JP2019124190A - Compression ignition type engine - Google Patents

Abstract

To actualize more sufficient improvement of heat insulation by water injection by preventing or suppressing a temperature drop at the outer peripheral edge of a piston crown surface when injecting high-density and high-temperature water into a combustion chamber.SOLUTION: A cylinder block 3, a cylinder head 4, and a piston 5c define a combustion chamber 6. A water injection device 22 is provided for injecting high-density and high-temperature water to a piston crown surface 5a as a surface facing a combustion chamber 6 out of the piston 5. On the piston crown surface 5a, a heat insulating material layer 7 is formed. The heat insulating material layer 7 is formed so that the thermal capacity at the outer peripheral edge of the piston crown surface 5a is greater than the thermal capacity at the center of the piston crown surface (for example, the center is a thin site 7a and the outer peripheral edge is a thick site 7b).SELECTED DRAWING: Figure 4

Description

  The present invention relates to a compression ignition engine.

  In the engine, as shown in Patent Document 1, there is one in which a heat insulating material layer is formed on a piston crown surface (top surface). By forming the heat shield material layer as one including, for example, hollow particles, heat insulation and thermal responsiveness (responsiveness of temperature change to surrounding heat change) are compatible, and burnability improvement and cold loss reduction are achieved. On top of the In Patent Document 2, while forming a heat shield layer on the piston crown surface, thermal insulation is further enhanced by injecting high-density high-temperature water such as supercritical water or subcritical water into the combustion chamber. Things are disclosed.

Unexamined-Japanese-Patent No. 2015-081527 JP 2017-025775 A

  By the way, the outer peripheral edge close to the cylinder inner wall surface in the piston crown surface is easier to dissipate heat than the central region. For this reason, as described in Patent Document 2, when attempting to inject high-density, high-temperature water into the combustion chamber to improve heat insulation, the outer peripheral edge portion of the piston crown surface which is relatively low temperature This is likely to cause the problem of condensation of water and a considerable decrease in temperature, and further improvement is desired in terms of heat insulation improvement.

  The present invention has been made in consideration of the above circumstances, and its object is to prevent a temperature drop at the outer peripheral portion of the piston crown surface when high-density, high-temperature water is injected into the combustion chamber. An object of the present invention is to provide a compression-ignition engine capable of suppressing the heat insulation improvement by water injection more sufficiently.

In order to achieve the above object, the following solutions are adopted in the present invention. That is, as described in claim 1,
The cylinder block, the cylinder head and the piston define a combustion chamber,
There is provided a water injection device for injecting high-temperature high-temperature water to a piston crown surface which is a surface facing the combustion chamber among the pistons,
A heat insulating material layer is formed on the piston crown surface,
The heat shield layer is formed such that the heat capacity at the outer peripheral edge of the piston crown surface is larger than the heat capacity at the central portion of the piston crown surface.
It is done like that.

  According to the solution method, the heat capacity of the heat shield layer at the outer peripheral edge of the piston crown surface is larger than the heat capacity of the heat shield layer at the central portion, so the temperature decrease at this outer peripheral edge Can be sufficiently prevented, and a situation in which the injected water condenses at this outer peripheral portion can be prevented or suppressed, and the effect of the heat insulation improvement by the water injection can be obtained more sufficiently.

Preferred embodiments based on the above solution method are as described in claim 2 and the following claims. That is,
The heat shield layer is formed such that the thickness at the outer peripheral edge of the piston crown surface is larger than the thickness at the central portion of the piston crown surface (corresponding to claim 2) . In this case, the heat capacity of the heat shield material layer can be made different between the central portion and the outer peripheral portion by a simple method of making the thickness different.

  The thickness of the heat shield layer is set to a predetermined thickness range of 500 μm to less than 1500 μm, and the thickness of the outer peripheral portion of the piston crown surface and the thickness at the central portion of the piston crown surface are the predetermined thickness It is made to be different within the range (corresponding to claim 3). In this case, the thickness of the heat shield layer necessary to prevent or suppress the jetted water from condensing at the outer peripheral portion of the piston crown surface is provided.

  The high-density high-temperature water is made to be supercritical water or subcritical water (corresponding to claim 4). In this case, the heat insulation improvement by the injected water can fully be obtained. In addition, even if the water jetted into the water condenses, its latent heat is extremely small, so the temperature drop at the outer peripheral edge of the piston crown surface can be made extremely small.

A cavity is formed at the center of the piston crown surface,
The boundary between the thick portion and the thin portion of the heat shield layer is set outside the outer peripheral edge of the cavity and in the vicinity of the outer peripheral edge,
It corresponds (claim 5 correspondence). In this case, the cavity portion which becomes high temperature by combustion is made a thin portion of the heat shield layer, and the other portion is made a thick portion of the heat shield layer, and the condensation of the injected water is sufficiently prevented or suppressed. Can.

A cavity is formed at the center of the piston crown surface,
The boundary between the thick portion and the thin portion of the heat shield layer is near the intermediate position between the outer peripheral edge of the cavity and the outer peripheral edge of the piston crown surface or on the outer peripheral edge side of the piston crown surface than the intermediate position Is set to,
(Claim 6 corresponds). In this case, it is preferable in terms of reducing the amount of heat shielding material constituting the heat shielding material layer.

  The heat shield layer is configured to include hollow particles (corresponding to claim 7). In this case, it is possible to improve the heat responsiveness and obtain the combustion improvement by the heat insulating property, and to prevent the situation where the piston crown surface is maintained unnecessarily high temperature.

  HCCI combustion is performed at least in a predetermined operating range (corresponding to claim 8). In this case, by performing HCCI combustion, it is possible to sufficiently improve the fuel consumption together with the reduction of the cooling loss due to the heat shield material layer. In addition, the heat shield layer can sufficiently secure the heat of compression required for HCCI combustion.

  According to the present invention, it is possible to more sufficiently improve the heat insulation by water injection by preventing or suppressing the temperature decrease at the outer peripheral edge portion of the piston crown surface.

BRIEF DESCRIPTION OF THE DRAWINGS The figure which showed the structure of the engine system concerning one Embodiment of this invention. Water phase diagram for explaining supercritical water. Phase diagram of water to explain subcritical water. The schematic sectional view which expands and shows a part of engine body. 1 is a schematic cross-sectional view of a fuel injection device. (A) Conceptual view showing spread of fuel spray when fuel injection interval is long, (b) Conceptual view showing spread of fuel spray when fuel injection interval is short. (A) A conceptual view showing the spread of the fuel spray when the lift amount of the injector is small, and (b) a conceptual view showing the spread of the fuel spray when the lift amount of the injector is large. A block diagram showing a control system of an engine. The figure which showed the control area | region of the engine. (A) A diagram showing the injection pattern of the low load injection mode and the shape of the mixture layer, (b) A view showing the injection pattern of the switching region injection mode and the shape of the mixture layer, (c) of the medium load injection mode The figure which shows the injection pattern and the shape of an air-fuel | gaseous mixture layer. The figure which showed the heat release rate in a high load area | region, a fuel injection rate, and a water injection rate. The figure which showed a mode that the heat insulation layer was formed by supercritical water injection. The top view which looked at a piston crown surface from upper direction shows distribution of the site | part from which the thickness of a heat shield material layer differs. The top view which looked at a piston crown surface from upper direction, and shows the example which made distribution of the site | part from which the thickness of a heat shield material layer differs from the case of FIG. The figure which shows the relationship between cylinder wall surface temperature, engine load, and the saturation line of water. The figure which shows the relationship between the heat conductivity of a heat shield material layer, engine load, and the limit thickness of a heat shield material layer. The figure which shows the structure of a heat shielding material layer typically.

(1) Overall Configuration of Engine System FIG. 1 is a view showing the configuration of an engine system to which a control device for an engine according to a first embodiment of the present invention is applied. The engine system of the present embodiment includes a four-stroke engine body 1, an intake passage 30 for introducing combustion air into the engine body 1, and an exhaust passage 40 for discharging exhaust gas generated by the engine body 1. And The engine body 1 is, for example, a four-cylinder engine having four cylinders 2. Although the type of fuel supplied to the engine body 1 is not limited, in the present embodiment, a fuel containing gasoline is used. The engine system of the present embodiment is mounted on a vehicle, and the engine body 1 is used as a drive source of the vehicle.

  An air cleaner 31 and a throttle valve 32 are provided in the intake passage 30 sequentially from the upstream side, and air after passing through the air cleaner 31 and the throttle valve 32 is introduced into the engine main body 1.

  The throttle valve 32 opens and closes the intake passage 30. However, in the present embodiment, during operation of the engine, the throttle valve 32 is basically maintained at or almost fully open, and is closed only under limited operating conditions such as when the engine is stopped. Shut off the intake passage 30.

  The exhaust passage 40 is provided with a three-way catalyst 41 for purifying exhaust gas, a heat exchanger (temperature raising booster) 42, a condenser (condenser) 43, and an exhaust shutter valve 44 in this order from the upstream side. The heat exchanger 42 and the condenser 43 constitute a part of an exhaust heat recovery apparatus (water processing apparatus) 60 described later.

  The exhaust shutter valve 44 is for promoting the reflux of the EGR gas to the intake passage 30.

  That is, in the engine system of the present embodiment, an EGR passage 51 is provided which communicates a portion of the intake passage 30 downstream of the throttle valve 32 and a portion of the exhaust passage 40 upstream of the three-way catalyst 41. The exhaust gas is partially recirculated to the intake passage 30 as EGR gas. The exhaust shutter valve 44 is a valve capable of opening and closing the exhaust passage 40. When the EGR is performed and the pressure in the exhaust passage 40 is low, the exhaust shutter valve 44 is operated on the valve closing side to operate upstream of the EGR passage 51. The pressure of the side part is increased to promote the reflux of the EGR gas.

  The EGR passage 51 is provided with an EGR valve 52 that opens and closes the EGR passage 51, and the amount of EGR gas recirculated to the intake passage 30 is adjusted by the valve opening amount of the EGR valve 52. Further, in the present embodiment, the EGR passage 51 is provided with an EGR cooler 53 for cooling the EGR gas passing therethrough, and the EGR gas is cooled by the EGR cooler 53 and then recirculated to the intake passage 30. Ru.

  The exhaust heat recovery device 60 is for producing supercritical water using the thermal energy of the exhaust gas. That is, in the engine system of the present embodiment, supercritical water is injected from the water injection device 22 into each cylinder 2 as described later, and the supercritical water is generated using exhaust gas. Is configured as.

  The exhaust heat recovery device 60 includes, in addition to the heat exchanger 42 and the condenser 43, an exhaust condensed water passage 61 connecting the water injection device 22 and the condenser 43, a water tank 62, and a water injection pump 63. There is.

  The condenser 43 is for condensing water (steam) in the exhaust gas passing through the exhaust passage 40. The water tank 62 stores condensed water inside. The condensed water generated by the condenser 43 is introduced into the water tank 62 through the exhaust condensed water passage 61 and stored in the water tank 62.

  The water injection pump 63 is for pumping the condensed water in the water tank 62 to the water injection device 22 via the heat exchanger 42. The condensed water in the water tank 62 is heated and pressurized at the time of pressure feeding by the water injection pump 63. For example, the condensed water is heated to about 350 K and pressurized to about 250 bar by the water injection pump 63.

  The heat exchanger 42 is for performing heat exchange between the condensed water pressure-fed from the water injection pump 63 and the exhaust gas passing through the exhaust passage 40. The heat exchanger 42 is an indirect heat exchanger, and the condensed water receives heat energy from the exhaust gas when passing through the heat exchanger 42. By passing through the heat exchanger 42, the condensed water is heated and pressurized from the state pressurized by the water injection pump 63 to become supercritical water.

  Supercritical water is water whose temperature and pressure are higher than the critical point of water, and has a high density close to a liquid while the molecule is vigorously moving like a gas. In other words, supercritical water is water that does not require latent heat to change to gas or liquid water. Although the details will be described later, in the present embodiment, a water insulating layer is formed on the wall surface of the combustion chamber 6 formed in the cylinder 2 by injecting water of such a property into the cylinder 2.

  This will be specifically described with reference to FIG. FIG. 2 shows a phase diagram of water when the horizontal axis is enthalpy and the vertical axis is pressure. In FIG. 2, a region Z2 is a region of liquid, a region Z3 is a region of gas, and a region Z4 is a region in which liquid and gas coexist. The lines LT350, LT400... LT1000 indicated by solid lines are isotemperature lines connecting points having the same temperature, and the numbers indicate the temperature (K). For example, LT 350 is a 350 K isothermal line, and LT 1000 is a 1000 K isothermal line. The point X1 is a critical point, the area Z1 is an area where the temperature and pressure are higher than the critical point X1, and the supercritical water is water contained in the area Z1. Specifically, while the critical point of water is a point of temperature: 647.3 K and pressure: 22.12 MPa, supercritical water has a temperature and pressure above these, ie, a temperature of 647.3 K and a pressure above It is water of 22.12 MPa or more.

  In FIG. 2, lines LR0.01, LR0.1,..., LR500 indicated by broken lines are isodensity lines connecting points having the same density, and the numbers indicate the density (kg / m 3). ing. For example, LR 0.01 is an isodensity line whose density is 0.01 kg / m 3, and LR 1000 is an isodensity line whose density is 500 kg / m 3. As apparent from the comparison between the isopycse line LR and the regions Z1 and Z3, the density of the water contained in the region Z1, ie, the supercritical water is a value close to that of the liquid water, about 50 kg / m3 to 500 kg / m3. The value is much higher than the density of the gas.

  As supercritical water generated by the engine system and injected into the cylinder 2, it is preferable to use supercritical water having a density of 250 kg / m 3 or more.

  Also, as indicated by arrow Y1 in FIG. 2, normal liquid water requires a large enthalpy to change to a gas. That is, normal liquid water requires a relatively large latent heat to turn into a gas. On the other hand, as indicated by the arrow Y2, supercritical water requires almost no enthalpy or latent heat to change to normal gas water.

Here, as is clear from FIG. 2, the water contained in the area near the area Z1 has a high density and a small latent heat for changing to a gas, and has a property close to that of supercritical water. Therefore, in the present embodiment, as described above, the supercritical water is generated by the exhaust heat recovery apparatus 60 and the supercritical water is injected into the cylinder 2. However, instead of the supercritical water, the supercritical water is included in the area near the area Z1. Subcritical water, which is water, may be generated and injected into the cylinder 2. For example, subcritical water may be generated and jetted in the region Z10 shown in FIG. 3 and included in the region Z10 having a temperature of 600 K or more and a density of 250 kg / m 3 or more.
(2) Configuration of Engine Body (2-1) Overall Configuration The configuration of the engine body 1 will be described next.

  FIG. 4 is a cross-sectional view showing a part of the engine body 1 in an enlarged manner. As shown in FIG. 4, the engine body 1 is capable of reciprocating (up and down) to the cylinder block 3 in which the cylinders 2 are formed, the cylinder head 4 provided on the upper surface of the cylinder block 3, and the cylinders 2. And a piston 5 fitted.

  A combustion chamber 6 is formed above the piston 5. The combustion chamber 6 is a so-called pent roof type, and the ceiling surface (the lower surface of the cylinder head 4) of the combustion chamber 6 has a triangular roof shape consisting of two inclined surfaces on the intake side and the exhaust side.

  In the present embodiment, the combustion chamber 6 is provided on the wall surface (inner side surface) of the combustion chamber 6 in order to reduce the cooling loss by suppressing the heat of the combustion gas in the combustion chamber 6 being released to the outside of the combustion chamber 6. The heat shield material layer whose thermal conductivity is lower than that of the inner surface of is provided. Specifically, the heat insulating material layer 71 is formed on the wall surface of the cylinder 2, which constitutes the inner side surface of the combustion chamber 6, the lower surface of the cylinder head 4, and the surfaces of the valve heads of the intake valve 19 and the exhaust valve 19. And the heat shield layer 7 is provided on the crown surface 5 a of the piston 5. In the present embodiment, as shown in FIG. 4, the heat insulating material layer 71 provided on the wall surface of the cylinder 2 is located above the piston ring 5 b (cylinder head 4) in a state where the piston 5 is positioned at top dead center. It is limited to the part which becomes the side), and piston ring 5b does not slide on the heat insulation material layer 71 top.

  The heat shield layers 7 and 71 may be made of a material having a low thermal conductivity as described above, and the specific material is not limited. However, it is preferable to use a material having a volume specific heat smaller than that of the inner surface of the combustion chamber 6 as the heat shield layers 7 and 71. That is, when the engine body 1 is cooled by the cooling water, the gas temperature in the combustion chamber 6 fluctuates with the progress of the combustion cycle, while the temperature of the inner side surface of the combustion chamber 6 is maintained substantially constant. Therefore, the cooling loss increases with this temperature difference. Therefore, if the heat shield layers 7 and 71 are formed of a material having a small volume specific heat, the temperature of the heat shield layers 7 and 71 changes following the fluctuation of the temperature of the gas in the combustion chamber 6, so the cooling loss Can be kept small.

  The heat shield layers 7 and 71 are formed, for example, by coating a ceramic material such as ZrO 2 by plasma spraying on the inner side surface of the combustion chamber 6. In addition, a large number of pores may be included in the ceramic material, thereby further reducing the thermal conductivity and volume specific heat of the heat shield layers 7 and 71. The details of the heat shield layers 7 and 71 will be described later.

  The crown surface 5 a of the piston 5 is formed with a cavity 10 in which a region including the central portion thereof is recessed on the opposite side (downward) to the cylinder head 4. The cavity 10 is formed to have a volume that occupies most of the combustion chamber 6 when the piston 5 rises to the top dead center.

  The heat shield material layer 7 formed on the piston crown surface 5a is formed so that its central portion is thin and its outer peripheral portion is thick, and its heat capacity is larger at the outer peripheral portion than at its central portion. It is set to become. In FIG. 4, the thin portion of the heat shield layer 7 is indicated by reference numeral 7 a and the thick portion is indicated by reference numeral 7 b. The boundary line α between the thin portion 7a and the thick portion 7b is set outside the outer peripheral edge of the cavity 10 and in the vicinity thereof as shown in FIG. 13 in the present embodiment. The thickness of the heat shield layer 7 will be described later.

  In this embodiment, the geometric compression ratio of the engine body 1, that is, the ratio of the volume of the combustion chamber 6 when the piston 5 is at the bottom dead center to the volume of the combustion chamber 6 when the piston 5 is at the top dead center Is set to 18 or more and 35 or less (for example, approximately 20).

  The cylinder head 4 has an intake port 16 for introducing air supplied from the intake passage 30 into the combustion chamber 6, and an exhaust port 17 for introducing exhaust gas generated in the combustion chamber 6 to the exhaust passage 40; An intake valve 18 for opening and closing an opening on the combustion chamber 6 side of the intake port 16 and an exhaust valve 19 for opening and closing an opening on the combustion chamber 6 side of the exhaust port 17 are provided. In the example shown in FIG. 4, a heat shield material layer 181 composed of the same components as the heat shield material layers 7 and 71 is also formed on the inner side surface of the intake port 16.

  The intake valve 18 is opened and closed by an intake valve opening and closing mechanism. The intake valve opening and closing mechanism is provided with an intake opening and closing timing changing mechanism 18a (see FIG. 8) capable of changing the opening and closing timing of the intake valve 18, and the opening and closing timing of the intake valve 18 is changed according to the operating conditions and the like. It is supposed to be.

  Further, a fuel injection device 21 for injecting fuel into the combustion chamber 6 and a water injection device 22 for injecting critical water into the combustion chamber 6 are attached to the cylinder head 4. As shown in FIG. 4, the tip (end on the combustion chamber 6 side) of the fuel injection device 21 and the water injection device 22 is located near the central axis of the cylinder 2 and faces the approximate center of the cavity 10. It is located adjacent to.

  Note that, in the present embodiment, premixed compression autoignition combustion is performed in which the mixture of fuel and air is mixed beforehand in the entire operation region and this mixture is self-ignited near the compression top dead center (TDC). It is configured as follows. Along with this, in the example shown in FIG. 4, the ignition plug for igniting the mixture in the combustion chamber 6 is not provided in the engine body 1, but in the case of cold start etc. In the case where ignition is necessary for the purpose, a spark plug may be provided on the engine body 1 as appropriate.

  As described above, the water injection device 22 injects the supercritical water pumped from the water injection pump 63 into the combustion chamber 6. The water injection device 22 has an injection port at its tip, and the amount of injection water is changed by changing the opening period of the injection port. As the water injection device 22, for example, a device for injecting a fuel into the combustion chamber 6 which is used for a conventional engine can be applied, and the detailed description of the structure is omitted. The water injection device 22 injects supercritical water into the combustion chamber 6 at, for example, about 20 MPa.

  As described above, the water injection device 22 is disposed such that its tip end is located near the central axis of the cylinder 2 and faces the substantially central portion of the cavity 10. Along with this, from the tip of the water injection device 22, supercritical water is injected toward the piston crown surface 5a.

The fuel injection device 21 injects the fuel pressure-fed by a fuel pump (not shown) into the combustion chamber 6. In the present embodiment, an open-valve fuel injection device 21 is used.
(2-2) Detailed Configuration of Fuel Injection Device The fuel injection device (fuel injection valve) 21 will be described in detail.

  FIG. 5 is a schematic cross-sectional view of the fuel injection device 21. As shown in FIG. As shown in FIG. 5, the fuel injection device 21 includes a fuel pipe 21c having a nozzle port 21b formed at its tip (end on the combustion chamber 6 side), and a nozzle port 21b disposed inside the fuel pipe 21c. And an open / close valve 21a. The outer opening valve 21a is connected to a piezo element 21d that deforms in accordance with the applied voltage. The outer opening valve 21a is in contact with the nozzle port 21b in a state where voltage is not applied to the piezoelectric element 21d and closes the nozzle port 21b, and the piezoelectric element 21d is deformed in response to voltage application. The nozzle 21 b is opened by protruding from the end 21 b toward the tip end.

  The portion of the nozzle opening 21b and the outer opening valve 21a that abuts against the nozzle opening 21b has a tapered shape whose diameter increases toward the tip end, and from the nozzle opening 21b, the central axis of the nozzle opening 21b, that is, the cylinder 2. The fuel is injected in the form of a cone (more specifically, a hollow cone) around substantially the central axis. For example, the taper angle of this cone is 90 ° to 100 ° (the taper angle of the hollow portion in the hollow cone is about 70 °).

  The opening period and lift amount of the outer opening valve 21a (lift amount is the projection amount from the closed position of the outer opening valve 21a and the opening amount of the nozzle port 21b) is the application period of voltage to the piezoelectric element 21d. And it changes according to the size of the voltage. Then, depending on the lift amount of the outer opening valve 21a, the penetration of the fuel spray injected from the nozzle port 21b, the amount of fuel injected per unit time, and the particle size of the fuel spray change. Specifically, when the lift amount is large and the opening amount of the nozzle 21 b is large, the penetration of the fuel spray becomes large, and the injected fuel amount per unit time becomes large and the particle diameter of the fuel spray becomes large.

  According to the above configuration, the fuel injection device 21 can perform multistage injection of about 20 times in 1 to 2 msec. Further, the fuel injection device 21 changes the fuel injection interval and the lift amount separately, thereby independently spreading the fuel spray in the radial direction orthogonal to the axial direction and spreading the fuel spray in the axial direction. Control is possible.

  This will be specifically described using FIGS. 6 and 7.

  FIG. 6 is a diagram conceptually showing the difference in the spread of the fuel spray accompanying the difference in the injection interval of the fuel injection device 21. As shown in FIG. Specifically, FIG. 6 (a) is a view when the fuel injection interval is increased, and FIG. 6 (b) is a view when the injection interval is shortened while keeping the lift amount constant with that in FIG. 6 (a). Of the

  As apparent from the comparison of FIGS. 6 (a) and 6 (b), the axial spread of the fuel spray is promoted as the fuel injection interval is shorter.

  That is, the fuel spray injected in a hollow cone shape from the fuel injection device 21 flows in the combustion chamber 6 at high speed. At this time, a negative pressure region is generated along the axis of the fuel injection device 21 inside the hollow cone by the Coanda effect, and the fuel spray is drawn toward this negative pressure region. Here, as shown in FIG. 6A, when the fuel injection interval is long, the pressure in the negative pressure region recovers from the predetermined fuel injection to the next fuel injection, so the negative pressure region is the fuel It does not extend much along the axis of the injector 21. On the other hand, when the fuel injection interval is short, the pressure recovery in the negative pressure region is suppressed, and the negative pressure region extends along the axial direction of the fuel injection device 21. Therefore, when the fuel injection interval is short, the fuel spray extends more along the axial direction of the fuel injection device 21.

  FIG. 7 is a diagram conceptually showing the difference in the spread of the fuel spray accompanying the difference in the lift amount of the fuel injection device 21. As shown in FIG. Specifically, FIG. 7 (a) is a view when the lift amount is reduced, and FIG. 7 (b) is a view when the lift amount is increased while making the injection interval the same as FIG. It is.

  As apparent from the comparison of FIGS. 7A and 7B, the spread of the fuel spray is promoted in the radial direction (the direction orthogonal to the axial direction of the fuel injection device 21) when the lift amount of the fuel is large. Ru.

That is, when the lift amount is large, the particle size of the fuel spray is increased as described above, and the momentum of the fuel spray is increased. Therefore, when the lift amount is large, the fuel spray is less likely to be attracted to the negative pressure region, and spreads outward in the radial direction.
(3) Control System (3-1) System Configuration FIG. 8 is a block diagram showing a control system of the engine. As shown in the figure, the engine system of the present embodiment is generally controlled by a PCM (power train control module, control means) 100. The PCM 100 is, as is well known, a microprocessor comprising a CPU, a ROM, a RAM, and the like.

  The PCM 100 is electrically connected to various sensors for detecting the operating state of the engine.

  For example, the cylinder block 3 is provided with a crank angle sensor SN1 that detects the rotational angle and rotational speed of the crankshaft, that is, the engine speed. Further, an air flow sensor SN2 is provided at a portion of the intake passage 30 between the air cleaner 31 and the throttle valve 32 to detect the amount of air (fresh air) sucked into each cylinder 2 through the air cleaner 31. ing. The vehicle is also provided with an accelerator opening sensor SN3 that detects the opening (accelerator opening) of an accelerator pedal (not shown) operated by the driver.

  The PCM 100 controls each part of the engine while performing various determinations and calculations based on input signals from the various sensors. That is, the PCM 100 is electrically connected to the fuel injection device 21, the water injection device 22, the throttle valve 32, the exhaust shutter valve 44, the EGR valve 52, the water injection pump 63, etc. The drive control signal is output to each of these devices.

In FIG. 9, the abscissa represents the engine speed, and the ordinate represents the control map of the engine load. In the present embodiment, a low load area A1 whose engine load is equal to or less than a preset reference load Tq1 and a high load area A2 whose engine load is higher than the reference load Tq1 are set as the control area. The control content of each area A1 and A2 will be described below.
(3-2) Low Load Region In the low load region A1, since the required engine torque is small, the effective compression ratio can be reduced. Therefore, in the low load region A1, the effective compression ratio is set to a small value in order to suppress the pumping loss to increase the energy efficiency. For example, the effective compression ratio can be reduced to a value less than 15. Specifically, the intake valve 18 is closed at a timing that is retarded and relatively later than the intake bottom dead center by the intake opening and closing timing changing mechanism 18a, whereby the effective compression ratio is reduced.

  In the low load region A1, since the calorific value of the air-fuel mixture is small and the combustion temperature is relatively low, NOx (so-called Raw NOx) generated by the combustion can be reduced. Therefore, in the region A1, it is not necessary to purify NOx by the three-way catalyst 41, and it is not necessary to set the air fuel ratio to the theoretical air fuel ratio that enables NOx purification by the three-way catalyst. Therefore, in the low load range A1, the air-fuel ratio of the air-fuel mixture is made lean, that is, the excess air ratio λ> 1 in order to improve the fuel efficiency.

  The EGR gas is recirculated into the cylinder 2 in the low load region A1. That is, in the low load region A1, the EGR valve 52 is opened, and a part of the exhaust gas in the exhaust passage 40 is recirculated to the intake passage 30 as EGR gas. Further, in the operating region where the engine load is very low and the pressure in the exhaust passage 40, that is, the pressure on the upstream side of the EGR passage 51 is low, the exhaust shutter valve 44 is controlled to the closing side to promote the recirculation of EGR gas.

  In the present embodiment, in the low load area A1, the EGR gas is recirculated such that G / F, which is the ratio of the total gas weight in the combustion chamber 6 to the fuel amount, is 35 or more. Further, the EGR rate (the ratio of the weight of the EGR gas to the total weight of the gas in the cylinder 2) is increased as the engine load is higher.

  In the low load region A1, the injection of supercritical water into the combustion chamber 6 by the water injection device 22 is stopped. And driving of the pump 63 for water injection is stopped in connection with this.

  In the low load range A1, all fuel is injected into the combustion chamber 6 by the fuel injection device 21 in the second half of the compression stroke (from 90 ° CA before compression top dead center to compression top dead center). For example, all the fuel is injected into the combustion chamber 6 in the vicinity of 30 ° CA before the compression top dead center.

  The injection mode in the low load area A1 is a mode in which no fuel adheres to the crown surface 5a of the piston 5 and a layer of air is formed on the outer peripheral portion of the combustion chamber 6. Here, the outer peripheral portion of the combustion chamber 6 refers to the surface of the crown surface 5 a of the piston 5 (the surface of the heat shield layer 7 on the crown surface 5 a) and the heat shield layer 71 on the inner surface of the cylinder 2. And the lower surface of the cylinder head 4.

  Specifically, a first area A1a having a lower engine load in the low load area A1, a second area A1b having a higher engine load than this, and a switching area A1c between the first area A1a and the second area A1b In each case, the injection mode is set to the low load injection mode, the medium load injection mode, and the switching area injection mode described below.

FIG. 10 is a view showing fuel injection modes of each of the low load injection mode, the medium load injection mode and the switching area injection mode, and the shape of the air-fuel mixture layer corresponding thereto.
(Low load injection mode)
FIG. 10A shows the low load injection mode implemented in the first region A1a. The low load injection mode is a mode in which injection with a small lift amount of the fuel injection device 21 and a short injection interval is continuously performed a plurality of times. The number of injections is not limited to the example shown in the drawing and can be changed as appropriate.

  As described above, when the injection interval is short, the fuel spray becomes axially long. When the lift amount is small, the outward spread of the fuel spray in the radial direction is suppressed. Therefore, in the low load injection mode, the layer of the fuel spray and the mixture thereof with the air have a longitudinally elongated shape in which the axial length is relatively long with respect to the radial direction.

Thus, in the first region A1a, the layer of the vertically elongated mixture is formed in the combustion chamber 6, whereby the layer of air is formed in the radially outer portion of the combustion chamber 6. Furthermore, the fuel injection amount is small in the first region A1a. Therefore, the axial length of the air-fuel mixture layer is short even if it has a vertically long shape, and the contact between the crown surface 5a of the piston 5 and the mixture is avoided, and an air layer is formed on the entire surface of the crown surface 5a of the piston 5 Be done.
(Medium load injection mode)
FIG. 10C shows the medium load injection mode implemented in the second region A1 b. The medium load injection mode is a mode in which the lift amount of the fuel injection device 21 is larger than the lift amount of the low load injection mode and the injection interval is longer than the low load injection mode. The number of injections is not limited to the example shown in the drawing and can be changed as appropriate.

  As described above, when the injection interval is long, the fuel spray is axially shortened. When the lift amount is large, the fuel spray spreads radially outward. Therefore, in the medium load injection mode, the layer of the fuel spray and the air-fuel mixture has a laterally elongated shape in which the radial length is relatively long in the axial direction.

In this manner, in the second area A1 b and in the area where the fuel injection amount is relatively large, the layer of the horizontally elongated mixture is formed in the combustion chamber 6, and the crown surface 5a of the piston 5 and the mixture Contact with the piston 5 is avoided, and an air layer is formed on the surface of the crown surface 5 a of the piston 5. Further, the dimension of the combustion chamber 6 near the compression top dead center is longer in the radial direction than in the axial direction, and there is a space in the radial direction. Therefore, although the layer of the mixture in the second region A1b has a laterally elongated shape as described above, the layer of the mixture does not reach the outer periphery of the combustion chamber 6 in the radial direction, ie, the inner surface of the cylinder 2. An air layer is also formed on the radially outer portion.
(Switching area injection mode)
FIG. 10B shows the switching area injection mode implemented in the third area A1 c. The switching area injection mode is a mode in which the low load injection mode and the medium load injection mode are combined. For example, as shown in FIG. 10 (b), after performing the injection in the medium load injection mode (after making the injection having a large lift amount and a long injection interval continue a plurality of times), the injection in the low load injection mode is performed (The injection with a small amount of lift and a short injection interval is made to continue several times). Instead of this, after the injection in the low load injection mode is performed, the injection in the medium load injection mode may be performed. Further, the number of injections is not limited to the example shown in the drawing and can be changed as appropriate.

  In the switching area injection mode, the spread of the mixture layer, particularly in the radial direction, is adjusted by the combination of the low load injection mode and the medium load injection mode. As a result, the mixture layer has a shape longer than the mixture layer in the low load injection mode and shorter than the mixture layer in the medium load injection mode.

  Thus, in the third area A1 c and in the boundary area between the first area A1 a and the second area A1 b, the shape of the mixture layer is adjusted to an appropriate shape, and the surface of the crown surface 5 a of the piston 5 and An air layer is formed on the radially outer portion of the combustion chamber 6.

  The switching area injection mode can be omitted.

  As described above, in the low load area A1, an air layer, that is, a heat insulating layer made of air is formed on the outer peripheral portion of the combustion chamber 6. And a cooling loss is suppressed by this air layer and fuel consumption performance is improved.

Here, with the formation of the air layer in the outer peripheral portion of the combustion chamber 6 as described above, the air-fuel ratio of the inner peripheral portion of the combustion chamber 6, ie, the central portion of the combustion chamber 6 becomes higher than that of the outer peripheral portion. However, as described above, in the low load region A1, there is surplus air whose excess air ratio λ is larger than 1 and does not contribute to combustion. Therefore, the air necessary for combustion is secured in the central portion of the combustion chamber 6, and the air-fuel ratio of this portion is kept within the appropriate range.
(3-3) High Load Region In the high load region A2, the effective compression ratio is made larger than the effective compression ratio in the low load region A1 in order to secure the engine torque. In the present embodiment, the effective compression ratio is set to 15 or more in the high load area A2. Specifically, the closing timing of the intake valve 18 is made more advanced than the closing timing in the low load range A1 by the intake opening and closing timing changing mechanism 18a, whereby the effective compression ratio is higher than the low load range A1. Be

  In the high load range A2, the air-fuel ratio is made the stoichiometric air-fuel ratio so that NOx purification by the three-way catalyst becomes possible. That is, the excess air ratio λ is set to one. Further, in the high load area A2, the EGR valve 52 is closed to stop the reflux of the EGR gas, and the G / F is set to a value smaller than 35.

  Here, in the high load area A2, the amount of fuel injected into the combustion chamber 6 is large, and the amount of air necessary to burn the fuel is also large, so the excess air is insufficient. In particular, in the present embodiment, by setting the excess air ratio λ to 1, there is no excess air. Therefore, when an air layer is formed on the outer peripheral portion of the combustion chamber 6 as in the low load region A1, an air-fuel ratio having an excessively high air-fuel ratio may be generated in the combustion chamber 6 and smoke may be deteriorated. . Further, in the high load region A2, the temperature in the combustion chamber 6 becomes high because the amount of injected fuel is large and the amount of heat generation is large. Furthermore, in the present embodiment, the temperature in the combustion chamber 6 is further increased due to the high effective compression ratio. Therefore, smoke is likely to be generated in the high load area A2, and a large amount of smoke may be generated if a rich area, that is, an area having a small excess air ratio is formed under such conditions.

  So, in this embodiment, an air layer like low load area A1 is not formed in high load area A2, but a heat insulation layer by supercritical water injection is formed instead, as mentioned later. Further, in order to suppress the deterioration of the smoke, the fuel is injected such that the combustion is started in a state where the air-fuel mixture in the combustion chamber 6 is more homogenized (the air-fuel ratio is made uniform).

  Further, in the high load area A2, since the calorific value is large and the effective compression ratio is high as described above, the absolute value of the in-cylinder pressure (pressure in the combustion chamber 6) when combustion starts before the compression top dead center. The increase rate of the value and the in-cylinder pressure becomes very high, and the combustion noise becomes large. Therefore, in the present embodiment, the fuel is injected so as to start combustion when the retarded side, ie, the piston 5 is lowered below the compression top dead center and the in-cylinder pressure is decreased.

  Specifically, in the high load area A2, fuel injection in the high load injection mode as shown in FIG. 11 is performed. That is, the first injection Q1 for injecting a relatively large amount of fuel is performed in the first half of the compression stroke (from intake bottom dead center to 90 ° CA before compression top dead center), and a part of the remaining fuel is injected in the second half of the compression stroke The second injection Q2 is performed, and then the third injection Q3 is performed to inject the remaining fuel slightly ahead of the compression top dead center.

  The first injection Q1 is an injection for homogenizing the air-fuel mixture, and the first injection Q1 is carried out, and a large amount of fuel is injected in the first half of the compression stroke and mixed with the air, whereby combustion before the start of combustion is started. The mixture in the chamber 6 is homogenized. The first injection Q1 is started, for example, near 150 ° CA before compression top dead center.

  The third injection Q3 is an injection for self-igniting the air-fuel mixture on the more retarded side, and the third injection Q3 is performed in the latter half of the compression stroke to form a homogeneous mixture generated by the first injection Q1. Fires after compression top dead center. The third injection Q3 is started, for example, around 15 ° CA before the compression top dead center.

  The second injection Q2 is an injection for enhancing the combustion stability. That is, when all the remaining fuel is injected by the third injection Q3 at a relatively retarded timing near the compression top dead center, the inside of the combustion chamber 6 is lowered along with the descent of the piston 5 before the start of the combustion. There is a risk that the temperature may drop below the temperature at which it can burn and misfire. So, in this embodiment, 2nd injection Q2 is implemented before 3rd injection Q3, and it is made for temperature in the combustion chamber 6 to be maintained above the temperature which can be combusted also after compression top dead center. The second injection Q2 is performed, for example, near 30 ° CA before compression top dead center.

  In the high load area A2, the water injection device 22 burns so that a high temperature, high density water layer 50, for example, a high density steam layer (see FIG. 12) is formed on the outer peripheral portion of the combustion chamber 6. Supercritical water is injected into the chamber 6. Here, the high-temperature, high-density water layer is a higher-temperature, high-density layer with respect to the water layer when the pressure is increased to such an extent that normal temperature water can be injected under the pressure in the cylinder. It means that there is.

  Specifically, as shown in FIG. 11, after the end of the third injection Q3, the combustion is between the second half of the compression stroke and the first half of the expansion stroke (compression top dead center to compression top dead center 90 ° CA) Before the mixture is ignited in the chamber 6, the water injection W1 is performed. In the present embodiment, as shown in FIG. 11, the water injection W1 is performed before the compression top dead center.

  As described above, the supercritical water injected from the water injection device 22 is injected toward the piston crown surface 5a. Therefore, as the water injection W1 is performed, high temperature, high density water adheres to the surface of the piston crown surface 5a in the high load area A2, as shown in FIG. Is formed. In particular, in the present embodiment, since the water injection W1 is performed before the compression top dead center and while the piston 5 is rising, the water injection W1 can be reliably attached to the piston crown surface 5a. In addition, the shaded part in FIG. 12 shows typically the mode of injection of water injection W1.

  Here, as a material for forming the heat insulating layer 50, it is conceivable to use normal liquid water instead of supercritical water (or subcritical water). However, normal liquid water becomes steam (gaseous water) when injected into the high temperature combustion chamber 6. And, as mentioned above, water vapor has a low density. Therefore, even if normal liquid water is jetted to form a heat insulating layer by steam, the weight (number of molecules) of water contained in the heat insulating layer is small, and the heat insulating effect is small. In addition, as described above, ordinary liquid water requires latent heat when converted to water vapor. Therefore, when normal liquid water is injected, the temperature of the mixture decreases with evaporation, and the thermal efficiency is degraded.

  Therefore, in the present embodiment, as described above, the supercritical water which has a high density and does not require latent heat is injected into the cylinder 2, and the supercritical water injected before ignition is in a high temperature / high density state. As present, supercritical water is injected into the high temperature and high pressure combustion chamber 6 in the second half of the compression stroke and the first half of the expansion stroke. Then, the heat insulating layer 50 is formed on the outer peripheral portion of the combustion chamber 6 before ignition of the mixture while making the mixture in the combustion chamber 6 homogeneous.

In the present specification, the ignition timing, that is, the timing at which the mixture is ignited refers to the timing at which the heat generation rate rises rapidly, as shown in FIG. That is, as shown in FIG. 11, when the temperature and pressure reaches a predetermined value, the mixture of fuel and air is a low temperature oxidation reaction (cold flame reaction) accompanied by a slight heat generation exceeding cooling loss etc. The reaction rate increases slowly, or increases slowly and then declines, and then causes a reaction with a high calorific value and a hot flame (hot flame reaction). And here, the timing when this heat flame reaction is started is called ignition timing. The heat flame reaction is known to occur when the temperature of the air-fuel mixture reaches about 1500 K or more. Therefore, the timing at which the temperature of the air-fuel mixture becomes 1500 K or higher may be used as the ignition timing.
(4) Operation, Etc. As described above, in the low load region A1 according to the present embodiment, the air layer is formed by the excess air in the outer peripheral portion of the combustion chamber 6, thereby maintaining the appropriate combustion while maintaining the cooling loss. It can be suppressed. Then, in the high load area A2, the heat insulating layer 50 is formed by water on the outer peripheral portion of the combustion chamber 6, thereby suppressing the air-fuel ratio of the air-fuel mixture to an appropriate range to suppress the generation of smoke while suppressing the cooling loss. be able to. In particular, since the heat insulating material 7 is formed on the wall surface of the combustion chamber 6 in addition to the heat insulating layer made of air and water, the cooling loss can be effectively reduced.

  Moreover, the heat insulating layer 50 in the high load area A2 is formed by supercritical water injection, and the supercritical water is injected into the high temperature and high pressure combustion chamber 6 in the latter half of the compression stroke and the first half of the expansion stroke. The water is configured to be present in a high temperature, high density state. Therefore, the density of water in the heat insulation layer 50 can be increased to obtain a high heat insulation effect, and the cooling loss can be more reliably suppressed, and the fuel consumption performance can be more reliably improved.

  Further, in the present embodiment, the supercritical water is generated using the thermal energy of the exhaust gas, and in the high load area A2 where the temperature of the exhaust gas is high, the thermal energy of the exhaust gas is effectively used to generate supercritical water. While it is possible to increase the energy efficiency, in the low load area A1 where the engine load is low, the necessary amount of supercritical water may not be generated because the temperature of the exhaust gas is low. In addition, in the low load area A1, when the insufficient energy is compensated by the separately provided heater or the like, the energy efficiency is deteriorated. On the other hand, in the present embodiment, the heat insulating layer is formed by air in the low load area A1, and the heat insulating layer 50 is formed by supercritical water injection only in the high load area A2. Cooling loss can be suppressed in a wide load range.

  Here, the heat shield material layer 7 formed on the piston crown surface 5a is a portion 7b where the thickness of the outer peripheral edge where the temperature is relatively low is large. Since the heat capacity of the outer peripheral edge portion to be the thick portion 7b is larger than that of the thin portion 7a in the central portion where the temperature is high, the heat insulating property is improved. Therefore, when the water injection is performed, the condensation of the water at the outer peripheral edge of the piston crown surface 5a is prevented or suppressed, and the cooling loss at the outer peripheral edge of the piston crown surface 5a is prevented or suppressed. (It greatly promotes the advantage of the heat insulation improvement by water injection).

  Here, it is conceivable to make the heat shield layer 7 as thick as the thick portion 7b as a whole, but in this case, the amount of use of the material constituting the heat shield layer 7 becomes large, and the cost etc. It is not desirable in terms of points. FIG. 14 shows a case where the area range of the thick portion 7 b is narrowed to further reduce the amount of the material constituting the heat shield layer 7. That is, in the example of FIG. 14, the boundary line α between the thin portion 7 a and the thick portion 7 b is set near an intermediate position between the outer peripheral edge of the cavity 10 and the outer peripheral edge of the piston crown surface 5 a. The boundary line α can also be set closer to the outer peripheral edge side of the piston crown surface 5a than the intermediate position. Furthermore, the thickness of the heat shield layer 7 can be gradually increased from the outer peripheral edge of the thin portion 7a toward the outer peripheral edge side of the piston crown surface 5a, and the thickness can be set to finally reach the thick portion 7b. .

  In order to prevent the water jetted toward the piston crown surface 5a from condensing at the outer peripheral edge of the piston crown surface 5a, the temperature of the outer peripheral edge may be maintained at a temperature higher than the saturation line of water. become. FIG. 15 shows the relationship between the wall surface temperature in the cylinder (that is, the temperature of the outer peripheral portion of the piston crown surface 5a which is a low temperature in the cylinder), the engine load, and the water saturation line. If the wall surface temperature is higher than 450 ° K, water will not condense in the engine load range of 100 kpa or more.

  The thickness of the heat shield layer 7 is preferably set within a predetermined thickness range of 500 μm to 1500 μm so as to ensure sufficient heat insulation even at low loads. The thin portion 7a and the thick portion 7b will be different within the above-mentioned predetermined thickness range. FIG. 16 shows the relationship between the critical thickness of the heat shield layer 7, the thermal conductivity of the heat shield layer 7, and the engine load. That is, it is necessary to maintain the piston crown surface 5a (of the outer peripheral portion where the temperature is particularly low) above a predetermined temperature (for example, 450.degree. K described in FIG. 15) during steady operation with a certain load. The critical thickness (minimum required thickness) of the heat shield material layer 7 is shown. For example, when the thermal conductivity of the heat shield layer 7 is 0.15 W / mK and the engine load is 100 kpa, the thickness of the heat shield layer 7 needs to be 500 μm or more. Of course, since the temperature in the cylinder increases as the engine load increases, the required thickness of the heat shield layer 7 can be reduced. The thermal conductivity of the heat shield layer 7 is selected in the range of about 0.1 to 0.23 W / mK in consideration of usable materials, etc. in terms of the material etc. constituting the heat shield layer 7 It is practical, and in this case, the thickness of the heat shield layer 7 may be set in a thickness range of 500 μm to 1500 μm. Of course, it is possible to make the thickness of the heat shield material layer 1500 μm or more, but it is preferable to set the thickness range as described above because it is unnecessary to increase the thickness if the cost is unnecessarily increased.

  Next, although the material and the like of the heat shield layer 7 will be described with reference to FIG. 17, the same material can be used for the heat shield layers 71 and 181. The heat shield material layer 7 of the present embodiment includes hollow particles 123 mainly composed of an inorganic oxide, and a dense binder material 125. In the heat shield layer 7, the dense binder material 125 covers the hollow particles 123, bonds the hollow particles 123 to each other, and bonds the hollow particles 123 to the piston crown surface 5a, whereby the piston crown surface 5a is formed. Layer structure. The dense binder material 125 bonds the hollow particles 123 so as to fill in the gaps between the hollow particles 123, and the dense binder material 125 is non-powdery and is itself densely configured. There is. For this reason, there is no gap through which the fuel can pass between the hollow particles 123 or in the dense binder 125 itself, and as a result, the fuel and water injected into the combustion chamber 6 penetrate into the heat shield layer 7 Can be prevented.

  In the present embodiment, the dense binder material 125 is non-powdery as described above, and the material is not particularly limited as long as the dense binder material 125 itself is densely formed, and, for example, a silicone resin can be used. . As the silicone resin, for example, a silicone resin composed of a highly branched three-dimensional polymer represented by methyl silicone resin and methyl phenyl silicone resin can be suitably used, and specific examples thereof include polyalkylphenyl siloxanes. It can be mentioned.

  In the present embodiment, the hollow particles 123 mainly composed of an inorganic oxide include Si-based oxide components (eg, silica (SiO 2)) such as fly ash balloons, silas balloons, silica balloons, airgel balloons, etc. or Al-based oxides. It is preferable to use ceramic-based hollow particles containing metal components (for example, alumina (Al.sub.2O.sub.3)) (the respective materials and particle sizes are set as disclosed in Table 1 of Patent Document 1). Can) The hollow particles 123 preferably have a median diameter of 5 μm to 30 μm, and hollow heavy particles having a median diameter of less than 5 μm may be further added.

  For example, the chemical composition of fly ash balloon is SiO2: 40.1 to 74.4%, Al2O3: 15.7 to 35.2%, Fe2O3: 1.4 to 17.5%, MgO: 0.2 to 7 .4%, CaO: 0.3-10.1% (more than% by mass). The chemical composition of the shirasu balloon is SiO2: 75 to 77%, Al2O3: 12 to 14%, Fe2O3: 1 to 2%, Na2O: 3 to 4%, K2O: 2 to 4%, IgLoss: 2 to 5% (or more) Is mass%).

  The heat shielding material layer 7 contains such hollow particles 123 at a volume ratio of 60 vol% or more and 75 vol% or less. Since the content of the hollow particles 123 as a component of the heat shield layer 7 is as large as 60 vol% or more in volume ratio, many air layers can be contained in the heat shield layer 7. For this reason, the heat conductivity and volume specific heat of the heat shield material layer 7 can be reduced, and the heat insulation performance of the heat shield material layer 7 can be improved. In addition, since the volume ratio of the hollow particles 123 in the heat shield layer 7 is 75 vol% or less, the amount of the dense binder material 125 for bonding the hollow particles 123 can be sufficiently ensured, and the film is durable. It is possible to form The volume ratio of the hollow particles 123 to be contained is calculated from the mass of the hollow particles 123 added to the dense binder material 125 by measuring the apparent density of the particles and the density of the dense binder material 125. can do. In order to improve the strength or hardness of the heat shield layer 7, the heat shield layer 7 may contain a filler material.

The heat shield layer 7 can be formed by thermal spraying or coating. Moreover, the material (structure) of the heat shield material layer 7 is not limited to what was mentioned above. However, by using the hollow particles 123, not only the heat insulating property is simply improved, but also the thermal responsiveness (responsiveness to thermal change) is improved, and the piston 5 and its crown surface 5a remain hot. It is preferable to prevent maintenance (for example, advantageous for knocking prevention and the like). By the way, when a heat insulating material is good and thermal responsiveness is poor (for example, a solid ceramic material) as the heat shield material layer 7, the state where the piston 5 and its crown surface 5a remain high temperature is maintained. Increase the risk of
(5) Modification Although the above-mentioned embodiment was described, the present invention is limited to this and can be suitably changed in the range indicated to the claim. For example, although the case where the effective compression ratio of the high load area A2 is set to 15 or more has been described, the effective compression ratio of the high load area A2 is not limited to this. However, as described above, when the engine load is high and the effective compression ratio is high, smoke tends to deteriorate. Therefore, in this case, if the heat insulating layer is formed not by air but by supercritical water injection, it is possible to suppress the cooling loss while suppressing the deterioration of the smoke. Further, the effective compression ratio of the low load area A1 is not limited to the above. Water injection may be performed even in the low load region.

  Further, instead of or in addition to the exhaust heat recovery apparatus 60, supercritical water may be generated using a heater or the like separately provided as described above. However, if the exhaust heat recovery device 60 is used as described above, the energy efficiency can be increased. In addition, since the temperature of the exhaust gas is increased by having the heat shield layer 7, supercritical water or critical water can be easily generated when the exhaust heat recovery apparatus 60 is used.

  Moreover, although the case where supercritical water was injected as water into the combustion chamber 6 was described in the above embodiment, as described above, water that is subcritical water and has properties close to supercritical water is supercritical It may be injected into the combustion chamber 6 instead of water. Even in this case, a heat insulating layer having a high heat insulating effect can be formed, and cooling loss can be suppressed.

  Further, the combustion mode is not limited to the self-ignition combustion, and the combustion may be started by igniting the air-fuel mixture by the spark plug. In addition, a fuel not containing gasoline may be used as the fuel, or a diesel engine may be used.

  The heat capacity of the heat shield layer 20 can be changed by changing the material (structure) instead of or in addition to changing the thickness (for example, changing the diameter or material of the hollow particles 23) Or change its content rate). Of course, the object of the present invention is not limited to what is explicitly stated, but implicitly also includes providing what is substantially preferred or expressed as an advantage.

  The present invention is suitable for application to a compression ignition engine.

1: Engine body 2: Cylinder 3: Cylinder block 4: Cylinder head 5: Piston 5a: Piston crown surface 7: Heat shield layer 7a: Thin portion 7b: Thick portion α: Boundary line (boundary between thin portion and thick portion )
10: Cavity 21: Fuel injection device 22: Water injection device 42: Heat exchanger (heating and pressure booster)
43: Condenser (Condenser)
50: Heat insulation layer (formed by water injection)
60: Exhaust heat recovery system (water processing system)
100: PCM (control means)

Claims (8)

The cylinder block, the cylinder head and the piston define a combustion chamber,
There is provided a water injection device for injecting high-temperature high-temperature water to a piston crown surface which is a surface facing the combustion chamber among the pistons,
A heat insulating material layer is formed on the piston crown surface,
The heat shield layer is formed such that the heat capacity at the outer peripheral edge of the piston crown surface is larger than the heat capacity at the central portion of the piston crown surface.
A compression-ignition engine characterized by In claim 1,
The compression / ignition engine according to claim 1, wherein a thickness of the heat shield layer at an outer peripheral edge of the piston crown surface is larger than a thickness at a central portion of the piston crown surface. In claim 2,
The thickness of the heat shield layer is set to a predetermined thickness range of 500 μm to less than 1500 μm, and the thickness of the outer peripheral portion of the piston crown surface and the thickness at the central portion of the piston crown surface are the predetermined thickness A compression-ignition engine characterized in that it is different within a range. In any one of claims 1 to 3,
The compression ignition engine, wherein the high-density, high-temperature water is used as supercritical water or subcritical water. In claim 2 or claim 3,
A cavity is formed at the center of the piston crown surface,
The boundary between the thick portion and the thin portion of the heat shield layer is set outside the outer peripheral edge of the cavity and in the vicinity of the outer peripheral edge,
A compression-ignition engine characterized by In claim 2 or claim 3,
A cavity is formed at the center of the piston crown surface,
The boundary between the thick portion and the thin portion of the heat shield layer is near the intermediate position between the outer peripheral edge of the cavity and the outer peripheral edge of the piston crown surface or on the outer peripheral edge side of the piston crown surface than the intermediate position Is set to,
A compression-ignition engine characterized by In any one of claims 1 to 6,
The compression ignition engine, wherein the heat shield layer is configured to include hollow particles. In any one of claims 1 to 7,
A compression ignition engine, wherein HCCI combustion is performed at least in a predetermined operation region.