JP2022136399A - Combustion chamber structure for engine - Google Patents

Abstract

To increase combustion speed utilizing tumble flow, in a combustion chamber with a pent-roof type ceiling surface.SOLUTION: A combustion chamber 5 for an engine is partitioned by a crown surface 40 of a piston 4, the inner wall surface of a cylinder 20, and a pent-roof type combustion chamber ceiling surface 5U. The crown surface 40 includes an exhaust side inclined surface 43 rising from the exhaust side to the center of the crown surface 40, an intake side inclined surface 44 rising from the intake side to the center of the crown surface 40, a flat surface 45 provided between the upper end of the exhaust side inclined surface 43 and the upper end of the intake side inclined surface 44, a cavity 80 recessed in an area including the flat surface 45, and a small cavity 90 formed by hollowing an area at an IN side edge 82 of the cavity 80 and at the upper end of the intake side inclined surface 44 into a bowl shape, wherein the small cavity 90 has an EX side peripheral edge 91 and an IN side peripheral edge 92 as the outer peripheral edge. The curvature factor of a second curve forming the IN side peripheral edge 92 is set to be higher than the curvature factor of a first curve forming the EX side peripheral edge 91.SELECTED DRAWING: Figure 9

Description

The present invention relates to a combustion chamber structure for an engine having a combustion chamber defined by a pent-roof type ceiling surface and a piston crown surface with a cavity.

BACKGROUND ART For the purpose of improving thermal efficiency, improving fuel consumption performance, etc., research is being conducted daily on the structure of an engine's combustion chamber, especially the structure of a piston. Patent Literature 1 discloses a technique for specifying the relationship between the height of a raised portion provided on the crown surface of a piston with a cavity and the inner diameter of the cylinder in order to suppress the deceleration of the tumble flow.

JP 2018-162731 A

In a combustion chamber having a pent roof type ceiling, the intake port is generally a tumble port. In order to improve the thermal efficiency and fuel efficiency of the combustion chamber, it is essential to maintain the tumble flow until the latter half of the compression stroke and increase the combustion speed of the air-fuel mixture. However, the fact is that no combustion chamber structure has yet been proposed that satisfies this requirement at a higher level.

SUMMARY OF THE INVENTION An object of the present invention is to provide a combustion chamber structure for an engine, which is capable of increasing the combustion speed by utilizing a tumble flow in a combustion chamber having a pent roof ceiling.

An engine combustion chamber structure according to one aspect of the present invention is defined by a crown surface of a piston, an inner wall surface of a cylinder in which the piston is slidably accommodated, and a pent roof-shaped ceiling surface formed on a cylinder head. In the combustion chamber structure for an engine, the ceiling surface has an intake port opening for supplying intake air to the combustion chamber and an exhaust port opening for discharging exhaust gas from the combustion chamber. When the side on which the intake port is arranged is defined as the intake side and the side where the exhaust port is arranged is defined as the exhaust side, the crown surface extends from the exhaust side toward the center of the crown surface. provided between an ascending exhaust-side inclined surface, an intake-side inclined surface ascending from the intake side toward the central portion of the crown surface, and an upper end of the exhaust-side inclined surface and an upper end of the intake-side inclined surface; a plane extending in a direction orthogonal to the axial direction of the cylinder at the center of the crown surface; a first cavity recessed in an area including the plane; an edge of the first cavity on the intake side; a second cavity formed by recessing an upper end region of the intake-side inclined surface in a bowl shape, wherein the second cavity is positioned as an outer peripheral edge on the first peripheral edge located on the exhaust side and the intake side. and a second peripheral edge, in top view, the first peripheral edge has a first curve, the second peripheral edge has a second curve, and the second curve has a larger curvature than the first curve. It is characterized by

According to this combustion chamber structure, since the intake port is formed in the pent roof type ceiling surface, the combustion chamber forms a tumble flow. Since the first cavity and the second cavity are recessed in the crown surface of the piston, the tumble flow can be maintained by utilizing the space of these cavities even in the latter half of the compression stroke. In particular, since the second cavity is provided in such a manner that the intake-side edge of the first cavity and the upper end of the intake-side inclined surface are recessed, the cavity provided by the crown surface extends toward the intake side. A large size can be achieved. Therefore, it becomes easier to maintain the tumble flow until the latter half of the compression stroke. Further, the second curve on the intake side of the second cavity has a configuration with a greater curvature than the first curve on the exhaust side. Therefore, the second cavity has a shape protruding toward the intake side. This makes it easier for the flame of the air-fuel mixture to move toward the intake side due to the tumble flow. Therefore, it is possible to increase the combustion speed, improve the thermal efficiency, and improve the fuel efficiency.

The combustion chamber structure described above further includes an ignition portion arranged on the ceiling surface for realizing flame propagation combustion in the combustion chamber, wherein the ignition portion forms the first cavity in a cross-sectional view along the cylinder axis. It is desirable that it be located above the region and offset from the center line of the cylinder toward the exhaust side.

According to this combustion chamber structure, the air-fuel mixture can be ignited on the upstream side of the tumble flow flowing through the cavity from the exhaust side to the intake side, and the flame can be carried on the tumble flow. In addition, due to the formation of the second cavity, the cavity provided by the crown surface has a form substantially offset toward the intake side. This makes it easier for the flame generated on the exhaust side to travel toward the intake side due to the tumble flow. Therefore, it is possible to increase the combustion speed, improve the thermal efficiency, and improve the fuel efficiency.

The combustion chamber structure described above preferably further includes a supercharger for supercharging intake air supplied from the intake port to the combustion chamber.

According to this combustion chamber structure, it is possible to obtain the effect of improving the fuel consumption performance by increasing the combustion speed while increasing the intake air amount by operating the supercharger to improve the torque.

In the combustion chamber structure described above, it is preferable that a fuel injection section for injecting fuel into the combustion chamber is arranged on the intake side of the combustion chamber.

According to this combustion chamber structure, the fuel sprayed from the fuel injection part can be easily put on the tumble flow circulating in the combustion chamber, and a homogeneous air-fuel mixture can be formed in the combustion chamber.

In the above-described combustion chamber structure, the engine is a vertically mounted engine that includes six cylinders arranged in series, and the direction of arrangement of the six cylinders is arranged along the longitudinal direction of the vehicle on which the engine is mounted. It is desirable to have

According to this combustion chamber structure, it is possible to increase the combustion speed and improve the fuel consumption performance of an in-line 6-cylinder longitudinal engine.

ADVANTAGE OF THE INVENTION According to the present invention, it is possible to provide a combustion chamber structure of an engine that can increase the combustion speed by utilizing a tumble flow in a combustion chamber having a pent roof ceiling surface and improve fuel efficiency. .

FIG. 1 is a schematic perspective view of the main body of an in-line 6-cylinder longitudinal engine to which the engine combustion chamber structure according to the present invention is applied. FIG. 2 is a schematic configuration diagram of an engine system including the engine main body of FIG. FIG. 3 is a cross-sectional view of the vicinity of the combustion chamber, showing the structure of one cylinder provided in the engine body. FIG. 4 is a perspective view of a piston; 5 is an enlarged view of a main part of FIG. 3. FIG. FIG. 6 is a schematic diagram for explaining the relationship between the cylinder axis, the ignition position, and the center point of the cavity. FIG. 7 is a schematic diagram of a combustion chamber, and is a diagram for explaining flame propagation due to a tumble flow. 8(A) and (B) are plan views of the combustion chamber and diagrams schematically showing the propagation of flames, (A) showing the combustion chamber structure of the comparative example, and (B) Each case of the combustion chamber structure of the embodiment is shown. FIG. 9 is a plan view of the crown surface of the piston. 10 is a cross-sectional view taken along the line XX of FIG. 9. FIG. 11 is an arrow view in the direction of line XI in FIG. 9. FIG. FIG. 12A is a diagram showing flame propagation when using the second cavity of the comparative example, and FIG. 12B is a diagram showing flame propagation when using the second cavity of the example.

[Appearance of engine]
A combustion chamber structure of an engine according to an embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic perspective view showing the appearance of an engine body 1 to which a combustion chamber structure according to an embodiment of the invention is applied. The engine body 1 shown here is a 4-stroke in-line 6-cylinder gasoline engine with a turbocharger mounted on a vehicle such as an automobile as a power source for driving the vehicle.

FIG. 1 also includes direction indications corresponding to the longitudinal direction of the vehicle in which the engine body 1 is mounted. The six cylinders provided in the engine body 1 are arranged in the longitudinal direction of the vehicle. That is, the engine body 1 is arranged vertically in the vehicle. The powertrain layout of the vehicle can be, for example, an FR layout. Each cylinder is connected to an intake system and an exhaust system in a 4-valve format of 2 intake valves and 2 exhaust valves. FIG. 1 shows that six sets of intake ports 6, each paired with a first intake port 6A and a second intake port 6B, are arranged in the longitudinal direction of the vehicle.

The combustion type of the engine body 1 is SI (Spark Ignition) combustion accompanied by flame propagation, which is started by forcibly igniting the air-fuel mixture in the combustion chamber. The engine body 1 may perform SPCCI (Spark Controlled Compression Ignition) combustion, which is a combination of SI combustion and CI (Compression Ignition) combustion that compressively ignites an air-fuel mixture.

[Overall structure of the engine]
FIG. 2 is a schematic configuration diagram of an engine system E including the engine body 1 shown in FIG. The engine system E includes the above-described engine body 1, an intake passage 50 for introducing air (intake) into the engine body 1, an exhaust passage 60 for discharging exhaust from the engine body 1 to the outside, and an intake passage for part of the exhaust gas. and an EGR device 70 that flows back to 50 . Furthermore, the engine system E includes a turbocharger 59 that supercharges the intake air.

An engine body 1 includes a cylinder block 2 , a cylinder head 3 and pistons 4 . The cylinder block 2 has six cylinders 20 arranged in series. It should be noted that only one cylinder 20 is shown in FIG. The cylinder head 3 is attached to the upper surface of the cylinder block 2 and closes the upper opening of the cylinder 20 . The piston 4 is housed in each cylinder 20 so as to be reciprocally slidable, and is connected to the crankshaft 15 via a connecting rod. As the piston 4 reciprocates, the crankshaft 15 rotates about its central axis.

A combustion chamber 5 is formed above the piston 4 . Fuel containing gasoline as a main component is supplied to the combustion chamber 5 by injection from an injector 11 which will be described later. The mixture of the supplied fuel and air is combusted in the combustion chamber 5, and the piston 4, which is pushed down by the expansion force of the combustion, reciprocates vertically. The geometric compression ratio of the cylinder 20, that is, the ratio of the volume of the combustion chamber 5 when the piston 4 is at the top dead center to the volume of the combustion chamber 5 when the piston 4 is at the bottom dead center, is 11 or more and 13 or less. is set in the range of Incidentally, the compression ratio of a normal turbocharged engine is about 10.5, and the above compression ratio is higher than normal.

An intake port 6 and an exhaust port 7 communicating with the combustion chamber 5 are formed in the cylinder head 3 . An intake port 6 is a port for supplying intake air to the combustion chamber 5 , and an exhaust port 7 is a port for discharging exhaust gas from the combustion chamber 5 . An intake side opening, which is the downstream end of the intake port 6, and an exhaust side opening, which is the upstream end of the exhaust port 7, are formed on the combustion chamber ceiling surface 5U (ceiling surface) formed by the lower surface of the cylinder head 3. there is The cylinder head 3 is assembled with an intake valve 8 that opens and closes the intake side opening and an exhaust valve 9 that opens and closes the exhaust side opening. An intake valve drive mechanism 18 for driving the intake valve 8 is provided with an intake opening/closing timing changing mechanism 18a for changing the opening/closing timing of the intake valve 8. As shown in FIG.

A spark plug 10 (ignition section) and an injector 11 (fuel injection section) are assembled so as to face the combustion chamber 5 . The ignition plug 10 is arranged on the combustion chamber ceiling surface 5</b>U and ignites a mixture of fuel and air to achieve flame propagation combustion within the combustion chamber 5 . As will be described in detail later, the spark plug 10 is arranged at a position offset from the center line of the cylinder toward the intake side where the intake port 6 is arranged.

The injector 11 injects fuel supplied from a fuel system (not shown) into the combustion chamber 5 . The injector 11 is arranged on the intake side at the periphery of the combustion chamber ceiling surface 5U. Such an arrangement makes it easier for the fuel sprayed from the injector 11 to be put on the tumble flow circulating in the combustion chamber 5 , so that the fuel can easily spread throughout the combustion chamber 5 . That is, a homogeneous air-fuel mixture can be formed within the combustion chamber 5 .

The intake passage 50 is a path that communicates with the intake port 6 and supplies intake air to the combustion chamber 5 of each cylinder 20 . Air taken in from the upstream end of the intake passage 50 is introduced into the combustion chamber 5 through the intake passage 50 and the intake port 6 . An air cleaner 51, a compressor 52 (a turbocharger 59), an intercooler 53, a throttle valve 54 and a surge tank 55 are arranged in the intake passage 50 in this order from the upstream side.

The air cleaner 51 cleans the intake air by removing foreign substances in the intake air. The compressor 52 rotates about its axis and supercharges the intake air. The intercooler 53 cools the intake air compressed by the turbocharger 59 . The throttle valve 54 opens and closes the intake passage 50 in conjunction with depression of an accelerator (not shown) to adjust the flow rate of intake air in the intake passage 50 . Surge tank 55 provides space for evenly distributing intake air to multiple cylinders 20 .

The exhaust passage 60 is a path that communicates with the exhaust port 7 and discharges the burned gas (exhaust gas) generated in the combustion chamber 5 to the outside of the vehicle. A turbine 61 and a catalytic converter 63 are arranged in the exhaust passage 60 . The turbine 61 is rotated by the exhaust flow flowing through the exhaust passage 60 . The catalytic converter 63 incorporates a three-way catalyst or the like for purifying harmful components (HC, CO, NOx) contained in the exhaust gas.

The turbocharger 59 includes the compressor 52 and the turbine 61 described above, and a common rotary shaft connecting them. The turbocharger 59 supercharges (compresses) the intake air introduced into the combustion chamber 5 and sends the intake air to the downstream side of the intake passage 50 . The compressor 52 rotates as the turbine 61 rotates due to the exhaust flow, and performs a supercharging operation.

This embodiment exemplifies a twin-scroll turbocharger. Of the six cylinders 20 provided in the engine body 1, the exhaust of the first to third cylinders 20 is in the first exhaust manifold 64a, and the exhaust of the fourth to sixth cylinders 20 is in the second exhaust manifold 64b. led to. Further, the exhaust passage 60 is provided with a bypass passage 65 for bypassing the turbine 61 and a wastegate valve 66 for allowing the exhaust discharged from the first and second exhaust manifolds 64 a and 64 b to flow into the bypass passage 65 . is set. In this embodiment, the turbocharger 59 using exhaust pressure as a power source is exemplified, but instead of this, a supercharger using the engine itself as a power source or an electric supercharger may be used.

The EGR device 70 is a device that realizes so-called high-pressure EGR, and includes an EGR passage 71 , an EGR cooler 72 and an EGR valve 73 . The EGR passage 71 connects the exhaust passage 60 and the intake passage 50 . The EGR cooler 72 cools the exhaust gas (EGR gas) recirculated from the exhaust passage 60 to the intake passage 50 through the EGR passage 71 by heat exchange. The EGR valve 73 adjusts the flow rate of exhaust gas flowing through the EGR passage 71 .

The engine system E includes various sensors. FIG. 2 illustrates the engine speed sensor SN1, the airflow sensor SN2, and the supercharging pressure sensor SN3. The engine speed sensor SN1 is attached to the cylinder block 2 and detects the engine speed based on the rotation angle of the crankshaft 15 . The airflow sensor SN2 is arranged downstream of the air cleaner 51 and detects the flow rate of intake air flowing through the intake passage 50 . A supercharging pressure sensor SN3 detects the pressure of intake air after supercharging.

[Detailed structure of combustion chamber and piston]
FIG. 3 is a sectional view near the combustion chamber 5 showing the structure of one cylinder 20 provided in the engine body 1. As shown in FIG. 4 is a perspective view of the piston 4. FIG. An inner wall surface of the cylinder 20 is composed of a cylinder liner 21 . A water jacket 22 is provided inside the cylinder block 2 so as to surround the cylinder 20 . The piston 4 is housed inside the cylinder 20 so as to be slidable with respect to the cylinder liner 21 .

The combustion chamber 5 includes an inner wall surface (cylinder liner 21) of the cylinder 20, a crown surface 40 of the piston 4, and a combustion chamber ceiling surface 5U formed on the bottom surface of the cylinder head 3 (each valve of the intake valve 8 and the exhaust valve 9). (including faces) and The combustion chamber ceiling surface 5U is a ceiling surface having an upwardly convex pent roof shape. An opening of the intake port 6 and an opening of the exhaust port 7 are formed in the pent roof type combustion chamber ceiling surface 5U. The intake port 6 of this embodiment is a tumble port capable of forming a tumble flow Ft (FIG. 5).

3, 4 and other figures are labeled with XYZ orientation. The Z direction corresponds to the AX direction of the cylinder axis, the X direction corresponds to the longitudinal direction of the engine body 1, which is the extension direction of the crankshaft 15, and the Y direction corresponds to the direction perpendicular to both the Z and X directions. In the following description, the F side (+X) and R side (-X) mean the front side and rear side in the installation direction of the engine body 1, and the intake side (IN side) means the side where the intake port 6 is arranged. ; +Y), the exhaust side (EX side; -Y) in the sense of the side on which the exhaust port 7 is arranged, and the upper side (+Z) and the lower side (-Z) in the sense of the upper side and the lower side on the cylinder axis AX. Sometimes.

The ignition plug 10 is vertically assembled to the cylinder head 3 so that the ignition portion 10A protrudes into the combustion chamber 5 from the combustion chamber ceiling surface 5U. The injector 11 is mounted substantially horizontally on the cylinder head 3 so that the fuel injection head at its tip faces the combustion chamber 5 from the intake side.

The shape of the piston 4 will be described in detail with reference to FIGS. 9 to 11 as well. 9 is a plan view of the crown surface 40 of the piston 4, FIG. 10 is a sectional view taken along line XX of FIG. 9, and FIG. 11 is a view taken along line XI of FIG. The piston 4 includes a piston head 4A and a skirt portion 4B connected to the lower side (-Z side) of the piston head 4A. The piston head 4A is formed of a cylindrical body, and has a crown surface 40 forming a part (bottom surface) of the wall surface of the combustion chamber 5 on its upper surface. A cavity 80 recessed in a spherical crown shape is formed in the central region of the crown surface 40 . Further, the piston head 4A has a side peripheral surface 4C that comes into sliding contact with the cylinder liner 21. As shown in FIG. The side peripheral surface 4C is provided with a plurality of ring grooves into which piston rings are fitted. The skirt portion 4B is arranged on the +Y side and the -Y side of the piston head 4A, and suppresses the swinging motion of the piston 4 during the reciprocating motion. A piston boss 4D defining a pin hole extending in the X direction is provided in the center of the skirt portion 4B in the Y direction. A piston pin for connection with the connecting rod 12 is inserted through the piston boss 4D.

The crown surface 40 is a substantially circular surface facing the combustion chamber ceiling surface 5U in the Z direction. The crown surface 40 includes an exhaust-side bottom portion 41 , an intake-side bottom portion 42 , an exhaust-side inclined surface 43 , an intake-side inclined surface 44 , a flat surface 45 , an inter-recess flat surface 46 , an F-side sidewall 47 and an R-side sidewall 48 . In addition to these, the crown surface 40 is provided with a cavity 80 (first cavity) and a small cavity 90 (second cavity) as portions positively recessed in the -Z direction. Excluding the portion of the cavity 80, the exhaust-side bottom 41 and the intake-side bottom 42 are base surfaces with the lowest height in the +Z direction in the crown surface 40, and the plane 45 and the inter-recess plane 46 have the lowest height in the +Z direction. It is the highest top surface.

The exhaust-side bottom portion 41 and the intake-side bottom portion 42 are planes extending in the XY directions perpendicular to the cylinder axis AX, and are at the same height in the Z direction. The exhaust-side bottom portion 41 and the intake-side bottom portion 42 may be surfaces that are slightly inclined with respect to the XY directions, or surfaces that are slightly convex or concave. The exhaust-side bottom portion 41 is arranged near the EX-side (−Y) edge of the crown surface 40 . The intake-side bottom portion 42 is arranged near the IN-side (+Y) edge of the crown surface 40 .

The exhaust-side bottom portion 41 is an arcuate plane having an arc at the −Y side outer peripheral edge (side peripheral surface 4C) of the crown surface 40 and a straight line extending in the X direction as a chord. The intake side bottom portion 42 is an arcuate plane having an arc at the +Y side outer peripheral edge of the crown surface 40 and a straight line extending in the X direction as a chord. The exhaust-side bottom portion 41 and the intake-side bottom portion 42 are squish areas where squish flows are formed when the piston 4 moves toward compression top dead center. In this embodiment, the surface area of the intake side bottom portion 42 is set larger than the surface area of the exhaust side bottom portion 41 .

The exhaust-side inclined surface 43 is an inclined surface that gradually rises from the exhaust-side bottom portion 41 toward the center of the crown surface 40 in the Y direction (the center portion of the crown surface 40 in the radial direction). The lower end of the exhaust-side inclined surface 43 is connected to the +Y edge of the exhaust-side bottom portion 41, and the upper end is connected to the -Y edge of the plane 45 and the inter-recess plane . The exhaust-side inclined surface 43 includes a pair of recess portions 431 on the +X side and the −X side, and inter-recess portions 432 positioned between the recess portions 431 . The recess portion 431 is a substantially semicircular recess for avoiding interference with the pair of exhaust valves 9 . The inter-recess portion 432 is a substantially triangular portion located between the pair of recess portions 431 and is a portion higher than the recess portion 431 . The inclination angles of the recess portion 431 and the inter-recess portion 432 with respect to the Y direction are set to be the same. In addition, the inclination angle may be slightly different.

The intake-side inclined surface 44 is an inclined surface that gradually rises from the intake-side bottom portion 42 toward the central portion of the crown surface 40 in the Y direction. The lower end of the intake-side inclined surface 44 is connected to the −Y edge of the intake-side bottom portion 42 , and the upper end is connected to the +Y edge of the flat surface 45 . In this embodiment, the intake-side inclined surface 44 is illustrated as an inclined plane without a recess, but if interference with the intake valve 8 occurs, a recess similar to the exhaust-side recess 431 is provided. .

The plane 45 is a plane extending in the XY direction perpendicular to the cylinder axis AX at the center of the crown surface 40 in the Y direction. The plane 45 continues between the upper end of the exhaust-side inclined surface 43 and the upper end of the intake-side inclined surface 44 . The plane 45 has an EX edge 451 as a side on the -Y side and an IN edge 452 as a side on the +Y side. The EX edge 451 is connected to each upper end of the pair of recessed portions 531 . The IN edge 452 is connected to the upper end of the intake-side inclined surface 44 . Since the cavity 80 exists in the radially central region of the crown surface 40, the plane 45 is divided into the +X side and the -X side. The flat surface 45 may be a surface slightly inclined with respect to the XY directions or a slightly convex or concave curved surface within a range that does not substantially hinder the flow of the tumble flow Ft.

The inter-recess plane 46 is a plane arranged so as to be sandwiched near the upper end between the pair of recess portions 431 of the exhaust-side inclined surface 43 . The inter-recess plane 46 is also a plane extending in the XY directions and is a plane that exists in the same plane as the plane 45 , that is, a plane positioned at the same Z-direction height as the plane 45 . The inter-recess plane 46 extends in the -Y direction from near the center of the EX edge 451 of the plane 45 in the X direction.

F-side sidewall 47 is the sidewall extending downward from the +X edge of plane 45 . R-side sidewall 48 is a sidewall extending downward from the −X edge of plane 45 . The F-side sidewall 47 and the R-side sidewall 48 straddle the +X edge and -X edge of the exhaust-side inclined surface 43 and the intake-side inclined surface 44, respectively.

Referring also to the top view of the crown surface 40 shown in FIG. 9, the cavity 80 is recessed in the plane 45 in the shape of a spherical crown, and has a circular opening edge in at least the region of the plane 45 when viewed from above. The cavity 80 is located in the center of the crown surface 40 in the X direction, but is arranged at a position eccentric to the +Y side in the Y direction. That is, the plane 45 extends in the X direction at the center in the Y direction, but the cavity 80 is slightly offset in the +Y direction. The EX side edge 81 of the cavity 80 is at substantially the same position as the EX edge 451 of the plane 45 , but the IN side edge 82 protrudes more to the +Y side than the IN edge 452 and extends to the upper end portion of the intake side inclined surface 44 . I'm on my way. This offset will be described in detail later.

Small cavity 90 is a depression located on the +Y side of cavity 80 and smaller than cavity 80 . The small cavity 90 is formed by recessing the IN side edge 82 of the cavity 80 and the vicinity of the upper end of the intake side inclined surface 44 into a bowl shape. In other words, the small cavity 90 is a small recess that is continuous with the air intake side of the cavity 80 and is located at a position unevenly distributed on the air intake side of the crown surface 40 . In addition, the small cavity 90 is located in the center of the crown surface 40 in the X direction. The shape of the small cavity 90 when viewed from above is substantially elliptical.

The small cavity 90 includes, as outer peripheral edges, an EX side peripheral edge 91 (first peripheral edge) on the −Y side and an IN side peripheral edge 92 (second peripheral edge) on the +Y side. When viewed from above, the EX side peripheral edge 91 has a first curved line convex to the -Y side, and the IN side peripheral edge 92 has a second curved line convex to the +Y side. The second curve of the IN side peripheral edge 92 has a larger curvature than the curvature of the first curve of the EX side peripheral edge 91 . That is, the degree of expansion of the IN side peripheral edge 92 toward the +Y side is greater than the degree of expansion of the EX side peripheral edge 91 toward the -Y side. By setting such a curvature, the small cavity 90 has a shape that can narrow the in-cylinder flow path from the IN-side peripheral edge 92 toward the EX-side peripheral edge 91 .

[Ignition part and cavity offset]
Next, the offset structure of the ignition portion 10A and the cavity 80 of the spark plug 10 will be described with reference to FIGS. 5 and 6. FIG. FIG. 5 is an enlarged view of a main part of FIG. 3, and FIG. 6 is a schematic diagram for explaining the offset structure. 5 and 6 show a cylinder center line C1 which is the axial center of the cylinder 20, a position C2 where the ignition section 10A ignites the air-fuel mixture, and a cavity center point C3 which is the radial center of the opening edge of the cavity 80. It is shown. In this embodiment, the cavity 80 is a spherical crown-shaped recess, so the cavity center point C3 is also the deepest part of the cavity 80 .

5 also shows the tumble flow Ft flowing through the combustion chamber 5. As shown in FIG. The tumble flow Ft flows into the combustion chamber 5 from the opening of the intake port 6 on the pent roof-type combustion chamber ceiling surface 5U. The tumble flow Ft once moves toward the exhaust side along the inflow direction, then reverses the flow direction and moves from the exhaust side to the intake side on the cavity 80 . Then, the tumble flow Ft flows upward from the intake side end of the cavity 80 to the combustion chamber ceiling surface 5U, and then returns to the exhaust side. Thereafter, similar vertical eddy currents are repeated.

The ignition position C2 (ignition portion 10A) is arranged at a position offset to the exhaust side from the cylinder center line C1 in a cross-sectional view along the cylinder axis AX. However, the ignition position C2 is arranged on the combustion chamber ceiling surface 5U above the region where the cavity 80 is formed. On the other hand, the cavity center point C3 is arranged at a position offset to the intake side from the cylinder center line C1 in a cross-sectional view along the cylinder axis AX. The ignition position C2 and the cavity center point C3 are arranged on a straight line that passes through the cylinder center line C1 and connects the intake side and the exhaust side.

5 and 6, d1 indicates the offset amount of the ignition position C2 with respect to the cylinder center line C1, and d2 indicates the offset amount of the cavity center point C3 with respect to the cylinder center line C1. In this embodiment, the amount of offset of the ignition position C2 toward the exhaust side is greater than the amount of offset of the cavity center point C3 toward the intake side. That is, the offset amounts of the ignition position C2 and the cavity center point C3 are set so as to satisfy the relationship of d1>d2. Note that the relationship between d1 and d2 is exaggerated in FIG. For example, in a cylinder 20 with a bore diameter of about 100 mm, d1=3.7 mm and d2=3.4 mm.

Next, advantages of the offset structure of the ignition part 10A and the cavity 80 will be described. FIG. 7 is a schematic diagram of a cross section of the combustion chamber 5 along the cylinder axis AX, and is a diagram for explaining propagation of flame by the tumble flow Ft. A cavity 80 is recessed in the crown surface 40 of the piston 4 . Focusing on the part that flows along the wall surface (bottom surface) of the cavity 80, the tumble flow Ft formed in the combustion chamber 5 becomes a flow from the exhaust side to the intake side as shown in FIG. The ignition portion 10A of the spark plug 10 forcibly ignites the air-fuel mixture in the combustion chamber 5 to form a flame ball 31 near the ignition portion 10A. As the tumble flow Ft acts on the flame ball 31, the flame propagates to the intake side as indicated by the arrow F1 in FIG.

In such a combustion chamber 5, the ignition position C2 by the ignition part 10A is offset from the cylinder centerline C1 to the exhaust side. Therefore, the air-fuel mixture can be ignited on the upstream side of the tumble flow Ft flowing in the cavity 80 to form the flame ball 31, and the flame can be put on the tumble flow Ft and propagated to the intake side. Further, the cavity center point C3 is offset to the intake side with respect to the cylinder center line C1. This makes it easier for the flame generated on the exhaust side to travel toward the intake side due to the tumble flow Ft. In other words, the area along the cavity 80 in which the flame can travel toward the intake side is lengthened by the offset amount, and the flame can be propagated to the intake side quickly. Therefore, it is possible to increase the combustion speed, improve the thermal efficiency, and improve the fuel efficiency.

Also, the offset amounts of the ignition position C2 and the cavity center point C3 with respect to the cylinder center line C1 are set so as to satisfy the relationship of d1>d2. Therefore, the flame ball 31 can be generated on the upstream side of the tumble flow Ft flowing in the cavity 80 portion, and the flame can be propagated to the intake side. Therefore, the burning speed can be made faster.

FIGS. 8A and 8B are plan views of the combustion chamber and diagrammatically show how the flame propagates. These figures show the flame propagation at a crank angle of 380 degrees. FIG. 8A shows the flame propagation situation in the case of the combustion chamber structure of the comparative example in which the ignition position C2 and the cavity center point C3 are not offset with respect to the cylinder centerline C1. A flame 32 is depicted here propagating towards the periphery of the combustion chamber 5 . Focusing on the intake side propagating portion 33 toward the intake side of the flame 32, it can be seen that the flame propagates only up to the vicinity of the reference circle indicated by the dotted line in the figure.

On the other hand, FIG. 8B shows the flame propagation situation in the case of the combustion chamber structure of the embodiment in which the ignition position C2 and the cavity center point C3 are offset from the cylinder center line C1 as shown in FIG. showing. FIG. 8(B) shows a state in which the flame 34 whose source is the flame ball 31 shown in FIG. Focusing on the intake side propagating portion 35 toward the intake side of the flame 34, it can be seen that the flame propagates further to the intake side beyond the position of the reference circle indicated by the dotted line in the figure. This indicates that the flame 34 was quickly propagated to the intake side by the tumble flow Ft. In other words, it has been demonstrated that the combustion rate is increased in the combustion chamber structure of the example.

[Regarding the contrivance of the shape of the crown of the piston]
The contribution of the tumble flow Ft is indispensable for realizing the flame propagation as described above. That is, it is important that the strong tumble flow Ft remains in the combustion chamber 5 even after the ignition unit 10A ignites the air-fuel mixture, that is, that the tumble flow Ft is maintained until the latter half of the compression stroke. The crown surface 40 of the piston 4 according to the present embodiment is designed to maintain the tumble flow Ft until the latter half of the compression stroke, and various contrivances are made to the shape thereof. In the following, this shape contrivance will be described with reference to FIGS. 9 to 11 in addition to the above drawings.

<Aspect of Cavity Offset>
Cavity 80 is positioned in crown surface 40 so as to be offset to the IN side (+Y). Specifically, the cavity 80 is arranged on the crown surface 40 such that the EX side edge 81 of the cavity 80 coincides with the upper end of the exhaust side inclined surface 43 (the EX edge 451 of the plane 45). On the other hand, the IN side edge 82 of the cavity 80 protrudes further to the +Y side than the IN edge 452 of the plane 45 and reaches the upper end portion of the intake side inclined surface 44 . Note that the EX side edge 81 may be positioned closer to the intake side than the upper end of the exhaust side inclined surface 43 .

By recessing the cavity 80 into the crown surface 40, the tumble flow Ft can maintain its flow using the space of the cavity 80 even in the latter half of the compression stroke. Therefore, it becomes easier to maintain the tumble flow Ft until the latter half of the compression stroke. The tumble flow Ft becomes a flow from the exhaust side to the intake side in the cavity 80 (see FIG. 5). Here, when the EX-side edge 81 of the cavity 80 reaches the exhaust-side inclined surface 43 of the crown surface 40 , a concave portion corresponding to the recess of the cavity 80 appears on the exhaust-side inclined surface 43 . In this case, if the tumble flow Ft remains until the latter half of the compression stroke, the tumble flow Ft will forcefully enter the cavity 80 from the recess. The momentum of this tumble flow Ft may hinder flame propagation toward the radially outer side of the combustion chamber 5 on the exhaust side. As a result, self-ignition of unburned gas may occur on the exhaust side of the combustion chamber 5, resulting in knocking.

However, in this embodiment, the cavity 80 is arranged on the crown surface 40 such that the EX side edge 81 of the cavity 80 coincides with the upper end of the exhaust side inclined surface 43 . For this reason, the exhaust-side inclined surface 43 does not have the above-described concave portion exposing a part of the cavity 80 , and the exhaust-side inclined surface 43 serves as a wall for restricting the inflow of the tumble flow Ft into the cavity 80 . Therefore, a vigorous tumble flow Ft directed from the exhaust side into the cavity 80 is not formed, and flame propagation radially outward on the exhaust side is not hindered. As a result, knocking caused by self-ignition of unburned gas is suppressed.

Further, the cavity 80 is arranged on the crown surface 40 so that the IN side edge 82 of the cavity 80 reaches the upper end portion of the intake side inclined surface 44 . For this reason, the size of the cavity 80 can be set to be large enough to reach the upper end portion of the intake-side inclined surface 44 . This makes it easier to maintain the tumble flow Ft until the latter half of the compression stroke. Further, as a result, the cavity 80 is arranged at a position eccentric to the intake side of the crown surface 40, so that the tumble flow Ft can be easily guided along the cavity 80 toward the intake side. As a result, flame propagation from the flame ball 31 to the intake side can be promoted, and the combustion speed can be increased. Therefore, the thermal efficiency can be improved and the fuel consumption performance can be improved.

<Arrangement of small cavities>
A small cavity 90 (second cavity) is arranged in addition to the cavity 80 (first cavity) on the crown surface 40 of the present embodiment. The small cavity 90 is formed by recessing the IN side edge 82 of the cavity 80 and the upper end region of the intake side inclined surface 44 into a bowl shape. As described above, the small cavity 90 includes, as outer peripheral edges, an EX side peripheral edge 91 having a first curved line convex to the -Y side and an IN side peripheral edge 92 having a second curved line convex to the +Y side.

The EX side peripheral edge 91 enters the inner side (−Y side) of the cavity 80 from the IN side edge 82 of the cavity 80 . On the other hand, the IN-side peripheral edge 92 bulges to the +Y side from the IN-side edge 82 (near the upper end of the intake-side inclined surface 44 ) to near the middle stage of the intake-side inclined surface 44 . When viewed from above, the curvature of the second curve of the IN-side peripheral edge 92 is larger than the curvature of the first curve of the EX-side peripheral edge 91 . As shown in FIG. 11, the IN side peripheral edge 92 is curved downward (−Z) in a side view. The EX side peripheral edge 91 is also the same.

According to this embodiment, since the small cavity 90 is connected to the IN side edge 82 of the cavity 80, the recess of the cavity provided in the crown surface 40 extends to the intake side. Therefore, it is possible to increase the size of the cavity, making it easier to maintain the tumble flow Ft until the latter half of the compression stroke.

Furthermore, the second curve of the IN-side peripheral edge 92 of the small cavity 90 has a larger curvature than the first curve of the EX-side peripheral edge 91 . Therefore, the small cavity 90 has a shape that protrudes further toward the intake side. This makes it easier for the flame of the air-fuel mixture to travel toward the intake side due to the tumble flow Ft. Therefore, it is possible to increase the combustion speed, improve the thermal efficiency, and improve the fuel efficiency.

The relationship between the first curve and the second curve will be described with reference to a comparative example with reference to FIG. 12 . FIG. 12(A) shows flame propagation when using the small cavity 900 of the comparative example, and FIG. 12(B) shows flame propagation when using the small cavity 90 according to the embodiment of the present invention. It is a diagram. The small cavity 900 of the comparative example and the small cavity 90 of this embodiment are the same in that they are arranged near the IN side edge 82 of the cavity 80 so as to be connected to the cavity 80 .

In the small cavity 900 of the comparative example, the first curved line forming the EX side peripheral edge 901 and the second curved line forming the IN side peripheral edge 902 have the same curvature. That is, the small cavity 900 has an elliptical shape when viewed from above. A flame ball 31 (FIG. 7) generated by the ignition operation of the ignition portion 10A offset to the exhaust side becomes a flame FA propagating in the cavity 80 from the exhaust side to the intake side due to the action of the tumble flow Ft. The flame FA is guided from the cavity 80 to the small cavity 900 toward the intake side. The flame FA is guided near the IN side peripheral edge 902 of the small cavity 900 .

On the other hand, in the small cavity 90 of the present embodiment, the second curve forming the IN side periphery 92 (second periphery) is more curved than the first curve forming the EX side periphery 91 (first periphery). It has a large curvature. Therefore, it is easy to set the shape of the IN side peripheral edge 92 to protrude further toward the intake side than the IN side peripheral edge 902 of the comparative example. The flame FB starting from the flameball 31 and directed toward the intake side with the assistance of the tumble flow Ft is guided along the cavity 80 and the small cavity 90 . In the case of this embodiment, the flame FB can go toward the intake side up to the vicinity of the IN-side peripheral edge 92 that protrudes further toward the intake side. That is, the flame FB tends to move toward the intake side more quickly than the flame FA of the comparative example. As a result, the burning speed can be increased.

[Modification]
Although the embodiments of the present invention have been described above, the present invention is not limited to these, and the following modified embodiments are possible.

(1) In the above embodiment, the offset amount (d1) of the ignition section 10A toward the exhaust side with respect to the cylinder center line C1 is set larger than the offset amount (d2) of the cavity center point C3 toward the intake side. (Fig. 6; d1>d2). This aspect is just an example, and there is no limit to the relationship between d1 and d2 as long as the ignition portion 10A is offset toward the exhaust side and the cavity center point C3 is offset toward the intake side. For example, d1=d2 or d1<d2 may be set.

(2) In the above embodiment, the spherical crown-shaped cavity 80 is exemplified in which the shape of the outer circumference of the cavity 80 when viewed from above is circular except for the portion of the IN side edge 82 . The term "spherical crown-shaped cavity" as used in the present invention is not limited to a circular outer peripheral shape when viewed from above, and includes, for example, a shape approximating a circle and an elliptical shape. If the cavity 80 has a non-circular shape, the center of the approximate circle can be treated as the cavity center point C3. In the case of an elliptical shape, the intersection of the long axis and the short axis can be treated as the cavity center point C3.

Furthermore, the recessed shape of the cavity 80 is not limited to a complete spherical crown shape, and may be a bowl-shaped recessed shape. For example, a part of the bottom surface 83 of the cavity 80 may be flat, or the deepest part of the bottom surface 83 may be shifted from the cavity center point C3.

(3) In the above embodiment, the cavity 80 is arranged on the crown surface 40 such that the EX side edge 81 of the cavity 80 coincides with the upper end of the exhaust side inclined surface 43 (the EX edge 451 of the plane 45). I gave an example. As described above, the EX side edge 81 may be positioned closer to the intake side (+Y side) than the EX edge 451, or conversely, the EX side edge 81 may be positioned closer to the exhaust side (-Y side) than the EX edge 451 is. side). Further, the cavity 80 may be arranged on the crown surface 40 so that the IN side edge 82 coincides with the upper end of the intake side inclined surface 44 (the IN edge 452 of the plane 45).

(4) In the above embodiment, the EX-side peripheral edge 91 and the IN-side peripheral edge 92 of the small cavity 90 are all curved. The EX side peripheral edge 91 and the IN side peripheral edge 92 may partially include a non-curved portion, for example, a straight portion. For example, the extension starting portion from the IN side edge 82 of the cavity 80 and the most projecting portion of the EX side peripheral edge 91 and the IN side peripheral edge 92 may be linear. That is, in a rough shape comparison, it is sufficient to satisfy the relationship of curvature of the first curve<curvature of the second curve.

1 engine body 11 intake valve 12 exhaust valve 15 injector (fuel injection part)
16 spark plug (igniter)
20 Cylinder 4 Piston 40 Crown 42 Intake-side bottom 43 Exhaust-side inclined surface 44 Intake-side inclined surface 45 Plane 5 Combustion chamber 5U Combustion chamber ceiling (pent roof type ceiling)
59 turbocharger (supercharger)
6 intake port 7 exhaust port 80 cavity (first cavity)
82 IN side edge (edge on intake side of first cavity)
90 small cavity (second cavity)
91 EX side peripheral edge (first peripheral edge)
92 IN side peripheral edge (second peripheral edge)
AX Cylinder shaft Ft Tumble flow

Claims (5)

A combustion chamber structure for an engine comprising a combustion chamber defined by a crown surface of a piston, an inner wall surface of a cylinder in which the piston is slidably accommodated, and a pent roof-shaped ceiling surface formed on a cylinder head. hand,
An intake port opening for supplying intake air to the combustion chamber and an exhaust port opening for discharging exhaust gas from the combustion chamber are formed on the ceiling surface. When the side on which the exhaust port is arranged is the exhaust side,
The crown surface is
an exhaust-side inclined surface that rises from the exhaust side toward the central portion of the crown surface;
an intake-side inclined surface rising from the intake side toward the central portion of the crown surface;
a plane provided between the upper end of the exhaust-side inclined surface and the upper end of the intake-side inclined surface and extending in a direction perpendicular to the axial direction of the cylinder at the center of the crown surface;
a first cavity recessed in a region including the plane;
a second cavity formed by recessing the edge of the first cavity on the intake side and the upper end region of the intake-side inclined surface into a bowl shape,
The second cavity includes, as outer peripheral edges, a first peripheral edge located on the exhaust side and a second peripheral edge located on the intake side,
When viewed from above, the first peripheral edge has a first curved line, the second peripheral edge has a second curved line,
The combustion chamber structure of an engine, wherein the second curve has a larger curvature than the first curve. In the engine combustion chamber structure according to claim 1,
further comprising an ignition unit arranged on the ceiling surface and realizing flame propagation combustion in the combustion chamber;
The ignition part is arranged at a position above the formation region of the first cavity and offset from the center line of the cylinder to the exhaust side in a cross-sectional view along the cylinder axis. room structure. In the engine combustion chamber structure according to claim 1 or 2,
A combustion chamber structure for an engine, further comprising a supercharger for supercharging intake air supplied from the intake port to the combustion chamber. In the engine combustion chamber structure according to any one of claims 1 to 3,
A combustion chamber structure for an engine, wherein a fuel injection section for injecting fuel into the combustion chamber is disposed on the intake side of the combustion chamber. In the engine combustion chamber structure according to any one of claims 1 to 4,
The combustion chamber structure of an engine, wherein the engine is a longitudinally mounted engine that includes six cylinders arranged in series, and the arrangement direction of the six cylinders is arranged along the longitudinal direction of a vehicle on which the engine is mounted. .

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