JP6526456B2 - diesel engine - Google Patents

Description

  The present invention relates to a diesel engine.

  There is disclosed a technique for reducing the heat transfer from the combustion gas in the combustion chamber by forming the heat shield layer on the inner wall of the combustion chamber of the internal combustion engine to reduce the cooling loss and to improve the fuel efficiency.

  For example, a configuration is disclosed in which the side wall of the cavity provided on the piston top surface and the wall surface of the cylinder head facing the squish area on the piston top surface are covered with a heat insulating layer whose thermal conductivity is lower than that of other parts (Patent Document 1) ). This is partially shielded because shielding the entire combustion chamber degrades the air filling efficiency, but in heat dissipation from the combustion chamber to the wall, most of the heat dissipation through the wall surface of the cylinder head seen in the cavity side wall and squish area It is due to the result of analysis that Moreover, the structure which covers the piston top surface and the vicinity of the lip | rip part of a cavity with a heat insulation layer is also disclosed (patent document 2). The reason for selecting the piston top surface and the vicinity of the lip of the cavity as the portion to provide the heat insulating layer is not described. There is also disclosed a configuration in which a heat insulating layer covers a top surface of the piston head facing the inner wall surface of the cylinder head, an inner wall surface portion of the cylinder head facing the top surface, and a part of the side wall of the cavity. ). This is because, in the expansion stroke after the piston reaches the top dead center, the flame ejected to the squish area by the reverse squish phenomenon is extinguished and causes the emission of soot and hydrocarbons (HC) to increase. By shielding the squish area of the piston and the wall surface of the cylinder head desired there, it is possible to suppress the extinction and reduce the emission of soot and hydrocarbons (HC).

JP 2011-256757 A JP, 2013-87721, A Unexamined-Japanese-Patent No. 4-191413

  However, in the prior art, although the cooling loss can be reduced, the intake air is heated, which may cause the deterioration of the filling efficiency. Moreover, the heat shield in the spray flame collision area where the heat flux is high is insufficient, and the improvement of the thermal efficiency becomes insufficient.

  According to one aspect of the present invention, the bottom of the cavity including the lowest part of the concave cavity provided in the piston head is provided with a heat insulating layer having a lower thermal conductivity than the other part of the piston, It is a diesel engine characterized in that the heat insulation layer is not provided on the inner wall surface of the head.

  Here, in addition to the cavity bottom, it is preferable to provide the heat insulating layer on the cavity side wall.

  Preferably, the cavity has a plurality of recessed portions with respect to the top surface of the piston head.

  According to the present invention, it is possible to reduce the cooling loss in the combustion chamber of the diesel engine and to improve the charging efficiency of the intake air.

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows the structure of the diesel engine in embodiment of this invention. It is sectional drawing which shows the structure of the piston head in embodiment of this invention. It is a figure explaining the definition of the surface field of the piston head in an embodiment of the present invention. It is a figure explaining the definition of the surface field of the piston head in an embodiment of the present invention. It is a figure explaining the definition of the surface field of the piston head in an embodiment of the present invention. It is a figure which shows the result of computational fluid dynamics analysis of the heat transfer amount in a piston upper surface. It is a figure which shows the result of a computational fluid dynamics analysis of the heat transfer amount in the cavity of a piston. It is a figure which shows the result of a computational fluid dynamics analysis of the heat transfer amount in the inner wall face of a cylinder head. It is a figure which shows the result of computational fluid dynamics analysis of the heat transfer amount of the piston head in a combustion process. It is a figure which shows each area | region of the piston head which formed the heat insulation layer. It is a figure which shows the improvement rate of the thermal efficiency at the time of forming a heat insulation layer in each area | region of a piston head. It is a figure which shows the improvement rate of the thermal efficiency at the time of forming a heat insulation layer in each area | region of a piston head. It is sectional drawing which shows another example of a structure of the diesel engine in embodiment of this invention. It is sectional drawing which shows another example of a structure of the piston head in embodiment of this invention.

  As shown in FIG. 1, a diesel engine 100 according to an embodiment of the present invention includes a cylinder bore 12 formed in a cylinder block 10, a piston 14 reciprocating in the cylinder bore 12, and a gasket 16 above the cylinder block 10. It comprises the attached cylinder head 18. In the present embodiment, the diesel engine 100 will be described as a direct injection diesel engine.

  The cylinder head 18 is provided with an intake port 22 and an exhaust port 24 communicating with the combustion chamber 20. An intake valve 26 is provided at the intake port 22. The exhaust port 24 is provided with an exhaust valve 28. Further, the cylinder head 18 is provided with a fuel injection valve (injector) 29 for injecting fuel into the combustion chamber 20.

  A space surrounded by the cylinder bore 12, the cylinder head 18 and the piston 14 forms a combustion chamber 20 of the diesel engine 100. In addition, a concave cavity 14 b is formed in the piston head 14 a of the piston 14.

  In the diesel engine 100, fuel is injected from the fuel injection valve 29 into the combustion chamber 20 at a timing (crank angle = 0 °) at which the piston 14 is located near the compression top dead center. The fuel injected into the combustion chamber 20 is self-ignited and burned. The combustion of the fuel causes the pressure in the combustion chamber 20 to rise, and the piston 14 is pushed down and converted into a reciprocating motion.

  In the present embodiment, as shown in the enlarged view of FIG. 2, a heat insulating layer having a thermal conductivity lower than that of other portions of the piston 14 at the cavity bottom 30 in the concave cavity 14 b provided in the piston head 14 a of the piston 14 40 are provided. Furthermore, the heat insulating layer 40 is not provided on the top surface 32 of the piston head 14 a and the inner wall surface of the cylinder head 18.

  Further, it is more preferable to provide the heat insulating layer 40 on the cavity side wall 36 in addition to the cavity bottom 30. Further, in addition to the top surface 32 of the piston head 14 a and the inner wall surface of the cylinder head 18, it is preferable not to provide the heat insulating layer 40 in the cavity crest 34.

  Here, as shown in FIGS. 3A to 3C, each part of the piston head 14a is defined. As shown in FIG. 3A, the cavity bottom 30 is directed toward the center of the cavity 14b with respect to a line OP horizontally drawn from the center O of the arc (curve) of the most concave portion (bottom) of the cavity 14b. From the point A advanced 45 ° to the point B where the curve forming the bottom of the cavity 14b and the straight line forming the cavity crest 34 meet. Further, when the cavity crest 34 has a straight line and is formed by a convex curve, a point of inflection with a circular arc (curve) forming the bottom of the cavity 14 is taken as a point B.

  When the bottom of the cavity 14b is formed in a straight line as shown in FIG. 3 (b), the cavity bottom 30 is a boundary between the straight line from the point A and the straight line or curve of the cavity peak 34. It is an area up to point B.

  Further, as shown in FIG. 3C, when the range of the arc (curve) of the bottom of the cavity 14b is large and the boundary with the cavity crest 34 is reversed, the cavity bottom 30 is an arc from the point A to the bottom. The region up to the point B which is the boundary between (curve) and the straight line or curve of the cavity crest 34 is used.

  The top surface 32 is an area from the outermost peripheral portion of the piston head 14a to a position where it starts to be recessed as the concave cavity 14b. The cavity crest 34 is a region from the cavity bottom 30 to the outermost center of the cavity 14 b. The cavity side wall 36 is a region from the top surface 32 to a point A which starts to be recessed as the cavity 14 b and to the start point of the cavity bottom 30.

  The heat insulating layer 40 may be a material having a thermal conductivity lower than that of the material constituting the main body of the piston 14. The heat insulating layer 40 is preferably made of a ceramic material. For example, the heat insulating layer 40 is preferably made of an inorganic oxide such as zirconia, silica, alumina, titania, silicon carbide, silicon nitride, or aluminum nitride. The inorganic oxide can be formed by a thermal spraying method or an anodic oxidation method.

  The heat insulating layer 40 may have a film thickness such that the heat from the fuel burning in the combustion chamber 20 is not easily transmitted to the main body of the piston 14, but for example, 300 μm or more and 1 mm or less is preferable.

  FIG. 4 shows the results of numerical fluid dynamics analysis of the amount of heat transfer on the upper surface of the piston 14 in the intake stroke where the crank angle of the piston 14 is −330 ° and −300 ° when the compression top dead center is 0 °. Indicates Moreover, FIG. 5 shows the result of having conducted the numerical fluid dynamics analysis about the heat transfer amount in the cavity 14b of piston 14 in the intake stroke of crank angle -330 degrees and -300 degrees. Moreover, FIG. 6 shows the result of having conducted the numerical fluid dynamics analysis of the heat transfer amount in the inner wall face of the cylinder head 18 in the intake stroke whose crank angle of piston 14 is -330 degrees and -300 degrees. In FIG. 6, the four small circles on the inner wall surface of the cylinder head 18 correspond to locations where the intake valve 26 and the exhaust valve 28 are provided.

  From the analysis results of the heat transfer amount shown in FIG. 4 to FIG. 6, the heat transfer from the wall surface to the gas is large at the time of intake on the top surface 32 of the piston head 14a and the inner wall surface of the cylinder head 18 It can be said that it is a site. This is because the distance between the top surface 32 of the piston 14 and the inner wall surface of the cylinder head 18 is short near the exhaust top dead center where the intake starts (the crank angle is near -360 °), and passes through the intake valve 26 to the combustion chamber. This is because the intake gas introduced into the inside 20 strongly collides with the top surface 32 of the piston 14 and the inner wall surface of the cylinder head 18 to easily receive heat. Similarly to the top surface 32 of the piston 14, the intake gas is likely to collide with the cavity crest 34 as well, and the intake gas also tends to receive heat from the cavity crest 34.

  If the intake gas is excessively heated, the filling efficiency of the intake gas into the combustion chamber 20 may be reduced, and the heat insulating layer 40 is provided on the top surface 32 of the piston head 14a and the inner wall surface of the cylinder head 18. Is an unsuitable area. Furthermore, it is preferable not to provide the heat insulating layer 40 also in the cavity crest 34.

  FIG. 7 shows the result of numerical fluid dynamics analysis of the amount of heat transfer of the piston head 14 a in the combustion stroke of the fuel gas introduced into the combustion chamber 20. The region where the amount of heat transfer from the combustion gas to the piston head 14a is particularly large is the cavity sidewall 36 and then the cavity bottom 30. Therefore, in order to obtain a sufficient heat shielding effect in the combustion process, it is preferable to thermally shield the cavity side wall 36 and the cavity bottom 30 by the heat insulating layer 40.

  FIG. 8 is a perspective view of a part of the piston head 14a. The improvement rate of the thermal efficiency when the heat insulating layer 40 is formed in any of the regions A to H of the piston head 14a shown in FIG. 8 is measured, and the heat insulating layer 40 is formed in any of the regions A to H. We examined whether it was effective. The heat insulating layer 40 was formed by thermal spraying of a ceramic having a thermal conductivity λ of 3.2 W / mK (for example, thermal spraying of a metal oxide such as zirconia). The heat insulating layer 40 was not formed on the inner wall surface of the cylinder head 18. The thermal efficiency was determined under high speed full load operating conditions.

  FIG. 9 is a graph showing the improvement rate (%) of the thermal efficiency when the heat insulating layer 40 with a film thickness of 300 μm is formed in the regions D and E, the regions C to E, and the regions C to G. FIG. 10 is a graph showing the improvement rate (%) of the thermal efficiency when the heat insulating layer 40 with a film thickness of 500 μm is formed in the regions D and E, the regions C to E, and the regions C to G. In FIG. 9 and FIG. 10, when the heat insulation layer 40 with a film thickness of 500 μm is formed in the regions C to G, the relative value is shown, assuming that the improvement rate of the thermal efficiency is 1.0. Here, the region D and the region E correspond to the cavity sidewall 36. Region C corresponds to the cavity bottom 30. The area F and the area G correspond to the top surface 32 of the piston head 14a.

  When the thermal insulation layer 40 is formed only in the regions D and E, the improvement of the thermal efficiency is small, but when the thermal insulation layer 40 is formed in the regions C to E including the region C (cavity bottom 30), the thermal efficiency is largely Improved. On the other hand, when the heat insulating layer 40 is formed on the regions F and G (the top surface 32 of the piston head 14a) in addition to the regions C to E, compared to the case where the heat insulating layer 40 is formed only on the regions C to E There was almost no change in the thermal efficiency.

  As described above, when the heat insulating layer 40 is formed in the region F and the region G (the top surface 32 of the piston head 14a), the gas may be heated during the intake stroke, and the filling efficiency of the intake may decrease. And since the thermal efficiency is not greatly improved, the heat insulating layer 40 is not formed on the region F and the region G (the top surface 32 of the piston head 14a), and the regions C to E (cavity bottom 30 and cavity) It is preferable to form the heat insulating layer 40 only on the side wall 36).

  As shown in the cross sectional view of FIG. 11 and the enlarged cross sectional view of FIG. 12, the effect of the present invention becomes more remarkable in the configuration in which the cavity side wall 36 is provided with the plurality of steps 32b, 36a, 36b. Even when the distance between the piston head 14a and the inner wall surface of the cylinder head 18 is increased by providing the plurality of steps 32b, 36a, 36b in the cavity side wall 36, the fuel is directed from the center to the periphery of the combustion chamber 20 as well. The velocity of the gas is reduced, the flow of the fuel gas along the wall of the cavity 14 b becomes remarkable, and the time for which the fuel gas stays in the cavity bottom 30 is extended. As a result, it becomes susceptible to the heat insulation of the heat insulation layer 40 in the cavity bottom 30, and the improvement of the thermal efficiency by providing the heat insulation layer 40 in the cavity bottom 30 becomes more remarkable. In the present invention, the steps 36a and 36b in the first step are regarded as the cavity side wall 36 and the steps 32b in the second and subsequent steps are considered to be the top surface 32 of the piston head 14a.

  Reference Signs List 10 cylinder block, 12 cylinder bore, 14 piston, 14a piston head, 14b cavity, 16 gasket, 18 cylinder head, 20 combustion chamber, 22 intake port, 24 exhaust port, 26 intake valve, 28 exhaust valve, 29 fuel injection valve, 30 Cavity bottom, 32 top surface, 34 cavity crest, 36 cavity side wall, 36a, 36b step, 40 thermal barrier, 100 diesel engine.

Claims (1)

A piston is provided on at least a portion of the cavity sidewall and the cavity bottom of a concave cavity which is provided on a piston head and connected to a central cavity peak through a cavity bottom which is the lowest from a cavity sidewall on the periphery of the piston head And a heat insulating layer having a thermal conductivity lower than that of the other portion of the cylinder head, and the heat insulating layer is not provided on the top surface of the piston head and the cavity ridge portion and the inner wall surface of the cylinder head.
The cavity side wall has a three-step difference including a first step, a second step, and a third step from the top surface of the piston head located at the periphery of the piston head to the cavity bottom.
The heat insulating layer is not provided in a region from the first step to the front of the second step, and the second step and the third step are continuously extended from the second step to the cavity bottom. A diesel engine provided with the thermal insulation layer .