JP2017025929A - Combustion chamber structure of diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress degradation of unburned HC by pilot injection in idling, while suppressing degradation of soot in a medium-high load region.SOLUTION: A piston 4 has a reentrant type cavity 6, and a fuel injection nozzle of multi-injection holes is disposed on a center line of the cavity. In the cavity 6, a lip portion 42 of a smallest diameter is disposed at a lower part with respect to a piston crown surface 43, and a pocket portion 44 is formed at an upper part with respect to the lip portion 42. The pocket portion 44 is located on a height position where pilot injection injected before a top dead point is directed in idling of an engine, and a position of a wall surface 45 is determined so that a spray tip does not strongly collide therewith. A fuel of the pilot injection temporarily stays in the pocket portion 44, and then flows into the cavity 6 by squish flow thereafter.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a combustion chamber structure of a direct injection type diesel engine provided with a reentrant type cavity on a piston crown surface and a fuel injection nozzle having multiple injection holes arranged on the center line of the cavity.

  In a direct injection type diesel engine, a technique for injecting a small amount of fuel before top dead center as a so-called pilot injection is known prior to main injection injected at a predetermined injection timing after top dead center. Attempts have been made to further increase the number of pilot injections in order to reduce noise in a low load range (so-called diesel knock noise) and to improve combustion stability.

  Thus, when pilot injection is attempted in multiple stages in a low load range such as when idling, the first injection is performed at a stage where the in-cylinder pressure before top dead center is relatively low. And since each injection quantity is very small, since the needle of the fuel injection nozzle is injected without being fully lifted, the penetration (penetration force or spray reach distance) increases. Therefore, in a general reentrant type cavity, the initial spray of the pilot injection collides with the inner wall surface of the cavity and is scattered outside the cavity or attached to the inner wall surface of the cavity. .

  On the other hand, if the large-diameter cavity combustion chamber is set with a large inlet diameter in consideration of the pilot injection penetration as described above, the cavity inevitably flattens, and therefore the cavity in the cavity after the top dead center The swirl speed and squish speed are reduced, and soot is easily deteriorated due to insufficient mixing in the cavity in the middle and high load range.

  In addition, Patent Document 1 discloses a configuration in which an “edge portion” is provided at the opening edge of the cavity, and an inlet lip portion is formed at a position one step down from the top surface of the piston. In this technique, the spray is positively collided with the “turned portion” above and directed upward, and the above-described deterioration of unburned HC due to pilot injection before top dead center cannot be improved.

Japanese Patent No. 4906005

The combustion chamber structure of the diesel engine according to this invention is
In a diesel engine comprising: a piston having a reentrant cavity on a crown surface; and a multi-injection fuel injection nozzle arranged on the center line of the cavity,
The lip portion having the smallest diameter at the inlet portion of the reentrant type cavity is located below the opening edge,
The pocket part formed above the lip part and on the outer peripheral side is at a height position where the earliest spray directed before top dead center at the time of idling is directed,
When idling, the position of the wall surface of the pocket portion is set so that the tip of the earliest spray enters the pocket portion and does not collide with the wall surface of the pocket portion,
The fuel temporarily staying in the pocket portion is configured to move into the cavity and burn by a squish flow toward the cavity as the piston rises.

  According to such a configuration, the fuel injected before the top dead center as a pilot injection during idling stays in the pocket portion without strongly colliding with the wall surface of the pocket portion. The fuel in the pocket portion is transferred into the cavity by a squish flow from the piston crown surface into the cavity as the piston rises thereafter. And it recombusts by the main combustion started by the main injection injected in a cavity at predetermined injection timing. Therefore, unburned HC caused by pilot injection is suppressed.

  In addition, there is a lip part below the pocket part, and the gas flow due to swirl and squish inside the cavity can be maintained at a high level compared to a flat reentrant cavity with a large diameter. Deterioration of soot is suppressed.

  In one preferred embodiment, the inlet diameter at the opening edge of the cavity is 58% or more with respect to the bore diameter of the cylinder, and the volume of the pocket portion is 3% or more with respect to the total combustion chamber volume at the piston top dead center position. And 13% or less.

  More desirably, the distance in the piston axial direction from the piston crown surface to the lip portion is not less than 10 percent and not more than 37 percent with respect to the height from the piston crown surface to the cavity bottom portion.

The structure explanatory view of the diesel engine provided with the combustion chamber structure of one example. The cylinder and piston half-sectional view which shows the combustion chamber structure of this Example. Explanatory drawing of the shape of the wall surface of a pocket part. Explanatory drawing of the dimension of each part of a pocket part. Explanatory drawing of the squish flow during piston ascent. The characteristic view which showed the relationship between aperture ratio (de / B) and HC discharge | emission amount. The characteristic view which showed the relationship between a volume ratio (vs / VC) and soot discharge. The characteristic view which showed the relationship between height ratio (h2 / h1) and soot discharge. The half sectional view showing the modification provided with two nozzle holes. The half sectional view showing the 2nd modification provided with two nozzle holes. The half sectional view showing the 3rd modification provided with two nozzle holes group.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is an explanatory diagram showing a configuration of a direct injection diesel engine 1 according to the present invention together with its intake and exhaust system. A piston 3 is slidably fitted into a cylinder 3 formed in a cylinder block 2. The cylinder head 5 fixed to the upper surface of the cylinder block 2 covers the upper end opening of the cylinder 3.

  A reentrant cavity 6 is recessed in the top surface of the piston 4. The cavity 6 has a rotating body shape around the central axis of the piston 4. That is, the piston 4 is formed into a perfect circle in a plan view and is formed at the center of the piston 4. On the cylinder head 5 side, a fuel injection nozzle 7 having multiple injection holes is disposed at the center position of the cylinder 3 corresponding to the center of the cavity 6. In this embodiment, the fuel injection nozzle 7 is arranged along the central axis of the cylinder 3, that is, vertically.

  The cylinder head 5 is provided with a pair of intake valves 8 and a pair of exhaust valves 9, which open and close the front end openings of the intake port 10 and the exhaust port 11, respectively. The intake valve 8 and the exhaust valve 9 are arranged in a vertical posture in which each valve stem is parallel to the central axis of the cylinder 3. The cylinder head 5 is provided with a glow plug 12 adjacent to the fuel injection nozzle 7.

  The fuel injection nozzle 7 of each cylinder is connected to a common rail 13 schematically shown. When a needle (not shown) of the fuel injection nozzle 7 is lifted by a drive signal from an engine control unit (not shown), the high pressure fuel The high-pressure fuel supplied into the common rail 13 by the pump 14 is injected. The fuel pressure in the common rail 13 is regulated to a predetermined pressure according to the operating conditions by the engine control unit via the pressure regulating valve 15.

  The diesel engine 1 of this embodiment includes a turbocharger 18, a turbine 19 of the turbocharger 18 is disposed in the passage of the exhaust passage 21, and the compressor 20 is disposed in the passage of the intake passage 22. Has been. A pre-catalytic converter 23 and a main catalytic converter 24 are arranged in series downstream of the turbine 19 in the exhaust passage 21. An air flow meter 25 and an air cleaner 26 are provided on the upstream side of the compressor 20 in the intake passage 22, and an intercooler 27 is disposed between the collector portion 28 on the downstream side of the compressor 20. . Further, as an exhaust gas recirculation device, an exhaust gas recirculation passage 29 that communicates the position of the exhaust passage 21 upstream of the turbine 19 and the intake collector portion 28, and the exhaust gas recirculation rate to a predetermined exhaust gas recirculation rate according to engine operating conditions. And an exhaust gas recirculation control valve 30 provided for control.

  FIG. 2 shows a more specific cross-sectional shape of the cavity 6. As a reentrant type, the cavity 6 has a central protrusion 41 bulging in the shape of a mountain at the center of the bottom surface, and the diameter of the inlet portion is relatively smaller than the maximum diameter at the height position near the middle. ing. In particular, the lip portion 42 having the smallest diameter at the inlet portion is located at a position that is lowered in the piston axial direction from the piston crown surface 43 that remains in an annular shape, and expands to the outer peripheral side of the piston 4 above the lip portion 42. Or the pocket part 44 which consists of the retreated space is formed. That is, the diameter of the opening edge 44 a of the pocket portion 44 in the piston crown surface 43 is larger than the diameter of the lip portion 42, and the pocket portion is formed by the outer peripheral wall 45 extending from the opening edge 44 to the lower lip portion 42. 44 is formed. Specifically, as shown in FIG. 4, a space defined between a virtual cylindrical surface 46 formed by extending the tip of the lip portion 42 in parallel with the central axis of the piston 4 and the wall surface 45 becomes a pocket portion 44. .

  The pocket portion 44 is at a height position at which the spray, particularly the earliest spray, directed before top dead center as pilot injection when the engine is idling is directed. A virtual line F in the figure indicates the spray center axis of the earliest spray. As described above, the pilot spray sprayed before the top dead center has a relatively large penetration as described above, but the cylinder pressure at the corresponding crank angle at the time of idling or the needle lift amount of the fuel injection nozzle 7 The position of the wall surface 45 of the pocket portion 44 is set so that the tip of the earliest spray enters the pocket portion 44 and does not collide with the outer peripheral wall surface 45 with respect to the penetration determined from the above.

  The wall surface 45 has a shape in which an inclined surface 45a extending from the upper surface of the lip portion 42 to the outer peripheral side and an inclined surface 45b extending from the opening edge 44a to the lower side of the piston 4 are smoothly continued by an intermediate transition portion 45c. I am doing. In particular, as shown in FIG. 3, it has a curved shape along an arc or ellipse C that extends symmetrically about the spray center axis F toward the pocket 44.

  In the diesel engine 1 provided with the pocket portion 44 described above, the fuel sprayed before the top dead center as a pilot injection during engine idling basically basically collides and diffuses strongly against the wall surface 45 of the pocket portion 44. Without temporarily staying in the pocket 44. Thereafter, when the piston 4 rises toward the top dead center, the air existing in the gap between the piston crown surface 43 and the lower surface of the cylinder head 5 becomes a squish flow S (see FIG. 5) and enters the cavity 6. To flow. The fuel from the pilot injection staying in the pocket 44 moves into the cavity 6 by the squish flow S. Thereafter, combustion by the main injection injected at a predetermined injection timing after the pilot injection is started in the cavity 6, and the fuel by the pilot injection is recombusted by this main combustion.

  Thus, in the above-described embodiment, fuel due to pilot injection with large penetration does not diffuse excessively or adhere to the wall surface of the cavity 6 and suppresses an increase in unburned HC during idling due to pilot injection. it can. In addition, the pocket portion 44 is formed in a shape in which only the opening edge portion of the cavity 6 is enlarged in diameter, and the volume thereof is the minimum necessary. Therefore, the shape of the cavity 6 itself, particularly the portion below the lip portion 42 is provided. Is not required to be excessively flattened, and the gas flow by the swirl or squish flow inside the cavity 6 can be maintained at a high level. Accordingly, there is no deterioration of soot in the middle and high load range.

  In particular, in the above embodiment, the wall surface 45 of the pocket portion 44 is formed in a curved shape along an arc or ellipse centered on the spray center axis F, so that collision with the spray tip is effectively avoided. The volume of the pocket portion 44 can be reduced. Moreover, even if the spray collides, it does not diffuse to the outer peripheral side.

  As shown in FIG. 4, the ratio (de / B) of the diameter of the opening edge 44 a of the pocket portion 44, that is, the inlet diameter de to the bore diameter B of the cylinder 3 is preferably 58% or more. The ratio (vs / VC) of the volume vs. the total combustion chamber volume VC at the piston top dead center position is preferably 3% or more and 13% or less. The total combustion chamber volume VC is the volume of the entire space obtained by adding the volume of the cavity 6 to the gap volume between the piston 4 and the lower surface of the cylinder head 5.

  FIG. 6 summarizes the results of actual measurement of the relationship between the former caliber ratio (de / B) and HC emissions during idling for various engines, and the caliber ratio (de / B) is 58% or more. In some cases, the result was that the HC emissions were sufficiently reduced. This means that if the aperture ratio (de / B) is 58% or more, the spray of pilot injection does not collide strongly with the wall surface 45 of the pocket portion 44.

  FIG. 7 summarizes the results of actual measurement of various engines on the relationship between the latter volume ratio (vs / VC) and the soot discharge amount in the middle and high load range. As shown in the figure, when the volume of the pocket portion 44 is larger than 13%, the cavity 6 is inevitably shallow under a certain compression ratio constraint. As a result, soot is deteriorated. When the ratio is smaller than 3 percent, the shape of the shallow cavity that does not substantially include the pocket portion 44 is approached, and soot deterioration still occurs.

  Therefore, it is desirable that the aperture ratio (de / B) is 58% or more and the volume ratio (vs / VC) is in the range of 3% to 13%.

  Further, as the height position (position in the piston axial direction) of the lip portion 42 defining the pocket portion 44, as shown in FIG. 4, the distance h2 in the piston axial direction from the piston crown surface 43 to the lip portion 42 is The height h1 from the piston crown 43 to the bottom of the cavity 6 is preferably 10% or more and 37% or less.

  FIG. 8 is a summary of the results of actual measurement of various engines on the relationship between the height ratio (h2 / h1) and the soot emission amount in the medium and high load range. As shown in the figure, when the height ratio (h2 / h1) was not within the range of 10% to 37%, soot deterioration was observed.

  9 to 11 each show a modification of the present invention. That is, in the above embodiment, the sprays from the plurality of nozzle holes of the fuel injection nozzle 7 are directed to the same height position (position in the cylinder axial direction). The fuel injection nozzle 7 includes a first nozzle hole group in which the spray center axis F1 is directed to a relatively upper position, and a second nozzle hole group in which the spray center axis F2 is directed to a relatively lower position. ,have. In other words, the first nozzle hole group and the second nozzle hole group have different inclination angles (so-called bevel angles) with respect to the cylinder center axis.

  The embodiment of FIG. 9 is an example in which the nozzle hole diameter of the first nozzle hole group and the nozzle hole diameter of the second nozzle hole group are equal to each other. In this case, since both have the same penetration, The pocket portion 44 is oriented.

  The embodiment of FIG. 10 is an example in which the nozzle hole diameter of the second nozzle hole group is larger than the nozzle hole diameter of the first nozzle hole group. The penetration of the first nozzle hole group is larger than the penetration of the first nozzle hole group. Therefore, the first nozzle hole group is directed to the pocket portion 44, and the second nozzle hole group is directed to the vicinity of the lip portion 42.

  The embodiment of FIG. 11 is an example in which the nozzle hole diameter of the second nozzle hole group is smaller than the nozzle hole diameter of the first nozzle hole group, and the first nozzle hole group under conditions where the injection quantity is small. The penetration of the second nozzle hole group is larger than the penetration of Therefore, similarly to FIG. 9, both are configured to face the pocket portion 44.

Claims (4)

In a diesel engine comprising: a piston having a reentrant cavity on a crown surface; and a multi-injection fuel injection nozzle arranged on the center line of the cavity,
The lip portion having the smallest diameter at the inlet portion of the reentrant type cavity is located below the opening edge,
The pocket part formed above the lip part and on the outer peripheral side is at a height position where the earliest spray directed before top dead center at the time of idling is directed,
When idling, the position of the wall surface of the pocket portion is set so that the tip of the earliest spray enters the pocket portion and does not collide with the wall surface of the pocket portion,
A combustion chamber structure of a diesel engine configured such that fuel temporarily staying in the pocket portion is moved into the cavity and burned by a squish flow toward the cavity as the piston rises.   The combustion chamber structure of a diesel engine according to claim 1, wherein an outer peripheral wall surface of the pocket portion immediately above the lip portion has an arc shape in cross section. The inlet diameter at the opening edge of the cavity is at least 58 percent of the bore diameter of the cylinder;
The combustion chamber structure of a diesel engine according to claim 1 or 2, wherein the volume of the pocket portion is not less than 3 percent and not more than 13 percent with respect to the total combustion chamber volume at the piston top dead center position.   The combustion of the diesel engine according to claim 3, wherein the distance in the piston axial direction from the piston crown surface to the lip portion is not less than 10 percent and not more than 37 percent with respect to the height from the piston crown surface to the cavity bottom portion. Chamber structure.

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