JP5585533B2 - gasoline engine - Google Patents

Description

  The present invention relates to a gasoline engine in which a piston is reciprocated by burning an air-fuel mixture of at least a part of gasoline and air in a combustion chamber.

  Conventionally, in the field of gasoline engines, a combustion mode 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 compression self-ignition combustion Research is underway to apply this 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 locations in the combustion chamber at the same time, and is said to have a shorter combustion period and higher thermal efficiency than combustion by spark ignition.

  As a specific example of a 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, a combustion chamber is provided in a part of the operating region of the engine. It is disclosed that the air-fuel mixture is combusted by self-ignition while performing so-called split injection in which fuel is injected in a plurality of times. Specifically, the first fuel injected prior to the compression stroke forms a premixed region in the combustion chamber in which the fuel concentration (air-fuel ratio of the air-fuel mixture) is uniform, and a predetermined timing in the latter half of the compression stroke. As a result of the second fuel injection performed in the above step, a stratified region of the air-fuel mixture is formed around the spark plug. When the piston reaches the vicinity of the compression top dead center, spark ignition is performed by a spark plug as so-called ignition assist. Then, the spark-ignition causes the rich air-fuel mixture in the stratified region to burn, and the air-fuel mixture in the pre-mixed region burns by auto-ignition triggered by the high temperature of the combustion chamber due to the combustion. .

JP 2009-74488 A

  However, when the fuel is divided and injected so that a stratified region in which the fuel is unevenly distributed is formed in the premixed region where the fuel concentration is uniform as in Patent Document 1, the ignition assist is performed in that state. In particular, when the engine load increases to some extent and the total fuel to be injected increases, the mixture in both the premixed region and the stratified region burns almost simultaneously, so that the in-cylinder pressure rises rapidly, Combustion noise may occur. Further, since combustion occurs in a state where oxygen is extremely insufficient partially, a large amount of soot (carbonaceous particles) may be generated.

  The present invention has been made in view of the circumstances as described above, and an object thereof is to provide a gasoline engine capable of appropriately performing compression self-ignition combustion without causing an increase in combustion noise and soot. To do.

In order to solve the above-mentioned problems, the present invention provides a gasoline engine that reciprocates a piston by combusting an air-fuel mixture of at least a part of gasoline and air in a combustion chamber. A concave cavity provided in the center of the crown surface, a multi-hole injector that injects the fuel in a plurality of directions from the center of the combustion chamber ceiling facing the cavity, and fuel injection by the injector Control means for controlling the operation, and the control means injects the fuel from the injector in at least two times of the pre-stage injection and the post-stage injection in the predetermined specific operation region of the engine and The pre-injection is performed during the compression stroke and the post-injection. The fuel is injected before, and the injected fuel forms an air-fuel mixture richer than the inside of the cavity in the outer peripheral portion of the combustion chamber located outside the piston cavity in the bore radial direction. In the latter stage injection, fuel is injected at a predetermined time from the latter stage of the compression stroke to the early stage of the expansion stroke , and combustion of the air- fuel mixture based on the earlier stage injection is caused by the injected fuel in the outer peripheral portion of the combustion chamber. In the state that has already occurred, a richer air-fuel mixture is formed inside the cavity than at the time of execution of the preceding stage injection, and at least part of the low speed side in the specific operation region, the preceding stage injection. Is set in the latter half of the compression stroke, and the timing of the latter-stage injection is set to any one from the compression top dead center to the initial stage of the expansion stroke (claim 1).

  According to the present invention, the injector is provided so as to face the cavity provided in the central portion of the piston, and the fuel is divided into at least two injection timings (pre-stage injection and post-stage injection) after the compression stroke from the injector. Is injected into separate spaces in the combustion chamber (the outer periphery of the combustion chamber and the inside of the cavity) to form an air-fuel mixture, so that the air-fuel mixture is independently formed near the compression top dead center. It can self-ignite and burn. For this reason, there is no case where the separately injected fuel is mixed and combusted at the same time, and an increase in combustion noise due to a sudden rise in in-cylinder pressure and an increase in soot (carbonaceous particles) due to local oxygen shortage are effective. Can be prevented.

  In the present invention, preferably, an annular concave portion having a circular shape in plan view is formed on the piston crown surface located on the outer side in the bore radial direction with respect to the cavity. Item 2).

  According to this configuration, when the fuel injected by the preceding injection is received in the annular recess in the outer peripheral portion of the piston, the mixing based on the preceding injection is performed in the outer peripheral portion of the combustion chamber corresponding to the installation portion of the annular recess. You can be sure of it. As a result, the air-fuel mixture based on the preceding injection can be clearly separated from the air-fuel mixture formed in the cavity based on the subsequent post-injection, and the combustion independence of these air-fuel mixtures is more reliably ensured. Can do.

  In the above-described configuration, more preferably, an upright wall portion that guides the fuel injected by the preceding stage injection upward is further formed outside the annular recess in the bore radial direction.

  According to this configuration, the fuel of the pre-injection injected toward the annular recess is rolled up along the vertical wall portion, so that the fuel is sufficiently dispersed, vaporized and atomized, and mixed in the outer peripheral portion of the combustion chamber Qi formation can be effectively promoted.

  In the present invention, preferably, the cavity is formed in a constricted shape so that an opening area at an upper end thereof is smaller than a maximum cross-sectional area inside the cavity.

  According to this configuration, the air-fuel mixture based on the fuel injected by the post-injection can be reliably retained in the cavity, and the air-fuel mixture based on the post-stage injection is mixed with the air-fuel mixture based on the previous stage injection. Can be clearly separated from the film.

  In the present invention, preferably, the injector has eight or more injection holes at the tip thereof, and the fuel is radially radiated through the eight injection holes so as to expand outward in the bore radial direction as it approaches the piston crown surface. Injecting (claim 5).

  According to this configuration, an air-fuel mixture having a substantially uniform air-fuel ratio in the circumferential direction can be formed in a very short time after the above-described front-stage injection and rear-stage injection are performed. Can be done.

  In the present invention, preferably, the specific operation region includes at least a high load region (Claim 6).

  According to this configuration, in the high load region where the total fuel injection amount from the injector is increased and combustion noise is particularly concerned, the combustion noise can be reduced by forming the mixture mixture as described above. The amount of soot generated can be suppressed while effectively reducing.

  In the above-described configuration, more preferably, in the specific operation region, external EGR for returning the exhaust gas generated in the combustion chamber to the intake passage through the EGR passage is performed (Claim 7).

  According to this configuration, it is possible to effectively reduce pumping loss and improve fuel efficiency while avoiding occurrence of abnormal combustion due to excessively high temperature in the combustion chamber.

  In the above-described configuration, more preferably, at least in the high rotation region in the specific operation region, preliminary injection for injecting fuel during the intake stroke is performed prior to the front-stage injection and the rear-stage injection.

  According to this configuration, since the amount of fuel to be injected in the front-stage injection and the rear-stage injection is reduced, even when the total amount of fuel to be injected is large and the piston speed is high, the mixture separation by the front-stage injection and the rear-stage injection is performed. Formation can be made accurately.

  As described above, according to the present invention, it is possible to provide a gasoline engine capable of appropriately performing compression self-ignition combustion without causing an increase in combustion noise and soot.

It is a figure showing the whole gasoline engine composition concerning one embodiment of the present invention. FIG. 2 is a partially enlarged view of FIG. 1. 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. (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.

(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. In addition, the fuel supplied to the engine body 1 may be anything that contains gasoline as a main component, and the contents may be all gasoline, or gasoline containing ethanol (ethyl alcohol) or the like. But you can.

  The piston 5 is connected to the crankshaft 7 via a connecting rod 8, and the crankshaft 7 rotates around the central axis in accordance with the reciprocating motion of the piston 5.

  A combustion chamber 6 is formed above the piston 5, an intake port 9 and an exhaust port 10 are opened in the combustion chamber 6, and an intake valve 11 and an exhaust valve 12 that open and close the ports 9 and 10 are connected to the cylinder head 4. Are provided respectively. The engine shown in the figure is a so-called double overhead camshaft (DOHC) engine, in which two intake ports 9 and two exhaust ports 10 are provided for each cylinder, and two intake valves 11 and two exhaust valves 12 are provided. It is provided one by one.

  Here, in a narrow sense, the “combustion chamber” refers to a space formed above the piston 5 when the piston 5 rises to the top dead center. Refers to the space formed above the piston 5 regardless of the vertical position (combustion chamber in a broad sense).

  The intake valve 11 and the exhaust valve 12 are driven to open and close 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. The

  A CVVL 15 is incorporated in the valve operating mechanism 13 for the intake valve 11. The CVVL 15 is called a continuous 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). (For example, 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). JP, 2007-85241, A).

  The valve mechanism 14 for the exhaust valve 12 incorporates a VVL 16, which 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 execute or stop the valve opening operation during the intake stroke with respect to the pair of exhaust valves 12 of each cylinder 2. It is configured.

  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 by the action of the VVL 16, the high-temperature exhaust gas flows backward from the exhaust port 10 to the combustion chamber 6, the temperature of the combustion chamber 6 is increased, and the combustion chamber 6 is heated. The amount of air (fresh air) introduced is reduced. Hereinafter, the exhaust gas remaining operation by the resumption valve of the exhaust valve 12 (opening during the intake stroke) is distinguished from the exhaust gas recirculation operation (external EGR) by the external EGR device 30 to be described later. Called.

  The cylinder head 4 of the engine body 1 is provided with one set of spark plugs 20 and injectors 21 for each cylinder 2.

  The injector 21 is provided so as to face the combustion chamber 6 from the ceiling surface (the lower surface of the cylinder head 4 covering the combustion chamber 6). A fuel supply pipe 23 is connected to the injector 21, and fuel (fuel mainly composed of gasoline) supplied through the fuel supply pipe 23 is injected from the tip of the injector 21.

  The injector 21 is a so-called multi-hole injector, and has twelve nozzle holes at its tip. These nozzle holes are installed at the center of the ceiling of the combustion chamber 6, and each nozzle hole is perforated so that its opening end faces obliquely downward on the outside in the bore diameter direction. Yes. For this reason, when fuel is injected from each injection hole of the injector 21, the fuel is injected radially so as to expand outward in the bore radial direction as it approaches the crown surface (upper surface) of the piston 5.

  The spark plug 20 is disposed adjacent to the injector 21 so as to face the combustion chamber 6 from above, and discharges sparks from the tip in response to power supply from an ignition circuit (not shown).

  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. In other words, intake air (fresh air) from the outside is supplied to the combustion chamber 6 through the intake passage 28 and exhaust gas (burned gas) generated in the combustion chamber 6 is discharged to the outside through the exhaust passage 29. It has become so.

  The intake passage 28 is provided with a common passage portion 28c composed of a single passage, a surge tank 28b provided at the downstream end of the common passage portion 28c, and a branch 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. Specifically, the external EGR device 30 is provided in an EGR passage 31 that connects the common passage portions 28c and 29c of the intake passage 28 and the exhaust passage 29 with each other, and an exhaust gas that passes through the EGR passage 31 in the middle of the EGR passage 31. An EGR valve 32 that controls the gas flow rate and a water-cooled EGR cooler 33 that cools the temperature of the exhaust gas that passes through the EGR passage 31 are provided.

  A throttle valve 25 that adjusts 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 exhaust gas remaining in the combustion chamber 6 is adjusted by the VVL 16, and further, the intake path 28 is set by the external EGR device 30. The amount of exhaust gas recirculated 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.

  In the common passage portion 29c of the exhaust passage 29, a catalytic converter 35 for exhaust gas purification is provided. 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.

  FIG. 2 is an enlarged view for specifically explaining the shape of the crown surface of the piston 5. As shown in FIG. 2 and FIG. 1 above, a concave cavity 40 is provided at the center of the crown surface of the piston 5. The cavity 40 has an upward opening 40a facing the injector 21 at the upper end, and the area (opening area) of the opening 40a is the maximum cross-sectional area inside the cavity 40 (each height of the cavity 40). The maximum horizontal cross-sectional area at the position) is set. That is, the cavity 40 is formed in a constricted shape so that the inner diameter becomes narrower toward the upper side in the range from the opening 40a to a predetermined depth.

  An annular recess 41 having an annular shape in plan view is provided on the crown surface of the piston 5 located outside the cavity 40 in the bore radial direction so as to surround the cavity 40. The annular recess 41 is formed such that the height decreases toward the outer side in the bore radial direction, and the maximum depth (the depth of the outermost peripheral portion) is set shallower than the depth of the cavity 40. Yes.

  Further, an annular standing wall portion 42 protruding upward from the annular recess 41 is provided on the outermost peripheral portion of the piston 5 positioned further outside the annular recess 41 in the bore radial direction. The protruding height of the standing wall portion 42 is set to be the same as the portion (lip portion) surrounding the opening 40 a at the upper end of the cavity 40.

  Returning to FIG. 1, a water jacket (not shown) through which cooling water flows is provided inside the cylinder block 3 and the cylinder head 4 of the engine body 1, and the temperature of the cooling water in the water jacket. The cylinder block 3 is provided with a water temperature sensor SW1 for detecting the above.

  The cylinder block 3 is also provided with a crank angle sensor SW2. The crank angle sensor SW2 outputs a pulse signal according to the rotation of a crank plate (not shown) that rotates integrally with the crankshaft 7, and based on this pulse signal, the rotation angle (crank angle) of the crankshaft 7 is output. ) And rotation speed (engine rotation speed) are detected.

  The cylinder head 4 is provided with a cam angle sensor SW3 for detecting the camshaft angle in the valve mechanism 14. The cam angle sensor SW3 is a pulse signal for determining a cylinder (determining whether each cylinder is in an intake stroke, compression stroke, expansion stroke, or exhaust stroke) according to passage of teeth of a signal plate that rotates integrally with the camshaft. Is output.

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

  Various information is input to the ECU 50 from various sensors provided in the engine. That is, the ECU 50 is electrically connected to the water temperature sensor SW1, the crank angle sensor SW2, and the cam angle sensor SW3 provided in the engine, and based on the input signals from these sensors SW1 to SW3, Various information such as cooling water temperature, crank angle, engine speed, and cylinder discrimination is acquired.

  The ECU 50 also receives information from various sensors provided in the vehicle. For example, the vehicle is provided with an accelerator opening sensor SW4 that detects the opening degree (accelerator opening degree) of an accelerator pedal (not shown) that is depressed by the driver, which is detected by the accelerator opening degree sensor SW4. The accelerator opening is input to the ECU 50.

  A more specific function of the ECU 50 will be described. The ECU 50 includes, as main functional elements, a determination unit 51, an injector control unit 52, an intake control unit 53, an internal EGR control unit 54, and an external EGR control unit 55. , And ignition control means 56.

  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 which operating region in the control map of FIG. It is determined whether or not the engine is operated.

  Specifically, in the control map of FIG. 4, the first operation region A1 is set over the entire rotational speed region in the region where the load T is relatively low. Further, in a region where the load T is higher than the first operation region A1 and the rotational speed Ne is lower than a predetermined value, the second operation region A2 is set and the load T is higher than that in the first operation region A1. The third operation region A3 is set in a region where the rotational speed Ne is higher than the predetermined value. Further, between the first operation region A1 and the second operation region A2 and between the first operation region A1 and the third operation region A3, there are a fourth operation region A4 and a fifth operation region A5, respectively. Is set. Note that the control map of FIG. 4 in which the first to fifth operation regions A1 to A5 are set basically has a temperature at which the coolant temperature detected by the engine coolant temperature sensor SW1 becomes a predetermined value (for example, 80 ° C.) or more. It is a thing at the time of an intermediate state. The description of the control map when the engine is in a cold state is omitted here.

  During operation of the engine, which operating region (A1 to A5) in FIG. 4 corresponds to the operating point of the engine (point on the control map specified from each value of the load T and the rotational speed Ne). Accordingly, an appropriate combustion mode is selected.

  Although details will be described later, in this embodiment, the combustion mode in which the air-fuel mixture is self-ignited by the compression action of the piston 5 even when the operating point of the engine is in any of the first to fifth operating regions A1 to A5. (Compression self-ignition combustion) is selected. However, in each operation region A1 to A5, the fuel injection timing from the injector 21, the air-fuel ratio of the air-fuel mixture based on the injected fuel, the presence or absence of internal EGR or external EGR, and the like are different. For example, in the first operating region A1, fuel is injected during the intake stroke and the internal EGR is executed. In the second and third operating regions A2 and A3, a plurality of times (from the compression stroke to the initial stage of the expansion stroke) ( Alternatively, fuel is injected in a plurality of times from the intake stroke to the initial stage of the expansion stroke, and external EGR is executed. Among these, the second and third operation regions A2 and A3 in which the compressed self-ignition combustion is executed while dividing the fuel are divided are equivalent to the “specific operation regions” according to the present invention.

  The injector control means 52 controls the amount and timing of fuel injected from the injector 21 into the combustion chamber 6. Specifically, the injector control means 52 includes a load T (target torque) calculated from a detection value (accelerator opening) of the accelerator opening sensor SW4, an engine rotational speed Ne specified from the crank angle sensor SW2, and the like. Based on the information, the target fuel injection amount and injection timing are calculated, and the valve opening timing and valve opening period of the injector 21 are controlled based on the calculation result.

  The intake control means 53 performs control to drive the CVVL 15 and change the lift amount (opening amount) of the intake valve 11. For example, when the engine load T is high and the amount of fuel injected from the injector 21 is large, the intake control means 53 is configured so that a large amount of air (fresh air) is introduced into the combustion chamber 6 accordingly. Increase the lift amount. On the other hand, when the engine load T is low and the amount of fuel injected from the injector 21 is small, the amount of fresh air can be reduced, so the lift amount of the intake valve 11 is reduced.

  The internal EGR control means 54 performs an operation (internal EGR) for causing the exhaust gas to remain (reverse flow) in the combustion chamber 6 by driving the VVL 16 to execute or stop the valve opening during the intake stroke of the exhaust valve 12. The presence or absence is switched. In the present embodiment, since two exhaust valves 12 are provided per cylinder, the combustion chamber is switched by switching the number of exhaust valves 12 that are opened during the intake stroke between 0, 1, and 2. The amount of exhaust gas remaining in 6 (internal EGR amount) can be changed stepwise.

  The external EGR control means 55 switches the presence / absence of an operation (external EGR) for returning the exhaust gas from the exhaust passage 29 to the intake passage 28 by adjusting the opening degree of the EGR valve 32 provided in the EGR passage 31. At the same time, the exhaust gas recirculation amount (external EGR amount) by the external EGR is controlled.

  The ignition control means 56 controls the timing (ignition timing) at which the spark plug 20 performs spark discharge. However, in this embodiment, as described above, in any of the first to fifth operation regions A1 to A5 shown in FIG. 4, the combustion mode (compressed self-ignition combustion) that self-ignites the air-fuel mixture irrespective of spark ignition. ) Is selected, the spark ignition from the spark plug 20 is stopped at least when the engine is warm. That is, the spark plug 20 operates only when it is difficult to self-ignite the air-fuel mixture (for example, when forced combustion by spark ignition is necessary), such as when the engine is started or when it is operated in a cold state. Basically it does not work except for.

(3) Combustion mode in each operation region Next, based on the control of the ECU 50 having the above function, how each combustion region (A1, A2, A3, A4, A5) shown in FIG. It will be explained whether a suitable combustion mode is selected. As a premise of this explanation, it is assumed that the engine coolant temperature is sufficiently warm (that is, the engine is warm). In this case, the compression self-ignition combustion is selected regardless of the operation range of the engine in any one of the operation regions A1 to A5. 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, and the like depending on the operation regions A1 to A5. Therefore, the ECU 50 controls the injector 21, CVVL15, VVL16, EGR valve 32, and the like while sequentially determining the operating point of the engine.

  That is, when the engine is started, the ECU 50 determines 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. Then, the following control is executed depending on which of the determined operation regions is one of the first to fifth operation regions A1 to A5 in FIG.

(I) 1st operation area A1
FIG. 5 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, internal EGR is performed in which the exhaust gas generated in the combustion chamber 6 flows back to the combustion chamber 6 by driving the VVL 16 and opening the exhaust valve 12 during the intake stroke. The That is, the exhaust valve 12 is normally opened only in the exhaust stroke (lift curve EX in FIG. 5), but by opening the exhaust valve 12 in the intake stroke based on driving of the VVL 16 (lift curve EX ′), Exhaust gas is caused to flow backward from the exhaust port 10 to the combustion chamber 6. Thus, by causing the high-temperature exhaust gas to flow backward (residual) into the combustion chamber 6, the combustion chamber 6 is heated to promote self-ignition of the air-fuel mixture. Note that the amount of exhaust gas remaining in the combustion chamber 6 (internal EGR amount) 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 internal EGR based on the restart valve (opening during the intake stroke) of the exhaust valve 12 is executed, so the external EGR is stopped. 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.

  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 2 or more. Is set to a lean value. Therefore, the 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. When leaning to λ ≧ 2, the NOx purification action by the three-way catalyst can hardly be expected, but the NOx amount (raw NOx amount) generated by combustion at λ ≧ 2 is greatly reduced. Even if a special catalyst (for example, NOx trap catalyst) is not provided in addition to the original catalyst, the amount of NOx contained in the exhaust gas can be suppressed to a sufficiently small value.

(Ii) Second operation area A2
In the second operation region A2 where the load T is higher than that in the first operation region A1 and the rotation speed Ne is set to a relatively low region, the control as shown in FIG. 6 is executed. That is, in the second operation region A2, the fuel is injected from the injector 21 in two injection timings (P1, P2) set in the vicinity of the compression top dead center and a predetermined time in the previous compression stroke. Split injection is performed. 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.

  Specifically, in the second operation region A2, the timing of the front injection P1 is based on the compression top dead center (TDC) before the top dead center (BTDC) 60 to 50 ° CA (CA represents the crank angle). ) And the timing of the post-injection P2 is set within a period of about 0 to 10 ° CA after top dead center (ATDC). Further, regarding the ratio of each injection amount by the front stage injection P1 and the rear stage injection P2, the front stage injection P1 is about 60% and the rear stage injection from the pattern in which the front stage injection P1 is set to 10% or less and the rear stage injection P2 is set to 90% or more. Up to a pattern in which P2 is set to about 40%, it is set at an appropriate ratio depending on the operating conditions and the like.

  The total injection amount by the front stage injection P1 and the rear stage injection P2 is increased from that in the first operation area A1 (injection quantity by the fuel injection P) in accordance with the high load corresponding to the second operation area 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 in the figure, two peaks with different timings are obtained by the self-ignition of the mixture of fuel and air (fresh air) divided and 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 time, thereby causing in-cylinder pressure. 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 an 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 center of the ceiling of the combustion chamber 6 and the crown surface of the piston 5 is formed in a special shape having a cavity 40 and the like. The injected fuel does not burn together, and it is possible to avoid an increase in combustion noise and a large amount of soot as described above (detailed mechanism will be described later).

  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 driven. 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 in place 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 manner is to prevent excessive combustion of the combustion chamber 6 and 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, and the total amount of injected fuel is large. Therefore, the amount of heat generated with combustion increases, and the combustion chamber 6 tends to be heated. It is in. For this reason, if the internal EGR is continued even in the second operation region A2, the combustion chamber 6 becomes increasingly hot and abnormal combustion such as pre-ignition or knocking may occur. Therefore, the internal EGR is switched to the external EGR, and the exhaust gas that has passed through the EGR passage 31 with the EGR cooler 33 (that is, cooled by the EGR cooler 33) is recirculated to the intake passage 28. High temperature is prevented 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 that is realized based on the control as described above will be described more specifically with reference to FIGS. FIG. 8A shows a state when the upstream injection P1 is performed from the injector 21. FIG. The piston 5 at this time is located at about 60 to 50 ° CA before compression top dead center (BTDC) as described above. When fuel is injected radially from the plural (12) nozzle holes provided at the tip of the injector 21 toward the crown surface of the piston 5 at such a position, the spray of the fuel is sprayed on the crown of the piston 5. It faces the annular recess 41 provided on the outer side of the surface in the bore radial direction.

  The fuel (spray) injected toward the annular recess 41 of the piston 5 is then dispersed while being guided upward by the standing wall portion 42 provided on the outermost peripheral portion of the piston 5, and based on the dispersed fuel, As shown in FIG. 8B, the air-fuel mixture X1 is formed in the outer peripheral portion of the combustion chamber 6 (mainly inside the annular recess 41 and the space above it). 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 a local air-fuel ratio only in the outer peripheral portion 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 theoretical air-fuel ratio is locally formed on the outer peripheral portion of the combustion chamber 6.

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

  The air-fuel mixture X1 formed on the outer peripheral portion 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. It burns by self-ignition as shown in (compression self-ignition). In the figure, the region where the air-fuel mixture X1 is burning is shown in black or gray. The region Y1 where the air-fuel mixture X1 burns is limited to the outer peripheral portion of the combustion chamber 6 corresponding to the region where the air-fuel mixture X1 is formed.

  When combustion based on the front injection P1 as described above starts, the rear injection P2 as shown in FIG. 8 (d) is executed around that time (in the example shown, substantially simultaneously with the start of combustion based on the front injection P1). . As described above, the timing of the post-injection P2 is about ATDC 0 to 10 ° CA at approximately the same time as or immediately after the time when the piston 5 reaches its rising end (compression top dead center). When fuel is injected from the injector 21 at such a timing (near the compression top dead center), the spray of the fuel goes to the inside of the cavity 40 provided in the central portion of the crown surface of the piston 5. . 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. The air-fuel mixture X2 is formed at the center 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 latter-stage injection P2 forms the inside of the cavity 40 in the cavity 40, which is richer than when the first-stage injection P1 is executed (more specifically, based on the fuel injected by the first-stage injection P1. A mixture X2 (richer than the air-fuel ratio of an extremely thin mixture) is formed.

  The air-fuel mixture X2 based on the rear injection P2 as described above is formed in a state where the piston 5 is close to the compression top dead center and combustion of the air-fuel mixture X1 based on the front injection P1 has already occurred. For this reason, as shown in FIG. 8F, the air-fuel mixture X2 reaches self-ignition and burns in a short time after the post-injection P2. The region Y2 where the air-fuel mixture X2 burns is limited to the central portion 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 (fuel region Y1) of the combustion chamber 6 corresponding to the installation portion of the annular recess 41, whereas the air-fuel mixture X2 based on the rear-stage injection P2 Then, combustion occurs in the central portion of the combustion chamber 6 corresponding to the installation portion of the cavity 40 (combustion region Y2 located closer to the center in the bore radial direction than the fuel region Y1).

  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 fuels are not mixed and combusted at the same time. Therefore, an increase in combustion noise due to a sudden rise in in-cylinder pressure and local oxygen There is no worry of increasing soot due to lack. 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.

(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. 7 is executed. That is, in the third operation region A3, the fuel from the injector 21 is injected in three times from the intake stroke to the vicinity of the compression top dead center (P0, P1, P2). Of these, the second fuel injection P1 executed during the compression stroke and the third fuel injection P2 executed near the compression top dead center after that (in the illustrated example, very early in the expansion stroke) are: Each corresponds to the front injection P1 and the rear injection P2 (FIG. 6) in the second operation region A2 described above. On the other hand, the first fuel injection P0 executed during the intake stroke is an injection that is not performed in the second operation region A2, and is specific to the third operation region A3. Therefore, hereinafter, the first fuel injection P0 in the third operation region A3 is referred to as preliminary injection, the second fuel injection P1 is referred to as pre-injection, and the third fuel injection P2 is referred to as post-injection.

  The pre-stage injection P1 and the post-stage injection P2 executed in the third operation area A3 play a role similar to that in the second operation area. That is, the pre-stage injection P1 executed during the compression stroke forms an air-fuel mixture X1 that is unevenly distributed in the outer periphery of the combustion chamber 6 as shown in FIG. 8B, and is executed near the compression top dead center. By the post-injection P2, an air-fuel mixture X2 that is unevenly distributed at the center of the combustion chamber 6 as shown in FIG.

  However, in the third operation region A3, the engine rotation speed Ne is higher than that in the second operation region A2, and the piston speed is faster. Therefore, the injected fuel from the injector 21 is in the vicinity of the vicinity of the crown surface of the piston 5. The piston 5 moves relatively large. Considering this, the timing of the front injection P1 and the rear injection P2 is set slightly earlier than in the second operation region A2. As a result, regardless of the difference in engine speed Ne, the air-fuel mixtures X1, X2 based on the injections P1, P2 are separately formed at the same locations as shown in FIGS. 8B and 8E. These air-fuel mixtures are self-ignited and burned independently (FIGS. 8C and 8F).

  On the other hand, in the third operation region A3, in addition to the front injection P1 and the rear injection P2, the preliminary injection P0 executed during the intake stroke is added because the third operation has a relatively high engine speed Ne. This is because in the region A3, it is difficult to inject a required amount of fuel within a short period of time. That is, in the third operating region A3 where the engine rotational speed Ne is high, unlike the second operating region A2 where the rotational speed Ne is relatively low, the piston speed is fast, so the required amount of fuel is injected over the same time. Even if it tries to do so, the position of the piston 5 may change significantly during that time, and the positional relationship between the piston 5 and the fuel spray may be destroyed.

  For example, in the third operation region A3, if the same amount of fuel as in the second operation region A2 is to be injected by the pre-injection P1, the fuel injection operation from the injector 21 (from the injection port of the injector 21 by opening it) It is necessary to continue the fuel injection operation) for the same period as in the second operation region A2, but in the third operation region A3 where the piston speed is high, the piston 5 If the position of the valve is greatly raised, the proper positional relationship as shown in FIG. 8A may be lost, and the spray direction may deviate from the annular recess 41 of the piston 5. When such a situation occurs, the air-fuel mixture X1 cannot be reliably unevenly distributed in the outer peripheral portion of the combustion chamber 6 corresponding to the installation portion of the annular recess 41, and the subsequent injection P1 after the compression top dead center. It is considered that the air-fuel mixture X1, X2 based on the post-injection P2 cannot be burned sufficiently independently.

  Therefore, in order to surely avoid the above situation, in the third operation region A3, first, the preliminary injection P0 is executed during the intake stroke to inject a small amount of fuel, and during the subsequent compression stroke and the compression top dead center. In the vicinity, the front injection P1 and the rear injection P2 are executed. As a result, the amount of fuel to be injected by the pre-stage injection P1 and the post-stage injection P2 is reduced, so the time required for the injection of the fuel is also reduced, and the above-described problems (the position of the piston 5 changes greatly during the fuel injection). ) Is avoided.

  As described above, when the fuel is injected in three times, that is, the preliminary injection P0, the pre-stage injection P1, and the post-stage injection P2, the pre-stage injection is performed in the extremely lean mixture previously formed in the combustion chamber 6 by the preliminary injection P0. Fuel from P1 and post-injection P2 is additionally injected. Then, in the combustion chamber 6, a rich air-fuel mixture of about λ = 1 based on the preliminary injection P0, the front injection P1, and the rear injection P2 is formed in the outer peripheral portion and the central portion (inside the cavity 40), In other regions, an extremely lean air-fuel mixture (for example, an air-fuel mixture that greatly exceeds λ = 2) based on the preliminary injection P0 alone is formed.

  Even in this case, the fuel injected by the front-stage injection P1 and the fuel injected by the rear-stage injection P2 are basically not mixed and are separated at different locations (the outer periphery of the combustion chamber 6 and the interior of the cavity 40). In) will burn independently. As a result, the combustion chamber 6 as a whole undergoes combustion accompanied by heat generation having two peaks at different times, as conceptually shown in the waveform Qc of FIG. In addition, when the air-fuel ratio mixture as described above burns, the generated exhaust gas can be sufficiently purified by a three-way catalyst, or the amount of NOx produced is extremely small. Is cleared without problems.

  As shown in FIG. 7, in the third operation region A3, the same control as that in the second operation region A2 is executed except for the control related to the fuel split injection 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.

(Iv) Fourth operation region A4 and fifth operation region A5
The fourth operation region A4 and the fifth operation region A5 are regions located between the first operation region A1 and the second operation region A2 or between the first operation region A1 and the third operation region A3. Thus, intermediate control of each control content described in the above (i) to (iii) is executed.

  Specifically, in the fourth operation region A4, the fuel injection timing from the injector 21 is retarded from the fuel injection timing in the first operation region A1, and is set to, for example, once during the compression stroke. The injection amount at that time is set so that the excess air ratio λ is 2 or more, as in the first operation region A1. In addition, external EGR for returning the exhaust gas to the intake passage 28 through the EGR passage 31 is executed.

  On the other hand, in the fifth operation region A5, the control for injecting the fuel is executed in two steps near the predetermined timing during the compression stroke and after the compression top dead center. The injection amount at that time is set so that the excess air ratio λ is locally about λ = 1. Further, the internal EGR for causing the exhaust gas to flow backward to the combustion chamber 6 is performed by opening the exhaust valve 12 during the intake stroke.

(4) Operational Effects As described above, in the gasoline engine of the present embodiment, the concave cavity 40 is provided in the central portion of the crown surface of the piston 5 and the central portion of the ceiling of the combustion chamber 6 facing the cavity 40. The cylinder head 4 is provided with a multi-hole type injector 21 that injects fuel (fuel mainly composed of gasoline) radially. Among the engine operating ranges, in the second and third operating ranges A2 and A3 (specific operating ranges) including the high load range, the pre-stage injection P1 set during the compression stroke and the compression top dead center after that are set. Control for injecting fuel is performed at least twice with the post-injection P2 set in the vicinity. When the pre-injection P1 is executed, at that time, the outer peripheral portion of the combustion chamber 6 located outside the cavity 40 of the piston 5 in the bore radial direction is richer than the inside of the cavity 40 (λ = 1). When the air-fuel mixture X1 is formed and the post-injection P2 is executed, the inside of the cavity 40 is richer than the time when the pre-injection P1 is executed (same as λ = 1, similar to the air-fuel mixture X1) The air-fuel mixture X2 is formed. According to such a configuration, in the second and third operation regions A2 and A3 (specific operation regions) where the load T is relatively high, an appropriate compression self without increasing combustion noise and soot (carbonaceous particles). Ignition combustion can be performed.

  That is, in the above-described embodiment, the injector 21 is provided so as to face the cavity 40 provided in the central portion of the crown surface of the piston 5, and at least two injection timings after the compression stroke (pre-stage injection P1) from the injector 21. In addition, as shown in FIGS. 8A to 8F, the fuel is injected separately into the second-stage injection P2), and as shown in FIGS. 8A to 8F, separate spaces in the combustion chamber 6 (the outer peripheral portion of the combustion chamber 6 and the cavity 40). Since the air-fuel mixtures X1 and X2 are formed separately inside, the air-fuel mixtures X1 and X2 can be self-ignited and burned independently in the vicinity of the compression top dead center. For this reason, it is possible to effectively prevent an increase in combustion noise due to a sudden rise in in-cylinder pressure and an increase in soot due to a local lack of oxygen, without the fuel that is separately injected mixed and combusted simultaneously. Can do.

  In particular, in the second and third operation regions A2 and A3 where the load T is relatively high, the total fuel that needs to be injected from the injector 21 increases, and therefore, the fuel that is divided and injected is mixed and burned simultaneously. As a result, the in-cylinder pressure suddenly rises and a considerable amount of combustion noise is generated, and a large amount of soot may be generated. However, as in the above embodiment, when the separately injected fuel is burned independently while being separated into separate spaces, the above problems can be effectively avoided and combustion noise can be reduced. It is possible to improve emission while reducing.

  Further, in the above-described embodiment, the annular concave portion 41 having a circular shape in a plan view is formed on the crown surface of the piston 5 that is located on the outer side in the bore radial direction with respect to the cavity 40, and the height decreases toward the outer side in the bore radial direction. Therefore, the spatial separation of the air-fuel mixture X1, X2 based on the preceding injection P1 and the succeeding injection P2 can be achieved more reliably.

  That is, if the annular recess 41 is provided outside the cavity 40 in the bore diameter direction, the fuel injected by the preceding injection P1 is received by the annular recess 41, so that the installation portion of the annular recess 41 is provided. The air-fuel mixture X1 based on the preceding stage injection P1 can be reliably retained on the outer peripheral portion of the corresponding combustion chamber 6. As a result, the air-fuel mixture X1 based on the preceding injection P1 can be clearly separated from the air-fuel mixture X2 formed in the cavity 40 based on the subsequent post-injection P2, and the combustion of the air-fuel mixtures X1 and X2 is independent of combustion. Sex can be secured more reliably.

  In particular, in the above-described embodiment, since the standing wall portion 42 that guides the fuel injected by the front injection P1 upward is further provided outside the annular recess 41 in the bore radial direction, The fuel of the pre-injection P1 injected toward the upper side is wound up upward along the standing wall portion 42, so that the fuel is sufficiently dispersed, vaporized and atomized, and the formation of the air-fuel mixture X1 in the outer peripheral portion of the combustion chamber 6 is effective. Can be promoted.

  Moreover, in the said embodiment, since the cavity 40 is formed in the shape of a constriction and the area of the opening part 40a of the upper end is set smaller than the largest cross-sectional area inside the cavity 40, it injected by the back | latter stage injection P2. The mixture X2 based on the fuel can be reliably kept inside the cavity 40, and the mixture X2 based on the subsequent injection P2 is clearly separated from the mixture X1 based on the preceding injection P1. Can be formed.

  Further, in the above-described embodiment, the exhaust gas generated in the combustion chamber 6 is taken into the intake region in the operation region (second and third operation regions A2, A3) in which the split injection of fuel is performed by the front injection P1 and the rear injection P2. Since the external EGR that recirculates from the passage 28 to the exhaust passage 29 through the EGR passage 31 is executed, the pumping loss can be effectively reduced and the fuel consumption can be improved. The pumping loss can be similarly reduced by the internal EGR that causes the exhaust gas to remain in the combustion chamber 6. However, in the second and third operation regions A2 and A3 having a relatively high load, the fuel injection amount is reduced. Therefore, if the high-temperature exhaust gas is left in the combustion chamber 6 as it is, the combustion chamber 6 becomes excessively hot and abnormal combustion such as pre-ignition or knocking may occur. On the other hand, in the above embodiment, since the external EGR that recirculates the exhaust gas through the EGR passage 31 is executed instead of the internal EGR, the occurrence of abnormal combustion due to excessively high temperature in the combustion chamber 6 is avoided. The pumping loss can be reduced.

  Moreover, in the said embodiment, in the 3rd driving | operation area | region A3 located in the relatively high rotation side among 2nd, 3rd driving | running area | region A2, A3 in which the said division | segmentation injection is performed, the said front stage injection P1 and a back | latter stage injection. Prior to P2, preliminary injection P0 for injecting fuel during the intake stroke is performed. According to such a configuration, even when the total amount of fuel to be injected is large and the piston speed is high, the air-fuel mixture X1 and X2 can be accurately separated and formed by the front injection P1 and the rear injection P2.

  In other words, in the third operation region A3 including the high rotation and high load region, if a large amount of fuel to be injected is to be injected forcibly by only two injections, the first injection P1 and the second injection P2, the piston is injected during the injection. It is considered that the air-fuel mixture X1 and X2 cannot be formed at a desired location because the position 5 greatly changes. On the other hand, when the preliminary injection P0 is executed before the front stage injection P1 and the rear stage injection P2 as in the above embodiment, the fuel injection amount by the front stage injection P1 and the rear stage injection P2 is reduced. Therefore, the air-fuel mixture X1, X2 can be separated and formed accurately while ensuring sufficient engine output according to the load.

  In the above embodiment, in the second operation region A2, the pre-stage injection P1 is executed within a period of about 60 to 50 ° CA before the compression top dead center, and within a period of about 0 to 10 ° CA after the compression top dead center. In the third operation region A3 where the rotational speed Ne is higher than that in the second operation region A2, the timing slightly higher than the above timing is considered in consideration of the increase in the piston speed. However, the timing of each of the fuel injections P1 and P2 is determined by the injection angle of fuel from the injector 21 (expansion angle with respect to the center axis of the cylinder) and the crown surface of the piston 5. It should be set appropriately according to the shape and the like, and the above embodiment is merely an example.

  For example, when the injection angle of the fuel from the injector 21 is smaller than the example of the above embodiment, an angle closer to the vertical downward direction (that is, an angle that does not greatly expand outward in the bore radial direction unless it is far from the injector 21). Therefore, assuming that the shape of the crown surface of the piston 5 is the same as that in the above embodiment, the front injection P1 and the rear injection P2 must be executed at an earlier timing than in the above embodiment. In this case, the fuel from the injections P1 and P2 cannot be unevenly distributed at desired locations (the outer periphery of the combustion chamber 6 and the inside of the cavity 40).

  However, even if there is an influence due to such a difference in the injection angle and the like, if the injection angle that can be set and the arrangement balance of the cavity 40 and the annular recess 41 are taken into consideration, the post-injection P2 is at least from the latter stage of the compression stroke It is necessary to execute at any timing until the beginning of the expansion stroke, and the front injection P1 needs to be executed before the post injection P2 and during the compression stroke. Here, the latter term of the compression stroke refers to a later stage when the compression stroke is divided into an initial stage, a middle stage, and a late stage, and indicates a range of 60 to 0 ° CA before compression top dead center (BTDC). Similarly, the initial stage of the expansion stroke is an initial stage when the expansion stroke is divided into an initial stage, an intermediate stage, and a late stage, and indicates a range of 0 to 60 ° CA after compression top dead center (ATDC).

  Further, in the above embodiment, when the engine is warm, the compression is performed so that the air-fuel mixture is self-ignited without using the spark plug 20 regardless of the operating range of the engine A1 to A5 shown in FIG. Although self-ignition combustion is performed, for example, particularly on the high rotation side in the third operation region A3, the combustion temperature tends to be low due to external EGR, and the heat receiving period of the fuel is short. There is a risk that self-ignition will not occur. Therefore, in such an operation region, as a measure for accelerating the self-ignition of the air-fuel mixture (ignition assist), spark ignition by the spark plug 20 may be performed in the vicinity of the compression top dead center.

  Moreover, in the said embodiment, although the injector 21 was a multi-hole type injector and 12 nozzle holes were provided in the front-end | tip part, the number of nozzle holes is not restricted to 12, It is possible even if more than 12 nozzle holes. It may be less. 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. If the number of nozzle holes is eight or more, it is possible to form an air-fuel mixture having a substantially uniform air-fuel ratio in the circumferential direction in a very short time after performing the preceding injection P1 and the following injection P2. It is possible to properly perform combustion by self-ignition (compression self-ignition combustion).

5 Piston 6 Combustion chamber 21 Injector 31 EGR passage 40 Cavity 41 Annular recess 42 Standing wall 50 ECU (control means)
A2 Second operation area (specific operation area)
A3 3rd operation area (specific operation area)
P0 Pre-injection P1 Pre-stage injection P2 Post-stage injection

Claims (8)

A gasoline engine that reciprocates a piston by burning a mixture of fuel and air, at least part of which is gasoline, in a combustion chamber,
A concave cavity provided in the center of the crown of the piston;
A multi-hole injector that injects the fuel in a plurality of directions from the center of the combustion chamber ceiling facing the cavity;
Control means for controlling the fuel injection operation by the injector,
The control means injects fuel from the injector in at least two times of front-stage injection and rear-stage injection in a predetermined specific operation region of the engine, and automatically mixes the injected fuel and air. Burned by ignition,
The pre-stage injection injects fuel during the compression stroke and before the post-stage injection, and by the injected fuel, the outer circumference of the combustion chamber located outside the piston cavity in the bore radial direction, A richer air-fuel mixture than the inside of the cavity,
In the latter-stage injection, fuel is injected at a predetermined time from the latter stage of the compression stroke to the initial stage of the expansion stroke , and combustion of the air- fuel mixture based on the earlier- stage injection has already occurred in the outer peripheral portion of the combustion chamber by the injected fuel. In this state, a richer air-fuel mixture is formed inside the cavity than when the preceding injection is performed,
In at least a part on the low rotation side in the specific operation region, the timing of the preceding injection is set to the latter half of the compression stroke, and the timing of the latter injection is set to any one from the compression top dead center to the initial stage of the expansion stroke. gasoline engine, characterized in that that. The gasoline engine according to claim 1, wherein
A gasoline engine, characterized in that an annular concave portion having a circular shape in plan view is formed on a piston crown surface located on the outer side in the bore radial direction from the cavity. The gasoline engine according to claim 2, wherein
A gasoline engine characterized in that a standing wall portion for guiding the fuel injected by the preceding injection upward is formed further outside the annular recess in the bore radial direction. The gasoline engine according to any one of claims 1 to 3,
The gasoline engine according to claim 1, wherein the cavity is formed in a constricted shape so that an opening area at an upper end thereof is smaller than a maximum cross-sectional area inside the cavity. The gasoline engine according to any one of claims 1 to 4,
The injector has eight or more injection holes at its tip, and injects fuel radially through the eight injection holes so as to expand outward in the bore diameter direction as it approaches the piston crown surface. Characteristic gasoline engine. The gasoline engine according to any one of claims 1 to 5,
The gasoline engine characterized in that the specific operation region includes at least a high load region. The gasoline engine according to claim 6,
In the specific operation region, a gasoline engine is characterized in that external EGR is performed to recirculate exhaust gas generated in the combustion chamber to the intake passage through the EGR passage. The gasoline engine according to claim 6 or 7,
A gasoline engine characterized in that preliminary injection for injecting fuel during an intake stroke is executed prior to the front-stage injection and the rear-stage injection at least in a high-speed region in the specific operation region.