Compression-ignition internal combustion engine

A compression-ignition internal combustion engine includes a fuel injection nozzle including a tip end portion exposed in a combustion chamber and a nozzle hole formed at the tip end portion; and a passage forming member forming a flow guide passage through which fuel injected from the nozzle hole passes. The passage forming member includes a passage wall portion located radially outward of the flow guide passage. The passage wall portion includes a first layer that is a base portion connected to a cylinder head, and a second layer located radially outward or radially inward of the first layer. The toughness of the first layer is higher than the toughness of the second layer. The thermal conductivity of the second layer is lower than the thermal conductivity of the first layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of Japanese Patent Application No. 2018-129991, filed on Jul. 9, 2018, which is incorporated by reference herein in its entirety.

BACKGROUND

Technical Field

The present disclosure relates to a compression-ignition internal combustion engine.

Background Art

For example, US 2016/0097360 A1 discloses a technique for controlling a compression-ignition internal combustion engine to promote premixing of fuel and charged air in a combustion chamber of the engine.

According to the technique described above, a duct configured by a hollow pipe is arranged in the vicinity of an opening (i.e., nozzle hole) of a tip end portion of a fuel injection device that is exposed in the combustion chamber. The fuel that is injected from the opening passes through this duct and is injected into the combustion chamber from the duct.

SUMMARY

The duct of the compression-ignition internal combustion engine disclosed in US 2016/0097360 A1 is exposed in the combustion chamber. Because of this, there is a concern that, as a result of the duct being exposed to a high-temperature combustion gas, the temperature of the duct may become higher. In addition, it is assumed that various kinds of weights or loads may be repeatedly applied to the duct due to an effect (such as, an effect of a vibration produced by the internal combustion engine itself, an effect of an in-cylinder pressure that goes up and down during a cycle, or an effect of fuel injection pressure).

The present disclosure has been made to address the problem described above, and an object of the present disclosure is to provide a compression-ignition internal combustion engine that includes a passage wall portion of a flow guide passage through which a fuel that is injected from a nozzle hole of a fuel injection nozzle or an in-cylinder gas passes, and that can enhance the reliability of shape retention of the passage wall portion and also reduce an increase of a wall surface temperature of the flow guide passage.

A compression-ignition internal combustion engine according to one aspect of the present disclosure includes: a fuel injection nozzle including a tip end portion exposed in a combustion chamber and a nozzle hole formed at the tip end portion; and a passage forming member forming a flow guide passage through which fuel injected from the nozzle hole passes. The passage forming member includes a passage wall portion located radially outward of the flow guide passage. The passage wall portion includes a first layer that is a base portion connected to a cylinder head, and a second layer located radially outward or radially inward of the first layer. A toughness of the first layer is higher than a toughness of the second layer. A thermal conductivity of the second layer is lower than a thermal conductivity of the first layer.

The second layer may be located radially outward of the first layer.

A gap may be formed between an outlet of the nozzle hole and an inlet of the flow guide passage. A heat capacity per unit volume of the second layer may also be smaller than a heat capacity per unit volume of the first layer.

One or more communication holes that cause the flow guide passage to communicate with the combustion chamber may be formed in the passage wall portion. A heat capacity per unit volume of the second layer may be smaller than a heat capacity per unit volume of the first layer.

The passage forming member may further include a support portion interposed between the first layer and the cylinder head. The passage wall portion may also be composed of the first layer and the second layer and be formed into a cylindrical shape.

The passage forming member may be integrally formed with the cylinder head.

The passage forming member may be fastened to a combustion chamber ceiling of the cylinder head.

A compression-ignition internal combustion engine according to another aspect of the present disclosure includes: a fuel injection nozzle including a tip end portion exposed in a combustion chamber at a central part of a combustion chamber ceiling and a nozzle hole formed at the tip end portion; and a piston arranged in a cylinder and including a top portion where a flow guide passage through which gas in the cylinder passes is formed. The flow guide passage extends from an inlet exposed in the combustion chamber on a side of a wall of a bore of the cylinder toward an outlet exposed in the combustion chamber on a side of a center of the bore. The piston includes a passage wall portion located on a side of the combustion chamber ceiling with respect to the flow guide passage. The passage wall portion includes a first layer that is a base portion connected to the piston, and a second layer located on a side of the piston or a side of the combustion chamber ceiling with respect to the first layer. A toughness of the first layer is higher than a toughness of the second layer. A thermal conductivity of the second layer is lower than a thermal conductivity of the first layer.

A heat capacity per unit volume of the second layer may be smaller than a heat capacity per unit volume of the first layer.

According to the compression-ignition internal combustion engine in one aspect of the present disclosure, the passage wall portion of the flow guide passage through which the fuel that is injected from the nozzle hole passes includes the first layer and the second layer located radially outward or radially inward of the first layer. Also, the first layer is connected to the cylinder head, and the toughness of the first layer is higher than the toughness of the second layer. As a result, even if the weight or load described above is repeatedly applied to the passage wall portion, the shape of the passage wall portion can be easy to be maintained over a long time. In addition, the thermal conductivity of the second layer is lower than the thermal conductivity of the first layer. As a result, the heat transferred to the outer wall of the passage wall portion from a high-temperature combustion gas around the passage wall portion can be prevented from being transferred to the inner wall of the passage wall portion (i.e., the wall surface of the flow guide passage). As just described, according to one aspect of the present disclosure, the reliability of the shape retention of the passage wall portion can be favorably enhanced, and an increase of the wall surface temperature of the flow guide passage can be favorably reduced.

Furthermore, according to the compression-ignition internal combustion engine in another aspect of the present disclosure, the flow guide passage is formed, on the top portion of the piston, so as to extend from the inlet exposed in the combustion chamber on the side of the wall of the bore of the cylinder toward the outlet exposed in the combustion chamber on the side of the center of the bore. The piston includes the passage wall portion located on the side of the combustion chamber ceiling with respect to this flow guide passage. The passage wall portion includes the first layer and the second layer located on the side of the piston or the side of the combustion chamber ceiling with respect to this first layer. Also, the first layer is connected to the piston, and the toughness of the first layer is higher than the toughness of the second layer. As a result, even if the weight or load described above is repeatedly applied to the passage wall portion, the shape of the passage wall portion can be easy to be maintained over a long time. In addition, the thermal conductivity of the second layer is lower than the thermal conductivity of the first layer. As a result, the heat transferred to the wall of the passage wall portion on the combustion chamber ceiling side from a high-temperature combustion gas around the passage wall portion can be prevented from being transferred to the wall of the passage wall portion on the piston side (i.e., the wall surface of the flow guide passage). As just described, according to another aspect of the present disclosure, similarly to one aspect described above, the reliability of the shape retention of the passage wall portion can be favorably enhanced, and an increase of the wall surface temperature of the flow guide passage can be favorably reduced.

DETAILED DESCRIPTION

In the following embodiments of the present disclosure, the same components in the drawings are denoted by the same reference numerals, and redundant descriptions thereof are omitted or simplified. Moreover, it is to be understood that even when the number, quantity, amount, range or other numerical attribute of an element is mentioned in the following description of the embodiments, the present disclosure is not limited to the mentioned numerical attribute unless explicitly described otherwise, or unless the present disclosure is explicitly specified by the numerical attribute theoretically. Furthermore, structures or steps or the like that are described in conjunction with the following embodiments are not necessarily essential to the present disclosure unless explicitly shown otherwise, or unless the present disclosure is explicitly specified by the structures, steps or the like theoretically.

1. First Embodiment

A first embodiment according to the present disclosure and modification examples thereof will be described with reference toFIGS. 1 to 5.

1-1. Configuration In and Around Combustion Chamber

FIG. 1is a longitudinal sectional view that schematically illustrates the configuration in and around a combustion chamber12of a compression-ignition internal combustion engine (hereunder, simply abbreviated as an “internal combustion engine”)10according to the first embodiment of the present disclosure. As an example, the internal combustion engine10shown inFIG. 1is a diesel engine.

As shown inFIG. 1, the internal combustion engine10is provided with a cylinder block14, pistons16and a cylinder head18. The pistons16reciprocate inside the respective cylinders formed in the cylinder block14. The cylinder head18is arranged on the cylinder block14. The combustion chamber12is mainly defined by a cylinder bore surface14aof the cylinder block14, a top surface16aof the piston16, a surface of a combustion chamber ceiling18aof the cylinder head18, and bottom surfaces of intake and exhaust valves (not shown).

The internal combustion engine10is further provided with a fuel injection nozzle20and ducts30. The fuel injection nozzle20is arranged at the center of the combustion chamber ceiling18a. The fuel injection nozzle20has a tip end portion20athat is exposed in the combustion chamber12. A plurality of (for example, eight) nozzle holes22are formed at the tip end portion20a. These eight nozzle holes22are formed such that fuel is injected in a radial manner toward the cylinder bore surface14a.

The ducts30are respectively provided with respect to eight nozzle holes22. Because of this, the number of ducts in the example shown inFIG. 1is eight. Each of the ducts30is formed into a cylindrical shape. A flow guide passage32is formed in the interior of each of the ducts30. The fuel injected from each of the nozzle holes22is injected in the combustion chamber12after passing through the corresponding flow guide passage32. It should be noted that the number of “flow guide passages” according to one aspect of the present disclosure may not always be the same as that of nozzle holes, and may be provided only for a part of a plurality of nozzle holes. Hereunder, the concrete structure in and around the ducts30will be described in detail with reference toFIGS. 2 and 3.

1-1-1. Example of Concrete Shape In and Around Duct

FIG. 2is an enlarged longitudinal sectional view that schematically illustrates one duct30inFIG. 1and around this duct30.FIG. 3is a transverse sectional view of the duct30shown inFIG. 1. According to the example shown inFIG. 2, the duct30is fixed to (i.e., suspended from) the combustion chamber ceiling18aof the cylinder head18with a support portion34interposed therebetween. The duct30is arranged such that the central axis line of the flow guide passage32is aligned with an axis line L1of the nozzle hole22. In other words, the duct30is formed so as to extend straight along the axis line L1of the nozzle hole22. In addition, as shown inFIG. 3, the flow passage cross-section of the duct30is a circle as an example, and thus, the duct30(more specifically, a passage wall portion36described below) is formed into a cylindrical shape.

According to the present embodiment, the duct30suspended from the combustion chamber ceiling18awith the support portion34interposed therebetween corresponds an example of the “passage forming member” that forms the flow guide passage32. The duct30includes the passage wall portion36located radially outward of the flow guide passage32, and the support portion34described above. The passage wall portion36has a double-layered structure composed of a first layer36aand a second layer36b.

The first layer36acorresponds to a base portion (base layer) connected to the combustion chamber ceiling18aof the cylinder head18with the support portion34interposed therebetween. That is to say, the first layer36aof the duct30is supported by the support portion34. According to the example shown inFIG. 2, although the first layer36aand the support portion34are integrally formed with the combustion chamber ceiling18a, any two or all of them may alternatively be separated from each other. In other words, the first layer36ahas only to be integrally or separately connected to the cylinder head18.

The second layer36bis located radially outward (i.e., on the outer peripheral side) of the first layer36a. Also, according to the example shown inFIG. 2, the second layer36bis formed so as to cover not only the first layer36abut also the support portion34. In addition, according to the example shown inFIG. 2, the first layer36aand the second layer36bare both formed into a cylindrical shape. Moreover, the first layer36ais formed so as to extend over the whole passage wall portion36in the longitudinal direction of the flow guide passage32and to cover the whole first layer36a. Furthermore, the second layer36bcovers the whole first layer36aalso in the circumferential direction thereof.

Moreover, according to the example shown inFIG. 2, the outer surface of the tip end portion20ahaving the nozzle hole22is not in contact with the duct30. In other words, a gap G is formed between the outlet of the nozzle hole22and the inlet of the flow guide passage32. In addition, not only the outlet of the duct30(flow guide passage32) but also the inlet thereof is exposed in the combustion chamber12. Gas (i.e., working gas) in the combustion chamber12uses this gap G to flow into the flow guide passage32as well as the fuel injected from the nozzle hole22.

1-1-2. Specific Example of Material of Duct Having Double-Layered Structure

The first layer36aand the second layer36bof the duct30meet the following relationships with respect to the toughness and thermal conductivity of materials thereof. That is to say, the toughness of the first layer36athat is the base layer of the duct30is higher than the toughness of the second layer36bthat is the outer layer thereof. Also, the thermal conductivity of the second layer36bis lower than the thermal conductivity of the first layer36a. An example of the material of the first layer36athat meets these relationships is a metal (such as, aluminum or iron), and an example of the material of the second layer36bis a silicon nitride (Si3N4). It should be noted that the “toughness” mentioned here means the properties of tenacity with respect to the fracture of a material, and one of specific indexes thereof is fracture toughness.

To be more specific, the second layer36bcan be obtained as a result of a coating of the silicon nitride being formed on the first layer36ausing, for example, thermal spraying. Since the thermal conductivity of the second layer36bis lower than the thermal conductivity of the first layer36aas described above, the second layer36bfunctions as a heat-shielding film.

1-2-1. Advantageous Effects by Use of Duct (Flow Guide Passage)

According to the compression-ignition internal combustion engine10, fuel is injected from the fuel injection nozzle20when air charged into the combustion chamber12is in a compressed state. It is favorable that, after the injected fuel is mixed with the charged air and homogenization of the fuel concentration is promoted, compression-ignition combustion is performed. However, in an example without including the duct30, there is a concern that fuel injected from the fuel injection nozzle20may receive heat of the combustion chamber12to quickly overheat, and, as a result, a self-ignition of the fuel may be performed before the fuel is sufficiently mixed with the charged air. As a result, smoke may be produced due to excessively rich fuel burning, or the thermal efficiency may be decreased due to prolongation of an afterburning time.

According to the internal combustion engine10of the first embodiment, in order to address the issue described above, the duct(s)30is arranged in the combustion chamber12. According to this kind of configuration, the spray of fuel injected from the nozzle hole22of the fuel injection nozzle20is introduced into the interior of the duct30(i.e., into the flow guide passage32). In addition, since the inlet of the duct30is exposed in the combustion chamber12, the charged air in the combustion chamber12is also guided to the interior of the duct30from the inlet thereof. As a result, in the interior of the duct30whose temperature is basically lower than that in the vicinity thereof, the spray of the fuel and the charged air are mixed while being cooled, and thus, homogenization of the fuel concentration is promoted without the fuel spray being self-ignited early. Moreover, after the air-fuel mixture is sufficiently premixed, it is injected from the outlet of the duct30. The injected air-fuel mixture receives heat from the combustion chamber12to be self-ignited and burn.

As described above, with the installation of the duct(s)30(flow guide passage(s)32), in the course of the spray of the fuel which is injected passing through the duct30, premix of the fuel spray and the charged air can be promoted while the occurrence of self-ignition is reduced. As a result, it becomes possible to reduce the occurrence of smoke due to the fact that the excessively rich fuel before homogenized is self-ignited. In addition, with the installation of the duct(s)30, since the occurrence of self-ignition is reduced during the fuel passing through the duct30, the timing of self-ignition can be retarded. Because of this, the afterburning time is shortened, and the thermal efficiency can thus be improved.

1-2-2. Issue Concerning Installation of Duct (Flow Guide Passage)

A duct as in the duct30is exposed in a combustion chamber. That is to say, this kind of duct is arranged at a location in which the temperature thereof is easy to become higher due to the fact that the duct is exposed to a high-temperature combustion gas. If the temperature of the wall surface of a flow guide passage (i.e., the inner wall of the duct) becomes high due to the heat received from combustion gas, the fuel spray passing through the duct is heated due to the heat received from the wall surface of the flow guide passage. As a result, the ignition delay is shortened (i.e., the above-described effect of retarding the self-ignition timing decreases), and thus, the combustion is started when the mixing of the fuel spray and the charged air is insufficient. Because of this, there is a concern that it may become difficult to properly reduce the occurrence of smoke.

Furthermore, it is assumed that various kinds of weights or loads may be repeatedly applied to the duct due to an effect (such as, an effect of a vibration produced by the internal combustion engine itself, an effect of an in-cylinder pressure that goes up and down during a cycle, or an effect of fuel injection pressure). Thus, it is required for countermeasures regarding reduction of temperature increase of the wall surface of a flow guide passage (i.e., the inner wall of a duct) to be made such that, even if a weight or load is repeatedly applied to the duct, the shape of the duct can be more surely maintained over a long time.

1-2-3. Adoption of Duct Having Double-Layered Structure

In view of the issue described above, according to the passage wall portion36of the duct30of the present embodiment, the first layer36ais configured as a base portion of the duct30that is connected to the cylinder head18(combustion chamber ceiling18a) with the support portion34interposed therebetween. Moreover, the materials of this first layer36aand the second layer36bare selected such that the toughness of the first layer36abecomes higher than the toughness of the second layer36b. As a result, even if the weight or load described above is repeatedly applied to the duct30, the shape of the duct30(passage wall portion36) can be easy to be maintained over a long time.

Furthermore, the materials of the first layer36aand the second layer36bare selected such that the thermal conductivity of the second layer36blocated on the outer peripheral side of the first layer36abecomes lower than the thermal conductivity of the first layer36a. As a result, the heat transferred to the outer wall of the passage wall portion36(i.e., the outer wall of the second layer36b) from a high temperature combustion gas around the duct30can be prevented from being transferred to the inner wall of the passage wall portion36(i.e., the wall surface of the flow guide passage32). Because of this, when the fuel passes through the flow guide passage32located on the inner side of the passage wall portion36, an increase of the temperature of the fuel can be reduced. As a result, a decrease of the effect of retarding the self-ignition timing can be reduced.

As described so far, according to the internal combustion engine10of the present embodiment, the reliability of shape retention of the duct30(passage wall portion36) can be favorably enhanced, and also an increase of the wall surface temperature of the flow guide passage32can be favorably reduced.

Furthermore, according to the duct30of the present embodiment, the support portion34is also covered by the second layer36b. Because of this, the transfer of heat to the first layer36a(i.e., the portion that serves as the inner wall of the flow guide passage32) from a high-temperature combustion gas with the support portion34interposed therebetween can also be effectively reduced.

1-3. Modification Examples Concerning First Embodiment

1-3-1. Another Example of Double-Layered Structure for Duct

FIG. 4is a schematic diagram for describing another example of the configuration of the first and second layers of the passage wall portion. It should be noted thatFIG. 4shows only one of ducts40, and this also applies toFIGS. 5 to 7. According to the example shown inFIG. 4, a duct40(i.e., passage forming member) includes a passage wall portion42along with the support portion34. The passage wall portion42includes a first layer42aand a second layer42blocated radially outward of the first layer42a.

According to the example of the duct30shown inFIG. 2, the first layer36ais formed so as to extend over the whole passage wall portion36in the longitudinal direction of the flow guide passage32, and the second layer36bis formed so as to cover the whole first layer36a. In contrast to this, according to the example of the duct40shown inFIG. 4, the first layer42adoes not extend over the whole passage wall portion42in the longitudinal direction of the flow guide passage32, and, at an end portion of the flow guide passage32on its outlet side, the inner wall of the flow guide passage32is configured by the second layer42b.

As shown by the example described above, the “first layer” according to one aspect of the present disclosure may not always extend over the whole passage wall portion in the longitudinal direction of the flow guide passage, and this also applies to the “second layer”. In other words, the double-layered structure may be provided not for the whole duct (passage wall portion) but for only a part of the duct, provided that, in order to enhance the reliability of shape retention of the first layer, the connection between the first layer and the cylinder head is not broken by the second layer. In addition, this also applies to other second to sixth embodiments described below.

1-3-2. Still Another Example of Double-Layered Structure for Duct

FIG. 5is a schematic diagram for describing still another example of the configuration of the first and second layers of the passage wall portion. According to the example shown inFIG. 5, a duct50(i.e., passage forming member) includes a passage wall portion52along with a support portion54. The passage wall portion52includes a first layer52aand a second layer52blocated radially inward of the first layer52a, contrary to the example of the duct30shown inFIG. 2.

According to the configuration in which the second layer52bcorresponding to the heat-shielding film as described above is arranged on the inner side of the first layer52a(i.e., base layer), heat that is transferred to the outer wall of the passage wall portion52(i.e., the outer wall of the first layer52a) from a high-temperature combustion gas around the duct50can also be prevented from being transferred to the inner wall of the passage wall portion52(i.e., the wall surface of the flow guide passage32). When the ease of production of the passage wall portion is also taken into consideration, the configuration in which the second layer36bis located radially outward as in the duct30shown inFIG. 2is superior. However, in terms of achieving the advantageous effects of reducing an increase of the wall surface temperature of the flow guide passage32, the configuration as shown inFIG. 5may alternatively be used.

2. Second Embodiment

Then, a second embodiment according to the present disclosure will be described with reference toFIG. 6.

2-1. Difference from First Embodiment

FIG. 6is a schematic diagram for describing the configuration of a duct60according to the second embodiment of the present disclosure. An internal combustion engine according to the present embodiment is different, in the following points, from the internal combustion engine10according to the first embodiment.

The duct60shown inFIG. 6includes a passage wall portion62along with the support portion34. The passage wall portion62includes a first layer62aand a second layer62b. The shape and material of the first layer62ais the same as those of the first layer36ashown inFIG. 2. On the other hand, the second layer62bhas the same shape as the second layer36bshown inFIG. 2but the second layer62band the second layer36bare different in material as described below.

More specifically, an example of the material of the second layer62bis zirconia (ZrO2). The second layer62bhaving the zirconia as a raw material can be obtained by forming a coat of zirconia on the first layer62ausing, for example, thermal spraying. The second layer62band the first layer62awhose materials are selected in this way meet the following relationships with respect to the toughness and thermal conductivity and heat capacity per unit volume of these materials. That is to say, the relationships with respect to the toughness and thermal conductivity in the second embodiment are the same as those in the first embodiment, and thus, the toughness of the first layer62ais higher than that of the second layer62band the thermal conductivity of the second layer62bis lower than that of the first layer62a. On that basis, the heat capacity per unit volume of the second layer62bis smaller than that of the first layer62a.

According to the internal combustion engine of the present embodiment that includes the duct(s)60described so far, the reliability of shape retention of the duct60(passage wall portion) can also be favorably enhanced, and an increase of the wall surface temperature of the flow guide passage32can also be favorably reduced. On that basis, according to the present embodiment, an additional issue described below can also be addressed.

That is to say, in an internal combustion engine including a duct as in the duct30or60, a charged air (working gas) around the duct is suctioned into the interior (flow guide passage) of the duct from a gap between a nozzle hole and the inlet of the duct (the gap G shown inFIGS. 2 and 6corresponds to this gap). An increase of the temperature of the inner wall of the first layer36a(i.e., the wall surface of the flow guide passage32) can be reduced by the use of the duct30according to the first embodiment that includes the second layer36bwith a low thermal conductivity. If, however, the heat capacity per unit volume of the material of the second layer36bis great (for example, silicon nitride), the temperature of the outer wall of the duct30(i.e., the outer peripheral wall of the second layer36b) always becomes higher. As a result, when the duct30suctions a charged air around the duct30, the charged air is heated by the outer wall. Because of this, there is a concern that the effect of reducing the self-ignition using the duct (i.e., the effect of retarding the self-ignition timing) may not be sufficiently achieved.

In view of the additional issue described above, according to the duct60(passage wall portion62) of the present embodiment, the materials of the first layer62aand the second layer62bare selected such that the second layer62bcorresponding to the outer wall of the duct60becomes smaller in heat capacity per unit volume than the first layer62a. As a result, the temperature of the second layer62bbecomes easy to increase and decrease in association with the in-cylinder gas temperature increasing and decreasing during one cycle. This can prevent the temperature of the second layer62bfrom always becoming high. Thus, according to the duct60of the present embodiment, heating of a charged air that is suctioned into the duct60via the gap G (seeFIG. 6) can be reduced while the advantageous effects of reduction of temperature increase of the wall surface of the flow guide passage32(i.e., the inner wall of the first layer62a) is achieved similarly to the first embodiment. Because of this, the effect of reducing the self-ignition using the duct60(i.e., the effect of retarding the self-ignition timing) can be more effectively achieved as compared to that of the first embodiment.

Then, a third embodiment according to the present disclosure will be described with reference toFIG. 7.

3-1. Difference from Second Embodiment

FIG. 7is a schematic diagram for describing the configuration of a duct70according to the third embodiment of the present disclosure. An internal combustion engine according to the present embodiment is different from the internal combustion engine according to the second embodiment in the following points.

Specifically, according to the second embodiment, the gap G is formed between the outlet of the nozzle hole22and the inlet of the duct60(i.e., the inlet of the flow guide passage32) as shown inFIG. 6. In contrast to this, according to the present embodiment, as shown inFIG. 7, this kind of gap G is not provided, and the outer wall of the tip end portion20ahaving the nozzle hole22is in contact with the inlet of the duct70(i.e., inlet of the flow guide passage32). In addition, a passage wall portion72of the duct70protrudes from the outer wall of the tip end portion20aalong the axial line L1of the nozzle hole22.

The passage wall portion72includes a first layer72aand a second layer72b. The material of the first layer72ais the same as that of the first layer62a, and the material of the second layer72bis the same as that of the second layer62b. However, as shown inFIG. 7, in the passage wall portion72, a desired number of (for example, three) communication holes74are formed in order to cause the flow guide passage32to communicate with the combustion chamber12. The communication holes74penetrate through the first layer72aand the second layer72b. According to the duct(s)70including this kind of communication holes74, the charged gas around the duct70flows into the flow guide passage32as well as the fuel injected from the corresponding the nozzle hole(s)22, through these communication holes74.

As described so far, the materials of the first layer72aand second layer72bof the duct70according to the present embodiment are the same as those of the first layer62aand second layer62baccording to the second embodiment. Because of this, according to the duct(s)70of the present embodiment, similar advantageous effects to those of the second embodiment can also be achieved. That is to say, the effects of reduction of temperature increase of the wall surface of the flow guide passage32(i.e., the inner wall of the first layer72a) are achieved, and heating of the charged gas that is suctioned into the duct70through the communication holes74is reduced.

It should be noted that, although the duct(s)70according to the third embodiment described above uses the communication holes74, a duct that is arranged so as to have the gap G in addition to this communication hole74can also achieve similar effects to those of the second and third embodiments.

Then, a fourth embodiment according to the present disclosure will be described with reference toFIGS. 8 and 9.

4-1. Difference from Second Embodiment

FIG. 8is a longitudinal cross-sectional view that schematically illustrates the configuration in and around a combustion chamber82of a compression-ignition internal combustion engine80according to the fourth embodiment of the present disclosure.FIG. 9is a transverse cross-sectional view obtained by cutting a passage wall portion88along an A-A line inFIG. 8. The internal combustion engine80according to the present embodiment is different from the internal combustion engine according to the second embodiment in the following points.

Specifically, the internal combustion engine80is equipped with a cylinder head84having a combustion chamber ceiling84a. In the combustion chamber ceiling84a, a flow guide passage86having the similar function to that of the flow guide passage32shown inFIG. 6is formed. In other words, according to the present embodiment, a “passage forming member” forming the flow guide passage86is integrally formed with the cylinder head84(combustion chamber ceiling84a).

As shown inFIGS. 8 and 9, the combustion chamber ceiling84aincludes a passage wall portion88located radially outward of the flow guide passage86. The passage wall portion88includes a first layer88aand a second layer88b. The first layer88ais a base portion that is connected to the cylinder head84(combustion chamber ceiling84a). That is to say, the first layer88ais integrally formed with the cylinder head84. In addition, the first layer88ais formed so as to protrude to the side of the combustion chamber12from a base surface84a1of the combustion chamber ceiling84a.

The second layer88bis located radially outward of the first layer88a. According to the example shown inFIG. 9, the second layer88bis formed so as to cover the first layer88athat protrudes from the base surface84a1of the combustion chamber ceiling84a. In addition, according to this example, the second layer88bis formed so as to also cover an end surface88a1of the first layer88alocated on the inlet side of the flow guide passage86.

The materials of the first layer88aand second layer88bof the passage wall portion88according to the present embodiment are the same as those of the first layer62aand second layer62baccording to the second embodiment, as an example. In addition, according to the present embodiment, the gap G is also formed between the outlet of the nozzle hole22and the inlet of the flow guide passage86. The internal combustion engine80may include communication holes similar to the communication holes74(seeFIG. 7) instead of this kind of gap G or in addition thereto.

According to the internal combustion engine80including the passage wall portion88described so far, similar advantageous effects to those of the internal combustion engine according to the second embodiment including the duct(s)60can also be achieved. In addition, according to the example shown inFIG. 8, the second layer88bis formed so as to also cover the end surface88a1of the first layer88alocated on the inlet side of the flow guide passage86. As a result, an increase of the wall surface temperature of the flow guide passage86due to a heat input into the end surface88a1from a high temperature combustion gas can also be reduced.

It should be noted that, as the material of the second layer88bof the duct60according to the present embodiment, silicon nitride (i.e., the example of the material that does not meet the above-described relationship with respect to the heat capacity) that is the same as the material of the second layer36baccording to the first embodiment may be used. In addition, in this example (i.e., in the example in which the effect of reducing the heating of a charged air suctioned into a duct through the gap G (seeFIG. 6) or a communication hole is not required), the second layer88bmay alternatively be arranged radially inward of the first layer88a, instead of the example shown inFIG. 8. This also applies to a fifth embodiment described below.

Then, a fifth embodiment according to the present disclosure will be described with reference toFIG. 10.

5-1. Difference from Fourth Embodiment

FIG. 10is a longitudinal cross-sectional view that schematically illustrates the configuration in and around a combustion chamber92of a compression-ignition internal combustion engine90according to the fifth embodiment of the present disclosure. The internal combustion engine90according to the present embodiment is different from the internal combustion engine80according to the fourth embodiment in the following points.

Specifically, the internal combustion engine90is equipped with a cylinder head94having a combustion chamber ceiling94a. In the combustion chamber ceiling94a, a passage forming member98that forms a flow guide passage96having the similar function to that of the flow guide passage86shown inFIG. 8is fastened using a fastener (not shown). That is to say, according to the present embodiment, the passage forming member98is separately arranged from the cylinder head94. The passage forming member98includes a passage wall portion100having a first layer100aand a second layer100b. The passage wall portion100is configured similarly to the passage wall portion88shown inFIG. 8. In addition, the first layer100ais connected to the cylinder head94via a fastening surface located between the passage wall portion100and the cylinder head94.

As described so far, the passage wall portion100according to the present embodiment is formed in the passage forming member98separately arranged from the cylinder head94. According to the internal combustion engine90having this kind of configuration, similar advantageous effects to those of the internal combustion engine according to the second embodiment having the duct60can also be achieved.

Then, a sixth embodiment according to the present disclosure and modification examples thereof will be described with reference toFIGS. 11 to 16.

6-1. Configuration In and Around Combustion Chamber

FIG. 11is a longitudinal cross-sectional view that schematically illustrates the configuration in and around a combustion chamber112of a compression-ignition internal combustion engine110according to the sixth embodiment of the present disclosure. The following explanation will be focused on the difference of the internal combustion engine110according to the present embodiment with respect to the internal combustion engine10according to the first embodiment.

As shown inFIG. 11, the internal combustion engine110is equipped with a piston116arranged in the interior of a cylinder114. A cavity118is formed at a central part of the piston116. This cavity118is also a part of the combustion chamber112. A fuel injection nozzle120is arranged at the center of a combustion chamber ceiling120aof a cylinder head120.

The top portion of the piston116is provided with a flow guide plate122. The flow guide plate122is fixed to the piston116at a predetermined distance (gap) from the cavity118formed at the top surface of the piston116. In the following, a configuration of the piston116with the flow guide plate122fixed thereto will be described in more detail with reference toFIGS. 12 and 13.

FIG. 12is a view of the piston116with the flow guide plate122shown inFIG. 11fixed thereto which is seen from the side of the top surface of the piston116.FIG. 13is an enlarged view that illustrates the configuration around the flow guide plate112shown inFIG. 11. As shown in these views, the flow guide plate122has an annular ring shape with a conical surface and covers a conical surface124included in surfaces of the cavity118that is downwardly inclined toward the outer peripheral side of the piston116. The flow guide plate122extends at a constant distance from the conical surface124and is fixed to the piston116by support portions126.

The support portions126are located between adjacent fuel sprays F and radially extend from an inner edge of the flow guide plate122having the annular ring shape toward an outer edge thereof. According to this kind of configuration, below each fuel spray F, a flow guide passage132having an inlet128located on the outer edge side (that is, the side of the wall of the bore of the cylinder114) and an outlet130located on the inner edge side (that is, the side of the center of the bore of the cylinder114) is formed in the gap between the flow guide plate122and the conical surface124. The inlet128and the outlet130are exposed in the combustion chamber112.

6-1-1. Flow Guide Plate (Passage Wall Portion) Having Double-Layered Structure

The flow guide plate122is located on the side of the combustion chamber ceiling120awith respect to the flow guide passage132. According to the internal combustion engine100of the present embodiment, this flow guide plate122corresponds to an example of the “passage wall portion” according to another aspect of the present disclosure. As shown inFIG. 13, the flow guide plate (passage wall portion)122has a double-layered structure composed of a first layer122aand a second layer122b.

The first layer122acorresponds to a base portion (base layer) connected to the piston116with the support portions126interposed therebetween. That is to say, the first layer122aof the flow guide plate (passage wall portion)122is supported by the support portions126.

The second layer122bis located on the side of the combustion chamber ceiling120awith respect to the first layer122a. In more detail, as an example, the second layer122bis formed so as to cover the whole first layer122a. In addition, as an example, the materials of the first layer122aand the second layer122bare the same as those of the first layer36aand the second layer36baccording to the first embodiment. That is to say, the toughness of the first layer122ais higher than the toughness of the second layer122b, and the thermal conductivity of the second layer122bis lower than the thermal conductivity of the first layer122a.

6-2-1. Advantageous Effects of Using Flow Guide Plate (Passage Wall Portion)

First, effects and advantages of the flow guide plate122will be described with reference toFIGS. 14 and 15.FIG. 14is a schematic diagram for illustrating a flow of air in a combustion chamber of a compression-ignition internal combustion engine having a piston200according to a comparative example without any flow guide plate.FIG. 15is a schematic diagram for illustrating a flow of air in the combustion chamber112of the compression-ignition internal combustion engine110having the piston116according to the sixth embodiment with the flow guide plate122shown inFIG. 11fixed thereto.

First, in the comparative example, the flow of air in the combustion chamber of the internal combustion engine having the piston200without the flow guide plate122will be described. As shown inFIG. 14, in the internal combustion engine without the flow guide plate122, in-cylinder gas (in more detail, fresh air in the combustion chamber) is taken in an upstream part of the fuel spray F while being mixed with a high-temperature burnt gas. As a result, there is a concern that, since the fuel spray F is mixed with the burnt gas at high temperature after ignition, the injected fuel may ignite too early. Because of this, an issue (such as, occurrence of smoke as a result of combustion of rich fuel or a decrease in thermal efficiency as a result of extension of the afterburning period) may occur.

In contrast to the above, in order to address the issue described above, the internal combustion engine110according to the present embodiment includes the piston116provided with the flow guide plate122. As shown inFIG. 15, the flow guide passage132is formed in the gap between the conical surface124of the piston116and the flow guide plate122. The fuel spray F injected from the fuel injection nozzle20is dispersed into the cavity118along an upper surface of the flow guide plate122(i.e., the surface located on the combustion chamber ceiling120a). In association with this, fresh air in the combustion chamber112is introduced into the flow guide passage132through the inlet128. The flow guide passage132is isolated from the fuel spray F by the flow guide plate122. Because of this, the fresh air introduced in the flow guide passage132through the inlet128exits the outlet130while being not mixed with much burnt gas at high temperature. As a result, the fresh air maintained at low temperature is taken in the upstream part of the fuel spray F, and it thus takes a certain time for the injected fuel to ignite. Therefore, combustion of rich fuel can be prevented, and occurrence of smoke or a decrease in thermal efficiency as a result of extension of the afterburning period can thus be prevented.

Furthermore, since the internal combustion engine110according to the present embodiment includes the flow guide passage132located on the lower side (that is, the side of the piston116) of the fuel sprays F, a low temperature fresh air exiting the outlet130can be efficiently taken in the upstream part of the fuel sprays F.

6-2-2. Issue on Installation of Flow Guide Plate (Passage Wall Portion)

A flow guide plate as in the flow guide plate122is exposed in a combustion chamber. That is to say, similarly to the example of the duct30according to the first embodiment, the flow guide plate122is arranged at a location in which the temperature thereof is easy to become higher due to the fact that the flow guide plate122is exposed to a high-temperature combustion gas. If the temperature of the wall surface itself of a flow guide passage (i.e., the wall surface itself of the flow guide plate located on the side of a piston) becomes higher due to the heat received from combustion gas, fresh air that passes through the flow guide plate is heated by the heat received from the flow guide plate. As a result, ignition delay is shortened (that is, the effect of retarding the self-ignition timing decreases), and thus, the combustion may be started before the fuel spray is sufficiently mixed with the charged air. Because of this, there is a concern that it may become difficult to properly reduce the occurrence of smoke.

In addition, in an example of the flow guide plate (passage wall portion), similarly to the example of the duct, it is required for countermeasures regarding reduction of temperature increase of the flow guide plate to be made such that, even if a weight or load is repeatedly applied to the flow guide plate, the shape of the flow guide plate can be more surely maintained over a long time.

6-2-3. Application of Flow Guide Plate (Passage Wall Portion) Having Double-Layered Structure

In view of the issue described above, according to the flow guide plate (passage wall portion)122of the present embodiment, the first layer122ais configured as a base portion that is connected to the piston116with the support portions126interposed therebetween. Also, the materials of the first layer122aand second layer122bare selected such that the toughness of the first layer122abecomes higher than the toughness of the second layer122b. As a result, even if the weight or load described above is repeatedly applied to the flow guide plate122, the shape of the flow guide plate122can be more surely maintained over a long time.

Moreover, the materials of those layers122aand122bof the flow guide plate122are selected such that the thermal conductivity of the second layer122bbecomes lower than the thermal conductivity of the first layer122a. As a result, the heat transferred to the wall of the flow guide plate122located on the side of the combustion chamber ceiling120a(i.e., the outer wall of the second layer122b) from a high temperature combustion gas around the flow guide plate122can be prevented from being transferred to the wall of the flow guide plate122located on the side of the piston116(i.e., the wall surface of the flow guide passage132). Because of this, when the in-cylinder gas (fresh air) passes through the flow guide passage132located on the side of the piston116of the flow guide plate122, an increaser of temperature of the fresh air can be reduced. As a result, a decrease of the effect of retarding the self-ignition timing can be reduced.

As described so far, according to the internal combustion engine110of the present embodiment, the reliability of maintaining the shape of the flow guide plate122(passage wall portion) can be favorably enhanced, and an increase of the wall surface temperature of the flow guide passage132can be favorably reduced.

Furthermore, as the material of the second layer122b, a material that is smaller in heat capacity per unit volume than that of the first layer122amay alternatively be selected similarly to the second layer62baccording to the second embodiment. As a result, the temperature of the second layer122bcan be prevented from always being high, and thus, an increase of the wall surface temperature of the flow guide passage132can be reduced more effectively.

6-3. Modification Examples Concerning Sixth Embodiment

6-3-1. Another Example of Double-Layered Structure for Passage Wall Portion

FIG. 16is a diagram for describing another example of the configuration of the first layer and second layer of the flow guide plate (passage wall portion). According to the example shown inFIG. 16, a flow guide plate140(passage wall portion) includes a first layer140athat is a base portion and a second layer140blocated on the side of the piston116with respect to the first layer140a. The double-layered structure for the passage wall portion may be changed as just described.

6-3-2. Another Example of Configuration of Passage Wall Portion

The flow guide passage132according to the sixth embodiment described above is formed between the flow guide plate122and the cavity118. However, a “flow guide passage” formed in a top portion of a piston according to another aspect of the present disclosure may be a through hole that is directly formed at a wall portion having a cavity of the piston, instead of the configuration described above. In this example, a part of a wall portion of the cavity having a double-bottom shape that is located on the side of the combustion chamber ceiling corresponds to an example of the “passage wall portion” according to another aspect of the present disclosure.

7. Other Embodiments

7-1. Other Examples of Selection of Material of Second Layer

In another example of the “second layer” that satisfies the above-described relationships regarding not only the toughness and the thermal conductivity but also the heat capacity per unit volume, the following may be used instead of zirconia (ZrO2) described above. That is to say, where an aluminum alloy is used as a material of the “first layer”, the second layer may be an anodized aluminum film formed by performing anodizing treatment on the surface of the first layer. According to the anodized aluminum film, a porous structure having pores that are formed in the process of the anodizing treatment is achieved, and thus, the second layer serves as a heat-shielding film that is lower in thermal conductivity and smaller in heat capacity per unit volume than the first layer.

Moreover, in still another example of the “second layer”, a ceramics-sprayed film obtained by performing thermal spraying of another ceramics (such as, zircon (ZrSiO4), silica (SiO2), silicon nitride (Si3N4), yttria (Y2O3) or titanium oxide (TiO2)) may be used instead of zirconia (ZrO2) described above. These sprayed-films have internal air bubbles that are formed in the process of the thermal spraying, and thus serve as heat-shielding films having lower heat capacities per unit volume than metal (such as, aluminum or iron used as the material of the first layer), similarly to the anodized aluminum film.

Furthermore, in yet another example of the “second layer”, a heat-insulating film (heat-shielding film) having the following structure may be used, as long as the whole second layer satisfies the above-described relationships regarding the toughness, the thermal conductivity and the heat capacity per unit volume. That is to say, this heat-shielding film includes a first heat insulator and a second heat insulator. The first heat insulator has a thermal conductivity lower than that of the base material (i.e., first layer) and also has a heat capacity per unit volume smaller than that of the base material. The second heat insulator has a thermal conductivity lower than or equal to the base material. In addition, the first heat insulator has a thermal conductivity lower than that of the second heat insulator, and the first heat insulator has a heat capacity per unit volume smaller than that of the second heat insulator. On that basis, specific examples of the first heat insulator include hollow ceramic beads, hollow glass beads, heat-insulating material having a microporous structure, silica aerogel, or any desired combination thereof. Also, specific examples of the second heat insulator include zirconia, silicon, titanium, zirconium, other ceramics, ceramic fibers, or any desired combination thereof. It should be noted that the details of heat-shielding films having these kinds of configurations are described in JP 5629463 B.

7-2. Another Example of Compression-Ignition Internal Combustion Engine

According to the first to sixth embodiments described above, diesel engines are used as an example of compression-ignition internal combustion engines. However, in another example, a compression-ignition internal combustion engine according to the present disclosure may be a premixed compression-ignition internal combustion engine that uses gasoline as its fuel, instead of the diesel engine.

7-3. Examples of Multi-Layered Structure Other Than Double-Layered

In other examples, a passage wall portion of a flow guide passage according to the present disclosure may not always have a double-layered structure as in the first to sixth embodiments described above and may have a multi-layered structure of triple or more multiple layers, as long as it includes a “first layer” and a “second layer” according to the present disclosure. That is to say, for example, the passage wall portion may have a triple-layered structure including a hollow layer located between the “first layer” and the “second layer”. In addition, for example, in order to increase the toughness of the passage wall portion or decrease the amount of heat transfer, the passage wall portion may has a third layer made of a different material located between the “first layer” and the “second layer”, or located on a side of the “first layer” opposite to the “second layer”, or located on a side of the “second layer” opposite to the “first layer”. Examples of these kinds of the third layers include a layer having a material for strengthening the bonding between the first layer and the second layer or a material for strengthening the coating of the second layer on the first layer.

7-4. Another Example of Passage Wall Portion

“Passage wall portions” according to the present disclosure and having a first layer connected to a cylinder head also include a passage wall portion without any of the gap G (seeFIG. 2) and the communication hole74(seeFIG. 7) contrary to the first to fifth embodiments described above. That is to say, by the use of this kind of passage wall portion, the passage wall portion may alternatively be configured so as to include a “first layer” and a “second layer” in order to reduce an increase of the wall surface temperature of a flow guide passage.

The embodiments and modification examples described above may be combined in other ways than those explicitly described above as required and may be modified in various ways without departing from the scope of the present disclosure.