JP2008038650A - Fuel direct injection diesel engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To equally mix fuel and air in all circumference direction of a cavity in a fuel direct injection diesel engine provided with a vent roof type piston. <P>SOLUTION: Since an angle α formed by a first fuel injection axis Li1 on which the height of a top surface of a piston 13 is oriented to a highest position and a second and third fuel injection axes Li2, Li3 adjacent in a direction in which the height of the top surface of the piston 13 gets low in a surface crossing a piston axis Lp at right angles in a view in the piston axis Lp direction is set smaller than an angle β formed by the second and third fuel injection axes Li2, Li3, the volume of a cavity 25 shared by fuel injected along each fuel injection axis Li1 - Li2 is made fixed, mixing condition of fuel and air in the cavity 25 is equalized in a circumference direction, and increase of engine output and reduction of exhaust gas harmful substance can be materialized. Since it is not necessary to change the diameter and number of injection orifices of an injector for that, increase of working cost of the injector can be avoided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention provides a piston whose top surface changes in the circumferential direction, and a plurality of fuel injection shafts spaced circumferentially from the piston central axis in a cavity recessed in the central portion of the piston. The present invention relates to a direct fuel injection diesel engine including a fuel injector that injects fuel along the fuel injection.

  In general, the top surface of a piston of a direct fuel injection diesel engine is formed flat, but a direct fuel injection diesel engine in which the top surface of the piston protrudes in a pent roof shape is known from Patent Document 1 below.

The cavity recessed in the top surface of the pent roof type piston is divided into a plurality of areas along its circumferential direction, and fuel is injected into each of the areas through the fuel injector injection nozzle located above the center of the cavity. By forming a plurality of injection holes, the amount of fuel corresponding to the size of each area is injected to make the mixed state of fuel and air uniform in the circumferential direction of the cavity. 1 is known.
JP-A-62-255524

  By the way, in order to inject an amount of fuel according to the size of each area, the above-mentioned conventional ones have different injection nozzle diameters of fuel injectors or have different directions in the vertical direction with respect to one area. A plurality of nozzle holes directed to it are formed.

  It is known that exhaust harmful substances decrease when the diameter of the nozzle hole is reduced, and exhaust harmful substances increase when the diameter of the nozzle hole is increased, but when the diameter of the injection nozzle of the fuel injector is made different. However, since it is necessary to provide a nozzle hole with a large diameter, there is a concern about an increase in exhaust harmful substances. Moreover, if the nozzle holes with different diameters are formed in the injection nozzle of the fuel injector, there is a problem that the processing cost of the injection nozzle increases.

  In addition, since the spray emitted from the nozzle holes travels while spreading, if a plurality of nozzle holes are arranged up and down in one area, the sprays from the nozzle holes may overlap each other and the fuel distribution may become non-uniform. .

  The present invention has been made in view of the above circumstances, and in a direct fuel injection diesel engine equipped with a pent roof type piston, fuel and air can be uniformly mixed in all directions in the circumferential direction of the cavity. Objective.

  In order to achieve the above object, according to the first aspect of the present invention, there is provided a piston in which the height of the top surface changes in the circumferential direction, and a cavity recessed in the central portion of the piston. A fuel direct injection diesel engine having a fuel injector that injects fuel along a plurality of fuel injection shafts spaced circumferentially from the central axis, and a piston in a circumferential direction as viewed from the direction of the piston central axis The fuel injection shaft oriented between the height of the top surface of the piston from the lowest position to the highest position is more than the angle formed by the adjacent fuel injection shaft in the direction in which the height of the piston top surface decreases in the circumferential direction. A direct fuel injection diesel engine is proposed in which the angle formed with the adjacent fuel injection shaft is smaller in the direction in which the height of the top surface of the piston becomes higher in the circumferential direction.

  Further, according to the invention described in claim 2, in addition to the configuration of claim 1, the inside of the cavity is divided into the same number of areas as the plurality of fuel injection shafts having the same volume along the circumferential direction. A fuel direct injection diesel engine is proposed in which the fuel injection shaft is arranged for each area and the fuel injection amount along each fuel injection shaft is set equal.

  According to the invention described in claim 3, in addition to the configuration of claim 1, the inside of the cavity is an area twice as many as the plurality of fuel injection shafts having the same volume along the circumferential direction. There is proposed a direct fuel injection diesel engine characterized by dividing and allocating two areas for each fuel injection axis and arranging each fuel injection axis on a boundary line between the two areas.

  According to the invention described in claim 4, in addition to the configuration of any one of claims 1 to 3, it has a plurality of intake ports extending in a direction substantially orthogonal to the cylinder arrangement direction. A direct fuel injection diesel engine is proposed in which these intake ports are straight ports having a throat portion formed in a straight line.

  According to a fifth aspect of the present invention, in addition to any one of the first to fourth aspects, a cross section of the cavity that passes through the nth fuel injection shaft is defined as a fuel injection cross section Sn, and the fuel The intersection of the injection cross section Sn and the opening periphery of the cavity is defined as a first specific point An, and the second specific point Bn passes through the first specific point An and is parallel to the lower surface of the cylinder head in the fuel injection cross section Sn. The third specific point Cn exists on the bottom wall of the cavity in the fuel injection cross section Sn, and the second specific point Bn is closer to the piston center axis than the first specific point An. The third specific point Cn is closer to the piston central axis than the position of the maximum outer diameter of the bottom wall of the cavity, and the first and second specific points An and Bn are the cylinders in the fuel injection cross section Sn. head A path AnBn connected by a line along the lower surface, a path AnCn connecting the first and third fixed points An and Cn along the wall surface of the cavity in the fuel injection cross section Sn, and the second and third specific points Bn and Cn A fuel direct-injection diesel engine is proposed in which the cross-sectional shape surrounded by the path BnCn connecting the two straight lines is substantially equal in each fuel injection cross section Sn.

  In order to achieve the above object, according to the configuration of claim 1, as viewed from the direction of the piston central axis, the fuel is directed while the height of the top surface of the piston in the circumferential direction goes from the lowest position to the highest position. The injection axis is adjacent in the circumferential direction where the height of the top surface of the piston is higher than the angle formed by the adjacent fuel injection shaft in the direction where the height of the top surface of the piston is low in the circumferential direction. Since the angle formed with the fuel injection shaft is smaller, the volume of the cavity shared by the fuel injected along each fuel injection shaft is kept constant, and the mixed state of fuel and air in the cavity is determined in the circumferential direction. It is possible to improve the engine output and reduce exhaust harmful substances. And since it is not necessary to change the diameter of the nozzle hole of an injector and the number of nozzle holes for that purpose, it can avoid that the process cost of an injector raises.

  According to the configuration of claim 2, the inside of the cavity is divided into the same number of areas as the plurality of fuel injection shafts having the same volume along the circumferential direction, the fuel injection shafts are arranged in each area, and each fuel injection Since the fuel injection amount along the axis is set equal, the volume of the cavity shared by the fuel injected along each fuel injection axis is made constant, and the mixed state of the fuel and air in the cavity is made uniform in the circumferential direction. The engine output can be improved and exhaust harmful substances can be reduced. And since it is not necessary to change the diameter of the nozzle hole of an injector and the number of nozzle holes for that purpose, it can avoid that the process cost of an injector raises.

  According to the configuration of claim 3, the inside of the cavity is divided into twice as many areas as the plurality of fuel injection shafts having the same volume along the circumferential direction, and two areas are assigned to each fuel injection shaft. Since each fuel injection shaft is arranged on the boundary line between the two areas, the volume of the cavity shared by the fuel injected along each fuel injection shaft is made constant, and the mixed state of fuel and air in the cavity is circular. Uniform in the circumferential direction can improve engine output and reduce exhaust harmful substances. And since it is not necessary to change the diameter of the nozzle hole of an injector and the number of nozzle holes for that purpose, it can avoid that the process cost of an injector raises.

  According to the fourth aspect of the present invention, since the plurality of intake ports extending in a direction substantially orthogonal to the cylinder arrangement direction are straight ports having a throat portion formed in a straight line, a swirl is generated in the cavity. It is possible to maintain a uniform mixed state of fuel and air.

  According to the fifth aspect of the present invention, a plurality of fuel injection shafts are provided from a fuel injector disposed on the piston central axis in a cavity recessed in the central portion of the piston whose top surface changes in the circumferential direction. , The cross section of the cavity passing through the nth fuel injection axis is defined as the fuel injection cross section Sn, and is defined by the first to third specific points An, Bn, Cn on the fuel injection cross section Sn. Since the cross-sectional shape of each cavity is set to be substantially equal in each fuel injection cross section Sn, the mixed state of the fuel and air in each fuel injection cross section Sn is made uniform even if the intervals of the plurality of fuel injection shafts are set to be uniform. As a result, engine output can be improved and exhaust harmful substances can be further reduced. Further, since the opening edge of the cavity in the portion where the top surface of the piston is inclined is not sharpened, the heat stress surface is also superior.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  1 to 6 show a first embodiment of the present invention. FIG. 1 is a longitudinal sectional view of an essential part of a diesel engine, FIG. 2 is a sectional view taken along line 2-2 in FIG. 1, and FIG. FIG. 4 is a top perspective view of the piston, FIG. 5 is a diagram for explaining the arrangement of the fuel injection shaft, and FIG. 6 is a graph showing the volume per unit angle of the cavity.

  As shown in FIGS. 1 to 3, the direct fuel injection type diesel engine includes a piston 13 slidably fitted into a cylinder 12 formed in a cylinder block 11, and the piston 13 includes a piston pin 14 and a piston 13. It is connected to a crankshaft (not shown) via a connecting rod 15. Two intake valve holes 17, 17 facing the top surface of the piston 13 and two exhaust valve holes 18, 18 are opened on the lower surface of the cylinder head 16 coupled to the upper surface of the cylinder block 11. The intake ports 19 and 19 communicate with the intake valve holes 17 and 17, and the exhaust ports 20 and 20 communicate with the exhaust valve holes 18 and 18, respectively. The intake valve holes 17 and 17 are opened and closed by intake valves 21 and 21, and the exhaust valve holes 18 and 18 are opened and closed by exhaust valves 22 and 22. A fuel injector 23 is provided so as to be positioned on the piston central axis Lp, and a glow plug 24 is provided adjacent to the fuel injector 23.

  The intake ports 19 and 19 are straight ports extending substantially linearly in a direction perpendicular to the cylinder row line Lc (see FIG. 2), and the cylinders pass through the intake valve holes 17 and 17 from these intake ports 19 and 19. 12 is configured so that a swirl is not generated in the intake air supplied to 12.

  As is apparent from FIGS. 1 and 4, the top surface of the piston 13 and the lower surface of the cylinder head 16 facing the piston 13 are not flat but inclined in a pent roof shape having a triangular cross section. In addition, the curvature of the exhaust port 20 can be reduced, and the diameters of the intake valve holes 17 and 17 and the exhaust valve holes 18 and 18 can be ensured to increase the intake efficiency and the exhaust efficiency.

  A cavity 25 centered on the piston center axis Lp is recessed in the top surface of the piston 13. On the radially outer side of the cavity 25, a pair of inclined surfaces 13 b, 13 b that incline downward from the top portions 13 a, 13 a extending linearly in parallel with the piston pin 14 toward the intake side and the exhaust side, and inclined surfaces 13 b, 13 b A pair of flat surfaces 13c, 13c that are formed in the vicinity of the lower end of the cylinder and orthogonal to the piston center axis Lp, and a pair of cutout portions 13d, 13d in which both ends of the top portions 13a, 13a are cut out flat are formed.

  Due to the shape of the pent roof type piston, the depth of the cavity 25 increases in the direction of the piston pin 14 and decreases in the direction orthogonal to the piston pin 14, so that the volume of the cavity 25 per unit angle around the piston central axis Lp is It increases in the direction of the piston pin 14 and decreases in the direction orthogonal to the piston pin 14 (see FIG. 6).

  As shown in FIG. 5, the fuel injectors 23 arranged along the piston center axis Lp are spaced apart at unequal intervals in the circumferential direction around a fuel injection point Oinj, which is a virtual point on the piston center axis Lp. The same amount of fuel is injected in each of the six directions. Of the six fuel injection shafts, two first fuel injection shafts Li1 overlap with the piston pin 14 when viewed in the direction of the piston central axis Lp, and are angled clockwise from the two first fuel injection shafts Li1. Two second fuel injection shafts Li2 are arranged at a position rotated by α, and two third fuel injection shafts Li3 are arranged at a position rotated by an angle α counterclockwise from the two first fuel injection shafts Li1. The The second fuel injection axis Li2 and the third fuel injection axis Li3 form an angle β, and the angle β is set larger than the angle α.

  In other words, on the surface orthogonal to the piston center axis Lp when viewed in the direction of the piston center axis Lp, the height of the top surface of the piston 13 is from the lowest position (the direction orthogonal to the piston pin 14) to the highest position (the direction of the piston pin 14). From the angle β formed by the second and third fuel injection shafts Li2 and Li3 that are oriented in the direction toward the first and second fuel injection shafts Li2 and Li3 adjacent to each other in the direction in which the height of the top surface of the piston 13 decreases. Also, the angle α formed with the adjacent first fuel injection axis Li1 in the direction in which the height of the top surface of the piston 13 is increased is set smaller.

  The cavity 25 is partitioned into 12 regions a..., B..., C... Having the same volume by six straight lines A, B, C, D, E, and F passing through the piston center line Lp. The straight line A coincides with the piston pin 14 direction, and the straight line D coincides with the piston pin 14 orthogonal direction. Three regions a, b, and c are included in a range of 90 ° between the direction of the piston pin 14 and the direction orthogonal thereto, and the angle of the region a where the top surface of the piston 13 is located in the high direction is The angle of the area | region c where it is small and the top surface of the piston 13 is located in a low direction is large, and the angle of the area | region b in the meantime becomes the intermediate value.

  The fuel injected along the first fuel injection axis Li1 shares two regions a and a on both sides thereof, and the fuel injected along the second and third fuel injection axes Li2 and Li3 is on both sides thereof. The two regions b and c are shared. As described above, since the volumes of the regions a, b, and c are set to be equal, the volumes of the two regions a and a shared by the fuel injected along the first fuel injection axis Li1, Since the volumes of the two regions b and c shared by the fuel injected along the third fuel injection axes Li2 and Li3 are equal, the mixed state of air and fuel in each part of the cavity 25 is made uniform in the circumferential direction. The combustion state of the air-fuel mixture can be improved to increase the engine output and reduce harmful exhaust substances.

  In the above description, the cavity 25 is divided into 12 regions a..., B..., C... Having the same volume by six straight lines A, B, C, D, E, and F, and the boundary (straight line) between the two regions. The first to third fuel injection shafts Li1 to Li3 are arranged on (A, C, and E), but the cavity 25 is divided into six regions having the same volume by three straight lines B, D, and F, Even if it is considered that the first to third fuel injection shafts Li1 to Li3 are arranged in the respective regions, the same is true.

  At this time, since the intake ports 19 and 19 are configured as straight ports, no swirl is generated in the intake air supplied from the intake ports 19 and 19 to the cylinder 12, so that the air and fuel are uniformly mixed. Will not collapse.

  In addition, since it is not necessary to change the diameter of the injection holes of the fuel injector 23 and the number of injection holes in the vertical direction, it is possible to avoid an increase in the processing cost of the fuel injector.

  The fuel injection point at which the fuel injector 23 actually injects fuel is slightly shifted radially outward from the piston center axis Lp. However, the fuel injection point Oinj is defined by the first to third fuel injection shafts Li1 to Li3. It is defined as a point that intersects the piston center axis Lp.

  7 to 10 show a second embodiment of the present invention. FIG. 7 is a sectional view taken along line 7-7 in FIG. 3, FIG. 8 is a sectional view taken along line 8-8 in FIG. 3, and FIG. 3 is a cross-sectional view taken along line 9-9, and FIG. 10 is a graph showing the rate of change of the cavity volume in the range of 30 ° to the left and right of the fuel injection axis when the direction of the fuel injection axis is changed in the circumferential direction.

  The second embodiment of the present invention is characterized by the cross-sectional shape of the cavity 25. The cavity 25 of the first embodiment is deepest in the direction of the piston pin 14 and shallowest in a direction perpendicular to the piston pin 14, and thus the volume per unit angle of the cavity 25 changes in the circumferential direction ( (See FIG. 6). In contrast, the cavity 25 of the second embodiment has a substantially constant depth in the circumferential direction.

  Hereinafter, the shape of the cavity 25 will be described in detail. 7 is a cross section in a direction orthogonal to the piston pin 14, and FIG. 8 is a cross section (second fuel injection shaft Li2 and third fuel injection shaft in a direction intersecting the piston pin 14 at an angle α. 9 is a cross section in the direction along the piston pin 14 (cross section including the first fuel injection axis Li1).

  What is important here is that the cross sections in FIGS. 7 to 9 are cross sections in a direction perpendicular to the top surface of the piston 13 through the fuel injection point Oinj. The cross section in the direction perpendicular to the piston pin 14 in FIG. 7 and the cross section in the direction of the piston pin 14 in FIG. 9 have their cut surfaces orthogonal to the top surface of the piston 13 and include the piston central axis Lp. On the other hand, a cross section in a direction intersecting with the piston pin 14 in FIG. 8 at an angle α passes through the second fuel injection shaft Li2 or the third fuel injection shaft Li3, and the top surface of the piston 13 (that is, the inclined surface 13b, It is orthogonal to 13b) and has a cross section that does not include the piston central axis Lp. That is, in FIG. 3, the cut surface along line 7-7 and the cut surface along line 9-9 are orthogonal to the paper surface, but the cut surface along line 8-8 is not orthogonal to the paper surface, The piston 13 is orthogonal to the inclined surfaces 13b and 13b.

  The feature of the present embodiment is that the shape of the cavity 25 substantially matches in any cross section that passes through the fuel injection point Oinj and is orthogonal to the top surface of the piston 13. The cross-sectional shape of the cavity 25 is divided into two parts on the left and right with the fuel injection point Oinj in between. The two parts are connected in a straight line in the cross section in the direction of the piston pin 14 in FIG. A cross section in a direction perpendicular to the pin 14 and a cross section in a direction intersecting with the piston pin 14 in FIG. 8 at an angle α are connected in a mountain shape according to the pent roof shape of the piston 13. However, the main part of the cross-sectional shape of the cavity 25, that is, the shape of the part shown by shading in FIGS.

  As is apparent from FIGS. 7 to 9, the cavity 25 formed around the piston center axis Lp includes a peripheral wall portion 25 a extending linearly downward from the top surface of the piston 13, and a piston center from the lower end of the peripheral wall portion 25 a. A curved wall portion 25b that curves in a concave shape toward the axis Lp, a bottom wall portion 25c that linearly extends obliquely upward from the radial inner end of the curved wall portion 25b toward the piston central axis Lp, and a piston central axis Lp The top portion 25d is continuous with the radially inner end of the bottom wall portion 25c.

  Lines extending downward and parallel to a distance Ha from lines L-R1 and L-R2 indicating the lower surface of the cylinder head 16 facing the cavity 25 are defined as piston top surface basic lines L-a1 and L-a2. Similarly, lines extending downward in parallel by a distance Hbc from the lines L-R1 and L-R2 indicating the lower surface of the cylinder head 16 are defined as cavity bottom surface basic lines L-bc1 and L-bc2, and the lower surface of the cylinder head 16 is illustrated. The lines extending downward in parallel from the lines L-R1 and L-R2 by a distance Hd are defined as cavity top basic lines L-d1 and L-d2.

  Intersection points between an arc having a radius Ra centered on the fuel injection point Oinj and the piston top surface basic lines L-a1, L-a2 are defined as a1, a2. Similarly, the intersections of the arc of radius Rb centered on the fuel injection point Oinj and the cavity bottom surface basic lines L-bc1, L-bc2 are b1, b2, the arc of radius Rc centered on the fuel injection point Oinj and the aforementioned The intersections of the cavity bottom basic lines L-bc1 and L-bc2 are c1 and c2, and the intersection of the arc of radius Rd centered on the fuel injection point Oinj and the cavity top basic lines Ld1 and Ld2 is d1. , D2. The intersections e1 and e2 are points where perpendiculars drawn from the intersections d1 and d2 to the piston top surface basic lines L-a1 and L-a2 intersect the piston top surface basic lines L-a1 and L-a2.

  The peripheral wall portion 25a of the cavity 25 is above the straight lines a1b1, a2, and b2, the bottom wall portion 25c of the cavity 25 is coincident with the straight lines c1d1, c2, and d2, and the curved wall portion 25b of the cavity 25 is the straight lines a1b1, a2b2, and straight lines. Connect c1d1 and c2d2 smoothly.

  Thus, the shaded cross-sectional shape determined by the intersection points a1, c1, d1, e1 or the intersection points a2, c2, d2, e2 is equal in any cross section orthogonal to the top surface of the piston 13 through the fuel injection point Oinj. Thus, the shape of the cavity 25 is set.

  The intersection points a1 and a2 correspond to the first specific point An of the present invention, the intersection points e1 and e2 correspond to the second specific point Bn of the present invention, and the intersection points d1 and d2 correspond to the third specific point Cn of the present invention. It corresponds to.

  8 and FIG. 9, the cross section passing through the first to third fuel injection axes Li1 to Li3 is shown in the shaded portion in the cross section in the direction of the piston pin 14 (fuel injection cross section S1) shown in FIG. The shaded portion in the cross section (fuel injection cross section S2) in the direction intersecting the piston pin 14 at an angle α has the same shape.

  When viewed in a direction perpendicular to the piston center axis Lp, the six first to third fuel injection shafts Li1 to Li3 are inclined obliquely downward, and the downward degree is small for the first fuel injection shaft Li1, The second and third fuel injection shafts Li2 and LI3 are larger.

  In the cross section in the direction of the piston pin 14 shown in FIG. 9, the point where the first fuel injection axis Li1 intersects the cavity 25 is defined as the fuel collision point P1, and the cross section in the direction intersecting the piston pin 14 shown in FIG. , A point where the second fuel injection axis Li2 and the third fuel injection axis Li3 intersect the cavity 25 is defined as a fuel collision point P2. The two fuel collision points P1 and P2 exist at the same position on the cross-section of the same shape shaded. Therefore, the position of the fuel collision point P2 is lower than the position of the fuel collision point P1, and the second fuel injection axis Li2 and the third fuel injection axis Li3 extending from the fuel injection point Oinj are further downward than the first fuel injection axis Li1. The fuel will be injected into the tank.

  A distance D1 from the fuel injection point Oinj to the fuel collision point P1 is equal to a distance D2 from the fuel injection point Oinj to the fuel collision point P2. The fuel collision angle α1 formed by the tangent line of the cavity 25 at the fuel collision point P1 and the first fuel injection axis Li1 is the fuel collision angle α2 formed by the tangent line of the cavity 25 at the fuel collision point P2 and the second fuel injection axis Li2. Match.

  As described above, according to the present embodiment, in an arbitrary cross section passing through the fuel injection point Oinj and orthogonal to the top surface of the piston 13, only a part of the vicinity of the fuel injection point Oinj (intersection points e1, d1, d2, Except for the region surrounded by e2, the cavity 25 has the same cross-sectional shape. Particularly, the two cross sections including the first to third fuel injection shafts Li12 to Li3 (see FIGS. 8 and 9) have the same cross-sectional shape of the cavity 25, and the fuel injection point in the two cross sections. The distances D1 and D2 from Oinj to the fuel collision points P1 and P2 are set to be equal, and the fuel collision angles α1 and α2 at the fuel collision points P1 and P2 are set to be equal. It is possible to further uniform the state in the circumferential direction and improve the combustion state of the air-fuel mixture to increase the engine output and further reduce the exhaust harmful substances.

  Further, in the cross section where the top surface of the piston 13 shown in FIGS. 7 and 8 is inclined, the angle formed by the edge of the opening of the cavity 25 (the portion of the intersection point a2) is flat when the top surface of the piston 13 shown in FIG. Therefore, the heat load of the portion can be reduced and the heat resistance can be improved.

  Of the cross section of the cavity 25 passing through the fuel injection point Oinj, the cross section that greatly affects the mixing of fuel and air is not a cross section including the piston central axis Lp but a cross section orthogonal to the top surface of the piston 13. This is because the circumferential diffusion of the fuel fine particles in the cavity 25 occurs in the direction along the top surface of the piston 13, and the cross section orthogonal to the diffusion direction is a cross section orthogonal to the top surface of the piston 13. In the present embodiment, the shape of the cavity 25 is substantially matched in an arbitrary cross section passing through the fuel injection point Oinj and orthogonal to the top surface of the piston 13, so that the mixed state of the fuel and air in each part of the cavity 25 is changed. It can be made more uniform.

  Since the intersection points d1 and d2 are located at the boundary between the bottom wall portion 25c and the top portion 25d of the cavity 25, the intersection points d1 and d2 and the intersection points e1 and e2 are made as close as possible to the piston center axis Lp. The ratio occupied in the fuel injection cross section Sn can be increased, and the variation in the mixed state of the fuel and air in each cross section in the circumferential direction of the cavity 25 can be minimized.

  FIG. 10 shows that when the direction of the fuel injection shaft is moved in the range of 60 ° to the left and right around the piston center axis Lp with the direction of the piston pin 14 as a reference (0 °), each 30 ° to the left and right of the fuel injection shaft. The rate of change of the volume of the cavity 25 in the range is shown. The solid line corresponds to the present embodiment in which the cross-sectional shape of the cavity 25 in an arbitrary cross section orthogonal to the top surface of the piston 13 passes through the fuel injection point Oinj, and the broken line corresponds to the conventional example. As can be seen from the figure, the volume change rate exceeds 20% in the conventional example, whereas the volume change rate is suppressed to less than 10% in the present embodiment.

  In the present embodiment, the shape of the cavity 25 is substantially matched in any cross section orthogonal to the top surface of the piston 13 through the fuel injection point Oinj. A slight change in shape such that the rate of change in volume is less than 10%, for example, when the fuel injection cross section Sn passes through the piston central axis Lp, or when the fuel injection cross section Sn is orthogonal to the top surface of the piston 13. Defined as allowing a slight tilt.

  As described above, in the second embodiment, the volume per unit angle of the cavity 25 hardly changes in the circumferential direction, so that the first fuel injection shaft Li1 becomes the second and third fuel injection shafts Li2 and Li3. The difference between the angle α formed between the second fuel injection shaft Li2 and the third fuel injection shaft Li3 can be reduced.

  By the way, although 1st, 2nd embodiment is equipped with six fuel-injection shafts Li1-Li3, the number of fuel-injection shafts is not limited to six.

  The third embodiment shown in FIG. 11 (A) includes four fuel injection shafts Li, and the piston pin 14 is arranged on a surface orthogonal to the piston central axis Lp when viewed in the piston central axis Lp direction. The angle α formed by the two fuel injection axes Li sandwiching the direction is set smaller than the angle β formed by the two fuel injection axes Li sandwiching the direction orthogonal to the piston pin 14.

  The fourth embodiment shown in FIG. 11 (B) includes eight fuel injection shafts Li1 to Li3. Two first fuel injection shafts Li1 along the direction of the piston pin 14 and the piston pin 14 are provided. Two third fuel injection shafts Li3 along the direction orthogonal to each other and four second fuel injection shafts Li2 disposed between them are provided, and the piston central shaft is seen in the piston central shaft Lp direction. On the surface orthogonal to Lp, the angle α formed by the first and second fuel injection axes Li1 and Li2 is set smaller than the angle β formed by the second and third fuel injection axes Li2 and Li3.

  Also according to the third and fourth embodiments, the same effects as those of the first and second embodiments described above can be achieved.

  The embodiments of the present invention have been described above, but various design changes can be made without departing from the scope of the present invention.

  For example, in the embodiment, the first fuel injection shaft Li1 is directed in the direction in which the height of the top surface of the piston 13 is the highest, but it is not necessarily directed in the highest direction.

  In the first embodiment, the cone angles formed by the first to third fuel injection shafts Li1 to Li3 with respect to the piston center line Lp are set to be the same. For example, in order to prevent spray spilling, As a modification of the first embodiment, the cone angle may be varied for each of the first to third fuel injection shafts Li1 to Li3.

Diesel engine longitudinal section 2-2 sectional view of FIG. 3-3 line view of FIG. Top perspective view of piston The figure explaining arrangement of a fuel injection axis Diagram showing volume per unit angle of cavity Sectional view along line 7-7 in FIG. Sectional view taken along line 8-8 in FIG. Sectional view along line 9-9 in FIG. A graph showing the rate of change of cavity volume in the range of 30 ° to the left and right of the fuel injection shaft when the direction of the fuel injection shaft is changed in the circumferential direction. The figure which shows the 3rd, 4th embodiment of this invention

Explanation of symbols

13 Piston 19 Intake port 23 Fuel injector 25 Cavity 25c Bottom wall portion Li1 Fuel injection axis Li2 Fuel injection axis Li3 Fuel injection axis Lc Cylinder row Lp Piston center axis α Angle β Angle

Claims (5)

The piston (13) whose height of the top surface changes in the circumferential direction, and the cavity (25) recessed in the central portion of the piston (13), from above the piston central axis (Lp) in the circumferential direction. In a fuel direct injection diesel engine comprising a fuel injector (23) for injecting fuel along a plurality of spaced apart fuel injection shafts (Li1, Li2, Li3),
When viewed from the direction of the piston central axis (Lp), the fuel injection axis (Li2) oriented while the height of the top surface of the piston (13) in the circumferential direction goes from the lowest position to the highest position is the circumferential direction. The height of the top surface of the piston (13) in the circumferential direction is higher than the angle (β) formed with the adjacent fuel injection shaft (Li3) in the direction in which the height of the top surface of the piston (13) decreases. A direct fuel injection diesel engine characterized in that an angle (α) formed with a fuel injection shaft (Li1) adjacent in a direction of increasing has a smaller relationship.   The cavity (25) is divided into the same number of areas as the plurality of fuel injection shafts (Li1, Li2, Li3) having the same volume along the circumferential direction, and the fuel injection shafts (Li1, Li2) for each area. , Li3), and the fuel injection amount along each fuel injection shaft (Li1, Li2, Li3) is set equal.   The inside of the cavity (25) is divided into twice as many areas as the plurality of fuel injection shafts (Li1, Li2, Li3) having the same volume along the circumferential direction, and each fuel injection shaft (Li1, Li2, 2. The direct fuel injection according to claim 1, wherein two areas are allocated to Li 3) and each fuel injection shaft (Li 1, Li 2, Li 3) is arranged on a boundary line between the two areas. diesel engine.   It has a plurality of intake ports (19) extending in a direction substantially orthogonal to the direction of the cylinder row (Lc), and these intake ports (19) are straight ports having a throat portion formed in a straight line. The direct fuel injection diesel engine according to any one of claims 1 to 3, wherein the direct fuel injection diesel engine is characterized. A section of the cavity (25) passing through the nth fuel injection axis (Li1, Li2, Li3) is defined as a fuel injection section Sn,
The intersection of the fuel injection cross section Sn and the opening periphery of the cavity (25) is defined as a first specific point An,
A second specific point Bn exists on a line passing through the first specific point An and parallel to the lower surface of the cylinder head (16) in the fuel injection cross section Sn,
A third specific point Cn exists on the bottom wall portion (25c) of the cavity (25) in the fuel injection cross section Sn,
The second specific point Bn is closer to the piston central axis (Lp) than the first specific point An,
The third specific point Cn is located closer to the piston center axis (Lp) than the maximum outer diameter position of the bottom wall portion (25c) of the cavity (25),
A path AnBn connecting the first and second specific points An and Bn along a line along the lower surface of the cylinder head (16) in the fuel injection section Sn, and the first and third fixed points An and Cn are connected to the fuel injection section Sn. The cross-sectional shape surrounded by the path AnCn connecting along the wall surface of the cavity (25) and the path BnCn connecting the second and third specific points Bn and Cn with the shortest straight line is substantially equal in each fuel injection cross section Sn. The direct fuel injection diesel engine according to any one of claims 1 to 4, wherein

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