JP7686474B2 - Combustion chamber structure of direct injection diesel engine - Google Patents

Description

One aspect of the present invention relates to a combustion chamber structure for a direct injection diesel engine.

As a combustion chamber structure for a direct injection diesel engine, a structure is known in which, in contrast to a reentrant combustion chamber shape, a step is provided from the squish area to the combustion space, and a lip structure is provided at the combustion chamber entrance (see, for example, Patent Document 1).

Patent No. 4906055

There is a demand for a combustion chamber structure that is more fuel efficient than the combustion chamber structure described in Patent Document 1 above.

One aspect of the present invention was made in consideration of the above situation, and aims to provide a combustion chamber structure for a direct injection diesel engine that is more fuel efficient.

The combustion chamber structure of a direct injection diesel engine according to one embodiment of the present invention is a combustion chamber structure for a direct injection diesel engine that has a cavity recessed downward relative to the piston top surface, and in which fuel is injected radially from the center of the cylinder ceiling into the cavity to cause self-ignition, and includes a first rising portion rising upward from the bottom surface of the cavity, a first horizontal portion that is continuous with the upper end of the first rising portion and extends approximately horizontally in the outer periphery of the cavity, and a second rising portion that is continuous with the first horizontal portion and rises upward. The combustion chamber has a first chamber defined by the bottom surface of the cavity and the first rising portion, a second chamber defined by the first horizontal portion and the second rising portion, and a third chamber defined by the second horizontal portion and the third rising portion, and the volume ratio of the third chamber to the entire combustion chamber is 2% or more and 40% or less.

In the combustion chamber structure of the direct injection diesel engine according to one embodiment of the present invention, so-called lip portions consisting of rising portions (first rising portion, second rising portion, third rising portion) and horizontal portions (first horizontal portion, second horizontal portion, piston top surface) are formed in three locations. Therefore, the combustion chamber structure is formed with a first chamber defined by the bottom surface of the cavity and the first rising portion, a second chamber defined by the first horizontal portion and the second rising portion (the second chamber formed on the outer periphery side and above the first chamber), and a third chamber defined by the second horizontal portion and the third rising portion (the third chamber formed on the outer periphery side and above the second chamber). In the conventional combustion chamber structure, there are only two so-called lip portions, and no combustion chamber corresponding to the third chamber of the present invention is formed. In contrast, in the present invention, in which the third chamber is formed outside and above the second chamber, the fuel spray flowing in the outer circumferential direction can reach the third rising portion (the vertical wall of the uppermost combustion chamber) that reaches the piston top surface through the third chamber, and the third chamber, which was not formed in the past, can be actively used as a space for mixing the fuel spray and air. That is, in the present invention, a space (third chamber) for mixing the fuel spray and air is formed on the outer circumferential side of the cavity, so that the opportunities for the fuel and oxygen to come into contact and react (burn) can be increased, and combustion can be promoted. In addition, in the present invention, the volume ratio of the third chamber to the entire combustion chamber is set to 2% or more and 40% or less, so that while promoting combustion on the outer circumferential side of the cavity, it is also possible to prevent the third chamber from becoming too large and causing combustion to become sluggish. As described above, according to one aspect of the present invention, a combustion chamber structure for a direct injection diesel engine that is more advantageous in terms of fuel efficiency can be provided.

The length of the third rising portion that rises upward may be 3% or more and 9.5% or less of the length of the bottom surface of the cavity in a direction intersecting the up-down direction. In this way, by setting the length of the third rising portion that divides the third chamber, it is possible to ensure a sufficient size of the third chamber to promote combustion on the outer periphery of the cavity, while also preventing the third chamber from becoming too large and slowing down combustion.

The length of the second horizontal portion in a direction intersecting the vertical direction may be 10% or more and 18% or less of the length of the bottom surface of the cavity in a direction intersecting the vertical direction. In this way, by setting the length of the second horizontal portion that divides the third chamber, it is possible to ensure a sufficient size of the third chamber to promote combustion on the outer periphery of the cavity, while also preventing the third chamber from becoming too large and slowing down combustion.

According to one aspect of the present invention, it is possible to provide a combustion chamber structure for a direct injection diesel engine that is more fuel efficient.

FIG. 1 is a schematic diagram of a diesel engine equipped with exhaust gas recirculation. FIG. 1 is a diagram showing a schematic diagram of a combustion chamber structure of a diesel engine. FIG. 2 is a diagram illustrating a combustion chamber structure. 1A and 1B are diagrams illustrating the structural difference between a two-stage lip structure and a three-stage lip structure. 1 is a diagram illustrating a difference in combustion state between a two-stage lip structure and a three-stage lip structure. FIG. 1 is a diagram illustrating a difference in combustion state between a two-stage lip structure and a three-stage lip structure. FIG. 4 is a graph showing the relationship between the volume ratio of the third stage combustion chamber and the fuel efficiency improvement effect.

The combustion chamber structure of a diesel engine according to an embodiment will be described below with reference to the drawings. In the following description, the same or equivalent elements will be given the same reference numerals, and duplicate descriptions will be omitted.

Figure 1 is a schematic diagram of a diesel engine 1 equipped with a mechanism for recirculating exhaust gas. The diesel engine 1 is a direct injection diesel engine, and may be one that employs so-called exhaust gas recirculation (EGR), in which, for example, a portion of the exhaust gas is extracted from the exhaust side and returned to the intake side, and the exhaust gas returned to the intake side is used to suppress the combustion of fuel in the engine, lowering the combustion temperature and reducing the generation of NOx.

In a diesel engine 1, an exhaust passage 3 through which exhaust gas 2 flows is connected to an intake passage 4 by an EGR pipe 5. A portion of the exhaust gas 2 is recirculated together with intake air 7 via an EGR valve 6 provided midway through the EGR pipe 5 and sent into the cylinders of the diesel engine 1, lowering the combustion temperature in the cylinders and reducing NOx.

A multi-hole injector 8 that injects fuel (diesel) into the cylinder is provided on the cylinder ceiling 11, which is the ceiling of each cylinder of the diesel engine 1. A cavity 10 that is recessed downward is formed on the top surface of the piston 9, and fuel is injected radially from the tip of the injector 8 onto the inner surface of the cavity 10, causing self-ignition due to the high temperature inside the cylinder at the end of the compression stroke.

The injection operation of the injector 8 in the diesel engine 1 is controlled by a fuel injection command 8a from a control device 12, which constitutes an engine control computer, and the fuel injection command 8a is output to the injector 8 near the top dead center of the compression stroke to inject fuel.

The control device 12 also receives an accelerator opening signal 13a from an accelerator sensor 13 that detects the accelerator opening as a load on the diesel engine 1, and a rotation speed signal 14a from a rotation sensor 14 that detects the engine speed of the diesel engine 1. In this way, the operating state of the diesel engine 1 is constantly monitored to perform various engine controls.

As shown in FIG. 1, the diesel engine 1 has a crankshaft 15, an exhaust port 16, an exhaust valve 17, an exhalation port 18, and an exhalation valve 19. The exhalation valve 19 and the exhaust valve 17 are opened and closed at appropriate times according to the stroke of each cylinder via a push rod or a rocker arm by a cam provided on an engine-driven camshaft (not shown).

Next, the combustion chamber structure 100 of the diesel engine 1 including the above-mentioned cavity 10 will be described with reference to Figures 2 to 6.

As shown in FIG. 2, the combustion chamber structure 100 is a combustion chamber structure for a direct injection diesel engine that has a cavity 10 recessed downward from the piston top surface 9a, which is the top surface of the piston 9, and injects fuel radially into the cavity 10 from the center of the cylinder ceiling 11 (see FIG. 1) to cause self-ignition.

The combustion chamber structure 100 includes a first rising portion 102, a first horizontal portion 103, a second rising portion 104, a second horizontal portion 105, and a third rising portion 106.

The first rising portion 102 is a wall-like portion that rises upward from the radial outer end of the bottom surface 101 so as to surround the bottom surface 101 (bottom) of the cavity 10, which is circular in plan view. The first horizontal portion 103 is a portion that is continuous with the upper end of the first rising portion 102 and extends approximately horizontally in the outer circumferential direction (radial outer side) of the cavity 10. The second rising portion 104 is a wall-like portion that is continuous with the radial outer end of the first horizontal portion 103 and rises upward. The second horizontal portion 105 is a portion that is continuous with the upper end of the second rising portion 104 and extends approximately horizontally in the outer circumferential direction (radial outer side) of the cavity 10. The third rising portion 106 is a wall-like portion that is continuous with the radial outer end of the second horizontal portion 105 and rises upward. The upper end of the third rising portion 106 reaches the piston top surface 9a. The third raised portion 106 functions as a flame prevention wall to prevent flame impingement on the liner.

The first lip portion 201 is formed by the first raised portion 102 and the first horizontal portion 103. The second lip portion 202 is formed by the second raised portion 104 and the second horizontal portion 105. The third lip portion 203 is formed by the third raised portion 106 and the piston top surface 9a.

The width dimension D' (in a direction intersecting the vertical direction) of the third rising portion 106, which is a flame prevention wall in the cavity 10, may be 95% or less of the length of the bore. In addition, the length L' (length of the second horizontal portion 105 in a direction intersecting the vertical direction) of the portion extending substantially horizontally of the second horizontal portion 105 may be 10% or more and 18% or less of the diameter d (length of the bottom surface 101 in a direction intersecting the vertical direction) of the bottom surface 101 of the cavity 10. In addition, the length h' rising upward of the third rising portion 106 may be 3% or more and 9.5% or less of the diameter d of the bottom surface 101 of the cavity 10 described above.

3 is a diagram illustrating the combustion chamber structure 100. As shown in FIG. 3, the combustion chamber included in the combustion chamber structure 100 is configured to include a first stage combustion chamber V1 (first chamber), a second stage combustion chamber V2 (second chamber), and a third stage combustion chamber V3 (third chamber). The first stage combustion chamber V1 is an area defined by the bottom surface 101 and the first rising portion 102 of the cavity 10. The second stage combustion chamber V2 is an area defined by the first horizontal portion 103 and the second rising portion 104 (area shown by diagonal lines in FIG. 3). The third stage combustion chamber V3 is an area defined by the second horizontal portion 105 and the third rising portion 106 (area shown by solid color in FIG. 3). The volume ratio of the third stage combustion chamber V3 to the entire combustion chamber is, for example, 2% or more and 40% or less, and preferably 4% or more and 35% or less. In addition, the volume ratio of the combined area of the second stage combustion chamber V2 and the third stage combustion chamber V3 to the entire combustion chamber is, for example, 50% or more and 80% or less.

In this way, in the combustion chamber structure 100 according to this embodiment, three lip portions are formed, and an additional step (third-stage combustion chamber V3) is provided in the squish area S (see FIG. 1) of a conventional combustion chamber (a combustion chamber having two lip portions), and the shape of the second lip portion 202, which is the lip portion within the step, is offset downward by a dimension equivalent to the step depth. By providing the step in this way, the volume inside the cavity 10 increases, but the bottom position of the cavity 10 is offset upward by the amount of the increase in volume, so that the volume is adjusted to be equivalent to that of a conventional combustion chamber. This reduces the squish flow, reduces cooling loss, and improves fuel efficiency. Note that if the squish flow is extremely reduced, it is considered that combustion efficiency will deteriorate, but if it is assumed that EGR will be reduced, smoke can be effectively suppressed even with a low squish flow. In the following, a structure in which only two lip portions are formed as in the conventional combustion chamber structure may be referred to as a two-stage lip structure, and a structure in which three lip portions are formed as in the combustion chamber structure 100 according to this embodiment may be referred to as a three-stage lip structure.

Figure 4 is a diagram explaining the structural difference between the two-stage lip structure and the three-stage lip structure. In Figure 4, the structure around the lip portion of the cavity 10 is illustrated, with the broken line indicating the structure in the case of the two-stage lip structure and the solid line indicating the structure in the case of the three-stage lip structure. As shown in Figure 4, in the three-stage lip structure, three lip portions are formed, the first lip portion 201, the second lip portion 202, and the third lip portion 203, and the area that is at the same height as the piston top surface 9a in the two-stage lip structure is offset downward to form the second horizontal portion 105. In addition, in the three-stage lip structure, the bottom of the cavity 10 is offset upward by the amount of the increase in volume due to the above-mentioned downward offset, so that the volume is the same as that of the two-stage lip structure.

Figure 5 is a diagram explaining the difference in combustion conditions between a two-stage lip structure and a three-stage lip structure. In Figure 5, the left side shows the combustion conditions of the two-stage lip structure at each crank angle (8°, 12°, 16°), and the right side shows the combustion conditions of the three-stage lip structure at each crank angle (8°, 12°, 16°). In Figure 5, the fuel flow rate is indicated by the color intensity (see the legend in Figure 5). Note that, in this explanation, the two-stage lip structure is assumed to be composed of a first rising portion 102, a first horizontal portion 103, and a second rising portion 104.

As shown in the left diagram of FIG. 5, in the case of a two-stage lip structure, when the crank angle is 8°, the fuel spray that collides with the inner lip portion is promoted to flow only upward in the upper combustion chamber divided by the first horizontal portion 103 and the second rising portion 104, and a squish flow, which is a vertical swirling flow, is generated. In this case, the fuel spray spreads only within the range of the swirling flow. Therefore, when the crank angle is 12°, the fuel spray spreads only to about half of the area of the piston top surface 9a, which is the uppermost stage. In this way, with a two-stage lip structure, there is no space on the outer periphery side of the cavity 10, and it can be said that combustion is rate-limiting. Furthermore, when the crank angle is 16°, the fuel injected from the nozzle nozzle (the part shown dark in the diagram) and the fuel that collides with the inner lip portion and spreads inside the combustion chamber (the part shown light in the diagram) interfere with each other, and the fuel spray inhibits the use of air in the combustion chamber, causing the combustion to deteriorate.

On the other hand, as shown in the right diagram of FIG. 5, in the case of the three-stage lip structure, when the crank angle is 8°, the fuel spray that collides with the inner lip portion promotes an upward flow and maintains an outward flow in the upper combustion chamber partitioned by the first horizontal portion 103 and the second rising portion 104. This is because the three-stage lip structure ensures sufficient space on the outer periphery of the cavity 10. As a result, when the crank angle is 12°, in the top combustion chamber, the outward flow of the fuel spray flows radially outward and spreads until it reaches the vertical wall of the top combustion chamber. Note that while the outward flow of the fuel spray is stronger in the three-stage lip structure than in the two-stage lip structure, the vertical vortex caused by the downward movement of the middle combustion chamber is weaker, so the squish flow, which is a vertical swirling flow generated by the upward fuel spray, is weaker in the three-stage lip structure than in the two-stage lip structure. For this reason, the fuel spray does not remain within the range of the squish flow, but also spreads to the inner area of the combustion chamber. With this type of three-stage lip structure, under conditions where there is little EGR and sufficient oxygen, there is a space where the fuel spray spreads into the air and mixes with the air, and there is also sufficient opportunity for the fuel and oxygen to come into contact and react (burn), which activates combustion and increases the rate of heat release (ROHR) at crank angles of 8° to 12°. As mentioned above, with the three-stage lip structure, the fuel spray spreads outward, so less fuel is distributed inside the combustion chamber. As a result, at a crank angle of 16°, there is less interference between the fuel injected from the nozzle orifice (the part shown dark in the figure) and the fuel that collides with the inner lip and spreads inside the combustion chamber (the part shown light in the figure).

Figure 6 is a diagram that explains the difference in combustion conditions between a two-stage lip structure and a three-stage lip structure. In Figure 6, the horizontal axis shows the crank angle, and the vertical axis shows the rate of heat release due to combustion (ROHR). As shown in Figure 6, the three-stage lip structure is able to stimulate combustion when the crank angle is between 8° and 16°, compared to the two-stage lip structure.

Next, the effects of the combustion chamber structure 100 of the direct injection diesel engine according to this embodiment will be described.

The combustion chamber structure 100 of the direct injection diesel engine according to this embodiment has a cavity 10 recessed downward relative to the piston top surface 9a, and is a combustion chamber structure of a direct injection diesel engine in which fuel is radially injected from the center of the cylinder ceiling portion 11 into the cavity 10 to cause self-ignition. The combustion chamber structure includes a first rising portion 102 rising upward from the bottom surface 101 of the cavity 10, a first horizontal portion 103 that is continuous with the upper end of the first rising portion 102 and extends approximately horizontally in the outer circumferential direction of the cavity 10, a second rising portion 104 that is continuous with the first horizontal portion 103 and rises upward, and a second rising portion 104 that is continuous with the first horizontal portion 103 and rises upward. The combustion chamber has a first stage combustion chamber V1 defined by the bottom surface 101 of the cavity 10 and the first rising portion 102, a second stage combustion chamber V2 defined by the first horizontal portion 103 and the second rising portion 104, and a third stage combustion chamber V3 defined by the second horizontal portion 105 and the third rising portion 106, and the volume ratio of the third stage combustion chamber V3 to the entire combustion chamber is 2% or more and 40% or less.

In the combustion chamber structure 100 of the direct injection diesel engine according to this embodiment, so-called lip portions consisting of rising portions (first rising portion 102, second rising portion 104, third rising portion 106) and horizontal portions (first horizontal portion 103, second horizontal portion 105, piston top surface 9a) are formed in three places. Therefore, as the structure of the combustion chamber, a first stage combustion chamber V1 defined by the bottom surface 101 of the cavity 10 and the first rising portion 102, a second stage combustion chamber V2 defined by the first horizontal portion 103 and the second rising portion 104, and a third stage combustion chamber V3 defined by the second horizontal portion 105 and the third rising portion 106 are formed. In the conventional combustion chamber structure, there are only two so-called lip portions, and a combustion chamber corresponding to the third stage combustion chamber V3 of this embodiment is not formed. In contrast, in the configuration according to the present embodiment, in which the third-stage combustion chamber V3 is formed outside and above the second-stage combustion chamber V2, the fuel spray flowing in the outer circumferential direction can reach the third rising portion 106 (the vertical wall of the uppermost combustion chamber) that reaches the piston top surface 9a via the third-stage combustion chamber V3, and the third-stage combustion chamber V3, which has not been conventionally configured, can be actively used as a space for mixing the fuel spray and air. That is, in the configuration according to the present embodiment, a space (third-stage combustion chamber V3) for mixing the fuel spray and air is formed on the outer circumferential side of the cavity 10, so that the opportunities for contact and reaction (combustion) between the fuel and oxygen can be increased, and combustion can be promoted. In addition, in the configuration according to the present embodiment, the volume ratio of the third-stage combustion chamber V3 to the entire combustion chamber is set to 2% or more and 40% or less, so that while promoting combustion on the outer circumferential side of the cavity 10, it is also possible to prevent the third-stage combustion chamber V3 from becoming too large and causing slow combustion. As described above, the combustion chamber structure 100 according to this embodiment can provide a combustion chamber structure for a direct injection diesel engine that is more fuel efficient.

Figure 7 is a graph showing the relationship between the volume ratio of the third stage combustion chamber V3 and the fuel economy improvement effect. In Figure 7, the horizontal axis shows the volume ratio of the third stage combustion chamber V3 in the entire combustion chamber, and the vertical axis shows fuel consumption (i.e., fuel economy improvement effect). A volume ratio of the third stage combustion chamber V3 of 0 (horizontal axis is 0) is a configuration without the third stage combustion chamber V3, which is a configuration equivalent to a conventional two-stage lip structure. Using this state as a reference, a state in which fuel consumption is less than when the volume ratio of the third stage combustion chamber V3 is 0 is a state in which fuel economy is improved, and a state in which fuel consumption is greater is a state in which fuel economy is worsened.

As shown in FIG. 7, when the volume ratio of the third stage combustion chamber V3 is about 2-3%, the fuel consumption reduction (fuel economy improvement) effect is confirmed, and when the volume ratio of the third stage combustion chamber V3 is about 4%, the fuel consumption reduction (fuel economy improvement) effect is clearly recognized. When the volume ratio of the third stage combustion chamber V3 is further increased, the fuel economy improves as the volume ratio of the third stage combustion chamber V3 increases up to about 8%. When the volume ratio of the third stage combustion chamber V3 is further increased, the fuel economy does not increase or decrease significantly even if the volume ratio of the third stage combustion chamber V3 is increased up to about 8-11%. When the volume ratio of the third stage combustion chamber V3 is further increased, the fuel economy improvement effect is recognized up to about 35% of the volume ratio of the third stage combustion chamber V3, and when the volume ratio of the third stage combustion chamber V3 is about 40%, the fuel economy becomes the same as the standard (volume ratio of the third stage combustion chamber V3 is 0). If the volume ratio of the third combustion chamber V3 becomes larger than 40%, fuel consumption will increase more than the standard and fuel economy will deteriorate. In this way, by increasing the volume ratio of the third combustion chamber V3 to a certain extent, the space in which the air and fuel mix will become larger and fuel economy will improve, but if it is increased too much, combustion will become slow and the effect of improving fuel economy will be small.

The length of the third rising portion 106 rising upward may be 3% or more and 9.5% or less of the length of the bottom surface 101 of the cavity 10 in a direction intersecting the vertical direction. In this way, by setting the length of the third rising portion 106 that divides the third stage combustion chamber V3, it is possible to ensure a sufficient size of the third stage combustion chamber V3 to promote combustion on the outer periphery side of the cavity 10, while also preventing the third stage combustion chamber V3 from becoming too large and slowing down combustion.

The length of the second horizontal portion 105 in a direction intersecting the vertical direction may be 10% or more and 18% or less of the length of the bottom surface 101 of the cavity 10 in a direction intersecting the vertical direction. In this way, by setting the length of the second horizontal portion 105 that divides the third stage combustion chamber V3, it is possible to ensure a sufficient size of the third stage combustion chamber V3 to promote combustion on the outer periphery of the cavity 10, while also preventing the third stage combustion chamber V3 from becoming too large and slowing down combustion.

1... diesel engine, 9a... piston top surface, 10... cavity, 11... cylinder ceiling, 100... combustion chamber structure, 101... bottom surface, 102... first rising portion, 103... first horizontal portion, 104... second rising portion, 105... second horizontal portion, 106... third rising portion, V1... first stage combustion chamber (first chamber), V2... second stage combustion chamber (second chamber), V3... third stage combustion chamber.

Claims (3)

A direct injection diesel engine having a cavity recessed downward from a piston top surface, in which fuel is radially injected from the center of a cylinder ceiling into the cavity to cause self-ignition,
a multi-hole injector provided on a cylinder ceiling for injecting fuel into the cylinder;
a control device that controls the injection operation of the injector based on a fuel injection command;
A combustion chamber structure,
The combustion chamber structure includes:
a first rising portion rising upward from a bottom surface of the cavity;
a first horizontal portion that is continuous with an upper end of the first rising portion and extends substantially horizontally in an outer circumferential direction of the cavity;
a second rising portion that is continuous with the first horizontal portion and rises upward;
a second horizontal portion that is continuous with an upper end of the second rising portion and extends substantially horizontally in an outer circumferential direction of the cavity;
a third rising portion that is continuous with the second horizontal portion and rises upward to reach the piston top surface,
The combustion chamber is
a first chamber defined by a bottom surface of the cavity and the first raised portion;
a second chamber defined by the first horizontal portion and the second rising portion;
a third chamber defined by the second horizontal portion and the third rising portion,
A volume ratio of the third chamber to the entire combustion chamber is 2% or more and 40% or less,
The control device includes:
a direct injection diesel engine in which the injection operation of the injector is controlled so that, when the crank angle is 8°, the fuel spray collides with an inner lip portion, and, when the crank angle is 12°, the outward flowing fuel spray in the uppermost third chamber flows radially outward and spreads until it reaches the third rising portion, which is the vertical wall of the third chamber, and, when the crank angle is 16°, the fuel injected from the nozzle orifice and the fuel that collides with the inner lip portion and spreads into the combustion chamber are less likely to interfere with each other . 2. The direct injection diesel engine according to claim 1, wherein an upwardly rising length of the third rising portion is 3% to 9.5% of a length of the bottom surface of the cavity in a direction intersecting the up-down direction. 3. The direct injection diesel engine according to claim 1, wherein the length of the second horizontal portion in a direction intersecting the vertical direction is 10% or more and 18% or less of the length of the bottom surface of the cavity in the direction intersecting the vertical direction .

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