JP2007192186A - Internal combustion engine and its piston - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve combustion stability by forming an air-fuel mixture in the vicinity of an ignition plug stably and to minimize reduction of tumbling flow. <P>SOLUTION: A bottom face 25 of a substantially rectangular recessed part 22 on a crown face of a piston 4 forms a curved face being substantially parallel with a central axis of a piston pin along the tumbling flow. A step part 27 being parallel with the central axis of the piston pin is formed in a substantially central part of the bottom face 25 just below the ignition plug, and a rising part 27a is formed on an intake valve side. A rising stream going upward occurs when a spray passes above the step part 27 at stratified lean combustion time, and a part of the spray is guided onto an upper ignition plug side, thereby achieving stable combustion. Since cut-out parts 28, 28 are provided at both ends of the step part 27 and the bottom face 25 is continuous without a step, flow of the tumbling flow at homogeneous combustion time is not obstructed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a piston used in a direct injection type internal combustion engine using a tumble flow.

In an internal combustion engine using a tumble flow as the gas flow in the cylinder, a cylindrical surface parallel to the piston pin central axis is provided on the piston crown so that the tumble flow generated in the intake stroke is well preserved in the compression stroke. A recessed piston is disclosed in Patent Document 1, for example. According to such a configuration, a circular or elliptical space is formed between the combustion chamber recess recessed on the cylinder head side when viewed from the piston pin axial direction, and the tumble along the vertical direction of the cylinder is formed. The flow turns smoothly.
JP 2000-186556 A

  In a direct injection internal combustion engine, in general, stratified lean combustion is realized by injecting fuel in the latter half of the compression stroke. However, in the configuration using the piston as described above, the fuel injection disposed on the side of the cylinder is performed. It is difficult to stably guide the fuel spray injected from the valve to the electrode part of the spark plug, and the combustion is not stable. Therefore, for example, even if an attempt is made to retard the ignition timing in order to reduce HC immediately after start-up, significant retarding becomes difficult due to restrictions on the combustion stability limit.

  In addition, in order to supply the fuel spray to the vicinity of the spark plug, if the fuel spray collides with the piston crown surface and reflects to the spark plug side, a part of the fuel adheres to the piston crown surface and the HC increases. It becomes a factor of.

  The present invention relates to a piston used in a direct injection type internal combustion engine in which a spark plug is disposed substantially at the center of a cylinder top surface and a fuel injection valve is disposed on the side of the combustion chamber on the intake valve side. In the piston of the internal combustion engine in which a curved surface substantially parallel to the piston pin central axis is recessed along the tumble flow in the cylinder on the piston crown surface, the curved surface that is in the vicinity immediately below the spark plug A ridge or step extending in parallel to the piston pin central axis is provided at substantially the center of the curved surface so as to form a rising portion on the intake valve side rising from the intake valve side portion of the curved surface. Then, both ends of the protruding portion or stepped portion are notched, and the intake valve side portion and the exhaust valve side portion of the curved surface are smoothly continuous at the notch portion.

  Desirably, the rising portion on the intake valve side is located in the vicinity immediately below the spark plug.

  In one specific aspect, a convex portion that swells toward the cylinder head is provided on substantially the entire surface of the piston crown surface, and the curved surface is left so as to leave a portion of the convex portion close to the end in the piston pin axial direction. Is recessed in a rectangular shape.

  And the said protrusion or step part is formed in the island shape in the said curved surface away from the side wall surface of this rectangular recessed part.

  That is, in the configuration provided with the rising portion consisting of the ridge portion or the step portion as described above, when the fuel is injected at the latter stage of the compression stroke for the stratified lean combustion, the spray passes above the rising portion. An upward flow upward along the rising portion is locally generated, and a part of the spray travels upward toward the spark plug. Accordingly, an appropriate air-fuel mixture can be stably formed in the vicinity of the spark plug, and stable combustion can be obtained.

  On the other hand, when a tumble flow is generated in the cylinder, for example, when homogeneous combustion is performed with a large amount of exhaust gas recirculation, the tumble flow swirls along the curved surface of the piston crown surface. Since both ends of the strip or step are notched, the tumble flow smoothly flows through the notches on both sides. Therefore, the decrease in the tumble flow due to the provision of the protrusion or step is minimized.

  According to this invention, the fuel spray injected from the fuel injection valve on the cylinder side can be stably guided to the electrode portion of the spark plug, and stable stratified lean combustion can be obtained. Therefore, for example, when the ignition timing is retarded to reduce HC immediately after starting, the combustion stability limit of the ignition timing becomes more retarded, and a large ignition timing retard is possible. At the same time, the tumble flow can be minimized.

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

  First, the configuration of a direct injection type internal combustion engine in which the piston 4 of the present invention is used will be described with reference to FIG. As illustrated, a plurality of cylinders 3 are arranged in series on the cylinder block 1, and a cylinder head 2 is fixed so as to cover the upper surface thereof. A piston 4 is slidably fitted in the cylinder 3. The combustion chamber 11 recessed in the cylinder head 2 has a so-called pent roof type, and a pair of intake valves 5 are provided on one inclined surface, and a pair of exhaust valves 6 are provided on the other inclined surface. Has been placed. A spark plug 7 is disposed at a substantially central position of the cylinder 3 surrounded by the pair of intake valves 5 and the pair of exhaust valves 6.

  The cylinder head 2 is formed with a pair of intake ports 8 corresponding to the pair of intake valves 5, and an exhaust port 9 corresponding to the pair of exhaust valves 6.

  The electromagnetic fuel injection valve 10 that directly injects fuel into the cylinder is disposed on the bottom surface of the cylinder head 2 near the side wall of the cylinder 3 on the intake valve 5 side, and is mounted with its center axis inclined downward. It has been. In particular, the fuel injection valve 10 is disposed between the two intake valves 5 and is configured to inject fuel at an inclination angle close to horizontal toward the center of the cylinder 3 where the spark plug 7 is located. .

  The intake port 8 has a shape suitable for generating a so-called forward tumble flow in the cylinder 3, but further, a partition partitioning the inside of the port into upper and lower flow paths and one of the flow paths. Tumble generating means such as a tumble control valve for opening and closing the path can be provided as necessary.

  The basic operation of the internal combustion engine will be briefly described. First, a homogeneous air-fuel mixture is formed in the cylinder 3 at the time of full load of the engine or in a region where the air-fuel ratio is relatively small in the lean combustion region. Homogeneous combustion is performed. During the homogeneous combustion, fuel is injected and supplied into the cylinder 3 during the intake stroke, and is actively diffused by the tumble flow generated in the cylinder 3.

  On the other hand, in a lean combustion region where the air-fuel ratio is very large in a low load region, stratified lean combustion is performed that enables reliable ignition by stratification of the air-fuel mixture. During this stratified lean combustion, fuel is injected from the fuel injection valve 10 toward the space between the combustion chamber 11 wall surface and the piston 4 in the latter half of the compression stroke. Since the injected fuel forms an ignitable air-fuel mixture around the spark plug 7 as will be described later, ignition and combustion are possible by igniting at an appropriate timing. In addition, when it has a means to variably control the tumble flow such as a tumble control valve, it is desirable to suppress the tumble flow during stratified lean combustion.

  Next, FIG. 2 shows the configuration of the piston 4 according to the present invention, particularly the configuration of its top. As shown in the figure, the piston 4 has a convex portion 21 that swells from the surrounding reference surface 20 toward the cylinder head 1 on substantially the entire crown surface, and an upper surface at the center of the convex portion 21. A concave portion 22 having a rectangular shape as viewed from above is provided. The convex portion 21 enters the combustion chamber 11 on the cylinder head 1 side at the top dead center position and swells in a dome shape so as to occupy a part of the volume thereof, but the concave portion 22 is largely provided. Therefore, actually, the convex portions 21a and 21b occupying the crescent-shaped regions in the crown surface are partially left at both ends in the piston pin axial direction. Then, crescent-shaped squish areas 23 and 24 are formed as the same plane as the reference surface 20 at two locations on both ends in the direction orthogonal to the piston pin (that is, the ends on the intake valve side and the exhaust valve side). .

  The concave portion 22 is formed of a bottom surface 25 having a curved surface substantially parallel to the piston pin central axis, and a pair of side wall surfaces 26 and 26 extending along a direction orthogonal to the piston pin. The bottom surface 25 may be a simple cylindrical surface having a constant radius of curvature, or may be a curved surface having a partially different radius of curvature, but as a whole, follows the flow of the tumble flow in the cylinder 3. Is so curved.

  Here, in this embodiment, there is a slight difference in height between the intake valve side portion 25a and the exhaust valve side portion 25b of the bottom surface 25 at a substantially central portion in the longitudinal direction of the bottom surface 25 made of the curved surface. Thus, the step part 27 extended linearly in parallel with the piston pin center axis line is provided. In the stepped portion 27, the exhaust valve side portion 25b is relatively higher than the intake valve side portion 25a, and as a result, rises upward from the intake valve side portion 25a as shown in FIG. A rising portion 27a on the intake valve side is formed. The rising portion 27 a on the intake valve side is located immediately below the electrode portion of the spark plug 7. Note that the upper portion of the rising portion 27 a on the intake valve side, that is, the portion near the spark plug 7 stands substantially vertically toward the spark plug 7.

  Further, the stepped portion 27 extending along the central axis of the piston pin is notched at both ends, and the pair of notched portions 28 and 28 has an intake valve side portion 25a and an exhaust valve side portion 25b of the bottom surface 25. Are smoothly continuous without causing a step. In other words, the stepped portion 27 is formed away from the side wall surfaces 26, 26 of the recess 22, and is formed in an independent island shape in the bottom surface 25 formed of a curved surface.

  In the configuration of the embodiment as described above, as shown in the explanatory diagram of FIG. 3, when fuel is injected in the late stage of the compression stroke for stratified lean combustion, the spray F has a traveling direction of the step portion 27. Directed toward the spark plug 7 along the rising portion 27a on the intake valve side, or upwards along the rising portion 27a on the intake valve side of the step portion 27 as the spray F passes above the step portion 27. An upward flow is generated locally, and a part of the spray F is guided to the upper spark plug 7 side. Therefore, an appropriate air-fuel mixture can be stably formed in the vicinity of the spark plug 7 and stable combustion can be obtained.

  On the other hand, in the homogeneous combustion, as described above, combustion is performed using the tumble flow in the cylinder. However, as shown in the explanatory view of FIG. It flows on the bottom surface 25 of the concave portion 22 from the exhaust valve side to the intake valve side and turns in the cylinder 3. Here, notch portions 28 and 28 are provided on both sides of the step portion 27, and the notch portions 28 and 28 form a continuous surface without a step, so that the tumble flow smoothly flows and the step portion 27 is provided. The decrease in tumble flow due to is minimized.

  Further, if the stepped portion 27 is provided continuously between the side wall surfaces 26, 26 of the recess 22, the piston crown portion is thin on the intake valve side and overall on the exhaust valve side. Although it becomes thicker and heat distortion is likely to occur at the time of piston casting, in the above configuration, since the step portion 27 having a relatively small volume is only partially present, the heat distortion generated when the piston is manufactured by casting is increased. It will be small. Further, the compression ratio of the engine is not greatly reduced by increasing the overall thickness of the piston crown.

  5 and 6 show a second embodiment of the present invention. In the piston 4 of this embodiment, the bottom surface 25 of the concave portion 22 is continuous without a step, and the piston pin center axis line is replaced with a substantially central portion in the longitudinal direction in place of the step portion 27 described above. A protrusion 31 extending linearly in parallel is provided. As shown in FIG. 7, the protrusion 31 has a substantially trapezoidal cross section, and the intake valve side rising part 31 a and the exhaust valve side rising part rising from the bottom surface 25 to the protrusion 31. Reference numerals 31b each form a smoothly continuous circular arc surface. As in the previous embodiment, the rising portion 31a on the intake valve side is located directly below the electrode portion of the spark plug 7. Further, the upper part of the rising part 31 a on the intake valve side, that is, the part on the tip side of the ridge part 31 stands upright substantially vertically toward the spark plug 7.

  The protruding portion 31 extending along the central axis of the piston pin is notched at both ends. The pair of notched portions 32 and 32 has an intake valve side portion 25a and an exhaust valve side portion of the bottom surface 25. 25b is smoothly continuous. In other words, the protruding portion 31 is formed away from the side wall surfaces 26, 26 of the recess 22, and is formed in an independent island shape in the bottom surface 25 formed of a curved surface.

  In the configuration of the embodiment as described above, as shown in the explanatory diagram of FIG. 7, when fuel is injected in the latter stage of the compression stroke for stratified lean combustion, the traveling direction of the spray F is the ridge 31. Is directed toward the ignition plug 7 along the rising portion 31a on the intake valve side, or along the rising portion 31a on the intake valve side of the protruding portion 31 by the spray F passing above the protruding portion 31. Thus, an upward flow upward is generated locally, and a part of the spray F is guided to the upper spark plug 7 side. Therefore, an appropriate air-fuel mixture can be stably formed in the vicinity of the spark plug 7 and stable combustion can be obtained.

  On the other hand, in the homogeneous combustion, as described above, combustion is performed using the tumble flow in the cylinder. As shown in the explanatory view of FIG. 8, the tumble flow indicated by the arrow T starts from the curved surface. It flows on the bottom surface 25 of the concave portion 22 from the exhaust valve side to the intake valve side and turns in the cylinder 3. Here, since the notched portions 32 are provided on both sides of the ridge 31, the tumble flow smoothly flows, and the decrease in the tumble flow due to the provision of the ridge 31 is minimized.

  If the stepped portion is provided over the entire width of the bottom surface 25 of the recess 22 so that only the rising portion 31a on the intake valve side occurs, as described above, the thickness of the piston crown is thin on the intake valve side, and the exhaust valve side However, in the above configuration, there is only a protrusion 31 having a relatively small volume, so that when the piston is manufactured by casting, thermal distortion occurs. There is no. Further, the compression ratio of the engine is not reduced by increasing the thickness of the piston crown.

  FIG. 9 shows the results of an experiment for improving the stability during stratified lean combustion by providing the above-mentioned ridges 31, and in particular, the ignition timing for the early activation of the catalyst immediately after the cold start. The characteristic when this is retarded is shown in comparison between the comparative example not including the protrusion 31 and the example including the protrusion 31. The experimental conditions are a 1250 rpm fast idle state, an air-fuel ratio of 16, and a water temperature of 20 ° C.

  (C) of the figure shows the relationship between the variation σPi of the in-cylinder pressure indicating the combustion fluctuation and the ignition timing. In general, the combustion fluctuation increases as the ignition timing is retarded. Here, L1 is the combustion stability limit. In the comparative example, the ignition timing exceeds the combustion stability limit L1 in the vicinity of 10 ° CA after the top dead center, and cannot be delayed more than this. On the other hand, in the embodiment provided with the ridge 31, the in-cylinder pressure variation σPi is relatively small and can be retarded to about 18 ° CA after top dead center.

  (A) of the figure shows the characteristics of HC generated in the engine, and in general, HC decreases as the ignition timing is retarded. When compared at the same ignition timing, the amount of HC generated in the example is larger than that in the comparative example, but assuming that the combustion is retarded to the combustion stability limit, the ignition timing near 10 ° CA after top dead center Compared to the amount of HC generated in the comparative example (indicated by point A1) under the above, the amount of HC generated in the example (indicated by point A2) under the ignition timing near 18 ° CA after top dead center However, it is about 40% lower.

  Further, (b) in the figure shows the exhaust temperature at the inlet of the catalytic device attached to the exhaust manifold of the internal combustion engine, and generally the exhaust temperature tends to increase with the retard of the ignition timing. Even when compared at the same ignition timing, the exhaust gas temperature in the example is higher than that in the comparative example. However, if it is assumed that the engine is retarded to the combustion stability limit, ignition at around 10 ° CA after top dead center Compared to the exhaust temperature of the comparative example under time (indicated by point B1), the exhaust temperature of the example (indicated by point B2) under the ignition timing near 18 ° CA after top dead center is 200. Highly obtained near ℃.

  As described above, according to the present invention, as a result of improving the combustion stability, ignition timing can be greatly retarded immediately after starting, etc., the amount of HC generated can be reduced, the exhaust temperature can be increased, and the catalyst Early activation can be achieved.

  FIG. 10 shows the strength of the tumble flow generated in the cylinder 3, that is, the change in the tumble ratio during the cycle. The tumble ratio on the vertical axis indicates that the tumble flow is stronger toward the bottom of the figure. Basically, however, the tumble strength increases during the intake stroke and tends to attenuate toward the compression top dead center. Here, the comparative example 1 is a characteristic when the protrusion 31 is not provided, and the comparative example 2 is a characteristic when the protrusion 31 is provided in a form in which both ends are continuous with the side wall surfaces 26 and 26. As shown in the figure, in the comparative example 2 that does not include the notched portions 32 and 32, the tumble ratio is greatly reduced as compared with the comparative example 1 that does not include the protruding portion 31. On the other hand, in the Example which provided the protrusion part 31 in the form which has a pair of notch parts 32 and 32, it becomes a characteristic close to the comparative example 1, and the fall of a tumble ratio is suppressed.

Sectional drawing which shows the structure of the cylinder injection type internal combustion engine in which the piston which concerns on this invention is used. The perspective view of the top part of the piston which concerns on this invention. Cross-sectional explanatory drawing of the principal part which shows the mode at the time of fuel injection. The perspective explanatory view showing the flow of the tumble flow. Sectional drawing of the internal combustion engine which shows a 2nd Example. The perspective view of the piston top part of this Example. Cross-sectional explanatory drawing of the principal part which shows the mode at the time of fuel injection. The perspective explanatory view showing the flow of the tumble flow. The characteristic view which showed the relationship between ignition timing, (a) HC generation amount, (b) catalyst inlet exhaust temperature, and (c) dispersion | variation (sigma) Pi of in-cylinder pressure by contrast in an Example and a comparative example. The characteristic view which showed the change in the cycle of tumble ratio in the Example and the comparative example by contrast.

Explanation of symbols

4 ... Piston 22 ... Recess 25 ... Bottom (curved surface)
27 ... Step part 28 ... Notch part 31 ... Projection part 32 ... Notch part

Claims (5)

A piston used in an in-cylinder direct injection internal combustion engine in which a spark plug is disposed at a substantially central portion of a cylinder top surface and a fuel injection valve is disposed at a side of a combustion chamber on an intake valve side. In the piston of the internal combustion engine in which a curved surface substantially parallel to the piston pin central axis is recessed so that the surface follows the tumble flow in the cylinder,
A ridge extending in parallel with the piston pin central axis so as to form a rising portion on the intake valve side that rises from the intake valve side portion of the curved surface at a substantially central portion of the curved surface that is directly below the spark plug. Part or step,
Both ends of the protruding portion or stepped portion are cut out, and the intake valve side portion and the exhaust valve side portion of the curved surface are smoothly continuous at the cutout portion. Piston.   2. The piston of an internal combustion engine according to claim 1, wherein a rising portion on the intake valve side is located in the vicinity of directly below the spark plug.   A convex portion bulging toward the cylinder head is provided on substantially the entire surface of the piston crown surface, and the curved surface is recessed in a rectangular shape so as to leave a portion of the convex portion near the end in the piston pin axial direction. The piston of the internal combustion engine according to claim 1 or 2, wherein   4. The piston for an internal combustion engine according to claim 3, wherein the protruding portion or the stepped portion is formed in an island shape in the curved surface apart from the side wall surface of the rectangular concave portion.   The internal combustion engine provided with the piston in any one of Claims 1-4.

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