JP2023034875A - engine piston structure - Google Patents

Abstract

To achieve both securing of piston strength and a reduction in a fuel loss due to unburnt mixed gas in a combustion chamber during compressed ignition.SOLUTION: A piston 20 comprises a crest face 21a compartmentalizing a combustion chamber 8 and a crown section 21 including a rear face 21b opposite to the crest face. The crown section has a first hollow space 26 and a second hollow space 27 which are compartmentalized from each other with the first hollow space positioned at a center side and the second hollow space positioned at an outer side of the first hollow space. The first and second hollow spaces respectively include ceiling faces 26a and 27a at a side of the combustion chamber and bottom faces 26b and 27b at an opposite side of the combustion chamber. The bottom face of the first hollow space has an introduction hole 30 introducing oil injected through an oil jet 15 toward the rear face into the first hollow space and a discharge hole 31 to discharge the oil. A low heat conductive member 34 having a heat conductivity lower than a base material of the piston is placed between the ceiling face and the crest face of the second hollow space.SELECTED DRAWING: Figure 2

Description

The present disclosure relates to engine piston structures.

Since the piston of the engine is in contact with the hot combustion chamber, the temperature is likely to rise. For this reason, a configuration is known in which a cooling channel is provided inside the piston and the piston is cooled by introducing oil injected from an oil jet into the cooling channel.

For example, the piston of the engine disclosed in US Pat. The crown portion has a crown surface that defines the combustion chamber and a back surface opposite to the crown surface. The cooling channel is formed inside the crown portion, ie, between the crown surface and the back surface, in a C-ring shape along the outer circumference of the crown portion.

JP 2019-052628 A

By the way, as an engine combustion method, there is CI (Compression Ignition) combustion. CI combustion is combustion initiated by compression self-ignition of the air-fuel mixture in the combustion chamber. The inventors of the present application investigated the starting position of the compressed self-ignition of the air-fuel mixture in the combustion chamber, and discovered that the compressed self-ignition of the air-fuel mixture starts on the central side of the combustion chamber rather than on the outer peripheral side.

In particular, when the piston according to Patent Document 1 is used, the temperature of the air-fuel mixture on the outer peripheral side of the combustion chamber tends to be deprived of heat by the oil flowing through the cooling channel, and the temperature of the air-fuel mixture tends to drop. For this reason, the temperature of the air-fuel mixture on the central side of the combustion chamber becomes relatively higher than the temperature of the air-fuel mixture on the outer peripheral side, and the air-fuel mixture is compressed more significantly than when a piston without cooling channels is used. Self-ignition is more likely to start on the central side of the combustion chamber.

When the compression self-ignition of the air-fuel mixture starts at the center side of the combustion chamber, the air-fuel mixture on the outer peripheral side of the combustion chamber becomes difficult to burn, resulting in an increase in unburned loss.

Further, when the cooling channel is provided inside the crown portion of the piston, the rigidity of the piston decreases, making it difficult to maintain the strength of the piston.

The present disclosure has been made in view of such points, and an object thereof is to achieve both maintenance of piston strength and reduction of unburned air-fuel mixture loss in the combustion chamber during compression ignition.

A piston structure for an engine according to a first disclosure includes a piston having a crown portion including a crown surface that defines a combustion chamber and a back surface opposite to the crown surface, and an oil jet that injects oil toward the back surface. wherein the interior of the crown portion is provided with a first cavity located on the central side and a second cavity located on the outer peripheral side of the first cavity, which are separated from each other. The first cavity and the second cavity each include a ceiling surface on the side of the combustion chamber and a bottom surface on the side opposite to the combustion chamber, and the bottom surface of the first cavity contains the oil. An introduction hole for introducing the oil injected from the jet toward the back surface into the first cavity and a discharge hole for discharging the oil are provided, and the ceiling surface and the crown of the second cavity are provided. A low heat conductive member having a lower heat conductivity than the base material of the piston is provided between the surface and the base material.

According to such a configuration, the oil jetted from the oil jet toward the rear surface of the crown portion is introduced into the central first cavity through the introduction hole. Oil is discharged to the outside of the crown portion through a discharge hole provided in the bottom surface of the first cavity. The air-fuel mixture on the central side of the combustion chamber loses heat by the oil flowing through the first cavity, and tends to drop in temperature. On the other hand, the temperature of the air-fuel mixture on the outer peripheral side of the combustion chamber is shielded by the air layer formed in the second cavity, making it difficult for the temperature to drop. As a result, the temperature of the air-fuel mixture on the outer peripheral side of the combustion chamber becomes higher than the temperature of the air-fuel mixture on the central side of the combustion chamber.

Therefore, at the time of compression ignition, compression self-ignition of the air-fuel mixture is first started on the outer peripheral side of the combustion chamber, and then compression self-ignition of the air-fuel mixture occurs on the central side of the combustion chamber. This reduces the unburned loss of the air-fuel mixture in the combustion chamber during compression ignition.

Here, if the heat of the air-fuel mixture on the outer peripheral side of the combustion chamber is simply to be shielded, the second cavity may be simply enlarged, but this is not preferable from the viewpoint of piston strength. On the other hand, if the second cavity is made small, it becomes difficult to shield the air-fuel mixture on the outer peripheral side of the combustion chamber from heat. By providing the low thermal conductivity member between the ceiling surface and the crown surface of the second cavity, the low thermal conductivity member functions as a rigid member compared to the case where the second cavity is simply enlarged, so the strength of the piston can be maintained. can be planned. Furthermore, compared to the case where the second cavity is made small, the temperature of the air-fuel mixture on the outer peripheral side in the combustion chamber can be raised by the low heat-conducting member and the air layer.

As described above, it is possible to maintain the strength of the piston and reduce the unburned loss of the air-fuel mixture in the combustion chamber during compression ignition.

A piston structure for an engine according to a second disclosure includes a piston having a crown portion including a crown surface that defines a combustion chamber and a back surface opposite to the crown surface, and an oil jet that injects oil toward the back surface. wherein the interior of the crown portion is provided with a first cavity located on the central side and a second cavity located on the outer peripheral side of the first cavity, which are separated from each other. The first cavity and the second cavity each include a ceiling surface on the side of the combustion chamber and a bottom surface on the side opposite to the combustion chamber, and the first cavity and the second cavity are divided. The wall is provided with a communication hole that communicates the first cavity and the second cavity, and the bottom surface of the first cavity receives the oil that is jetted from the oil jet toward the back surface. An introduction hole for introducing oil into the first cavity is provided, and a discharge hole for discharging the oil is provided in the bottom surface of at least one of the first cavity and the second cavity, and the oil is discharged from the second cavity. A low thermal conductive member having a lower thermal conductivity than the base material of the piston is provided between the ceiling surface and the crown surface of the piston.

According to such a configuration, the oil jetted from the oil jet toward the rear surface of the crown portion is introduced into the central first cavity through the introduction hole. The air-fuel mixture on the central side of the combustion chamber loses heat by the oil flowing through the first cavity, and tends to drop in temperature. On the other hand, the temperature of the air-fuel mixture on the outer peripheral side of the combustion chamber is shielded by the air layer formed in the second cavity, making it difficult for the temperature to drop. As a result, the temperature of the air-fuel mixture on the outer peripheral side of the combustion chamber becomes higher than the temperature of the air-fuel mixture on the central side of the combustion chamber.

Therefore, at the time of compression ignition, compression self-ignition of the air-fuel mixture is first started on the outer peripheral side of the combustion chamber, and then compression self-ignition of the air-fuel mixture occurs on the central side of the combustion chamber. This reduces the unburned loss of the air-fuel mixture in the combustion chamber during compression ignition.

Here, if the heat of the air-fuel mixture on the outer peripheral side of the combustion chamber is simply to be shielded, the second cavity may be simply enlarged, but this is not preferable from the viewpoint of piston strength. On the other hand, if the second cavity is made small, it becomes difficult to shield the air-fuel mixture on the outer peripheral side of the combustion chamber from heat. By providing the low thermal conductivity member between the ceiling surface and the crown surface of the second cavity, the low thermal conductivity member functions as a rigid member compared to the case where the second cavity is simply enlarged, so the strength of the piston can be maintained. can be planned. Furthermore, compared to the case where the second cavity is made small, the temperature of the air-fuel mixture on the outer peripheral side in the combustion chamber can be raised by the low heat-conducting member and the air layer.

As described above, it is possible to maintain the strength of the piston and reduce the unburned loss of the air-fuel mixture in the combustion chamber during compression ignition.

Furthermore, the oil in the first cavity on the center side flows into the second cavity on the outer peripheral side through the communication hole and is discharged to the outside of the crown portion through the discharge hole provided in the bottom surface of the first cavity or the second cavity. be done. As a result, the oil is circulated inside the crown to cool the crown.

In one embodiment, the ceiling surface of the second cavity is positioned closer to the combustion chamber than the ceiling surface of the first cavity.

According to such a configuration, the air layer in the second cavity on the outer peripheral side is closer to the combustion chamber, so the air layer on the outer peripheral side of the combustion chamber is more likely to shield the heat, and the temperature of the air-fuel mixture on the outer peripheral side is more difficult to decrease.

In one embodiment, the low thermal conductivity member covers the second cavity from the combustion chamber side.

According to such a configuration, the strength of the piston can be improved by increasing the surface area of the low heat conductive member.

In one embodiment, the second cavity has an annular shape surrounding the entire circumference of the first cavity, and the low thermal conductivity member has an annular shape corresponding to the second cavity.

According to this configuration, it is possible to maintain the strength of the piston over the entire circumference and reduce the unburned loss of the air-fuel mixture in the combustion chamber during compression ignition.

According to the present disclosure, it is possible to both maintain the strength of the piston and reduce the unburned loss of the air-fuel mixture in the combustion chamber during compression ignition.

1 is a front sectional view showing an engine according to an embodiment of the present disclosure; FIG. FIG. 2 is a front sectional view showing a piston. FIG. 3 is a plan sectional view showing a piston. FIG. 4 is an enlarged view of part IV in FIG. FIG. 5 is a view corresponding to FIG. 4 according to another embodiment.

An embodiment of the present disclosure will be described in detail below with reference to the drawings. The following description of preferred embodiments is merely exemplary in nature and is in no way intended to limit the disclosure, its applicability or its uses.

(Engine configuration)
FIG. 1 is a front sectional view showing an engine 1. FIG. The engine 1 performs CI (Compression Ignition) combustion in a partial operating range (for example, a low load range). In addition, the engine 1 performs SI (Spark Ignition) combustion in a partial operating range (for example, a high load range). CI combustion is combustion initiated by compression self-ignition of the air-fuel mixture in the combustion chamber. SI combustion is combustion with flame propagation initiated by forced ignition of the mixture in the combustion chamber.

CI combustion includes various combustion methods such as HCCI (Homogeneous Charge Compression Ignition) combustion.

The engine 1 includes a cylinder block 2 and a cylinder head 3. A plurality of cylinder bores 4 are formed in the upper part of the cylinder block 2 so as to be aligned in the cylinder row direction. A crankcase 5 is formed below the cylinder block 2 . Note that FIG. 1 shows only one cylinder bore 4 .

A piston 20 is slidably (reciprocally) inserted in the cylinder bore 4 . A crankshaft 7 is connected to the piston 20 via a connecting rod 6 . The crankshaft 7 is housed in the crankcase 5 . The piston 20 defines the combustion chamber 8 together with the cylinder bore 4 and the cylinder head 3 .

Hereinafter, the axial direction (reciprocating direction) of the piston 20 may be referred to as the vertical direction. The side of the combustion chamber 8 in the axial direction of the piston 20 may be called "upper side". The side opposite to the combustion chamber 8 in the axial direction of the piston 20 may be referred to as the "lower side". When the engine 1 is tilted, the vertical direction does not necessarily match the vertical direction.

An intake port 9 is formed in each cylinder bore 4 in the cylinder head 3 . An intake valve 10 is arranged in the intake port 9 . The intake valve 10 opens and closes between the combustion chamber 8 and the intake port 9 at a predetermined timing by a valve mechanism (not shown). Further, an exhaust port 11 is formed for each cylinder bore 4 in the cylinder head 3 . An exhaust valve 12 is arranged in the exhaust port 11 . The exhaust valve 12 opens and closes between the combustion chamber 8 and the exhaust port 11 at a predetermined timing by a valve mechanism (not shown).

An injector (not shown) is attached to the cylinder head 3 for each cylinder bore 4 . The injector injects fuel into the combustion chamber 8 . The fuel is gasoline, for example. A spark plug (not shown) is attached to each cylinder bore 4 in the cylinder head 3 . The spark plug forcibly ignites the mixture of fuel and air in the combustion chamber 8 during SI combustion. The lower surface of the cylinder head 3, that is, the ceiling surface of the combustion chamber 8 has a so-called pent roof shape.

A main gallery 13 extends in the cylinder row direction inside the cylinder block 2 . Oil pumped from an oil pump (not shown) flows through the main gallery 13 . An oil jet 15 is attached to the crankcase 5 at a position that does not interfere with the connecting rod 6 and the crankshaft 7 .

The oil jet 15 has a nozzle portion 15a and a valve portion 15b. The tip of the nozzle portion 15a faces the rear surface 21b of the crown portion 21 of the piston 20, which will be described later. The valve portion 15b incorporates a ball-shaped valve body (not shown) and a spring (not shown). A spring biases the valve body to the closed state. The oil pump supplies oil to the oil jets 15 via the main gallery 13 . Specifically, the oil is supplied to the oil jet 15 via a branch path branched from the main gallery 13 .

The oil jet 15 is configured such that the valve body of the valve portion 15b is kept closed by the biasing force of the spring when the hydraulic pressure of the oil flowing through the main gallery 13 (discharge pressure of the oil pump) is below a predetermined value. there is On the other hand, when the hydraulic pressure of the oil flowing through the main gallery 13 (discharge pressure of the oil pump) is equal to or higher than a predetermined value, the valve body of the valve portion 15b of the oil jet 15 opens against the biasing force of the spring. is configured to inject oil toward the rear surface 21b of the crown portion 21, which will be described later (see L in FIG. 1).

A controller (not shown) is connected to the oil pump. The control unit is, for example, an ECU (Engine Control Unit).

(Piston structure)
The structure of piston 20 will be described with reference to FIGS. FIG. 2 is a front cross-sectional view of the piston 20, showing a section II-II of FIG. FIG. 3 is a plan cross-sectional view of the piston 20, showing the III-III section of FIG.

As shown in FIG. 2 , the piston 20 has a crown portion 21 and a skirt portion 22 . The crown portion 21 is substantially disc-shaped. The crown portion 21 is a portion that slides on the cylinder bore 4 . The crown portion 21 includes a crown surface 21a and a back surface 21b opposite to the crown surface 21a. The crown surface 21 a defines the combustion chamber 8 together with the cylinder head 3 and the cylinder bore 4 . The crown surface 21 a protrudes toward the ceiling surface of the combustion chamber 8 . A cavity 23 is formed in the central portion of the crown surface 21a. An annular groove portion 24 is formed in the outer circumference 21 c of the crown portion 21 . A piston ring (not shown) is attached to the groove 24 . The back surface 21b is formed so that the central portion protrudes downward from the outer peripheral portion.

The skirt portion 22 extends downward from the outer circumference 21c of the crown portion 21 . The skirt portion 22 has a pin hole 25 for inserting a piston pin (not shown). The piston pin is for connecting the connecting rod 6 and the piston 20 .

The crown portion 21 can be divided into upper and lower parts along line III-III in FIG.

As shown in FIGS. 2 and 3, a first cavity 26 and a second cavity 27 that are partitioned from each other are provided inside the crown portion 21, that is, between the crown surface 21a and the back surface 21b. The first cavity 26 is positioned closer to the center than the second cavity 27 in the radial direction of the piston 20 . The second cavity 27 is located on the outer peripheral side relative to the first cavity 26 in the radial direction of the piston 20 .

As shown in FIG. 3 , the first cavity 26 is formed in a circular shape at the central portion of the crown portion 21 when viewed in the axial direction of the piston 20 . The second cavity 27 is formed in an annular shape surrounding the entire circumference of the first cavity 26 on the outer peripheral side of the crown portion 21 when viewed in the axial direction of the piston 20 .

As shown in FIG. 3, the first cavity 26 and the second cavity 27 are partitioned by an inner peripheral wall portion 28a formed in an annular shape in the central portion of the crown portion 21 and an outer peripheral wall portion 28b of the crown portion 21 itself. It is The inner peripheral wall portion 28a partitions the first cavity 26 and the second cavity 27 (hereinafter referred to as "partition wall 28a"). The outer peripheral wall portion 28 b separates the second cavity 27 from the outside of the crown portion 21 .

As shown in FIGS. 2 and 3, a communication hole 29 is provided in the partition wall 28a. The communication hole 29 penetrates the partition wall 28a and communicates the first cavity 26 and the second cavity 27 with each other. The penetrating direction of the communication hole 29 is the radial direction of the piston 20 . An opening of the communication hole 29 on the side of the first cavity 26 is called an "inlet 29a". An opening of the communication hole 29 on the side of the second cavity 27 is called an "outlet 29b".

As shown in FIG. 2 , the first cavity 26 includes a ceiling surface 26 a on the side of the combustion chamber 8 and a bottom surface 26 b on the side opposite to the combustion chamber 8 . The surface area of the ceiling surface 26a and the surface area of the bottom surface 26b in the first cavity 26 are approximately equal to each other. Similarly, the second cavity 27 includes a ceiling surface 27 a on the side of the combustion chamber 8 and a bottom surface 27 b on the side opposite to the combustion chamber 8 . The surface area of the ceiling surface 27a and the surface area of the bottom surface 27b in the second cavity 27 are approximately equal to each other.

As shown in FIGS. 2 and 3, the bottom surface 26b of the first cavity 26 is provided with an introduction hole 30. As shown in FIGS. The introduction hole 30 introduces the oil injected from the oil jet 15 toward the back surface 21 b of the crown portion 21 into the first cavity 26 . A discharge hole 31 is provided in the bottom surface 26 b of the first cavity 26 . The discharge hole 31 discharges oil from the first cavity 26 .

2 and 3, the distance between the discharge hole 31 and the inlet 29a of the communication hole 29 in the first cavity 26 is shorter than the distance between the introduction hole 30 and the inlet 29a of the communication hole 29. As shown in FIGS.

As shown in FIG. 2, the bottom surface 27b of the second cavity 27 and the lower end (bottom surface) 29c of the communication hole 29 are substantially flush with each other. The ceiling surface 26a of the first cavity 26 and the upper end portion (ceiling surface) 29d of the communication hole 29 are substantially flush with each other.

As shown in FIG. 2 , the ceiling surface 27a of the second cavity 27 is located closer to the combustion chamber 8 than the ceiling surface 26a of the first cavity 26 is. That is, the ceiling surface 27a of the second cavity 27 is located higher than the ceiling surface 26a of the first cavity 26. As shown in FIG.

As shown in FIG. 3 , the surface area of the ceiling surface 27 a of the second cavity 27 is larger than the surface area of the ceiling surface 26 a of the first cavity 26 .

As shown in FIG. 2, a low thermal conductivity member 34 is provided between the ceiling surface 27a and the crown surface 21a of the second cavity 27. As shown in FIG. The low thermal conductivity member 34 has a lower thermal conductivity than the base material (main material) of the piston 20 . For example, if the base material of the piston 20 is iron, aluminum, or the like, ceramic or the like may be applied to the low thermal conductive member 34 .

The low thermal conductivity member 34 is formed in an annular shape when viewed in the axial direction of the piston 20 so as to correspond to the second cavity 27, and covers the entire second cavity 27 from above (combustion chamber 8 side). . In addition, the lower surface of the low thermal conductive member 34 constitutes the ceiling surface 27a of the second cavity 27. As shown in FIG.

As described above, the crown portion 21 can be divided into upper and lower parts along line III-III in FIG. It can be attached between the surface 21a. In this embodiment, a low thermal conductive member 34 is attached to the lower side of the meat portion 21e between the ceiling surface 27a and the crown surface 21a of the second cavity 27. As shown in FIG.

(Effect)
Advantages of the structure of the piston 20 according to this embodiment will be described mainly with reference to FIG.

The oil jetted from the nozzle portion 15a of the oil jet 15 toward the back surface 21b of the crown portion 21 of the piston 20 flows through the introduction hole 30 into the first cavity 26 on the central side, as indicated by L in FIG. be introduced.

The air-fuel mixture 41 on the central side of the combustion chamber 8 is deprived of heat by the oil flowing through the first cavity 26 and tends to drop in temperature. On the other hand, the air layer 40 formed in the second cavity 27 blocks heat from the air-fuel mixture 42 on the outer peripheral side of the combustion chamber 8, making it difficult for the temperature to drop. As a result, the temperature of the air-fuel mixture 42 on the outer peripheral side of the combustion chamber 8 becomes higher than the temperature of the air-fuel mixture 41 on the central side of the combustion chamber 8 . That is, the temperature distribution of the combustion chamber 8 is realized such that the temperature of the air-fuel mixture 42 on the outer peripheral side is higher than the temperature of the air-fuel mixture 41 on the central side.

Therefore, at the time of compression ignition, compression self-ignition of the air-fuel mixture 42 is first started on the outer peripheral side of the combustion chamber 8 , and then compression self-ignition of the air-fuel mixture 41 occurs on the central side of the combustion chamber 8 . This reduces the unburned loss of the air-fuel mixtures 41 and 42 (especially the air-fuel mixture 42 on the outer peripheral side) in the combustion chamber 8 during compression ignition.

Here, if the heat of the air-fuel mixture 42 on the outer peripheral side of the combustion chamber 8 is simply to be shielded, the second cavity 27 (air layer 40) may be simply enlarged. I don't like it. On the other hand, if the second cavity 27 is made smaller, it becomes difficult to shield the air-fuel mixture 42 on the outer peripheral side of the combustion chamber 8 from heat. When the second cavity 27 is simply enlarged by providing the low heat conductive member 34 between the ceiling surface 27a and the crown surface 21a of the second cavity 27, that is, by making the air layer 40 and the low heat conductive member 34 hybrid. Compared to , since the low thermal conductivity member 34 functions as a rigid member, the strength of the piston 20 can be maintained. Furthermore, compared to the case where the second cavity 27 is made small, the temperature of the air-fuel mixture 42 on the outer peripheral side in the combustion chamber 8 can be increased by the low heat conduction member 34 and the air layer 40 .

As described above, it is possible to both maintain the strength of the piston 20 and reduce the unburned loss of the air-fuel mixtures 41 and 42 in the combustion chamber 8 during compression ignition.

Further, part of the oil in the first cavity 26 flows through the communication hole 29 into the second cavity 27 on the outer peripheral side. The rest of the oil in the first cavity 26 is discharged outside the crown portion 21 through a discharge hole 31 provided in the bottom surface 26 b of the first cavity 26 . As a result, the oil is circulated inside the crown portion 21 to cool the crown portion 21 .

Since the ceiling surface 27a of the second cavity 27 is positioned closer to the combustion chamber 8 (at a higher position) than the ceiling surface 26a of the first cavity 26, the air layer 40 of the second cavity 27 on the outer peripheral side is formed by the combustion chamber 8. come closer. As a result, the mixture 42 on the outer peripheral side of the combustion chamber 8 is more easily shielded from heat by the air layer 40, and the temperature thereof is less likely to drop.

In addition, since the ceiling surface 27a of the second cavity 27 is located closer to the combustion chamber 8 (at a higher position) than the ceiling surface 26a of the first cavity 26, the oil flowing into the second cavity 27 through the communication hole 29 does not fill the second cavity 27 and does not reach its ceiling surface 27a. Therefore, even if oil flows into the second cavity 27, the air layer 40 can be formed near the ceiling surface 27a of the second cavity 27. As shown in FIG.

Since the low thermal conductivity member 34 covers the entire second cavity 27 from above, the strength of the piston 20 can be improved by increasing the surface area of the low thermal conductivity member 34 .

Since the second cavity 27 is formed in an annular shape surrounding the entire circumference of the first cavity 26 (see FIG. 3), compression self-ignition of the air-fuel mixture 42 from the outer peripheral side can be achieved over the entire circumference of the combustion chamber 8. will start. Further, since the low thermal conductivity member 34 is formed in an annular shape corresponding to the second cavity 27, the strength of the piston 20 can be maintained over the entire circumference. That is, it is possible to maintain the strength of the piston 20 over the entire circumference and reduce the unburned loss of the air-fuel mixtures 41 and 42 in the combustion chamber 8 during compression ignition.

(Other embodiments)
Although the present disclosure has been described in terms of preferred embodiments, such descriptions are not intended to be limiting, and various modifications are of course possible.

In the above embodiment, the discharge hole 31 is provided only in the bottom surface 26b of the first cavity 26, but the invention is not limited to this. 2 to 4, the discharge hole 32 may be provided not only in the bottom surface 26b of the first cavity 26 but also in the bottom surface 27b of the second cavity 27. As shown in FIGS. That is, both the bottom surface 26 b of the first cavity 26 and the bottom surface 27 b of the second cavity 27 may be provided with the discharge holes 31 and 32 . Further, when the communication hole 29 is provided in the partition wall 28a between the first cavity 26 and the second cavity 27, the discharge hole 31 is not provided in the bottom surface 26b of the first cavity 26, and the bottom surface 27b of the second cavity 27 is Only the discharge hole 32 may be provided.

Although not shown, the bottom surface 27b of the second cavity 27 may be located on the opposite side of the combustion chamber 8 from the lower end portion (bottom surface) 29c of the communication hole 29 .

The low thermal conductivity member 34 does not have to cover the entire second cavity 27 from above. Also, the low thermal conductivity member 34 does not have to be formed in an annular shape corresponding to the second cavity 27 . The low thermal conductivity member 34 may face only part of the second cavity 27 .

The second cavity 27 may be formed in a quadrangular ring or the like that surrounds the first cavity 26 over the entire circumference instead of being annular. Also, the second cavity 27 does not need to surround the entire circumference of the first cavity 26, and may be formed, for example, in a C-ring shape (arcuate shape). Furthermore, the second cavity 27 may be provided only in a partial region on the outer peripheral side of the first cavity 26 .

FIG. 5 is a view corresponding to FIG. 4 according to another embodiment. In this embodiment, as shown in FIG. 5, the partition wall 28a between the first cavity 26 and the second cavity 27 is not provided with the communicating hole 29. As shown in FIG. That is, the second cavity 27 is a closed space. In this case, the oil in the first cavity 26 is prevented from flowing into the second cavity 27 by the partition wall 28a, and is discharged outside the crown portion 21 through the discharge hole 31 provided in the bottom surface 26b of the first cavity 26. be done.

The engine 1 may perform CI combustion in all operating ranges without performing SI combustion at all.

INDUSTRIAL APPLICABILITY The present disclosure can be applied to the piston structure of an engine, so it is extremely useful and has high industrial applicability.

1 engine 8 combustion chamber 13 main gallery 15 oil jet 20 piston 21 crown portion 21a crown surface 21b rear surface 21e meat portion 26 first cavity 26a ceiling surface 26b bottom surface 27 second cavity 27a ceiling surface 27b bottom surface 28a partition wall (inner peripheral wall portion)
28b outer peripheral wall portion 29 communication hole 29a inlet 29b outlet 29c lower end portion 29d upper end portion 30 introduction hole 31 discharge hole 32 discharge hole 34 low thermal conductive member 40 air layer 41 mixture 42 mixture

Claims (5)

A piston structure for an engine comprising: a piston having a crown portion including a crown surface defining a combustion chamber and a back surface opposite to the crown surface; and an oil jet for injecting oil toward the back surface,
In the interior of the crown portion, a first cavity located on the central side and a second cavity located on the outer peripheral side of the first cavity are provided, which are separated from each other.
The first cavity and the second cavity each include a ceiling surface on the side of the combustion chamber and a bottom surface on the side opposite to the combustion chamber,
The bottom surface of the first cavity is provided with an introduction hole through which the oil injected toward the back surface from the oil jet is introduced into the first cavity, and a discharge hole through which the oil is discharged. ,
A piston structure for an engine, wherein a low thermal conductivity member having a lower thermal conductivity than a base material of the piston is provided between the ceiling surface and the crown surface of the second cavity. A piston structure for an engine comprising: a piston having a crown portion including a crown surface defining a combustion chamber and a back surface opposite to the crown surface; and an oil jet for injecting oil toward the back surface,
In the interior of the crown portion, a first cavity located on the central side and a second cavity located on the outer peripheral side of the first cavity are provided, which are separated from each other.
The first cavity and the second cavity each include a ceiling surface on the side of the combustion chamber and a bottom surface on the side opposite to the combustion chamber,
A partition wall between the first cavity and the second cavity is provided with a communication hole that communicates the first cavity and the second cavity,
The bottom surface of the first cavity is provided with an introduction hole for introducing the oil injected toward the back surface from the oil jet into the first cavity,
a discharge hole for discharging the oil is provided in the bottom surface of at least one of the first cavity and the second cavity,
A piston structure for an engine, wherein a low thermal conductivity member having a lower thermal conductivity than a base material of the piston is provided between the ceiling surface and the crown surface of the second cavity. The engine piston structure according to claim 1 or 2,
The piston structure of the engine, wherein the ceiling surface of the second cavity is positioned closer to the combustion chamber than the ceiling surface of the first cavity. A piston structure for an engine according to any one of claims 1 to 3,
The piston structure for an engine, wherein the low thermal conductivity member covers the second cavity from the combustion chamber side. A piston structure for an engine according to claim 4,
The second cavity has an annular shape surrounding the entire circumference of the first cavity,
The piston structure for an engine, wherein the low thermal conductivity member is annular corresponding to the second cavity.

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