JP5440540B2 - gasoline engine - Google Patents

Description

  The present invention includes an injector capable of supplying fuel, at least part of which is made of gasoline, in a fuel chamber formed in a cylinder, and burns an air-fuel mixture of the fuel and air in the combustion chamber by self-ignition in a specific operation region. This relates to a gasoline engine that reciprocates a piston.

  Conventionally, in the gasoline engine field, a combustion mode (spark ignition combustion) in which an air-fuel mixture is forcibly ignited by spark ignition of a spark plug has been common, but in recent years, instead of such spark ignition combustion, so-called Research is underway to apply compression auto-ignition combustion to gasoline engines. The compression self-ignition combustion is to compress the air-fuel mixture generated in the combustion chamber (inside the cylinder) with a piston, and to self-ignite the air-fuel mixture regardless of spark ignition in a high temperature / high pressure environment. Compressed self-ignition combustion is combustion in which self-ignition occurs at various points in the combustion chamber at the same time, and it is said that high thermal efficiency can be obtained compared to combustion by spark ignition.

  As a specific example of the gasoline engine to which the compression self-ignition combustion is applied, for example, one disclosed in Patent Document 1 below is known. In Patent Document 1, the geometric compression ratio of the cylinder is set to 14 or more. It is disclosed that the air-fuel mixture is raised to a temperature at which self-ignition is possible.

JP 2007-154859 A

  There is still a high demand for improvement in thermal efficiency from the viewpoint of fuel efficiency, and there is a need to further increase thermal efficiency by, for example, reliable realization of compression self-ignition combustion.

  This invention is made | formed in view of the above situations, and it aims at providing the gasoline engine which can improve thermal efficiency more.

  In order to solve the above-described problems, the present invention includes an injector capable of supplying fuel, at least part of which is made of gasoline, in a fuel chamber formed in a cylinder, and the fuel and air in the combustion chamber in a specific operation region of the engine. A gasoline engine that reciprocates the piston by burning the mixture with self-ignition, the geometric compression ratio of the cylinder is set to 14 or more, and the ceiling surface of the combustion chamber has a diameter of It has a conical surface shape that is inclined so that its height decreases from the center in the direction toward the outer side in the radial direction, and the crown surface of the piston has a cavity curved in a concave shape with a predetermined curvature at the center. And extending parallel to the ceiling surface of the combustion chamber so as to decrease in height as it goes radially outward from the opening edge of the cavity. The injector has a tip portion formed with an injection hole capable of injecting fuel toward the crown side of the piston, and the tip portion is located near the top of the ceiling surface of the combustion chamber. A gasoline engine characterized by being provided is provided (claim 1).

  According to the present invention, the cylinder can have a high compression ratio, and the amount of heat released from the mixture to the wall of the combustion chamber, i.e., the cooling loss, can be suppressed to a low level. The temperature can be increased to a possible temperature, and the thermal efficiency can be further increased as the cooling loss is reduced. That is, the volume of the combustion chamber at the compression top dead center decreases with a high compression ratio. However, in the present invention, the cavity is formed in the radial center of the crown surface of the piston. The clearance between the combustion chamber ceiling surface and the piston crown surface is secured, and most of the air-fuel mixture can be burned in this cavity while being separated from the wall surface of the combustion chamber. It can be kept smaller. In addition, the air-fuel mixture can be burned efficiently while suppressing the adhesion of fuel to the combustion chamber wall surface.

  In addition, the ceiling surface of the combustion chamber has a conical surface, and the valve area of the intake and exhaust valves provided on the ceiling surface, as well as the intake and exhaust amounts, can be secured, and the cavity on the crown surface of the piston can be secured. Since the portion on the outer side in the radial direction (hereinafter sometimes referred to as the outer peripheral portion of the piston crown surface) is configured in parallel with the conical surface of the combustion chamber, the diameter of the combustion chamber is larger than that of the cavity. The volume of the outer part in the direction (hereinafter sometimes referred to as the outer peripheral part of the combustion chamber) and the clearance dimension between the ceiling surface of the combustion chamber and the piston crown surface are secured to achieve proper combustion of the air-fuel mixture in this outer part. However, S / V_TDC (SV ratio), which is the ratio of the combustion chamber surface area S to the combustion chamber volume V_TDC at the compression top dead center, can be reduced, and the amount of heat released to the combustion chamber wall surface can be kept small. . Specifically, when the volume and clearance size of the outer peripheral portion of the combustion chamber are set to predetermined amounts necessary for combustion under conditions where the combustion chamber diameter and compression ratio are constant, respectively, the combustion chamber ceiling surface and piston crown surface When the outer peripheral part of the piston is conical, the length of the outer peripheral part of the piston crown surface can be made longer than when the outer peripheral part is made spherical, and the radial length of the cavity is accordingly increased. Therefore, under the condition where the cavity volume is constant, the curvature of the cavity can be increased and the SV ratio can be reduced.

  In the present invention, the injector is disposed in a portion of the combustion chamber before the compression top dead center and injected fuel at a portion radially outside the cavity in at least a part of the specific operation region. The fuel is injected in multiple steps including a front injection that injects fuel at the arrival timing and a rear injection that injects fuel at a timing after the preceding injection and the injected fuel arrives in the cavity. (Claim 2).

  In this way, an air-fuel mixture is formed separately inside and outside the cavity, so that the air-fuel mixture is prevented from being mixed and combusted at the same time. It is possible to effectively prevent an increase in combustion noise due to a sudden rise in pressure and a large amount of soot due to local oxygen shortage.

  Further, in the present invention, an intake port that communicates with the combustion chamber via a plurality of intake side openings formed on a ceiling surface of the combustion chamber, and a valve body that can open and close the plurality of intake side openings, respectively. A plurality of intake valves each including a stem extending from the valve body and moving the valve body in the opening / closing direction of the intake side opening; and a plurality of exhaust side openings formed on a ceiling surface of the combustion chamber. An exhaust port communicating with the combustion chamber, a valve body that can open and close the plurality of exhaust side openings, and a stem that extends from the valve body and moves the valve bodies in the opening and closing direction of the exhaust side opening, respectively. And at least one of the plurality of intake valves and the plurality of exhaust valves has a combustion chamber side surface of each valve body and a ceiling surface of the conical surface of the combustion chamber Extending parallel to the tangent surface, and each stem is It is preferably arranged so as to extend in the radial direction of a circle centered on the top of the ceiling surface of the combustion chamber in a plan view when the piston crown surface is viewed from the top of the ceiling surface of the combustion chamber. 3).

  In this way, in the state where the intake valve or the exhaust valve is closed, the ceiling surface of the combustion chamber can be made smoother and the surface area of the combustion chamber ceiling surface can be made smaller.

In the above configuration, a spark plug capable of igniting the air-fuel mixture in the combustion chamber is provided,
The plurality of exhaust valves are adjacent to each other, a surface of each valve body on the combustion chamber side extends in parallel with a contact surface with a ceiling surface of the conical combustion chamber, and each stem thereof, In plan view, it is preferably arranged so as to extend in the radial direction of a circle centering on the top of the ceiling surface of the combustion chamber, and the spark plug is preferably arranged between the exhaust valves ( Claim 4).

  According to this configuration, while the spark plug is disposed between the exhaust valves, the exhaust valve body extends in parallel with the contact surface of the ceiling surface of the combustion chamber. An opening area can be ensured.

  Further, each of the plurality of intake valves has a valve body extending in parallel with a contact surface with the ceiling surface of the conical combustion chamber, and each stem has a piston crown from the top of the ceiling surface of the combustion chamber. In plan view of the combustion chamber, the exhaust chamber is disposed so as to extend in the radial direction of a circle centered on the top of the ceiling surface of the combustion chamber, and the opening area of the intake side opening is the exhaust side opening It is preferable that the opening area is set to be larger than the opening area.

  In this way, the area of the valve body of the intake valve and thus the opening area of the exhaust side opening can be increased, and the amount of intake air flowing into the combustion chamber and thus the engine torque can be ensured.

  As described above, according to the present invention, it is possible to realize a gasoline engine that can further increase thermal efficiency.

It is a figure showing the whole gasoline engine composition concerning one embodiment of the present invention. It is a schematic sectional drawing which expands and shows the combustion chamber periphery shown in FIG. FIG. 3 is a schematic plan view around the combustion chamber shown in FIG. 2. It is the figure which showed typically the combustion chamber shown in FIG. It is the figure which showed typically the combustion chamber of the comparative example of this invention. (A)-(e) It is the graph which compared SV of this invention and the comparative example. It is a block diagram which shows the control system of the said engine. It is a figure which shows an example of the control map for selecting the combustion form according to the driving | running state of an engine. It is a time chart for demonstrating the control content in the 1st driving | operation area | region (A1) of FIG. It is a time chart for demonstrating the control content in the 2nd driving | running area | region (A2) of FIG. It is a time chart for demonstrating the control content in the 3rd driving | running area | region (A3) of FIG. It is a time chart for demonstrating the control content in the 4th driving | running area | region (A4) of FIG. (A)-(f) is a figure for demonstrating typically the fuel injection performed in the said 2nd driving | operation area | region (A2), and combustion of the air-fuel mixture based on it. (A)-(f) is a figure for demonstrating typically fuel injection performed in the said 3rd driving | operation area | region (A3), and combustion of the air-fuel mixture based on it. It is a figure for demonstrating typically in which area | region each of the multistage fuel injection performed in said 3rd operation area | region (A3) burns.

(1) Overall Configuration of Engine FIG. 1 is a diagram showing an overall configuration of an engine according to an embodiment of the present invention. The engine shown in the figure is a reciprocating piston type multi-cylinder gasoline engine mounted on a vehicle as a power source for driving driving. An engine body 1 of this engine includes a cylinder block 3 having a plurality of cylinders 2 (only one of which is shown in the drawing) arranged in a direction orthogonal to the paper surface, and a cylinder head 4 provided on the upper surface of the cylinder block 3. And a piston 5 inserted in each cylinder 2 so as to be slidable back and forth. The fuel supplied to the engine body 1 is mainly composed of gasoline. The fuel only needs to have gasoline as a main component, and the contents thereof may be all gasoline, or may be gasoline containing ethanol (ethyl alcohol) or the like. Hereinafter, as appropriate, the axial direction of the piston 5 and the sliding direction thereof are referred to as the vertical direction, the cylinder head 4 side is referred to as the upper side, and the cylinder block 3 side is referred to as the lower side.

  The geometric compression ratio of the engine body 1, that is, the cylinder 2 is set to a high value of 14 or more for the purpose of improving the theoretical thermal efficiency, stabilizing the CI combustion (compression self-ignition combustion) described later, and the like. The geometric compression ratio is considered to be about 20 from the practical viewpoint. Therefore, the geometric compression ratio is set to an appropriate value in the range of 14 or more and 20 or less.

  The piston 5 is connected to the crankshaft 7 via a connecting rod 8. In response to the reciprocating motion of the piston 5, the crankshaft 7 rotates about its central axis. A combustion chamber 6 is formed above the piston 5.

  The cylinder head 4 is provided with one injector 21 for each cylinder 2 for injecting fuel (fuel mainly composed of gasoline) into the combustion chamber 6 from the tip. This injector 21 is a so-called multi-hole injector. In the present embodiment, the injector 21 has twelve injection holes at its distal end portion I1.

  A fuel supply pipe 23 is connected to the injector 21, and the injector 21 injects fuel supplied through the fuel supply pipe 23. Connected to the upstream side of the fuel supply pipe 23 is a high-pressure fuel pump such as a plunger pump linked to the crankshaft 7, and between the high-pressure fuel pump and the fuel supply pipe 23. Is provided with a common rail for accumulating pressure common to all cylinders. Then, the fuel accumulated in the common rail is supplied to the injectors 21 of the respective cylinders 2, whereby high pressure fuel of 30 MPa or more is injected from each injector 21. Since the upper limit value of the fuel injection pressure is considered to be about 120 MPa from a practical viewpoint, the injection pressure from the injector 21 is set to an appropriate value in the range of 30 MPa to 120 MPa.

  A spark plug 20 for supplying ignition energy to the air-fuel mixture in the combustion chamber 6 is attached to the cylinder head 4, one for each cylinder 2. The spark plug 20 is attached to the cylinder head 4 in such a posture that the ignition point faces the combustion chamber 6. The spark plug 20 discharges a spark from its tip in response to power supply from an ignition circuit (not shown) to ignite the air-fuel mixture in the combustion chamber 6.

  The cylinder head 4 is formed with an intake port 9 and an exhaust port 10 communicating with the combustion chamber 6. That is, the combustion chamber 6 is formed with an intake side opening 61 communicating with the intake port 9 and an exhaust side opening 62 opening with the exhaust port 10. The cylinder head 4 is provided with an intake valve 11 and an exhaust valve 12 for opening and closing the openings 61 and 62, respectively. The illustrated engine is a so-called double overhead camshaft (DOHC) engine. Two intake side openings 61 and two exhaust side openings 62 are provided for each cylinder 2, and two intake valves 11 and two exhaust valves 12 are also provided.

  The intake valve 11 and the exhaust valve 12 have so-called valve bodies 11a and 12a for opening and closing the umbrella-shaped openings 61 and 62 and so-called stems 11b and 12b extending vertically from the valve bodies 11a and 12a. It is a poppet valve.

  The intake valve 11 and the exhaust valve 12 are respectively connected to the stems 11b in conjunction with the rotation of the crankshaft 7 by valve mechanisms 13 and 14 including a pair of camshafts (not shown) disposed in the cylinder head 4. , 12b are driven to open and close the intake side opening 61 and the exhaust side opening 62.

  A CVVL 15 is incorporated in the valve operating mechanism 13 for the intake valve 11. The CVVL 15 is called a continuously variable valve lift mechanism and continuously (steplessly) changes the lift amount of the intake valve 11. The CVVL 15 is provided so that the lift amounts of all the intake valves 11 of the engine can be changed. When the CVVL 15 is driven, the lift amounts of the pair of intake valves 11 in each cylinder 2 are changed simultaneously. It has become.

  The CVVL 15 having such a configuration is already known, and as a specific example thereof, a link mechanism that reciprocally swings the cam for driving the intake valve 11 in conjunction with the rotation of the camshaft, and the arrangement of the link mechanism (lever ratio). And a stepping motor that changes the swing amount of the cam (the amount by which the intake valve 11 is pushed down) by electrically driving the control arm (for example, JP, 2007-85241, A).

  The valve mechanism 14 for the exhaust valve 12 incorporates a VVL 16 that is an ON / OFF type variable valve lift mechanism (variable valve lift mechanism) that enables or disables the function of depressing the exhaust valve 12 during the intake stroke. ing. That is, the VVL 16 has a function of enabling the exhaust valve 12 to be opened not only in the exhaust stroke but also in the intake stroke, and switching between performing or stopping the valve opening operation of the exhaust valve 12 during the intake stroke. Yes. The VVL 16 is provided corresponding to all the exhaust valves 12 of the engine, and can individually perform or stop the valve opening operation during the intake stroke with respect to the pair of exhaust valves 12 of each cylinder 2.

  The VVL 16 having such a configuration is already known, and as a specific example thereof, the exhaust valve 12 is pushed down during the intake stroke separately from a normal cam for driving the exhaust valve 12 (a cam for pushing the exhaust valve 12 during the exhaust stroke). Examples include a sub cam and a so-called lost motion mechanism that enables or disables transmission of the driving force of the sub cam to the exhaust valve 12 (see, for example, JP-A-2007-85241).

  When the exhaust valve 12 is opened during the intake stroke in addition to during the exhaust stroke due to the action of the VVL 16, the high-temperature exhaust flows back from the exhaust port 10 to the combustion chamber 6 during the exhaust stroke, and the combustion chamber 6 A large amount of exhaust remains. That is, a large amount of internal EGR gas is secured. On the other hand, when the exhaust valve 12 is opened only during the exhaust stroke, the internal EGR gas amount is suppressed to a small amount or not.

  An intake passage 28 and an exhaust passage 29 are connected to the intake port 9 and the exhaust port 10 of the engine body 1, respectively.

  The intake passage 28 is provided with a common passage portion 28c composed of a single passage, a surge tank 28b provided at an upstream end portion of the common passage portion 28c, and branched for each cylinder 2. The surge tank 28b And a branch passage portion 28 a that connects the intake port 9 of each cylinder 2.

  The exhaust passage 29 is provided for each cylinder 2 by branching to a common passage portion 29c composed of a single passage, and connects the upstream end of the common passage portion 29c and the exhaust port 10 of each cylinder 2. And a branch passage portion 29a.

  An external EGR device 30 is provided between the intake passage 28 and the exhaust passage 29 to recirculate a part of the exhaust gas passing through the exhaust passage 29 to the intake passage 28. The external EGR device 30 is provided with an EGR passage 31 communicating with the common passage portions 28c and 29c of the intake passage 28 and the exhaust passage 29 and a flow rate of exhaust gas that is provided in the middle of the EGR passage 31 and passes through the EGR passage 31. An EGR valve 32 to be controlled and a water-cooled EGR cooler 33 for cooling the exhaust gas passing through the EGR passage 31 are provided.

  A throttle valve 25 for adjusting the amount of intake air passing through the intake passage 28 is provided in the common passage portion 28 c of the intake passage 28. However, in this embodiment, the lift amount of the intake valve 11 is adjusted by the CVVL 15, the amount of internal EGR gas in the combustion chamber 6 is adjusted by the VVL 16, and further, the return to the intake passage 28 is returned by the external EGR device 30. The amount of exhaust gas is adjusted. Therefore, it is possible to adjust the amount of air (fresh air) introduced into the combustion chamber 6 without operating the throttle valve 25 based on these operations. For this reason, the throttle valve 25 is kept fully open or close to it except when the engine is stopped.

  A catalyst converter 35 for purifying exhaust gas is provided in the common passage portion 29 c of the exhaust passage 29. For example, a three-way catalyst is incorporated in the catalytic converter 35, and harmful components in the exhaust gas passing through the exhaust passage 29 are purified by the action of the three-way catalyst.

Various sensors are attached to the engine body. For example, a water temperature sensor SW1 for detecting the temperature of the engine coolant, a rotation angle (crank angle) of the crankshaft 7, and a crank angle sensor SW2 for detecting the engine speed, and the camshaft angle are detected to detect the cylinder temperature. A cam angle sensor SW3 that outputs a signal for determination (determination of whether each cylinder is in an intake stroke, compression stroke, expansion stroke, or exhaust stroke) is attached to the engine body.
(2) Detailed Structure of Combustion Chamber FIG. 2 is a schematic sectional view around the combustion chamber 6. FIG. 3 is a schematic plan view of the combustion chamber 6 as viewed from above. As shown in FIG. 2 and the like, a portion of the ceiling surface 60 of the combustion chamber 6, that is, the bottom surface of the cylinder head 4, which faces the piston 5 is radially outward with the center in the radial direction, that is, the point on the axis u 1 of the cylinder 2. It has a conical surface shape whose height decreases as it goes to.

  The injector 21 is disposed such that its tip end is located near the top of the combustion chamber ceiling surface 60 (slightly below this top) and its axis coincides with the axis u 1 of the cylinder 2. The injection holes 21a of the injectors 21 are formed at equal intervals on a circumference centered on the axis 21 of the injector 21, that is, the cylinder 2. The axis of each injection hole 21a, that is, the opening direction of each injection hole 21a is radially outward. It faces diagonally downward (the crown side of the piston 4). When fuel is injected from each injection hole 21 a of the injector 21, the fuel spreads radially downward and radially outward substantially parallel to the ceiling surface 60 of the combustion chamber 6.

  As shown in FIG. 3, the intake side opening 61 and the exhaust side opening 62 are opened on the ceiling surface 60 of the combustion chamber 6 side by side in the circumferential direction. The two intake side openings 61 and the two exhaust side openings 62 are provided on both sides of a straight line passing through the center of the ceiling surface 60 of the combustion chamber 6. In the example shown in FIG. 3, the two intake side openings 61 are provided on the right side of the combustion chamber ceiling surface 60, and the two exhaust side openings 62 are provided on the left side of the combustion chamber ceiling surface 60. . The opening area of the intake side opening 61 is set to be larger than the opening area of the exhaust side opening 62, and more intake air is introduced into the combustion chamber 6 so that high engine torque can be obtained. It is configured.

  Returning to FIG. 2, the intake valve 11 and the exhaust valve 12 are such that the planar valve surface 11 c on the combustion chamber 6 side of each valve body 11 a, 12 a is in contact with the conical combustion chamber ceiling surface 60. It is arrange | positioned so that it may extend along. Accordingly, the stems 11b and 12b of the valves 11 and 12 extend in a vertical direction from the conical surface of the combustion chamber ceiling surface 60, respectively. That is, as shown in FIG. 3, these stems 11 b and 12 b extend radially in the radial direction of a circle centered on the top of the combustion chamber ceiling surface 60 in a plan view as viewed from the direction of the axis u <b> 1 of the cylinder 2. . As described above, the valve surfaces 11c and 12c of the valves 11 extend along the contact surface of the combustion chamber ceiling surface 60, so that when the valves 11 are closed, the combustion chambers including the valve surfaces 11c and 12c are included. The ceiling surface of 6 has a shape in which unevenness is suppressed to a small size.

  The spark plug 20 is disposed between the exhaust valves 12 in the cylinder head 9. As described above, the opening area of the intake valve 11 is set larger than that of the exhaust valve 12. Therefore, by arranging the spark plug 20 on the exhaust valve 12 side having a small opening area in this way, the opening area of the intake valve 11 and thus the engine torque is secured while securing the mounting position of the spark plug 20.

  Returning to FIG. 2, the crown surface of the piston 5 that constitutes the bottom surface of the combustion chamber 6 is formed at the center portion thereof and has a cavity 40 that is concavely curved downward with a predetermined curvature, and an opening edge of the cavity 40. The reference surface 41 is inclined downwardly from 40a toward the radially outer side.

  The inner peripheral surface 40b of the cavity 40 has a shape that forms a part of a spherical surface centered on a point on the axis 2 of the cylinder 2 and the injector 21, and the cavity 40 and the injector 21 face each other. Yes. The reference surface 41 extends in parallel with the conical surface of the combustion chamber ceiling surface 60. That is, the crown surface of the piston 5 has a shape in which the cavity 40 is formed at the center of a conical surface extending parallel to the combustion chamber ceiling surface.

  The spark plug 20 is attached to the cylinder head 9 so that the ignition point is radially inward of the opening edge 40a at the upper end of the cavity 40.

  The reason why the ceiling surface 60 and bottom surface of the combustion chamber 6, that is, the crown surface of the piston 5, is formed as described above is as follows.

  In order to secure the opening area of the intake side opening 61 and the exhaust side opening 62 and thus the amount of intake air flowing into the combustion chamber 6 and the amount of exhaust exhausted from the combustion chamber 6, these openings 61, 62 are provided. The open combustion chamber ceiling surface 60 needs to have a shape that protrudes upward, that is, a shape that decreases in height as it goes radially outward with the radial center as the top.

  In order to further increase the geometric compression ratio of the cylinder 2, that is, to reduce the volume of the combustion chamber 6 at the compression top dead center, with the combustion chamber ceiling surface 60 having a convex shape as described above, a piston is used. It is necessary to make the shape of the crown 5 convex upward and to extend in parallel with the combustion chamber ceiling surface 60.

  However, if the entire crown surface of the piston 5 is simply extended in parallel with the combustion chamber ceiling surface 60, the vertical height of the combustion chamber 6 at the compression top dead center, that is, the ceiling surface 60 of the combustion chamber 6 and the piston 5. The overall clearance between the crown surface of the cylinder and the combustion chamber is shortened as a whole, so that the combustion does not spread properly. In addition, when performing multistage CI combustion, which will be described later, the air-fuel mixture based on each injection cannot be separated and burned. The problem arises.

  Therefore, in the present gasoline engine, the portion on the radially outer side of the crown surface of the piston 5 is constituted by the reference surface 41 parallel to the combustion chamber ceiling surface 60 as described above. By forming the hollow cavity 40, the air-fuel mixture is combusted mainly in the cavity 40 to achieve proper combustion, and the air-fuel mixture based on each injection in the multi-stage CI combustion is separated to burn them simultaneously. I try to suppress this. If the cavity 40 is provided in this way, in addition to the above-described effect, the cooling loss can be further suppressed by reducing the portion of the combustion chamber 6 having a relatively low temperature, that is, the portion contacting the wall surface of the cylinder 3. .

  However, it is necessary that proper combustion is performed also in the portion 6b radially outside the cavity 40 in the combustion chamber 6 (hereinafter, the cavity 40 is formed at the center in the radial direction in the combustion chamber 6 as appropriate). This portion is simply referred to as the central portion 6a of the combustion chamber 6, and the radially outer portion of the central portion 6a is simply referred to as the outer peripheral portion 6b of the combustion chamber 6). Specifically, as will be described later, in multi-stage CI combustion, a part of the injection amount corresponding to the engine torque is injected separately into the cavity 40 and the part radially outside the cavity 40, and the combustion chamber 6 Appropriate combustion that contributes to the engine torque is required in the outer peripheral portion 6b in addition to the central portion 6a. Even in the case of combustion by spark ignition, in order to obtain high fuel efficiency, combustion needs to be completed in a short time in the outer peripheral portion 6b of the combustion chamber 6. Therefore, in this gasoline engine, the clearance between the reference surface 41 and the combustion chamber ceiling surface 60 radially outside the cavity 40 of the crown surface of the piston 5 is provided at the compression top dead center while providing the cavity 40. A predetermined amount of the dimension d (see FIG. 4) is secured, and the volume V2 (see FIG. 4) of the region sandwiched between these is secured. For example, in the multistage CI combustion, in order to realize proper combustion in the outer peripheral portion 6b of the combustion chamber 6, the clearance dimension d is required to be about 2.4 mm at the compression top dead center, and the volume thereof is 11 cc to 11 cc. About 15cc is required.

  Here, in order to reduce the cooling loss and increase the thermal efficiency, it is necessary to reduce the SV ratio (S / V_TDC), which is the ratio of the surface area S of the combustion chamber 6 to the total volume V_TDC of the combustion chamber 6 at the compression top dead center. There is. Generally, it is said that the SV ratio is smaller when the ceiling surface 60 of the combustion chamber 6 is a spherical surface than when the ceiling surface 60 is a planar surface. However, as a result of intensive studies, the inventors have formed the cavity 40 in the center of the crown surface of the piston 5 as described above, and the radially outer portion of the crown surface of the piston 5 (hereinafter referred to as appropriate). The portion of the crown surface of the piston 5 that is radially outward from the cavity 40 is simply referred to as the outer peripheral portion of the crown surface of the piston 5) 41 and parallel to the ceiling surface 60 of the combustion chamber 6, and the outer periphery of the combustion chamber 6. Under the condition that a predetermined amount of the clearance d and the volume V2 of the portion 6b is ensured, the combustion chamber ceiling surface 60 and the outer peripheral portion 41 of the crown surface of the piston 5 parallel thereto are more conical than the spherical surface. It has been found that the SV ratio becomes smaller when it is configured.

  That is, under the above-described conditions, the combustion chamber ceiling surface 60 as shown in FIG. 4 and the outer peripheral portion 41 of the crown surface of the piston 5 parallel to the combustion chamber have a conical surface as shown in FIG. The radial length L2 of the opening edge 40a at the upper end of the cavity 40 as compared with the case where the radially outer portion of the crown surface of the piston 5 parallel to the chamber ceiling surface 60 is a spherical surface (see FIGS. 4 and 5). Since the curvature of the cavity 40 (1 / r and r are shown in FIGS. 4 and 5) can be increased, the SV ratio is reduced. 4 and 5 are diagrams schematically showing the structure of the combustion chamber 6 in order to compare the difference in shape.

  Specifically, when the clearance dimension d and the volume V2 of the outer peripheral portion 6b of the combustion chamber 6 are constant, the outer peripheral portion 41 of the combustion chamber ceiling surface 60 and the crown surface of the piston 5 is conical. The radial length L1 of the outer peripheral portion 6b of the combustion chamber 6 is longer than that of the spherical surface. Therefore, when the volume V2 of the outer peripheral portion 6b of the combustion chamber 6 is constant, the length L2 in the radial direction of the opening edge 40a at the upper end of the cavity 40 becomes smaller when the respective parts are formed in a conical surface shape. Under the condition that the volume of the cavity 40 is constant, the curvature of the cavity 40 (1 / r, r for FIGS. 4 and 5) increases.

  FIGS. 6A to 6E show a case where the ceiling surface 60 of the combustion chamber 6 and the outer peripheral portion 41 of the crown surface of the piston 5 are conical (under solid lines) under the above conditions, and these are spherical. The results obtained by examining the SV ratio of the combustion chamber 6 are shown in FIG. 6 (a) to 6 (e), the horizontal axis represents the ratio (V1 / V_TDC) of the volume V1 of the cavity 40 to the volume V_TDC of the entire combustion chamber 6 at the compression top dead center, and the vertical axis represents the combustion chamber. SV ratio of 6 as a whole. 6 (a) to 6 (e) show the horizontal plane of the combustion chamber ceiling surface 60 (with the axis u1 of the cylinder 2) so that the opening areas of the intake side opening 61 and the exhaust side opening 62 are constant. FIG. 4 is a comparison result when the inclination angle α with respect to the parallel plane is constant (in the case of a spherical shape, the inclination angle of the tangent to the combustion chamber ceiling surface 60 with respect to the horizontal plane, see FIGS. 4 and 5). Specifically, FIG. 6A shows the result when the tilt angle α is 10 degrees, FIG. 6B shows the result when the tilt angle α is 20 degrees, and FIG. FIG. 6D shows the result when the tilt angle α is 30 degrees, FIG. 6D shows the result when the tilt angle α is 40 degrees, and FIG. 6E shows the result when the tilt angle α is 50 degrees. 6A to 6E, the displacement is 2000 cc, the geometric compression ratio is 20, the bore diameter of the cylinder 2 is 86 mm, and the clearance d of the outer peripheral portion of the combustion chamber 6 is d = 2.4 mm. Is the result of the case. Here, the ratio (V1 / V_TDC) of the volume V1 of the cavity 40 to the volume V_TDC of the entire combustion chamber 6 at the compression top dead center is preferably 7/10 to 9/10.

  As shown in FIGS. 6 (a) to 6 (e), the combustion chamber ceiling surface 60 and the outer peripheral portion 41 of the crown surface of the piston 5 are conical. The SV ratio is smaller than in the case of FIG. When α is 20 degrees or less, the difference in SV ratio is very small. To obtain the effect of reducing the SV ratio by making the combustion chamber ceiling surface 60 conical, α should be 20 degrees or more. Is preferred.

(3) Control System FIG. 7 is a block diagram showing an engine control system. The ECU 50 shown in the figure is a device for comprehensively controlling each part of the engine, and includes a well-known CPU, ROM, RAM, and the like.

  Various information is input to the ECU 50 from various sensors such as the water temperature sensor SW1, the crank angle sensor SW2, and the cam angle sensor SW3 provided in the engine body. The ECU 50 also receives information from various sensors provided in the vehicle. For example, information on the accelerator opening is input to the ECU 50 from an accelerator opening sensor SW4 that detects the opening of an accelerator pedal (not shown) that is depressed by the driver.

  The ECU 50 includes a determination unit 51, an injector control unit 52, an intake control unit 53, an internal EGR control unit 54, an external EGR control unit 55, and an ignition control unit 56 as main functional elements.

  Based on the engine speed Ne and the load T (target torque) specified from the detected values of the crank angle sensor SW2 and the accelerator opening sensor SW4, the determination means 51 determines whether the current engine operating range is as shown in FIG. It is determined which operating region is in the control map.

  In the control map of FIG. 8, in the region where the engine load T is relatively low (low load region), the first operation region (specific operation region) A1 is set over the entire rotational speed region. Further, in the middle load region where the load T is higher than the first operation region A1, the second operation region (specific operation region) A2 and the third operation region (specific operation region) A3 are set in order from the low rotation side. Yes. That is, in the medium load region of the engine, the second operation region A2 is set in a region where the rotation speed Ne is lower than a predetermined value (for example, about 2000 to 3000 rpm), and the rotation speed Ne is higher than the second operation region A2. The third operation region A3 is set in the high region. Further, in the high load region where the load T is higher than those in the second and third operation regions A2 and A3, the fourth operation region A4 is set over the entire rotation speed region.

  During the operation of the engine, which operating region (A1 to A4) in FIG. 8 corresponds to the operating point of the engine (the point on the control map specified from each value of the load T and the rotational speed Ne). Is determined each time, and appropriate control corresponding to each operation region is executed.

  The contents of control based on the control map of FIG. 8 will be briefly described. Of this control map, all of the partial load areas excluding the fourth operation area A4 set on the highest load side, that is, the first operation area A1, the second operation area A2, and the third operation area A3. Is defined as the execution region (specific region) of CI combustion (compression self-ignition combustion) in which the air-fuel mixture is self-ignited by the compression action of the piston 5. However, in each of the regions A1 to A3, the form of fuel injection from the injector 21, the presence or absence of ignition assist using the spark plug 20, and the presence or absence of internal EGR or external EGR are different (details will be described later). . Here, the control for CI combustion executed in each of the regions A1 to A3 is referred to as “lean HCCI mode”, “multistage CI mode”, and “SA-multistage CI mode”, respectively.

  On the other hand, in the fourth operation region A4 set on the higher load side than the first to third operation regions A1 to A3, mixing is performed not by CI combustion but by spark ignition using the spark plug 20 (spark ignition). A combustion mode (hereinafter abbreviated as SI combustion) in which the gas is burned by flame propagation is selected. However, SI combustion in the fourth operation region A4 is different from general SI combustion in that the air-fuel mixture is burned by rapid flame propagation while setting the fuel injection timing and ignition timing later. The control in the fourth operation region A4 for realizing such combustion (which will be described in detail later) is referred to herein as “rapid retarded SI mode”.

  In addition, the control map which consists of these 1st-4th operation area | regions A1-A4 is fundamentally in the time of the warm state where the coolant temperature detected by engine water temperature sensor SW1 becomes more than predetermined value (for example, 80 degreeC). Is. The description of the control map when the engine is in a cold state is omitted here.

  Returning to FIG. 7 again, the injector control means 52 controls the injection amount and timing of fuel injected from the injector 21 into the combustion chamber 6. Specifically, the injector control means 52 calculates a target fuel injection amount and injection timing based on the load T, the engine speed Ne, and the like, and drives the injector 21 based on the calculation result.

  The intake control means 53 drives the CVVL 15 to change the lift amount (valve opening amount) of the intake valve 11. For example, the intake control means 53 increases the lift amount of the intake valve 11 to introduce a large amount of air (fresh air) into the combustion chamber 6 when the engine load T is high. On the other hand, the intake control means 53 reduces the lift amount of the intake valve 11 when the engine load T is low.

  The internal EGR control means 54 adjusts the amount of internal EGR gas remaining in the combustion chamber 6 by driving or stopping the VVL 16 to open or stop the exhaust valve 12 during the intake stroke. In this embodiment, since two exhaust valves 12 with VVL 16 are provided per cylinder, by switching the number of exhaust valves 12 opened during the intake stroke between 0, 1, and 2, It is possible to change the amount of internal EGR gas stepwise.

  The external EGR control means 55 adjusts the amount of exhaust gas recirculated from the exhaust passage 29 to the intake passage 28, that is, the amount of external EGR, by changing the opening degree of the EGR valve 32 provided in the EGR passage 31.

  The ignition control means 56 controls the timing (ignition timing) at which the spark plug 20 performs spark discharge.

(3) Specific control procedure for each operation region Next, the control performed by the ECU 50 in each operation region A1 to A4 will be specifically described.

  First, when the engine is started, the ECU 50 controls the engine operating point (load T and rotational speed Ne) based on the detected values of the crank angle sensor SW2 and the accelerator opening sensor SW4 as shown in FIG. It is sequentially determined which operation region in the map corresponds. And according to the determined driving | operation area | region, the following control is performed, respectively.

  As a premise of this explanation, it is assumed that the engine coolant temperature is sufficiently warm (that is, the engine is warm). In the present embodiment, the ECU 50 performs the control that realizes the compression self-ignition combustion regardless of the operation range A1 to A4 when the engine is operating in the warm state. However, in order to perform appropriate compression self-ignition combustion, it is necessary to change the fuel injection timing from the injector 21, the presence / absence of internal EGR or external EGR, the presence / absence of ignition from the spark plug 20, and the like depending on the operation region. is there. Therefore, the ECU 50 controls the injector 21, spark plug 20, CVVL15, VVL16, EGR valve 32, and the like while sequentially determining the operating point of the engine.

(I) 1st operation area A1
FIG. 9 is a diagram showing the fuel injection timing when the engine is operated in the first operation region A1, the lift characteristics of the intake and exhaust valves 11 and 12, and the heat generation rate (J / deg) generated by combustion based thereon. is there. As shown in the figure, in the first operation region A1, a general premixed compression self-ignition combustion is performed in which the air-fuel mixture injected before the compression stroke is self-ignited by the compression action of the piston 5. Executed. Specifically, in the first operation region A1, fuel is injected (P) from the injector 21 into the combustion chamber 6 at a predetermined time during the intake stroke, and the fuel injected by the fuel injection P is combusted from the intake passage 28. The mixture with the air (fresh air) introduced into the chamber 6 becomes high temperature and high pressure by the compression action of the piston 5, and self-ignites near the compression top dead center (TDC between the compression stroke and the expansion stroke). Then, based on such self-ignition, combustion accompanied by heat generation as shown by the waveform Qa occurs.

  However, since the load T is relatively low in the first operation region A1 and the amount of fuel injected from the injector 21 is small, misfire may occur unless the in-cylinder temperature is intentionally increased. Therefore, in the first operation region A1, the VVL 16 is driven to open the exhaust valve 12 during the intake stroke, whereby the exhaust gas generated in the combustion chamber 6 is caused to flow back to the combustion chamber 6 and the combustion chamber 6 A large amount of internal EGR is secured. That is, the exhaust valve 12 opens in the intake stroke (lift curve EX ′) in addition to the exhaust stroke (lift curve EX in FIG. 9). Thus, when a large amount of high-temperature internal EGR gas is ensured, the temperature of the air-fuel mixture in the combustion chamber 6 becomes high, and self-ignition of the air-fuel mixture is promoted. The amount of internal EGR gas is set to be larger on the low load side and smaller on the high load side. As a control for that, for example, in the low load region (region close to no load) in the first operation region A1, the number of the exhaust valves 12 opened during the intake stroke is two, and the load becomes higher than that, The number of valve openings is reduced to one.

  As described above, in the first operation region A1, the introduction of the external EGR gas is stopped as the internal EGR gas amount based on the restart valve (opening during the intake stroke) of the exhaust valve 12 is increased. That is, when the opening degree of the EGR valve 32 provided in the EGR passage 31 is set to be fully closed, the recirculation of the exhaust gas from the exhaust passage 29 to the intake passage 28 is stopped. Further, the ignition of the air-fuel mixture by the spark plug 20 is stopped.

  In the first operation region A1, the excess air ratio λ, which is a value obtained by dividing the air-fuel ratio (actual air-fuel ratio) of the air-fuel mixture in the combustion chamber 6 by the stoichiometric air-fuel ratio (14.7), is about λ = 2 or more. Set to a significantly leaner value. Therefore, control for increasing or decreasing the lift amount of the intake valve 11 (lift curve IN) is executed by driving the CVVL 15, and the amount of fresh air introduced into the combustion chamber 6 is considerably excessive with respect to the fuel injection amount from the injector 21. It is controlled to become. In this way, when the air-fuel mixture set to be significantly lean is burned, the combustion temperature is greatly lowered, so that the cooling loss can be reduced and the thermal efficiency (fuel consumption) can be improved. In addition, when leaning to λ = 2 or more, the NOx purification action by the three-way catalyst can hardly be expected, but if λ = 2 or more, the amount of NOx generated by combustion (raw NOx amount) is greatly reduced. Therefore, the amount of NOx contained in the exhaust gas can be suppressed to a sufficiently small value without providing a special catalyst (for example, a NOx trap catalyst) other than the three-way catalyst.

  Thus, in the first operating region A1, compression self-ignition combustion is performed with a large lean of λ = 2 or more. As described above, in the gasoline engine, the cooling loss is set while the compression ratio is set high. Therefore, it is possible to reliably increase the temperature in the combustion chamber 6 in the vicinity of the compression top dead center, realize appropriate compression self-ignition combustion, and further increase the thermal efficiency. Can do.

(Ii) Second operation area A2
In the second operation region A2 where the load T is higher than the first operation region A1 and the rotation speed Ne is relatively low, the control as shown in FIG. 10 is executed. That is, in the second operation region A2, split injection is performed in which fuel is injected from the injector 21 in two steps (P1, P2) across the compression top dead center. In the following description, the first fuel injection P1 executed during the compression stroke is performed as the pre-stage injection, and the second fuel injection P2 executed near the compression top dead center after that (in the illustrated example, very early in the expansion stroke). This is called post-injection. Further, the ignition of the air-fuel mixture by the spark plug 20 is stopped.

  Specifically, in the present embodiment, the timing (more precisely, the start timing) of the front injection P1 in the multistage CI mode is based on the compression top dead center (TDC between the compression stroke and the expansion stroke). It is set within a period of about 60 to 50 ° CA (CA represents the crank angle) before its top dead center (BTDC), and the timing (start timing) of the post-injection P2 is 0 to 10 after top dead center (ATDC). It is set within a period of about CA. Moreover, the ratio of each injection amount by the front | former stage injection P1 and the back | latter stage injection P2 is set to about 3: 7-7: 3.

  The total injection amount by the front injection P1 and the rear injection P2 is increased in comparison with the first operation region A1 (the injection amount by the fuel injection P) in accordance with the high load corresponding to the second operation region A2. Further, the CVVL 15 is driven to increase the lift amount of the intake valve 11 (lift curve IN) in order to introduce a large amount of fresh air corresponding to the fuel injection amount set to increase in this way into the combustion chamber 6. Then, as shown in the waveform Qb of FIG. 10, two peaks with different timings are obtained by the self-ignition of the mixture of fuel and air (fresh air) separately injected as described above near the compression top dead center. Combustion accompanied by heat generation occurs. Note that such a shape of the waveform Qb is conceptual only, and there are naturally cases where two peaks do not appear clearly.

  The reason why the fuel is injected separately into the front injection P1 and the rear injection P2 as described above is in consideration of problems such as combustion noise. That is, in the second operation region A2 where the fuel injection amount is large, if the fuel is injected at one time, a sudden combustion occurs in which all of the injected large amount of fuel burns in a short period of time. May suddenly increase, resulting in a marked increase in combustion noise. Therefore, by dividing and injecting fuel as described above, relatively mild combustion is continuously caused to avoid the increase in combustion noise as described above.

  However, even if the fuel injection is divided into a plurality of times, depending on the arrangement of the injector 21 and the shape of the piston 5, the fuels injected at each time may be mixed and the mixed fuel may burn almost simultaneously. . In this way, if combustion occurs in a mixture of fuels with different injection timings, not only does combustion noise become excessive, but the oxygen required for combustion is significantly insufficient locally, resulting in a large amount of soot (carbonaceous matter). Particles) may occur.

  In this embodiment, the injector 21 is disposed at the radial center of the ceiling of the combustion chamber 6, and the cavity 40 is formed at the radial center of the crown surface of the piston 5. The injected fuel does not burn at the same time, and it is possible to avoid an increase in combustion noise and a large amount of soot (the detailed mechanism will be described later). Further, since a predetermined amount of the volume of the combustion chamber 6 in the radially outer portion than the cavity 40, that is, the outer peripheral portion 6b of the combustion chamber 6, is secured, the air-fuel mixture formed by the upstream injection P1 in the outer peripheral portion 6b. Can be reliably burned.

  In the second operation region A2, in addition to the fuel split injection control as described above, the VVL 16 is driven so as to invalidate the function of depressing the exhaust valve 12 during the intake stroke, and the intake stroke of the exhaust valve 12 is determined. The valve opening inside is stopped. As a result, the exhaust gas hardly flows back into the combustion chamber 6 and internal EGR is prohibited.

  On the other hand, in the second operation region A2, external EGR is executed instead of the prohibited internal EGR as described above. That is, the operation of recirculating the exhaust gas from the exhaust passage 29 to the intake passage 28 is performed by opening the EGR valve 32 provided in the EGR passage 31 to a predetermined opening degree.

  The reason for switching from the internal EGR to the external EGR in this way is to ensure the amount of fresh air in the combustion chamber 6 and to avoid abnormal combustion. That is, in the second operation region A2, the engine load T is higher than that in the first operation region A1, a larger amount of fresh air is required, and pre-ignition and knocking are accompanied by a large amount of total fuel injected. Abnormal combustion may occur. Therefore, the volume of the EGR gas is reduced by switching from the internal EGR to the external EGR and returning the exhaust gas that has passed through the EGR passage 31 with the EGR cooler 33 (that is, cooled by the EGR cooler 33) to the intake passage 28. Thus, the amount of fresh air in the combustion chamber 6 is ensured, the temperature of the combustion chamber 6 is prevented from being increased, and abnormal combustion as described above is avoided. However, even in the second operation region A2, the external EGR is stopped in the vicinity of the full load of the engine in order to ensure a large amount of fresh air.

  Here, the combustion mode in the second operation region A2 realized based on the control as described above will be described more specifically with reference to FIGS. 13 (a) to (f). FIG. 13A shows a state when the front injection P1 is performed from the injector 21. FIG. As described above, the piston 5 at this time is positioned at about 50 to 60 ° CA before compression top dead center (BTDC). When fuel is injected radially from the plural (12) nozzle holes provided at the tip end portion I1 of the injector 21 toward the crown surface of the piston 5 at such a position, the spray of the fuel is sprayed into the combustion chamber 6. It goes to the outer peripheral part 6b.

  The fuel (spray) injected toward the outer peripheral portion 6b of the combustion chamber 6 is then dispersed. Based on the dispersed fuel, as shown in FIG. 13B, the outer peripheral portion 6b of the combustion chamber 6 is mainly used. The air-fuel mixture X1 is formed. The air-fuel ratio of the air-fuel mixture X1 formed here is set to a theoretical air-fuel ratio (excess air ratio λ = 1) as the local air-fuel ratio of only the outer peripheral portion 6b of the combustion chamber 6. That is, the injection timing and the injection amount of the pre-stage injection P1 are set so that the air-fuel mixture X1 having a concentration about the stoichiometric air-fuel ratio is locally formed on the outer periphery of the combustion chamber 6.

  Of course, the pre-injection P1 may cause a small amount of fuel to exist outside the outer peripheral portion of the combustion chamber 6 (for example, inside the cavity 40), but the concentration of the fuel is higher than that of the outer peripheral portion 6b of the combustion chamber 6. It is extremely thin. In other words, when the pre-stage injection P1 is executed, the air-fuel mixture X1 that is extremely richer than the inside of the cavity 40 is formed in the outer peripheral portion 6b of the combustion chamber 6.

  The air-fuel mixture X1 formed in the outer peripheral portion 6b of the combustion chamber 6 as described above is compressed by the rise of the piston 5 to become high temperature and pressure, and when the piston 5 reaches the vicinity of the compression top dead center, FIG. ) To burn by self-ignition as shown in (). In the figure, the region where the air-fuel mixture X1 is burning is shown in black or gray. The region Y2 where the air-fuel mixture X1 burns is limited to the outer peripheral portion 6b of the combustion chamber 6 corresponding to the region where the air-fuel mixture X1 is formed.

  When the combustion based on the front injection P1 as described above starts, the rear injection P2 as shown in FIG. 13 (d) is executed almost at the same time or after a short period of time. As described above, the timing of the post-injection P2 is about 0 to 10 ° CA after top dead center (ATDC) shortly after the piston 5 starts to descend. Thus, when the fuel is injected from the injector 21 at the timing when the piston 5 is close to the top dead center, the spray of the fuel goes to the inside of the cavity 40 provided in the radial center portion of the crown surface of the piston 5. It will be. Then, the fuel (spray) injected toward the inside of the cavity 40 is dispersed while being guided upward along the peripheral wall of the cavity 40, and based on the dispersed fuel, as shown in FIG. 13 (e). The air-fuel mixture X2 is formed in the radial center portion 6a of the combustion chamber 6 (mainly inside the cavity 40). The local air-fuel ratio of the air-fuel mixture X2 is also set to about the stoichiometric air-fuel ratio (excess air ratio λ = 1), similar to the air-fuel mixture X1 based on the pre-stage injection P1 described above. In other words, the rich air-fuel mixture X2 is formed in the radial center portion 6a of the combustion chamber 6 by the post-stage injection P2 than when the pre-stage injection P1 is performed.

  The mixture X2 based on the post-stage injection P2 as described above is formed in a state where the piston 5 is close to the compression top dead center and the combustion of the mixture X1 based on the pre-stage injection P1 has already occurred. For this reason, as shown in FIG. 13 (f), the air-fuel mixture X2 reaches self-ignition in a very short time after the post-injection P2, and burns. The region Y2 where the air-fuel mixture X2 burns is limited to the radial center portion 6a of the combustion chamber 6 corresponding to the region where the air-fuel mixture X2 is formed. That is, the air-fuel mixture X1 based on the above-described front-stage injection P1 burns in the outer peripheral portion 6b (fuel region Y1) of the combustion chamber 6, whereas the air-fuel mixture X2 based on the rear-stage injection P2 corresponds to the installation portion of the cavity 40. Combustion is performed in the radial center portion 6a (combustion region Y2) of the combustion chamber 6 that performs.

  As described above, in the second operation region A2, a relatively large amount of fuel corresponding to the load T is injected in a plurality of times (the front-stage injection P1 and the rear-stage injection P2), so that the air-fuel mixture (X1) , X2) and self-igniting and burning them independently. In the second operation region A2 in which such control is performed, the separately injected fuel is not mixed and combusted at the same time. Therefore, an increase in combustion noise due to a sudden rise in the in-cylinder pressure, and local oxygen There is no worry that a large amount of soot will occur due to shortage. Moreover, the air-fuel mixtures X1 and X2 based on the front-stage injection P1 and the rear-stage injection P2 are each locally set to an excess air ratio of about λ = 1, so that the exhaust gas generated by combustion in such an environment is used. If it exists, harmful components can be sufficiently purified only by the three-way catalyst.

  In particular, while the volume is secured in the central portion 6a of the combustion chamber 6 including the cavity 40 and the outer peripheral portion 6b of the combustion chamber 6, the SV ratio of the combustion chamber 6 is suppressed to be small and the cooling loss is reduced. The temperature in the chamber 6 can be more reliably increased to a temperature at which the air-fuel mixture can self-ignite, and the air-fuel mixture formed by the injections P1 and P2 can be self-ignited more reliably.

(Iii) Third operation area A3
In the third operation region A3 where the load T is higher than that in the first operation region A1 and the rotational speed Ne is higher than that in the second operation region A2, the control as shown in FIG. 10 is executed. That is, in the third operation region A3, the fuel from the injector 21 is injected in a plurality of times as in the second operation region A2, but unlike the second operation region A2, the pre-stage injection is performed. An ignition assist for promoting self-ignition is executed between P1 and the subsequent injection P2. In the control map of FIG. 5, “SA + multi-stage CI” is indicated in the third operation region A3 to indicate that the compression self-ignition combustion based on such divided injection and ignition assist (Spark Assist) is executed. doing.

  Specifically, as the ignition assist, the pre-stage and post-stage injections P1, P2 are performed at a predetermined time between the pre-stage injection P1 executed during the compression stroke and the post-stage injection P2 executed near the compression top dead center. A small amount of fuel is injected from the injector 21 (Pa), and spark ignition S is performed by the spark plug 20 immediately after the injection Pa and at the end of the compression stroke before the post-injection P2. The Then, such ignition assist causes combustion with a small amount of heat generation as shown by waveform Qc ′ in FIG. 10, and the combustion chamber 6 is heated to a high temperature due to the combustion, as shown in the following waveform Qc. In addition, the air-fuel mixture based on the front injection P1 and the rear injection P2 burns by self-ignition. In the following, the small amount of fuel injection Pa (ignition assist fuel injection) executed for the ignition assist is referred to as the assist injection Pa, and the spark ignition S (ignition assist) executed for the ignition assist. The spark ignition) is referred to as assist ignition S.

  The assist ignition S is performed at a timing immediately after the tip of fuel (spray) injected as assist injection Pa from the tip end (injection port) of the injector 21 passes through the opening edge 62b at the lower end of the injector storage portion 62. Is done.

  In the third operation region A3, the same control as in the second operation region A2 is executed except for the control related to the ignition assist as described above. For example, in the third operation region A3, the internal EGR that opens the exhaust valve 12 during the intake stroke (returns the exhaust gas to the combustion chamber 6) is prohibited, and the exhaust gas is passed through the EGR passage 31 to the intake passage 28. External EGR to reflux is performed. However, external EGR is prohibited in the vicinity of the full load of the engine in order to ensure a large amount of fresh air.

  FIGS. 14A to 14H are diagrams schematically showing the state of combustion in the third operation region A3 in which the compression self-ignition combustion based on the above-described ignition assist is performed. As shown in FIG. 14 (a), in the third operation region A3, substantially the same timing as the timing of the preceding injection P1 (FIG. 14 (a)) in the second operation region A2 (about BTDC 50 to 60 ° CA). Thus, the front stage injection P1 is executed, and by this front stage injection P1, the air-fuel mixture X1 having an air-fuel ratio of about the theoretical air-fuel ratio (λ = 1) is formed in the outer peripheral portion of the combustion chamber 6. However, strictly speaking, the timing of the upstream injection P1 is set to be slightly earlier than the timing of the upstream injection P1 in the second operation region A2. This is because in the third operation region A3, the engine rotation speed Ne is higher and the piston speed is faster than in the second operation region A2. In other words, if the piston speed is high, the piston 5 moves relatively large until the fuel injected from the injector 21 reaches the vicinity of the crown surface of the piston 5, so that fuel (spray) is injected at the same position on the piston 5. If it is to be delivered, it is necessary to advance the injection timing from the injector 21 even slightly. The same applies to the later-stage injection P2 described later.

  After the pre-stage injection P1, the ignition assist shown in FIG. 14 (c) is executed. That is, after the pre-stage injection P1, when the piston 5 is raised to some extent (for example, about BTDC 30 to 10 ° CA), the assist injection Pa that is the fuel injection for the ignition assist is executed and immediately after that, the assist Assist ignition S that is spark ignition is executed. Then, based on the fuel injected by the assist injection Pa, an air-fuel mixture containing sufficiently atomized fuel is formed around the electrode of the spark plug 20, and the air-fuel mixture causes the assist ignition S to flow. By forming a flame as a fire type, a combustion region Ya of the air-fuel mixture is locally formed around the electrode of the spark plug 20 as shown in FIG.

  When the flame (combustion region Ya) is generated by the ignition assist as described above, the high temperature of the combustion chamber 6 due to the flame and the compression action due to the rising of the piston 5 are combined, and the outer peripheral portion of the combustion chamber 6 is combined. The air-fuel mixture X1 based on the pre-stage injection P1 that has been formed burns by self-ignition near the compression top dead center, as shown in FIG. 14 (e). The combustion region Y1 of the air-fuel mixture X1 is limited to the outer peripheral portion 6b of the combustion chamber 6, and is a region spaced radially outward from the electrode of the spark plug 20.

  When the combustion based on the pre-stage injection P1 as described above starts, the combustion proceeds thereafter in the same manner as in the second operation region A2. That is, the post-injection P2 as shown in FIG. 14 (f) is executed almost simultaneously with the start of combustion based on the pre-injection P1, or after a short period of time, and based on the post-injection P2, the combustion chamber 6 An air-fuel mixture X2 having an air-fuel ratio of about the theoretical air-fuel ratio (λ = 1) is formed in the radial center portion 6a (mainly inside the cavity 40). Then, the air-fuel mixture X2 is self-ignited in a very short time, and forms a fuel region Y2 of the air-fuel mixture X2 in the radial center portion 6a of the combustion chamber 6.

  As described above, in the third operation region A3, the ignition assist is executed using the spark plug 20 between the front injection P1 and the rear injection P2, and the combustion chamber 6 is heated to a high temperature by the ignition assist. Following the ignition assist, the air-fuel mixtures X1 and X2 based on the front injection P1 and the rear injection P2 are each burned by self-ignition. As described above, in the third operation region A3 in which self-ignition of the air-fuel mixture is promoted by the ignition assist, the engine rotation speed Ne is higher than that in the above-described second operation region A2, and the fuel heat receiving period is shortened. In spite of the situation, it is ensured that the air-fuel mixture burns by self-ignition and misfires are avoided. In addition, as in the second operation region A2, the air-fuel mixture X1, X2 based on the front injection P1 and the rear injection P2 is self-ignited and burned independently in separate spaces, so that combustion noise increases and soot is generated. Is also avoided. In particular, as in the case of the second operation region A2, in this gasoline engine, volumes are secured in the central portion 6a of the combustion chamber 6 including the cavity 40 and the outer peripheral portion 6b of the combustion chamber 6 on the radially outer side. Since the SV ratio of the combustion chamber 6 is kept small and the cooling loss is reduced, the air-fuel mixture formed by the injections P1 and P2 is more reliably determined in the central portion 6a and the outer peripheral portion 6b of the combustion chamber 6. Moreover, it can be burned efficiently.

  FIG. 15 schematically shows the positional relationship between the combustion region Ya of the air-fuel mixture based on the ignition assist and the combustion regions Y1 and Y2 of the air-fuel mixture based on the front injection P1 and the rear injection P2 in the third operation region A3. It is a top view for showing to. According to the combustion mode in the third operation region A3 described above, as shown in FIG. 12, first, the combustion region Ya based on the ignition assist is formed only in the vicinity of the spark plug 20, and the combustion chamber 6 that does not substantially overlap therewith is formed. A combustion region Y1 based on the front-stage injection P1 is formed on the outer periphery, and further, a combustion region Y2 based on the rear-stage injection P2 is formed radially inward from the combustion region Y1. These combustion regions are formed in the order of Ya → Y1 → Y2, and their generation positions or generation times hardly overlap each other.

(IV) Fourth operation region A4
In the fourth operation region A4 set in the high load region, a large amount of fuel is injected. Therefore, if compression self-ignition combustion is performed, combustion noise increases remarkably and knocking occurs. is there. Therefore, in this fourth operation region A4, spark ignition combustion (SI combustion) is performed in which the air-fuel mixture is ignited and flame is propagated instead of compression self-ignition combustion.

  Here, also in SI combustion, when the temperature in the combustion chamber 6 is excessively high, knocking occurs. In particular, in this gasoline engine, the compression ratio is set to a very high value. Therefore, the temperature in the combustion chamber 6 tends to increase.

  Therefore, in this gasoline engine, in this fourth operating region A4, in order to more reliably avoid abnormal combustion such as knocking, the fuel is injected considerably before the compression top dead center (for example, during the intake stroke). Instead of normal SI combustion in which spark ignition is performed in the vicinity of the dead center, as shown in FIG. 11, fuel is injected from the injector 21 during the compression stroke (P3, P4), and an ignition plug is inserted after the fuel injection P3, P4. 20 is ignited with a spark, and a rapid retarded SI combustion mode is executed in which the air-fuel mixture is combusted by flame propagation in a short time from the timing when the compression top dead center is passed (the initial stage of the expansion stroke). FIG. 11 is a diagram showing the fuel injection timing, the lift characteristics of the intake and exhaust valves 11 and 12, and the heat generation rate (J / deg) generated by combustion based on the fuel injection timing and the lift characteristics when the rapid retarded SI combustion mode is executed. .

  Specifically, as shown in FIG. 11, in this rapid retarded SI combustion mode, the high pressure of 30 MPa or more is supplied from the injector 21 in two injection timings (P3, P4) set in the latter half of the compression stroke. Then fuel is injected. As the timing of each fuel injection P3, P4, for example, the period from the start timing of the first injection P1 to the completion timing of the second injection P2 is approximately 20 to 0 ° CA before compression top dead center (BTDC). It is set to be within a certain period.

  In the rapid retarded SI combustion mode in which such injection control is performed, the injection period can be shortened by injecting fuel at a very high injection pressure of 30 MPa or more (for example, 40 MPa) as described above. At the same time, the fuel spray can be atomized, and a large amount of fuel can be sufficiently vaporized and atomized in a short time to form a relatively homogeneous (or weakly stratified) mixture. In addition, since the injection pressure is high, large turbulent energy can be maintained until the combustion chamber 6 passes a certain amount of compression top dead center at which the temperature and pressure of the combustion chamber 6 are the highest. Therefore, it is possible to start combustion by spark ignition within a short time after the fuel is injected and while the attenuation of the turbulent energy accompanying the fuel injection is small (in a state where the turbulent energy is large). The combustion period can be shortened by large turbulent energy. As the combustion period is shortened, the air-fuel mixture can be burned out by proper flame propagation before combustion is caused. That is, high thermal efficiency and engine torque can be maintained while avoiding abnormal combustion such as knocking. Further, since the combustion temperature does not rise excessively and combustion does not start with insufficient vaporization and atomization of fuel, an increase in NOx and soot is avoided, and the emission property is also maintained well.

  In particular, in this gasoline engine, the volume is secured in the cavity 40 and the radially outer portion thereof, and the SV ratio of the combustion chamber 6 is suppressed to be small and the cooling loss is reduced. Combustion can be completed rapidly, and high engine torque can be secured.

  In addition, the fuel is injected in two parts and ignition is performed after the second fuel injection. The fuel is atomized by the first fuel injection P3, and the fuel is injected at the time of ignition by the second fuel injection. Turbulent energy can be increased. The turbulent energy is also increased by being ejected from as many as 12 nozzles.

  Here, if the ignition timing is further advanced than in the example of FIG. 11, the combustion start timing θig approaches the compression top dead center accordingly, and therefore further improvement in thermal efficiency and output torque can be expected. Since the knocking is likely to occur if the acceleration is accelerated, the ignition timing is set to the advance side as much as possible under the restriction that knocking does not occur. For this reason, the ignition timing is set, for example, within a range of about 0 to 20 ° CA after compression top dead center (ATDC).

  In the rapid retarded SI combustion mode, the new air-fuel ratio is set so that the average air-fuel ratio of the entire combustion chamber 6 becomes the stoichiometric air-fuel ratio (excess air ratio λ = 1) with respect to the total injection amount by the fuel injections P1 and P2. The volume is controlled. Specifically, in the second operation region A2, the CVVL 15 is driven according to the increase in the load T, the lift amount of the intake valve 11 is increased, and accordingly, fresh air corresponding to the fuel injection amount is introduced. (Lift curve IN in FIG. 11). In this embodiment, the lift amount is increased while the lift peak position of the intake valve 11 is fixed.

  Further, in the rapid retarded SI combustion mode, external EGR for returning the exhaust gas to the intake passage 28 through the EGR passage 31 is executed. Note that external EGR is prohibited in the vicinity of the full load of the engine in order to secure a larger amount of fresh air.

(5) Operational effects and the like As described above, in the gasoline engine of the present embodiment, the combustion chamber ceiling surface 60 is conical, the cavity 40 is formed in the center in the radial direction of the crown surface of the piston 5, and A portion of the crown surface of the piston 5 that is radially outward from the cavity 40 is a conical reference surface 41 parallel to the combustion chamber ceiling surface 60, and the intake side opening 61 and the exhaust side opening 62 are opened. Since the SV ratio of the combustion chamber 6 is kept small and the cooling loss is reduced while the area and the volume of the radially central portion 6a and the radially outer portion 6b of the combustion chamber 6 are ensured, the cooling loss is reduced. Efficient combustion can be realized in the central portion 6a and the radially outer portion 6b, and thermal efficiency can be further increased.

  In particular, in the second operation region A2 and the third operation region A3, the fuel is individually injected into the central portion 6a of the combustion chamber 6a including the cavity 40 and the outer peripheral portion 6b on the radially outer side. Combustion is carried out individually in each part, and clearances d and volumes V1, V2 between the ceiling surface 60 of the combustion chamber 6 and the crown surface of the piston 5 in each part are secured, so that Therefore, combustion noise and SOOT generation can be suppressed while ensuring engine torque.

  The valve surfaces 11c and 12c of the valves 11 and 12 extend along the contact surface of the combustion chamber ceiling surface 60, and the stems 11b and 11c of the valves 11 and 12 are centered on the top of the combustion chamber ceiling surface 60. The ceiling surface 60 of the combustion chamber 6 including the valve surfaces 11c and 12c while stably driving the valves 11 and 12 while extending radially in the radial direction of the circle Therefore, the combustion can be appropriately expanded in the combustion chamber 6 as compared with the case where the unevenness is large, and the surface area of the combustion chamber 6 can be more reliably reduced to obtain high thermal efficiency. it can. Further, the spark plug 20 is disposed between the exhaust valves 12, and the area of the intake valve 11 is ensured to be larger, so that the intake air amount and thus the engine torque can be increased.

  Here, the present invention is widely applicable to engines having a geometric compression ratio of 14 or more and performing compression auto-ignition combustion in a predetermined operation region, and specific combustion modes in each operation region are as follows. It is not restricted to the above. For example, in all of the second operation region, the third operation region, and the fourth operation region, normal spark ignition type combustion is performed in which fuel is injected once in the intake stroke and spark ignition is performed near the compression top dead center. May be.

  Further, the number of nozzle holes of the injector 21 is not limited to 12, and may be more or less than 12. However, if the number of injection holes is too small, the concentration of the fuel injected from the injector 21 varies greatly in the circumferential direction. For this reason, it is desirable that the number of nozzle holes be eight or more.

5 Piston 6 Combustion Chamber 21 Injector 40 Cavity 41 Reference Surface 50 ECU (Control Unit)
A1 1st operation area (specific operation area)
A2 3rd operation area (specific operation area)
A3 3rd operation area (specific operation area)

Claims (5)

A fuel chamber formed in the cylinder is provided with an injector capable of supplying at least a part of gasoline fuel, and a mixture of the fuel and air is combusted by self-ignition in the combustion chamber in a specific operation region of the engine. A gasoline engine that reciprocates a piston,
The geometric compression ratio of the cylinder is set to 14 or more,
The ceiling surface of the combustion chamber has a conical surface shape that is inclined so that its height decreases as it goes radially outward with the radial center as a top,
The crown surface of the piston has a cavity curved in a concave shape with a predetermined curvature at the center portion thereof, and is inclined so that the height decreases toward the outer side in the radial direction from the opening edge of the cavity. A reference surface extending parallel to the ceiling surface of the room,
The injector has a tip portion formed with an injection hole capable of injecting fuel toward the crown side of the piston, and is provided so that the tip portion is located near the top of the ceiling surface of the combustion chamber. A gasoline engine characterized by The gasoline engine according to claim 1,
The injector is configured so that, in at least a part of the operation region of the specific operation region, fuel injected before reaching the compression top dead center reaches a portion radially outside the cavity in the combustion chamber. The fuel is injected in a plurality of times including a front-stage injection for injecting fuel and a rear-stage injection for injecting the fuel when the injected fuel reaches the cavity after the front-stage injection. A gasoline engine. The gasoline engine according to claim 1 or 2,
An intake port communicating with the combustion chamber via a plurality of intake side openings formed in the ceiling surface of the combustion chamber, a valve body capable of opening and closing the plurality of intake side openings, and extending from the valve body A plurality of intake valves each having a stem for moving the valve body in the opening / closing direction of the intake side opening, and the combustion chamber via a plurality of exhaust side openings formed on a ceiling surface of the combustion chamber; A plurality of exhaust ports, a plurality of valve bodies capable of opening and closing the plurality of exhaust side openings, and a stem extending from the valve body and moving the valve bodies in the opening and closing direction of the exhaust side openings, respectively. An exhaust valve,
At least one of the plurality of intake valves and the plurality of exhaust valves has a surface on the combustion chamber side of each valve body extending in parallel with a contact surface with the ceiling surface of the conical surface combustion chamber, and The stem is disposed so as to extend in a radial direction of a circle centering on the top of the ceiling surface of the combustion chamber in a plan view when the piston crown surface is viewed from the top of the ceiling surface of the combustion chamber. A gasoline engine. The gasoline engine according to claim 3,
A spark plug capable of igniting the air-fuel mixture in the combustion chamber;
The plurality of exhaust valves are adjacent to each other, a surface of each valve body on the combustion chamber side extends in parallel with a contact surface with a ceiling surface of the conical combustion chamber, and each stem thereof, Arranged in a plan view so as to extend in the radial direction of a circle centered on the top of the ceiling surface of the combustion chamber;
The gasoline engine, wherein the spark plug is disposed between the exhaust valves. The gasoline engine according to claim 4, wherein
Each of the plurality of intake valves has a valve body extending in parallel with a contact surface with the conical surface of the combustion chamber, and each stem has a piston crown surface from the top of the combustion chamber ceiling surface. It is arranged so as to extend in the radial direction of a circle centered on the top of the ceiling surface of the combustion chamber in a plan view as seen,
The gasoline engine according to claim 1, wherein an opening area of the intake side opening is set larger than an opening area of the exhaust side opening.