JP2020037898A - 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 fuel so that at least part of the fuel stays in a cavity C of a piston 5 to form richer air-fuel mixture inside the cavity C than outside it. The plasma production plug 16 discharges the non-equilibrium plasma to an opening edge C1 of the cavity C after the fuel is injected by the injector but before the air-fuel mixture in the cavity C 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.

  Patent Document 2 below is also known as a disclosure of an engine similar to the above.

Japanese Patent No. 6237329 Japanese Patent No. 6149759

  According to Patent Documents 1 and 2, the supply of ozone as an active species to the air-fuel mixture has the advantage that combustion at low temperatures is particularly promoted and compression ignition combustion is stabilized.

  However, in the above Patent Documents 1 and 2, the compression ignition combustion is performed in a state in which the mixture and the ozone are entirely mixed. Therefore, when 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 has as its object to provide a premixed compression ignition engine capable of sufficiently increasing the combustion speed while suppressing combustion noise to an appropriate level. I do.

  In order to solve the above problems, a homogeneous charge compression ignition engine according to the present invention has an injector for injecting fuel into a combustion chamber, a crown defining a bottom surface of the combustion chamber, and a center of the crown. A piston having a cavity formed in a portion thereof, a plasma generating plug having an electrode near the center of the ceiling of the combustion chamber overlapping the cavity in plan view, and discharging non-equilibrium plasma from the electrode, and injection from the injector A control device that controls the injector and the plasma generation plug so that premixed compression ignition combustion in which compressed fuel is mixed and ignited with air is realized, and the control device is injected from the injector. At least a portion of the fuel remains in the cavity, and a rich mixture is formed in the cavity than outside the cavity. After the fuel is injected by the injector and before the mixture in the cavity is ignited, the non-equilibrium plasma is discharged from the electrode of the plasma generation plug toward the opening edge of the cavity. Thus, the plasma generating plug is controlled (claim 1).

  According to the present invention, after fuel injection by the injector, before the air-fuel mixture formed in the cavity ignites, a non-equilibrium is formed from the plasma generation plug located near the center of the ceiling of the combustion chamber toward the opening edge of the cavity. Since the plasma is discharged, active species such as ozone and OH generated by the action of the non-equilibrium plasma promote the combustion of the outer peripheral portion of the air-fuel mixture in the cavity, so that the outer peripheral portion of the air-fuel mixture is earlier than the central portion. It can cause a peculiar combustion of burning. The outer peripheral portion of the air-fuel mixture has a larger volume than the central portion, but tends to have a lower temperature. For this reason, by burning the air-fuel mixture from the outer periphery first, it is possible to secure a large amount of heat generation in a short period of time without causing a remarkable pressure increase and accompanying excessive combustion noise.

  On the other hand, since the central portion of the air-fuel mixture burns with a delay toward the outer peripheral portion, the lowering speed of the piston during this central portion combustion is considerably high. However, since the temperature of the center of the air-fuel mixture is originally high, the center of the air-fuel mixture is relatively low even in such a situation where the piston is rapidly lowered (therefore, the combustion chamber is rapidly expanding). Burns at a fast rate. As a result, it is possible to avoid a large decrease in the combustion speed in the latter half of the combustion, to burn the entire mixture in a relatively short period of time, and to obtain sufficiently high thermal efficiency.

  Preferably, the control device causes the plasma generating plug to discharge non-equilibrium plasma from a point in time when ／ of the compression stroke has elapsed until the mixture in the cavity is ignited (claim 2).

  According to this configuration, immediately before the mixture in the cavity is ignited, the active species generated by the action of the non-equilibrium plasma can be appropriately supplied to the outer periphery of the mixture, and the outer periphery of the mixture is supplied first. Thus, the above-described combustion mode in which the fuel is combusted can be reliably realized.

  Preferably, the control device forms a relatively rich mixture in the cavity and generates the non-equilibrium plasma when the engine is operated in a low load region where the engine load is less than a predetermined load. The control is performed (claim 3).

  Since the required fuel injection amount is smaller in the low load range of the engine than in the high load range, much of the fuel can be introduced into the cavity without hindrance, and a relatively rich air-fuel mixture can be appropriately introduced into the cavity. Can be formed.

  In the above configuration, more preferably, the injector radially injects fuel from near the center of the ceiling of the combustion chamber toward the cavity, and the control device controls the amount of fuel injected from the injector. Fuel is injected into the injector between the time when half the compression stroke has elapsed and the time when three-fourths of the compression stroke has elapsed so that the portion remains in the cavity until the time of discharge of the non-equilibrium plasma. (Claim 4).

  According to this configuration, the fuel injected from the injector can be introduced into the cavity before the spray of the fuel spreads sufficiently in the radial direction, and most of the fuel can be retained in the cavity. Thereby, a relatively rich air-fuel mixture can be appropriately formed inside the cavity, and the air-fuel mixture existing outside the cavity can be reduced as much as possible.

  In the above configuration, more preferably, the control device is configured to inject the fuel in a plurality of times from a point in time when a half of the compression stroke has elapsed to a point in time when a quarter of the compression stroke has elapsed. The injector is controlled (claim 5).

  As described above, when the fuel is injected in a plurality of injections, the penetration (penetration force) of the spray in each injection becomes weak, and thus the distribution of the air-fuel mixture as described above (relative in the cavity). (A state in which a rich air-fuel mixture is formed) can be appropriately maintained until immediately before ignition of the air-fuel mixture.

  Preferably, the engine further includes a throttle valve that adjusts a flow rate of the air introduced into the combustion chamber, and the control device is configured to control the excess air ratio in the combustion chamber to be less than 2 during operation in the low load range. The throttle valve and the injector are controlled so that the pressure increases.

  According to this configuration, the combustion temperature of the air-fuel mixture during operation in a low load range can be significantly reduced, so that the amount of NOx generated during combustion can be sufficiently reduced to eliminate the need for a NOx catalyst. Can be.

  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 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. 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. 5 is a time chart for explaining the details of combustion control performed during operation of the engine in a low load range. It is a figure for explaining behavior of fuel injected from an injector at the time of operation in a low load range, (a) shows a situation during fuel injection, and (b) shows a situation after combustion injection, respectively. . It is a figure which shows typically the situation in a combustion chamber when non-equilibrium plasma is discharged from a plasma generation plug. 4 is a graph of a heat release rate for explaining an effect of the embodiment. It is a figure for explaining the modification of the plasma generation plug used by the above-mentioned embodiment, (a) is a side view and (b) is a bottom view.

(1) Overall Configuration of Engine FIG. 1 is a schematic plan view showing a configuration of a homogeneous charge compression ignition 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.

  FIG. 2 is a cross-sectional view of the engine body 1 along the intake and exhaust directions, and FIG. 3 is a cross-sectional view of the engine body 1 along a direction (cylinder row direction) orthogonal to the intake and exhaust directions. In FIG. 2, IN indicates the intake side, and EX indicates the exhaust side. As shown in FIGS. 2 and 3, the engine main 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.

  As shown in FIGS. 1 and 2, in the cylinder head 4, for each cylinder 2, an intake port 9 for introducing air supplied from an intake passage 50 into the combustion chamber 6, and a gas generated in the combustion chamber 6. An exhaust port 10 for leading exhaust gas to an exhaust passage 60; an intake valve 11 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. Are provided respectively.

  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. 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 FIGS. 2 and 3 described above, 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 on the outer peripheral edge thereof and a reference surface 21. And a protruding portion 20 protruding upward (on the side approaching the cylinder head 4). 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.

  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. 5A and 5B are enlarged views 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 of (here, a plurality of) protruding radially from the tip of the center electrode 32. (Four) discharge electrodes 33. The discharge electrode 33 is provided so as to be exposed to the combustion chamber 6 at a position corresponding to the center of the ceiling surface 28 of the combustion chamber 6. The tip of the center electrode 32 projecting toward the combustion chamber 6 with respect to the plug body 31 is covered with an insulator 34 (insulator) made of alumina or the like.

  In the plan view shown in FIG. 4, the plasma generating plug 16 is disposed at a position where its center (the central axis of the center electrode 32) substantially coincides with the cylinder axis X and the discharge electrode 33 overlaps the bottom surface 22 of the cavity C. Have been. The four discharge electrodes 33 are formed so as to extend in a direction non-parallel to both the intake / exhaust direction and the cylinder row direction, more specifically, to extend in a direction intersecting the intake / exhaust direction and the cylinder row direction at an angle of about 45 degrees. Have been. As will be described later in detail, a predetermined pulse voltage is applied to the center electrode 32, and the non-equilibrium plasma is discharged from each discharge electrode 33 toward the combustion chamber 6 according to the applied voltage. It has become.

  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. 6 is a 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.

  FIG. 7 is an enlarged sectional view showing the combustion chamber 6 and its peripheral portion. As shown in this figure, 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, 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 (on the cylinder head 4 side) when the piston 5 is at the top dead center (see FIG. 14 described later). ), So that the piston ring 5a 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 lower volume specific heat 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 completely covers the combustion chamber 6 when the piston 5 is at the top dead center. However, the heat shield layer 19 is limited only to the opening edge C1 of the cavity C of the piston 5. 19 is not formed. The heat shield layer 19 has a higher insulating property than the piston 5 made of, for example, an aluminum alloy. Therefore, when the non-equilibrium plasma is discharged from the discharge electrode 33 of the plasma generation plug 16, the non-equilibrium plasma is naturally guided to the opening edge C <b> 1 of the cavity C not covered by the heat shielding layer 19 (described later). FIG. 14). Thus, at the time of plasma discharge, the anode and the cathode are constituted by the discharge electrode 33 and the opening edge C1 of the cavity C. That is, the discharge electrode 33 corresponds to the anode, and the opening edge C1 of the cavity C corresponds to the cathode.

  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. 8 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 above 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 at which the non-equilibrium plasma is to be discharged 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. 9 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. 9, 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 voltage applied to the plasma generation plug 16 is controlled by the PCM 100 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 discharged from the four discharge electrodes 33 of the plasma generation plug 16 toward the combustion chamber 6 (the opening edge C1 of 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. 11 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 low load region A1 and the high load region A2 are as follows.

(A) Control in low load range FIG. 12 is a time chart for illustrating the contents of the combustion control executed by the PCM 100 during operation in the low load range A1. The timing of fuel injection and plasma discharge executed at an operation point (for example, 1/3 load, 2500 rpm) is shown. As shown in the figure, during operation in the low load region A1, the PCM 100 injects fuel from the injector 15 at a predetermined timing during the latter half of the compression stroke excluding the vicinity of the compression top dead center. The non-equilibrium plasma is discharged from the plasma generation plug 16 from the end of the process to the time when the mixture is ignited.

  More specifically, in the low load range A1, the fuel is injected from the injector 15 in a plurality of times during a period from the time when half of the compression stroke elapses to the time when 3/4 of the compression stroke elapses. Here, the point in time at which half of the compression stroke has elapsed means BTDC 90 ° CA advanced by 90 ° from the compression top dead center (TDC on the right side in FIG. 12) (“° CA” indicates the crank angle. ), And the time when 3/4 of the compression stroke has elapsed means BTDC 45 ° CA advanced by 45 ° from the compression top dead center. In other words, in the present embodiment, the injection operation by the injector 15 is controlled so that the first split injection starts after BTDC 90 ° CA and the last split injection ends by BTDC 45 ° CA. In the example of FIG. 12, the number of fuel split injections is two, and the first injection start timing is slightly retarded from BTDC 90 ° CA, and the second injection end timing is slightly advanced from BTDC 45 ° CA. Each is set on the corner side. In the following, the above-described period during which fuel injection is performed (the period from BTDC 90 ° CA to BTDC 45 ° CA) may be referred to as “１ ／ to ／ of the compression stroke”.

  Discharge of the non-equilibrium plasma from the plasma generating plug 16 is performed from the time when ／ of the compression stroke has elapsed (BTDC 45 ° CA) to the time when the air-fuel mixture is ignited. In the present embodiment, at the time of operation in the low load range A1, the air-fuel mixture ignites at the latest at ATDC5 ° CA which is delayed by 5 ° from the compression top dead center. For this reason, the latest time to end the plasma discharge is set to ATDC5 ° CA. In other words, in the present embodiment, the discharge operation by the plasma generation plug 16 is controlled so that the plasma discharge is started and ended at least from BTDC 45 ° CA to ATDC 5 ° CA. In the example of FIG. 12, the discharge of the non-equilibrium plasma from the plasma generation plug 16 is continuously performed from BTDC 25 ° CA to the compression top dead center (ATDC 0 ° CA), and almost at the compression top dead center (plasma discharge). At about the same time as the end of the mixture) the mixture is ignited. 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%).

  FIG. 13 is a diagram for explaining the behavior of the fuel injected from the injector 15 during operation in the low load range A1. In FIG. 13, for convenience, the plasma generating plug 16 is not shown, and the injector 15 located on the front side of the drawing is illustrated at the original position of the plasma generating plug 16. As shown in the figure, the fuel injected from the injector 15 during 1/2 to 3/4 of the compression stroke is introduced into the cavity C of the piston 5 and stays inside 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 disposed at a relatively late timing of 1/2 to 3/4 of the compression stroke ( That is, when the fuel is injected in a cone shape from the injector 15 (in a state where the injector 15 is relatively close to the piston), the injected fuel is introduced into the cavity C before sufficiently expanding in the radial direction (see FIG. a)). Moreover, in the low load region A1 where the required torque is low, the injection amount of the fuel is small and the penetration (penetration force) of the spray is weak, so most of the fuel once introduced into the cavity C leaks out of the cavity C. Without stopping, it stays in the cavity C until the compression top dead center (immediately before ignition). Thus, in the low load region A1, 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 (see FIG. 13B). ).

  In particular, in this embodiment, the fuel is injected in a plurality of times (for example, two times) in 1/2 to 3/4 of the compression stroke, so that the same amount of fuel is injected in one shot. Thus, the penetration of the spray becomes weaker. This means that the amount of fuel leaking to the outside of the cavity C is extremely small, that is, the above-described distribution of the air-fuel mixture (a state in which the air-fuel mixture is formed only in the cavity C) is further obtained. Means easier.

  As described above, during the operation in the low load range A1, fuel is basically supplied only to the inside of the cavity C, so that the air-fuel ratio (A / F) of the air-fuel mixture in the entire combustion chamber 6 is considerably lean. Become. Specifically, in the low load region A1, 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 (14.7). In other words, in the low load range A1, the throttle valve 54 is opened to a relatively 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 low load region A1, the EGR valve 73 is opened, and a predetermined amount of EGR gas is introduced into the combustion chamber 6. Accordingly, in the low load region A1, 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), is increased. , 35 or more.

FIG. 14 is a diagram schematically showing a state in the combustion chamber 6 when the non-equilibrium plasma is discharged from the plasma generation plug 16. As shown in this figure, the non-equilibrium plasma discharged from the discharge electrode 33 of the plasma generating plug 16 is guided to the opening edge C1 of the cavity C having low insulation because it is not covered by the heat shield layer 19. Here, in the low load region A1, as described above, the non-equilibrium plasma is discharged after the fuel injection is completed and before the air-fuel mixture is ignited (at least between BTDC 45 ° CA and ATDC 5 ° CA). Therefore, at the time of this discharge, an air-fuel mixture already exists inside the non-equilibrium plasma (that is, inside the cavity C). On the other hand, on a discharge path connecting the discharge electrode 33 and the opening edge C1 of the cavity C, a lean environment in which the concentration of fuel is sufficiently low is realized, so that the non-equilibrium plasma discharge causes ozone (O ₃ ) or the like. Active species such as OH are generated. This active species promotes the ignition and combustion of the gas mixture present inside the non-equilibrium plasma, particularly the gas mixture located near the non-equilibrium plasma (the gas mixture on the outer peripheral side).

  After the plasma discharge by the plasma generating plug 16, the air-fuel mixture is ignited with little time interval (for example, near the compression top dead center), and compression ignition combustion is started. At this time, since the combustion is promoted by the presence of the above-described active species, the combustion speed of the air-fuel mixture is increased in spite of the lean environment of λ> 2, and the combustion is performed in a relatively short time. Will end.

(B) Control in high load range Although not shown in detail, in the high load range A2 where the engine load is higher than the low load range A1 (that is, the required amount of fuel increases), the fuel injection from the injector 15 is not performed. The non-equilibrium plasma is discharged from the plasma generation plug 16 in the intake stroke as well as in the compression stroke, and near the compression top dead center and before the mixture is ignited.

  For example, in the high load region A2, fuel is injected from the injector 15 at a predetermined time during the intake stroke and at a time corresponding to 1/2 to 3/4 of the compression stroke. As a result, in the high load region A2, an air-fuel mixture is formed both inside and outside the cavity C.

  Further, in the high load region A2, the non-equilibrium plasma is discharged from the plasma generation plug 16 between the BTDC 5 ° CA and the ATDC 5 ° CA. The non-equilibrium plasma discharged in the vicinity of the compression top dead center leads to the generation of active species such as ozone and OH. This active species is generated by the reverse squish flow (piston 5 along the intake-side / exhaust-side inclined surfaces 24, 25, and moves mainly to the outer peripheral portion of the combustion chamber 6. The moved active species promotes the combustion of the air-fuel mixture on the outer peripheral portion of the combustion chamber 6 where the combustion speed tends to be low due to the low temperature, and contributes to shortening the combustion period of the air-fuel mixture in the entire combustion chamber 6. .

  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 range A2 and the low load range A1 (in all operating ranges of the engine), the control of compression ignition combustion of the air-fuel mixture is performed under a lean environment of λ> 2. . 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.

(4) Operational Effects As described above, in the present embodiment, when the engine is operated in the low load range A1, the injector 15 is formed such that the air-fuel mixture is formed only in the cavity C of the piston 5 in a limited manner. And the non-equilibrium plasma is discharged from the discharge electrode 33 of the plasma generation plug 16 toward the opening edge C1 of the cavity C after the fuel injection by the injector 15 and before the mixture in the cavity C ignites. Thus, the plasma generation plug 16 is controlled. According to such a configuration, 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.

  That is, in the above-described embodiment, after the fuel is injected by the injector 15 and before the mixture in the cavity C ignites, the opening edge of the cavity C is cut from the plasma generation plug 16 located at the center of the ceiling surface 28 of the combustion chamber 6. Since the non-equilibrium plasma is discharged toward C1, the combustion of the outer peripheral portion of the air-fuel mixture in the cavity C is promoted by active species such as ozone and OH generated by the action of the non-equilibrium plasma. Can cause a peculiar combustion in which it burns before the center. The outer peripheral portion of the air-fuel mixture has a larger volume than the central portion, but tends to have a lower temperature. For this reason, by burning the air-fuel mixture from the outer periphery first, it is possible to secure a large amount of heat generation in a short period of time without causing a remarkable pressure increase and accompanying excessive combustion noise.

  On the other hand, since the central portion of the air-fuel mixture burns with a delay toward the outer peripheral portion, the lowering speed of the piston 5 during this central portion combustion is considerably high. However, since the temperature of the center of the air-fuel mixture is originally high, the center of the air-fuel mixture is maintained even under such a situation where the piston 5 is rapidly lowered (therefore, the combustion chamber 6 is rapidly expanding). Burns at a relatively fast rate. As a result, it is possible to avoid a large decrease in the combustion speed in the latter half of the combustion, to burn the entire mixture in a relatively short period of time, and to obtain sufficiently high thermal efficiency.

  FIG. 15 is a diagram for explaining the above operation and effect. In FIG. 15, the waveform of the heat generation rate when the air-fuel mixture is burned by the method of the above embodiment is indicated by W1. As shown in the waveform W1, when the air-fuel mixture is burned first from the outer peripheral portion using the non-equilibrium plasma (active species) (that is, according to the method of the above embodiment), the upper limit in consideration of the combustion noise is used. Below the "combustion noise limit" line, which is the heat generation rate (i.e., within a range where excessive combustion noise does not occur), a sufficiently peaky combustion waveform that rises and falls relatively rapidly can be obtained. 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. 15 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 the active species is not supplied to the outer peripheral portion of the air-fuel mixture, the central portion of the air-fuel mixture having a relatively high temperature burns first, and then the outer peripheral portion of the air-fuel mixture becomes Since the fuel is burned, it is inevitable that the burning speed in the latter half of the burning decreases, and the burning period becomes longer as a whole. That is, since the center of the mixture having a high temperature burns first, the combustion speed in the first half of combustion becomes sufficiently high, but since the volume of the center is smaller than that of the outer periphery, the amount of heat generation itself is small. Does not become so large. The outer peripheral portion of the air-fuel mixture having a large volume finally burns in the latter half of the combustion, but at this time, the temperature of the outer peripheral portion of the air-fuel mixture is originally low because the piston 5 has been lowered (expansion of the combustion chamber 6). Combined with this, the combustion rate must be greatly reduced. As described above, when the plasma discharge (supply of the active species) is not performed, the outer peripheral portion of the low-temperature and large-volume air-fuel mixture is burned with a delay, and as a result, the combustion period is prolonged as a whole and the thermal efficiency is reduced. It is understood to invite.

  If the active species is supplied to the entire air-fuel mixture to promote the combustion of both the central part and the outer peripheral part of the air-fuel mixture, for example, as shown by a waveform W3 in FIG. It is possible to further shorten the combustion period and increase the thermal efficiency. However, in this case, the combustion at the center of the air-fuel mixture is likely to be sharper due to the originally high temperature, and the heat generation rate in the early stage of the combustion rises sharply as the combustion noise limit is exceeded. turn into. 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, the combustion speed can be increased as much as possible within a range in which the combustion noise does not become excessive, and therefore, the merchantability and the thermal efficiency can be compatible at a high level.

  Further, in the above embodiment, during operation in the low load range A1, the excess air ratio λ in the combustion chamber 6 is set to be larger than 2, so that the combustion temperature of the air-fuel mixture can be significantly reduced, and the air-fuel ratio associated with combustion can be reduced. 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 above-described embodiment in which the air-fuel mixture is burned in a significantly lean environment of λ> 2, high-temperature combustion exceeding the NOx limit occurs. This can be avoided, and the NOx generation amount can be reduced so that the NOx catalyst can be made unnecessary.

  Further, in the above-described embodiment, fuel is supplied from the injector 15 disposed near the center of the ceiling surface 28 of the combustion chamber 6 in a period of ／ to ／ of the compression stroke (between BTDC 90 ° CA and BTDC 45 ° CA). Since the fuel is injected in a cone shape (radial shape), the injected fuel can be introduced into the cavity C before the spray of the fuel spreads sufficiently in the radial direction, and the air-fuel mixture exists only in the cavity C. (A state in which all or most of the air-fuel mixture exists in the cavity C) can be appropriately created.

  In particular, in the above-described embodiment, the fuel is injected in a plurality of times in 1/2 to 3/4 of the compression stroke, so that the penetration (penetration force) of the spray in each injection can be reduced. It is possible to properly maintain the distribution of the air-fuel mixture (a state in which the air-fuel mixture is formed only in the cavity C) until immediately before the ignition of the air-fuel mixture.

  Further, in the above embodiment, the non-equilibrium plasma is discharged from the plasma generating plug 16 during a period from the lapse of ／ of the compression stroke (BTDC 45 ° CA) which is the latest timing of the end of the fuel injection until the fuel-air mixture ignites. Therefore, immediately before the mixture in the cavity C ignites, the active species generated by the action of the non-equilibrium plasma can be appropriately supplied to the outer periphery of the mixture, and the outer periphery of the mixture is burned first. The above-described combustion mode can be reliably realized.

(5) Modification In the above-described embodiment, during operation in the low-load region A1, the mixture is formed only in the cavity C (that is, all or most of the mixture exists in the cavity C). The fuel is injected from the injector 15 during 1/2 to 3/4 of the compression stroke so that the air-fuel mixture hardly exists outside the cavity C. However, at least the fuel is formed inside the cavity C. The fuel may be injected such that the air-fuel ratio of the air-fuel mixture is smaller (richer) than the air-fuel ratio outside the cavity C (a region radially outside the opening edge C1 of the cavity C). May be different from that of the above embodiment. That is, as long as a rich air-fuel mixture is formed inside the cavity C, the position of the injector 15 and the fuel injection timing from the injector 15 may be appropriately changed.

  In the above 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. (See FIG. 3), on the contrary, 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.

  In the above embodiment, the plasma generating plug 16 has a plurality of discharge electrodes 33 radially protruding from the tip of the center electrode 32 (see FIG. 5). Instead, for example, FIG. As shown in (b), a plasma generating plug 116 having a shallow dish-shaped discharge electrode 133 may be used.

  More specifically, the plasma generating plug 116 according to the above-described modification includes a cylindrical plug main body 131, a center electrode 132 inserted into the plug main 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 an open-open type in which a ring-shaped nozzle port 44 is formed when the valve is opened (see FIG. 6). 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 compresses and ignites fuel containing gasoline as a main component while mixing 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 S (Piston) crown surface C (Piston) cavity C1 (Cavity) opening edge 15 Injector 16, 116 Plasma generating plug 33, 133 Discharge electrode 54 Throttle valve 100 PCM (Control device)
A1 Low load area

Claims (6)

An injector for injecting fuel into the combustion chamber,
A piston having a crown defining the bottom surface of the combustion chamber and having a cavity formed in the center of the crown,
A plasma generation plug having an electrode near the center of the ceiling of the combustion chamber overlapping the cavity in plan view and discharging non-equilibrium plasma from the electrode;
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 control device controls the injector so that at least a part of the fuel injected from the injector remains in the cavity and a rich mixture is formed in the cavity than outside the cavity, and the injector is controlled by the injector. Controlling the plasma generating plug so that non-equilibrium plasma is discharged from the electrode of the plasma generating plug toward the opening edge of the cavity before the fuel-air mixture in the cavity is ignited after the fuel injection by And a premixed compression ignition engine. The premixed compression ignition engine according to claim 1,
The control device discharges non-equilibrium plasma to the plasma generating plug from a point in time when 3/4 of a compression stroke has elapsed to a point in time when the air-fuel mixture in the cavity is ignited. Expression engine. The premixed compression ignition engine according to claim 1 or 2,
The control device forms a relatively rich mixture in the cavity and generates the non-equilibrium plasma when the engine is operated in a low load region where the engine load is less than a predetermined load. A premixed compression ignition engine. The premixed compression ignition engine according to claim 3,
The injector radially injects fuel toward the cavity from near the center of the ceiling of the combustion chamber,
The control device determines that 3/4 of the compression stroke starts after 1/2 of the compression stroke has elapsed so that most of the fuel injected from the injector remains in the cavity until the non-equilibrium plasma is discharged. A premixed compression ignition engine, characterized in that fuel is injected into the injector until a lapse of time. The premixed compression ignition engine according to claim 4,
The control device controls the injector such that fuel is injected in a plurality of times from a point in time when 1/2 of the compression stroke has elapsed to a point in time when 3/4 of the compression stroke has elapsed. Features a homogeneous charge compression ignition engine. The homogeneous charge compression ignition engine according to any one of claims 3 to 5,
Further comprising a throttle valve for adjusting the flow rate of the air introduced into the combustion chamber,
The control device controls the throttle valve and the injector such that an excess air ratio in the combustion chamber becomes larger than 2 during an operation in the low load range, wherein the engine is a premixed compression ignition engine. .

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