JP2018184909A - Premixed compression ignition type engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a premixed compression ignition type engine for securing good thermal efficiency while suppressing combustion noise by appropriately slowing combustion according to operating conditions.SOLUTION: On the radial center of a combustion chamber wall surface encircling a combustion chamber 10, a heat insulation layer 45 is arranged which has a smaller heat conductivity than a parent material for the combustion chamber wall surface. When the engine is operated in a prefixed high speed region, water is injected from a water injection device (57) at a timing such that the water can be unevenly distributed onto the radial center of the combustion chamber 10 right before combustion is started. When the engine is operated in a low speed region where an engine speed is lower than in the high speed region, the water is injected at a timing different from that in the high speed region so that the amount of the water on the radial center becomes smaller than in the high speed region, or water injection is stopped.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an engine capable of premixed compression ignition combustion (HCCI combustion) in which fuel is self-ignited while being mixed with air.

  In recent years, as a gasoline engine for vehicles such as automobiles, research on a premixed compression ignition type engine in which a fuel (air mixture) previously mixed with air is self-ignited in a combustion chamber has been advanced. The premixed compression ignition type engine is characterized in that the air-fuel mixture simultaneously starts combustion in various places in the combustion chamber. For this reason, it is expected that the thermal efficiency of the engine and hence the fuel efficiency can be greatly improved.

  On the other hand, as the air-fuel mixture burns simultaneously and frequently at various locations in the combustion chamber, a new problem arises that the pressure in the combustion chamber, that is, the in-cylinder pressure rises rapidly. For example, a large combustion noise is generated due to a sudden rise in in-cylinder pressure, and a load applied to an engine component such as a crank mechanism for reciprocating a piston increases. In addition, the flame generated in the combustion chamber is pressed against and contacted with the wall surface of the combustion chamber due to an abrupt pressure increase in the combustion chamber, whereby heat radiation through the wall surface of the combustion chamber is promoted and cooling loss increases.

  In Patent Document 1, as an engine capable of coping with some of the above problems, a thermal spray coating having a function of blocking heat conduction is coated on the combustion chamber wall surface, and the combustion chamber wall surface is provided with a heat shielding property. A premixed compression ignition engine is disclosed.

Japanese Patent Laid-Open No. 2006-98407

  However, as described in Patent Document 1, simply providing a coating layer having a heat shielding property on the entire wall surface of the combustion chamber causes the above-mentioned problem that is a concern in a premixed compression ignition type engine, that is, a rapid increase in in-cylinder pressure. It is not possible to sufficiently cope with problems such as combustion noise and increased cooling loss.

  Here, in order to suppress the rapid increase in the in-cylinder pressure, it is effective to slow down the combustion. However, if combustion is slowed down regardless of operating conditions, exhaust loss (energy that is discharged without being converted into work) increases, and the merit of improving thermal efficiency, which is an advantage of premixed compression ignition engines, is achieved. It cannot be fully enjoyed.

  The present invention has been made in view of the circumstances as described above, and is capable of ensuring good thermal efficiency while suppressing combustion noise by moderately slowing down combustion according to operating conditions. An object is to provide a mixed compression ignition engine.

  In order to solve the above problems, the present invention includes a piston accommodated in a cylinder so as to be capable of reciprocating, and a fuel injection device for injecting fuel containing gasoline into a combustion chamber defined by the cylinder and the piston. And an engine capable of premixed compression ignition combustion in which fuel injected from the fuel injection device is combusted by self-ignition while being mixed with air. When a heat insulating layer having a lower thermal conductivity than the base material of the combustion chamber wall is disposed at the radial center of the combustion chamber wall surrounding the chamber, the water injection is performed when the engine is operated in a predetermined high speed region. The apparatus injects water at a timing at which water can be unevenly distributed in the radial center of the combustion chamber immediately before the start of combustion, and the engine is operated in a low speed region where the rotational speed is lower than the high speed region. And The water injection device injects water at a timing different from that in the high speed region or stops water injection so that the amount of water in the central portion in the radial direction is smaller than that in the high speed region. (Claim 1).

  According to the present invention, since the heat insulating layer is disposed at the center portion of the combustion chamber wall surface, the effect of the heat insulating layer can suppress the escape of heat from the combustion chamber to the outside through the center portion of the combustion chamber wall surface. . As a result, immediately before the start of combustion, that is, when the piston rises to the vicinity of compression top dead center and the combustion chamber is sufficiently heated, a significant temperature distribution (a predetermined amount or more in the radial direction) is generated inside the combustion chamber. A state in which a temperature difference occurs). That is, for the central part of the combustion chamber in which the heat insulating layer is provided on the wall surface, heat dissipation is suppressed and the temperature is increased, while for the outer peripheral part of the combustion chamber in which the heat insulating layer is not provided on the wall surface, the heat insulating effect by the heat insulating layer is provided. Since it cannot be obtained, the temperature is relatively low. In this way, the temperature at the center of the combustion chamber is sufficiently higher than the outer periphery, and a significant temperature distribution is obtained.

  As the above temperature distribution (temperature difference) is formed in the combustion chamber, a difference occurs in the timing at which the fuel self-ignites in the combustion chamber, and the combustion can be slowed down. That is, combustion proceeds in order from the inside to the outside in the radial direction, such that the fuel self-ignites first at the center of the combustion chamber where the temperature is high, and then the fuel self-ignites at the outer periphery of the relatively low temperature combustion chamber. It is possible to realize a combustion mode of performing. Thereby, combustion can be slowed down as a whole, and rapid increase in in-cylinder pressure can be suppressed.

  As a result of the suppression of the rapid increase in the in-cylinder pressure as described above, the maximum value of the in-cylinder pressure increase rate (dp / dθ) is reduced, and an engine that is excellent in commerciality with low combustion noise can be realized. it can. Moreover, since the maximum value of the combustion temperature is also reduced, clean combustion with a small amount of NOx generated can be realized.

  In addition, according to the present invention, which allows combustion to proceed in order from the inner side to the outer side in the radial direction, it is possible to avoid the flame from reaching the wall surface of the outer peripheral portion of the combustion chamber, which is at a relatively low temperature, at once. Since the region where the initial combustion occurs is insulated by the heat insulating layer, it is possible to sufficiently suppress the combustion heat from escaping to the outside. Thereby, a cooling loss can be reduced and the thermal efficiency of an engine can be improved effectively.

  However, in the high-speed region where the rotational speed is high, the amount of change in the crank angle per unit time is large, so even if the combustion becomes somewhat steep, the maximum value of the pressure increase rate (dp / dθ) and the flame collision area are not much. Does not grow. For this reason, even if the temperature difference in the combustion chamber is reduced in the high-speed region, the increase in combustion noise and cooling loss is not a problem. Rather, as a result of burning a lot of fuel in a short time, exhaust loss is reduced. This is thought to lead to improved thermal efficiency. In other words, if the temperature difference in the combustion chamber is sufficiently large in the high-speed region, the combustion is unnecessarily slowed down, and the thermal efficiency may be deteriorated.

  On the other hand, in the present invention, during the operation in the high speed region, water injection that causes water to be unevenly distributed in the central portion of the combustion chamber is executed before the start of combustion (self-ignition). By centrally cooling the central portion of the combustion chamber that tends to become high temperature, the temperature difference between the central portion and the outer peripheral portion of the combustion chamber is reduced. As a result, there is no significant difference in the ignition timing at the center / outer periphery of the combustion chamber, and relatively steep combustion occurs in which a large amount of fuel burns in a short time. As a result, it is possible to avoid unnecessarily slowing down the combustion in a high speed region where the rate of increase (dp / dθ) of the in-cylinder pressure tends to decrease, and to ensure good thermal efficiency.

  On the other hand, in the low speed region where the rotational speed is lower than that of the high speed region, the maximum value of the pressure increase rate (dp / dθ) tends to be excessive, contrary to the high speed region, and it is necessary to slow down the combustion sufficiently. . In contrast, in the present invention, during operation in the low speed region, the amount of water in the center of the combustion chamber is controlled to decrease, so the temperature difference between the central portion and the outer periphery of the combustion chamber is in the high speed region. It can be expanded more than the time, and the slowing of combustion can be promoted. As a result, it is possible to suppress a sudden rise in the in-cylinder pressure that is likely to be a problem in a low speed region, and to suppress an increase in combustion noise and cooling loss.

  Preferably, the water injection device is provided so as to inject water from the vicinity of the center of the ceiling surface of the combustion chamber facing the piston toward the piston, and during operation in the high speed region, the water injection device is Water is injected in the latter half of the compression stroke, and the fuel injection valve injects fuel in the first half of the intake stroke or the compression stroke.

  As described above, when water is jetted from the vicinity of the center of the ceiling surface of the combustion chamber toward the piston at the later stage of the compression stroke during operation in the high speed region, the injected water is centered on the crown surface of the piston. This jet water can properly create a state in which water is unevenly distributed in the center of the combustion chamber.

  In addition, in the high speed region, fuel is injected during the intake stroke or the first half of the compression stroke, so that the injected fuel can be sufficiently dispersed in the combustion chamber before the start of combustion, and the fuel concentration in the combustion chamber (Air-fuel ratio) can be made uniform. This, combined with the effect of the water injection performed in the high speed region (that is, the effect of cooling the central portion of the combustion chamber to reduce the temperature difference) leads to a reduction in the ignition timing difference in the combustion chamber. Therefore, it is possible to sufficiently eliminate the slow combustion and to realize combustion with high thermal efficiency.

  In the above configuration, more preferably, the water injection device increases the amount of water injected in the latter half of the compression stroke as the rotational speed increases during operation in the high speed region.

  According to this configuration, the condition that the rate of increase in cylinder pressure (dp / dθ) is likely to be smaller can increase the cooling effect of water on the center of the combustion chamber, and reduce the temperature difference in the combustion chamber. The sharpening of combustion can be promoted.

  Preferably, during the operation in the low speed region, the water injection device injects water at a timing at which water can be unevenly distributed on the outer peripheral portion of the combustion chamber immediately before the start of combustion.

  As described above, when water injection that causes water to be unevenly distributed in the outer peripheral portion of the combustion chamber is performed in the low speed region, the outer peripheral portion of the combustion chamber is cooled by the injected water, thereby lowering the temperature of the outer peripheral portion. In addition, the temperature difference between the central portion and the outer peripheral portion of the combustion chamber increases. Thereby, since the progress of combustion becomes sufficiently slow, it is possible to suppress a rapid increase in in-cylinder pressure, and to effectively suppress an increase in combustion noise and cooling loss.

  In the above configuration, more preferably, the water injection device is provided to inject water radially from the vicinity of the center of the ceiling surface of the combustion chamber facing the piston toward the piston, and operates in the low speed region. The water injection device injects water in the first half or the middle of the compression stroke, and the fuel injection valve injects fuel in the first half of the intake stroke or the compression stroke.

  Thus, when water is injected radially from near the center of the ceiling surface of the combustion chamber during the first or middle stage of the compression stroke during operation in the low speed region, the injected water is injected into the outer periphery of the piston crown surface. It can be directed to the peripheral wall of the part or the cylinder, and a state in which water is unevenly distributed on the outer peripheral part of the combustion chamber can be appropriately created by this jet water. As a result, the temperature difference between the central part and the outer peripheral part of the combustion chamber increases as described above. Therefore, even if fuel is injected at an earlier timing such as the first half of the intake stroke or the compression stroke, a significant difference is caused in the ignition timing of the fuel. Can be provided, and combustion can be sufficiently slowed down.

  Preferably, the premixed compression ignition type engine further includes a heat shield layer that covers the heat insulating layer and a portion of the combustion chamber wall located radially outside the heat insulating layer, and the heat shield layer has a heat conduction layer. It is comprised by the member whose rate is smaller than the base material of the said combustion chamber wall surface, and whose volume specific heat is smaller than the said heat insulation layer (Claim 6).

  In this way, when a heat shield layer covering both the heat insulating layer and the radially outer wall surface is provided, the heat shield layer can prevent heat from escaping through the combustion chamber wall surface, thereby reducing cooling loss. can do. In particular, the central portion of the combustion chamber that is likely to become high temperature is double-insulated by the heat-insulating layer and the heat-insulating layer, so that the cooling loss reduction effect can be further enhanced. Moreover, the heat shield layer has a smaller volume specific heat (lower heat storage property) than the heat insulation layer, and is likely to fluctuate following the ambient temperature. For this reason, the temperature of the heat shield layer can be quickly reduced, for example, with the introduction of fresh air, between the end of combustion and the next combustion. This avoids a situation in which the temperature of the combustion chamber rises excessively, so that it is possible to effectively prevent the occurrence of abnormal combustion or the like while reducing the cooling loss by insulating the combustion chamber. it can.

  As described above, according to the premixed compression ignition type engine of the present invention, combustion can be moderated moderately according to operating conditions, and thermal efficiency can be ensured satisfactorily while suppressing combustion noise. .

It is a figure showing the whole premixed compression ignition type engine composition concerning one embodiment of the present invention. It is sectional drawing of an engine main body. It is the schematic plan view which looked at the crown surface of the piston from the upper part. It is a figure for demonstrating the effect by having provided the heat insulation layer and the heat-shielding layer in the crown surface of the piston. It is a block diagram which shows the control system of an engine. It is a map figure which shows the difference in control according to the driving | running condition of an engine. It is a time chart for demonstrating the aspect of the fuel injection and water injection in a low speed and high load area | region (1st operation area | region). It is a time chart for demonstrating the aspect of the fuel injection and water injection in a high-speed and high load area | region (2nd operation area | region). It is a time chart for demonstrating the aspect of fuel injection and water injection in a low speed and low load area | region (3rd driving | operation area | region). It is operation | movement explanatory drawing for demonstrating the effect | action by the water injection in the said 1st driving | operation area | region. It is operation | movement explanatory drawing for demonstrating the effect | action by the water injection in the said 2nd driving | operation area | region. It is operation | movement explanatory drawing for demonstrating the effect | action by the water injection in the said 3rd driving | operation area | region.

(1) Overall Configuration of Engine FIGS. 1 and 2 are views showing a preferred embodiment of a premixed compression ignition engine (hereinafter also simply referred to as an engine) to which the control device of the present invention is applied. The engine shown in the figure is a four-cycle gasoline engine mounted on a vehicle as a driving power source, and includes an in-line multi-cylinder engine main body 1 including four cylinders 2 arranged in a row, and an engine main body 1. The intake passage 20 through which the intake air introduced into the exhaust gas flows, the exhaust passage 30 through which the exhaust gas discharged from the engine body 1 flows, and the water taken out from the exhaust gas flowing through the exhaust passage 30 is supplied to each cylinder 2 of the engine body 1. And a water supply system 50 for supplying the water.

  As shown in FIG. 2, the engine body 1 includes a cylinder block 3 in which the cylinder 2 is formed, a cylinder head 4 attached to the upper surface of the cylinder block 3 so as to close the cylinder 2 from above, and each cylinder 2. And a piston 5 accommodated in a reciprocating manner.

  A combustion chamber 10 is defined above the piston 5. The combustion chamber 10 is supplied with fuel (a fuel mainly composed of gasoline) injected from a fuel injection valve 11 described later. The supplied fuel burns in the combustion chamber 10, and the piston 5 pushed down by the expansion force due to the combustion reciprocates in the vertical direction.

  Below the piston 5, a crankshaft 15 that is an output shaft of the engine body 1 is disposed. The crankshaft 15 is connected to the piston 5 via the connecting rod 14 and rotates around the central axis according to the reciprocating motion of the piston 5. The cylinder block 3 is provided with a crank angle sensor SN1 that detects a rotation angle (crank angle) and a rotation speed (engine rotation speed) of the crankshaft 15.

  FIG. 3 is a schematic plan view of the crown surface 40 of the piston 5 as viewed from above. As shown in FIG. 3 and FIG. 2, the crown surface 40 of the piston 5 is formed with a cavity C in which the central portion is recessed on the opposite side (downward) from the cylinder head 4. The cavity C is formed so as to form a deep dish-shaped recess in a circular area in a plan view at the center of the crown surface 40. That is, the cavity C has a substantially circular and flat bottom surface portion 41 and an inclined peripheral surface portion 42 that rises upward from the outer peripheral edge of the bottom surface portion 41. The crown surface 40 of the portion positioned radially outward from the opening edge C1 of the cavity C, which is the upper end of the peripheral surface portion 42, is a squish portion 43, and the squish portion 43 has a height that decreases toward the radially outer side. It is set as the ring-shaped inclined surface in plane view inclined in the direction.

  A heat insulating layer 45 is disposed on the bottom surface portion 41 of the cavity C. Moreover, the heat shield layer 46 is disposed so as to cover the crown surface 40 on which the heat insulating layer 45 is disposed from the top. Details of the heat insulation layer 45 and the heat shield layer 46 will be described later.

  The cylinder head 4 has a fuel injection valve 11 for injecting fuel mainly composed of gasoline supplied from a fuel pump (not shown) into the combustion chamber 10 of each cylinder 2 (corresponding to “fuel injection device” in the claims). Is provided for each cylinder 2 (four in total). Each fuel injection valve 11 is provided so as to be adjacent to a water injection valve 57 to be described later in a slightly inclined posture with respect to the central axis of the cylinder 2. As shown in FIG. 1, a fuel rail 16 is provided above the fuel injection valve 11 to store the fuel supplied from the fuel pump in a pressure accumulation state. The fuel stored in the fuel rail 16 is supplied to each fuel injection valve 11 through the same number (four) of distribution pipes 17 as the fuel injection valves 11.

  The fuel injection valve 11 has a tip portion exposed to the combustion chamber 10 in the vicinity of the central axis of the cylinder 2 and can inject fuel radially through a plurality of injection holes (not shown) provided in the tip portion. Is possible. The tip of the fuel injection valve 11 is disposed at a position where fuel can be supplied to the cavity C of the piston 5 when the piston 5 is in the vicinity of the top dead center. Fuel is injected from the fuel injection valve 11 during the intake stroke or the compression stroke, and the injected fuel is mixed with air (intake air) introduced into the combustion chamber 10 and then, for example, in the vicinity of the compression top dead center. Ignite.

  That is, in the engine of the present embodiment, instead of spark ignition combustion (combustion in which an air-fuel mixture is forcibly ignited by spark ignition) generally employed when gasoline is used as a fuel, an air-fuel mixture of fuel and air is used as a piston. HCCI combustion (premixed compression ignition combustion) that is self-ignited in accordance with compression by 5 is performed in all operating regions of the engine. For this reason, the spark plug is basically unnecessary in the engine of the present embodiment, but spark ignition combustion is executed instead of HCCI combustion in a situation where self-ignition is difficult, for example, immediately after the engine is cold started. Even after warm-up, so-called spark assist may be executed to promote HCCI combustion, and a spark plug may be provided for such a purpose.

  In order to enable the HCCI combustion as described above, the compression ratio of each cylinder 2 is set higher in the engine of the present embodiment than in a general gasoline engine that employs spark ignition combustion. Specifically, in this embodiment, the geometric compression ratio of each cylinder 2, that is, the volume of the combustion chamber 10 when the piston 5 is at the top dead center and the combustion chamber 10 when the piston 5 is at the bottom dead center. The volume ratio is set to 16 to 35, more preferably 18 to 30.

  As shown in FIG. 2, the cylinder head 4 receives, for each cylinder 2, the intake port 6 for introducing the air supplied from the intake passage 20 into the combustion chamber 10 and the exhaust gas generated in the combustion chamber 10. An exhaust port 7 for leading to the exhaust passage 30, an intake valve 8 that opens and closes the opening of the intake port 6 on the combustion chamber 10 side, and an exhaust valve 9 that opens and closes the opening of the exhaust port 7 on the combustion chamber 10 side, respectively. Is provided.

  As shown in FIG. 1, the intake passage 20 includes a single tubular common intake pipe 22 and an intake manifold 21 formed to branch from the downstream end of the common intake pipe 22. The downstream end of each branch pipe of the intake manifold 21 is connected to the engine body 1 (cylinder head 4) so as to communicate with the combustion chamber 10 of each cylinder 2 via the intake port 6. In the present specification, the upstream (or downstream) in the intake passage 20 refers to the upstream (or downstream) in the flow direction of the intake air flowing through the intake passage 20.

  The common intake pipe 22 is provided with an air cleaner 25 for removing foreign substances contained in the intake air and an openable / closable throttle valve 27 for adjusting the flow rate of the intake air flowing through the common intake pipe 22 in this order from the upstream side. Yes. Furthermore, an air flow sensor SN2 that detects the flow rate of the intake air flowing through the common intake pipe 22 is provided on the downstream side of the throttle valve 27 in the common intake pipe 22.

  In the engine of this embodiment, since HCCI combustion is performed in all operating regions, the throttle valve 27 is basically maintained at an opening corresponding to full opening except during deceleration operation or when the engine is stopped. .

  The exhaust passage 30 includes a single tubular common exhaust pipe 32 and an exhaust manifold 31 formed so as to branch from the upstream end of the common exhaust pipe 32. The upstream end of each branch pipe of the exhaust manifold 31 is connected to the engine body 1 (cylinder head 4) so as to communicate with the combustion chamber 10 of each cylinder 2 via the exhaust port 7. In the present specification, upstream (or downstream) in the exhaust passage 30 means upstream (or downstream) in the flow direction of the exhaust gas flowing through the exhaust passage 30.

  In the common exhaust pipe 32, a catalyst device 35, a heat exchanger 54, and a condenser 51 are provided in this order from the upstream side.

  The catalyst device 35 is for purifying harmful components contained in the exhaust gas. For example, the catalyst device 35 has a built-in catalyst composed of any one or a combination of a three-way catalyst, an oxidation catalyst, and an NOx catalyst. In addition to such a catalyst, a filter for collecting PM contained in the exhaust gas may be included.

  The condenser 51 condenses water vapor contained in the exhaust gas, and the heat exchanger 54 raises the temperature of the condensed water generated by the condenser 51. The heat exchanger 54 and the condenser 51 are elements that constitute a part of the water supply system 50 (details will be described later).

(2) Details of heat insulation layer / heat insulation layer The above-described heat insulation layer 45 and heat insulation layer 46 provided on the crown surface 40 of the piston 5 are desired for the combustion chamber 10 during combustion of the air-fuel mixture in the combustion chamber 10. It is provided to give a temperature distribution. Here, the desired temperature distribution means that the radially central portion of the combustion chamber 10 (hereinafter simply referred to as the central portion) has a high temperature, and the outer peripheral portion of the combustion chamber 10 positioned radially outward from the central portion. The temperature distribution is low. In order to realize such a temperature distribution, each of the heat insulating layer 45 and the heat shielding layer 46 is configured as follows.

  As shown in FIGS. 2 and 3, the heat insulating layer 45 is a substantially disk-shaped member having a predetermined thickness in the vertical direction (the axial direction of the cylinder 2), and is formed on the cavity surface C of the crown surface 40 of the piston 5. It arrange | positions so that the center part corresponding to the bottom face part 41 may be occupied. Of course, the shape of the heat insulating layer 45 in a disc shape is merely an example, and a member having a polygonal shape or other shapes in plan view may be used.

  The heat shield layer 46 is provided so as to cover almost the entire crown surface 40 of the piston 5 including the cavity C. That is, the heat shielding layer 46 has a diameter smaller than the bottom surface portion 41 of the cavity C in which the heat insulating layer 45 is disposed, the peripheral surface portion 42 of the cavity C rising upward from the outer peripheral edge of the bottom surface portion 41, and the opening edge C1 of the cavity C. It arrange | positions so that the squish part 43 located in the direction outer side may be covered continuously. In particular, on the bottom surface portion 41 of the cavity C, a heat shielding layer 46 is disposed so as to further cover the heat insulating layer 45 from above, and the heat insulating layer 45 and the heat insulating layer 46 are laminated in this order from the bottom. The heat shield layer 46 has a substantially constant thickness at any position on the crown surface 40, but the thickness is smaller than that of the heat insulating layer 45.

  Here, the wall surface surrounding the combustion chamber 10, that is, the inner peripheral surface of the cylinder block 3 that defines the peripheral surface of the combustion chamber 10 (the peripheral wall of the cylinder 2), and the lower surface of the cylinder head 4 that defines the ceiling surface of the combustion chamber 10. And the wall surfaces including the crown surface 40 of the piston 5 that defines the bottom surface of the combustion chamber 10 are collectively referred to as “combustion chamber wall surfaces”. In this embodiment, an aluminum alloy such as AC8A is used as a base material for the parts (that is, the cylinder block 3, the cylinder head 4, the piston 5 and the like) constituting the combustion chamber wall surface.

  On the other hand, as the material of the heat insulating layer 45, a material having a thermal conductivity smaller than that of the base material of the combustion chamber wall surface and a larger volume specific heat (in other words, a higher heat storage property) is selected. Further, as the material of the heat shield layer 46, a material having a thermal conductivity smaller than that of the base material of the combustion chamber wall surface and a smaller volume ratio (in other words, a lower heat storage property) is selected.

  Furthermore, when the heat insulating layer 45 and the heat insulating layer 46 are directly compared, the relationship between the two is such that both the thermal conductivity and the volume specific heat of the heat insulating layer 46 are smaller than that of the heat insulating layer 45. .

  The reason why the thermal conductivity of the heat insulating layer 45 and the heat shield layer 46 is smaller than that of the base material of the combustion chamber wall surface is to suppress the escape of heat from the combustion chamber 10 through the crown surface 40 of the piston 5. However, since the heat insulating layer 46 is smaller in thickness than the heat insulating layer 45, it may not be possible to ensure sufficient heat insulating properties if the heat insulating layer 46 is given the same thermal conductivity as the heat insulating layer 45. . Therefore, as described above, the thermal conductivity of the heat shield layer 46 is set to a value smaller than that of the heat insulating layer 45.

  Moreover, the volume specific heat of the heat insulation layer 45 is larger than that of the base material of the combustion chamber wall surface in order to maintain the central portion of the crown surface 40 at a high temperature. On the other hand, when the heat shield layer 46 has a large volume specific heat (heat storage property) at the same level as the heat insulating layer 45, not only the central portion of the crown surface 40 but also the outer peripheral portion is maintained at a high temperature, Again, it becomes difficult to form a significant temperature distribution. Therefore, as described above, the volume specific heat of the heat shield layer 46 is set to be smaller than that of the heat insulating layer 45.

  As a material of the heat insulation layer 45 satisfying the above requirements, for example, a ceramic material can be exemplified. In general, a ceramic material is suitable as the heat insulating layer 45 because it has a low thermal conductivity but a large volumetric specific heat and excellent heat resistance. Specifically, a preferred ceramic material is zirconia. In addition, ceramic materials such as silicon nitride, silica, cordierite, and mullite can also be exemplified. In addition, as a method of fixing the ceramic heat insulating layer 45 to the bottom surface portion 41 of the cavity C, for example, a method of providing a housing recess in the bottom surface portion 41 and press-fitting the heat insulating layer 45 into the housing recess, or casting-molding A method of welding to the bottom surface portion 41 can be exemplified.

  Moreover, as a material of the thermal insulation layer 46 which satisfy | fills said requirements, a heat resistant silicone resin can be illustrated, for example. Examples of the silicone resin include silicone resins made of a three-dimensional polymer having a high degree of branching, represented by methyl silicone resin and methylphenyl silicone resin. For example, polyalkylphenylsiloxane is preferable. Such silicone resin may contain hollow particles such as a shirasu balloon. The heat insulating layer 46 made of silicone resin can be fixed to the crown surface 40 of the piston 5 by, for example, a coating process.

  Table 1 below shows preferable examples of material selection for the base material of the combustion chamber wall surface, the heat insulating layer 45, and the heat shielding layer 46. Specifically, Table 1 shows examples of preferable thermal conductivity / volume specific heat of the base material of the combustion chamber wall surface, the heat insulating layer 45, and the heat insulating layer 46, and preferable film thickness examples of the heat insulating layer 45 and the heat insulating layer 46. Respectively.

  As illustrated in Table 1 above, the thermal barrier layer 46 has a “low” thermal conductivity and a “small” volume specific heat. Such a heat shield layer 46 can sufficiently block heat transfer, but has a poor function of storing heat. Since the heat shield layer 46 is coated on the entire crown surface 40 of the piston 5, it is possible to prevent heat from escaping from the combustion chamber 10 through the crown surface 40 as indicated by an arrow D <b> 1 in FIG. 4. However, since the heat storage capacity of the heat shield layer 46 is low, the region where only the heat shield layer 46 is coated tends to change in temperature following the room temperature of the combustion chamber 10.

  On the other hand, the heat conductivity of the heat insulating layer 45 is “small”, which is larger than the heat conductivity of the heat shield layer 46 but sufficiently smaller than the heat conductivity of the base material. On the other hand, the volume specific heat of the heat insulating layer 45 is “large”, which is larger than the volume specific heat of both the heat shield layer 46 and the piston 5 (base material of the combustion chamber wall surface). Moreover, the thickness of the heat insulating layer 45 is about 10 times larger than that of the heat shield layer 46. Such a heat insulating layer 45 can sufficiently block heat transfer and has an excellent function of storing heat. By additionally disposing such a heat insulating layer 45 in the center portion of the crown surface 40, not only the center portion of the crown surface 40 is highly insulated, but also as shown by arrows D2 and D3 in FIG. The heat around the heat insulating layer 45 is stored in the heat insulating layer 45.

  Depending on the properties of the heat insulating layer 45 and the heat shield layer 46, the combustion chamber 10 has a temperature distribution in which the temperature at the center is higher than that at the outer periphery, particularly near the compression top dead center ( (See the upper graph in FIG. 4).

  That is, in this embodiment, since the heat shield layer 46 is formed on the entire crown surface 40 of the piston 5, even if the inside of the combustion chamber 10 is heated near the compression top dead center due to compression or combustion by the piston 5. The heat is suppressed from escaping to the outside through the crown surface 40 of the piston 5, and the combustion chamber 10 is maintained at a high temperature. At this time, since the outer peripheral portion of the combustion chamber 10 has a larger contact area with the wall surface of the combustion chamber than the central portion, the temperature of the outer peripheral portion of the combustion chamber 10 is naturally higher than that of the central portion. It tends to decrease. For this reason, when the temperature of the combustion chamber 10 in the vicinity of the compression top dead center generally rises as described above, the temperature difference between the central portion and the outer peripheral portion of the combustion chamber 10 increases. However, since the heat shield layer 46 covers the entire crown surface 40, the temperature difference is not positively generated, and the temperature difference does not become sufficiently large.

  On the other hand, in this embodiment, since the heat insulation layer 45 is additionally arranged only in the central portion of the crown surface 40 (the bottom surface portion 41 of the cavity C), the heat transfer blocking function and the heat storage function by the heat insulation layer 45 are provided. Is added, the temperature of the central portion of the combustion chamber 10 is further increased, and the temperature difference between the central portion and the outer peripheral portion is increased. As described above, in the present embodiment, the central portion of the combustion chamber 10 is heated to a temperature sufficiently higher than that of the outer peripheral portion due to the synergistic effect of the heat insulating layer 45 and the heat shielding layer 46, so that the environment of the central portion is self-ignited. It is possible to make the environment more likely to occur.

  In addition, since the heat insulation layer 46 covering the crown surface 40 as a whole has a sufficiently small volume specific heat, after the combustion is completed in the combustion chamber 10, the temperature is promptly increased with the introduction of fresh air, for example. descend. As described above, a situation in which the temperature of the piston 5 and the combustion chamber 10 excessively increases is avoided by the temperature change of the heat shield layer 46 with a high response in the combustion cycle.

(3) Specific Configuration of Water Supply System As shown in FIG. 1, the water supply system 50 includes the above-described condenser 51 and heat exchanger 54 provided in the exhaust passage 30 and the condensed water generated by the condenser 51. A water tank 52 to be stored, a water pump 53 that pumps condensed water stored in the water tank 52 toward the heat exchanger 54, and a high temperature / high pressure that is pressurized by the water pump 53 and heated by the heat exchanger 54. Pressure accumulation rail 56 for accumulating water in a pressure accumulating state while maintaining the temperature of the water, and one for each cylinder 2 (four in total) for supplying the water accumulated in the pressure accumulation rail 56 to the combustion chamber 10 of each cylinder 2 The water injection valve 57 (corresponding to the “water injection device” in the claims), the first water pipe 61 connecting the condenser 51 and the water tank 52, and the water tank 52 and the heat exchanger 54 are connected. Second water pipe 62 and heat It has the 3rd water piping 63 which connects the exchanger 54 and the pressure accumulation rail 56, and the some (four) distribution piping 64 which connects the pressure accumulation rail 56 and each water injection valve 57. FIG.

  The condenser 51 is a heat exchanger for condensing water vapor contained in the exhaust gas flowing through the common exhaust pipe 32, and cools the exhaust gas by heat exchange with a predetermined refrigerant (for example, engine cooling water). Thus, the water vapor contained in the exhaust gas is condensed. The condensed water generated by the condenser 51 flows out downstream through the first water pipe 61 and is stored in the water tank 52.

  The water feed pump 53 is provided in the middle of the second water pipe 62 and feeds the condensed water stored in the water tank 52 toward the heat exchanger 54 while pressurizing the condensed water. In FIG. 1, a single pump 53 is illustrated as a water pump. However, the water pump is configured to feed water stored in the water tank 52 at a relatively low pressure, and water fed from the feed pump. A multi-stage pump combined with a high-pressure pump that pressurizes and raises the pressure to a desired pressure may be used.

  The heat exchanger 54 is provided so as to heat the water supplied from the water pump 53 by heat exchange with the exhaust gas before flowing into the condenser 51. Although not shown in detail, the heat exchanger 54 includes a small-diameter and long-shaped thin tube 54a inserted into a portion of the common exhaust pipe 32 positioned between the catalyst device 35 and the condenser 51, and the thin tube 54a. And a heat retaining case 54b provided so as to cover the common exhaust pipe 32 in the portion where the is inserted.

  The water heated by the heat exchanger 54 is sent to the downstream side through the third water pipe 63 and stored in the pressure accumulation rail 56. The pressure accumulation rail 56 is provided with a water pressure sensor SN3 that detects the pressure of the internal water.

  The water stored in the pressure accumulation rail 56 is heated to 100 ° C./2 MPa or more through the heating by the heat exchanger 54 and the pressurization by the water supply pump 53 as described above. Since the pressure is as high as 2 MPa or higher, water does not boil even when heated to 100 ° C. or higher, and maintains a liquid state. Then, the water stored in the pressure accumulation rail 56 in such a state is injected into the combustion chamber 10 of each cylinder 2 through the water injection valve 57 when necessary. That is, in this embodiment, water injected from the water injection valve 57 to the cylinder 2 is high-temperature / high-pressure liquid water having a temperature of 100 ° C. or higher and a pressure of 2 MPa or higher.

  The water injection valve 57 is attached to the cylinder head 4 in such a posture that its axial center substantially coincides with the central axis of the cylinder 2. The water injection valve 57 has a tip portion exposed to the combustion chamber 10 in the vicinity of the center of the ceiling surface of the combustion chamber 10 so as to face the cavity C of the piston 5 from directly above, and a plurality of nozzle holes provided in the tip portion. It is possible to spray water radially through (not shown).

(4) Engine Control System FIG. 5 is a block diagram showing the engine control system. The PCM 100 shown in this figure is a microprocessor for overall control of the engine, and includes a known CPU, ROM, RAM, and the like.

  Detection signals from various sensors are input to the PCM 100. For example, the PCM 100 is electrically connected to the above-described crank angle sensor SN1, airflow sensor SN2, and water pressure sensor SN3, and information detected by these sensors (that is, crank angle, engine speed, intake air flow rate, water pressure). Etc.) are sequentially input to the PCM 100 as electrical signals.

  Further, the vehicle is provided with an accelerator sensor SN4 that detects an opening degree of an accelerator pedal (not shown) operated by a driver driving the vehicle, and a detection signal from the accelerator sensor SN4 is also input to the PCM 100. .

  The PCM 100 controls each part of the engine while executing various determinations and calculations based on input signals from the sensors. That is, the PCM 100 is electrically connected to the fuel injection valve 11, the throttle valve 27, the water pump 53, the water injection valve 57, and the like, and each of these devices has a control signal based on the result of the above calculation. Is output.

  As functional elements related to the control, the PCM 100 includes a fuel injection control unit 101 and a water injection control unit 102.

  The fuel injection control unit 101 is detected by the engine rotational speed detected by the crank angle sensor SN1, the engine load (requested torque) specified from the detected value (accelerator opening) of the accelerator sensor SN4, and the airflow sensor SN2. The fuel injection amount and injection timing from the fuel injection valve 11 are determined based on the intake flow rate, and the fuel injection valve 11 is controlled according to the determination.

  Based on the internal pressure of the pressure accumulation rail 56 (the pressure of water stored in the pressure accumulation rail 56) detected by the water pressure sensor SN3, the water injection control unit 102 holds the pressure at a required pressure (2 MPa) or more. Then, the water supply pump 53 is driven. Further, the water injection control unit 102 determines the water injection amount and the injection timing from the water injection valve 57 based on the engine load and the rotation speed, and controls the water injection valve 57 according to the determination.

(5) Control according to operation conditions Next, the control of the fuel injection valve 11 and the water injection valve 57 by the PCM 100 (the fuel injection control unit 101 and the water injection control unit 102) will be described in detail.

  FIG. 6 is a map for explaining the difference in control according to the engine operating conditions (load / rotational speed). As shown in the figure, the engine operating region is roughly divided into four operating regions A1 to A4 depending on the control of the fuel injection valve 11 and the water injection valve 57. Assuming that the first operation region A1, the second operation region A2, the third operation region A3, and the fourth operation region A4, respectively, the first operation region A1 has a rotational speed that is less than a predetermined value Rx and a load that is greater than or equal to a predetermined value Lx. The second operation region A2 is a high-speed / high-load operation region in which the rotational speed is equal to or higher than the predetermined value Rx and the load is equal to or higher than the predetermined value Lx. The third operation region A3 Is a low-speed / low-load operation region in which the rotation speed is less than the predetermined value Rx and the load is less than the predetermined value Lx. The fourth operation region A4 is a rotation speed that is greater than or equal to the predetermined value Rx and the load is the predetermined value Lx. This is a high-speed, low-load operating range that is less than The first operation region A1 corresponds to the “low speed region” in the claims, and the second operation region A2 corresponds to the “high speed region” in the claims. The predetermined value Lx, which is the load threshold value, is shown as being constant regardless of the rotational speed in FIG. 4, but may be a value that changes according to the rotational speed. Similarly, the predetermined value Rx that is the threshold value of the rotation speed is illustrated as being constant regardless of the load in FIG. 4, but may be a value that changes according to the load.

  Among the operation regions A1 to A4 shown in FIG. 6, at least in the first, second, and third operation regions A1, A2, and A3, the fuel injection valve is controlled by the fuel injection control unit 101 and the water injection control unit 102. 11 and the water injection from the water injection valve 57 are both executed. Operations of the fuel injection valve 11 and the water injection valve 57 in the first, second, and third operation regions A1, A2, and A3 will be described in order in the following (a), (b), and (c).

  Here, in the following explanations (a) to (c), terms such as “early period”, “middle period”,... Of the intake stroke or compression stroke are used as terms for specifying the timing of fuel injection or water injection. This is based on the following assumptions. That is, in this specification, each period when an arbitrary stroke such as an intake stroke or a compression stroke is divided into three equal parts is defined as “first period”, “middle period”, and “late period” in order from the front. For this reason, for example, (i) the first half, (ii) the middle, and (iii) the second half of the compression stroke are (i) before compression top dead center (BTDC) 180 to 120 ° CA, and (ii) BTDC 120 to 60 °, respectively. CA, (iii) refers to each range of BTDC 60 to 0 ° CA. Similarly, in this specification, each period when an arbitrary stroke such as an intake stroke or a compression stroke is divided into two equal parts is defined as “first half” and “second half” in order from the front. For this reason, for example, (iv) first half and (v) second half of the compression stroke indicate ranges of (iv) BTDC 180 to 90 ° CA and (v) BTDC 90 to 0 ° CA, respectively.

(A) Control in the first operation region In the first operation region A1 of low speed and high load, as shown in FIG. 7, the fuel injection If1 from the fuel injection valve 11 is within the period from the middle stage to the latter stage of the intake stroke. And the water injection Iw1 from the water injection valve 57 is executed within the period from the first period to the middle period of the compression stroke.

  By performing the fuel injection If1 at the timing as described above (the middle stage to the latter stage of the intake stroke), the fuel distribution before ignition is sufficiently uniformized. That is, when fuel is injected from the fuel injection valve 11 within a period from the middle to the latter stage of the intake stroke, the injected fuel is vaporized and atomized while being agitated by the intake air flow or the like, and the compression top dead center (FIG. 7). Until the TDC) is uniformly dispersed in the combustion chamber 10.

  The amount of fuel injected by the fuel injection If1 is set to a relatively large value that meets the conditions of the first operating region A1 where the load is high. The injection pulse width of the fuel injection If1 (the valve opening period of the fuel injection valve 11) is increased or decreased according to the determined injection amount, and the pulse width is increased as the injection amount increases. The same applies to the water injection Iw1.

  The timing of the water injection Iw1 (from the first to the middle of the compression stroke) is determined with the intention that the water injected from the water injection valve 57 is unevenly distributed in the outer peripheral portion of the combustion chamber 10. That is, when water is injected from the water injection valve 57 during the period from the first period to the middle period of the compression stroke, the injected water is as shown in the lower diagram of FIG. 10 (FIG. 7 also shows the corresponding figure). (Schematically shown), which flies radially toward a region outside the cavity C (that is, the squish portion 43) on the crown surface 40 of the piston 5 or toward the peripheral wall of the cylinder 2. The flying water evaporates after adhering to the squish portion 43 of the crown surface 40 or the peripheral wall of the cylinder 2, and as a result, the piston 5 rises to near the compression top dead center as shown in the middle diagram of FIG. At this point, a relatively concentrated water (mainly water vapor) is present in the outer peripheral portion of the combustion chamber 10, that is, the concentration of water present in the outer peripheral portion of the combustion chamber 10 is higher than that in the central portion of the combustion chamber 10. A sufficiently dark state is obtained. And as a result of the wall surface temperature and gas temperature of the outer peripheral part of the combustion chamber 10 falling by the cooling effect of the water unevenly distributed in the outer peripheral part of the combustion chamber 10 as described above, immediately before the start of combustion (near the compression top dead center) As the temperature distribution of the combustion chamber 10, a distribution as shown in the upper graph of FIG. 10 is obtained.

  In the above graph, the solid line waveform indicates the temperature distribution when water injection is performed, and the broken line waveform indicates the temperature distribution when water injection is not performed. From the comparison between the two, the temperature (wall surface temperature and gas temperature) of the outer peripheral portion of the combustion chamber 10 intensively decreases due to the effect of the water injection Iw1, and the difference between the temperature of the outer peripheral portion and the temperature of the central portion increases. It is understood that In the present embodiment, since the heat insulating layer 45 and the heat shielding layer 46 described above serve to increase the temperature of the central portion of the combustion chamber 10, even if water injection is not performed, the central portion and the outer periphery A certain temperature difference has already occurred between these parts. The water jet Iw1 serves to further expand this temperature difference.

  As a cooling effect on the outer peripheral portion of the combustion chamber 10 by the injection water Iw1, a higher level is required on the high load side where the fuel injection amount is large and the combustion chamber 10 is likely to become high temperature. For this reason, the amount of water injection by the water injection Iw1 is generally set to a larger value on the higher load side in the first operation region A1.

(B) Control in the second operation region In the second operation region A2 of high speed and high load, as shown in FIG. 8, the fuel injection If2 from the fuel injection valve 11 is within the period from the middle stage to the latter stage of the intake stroke. And water injection Iw2 from the water injection valve 57 is executed at the latter stage of the compression stroke.

  Since the second operation region A2 is in the same load region as the first operation region A1, the fuel injection amount by the fuel injection If2 is substantially equal to the fuel injection If1 (FIG. 7) in the first operation region A1. However, since the rotation speed is higher in the second operation region A2 than in the first operation region A1 (in other words, the amount of change in the crank angle per unit time is large), the crank angle required to inject the same amount of fuel is the first. It becomes longer than in the case of one operation region A1. This is why the pulse width of the fuel injection If2 in the second operation region A2 is longer than the pulse width of the fuel injection If1 in the first operation region A1.

  The timing of the water injection Iw2 (the latter stage of the compression stroke) is determined with the intention of containing the water injected from the water injection valve 57 in the cavity C of the combustion chamber 10. That is, when water is injected from the water injection valve 57 in the latter stage of the compression stroke, the injection water is as shown in the lower diagram of FIG. 11 (FIG. 8 also shows a schematic diagram corresponding thereto). It flies radially toward the inside of the cavity C. The flying water evaporates after adhering to the wall surface of the cavity C. As a result, as shown in the middle diagram of FIG. 11, when the piston 5 rises to near the compression top dead center, A state in which relatively high concentration water (mainly water vapor) is present in the part, that is, a state in which the concentration of water existing in the central part of the combustion chamber 10 is sufficiently thicker than the outer peripheral part of the combustion chamber 10 is obtained. . As a result of the cooling effect of the water unevenly distributed in the central portion of the combustion chamber 10 as described above, the wall surface temperature and gas temperature of the central portion (cavity C) of the combustion chamber 10 are lowered, so As the temperature distribution of the combustion chamber 10 in the vicinity of the dead center), a distribution as shown in the upper graph of FIG. 11 is obtained. In the present embodiment, the central portion of the combustion chamber 10 is heated to a temperature higher than that of the outer peripheral portion due to the effects of the heat insulating layer 45 and the heat shielding layer 46 described above (see the waveform shown by the broken line in the graph). When the temperature at the center portion of 10 is lowered, the temperature difference between the center portion and the outer peripheral portion is reduced (refer to the solid line waveform in the graph).

  As a cooling effect on the central portion of the combustion chamber 10 by the jet water Iw2, a higher level is required as the crank angle change amount per unit time is larger (so that the crank angle range in which combustion continues tends to be longer). . For this reason, the amount of water injected by the water injection Iw2 is generally set to a larger value on the higher speed side in the second operation region A2.

(C) Control in the third operation region In the third operation region A3 of low speed and low load, as shown in FIG. 9, the fuel injection If3 from the fuel injection valve 11 is executed in the latter half of the compression stroke, Water injection Iw3 from the injection valve 57 is executed immediately after the compression top dead center.

  Since the load in the third operation region A3 is lower than that in the first operation region A1, the fuel injection amount by the fuel injection If3 is set to a value smaller than the fuel injection If1 (FIG. 7) in the first operation region A1. . As described above, since the fuel injection amount is small in the third operation region A3, the penetration (penetration force) of the fuel injected from the fuel injection valve 11 is relatively weak.

  The timing of the fuel injection If3 (the second half of the compression stroke) is determined with the intention of accommodating the fuel injected from the fuel injection valve 11 in the cavity C of the piston 5. That is, when fuel is injected from the fuel injection valve 11 in the latter half of the compression stroke, the injected fuel is as shown in the lower diagram of FIG. 12 (FIG. 9 also schematically shows the corresponding figure). It flies radially toward the inside of the cavity C of the piston 5. Most of the fuel flying into the cavity C stays inside the cavity C in combination with the small injection amount and weak fuel penetration. As a result, a state in which the concentration of the fuel existing inside the cavity C is sufficiently thicker than that outside the cavity C is obtained, and a locally rich air-fuel mixture is formed inside the cavity C. This rich air-fuel mixture reaches self-ignition without difficulty when the piston 5 reaches near the compression top dead center, and HCCI combustion is started.

  The timing of the water injection Iw3 (immediately after the compression top dead center) is determined with the intention of repeating the water injection during the period of combustion (HCCI combustion) started near the compression top dead center as described above. More specifically, the timing of water injection Iw3 is set to a timing at which water injection is started after the time when combustion starts and water injection is ended before the time when combustion is ended. The water jetted from the water jet valve 57 at such a timing flies radially toward the inside of the cavity C as shown in the upper diagram of FIG. At this time, since combustion has already occurred in the central portion of the combustion chamber 10 (mainly the internal space of the cavity C), the water injected by the water injection Iw3 is supplied into this combustion region.

(6) Effect As described above, in the premixed compression ignition type engine of the present embodiment, the heat insulating layer 45 having a lower thermal conductivity than the base material of the combustion chamber wall surface is provided at the center of the crown surface 40 of the piston 5. The water injection valve 57 is disposed at the latter stage of the compression stroke so that water is unevenly distributed in the center of the combustion chamber 10 immediately before the start of combustion during operation in the second operation region A2 of high speed and high load. Water is injected so that the water is unevenly distributed on the outer peripheral portion of the combustion chamber 10 immediately before the start of combustion during the operation in the first operation region A1 on the lower speed side (low speed / high load) than the second operation region A2. In addition, water is injected from the water injection valve 57 in the first or middle period of the compression stroke. According to such a configuration, there is an advantage that the combustion can be moderated moderately according to the operating conditions, and the thermal efficiency can be secured satisfactorily while suppressing the combustion noise.

  That is, in the above-described embodiment, the heat insulating layer 45 is disposed at the center of the crown surface 40 (combustion chamber wall surface) of the piston 5, so that the effect of the heat insulating layer 45 causes the combustion chamber 10 to pass through the center of the crown surface 40. Heat can be prevented from escaping to the outside. As a result, immediately before the start of combustion, that is, when the piston 5 rises to the vicinity of the compression top dead center and the combustion chamber 10 is sufficiently heated, a significant temperature distribution (in the radial direction) is set inside the combustion chamber 10. A state in which a temperature difference more than a fixed amount occurs) can be created. More specifically, about the center part of the combustion chamber 10 in which the heat insulating layer 45 is provided on the wall surface, heat dissipation is suppressed and the temperature is increased, while on the outer peripheral part of the combustion chamber 10 in which the heat insulating layer 45 is not provided on the wall surface, Since the heat insulation effect by the heat insulation layer 45 cannot be obtained, the temperature becomes relatively low. In this way, the temperature of the central portion of the combustion chamber 10 is sufficiently higher than that of the outer peripheral portion, and a significant temperature distribution is obtained.

  As the above temperature distribution (temperature difference) is formed in the combustion chamber 10, a difference occurs in the timing at which the fuel self-ignites in the combustion chamber 10, and the combustion can be slowed down. That is, the fuel self-ignites first in the center portion of the combustion chamber 10 having a high temperature, and then the fuel self-ignites in the outer peripheral portion of the combustion chamber 10 having a relatively low temperature. It is possible to realize a combustion mode in which the gas advances. Thereby, combustion can be slowed down as a whole, and rapid increase in in-cylinder pressure can be suppressed.

  As a result of the suppression of the rapid increase in the in-cylinder pressure as described above, the maximum value of the in-cylinder pressure increase rate (dp / dθ) is reduced, and an engine excellent in commerciality with low combustion noise can be realized. it can. Moreover, since the maximum value of the combustion temperature is also reduced, clean combustion with a small amount of NOx generated can be realized.

  In addition, according to the above-described embodiment in which combustion can proceed in order from the inner side to the outer side in the radial direction, it is possible to avoid a flame from reaching the wall surface of the outer peripheral portion of the combustion chamber 10 that is relatively low in temperature. In addition, since the region where the initial combustion occurs is insulated by the heat insulating layer 45, it is possible to sufficiently suppress the escape of combustion heat to the outside. Thereby, a cooling loss can be reduced and the thermal efficiency of an engine can be improved effectively.

  However, in the second operating region A2 where the rotational speed is high, the amount of change in the crank angle per unit time is large, so even if the combustion becomes somewhat steep, the maximum value of the rate of pressure increase (dp / dθ) or the collision of flame The area is not so large. For this reason, even if the temperature difference in the combustion chamber 10 is reduced in the second operation region A2, the increase in combustion noise and cooling loss is not a problem. Rather, as a result of burning a lot of fuel in a short time, It is thought that exhaust loss is reduced and thermal efficiency is improved. In other words, if the temperature difference in the combustion chamber 10 is sufficiently large in the second operation region A2, the combustion is unnecessarily slowed, and the thermal efficiency may be deteriorated.

  On the other hand, in the above embodiment, since the water injection Iw2 that causes water to be unevenly distributed in the central portion of the combustion chamber 10 is executed before the start of combustion (self-ignition) in the second operation region A2 of high speed and high load. The central portion of the combustion chamber 10 that tends to be hotter than the outer peripheral portion of the combustion chamber 10 is intensively cooled, so that the temperature difference between the central portion and the outer peripheral portion of the combustion chamber 10 is reduced. As a result, there is no significant difference in the ignition timing between the central portion / outer peripheral portion of the combustion chamber 10, and relatively steep combustion in which a large amount of fuel is combusted in a short time occurs. As a result, in the high speed region (second operation region A2) in which the rate of increase in in-cylinder pressure (dp / dθ) tends to decrease, it is possible to avoid unnecessarily slowing down combustion and to ensure good thermal efficiency. be able to.

  Moreover, although the center part of the combustion chamber 10 is a part which becomes easy to become high temperature especially on the conditions of high rotation and high load like 2nd operation area | region A2, in the said 2nd operation area | region A2, the center part of the combustion chamber 10 is concerned. According to the above embodiment, in which the temperature of the combustion chamber 10 is directly cooled by the jet water, it is reliably avoided that the temperature of the central portion of the combustion chamber 10 is excessively increased, so that the cooling loss is reduced and the thermal efficiency of the engine is improved. be able to.

  On the other hand, in the first operation region A1 of low speed and high load that is on the lower speed side than the second operation region A2, the maximum value of the rate of pressure increase (dp / dθ) is excessive, contrary to the second operation region A2. It is easy and needs to slow down combustion sufficiently. On the other hand, in the above embodiment, since the water injection Iw1 that causes water to be unevenly distributed in the outer peripheral portion of the combustion chamber 10 is executed before the start of combustion (self-ignition) in the first operation region A1 of low speed and high load. The outer peripheral part of the combustion chamber 10 is cooled by the jetted water, and as a result, the temperature of the outer peripheral part is lowered. As a result, the temperature difference between the central part and the outer peripheral part of the combustion chamber 10 increases. Thereby, since the progress of combustion becomes sufficiently slow, it is possible to suppress a rapid increase in in-cylinder pressure, and to effectively suppress an increase in combustion noise and cooling loss.

  Moreover, in the said embodiment, since water is radially injected toward the piston 5 from the water injection valve 57 provided in the center vicinity of the ceiling surface of the combustion chamber 10, in 1st operation area | region A1 of low speed and high load. By performing the water injection Iw1 in the first or middle period of the compression stroke, the injected water can be directed to the outer peripheral portion (squish portion 43) of the crown surface 40 of the piston 5 or the peripheral wall of the cylinder 2. A state in which water is unevenly distributed on the outer peripheral portion of the combustion chamber 10 can be appropriately created by water.

  On the other hand, in the second operation region A2 of high speed and high load, the water injection Iw2 is executed at the latter stage of the compression stroke, so that the injected water can be directed to the central part (cavity C) of the piston 5, A state in which water is unevenly distributed in the central portion of the combustion chamber 10 can be appropriately created by the jet water.

  Further, in the above embodiment, during the operation in the second operation region A2 of high speed and high load, the fuel is injected from the fuel injection valve 11 during the intake stroke (fuel injection If2), so that the injected fuel starts burning. Can be sufficiently dispersed in the combustion chamber 10 until the fuel concentration (air-fuel ratio) in the combustion chamber 10 can be made uniform. This, together with the effect of the water injection Iw2 performed in the second operation region A2 (that is, the effect of cooling the central portion of the combustion chamber 10 to reduce the temperature difference), Since this leads to a reduction in the ignition timing difference, it is possible to sufficiently eliminate the slowing of combustion and realize combustion with high thermal efficiency.

  Moreover, in the said embodiment, since the quantity of the said water injection Iw2 implemented at the latter stage of a compression stroke is increased, so that the rotational speed is high at the time of driving | operation in 2nd operation area | region A2 of high speed and high load, cylinder pressure The higher the rate of increase (dp / dθ) is, the more the cooling effect on the central portion of the combustion chamber 10 can be enhanced, and the temperature difference in the combustion chamber 10 is reduced to promote sharpening of combustion. Can do.

Further, in the above embodiment, during the operation in the third operation region A3 with low speed and low load, the fuel injection If3 is executed at a relatively late time such as the latter half of the compression stroke, so the fuel injection is small and the fuel penetration is weak. By the synergistic effect with this, a state in which the fuel is unevenly distributed only in a part of the combustion chamber 10 (particularly, inside the cavity C) can be obtained before the fuel self-ignites. When the fuel self-ignites in such a state, the combustion region (region where the flame spreads) is limited to a relatively narrow range, so the area where the flame contacts the combustion chamber wall surface can be reduced, and cooling loss is effectively reduced. can do. However, there is a possibility that an excessively rich air-fuel mixture may be formed in the region where the fuel is unevenly distributed (in the cavity C), and there is a concern that the amount of soot generated increases accordingly. On the other hand, in the above-described embodiment, water is supplied to the high-temperature combustion chamber 10 where the combustion is continuing and is vaporized by the water injection Iw3 that is started immediately after the compression top dead center, and the vaporized water is combusted. Since the effect of increasing OH radicals contributing to the reaction is brought about, the strong oxidation action by the OH radicals promotes the oxidation of carbon (C) (converted to CO ₂ or the like). As a result, the amount of soot generated is reduced. It can be effectively suppressed.

  Further, in the above-described embodiment, the heat insulating layer 46 that covers both the heat insulating layer 45 at the center and the radially outer crown surface 40 is provided on the crown surface 40 of the piston 5. Can be suppressed by the heat shield layer 46, and cooling loss can be reduced. In particular, since the central portion of the combustion chamber 10 that is likely to become high temperature is thermally insulated twice by the heat shield layer 46 and the heat insulating layer 45, the effect of reducing the cooling loss can be further enhanced. In addition, the heat insulating layer 46 has a smaller volume specific heat (lower heat storage property) than the heat insulating layer 45 and is likely to fluctuate following the ambient temperature. For this reason, the temperature of the heat shield layer 46 can be quickly reduced, for example, with the introduction of fresh air, between the end of combustion and the next combustion. As a result, a situation in which the temperature of the combustion chamber 10 excessively rises is avoided, so that the occurrence of abnormal combustion or the like is effectively prevented while the cooling loss is reduced by the heat insulation of the combustion chamber 10. be able to.

(7) Modified Example In the above embodiment, the heat insulating layer 45 is disposed at the center of the crown surface 40 of the piston 5, and the heat insulating layer 45 and the radially outer crown surface 40 are covered together. Although the layer 46 is disposed, the heat shield layer 46 may not be disposed immediately above the heat insulating layer 45. Alternatively, only the heat insulating layer 45 may be left and the heat shield layer 46 may be omitted. Conversely, the heat shield layer 46 may be disposed not only on the crown surface 40 of the piston 5 but also on the ceiling surface of the combustion chamber 10 and the peripheral wall of the cylinder 2 (inner peripheral surface of the cylinder block 3).

  Moreover, in the said embodiment, although the heat insulation layer 45 was arrange | positioned only in the center part (bottom part 41 of the cavity C) of the crown surface 40 of piston 5, the heat insulation layer 45 is arrange | positioned at the radial direction center part of a combustion chamber wall surface. For example, the heat insulating layer 45 can be arranged also in the center of the ceiling surface of the combustion chamber 10 (the lower surface of the cylinder head 4 in the portion located between the intake / exhaust ports 6 and 7 in FIG. 2). .

  Moreover, in the said embodiment, although the heat conductivity of the thermal insulation layer 46 was set to the value smaller than that of the heat insulation layer 45, the thermal conductivity of these both layers 45 and 46 may be the same, or a thermal insulation layer The thermal conductivity of 46 may be set to a value larger than that of the heat insulating layer 45.

  Moreover, in the said embodiment, the multi-injection type injection valve in which the several injection hole was formed in the front-end | tip part was used as the water injection valve 57, However, The water injection valve which can be used in this invention is not restricted to this. It is also possible to use a so-called outer-opening type injection valve as a water injection valve. The outward opening type injection valve includes, for example, a cylindrical valve body and a needle valve inserted into the valve body so as to be able to advance and retract. The distal end portion of the needle valve is accommodated in a state where the outer peripheral surface thereof can be in close contact with the inner peripheral surface of the distal end portion of the valve body. In such an outside-opening type injection valve, the needle valve is driven in the protruding direction when the valve is opened, so that a continuous ring-shaped slit is formed between the tip of the needle valve and the inner peripheral surface of the valve body. A nozzle port is formed, and water is sprayed in a cone shape through this nozzle port (such a cone-shaped water spray is also a mode in which water is sprayed radially). The same applies to the fuel injection valve 11 in which an outwardly opening type injection valve can be used.

  Moreover, in the said embodiment, in order to produce the state where water is unevenly distributed in the outer peripheral part of the combustion chamber 10 just before the start of combustion in the first operation region A1 of low speed and high load, the vicinity of the center of the ceiling surface of the combustion chamber 10 The water is injected radially from the water injection valve 57 disposed in the first half or middle of the compression stroke (water injection Iw1), but the timing of the water injection Iw1 in the first operation region A1 starts combustion. Any timing may be used as long as it can create a state in which water is unevenly distributed in the outer peripheral portion of the combustion chamber 10 immediately before the operation is performed. For example, if the mounting position of the water injection valve 57 changes, the timing suitable for uneven distribution of water on the outer peripheral portion of the combustion chamber 10 can also change, so the timing of the water injection Iw1 depends on the mounting position of the water injection valve 57 and the like. May be adjusted accordingly. However, in any case, the water injection Iw1 in the first operation region A1 needs to be started after the fuel injection If1 by the fuel injection valve 11 is completed until the fuel self-ignites.

  Further, in the first operation region A1 with low speed and high load, the amount of water present in the central portion of the combustion chamber 10 is reduced at least as compared with the second operation region A2 with high speed and high load. The injection valve 57 may be controlled, and the injection mode can be variously changed as long as it is controlled. For example, in at least a part of the first operation region A1, water injection in the first or middle period of the compression stroke (water injection for unevenly distributing water in the outer peripheral portion of the combustion chamber 10) and water injection (combustion in the second half of the compression stroke) Water injection for causing water to be unevenly distributed in the center of the chamber 10) may be performed in small amounts. Alternatively, the water injection may be executed only in a part of the first operation area A1, and the water injection may be stopped in the remaining part of the area A1. Furthermore, water injection may be stopped throughout the first operation region A1.

  In the above-described embodiment, the fuel is injected from the fuel injection valve 11 in the middle or later stage of the intake stroke in the first operation region A1 of low speed and high load (fuel injection If1). The timing of the fuel injection If1 at A1 may be any timing as long as the fuel is relatively uniformly dispersed in the combustion chamber 10 until the compression top dead center, and can be appropriately changed as long as the timing is reached. For example, the fuel injection If1 may be executed in the first half of the compression stroke. However, the timing of the fuel injection If1 in the first operation region A1 is set at least earlier than the timing of the fuel injection If3 in the third operation region A3. The same applies to the fuel injection If2 in the second operation region A2 with high speed and high load.

  In the above-described embodiment, the water injection valve is used to create a state in which water is unevenly distributed in the central portion (inside the cavity C) of the combustion chamber 10 immediately before the start of combustion in the high-speed / high-load second operation region A2. Although the water is injected radially from 57 in the latter half of the compression stroke (water injection Iw2), the timing of the water injection Iw2 in the second operation region A2 is the center of the combustion chamber 10 immediately before the start of combustion. Any timing can be used as long as the water can be unevenly distributed, and appropriate changes can be made as long as the timing is reached. For example, depending on the scheduled timing of self-ignition, water may be injected at a timing such that a part of the period of the water injection Iw2 is included in the initial stage of the expansion stroke.

  Moreover, in the said embodiment, in the 3rd driving | running area | region A3 of low speed and low load, the period of the said water injection Iw3 is carried out during a combustion period by starting the water injection Iw3 immediately after a compression top dead center, and ending after that. Although water was injected in such a manner that it was completely included, the timing of water injection Iw3 in this third operation region A3 is that the action of increasing OH radicals by the water injected into the combustion chamber 10 (water vapor after vaporization). Any timing that extends during the combustion period may be used, and appropriate changes can be made as long as the timing is reached. However, for this purpose, the water injection Iw3 in the third operation region A3 needs to be performed from the end of the fuel injection If3 to the end of combustion, and the timing is the latter part of the compression stroke. Within the period from the beginning of the expansion stroke.

  In the above embodiment, as the water injected from the water injection valve 57 to the combustion chamber 10, relatively high temperature / high pressure water having a temperature of 100 ° C. or higher and a pressure of 2 MPa or higher is used. The temperature, pressure, and the like can be appropriately changed depending on which of the water jet contribution effects required in each of the operation regions A1, A2, and A3 described above is important.

  For example, water whose temperature and pressure have been increased to a so-called supercritical or subcritical state (that is, supercritical water or subcritical water) can be injected into the combustion chamber 10. Here, supercritical water is water having a temperature and pressure higher than the critical point of water (647 K, 22 MPa), and subcritical water is water having properties close to supercritical water. .

  Supercritical water / subcritical water requires little enthalpy (latent heat) to change to gaseous water. Moreover, the supercritical water / subcritical water injected into the combustion chamber 10 works to push down the piston 5 by rapidly expanding in the combustion chamber 10. Therefore, when supercritical water / subcritical water having such a property is injected into the combustion chamber 10, there is a possibility that the fuel consumption can be improved by reducing the fuel injection amount by the work increment accompanying the injection.

  In particular, in the third operation region A3, water is injected immediately after the compression top dead center. Therefore, if supercritical water / subcritical water is used as the injection water, it is considered that a sufficient fuel efficiency improvement effect can be obtained. That is, when supercritical water / subcritical water is injected immediately after the compression stroke in the third operation region A3, the injected supercritical water / subcritical water rapidly expands during the expansion stroke, so that the piston 5 Therefore, the amount of fuel injection can be reduced by the amount corresponding to the depressing work (the increase in the output torque). In addition, when supercritical water / subcritical water is generated using the heat of the exhaust gas, so-called exhaust heat recovery that converts the heat of the exhaust gas into output is achieved, so that a sufficient fuel economy improvement effect is obtained. It becomes possible.

  However, when supercritical water / subcritical water is used also in the first operation region A1 on the high load side, for example, the cooling effect by water injection expected in the first operation region A1 (that is, the outer peripheral portion of the combustion chamber 10 is The effect of cooling and slowing down combustion) is thought to decrease. However, even supercritical water / subcritical water is lower than the temperature of the combustion chamber 10 in the vicinity of the compression top dead center, so that a reasonable cooling effect can be expected. In addition, since water is an inert gas, it is considered that the same slowing effect can be obtained even when supercritical water / subcritical water is injected due to the effect of the uneven distribution of water as the inert gas. That is, when supercritical water / subcritical water is injected, a sufficient amount of water can be supplied to the outer peripheral portion of the combustion chamber 10 in a very short time, so that the concentration of the inert gas in the outer peripheral portion is sufficiently increased. The action brought about by the concentration difference makes it possible to slow down the combustion (even if the cooling effect is somewhat reduced).

  Moreover, although the said embodiment demonstrated the example which applied this invention to the gasoline engine by which the HCCI combustion which compresses and self-ignites the mixture of gasoline and air is performed in all the operation areas, this invention is applied. Possible engines are not limited to such engines. For example, a gasoline engine in which HCCI combustion is performed in a part of the operation region and spark ignition combustion is performed in the remaining operation region, or an engine that causes HCCI combustion of fuel containing subcomponents (alcohol or the like) other than gasoline. The present invention is also applicable.

2 cylinders 5 pistons 10 combustion chambers 11 fuel injection valves (fuel injection devices)
40 Crown (combustion chamber wall)
45 Heat insulation layer 46 Heat insulation layer 57 Water injection valve (water injection device)
A1 First operating range (low speed range)
A2 Second operating range (high speed range)
Lx (load) predetermined value Iw1 water injection (in the first operation region) Iw2 water injection (in the second operation region) If1 (in the first operation region) fuel injection If2 (in the second operation region) fuel injection

Claims (6)

A piston accommodated reciprocally in a cylinder, a fuel injection device for injecting fuel containing gasoline into a combustion chamber defined by the cylinder and the piston, and a water injection device for injecting water into the combustion chamber An engine capable of premixed compression ignition combustion in which fuel injected from the fuel injection device is mixed with air and burned by self-ignition,
A heat insulating layer having a lower thermal conductivity than the base material of the combustion chamber wall surface is disposed at the radial center of the combustion chamber wall surface surrounding the combustion chamber,
When the engine is operated in a predetermined high-speed region, the water injection device injects water at a timing at which water can be unevenly distributed in the radial center of the combustion chamber immediately before the start of combustion. ,
When the engine is operated in a low speed region where the rotational speed is lower than that in the high speed region, the water injection device is configured so that the amount of water in the radially central portion is smaller than that in the high speed region. A premixed compression ignition type engine characterized by injecting water at a timing different from that in the region or stopping water injection. The premixed compression ignition type engine according to claim 1,
The water injection device is provided to inject water from the vicinity of the center of the ceiling surface of the combustion chamber facing the piston toward the piston,
During operation in the high speed region, the water injection device injects water in the latter half of the compression stroke, and the fuel injection valve injects fuel in the first half of the intake stroke or the compression stroke. Compression ignition engine. The premixed compression ignition engine according to claim 2,
The premixed compression ignition engine characterized in that the water injection device increases the injection amount of the water injected later in the compression stroke when the rotation speed is high during operation in the high speed region. The premixed compression ignition type engine according to any one of claims 1 to 3,
The water injection device injects water at a timing at which water can be unevenly distributed on the outer peripheral portion of the combustion chamber immediately before the start of combustion during operation in the low speed region. Ignition engine. The premixed compression ignition engine according to claim 4,
The water injection device is provided to inject water radially from the vicinity of the center of the ceiling surface of the combustion chamber facing the piston toward the piston,
During operation in the low speed region, the water injection device injects water in the first half or the middle of the compression stroke, and the fuel injection valve injects fuel in the first half of the intake stroke or the compression stroke. Premixed compression ignition engine. The premixed compression ignition type engine according to any one of claims 1 to 5,
A heat shield layer that covers the heat insulating layer and a portion of the combustion chamber wall located radially outside the heat insulating layer;
The preheated compression ignition engine, wherein the thermal barrier layer is composed of a member having a thermal conductivity smaller than that of the base material of the combustion chamber wall surface and having a volume specific heat smaller than that of the heat insulating layer.

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