JP2020037894A - Premixed compression ignition engine - Google Patents

Abstract

To provide a premixed compression ignition engine for sufficiently increasing a combustion speed while holding combustion noise down to an appropriate level.SOLUTION: The premixed compression ignition engine includes an injector for injecting fuel into a combustion chamber 6, and a plasma production plug 16 for discharging non-equilibrium plasma to the combustion chamber 6. The injector injects the fuel so that part of air-fuel mixture made by mixing fuel and air are is formed in an outer peripheral portion of the combustion chamber 6. The plasma production plug 16 having an extending electrode on the outer periphery side of the combustion chamber 6 performs discharge so that the non-equilibrium plasma is produced during a time after the fuel injection but before the air-fuel mixture is ignited.SELECTED DRAWING: Figure 14

Description

  The present invention relates to a premixed compression-ignition engine in which fuel injected into a combustion chamber is mixed with air and then compression-ignited, in particular, a premixed compression-ignition engine capable of supplying a non-equilibrium plasma for promoting combustion to the combustion chamber. The present invention relates to a mixed compression ignition type engine.

  As an example of a premixed compression ignition engine using non-equilibrium plasma, the one disclosed in Patent Document 1 is known. The premixed compression ignition engine disclosed in Patent Literature 1 supplies an injector that injects gasoline-containing fuel into a combustion chamber and ozone as an active species to the combustion chamber by generating non-equilibrium plasma (streamer discharge). An ozone generator.

  Also, Patent Document 2 below is known as an engine that discloses the same engine as described above.

Japanese Patent No. 6237329 Japanese Patent No. 6149759

  According to Patent Documents 1 and 2, there is an advantage that the combustion at a low temperature is particularly promoted and the compression ignition combustion is stabilized by supplying ozone as the active species to the air-fuel mixture.

  However, in Patent Documents 1 and 2, since the compression ignition combustion is performed in a state where the air-fuel mixture and the ozone are entirely mixed, if a large amount of ozone is supplied carelessly, the combustion becomes excessively sharp. Loud combustion noise may be generated. On the other hand, when the supply amount of ozone is reduced in order to suppress combustion noise, there is a problem that the combustion speed becomes slow particularly in the latter half of the combustion when the temperature of the combustion chamber becomes low, and the thermal efficiency decreases.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a premixed compression ignition engine capable of sufficiently increasing a combustion speed while suppressing combustion noise to an appropriate level. I do.

  In order to solve the above-mentioned problem, a homogeneous charge compression ignition type engine of the present invention includes an injector for injecting fuel into a combustion chamber, a piston having a crown defining a bottom surface of the combustion chamber, A plasma generating plug having an electrode near the center of the ceiling surface and discharging non-equilibrium plasma from the electrode into the combustion chamber, and premixed compression ignition combustion for compressing and igniting fuel injected from the injector while mixing it with air A control device that controls the injector and the plasma generation plug so that is realized, the electrode of the plasma generation plug has a shape extending from the tip of the plasma generation plug to the outer peripheral side of the combustion chamber, The control device may be configured such that a part of a mixture of fuel and air injected from the injector near a compression top dead center is an outer peripheral portion of the combustion chamber. Controlling the injector to be formed, and after the fuel injection by the injector, before the mixture is ignited, the plasma generating plug is discharged so that non-equilibrium plasma is discharged from the electrode of the plasma generating plug. Is controlled (claim 1).

  In the present invention, the electrode of the plasma generation plug has a shape extending to the outer peripheral side of the combustion chamber, and the air-fuel mixture is formed on the outer peripheral portion of the combustion chamber and the air-fuel mixture is injected into the combustion chamber before the air-fuel mixture is injected. Until the ignition, the non-equilibrium plasma is discharged from the electrode of the plasma generating plug. For this reason, active species such as ozone and OH generated by the action of the non-equilibrium plasma can be supplied to the air-fuel mixture before ignition existing in the outer peripheral portion of the combustion chamber.

  Here, the outer peripheral portion of the combustion chamber tends to be lower in temperature than the central portion of the combustion chamber. Therefore, if the active species is not supplied to the outer peripheral portion of the combustion chamber, the air-fuel mixture at the outer peripheral portion ignites with a considerable delay with respect to the air-fuel mixture at the central portion of the combustion chamber, and the combustion after ignition The speed will be quite slow. In contrast, in the present invention, as described above, the active species is supplied to the outer peripheral portion of the combustion chamber before the mixture is ignited, so that the effect of the active species reduces the ignition delay of the air-fuel mixture in the outer peripheral portion. At the same time, the combustion of the air-fuel mixture can be promoted to increase the combustion speed (that is, the combustion speed in the latter half of the combustion). In addition, since the combustion speed of the air-fuel mixture in the central portion of the combustion chamber that ignites first does not particularly change, the pressure rise that occurs in the first half of combustion does not become remarkable. Therefore, the entire air-fuel mixture can be burned in a short period of time while suppressing the combustion noise to an appropriate level, and a sufficiently high thermal efficiency can be obtained.

  In the above configuration, preferably, the control device causes the injector to perform a first-stage injection for injecting fuel into the combustion chamber during an intake stroke and a second-stage injection for injecting fuel into the combustion chamber during a compression stroke. (Claim 2).

  According to this configuration, the fuel concentration of the air-fuel mixture in the central portion of the combustion chamber is appropriately increased by the latter-stage injection, and the air-fuel mixture is reliably ignited, thereby reliably burning the air-fuel mixture of the entire combustion chamber. it can. As described above, the air-fuel mixture existing in the outer peripheral portion of the combustion chamber, which is relatively difficult to ignite, can be burned early by the action of the non-equilibrium plasma.

  In the above configuration, preferably, the control device is configured to discharge the plasma from the electrode of the plasma generation plug during a period from the end of the pre-injection to the start of the post-injection, The generation plug is controlled (claim 3).

  According to this configuration, when the pressure in the combustion chamber is relatively low, discharge is performed from the electrode of the plasma generation plug, so that a large amount of active species can be more reliably supplied to the outer peripheral portion of the combustion chamber. Further, the contact between the fuel supplied to the combustion chamber by the latter-stage injection and the non-equilibrium plasma can be avoided, and the generation of substances that suppress combustion, such as formaldehyde, can be suppressed.

  In the above configuration, preferably, the control device is configured such that the plasma generation plug is configured such that a discharge period from the electrode of the plasma generation plug is longer at a crank angle than when the engine speed is higher than when the engine speed is lower. Is controlled (claim 4).

  When the engine speed is high, the amount of fuel injected from the injector per unit time is larger than when the engine speed is low, so that the fuel is more likely to adhere to the outer peripheral wall surface of the combustion chamber. Therefore, when the engine speed is high, the air-fuel mixture on the outer peripheral portion of the combustion chamber tends to be less likely to burn than when the engine speed is low. On the other hand, according to this configuration, a large amount of active species can be supplied to the outer peripheral portion of the combustion chamber when the engine speed is high, and the air-fuel mixture in the outer peripheral portion of the combustion chamber can be more appropriately burned. Can be. On the other hand, when the engine speed is low, the discharge period of the plasma generating plug, that is, the driving period is suppressed to be short, so that the energy consumption accompanying the driving of the plasma generating plug can be suppressed.

  In the above configuration, preferably, when the engine speed is higher than a predetermined reference speed, during the period from the end of the first-stage injection to the start of the second-stage injection and the second-stage injection, Non-equilibrium plasma is discharged from the electrode of the plasma generation plug during the period from the end to the time when the air-fuel mixture ignites, and when the engine speed is equal to or less than the reference speed, the post-injection ends. The plasma generation plug is controlled so that the discharge from the electrode of the plasma generation plug is stopped during a period from when the air-fuel mixture ignites (claim 5).

  According to this configuration, when the engine speed is higher than the reference speed and the time between the first injection and the second injection is short, and when the engine speed is high as described above, mixing of the outer peripheral portion of the combustion chamber is prevented. When the gas is relatively difficult to burn, the discharge time of the plasma generating plug can be secured, so that the amount of active species supplied to the air-fuel mixture at the outer peripheral portion of the combustion chamber can be secured. When the engine speed is lower than the reference speed and the time between the first injection and the second injection is relatively long, a large amount of active species is mixed by discharging from the plasma generating plug during these injections. While driving the plasma generating plug, the number of opportunities for driving the plasma generating plug can be suppressed to reduce the energy consumption associated with the driving.

  In the above configuration, preferably, an insulator is provided in a predetermined region including a central portion of the ceiling surface of the combustion chamber (claim 6).

  According to this configuration, discharge can be prevented from being performed from the electrode of the plasma generation plug toward the region including the center of the ceiling surface of the combustion chamber, and the discharge can be more reliably performed from the electrode toward the outer peripheral portion of the combustion chamber. Can be done.

  As described above, according to the homogeneous charge compression ignition engine of the present invention, the combustion speed can be sufficiently increased while suppressing the combustion noise to an appropriate level.

1 is a schematic plan view showing a configuration of a homogeneous charge compression ignition engine according to one embodiment of the present invention. FIG. 3 is a cross-sectional view of the engine body taken along a direction of intake and exhaust. FIG. 2 is a cross-sectional view of the engine body taken along a cylinder row direction. It is a top view which shows the shape of the crown surface of a piston. It is a schematic diagram for explaining the tumble flow formed in a combustion chamber. It is a figure which expands and shows the front-end | tip part of a plasma generation plug, (a) is a side view, (b) is a bottom view. It is sectional drawing of an injector single body. It is sectional drawing which expands and shows a combustion chamber and its peripheral part. It is a top view which shows the ceiling surface of a combustion chamber. FIG. 2 is a block diagram showing a control system of the engine. 5 is a graph for explaining conditions for generating non-equilibrium plasma. 4 is a time chart showing a voltage application pattern to a plasma generation plug. FIG. 4 is a map diagram for explaining a difference in control according to an operating condition of an engine. 4 is a time chart for explaining the contents of combustion control executed during operation of the engine in a high-load low-speed range. 5 is a time chart for explaining the contents of combustion control performed during operation of the engine in a high-load, high-speed range. It is a figure for explaining behavior of fuel injected from an injector at the time of operation in a high load area, (a) shows a situation during execution of pre-stage injection, (b) shows a situation during execution of post-stage injection, (C) shows the situation near the compression top dead center, respectively. It is a figure which shows typically the situation in a combustion chamber when main discharge is implemented. It is a figure which shows typically the situation in a combustion chamber at the time of performing additional discharge. 4 is a graph of a heat generation rate for describing an effect of the embodiment. It is a figure for explaining the modification of the plasma generation plug used by the above-mentioned embodiment. It is a figure for explaining the modification of the plasma generation plug used by the above-mentioned embodiment. It is a figure for explaining the modification of the plasma generation plug used by the above-mentioned embodiment, (a) is a top view corresponding to Drawing 4, and (b) is a side view of the tip part of a plasma generation plug.

(1) Overall Configuration FIG. 1 is a schematic plan view showing a configuration of a homogeneous charge compression ignition type engine (hereinafter, also simply referred to as an engine) according to an embodiment of the present invention. The engine shown in this figure is a four-cycle gasoline direct injection engine mounted on a vehicle as a power source for traveling, and includes an in-line multi-cylinder engine body 1 including four cylinders 2 arranged in a row, and an engine. An intake passage 50 through which intake air introduced into the main body 1 flows, an exhaust passage 60 through which exhaust gas discharged from the engine main body 1 flows, and a part of the exhaust gas flowing through the exhaust passage 60 are returned to the intake passage 50. An EGR device 70 is provided.

  The engine body 1 is provided with an intake valve 11 described later on one side and a discharge valve 12 described later on the other side with a plane passing through the center axis of the cylinder 2 interposed therebetween. In the present embodiment, an intake valve 11 and an exhaust valve 12 are provided on one side and the other side, respectively, with a surface along the arrangement direction of the cylinders 2 (hereinafter, referred to as a cylinder row direction) interposed therebetween. Hereinafter, a direction orthogonal to the cylinder row direction is referred to as an intake / exhaust direction, and in this intake / exhaust direction, a side on which the intake valve 11 is provided is referred to as an intake side, and a side on which the exhaust valve 12 is provided is referred to as an exhaust side.

  FIG. 2 is a diagram schematically illustrating a cross section of the engine body 1 along the intake and exhaust directions. FIG. 3 is a cross section of the engine body 1 along a direction (cylinder arrangement direction) orthogonal to the intake and exhaust directions. It is the figure which showed typically. In FIG. 2, IN indicates the intake side, and EX indicates the exhaust side. As shown in FIGS. 2 and 3, the engine body 1 is mounted on a cylinder block 3 in which the four cylinders 2 are formed, and on an upper surface of the cylinder block 3 so as to close each cylinder 2 from above. A cylinder head 4 and a piston 5 inserted into each cylinder 2 so as to be able to slide back and forth.

  Above the piston 5, a combustion chamber 6 surrounded by the peripheral surface of the cylinder 2, the crown surface S of the piston 5, and the lower surface of the cylinder head 4 is formed. A ceiling surface 28 which is a portion of the lower surface of the cylinder head 4 that covers the combustion chamber 6 is formed in a so-called pent roof shape (triangular roof shape). In other words, the height of the ceiling surface 28 of the combustion chamber 6 decreases as the distance from the cylinder axis X (the center axis of the cylinder 2) to the intake side increases in the cross-sectional view shown in FIG. 2 (that is, the cross-sectional view along the intake and exhaust directions). And an inclined surface whose height decreases as the distance from the cylinder axis X to the exhaust side increases.

  Fuel mainly composed of gasoline is supplied to the combustion chamber 6 by injection from an injector 15 described later. Then, the supplied fuel is burned by compression ignition while being mixed with air in the combustion chamber 6, and the piston 5 pushed down by the expansion force due to the combustion reciprocates vertically. The fuel injected into the combustion chamber 6 only needs to contain gasoline as a main component, and may contain, for example, sub-components such as bioethanol in addition to gasoline.

  Below the piston 5, a crankshaft 7, which is an output shaft of the engine body 1, is provided. The crankshaft 7 is connected to the piston 5 via a connecting rod 8, and is driven to rotate around the central axis according to the reciprocating motion (vertical motion) of the piston 5.

  The cylinder block 3 is provided with a crank angle sensor SN1 for detecting a rotation angle (crank angle) of the crank shaft 7 and a rotation speed (engine rotation speed) of the crank shaft 7.

  The geometric compression ratio of the cylinder 2, that is, the ratio of the volume of the combustion chamber 6 when the piston 5 is at the top dead center to the volume of the combustion chamber when the piston 5 is at the bottom dead center, presumes gasoline-containing fuel. A value suitable for mixed compression ignition combustion is set to 15 or more and 30 or less.

  FIG. 4 is a plan view showing the crown surface S of the piston 5 together with a tip portion (discharge electrode 33) of the plasma generation plug 16 described later. As shown in FIG. 4 and the like, the cylinder head 4 has, for each cylinder 2, an intake port 9 for introducing air supplied from an intake passage 50 into a combustion chamber 6, and an exhaust gas generated in the combustion chamber 6. , An exhaust valve 10 for opening and closing an opening of the intake port 9 on the combustion chamber 6 side, and an exhaust valve 12 for opening and closing an opening of the exhaust port 10 on the combustion chamber 6 side. Each is provided. In the present embodiment, two intake ports 9 and two exhaust ports 10 are provided for each cylinder, and two intake valves 11 and two exhaust valves 12 are provided for each cylinder. The intake valve 11 and the exhaust valve 12 are opened and closed by a valve operating mechanism (not shown) including a pair of camshafts and the like provided on the cylinder head 4 in conjunction with the rotation of the crankshaft 7.

  FIG. 5 is a diagram schematically showing a state in the combustion chamber 6 when the intake valve 11 is opened. The intake port 9 is formed to extend upward from the ceiling surface 28 of the combustion chamber 6, and a tumble flow is formed in the combustion chamber 6 with the opening of the intake valve 11. In the present embodiment, the intake port 9 is connected to the ceiling surface 28 of the combustion chamber 6 at an angle close to a right angle (the inner peripheral surface of the opening of the intake port 9 on the combustion chamber 6 side and the ceiling of the combustion chamber 6). The angle with the surface 28 is closer to 90 degrees). In addition, the area of the opening of the intake port 9 on the combustion chamber 6 side is increased. Thereby, the vortex diameter of the tumble flow becomes relatively small. Specifically, as shown by an arrow Y1, after flowing into the combustion chamber 6 from the intake port 9, the flow of the main intake air flowing toward the exhaust side portion in the combustion chamber 6 is directed downward (toward the crown surface S of the piston 5). ). Also, as indicated by arrow Y2, a relatively large amount of intake air flows from the intake port 9 to the intake side. The main intake flow Y1 tends to move toward the intake side of the combustion chamber 6 near the crown surface S of the piston 5, but this flow is obstructed by the flow indicated by the arrow Y2. Thus, in the present embodiment, the tumble flow formed in the combustion chamber 6 becomes a small-diameter vortex passing near the center of the combustion chamber 6. Accordingly, during the opening of the intake valve 11 and immediately after the intake valve 11 is closed, the gas flow near the outer peripheral edge of the ceiling surface 28 of the combustion chamber 6 is suppressed to be small. In FIG. 5, a broken arrow Y11 connects the intake port 9 to the ceiling surface 28 of the combustion chamber 6 in a state where the intake port 9 is laid down more than the state shown by the solid line in FIG. 4 illustrates a tumble flow according to a comparative example when the area of the opening portion is smaller than that of the present embodiment.

  As shown in FIG. 4 and the like, the crown surface S of the piston 5 that defines the bottom surface of the combustion chamber 6 has a flat reference surface 21 located at the outer peripheral edge thereof and a position higher than the reference surface 21 (in the cylinder head 4). (A side approaching). The raised portion 20 is formed so as to be higher along the ceiling surface 28 of the pent roof-shaped combustion chamber 6 as it approaches the cylinder axis X in a sectional view of FIG. 2 (that is, a sectional view along the intake and exhaust directions). Is formed. A cavity C that is depressed downward (on the side opposite to the cylinder head 4) is formed in the center of the raised portion 20, in other words, in the center of the crown surface S of the piston 5.

  The cavity C has a flat bottom surface portion 22 having a substantially circular shape in a plan view, and a peripheral surface portion 23 which rises from an outer peripheral edge of the bottom surface portion 22 while being inclined upward and radially outward. The opening edge C1 of the cavity C, which is the upper end of the peripheral surface portion 23, is formed so as to have a substantially elliptical shape in plan view, and is larger in the cylinder row direction (direction perpendicular to the intake / exhaust direction) than in the intake / exhaust direction. It is formed to have a longer dimension.

  The raised portion 20 has an intake-side inclined surface 24 formed on the intake side of the cavity C, and an exhaust-side inclined surface 25 formed on the exhaust side of the cavity C. The intake-side inclined surface 24 is formed such that its height becomes lower as it goes away from the opening edge C1 on the intake side of the cavity C toward the intake side (radially outward), and the exhaust-side inclined surface 25 is It is formed so that the height decreases as the distance from the opening edge C1 on the exhaust side to the exhaust side increases (radially outward).

  A pair of peaks 26 is formed in a region outside the cavity C in the cylinder row direction and located between the intake-side inclined surface 24 and the exhaust-side inclined surface 25. The pair of peaks 26 are formed in a substantially planar shape at the highest position in the crown surface S.

  Here, the region of the crown surface S of the piston 5 radially outside the opening edge C1 of the cavity C (that is, the intake-side / exhaust-side inclined surfaces 24, 25, the peak 26, and the reference surface 21) is referred to as "the crown surface S. The radially outer region and the region on the crown surface S of the piston 5 radially inside the opening edge C1 of the cavity C (that is, the forming surfaces 22 and 23 of the cavity C) are called the “radially inner regions of the crown surface S”. In the present embodiment, since the cavity C is formed so as to be depressed on the side opposite to the ceiling surface 28 of the combustion chamber 6, naturally, the radially outer region of the crown surface S and the ceiling surface of the combustion chamber 6 opposed thereto are formed. 28 is smaller than the vertical distance between the radially inner region of the crown surface S and the opposing ceiling surface 28 of the combustion chamber 6. Therefore, when the piston 5 approaches the top dead center, a squish flow, which is a gas flow flowing from the outside to the inside in the radial direction, is formed in the space between the radially outer region of the crown surface S and the ceiling surface 28. Is done. Hereinafter, a portion of the combustion chamber 6 where the squish flow is formed, that is, a portion between a radially outer region of the crown surface S (a region radially outward of the opening edge C1 of the cavity C) and the ceiling surface 28. Is called a squish area.

  As shown in FIGS. 1 to 3, the cylinder head 4 includes an injector 15 for injecting fuel (mainly gasoline) into the combustion chamber 6 and a plasma generating plug 16 for discharging non-equilibrium plasma into the combustion chamber 6. One set is provided for each cylinder 2. Note that non-equilibrium plasma refers to thermally non-equilibrium plasma in which the energy of electrons, ions, molecules, and the like is not uniform (the energy of electrons is larger than the energy of ions, molecules, and the like). Non-equilibrium plasma is also called low-temperature plasma because it does not involve a rise in temperature. The supply of the non-equilibrium plasma having such a property hardly raises the gas temperature in the combustion chamber 6, but leads to reforming the gas component in the combustion chamber 6 (details will be described later).

  FIGS. 6A and 6B are enlarged views of the tip of the plasma generation plug 16. FIG. 7 is a cross-sectional view of the injector 15 alone. FIG. 8 is an enlarged cross-sectional view showing the combustion chamber 6 and its peripheral portion.

  As shown in FIGS. 6A and 6B, FIG. 7 is an enlarged view of the tip of the plasma generation plug 16. As shown in the figure, the plasma generating plug 16 includes a cylindrical plug main body 31, a center electrode 32 inserted into the plug main body 31, and a plurality (four in this example) extending radially from the tip of the center electrode 32. ) Discharge electrodes 33). In the present embodiment, these discharge electrodes 33 correspond to “electrodes” in the claims. As shown in FIG. 6A, the tip of the center electrode 32 protruding from the plug body 31 is covered with an insulator 34 (insulator) made of alumina or the like.

  As shown in FIG. 8 and the like, the plasma generating plug 16 is provided on the cylinder head 4 in a state where the tip portion of the center electrode 32 and the entire discharge electrode 33 are exposed to the combustion chamber 6. The plasma generation plug 16 has its center axis (the center axis of the center electrode 32) substantially coincident with the cylinder axis X, and the center electrode 32 and the discharge electrode 33 are located near the center of the ceiling surface 28 of the combustion chamber 6. , Provided on the cylinder head 4.

  The discharge electrode 33 radially extends from the tip of the center electrode 32, that is, the position corresponding to the center axis of the plasma generation plug 16, that is, the position corresponding to the cylinder axis X, and burns from the position corresponding to the center axis of the plasma generation plug 16. The outer periphery of the chamber 6 extends. The discharge electrode 33 is arranged at a position overlapping the bottom surface 22 of the cavity C in plan view (when viewed from the direction along the cylinder axis X). The four discharge electrodes 33 extend substantially toward the center of the valve surfaces of the intake valve 11 and the exhaust valve 12, respectively, and extend in a direction non-parallel to both the intake and exhaust directions and the cylinder row direction. The direction extends in a direction crossing the direction and the cylinder row direction at an angle of about 45 degrees.

  The four discharge electrodes 33 of the plasma generation plug 16 have a shape that is inclined downward (in a direction approaching the crown surface S of the piston 5) toward the outer peripheral side of the combustion chamber 6. In the present embodiment, the inclination angle θ1 of each discharge electrode 33 with respect to a plane perpendicular to the cylinder axis X is larger than the inclination angle θ2 of the ceiling surface 28 of the combustion chamber 6 with respect to this plane. Therefore, the distance between each of the discharge electrodes 33 and the ceiling surface 28 of the combustion chamber 6 increases toward the tip of the discharge electrode 33 (the outer peripheral side of the combustion chamber 6).

When a predetermined pulse voltage is applied to the center electrode 32 (details will be described later), discharge is performed from each of the discharge electrodes 33 to the earth-side electrode provided in the combustion chamber 6 according to the applied voltage. As a result, non-equilibrium plasma is generated along the discharge path connecting the discharge electrode 33 and the ground electrode. Then, charged plasma particles (ions and electrons) collide with gas molecules by the action of an electric field to generate active species such as ozone (O ₃ ) and OH, and these active species are grounded from the discharge electrode 33. A flow (induced flow) moving toward the side electrode is formed.

  As shown in FIG. 3, the injector 15 is mounted such that a tip end portion where a fuel outlet (a nozzle port 44 described later) is formed is located near the center of the ceiling surface 28 of the combustion chamber 6. The tip end of the injector 15 is slightly offset from the cylinder axis X to one side in the cylinder row direction in the cross-sectional view of FIG. 3, and is disposed at a position overlapping the cavity C in a plan view. In other words, the tip of the injector 15 is arranged so as to be arranged in the cylinder row direction close to the tip (discharge electrode 33) of the plasma generation plug 16 located at the center of the ceiling surface 28 of the combustion chamber 6. .

  FIG. 7 is a cross-sectional view of the injector 15 alone. As shown in the figure, the injector 15 is a so-called open-type injector, and has a cylindrical valve body 41, a needle valve 42 inserted into the valve body 41 so as to be able to advance and retreat, and a valve according to an applied voltage. And a driving unit 43 including a piezo element that is deformed. The needle valve 42 has a substantially frustoconical tip portion 42a whose outer diameter decreases toward the tip end.

  When the injector 15 is closed, the needle valve 42 is housed in the valve body 41 such that the peripheral surface of the maximum diameter portion of the distal end portion 42a is in close contact with the inner peripheral surface of the distal end portion of the valve body 41. In such an open-type injector 15, the needle valve 42 is driven in the protruding direction when the valve is opened, so that the ring-shaped slit continuous between the distal end portion 42 a of the needle valve 42 and the valve body 41. Is formed. Therefore, when the injector 15 is opened, the fuel is injected in a cone shape (specifically, a hollow cone shape) through the nozzle port 44. Note that, in this specification, such cone-shaped fuel injection is also one mode of radially injecting fuel.

  The lift amount of the needle valve 42 changes according to the magnitude of the voltage applied to the piezo element and the application period. In accordance with such a change in the lift amount, it is possible to adjust the spread of the spray of the fuel injected from the nozzle port 44 and the penetration (penetration force) of the spray.

  As shown in FIG. 8, each wall surface that partitions the combustion chamber 6, that is, the peripheral surface of the cylinder 2, the crown surface S of the piston 5, the ceiling surface 28 of the combustion chamber 6, and each of the intake valve 11 and the exhaust valve 12 On the lower surface of the valve head, a heat shield layer 19 is provided. The heat shield layer 19 provided on the peripheral surface of the cylinder 2 is limited to a position above the piston ring 5a when the piston 5 is at the top dead center (on the side of the cylinder head 4). It does not slide on the heat shield layer 19.

  The heat shield layer 19 is made of a material having a lower thermal conductivity and a smaller specific heat of volume than any of the cylinder block 3, the cylinder head 4, the piston 5, and the intake / exhaust valves 11 and 12. This is for suppressing the heat of the combustion gas generated in the combustion chamber 6 from being released to the outside of the combustion chamber 6 and reducing the cooling loss of the engine. In addition, as the heat shielding layer 19, a material in which silica-based porous particles are contained in a silicone-based main material can be suitably used.

  As described above, the heat shield layer 19 almost entirely covers the combustion chamber 6 when the piston 5 is at the top dead center. The heat shield layer 19 is not formed on the opening edge C1 of the cavity C of No. 5. The heat shield layer 19 has a higher insulating property than the piston 5 made of, for example, an aluminum alloy. Therefore, the insulation between the region D and the opening edge C1 of the cavity C is lower than that of the other region of the wall surface of the combustion chamber 6. Hereinafter, the region D having low insulation properties formed on the ceiling surface 28 of the combustion chamber 6 is referred to as a ceiling-side non-insulating region D. Thus, when the non-equilibrium plasma is discharged from the discharge electrode 33 of the plasma generating plug 16, the non-equilibrium plasma is transferred to the ceiling-side non-insulating region D or the opening edge C 1 of the cavity C which is not covered by the heat shielding layer 19. It is led. Specifically, the piston 5 is located relatively below, and the distance between the tip of the discharge electrode 33 and the ceiling-side non-insulating region D is longer than the distance between the tip of the discharge electrode 33 and the opening edge C1 of the cavity C. Is shorter, the non-equilibrium plasma is guided to the ceiling-side non-insulated region D, the piston 5 is located relatively above, and the distance between the tip of the discharge electrode 33 and the ceiling-side non-insulated region D is greater than the discharge electrode. When the distance is longer than the distance between the tip of 33 and the opening edge C1 of the cavity C, the non-equilibrium plasma is guided to the opening edge C1 of the cavity C. Thus, at the time of plasma discharge, the discharge electrode 33 functions as an anode, and the opening edge C1 of the ceiling-side non-insulating region D or the cavity C functions as a cathode, that is, an earth-side electrode.

  FIG. 9 is a diagram schematically illustrating the ceiling surface 28 of the combustion chamber 6. As shown in FIG. 9, the ceiling-side non-insulating region D is provided in a ring shape with the center axis of the plasma generation plug 16 as the center. In the present embodiment, the ceiling-side non-insulating region D is provided on the outer peripheral side of the intake valve 11 and the exhaust valve 12 and near the outer peripheral edge of the ceiling surface 28 of the combustion chamber 6. Specifically, the ceiling surface 28 of the combustion chamber 6 has a portion near the outer peripheral edge, which is inclined downward toward the outside in the radial direction, and a portion extending from the lower edge of the portion in a direction orthogonal to the cylinder axis X. A step 28a is formed by this, and the step 28a is a ceiling-side non-insulating region D. When the piston 5 is at the center position between the bottom dead center position and the top dead center position (the position where the piston 5 has risen by approximately の of its stroke) and the position up to the top dead center, the plasma generation plug 16 The distance between the tip of the discharge electrode 33 and the opening edge C1 of the cavity C is shorter than the distance between the tip of the discharge electrode 33 and the ceiling non-insulated region D. Is provided.

  Returning to FIG. 1, the intake and exhaust system of the engine will be described. The intake passages 50 are connected to four independent intake passages 51 communicating with the respective intake ports 9 of the four cylinders 2 and to upstream ends (upstream ends in the intake air flow direction) of the respective independent intake passages 51. It has a surge tank 52 and a single tubular common intake passage 53 extending upstream from the surge tank 52. An openable / closable throttle valve 54 for adjusting the flow rate of the intake air introduced into the engine body 1 is provided at an intermediate portion of the common intake passage 53. The surge tank 52 is provided with an air flow sensor SN2 for detecting a flow rate of intake air introduced into the engine body 1.

  The exhaust passage 60 has four independent exhaust passages 61 communicating with the respective exhaust ports 10 of the four cylinders 2, and one downstream end (the downstream end in the exhaust gas flow direction) of each independent exhaust passage 61. And a single tubular common exhaust passage 63 extending downstream from the collecting portion 62. The common exhaust passage 63 is provided with a catalytic converter 65 for purifying exhaust gas. The catalytic converter 65 includes, for example, an oxidation catalyst that oxidizes HC and CO contained in exhaust gas to make it harmless, and a GPF (gasoline particulate) that collects particulate matter (PM) contained in exhaust gas.・ Filter) is built in.

  The EGR device 70 includes an EGR passage 71 that connects the common exhaust passage 63 and the surge tank 52, an EGR cooler 72 that cools exhaust gas (EGR gas) that is recirculated to the intake passage 50 through the EGR passage 71, and an EGR gas An EGR valve 73 is provided in the EGR passage 71 so as to be openable and closable to adjust the flow rate.

(2) Control System of Engine FIG. 10 is a block diagram showing a control system of the engine. The PCM 100 shown in FIG. 1 is a microprocessor for controlling the engine as a whole, and includes a well-known CPU, ROM, RAM, and the like. Note that the PCM 100 corresponds to an example of a “control device” in the claims.

  Detection signals from various sensors are input to PCM 100. For example, the PCM 100 is electrically connected to the above-described crank angle sensor SN1 and air flow sensor SN2, and information detected by these sensors (that is, the crank angle, the engine rotation speed, the intake air flow rate, etc.) is used as an electric signal by the PCM 100. Are sequentially input to

  Further, the vehicle is provided with an accelerator sensor SN3 for detecting an opening degree of an accelerator pedal (not shown) operated by a driver who drives the vehicle. A detection signal from the accelerator sensor SN3 is also input to the PCM 100. .

  The PCM 100 controls various parts of the engine while executing various determinations and calculations based on input signals from the various sensors. That is, the PCM 100 is electrically connected to the injector 15, the plasma generation plug 16, the throttle valve 54, the EGR valve 73, and the like, and outputs a control signal to each of these devices based on the result of the calculation and the like. I do.

  For example, the PCM 50 calculates an engine load (requested torque) based on the accelerator opening and the like detected by the accelerator sensor SN3, and calculates the calculated engine load, the intake air flow detected by the air flow sensor SN2, and the crank angle sensor. The amount of fuel to be injected into the cylinder 2 (target injection amount) and the fuel injection timing are determined based on the engine rotation speed detected by SN1, and the injector 15 is controlled according to the determination.

  Further, the PCM 100 determines a timing and a discharge period for discharging the non-equilibrium plasma from the plasma generation plug 16 based on the engine load and the engine rotation speed, and controls the plasma generation plug 16 according to the determination.

  FIG. 11 is a graph for explaining the conditions for generating the non-equilibrium plasma, and shows the relationship between the conditions of the pulse voltage (pulse width and applied voltage) applied to the plasma generation plug 16 and the type of the generated plasma. Is shown. The horizontal axis of the graph indicates the pulse width, and the vertical axis indicates the peak value of the applied voltage. The scale of each axis is a logarithmic scale. As shown in the graph of FIG. 11, in order to generate non-equilibrium plasma, it is necessary to set the pulse width to 0.01 μsec or more and less than 1 μsec. On the other hand, when the pulse width is increased to 1 μsec or more, thermal equilibrium plasma is generated. As described above, when a pulse voltage with a short pulse width is applied, non-equilibrium plasma is generated because, under conditions of a short pulse width, only electrons react and ions and molecules hardly react.

  From the above findings, in the present embodiment, the PCM 100 controls the voltage applied to the plasma generation plug 16 under the conditions shown in FIG. That is, the PCM 100 controls the power supply from the power supply unit (not shown) to the center electrode 32 such that a pulse voltage having a peak voltage of 10 kV and a pulse width of 0.1 μsec is applied to the center electrode 32 of the plasma generation plug 16. Control the supply. At this time, the PCM 100 repeatedly applies a pulse voltage at a frequency of 100 kHz. Thus, non-equilibrium plasma is generated between the four discharge electrodes 33 of the plasma generation plug 16 and the opening edge C1 of the ceiling-side non-insulated region D or the cavity C of the piston 5.

  In addition, the peak voltage of the pulse voltage for generating the non-equilibrium plasma may be changed in a range of 1 kV to 30 kV according to the operating conditions. For example, it is conceivable to set the peak voltage to be higher as the operating conditions increase the pressure (in-cylinder pressure) of the combustion chamber 6.

When the non-equilibrium plasma is generated in the combustion chamber 6, various substances are generated according to the environment around the discharge electrode 33 of the plasma generation plug 16. In particular, when the surroundings of the discharge electrode 33 are in a lean environment having a large air-fuel ratio, the combustion of the air-fuel mixture such as ozone (O ₃ ) and OH in the combustion chamber 6 by the action of the non-equilibrium plasma. Active species (radicals), which are substances to be promoted, are generated.

(3) Control According to Operating Conditions FIG. 13 is a map diagram for explaining a difference in control according to operating conditions (load / rotational speed) of the engine. The operation map shown in the figure is roughly divided into a low load region A1 having a load less than the predetermined load Ts and a high load region A2 having a load equal to or more than the predetermined load Ts. The PCM 100 determines each time whether the operating point of the engine is included in the low load area A1 or the high load area A2 based on the detection values of the sensors SN1 to SN3, etc., and combustion suitable for the determined operating area is performed. Control each part of the engine to be realized. For example, when operating in the high load region A2, the PCM 100 operates such that a mixture is formed over substantially the entire combustion chamber 6 (both inside and outside the cavity C) and the mixture is burned by compression ignition. The injector 15 and the plasma generation plug 16 are controlled. On the other hand, during operation in the low load region A1, the PCM 100 controls the injector 15 and the plasma generation plug 16 such that a mixture is formed only inside the cavity C and the mixture is burned by compression ignition. .

  Specific examples of the combustion control in the high load area A2 and the low load area A1 are as follows.

(A) Control in High Load Range FIGS. 14 and 15 are time charts illustrating the contents of the combustion control executed by the PCM 100 during operation in the high load range A2. FIG. 14 shows an example of operation in a high-load low-speed range A21 in which the engine speed is equal to or less than a preset reference speed N1 in the high-load range A2. FIG. 13 shows an example of operation in a high-load high-speed area A22 in which the engine speed is higher than the reference engine speed N1 in the high-load area A2. As shown in FIGS. 14 and 15, during operation in the high load region A2, the PCM 100 injects fuel from the injector 15 during the intake stroke and the compression stroke, and discharges the non-equilibrium plasma from the plasma generation plug 16. Let it.

(Control at high load and low speed range)
In the high-load low-speed range A21, fuel is injected from the injector 15 in the first half of the intake stroke and in the second half of the compression stroke, respectively. Hereinafter, fuel injection performed in the first half of the intake stroke is referred to as first-stage injection, and combustion injection performed in the second half of the compression stroke is referred to as second-stage injection. The pre-injection is performed collectively (without division) from exhaust top dead center (TDC on the left side in FIG. 14), which is the start timing of the intake stroke, to a point in time when half of the intake stroke has elapsed. . The latter-stage injection is executed between the time when 1/2 of the compression stroke has elapsed and the time when 3/4 of the compression stroke has elapsed.

  The timing of the latter injection will be described in more detail. The time point at which の of the compression stroke has elapsed means BTDC 90 ° CA advanced by 90 ° from the compression top dead center (TDC on the right side of FIG. 14) (“° CA” represents a crank angle), The time point at which ／ of the compression stroke has elapsed is BTDC 45 ° CA advanced by 45 ° CA from the compression top dead center. In other words, in the present embodiment, the fuel is injected from the injector 15 during the period from the BTDC 90 ° CA to the BTDC 45 ° CA as the post injection. Hereinafter, the period during which the second-stage injection is performed (the period from BTDC 90 ° CA to BTDC 45 ° CA) may be referred to as “１ ／ to ／ of the compression stroke”.

  FIG. 16 is a diagram for explaining the behavior of the fuel injected from the injector 15 during operation in the high load range A2. In FIG. 16, for convenience, the illustration of the plasma generation plug 16 is omitted, and the injector 15 located on the front side of the drawing is illustrated at the original position of the plasma generation plug 16. As shown in FIG. 16A, the fuel injected in the first half of the intake stroke by the pre-injection is sprayed in a cone shape from the injector 15 arranged near the center of the ceiling surface 28 of the combustion chamber 6, and the fuel is combusted. It diffuses into the chamber 6. At this time, part of the fuel adheres to the peripheral surface of the cylinder 2 to form droplets.

  As shown in FIG. 16 (b), at the time when the second-stage injection is performed, the fuel injected into the combustion chamber 6 by the first-stage injection is dispersed and present in various parts of the combustion chamber 6. However, as shown by the dark colored area in FIG. 16B, the fuel adhering to the peripheral surface of the cylinder 2 in the state of droplets as described above is provided on the outer peripheral side portion of the combustion chamber 6. By evaporating the portion, a relatively rich mixture (having a higher fuel concentration than other regions) is formed. In FIG. 16B, the region other than the dark colored region is lightly colored, indicating that a relatively lean (low fuel concentration) air-fuel mixture exists.

  On the other hand, the fuel injected by the latter injection in 1/2 to 3/4 of the compression stroke is introduced into the cavity C of the piston 5 and stays in the cavity C until the compression top dead center. That is, in the present embodiment, since the injector 15 is arranged near the center of the ceiling surface 28 of the combustion chamber 6 facing the cavity C, the injector 15 is formed in a cone shape at a timing of 1/2 to 3/4 of the compression stroke. When the fuel is injected, the injected fuel is introduced into the cavity C before sufficiently expanding in the radial direction (see FIG. 16B).

  As shown in FIG. 16 (c), most of the fuel once introduced into the cavity C does not leak out of the cavity C and stays in the cavity C until the compression top dead center (immediately before ignition). Will be. Accordingly, near the compression top dead center, the air-fuel mixture above the cavity C is maintained in a relatively lean state.

  As described above, in the present embodiment, in the combustion chamber 6 near the compression top dead center, a relatively rich air-fuel mixture (air-fuel mixture having a high fuel concentration) based on the pre-injection is applied to the outer peripheral portion of the combustion chamber 6, that is, the squish area. (Outside of the cavity C), and a richer air-fuel mixture (air-fuel mixture having a high fuel concentration) based on the second-stage injection is formed inside the cavity C.

  As shown in FIG. 14, in the high-load low-speed range A21, the discharge of the non-equilibrium plasma from the plasma generation plug 16 is performed during a period from the end of the first-stage injection to the start of the second-stage injection. Hereinafter, a discharge performed during a period from the end of the first-stage injection to the start of the second-stage injection is referred to as a main discharge.

  FIG. 17 is a diagram schematically showing a situation in combustion chamber 6 when the main discharge is performed. In the present embodiment, the main discharge is performed over a period from the bottom dead center (BDC) of the intake to a predetermined timing near the closing timing of the intake valve 11, and during this period, the main discharge is continuously applied to the center electrode 32 and the center electrode 32. A pulse voltage is applied. The intake valve 11 opens before the intake bottom dead center, and the main discharge is started in a state where the intake valve 11 is open. For example, the intake valve 11 opens at about 20 ° CA before the exhaust top dead center and closes at about 60 ° A after the intake bottom dead center.

  In the present embodiment, the start timing of the main discharge is kept constant regardless of the engine speed. On the other hand, as the engine speed increases, the end timing of the main discharge is retarded at the crank angle. That is, when the engine speed is low, the main discharge is performed during the period shown by the solid line in FIG. 14, and when the engine speed is high, the main discharge is performed during the period shown by the broken line in FIG.

  As described above, when the piston 5 is located below the center between the top dead center position and the bottom dead center position, the distance between the tip of the discharge electrode 33 and the ceiling non-insulated region D is larger than the distance of the plasma generation plug 16. It is shorter than the distance between the tip of the discharge electrode 33 and the opening edge C1 of the cavity C. Thus, in the main discharge, the non-equilibrium plasma is discharged along the line L1 connecting the discharge electrode 33 and the ceiling-side non-insulated region D as shown in FIG. That is, the non-equilibrium plasma is emitted from the tip of the discharge electrode 33 toward the outer periphery of the combustion chamber 6 and is guided to the ceiling non-insulating region D having a low insulation level.

The main discharge is performed before the latter stage injection, in a state where the fuel concentration in the combustion chamber 6 is low and active species such as ozone (O ₃ ) and OH are easily generated. Further, the main discharge is performed near the intake bottom dead center and at a time when the pressure in the combustion chamber 6 is relatively low. In plasma discharge, more active species such as ozone (O ₃ ) and OH are formed when the pressure of the field where the discharge is performed, that is, when the atmospheric pressure is low, than when the pressure is high. Therefore, many active species are generated in the combustion chamber 6 by performing the main discharge at the timing as described above.

Active species such as ozone (O ₃ ) are generated on the discharge path and flow from the discharge electrode 33 toward the earth-side electrode as described above. Therefore, in the main discharge, active species such as ozone (O ₃ ) are generated around the line L1 connecting the discharge electrode 33 of the plasma generation plug 16 and the ceiling-side non-insulated region D, and the ceiling-side non-insulated from the discharge electrode 33. The active species flows toward the region D.

  Here, a tumble flow is formed in the combustion chamber 6 with the opening of the intake valve 11. However, as shown by the dashed line and the solid line in FIG. 5, the tumble flow is a flow mainly directed downward from the intake valve 11, and the flow near the outer peripheral edge of the ceiling surface of the combustion chamber 6 where the ceiling-side non-insulating region D is located. Is relatively weak. In particular, in the present embodiment, the flow near the outer peripheral edge of the ceiling surface of the combustion chamber 6 is extremely weak as described above with respect to the solid line in FIG. Therefore, the generated active species travels from the discharge electrode 33 to the ceiling-side non-insulating region D, and thereafter, in the vicinity of this region D, that is, in the region near the outer peripheral edge of the ceiling surface 28 of the combustion chamber 6 (the region indicated by M1 in FIG. 17). ) Stays active species.

  After the main discharge, post-stage injection is performed, and a rich air-fuel mixture is formed in the cavity C. Thereafter, near the compression top dead center, the air-fuel mixture present in the cavity C having a high fuel concentration and a relatively low temperature from the wall surface of the combustion chamber 6 ignites and burns. Subsequently, the air-fuel mixture present in the outer peripheral portion of the combustion chamber 6, that is, in the squish area, ignites and burns. At this time, the ignition and combustion of the air-fuel mixture present in the squish area are promoted by the active species remaining near the outer peripheral edge of the combustion chamber 6. Specifically, the ignition of the air-fuel mixture present in the squish area starts earlier than in the case where there is no active species, and the combustion speed of the air-fuel mixture is increased. Here, near the compression top dead center, a flow toward the inner periphery of the combustion chamber 6 occurs in the squish area as the piston rises. Therefore, the active species that have stayed in the vicinity of the outer peripheral edge of the combustion chamber 6 due to this flow spread over the entire squish area. Then, ignition and combustion of the air-fuel mixture are promoted by the active species in the entire squish area.

(Control at high load and high speed range)
As shown in FIG. 15, in the high-load high-speed range A22, fuel is injected from the injector 15 in the first half of the intake stroke and in the second half of the compression stroke, similarly to the high-load low-speed range A21. This fuel injection control is the same in the high-load high-speed region A22 and the high-load low-speed region A21, and a description thereof will be omitted.

  On the other hand, as shown in FIG. 15, in the high-load high-speed region A22, the discharge from the plasma generation plug 16 includes, in addition to the main discharge performed during the period from the end of the first-stage injection to the start of the second-stage injection, After the end of the injection, additional discharge is performed over a predetermined period including the compression top dead center. The control content of the main discharge is the same in the high-load high-speed range A22 and the high-load low-speed range A21, and the description thereof will be omitted.

  FIG. 18 is a diagram schematically showing a situation in combustion chamber 6 when additional discharge is performed. As described above, the second-stage injection is performed after a half of the compression stroke has elapsed. Accordingly, at the time of performing the additional discharge after the end of the second-stage injection, the distance between the tip of the discharge electrode 33 and the opening edge C1 of the cavity C is larger than the distance between the tip of the discharge electrode 33 and the ceiling-side non-insulated region D. Is also short. Therefore, in the additional discharge, the non-equilibrium plasma is discharged mainly from the discharge electrode 33 toward the opening edge C1 of the cavity C. Specifically, the non-equilibrium plasma flows along the line L2 connecting the discharge electrode 33 and the opening edge C1 of the cavity C from the tip of the discharge electrode 33 toward the opening edge C1 of the cavity C on the outer peripheral side of the combustion chamber 6. Discharges downward and upward.

  In the present embodiment, the additional discharge is performed from a predetermined timing in the latter half of the compression stroke when the squish flow starts to weaken until the fuel-air mixture ignites. In the case of the present embodiment, a squish flow formed in a squish area (a region located radially outward from the opening edge C1 of the cavity C) as the piston 5 rises, that is, a gas flow flowing from the radial outside to the inside, BTDC10 ° CA advanced by 10 ° from the compression top dead center begins to weaken. Accordingly, the earliest timing for starting the additional discharge is set to BTDC10 ° CA. Further, in the present embodiment, at the time of operation in the high-load high-speed range A22, the air-fuel mixture ignites at least 10 ATCA of ATDC which is delayed by 10 ° from the compression top dead center. For this reason, the latest time to end the plasma discharge is set to ATDC10 ° CA. In other words, in the present embodiment, the plasma discharge is started and terminated from BTDC 10 ° CA at which the squish flow starts to weaken to ATDC 10 ° CA which is the latest timing of the mixture ignition. In the example of FIG. 12, the discharge of the non-equilibrium plasma from the plasma generation plug 16 is continuously performed between BTDC5 ° CA and ATDC5 ° CA. In this specification, the ignition point of the air-fuel mixture refers to the start point of the hot flame reaction of the fuel. The start time of the hot flame reaction can be defined as the time when about 10% by mass of the total supplied fuel has burned (MFB 10%).

As shown in FIG. 16C, near the compression top dead center, a relatively rich air-fuel mixture is formed in the squish area and the inside of the cavity C by the first-stage injection and the second-stage injection. On the other hand, the discharge path L2 connecting the tip of the discharge electrode 33 and the opening edge C1 of the cavity C is located between the two air-fuel mixture regions (between the squish area and the cavity C). The mixture present is relatively lean. Further, as described above, the fuel related to the second-stage injection remains in the cavity C, and the air-fuel mixture existing above the cavity C is relatively lean. Therefore, even by the additional discharge, active species such as ozone (O ₃ ) and OH are generated around the discharge path L2 connecting the tip of the discharge electrode 33 and the opening edge C1 of the cavity C. In this manner, the active species are additionally generated around the line L2 near the compression top dead center, and the piston 5 starts to descend. When the piston 5 starts descending, a reverse squish flow that flows from the inside to the outside in the radial direction starts to be formed in the squish area. Therefore, most of the active species generated around the line L2 move to the squish area on the reverse squish flow.

  As described above, in the high-load, high-speed range A22, the active species M2 generated by the additional discharge in addition to the active species M1 generated by the main injection are supplied to the squish area, and the mixture present in the squish area is ignited. Combustion is promoted.

  In this embodiment, the air-fuel mixture ignites almost simultaneously with the end of the additional discharge (that is, in the vicinity of ATDC5 ° CA). For example, the mixture is ignited at about ATDC 5 to 10 ° CA, and compression ignition combustion is started.

(Control of intake)
As described above, during operation in the high load range A2, an air-fuel mixture is formed mainly in the cavity C and the squish area (that is, fuel is distributed over a wide range in the combustion chamber 6). On the other hand, the amount of air introduced into the combustion chamber 6 is considerably increased. As a result, the air-fuel ratio (A / F) in the combustion chamber 6 is significantly leaner than the stoichiometric air-fuel ratio (14.7). Is set to Specifically, in the high load region A2, the average air-fuel ratio in the entire combustion chamber 6, that is, the ratio (mass ratio) between the amount of fuel injected from the injector 15 and the amount of air in the combustion chamber 6 during one cycle is reduced. , Is set to a value larger than twice the stoichiometric air-fuel ratio. In other words, in the high load region A2, the throttle valve 54 is opened to a sufficiently high opening degree so that the excess air ratio λ is larger than 2 (air corresponding to λ> 2 is introduced into the combustion chamber 6). . In the high load region A2, the EGR valve 73 is opened except at least near the maximum load, and a predetermined amount of EGR gas is introduced into the combustion chamber 6. For example, in the high load region A2, the gas air-fuel ratio (G / F), which is the ratio of the amount of fuel injected from the injector 15 to the total amount of gas in the combustion chamber 6 (total amount of air and EGR gas), It is set to 30 or more. Note that even if an attempt is made to secure an air amount equivalent to λ> 2 in the high load region A2, there is a possibility that the air amount will be insufficient with only natural intake, but in such a case, a turbocharger may be added.

  As described above, in the high load area A2, the combustion chamber 6 is in a very lean state where the excess air ratio λ is larger than 2; And the combustion of the air-fuel mixture in a lower temperature environment than the inside of the cavity C) is promoted. Therefore, the combustion speed of the air-fuel mixture is increased even in a lean environment of λ> 2. Combustion will end in time.

(B) Control in the low load range Although not shown in detail, in the low load range A1 in which the engine load is lower than the high load range A2 (that is, the required amount of fuel is reduced), the fuel injection from the injector 15 is not performed. The non-equilibrium plasma is discharged from the plasma generation plug 16 only during the latter half of the compression stroke and between the end of the combustion injection and the compression top dead center.

  For example, in the low load range A1, fuel is injected from the injector 15 in a plurality of times in 1/2 to 3/4 of the compression stroke. As a result, in the low load region A1, an air-fuel mixture is formed only inside the cavity C. That is, all or most of the air-fuel mixture is formed inside the cavity C, and almost no air-fuel mixture is formed outside the cavity C.

  Further, in the low load region A1, a discharge is generated from the plasma generation plug 16 after 3/4 of the compression stroke (BTDC 45 ° CA) and after the end of fuel injection and before the compression top dead center. By performing discharge at this timing, in the low load region A1, non-equilibrium plasma and active species are generated around the line connecting the tip of the discharge electrode 33 and the opening edge C1 of the cavity C, as in the above-described additional injection. At the same time, active species are mainly supplied to the squish area. As a result, the combustion speed of the air-fuel mixture in the squish area where the temperature tends to be lower than that in the center of the cavity C is increased, and the combustion period of the air-fuel mixture is shortened.

  The air-fuel ratio (A / F) of the air-fuel mixture in the entire combustion chamber 6 is set to a value that is more than twice as large as the stoichiometric air-fuel ratio (that is, λ> 2). As described above, in the present embodiment, in both the high-load region A2 and the low-load region A1 (in all the operating regions of the engine), the control of performing the compression ignition combustion of the air-fuel mixture under a lean environment of λ> 2 is executed. .

(4) Operational Effects As described above, in the present embodiment, the discharge electrode 33 of the plasma generation plug 16 is positioned near the center axis (cylinder axis X) of the plasma generation plug near the center of the ceiling of the combustion chamber. It has a shape extending to the outer peripheral side of the combustion chamber 6. When the engine is operating in the high load region A2, the pre-injection is performed to form a mixture in the inside of the cavity C and the squish area radially outside the cavity C, that is, the outer peripheral portion of the combustion chamber 6. After the pre-injection is performed and before the mixture is ignited, a discharge is generated from the plasma generating plug (main discharge is performed) and the non-equilibrium plasma is generated in the combustion chamber 6. Is generated. Therefore, in the homogeneous charge compression ignition engine, there is an advantage that the combustion speed can be sufficiently increased while suppressing the combustion noise to an appropriate level.

Specifically, non-equilibrium plasma and active species such as ozone (O ₃ ) can be generated in the combustion chamber 6 by discharging from the plasma generation plug 16. Here, since the discharge electrode 33 has the above-described shape, the non-equilibrium plasma is discharged from the discharge electrode 33 toward the outer peripheral side of the combustion chamber 6, and the active species such as ozone (O ₃ ) is discharged. Can be moved to the outer peripheral side of the combustion chamber 6. Therefore, many active species can be supplied to the air-fuel mixture in the outer peripheral portion of the combustion chamber 6, that is, in the squish area. The active species promotes the combustion of the air-fuel mixture in the squish area, thereby increasing the combustion speed of the air-fuel mixture and shortening the combustion period.

  Here, the temperature of the squish area located on the outer peripheral portion of the combustion chamber 6 tends to be lower than that of the central portion of the combustion chamber 6 (inside the cavity C). Therefore, if the active species is not supplied to the squish area, the air-fuel mixture in the squish area ignites with a considerable delay with respect to the air-fuel mixture in the cavity C, and the combustion speed after the ignition becomes considerably slow. I will. On the other hand, in the embodiment, since the active species is supplied to the squish area, the ignition delay of the air-fuel mixture in the squish area can be reduced, and the combustion speed of the air-fuel mixture (that is, the combustion speed in the latter half of combustion). Can be accelerated. In addition, since the combustion speed of the air-fuel mixture in the cavity C, which is ignited first, does not substantially change, the pressure rise occurring in the first half of the combustion does not become remarkable. Therefore, the entire air-fuel mixture can be burned in a short period of time while suppressing the combustion noise to an appropriate level, and a sufficiently high thermal efficiency can be obtained.

  FIG. 19 is a diagram for explaining the operation and effect. In FIG. 19, the waveform of the heat generation rate when the air-fuel mixture is burned by the method of the embodiment is indicated by W1. As shown in the waveform W1, when the combustion speed of the air-fuel mixture in the squish area is increased by using the non-equilibrium plasma (active species) (that is, according to the method of the above embodiment), the combustion noise is considered. Below the upper limit of the heat generation rate, the "combustion noise limit" line (that is, within a range where excessive combustion noise does not occur), it is possible to obtain a sufficiently peaky combustion waveform that rises and falls relatively sharply. . This makes it possible to achieve near-ideal combustion with a short combustion period, excellent thermal efficiency, and without excessive combustion noise.

  A waveform W2 in FIG. 19 shows a waveform of the heat generation rate when no active species is supplied based on the non-equilibrium plasma. As shown in the waveform W2, when no active species is supplied to the squish area, a high combustion speed is obtained in the first half of the combustion, which is a stage in which the air-fuel mixture in the cavity C having a high temperature burns. In the latter half of the combustion, which is the stage where the air-fuel mixture in the squish area where the temperature is low is burned, the combustion speed is significantly reduced, and the combustion period is prolonged as a whole. That is, in the latter half of the combustion, since the piston 5 is being lowered (expansion of the combustion chamber 6), the combustion in the air-fuel mixture in the squish area is combined with the fact that the temperature in the squish area was originally low. The speed must be greatly reduced. As described above, it is understood that, when the plasma discharge (supply of the active species) is not performed, the combustion of the air-fuel mixture in the squish area is slowed down, resulting in a longer combustion period as a whole and a decrease in thermal efficiency. Is done.

  If the active species is supplied to the entire combustion chamber 6 to promote combustion of both the mixture in the squish area and the mixture in the cavity C, for example, the waveform shown in FIG. As shown by W3, it becomes possible to further shorten the combustion period and increase the thermal efficiency. However, in this case, the combustion of the air-fuel mixture in the cavity C, which is likely to be steep due to the originally high temperature, becomes even steeper. Become. In this case, even if it is excellent in terms of thermal efficiency, the combustion noise becomes too large and it is not possible to maintain commerciality. On the other hand, in the above-described embodiment, basically, only the combustion of the air-fuel mixture in the squish area is promoted (increased in speed), so that the combustion speed can be increased as much as possible without the combustion noise becoming excessive. In addition, the merchantability and the thermal efficiency can be compatible at a high level.

  Further, in the above embodiment, the excess air ratio λ in the combustion chamber 6 is made larger than 2 during the operation in the high load region A2, so that the combustion temperature of the air-fuel mixture can be significantly reduced, and The generation amount of NOx can be sufficiently suppressed. Here, the “NOx limit” line in FIG. 15 indicates the lower limit heat generation rate at which the generation amount of NOx can be suppressed so that a catalyst (NOx catalyst) for reducing NOx is unnecessary. As is clear from the comparison between the NOx limit line and the waveform W1, according to the embodiment in which the air-fuel mixture is burned under a significantly lean environment of λ> 2, high-temperature combustion exceeding the NOx limit is prevented. This can be avoided, and the NOx generation amount can be reduced so that the NOx catalyst can be made unnecessary.

  In the above-described embodiment, in the high load region A2, in addition to the first-stage injection in which fuel is injected from the injector 15 during the intake stroke, the second-stage injection in which fuel is injected from the injector 15 during the compression stroke is performed. Therefore, an air-fuel mixture having a moderately high fuel concentration can be formed inside the cavity C located at the central portion of the combustion chamber 6 while forming the air-fuel mixture in the squish area on the outer peripheral portion of the combustion chamber 6. Therefore, the air-fuel mixture in the central portion of the combustion chamber 6 is reliably ignited, whereby the air-fuel mixture in the entire combustion chamber can be reliably burned. As described above, the air-fuel mixture existing in the outer peripheral portion of the combustion chamber, which is relatively difficult to ignite, can be burned early by the action of the non-equilibrium plasma.

In particular, the latter-stage injection is performed during a period from the time when half of the compression stroke has elapsed to the time when three-fourths of the compression stroke has elapsed (between BTDC 90 ° CA and BTDC 45 ° CA). The fuel injected from the injector 15 by the injection can be reliably held in the cavity C up to the compression top dead center. Further, the fuel concentration of the air-fuel mixture above the cavity C can be suppressed to a low level, thereby suppressing the generation of suppressive species such as formaldehyde during the execution of the additional discharge and reducing the active species such as ozone (O ₃ ). Many can be generated. Therefore, combustion of the air-fuel mixture can be appropriately promoted.

  In the above-described embodiment, in the high load range A2, the main discharge is performed during a period from the end of the pre-injection performed during the intake stroke to the start of the post-injection performed during the compression stroke. You. That is, the main discharge is performed when the pressure in the combustion chamber is relatively low. Therefore, a large amount of active species can be more reliably supplied to the squish area. In addition, when the fuel comes into contact with the non-equilibrium plasma, a substance that suppresses combustion such as formaldehyde (hereinafter, referred to as a suppressive species) is likely to be generated. On the other hand, by performing the main discharge during the period before the second-stage injection is started, it is possible to avoid contact between the fuel supplied to the combustion chamber 6 by the second-stage injection and the non-equilibrium plasma. Therefore, it is possible to prevent the combustion of the air-fuel mixture from being hindered by a suppressing species such as formaldehyde.

  Further, in the above embodiment, the end timing of the main discharge is delayed more when the engine speed is high than when the engine speed is low, so that the main discharge is performed during the execution period, that is, in the crank angle from the discharge electrode 33 of the plasma generation plug 16. The discharge period is longer when the engine speed is high than when it is low. Therefore, the combustion of the air-fuel mixture in the squish area can be more reliably promoted when the engine speed is high, and the energy consumption accompanying driving of the plasma generation plug can be suppressed from becoming excessive. Specifically, when the engine speed is high, the amount of fuel injected from the injector per unit time is larger than when the engine speed is low, so that the penetration of the fuel is increased and the fuel is deposited on the inner peripheral surface of the cylinder 2. Easy to adhere. The fuel adhering to the peripheral surface of the cylinder 2 evaporates substantially with the rise of the piston 5, but is not sufficiently mixed with air and is difficult to burn. On the other hand, in the embodiment, as described above, the discharge period is extended when the engine speed is high, so that many active species are supplied to the squish area, that is, the area close to the peripheral surface of the cylinder 2. And combustion of the air-fuel mixture in this region can be promoted.

  Further, in the above-described embodiment, in the high-load high-speed range A22 in which the engine speed is higher than the reference speed N1, additional discharge is performed during the period from the end of the second-stage injection to the ignition of the air-fuel mixture, and the combustion is performed. Active species can be further added to the outer peripheral portion of the chamber 6, that is, the squish area. Therefore, the combustion of the air-fuel mixture in the outer peripheral portion of the combustion chamber 6, which is likely to be difficult to burn due to the high engine speed as described above, can be promoted.

(5) Modification In the above-described embodiment, the injector 15 is disposed near the center of the ceiling surface 28 of the combustion chamber 6, and the squish area and the inside of the cavity C are relatively positioned during operation in the high load area A2. In order to form a rich air-fuel mixture, fuel is injected from the injector 15 in the first half of the intake stroke and in 1/2 to 3/4 of the compression stroke, respectively. Is not limited to this. That is, the position of the injector 15 and the timing of fuel injection from the injector 15 may be appropriately changed. For example, the post injection may be omitted.

  In the above-described embodiment, the case where the main discharge is performed during the period from the bottom dead center of the intake air to the vicinity of the closing timing of the intake valve 11 has been described. Other periods may be used as long as they are within the period before the operation. Further, the execution period of the additional discharge is not limited to the period of the above-described embodiment, and may be set to another period as long as it is within a period until the air-fuel mixture is ignited after the execution of the second-stage injection. Further, the additional discharge may be omitted.

  In the above-described embodiment, the plasma generation plug 16 is arranged at the center of the ceiling surface 28 of the combustion chamber 6, and a position slightly offset from the position of the plasma generation plug 16 (the center of the ceiling surface 28) to one side in the cylinder row direction. The injector 15 is arranged at the center of the ceiling surface 28, and the plasma generating plug 16 is arranged at a position offset from the center of the ceiling surface 28. May be. Alternatively, both the injector 15 and the plasma generation plug 16 may be arranged at positions offset from the center of the ceiling surface 28. In this case, the position of the plasma generation plug 16 is preferably offset from the center of the ceiling surface 28 (cylinder axis X) to the intake side. When the position of the plasma generation plug 16 is offset toward the intake side, the non-equilibrium plasma discharged from the discharge electrode 33 of the plasma generation plug 16 can be further strengthened on the intake side. Thereby, in the combustion chamber 6 where the intake side tends to be lower in temperature than the exhaust side, the combustion speed of the air-fuel mixture is increased in a well-balanced manner (the combustion speed becomes equal on the intake side and the exhaust side), and the combustion period Can be expected to be more shortened.

  On the other hand, if the offset amount of the plasma generation plug 16 to the intake side is large, the amount of the active species supplied to the intake side becomes too large, and the combustion speed is reversed between the intake side and the exhaust side (the intake side is faster). ). When there is a concern about such a reversal of the combustion speed, it is effective to make the vertical gap of the squish area smaller on the exhaust side than on the intake side, for example, as shown in FIG. By doing so, the strength of the reverse squish flow generated by the lowering of the piston 5 becomes stronger on the exhaust side, so that the supply amount of the active species is reduced because the radial distance from the plasma generation plug 16 is large. In the squish area on the exhaust side, the supply amount of active species can be corrected in the increasing direction, and the above-described combustion speed reversal phenomenon can be avoided while the plasma generating plug 16 is largely offset to the intake side. it can.

  It should be noted that even when the plasma generating plugs 16 are offset in the cylinder row direction (the direction orthogonal to the intake / exhaust direction), the imbalance of the combustion speed accompanying the offset can be corrected by the shape of the piston 5. For example, when the plasma generation plug 16 is disposed at a position offset from the center of the ceiling surface 28 of the combustion chamber 6 (cylinder axis X) to one side in the cylinder row direction, a pair of ridges 26 of the piston 5 (FIG. 3, FIG. 4) and the ceiling surface 28 may be made smaller on the other side in the cylinder row direction than on the other side. Alternatively, the same effect can be obtained by making the area of the pair of peaks 26 larger on one side in the cylinder row direction than on the other side.

  In the embodiment, the heat insulating layer 19 covers almost the entire surface of the combustion chamber 6 excluding the ceiling-side non-insulating region D and the opening edge C1 of the cavity C, so that the combustion chamber 6 is not covered (exposed). Although the ceiling-side non-insulating region D and the opening edge C1 of the cavity C function as the earth-side electrode during the plasma discharge, the portion functioning as the earth-side electrode is not limited to this. However, if an insulator is provided at least in a portion of the ceiling surface 28 of the combustion chamber 6 including the central portion thereof and overlapping the discharge electrode 33, the non-equilibrium plasma is generated from the discharge electrode 33 toward the outer peripheral side of the combustion chamber 6. To discharge more active species to the outer peripheral portion of the combustion chamber.

  Further, as shown in FIG. 21, the upper surface of each discharge electrode 33 (the surface on the ceiling surface 28 side of the combustion chamber 6) may be covered with an insulator (insulator) made of alumina or the like. In this way, the non-equilibrium plasma is prevented from being discharged from the upper surface of each discharge electrode 33 upward, that is, toward the center of the ceiling surface 28 of the combustion chamber 6, and the outer periphery of the combustion chamber 6 is prevented from being discharged from the discharge electrode 33. The active species can be more reliably supplied to the side.

  In the above embodiment, the plasma generating plug 16 has a plurality of discharge electrodes 33 projecting radially from the tip of the center electrode 32 (see FIG. 6), but instead of this, for example, FIG. A plasma generating plug 116 having a shallow dish-shaped discharge electrode 133 as shown in FIG.

  More specifically, the plasma generating plug 116 according to the modified example has a cylindrical plug body 131, a center electrode 132 inserted inside the plug body 131, and a shallow portion extending radially outward from the tip of the center electrode 132. It has a dish-shaped discharge electrode 133 and an insulator 134 provided between the end face of the plug body 131 and the discharge electrode 133. The discharge electrode 133 is formed in a circular shape centered on the tip of the center electrode 132 when viewed from the bottom, and is formed so that its height becomes lower toward the outside in the radial direction when viewed in cross section. The insulator 134 is formed so as to cover almost the entire back surface (the surface on the cylinder head 4 side) of the discharge electrode 133.

  In the above-described embodiment, the injector 15 is of the open-open type in which the ring-shaped nozzle port 44 is formed when the valve is opened (see FIG. 7). A multi-hole type having a plurality of side-by-side injection holes may be used.

  In the above-described embodiment, an example in which the present invention is applied to a premixed compression ignition type gasoline engine that performs compression ignition while mixing fuel containing gasoline as a main component with air has been described. The present invention may be applied to a premixed compression ignition type diesel engine that performs compression ignition while mixing with air.

DESCRIPTION OF SYMBOLS 1 Engine main body 5 Piston 6 Combustion chamber 15 Injector 16, 116 Plasma generation plug 33, 133 Discharge electrode (electrode)
100 PCM (control device)
C Cavity (of the piston) C1 Opening edge of the (cavity) D Non-insulated area on the ceiling side S Crown surface (of the piston)

Claims (6)

An injector for injecting fuel into the combustion chamber,
A piston having a crown defining a bottom surface of the combustion chamber;
A plasma generation plug having an electrode near the center of the ceiling of the combustion chamber and discharging non-equilibrium plasma from the electrode into the combustion chamber;
A control device that controls the injector and the plasma generation plug so that premixed compression ignition combustion that performs compression ignition while mixing fuel injected from the injector with air is realized,
The electrode of the plasma generation plug has a shape extending from the tip of the plasma generation plug to the outer peripheral side of the combustion chamber,
The control device controls the injector so that a part of a mixture of fuel and air injected from the injector near the compression top dead center is formed on an outer peripheral portion of the combustion chamber, and controls a fuel generated by the injector. A premixed compression ignition engine, comprising: controlling the plasma generation plug so that non-equilibrium plasma is discharged from the electrode of the plasma generation plug before the mixture is ignited after injection. The premixed compression ignition engine according to claim 1,
The control device causes the injector to perform a pre-injection of injecting fuel into the combustion chamber during an intake stroke and a post-injection of injecting fuel into the combustion chamber during a compression stroke. Mixed compression ignition engine. The premixed compression ignition engine according to claim 2,
The control device controls the plasma generation plug so that discharge is performed from the electrode of the plasma generation plug during a period from the end of the pre-injection to the start of the post-injection, A homogeneous charge compression ignition engine. The premixed compression ignition engine according to claim 3,
The control device controls the plasma generation plug such that a discharge period from the electrode of the plasma generation plug at a crank angle is longer at a crank angle than when the engine rotation speed is low. Premixed compression ignition engine. The premixed compression ignition engine according to claim 3 or 4,
When the engine rotation speed is higher than a predetermined reference rotation speed, the control device controls the air-fuel mixture during a period from the end of the first-stage injection to the start of the second-stage injection and after the second-stage injection is completed. Non-equilibrium plasma is discharged from the electrode of the plasma generating plug during a period until the fuel is ignited, and when the engine speed is equal to or lower than the reference speed, the mixture is ignited after the end of the latter-stage injection. And controlling the plasma generation plug so that discharge from the electrode of the plasma generation plug is stopped during the period up to. The homogeneous charge compression ignition engine according to any one of claims 1 to 5,
A premixed compression ignition engine, wherein an insulator is provided in a predetermined area including a central portion of a ceiling surface of the combustion chamber.

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