JPH10131804A - Structure of piston with auxiliary chamber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of stress concentration with a guide groove and prevent the expansion of jet flow to the width direction of the groove by deviationwise guiding jet flow comming from an auxiliary chamber through a communication hole by use of the groove. SOLUTION: Jet flow E including fire flame jetting out from an auxiliary chamber in an auxiliary chamber main body 16 thorough a communication hole 21 is guided by an arc-shaped guide groove 25. The groove 25 is formed of part of cylindrical face whose arc shape is moderate rather that that of the hole 21 so that generation of stress at the groove 25 is prevent. The center line A-A of the cylindrical face of the hole 21 is offset from the center line B-B of the cylindrical face of the groove 25, and thereby the jet flow E is guided deviationwise to one of surfaces of the groove 25 and extends to a peripheral part 24 of a piston heat without its expansion in the width direction of the groove 25.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piston with a sub-chamber in which a sub-chamber main body having a sub-combustion chamber (hereinafter referred to as "sub-chamber") is provided in a cavity of the piston main body.

[0002]

2. Description of the Related Art Conventionally, a main combustion chamber (hereinafter referred to as a main combustion chamber) has been used as a combustion chamber.
A sub-chamber engine having a main chamber) and a sub-chamber communicating with the main chamber through a communication hole is known. In the sub-chamber, fuel is injected from the fuel injection nozzle, and the injected fuel is ignited. As a sub-chamber engine,
There is a diesel engine that injects a liquid fuel such as alcohol fuel or light oil from a fuel injection nozzle into a sub-chamber and burns the fuel. For diesel engines, Fukushitsushiki engine, the thermal efficiency is inferior to the direct-injection diesel engine, has the advantage that the amount of the NO _X is reduced. For example, Japanese Patent Application No. Hei 6
As disclosed in JP-A-257441 and Japanese Patent Application No. 7-4248, a sub-chamber is provided on the piston side.

[0003] A piston having a sub-chamber has a plurality of communication holes for communication between a sub-chamber provided in a cavity at the center of the piston and a main chamber formed on the cylinder side for the purpose of promoting combustion. The communication hole is radially formed in the sub-chamber main body so as to be circumferentially spaced from the center of the sub-chamber. The piston has a plurality of guide grooves radially formed in the piston head portion of the piston body corresponding to the opening of the communication hole on the main chamber side so as to extend the communication hole in addition to the communication hole.

[0004]

Such a piston with a sub-chamber has a tensile stress on the top surface of the piston due to a high combustion gas pressure generated in the main chamber and a residual stress generated in the piston in an engine operating state. Is generated. That is, most of the force acting on the piston based on the pressure of the combustion gas in the main chamber is ultimately supported by the piston pin, but the deformation of the piston at this time, particularly the deformation of the piston head, is caused by the piston pin. In the diametrical region of the piston head portion directly above the position, the bending hardly occurs, but the bending deformation becomes larger as the region is further away from the diametrical region. In response to such bending deformation of the piston head, a tensile stress is applied to the top surface of the piston,
Then, a compressive stress is generated inside the piston. The value of the tensile stress is greatest at the piston top surface.

On the other hand, in a conventional piston with a sub-chamber, the curvature of the guide groove is the same as that of the communication hole when viewed in cross section, so that the tensile stress acting in the top surface of the piston is curved. When acting on a portion of the guide groove having a large degree (i.e., curvature), stress concentration may occur such that the guide groove is torn along the groove bottom. When the piston with the sub chamber has the same degree of curvature of the communication hole and the guide groove, and the direction of jet of the jet from the communication hole coincides with the direction of the guide center line of the guide groove, the groove bottom of the guide groove is Since it is the portion where the stress concentration of the tensile stress is most likely to occur, it is the most severe portion in terms of stress. Although the main body of the piston is made of aluminum alloy, the sub-chamber main body is made of ceramic or heat-resistant alloy, so that the sub-chamber main body is thermally treated,
The guide groove, especially its bottom, becomes hot because the flame ejected through the communication hole is directly blown, and becomes the severest part in terms of heat. As described above, the groove bottom of the guide groove is in a complex state in which it is exposed to a high-temperature flame and easily subjected to stress concentration, and as a result, cracks are likely to occur. Regarding how to prevent cracks that easily occur at the groove bottom of the guide groove, including the piston with a sub-chamber described in each of the above-mentioned publications, the conventional one has a relationship between the communication hole and the guide groove. Is not taken into account.

On the other hand, by expanding the guide groove to the left and right of the central axis of the communication hole to make the curve of the guide groove gentle, the stress concentration coefficient of the tensile stress in the guide groove portion is reduced, and the It is also conceivable to reduce the temperature effect on the guide groove by lowering the jet temperature by jetting the jet through the communication hole widely, but with such a measure, the jet will spread too much in the guide groove,
On the contrary, it is difficult for the flame to reach the peripheral portion of the piston head portion, and the combustion efficiency may be reduced.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a piston body having a cavity at the center of the piston, a sub-chamber body provided in the cavity and forming a sub-chamber therein, and the sub-chamber and the cylinder side. A plurality of communication holes formed in the main body of the sub-chamber radially spaced from the center of the sub-chamber so as to communicate with the main chamber formed in the sub-chamber, and the main chamber of the communication holes In the structure of the sub chamber-equipped piston having a plurality of arc-shaped guide grooves radially formed in the piston head portion of the piston body corresponding to the opening on the side of the piston, the groove surface of the guide groove is larger than the curvature of the hole of the communication hole. By relaxing the curvature of the guide groove, stress concentration in the guide groove can be avoided, and the jet flow spouting from the sub chamber through the communication hole is deviated from the guide center line of the guide groove so that the jet flow is It is to provide a structure of a sub-chamber with a piston which can be prevented, placed in too spread to prevent a reduction in the combustion efficiency of the engine.

The present invention has the following configuration to achieve the above object. That is, the present invention provides a piston main body having a cavity at the center of the piston, a sub-chamber main body provided in the cavity and constituting a sub-chamber therein, and communicating the sub-chamber with a main chamber formed on the cylinder side. A plurality of communication holes radially formed in the main body of the sub-chamber so as to be circumferentially spaced from the center of the sub-chamber, and corresponding to the opening of the communication hole on the main chamber side. The piston head portion of the piston body includes a plurality of arcuate guide grooves formed radially, and the cross-sectional area of the arc-shaped guide groove on the opening side is smaller than the cross-sectional area of a portion of the communication hole corresponding to the arc-shaped guide groove. And a center line of the communication hole is offset in a circumferential direction of the sub-chamber main body with respect to a guide center line of the guide groove.

The structure of the piston with a sub-chamber according to the present invention is configured as described above and operates as follows. That is, according to the structure of the piston with the sub chamber, the cross-sectional area of the arc-shaped guide groove on the opening side is larger than the cross-sectional area of the portion corresponding to the arc-shaped guide groove of the communication hole, and the center line of the communication hole is formed in an arc shape. Since the sub-chamber main body is offset in the circumferential direction with respect to the guide center line of the guide groove, the groove wall surface of the arc-shaped guide groove has a gentler curved surface than the hole surface of the communication hole based on this area relationship, and the tensile stress , Stress concentration hardly occurs in the arc-shaped guide groove. Moreover, since the center line of the communication hole is offset in the circumferential direction of the sub-chamber main body with respect to the guide center line of the arc-shaped guide groove, the portion where the flame contained in the jet heats the arc-shaped guide groove most strongly is the guide groove. From the center of the groove. Further, the jet flows out while being restricted by one slope of the arcuate guide groove, and does not spread in the width direction of the arcuate guide groove. Therefore, the structure of the piston with the sub-chamber according to the present invention makes it possible to reduce the stress and thermal load on the arcuate guide groove.
Here, the guide center line of the arc-shaped guide groove flows most stably with respect to the arc surface, as in the case where the jet is jetted with its center placed directly above the groove bottom with respect to the arc-shaped guide groove. It is the center line of the flow of time. For example, when the arc-shaped guide groove is an arc-shaped groove, the guide center line exists in a left-right symmetric plane of the groove where a line that follows the curvature center line, area center, or groove bottom of the groove exists.

When the jet is ejected with its center corresponding to the groove bottom of the arc-shaped guide groove, the arc-shaped guide groove expands right and left around the groove bottom. However, according to the piston structure with the sub-chamber of the present invention, since the jet from the sub-chamber through the communication hole is jetted deviated from the guide center line of the arc-shaped guide groove, the jet flows along the slope of the arc-shaped guide groove. There is little flow over and the jet does not immediately descend the slope due to its momentum. The jet quickly reaches the periphery of the air-rich piston head before its width widens.

As a structure for biasing the jet flow spouted from the sub-chamber through the communication hole with respect to the guide center line of the arc-shaped guide groove, the center line of the communication hole is aligned with the direction of the guide center line of the arc-shaped guide groove. A structure is adopted in which the body is offset parallel or at an angle to the circumferential direction of the main body. With such an offset structure, the jet flows out along the slope of the arc-shaped guide groove, and may spread to the left and right with respect to the groove center as seen when the jet is jetted around the groove bottom of the arc-shaped guide groove. Absent.

As another structure for deflecting the jet flow spouted from the sub chamber through the communication hole with respect to the guide center line of the arc-shaped guide groove, the center line of the communication hole is swirled with respect to the guide center line of the arc-shaped guide groove. A structure located downstream of the flow is employed. With such a structure, the intake swirl flow flowing in the circumferential direction of the sub-chamber main body acts to blow the jet toward one slope of the arc-shaped guide groove.
The tendency to flow along the slope is strengthened, and it does not spread too much left and right with respect to the groove center.

In the sub chamber main body, a central communication hole is formed near the top dead center of the piston and into which a fuel injection nozzle provided in the cylinder head can enter. Since the fuel injection nozzle provided in the cylinder head can protrude into the central communication hole formed in the sub-chamber main body near the top dead center of the piston, the fuel injection nozzle is completely fitted into the central communication hole, or The fuel is injected into the sub-chamber in a state of covering so that the jet does not flow out from the central communication hole. The injected fuel ignites and squirts from the sub chamber into the main chamber through the communication hole.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the structure of a piston with a sub-chamber according to the present invention will be described below with reference to the drawings.
FIG. 1 is a cross-sectional view showing an example of an engine to which a piston with a sub-chamber is applied, FIG. 2 is a plan view showing a part of the embodiment of the piston with a sub-chamber according to the present invention used in FIG.
FIG. 3 is a line XX of the structure of the piston with the sub-chamber shown in FIG.
FIG. 4 is a cross-sectional view showing a part of the piston when it is cut vertically, and FIG. 4 is a partial view of the structure of the piston with a sub-chamber shown in FIG.

As shown in FIG. 1, an engine to which a piston with a sub-chamber is applied is fixed to a cylinder block 1 made of a metal material such as aluminum, an aluminum alloy, or cast iron with a gasket 3 interposed therebetween. It has a cylinder head 2 made of a metal material such as an aluminum alloy. Holes corresponding to the number of cylinders of the engine are formed in the cylinder block 1, and a cylinder liner forming the cylinder 4 is fitted in the hole. Cylinder 4
, A piston 5 is reciprocated via a piston ring 6. In addition, cylinder head 2
, An intake port 7 and an exhaust port 8 are formed corresponding to each cylinder 4, an intake valve 9 is arranged in a valve seat of the intake port 7, and an exhaust valve 10 is arranged in a valve seat of the exhaust port 8. ing.

In the cylinder 4, the cylinder head 2
A main chamber 13 as a main combustion chamber is formed between the lower surface 11 of the piston 5 and the top surface 12 of the piston 5. The fuel injection nozzle 14 provided in the cylinder head 2 has a multi-injection hole 15 at the tip. A sub-chamber main body 16 made of ceramic or heat-resistant alloy is provided in a central cavity 18 of the piston 5 by, for example, casting. The upper part of the sub-chamber main body 16 is cast so as to be exposed above the piston head 19 of the aluminum alloy piston 5 so as to face the main chamber 13. A sub-chamber 17 is formed inside the sub-chamber main body 16.

The main chamber 13 and the sub-chamber 17 communicate with each other at the upper center of the sub-chamber main body 16 so that the fuel injection nozzle 14 can enter when near the top dead center of the compression stroke of the piston 5. A central communication hole 20 having a diameter slightly larger than the outer diameter of the fuel injection nozzle 14 is formed. Fuel injection nozzle 1
The fuel injection nozzle 4 can inject fuel into the sub-chamber 17 from the multiple injection holes 15 provided at the tip of the fuel injection nozzle 14 when it enters the central communication hole 20 or reaches a position where the central communication hole 20 is sufficiently closed. . In order to mix the flame ignited in the sub chamber 17 and the unburned air-fuel mixture with air in the main chamber to perform lean combustion, a plurality of communication holes 21 communicating the main chamber 13 and the sub chamber 17 are provided. It is formed radially at a distance from the center of the chamber 17 in the circumferential direction. The communication hole 21 is formed to be inclined toward the cylinder head 2 from the opening 22 on the sub-chamber 17 side to the opening 23 on the main chamber side.

According to the piston with the sub-chamber, air flows obliquely into the sub-chamber 17 from the main chamber 13 through the communication hole 21 in the compression stroke of the engine to form an air flow in the sub-chamber 17, Fuel is injected from the multiple injection holes 15 of the fuel injection nozzle 14 during the air flow. Then, the mixture of the air and the fuel spray is performed in a short time, and the fuel is ignited and burned. At this time, the formation of the communication hole 21 is set so that the air flow generated in the sub chamber 17 becomes an optimum value. The ignited fuel is jetted into the main chamber 13 through the communication hole 21.

The piston 5 is extended so as to extend the communication hole 21.
The opening 23 on the side of the main chamber 13 of the communication hole 21
An arc-shaped guide groove 25 is formed in correspondence with. The arc-shaped guide groove 25 is formed by a part of a cylindrical surface as described later. In this example, the arc-shaped guide groove 25 is formed so as to be shallower toward the outside in the radial direction. The flame ejected from the communication hole 21 is guided by the arc-shaped guide groove 25 and spreads to the main chamber 13. That is, when the piston 5 is located at the top dead center, the communication hole 21 has a small size between the lower surface 11 of the cylinder head 2 and the top surface 12 of the piston 5, despite the small clearance. Effective area can be secured. That is, the arc-shaped guide grooves 25 formed on the top surface 12 of the piston 5 allow the jet gas such as the flame and the unburned mixture injected from the sub chamber 17 to the main chamber 13 through the communication hole 21 to the peripheral portion of the cylinder 4, that is, It is guided to the space above the peripheral portion 24 of the piston head portion 19, and the combustion in the main chamber 13 can be performed from the peripheral portion of the cylinder 4 having a large amount of air.
The fresh air remaining in 3 can be used effectively, and the air utilization rate can be increased.

The plan view of the piston shown in FIG. 2 shows only one of the plurality of communication holes 21 for simplicity. In the structure of this sub-chamber piston,
The center line AA of the communication hole 21 radiates obliquely upward toward the cylinder head 2 with respect to a plane perpendicular to the plane of the drawing including the line XX in FIG. Extending in the direction. On the other hand, the arc-shaped guide groove 25 is
Although parallel to the center line A-A, in FIG.
2 with respect to a plane perpendicular to the plane of the paper including X,
That is, it is formed as a part of a cylindrical surface centered on a line BB in a plane offset in the circumferential direction with respect to the center of the sub chamber 17. The arc-shaped guide groove 25 is usually formed as a part of a cylindrical surface as in the illustrated example.
The cross-sectional shape is not limited to an arc.

As shown in FIG. 4, the arrangement relationship between the communication hole 21 and the arcuate guide groove 25 is such that the cross-sectional area S of the arcuate guide groove 25 on the opening 23 side, that is, the cutting of the groove portion of the arcuate guide groove 25. The area S is set so as to be larger than the cross-sectional area T of a portion corresponding to the arc-shaped guide groove 25 of the communication hole 21, that is, a portion sinking into the arc-shaped guide groove 25. The radius of the cylindrical wall forming the groove of the arc-shaped guide groove 25 is set to be larger than the radius of the hole of the communication hole 21. Further, the center line AA of the communication hole 21 is offset with respect to the center line BB of the cylindrical surface of the arc-shaped guide groove 25. As can be understood from the description of FIG. 2, the arc-shaped guide groove 25 is inclined upward toward the cylinder head 2, and thus extends from the center of the sub chamber 17 to the peripheral portion 24 of the piston head 19. It extends as a tongue-shaped groove so that the width becomes narrower.

Therefore, the groove surface of the arc-shaped guide groove 25 is
For example, a curved surface is gentler than that formed as an arc-shaped guide groove having the same curvature as the curvature of the hole of the communication hole 21, and a stress is applied to the arc-shaped guide groove 25 due to a tensile stress generated on the top surface 12 of the piston 5. The potential for concentration is reduced. Further, as shown in FIG. 2, a jet E of a flame or the like spouting from the communication hole 21 is indicated by an imaginary line in FIG.
Flows on one side (upper side in FIG. 2)
Spreading of the jet to the left and right is suppressed. In addition, the communication hole 21
4 strongly heats the portion 26 closest to the communication hole 21 as shown in FIG. 4, but heats the groove bottom 27 of the arcuate guide groove 25 which is placed under the most severe stress condition to the highest temperature. Therefore, the possibility of causing damage such as cracks in the arc-shaped guide groove 25 is greatly reduced.

In the main chamber 13, the air sucked into the main chamber 13 through the intake port 7 when the intake valve 9 is opened may be swirled as intake swirl. When the intake swirl is swirling in the direction indicated by the arrow G, the jet direction of the jet E is offset with respect to the center line BB of the cylindrical surface of the arc-shaped guide groove 25 as shown in FIG. Is set to a direction that matches the turning direction G of the intake swirl. With this selection, the jet E is more likely to be shifted toward one of the slopes of the arc-shaped guide groove 25, and the jet E is less likely to spread left and right. As a result, the flame in the jet E easily reaches the peripheral portion of the cylinder 4, that is, the peripheral portion of the main chamber 13, and the combustion efficiency is improved.

Next, another embodiment of the structure of the sub chamber-provided piston according to the present invention will be described with reference to FIG. FIG. 5 is a plan view of the piston similar to that of FIG. 2, and for simplification, only one of the plurality of communication holes 21 is shown.
The same reference numerals are given to the same structure as the structure of the piston in FIG. 2, and the description will not be repeated.

In the structure of the piston with the sub-chamber shown in FIG. 5, the center line AA of the communication hole 21 is perpendicular to the plane including the line XX in FIG. It extends in a plane, that is, radially with respect to the center of the sub-chamber 17. On the other hand, the arc-shaped guide groove 28 is formed by a part of a cylindrical surface centered on the line CC like the arc-shaped guide groove 25 in the previous embodiment, and is similar to the cylinder head like the communication hole 21. It extends obliquely upward toward two sides. The position of the opening 23 where the communication hole 21 opens with respect to the arc-shaped guide groove 28 is deviated from the groove bottom position as in the case of the arc-shaped guide groove 25 shown in FIG. The center line C-C of the cylindrical surface 28
Is not parallel to the center line AA of FIG.
It is turned at an angle θ in a direction approaching the center line AA as the distance from the opening 23 increases. That is, the arc-shaped guide grooves 28 are arranged not only in parallel offset but also angularly offset with respect to the center line AA of the communication hole 21. The area relationship between the communication hole 21 and the arc-shaped guide groove 28 on the opening 23 side is the same as the relationship shown in FIG.

The groove surface of the arc-shaped guide groove 28 is formed as a gentle curved surface, similarly to the arc-shaped guide groove 25, and the possibility of stress concentration is reduced. Furthermore, as shown in FIG. 5, a jet F of a flame or the like spouting from the communication hole 21 is traversed by one side slope of the arcuate guide groove 28 at the beginning of the spout, as indicated by the imaginary line in FIG. 5 and indicated by hatching. Rather, it flows along one side of the slope so as to be an extension of the jet. As the jet F travels along the arc-shaped guide groove 28 toward the peripheral portion 24 of the piston head 19, the jet F also flows through the groove bottom 29. Therefore,
The jet flows without changing its width very much, and the spread of the jet to the left and right is suppressed.

[0027]

The structure of the piston with sub-chamber according to the present invention is constructed as described above, and has the following effects. That is, according to the structure of the piston with the sub-chamber according to the present invention, the cross-sectional area of the arc-shaped guide groove on the opening side is larger than the cross-sectional area of the portion corresponding to the arc-shaped guide groove of the communication hole, and the center line of the communication hole is formed. Are offset in the circumferential direction of the sub-chamber main body with respect to the guide center line of the arc-shaped guide groove, so that stress concentration hardly occurs in the arc-shaped guide groove even when a tensile stress acts. Moreover, since the center line of the communication hole is offset in the circumferential direction of the sub-chamber main body with respect to the guide center line of the arc-shaped guide groove, the portion where the flame contained in the jet is heated most strongly by the arc-shaped guide groove is as follows. , From the center of the guide groove. Therefore, the structure of the piston with the sub-chamber according to the present invention makes it possible to reduce the stress and the thermal load on the groove bottom of the arc-shaped guide groove, and it is possible to prevent the guide groove from being cracked.

Further, in the structure of the piston with the sub-chamber according to the present invention, the jet from the sub-chamber through the communication hole is jetted deviated from the guide center line of the arc-shaped guide groove. It is guided on one of the arc-shaped slopes without widening into the groove, so that the penetration can be extended to the outer periphery of the cylinder containing a large amount of intake air. Therefore, the air utilization rate in the main chamber is improved, and the mixing of the jet and the fresh air in the main chamber is promoted in a short time, the combustion speed is increased, the combustion period is shortened, the thermal efficiency is improved, and the output is improved. to reduce fuel consumption by, NO _X and H
The generation of particulates such as carbon, carbon, and smoke can be suppressed.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an engine to which a piston with a sub-chamber according to the present invention is applied.

FIG. 2 is an enlarged view showing a part of a piston top surface of an embodiment of a piston with a sub-chamber according to the present invention.

3 is a partial sectional view of a plane passing through line XX of the structure of the piston with a sub-chamber in FIG. 2;

4 is a partial view of the structure of the piston with a sub-chamber as viewed from the line YY in FIG. 3;

FIG. 5 is an enlarged view showing a part of a piston top surface of another embodiment of the structure of the piston with a sub-chamber according to the present invention.

[Explanation of symbols]

 2 Cylinder head 4 Cylinder 5 Piston 13 Main chamber 14 Fuel injection nozzle 16 Sub chamber main body 17 Sub chamber 18 Cavity 19 Piston head section 20 Central communication hole 21 Communication hole 22 Opening 23 Opening 25 Arc guide groove 28 Arc guide groove A- A center line of communication hole BB center line of cylindrical surface CC center line of cylindrical surface E jet F jet G intake swirl S cross-sectional area (arc-shaped guide groove) T cross-sectional area (corresponding to the arc-shaped guide groove of communication hole) part)

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F02B 23/06 F02B 23/06 L F02M 61/14 310 F02M 61/14 310P F16J 1/09 F16J 1/09

Claims (4)

[Claims] 1. A piston main body having a cavity in the center of a piston, a sub-chamber main body provided in the cavity and constituting a sub-chamber therein, and communicating the sub-chamber with a main chamber formed on a cylinder side. Therefore, a plurality of communication holes radially formed in the sub-chamber main body and circumferentially spaced from the center of the sub-chamber, and the piston corresponding to the opening of the communication hole on the main chamber side. The piston head portion of the main body includes a plurality of arc-shaped guide grooves formed radially, and a cross-sectional area of the arc-shaped guide groove on the opening side is larger than a cross-sectional area of a portion of the communication hole corresponding to the arc-shaped guide groove. Large, and the center line of the communication hole is offset in the circumferential direction of the sub-chamber main body with respect to the guide center line of the guide groove,
Structure of piston with subchamber consisting of 2. The guide center line of the guide groove is offset parallel to or at an angle to the circumferential direction of the sub-chamber main body with respect to the center line of the communication hole. Item 2. The structure of the piston with a sub-chamber according to Item 1. 3. The auxiliary according to claim 1, wherein the center line of the communication hole is located downstream of the intake swirl flow with respect to the guide center line of the guide groove. Piston structure with chamber. 4. The sub-chamber main body is provided with a central communication hole near a piston top dead center, into which a fuel injection nozzle provided in a cylinder head can enter. The structure of the piston with a sub-chamber according to any one of the preceding claims.