JP7556271B2 - Engine combustion chamber structure - Google Patents

Description

The present invention relates to a combustion chamber structure for an engine equipped with a combustion chamber having a pent roof type ceiling surface.

In order to improve thermal efficiency and fuel economy, research is being conducted daily on the structure of engine combustion chambers, particularly piston structures. For example, Patent Document 1 discloses a structure in which a piston crown surface has a cavity and an inclined surface that follows the shape of a pent roof-shaped ceiling surface in a combustion chamber. This combustion chamber structure suppresses the deceleration of the tumble flow, promotes combustion, and improves fuel economy. In addition, a straightforward and effective means of improving fuel economy is to set a high compression ratio. By increasing the compression ratio and implementing the above-mentioned combustion chamber shape innovations, it is possible to further improve fuel economy.

JP 2018-162733 A

In general, when manufacturing engines to be installed in commercially available automobiles, it is necessary to manufacture multiple types of engines with different displacements, for example, a lineup of displacements of 1.5L, 2.0L, and 2.5L. When the engine displacement differs, the bore diameter or stroke also differs. This causes the in-cylinder flow in the combustion chamber to change. In particular, the swirl flow that forms a swirling flow around the cylinder axis is affected by changes in the bore diameter or stroke. In other words, when the engine displacement differs, the flow of the swirl flow changes even if the engine speed and load are the same.

When the swirl ratio changes, the degree of progress of the flame propagation combustion assisted by the swirl flow also changes, causing differences in the combustion of the air-fuel mixture in the combustion chamber. In other words, fuel economy fluctuates. For this reason, even if attempts are made to improve fuel economy by increasing the compression ratio and modifying the combustion chamber shape, if the behavior of the swirl flow varies between engines with different displacements, it will not be possible to achieve the same combustion at the same engine speed and load, that is, combustion that improves fuel economy, in each engine. For this reason, for example, in combustion simulations, calibration according to the flow of the swirl flow is required for each displacement, which is a bottleneck in engine development.

The object of the present invention is to provide an engine combustion chamber structure that, when manufacturing multiple types of engines with different displacements, can achieve equivalent combustion in engines of each displacement at the same engine speed and the same load.

According to one aspect of the present invention, there is provided 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 in a cylinder head, the geometric compression ratio of the combustion chamber being set within a range of 13.5 to 15.5, the engine being provided with a swirl generating mechanism that generates a swirl flow in the combustion chamber, the ceiling surface being formed with an opening of an intake port that supplies intake air to the combustion chamber and an opening of an exhaust port that discharges exhaust gas from the combustion chamber, the side on which the intake port is provided being defined as the intake side and the side on which the exhaust port is provided being defined as the exhaust side, a fuel injection unit that injects fuel into the combustion chamber is provided on the intake side of the combustion chamber, the crown surface being formed with a swirl generating mechanism that generates a swirl flow in the combustion chamber, the ceiling surface being formed with an opening of an intake port that supplies intake air to the combustion chamber and an opening of an exhaust port that discharges exhaust gas from the combustion chamber, the side on which the intake port is provided being defined as the intake side and the side on which the exhaust port is provided being defined as the exhaust side, a convex raised portion formed on a surface of the crown surface, the convex raised portion including an exhaust-side bottom portion disposed near the exhaust-side edge of the crown surface and an intake-side bottom portion disposed near the intake-side edge, an exhaust-side inclined surface rising from the exhaust-side bottom portion toward the center of the crown surface , an intake-side inclined surface rising from the intake-side bottom portion toward the center of the crown surface , and a rectangular plane that is continuously provided so that no depression is present between an upper end of the exhaust-side inclined surface and an upper end of the intake-side inclined surface, and that extends in a direction perpendicular to the axial direction of the cylinder at the center of the crown surface and is long in the perpendicular direction , wherein the convex raised portion satisfies the relationship of 0.055<h/Lb<0.075, where h is the height of the raised portion and Lb is the bore diameter of the cylinder.

According to the above combustion chamber structure, the combustion chamber is divided by a crown surface having a convex protrusion, an inner wall surface of the cylinder, and a pent roof-shaped ceiling surface. In this combustion chamber structure, a plane extending in a direction perpendicular to the axial direction of the cylinder is provided at the center of the crown surface of the piston. Therefore, the swirl flow generated by the swirl generating mechanism flows stably along the plane. In addition, this combustion chamber structure satisfies the relationship of 0.055<h/Lb<0.075. According to such a combustion chamber structure, it is possible to make the swirl ratio of the swirl flow relatively high and uniform between engines with different displacements. Therefore, it is possible to realize the same combustion at the same engine speed and the same load in engines of each displacement. As a result, if a combustion form is determined for an engine of one displacement, it is possible to realize the same combustion in engines of other displacements, so that it is possible to improve the efficiency of engine development.
Furthermore, according to the above-described combustion chamber structure, a fuel injection section that injects fuel into the combustion chamber is disposed on the intake side of the combustion chamber, making it easier for the fuel sprayed from the fuel injection section to be entrained in a swirl flow, and a homogeneous mixture can be formed in the combustion chamber.

In the above combustion chamber structure, it is desirable that the surface area of the flat surface is greater than the sum of the surface area of the exhaust side inclined surface and the surface area of the intake side inclined surface.

This combustion chamber structure prevents the swirl flow from colliding with the exhaust side inclined surface, and allows the swirl flow to flow more stably along the plane. This contributes to making the swirl ratio of the swirl flow formed in engines of each displacement equal.

In the above combustion chamber structure, it is desirable that an ignition unit that realizes flame propagation combustion within the combustion chamber is disposed on the ceiling surface opposite the flat surface.

With this combustion chamber structure, the intake air compressed while maintaining a swirl flow is in a state of high turbulence energy at the position opposite the plane. By locating the ignition part in such a position, the combustion speed of the flame propagation combustion can be increased.

In the above combustion chamber structure, it is preferable that the swirl generating mechanism includes a swirl valve capable of opening and closing a portion of the intake port, and an actuator that drives the swirl valve.

With this combustion chamber structure, a swirl flow is formed in the combustion chamber by closing part of the intake port with the swirl valve, and the swirl ratio can be adjusted by changing the opening of the swirl valve with an actuator. This makes it possible to fine-tune the swirl ratio, which cannot be achieved by structural modifications to the combustion chamber alone.

According to the present invention, when manufacturing multiple types of engines with different displacements, it is possible to provide an engine combustion chamber structure that can achieve equivalent combustion in engines of each displacement at the same engine speed and the same load.

FIG. 1 is a schematic cross-sectional view showing an example of an engine to which a combustion chamber structure of an engine according to the present invention is applied. FIG. 2 is a schematic perspective view showing the structure of one cylinder of the engine. FIG. 3 is a schematic plan view showing the structure of the cylinder and the intake and exhaust system in its vicinity. FIG. 4 is a perspective view of the piston. FIG. 5 is a plan view of the piston crown. FIG. 6 is a side view of the piston crown. FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. 8A and 8B are schematic diagrams for explaining the flow of the swirl flow in engines with pistons having different crest heights. FIG. 9 is a table showing data relating to the swirl ratio of the engines according to the examples of the present invention and the comparative example. FIG. 10 is a graph showing the relationship between swirl ratio and peak height/stroke. FIG. 11 is a graph showing the relationship between the swirl ratio and the peak height/bore diameter. FIG. 12 is a perspective view of a piston, with various parameters related to the piston crown surface noted.

[Overall engine configuration]
Hereinafter, a combustion chamber structure of an engine according to an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a schematic cross-sectional view of an engine to which a combustion chamber structure according to an embodiment of the present invention is applied. The engine shown here is a multi-cylinder gasoline engine mounted on a vehicle such as an automobile as a power source for driving the vehicle. The engine includes an engine body 1 and auxiliary devices such as intake and exhaust manifolds and various pumps, not shown, attached thereto.

The engine body 1 has a cylinder block 3 in which a cylinder 2 is formed, a cylinder head 4 attached to the upper surface of the cylinder block 3 so as to close the cylinder 2 from above, and a piston 5 housed in the cylinder 2. The engine body 1 is typically a multi-cylinder type having multiple cylinders (e.g., four), but for simplicity, only one cylinder 2 is shown in FIG. 1.

Figure 2 shows a schematic perspective view of one cylinder 2. The piston 5 is a generally cylindrical body having an outer diameter corresponding to the bore diameter Lb of the cylinder 2, and is housed in the cylinder 2 so as to be able to slide back and forth at a predetermined stroke Ls. As will be described in detail later, a convex protrusion is formed on the crown surface 50, which is the upper surface of the piston 5, and a flat surface 55 with a crest height h corresponding to the length of the protrusion in the Z direction is provided. Below the piston 5, a crankshaft 7, which is the output shaft of the engine body 1, is provided. The crankshaft 7 is connected to the piston 5 via a connecting rod 8, and is driven to rotate around a central axis in response to the reciprocating motion of the piston 5.

A combustion chamber 6 is defined above the piston 5. The fuel is supplied to the combustion chamber 6 by injection from the injector 15 described below. The supplied fuel is mixed with air in the combustion chamber 6 and combusted, and the piston 5 is pushed down by the expansion force caused by the combustion, reciprocating in the vertical direction. The combustion chamber 6 is defined by the inner wall surface of the cylinder 2, the crown surface 50 of the piston 5, and the combustion chamber ceiling surface 6U (including the valve surfaces of the intake valve 11 and exhaust valve 12) formed on the bottom surface of the cylinder head 4. The combustion chamber ceiling surface 6U is a ceiling surface having an upwardly convex pent roof shape.

The geometric compression ratio of the cylinder 2, that is, the ratio between the volume of the combustion chamber 6 when the piston 5 is at top dead center and the volume of the combustion chamber 6 when the piston 5 is at bottom dead center, is desirably set to a high compression ratio of 13.5 or more. The preferred compression ratio range is 13.5 to 15.5. Setting such a high compression ratio can improve fuel efficiency.

The pent roof type combustion chamber ceiling surface 6U is provided with an intake port 9 and an exhaust port 10 that open toward the combustion chamber 6. The intake port 9 is a port that supplies intake air to the combustion chamber 6. In this embodiment, the intake port 9 is a tumble port that can form a tumble flow (vertical vortex). The flow direction of the tumble flow Ft is indicated in FIG. 2. The exhaust port 10 is a port that discharges exhaust gas after combustion from the combustion chamber 6. The combustion chamber ceiling surface 6U is provided with an intake valve 11 that opens and closes the intake port 9, and an exhaust valve 12 that opens and closes the exhaust port 10.

The valve type of the engine of this embodiment is a four-valve type with two intake valves and two exhaust valves, as shown in Figures 2 and 3. Figure 3 is a schematic plan view showing the structure of the cylinder 2 and the intake and exhaust system in its vicinity. The intake port 9 has a first intake port 9A and a second intake port 9B. The exhaust port 10 has a first exhaust port 10A and a second exhaust port 10B. One intake valve 11 is provided for each of the first intake port 9A and the second intake port 9B, and one exhaust valve 12 is provided for each of the first exhaust port 10A and the second exhaust port 10B.

As shown in FIG. 3, the second intake port 9B (i.e., a part of the intake port 9) of the first and second intake ports 9A and 9B is provided with a swirl valve 17 (swirl generating mechanism) capable of opening and closing the second intake port 9B. The swirl valve 17 is driven by an actuator 18 such as a stepping motor. When the swirl valve 17 is driven in the closing direction, the proportion of intake air flowing into the combustion chamber 6 from the first intake port 9A, which is not provided with the swirl valve 17, increases. This makes it possible to strengthen the swirling flow that swirls around the cylinder axis AX (the central axis of the combustion chamber 6), that is, the swirl flow. The flow direction of the swirl flow Fs is also indicated in FIG. 3. Conversely, if the swirl valve 17 is driven in the opening direction, the swirl flow Fs can be weakened. As described above, since the intake port 9 is a tumble port, the swirl flow Fs formed when the swirl valve 17 is closed becomes an oblique swirl flow mixed with the tumble flow Ft.

The cylinder head 4 is provided with an intake-side valve mechanism 13 that drives the intake valve 11, and an exhaust-side valve mechanism 14 that drives the exhaust valve 12. These valve mechanisms 13, 14 drive the intake valve 11 and exhaust valve 12 in conjunction with the rotation of the crankshaft 7. This drive causes the valve head of the intake valve 11 to open and close the opening of the intake port 9, and the valve head of the exhaust valve 12 to open and close the opening of the exhaust port 10. The valve mechanisms 13, 14 incorporate a variable valve timing mechanism (not shown) that changes the opening and closing timing.

An injector 15 (fuel injection unit) and a spark plug 16 (ignition unit) are attached to the cylinder head 4. The injector 15 injects fuel supplied from a fuel system (not shown) into the combustion chamber 6. The injector 15 is disposed on the periphery of the combustion chamber ceiling surface 6U, on the intake side where the intake port 9 is disposed. With this arrangement, the fuel sprayed from the injector 15 merges with the oblique swirl flow, and is easily distributed throughout the combustion chamber 6 by riding on the oblique swirl flow. In other words, a homogeneous mixture can be formed in the combustion chamber 6.

The spark plug 16 ignites the mixture of fuel injected from the injector 15 into the combustion chamber 6 and air introduced into the combustion chamber 6 through the intake ports 9 (9A, 9B). The spark plug 16 is attached to the cylinder head 4 along the cylinder axis AX. The ignition electrode portion of the spark plug 16 is exposed inside the combustion chamber 6 at the radial center of the combustion chamber ceiling surface 6U and faces the flat surface 55 of the crown surface 50 of the piston 5. When ignition energy is supplied from the spark plug 16 to the mixture in the combustion chamber 6, flame propagation combustion occurs in the combustion chamber 6 starting from the ignition point.

[Detailed structure of piston]
Next, the structure of the piston 5, particularly the structure of the crown surface 50, will be described in detail with reference to Figures 4 to 7. Figure 4 is a perspective view of the piston 5 shown in Figures 1 and 2, Figure 5 is a plan view of the crown surface 50 of the piston 5, Figure 6 is a side view of the crown surface 50, and Figure 7 is a cross-sectional view taken along line VII-VII in Figure 5.

In Fig. 4 to Fig. 7, to ensure clarity of explanation, directional indications of XYZ are used. The Z direction corresponds to the direction of the cylinder axis AX, the X direction corresponds to the front-rear direction of the engine body 1, which is the extension direction of the crankshaft 7, and the Y direction corresponds to the direction perpendicular to both the Z direction and the X direction. In each figure, the F side (+X) and R side (-X) are indicated as the front and rear sides in the installation direction of the engine body 1, the intake side (IN side; +Y) as the side where the intake port 9 is arranged, the exhaust side (EX side; -Y) as the side where the exhaust port 10 is arranged, and the top (+Z) and bottom (-Z) as the top and bottom sides on the cylinder axis AX are indicated.

The piston 5 includes a piston head 5A and a skirt portion 5B connected to the lower side (-Z side) of the piston head 5A. The piston head 5A is a cylinder, and has a crown surface 50 on the upper surface that constitutes a part (bottom surface) of the wall surface of the combustion chamber 6, and a side peripheral surface 5C that slides against the inner wall surface of the cylinder 2. The side peripheral surface 5C has a plurality of ring grooves into which piston rings are fitted. The skirt portion 5B is arranged on the +Y side and -Y side of the piston head 5A, and suppresses the oscillation of the piston 5 during reciprocating motion. A piston boss 5D that defines a pin hole extending in the X direction is provided at the center of the skirt portion 5B in the Y direction. A piston pin is inserted into the piston boss 5D for connection to the connecting rod 8.

The crown surface 50 is a substantially circular surface facing the combustion chamber ceiling surface 6U in the Z direction. The crown surface 50 includes an exhaust side bottom 51, an intake side bottom 52, an exhaust side inclined surface 53, an intake side inclined surface 54, a flat surface 55, an inter-recess flat surface 56, an F side side wall 57, and an R side side wall 58. Of these parts, the exhaust side bottom 51 and the intake side bottom 52 are base surfaces that are the lowest in height in the +Z direction on the crown surface 50, and the other parts form convex raised portions that are raised from the base surface in the +Z direction by a mountain height h.

The exhaust side bottom 51 and the intake side bottom 52 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 51 and the intake side bottom 52 may be surfaces that are slightly inclined with respect to the XY directions, or surfaces that have a slight convex or concave curve. The exhaust side bottom 51 is located near the edge of the EX side (-Y) of the crown surface 50. The intake side bottom 52 is located near the edge of the IN side (+Y) of the crown surface 50.

The exhaust side bottom 51 is a bow-shaped plane with an arc at the -Y side outer periphery (side periphery 5C) of the crown surface 50 and a straight line extending in the X direction as its chord. The intake side bottom 52 is a bow-shaped plane with an arc at the +Y side outer periphery of the crown surface 50 and a straight line extending in the X direction as its chord. The exhaust side bottom 51 and the intake side bottom 52 are squish areas where a squish flow is formed when the piston 5 moves toward the top dead center of compression. In this embodiment, the surface area of the intake side bottom 52 is set to be larger than the surface area of the exhaust side bottom 51.

The exhaust side inclined surface 53 is an inclined surface that gradually rises from the exhaust side bottom 51 (the exhaust side of the crown surface) toward the Y direction center of the crown surface 50 (the radial center of the crown surface 50). The lower end of the exhaust side inclined surface 53 is connected to the +Y edge of the exhaust side bottom 51, and the upper end is connected to the -Y edge of the flat surface 55 and the inter-recess flat surface 56. The exhaust side inclined surface 53 includes a pair of recesses 531 on the +X side and the -X side, and an inter-recess portion 532 located in these recesses 531. The recesses 531 are approximately semicircular depressions to avoid interference with the exhaust valves 12 arranged in the first and second exhaust ports 10A and 10B. The recessed portion 532 has a generally trapezoidal shape in plan view in the +Z direction ( FIG. 5 ), with the lower edge continuing to the exhaust side bottom 51 as the lower base and the upper edge continuing to the inter-recess plane 56 located at the pair of recessed portions 531 as the upper base. The recessed portions 531 and the recessed portion 532 are set to have the same inclination angle with respect to the Y direction. The inclination angles may be slightly different.

The intake side inclined surface 54 is an inclined surface that gradually rises from the intake side bottom 52 (the intake side of the crown surface) toward the center of the crown surface 50 in the Y direction. The lower end of the intake side inclined surface 54 is connected to the -Y edge of the intake side bottom 52, and the upper end is connected to the +Y edge of the plane 55. In this embodiment, in a plan view in the +Z direction, the lower and upper ends of the intake side inclined surface 54 are both edges that extend linearly in the X direction. A simple inclined plane is exemplified as the intake side inclined surface 54, but if interference with the intake valve 11 occurs, a recess similar to the exhaust side recess 531 is provided.

Plane 55 is a plane extending in the XY direction perpendicular to cylinder axis AX at the center of crown surface 50 in the Y direction. Plane 55 is a plane provided continuously between the upper end of exhaust side inclined surface 53 and the upper end of intake side inclined surface 54. Note that "continuous plane" means a plane without any cavity or other depression. Plane 55 may also be a surface with a slight inclination with respect to the XY direction, or a surface with a slight convex or concave curve, within a range that does not substantially impede the flow of tumble flow Ft or swirl flow Fs.

More specifically, the plane 55 has a generally rectangular shape that is long in the X direction when viewed in a plan view in the +Z direction. The plane 55 has a first EX edge 551 and a second EX edge 552 as its -Y side edges, and an IN edge 553 as its +Y side edge. The first EX edge 551 is connected to the upper end of the recessed portion 531 on the +X side. The second EX edge 552 is connected to the upper end of the recessed portion 531 on the -X side. The IN edge 553 is connected to the upper end of the intake side inclined surface 54. The +X side and -X side edges of the plane 55 have an arc shape that follows the circumference of the side peripheral surface 5C.

The inter-recess plane 56 is a plane disposed between a pair of recessed portions 531 of the exhaust-side inclined surface 53. The inter-recess plane 56 is also a plane extending in the XY direction and is a plane that exists in the same plane as the plane 55, that is, a plane that is located at the same height in the Z direction as the plane 55. The inter-recess plane 56 is a plane that is continuous with the plane 55. The plane 55 and the inter-recess plane 56 form the top surface of the raised portion on the crown surface 50, and are the surfaces that are highest in the +Z direction.

The inter-recess plane 56 is formed so as to extend to the -Y side from the center in the X direction of the -Y side edge of the plane 55, in other words, so as to extend to the -Y side from between the first EX edge 551 and the second EX edge 552. The EX edge 561 of the inter-recess plane 56 is connected to the upper end of the inter-recess portion 532 of the intake side inclined surface 54. The inter-recess plane 56 is located so as to be sandwiched near the upper end between the pair of recess portions 531, and has a roughly square shape when viewed from above in the +Z direction. Note that the inter-recess plane 56 may be omitted, and only the plane 55 may be disposed on the crown surface 50.

[Combustion chamber structure of engines with different displacements]
Automobile manufacturers often create a lineup of multiple vehicles with different engine displacements for a certain brand of automobile. For example, small passenger cars are offered with engine displacements of 1.5L, 2.0L, and 2.5L. That is, automobile manufacturers need to manufacture multiple engines with different displacements. If the engine displacement is different, the bore diameter Lb or stroke Ls will also be different. Then, the in-cylinder flow in the combustion chamber 6 will also change. In particular, the swirl flow Fs that forms a swirling flow around the cylinder axis AX is affected by the change in the bore diameter Lb or stroke Ls. The bore diameter Lb is the inner diameter of the cylinder 2 and is a length that is approximately equivalent to the diameter of the piston 5. The stroke Ls is the length that the piston 5 moves in the Z direction (extension direction of the cylinder axis AX) between TDC (top dead center) and BDC (bottom dead center).

Figures 8(A) and (B) are schematic diagrams for explaining the flow of the swirl flow Fs in engines with different displacements. Figure 8(A) shows a small cylinder 2A with a relatively small displacement, and Figure 8(B) shows a large cylinder 2B with a relatively large displacement. As mentioned above, the intake port 9 is a tumble port, so the swirl flow Fs is an oblique swirl flow that contains a tumble component. In this embodiment, the crown surface 50 is provided with a raised portion consisting of an exhaust side inclined surface 53, an intake side inclined surface 54, and a flat surface 55. The peak height h of the raised portion also affects the maintainability of the swirl flow.

The small cylinder 2A shown in FIG. 8(A) has a predetermined first bore diameter Lb1 and first stroke Ls1 to achieve that displacement. The raised portion of the crown surface 50 has a predetermined first peak height h1. The large cylinder 2B shown in FIG. 8(B) also has a predetermined second bore diameter Lb2, second stroke Ls2, and second peak height h2. In general, the larger the displacement, the larger each value, so the relationships are Lb1<Lb2, Ls1<Ls2, and h1<h2. In each cylinder 2A, 2B, the intake air flowing in from the first intake port 9A forms first and second swirl flows Fs1, Fs2.

The larger the bore diameter Lb, the longer the distance over which the swirl flow Fs collides with the inner wall surface of the cylinder 2 per revolution. Also, the longer the stroke Ls, the longer the distance over which the swirl flow Fs collides with the inner wall surface of the cylinder 2. Since Lb1<Lb2 and Ls1<Ls2, the second swirl flow Fs2 of the large cylinder 2B tends to be more easily attenuated than the first swirl flow Fs1 of the small cylinder 2A. On the other hand, the higher the peak height h, the more easily the swirl flow Fs collides with the exhaust side inclined surface 53 or the intake side inclined surface 54. Therefore, depending on what values are set for the first peak height h1 and the second peak height h2, the degree to which the first and second swirl flows Fs1 and Fs2 collide with the raised portions of each crown surface 50 and are attenuated varies. If the bore diameters Lb1, Lb2 and strokes Ls1, Ls2 are fixed for the purpose of setting the displacement, the selection of the first and second peak heights h1, h2 is crucial.

As described above, when the displacement is different, the damping factor of the swirl flow Fs differs. Therefore, even if the engine speed and load are the same, the behavior (swirl ratio) of the first and second swirl flows Fs1 and Fs2 often does not become the same. When the swirl ratio differs, the degree of progress of the flame propagation combustion assisted by the first and second swirl flows Fs1 and Fs2, respectively, also changes, and a difference occurs in the combustion of the mixture in each combustion chamber 6 of the cylinders 2A and 2B. In other words, it is not possible to achieve the same combustion at the same engine speed and the same load, in other words, the optimal combustion as intended, in engines with different displacements. The present invention provides an engine combustion chamber structure that can eliminate such problems.

Figure 9 shows the combustion chamber structure of engines with different displacements, and is a table showing data related to the swirl ratio. Samples 1 to 3 are engines with a displacement of 1.5 L, samples 4 to 6 are engines with a displacement of 2.0 L, and samples 7 to 9 are engines with a displacement of 2.5 L. The data listed are the compression ratio ε, bore diameter Lb, stroke Ls, crest height h, swirl ratio, first evaluation value R1, second evaluation value R2, third evaluation value R3, and fourth evaluation value R4.

The first evaluation value R1 and the second evaluation value R2 are mainly evaluation values related to the stroke Ls. Specifically, the first evaluation value R1 is an evaluation value showing the relationship between the peak height h and the stroke Ls, and is a value calculated by h/Ls. The second evaluation value R2 is an evaluation value showing the relationship between the swirl ratio and the stroke Ls, and is a value calculated by swirl ratio/Ls. On the other hand, the third evaluation value R3 and the fourth evaluation value R4 are mainly evaluation values related to the bore diameter Lb. Specifically, the third evaluation value R3 is an evaluation value showing the relationship between the peak height h and the bore diameter Lb, and is a value calculated by h/Lb. The fourth evaluation value R2 is an evaluation value showing the relationship between the swirl ratio and the bore diameter Lb, and is a value calculated by swirl ratio/Lb. In FIG. 9, data related to the swirl ratio of the engines of the embodiment and the comparative example of the present invention are mixed.

In actual engine design, the bore diameter Lb or stroke Ls is fixed and difficult to change. In the example of Figure 9, samples 1 to 3 have a bore diameter Lb = 74.5 mm and stroke Ls = 85.8 mm, samples 4 to 6 have a bore diameter Lb = 83.5 mm and stroke Ls = 91.2 mm, and samples 7 to 9 have a bore diameter Lb = 89 mm and stroke Ls = 100 mm. The compression ratio ε is also often set as a target value. For this reason, of the parameters listed in Figure 9, the only one that can be changed during engine manufacturing is the crest height h.

A desirable engine combustion chamber structure is to manufacture an engine that achieves a high geometric compression ratio of 13.5 to 15.5, has a high swirl ratio, and has a similar swirl ratio between engines of different displacements. In FIG. 9, attention is paid to sample 1 with a displacement of 1.5 L, sample 4 with a displacement of 2.0 L, and sample 7 with a displacement of 2.5 L. The compression ratios ε of these samples 1, 4, and 7 are high compression ratios of 14 to 15. In addition, the swirl ratios of samples 1, 4, and 7 are not the top data for each displacement grade, but are at a high level of about 2.3 to 2.7. The first evaluation value R1, which is determined by the shape of the cylinder 2, specifically the stroke Ls and the crest height h, is in the range of about 0.050 to 0.061 for samples 1, 4, and 7. Additionally, the third evaluation value R3, determined by the bore diameter Lb and the peak height h, is in the range of approximately 0.056 to 0.070 for samples 1, 4, and 7.

The first evaluation value R1, which can be said to indicate the shape characteristics of the combustion chamber 6, and the second evaluation value R2, which is an index showing the relationship with the swirl ratio, are generally 0.027 and equivalent for samples 1, 4, and 7. Similarly, the third evaluation value R3, which can be said to indicate the shape characteristics, and the fourth evaluation value R4, which is an index showing the relationship with the swirl ratio, are within a small range of about 0.029 to 0.032 for samples 1, 4, and 7. This indicates that even if the displacement (bore diameter Lb or stroke Ls) changes, it is possible to generate a substantially equivalent swirl flow Fs. In other words, by forming the combustion chamber 6 (peak height h) so that the first evaluation value R1 and the third evaluation value R3 of samples 1, 4, and 7 are obtained, it is possible to suppress the degree of variation in the swirl flow between engines with different displacements. Therefore, it is possible to achieve equivalent combustion at the same engine speed and load in engines of each displacement. In addition, a high compression ratio is also achieved, which also contributes to improving fuel efficiency.

Samples 2, 5, and 9 are in a group with a lower crest height h than samples 1, 4, and 7. The latter has h = 5 mm, while the former has h = 0.1 to 0.2 mm. In this group, the swirl ratio is high, but there is a large variation in the swirl ratio. In addition, the first evaluation value R1 of samples 2, 5, and 9 is generally the same, but the second evaluation value R2 has a relatively large variation. In addition, the third evaluation value R3 of samples 2, 5, and 9 is also generally the same, but the fourth evaluation value R4 has a relatively large variation. Furthermore, the compression ratio ε of samples 2, 5, and 9 is 10, and the intended high compression ratio is not achieved. Therefore, with the combustion chamber structure of samples 2, 5, and 9, it is difficult to obtain the same combustion between engines with different displacements, and it is also difficult to improve fuel efficiency.

On the other hand, samples 3, 6, and 8 belong to a group with a higher peak height h than samples 1, 4, and 7. For the former, h = about 8 to 11 mm. In this group, the compression ratio ε is generally high, but the swirl ratio is generally low, although there is little variation. The third evaluation value R3 of samples 3, 6, and 8 is found to vary relatively widely. A similar tendency is seen for the fourth evaluation value R4. However, because the swirl ratio value itself is small, the assistance of flame propagation combustion by the in-cylinder flow is low, and the combustion level drops. Therefore, even if a high compression ratio can be achieved, improvement in fuel efficiency cannot be expected.

Figure 10 is a graph showing the relationship between the first evaluation value R1 and the second evaluation value R2 described above, which are evaluation values that focus on the relationship between the mountain height h and the stroke Ls. The numbers attached to the plots in the figure correspond to the sample numbers in Figure 9.

As shown in FIG. 10, the region where the first evaluation value R1 is small is a region where the compression ratio ε is low because the peak height h is relatively low or the stroke Ls is relatively small. In this region, a high compression ratio of compression ratio ε = 13.5 to 15.5 cannot be achieved. Samples 2, 5, and 9 belong to this region, and cannot be adopted even if the variation in the second evaluation value R2 is small, because fuel efficiency will decrease. On the other hand, the region where the first evaluation value R1 is large is a region where the compression ratio ε is high because the peak height h is relatively high or the stroke Ls is relatively small. In this region, the high compression ratio listed above can be achieved. Samples 1, 4, and 7 and samples 3, 6, and 8 belong to this region.

A region where the second evaluation value R2 is large is a region where the swirl flow Fs is strong. A strong swirl flow Fs means that the combustion chamber 6 is provided in an environment where the swirl flow is difficult to attenuate. If the raised portion of the crown surface 50 has a high peak height h, the swirl flow Fs collides with the raised portion, causing significant attenuation. In other words, in a region where the peak height h is too high, the first evaluation value R1 becomes large and a high compression ratio can be achieved, but the high peak height h tends to affect the second evaluation value R2 to become small. In addition, there is a tendency for the second evaluation value R2 to vary widely between engines with different displacements. Samples 3, 6, and 8 belong to this region and cannot be used because a decrease in combustibility and variation are expected.

Figure 11 is a graph showing the relationship between the third evaluation value R3 and the fourth evaluation value R4 described above, which are evaluation values that focus on the relationship between the peak height h and the bore diameter Lb. As with Figure 10, the numbers attached to the plots correspond to the sample numbers in Figure 9.

As shown in FIG. 11, the region where the third evaluation value R3 is small is a region where the compression ratio ε is low because the crest height h is relatively low or the bore diameter Lb is relatively large. In this region, a high compression ratio of ε = 13.5 to 15.5 cannot be achieved. Samples 2, 5, and 9 belong to this region, and cannot be adopted even if the variation in the fourth evaluation value R4 is small, because the fuel efficiency performance decreases. On the other hand, the region where the third evaluation value R3 is large is a region where the crest height h is relatively high or the bore diameter Lb is relatively small, and therefore the compression ratio ε is high. In this region, the high compression ratio listed above can be achieved. Samples 1, 4, 7 and samples 3, 6, and 8 belong to this region.

A region where the fourth evaluation value R4 is large is a region where the swirl flow Fs is strong. A strong swirl flow Fs means that the combustion chamber 6 is provided in an environment where the swirl flow is difficult to attenuate. As mentioned above, if the raised portion of the crown surface 50 has a high crest height h, the swirl flow Fs collides with the raised portion, and the attenuation becomes significant. In other words, in a region where the crest height h is too high, the third evaluation value R3 becomes large and a high compression ratio can be achieved, but the high crest height h has an influence such that the fourth evaluation value R4 tends to become small. In addition, there is a tendency for the variation in the fourth evaluation value R4 to increase between engines with different displacements. Samples 3, 6, and 8 belong to this region, and cannot be adopted because a decrease in combustibility and variation are expected.

From the above, by setting the first evaluation value R1 in an optimum region that is neither too small nor too large, and by setting the third evaluation value R3 in an optimum region that is neither too small nor too large, it is possible to provide an engine combustion chamber structure that achieves a high compression ratio while having a high swirl ratio and that has a similar swirl ratio between different displacements. From the data shown in Figure 9, it can be seen that samples 1, 4, and 7 belong to the optimum region in terms of both stroke Ls and bore diameter Lb. In view of the values of the first evaluation value R1 of samples 1, 4, and 7 (approximately 0.050 to 0.061), the optimum region of the first evaluation value R1 is,
0.045<R1<0.065 (0.045<h/Ls<0.065)...(A)
can be set to.

In addition, in consideration of the values of the third evaluation value R3 of samples 1, 4, and 7 (approximately 0.056 to 0.070), the optimum region of the third evaluation value R3 is
0.055<R3<0.075 (0.055<h/Lb<0.075)...(B)
can be set to.

In other words, by setting R1 (h/Ls) within the range of formula (A) above, the attenuation of the swirl flow Fs caused by the high peak height h and the attenuation of the swirl flow Fs caused by the large stroke Ls can be made equivalent. Also, by setting R3 (h/Lb) within the range of formula (B) above, the attenuation of the swirl flow Fs caused by the high peak height h and the attenuation of the swirl flow Fs caused by the large bore diameter Lb can be made equivalent. Therefore, even if the engine displacement is different, it is possible to form an equivalent swirl flow Fs in the combustion chamber 6 and achieve equivalent combustion in the combustion chamber 6.

[Preferable shape of piston crown surface]
Next, a preferred shape of the crown surface 50 of the piston 5 will be described. Here, a shape of the crown surface 50 intended to maintain the tumble flow Ft until the latter half of the compression stroke will be described. Fig. 12 is a diagram showing various parameters related to the crown surface 50. In the diagram, the crest height h, the lateral width Lie and the front-rear width Lfr of the plane 55, the exhaust-side inclined surface angle Exd, the surface area S1 of the plane 55, the surface area S2 of the exhaust-side inclined surface 53, and the surface area S3 of the intake-side inclined surface 54 are shown.

As described above, the peak height h is the Z-direction height from the exhaust side bottom 51 or intake side bottom 52, which is the base surface of the crown surface 50, to the plane 55 and inter-recess plane 56, which are the top surfaces. The horizontal width Lie is the Y-direction width of the plane 55. The front-rear width Lfr is the X-direction width of the plane 55. The +X and -X side sides of the plane 55 are arc sides. The front-rear width Lfr is the X-direction width between the parts where these arc sides extend most to the +X or -X side. The exhaust side inclined surface angle Exd is the inclination angle of the exhaust side inclined surface 53 with respect to the Y direction. In this embodiment, the plane 55 is a horizontal plane along the Y direction, so the inclined surface angle Exd is the angle between the plane 55 and the exhaust side inclined surface 53.

The surface area S1 of the plane 55 is the area of the portion enclosed by the +X and -X side edges and the +Y and -Y side edges that define the plane 55, and is roughly the area calculated by multiplying the horizontal width Lie by the front-to-rear width Lfr. In the present embodiment, when the inter-recess plane 56 is connected to the plane 55, the surface area S1 is treated as the combined surface area of the plane 55 and the inter-recess plane 56.

The surface area S2 of the exhaust side inclined surface 53 is the sum of the surface area of the pair of recesses 531 and the surface area of the inter-recess portion 532. The step portion 53A existing between the recesses 531 and the inter-recess portion 532 is not included in the surface area S2. This is because the step portion 53A does not substantially affect the flow of the tumble flow Ft. The surface area S3 of the intake side inclined surface 54 is, in the example of FIG. 11, simply the area of the inclined plane that constitutes the intake side inclined surface 54. If a recess that avoids interference with the intake valve 11 is also formed on the intake side inclined surface 54, the surface area S3 is the sum of the surface area of the recess and the surface area of the inter-recess portion.

In this embodiment, in order to reduce the resistance of the piston crown surface 50 to the tumble flow Ft and maintain the tumble flow Ft until the latter half of the compression stroke, it is desirable to manufacture the engine by setting the above surface areas S1, S2, and S3 to have the following characteristics (1) to (3).
(1) The surface area S1 of the flat surface 55 is larger than the surface area S2 of the exhaust side inclined surface 53.
(2) Preferably, the surface area S1 of the flat surface 55 is greater than the surface area S3 of the intake side inclined surface 54.
(3) More preferably, the surface area S1 of the flat surface 55 is greater than the sum of the surface area S2 of the exhaust-side inclined surface 53 and the surface area S3 of the intake-side inclined surface 54.

The crown surface 50 has a plane 55 that is a "continuous plane," so the tumble flow Ft (see FIG. 2) can flow along the plane 55 without being hindered by depressions such as cavities. In addition, by setting the relationship between the surface areas S1 to S3 according to the above characteristics (1) to (3), the flow of the tumble flow Ft is not weakened by the presence of the exhaust side inclined surface 53 and the intake side inclined surface 54. In other words, the above structural design of the crown surface 50 reduces the resistance of the crown surface 50 to the tumble flow Ft, making it easier for the tumble flow Ft to continue its flow within the combustion chamber 6.

In other words, the proportion of the tumble flow Ft that collides with the exhaust side inclined surface 53 or the inner wall of the cylinder 2 and is lost is reduced, making it easier to maintain the tumble flow Ft until the latter half of the compression stroke. When the tumble flow Ft collapses, turbulent energy is generated. Maintaining the tumble flow Ft leads to maintaining the turbulent energy that the tumble flow Ft inherently possesses at a high level without loss due to the collision. Therefore, by collapsing the tumble flow Ft in the latter half of the compression stroke and generating high turbulent energy, it is possible to increase the combustion speed of the mixture in the combustion chamber 6.

In the combustion chamber 6, the ignition operation of the spark plug 16 is the starting point, and flame propagation combustion of the mixture occurs. Here, if the cylinder 2 is set to a high compression ratio, the pressure and temperature in the combustion chamber 6 rise excessively at the compression end of the piston 5, inducing abnormal combustion. The abnormal combustion is abrupt self-ignition of unburned fuel gas before the completion of flame propagation combustion, which causes knocking. However, by maintaining the tumble flow Ft until the latter half of the compression stroke and accelerating the combustion speed, it is possible to complete the combustion before the occurrence of self-ignition, which is a cause of knocking. And since knocking can be suppressed, it is possible to avoid control that deliberately suppresses engine output, such as control of the fuel injection timing of the injector 15 to retard the center of gravity of the combustion. As a result, a high compression ratio can be achieved in the cylinder 2, which makes it possible to improve fuel efficiency.

By satisfying at least the above characteristic (1), the maintainability of the tumble flow Ft can be improved. In addition, by satisfying characteristic (2), the maintainability of the tumble flow Ft can be further improved. Furthermore, after satisfying characteristics (1) and (2), it is desirable to set the surface area S1 of the plane 55 to be larger than the sum of the surface area S2 of the exhaust side inclined surface 53 and the surface area S3 of the intake side inclined surface 54, as in characteristic (3). This can further suppress the collision of the tumble flow Ft with the exhaust side inclined surface 53 and the collision of the tumble flow Ft with the IN side inner wall of the cylinder 2 due to the guide of the tumble flow Ft by the intake side inclined surface 54, and can further improve the maintainability of the tumble flow Ft. In addition, as for the swirl flow Fs, the swirl flow Fs is guided along the plane 55 having a large surface area, which contributes to the stable flow of the swirl flow Fs. In addition, it contributes to suppressing the variation in the flow in the cylinder between engines with different displacements.

Next, the shape characteristics of the crown surface 50 other than the above surface areas S1 to S3 will be described. First, the ratio Lie/h of the width Lie, which is the width in the Y direction of the flat surface 55, to the height h of the crown surface 50 is as follows in the range of the compression ratio = 13.5 to 15.5:
2.5<Lie/h<9.0...(C)
It is desirable to satisfy the following relationship.

In the combustion chamber 6 having a pent roof type combustion chamber ceiling surface 6U, the inclination angle of the exhaust side inclined surface 53 and the intake side inclined surface 54 of the crown surface 50 is generally in line with the inclination angle of the combustion chamber ceiling surface 6U. For this reason, the crest height h has a large effect on the width Lie of the plane 55. Increasing the crest height h leads to increasing the compression ratio. For example, if the crest height h is set high in order to improve fuel efficiency, the width Lie becomes narrow. In other words, the surface area S1 of the plane 55 becomes small. In this case, even if the fuel efficiency is improved, it becomes difficult to maintain the tumble flow Ft until the latter half of the compression stroke. In the end, in order to prevent knocking, suppression control of the engine output is required. However, by setting Lie/h within the range of the above formula (C), it is possible to achieve both improvement in fuel efficiency and improvement in engine output. From the viewpoint of making this balance more desirable, Lie/h is set to be in the range of compression ratio = 13.5 to 15.5,
5.0<Lie/h<9.0...(C1)
It is desirable to satisfy the following relationship.

Next, regarding Lfr/h, which is the ratio of the front-rear width Lfr, which is the width in the X direction of the plane 55, to the mountain height h, in the range of the compression ratio = 13.5 to 15.5,
12.0<Lfr/h<16.0...(D)
It is desirable to satisfy the following relationship.

As with the lateral width Lie, the higher the crest height h is set, the narrower the front-to-rear width Lfr becomes, and the smaller the surface area S1 of the flat surface 55 becomes. Therefore, it becomes difficult to maintain the tumble flow Ft until the latter half of the compression stroke, and it becomes necessary to suppress the engine output. However, by setting Lfr/h within the range of the above formula (D), it is possible to achieve both improved fuel economy and improved engine output. From the viewpoint of making it more desirable to achieve both, Lfr/h is set to:
13.5<Lfr/h<14.5...(D1)
It is desirable to satisfy the following relationship.

Here, it is desirable that the front-rear width Lfr of the plane 55 is larger than the lateral width Lie. As shown in FIG. 2, the tumble flow Ft is introduced into the combustion chamber 6 from the intake port 9, turns around at the inner wall surface on the EX side of the cylinder 2, and passes over the plane 55 toward the IN side. If the plane 55 has a front-rear width Lfr smaller than the lateral width Lie, the plane will not exist at the +X side and -X side ends of the crown surface 50. In this case, the tumble flow Ft is difficult to guide at the +X side and -X side ends, resulting in flow loss. On the other hand, by making the plane 55 have a front-rear width Lfr larger than the lateral width Lie, the tumble flow Ft can be guided even at the +X side and -X side ends, and the maintenance of the tumble flow Ft can be improved.

Regarding (Exd×S1)/h, which shows the relationship between the exhaust-side inclined surface angle Exd, which is the angle between the plane 55 and the exhaust-side inclined surface 53, the surface area S1 of the plane 55, and the mountain height h, in the range of compression ratio = 13.5 to 15.5,
5000<(Exd×S1)/h<18000...(E)
It is desirable to satisfy the following relationship.

Since the tumble flow Ft flows as described above, the smaller the exhaust-side inclined surface angle Exd, the more the degree to which the flow of the tumble flow Ft is changed at the boundary between the exhaust-side inclined surface 53 and the flat surface 55, or the degree to which the tumble flow Ft collides with the exhaust-side inclined surface 53 can be suppressed. However, unless the crest height h is set to a certain height, the compression ratio cannot be increased. In order to increase the crest height h and increase the surface area S1 of the flat surface 55, it is necessary to increase the exhaust-side inclined surface angle Exd. In consideration of these conflicting demands, in order to achieve both a high compression ratio and the maintenance of the tumble flow Ft, it is sufficient to set (Exd×S1)/h within the range of the above formula (E). From the viewpoint of making it more desirable to achieve both, (Exd×S1)/h is set to the following value within the compression ratio range of 13.5 to 15.5:
7000<(Exd×S1)/h<12000...(E1)
It is desirable to satisfy the following relationship.

[Action and Effect]
The combustion chamber structure of the engine according to the present embodiment described above provides the following effects. First, the combustion chamber 6 is defined by the crown surface 50 (piston 5) having a convex raised portion, the inner wall surface of the cylinder 2, and the pent roof type combustion chamber ceiling surface 6U. In this combustion chamber structure, the height h of the raised portion and the stroke Ls, which is the length of movement of the piston 5 from the top dead center to the bottom dead center, satisfy the relationship of 0.045<h/Ls<0.065. As described above, this combustion chamber structure makes it possible to make the swirl ratio of the swirl flow Fs formed by the swirl valve 17 (swirl generating mechanism) relatively high and uniform between engines with different displacements. Therefore, the same combustion can be achieved in engines of each displacement at the same engine speed and load. As a result, if a combustion form is determined for an engine of one displacement, the same combustion can be achieved in engines of other displacements, so that the efficiency of engine development can be improved.

In addition, because a continuous plane 55 is formed on the crown surface 50, the swirl flow Fs can be guided along the plane 55. In particular, if the surface area S1 of the plane 55 is set to be larger than the sum of the surface area S2 of the exhaust side inclined surface 53 and the surface area S3 of the intake side inclined surface 54, the swirl flow Fs can be made to flow stably along the plane 55. Stabilizing the flow of the swirl flow Fs contributes to making the swirl ratio of the swirl flow Fs formed at the same engine speed and load equal in engines of each displacement.

The spark plug 16 that realizes flame propagation combustion is disposed on the combustion chamber ceiling surface 6U that faces the plane 55 of the crown surface 50. In this embodiment, it is disposed at the center of the combustion chamber 6 (on the cylinder axis AX). The intake air that is compressed while maintaining the swirl flow Fs reaches a state of high turbulence energy at the position facing the plane 55. By disposing the spark plug 16 in such a position, the combustion speed of the flame propagation combustion can be increased.

The injector 15 is also disposed on the intake side of the combustion chamber 6. This makes it easier for the fuel sprayed from the injector 15 to be carried by the swirl flow Fs, allowing a homogeneous mixture to be formed in the combustion chamber 6.

As a swirl generating mechanism for forming the swirl flow Fs, a swirl valve 17 is provided that is driven by an actuator 18 such as a stepping motor and can open and close the second intake port 9B of the first and second intake ports 9A and 9B. Therefore, by changing the opening degree of the swirl valve 17, it is possible to fine-tune the swirl ratio, which cannot be achieved by structural design alone of the combustion chamber 6. In the embodiment, as described above, the swirl generating mechanism is embodied by the swirl valve 17 provided in the swirl valve 17. However, the means for generating the swirl flow Fs is not limited to this. For example, a helical port with a spiral shape of the intake port can be applied. The swirl flow Fs may also be formed by controlling the intake side valve mechanism 13 (variable valve timing mechanism) to cause a time difference in the closing timing of the two intake valves 11.

1 engine body 11 intake valve 12 exhaust valve 15 injector (fuel injection part)
16 Spark plug (ignition part)
17 Swirl valve (swirl generating mechanism)
2 Cylinder 5 Piston 50 Crown surface 51 Exhaust side bottom 52 Intake side bottom 53 Exhaust side inclined surface 54 Intake side inclined surface 55 Flat surface 6 Combustion chamber 6U Combustion chamber ceiling surface (pent roof type ceiling surface)
9 Intake port 10 Exhaust port AX Cylinder axis Fs Swirl flow Ft Tumble flow h Peak height Ls Stroke Lb Bore diameter

Claims (4)

A combustion chamber structure for an engine, comprising: a combustion chamber defined by a piston crown surface, an inner wall surface of a cylinder in which the piston is slidably housed, and a pent roof type ceiling surface formed in a cylinder head, the geometric compression ratio of the combustion chamber being set within a range of 13.5 to 15.5,
The engine includes a swirl generating mechanism that generates a swirl flow in the combustion chamber,
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 in the ceiling surface, and when the side on which the intake port is disposed is defined as the intake side and the side on which the exhaust port is disposed is defined as the exhaust side, a fuel injection unit for injecting fuel into the combustion chamber is disposed on the intake side of the combustion chamber,
On the crown surface,
an exhaust side bottom portion disposed near an end edge of the crown surface on the exhaust side, and an intake side bottom portion disposed near an end edge of the crown surface on the intake side, each of which extends in a direction perpendicular to an axial direction of the cylinder;
an exhaust-side inclined surface rising from the exhaust-side bottom portion toward the center of the crown surface , an intake-side inclined surface rising from the intake-side bottom portion toward the center of the crown surface , and a rectangular flat surface that is continuously provided so that no depression is present between an upper end of the exhaust-side inclined surface and an upper end of the intake-side inclined surface, and that extends in a direction perpendicular to the axial direction of the cylinder at the center of the crown surface and is long in the perpendicular direction ,
When the height of the raised portion is h and the bore diameter of the cylinder is Lb,
0.055<h/Lb<0.075
A combustion chamber structure for an engine, characterized in that the above relationship is satisfied. The combustion chamber structure of the engine according to claim 1,
A combustion chamber structure for an engine, wherein the surface area of the flat surface is greater than the sum of the surface area of the exhaust side inclined surface and the surface area of the intake side inclined surface. The combustion chamber structure of the engine according to claim 1 or 2,
An engine combustion chamber structure, wherein an ignition unit that realizes flame propagation combustion within the combustion chamber is disposed on the ceiling surface opposite the flat surface. The combustion chamber structure of the engine according to any one of claims 1 to 3 ,
The swirl generating mechanism includes a swirl valve capable of opening and closing a portion of the intake port, and an actuator that drives the swirl valve.

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