JPH11257077A - In-cylinder injection spark ignition engine - Google Patents

Abstract

(57) [Summary] In a cylinder injection type spark ignition engine intended for stratified combustion, stratification is efficiently performed with a simple and low-cost structure. In an internal-injection-type spark ignition engine provided with a spark plug in the center of an upper part of a cylinder, a fuel injection valve in a peripheral part of the cylinder, and a swirl generator in an intake pipe, a swirl suppressing plate is provided on a top surface of a piston. Inject fuel toward the piston surface at the beginning of the intake stroke or the compression stroke.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct injection type spark ignition engine capable of stratifying fuel efficiently at low cost.

[0002]

2. Description of the Related Art In a spark ignition type internal combustion engine, an in-cylinder injection system for directly injecting fuel into a cylinder has been attracting attention for the purpose of improving fuel efficiency and reducing pollution of exhaust gas. FIG. 23 shows a configuration diagram of a representative in-cylinder injection engine. In this system, the ignition plug 5 is usually arranged at the center of the upper part of the cylinder 2, and a concave groove 15 is provided at the top of the piston 3 on the side of the intake pipes 6a and 6b. Groove 1
The position of the side wall surface of the exhaust pipe 5 is installed below the ignition plug 5. The fuel injection valve 4 is installed on the intake pipe side above the cylinder 2.

The intake pipe 6b is provided with a swirl control valve 16 which can be opened and closed by a drive mechanism (not shown) according to the load of the engine. When the engine load is low load or medium load, the swirl control valve 16 is closed by the driving mechanism, and air is introduced into the cylinder 2 only from the intake pipe 6a during the intake stroke. A swirling flow (swirl) occurs.

[0004] In the latter half of the compression stroke, fuel atomized to a number of diameters + m from the fuel injection valve 4 is injected into the groove 15. The fuel injection amount at this time is approximately 1/25 to 1/50 of the intake air mass.

The fuel rotates in the groove 15 while being vaporized by the action of the swirl, and is carried to the vicinity of the spark plug 5. As a result, an air-fuel mixture having a relatively high fuel concentration is formed around the ignition plug, and even a lean air-fuel mixture having an average air-fuel ratio of 25 to 50 can be stably ignited and burned.
An in-cylinder injection spark ignition engine of this type is described, for example, in Japanese Patent Application Laid-Open No. 4-94416.

[0006]

By the way, in such a direct injection spark ignition engine, the fuel vaporization time is short in order to inject fuel in the latter stage of the compression stroke at the time of low load or medium load operation, so that the ignition timing is sufficient. In order to obtain a gaseous mixture that has been vaporized, very finely atomized fuel must be supplied into the cylinder. For this reason, it is necessary to increase the pressure of the fuel supplied to the fuel injection valve to usually 5 MPa or more,
It costs much to install a high-pressure fuel pump.

[0007]

In order to overcome the above-mentioned problems, in a first direct injection type spark ignition engine according to the present invention, a spark plug is disposed substantially at the center of an upper portion of a cylinder, and a fuel injection valve is disposed at an upper portion of the cylinder. Means for generating a swirl around the central axis of the cylinder in the intake and compression strokes, a structure for reducing the speed of the swirl around the central axis of the cylinder on the top surface of the piston, and an intake stroke at least during low load or medium load operation. Alternatively, means for injecting fuel toward the piston surface in the first half of the compression stroke is provided.

Further, in the second direct injection type spark ignition engine of the present invention, in the first direct injection type spark ignition engine, the swirl around the central axis of the cylinder is approximately 90 ° on the top surface of the piston. Are provided.

In the third direct injection type spark ignition engine according to the present invention, the first direct injection type spark ignition engine is provided with radial projections extending from the vicinity of the center of the piston to the peripheral portion.

In the fourth in-cylinder injection spark ignition engine according to the present invention, a corrugated projection is provided on the top surface of the piston in the first in-cylinder injection spark ignition engine.

Further, in the fifth direct injection type spark ignition engine of the present invention, an ignition plug is disposed substantially at the center of the upper part of the cylinder, a fuel injection valve is provided at the upper part of the cylinder, and the central axis of the cylinder in the intake and compression strokes. A means for generating a swirl around the piston, and a means for blocking the swirl around the central axis of the cylinder from the outer peripheral portion of the top surface of the piston to approximately the center of the piston, and providing an intake stroke or a compression stroke at least during low load or medium load operation. Means for injecting fuel toward the piston surface are provided in the first half.

According to the sixth in-cylinder injection type spark ignition engine of the present invention, in the above-mentioned fifth in-cylinder injection type spark ignition engine, the means for blocking the swirl around the cylinder center axis is provided on the upstream side of the swirl. From the viewpoint of concave curvature.

That is, in the first in-cylinder injection spark ignition engine described above, fuel is injected from the fuel injection valve toward the piston surface in the first half of the intake stroke or the compression stroke during low or medium load operation. The fuel is vaporized near the surface of the piston, and an air-fuel mixture having a high fuel concentration is formed in a lower portion of the combustion chamber.
In the intake stroke, swirl is generated by the swirl generation means around the central axis of the cylinder. The swirl at the lower part of the cylinder is attenuated during the compression stroke by the swirl deceleration means provided on the surface of the piston.

As a result, there is a difference in swirl strength between the upper cylinder and the lower cylinder. At the upper part of the cylinder where the swirl is strong, the centrifugal force acts strongly, so that the pressure difference between the center and the outer part is large. At the lower part of the cylinder, the swirl is weak, so the pressure difference between the center and the outer part due to the centrifugal force is small. For this reason, a flow from the lower part of the cylinder toward the upper part of the cylinder occurs at the center part of the cylinder, and a flow from the upper part of the cylinder toward the lower part of the cylinder occurs at the outer peripheral part. Due to this flow, the air-fuel mixture having a high vapor concentration generated in the lower part of the cylinder gathers at the center of the cylinder and rises from the center of the cylinder to the upper part of the cylinder. As a result, a mixture having a relatively high concentration gathers around the ignition plug provided at the center of the upper part of the cylinder.

In the above-described second in-cylinder injection type spark ignition engine, the first in-cylinder injection type spark ignition engine has a side surface that forms an angle of about 90 ° with the swirl direction on the piston top surface. The swirl near the piston surface can be efficiently attenuated. Further, in the third in-cylinder injection type spark ignition engine described above, in the first in-cylinder injection type spark ignition engine described above, by providing radial projections on the piston top surface, swirl near the piston surface can be efficiently reduced. In addition to being able to attenuate well, the fuel-rich mixture near the piston surface is carried toward the center of the cylinder along the long axis direction of the radial projection, and the fuel-rich mixture is more reliably removed. Can be collected at the center of the cylinder.

In the above-described fourth in-cylinder injection type spark ignition engine, the first in-cylinder injection type spark ignition engine is provided with a corrugated projection on the top surface of the piston. The nearby swirl can be efficiently attenuated.

In the above-described fifth in-cylinder injection spark ignition engine, fuel is injected from the fuel injection valve toward the piston surface in the first half of the intake stroke or the compression stroke during low or medium load operation. The fuel is vaporized near the surface of the piston, and an air-fuel mixture having a high fuel concentration is formed in a lower portion of the combustion chamber.
Further, in the intake stroke, the first swirl is generated around the central axis of the cylinder by the swirl generating means.

From the outer peripheral portion of the piston top surface toward the center,
A structure for blocking the first swirl is provided, and the end of the blocking structure is located near the cylinder center axis.
The swirl near the piston surface changes the flow direction by the blocking structure, and a second swirl having a center closer to the outer circumference than the center axis of the cylinder is generated near the piston surface. In the upper part of the cylinder, the pressure in the central part is low due to the first swirl, while in the lower part of the cylinder, the pressure is low due to the second swirl.
The pressure at the center of the cylinder increases. As a result, an upward flow from the piston surface toward the upper part of the cylinder is generated at the center of the cylinder.

Due to this upward flow, the mixture having a high vapor concentration generated near the piston surface is collected at the center of the cylinder and rises from the center of the cylinder to the upper part of the cylinder. As a result, a mixture having a relatively high concentration gathers around the ignition plug provided at the center of the upper part of the cylinder.

In the above-described sixth in-cylinder injection spark ignition engine, the swirl blocking means on the piston top surface in the fifth in-cylinder injection spark ignition engine is viewed from the upstream side of the swirl. By setting the concave curvature, the above-described second swirl can be produced more efficiently.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the schematic configuration of the engine of the present embodiment has two intake pipes 6a and 6b and two exhaust pipes 8 at the upper part of the cylinder 2. An intake valve 13 is provided at each intake pipe and exhaust pipe. And an exhaust valve 14. A swirl control valve 16 is provided in the intake pipe 6b.

The swirl control valve 16 is closed by a driving mechanism (not shown) at the time of low load or medium load operation, and is opened at the time of high load operation. A spark plug 5 is provided above the center of the cylinder 2 and an intake pipe 6 is provided.
A fuel injection valve 4 is provided between a and 6b.

The fuel injection valve 4 has a fuel flow of approximately 20 μm
It is atomized into the following size and injected into the cylinder 2 in a conical shape. The fuel injection valve 4 can inject fuel by a drive mechanism (not shown) in the first half of the intake stroke or the compression stroke.

On the top surface of the piston 3, a swirl suppressing plate 1 having a rectangular parallelepiped shape is provided radially. The side surface of the swirl suppressing plate 1 and the upper surface of the piston 3 form an angle of 90 °. When the piston 3 is at the top dead center, the height of the swirl suppressing plate 1 is set to the intake valve 13 and the exhaust valve 14.
It is determined within the range that does not collide with.

In this embodiment, during low load and medium load operation, the swirl control valve 16 is closed, and air is introduced into the cylinder 2 from the intake pipe 6a. As a result,
A swirl S around the axis of the cylinder is generated in the cylinder 2 as shown in FIG. Subsequently, at any time during the intake stroke or at the beginning of the compression stroke, the fuel 9 atomized to a diameter of about 20 μm or less is injected from the fuel injection valve 4 toward the surface of the piston 3. The injection amount at this time is approximately 1/25 to 1/50 of the mass of air sucked into the cylinder.

If a large amount of liquid film is formed on the piston surface, soot is generated. Therefore, the fuel injection speed at this time is desirably within a range where the atomized fuel 9 does not collide with the piston 3. Further, as shown in FIG. 2, it is desirable that the bottom of the cone-shaped spray 9 is parallel to the surface of the piston 3. Fuel 9 injected into cylinder
Is vaporized, and an air-fuel mixture 10 having a high fuel concentration is generated near the surface of the piston 3 as shown in FIG.

On the other hand, in the vicinity of the surface of the piston 3, the flow in the circumferential direction is blocked by the swirl suppressing plate 1 as shown in FIG. 4, and the flow is directed toward the center of the piston.

FIG. 5 is a diagram showing the distribution of the swirl number (angular velocity in the circumferential direction of air / crank angular velocity of the engine) in the height direction at this time. In the vicinity of the piston surface, the swirl number is small due to the flow blocking effect by the swirl suppression plate, and the effect of the swirl suppression plate does not work sufficiently in the upper part of the cylinder far from the piston surface, so the initial swirl number generated by intake Will be maintained.

That is, in the upper part of the cylinder, the centrifugal force due to the swirl acts more strongly than in the vicinity of the piston surface. Due to this difference in centrifugal force, the pressure in the upper part of the cylinder is lower than that in the lower part of the cylinder in the center part of the cylinder, and the pressure in the upper part of the cylinder is higher than the pressure in the lower part of the cylinder at the peripheral part.

As a result, as shown in FIG. 6, a flow F2 from the vicinity of the surface of the piston 3 to the upper part of the cylinder is generated at the center of the cylinder, and a flow F1 from the upper part of the cylinder to the vicinity of the piston surface is generated at the peripheral part of the cylinder. . The mixture 10 having a high fuel concentration near the surface of the piston 3 due to the flow of F1 is collected at the center of the piston 3 and then carried from the top surface of the piston to the top of the cylinder by the flow of F2.

As a result, as shown in FIG. 7, in the latter half of the compression stroke, the mixture 10 having a high fuel concentration is collected near the electrode of the ignition plug 5, and even if the mixture is lean, the fuel is stably ignited. Becomes possible.

In the above embodiment, the plate-shaped swirl suppressing plate 1 is provided to suppress the swirl near the surface of the piston 3. However, as shown in FIG. 8, the swirl suppressing plate 1 has a side surface perpendicular to the swirl direction. The same effect can be obtained with the step structure 1a. In the plate-shaped swirl suppressing plate, as shown in FIG. 9, since a stagnation region 20 of the air flow is formed on the downstream side, there is a possibility that liquid fuel may accumulate in the stagnation region 20. However, in the method in which a step is provided in the circumferential direction as shown in FIG. 8, a stagnation region does not occur, and there is no possibility that fuel is accumulated on the piston surface.

FIG. 10 shows an example in which the mounting angle of the swirl suppressing plate 1 is advanced with respect to the flow direction of the swirl S. When the swirl flow direction and the swirl suppression plate 1 are perpendicular to each other, a part of the flow may be directed to the outer peripheral direction of the piston and the flow on the piston surface may be disturbed. In the method of FIG. Since the colliding flow surely changes its direction toward the center of the piston, turbulence of the flow can be prevented.

FIG. 11 shows a method in which the swirl suppression plates 1 are provided in parallel with each other, and FIG. 12 shows a method in which a swirl suppression plate in the shape of a corrugated plate is provided on the piston surface. It can be attenuated effectively. In addition, as shown in FIG. 13, when the swirl suppressing plate 1 is mounted at right angles to the surface of the piston 3, when the flow velocity of the swirl S is high, or when the height of the swirl suppressing plate 1 is low, the swirl S May get over the swirl suppressing plate 1 and cannot effectively attenuate the swirl.

As shown in FIG. 14, the upper portion of the swirl suppressing plate is bent parallel to the surface of the piston 3, or the swirl suppressing plate 1 is provided at an acute angle to the flow as shown in FIG. Thus, the swirl near the piston surface can be more reliably attenuated.

Next, a second embodiment of the direct injection type spark ignition engine according to the present invention will be described. FIG. 16 shows the shape of the piston surface in the present invention. In the present invention, the secondary swirl generating plate 12 is provided on the surface of the piston 3. As shown in FIG. 17, the secondary swirl generating plate 12 has an arc shape in which an upstream side of a swirl S (hereinafter, referred to as a primary swirl) around a cylinder axis generated in an intake stroke is open, and is formed at a center axis of the piston. On the other hand, the center is provided outside the center.

The position 12L of the secondary swirl generating plate on the cylinder center side is determined so as to substantially coincide with the central axis of the cylinder. In the present invention, intake and fuel injection are performed in exactly the same manner as in the first embodiment.

That is, at the time of low load and medium load operation, the swirl control valve is closed to generate a swirl S around the axis of the cylinder. Subsequently, during one of the intake stroke or the first half of the compression stroke, Fuel is injected from the injection valve 4 toward the surface of the piston 3.

As a result, an air-fuel mixture having a high fuel concentration is generated near the surface of the piston 3. The primary swirl S near the piston surface is blocked by the secondary swirl generating plate 12 and rotates along the inner surface of the secondary swirl generating plate.

As a result, as shown in FIG. 18, inside the secondary swirl generating plate 12, the primary swirl and the center of rotation are different, and a secondary swirl S 'having a small turning radius is generated. Further, the mixture 10 having a high fuel concentration near the piston surface is taken into the secondary swirl generating plate 2 with the flow toward the inside of the secondary swirl generating plate 12. On the other hand, in the upper part of the cylinder 2, the secondary swirl generating plate 12
, The primary swirl S is stored without attenuation.

FIG. 19 shows the pressure on the cylinder center axis. In the upper part of the cylinder, the pressure at the center of the cylinder decreases due to the centrifugal force of the primary swirl around the cylinder center axis. On the other hand, near the surface of the piston, the rotation center axis of the secondary swirl is shifted outward from the center axis of the cylinder, so that the pressure at the center of the cylinder increases.

As a result, as shown in FIG. 20, near the center of the cylinder, an upward flow F2 is generated from the vicinity of the surface of the piston 3 toward the upper part of the cylinder 2. As a result, the fuel-air mixture 1 with a high fuel concentration taken in the secondary swirl generating plate 12.
0 rises to the vicinity of the electrode of the ignition plug 5, and the lean air-fuel mixture can be stably burned.

The same effect can be obtained even if the secondary swirl generating plate 12 is a straight plate extending in the radial direction from the center of the piston as shown in FIG. Also, as shown in FIG. 22, a secondary swirl can be effectively generated even in a shape in which only the vicinity of the center of the piston is bent.

[0044]

The effects of the present invention will be described using the results of numerical simulations. FIG. 24 shows the shape of the engine on which the numerical simulation was performed. Bore diameter 80mm,
Assuming an engine having a cylindrical cylinder having a height of 80 mm, four swirl suppressing plates having a height of 5 mm are provided radially on the surface of the piston.

[0045]

[Table 1]

Table 1 shows the calculation conditions. The calculation was performed by a method of spatially discretizing each of the conservation formulas of the mass, momentum and energy of the air-fuel mixture and the conservation formula of the fuel vapor by the finite volume method, and integrating over time by the FLIC method. The average air-fuel ratio was 40, the engine speed was 2000 rpm, and the charging efficiency was 100%.

The calculation was performed from the start point (bottom dead center) of the compression stroke to the end point (top dead center) of the compression stroke, and a rigid vortex having an initial swirl number of 2 was uniformly applied to the cylinder. In the initial fuel concentration distribution, as shown in FIG. 25, the lower 1/3 of the cylinder was a mixture having a uniform vapor concentration, and the upper 2/3 of the cylinder was only air.

FIG. 26 shows contour lines of the circumferential velocity of the air-fuel mixture in the longitudinal section at 160 ° CA before the top dead center. In the vicinity of the piston surface, the speed in the circumferential direction is rapidly attenuated by the swirl suppressing plate.

On the other hand, in the upper part of the cylinder, there is almost no influence of the swirl suppressing plate, and the initial swirl speed is maintained.
The swirl suppressing plate confirms that only the swirl near the surface of the piston is decelerating.

FIG. 27 shows isolines of the rising speed of the air-fuel mixture in the longitudinal section. It can be seen that, due to the pressure gradient caused by the difference in swirl strength, the ascending speed in the vicinity of the center axis of the cylinder is higher than in the peripheral portion.

FIGS. 28 (a) to 28 (c) show contour lines of the fuel-air ratio (fuel vapor mass / air mass) of the air-fuel mixture in the longitudinal section. The mixture that has a high concentration on the piston surface rises to the top of the cylinder due to the upward flow in the center of the cylinder, and the mixture near the piston surface flows from the outer periphery to the center of the piston near the piston surface. It can be confirmed that the air gathers toward the center of the cylinder. At 20 ° CA before top dead center near the ignition timing, the stoichiometric fuel-air ratio is almost at the center of the cylinder.
It can be seen that the mixture of (0.067) is collected, and the mixture has good ignitability.

According to the present invention, it is possible to stably supply an air-fuel mixture having good ignitability to the electrode portion of the ignition plug during low-load and medium-load operation, thereby improving the combustion stability during lean burn. it can. Also, in the present invention, since fuel can be injected at the beginning of the intake stroke or the compression stroke, a high-pressure fuel pump and a fuel injection valve for high fuel pressure, which are indispensable in the conventional method, become unnecessary, and stratified combustion is intended at low cost. In-cylinder injection spark ignition engine can be provided.

[Brief description of the drawings]

FIG. 1 is a perspective view of a direct injection type spark ignition engine showing a first embodiment of the present invention.

FIG. 2 is a longitudinal sectional view of the direct injection type spark ignition engine showing the first embodiment of the present invention.

FIG. 3 is a longitudinal sectional view of the direct injection type spark ignition engine showing the first embodiment of the present invention.

FIG. 4 is a plan view of a direct injection type spark ignition engine showing the first embodiment of the present invention.

FIG. 5 is a characteristic diagram of a swirl number distribution on a central axis in the first embodiment of the present invention.

FIG. 6 is a longitudinal sectional view of a direct injection type spark ignition engine showing a first embodiment of the present invention.

FIG. 7 is a longitudinal sectional view of a direct injection type spark ignition engine showing a first embodiment of the present invention.

FIG. 8 is a perspective view showing an alternative example of the piston in the first embodiment of the present invention.

FIG. 9 is a view showing a flow near the piston surface in the first embodiment of the present invention.

FIG. 10 is a plan view showing a modification of the piston in the first embodiment of the present invention.

FIG. 11 is a perspective view showing a modification of the piston in the first embodiment of the present invention.

FIG. 12 is a perspective view showing a modification of the piston in the first embodiment of the present invention.

FIG. 13 is a diagram showing a flow near the piston surface in the first embodiment of the present invention.

FIG. 14 is a diagram showing a flow near the piston surface in the first embodiment of the present invention.

FIG. 15 is a diagram showing a flow near the piston surface in the first embodiment of the present invention.

FIG. 16 is a perspective view of a piston shape of a direct injection type spark ignition engine showing a second embodiment of the present invention.

FIG. 17 is a plan view of a direct injection type spark ignition engine showing a second embodiment of the present invention.

FIG. 18 is a view showing an air flow in a second embodiment of the present invention.

FIG. 19 is a characteristic diagram showing a pressure distribution on a central axis in the second embodiment of the present invention.

FIG. 20 is a longitudinal sectional view of a direct injection type spark ignition engine showing a second embodiment of the present invention.

FIG. 21 is a plan view showing a modification of the piston according to the second embodiment of the present invention.

FIG. 22 is a plan view showing a modification of the piston according to the second embodiment of the present invention.

FIG. 23 is a schematic perspective view of a conventional direct injection type spark ignition engine.

FIG. 24 is a perspective view of a piston according to a calculation system of a numerical simulation.

FIG. 25 is a diagram showing a ratio between air and air-fuel mixture according to calculation conditions of a numerical simulation.

FIG. 26 is a contour diagram of circumferential velocity in a numerical simulation.

FIG. 27 is an upward velocity contour diagram obtained by numerical simulation.

FIGS. 28A to 28C are fuel-air ratio contour diagrams obtained by numerical simulation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Swirl suppressing plate, 2 ... Cylinder, 3 ... Piston, 4 ... Fuel injection valve, 5 ... Spark plug, 16 ... Swirl control valve, S ... Swirl.

Continued on the front page (72) Inventor Yoko Nakayama 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi Research Laboratory, Hitachi, Ltd. (72) Ryuhei Kawabe 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture No. Within Hitachi Research Laboratory, Hitachi, Ltd.

Claims (6)

[Claims] 1. An ignition plug is disposed substantially at the center of the upper part of a cylinder.
In a direct injection type spark ignition engine having a fuel injection valve provided at an upper part of a cylinder and having means for generating a swirling flow around the central axis of the cylinder during the intake and compression strokes, a swirl flow around the central axis of the cylinder is provided at a piston top surface. A structure to reduce the speed of
A cylinder injection type spark ignition engine characterized by comprising means for injecting fuel from the fuel injection valve toward the piston top surface at least in the first half of the intake stroke or the compression stroke during low-load or medium-load operation. . 2. The method according to claim 1, wherein the means for reducing the swirl flow on the top surface of the piston is approximately 90 degrees relative to the flow direction of the swirl flow.
An in-cylinder injection spark ignition engine, characterized in that the projection has a surface forming an angle of °. 3. The in-cylinder injection type spark according to claim 1, wherein the means for decelerating the swirling flow on the top surface of the piston is a projection radially extending from near the center of the piston toward the periphery of the piston. Ignition engine. 4. The in-cylinder injection spark ignition engine according to claim 1, wherein the means for decelerating the swirling flow on the piston top surface is a corrugated projection. 5. An ignition plug is disposed substantially at the center of the upper part of the cylinder,
In a cylinder injection type spark ignition engine provided with a fuel injection valve at the top of the cylinder and having a means for generating a swirling flow around the central axis of the cylinder in the intake and compression strokes, from the periphery of the piston top surface toward the center, A structure for blocking the swirling flow around the cylinder center axis is provided, and an end of the blockage structure is positioned near the cylinder center axis, and at least in the first half of the intake stroke or the compression stroke during low-load or medium-load operation, An in-cylinder injection spark ignition engine, comprising means for injecting fuel from a fuel injection valve toward a piston top surface. 6. The in-cylinder injection spark ignition according to claim 5, wherein the structure for blocking the swirl flow near the piston surface has a concave curvature when viewed from the upstream side of the swirl flow around the cylinder axis. organ.