Gas engine

Abstract

A gas engine (100) according to one aspect of the present disclosure comprises: a cylinder (11); a piston (12) that reciprocates inside the cylinder (11) and has a cavity (32) formed within a range including the center of a top surface; a fuel injector (18) that directly injects gas fuel toward the center of the top surface of the piston (12); and an ignition device (17) that ignites an air-fuel mixture of the gas fuel and air.

Description

gas engine

[0001] The present disclosure relates to gas engines.

[0002] The following Patent Document 1 describes a gas-fueled direct injection engine in which a cavity is formed on the top surface of the piston. The cavity described in Patent Document 1 is offset toward the intake port, thereby guiding gas fuel to the vicinity of the spark plug and forming a combustible mixture near the spark plug. Note that in the gas-fueled direct injection engine described in Patent Document 1, because the cavity is offset toward the intake port, it is thought that the gas fuel is injected toward a portion of the top surface of the piston closer to the intake port than the center.

[0003] Japanese Patent Application Laid-Open No. 2005-337073

[0004] In the gas engine, the gas fuel is guided by the cavity wall and then supplied directly to the spark plug. As a result, the gas fuel is not mixed sufficiently with the air, and the gas fuel becomes locally too rich in the combustion chamber, which can increase the amount of NOx generated.

[0005] Therefore, an object of the present disclosure is to provide a gas engine that can suppress the generation of NOx.

[0006] A gas engine according to one aspect of the present disclosure includes a cylinder, a piston that moves back and forth within the cylinder and has a cavity formed in an area including the center of the top surface, a fuel injector that injects gas fuel directly toward the center of the top surface of the piston, and an ignition device that ignites a mixture of gas fuel and air.

[0007] According to a gas engine according to one aspect of the present disclosure, the generation of NOx can be suppressed.

[0008] Fig. 1 is a schematic diagram of a gas engine according to a first embodiment. Fig. 2 is a cross-sectional view taken along the line II-II in Fig. 1. Fig. 3 is a view corresponding to Fig. 2 of a gas engine according to a second embodiment.

[0009] First Embodiment First, a gas engine 100 according to the first embodiment will be described. Fig. 1 is a schematic diagram of the gas engine 100 according to the first embodiment. Fig. 2 is a cross-sectional view taken along the line II-II in Fig. 1. The gas engine 100 uses a gas fuel such as hydrogen gas or natural gas as fuel.

[0010] As shown in Fig. 1, the gas engine 100 includes a cylinder 11, a piston 12, an intake port 13, an exhaust port 14, an intake valve 15, an exhaust valve 16, and an ignition device 17. Furthermore, as shown in Fig. 2, the gas engine 100 includes a fuel injector 18. These components will be described below in order.

[0011] <Cylinder> The cylinder 11 is formed by a cylinder block 21 and a cylinder head 22. The cylinder block 21 forms the inner surface of the cylinder 11, and the cylinder head 22 forms the top surface of the cylinder 11. Although one cylinder 11 is shown in FIG. 1, the gas engine 100 according to this embodiment is equipped with multiple cylinders 11. The vertical direction of the paper in FIG. 1 is the central axial direction of the cylinder 11. Hereinafter, the central axial direction of the cylinder 11 will be simply referred to as the "axial direction." Furthermore, below, the direction perpendicular to the central axis 23 of the cylinder 11 will be referred to as the "radial direction."

[0012] <Piston> The piston 12 is a member that reciprocates within the cylinder 11 and is connected to the crankshaft via a connecting rod 24. The piston 12 forms a combustion chamber 25 together with the cylinder 11. The piston 12 also has a cavity 32 located in a top surface 31. In this embodiment, the cavity 32 is formed in a range that includes the center of the top surface 31 of the piston 12. More specifically, the cavity 32 is circular when viewed in the axial direction, and the center of the bottom surface 33 is located on the central axis 23 of the cylinder 11. However, the shape and position of the cavity 32 are not limited to those described above.

[0013] The "center of top surface 31" of piston 12 mentioned above refers not only to a point on top surface 31 on central axis 23 of cylinder 11 (hereinafter referred to as the "center point"), but also to a portion that can be said to be substantially the center point, that is, a portion slightly displaced radially from the center point. For example, a portion that is displaced radially from the center point by 20% or less of the diameter of cylinder 11 is included in the "center of top surface 31."

[0014] Furthermore, the area of ​​a cross section of the cavity 32 perpendicular to the axial direction (hereinafter referred to as the "cross-sectional area") is 25% to 80% of the cross-sectional area of ​​the cylinder 11. In this embodiment, the cross section of the cavity 32 perpendicular to the axial direction is circular. In such a case, if the diameter of the cavity 32 is 50% of the inner diameter of the cylinder 11, the cross-sectional area of ​​the cavity 32 is 25% of the cross-sectional area of ​​the cylinder 11. Furthermore, if the diameter of the cavity 32 is approximately 90% of the inner diameter of the cylinder 11, the cross-sectional area of ​​the cavity 32 is 80% of the cross-sectional area of ​​the cylinder 11. Note that if the cross-sectional area of ​​the cavity 32 varies depending on the axial position, the "cross-sectional area of ​​the cavity 32" in the phrase "the cross-sectional area of ​​the cavity 32 is 25% to 80% of the cross-sectional area of ​​the cylinder 11" refers to the largest cross-sectional area of ​​the cavity 32.

[0015] <Air Intake Port> The air intake port 13 is a port that supplies air to the combustion chamber 25, and is formed in the cylinder head 22. In this embodiment, one air intake port 13 is formed for one cylinder 11. However, multiple air intake ports 13 may be formed for one cylinder 11.

[0016] <Exhaust Port> The exhaust port 14 is a port that discharges exhaust gas from the combustion chamber 25, and is formed in the cylinder head 22. In this embodiment, one exhaust port 14 is formed for one cylinder 11. However, multiple exhaust ports 14 may be formed for one cylinder 11.

[0017] <Air Intake Valve> The air intake valve 15 is a valve that opens and closes the air intake port 13. The air intake valve 15 may be configured so that the timing at which the air intake port 13 is opened and closed can be changed, or may be configured so that the timing at which the air intake port 13 is opened and closed is constant.

[0018] <Exhaust Valve> The exhaust valve 16 is a valve that opens and closes the exhaust port 14. The exhaust valve 16 may be configured so that the timing at which the exhaust port 14 is opened and closed can be changed, or may be configured so that the timing at which the exhaust port 14 is opened and closed is constant.

[0019] <Ignition Device> The ignition device 17 ignites the air-fuel mixture in the combustion chamber 25. In this embodiment, the ignition device 17 is provided in the cylinder block 21 so as to face the combustion chamber 25 and is located on the central axis 23 of the cylinder 11. The ignition device 17 may be a spark plug, a sub-chamber ignition device, or a device that injects pilot fuel, which is easier to ignite than the main fuel, and ignites the pilot fuel by self-ignition. Gas fuel is injected into the combustion chamber 25 from a fuel injector 18 (described later), and the injected gas fuel is mixed with air supplied from the intake port 13. This forms a gas-fuel-air mixture. Increasing the proportion of gas fuel in the mixture near the ignition device 17 (hereinafter referred to as "air-fuel mixture concentration") makes ignition by the ignition device 17 easier.

[0020] On the other hand, depending on the method used to increase the concentration of the air-fuel mixture near the ignition device 17, variations in the concentration of the air-fuel mixture may occur within the combustion chamber 25, resulting in a locally excessively rich mixture. In this case, the combustion temperature increases in the area where the excessively rich mixture exists, making it more likely that NOx will be generated. In other words, to suppress the generation of NOx, it is desirable to suppress variations in the concentration of the air-fuel mixture within the combustion chamber 25 and make the concentration of the mixture uniform. Therefore, as will be described later, the gas engine 100 according to this embodiment is configured to make the concentration of the air-fuel mixture uniform within the combustion chamber 25.

[0021] <Fuel injector> The fuel injector 18 is a device that injects gas fuel into the combustion chamber 25. As shown in FIG. 2 , the fuel injector 18 is provided in the cylinder block 21 so as to face the combustion chamber 25, and is located radially outward of the central axis 23 of the cylinder 11. The fuel injector 18 of this embodiment injects gas fuel directly toward the center of the top surface 31 of the piston 12. However, because the piston 12 moves within the cylinder 11, it is substantially impossible for the fuel injector 18 to continue to inject gas fuel directly toward the center of the top surface 31 of the piston 12. Therefore, the fuel injector 18 only needs to inject gas fuel directly toward the center of the top surface 31 of the piston 12 at a certain point during the injection of the gas fuel.

[0022] As described above, in this embodiment, the fuel injector 18 injects gas fuel directly toward the center of the top surface 31 of the piston 12, so that the gas fuel injected from the fuel injector 18 collides with the bottom surface 33 of the cavity 32. The gas fuel that collides with the bottom surface 33 of the cavity 32 creates vortices near the side surface 35, promoting mixing of the gas fuel and air. This forms a mixture with a uniform concentration, which spreads from the top of the cavity 32 to the ignition device 17. Therefore, with the gas engine 100 according to this embodiment, the concentration of the mixture can be uniformized, thereby suppressing the generation of NOx. Furthermore, uniforming the concentration of the mixture can also suppress abnormal combustion.

[0023] Furthermore, the gas fuel is mixed with air within the cavity 32. This prevents the air-fuel mixture from flowing into the squish area 26. The squish area 26 refers to the gap formed between the portion of the top surface 31 of the piston 12 radially outward of the cavity 32 and the underside of the cylinder block 21. By preventing the air-fuel mixture from flowing into the squish area 26, various effects can be achieved, such as preventing excessive dilution of the formed air-fuel mixture, reducing blow-by gas, reducing cooling loss, and preventing abnormal combustion caused by high-temperature particles derived from the lubricating oil generated by the piston rings. Of these, reducing blow-by gas and reducing cooling loss contribute to improving the thermal efficiency of the engine.

[0024] Furthermore, the fuel injector 18 of this embodiment has only one injection opening 34 for injecting gas fuel. Therefore, according to this embodiment, the directionality of the gas fuel injected from the fuel injector 18 is improved, and the gas fuel injected toward the center of the top surface 31 of the piston 12 can accurately reach the center of the top surface 31 of the piston 12. The shape (not the arrangement) of the injection opening 34 is not limited, and may be circular, rectangular, or annular.

[0025] Furthermore, the pressure of the gas fuel injected by the fuel injector 18 of this embodiment is less than 10 MPa. Generally, the pressure of the gas fuel to be injected is classified into "low pressure" which is equal to or greater than 0.1 MPa and less than 3 MPa, "medium pressure" which is equal to or greater than 3 MPa and less than 10 MPa, and "high pressure" which is equal to or greater than 10 MPa. In other words, the pressure of the gas fuel injected by the fuel injector 18 of this embodiment is low pressure or medium pressure.

[0026] In this way, by suppressing the pressure of the gas fuel injected by the fuel injector 18, it is possible to prevent the gas fuel from being discharged from the cavity 32 before being mixed with air. In other words, the concentration of the mixture can be made more uniform. Furthermore, it is also possible to suppress the inflow of the mixture into the squish area 26. Furthermore, since the pressure of the injected gas fuel is relatively low in this embodiment, it is possible to reduce the costs of auxiliary equipment such as a compressor that pressurizes the gas fuel and a tank that stores the pressurized gas fuel, compared to when the pressure of the injected gas fuel is high.

[0027] As described above, in this embodiment, the bottom surface area of ​​the cavity 32 is 25% to 80% of the cross-sectional area of ​​the cylinder 11. By making the bottom surface area of ​​the cavity 32 25% or more of the cross-sectional area of ​​the cylinder 11, it is possible to prevent the concentration of the air-fuel mixture in the cavity 32 from becoming too high. Furthermore, by making the bottom surface area of ​​the cavity 32 80% or less of the cross-sectional area of ​​the cylinder 11, it is possible to maintain the diffusibility of the gas fuel.

[0028] Furthermore, the fuel injector 18 of this embodiment starts injecting gas fuel when the crank angle of the piston 12 is between 90 degrees before bottom dead center (BBDC) and 140 degrees after bottom dead center (ABDC) and after the intake valve 15 closes the intake port 13. In this embodiment, the fuel injector 18 injects gas fuel at a slightly earlier timing, between 90 degrees before bottom dead center and 140 degrees after bottom dead center, which allows sufficient time for the formation of an air-fuel mixture and makes the concentration of the mixture more uniform. Furthermore, because the fuel injector 18 injects gas fuel after the intake valve 15 closes the intake port 13, it is possible to prevent the gas fuel from blowing back into the intake port 13.

[0029] Second Embodiment Next, a gas engine 200 according to the first embodiment will be described. Figure 3 is a view corresponding to Figure 2 of the gas engine 200 according to the second embodiment. The gas engine 200 according to the second embodiment differs from the gas engine 100 according to the first embodiment in the shape of the cavity 32. Other than that, the gas engine 200 has the same configuration as the gas engine 100 according to the first embodiment. Therefore, the following description will mainly focus on the shape of the cavity 32 of the gas engine 200 according to the second embodiment, and other descriptions will be omitted.

[0030] In this embodiment, the cavity 32 includes a protrusion 41. The protrusion 41 is located on the bottom surface 33 of the cavity 32. The protrusion 41 is convex toward the opening of the cavity 32, that is, toward the axially outward direction (upward in the plane of FIG. 3 ). The protrusion 41 in this embodiment has a substantially conical shape, with a hemispherical tip. The apex of the protrusion 41 is located at the center of the top surface 31 of the piston 12. Therefore, gas fuel injected from the fuel injector 18 toward the center of the top surface 31 of the piston 12 collides near the apex of the protrusion 41.

[0031] Therefore, according to this embodiment, the gas fuel injected from the fuel injector 18 collides with and diffuses near the top of the projection 41, and further diffuses by spreading radially outward along the surface of the projection 41. The diffused gas fuel then mixes with air due to vortexes formed near the side surface 35, forming a mixture with a more uniform concentration. Therefore, the gas engine 200 according to this embodiment can further suppress the generation of NOx.

[0032] Furthermore, the volume of the cavity 32 is set to an appropriate value depending on design conditions such as the engine compression ratio and the valve lifts of the intake valve 15 and the exhaust valve 16. Therefore, under the same design conditions, the cavity 32 including the protrusion 41 as in this embodiment will have a deeper recess than the cavity 32 not including the protrusion 41. Therefore, in the gas engine 200 according to this embodiment, the air-fuel mixture is more likely to be retained in the cavity 32, and the air-fuel mixture is less likely to flow into the squish area 26.

[0033] Furthermore, the cavity 32 of this embodiment includes a barb 42. The barb 42 is located at the opening of the cavity 32 and is inclined inward. In other words, the axially outer end portion of the side surface 35 of the cavity 32 corresponds to the barb 42, and this portion is formed so that the inner diameter decreases as it extends axially outward.

[0034] As described above, the cavity 32 of this embodiment includes the barbs 42, which can prevent the air-fuel mixture formed in the cavity 32 from flowing into the squish area 26. Furthermore, in this embodiment, the boundary between the bottom surface 33 and the side surface 35 of the cavity 32 is curved in cross section, which makes it easier for the air-fuel mixture formed in the cavity 32 to flow along the side surface 35. This, combined with the fact that the cavity 32 includes the barbs 42, can further prevent the air-fuel mixture from flowing into the squish area 26.

[0035] Although the cavity 32 in this embodiment includes both the protrusion 41 and the barb 42, it may include only one of them.

[0036] (Modification) The gas engines 100, 200 described above may be provided with a swirl flow generating mechanism that generates a swirl flow within the cylinder 11. The swirl flow here refers to a flow of fuel gas and air with the central axis of the cylinder 11 as its axis of rotation. Because fuel gas has a lower density than air, when a swirl flow is generated within the cylinder 11, the fuel gas moves toward the radially inner portion of the cylinder 11. As a result, a mixture containing fuel gas is less likely to be generated near the squish area 26, and the flow of the mixture into the squish area 26 can be further suppressed.

[0037] Furthermore, as described above, the gas engines 100, 200 can homogenize the concentration of the air-fuel mixture in the cavity 32. These gas engines 100, 200 are equipped with a swirl flow generating mechanism that generates a swirl flow in the cylinder 11, which suppresses the flow of the air-fuel mixture into the squish area 26, thereby further homogenizing the concentration of the air-fuel mixture in the cavity 32 and further suppressing the generation of NOx.

[0038] The above-described swirl flow generating mechanism is, for example, a helical air intake port 13. The helical air intake port 13 has an air intake passage formed in a spiral shape, and air passing through the air intake passage generates a swirl flow inside the cylinder 11. The swirl flow generating mechanism may also be an air intake port 13 that introduces air into the cylinder 11 in a tangential direction of the cylinder 11. In this case, the air introduced by the air intake port 13 swirls along the inner surface of the cylinder 11, thereby generating a swirl flow. Note that the swirl flow generating mechanism is not limited to the above configuration and may be configured in a manner other than the air intake port 13.

[0039] <Summary> The first item disclosed in this specification is a gas engine comprising a cylinder, a piston that reciprocates within the cylinder and has a cavity formed in an area including the center of the top surface, a fuel injector that injects gas fuel directly toward the center of the top surface of the piston, and an ignition device that ignites a mixture of gas fuel and air.

[0040] According to this configuration, the gas fuel is diffused within the cavity, thereby making it possible to homogenize the concentration of the mixture and suppress the generation of NOx.

[0041] A second item disclosed in this specification is a gas engine described in the first item, wherein the fuel injector starts injecting gas fuel when the piston crank angle is between 90 degrees before bottom dead center and 140 degrees after bottom dead center and after the intake valve closes the intake port.

[0042] According to this configuration, the concentration of the air-fuel mixture can be made more uniform, and the gas fuel can be prevented from blowing back into the intake port.

[0043] A third item disclosed in this specification is the gas engine according to the first or second item, wherein the cross-sectional area of ​​the cavity is 25% or more and 80% or less of the cross-sectional area of ​​the cylinder.

[0044] According to this configuration, it is possible to prevent the concentration of the air-fuel mixture in the cavity from becoming too high, and it is possible to maintain the diffusibility of the gas fuel.

[0045] A fourth item disclosed in this specification is the gas engine according to any one of the first to third items, wherein the center of the bottom surface of the cavity is positioned on the central axis of the cylinder.

[0046] According to this configuration, the gas fuel can be efficiently diffused within the cavity.

[0047] A fifth item disclosed in this specification is a gas engine described in any one of the first to fourth items, wherein the fuel injector has only one injection opening for injecting gas fuel.

[0048] According to this configuration, the gas fuel injected from the fuel injector can be made to reach the center of the top surface of the piston with high accuracy.

[0049] A sixth item disclosed in this specification is the gas engine according to any one of the first to fifth items, wherein the pressure of the gas fuel injected by the fuel injector is less than 10 MPa.

[0050] This configuration makes it possible to make the concentration of the air-fuel mixture more uniform.

[0051] A seventh item disclosed in this specification is a gas engine described in any one of items 1 to 6, wherein the cavity includes a protrusion located on the bottom surface and convex toward the opening.

[0052] This configuration makes it possible to make the concentration of the air-fuel mixture more uniform.

[0053] An eighth item disclosed in this specification is a gas engine according to any one of the first to seventh items, wherein the cavity is located in the opening portion and includes an inwardly inclined barb.

[0054] This configuration can prevent the air-fuel mixture from flowing into the squish area.

[0055] A ninth item disclosed in this specification is a gas engine described in any one of claims 1 to 8, which is provided with a swirl flow generating mechanism that generates a swirl flow within the cylinder.

[0056] This configuration can further prevent the air-fuel mixture from flowing into the squish area.

[0057] REFERENCE SIGNS LIST 11 Cylinder 12 Piston 13 Intake port (swirl flow generating mechanism) 15 Intake valve 17 Ignition device 18 Fuel injector 23 Central axis 31 Top surface 32 Cavity 33 Bottom surface 34 Injection opening 41 Protrusion 42 Barb 100 Gas engine 200 Gas engine

Claims

1. A gas engine comprising: a cylinder; a piston that reciprocates within the cylinder and has a cavity formed in an area including the center of the top surface; a fuel injector that injects gas fuel directly toward the center of the top surface of the piston; and an ignition device that ignites a mixture of gas fuel and air.

2. A gas engine as described in claim 1, wherein the fuel injector begins injecting gas fuel when the piston crank angle is between 90 degrees before bottom dead center and 140 degrees after bottom dead center and after the intake valve closes the intake port.

3. The gas engine according to claim 1, wherein the cross-sectional area of ​​the cavity is 25% or more and 80% or less of the cross-sectional area of ​​the cylinder.

4. The gas engine according to claim 1, wherein the center of the bottom surface of the cavity is located on the central axis of the cylinder.

5. A gas engine as claimed in claim 1, wherein said fuel injector has only one injection opening for injecting gas fuel.

6. The gas engine according to claim 1, wherein the pressure of the gas fuel injected by the fuel injector is less than 10 MPa.

7. A gas engine as set forth in claim 1, wherein said cavity includes a protrusion located on a bottom surface thereof and convex toward the opening.

8. A gas engine as set forth in claim 1, wherein said cavity includes an inwardly sloping barb located at an opening thereof.

9. The gas engine according to claim 1, further comprising a swirl flow generating mechanism for generating a swirl flow within the cylinder.

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