JP2012057566A - Combustion chamber structure for direct injection type diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a combustion chamber structure for a direct injection type diesel engine capable of restraining generation of black smoke by making effective use of air in a combustion chamber.SOLUTION: It is avoided that fuel concentration in a cavity 26 becomes locally high since the major portion of a fuel is full in a squish area and is guided to its circumferential direction so as to form an air-fuel mixture in the squish area by means of synergism of recessed sections 28 arranged on the top of a piston 25 and a swirl during a low speed operation of an engine. It is avoided that the fuel concentration in the squish area becomes locally high since the major portion of the fuel is guided to the cavity 26 by means of an inner peripheral wall of the cavity below the recessed sections 28 arranged on the top of the piston 25 and the balance of the fuel is guided to the squish area by means of the swirl during a high speed operation of the engine, and consequently, generation of black smoke can be reduced.

Description

  The present invention relates to a combustion chamber structure of a direct injection diesel engine.

  Conventionally, in a diesel engine of an automobile, a part of the exhaust gas is extracted from the exhaust side and returned to the intake side, and the exhaust gas returned to the intake side suppresses the combustion of fuel in the engine to reduce the combustion temperature. Some of them adopt so-called exhaust gas recirculation (EGR), which reduces the generation of NOx by lowering.

  8 to 12 show an example of a diesel engine having a mechanism for recirculating exhaust gas. In the diesel engine 1 shown here, an exhaust passage 3 and an intake passage 4 through which the exhaust gas 2 flows are shown. Are connected by an EGR pipe 5, and a part of the exhaust gas 2 is recirculated together with the intake air 7 through an EGR valve 6 provided in the middle of the EGR pipe 5, so that the inside of the cylinder of the diesel engine 1 NOx is reduced by lowering the combustion temperature in the cylinder.

  In addition, a porous injector 8 for injecting fuel (light oil) into the cylinder is provided at the ceiling (cylinder ceiling 23) of each cylinder of the diesel engine 1, and a combustion chamber is provided at the top of the piston 9. The cavity 10 is formed so as to form most of the cavity 10. The fuel 10 is injected radially from the tip of the injector 8 onto the inner peripheral surface of the cavity 10, and the self is caused by the high cylinder temperature at the end of the compression stroke. It comes to ignite.

  The details of the cavity 10 are as shown in FIG. 9, and an inlet lip portion 11 that forms the outer edge of the opening of the cavity 10, and the inlet lip portion 11 descends so as to draw a gentle S-curve. From the combustion chamber wall surface portion 12 projecting radially outward from the portion 11, an outer curved surface portion 13 that forms a gentle curved surface extending radially inward from the combustion chamber wall surface portion 12, and from the entire lower end of the outer curved surface portion 13 The cavity 10 is formed with a center cone 14 having a flat conical shape toward the piston center O.

  The injection operation of the injector 8 in the diesel engine 1 of FIG. 8 is controlled by a fuel injection command 8a from a control device 15 constituting an engine control computer (ECU: Electronic Control Unit), and compression top dead center. In the vicinity, a fuel injection command 8a is output to the injector 8 to inject fuel.

  The control device 15 includes an accelerator opening signal 16 a from an accelerator sensor 16 (load sensor) that detects the accelerator opening as a load of the diesel engine 1, and a rotation sensor 17 that detects the engine speed of the diesel engine 1. The rotational speed signal 17a and the like are input, and the operation state of the diesel engine 1 is constantly monitored so as to execute various engine controls.

  In FIG. 8, 18 is a crankshaft, 19 is an exhaust port, 20 is an exhaust valve, 21 is an intake port, 22 is an intake valve, and 23 is a cylinder ceiling, and the intake valve 22 and the exhaust valve 20 are shown. The valve is operated to open at an appropriate timing according to the stroke of each cylinder via a push rod or a rocker arm by a cam provided on an engine-driven cam shaft (not shown).

  As prior art document information related to the combustion chamber structure of this type of direct injection diesel engine, for example, the following Patent Document 1 and Patent Document 2 have already been proposed.

Japanese Patent Laid-Open No. 6-212973 JP 7-150944 A

  However, in the conventional direct injection diesel engine as shown in FIG. 8, if the amount of intake air 7 is reduced in order to increase the recirculation amount of the exhaust gas 2, the NOx can be reduced, but the combustion in the cylinder Black smoke is generated due to defects. Further, if the recirculation amount of the exhaust gas 2 is increased without reducing the amount of the intake air 7, the pumping loss is increased and the fuel consumption is deteriorated.

  Therefore, it is important to effectively use the air in the combustion chamber to suppress the generation of black smoke, and the following is now known.

  During low-speed engine operation, since the descending speed of the piston 9 is slow, the time for the piston 9 to descend is longer than the time for injecting the fuel. Will end.

  Therefore, as shown in FIG. 10, most of the fuel is injected into the cavity 10 to generate an air-fuel mixture, and the squish area S (the area between the top surface of the piston 9 and the cylinder ceiling 23 around the cavity 10). This air is considered not to be used effectively, and black smoke is likely to be generated. In the drawing, F schematically shows the diffusion state of the distribution by concentration of the injected fuel, and indicates that the concentration of fuel is higher in the inner distribution region.

  This is apparent from the experimental results that if the projecting dimension of the injector 8 from the cylinder ceiling portion 23 is shortened, the generation of black smoke during engine low speed operation is reduced. That is, when the projecting dimension of the injector 8 is shortened, a part of the fuel flows to the squish area S due to an increase in the relative distance from the injection hole of the injector 8 to the cavity 10, and the fuel concentration in the cavity 10 is reduced. This is because reduction is achieved.

  When the engine 9 is operating at high speed, the lowering speed of the piston 9 is fast, and therefore the time for the piston 9 to descend is shorter than the time for injecting the fuel. Therefore, the piston 9 is already in the first half of the fuel injection period. It goes down too much.

  Therefore, as shown in FIG. 11, most of the fuel flows into the squish area S without entering the cavity 10 to generate an air-fuel mixture, and it is considered that the air in the cavity 10 is not effectively used, and black smoke is generated. It is easy to occur. F schematically shows the diffusion state of the distribution according to the concentration of the injected fuel, and indicates that the fuel concentration is higher in the inner distribution region.

  This is apparent from the experimental results that if the projecting dimension of the injector 8 from the cylinder ceiling 23 is increased, the generation of black smoke during engine high-speed operation is reduced. That is, when the projecting dimension of the injector 8 is increased, a part of the fuel enters the cavity 10 due to a reduction in the relative distance from the injection hole of the injector 8 to the cavity 10, and the fuel concentration in the squish area S is increased. This is because reduction is achieved.

  Further, when the direct injection type diesel engine immediately after operation is disassembled and the piston 9 is observed, as shown in FIG. 12, the radial direction from the injector 8 above the piston 9 toward the arrow A is around the cavity 10 at the top of the piston 9. The fuel combustion trace 24 could be confirmed at the location where the fuel was injected, but the fuel combustion trace 24 could not be confirmed where the fuel was not injected from the injector 8. That is, the air-fuel mixture is not formed or the fuel concentration is low where the fuel combustion trace 24 is not confirmed, so it is considered that the air in the squish area S is not effectively used over the entire circumference.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a combustion chamber structure of a direct injection diesel engine capable of suppressing the generation of black smoke by effectively using the air in the combustion chamber.

In order to achieve the above object, the invention described in claim 1
This is a combustion chamber structure for a direct injection diesel engine in which a cavity is formed at the top of the piston so that it is recessed to form most of the combustion chamber, and fuel is injected in multiple directions from the center of the cylinder ceiling toward the top of the piston. And
In each part corresponding to the downstream side of the fuel injection direction of the piston top part, a concave part is formed which is recessed with respect to the end face of the piston top part and extends in the piston radial direction from the cavity side to extend in the piston circumferential direction.
The inner bottom surface of the recess is configured so as to gradually approach the end surface of the piston top portion from the upstream side to the downstream side in the swirl flow direction.

The invention described in claim 2
The fuel injected toward the top of the piston during low-speed operation of the engine is deflected by the swirl and reaches the recess, and the fuel injected toward the top of the piston during the high-speed operation of the engine is deflected by the swirl and below the recess. The inclination of the inner bottom surface of the recessed portion is set so as to reach the inner peripheral wall of the cavity.

  According to the combustion chamber structure of the direct injection diesel engine of the present invention, the following excellent effects can be obtained.

  (1) During low speed operation of the engine, the synergistic action of the recess provided on the top of the piston and the swirl causes most of the fuel to overflow into the squish area and be guided in the circumferential direction. The air generated in the squish area can be effectively used.

  (2) Moreover, by introducing fuel into the squish area, it is possible to avoid locally increasing the fuel concentration in the cavity and to reduce the generation of black smoke.

  (3) During high-speed operation of the engine, most of the fuel is guided into the cavity by the cavity inner peripheral wall below the recessed portion provided at the top of the piston, and the remaining fuel is guided in the circumferential direction of the squish area by the swirl. It is avoided that the fuel concentration of the fuel becomes high locally, and the generation of black smoke can be reduced.

  (4) The top clearance area of the piston is reduced by the amount of depression formed on the top of the piston, and the amount of air-fuel mixture drawn back to the squish area by reverse squish is reduced. A local increase in height is avoided, and the generation of black smoke can be reduced.

It is a top view which shows the piston in an example of the combustion chamber structure of the direct injection type diesel engine of this invention. It is a perspective view which shows the piston in an example of the combustion chamber structure of the direct injection type diesel engine of this invention. It is explanatory drawing about the side surface shape of the recessed part of the piston top part in FIG. It is explanatory drawing about the planar shape of the recessed part of the piston top part in FIG. It is explanatory drawing about the depth of the cavity of the piston top part in FIG. (A) is a conceptual diagram which shows the deflection | deviation of the fuel flow by the swirl at the time of engine low speed driving | operation, (b) is a conceptual diagram which shows the deflection | deviation of the fuel flow by the swirl at the time of engine high speed driving | operation. It is a conceptual diagram which shows the reach | attainment range of the fuel with respect to the piston top part in FIG. It is a conceptual diagram which shows an example of the diesel engine which employ | adopted exhaust gas recirculation. It is explanatory drawing about the shape of the cavity of the piston top part in FIG. It is explanatory drawing about the fuel concentration distribution in the cylinder at the time of engine low speed driving | operation. It is explanatory drawing about the fuel concentration distribution in the cylinder at the time of engine high-speed driving | operation. It is explanatory drawing about the fuel combustion trace in the piston top part of a direct injection type diesel engine.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  1 to 7 show an example of a combustion chamber structure of a direct injection type diesel engine according to the present invention. A cavity 26 is formed at the top of the piston 25 so as to form the majority of the combustion chamber. From the injector 8 (see FIG. 8) disposed above, fuel is injected in six directions toward the top of the piston 25.

  Each of the locations corresponding to the fuel injection direction downstream side of the top of the piston 25 (where the fuel combustion trace 24 in FIG. 12 appears) is recessed with respect to the piston top end surface 27 and from the cavity 26 side in the radial direction of the piston 25. A recessed portion 28 is formed that extends and extends in the circumferential direction of the piston 25. That is, if the number of injection holes in the injector 8 is six, the number of the recessed portions 28 is also six.

  The recessed portion 28 faces the center side of the cavity 26 and is continuous with the side wall surface 32 connected to the piston top end surface 27, the cavity inner peripheral wall 30, the swirl upstream edge of the side wall surface 32, and the end wall surface 31 continuous with the piston top end surface 27. It has a cavity inner peripheral wall 30, a lower edge portion of the end wall surface 31, a lower edge portion of the side wall surface 32, and an inner bottom surface 29 connected to the piston top end surface 27.

  The side surface shape of the recessed portion 28 viewed from the center side of the cavity 26 has a depth of an angle θ1 with respect to the piston top end surface 27 from the “starting point P” set at the outer edge portion of the cavity 26 of the piston top end surface 27. The side edge of the end wall surface 31 reaching h and the side edge of the end wall surface 31 are formed at an angle of θ2 with respect to the piston top end surface 27 so as to be located on the downstream side of the swirl from the “cutting start point P”. Further, it is determined by the side edge of the inner bottom surface 29 that reaches the “cutting end point Q” set in the outer edge portion of the cavity 26 of the piston top end surface 27 (see FIG. 3).

  The inner bottom surface 29 of the recessed portion 28 is configured so as to gradually approach the piston top end surface 27 and continue from the upstream side to the downstream side of the swirl flowing in the arrow B direction.

The depth h is a distance from the piston top end surface 27 to the deepest portion of the inner bottom surface 29 in the recessed portion 28.
The depth h is based on the depth H of the cavity 26 of the piston 25,
H x 20% ≤ h ≤ H x 40%
It is set to become.
The depth H is a distance from the piston top end face 27 to the deepest point on the inner bottom face of the cavity 26 (see FIG. 5).
θ1 is
45 ° ≦ θ1 ≦ 90 °
It is set to become.
θ2 is
10 ° ≦ θ2 ≦ 45 °
It is set to become.
The radius R1 of the concave curved surface between the end wall surface 31 and the inner bottom surface 29 is not specified.

The planar shape of the recessed portion 28 as viewed from above the cavity 26 is such that the edge of the end wall surface 31 that forms an angle θ3 with respect to the tangent L1 of the cavity inner peripheral wall 30 at the “starting point P” described above, and the end wall surface 31 Is determined at the edge of the side wall surface 32 that reaches the “cutting end point Q” set downstream of the swirl.
θ3 is
90 ° ≦ θ3 ≦ 180 °
It is set to become.
When 130 ° ≦ θ3 ≦ 180 °, the concave curved surface interposed between the end wall surface 31 and the side wall surface 32 can be eliminated.
The angle of θ4 formed by the side wall surface 32 with respect to the tangent line L2 of the cavity inner peripheral wall 30 at the “cutting end point Q” set on the downstream side of the swirl may be arbitrary.

  6 (a) and 6 (b), assuming that the direction of the injection hole of the injector 8 (see FIG. 8) overlaps with the radial direction of the piston 25 indicated by the arrow C, the injection is performed from the injection hole during engine operation. The fuel is affected by the swirl flowing in the direction of the arrow B in the cylinder, and is deflected to the downstream side of the swirl as indicated by the arrow A. FIG. 6 (a) shows the flow of fuel during low-speed engine operation, and FIG. 6 (b) shows the flow of fuel during high-speed engine operation. The arrow indicates that the swirl speed is higher than that during low-speed engine operation. The fuel flow represented by A is deflected further to the swirl downstream side.

  Therefore, as shown in FIG. 7, the fuel injected toward the top of the piston 25 during low-speed operation of the engine is deflected by the swirl and reaches the range X of the side wall surface 32 in the recessed portion 28, so that the piston is The inclination of the inner bottom surface 29 of the recessed portion 28 is set so that the fuel injected toward the top of the recessed portion 28 is deflected by the swirl and reaches the range Y of the cavity inner peripheral wall 30 below the recessed portion 28. .

  Next, the operation of the combustion chamber structure of the direct injection diesel engine of the present invention will be described.

  The fuel injected from the injector 8 (see FIG. 8) toward the top of the piston 25 during low-speed operation of the engine has a low swirl speed, so that the fuel is not largely deflected downstream of the swirl and the side wall surface 32 of the recessed portion 28 is not deflected. Range X is reached. For this reason, most of the fuel enters the recess 28, and the remaining fuel generates an air-fuel mixture in the cavity 26.

  A part of the fuel that has entered the recess 28 overflows the side wall surface 32 to the squish area S above the piston top end surface 27, and the remaining fuel flows along the inner bottom surface 29 and the side wall surface 32. It is guided in the circumferential direction of the combustion chamber and overflows to the squish area S above the piston top end face 27.

  If fuel is introduced into the squish area S where a large amount of air is present during engine low speed operation, an air-fuel mixture is generated in the entire circumferential direction of the squish area S, and the air in the entire circumferential direction of the squish area S. Can be used effectively. That is, by introducing the fuel into the squish area S, it is possible to avoid a local increase in the fuel concentration in the cavity 26 and to reduce the generation of black smoke.

  The fuel that is injected from the injector 8 (see FIG. 8) toward the top of the piston 25 during high-speed operation of the engine has a high swirl speed, so that it largely deflects downstream of the swirl, and the range of the cavity inner peripheral wall 30 below the recess 28 Y is reached. For this reason, most of the fuel enters the cavity 26 to generate an air-fuel mixture, and the remaining fuel enters the recess 28.

  If fuel is introduced into the cavity 26 where a large amount of air is present during high-speed operation of the engine as described above, the fuel concentration in the squish area S is avoided from being locally increased, and black smoke is generated. Can be reduced.

  Then, the fuel that has entered the recessed portion 28 is guided along the inner bottom surface 29 and the side wall surface 32 to the squish area S above the piston top end surface 27, and this fuel is dispersed throughout the squish area S in the circumferential direction so Since it is generated, it is avoided that the fuel concentration is locally increased in the squish area S, and the generation of black smoke can be reduced.

  Further, when the piston 25 descends, a phenomenon called reverse squish occurs in which the air-fuel mixture in the cavity 26 is pulled to the squish area S. In the combustion chamber structure of the direct injection diesel engine of the present invention, the concave portion 28 is formed at the top of the piston 25. The top clearance area of the piston 25 is reduced by the amount formed, and the amount of air-fuel mixture drawn back to the squish area S by the reverse squish is reduced. Therefore, locally high fuel concentration is avoided in the squish area S during high-speed engine operation, and the generation of black smoke can be reduced.

  It should be noted that the combustion chamber structure of the direct injection diesel engine of the present invention is not limited to the above-described embodiment, and it is needless to say that changes can be made without departing from the gist of the present invention.

25 Piston 26 Cavity 27 Piston top end surface 28 Recessed portion 29 Inner bottom surface 30 Cavity inner peripheral wall 31 End wall surface 32 Side wall surface

Claims (2)

This is a combustion chamber structure for a direct injection diesel engine in which a cavity is formed at the top of the piston so that it is recessed to form most of the combustion chamber, and fuel is injected in multiple directions from the center of the cylinder ceiling toward the top of the piston. And
In each part corresponding to the downstream side of the fuel injection direction of the piston top part, a concave part is formed which is recessed with respect to the end face of the piston top part and extends in the piston radial direction from the cavity side to extend in the piston circumferential direction.
A combustion chamber structure of a direct injection type diesel engine, wherein the inner bottom surface of the recessed portion is configured to gradually approach the end surface of the piston top portion from the upstream side to the downstream side in the swirl flow direction.   The fuel injected toward the top of the piston during low-speed operation of the engine is deflected by the swirl and reaches the recess, and the fuel injected toward the top of the piston during the high-speed operation of the engine is deflected by the swirl and below the recess. The combustion chamber structure of a direct-injection diesel engine according to claim 1, wherein the inclination of the inner bottom surface of the recessed portion is set so as to reach the inner peripheral wall of the other cavity.

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