JP2006328998A - Premixed compression self-ignition type internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain the generation of NOx and smoke, by forming a uniform lean mixture, by promoting the diffusion of fuel and mixing with air, in a premixed compression self-ignition type internal combustion engine. <P>SOLUTION: A recessed head cavity 29 is formed on an explosive surface 28 of a cylinder head 12 opposed to a top surface of a piston 11, and an injection nozzle 13 having a plurality of fuel injection ports 44 for injecting the fuel toward a peripheral wall surface 31 of the head cavity 29, is arranged in a bottom part of the head cavity 29. A recessed piston cavity 20 is formed on the top surface of the piston 11, and a bottom surface 30 of the head cavity 29 is formed as a flat surface in substantially parallel to the explosive surface 28 of the cylinder head 12. The peripheral wall surface 31 is inclined in an expansively opening shape toward the piston 11 side. An opening diameter ϕX of the head cavity 29 and an opening diameter ϕY of the piston cavity 20, are set in the relationship of ϕY ≥ 1.2 × ϕX. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a premixed compression self-ignition internal combustion engine.

  In recent years, in the field of internal combustion engines such as diesel engines, stricter exhaust regulations have been enforced year by year due to the growing environmental problems, reducing emissions of soot, smoke, CO, NOx and other air pollutants, and fuel consumption There is a strong demand for improvement.

  However, since diesel engines spray fuel on compressed air in a cylinder and burn it by self-ignition, light and dark mixing tends to occur in the mixture of air and fuel, and soot and smoke can be produced where the fuel concentration is high. It is easy to generate NOx near the theoretical ratio.

  In view of such circumstances, in recent years, a premixed compression self-ignition system has attracted attention. This premixed compression auto-ignition method injects fuel from the beginning to the middle of the compression stroke before the piston reaches top dead center, so that the fuel and air are uniformly mixed in advance in the combustion chamber. This is a technology that raises the temperature and raises the pressure by compression by the self-ignition at the ignition point of the fuel. According to this method, the generation of smoke can be suppressed by a high excess air ratio, and the generation of NOx can be suppressed because the flame temperature can be made uniform. Also known is a technique in which a normal spray combustion system is combined with a premixed compression self-ignition system to perform a plurality of stages of injection.

  However, in the premixed compression self-ignition system, fuel is injected when the piston is at a low position before reaching the top dead center, so that the fuel may directly collide with the cylinder liner. In the process of raising the piston, the temperature and pressure in the cylinder do not rise so much, so the fuel that has collided with the cylinder liner is likely to adhere as it is without evaporating. The adhering fuel may be scraped off by the piston to dilute the oil, or may be discharged as white smoke without contributing to the combustion as it is, and naturally also causes a deterioration in fuel consumption.

  On the other hand, in Patent Document 1 below, a fuel injection port is formed in which a concave dome (head cavity) is formed on the explosion surface of the cylinder head facing the top surface of the piston and opens toward the peripheral wall surface of the dome at the bottom of the dome. A diesel engine with an injection nozzle having the following is disclosed.

  When premixed compression auto-ignition is performed using this technique, the fuel collides with the peripheral wall of the dome before reaching the cylinder liner, and the direction is changed and atomized. It becomes possible to prevent adhesion.

JP 10-141622 A

  In Patent Document 1, the dome of the cylinder head is formed in a hemispherical shape, and most of the fuel that collides with the peripheral wall surface of this dome diffuses downward, but part of it diffuses upward. However, since there is little space above the fuel collision portion, there is a drawback that a fuel rich mixture is generated and smoke is likely to be generated.

  In Patent Document 1, a concave combustion chamber (piston cavity) is formed at the top of the piston, and the opening diameter of the fuel chamber is the same as the opening diameter of the dome. Therefore, when fuel is injected near the top dead center of the piston, there is a high possibility that the fuel will protrude from the combustion chamber. The protruding fuel is blown back to the center of the piston cavity by a strong squish flow in the compression stroke, so that the air-fuel mixture in the piston cavity becomes non-uniform and the combustion is worsened.

  In the premixed compression self-ignition internal combustion engine, the present invention prevents the injected fuel spray from directly colliding with the cylinder liner and promotes the diffusion of the fuel and the mixing with the air, thereby producing a uniform lean mixture. The purpose of this is to suppress the generation of NOx and smoke.

  According to the first aspect of the present invention, a concave head cavity is formed on the explosion surface of the cylinder head facing the top surface of the piston, and a plurality of fuels inject fuel toward the peripheral wall surface of the head cavity at the bottom of the head cavity. An injection nozzle having an injection port is provided, a concave piston cavity is formed on the top surface of the piston, and a bottom surface of the head cavity is formed on a flat surface substantially parallel to the explosion surface of the cylinder head. However, the opening diameter φX of the head cavity and the opening diameter φY of the piston cavity are set in a relationship of φY ≧ 1.2 × φX. It is characterized by.

  According to a second aspect of the present invention, in the first aspect of the present invention, the head cavity is disposed within a circumscribed circle of a plurality of valve seats provided in the cylinder head.

  According to a third aspect of the present invention, in the first aspect of the invention, the opening diameter φY of the piston cavity and the maximum outer diameter φZ on the side opposite to the cylinder head from the opening of the piston cavity are set in a relationship of φZ ≧ φY. It is characterized by being.

  According to a fourth aspect of the present invention, in the first aspect of the invention, when the piston is in a crank angle range of ± 40 ° from the top dead center, the fuel spray axis after colliding with the peripheral wall surface of the head cavity is , And is set to enter the piston cavity.

  According to a fifth aspect of the present invention, in the first aspect of the present invention, the diameter φd of the fuel injection port and the length L of the spray axis until the fuel injected from the fuel injection port collides with the peripheral wall surface. , L / d ≧ 50 is set.

  According to a sixth aspect of the present invention, in the first aspect of the present invention, the central portion of the piston cavity is provided with a projecting portion that projects toward the cylinder head, and the projecting portion expands toward the non-cylinder head. And an expansion angle C of the outer peripheral surface is set to C ≦ 140 °.

  According to a seventh aspect of the present invention, in the first aspect of the invention, the diameter φV of the portion where the fuel spray axis collides with the wall surface of the piston cavity after colliding with the peripheral wall surface of the head cavity, and the maximum outer diameter of the piston cavity. The relationship between φZ and φV ≧ 0.5φZ is set.

  According to an eighth aspect of the present invention, in the first aspect of the present invention, the central portion of the piston cavity is provided with a protruding portion that protrudes toward the cylinder head, and the protruding portion is located when the piston is at top dead center. The section defines a combustion chamber in the cylinder into a first combustion chamber in the head cavity and a second combustion chamber in the piston cavity, and between the first and second combustion chambers between the opening periphery of the head cavity. A communication port that communicates with the chamber is formed.

  According to the first aspect of the present invention, since the fuel collides with the peripheral wall surface of the head cavity, it is prevented from directly adhering to the cylinder liner, and the fuel is atomized by the collision. Is promoted. In addition, since the bottom surface of the head cavity is formed on a flat surface substantially parallel to the explosion surface of the cylinder head, a larger space is formed on the bottom surface side of the head cavity than the collision portion of the fuel as compared with the conventional case. And mixing of fuel and air in the head cavity can be promoted. Furthermore, since the opening diameter φY of the piston cavity is 1.2 times or more the opening diameter φX of the head cavity, when fuel is injected near the top dead center of the piston, the fuel can be reliably received in the piston cavity. It is possible to activate the flow of fuel in the piston cavity and promote the formation of a uniform air-fuel mixture. Therefore, generation | occurrence | production of smoke etc. can be suppressed suitably.

  Usually, two or four valve seats are formed on the cylinder head, and when the head cavity extends to the valve seat, the deposit on the valve seat increases, which is not preferable. On the other hand, if the head cavity is too small, the distance from the fuel injection port to the peripheral wall surface of the head cavity becomes short, and the injected fuel adheres to the peripheral wall surface of the head cavity in a liquid state, causing white smoke to be generated. Therefore, according to the invention of claim 2, the size of the head cavity can be appropriately set so as not to cause deposits and white smoke on the valve seat.

  According to the invention of claim 3, in the compression stroke, a strong squish flow into the piston cavity can be generated, and fuel agitation and mixing can be promoted in the piston cavity. In particular, as described above, the fuel injected from the fuel injection port collides with the peripheral wall surface of the head cavity, and the diffusion energy outward in the radial direction is lost, so that the atomized state floats in the cavity. Therefore, as described in claim 3, it is particularly effective to generate a strong squish flow into the piston cavity and reinforce the disturbance of the flow in the piston cavity.

  When retarding the fuel injection timing for NOx reduction or the like, the injection timing is usually within a range of a crank angle of 40 ° from the top dead center. Therefore, according to the invention of claim 4, even when the retard is performed, the fuel can be sprayed into the piston cavity, the flow of fuel in the piston cavity is activated, and the air-fuel mixture is suitably used. Can be formed.

  The fuel injected from the fuel injection port has a liquid column shape immediately after it exits from the injection port, and diffuses in the form of a mist around it. When the liquid columnar fuel adheres to the peripheral wall surface of the head cavity during low load operation such as when the engine is started, there is a high possibility that white smoke will be discharged without being involved in combustion. Therefore, according to the invention of claim 5, by appropriately setting the fuel injection port diameter φd and the distance L from the fuel injection port to the head cavity peripheral wall surface, the liquid columnar portion of the fuel injected from the fuel injection port Can be prevented from directly adhering to the peripheral wall surface, and as a result, generation of white smoke can be prevented.

  When the fuel is sprayed into the piston cavity, if the peak of the central protrusion is low (the expansion angle of the outer peripheral slope is large), the penetration force of the fuel that has collided with the central protrusion is lost, and only at the radially outer side In addition, the fuel concentration increases at the center of the piston cavity where the amount of air is small, and smoke and soot are likely to occur. Therefore, according to the sixth aspect of the present invention, the central protrusion can have a certain height and a steep outer peripheral inclined surface, and the fuel colliding with the central protrusion can be lost without losing the penetration force. The formation of the air-fuel mixture can be promoted by diffusing radially outward.

  When fuel is sprayed near the center of the piston cavity and burned, the flame contracts without using the outside air, and the phenomenon that the combustion contracts (thermal pinch) may occur, which causes the radially outward movement. Fuel diffusion is impeded. Therefore, in the invention of claim 7, the fuel can be preferably diffused radially outward without spraying the central portion of the piston cavity, and the mixture formation can be promoted.

  According to the eighth aspect of the present invention, the effect of throttling by the gap communicating with the first and second combustion chambers activates the turbulence of the fuel spray in the cylinder and is lost due to high-speed mixing of fuel and air and wall collision. The penetration force of the spray in the spray axis direction can be increased, and as a result, the mixing of fuel and air can be activated.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The internal combustion engine of the present embodiment is a premixed compression auto-ignition type diesel engine, and a basic operation process will be briefly described before the details of the structure are described. FIG. 1 is an explanatory diagram showing an outline of the operation process of the diesel engine of the present embodiment. In this diesel engine, a piston 11 is slidably fitted to a cylinder 10, and a fuel injection nozzle 13, an intake hole 14, and an exhaust hole 15 are provided in a cylinder head 12, and the intake hole 14 and the exhaust hole 15 are provided. These can be opened and closed by intake and exhaust valves 17 respectively. A combustion chamber 18 is formed between the top surface of the piston 11, the side surface of the cylinder 10, and the bottom surface of the cylinder head 12.

  In FIG. 1, (a) is an intake stroke, (b) is a compression stroke, (c) is an expansion stroke, and (d) is an exhaust stroke. In the intake stroke (a), the piston 11 moves from the top dead center to the bottom dead center, and air is taken into the combustion chamber 18 from the intake hole 14 during that time. In the compression stroke of (b), the intake air is compressed and fuel S is injected (pre-injected) from the fuel injection nozzle 13 from the initial stage to the middle stage of compression to form a mixture of air and fuel ( Premix). When the piston 11 is in the vicinity of the top dead center, the premixed gas is heated and pressurized by compression and self-ignited. Further, near the top dead center, the fuel S is injected again (main injection; shown in (c)), the fuel S immediately diffuses while evaporating, and self-ignites in a high-temperature and high-pressure atmosphere. The combustion by the main injection improves the output and burns HC and CO generated by the combustion of the premixed gas. In the expansion stroke (c), the piston 11 is lowered, and in the exhaust stroke (d), the exhaust valve 17 is opened, and the exhaust in the combustion chamber 18 is discharged from the exhaust hole 15 by the lift of the piston 11.

  Therefore, in the present embodiment, multiple stages of injection are performed, that is, pre-injection for premixing and main injection for normal compression self-ignition combustion (diesel combustion). And in order to perform such multi-stage injection, optimization of the shape of the said cylinder 10, the piston 11, and the fuel-injection nozzle 13 is achieved. Hereinafter, this point will be described in detail.

[Configuration of Piston 11]
2 and 3 are cross-sectional views of a cylinder top portion of a diesel engine to which the present invention is applied. In particular, FIG. 2 is a state where the piston 11 is below the top dead center (a state where pre-injection is performed), FIG. 3 shows a state where the piston 11 is near top dead center (a state where main injection is performed). As shown in FIG. 2, a concave piston cavity 20 is formed on the top surface of the piston 11. A central protrusion 21 that protrudes toward the cylinder head 12 is formed at the center of the bottom of the piston cavity 20, and the central protrusion 21 has a large outer diameter at the base (bottom) and a smaller outer diameter at the top. The outer peripheral surface 22 is an inclined surface that is inclined so as to expand toward the anti-cylinder head 12 side. C is attached to the expansion angle of the outer peripheral surface 22.

  The bottom surface 24 of the piston cavity 20 is formed in an arc surface that is gently connected to the inclined surface 22 of the central projecting portion 21. The outer peripheral surface 25 of the piston cavity 20 is formed as a circular arc surface continuous from the bottom surface 24 and extends to the top surface of the piston 11. The opening 26 of the piston cavity 20 is slightly inward in the radial direction from the outer peripheral surface 25, thereby forming a lip portion 27 that protrudes inward in the radial direction. In FIG. 2, φY is given to the opening diameter of the piston cavity 20, and φZ is given to the maximum outer diameter of the outer peripheral surface 25 of the piston cavity 20.

[Configuration of Cylinder Head 12]
FIG. 5 is an enlarged cross-sectional view of the cylinder head. As shown in FIGS. 2, 3, and 5, the explosive surface 28 of the cylinder head 12 facing the top surface of the piston 11 is provided with a concave head cavity 29. It is formed in a conical shape. That is, the bottom surface 30 of the head cavity 29 is formed as a flat surface substantially parallel to the explosion surface 28, and the peripheral wall surface 31 of the head cavity 29 is expanded toward the piston 11 side (lower side in FIG. 2). It is inclined to. In FIG. 2, φX is attached to the opening diameter of the head cavity 29, and A is attached to the opening angle of the peripheral wall surface 31.

  6 is a cross-sectional view taken along the line VI-VI in FIG. The cylinder head 12 is provided with two valve seats 33 for intake and exhaust, and each valve seat 33 is provided with a valve 17 for intake and exhaust. The head cavity 29 is surrounded by the four valves 17 and has a size that circumscribes each valve seat 33. However, even if it does not necessarily circumscribe each valve seat 33, it can be formed as large as possible within the range of the circumscribed circle.

  As shown in FIG. 5, in the present embodiment, a fuel collision portion 34 formed of a material different from the cylinder head 12 is provided on the peripheral wall surface 31 of the head cavity 29. The fuel collision portion 34 has a width over the entire depth of the head cavity 29 and is formed to have a uniform thickness over the width. As a raw material, what has heat resistance and heat insulation is used, for example, SUH heat-resistant steel etc. are used. However, the fuel collision part 34 may be omitted, and the peripheral wall surface 31 may be made of the material of the cylinder head 12 itself.

[Configuration of Fuel Injection Nozzle 13]
As shown in FIGS. 2 and 3, the fuel injection nozzle 13 is provided at the center of the bottom surface 30 of the head cavity 29. The injection nozzle 13 has a small-diameter portion 36 that accommodates a needle valve in the lower portion and a large-diameter portion 37 that is larger in diameter than the small-diameter portion 36 in the upper portion, and between the small-diameter portion 36 and the large-diameter portion 37. A stepped portion 38 is formed in the upper portion. The injection nozzle 13 is attached to a holding hole 39 formed in the cylinder head 12 via a holder 40. The holder 40 is formed in a cylindrical shape, and the inside of the cylinder is formed in a shape that matches the outer shape of the injection nozzle 13. Between the inner surface of the holder 40 and the small-diameter portion 36, a cylindrical first sealing material 41 is provided to keep the airtight therebetween, and an annular second seal is provided between the holder 40 and the step portion 38. A material 42 is interposed. The tip of the holder 40 is exposed on the bottom surface of the head cavity 29 through the holding hole 39. In addition, a bent portion 43 protrudes from the distal end portion of the holder 40 in the radially inward direction, and the first seal material 41 is received from the lower side by the bent portion 43.

  As shown in FIG. 5, a plurality of fuel injection ports 44 are formed at the tip of the injection nozzle 13. Four to ten fuel injection ports 44 are formed radially about the nozzle axis O1. Each injection port 44 opens toward the peripheral wall surface 31 (fuel collision part 34) of the head cavity 29, respectively.

[Relationship Between Head Cavity 29 and Fuel Injection Nozzle 13]
As shown in FIG. 2, the fuel injected from the fuel injection port 44 of the injection nozzle 13 diffuses in a radial direction conically or horizontally as a whole. If the diffusion angle (spray angle) of the spray axis X1 at this time is B, 120 ° ≦ B ≦ 180 ° is set. By setting in this way, the fuel directly collides with the peripheral wall surface 31. By this collision, the direction of the fuel is changed and the fuel is atomized.

  Therefore, when pre-injection is performed (FIGS. 1B and 2), it is possible to prevent the fuel from directly colliding with the cylinder liner 46 and adhering thereto. Further, since the fuel is atomized by the collision with the peripheral wall surface 31, the mixing with the air can be further promoted. In FIG. 2, as a comparison, the spray axis X <b> 2 when it does not collide with the peripheral wall surface 31 of the head cavity 29 is illustrated by a two-point difference line. In this case, the fuel collides with the cylinder liner 46.

  If the spray angle B is set to 120 ° or more, it is necessary to form the head cavity 29 deep in order to cause the fuel to collide with the peripheral wall surface 31 of the head cavity 29. This is because it involves a change in structure. The reason why the spray angle B is set to 180 ° or less is that if it is more than that, the flow coefficient in the injection nozzle 13 decreases. In addition, when performing the main injection, a large amount of fuel diffuses in the upper side (the bottom surface 30 side) in the head cavity 29, and the fuel concentration in the portion becomes high, which causes the generation of smoke.

  On the other hand, the expansion angle A of the peripheral wall surface 31 of the head cavity 29 is set to 60 ° ≦ A ≦ 120 °. The smaller the spread angle A, the lower the possibility that the fuel after the collision will adhere to the cylinder liner 46. However, reducing the expansion angle A hinders fuel diffusion when performing main injection (FIGS. 1C and 3). That is, it becomes difficult to promote the mixing of the fuel after the main injection and collides with the peripheral wall surface 31 and a large amount of air existing outside in the cylinder 10 in the radial direction, which hinders the formation of a uniform air-fuel mixture. Further, a large amount of fuel colliding with the peripheral wall surface 31 of the head cavity 29 is diffused to the upper side in the head cavity 29, and smoke is likely to be generated due to excessive fuel concentration in the head cavity 29. Therefore, in the present embodiment, by setting the expansion angle A as described above, appropriate mixture formation is realized in both the pre-injection and the main injection.

  Further, assuming that the intersection angle between the peripheral wall surface 31 of the head cavity 29 and the fuel spray axis X1 on the piston 11 side is D, 120 ° ≦ D <180 ° is set. When D is 120 ° or less, in the case of performing main injection, the diffusion of fuel to the upper side in the head cavity 29 increases, and smoke is likely to be generated. This is because the fuel hardly collides with the peripheral wall surface 31.

  The head cavity 29 has a bottom surface 30 formed on a flat surface substantially parallel to the explosion surface 28 of the cylinder head 12, and a corner 47 is formed at the boundary between the bottom surface 30 and the peripheral wall surface 31 as shown in FIG. 5. Has been. Due to the presence of the corner portion 47, a space R wider than the conventional technique (dome-shaped or conical head cavity) is secured on the upper side in the head cavity 29. Therefore, even if the fuel that collided with the peripheral wall surface 31 of the head cavity 29 diffuses to the upper side (the bottom surface 30 side) of the head cavity 29, mixing with air is promoted as compared with the prior art.

  The fuel injected from the fuel injection port 44 does not have to collide with the peripheral wall surface 31 as long as it is set so that at least the spray axis X1 collides with the peripheral wall surface 31. In other words, when the distance in the cylinder axial direction between the portion where the spray axis X1 collides with the peripheral wall surface 31 and the explosion surface 28 is E, it is only necessary to satisfy the condition of E> 0. The speed of the fuel droplets is highest on the spray axis X1, and the speed decreases due to the shearing action with the surrounding air as it goes radially outward, so the fuel below the spray axis X1 becomes mist-like. The penetrating force is reduced, so that the cylinder liner 46 is less likely to reach the cylinder liner 46 without colliding with the peripheral wall surface 31, and the cylinder liner 46 is diffused by the fuel colliding with the peripheral wall surface 31 being diffused as a downward component. This is because the penetrating ability to go to is further weakened.

  The fuel is in the form of a liquid column immediately after exiting from the fuel injection port 44 and then reaches the peripheral wall surface 31 while diffusing in a mist form. In the present embodiment, the distance (length) L of the spray axis X1 from the fuel injection port 44 to the peripheral wall surface 31 and the diameter φd of the fuel injection port 44 are set to have a relationship of L / φd ≧ 50. By setting in this way, the liquid column portion S1 (length is indicated by L1) of the fuel S directly collides with the peripheral wall surface 31 and does not adhere thereto.

  If the liquid columnar fuel adheres to the peripheral wall surface 31 of the head cavity 29 during low load operation such as when starting the engine, white smoke is discharged without being involved in combustion. Such a problem can be solved by setting the diameter φd. The above relationship between the distance L and the diameter φd is particularly effective when the fuel injection pressure is about 50 MPa or more.

  As described above, the fuel collision portion 34 having heat insulation and heat resistance is provided on the peripheral wall 31 of the head cavity 29. Although the cylinder head 12 is cooled by the cooling water, the temperature of the fuel collision part 34 is difficult to be lowered due to the heat insulating property. Therefore, the fuel that has collided with the fuel collision part 34 is easily evaporated by the heat of the fuel collision part 34, and the attached fuel is prevented from being discharged as white smoke.

  As shown in FIG. 6, the head cavity is provided so as to circumscribe the four valve seats 33 for intake and exhaust. That is, the valve seat 33 is formed to the maximum extent that does not erode. As a result, the distance from the fuel injection nozzle 13 to the peripheral wall surface 31 of the head cavity 29 can be made as long as possible, and the liquid column portion S1 of the fuel is prevented from adhering to the peripheral wall surface 31.

[Relationship between head cavity and piston cavity]
The opening diameter φX of the head cavity 29 is formed smaller than the diameter φY of the opening 26 of the piston cavity 20. Specifically, there is a relationship of φY ≧ 1.2 × φX. Therefore, in the case of performing main injection (FIGS. 1C and 3), most of the fuel injected from the injection nozzle 13 and colliding with the peripheral wall surface 31 of the head cavity 29 is introduced into the piston cavity 20. The squish flow into the piston cavity 20 during the compression stroke and the reverse squish flow outside the piston cavity 20 during the expansion stroke cause a strong turbulence in the flow of the fuel, and the air flows outside the piston 11 in the radial direction. Mixing is promoted more.

  The diameter φY of the opening 26 of the piston cavity 20 is formed smaller than the outer diameter (maximum outer shape) φZ of the outer peripheral surface 25 of the piston cavity 20, and the opening 26 has a lip portion protruding radially inward. Since 27 is formed, a stronger squish flow and a reverse squish flow can be obtained.

[Relationship between injected fuel and piston cavity 20]
The protrusion 21 formed in the piston cavity 20 is formed in a mountain shape as described above, and the expansion angle C of the outer peripheral surface 22 is set to C ≦ 140 °. In the case of performing the main injection (FIGS. 1C and 3), the fuel injected from the injection nozzle 13 and colliding with the head cavity 29 spreads further in a conical shape, and the outer peripheral surface 22 of the protrusion 21 in the piston cavity 20. Collide with the slope. Therefore, even after colliding with the projecting portion inclined surface 22, the fuel is further guided radially outward to promote mixing with the air.

  As shown in FIG. 3, the diameter φV of the portion where the fuel collides with the piston cavity 20 and the maximum outer diameter φZ of the piston cavity 20 are in a relationship of φV ≧ 0.5 × φZ. Therefore, when the main injection is performed, the fuel spreads radially outward in the piston cavity, and mixing with air is promoted. Therefore, the fuel is injected near the center of the piston cavity 20 to prevent the occurrence of “thermal pinch” in which the flame contracts without using radially outward air.

  The main injection may be delayed not only when the piston 11 is substantially at the top dead center but also after passing the top dead center mainly for NOx reduction. In such a case, if the injected fuel is injected out of the piston cavity 20, the generation of a uniform air-fuel mixture is hindered and smoke is increased. Therefore, in the present embodiment, when the piston 11 is within the range of the crank angle ± 40 ° from the top dead center, the fuel injection angle B and the peripheral wall surface are ensured so that the injected fuel surely enters the piston cavity 20. An expansion angle A of 31, an opening diameter φY of the piston cavity 20 and the like are set. As an example, FIG. 4 shows the arrangement of the pistons 11 when the crank angle is top dead center ± 30 °.

  In the present embodiment, when the fuel spray axis X1 that collides with the peripheral wall surface 31 spreads out to the maximum in the radial direction, that is, after the fuel spray axis X1 collides with the peripheral wall surface 31, The spray axis X 1 is set so as to enter the piston cavity 20, assuming the case of diffusion along the inclination of the wall surface 31.

[Second Embodiment]
FIG. 7 is a cross-sectional view of the cylinder top portion showing the second embodiment of the present invention. In the present embodiment, the top surface 21A of the central protrusion 21 of the piston cavity 20 is formed in a flat circular shape having an outer diameter φU that is slightly smaller than the opening diameter φX of the head cavity 29, and from the top surface of the piston 11. It is arranged at a position slightly retracted to the side opposite to the cylinder head 12. The central protrusion 21 divides the combustion chamber into a first combustion chamber 49 in the head cavity 29 and a second combustion chamber 50 in the piston cavity when the piston 11 is near top dead center. ing. The first combustion chamber 49 and the second combustion chamber 50 are communicated by a gap (communication port) 51 formed between the outer peripheral portion of the top surface 21A of the central protrusion 21 and the opening edge of the head cavity 29. .

  In the present embodiment, as the piston 11 descends from the top dead center, a jet flow from the first combustion chamber 49 to the second combustion chamber 50 is generated through the narrow communication port 51, and the turbulence of the flow in the cylinder 10 is activated. The Therefore, in the case of performing the main injection (FIG. 1 (c), FIG. 3), the fuel and air can be mixed at high speed, and the spray that is lost when the fuel collides with the peripheral wall surface 31 of the head cavity 29. The penetration force in the direction of the spray axis X1 can be increased. As a result, the mixing of fuel and air is activated.

  The present invention is not limited to the embodiment described above, and can be appropriately changed in design.

  The present invention can be suitably employed in a premixed compression auto-ignition diesel engine.

It is explanatory drawing which shows the outline | summary of the operation process of the diesel engine of this embodiment. It is sectional drawing of the cylinder top part of the diesel engine to which this invention is applied. It is sectional drawing of the cylinder top part of the diesel engine to which this invention is applied. It is sectional drawing of the cylinder top part of the diesel engine to which this invention is applied. FIG. 3 is an enlarged cross-sectional view showing a head cavity and a fuel injection nozzle 13. FIG. 6 is a cross-sectional view taken along the line VI-VI in FIG. 2. It is sectional drawing of the cylinder top part which shows 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Cylinder 11 Piston 12 Cylinder head 13 Fuel injection nozzle 20 Piston cavity 21 Protrusion part 22 Outer peripheral surface 26 Opening part 28 Explosive surface 29 Head cavity 30 Bottom surface 31 Perimeter wall surface 33 Valve seat 44 Fuel injection port 46 Cylinder liner 49 1st combustion chamber 50 Second combustion chamber 51 Communication port

Claims (8)

A concave head cavity is formed on the explosion surface of the cylinder head facing the top surface of the piston, and an injection nozzle having a plurality of fuel injection ports for injecting fuel toward the peripheral wall surface of the head cavity is provided at the bottom of the head cavity. A concave piston cavity is formed on the top surface of the piston,
The bottom surface of the head cavity is formed as a flat surface substantially parallel to the explosion surface of the cylinder head, and the peripheral wall surface is inclined so as to expand toward the piston side,
2. A premixed compression self-ignition internal combustion engine characterized in that an opening diameter φX of the head cavity and an opening diameter φY of the piston cavity are set in a relationship of φY ≧ 1.2 × φX.   2. The premixed compression self-ignition internal combustion engine according to claim 1, wherein the head cavity is disposed within a circumscribed circle of a plurality of valve seats provided in the cylinder head.   2. The premixing according to claim 1, wherein an opening diameter φY of the piston cavity and a maximum outer diameter φZ on the side opposite to the cylinder head from the opening of the piston cavity are set in a relationship of φZ ≧ φY. A compression self-ignition internal combustion engine.   When the piston is in a range of crank angle ± 40 ° from top dead center, the fuel spray axis after colliding with the peripheral wall surface of the head cavity is set to enter the piston cavity. The compression self-ignition internal combustion engine according to claim 1.   The diameter φd of the fuel injection port and the length L of the spray axis until the fuel injected from the fuel injection port collides with the peripheral wall surface are set such that L / d ≧ 50. The premixed compression self-ignition internal combustion engine according to claim 1.   The piston cavity has a projecting portion projecting toward the cylinder head side at the center portion, and the projecting portion has an outer peripheral surface that is inclined so as to expand toward the non-cylinder head side, and the expansion angle of the outer peripheral surface 2. The premixed compression self-ignition internal combustion engine according to claim 1, wherein C is set to C ≦ 140 °.   The diameter φV of the portion where the spray axis of the fuel after colliding with the peripheral wall surface of the head cavity collides with the wall surface in the piston cavity and the maximum outer diameter φZ of the piston cavity are set in a relationship of φV ≧ 0.5φZ. The premixed compression self-ignition internal combustion engine according to claim 1, wherein A projecting portion projecting toward the cylinder head is provided at the center of the piston cavity, and when the piston is at top dead center, the projecting portion defines a combustion chamber in the cylinder as a first in the head cavity. A compartment is formed in the combustion chamber and the second combustion chamber in the piston cavity, and a communication port for communicating the first and second combustion chambers is formed between the opening periphery of the head cavity. Item 2. A premixed compression self-ignition internal combustion engine according to item 1.

Images