JP5887877B2 - Start control device for compression self-ignition engine - Google Patents

Description

  The present invention is provided in a compression self-ignition engine that burns fuel injected from a fuel injection valve into a cylinder by self-ignition, and automatically stops the engine when a predetermined automatic stop condition is satisfied, and then Start control of a compression self-ignition engine that restarts the engine by injecting fuel from the fuel injection valve while applying a rotational force to the engine using a starter motor when the restart condition is satisfied Relates to the device.

Compressed self-ignition engines such as diesel engines are generally more widely used as in-vehicle engines in recent years because they are more thermally efficient than spark ignition engines such as gasoline engines and emit less CO ₂ I am doing.

In the compression self-ignition engine as described above, in order to further reduce CO ₂ , the engine is automatically stopped at the time of idle operation or the like, and then the engine is restarted when the vehicle is started. It is effective to adopt a so-called idle stop control technique, and various studies have been conducted on this.

  For example, in Patent Document 1 below, when a predetermined automatic stop condition is satisfied, the diesel engine is stopped, and when a predetermined restart condition is satisfied, fuel injection is performed while the starter motor is driven to restart the diesel engine. In a control device for a diesel engine to be started, it is disclosed to variably set a cylinder for injecting fuel first based on a piston stop position of a cylinder stopped in a compression stroke (compression stroke cylinder at stop).

  Specifically, in this document, when the diesel engine is automatically stopped, the stop position of the piston of the stop-time compression stroke cylinder in the compression stroke at that time is obtained, and the piston stop position is relatively close to the bottom dead center. In the case of being in the proper position, the first fuel is injected into the above-mentioned compression stroke cylinder at the time of stop so that the first compression top dead center of the engine as a whole is obtained. Combustion is resumed from the time of arrival (hereinafter referred to as “one compression start”).

  On the other hand, when the piston in the compression stroke at the time of stop is at the top dead center side from the appropriate position, the cylinder stopped in the intake stroke (intake stroke cylinder at the time of stop) shifts to the compression stroke and then the cylinder first By injecting this fuel, combustion is restarted from the time when the engine reaches the second compression top dead center (hereinafter referred to as “two-compression start”). As described above, the two-compression start in which fuel is injected into the stop intake stroke cylinder instead of the stop compression stroke cylinder is performed when the piston of the stop compression stroke cylinder is deviated from the appropriate position to the top dead center side. Because the compression allowance (stroke amount to top dead center) by the piston is small and the air in the cylinder does not sufficiently heat up, misfire may occur even if fuel is injected into the compression stroke cylinder at the time of stop. is there.

JP 2009-62960 A

  In the technique of the above-mentioned patent document 1, when the piston of the compression stroke cylinder at the time of stoppage is in the proper position, the engine can be restarted quickly by one compression start, but when it has deviated from the proper position to the top dead center side. Requires a two-compression start, which increases the time required for restart. That is, in the two-compression start, since the fuel is injected after waiting for the intake stroke cylinder at the time of stoppage to shift to the compression stroke, the energy from combustion is used until the engine reaches the second compression top dead center. Cannot be done, and the restart time becomes longer accordingly. For this reason, it has been desired to enable engine restart by one compression start as frequently as possible.

  By the way, in order to increase the frequency of the one-compression start, it is necessary to improve the ignitability of the fuel injected into the compression cylinder at the time of stop. For this purpose, it is effective to promote atomization of the fuel. To promote atomization of the fuel, the fuel pressure of the fuel injection valve (the pressure of the fuel supplied to the fuel injection valve) may be increased. However, after the engine is stopped by automatic stop control, the fuel is supplied to the fuel injection valve. Since the supply pump (which generally operates with the driving force of the engine) is also stopped, it is impossible to increase the fuel pressure. Of course, if the supply pump is replaced with an electric one, the supply pump can be driven even when the engine is stopped, but this increases the cost.

  The present invention has been made in view of the above circumstances, and by executing appropriate fuel injection control while atomizing fuel, the opportunity for quick restart by one compression start is further increased. It is an object of the present invention to provide a restart control device for a compression self-ignition engine capable of performing the same.

In order to solve the above problems, the present invention is provided in a compression self-ignition engine that burns fuel injected from a fuel injection valve into a cylinder by self-ignition, and when a predetermined automatic stop condition is satisfied. When the engine is automatically stopped and then a predetermined restart condition is satisfied, the engine is restarted by injecting fuel from the fuel injection valve while applying a rotational force to the engine using a starter motor. A start control device for a compression self-ignition engine to be started, wherein a specific range in which a piston of a stop-time compression stroke cylinder that is stopped in a compression stroke due to the automatic stop is set to a lower dead center side than a predetermined reference stop position Determining means for determining whether or not the engine is in the specified range, and it is determined that the piston of the compression stroke cylinder at the time of stop has stopped within the specific range, and the engine restart condition When the fuel injection valve is established, the fuel injection valve comprises: an injection control means for controlling the fuel injection valve to inject the first fuel into the stop-time compression stroke cylinder; and a fuel pressure adjusting means for adjusting a fuel pressure of the fuel injection valve. The valve is supplied with fuel from a supply pump mechanically driven by an engine, and the piston has a cavity recessed in a predetermined portion of the crown surface facing the fuel injection valve, compared to other portions. And the fuel pressure adjusting means adjusts the fuel pressure of the fuel injection valve between the time when the automatic engine stop condition is established and the time when fuel cut from the fuel injection valve is stopped. After the restart condition is satisfied, at least the first fuel injection to the compression stroke cylinder at the time of stop is performed, and the injection control means performs the peak heat generation rate after the compression top dead center. A main injection to cause main combustion, such as welcome, a plurality of times to cause pre-combustion as peaking of heat release rate before the start of the main injection is performed and the pre-injection, the plurality of times The pre-injection is performed at a timing at which all of the fuel from each pre-injection is contained in the cavity of the piston (claim 1).

According to the present invention, after the engine is automatically stopped, pre-injection is first executed and then main injection is executed at the time of one compression start in which the engine is restarted by fuel injection into the stop-time compression stroke cylinder. . The pre-injected fuel is combusted by self-ignition after a predetermined ignition delay (pre-combustion), and increases the in-cylinder temperature and pressure of the compression stroke cylinder at the time of stop, so when main injection is subsequently executed, The injected fuel will burn soon by self-ignition (main combustion). This main combustion is a combustion that reaches the peak of the heat generation rate after passing through the compression top dead center, and acts to push down the piston after passing through the compression top dead center, so that a positive torque is applied to the engine. , Increase its rotational speed.
In particular, in the present invention, a plurality of pre-injections are executed at a timing such that the injected fuel fits in the piston cavity, so that a rich mixture that easily ignites is reliably formed in the cavity of the piston crown surface. can do. As a result, the pre-injected fuel is self-ignited more reliably (pre-combustion), which promotes self-ignition of the main injected fuel (main combustion).

  Thus, since the ignitability of the fuel injected by the main injection is improved by the pre-injection (pre-combustion) before that, even if the compression allowance in the stop compression stroke cylinder is not so much, the stop compression stroke cylinder Combustion at is ensured.

  Further, in the present invention, since the control from the time when the automatic stop condition is satisfied to the time when the fuel cut is performed is performed to increase the fuel pressure of the fuel injection valve, the pre-injection and the main injection are performed. In addition, the fuel can be atomized by a relatively high fuel pressure, and the ignitability of the fuel can be further improved. In addition, since the fuel pressure is raised in advance before the fuel cut is performed, a general mechanical pump (a pump mechanically driven by the engine) is used as a supply pump for supplying fuel to the fuel injection valve. However, the control for increasing the fuel pressure can be performed without difficulty. Thereby, without incurring extra cost, it is possible to reliably increase the fuel pressure and improve the ignitability.

  As described above, according to the above embodiment in which the ignitability of the fuel is greatly improved, the specific range that is the piston stop position range where one compression start is possible can be expanded to the top dead center side. Opportunities can be increased to ensure quick startability.

  In the present invention, preferably, the fuel injection valve is of a multi-injection type having 8 to 12 injection holes.

  According to this structure, atomization of the injected fuel can be further promoted by a synergistic effect with the above-described operation by the fuel pressure increase control (control to increase the fuel pressure of the fuel injection valve in advance before the fuel cut). Can improve the ignitability.

In the present invention, preferably, the compression self-ignition engine is a diesel engine having a geometric compression ratio set to less than 16. ( Claim 3 )

  A diesel engine having a geometric compression ratio of less than 16 has a compression ratio lower than that of a diesel engine that has been widely used so far, and a compression allowance by a piston that stops in the middle of the compression stroke (compression from the piston stop position). Since there are many cases where the stroke amount to the top dead center is not sufficiently secured, the ignitability at the time of restart is relatively deteriorated. Therefore, the configuration of the present invention that improves the ignitability at the time of restart by increasing the fuel pressure and pre-injection can be suitably applied to a diesel engine having a geometric compression ratio of less than 16.

  As described above, according to the start-up control device for a compression self-ignition engine of the present invention, an appropriate fuel injection control is executed while atomizing the fuel, thereby allowing a quick restart opportunity by one compression start. Can be increased more.

It is a figure showing the whole diesel engine composition to which the starting control device concerning one embodiment of the present invention was applied. It is a system diagram which shows schematic structure of the fuel supply system of the said engine. It is a time chart which shows the change of each state quantity at the time of the automatic stop control of the engine. It is explanatory drawing for showing the effect | action by the engine automatic stop control, (a) shows the piston position of each cylinder immediately before an engine stop, (b) has shown the piston position after completion | finish of an engine stop. . It is a flowchart which shows the specific operation example of the said engine automatic stop control. It is a flowchart which shows the specific operation example of the restart control of the said engine. It is explanatory drawing which shows the behavior of the fuel pre-injected at the time of the said engine restart, Comprising: (a) shows the case where the frequency | count of pre-injection is 1 time, (b) shows the frequency | count of pre-injection in multiple times. Shows the case. It is the figure which showed the change of the equivalence ratio in a cylinder after performing pre-injection according to the frequency | count of pre-injection. It is a figure which shows the relationship between the equivalence ratio of air-fuel | gaseous mixture, and ignition delay time. It is a figure which shows what timing the fuel based on each injection burns when pre-injection and main injection are performed.

(1) Overall Configuration of Engine FIG. 1 is a diagram showing an overall configuration of a diesel engine to which a start control device according to an embodiment of the present invention is applied. The diesel engine shown in the figure is a four-cycle diesel engine mounted on a vehicle as a power source for driving driving. The engine body 1 of this engine is of a so-called in-line 4-cylinder type, and is provided on the upper surface of the cylinder block 3 having a cylinder block 3 having four cylinders 2A to 2D arranged in a line in a direction orthogonal to the paper surface. A cylinder head 4 and a piston 5 inserted in each of the cylinders 2A to 2D so as to be reciprocally slidable are provided.

  A combustion chamber 6 is formed above the piston 5, and light oil as fuel is supplied to the combustion chamber 6 by injection from a fuel injection valve 15 described later. The injected fuel (light oil) is self-ignited in the combustion chamber 6 that has been heated to a high temperature and pressure by the compression action of the piston 5 (compression self-ignition), and the piston 5 pushed down by the expansion force due to the combustion is moved vertically. It is designed to reciprocate.

  The piston 5 is connected to a crankshaft 7 via a connecting rod (not shown), and the crankshaft 7 rotates around the central axis in accordance with the reciprocating motion (vertical motion) of the piston 5. .

  Here, in the four-cycle four-cylinder diesel engine as shown in the figure, the piston 5 provided in each of the cylinders 2A to 2D moves up and down with a phase difference of 180 ° (180 ° CA) in crank angle. For this reason, the timing of combustion (fuel injection therefor) in each of the cylinders 2A to 2D is set to a timing shifted in phase by 180 ° CA. Specifically, if the cylinder numbers of the cylinders 2A, 2B, 2C, and 2D are 1, 2, 3, and 4, respectively, the first cylinder 2A → the third cylinder 2C → the fourth cylinder 2D → the second cylinder Combustion is performed in the order of 2B. Therefore, for example, if the first cylinder 2A is in the expansion stroke, the third cylinder 2C, the fourth cylinder 2D, and the second cylinder 2B are in the compression stroke, the intake stroke, and the exhaust stroke, respectively.

  The cylinder head 4 is provided with an intake port 9 and an exhaust port 10 that open to the combustion chambers 6 of the cylinders 2A to 2D, and an intake valve 11 and an exhaust valve 12 that open and close the ports 9 and 10, respectively. 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 and the like disposed in the cylinder head 4.

  The cylinder head 4 is provided with one fuel injection valve 15 for each of the cylinders 2A to 2D. Each fuel injection valve 15 is of a multiple injection hole type having a plurality of injection holes at the tip, and inside thereof, a fuel passage that communicates with each of the injection holes and an electromagnetic for opening and closing the fuel passage. And a needle-like valve body that is driven by the motor (not shown). Then, the valve body is driven in the opening direction by electromagnetic force generated by energization, so that fuel supplied at a high pressure from a common rail 40 (accumulation chamber), which will be described later, is directly injected toward the combustion chamber 6 from each of the nozzle holes. It has become so. In addition, the fuel injection valve 15 in this embodiment has many injection holes called 8-12 pieces.

  A cavity 5a is formed in the center portion of the crown surface (upper surface) of the piston 5 facing the fuel injection valve 15 so as to be recessed below other portions (peripheral edge portions of the crown surface). For this reason, when the fuel is injected from the fuel injection valve 15 with the piston 5 being near the top dead center, the fuel first enters the cavity 5a.

  Here, the engine body 1 of the present embodiment has a geometric compression ratio (ratio of the combustion chamber volume when the piston 5 is at bottom dead center and the combustion chamber volume when the piston 5 is at top dead center). Is set to 14. That is, the geometric compression ratio of a general vehicle-mounted diesel engine is often set to 18 or more, whereas in this embodiment, the geometric compression ratio is set to a considerably low value of 14. Has been.

  A water jacket (not shown) through which cooling water flows is provided inside the cylinder block 3 and the cylinder head 4, and a water temperature sensor SW1 for detecting the temperature of the cooling water in the water jacket is provided in the cylinder. It is provided in the block 3.

  The cylinder block 3 is provided with a crank angle sensor SW2 for detecting a rotation angle and a rotation speed of the crankshaft 7. The crank angle sensor SW2 outputs a pulse signal according to the rotation of the crank plate 25 that rotates integrally with the crankshaft 7.

  Specifically, a large number of teeth lined up at a constant pitch are projected on the outer peripheral portion of the crank plate 25, and a tooth missing portion 25a (teeth) for specifying a reference position is provided in a predetermined range on the outer peripheral portion. A portion where no is present) is formed. The crank plate 25 having the tooth missing portion 25a at the reference position rotates in this way, and a pulse signal based on the crank plate 25 is output from the crank angle sensor SW2, whereby the rotation angle (crank angle) of the crankshaft 7 and The rotational speed (engine rotational speed) is detected.

  On the other hand, the cylinder head 4 is provided with a cam angle sensor SW3 for detecting the angle of a camshaft (not shown) for valve actuation. The cam angle sensor SW3 outputs a pulse signal for cylinder discrimination according to the passage of the teeth of the signal plate that rotates together with the camshaft.

  That is, the pulse signal output from the crank angle sensor SW2 includes a no-signal portion generated every 360 ° CA corresponding to the above-mentioned tooth missing portion 25a. When the piston 5 is moving up, it is impossible to determine which cylinder corresponds to the compression stroke or the exhaust stroke. Therefore, a pulse signal is output from the cam angle sensor SW3 based on the rotation of the camshaft that rotates once every 720 ° CA, the timing at which the signal is output, and the timing of the non-signal portion of the crank angle sensor SW2 (tooth missing). The cylinder discrimination is performed on the basis of the passage timing of the section 25a.

  An intake passage 28 and an exhaust passage 29 are connected to the intake port 9 and the exhaust port 10, respectively. That is, intake air (fresh air) from the outside is supplied to the combustion chamber 6 through the intake passage 28 and exhaust gas (combustion gas) generated in the combustion chamber 6 is discharged to the outside through the exhaust passage 29. It is like that.

  Of the intake passage 28, a range from the engine body 1 to the upstream side by a predetermined distance is a branch passage portion 28a branched for each of the cylinders 2A to 2D, and the upstream end of each branch passage portion 28a is a surge tank 28b. It is connected to the. A common passage portion 28c including a single passage is provided on the upstream side of the surge tank 28b.

  The common passage portion 28c is provided with an intake throttle valve 30 for adjusting the amount of air (intake flow rate) flowing into the cylinders 2A to 2D. The intake throttle valve 30 is basically fully opened during operation of the engine or maintained at a high opening degree close thereto, and is configured to be closed only when necessary, such as when the engine is stopped, to block the intake passage 28. Has been.

  An air flow sensor SW4 for detecting the intake air flow rate is provided in the common passage portion 28c between the intake throttle valve 30 and the surge tank 28b.

  An alternator 32 is connected to the crankshaft 7 via a belt or the like. This alternator 32 incorporates a regulator circuit that controls the current of a field coil (not shown) and adjusts the amount of power generation. The alternator 32 has a target value of power generation (target power generation determined from the electric load of the vehicle, the remaining battery capacity, etc.). Based on the current), the driving force is obtained from the crankshaft 7 to generate power.

  The cylinder block 3 is provided with a starter motor 34 for starting the engine. The starter motor 34 has a motor body 34a and a pinion gear 34b that is rotationally driven by the motor body 34a. The pinion gear 34b meshes with a ring gear 35 connected to one end of the crankshaft 7 so as to be detachable. When the engine is started using the starter motor 34, the pinion gear 34b moves to a predetermined meshing position and meshes with the ring gear 35, and the rotational force of the pinion gear 34b is transmitted to the ring gear 35. Thus, the crankshaft 7 is driven to rotate.

  FIG. 2 is a system diagram showing a schematic configuration of a fuel supply system for supplying fuel to the fuel injection valve 15. As shown in FIG. 2 (partially as shown in FIG. 1), the fuel supply system of the engine of this embodiment includes a fuel tank 40 for storing fuel (light oil), a fuel tank 40, Connected via a fuel pipe 41, a supply pump 43 that pumps up the fuel in the fuel tank 40 and sends it to a high pressure state, and a fuel pump 41 that is provided in the middle of the fuel pipe 41 while heating the fuel pumped up from the fuel tank 40 A fuel filter 42 for filtering, a fuel supply pipe 44 through which fuel pumped from the supply pump 43 passes, a single common rail (accumulation chamber) 45 connected to the downstream end of the fuel supply pipe 44, a common rail 45, A plurality of (four) branch pipes 46 connecting the fuel injection valves 15 of the cylinders 2A to 2D and the pressure (fuel pressure) of the fuel stored in the common rail 45 are below a predetermined value. The fuel and pressure limiter 47 that discharges from the common rail 45 includes a return pipe 48 for returning fuel discharged from the common rail 45 to the fuel tank 40 through the pressure limiter 47 when it becomes.

  The supply pump 43 is a mechanical plunger pump that operates with the driving force of the engine. Specifically, the input shaft 43a of the supply pump 43 is connected to the camshaft of the engine body 1 via a gear mechanism or a chain. When the input shaft 43a rotates in conjunction with the camshaft, the plunger built in the supply pump 43 reciprocates, and fuel is pumped from the supply pump 43 according to the reciprocation. ing.

  In addition, the supply pump 43 allows a part of the fuel pushed out by the plunger to escape to the return pipe 48 through the regulator pipe 50, so that the pressure of the fuel pumped from the supply pump 43 toward the common rail 45 is within a certain range. A suction control valve 43b (hereinafter abbreviated as “SCV”) for adjusting to the above is incorporated.

  An injector return pipe 49 is connected to the fuel injection valve 15 of each of the cylinders 2 </ b> A to 2 </ b> D to return the remaining fuel that has not been injected to the fuel tank 40. The downstream end of the injector return pipe 49 is connected to the regulator pipe 50, and the fuel discharged through the injector return pipe 49 is returned to the fuel tank 40 through the regulator pipe 50 and the return pipe 48. .

  A check valve 55 is provided in the middle of the injector return pipe 49. The check valve 55 has a function of keeping the pressure in the injector return pipe 49 constant by opening when the pressure of the fuel on the upstream side exceeds a predetermined value.

  The common rail 45 is provided with a fuel pressure sensor SW5 for detecting the pressure (fuel pressure) of the fuel stored therein. In addition, since the pressure in the common rail 45 is the same as the pressure of the fuel supplied to the fuel injection valve 15, hereinafter, the pressure detected by the fuel pressure sensor SW5 is referred to as “fuel pressure of the fuel injection valve 15”. Also called.

(2) Control System As shown in FIG. 1, the engine configured as described above is centrally controlled by an ECU (engine control unit) 60 for each part. As is well known, the ECU 60 is a microprocessor including a CPU, a ROM, a RAM, and the like.

  Various information is input to the ECU 60 from various sensors. That is, the ECU 60 is electrically connected to the water temperature sensor SW1, the crank angle sensor SW2, the cam angle sensor SW3, the air flow sensor SW4, and the fuel pressure sensor SW5 provided in each part of the engine, and these sensors SW1 to SW5. Based on the input signal from the engine, various information such as engine coolant temperature, crank angle, rotation speed, cylinder discrimination information, intake air flow rate, fuel pressure, and the like are acquired.

  The ECU 60 also receives information from various sensors (SW6 to SW9) provided in the vehicle. That is, the vehicle includes an accelerator opening sensor SW6 for detecting the opening degree of the accelerator pedal 36 that is depressed by the driver, and a brake sensor for detecting ON / OFF of the brake pedal 37 (presence of braking). SW7, a vehicle speed sensor SW8 for detecting the traveling speed (vehicle speed) of the vehicle, and a battery sensor SW9 for detecting the remaining capacity of the battery (not shown) are provided. Based on the input signals from these sensors SW6 to SW9, the ECU 60 acquires information such as the accelerator opening, the presence or absence of a brake, the vehicle speed, and the remaining battery capacity.

  The ECU 60 controls each part of the engine while performing various calculations based on input signals from the sensors SW1 to SW9. Specifically, the ECU 60 is electrically connected to the fuel injection valve 15, the intake throttle valve 30, the alternator 32, the starter motor 34, the SCV 43b, and the like. Each outputs a control signal for driving.

  More specific functions of the ECU 60 will be described. For example, during normal operation of the engine, the ECU 60 controls the SCV 43b and the like to maintain the fuel pressure of the fuel injection valve 15 at a desired value, or controls the fuel injection valve 15 to fuel the required amount of fuel according to the operating conditions. In addition to having a function of executing basic control such as injecting from the injection valve 15 or generating the required amount of electric power according to the electric load of the vehicle or the remaining capacity of the battery by controlling the alternator 32, so-called idle stop As a function, it also has a function of automatically stopping or restarting the engine under a predetermined specific condition. For this reason, the ECU 60 includes an automatic stop control unit 61 and a restart control unit 62 as functional elements relating to the automatic stop or restart control of the engine.

  The automatic stop control unit 61 determines whether or not a predetermined engine automatic stop condition is satisfied during operation of the engine, and executes control to automatically stop the engine when it is satisfied. .

  For example, it is determined that the automatic stop condition is satisfied when a plurality of requirements such as the vehicle being in a stopped state are satisfied and it is confirmed that there is no problem even if the engine is stopped. Then, the engine is stopped by stopping fuel injection from the fuel injection valve 15 (fuel cut) or the like.

  The restart control unit 62 determines whether or not a predetermined restart condition is satisfied after the engine is automatically stopped, and executes control to restart the engine when the restart condition is satisfied.

  For example, when the driver needs to start the engine by depressing the accelerator pedal 36 to start the vehicle, it is determined that the restart condition is satisfied. Then, the engine is restarted by driving the starter motor 34 to apply rotational force to the crankshaft 7 and restarting fuel injection from the fuel injection valve 15.

(3) Automatic Stop Control Next, the details of the automatic engine stop control executed by the automatic stop control unit 61 of the ECU 60 will be described more specifically. As will be apparent from the description here, in this embodiment, the automatic stop control unit 61 of the ECU 60 functions as a fuel pressure adjusting means for increasing the fuel pressure of the fuel injection valve 15.

  FIG. 3 is a time chart showing changes in each state quantity at the time of engine automatic stop control. In this figure, the time point when the automatic engine stop condition is satisfied is set to t0. When the automatic stop condition is satisfied at this time t0, the target fuel pressure that is the target value of the fuel pressure of the fuel injection valve 15 is increased almost simultaneously. In the present embodiment, the target fuel pressure at the time of idling (before the start of automatic stop) is set to 40 MPa, and after the automatic stop condition is satisfied, the target fuel pressure is increased to 50 MPa, which is 10 MPa higher than that at the time of idling. .

  By increasing the target fuel pressure to 50 MPa, the actual fuel pressure (actual fuel pressure detected by the fuel pressure sensor SW5) gradually increases thereafter. In the present embodiment, the increase in the fuel pressure is realized by fully closing the SCV 43b built in the supply pump 43. That is, when the SCV 43b is fully closed, the flow of fuel that escapes from the supply pump 43 to the return pipe 48 through the regulator pipe 50 (broken arrow in FIG. 2) is cut off, and all the fuel from the supply pump 43 flows into the common rail 45. As a result, the pressure in the common rail 45 gradually increases. Then, by continuing such valve closing control of the SCV 43b for a predetermined period, the fuel pressure (actual fuel pressure) of the fuel injection valve 15 increases from 40 MPa to 50 MPa.

  When the fuel pressure rises to 50 MPa as described above, the intake throttle valve 30 is driven in the closing direction at a subsequent time point t1, and the opening degree thereof is the same as that during the normal operation set before the automatic stop condition is satisfied. From the opening (80% in the figure), it is finally reduced to fully closed (0%). Then, after the opening degree is fully closed, control (fuel cut) for stopping fuel injection from the fuel injection valve 15 is performed at time t2.

  Next, after the fuel cut is performed, the intake throttle valve 30 is opened again while the engine rotation speed gradually decreases. Specifically, when the last top dead center immediately before engine stop in all the cylinders 2A to 2D is set as the final TDC, the intake throttle is reduced when the top dead center passes one time before the final TDC (time point t4). The valve 30 is driven in the opening direction, and the opening degree is increased to a predetermined opening degree (for example, about 10 to 30%) exceeding 0%.

  Thereafter, after reaching the final TDC at time t5, the engine temporarily rotates backward due to the swinging back of the piston, but reaches the complete stop state at time t6 without exceeding the top dead center.

  Here, the control for opening the intake throttle valve 30 as described above is executed at the time t4 because the cylinder in the compression stroke when the engine is completely stopped, that is, the compression stroke cylinder at the time of stop (the third cylinder 2C in FIG. 3). As shown in FIG. 4 (b), the piston stop position is set within a specific range Rx set at the bottom dead center side relative to the reference stop position X located between the top dead center and the bottom dead center. Because. The reference stop position X may vary depending on the shape of the engine (displacement, bore / stroke ratio, etc.), the degree of progress of warm-up, and the like. For example, the reference stop position X may be any value between 90 and 75 ° CA before top dead center (BTDC) Can be set to any position. For example, when the reference stop position X is BTDC 80 ° CA, the specific range Rx is a range of BTDC 80 to 180 ° CA.

  If the piston 5 of the stop-time compression stroke cylinder 2C is stopped within the specific range Rx, then when the engine restart condition is satisfied, the stop-time compression stroke cylinder 2C is first (the first engine as a whole). ) The engine can be restarted quickly by one compression start injecting fuel. On the other hand, if the piston stop position of the stop-time compression stroke cylinder 2C is out of the specific range Rx, after the start of restart, the cylinder that reaches the compression stroke next to the stop-time compression stroke cylinder 2C, that is, the intake stroke when the engine is stopped. It is necessary to restart the engine by the two-compression start in which fuel is injected into the intake stroke cylinder (4th cylinder 2D in FIG. 3). The reason why the 1-compression start and the 2-compression start are properly used depending on the piston stop position is that the ignitability in the stop-time compression stroke cylinder 2C differs depending on the piston stop position. This will be described in “4) Restart control”.

  In the above-described two-compression start, fuel cannot be combusted until the intake stroke cylinder 2D at the time of stoppage shifts to the compression stroke. Therefore, the one-compression start is naturally more advantageous in terms of quick start. For this reason, in order to be able to execute one compression start with high frequency, it is necessary to keep the piston stop position of the stop-time compression stroke cylinder 2C within the specific range Rx as much as possible. Therefore, in this embodiment, as shown in FIG. 3, the intake throttle valve 30 is opened at time t4. That is, according to the control of FIG. 3, the opening degree of the intake throttle valve 30 is set to 0% until the top dead center (ii) one time before the final TDC (until time t4), and one of the final TDCs is reached. When the previous top dead center (ii) is passed (after time t4), the opening of the intake throttle valve 30 is increased to a predetermined opening exceeding 0%. As a result, the intake flow rate to the compression stroke cylinder 2C at the time of stopping, which reaches the intake stroke from the top dead center (ii) immediately before the final TDC (the time t4 to t5 becomes the intake stroke), is 2 prior to the final TDC. Cylinders that reach the intake stroke from top dead center (iii) (time points t3 to t4 become the intake stroke), in other words, the stop expansion stroke cylinder that is in the expansion stroke when the engine is completely stopped (the first cylinder in FIG. 3) The intake air flow rate for 2A) will increase.

  This point will be described in more detail with reference to FIGS. As described above, when the intake throttle valve 30 is opened when passing through the top dead center (ii) immediately before the final TDC, as described above, immediately before the engine automatically stops, the intake stroke cylinder 2C enters the stop-time compression stroke cylinder 2C. The intake air amount becomes larger than the intake air amount into the expansion stroke cylinder 2A at the time of stop. As a result, as shown in FIG. 4A, the pressing force by the compressed air acting on the piston 5 of the stop-time compression stroke cylinder 2C increases, while the compressed air acting on the piston 5 of the stop-time expansion stroke cylinder 2A. The pressing force due to becomes small. For this reason, when the engine is completely stopped, as shown in FIG. 4B, the stop position of the piston 5 of the stop-time compression stroke cylinder 2C is naturally close to the bottom dead center (the piston 5 of the stop-time expansion stroke cylinder 2A As a result, the piston 5 of the compression stroke cylinder 2C at the time of stop can be stopped in the specific range Rx on the lower dead center side with respect to the reference stop position X with a relatively high frequency. become able to. If the piston 5 is stopped within the specific range Rx, when the engine is restarted, the engine can be quickly restarted by one compression start in which fuel is injected into the compression stroke cylinder 2C at the time of stop.

  Next, an example of the control operation of the automatic stop control unit 61 that controls the engine automatic stop control as described above will be described with reference to the flowchart of FIG. When the processing shown in the flowchart of FIG. 5 starts, the automatic stop control unit 61 executes control for reading various sensor values (step S1). Specifically, the detection signals from the water temperature sensor SW1, the crank angle sensor SW2, the cam angle sensor SW3, the air flow sensor SW4, the fuel pressure sensor SW5, the accelerator opening sensor SW6, the brake sensor SW7, the vehicle speed sensor SW8, and the battery sensor SW9, respectively. Based on these signals, various information such as engine coolant temperature, crank angle, engine rotation speed, cylinder discrimination information, intake air flow, fuel pressure, accelerator opening, presence of brake, vehicle speed, remaining battery capacity, etc. get.

  Next, the automatic stop control unit 61 determines whether or not an automatic engine stop condition is satisfied based on the information acquired in Step S1 (Step S2). Here, three requirements are that the vehicle is in a stopped state, that the opening degree of the accelerator pedal 36 is zero (accelerator OFF), and that the brake pedal 37 is depressed more than a predetermined depression force (brake ON). When all of the above are prepared, it is determined that the automatic stop condition is satisfied. As for the requirement that the vehicle is in a stopped state, it is not always necessary to make a complete stop (vehicle speed = 0 km / h), and the vehicle stops when the vehicle speed falls below a predetermined low vehicle speed (for example, 3 km / less). You may determine with being in a state.

  When it is determined as YES in step S2 and it is confirmed that the automatic stop condition is satisfied, the automatic stop control unit 61 executes control for increasing the target fuel pressure (step S3). That is, as shown in the time chart of FIG. 3, the target fuel pressure is increased from 40 MPa to 50 MPa at the time t0 when the automatic stop condition is satisfied. Then, as the fuel pressure increases, the SCV 43b built in the supply pump 43 is fully closed, so that the actual fuel pressure (actual fuel pressure) of the fuel injection valve 15 gradually increases and finally reaches 50 MPa. .

  Next, the automatic stop control unit 61 determines whether there is really no problem by automatically stopping the engine in consideration of restrictions on the system (step S4). For example, when the engine is in a cold state or when the remaining capacity of the battery is extremely small, it may be difficult to restart the engine after automatically stopping the engine. Also, it is not appropriate to stop the engine when the load condition of the air conditioner (the difference between the actual temperature in the passenger compartment and the set temperature of the air conditioner) is large. Therefore, in step S4, it is confirmed whether or not such a system problem has been cleared. Specifically, based on the various information acquired in step S1, the cooling water temperature of the engine is higher than a predetermined value, the remaining capacity of the battery is higher than a predetermined value, and the load state of the air conditioner is relatively low. If all of the plurality of requirements are cleared, it is determined that there is no problem on the system.

  When it is determined YES in step S4 and it is confirmed that there is no system problem, the automatic stop control unit 61 acquires the fuel pressure of the fuel injection valve 15 from the fuel pressure sensor SW5, and based on the information, It is determined whether or not the fuel pressure has actually increased up to the fuel pressure (50 MPa) increased in step S3 (step S5).

  If it is determined YES in step S5 and an increase in fuel pressure is confirmed, the automatic stop control unit 61 performs control to set the opening of the intake throttle valve 30 to fully closed (0%) (step S6). . That is, as shown in the time chart of FIG. 3, the opening of the intake throttle valve 30 is closed at time t1 after the fuel pressure (actual fuel pressure) of the fuel injection valve 15 detected by the fuel pressure sensor SW5 has increased to 50 MPa. It starts to drive in the direction, and its opening is finally reduced to 0%.

  Next, the automatic stop control unit 61 performs a fuel cut that stops the supply of fuel from the fuel injection valve 15 (step S7). That is, at time t2 (FIG. 3) that is slightly delayed from time t1 at which the intake throttle valve 30 starts to be driven in the closing direction, all the drive signals for the fuel injection valves 15 of the cylinders 2A to 2D are turned off. The fuel cut is performed by maintaining the 15 valve bodies in the fully closed position.

  As described above, in this embodiment, when the automatic stop condition is satisfied, the presence or absence of a problem in the system and the fuel pressure are confirmed, and further, the intake throttle valve 30 is closed, and then the fuel cut is executed. For this reason, a time of approximately 0.2 to 0.3 s (seconds) is required between the establishment of the automatic stop condition (time t0) and the execution of the fuel cut (time t2), depending on the case.

  After executing the fuel cut as described above, the automatic stop control unit 61 performs engine rotation speed (top dead center rotation speed) when the piston 5 of any of the four cylinders 2A to 2D reaches top dead center. It is determined whether or not the value is within a predetermined range (step S8). As shown in FIG. 3, the engine rotational speed temporarily falls every time one of the four cylinders 2A to 2D reaches the compression top dead center (the top dead center between the compression stroke and the expansion stroke). It gradually decreases while repeating the up-down that it rises again after exceeding the compression top dead center. Therefore, the top dead center rotation speed can be measured as the rotation speed at the timing of the up / down valley of the engine rotation speed.

  The determination regarding the top dead center rotation speed in the above step S8 is performed in order to specify the passage timing of the top dead center immediately before the last top dead center (final TDC) immediately before engine stop (time t4 in FIG. 3). Done. In other words, there is a certain regularity in how the engine speed decreases during the process of automatic engine stop, so if you check the rotation speed at that time (top dead center rotation speed) when passing through the top dead center, It can be estimated how many times before the final TDC the top dead center. Therefore, the top dead center rotational speed is always measured, and it is a predetermined range set in advance, that is, a predetermined range obtained in advance by experiments or the like as the rotational speed when passing the top dead center immediately before the final TDC. The passage timing of the top dead center immediately before the final TDC is specified by determining whether or not the vehicle enters the position.

  When it is determined YES in step S8 and it is confirmed that the current time point is the top dead center passage timing one time before the final TDC (time point t4 in FIG. 3), the automatic stop control unit 61 determines the intake throttle valve 30. Is started to drive in the opening direction, and control is performed to increase the opening to a predetermined opening exceeding 0% (for example, about 10 to 30%) (step S9). As a result, the intake air flow rate for the stop-time compression stroke cylinder 2C that starts the intake stroke from the time point t4 is larger than the intake air flow rate for the stop-time expansion stroke cylinder 2A that was the intake stroke one cycle before (time points t3 to t4). .

  Thereafter, the automatic stop control unit 61 determines whether or not the engine has completely stopped by determining whether or not the engine rotation speed is 0 rpm (step S10). If the engine is completely stopped, the automatic stop control unit 61, for example, sets the opening of the intake throttle valve 30 to a predetermined opening (for example, 80%) set during normal operation. The automatic stop control is terminated.

  As described above, according to the automatic stop control shown in FIG. 5, the stop-time compression stroke cylinder is controlled by the control in step S9 that opens the intake throttle valve 30 when the top dead center passes one time before the final TDC (at time t4). Since there is a difference in the intake flow rate between 2C and the expansion stroke cylinder 2A at the time of stop, when the engine is completely stopped, the piston 5 of the compression stroke cylinder 2C at the time of stop has a relatively high specific range near the bottom dead center. It will be within Rx (FIG. 4B).

  When the engine is completely stopped by the automatic stop control, the fuel pressure of the fuel injection valve 15 gradually decreases with time as shown in the rightmost part of the actual fuel pressure column in FIG. As the engine stops, the supply pump 43 mechanically driven by the engine also stops, and fuel is not supplied to the common rail 45. On the other hand, the common rail 45 is externally supplied little by little through a pressure limiter 47 and the like. This is because the fuel leaks.

(4) Restart Control Next, the specific contents of the engine restart control executed by the restart control unit 62 of the ECU 60 will be described with reference to the flowchart of FIG. As will be apparent from the description here, in this embodiment, the restart control unit 62 of the ECU 60 determines whether or not the piston 5 of the stop-time compression stroke cylinder 2C is in the specific range Rx. A function as a means and a function as an injection control means for injecting fuel when the engine is restarted.

  When the process shown in the flowchart of FIG. 6 starts, the restart control unit 62 determines whether or not the engine restart condition is satisfied based on various sensor values (step S11). For example, the accelerator pedal 36 has been depressed to start the vehicle (accelerator ON), the engine coolant temperature has fallen below a predetermined value, the amount of decrease in the remaining battery capacity has exceeded an allowable value, When at least one of the requirements such as the stop time (elapsed time after automatic stop) exceeds a predetermined time is satisfied, it is determined that the restart condition is satisfied.

  When it is determined YES in step S11 and it is confirmed that the restart condition is satisfied, the restart control unit 62 determines the stop-time compression stroke cylinder 2C that has stopped in the compression stroke in accordance with the automatic engine stop control described above. The piston stop position is specified based on the crank angle sensor SW2 and the cam angle sensor SW3, and the specified piston stop position is in the specified range Rx (FIG. 4B) on the bottom dead center side with respect to the reference stop position X. It is determined whether or not (step S12).

  When it is determined YES in step S12 and it is confirmed that the piston stop position of the stop-time compression stroke cylinder 2C is within the specific range Rx, the restart control unit 62 supplies the first fuel to the stop-time compression stroke cylinder 2C. Control is performed to restart the engine by one compression start for injection (step S13). That is, by driving the starter motor 34 and applying rotational force to the crankshaft 7, fuel is injected from the fuel injection valve 15 into the compression stroke cylinder 2 </ b> C at the time of stop and self-ignition is performed. The combustion is restarted from the time when the dead point is reached, and the engine is restarted.

  Here, the piston stop position of the stop-time compression stroke cylinder 2C is considered to be within the specific range Rx in a relatively large number of cases due to the effect of the automatic stop control (FIGS. 2 and 4) described above. However, in some cases, the piston stop position may deviate from the specific range Rx (the piston 5 stops at the top dead center side with respect to the reference stop position X). In such a case, NO is determined in step S12.

  When it is determined NO in step S12 (that is, when the piston 5 of the stop-time compression stroke cylinder 2C is stopped at the top dead center side with respect to the specific range Rx), the restart control unit 62 stops in the intake stroke. The control for restarting the engine by the two-compression start that injects the first fuel into the stopped intake stroke cylinder 2D that has been performed is executed (step S14). That is, the engine is driven only by driving the starter motor 34 without injecting fuel until the piston 5 of the stop compression stroke cylinder 2C exceeds the top dead center and then the stop intake stroke cylinder 2D reaches the compression stroke. Force to rotate. At that time, fuel is injected from the fuel injection valve 15 into the stop-time intake stroke cylinder 2D, and the injected fuel is self-ignited, so that the combustion is resumed from the time when the engine reaches the second compression top dead center. Restart the engine.

  As described above, in the restart control of FIG. 6, the one-compression start (S13) and the two-compression start (S14) are selectively used according to the piston stop position of the stop-time compression stroke cylinder 2C. Hereinafter, the features of the 1 compression start and the 2 compression start will be described while comparing the two.

  As shown in FIG. 4B, the specific range Rx is set on the bottom dead center side with respect to a predetermined reference stop position X (for example, any position between BTDC 90 to 75 ° CA). When the piston 5 of the stop stroke compression cylinder 2C is stopped in such a specific range Rx near the bottom dead center, the compression allowance (stroke amount to the top dead center) by the piston 5 is large. As the piston 5 rises, the air in the cylinder 2C is sufficiently compressed to increase the temperature and pressure. For this reason, if the first fuel at the time of restarting is injected into the compression stroke cylinder 2C at the time of stop, this fuel will reach self-ignition and burn within the cylinder 2C relatively easily (1 compression start).

  On the other hand, if the piston 5 of the stop compression stroke cylinder 2C deviates from the specific range Rx to the top dead center side, the compression allowance by the piston 5 is small, and even if the piston 5 rises to the top dead center, Since the air is not sufficiently heated to a high temperature and pressure, misfire may occur even if fuel is injected into the compression stroke cylinder 2C when stopped. Therefore, in such a case, the engine is restarted by injecting fuel into the stop intake stroke cylinder 2D instead of the stop compression stroke cylinder 2C to cause self-ignition (two compression start).

  In the above-described two-compression start, until the piston 5 of the intake stroke cylinder 2D at the time of stop reaches the vicinity of the compression top dead center (that is, until the second compression top dead center is reached as the whole engine), it is based on the fuel injection. Combustion cannot be performed, and the time required for engine restart, that is, the time from the start of driving the starter motor 34 to the complete explosion of the engine (for example, the state where the rotational speed reaches 750 rpm) becomes long. Therefore, when restarting the engine, it is preferable to restart the engine by one compression start as much as possible.

  Therefore, in the present embodiment, the fuel injection valve 15 is caused to perform pre-injection at least when one compression start is performed in step S13. The pre-injection is a fuel injection that is preliminarily injected before the main injection when the fuel injection for diffusion combustion injected near or at the compression top dead center is used as the main injection. . The fuel by the pre-injection is used to surely cause diffusion combustion (hereinafter, this combustion is referred to as “main combustion”) that occurs mainly after compression top dead center based on the main injection. That is, in a stage earlier than the main injection, a small amount of fuel is injected by pre-injection, and the injected fuel is burned after a predetermined ignition delay (hereinafter, this combustion is referred to as “pre-combustion”). Increase the temperature and pressure to promote the subsequent main combustion.

  If the pre-injection as described above is executed for the compression stroke cylinder 2C at the time of stop, the in-cylinder temperature and pressure near the compression top dead center can be intentionally increased, so the piston stop position of the compression stroke cylinder 2C at the time of stop However, even if it approaches the top dead center side, the engine can be reliably restarted by one compression start. The reference stop position X that is the boundary of the specific range Rx is set in consideration of the improvement in ignitability by such pre-injection. That is, when there is no pre-injection, the reference stop position X must be set to the bottom dead center side than the example of FIG. 4B, but by improving the ignitability by pre-injection. Thus, it is possible to set the reference stop position X closer to the top dead center, and as a result, the reference stop position X can be set to a position far away from the bottom dead center, for example, BTDC 90 to 75 ° CA. It becomes possible. As a result, the specific range Rx expands to the top dead center side, so that the piston 5 of the compression stroke cylinder 2C at the time of stoppage falls within the specific range Rx with a higher frequency, and an opportunity to perform a quick restart by one compression start Will increase.

  Here, the pre-injection in the present embodiment is executed a plurality of times (for example, any number of 2 to 5 times) within a predetermined crank angle range before the compression top dead center. This is because if the same amount of fuel is used, it is richer in the cavity 5a provided on the crown surface of the piston 5 when the fuel is divided into a plurality of pre-injections than when the fuel is injected once. This is because a simple air-fuel mixture can be formed continuously and the ignition delay can be shortened.

  This point will be described in detail with reference to FIGS. FIGS. 7A and 7B are views for explaining the behavior of the fuel injected by the pre-injection in the cavity 5a.

  The pre-injection is performed before the main injection and at a timing when the injected fuel is accommodated in the cavity 5a. The timing is, for example, in the range of 20 to 0 ° CA before top dead center (BTDC). FIG. 7A shows a case where the pre-injection is performed only once within the crank angle range, and FIG. 7B shows a case where the pre-injection is performed a plurality of times within the crank range. .

  As shown in FIG. 7A, when the pre-injection is performed once, since the penetration of the spray F is strong, the spray F is rolled up along the wall surface of the cavity 5a. The fuel diffuses to the entire cavity 5a and further to the outside of the cavity 5a, and as a result, the frequency (spatial frequency) at which a rich air-fuel mixture is distributed in the cavity 5a decreases. On the other hand, as shown in FIG. 7 (b), when the pre-injection is performed a plurality of times, the injection amount per pre-injection is small and the penetration of the spray F is weak, so that it is near the bottom of the cavity 5a. As a result of a large amount of fuel remaining (double-hatching portion in the figure), the frequency of rich air-fuel mixture distribution in the cavity 5a increases.

FIG. 8 is a diagram for explaining how the equivalent ratio φ after injection changes when a predetermined amount of fuel is pre-injected using the fuel injection valve 15 having eight injection holes having eight injection holes. FIG. Specifically, A1 in the figure represents the change in the equivalence ratio φ after the injection of 6 mm ³ fuel at one timing of BTDC 14 ° CA, and A2 represents the fuel at three timings after BTDC 18 ° CA. the change in the equivalence ratio φ after each 2 mm ³ (total 6 mm ³⁾ injection, A3 is a fuel by 1.2 mm ³ at 5 times the timing of BTDC18 ° CA after (total 6 mm ³⁾ equivalent ratio after injection The change of φ is shown respectively. The vertical axis in the figure is a rich mixture ratio indicating how often an air-fuel mixture having an equivalence ratio φ> 0.75 is present in the cavity 5a, and the horizontal axis in the figure is the compression top dead center. The front crank angle.

  As shown in this figure, when the number of pre-injections is set to 1 (A1), the equivalent ratio φ immediately after injection is large, but the equivalent ratio cannot be maintained until the vicinity of compression top dead center where main injection is performed. I understand. This is because, as described above, the penetration of the spray is too strong, and the spray is wound up and diffused upward (on the cylinder head 4 side). On the other hand, if the number of pre-injections is increased to 3 times and 5 times (A2, A3), since the penetration of the spray is suppressed, a large amount of fuel is unevenly distributed at specific locations in the cavity 5a, and the state is compared. Continue for a long time. As a result, the change in the equivalence ratio φ also becomes gradual, and a large equivalence ratio is maintained up to the vicinity of the compression top dead center (BTDC 0 ° CA).

  Here, it is known that the larger the equivalence ratio φ of the air-fuel mixture (that is, the richer the fuel), the shorter the ignition delay time. FIG. 9 is a diagram showing the relationship between the equivalence ratio φ of the air-fuel mixture and the ignition delay time τ. More specifically, the atmospheric pressure air is compressed at a rotational speed of 120 rpm from the piston position of BTDC 75 ° CA. This is a result of calculating how the ignition delay time τ varies depending on the equivalent ratio φ when fuel is injected under the maximum temperature and pressure at that time. The rotational speed of 120 rpm was set as an example of the rotational speed that can be taken when the engine passes through the top dead center at the time of engine restart (approximately 100 to 120 rpm).

  According to FIG. 9, for example, when the equivalence ratio φ of the air-fuel mixture is 0.75, the ignition delay time τ is 15 ms. When the equivalence ratio φ is smaller than 0.75, the ignition delay time τ increases rapidly as the equivalence ratio φ becomes smaller (that is, the fuel is leaner). On the other hand, when the equivalence ratio φ is larger than 0.75, the ignition delay time τ becomes shorter as the equivalence ratio φ becomes larger (that is, as the fuel is richer), but the rate of change is moderate, and the equivalence ratio φ becomes 0. Even if it is slightly larger than .75, the ignition delay time τ does not change so much (for example, even if φ = 1, τ is shortened by only 1 ms).

  From this, even if compression is started from a piston position (position far away from the bottom dead center) such as the vicinity of the reference stop position X in FIG. If the air-fuel mixture is created before the compression top dead center (see FIG. 8) and held for about 15 ms, the air-fuel mixture may ignite. 15 ms is only 10 ° CA at a rotational speed of 120 rpm, so if it passes the first compression top dead center at the time of restart, there is a problem near the compression top dead center where the in-cylinder temperature / pressure becomes maximum. It is thought that the air-fuel mixture is ignited.

  From the above situation, in this embodiment, the pre-injection is performed a plurality of times instead of once. As shown in FIG. 8, if the number of pre-injections is set to a plurality of times, a rich air-fuel mixture with φ> 0.75 can be continuously created until the compression top dead center is reached. As a result, even when the piston 5 of the stop compression stroke cylinder 2C is located at a position far from the bottom dead center such as the vicinity of the reference stop position X (that is, when the compression allowance by the piston 5 is small), pre-injection is performed. It is considered that the ignitability of the remaining fuel is ensured and pre-combustion is surely caused. If pre-combustion occurs, the in-cylinder temperature / pressure of the stop-time compression stroke cylinder 2C is increased and the fuel from the subsequent main injection is easily ignited, so that one-compression start is reliably performed.

  In addition, in the present embodiment, control for increasing the fuel pressure of the fuel injection valve 15 is executed in the initial stage of the engine automatic stop control (between the time t0 when the automatic stop condition is satisfied and the time t2 when the fuel cut is performed). Therefore, atomization of fuel is promoted when fuel injection (pre-injection and main injection) is performed on the compression stroke cylinder 2C at the time of stop. As a result, the ignitability of the injected fuel is further improved, and the ignition delay time until the self-ignition is shortened. As a result, the startability by one compression start is improved.

  However, even if the fuel pressure is increased, after the engine is completely stopped, the fuel pressure after the increase cannot be maintained, and the fuel pressure gradually decreases. However, in this embodiment, when the engine stop time (elapsed time after the engine is completely stopped) exceeds a predetermined time, the restart condition is satisfied (step S11 in FIG. 6). At the time of restart, the fuel pressure is not extremely reduced, and the engine can be restarted without any problem. As a specific example, assuming that the maximum engine stop time is 2 minutes, the case where the fuel pressure is increased during the automatic engine stop control is compared with the case where the fuel pressure is not increased. In the latter case, that is, when the fuel pressure is not increased during the automatic engine stop control, the fuel pressure of the fuel injection valve 15 is the idling fuel pressure when the engine stop time reaches a maximum of 2 minutes. The pressure drops from 40 MPa to less than 30 MPa, and there is a possibility of causing misfires at the time of restart. On the other hand, in the former case, that is, when the fuel pressure is increased from 40 MPa to 50 MPa during the automatic engine stop control, even if the engine stop time reaches a maximum of 2 minutes, the fuel injection valve 15 The fuel pressure is maintained at 30 MPa or more, and it is considered that the ignitability at the time of restart is sufficiently ensured.

FIG. 10 is an explanatory diagram for demonstrating the effect of executing the pre-injection. Here, as an example, the pre-injection is executed three times, the change in the fuel injection rate (mm ³ / deg) at that time is shown in the lower stage, and the change in the heat generation rate (J / deg) is shown in the upper stage. Specifically, between BTDC 18 to 10 ° CA, fuel of 2 mm ³ is injected three times as a pre-injection (lower waveform Ip), and then more than the pre-injection as the main injection (at least The fuel was injected at a compression top dead center (BTDC 0 ° CA) (more than that of one pre-injection) (lower waveform Im). And, what kind of combustion occurs with such fuel injection is shown as a change in heat generation rate (upper waveforms Bp, Bm).

  As shown in FIG. 10, when three pre-injections (Ip) are executed, after completion of the last pre-injection, a predetermined ignition delay time elapses and pre-injection by self-ignition of the pre-injected fuel is performed. Combustion (Bp) occurs. This pre-combustion (Bp) occurs before the compression top dead center (BTDC 0 ° CA), and also reaches the peak of the heat generation rate before the compression top dead center. Thereafter, the heat generation rate once decreases, but main injection (Im) is started from the compression top dead center, and main combustion (Bm) due to self-ignition of the main injected fuel is subsequently generated. This main combustion (Bm) starts combustion with almost no ignition delay (diffusion combustion) based on main injection (Im) that is executed in a state in which the inside of the cylinder is at a high temperature and high pressure by pre-combustion (Bp).

  Further, according to FIG. 10, the pre-combustion and the main combustion are divided by the valley of the heat generation rate, and are independent combustions. That is, in the present embodiment, the pre-combustion (Bp) improves the in-cylinder environment of the stop compression stroke cylinder 2C to a state advantageous for fuel self-ignition (that is, the in-cylinder temperature and pressure near the compression top dead center). It can be understood that the combustion is not for generating torque for starting the engine as in the main combustion.

  The combustion control based on the pre-injection and the main injection may be executed as needed not only for the stop-time compression stroke cylinder 2C but also for the cylinder that reaches the compression stroke after that. When the engine is restarted, the ignitability is most severe in the combustion in the compression stroke cylinder 2C at the stop when the engine as a whole reaches the first compression top dead center (1 compression TDC). For the cylinders 2D and 2B that reach the dead center (2-compression TDC, 3-compression TDC), it is considered that the improvement in ignitability is not sufficient. That is, even in 2-compression TDC and 3-compression TDC, the engine speed is not sufficiently high, so the air in the cylinder may not be sufficiently heated and pressurized due to air leakage from the piston ring gap and cooling loss. Because there is. Therefore, from the viewpoint of reliably preventing misfire, combustion control based on pre-injection and main injection may be performed on the cylinders 2D and 2B (hereinafter referred to as “subsequent cylinders”).

  However, in the 2-compression TDC, the 3-compression TDC, etc., where the subsequent cylinders reach compression top dead center, the engine speed is faster than in the 1-compression TDC, where the compression stroke cylinder 2C when stopped reaches compression top dead center. The number of pre-injections to the subsequent cylinder is not necessarily the same as that to the compression stroke cylinder 2C at the time of stop. For example, when the number of times of pre-injection to the compression stroke cylinder 2C at the time of stop is a large number of times such as 4 times or 5 times, the number of times of pre-injection to the succeeding cylinders is reduced as the number of times of advancement increases, such as Accordingly, the timing and amount of each pre-injection may be adjusted.

  In addition, the combustion control based on the pre-injection and the main injection as described above is performed not only in the first compression start in which the first fuel is injected into the stop-time compression stroke cylinder 2C but also in the stop-time intake stroke cylinder 2D. The same can be done when the engine is restarted by the two-compression start (step S14 in FIG. 6).

(5) Operational effects and the like As described above, in the present embodiment, in a diesel engine having a so-called idle stop function that automatically stops or restarts the engine under a predetermined condition, Adopting a characteristic configuration.

  The automatic stop control unit 61 of the ECU (engine control unit) 60 indicates that there is no problem in the system after the automatic engine stop condition based on the driver's operation (accelerator and brake operation) and the vehicle speed is satisfied. After confirming, the fuel cut for stopping the fuel injection from the fuel injection valve 15 is executed, and the period (that is, from the time t0 when the automatic stop condition is satisfied to the time t2 of the fuel cut) is used. Then, control for increasing the fuel pressure of the fuel injection valve 15 is executed. When the restart condition is satisfied after the engine is automatically stopped, the restart control unit 62 of the ECU 60 determines that the piston 5 of the stop-time compression stroke cylinder 2C stopped in the compression stroke is lower than the predetermined reference stop position X. It is determined whether or not the fuel is within a specific range Rx (FIG. 4B) set on the side, and if it is within the specific range Rx, the first fuel is supplied from the fuel injection valve 15 to the stop-time compression stroke cylinder 2C. The engine is restarted by injection (1 compression start). In the first fuel injection to the compression stroke cylinder 2C at the time of stop, as shown in FIG. 10, for example, the main combustion (Bm) that causes the peak of the heat generation rate after the compression top dead center is reached. The injection (Im) and the pre-injection (Ip) for causing the pre-combustion (Bp) to reach the peak of the heat generation rate before the start of the main injection are executed.

  According to the above configuration, after the engine is automatically stopped, pre-injection for injecting a small amount of fuel is first executed at the time of one compression start in which the engine is restarted by fuel injection into the stop-time compression stroke cylinder 2C. Thereafter, main injection is executed. A small amount of the pre-injected fuel is burned by self-ignition after a predetermined ignition delay (pre-combustion), and the in-cylinder temperature / pressure of the stop compression stroke cylinder 2C is increased, so that main injection is subsequently performed. The injected fuel will soon burn by self-ignition (main combustion). This main combustion is a combustion that reaches the peak of the heat generation rate after passing the compression top dead center, and acts to push down the piston 5 after passing through the compression top dead center, so that a positive torque is applied to the engine. And increase its rotational speed.

  As described above, the ignitability of the fuel injected by the main injection is improved by the pre-injection (pre-combustion) before that, so that the compression allowance (stroke amount to the top dead center) in the compression stroke cylinder 2C at the time of stop is so much. Even if not, combustion in the compression stroke cylinder 2C at the time of stop is reliably performed.

  Further, in the above-described embodiment, the control for increasing the fuel pressure of the fuel injection valve 15 is executed using the period from the time when the automatic stop condition is satisfied to the time when the fuel cut is executed (time t0 to t2 in FIG. 3). Therefore, when performing the pre-injection and the main injection, the fuel can be atomized by a relatively high fuel pressure, and the ignitability of the fuel can be further improved. Moreover, since the fuel pressure is raised in advance before the fuel cut is performed, a general mechanical pump (a pump mechanically driven by the engine) is used as the supply pump 43 that supplies fuel to the fuel injection valve 15. While being used, the control for increasing the fuel pressure can be performed without difficulty. That is, if the fuel pressure is increased after the engine is stopped, a special (eg, electric) supply pump that can operate independently of the engine needs to be used. When the fuel pressure is increased, it is not necessary to use a special supply pump as described above, so that the fuel pressure can be reliably increased and the ignitability can be improved without incurring extra costs.

  As described above, according to the above-described embodiment in which the ignitability of the fuel is greatly improved, the specific range Rx that is the piston stop position range in which one compression start is possible can be expanded to the top dead center side. It is possible to secure a quick startability by increasing the number of opportunities.

  In particular, in the above embodiment, since the fuel injection valve 15 having a large number of injection holes of 8 to 12 is used, the fuel pressure increase control as described above (the fuel pressure of the fuel injection valve 15 is increased in advance before the fuel cut). By the synergistic effect with the action due to the control), atomization of the injected fuel can be further promoted, and the ignitability of the fuel can be enhanced.

  Furthermore, in the above embodiment, since the pre-injection is executed a plurality of times at a timing such that the fuel is contained in the cavity 5a provided on the crown surface of the piston 5, it is easy to ignite the inside of the cavity 5a (ignition delay time). (Short) can be reliably formed. As a result, the pre-injected fuel is self-ignited more reliably (pre-combustion), which promotes self-ignition (main combustion) of the main-injected fuel. Can be increased.

  In particular, in the configuration of the above embodiment in which the fuel pressure is increased in advance before the fuel cut, when the number of pre-injections is set to one, a relatively large amount of fuel is injected at a high fuel pressure by the pre-injection, The penetration of the spray becomes stronger, and the tendency of the injected fuel to diffuse to the outside of the cavity 5a becomes stronger. Then, the ratio of the rich air-fuel mixture formed in the cavity 5a is lowered, and there is a possibility that the ignitability is deteriorated. On the other hand, when the number of pre-injections is set to a plurality of times as in the above embodiment, the amount of injection per pre-injection is reduced and the penetration is weakened. A mixture can be formed, and the ignitability of the fuel can be effectively enhanced.

  In the above embodiment, the automatic stop control unit 61 (fuel pressure adjusting means) of the ECU 60 removes all the SCVs 43b built in the supply pump 43 from when the automatic stop condition is satisfied until the fuel cut is executed. Although the control for closing is executed, and thereby the fuel pressure of the fuel injection valve 15 is increased, the control of the fuel pressure adjusting means in the present invention only needs to be able to increase the fuel pressure with a response not so slow, It is not particularly limited to the control using the SCV 43b.

  Further, in the above-described embodiment, at least when the first fuel is injected into the stop compression stroke cylinder 2C, the pre-injection is performed a plurality of times (for example, any one of 2 to 5 times). When the piston stop position of the stroke cylinder 2C is substantially at the bottom dead center side within the specific range Rx, or when the restart condition is satisfied with almost no time after the engine is automatically stopped (that is, the engine stop time is considerable) When the piston 5 of the compression stroke cylinder 2C at the time of stoppage rises to the compression top dead center (at the time of 1 compression TDC), as shown in FIG. The number of times may be set to only once.

  Moreover, in the said embodiment, although the preferable form of this invention was demonstrated taking the example of the diesel engine provided with the engine main body 1 whose geometric compression ratio is 14, the engine which can apply the structure of this invention naturally. Is not limited to a geometric compression ratio of 14. For example, a diesel engine with a geometric compression ratio of less than 16 has a lower compression ratio and relatively poor ignitability as compared to a diesel engine that has been widely used so far. There is room for suitably applying the configuration of the present invention to improve the performance. On the other hand, it is considered that the geometric compression ratio of the diesel engine is required to be 12 or more from the limit of ignitability. From the above, the diesel engine to which the present invention can be suitably applied is a diesel engine having a geometric compression ratio of 12 or more and less than 16, more preferably a diesel engine having a geometric compression ratio of 13 or more and 15 or less. You can say that.

  The present invention is not limited to a diesel engine (an engine that burns light oil by self-ignition) as in the above embodiment as long as it is a compression self-ignition engine. For example, recently, an engine of a type that compresses fuel containing gasoline at a high compression ratio and self-ignites has been researched and developed, but the present invention also applies to such a compression self-ignition type gasoline engine. Such automatic stop / restart control can be suitably applied.

DESCRIPTION OF SYMBOLS 1 Engine main body 2A-2D Cylinder 5 Piston 5a Cavity 15 Fuel injection valve 34 Starter motor 61 Automatic stop control part (fuel pressure adjustment means)
62 Restart control unit (determination means, injection control means)
X Reference stop position Rx Specific range Ip Pre-injection Im Main injection Bp Pre-combustion Ip Main combustion

Claims (3)

Provided in a compression self-ignition engine that burns fuel injected from a fuel injection valve into a cylinder by self-ignition, and automatically stops the engine when a predetermined automatic stop condition is satisfied, and then restarts the engine A start control device for a compression self-ignition engine that restarts the engine by injecting fuel from the fuel injection valve while applying rotational force to the engine using a starter motor. ,
Determination means for determining whether or not the piston of the compression stroke cylinder at the time of stop that has been stopped in the compression stroke due to the automatic stop is in a specific range set on the bottom dead center side with respect to a predetermined reference stop position;
When it is determined that the piston of the stop compression stroke cylinder has stopped in the specific range and the engine restart condition is satisfied, the fuel injection valve is controlled to supply the first fuel to the stop compression stroke cylinder. Injection control means for injecting;
Fuel pressure adjusting means for adjusting the fuel pressure of the fuel injection valve,
The fuel injection valve receives fuel from a supply pump mechanically driven by an engine.
The piston has a cavity that is recessed from other parts at a predetermined part of the crown surface facing the fuel injection valve,
The fuel pressure adjusting means is a control for increasing the fuel pressure of the fuel injection valve from when the automatic engine stop condition is satisfied until a fuel cut for stopping fuel injection from the fuel injection valve is executed. Run
After the restart condition is satisfied, the injection control means causes main combustion that reaches the peak of the heat generation rate after passing the compression top dead center at least as the first fuel injection to the compression stroke cylinder at the time of stop. And a plurality of pre-injections that cause pre-combustion that reaches the peak of the heat generation rate before the start of the main injection ,
The start control device for a compression self-ignition engine, wherein the plurality of pre-injections are executed at a timing such that all of the fuel from each pre-injection falls within the cavity of the piston . The start control device for a compression self-ignition engine according to claim 1,
A start control apparatus for a compression self-ignition engine, wherein the fuel injection valve is of a multi-hole type having 8 to 12 holes. The start control device for a compression self-ignition engine according to claim 1 or 2 ,
The compression self-ignition engine is a diesel engine having a geometric compression ratio set to less than 16 and a start-up control device for a compression self-ignition engine.

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