the construction similar to that of the nozzle holder  12  and the round head portion  13  of the single injection hole nozzle  10 , and then explanation thereof is omitted. 
         [0041]    The compound angle of the spraying angles θ of the group injection holes  24 ,  24  is defined as compound spraying angle φ. Similarly to the single injection hole nozzle  10 , the angle between the center axes of the compound spraying angle φ (alternate long and two short dashes lines in  FIG. 1(   b )) downward about the nozzle axis P is defined as injection hole angle α. The angle between the center axes of the group injection holes  24 ,  24  (alternate long and short dash lines in  FIG. 1(   b )) is defined as opening angle γ. 
         [0042]    Compared with the single injection hole nozzle  10 , with regard to the group injection hole nozzle  20 , the diameter of each of the group injection holes  24 ,  24  can be made smaller. Namely, compared with the single injection hole nozzle  10 , the group injection hole nozzle  20  has smaller spray particle diameter, wider compound spraying angle, and larger spray tip range. Accordingly, the group injection hole nozzle  20  is improved its performance of mixture of air and fuel so as to reduce combustion noise and smoke. Furthermore, degree of freedom of design is improved, for example, the compound spraying angle φ is increased by changing the opening angle γ of the group injection holes  24 ,  24 . 
         [0043]    Explanation will be given on the problems of the group injection hole nozzle  20  referring  FIG. 2 .  FIG. 2  shows fuel injection by the group injection hole nozzle  20  when a piston  90  moves downward in vertical movement. A combustion chamber  100  is a space constituted by a top surface  111  of the piston  90 , a cylinder head  103 , a cylinder liner wall surface  104  and a cavity (region C in the drawing) at the downward movement process of the piston  90 . As mentioned above, the fuel injection by the group injection hole nozzle  20  has higher passing-through ability than the conventional single injection hole nozzle  10 . 
         [0044]    The spray by fuel injection touches a piston top surface  101  and then moves toward the cylinder liner wall surface  104 . Then, in crevice region (region B in  FIG. 2 ), the fuel colliding with the cylinder liner wall surface  104  makes lubricating oil burn and dilutes it. Furthermore, in the vicinity of the piston top surface  101  (region A in  FIG. 2 ) and in the crevice region, air is insufficient against fuel so as to generate smoke. 
         [0045]    Then, the embodiment of the present invention provides an art that a step is provided in the piston top surface so as to make spray not reach the cylinder liner wall surface  104 . Concretely, that is steps  115 ,  125 ,  135 ,  136 ,  145  and  146  shown in the embodiments 1 to 4 explained referring  FIGS. 3 to 6 . As another embodiment of the step, steps  155  and  165  are shown in the embodiments 5 and 6. 
       Embodiment 1 
       [0046]    Explanation will be given on the step  115  which is the embodiment 1 of the present invention referring  FIG. 3 . The step  115  is provided over all the perimeter of the piston top surface  111  so that the height thereof is increased outward along the diameter of the piston. With regard to the step  115 , a tilted part  115   b  is formed from a lower surface  115   a  to an upper surface  115   c  as a step surface of the step  115  (in other words, a boundary surface between the piston top surface  111  and the step  115 ). 
         [0047]    The angle between the lower surface  115   a  and the tilted part  115   b  and the angle between the tilted part  115   b  and the upper surface  115   c  are not limited. 
         [0048]    According to the construction, at the downward movement of the piston, the movement direction of spray of fuel injection touching the piston top surface  111  is changed upward by the step  115  so that the spray is prevented from touching the cylinder liner wall surface  104 . 
         [0049]    Accordingly, burn or dilution of lubricating oil on the cylinder liner wall surface  104  is prevented, and generation of smoke caused by localized insufficient of air in the vicinity of the piston top surface  111  and in the crevice region is prevented. 
       Embodiment 2 
       [0050]    Explanation will be given on the step  125  which is the embodiment 2 of the present invention referring  FIG. 4 . The step  125  is provided in a piston top surface  121 . The construction of the piston top surface  121  and the step  125  is similar to that of the piston top surface  111  and the step  115  of the embodiment 1, and the step  125  has a lower surface  125   a , a tilted part  125   b  and an upper surface  125   c.    
         [0051]    In this embodiment, with regard to the piston top surface  121 , when a boundary between the lower surface  125   a  and the tilted part  125   b  is defined as a step starting diameter d 1 , the step starting diameter d 1  is not less than 50% and not more than 75% of a bore diameter D. 
         [0052]    When the step  125  is in the vicinity of the cylinder liner wall surface  104 , the movement direction of spray cannot be changed upward. Then, by providing the step  125  within the predetermined region of the piston top surface  121  as this embodiment, the movement direction of spray touching the piston top surface  121  at the downward movement of the piston is changed upward certainly by the step  125 , whereby the spray is prevented from touching the cylinder liner wall surface  104 . 
         [0053]    Accordingly, the effect similar to the embodiment 1 can be obtained. 
       Embodiment 3 
       [0054]    Explanation will be given on the steps  135  and  136  which is the embodiment 3 of the present invention referring  FIG. 5 . The steps  135  and  136  are a double step provided in a piston top surface  131 . The construction of the piston top surface  131  and the steps  135  and  136  is similar to that of the piston top surface  111  and the step  115  of the embodiment 1, and each of the steps  135  and  136  has a lower surface, a tilted part and an upper surface. As shown in  FIG. 5 , the steps  135  and  136  are continuous, and the upper surface of the step  135  and the lower surface of the step  136  are the same surface. 
         [0055]    Each of the steps  135  and  136  is formed correspondingly to respective one of group injection holes  24 ,  24  of the group injection hole nozzle  20 . Namely, each of the steps  135  and  136  is formed at the position with which spray collides when fuel is injected from corresponding one of the group injection holes  24 ,  24 . In other words, each of the steps  135  and  136  is arranged on the extension of the axis of corresponding one of the group injection holes  24 ,  24 . In this embodiment, the tilt angle of each of the steps  135  and  136  is not limited. 
         [0056]    According to the construction, with regard to the fuel injection by the group injection hole nozzle  20 , fuel injected from each of the group injection holes  24 ,  24  is directed upward by corresponding one of the steps  135  and  136 , whereby the spray is prevented from touching the cylinder liner wall surface  104 . 
         [0057]    Accordingly, even if the opening angle γ of the group injection holes  24 ,  24  is large, the effect similar to the embodiment 1 can be obtained certainly. 
       Embodiment 4 
       [0058]    Explanation will be given on the steps  145  and  146  which is the embodiment 4 of the present invention referring  FIG. 6 . The steps  145  and  146  are a double step provided in a piston top surface  141 . The construction of the piston top surface  141  and the steps  145  and  146  is similar to that of the piston top surface  131  and the steps  135  and  136  of the embodiment 3. However, the step  146  is formed at the position with which spray of the group injection hole nozzle  20  collides at the upward movement process of a piston  140 , and the step  145  is formed at the position with which spray collides at the downward movement process of the piston  140 . In this embodiment, the tilt angle of each of the steps  145  and  146  is not limited. 
         [0059]    According to the construction, the movement direction of spray of the group injection hole nozzle  20  is changed aslant upward not only at the downward movement process of the piston  140  but also at the upward movement process by the corresponding step  146 , whereby the spray is prevented from touching the cylinder liner wall surface  104 . 
         [0060]    Accordingly, the effect similar to the embodiment 1 can be obtained certainly at the upward movement process of the piston  140 . 
         [0061]    Explanation will be given on the embodiments 5 and 6 of the step referring  FIGS. 7 and 8 . The mode (shape, arrangement and the like) of each of the steps  155  and  165  is adoptable to each of the above-mentioned embodiments 1 to 4, and by the adoption, the effect similar to each of the embodiments 1 to 4. 
       Embodiment 5 
       [0062]    Explanation will be given on the step  155  which is the embodiment 5 of the present invention referring  FIG. 7 . The step  155  is provided over all perimeter of a piston top surface  151  so that the height thereof is increased outward along the diameter of the piston. With regard to the step  155 , in the sectional view along the diameter of the piston  150 , a vertical part  155   b  is formed from a lower surface  155   a  to an upper surface  155   c  as a step surface of the step  155  (in other words, a boundary surface between the piston top surface  151  and the step  155 ). Namely, each of the angle between the lower surface  155   a  and the vertical part  155   b  and the angle between the vertical part  155   b  and the upper surface  155   c  is 90°. 
         [0063]    For example, the group injection hole nozzle  20  having large injection hole angle α has high passing-through ability of spray. Then, according to the construction of this embodiment, the movement direction of spray which touches the piston top surface  151  at the downward movement of the piston  150  is changed upward certainly by the step  155 , whereby the spray is prevented from touching the cylinder liner wall surface  104 . 
         [0064]    Accordingly, the effect similar to the embodiment 1 can be obtained certainly with the combustion chamber  100  provided therein with the group injection hole nozzle  20  having large injection hole angle α. 
       Embodiment 6 
       [0065]    Explanation will be given on the step  165  which is the embodiment 6 of the present invention referring  FIG. 8 . The step  165  is provided over all perimeter of a piston top surface  161  so that the height thereof is increased outward along the diameter of the piston. With regard to the step  165 , in the sectional view along the diameter of the piston  160 , a curved part  165   b  is formed from a lower surface  165   a  to an upper surface  165   c  as a step surface of the step  165  (in other words, a boundary surface between the piston top surface  161  and the step  165 ). In the sectional view along the diameter of the piston  160 , the curved part  165   b  is formed circular arc-like, and the angle between the curved part  165   b  and the upper surface  165   c  is 90°. 
         [0066]    According to the construction, in the cavity (region C in the drawing), the movement direction of back squish flow is changed upward smoothly (an arrow of a solid line in the drawing). Simultaneously, the movement direction of spray which touches the piston top surface  161  at the downward movement of the piston is changed upward by the step  165 . 
         [0067]    Accordingly, even if the squish flow is strong in the combustion chamber  100 , the effect similar to the embodiment 1 can be obtained certainly without any effect on the back squish flow. 
         [0068]    Explanation will be given on the combustion chamber  100  comprising the steps  115  to  165  which are the above-mentioned embodiments 1 to 6 referring  FIGS. 9 to 12 . 
         [0069]    Explanation will be given on the relation between the injection hole angle α and the opening angle γ of the group injection hole nozzle  20  referring  FIG. 9 . Normally, with regard to the group injection hole nozzle  20 , by increasing the opening angle γ, the compound spraying angle φ is increased so as to improve the space utilization rate, whereby the combustion is improved at low excess air state. However, when the opening angle γ is increased, the height of one of the steps  115  to  165  which are the above-mentioned embodiments 1 to 6 must be increased excessively so as to obtain the effect of the present invention. When the opening angle γ is increased, the injection hole angle α must be decreased. The decrease of the injection hole angle α makes the smoke worsened. 
         [0070]    In each of the embodiments 1 to 6, the sum of the injection hole angle α and the opening angle γ is not more than 180°. 
         [0071]    Accordingly, the effect of increase of the opening angle γ can be used while using the steps  115  to  165  combinedly effectively. For example, at medium or low speed, the worsening of smoke can be prevented while raising the torque. 
         [0072]    Explanation will be given on the plane construction of the steps  115  to  165  of the embodiments 1 to 6 referring  FIGS. 10 and 11 . In the combustion chamber  100 , the distance between each of the group injection holes  24 A to  24 F and the step  115  is equal to each other. Next, explanation will be given on the two cases to which the construction of the combustion chamber  100  is adopted. 
         [0073]    The upper side of  FIG. 10  shows the constructional plan view of the combustion chamber  100  and the group injection hole nozzle  20 , and the lower side thereof shows the constructional sectional view of the same. When viewed in plan, for example, the group injection hole nozzle  20  is provided therein with the six group injection holes  24 ,  24  ( 24 A to  24 F in  FIG. 10 ). In the combustion chamber  100 , for example, the piston top surface  111  forms the tilted step  115  of the above-mentioned embodiment 1. The center of the combustion chamber  100  may be in agreement with the center of the group injection hole nozzle  20 , and the group injection hole nozzle  20  may alternatively be offset from the center of the combustion chamber  100 . Namely, in this case, the distance between each of the group injection holes  24 A to  24 F and the combustion chamber  100  (Le and Lb in  FIG. 10 ) is not equal to each other. 
         [0074]    As the combustion chamber  100  that the center of the combustion chamber  100  is offset from the group injection hole nozzle  20 , for example, there is given the combustion chamber  100  that one intake valve and one exhaust valve are provided or that the number of the intake valve is different from that of the exhaust valve. Such a construction is adopted in the case that it is difficult to arrange the group injection hole nozzle  20  at the center of the combustion chamber  100 . 
         [0075]    In such the combustion chamber  100 , the distance between each of the group injection holes  24 A to  24 F and the step  115  (for example, LSe and LSb in  FIG. 10 ) is equal to each other. 
         [0076]    Accordingly, with regard to all the group injection holes  24 A to  24 F of the combustion chamber  100 , the effect similar to the embodiment 1 can be obtained certainly. 
         [0077]    The upper side of  FIG. 11  shows the constructional plan view of the combustion chamber  100  and the group injection hole nozzle  20 , and the lower side thereof shows the construction of the opening angles γa, γb and γe of the group injection hole nozzle  20 . When the center of the group injection hole nozzle  20 , which has the group injection holes at the six positions ( 24 A,  24 A) to ( 24 F,  24 F), is offset from the center of the combustion chamber (cavity C)  100 , each of the opening angles γ of the group injection holes ( 24 A,  24 A) to ( 24 F,  24 F) is different from each other corresponding to the distance to the wall surface of the cavity C (for example, Le and Lb in  FIG. 11 ). Namely, when the distance from the group injection holes  24 ,  24  to the wall surface of the combustion chamber  100  is short, the opening angle γ is large, and when the distance from the group injection holes  24 ,  24  to the wall surface of the cavity C is long, the opening angle γ is small. 
         [0078]    For example, the group injection holes  24 B,  24 B that the distance from the group injection holes  24 ,  24  to the wall surface of the cavity C is the longest (distance Lb) has the smallest opening angle γb, and the group injection holes  24 E,  24 E that the distance from the group injection holes  24 ,  24  to the wall surface of the cavity C is the shortest (distance Le) has the smallest opening angle γe. 
         [0079]    Concretely, the opening angle γ is set based on the quotient of the distance L between each group injection holes  24 ,  24  and the wall surface of the cavity C and the average distance Lm of the distances between all the group injection holes ( 24 A,  24 A) to ( 24 F,  24 F) and the wall surface of the cavity C (L/Lm). For example, the opening angle γ of the group injection holes  24 ,  24  that the distance between the group injection holes  24 ,  24  and the wall surface of the cavity C is the longest is parallel (γ=0 deg), the opening angle γ of the group injection holes  24 ,  24  of the shortest distance is 5 deg, and the opening angle γ of each of the other group injection holes  24 ,  24  is proportional (5*L/Lm). 
         [0080]    In such the combustion chamber  100 , the distance between each of the group injection holes ( 24 A,  24 A) to ( 24 F,  24 F) and the step  115  (for example, LSe and LSb in  FIG. 10 ) is equal to each other. Accordingly, the opening angle at the time that the spray from each of the group injection holes  24 ,  24  collides with the wall surface of the cavity C is substantially equal to each other, whereby the reflection at the wall surface part of the cavity C is substantially equalized. 
         [0081]    Accordingly, with regard to all the group injection holes ( 24 A,  24 A) to ( 24 F,  24 F) of the combustion chamber  100 , the effect similar to the embodiment 1 can be obtained certainly. 
         [0082]    Explanation will be given on the fuel injection at a top dead point of the combustion chamber  100  in which the steps of the embodiments 1 to 6 are constructed referring  FIG. 12 . As mentioned above, at the downward movement of the piston  110 , the combustion chamber  100  is the space constituted by the top surface  102  of the piston  90 , the cylinder head  103  and the cylinder liner wall surface  104 . However, at the top dead point of the piston  110 , the combustion chamber  100  is the cavity (region C in the drawing). 
         [0083]    In the combustion chamber  100 , for example, the piston top surface  111  forms the step  115  of the above-mentioned embodiment 1. In this case, at the top dead point of the piston  110 , the whole spray collides with the wall surface of the combustion chamber  100 , that is, the cavity. 
         [0084]    Accordingly, the whole spray is supplied into the combustion chamber  100  so as to obtain ideal combustion. 
       INDUSTRIAL APPLICABILITY 
       [0085]    The present invention can be used for a diesel engine having a group injection hole nozzle.