JP4416680B2 - Combustion chamber structure of direct injection diesel engine - Google Patents

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.

  FIG. 12 shows an example of a diesel engine equipped with a mechanism for recirculating exhaust gas. In the diesel engine 1 shown here, the exhaust gas 2 flows between the exhaust passage 3 and the intake passage 4. Are connected by an EGR pipe 5, and part of the exhaust gas 2 is recirculated together with the intake air 7 through the EGR valve 6 provided in the middle of the EGR pipe 5, and sent into the cylinder of the diesel engine 1. The combustion temperature in the cylinder is lowered to reduce NOx.

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

  The details of the cavity 10 are as shown in FIG. 13, and an inlet lip portion 11 forming an outer edge of the opening of the cavity 10, and the inlet lip portion 11 descends so as to draw a gentle S-curve. A reentrant portion 12 projecting radially outward from the portion 11, an outer curved surface portion 13 forming a gentle curved surface extending radially inward from the reentrant portion 12, and a piston center O 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.

  Further, the injection operation of the injector 8 in the diesel engine 1 of FIG. 12 is controlled by a fuel injection command 8a from a control device 15 constituting an engine control computer (ECU: Electronic Control Unit). In the vicinity of the dead point, a fuel injection command 8a is output to the injector 8 to inject fuel.

  Further, 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 to execute various engine controls.

  In FIG. 12, 18 is a crankshaft, 19 is an exhaust port, 20 is an exhaust valve, 21 is an intake port, and 22 is an intake valve. The intake valve 22 and the exhaust valve 20 are not shown. The cam provided on the camshaft is operated to open at an appropriate timing according to the stroke of each cylinder via a push rod and a rocker arm.

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.
JP-A-6-212973 JP 7-150944 A

  However, in the conventional direct injection type diesel engine as shown in FIG. 12, reducing NOx by recirculation of the exhaust gas 2 is a trade-off between generating black smoke due to poor combustion in the cylinder. Therefore, if the recirculation amount of the exhaust gas 2 is simply increased in order to realize a substantial reduction in NOx, problems such as generation of black smoke and deterioration of fuel consumption are caused.

  Therefore, the present inventor has conducted intensive research on the existing combustion chamber structure as shown in FIG. 13 while paying attention to the diffusion state of the fuel spray. As shown in FIG. 14 (compression top dead center), the reentrant portion 12 As shown in FIG. 15 (crank angle 6 °) and FIG. 16 (crank angle 12 °), the fuel injected toward the vehicle collides with the reentrant portion 12, and the flow of air generated by the fuel injection is an outer curved surface portion. As shown in FIG. 17 (crank angle 18 °), the air flow toward the outside in the latter half of the fuel injection remains above as shown in FIG. 17 (crank angle 18 °). As a result, a large vortex that recirculates in the cavity 10 as shown in FIG. 18 (crank angle 30 °) is formed. As a result, as shown in FIG. Remains within dark areas of the fuel has been found the fact that it is easy black smoke is generated locally formed here. In addition, the diagram shown by F in FIGS. 14 to 19 schematically shows the diffusion state of the distribution according to the concentration of the injected fuel, and indicates that the concentration of the fuel is higher in the inner distribution region. .

  Then, fuel diffusibility is improved by actively directing the fuel spray from the cavity 10 to the squish area S (the region between the top surface of the piston 9 and the cylinder ceiling 23 around the cavity 10), and The inventors have devised a technique for suppressing the generation of black smoke by favorably burning on the outer peripheral side where there is more oxygen than the inner peripheral side of the cylinder (the amount of oxygen is increased by the volume). Patent Document 2 and the like pay attention only to the mixing property of fuel and air, and it has not been possible to realize active diffusion of fuel from the cavity 10 to the squish area S.

  The present invention has been made in view of the above circumstances, and provides a combustion chamber structure of a direct injection type diesel engine that can avoid generation of black smoke and deterioration of fuel consumption as much as possible even if the amount of exhaust gas recirculation is increased. The purpose is to do.

The present invention provides a direct-injection diesel engine that includes a cavity that is recessed so as to form most of a combustion chamber on the piston top surface, and that self-ignites by injecting fuel radially from the center of the cylinder ceiling to the inner peripheral surface of the cavity. A combustion chamber structure, wherein a folded portion that rises from the bottom surface of the cavity toward the outside in the radial direction is formed over the entire circumference at a midway portion in the radial direction of the cavity, and the entire upper end portion of the folded portion A center cone having a flat conical shape from the center to the piston center is formed, the angle formed by the outer edge of the center cone and the upper edge of the folded portion is an acute angle , and the outer edge of the cavity opening is An inlet lip portion formed, and a reentrant portion that descends from the inlet lip portion so as to draw a gentle S-curve and protrudes radially outward from the inlet lip portion And an outer curved surface portion that is continuous with the bottom surface of the cavity through a gentle curved surface directed radially inward from the reentrant portion, and the curvature radius at the lower end portion of the folded portion that rises from the bottom surface of the cavity is that of the outer curved surface portion. The bottom surface of the cavity that is smaller than the radius of curvature and surrounds the periphery of the folded portion is formed to be linear in the radial direction, and the diameter dimension of the inlet lip portion is set in a dimension range of 40 to 70% of the piston diameter. The diameter of the center cone is set in a range of 20 to 40% of the piston diameter, and the angle of warpage of the folded portion with respect to the vertical line toward the outside in the radial direction is set in the range of angles larger than 0 ° and smaller than 60 °. , than the warp angle of the folded portion, and characterized by being configured such that towards the slope angle of the center cone relative to the horizontal is reduced Is shall.

  Thus, when fuel is injected radially from the center of the cylinder ceiling toward the inner peripheral surface of the cavity, the flow of air pushed out by the fuel spray hits the inner peripheral surface of the cavity, and the cavity flows from the inner peripheral surface to the cavity. The lower part of the slab is lowered to the inner side in the radial direction, and the folded part in the middle part in the radial direction causes an upward flow toward the outer side in the radial direction. As a result of such a series of air flows, a large vortex is formed in the area surrounded by the inner peripheral surface, the bottom surface, and the folded portion of the cavity, and this continues until the fuel injection is completed. .

  On the other hand, when the upward flow along the folded portion exceeds the edge portion, a pressure drop due to the upward flow occurs on the upper side of the edge portion, causing a rapid flow toward the center cone side. As the fuel injection ends, the air flow due to the fuel spray disappears, and the piston moves downward in the expansion stroke at that time, and the vortex in the direction opposite to the vortex is generated. Will form.

  These two opposite vortices form a strong flow toward the squish area (the area between the piston top surface around the cavity and the cylinder ceiling), which causes the fuel / air mixture to flow to the bottom of the cavity. Good for a squish area where there is a lot of oxygen without producing a fuel-rich region locally in the center (the cylinder's outer circumference has more oxygen than the inner circumference) As a result of diffusion, the reoxidation of soot is promoted and the generation of black smoke is remarkably suppressed.

In the present invention, the inlet lip portion that forms the outer edge of the opening of the cavity, and the reentrant portion that descends from the inlet lip portion so as to draw a gentle S-curve and protrudes radially outward from the inlet lip portion When, since an outer curved portion continuous to the bottom of the cavity through a gently curved surface extending from the said reentrant portion radially inwardly, the air flow is outside curved surface caused by the fuel injection toward the reentrant portion The portion is smoothly folded inward in the radial direction, and the upward flow toward the radially outer side is effectively formed by the folded portion in the middle in the radial direction.

In addition, the air this time of the upward flow satisfactorily formed, which is because the radius of curvature at the lower end of the folded portion rising from the bottom surface of the cavity is smaller Kuna' than the radius of curvature of the outer curved portion, folded into the semi radially inward The momentum of the flow is guided to the lower end of the folded portion without being scraped off.

Further, since the bottom surface of the cavity surrounding the folded portion is formed so as to form a straight line in the radial direction, leading a flow of air toward the radially inward linear bottom portion of this as runway straight It is possible to hit against the lower end of the folded portion.

  The diameter of the inlet lip should be set within the range of 40 to 70% of the piston diameter, and the diameter of the center cone should be set within the range of 20 to 40% of the piston diameter. The radius angle of the center cone with respect to the horizontal line is set to be larger than 0 ° and smaller than 60 °, and the angle of curvature of the center cone with respect to the horizontal line is more than the angle of curvature of the folded portion with respect to the vertical line. It is better to make smaller.

  According to the combustion chamber structure of the above-described direct injection type diesel engine of the present invention, it is possible to avoid the occurrence of a locally fuel-rich region at the bottom or center of the cavity, and a large amount of oxygen exists. As a result, fuel can be actively diffused into the squish area and burned well, so that the amount of exhaust gas recirculation can be achieved with existing direct injection diesel engines while avoiding the generation of black smoke and fuel consumption. It can be increased to a level that has not been achieved, and an excellent effect of being able to obtain a NOx reduction effect superior to that of the prior art can be achieved.

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

  FIGS. 1-11 shows an example of the form which implements this invention, and the part which attached | subjected the code | symbol same as FIG.12 and FIG.13 represents the same thing.

  With respect to the direct injection diesel engine 1 described above with reference to FIG. 12, in this embodiment, as shown in FIG. 1, the cavity 10 is disposed in the midway in the radial direction of the cavity 10 that is recessed in the top surface of the piston 9. A folded portion 24 that rises from the bottom surface 27 so as to bend outward in the radial direction is formed over the entire circumference, and a center cone that forms a flat conical shape from the entire upper end portion of the folded portion 24 to the piston center O is formed. 25, and the angle formed by the edge portion 26 formed by the outer edge portion of the center cone 25 and the upper end portion of the folded portion 24 is an acute angle (an angle smaller than a right angle).

  Here, the shape of the inner peripheral surface of the cavity 10 into which the fuel is injected is the same as that of the conventional cavity 10 described above with reference to FIG. 13, and the inlet lip portion 11 that forms the outer edge of the opening of the cavity 10, Through a reentrant portion 12 that descends from the inlet lip portion 11 so as to draw a gentle S-curve and projects radially outward from the inlet lip portion 11, and a gentle curved surface that extends radially inward from the reentrant portion 12 The outer curved surface portion 13 is continuous with the bottom surface 27 of the cavity 10.

  The radius of curvature at the lower end of the folded portion 24 rising from the bottom surface 27 of the cavity 10 is smaller than the radius of curvature of the curved surface from the reentrant portion 12 to the bottom surface 27 of the cavity 10, and surrounds the periphery of the folded portion 24. The bottom surface 27 of the cavity 10 is a flat surface parallel to the top surface of the piston 9 and is formed to be linear in the radial direction.

  However, when the bottom surface 27 of the cavity 10 is formed to be linear in the radial direction, it is not always necessary to use a flat surface parallel to the top surface of the piston 9, and a downward gradient or an upward gradient toward the piston center O. It is also possible to adopt a cone-shaped or bowl-shaped shape.

  The diameter D1 of the inlet lip 11 is set in a range of 40 to 70% of the piston diameter D0, and the diameter D2 of the center cone 25 is set in a range of 20 to 40% of the piston diameter D0. (See FIG. 2).

  Further, the angle of curvature α of the folded portion 24 with respect to the vertical line toward the outside in the radial direction is set in an angle range larger than 0 ° and smaller than 60 ° (see FIG. 3). The inclination angle β of the center cone 25 with respect to the horizontal line is made smaller (see FIG. 4).

  Thus, when the combustion chamber structure is configured in this way, when fuel is injected radially from the center of the cylinder ceiling 23 toward the reentrant portion 12 of the cavity 10 as shown in FIG. 5 (compression top dead center). As shown in FIG. 6 (crank angle 6 °), the flow of air pushed out by the fuel spray hits the reentrant portion 12, descends from the reentrant portion 12, and is smoothly folded radially inward by the outer curved surface portion 13. However, as shown in FIG. 7 (crank angle 12 °), this upward flow is converted into the flow of air by the fuel spray being continued as shown in FIG. 7 (crank angle 12 °). As a result, a large vortex is formed in the region surrounded by the inner peripheral surface, the bottom surface 27 and the folded portion 24 of the cavity 10 by such a series of air flows. Morphism so that is continued until the end.

  At this time, particularly in this embodiment, the radius of curvature at the lower end of the folded portion 24 rising from the bottom surface 27 of the cavity 10 is smaller than the radius of curvature of the outer curved surface portion 13 from the reentrant portion 12 to the bottom surface 27. The momentum of the air flow folded back inward in the radial direction is guided to the lower end portion of the folded portion 24 without being scraped, and the bottom surface 27 of the cavity 10 surrounding the folded portion 24 is linear in the radial direction. Since the straight bottom surface portion is used as a runway, the air flow toward the inside in the radial direction can be guided straight and hit the lower end portion of the folded portion 24.

  Further, when the upward flow along the folded portion 24 exceeds the edge portion 26, a pressure drop due to the upward flow occurs on the upper side of the edge portion 26, and a flow toward the center cone 25 side is abruptly caused. As shown in FIG. 8 (crank angle 18 °), the air flow toward the center cone 25 disappears when the fuel injection is terminated, and the piston 9 descends with an expansion stroke. As a result, the vortex is stretched in the vertical direction to form a vortex opposite to the vortex as shown in FIG. 9 (crank angle 30 °).

  A strong flow toward the squish area S (a region between the top surface of the piston 9 around the cavity 10 and the cylinder ceiling 23) is formed by these two opposite vortices, and a crank angle of 40 ° in FIG. ), The squish area S (where the outer peripheral side of the cylinder is located on the outer periphery side of the cylinder) where the fuel / air mixture does not produce a fuel-rich region locally at the bottom or center of the cavity 10. As a result, the reoxidation of soot is promoted and the generation of black smoke is remarkably suppressed. 5 to 10 schematically show the diffusion state of the distribution of the injected fuel according to the concentration distribution, and the inner distribution region indicates that the concentration of the fuel is higher. .

  Therefore, according to the above-described embodiment, it is possible to avoid the occurrence of a locally fuel-rich region at the bottom or center of the cavity 10 and to supply the fuel to the squish area S where a large amount of oxygen exists. The existing direct injection diesel engine 1 can recirculate the exhaust gas 2 (see Fig. 12) while avoiding the generation of black smoke and deterioration of fuel consumption as much as possible because it can be actively diffused and burned well. The level can be increased to a level that could not be achieved, and a NOx reduction effect superior to that of the prior art can be obtained.

  In fact, according to a verification experiment by the present inventor, as shown by a graph in FIG. 11, the light impermeability of exhaust gas 2 with respect to λ (the excess air ratio; the excess air ratio decreases as the EGR ratio increases) (due to poor combustion). The substitution value of the exhaust smoke concentration obtained by evaluating the degree of pollution of the exhaust gas 2 by the generated black smoke by the light impermeability) when the conventional combustion chamber structure is adopted and when the combustion chamber structure of the present invention is adopted As a result, in any of the experimental results carried out at different rotational speeds, the light impermeability of the exhaust gas 2 is lower when the combustion chamber structure of the present invention is adopted, that is, the exhaust gas is regenerated at the same EGR rate. It has been confirmed that the degree of contamination of the exhaust gas 2 by black smoke when the circulation is executed (at the same λ) is low.

  Note 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 various modifications can be made without departing from the scope of the present invention.

It is sectional drawing which shows an example of the form which implements this invention. It is explanatory drawing regarding the diameter dimension of an entrance lip | rip part and a center cone. It is explanatory drawing regarding the angle range of the curvature angle of a folding | returning part. It is explanatory drawing which shows the relationship between the curvature angle of a folding | turning part, and the inclination angle of a center cone. It is sectional drawing which shows the combustion injection condition in the compression top dead center of this example. It is sectional drawing which shows the combustion injection condition in the crank angle of 6 degrees of this embodiment. It is sectional drawing which shows the combustion injection condition in the crank angle of 12 degrees of this embodiment. It is sectional drawing which shows the combustion injection condition in the crank angle of 18 degrees of this form example. It is sectional drawing which shows the combustion injection condition in the crank angle of 30 degrees of this embodiment. It is sectional drawing which shows the combustion injection condition in the crank angle of 40 degrees of this embodiment. It is a graph which shows the result of the verification experiment by this inventor. It is the schematic of the diesel engine provided with the mechanism which recirculates exhaust gas. It is sectional drawing which shows the detail of the cavity of FIG. It is sectional drawing which shows the combustion injection condition in the conventional compression top dead center. It is sectional drawing which shows the combustion injection condition in the conventional crank angle of 6 degrees. It is sectional drawing which shows the combustion injection condition in the conventional crank angle of 12 degrees. It is sectional drawing which shows the combustion injection condition in the conventional crank angle of 18 degrees. It is sectional drawing which shows the combustion injection condition in the conventional crank angle of 30 degrees. It is sectional drawing which shows the combustion injection condition in the conventional crank angle of 40 degrees.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Diesel engine 9 Piston 10 Cavity 11 Inlet lip part 12 Reentrant part 13 Outer curved surface part 23 Cylinder ceiling part 24 Folding part 25 Center cone 26 Edge part 27 Bottom face

Claims (1)

A combustion chamber structure of a direct-injection diesel engine that has a cavity that is recessed to form the majority of the combustion chamber on the top surface of the piston and that self-ignites by injecting fuel radially from the center of the cylinder ceiling to the inner peripheral surface of the cavity. There,
A folded portion that rises from the bottom surface of the cavity toward the outside in the radial direction is formed over the entire circumference at a midway portion in the radial direction of the cavity, and is flat from the entire circumference of the upper end portion of the folded portion toward the center of the piston. Forming a conical center cone, the angle formed by the edge of the outer edge of the center cone and the upper end of the folded portion is an acute angle ;
Furthermore, an inlet lip portion forming the outer edge of the opening of the cavity, a reentrant portion descending from the inlet lip portion so as to draw a gentle S-curve and projecting radially outward from the inlet lip portion, and the reentrant portion An outer curved surface portion that is continuous with the bottom surface of the cavity through a gentle curved surface directed radially inward from
In addition, the curvature radius at the lower end portion of the folded portion rising from the bottom surface of the cavity is smaller than the curvature radius of the outer curved surface portion, and the bottom surface of the cavity surrounding the folded portion is formed so as to be linear in the radial direction. The diameter dimension of the inlet lip portion is set in a dimension range of 40 to 70% of the piston diameter, the diameter dimension of the center cone is set in a dimension range of 20 to 40% of the piston diameter, and The curvature angle to the outside in the radial direction is set in an angle range larger than 0 ° and smaller than 60 °, and the inclination angle of the center cone with respect to the horizontal line is smaller than the curvature angle of the folded portion. The combustion chamber structure of a direct injection diesel engine.

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