Engine stop control device

Abstract

To secure ignitability at restart while suppressing vibration during stop of an engine.SOLUTION: A stop control device for an engine includes: a determination section determining whether an automatic stop condition is satisfied; and a control section stopping the engine in accordance with the satisfaction of the automatic stop condition. The control section adjusts an opening degree of each of a throttle valve and an EGR valve so that oxygen concentration in an intake passage downstream of a connection part to an EGR passage increases to a predetermined target value Dx and a minimum value of an intake pressure falls within a predetermined target range Z, from execution of fuel cut until the engine is stopped completely.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an engine that has cylinders, an output shaft that rotates by receiving energy from combustion in the cylinders, an intake passage through which intake air introduced into the cylinders flows, an exhaust passage through which exhaust gas discharged from the cylinders flows, and an EGR passage connecting the intake passage and the exhaust passage, and relates to a stop control device that stops the engine when predetermined automatic stop conditions are met. [Background technology]

[0002] One example of the stop control device is disclosed in the following Patent Document 1. The engine stop control device described in Patent Document 1 includes a throttle valve provided in an intake passage, an EGR valve (EGR control valve) provided in an EGR passage, and a control unit that controls the throttle valve and the EGR valve. When there is a request to automatically stop the engine, the control unit opens the EGR valve and stops fuel injection into the cylinder, and closes the throttle valve and the EGR valve when the engine stops due to the injection stop (fuel cut). By such control, in Patent Document 1, EGR gas (exhaust gas) is introduced into the intake passage through the EGR passage before the engine stops, and the EGR gas is contained in the intake passage after the engine stops. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2004-100497 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned Patent Document 1, the throttle valve and EGR valve are closed when the engine stops (in other words, the EGR valve is open until the engine stops), so depending on the operating state immediately before the fuel cut, it is assumed that the engine will stop with an excessively large amount of EGR gas trapped in the intake passage. In such a case, the ratio of fresh air introduced into the cylinder when restarting the engine will be too low, and there is a risk that the ignition of the fuel will not be ensured.

[0005] In addition, in the above-mentioned Patent Document 1, the EGR valve is maintained in an open state from the time when the fuel is cut to the time when the engine is stopped, so that the intake pressure does not become negative even after the fuel is cut, and it is assumed that the period during which the engine rotates by inertia (in other words, the period from the time when the fuel is cut to the time when the engine is stopped) becomes long. If the period of inertia rotation becomes long, the period during which the engine speed is included in the resonance band (the rotation range that causes resonance with the vehicle body) becomes long, and there is a risk that unpleasant vibrations will be transmitted to the passengers in the vehicle.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide an engine stop control device that can suppress vibration when the engine is stopped while ensuring ignition performance when the engine is restarted. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides a stop control device that is applied to an engine that includes a cylinder, an output shaft that rotates by receiving energy from combustion in the cylinder, an intake passage through which intake air introduced into the cylinder flows, an exhaust passage through which exhaust gas discharged from the cylinder flows, and an EGR passage that connects the intake passage and the exhaust passage, the stop control device including an injector that supplies fuel to the cylinder, an EGR valve that is openably and closably provided in the EGR passage, a throttle valve that is openably and closably provided in the intake passage upstream of a connection between the EGR passage and the intake passage, and a stop control device that automatically stops the engine when an automatic stop condition for automatically stopping the engine is satisfied. and a control unit that controls each part of the engine including the injector, the throttle valve, and the EGR valve, and when the determination unit confirms that the automatic stop condition is established, the control unit executes a fuel cut to stop the supply of fuel by the injector, and executes adjustment control to adjust the opening degree of the throttle valve and the EGR valve so that the oxygen concentration in the intake passage downstream of the connection part increases to a predetermined target value and the minimum value of the intake pressure falls within a predetermined target range during the period from when the fuel cut is executed to when the rotation of the output shaft stops. The adjustment control includes a first control for opening the throttle valve and then closing it to maintain the state in which the oxygen concentration has increased to the target value, and a second control for opening the EGR valve and then closing it to keep the minimum value of the intake pressure within the target range, and the control unit closes the EGR valve through the second control at a later time than the throttle valve through the first control. This is characterized by the above (Claim 1).

[0008] Before the automatic stop condition is satisfied, exhaust gas is recirculated through the EGR passage (EGR operation) so that the oxygen concentration in the intake passage downstream of the connection with the EGR passage (hereinafter, also referred to as the final intake oxygen concentration) is significantly lower than the oxygen concentration in the atmosphere. Therefore, if no measures are taken to increase the final intake oxygen concentration, the ignition of fuel may be impaired when the engine is subsequently restarted, and the restart operation may become unstable. In contrast, in the present invention, the final intake oxygen concentration is increased to a target value before the engine is completely stopped, so that good ignition at the time of restart can be ensured and the restart operation can be stabilized.

[0009] In addition, in the present invention, the minimum intake pressure from fuel cut to complete stop is kept within a target range, so that the engine speed can be reduced at an appropriate speed after fuel cut, and vibrations when the engine is stopped can be suppressed.

[0010] For example, when the minimum value of the intake pressure is large, that is, when the negative pressure level of the intake pressure is small, the pumping loss of the engine is reduced, and the period during which the engine rotates by inertia after the fuel cut is extended. Here, in an engine mounted on a vehicle, a resonance band that causes resonance with the vehicle body exists on the lower rotation side than the idling rotation speed, and the vibration of the vehicle body is likely to increase in this resonance band. Therefore, if the period of the inertia rotation is extended, the period during which the engine rotation speed is included in the resonance band is extended, and there is a risk that unpleasant vibrations will be transmitted to the occupants. For this reason, it is desirable that the minimum value of the intake pressure is made small to a certain extent. However, if the minimum value of the intake pressure is made excessively small (the negative pressure level of the intake pressure is made excessively large), the speed at which the engine rotation speed decreases becomes too fast, and as a result, the shock generated when the engine completely stops (the shock transmitted to the vehicle body due to the reaction of the sudden stop) is likely to increase, and the occupants may feel uncomfortable.

[0011] In contrast, in the present invention, it is possible to keep the minimum intake pressure within a target range, and by setting an appropriate target range taking into account the above-mentioned circumstances, it is possible to reduce vibrations, etc. when the engine is stopped, and to improve the ride comfort of the vehicle.

[0012] Specifically, in the present invention, the control (adjustment control) for increasing the final intake oxygen concentration to a target value and keeping the minimum value of the intake pressure within a target range is as follows: Open the throttle valve and then close it The 1 control and 、E Open the GR valve and then close it The 2 control and The first 2 control R GR valve closed but The first control Rus Executed later than the throttle valve closing time Therefore, the final intake oxygen concentration and the intake pressure can be appropriately adjusted. .

[0013] That isSince the throttle valve is opened before being closed, the final intake oxygen concentration can be increased to the target value and then maintained in that state. This means that the inert EGR gas can be maintained in the intake air until the start of the restart. This appropriately suppresses the combustion temperature at the time of the restart, so that the amount of NOx generated can be suppressed while ensuring ignition at the time of the restart. In addition, since the EGR valve is closed later than the throttle valve (the EGR valve is kept open for a while after the throttle valve is closed), it is possible to prevent the minimum value of the intake pressure from becoming excessively small and falling below the target range. This increases the probability that the minimum value of the intake pressure will fall within the target range, thereby improving the vibration reduction effect when the engine is stopped.

[0014] good Preferably, the control unit adjusts the opening degree of the EGR valve so that the intake pressure does not fall below a predetermined reference pressure while the EGR valve is in an open state under the second control. 2 ).

[0015] This configuration stabilizes the pressure state before the intake pressure becomes negative due to the closing of the EGR valve, and stabilizes the behavior of the pressure drop thereafter. This makes it possible to keep the minimum intake pressure within the target range with a high probability, and further enhances the vibration reduction effect described above.

[0016] In the above-mentioned configuration, it is more preferable that the control unit closes the EGR valve by the second control when the engine speed drops to a predetermined reference speed (see claim 3 ).

[0017] According to this configuration, the timing to close the EGR valve can be determined in a simple manner based on the engine speed, and the intake pressure can be subsequently reduced to an appropriate level. Effect of the Invention

[0018] As described above, according to the engine stop control device of the present invention, it is possible to suppress vibrations when the engine is stopped while ensuring ignition performance when the engine is restarted. [Brief description of the drawings]

[0019] [Figure 1] 1 is a schematic system diagram showing a preferred embodiment of an engine to which a stop control device of the present invention is applied. [Diagram 2] FIG. 2 is a functional block diagram showing a control system of the engine. [Diagram 3] 5 is a flowchart showing the first half of an automatic stop control for automatically stopping the engine. [Figure 4] 10 is a flowchart showing a second half of the automatic stop control. [Diagram 5] 4 is a time chart showing an example of time changes of each state quantity when the automatic stop control is executed. [Figure 6] 6 is a time chart showing an example of time changes in each state quantity when automatic stop control is performed under conditions different from those in FIG. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] <Overall engine configuration> Fig. 1 is a schematic system diagram showing a preferred embodiment of an engine to which the stop control device of the present invention is applied. The engine shown in this figure is a four-stroke diesel engine mounted on a vehicle as a power source for running. The engine includes an engine body 1 driven by a supply of fuel containing diesel, an intake passage 30 through which intake air introduced into the engine body 1 flows, an exhaust passage 40 through which exhaust gas discharged from the engine body 1 flows, a supercharger 50 driven by the exhaust gas passing through the exhaust passage 40, and a high-pressure EGR device 60 and a low-pressure EGR device 70 that recirculate a part of the exhaust gas flowing through the exhaust passage 40 to the intake passage 30.

[0021] The engine body 1 is an in-line multi-cylinder type having a plurality of cylinders 2 (only one of which is shown in FIG. 1) arranged in a direction perpendicular to the plane of the paper of Fig. 1. The engine body 1 includes a cylinder block 3 in which the plurality of cylinders 2 are formed, a cylinder head 4 attached to the upper surface of the cylinder block 3 so as to close the upper end opening of each cylinder 2, and a plurality of pistons 5 housed in each cylinder 2 so as to be capable of reciprocating and sliding. In this embodiment, the side from the cylinder block 3 toward the cylinder head 4 is treated as the top and the opposite is treated as the bottom, but this is for convenience of explanation and is not intended to limit the installation posture of the engine.

[0022] A combustion chamber 6 is formed above the piston 5 of each cylinder 2. Each combustion chamber 6 is a space defined by the lower surface of the cylinder head 4, the side peripheral surface (cylinder liner) of the cylinder 2, and the crown surface 5a of the piston 5. Fuel is supplied to the combustion chamber 6 by injection from an injector 15, which will be described later. The mixture of the supplied fuel and air is combusted in the combustion chamber 6, and the piston 5, which is pushed down by the expansion force caused by the combustion, reciprocates up and down.

[0023] A crankshaft 7, which is the output shaft of the engine body 1, is provided at the bottom of the cylinder block 3 (below the pistons 5). The crankshaft 7 is connected to the pistons 5 of each cylinder 2 via connecting rods 8, and rotates around its central axis in response to the reciprocating motion (up and down movement) of the pistons 5.

[0024] A crank angle sensor SN1 and a water temperature sensor SN2 are attached to the cylinder block 3. The crank angle sensor SN1 detects the crank angle, which is the rotation angle of the crankshaft 7, and the engine speed, which is the rotation speed of the crankshaft 7. The water temperature sensor SN2 detects the temperature of the cooling water flowing inside the cylinder block 3 and the cylinder head 4, i.e., the engine water temperature.

[0025] In the cylinder head 4, an intake port 9 and an exhaust port 10 communicating with the combustion chamber 6 are formed for each cylinder 2. In addition, the cylinder head 4 is equipped with a combination of an intake valve 11, an exhaust valve 12, and an injector 15 for each cylinder 2. The intake valve 11 is a valve that opens and closes the opening of the intake port 9 on the combustion chamber 6 side. The exhaust valve 12 is a valve that opens and closes the opening of the exhaust port 10 on the combustion chamber 6 side. The injector 15 is an injection valve that injects fuel (diesel) into the combustion chamber 6, and is attached to the cylinder head 4 so as to inject fuel from the center of the ceiling surface of the combustion chamber 6 toward the crown surface 5a of the piston 5, for example.

[0026] A valve mechanism 13 that drives the intake valves 11 to open and close, and a valve mechanism 14 that drives the exhaust valves 12 to open and close are attached to the cylinder head 4. The combination of these valve mechanisms 13, 14 includes, for example, a pair of camshafts linked to the crankshaft 7, and drives the intake valves 11 and exhaust valves 12 of each cylinder 2 to open and close in conjunction with the rotation of the crankshaft 7.

[0027] The intake passage 30 is a tubular member that forms a flow passage for intake air introduced into the combustion chamber 6 of each cylinder 2. The intake passage 30 has an intake manifold 30a (including a plurality of branch pipes aligned in a direction perpendicular to the paper surface of FIG. 1) branched in correspondence with the plurality of cylinders 2 in a downstream portion close to the engine body 1. The intake manifold 30a is connected to the cylinder head 4 so as to communicate with the intake ports 9 of each cylinder 2. The portion of the intake passage 30 other than the intake manifold 30a is a single tubular member that forms a common passage connected to the intake manifold 30a.

[0028] In the intake passage 30, an air cleaner 31, a throttle valve 32, an intercooler 33, and a surge tank 34 are arranged in this order from the upstream side. The air cleaner 31 is a filter that removes foreign matter from the intake air. The throttle valve 32 is an electric butterfly valve that can adjust the flow rate of the intake air flowing through the intake passage 30. The intercooler 33 is a heat exchanger that cools the intake air compressed by the turbocharger 50 (a compressor 51, described in detail later). The surge tank 34 is a tank that provides a space for evenly distributing the intake air to each cylinder 2, and is connected to the upstream end of the intake manifold 30a.

[0029] An airflow sensor SN3 and an intake pressure sensor SN4 are disposed in the intake passage 30. The airflow sensor SN3 is disposed in a portion of the intake passage 30 downstream of the air cleaner 31, and detects the flow rate of the intake air passing through that portion, i.e., the intake air flow rate. The intake pressure sensor SN4 is disposed in the surge tank 34, and detects the pressure of the intake air passing through the surge tank 34, i.e., the intake pressure.

[0030] The exhaust passage 40 is a tubular member that forms a flow passage for exhaust gas discharged from the combustion chamber 6 of each cylinder 2. The exhaust passage 40 has an exhaust manifold 40a (including a plurality of branch pipes aligned in a direction perpendicular to the paper surface of FIG. 1) that branches out corresponding to the plurality of cylinders 2 in an upstream portion close to the engine body 1. This exhaust manifold 40a is connected to the cylinder head 4 so as to communicate with the exhaust ports 10 of each cylinder 2. The portion of the exhaust passage 40 other than the exhaust manifold 40a is a single tubular member that forms a common passage connected to the exhaust manifold 40a.

[0031] A catalytic converter 41 for purifying exhaust gas is provided in the exhaust passage 40. The catalytic converter 41 incorporates an oxidation catalyst 42 for oxidizing and detoxifying CO and HC in the exhaust gas, and a DPF (diesel particulate filter) 43 for collecting particulate matter contained in the exhaust gas.

[0032] The exhaust passage 40 is provided with an exhaust 2Sensor SN5 is installed. Exhaust O 2 The sensor SN5 is provided in a portion of the exhaust passage 40 between the turbine 52 and the catalytic device 41, and detects the concentration of oxygen contained in the exhaust gas passing through this portion, that is, the exhaust oxygen concentration.

[0033] The turbocharger 50 includes a compressor 51 arranged in the intake passage 30, a turbine 52 arranged in the exhaust passage 40, and a turbine shaft 53 connecting the compressor 51 and the turbine 52.

[0034] The turbine 52 is an impeller that rotates by receiving energy from the exhaust gas flowing through the exhaust passage 40. The turbine 52 is disposed in the exhaust passage 40 between the downstream end (exhaust collecting portion) of the exhaust manifold 40a and the catalytic device 41. The rotation of the turbine 52 is transmitted to the compressor 51 via a turbine shaft 53.

[0035] The compressor 51 is an impeller that compresses and sends out (supercharges) the intake air by rotating in conjunction with the rotation of the turbine 52. The compressor 51 is disposed in the intake passage 30 between the air cleaner 31 and the throttle valve 32.

[0036] The high-pressure EGR device 60 is a recirculation device for recirculating a part of the exhaust gas having a relatively high pressure before flowing into the turbine 52 to the intake passage 30 as EGR gas. The high-pressure EGR device 60 includes a first EGR passage 61 that connects the exhaust passage 40 and the intake passage 30, and a first EGR valve 62 and a first EGR cooler 63 provided in the first EGR passage 61. The first EGR valve 62 is an electric valve capable of adjusting the flow rate of the EGR gas flowing through the first EGR passage 61. The first EGR cooler 63 is a heat exchanger that cools the EGR gas flowing through the first EGR passage 61. The first EGR passage 61 and the first EGR valve 62 correspond to the "EGR passage" and the "EGR valve" in the present invention, respectively.

[0037] The location where the upstream end (exhaust passage 40 side) of the first EGR passage 61 is connected to the exhaust passage 40 is defined as a first EGR inlet portion 61a, and the location where the downstream end (intake passage 30 side) of the first EGR passage 61 is connected to the intake passage 30 is defined as a first EGR outlet portion 61b. In this case, the first EGR inlet portion 61a is located between the turbine 52 in the exhaust passage 40 and the downstream end (exhaust collection portion) of the exhaust manifold 40a. Also, the first EGR outlet portion 61b is located between the throttle valve 32 and the surge tank 34 in the intake passage 30. In other words, the first EGR passage 61 connects the exhaust passage 40 upstream of the turbine 52 and the intake passage 30 downstream of the throttle valve 32 to each other. The first EGR outlet portion 61b corresponds to the "connection portion between the EGR passage and the intake passage" in the present invention.

[0038] The low pressure EGR device 70 is a recirculation device for recirculating a portion of the exhaust gas having a relatively low pressure after passing through the turbine 52 to the intake passage 30 as EGR gas. The low pressure EGR device 70 includes a second EGR passage 71 that connects the exhaust passage 40 and the intake passage 30, and a second EGR valve 72 and a second EGR cooler 73 provided in the second EGR passage 71. The second EGR valve 72 is an electrically operated valve capable of adjusting the flow rate of the EGR gas flowing through the second EGR passage 71. The second EGR cooler 73 is a heat exchanger that cools the EGR gas flowing through the second EGR passage 71.

[0039] The location where the upstream end (exhaust passage 40 side) of the second EGR passage 71 is connected to the exhaust passage 40 is referred to as the second EGR inlet portion 71a, and the location where the downstream end (intake passage 30 side) of the second EGR passage 71 is connected to the intake passage 30 is referred to as the second EGR outlet portion 71b. In this case, the second EGR inlet portion 71a is located downstream of the turbine 52 and the catalytic device 41 in the exhaust passage 40. The second EGR outlet portion 71b is located between the air cleaner 31 and the throttle valve 32 in the intake passage 30 (more specifically, between the air cleaner 31 and the compressor 51). In other words, the second EGR passage 71 connects the exhaust passage 40 downstream of the turbine 52 and the intake passage 30 upstream of the throttle valve 32 to each other.

[0040] <Control system> 2 is a functional block diagram showing the engine control system. The ECU 100 shown in the figure is a device for controlling the engine in an integrated manner, and is composed of a microcomputer including a processor (CPU) for performing various arithmetic processing, memories such as ROM and RAM, and various input / output buses.

[0041] Information detected by each sensor of the engine is input to the ECU 100. For example, the ECU 100 detects the crank angle sensor SN1, the water temperature sensor SN2, the air flow sensor SN3, the intake pressure sensor SN4, and the exhaust O 2 The ECU 100 is electrically connected to the sensor SN5. Information detected by the sensors SN1 to SN5, that is, information such as the crank angle, engine speed, engine water temperature, intake flow rate, intake pressure, and exhaust oxygen concentration, is sequentially input to the ECU 100.

[0042] Further, detection information by various sensors provided in the vehicle is also input to the ECU 100. In this embodiment, the vehicle is provided with a vehicle speed sensor SN6, an accelerator sensor SN7, and a brake sensor SN8. The vehicle speed sensor SN6 is a sensor that detects the vehicle speed, which is the traveling speed of the vehicle, the accelerator sensor SN7 is a sensor that detects the accelerator opening, which is the opening of an accelerator pedal provided in the vehicle, and the brake sensor SN8 is a sensor that detects whether or not a brake pedal (brake operation) provided in the vehicle is being operated. The detection information (vehicle speed, accelerator opening, and brake operation information) by the sensors SN6 to SN8 is also sequentially input to the ECU 100.

[0043] The ECU 100 controls each part of the engine while executing various judgments and calculations based on the input information from the above-mentioned sensors SN1 to SN8. For example, the ECU 100 is electrically connected to the injector 15, the throttle valve 32, the first EGR valve 62, and the second EGR valve 72, and outputs control signals to these devices based on the results of the above-mentioned calculations.

[0044] As elements for implementing the above-mentioned control, the ECU 100 functionally includes a determination unit 101, an automatic stop control unit 102, and a restart control unit 103. The automatic stop control unit 102 corresponds to the "control unit" of the present invention.

[0045] The automatic stop control unit 102 is a control module that automatically stops a running engine when a specific condition is satisfied. The restart control unit 103 is a control module that restarts the engine that has been stopped by the automatic stop control unit 102. The determination unit 101 is a control module that performs various determinations and calculations required to execute the controls by the automatic stop control unit 102 and the restart control unit 103.

[0046] <Automatic stop control> Next, the automatic stop control for automatically stopping a running engine will be described in detail. Figures 3 and 4 are flowcharts showing the specific procedure of the automatic stop control. When the control according to this flowchart starts, the determination unit 101 of the ECU 100 determines whether or not a predetermined automatic stop condition is satisfied (step S1). The automatic stop condition is a condition that permits automatic stopping of the engine, and various conditions can be set depending on the type of vehicle, etc.

[0047] For example, in a vehicle whose driving power source is substantially only an engine (so-called engine vehicle), the automatic stop condition may be established when a plurality of conditions including (i) the vehicle is substantially stopped, (ii) the accelerator pedal is in an OFF state, and (iii) the brake pedal is in an ON state are all satisfied. In this case, the determination unit 101 determines whether the vehicle speed is substantially zero, whether the accelerator opening is substantially zero, and whether the brake pedal is being depressed, based on information input from the vehicle speed sensor SN6, the accelerator sensor SN7, and the brake sensor SN8. Then, when all of these determinations are YES (i.e., when all of the above conditions (i) to (iii) are established), it is determined that the automatic stop condition is established.

[0048] On the other hand, in a vehicle that also uses a motor as a power source for driving, that is, in a so-called hybrid vehicle that can run in EV mode using only the motor, the driving force of the engine may not be needed even while the vehicle is running, and when such a situation occurs, the automatic stop condition may be established. In this case, the determination unit 101 calculates the target torque of the driving source including the engine and the motor from the vehicle speed detected by the vehicle speed sensor SN6 and the accelerator opening detected by the accelerator sensor SN7, and determines whether or not engine output (positive output torque that contributes to driving) is needed based on various conditions including the calculated target torque. Then, when it is determined that engine output is not needed, it is determined that the automatic stop condition is established.

[0049] Regardless of which of the above patterns the automatic stop condition is based on, the engine load is not high immediately before the automatic stop condition is satisfied. In this state, the engine is operated with at least one of the first EGR valve 62 and the second EGR valve 72 open. That is, as a prerequisite for the automatic stop condition to be satisfied, the engine is in the process of performing EGR, which recirculates exhaust gas from the exhaust passage 40 to the intake passage 30, and the EGR rate (the proportion of EGR gas in the intake air) is relatively high. For this reason, when the automatic stop condition is satisfied, the oxygen concentration in the intake passage 30 is significantly lower than the oxygen concentration in the atmosphere at least in the vicinity of the engine body 1 (more specifically, the portion downstream of the first EGR outlet portion 61b), and the value is lower than the target value Dx of the oxygen concentration used in step S9 described later.

[0050] When it is determined that the automatic stop condition is satisfied as a result of the determination of YES in step S1, the automatic stop control unit 102 of the ECU 100 closes the throttle valve 32 (step S2). That is, the automatic stop control unit 102 closes the throttle valve 32 so that the opening degree of the throttle valve 32 decreases to a degree equivalent to a fully closed state (0%). Note that the opening degrees of the first EGR valve 62 and the second EGR valve 72 may differ depending on the operating conditions before the automatic stop condition is satisfied, but it is assumed that at least the first EGR valve 62 is in an open state (a state other than fully closed) when the automatic stop condition is satisfied, and that this state is maintained in step S2. In other words, in step S2, a control is executed to close the throttle valve 32 while maintaining the first EGR valve 62 in an open state.

[0051] Next, the automatic stop control unit 102 executes a fuel cut to stop fuel injection from the injector 15 of each cylinder 2 (step S3). After this fuel cut is executed, combustion in each cylinder 2 is stopped, and the engine speed gradually decreases.

[0052] Next, the determination unit 101 determines whether or not the permission condition for the oxygen concentration adjustment control is satisfied (step S4). The oxygen concentration adjustment control is a control for adjusting the oxygen concentration in the intake air present in the intake passage 30 (mainly the surge tank 34 and the intake manifold 30a) downstream of the first EGR outlet portion 61b, which is the connection portion between the first EGR passage 61 and the intake passage 30, to a predetermined target value, and here refers to the control of steps S5 to S10 described later. The permission condition for the oxygen concentration adjustment control is a condition that determines whether or not such an adjustment of the oxygen concentration to the target value is possible. In the following, the oxygen concentration to be adjusted, that is, the oxygen concentration in the intake air present in the intake passage 30 downstream of the first EGR outlet portion 61b (in other words, the oxygen concentration in the intake air after the EGR gas is recirculated from the first EGR passage 61), is also referred to as the "final intake oxygen concentration."

[0053] Specifically, in step S4, the satisfaction of the permission conditions for the oxygen concentration adjustment control is determined based on the engine water temperature and the atmospheric pressure. For example, the determination unit 101 determines that the permission conditions are satisfied when both a first condition that the engine water temperature detected by the water temperature sensor SN2 is equal to or higher than a predetermined threshold and a second condition that the atmospheric pressure estimated from the detection value of the intake pressure sensor SN4 is equal to or higher than a predetermined threshold are satisfied. In other words, the determination unit 101 permits the execution of the oxygen concentration adjustment control from step S5 onwards, which will be described later.

[0054] Conversely, if either the first or second condition is not satisfied, that is, when the engine water temperature is low or the atmospheric pressure is low, there is a possibility that the ignitability required for restarting the engine cannot be ensured, and it is necessary to prioritize the ignitability of the fuel over the adjustment of the oxygen concentration. In such a case, the determination unit 101 determines that the permission condition is not satisfied in step S4 and prohibits the oxygen concentration adjustment control. In this case, the automatic stop control unit 102 executes control to stop the engine while greatly increasing the oxygen concentration (step S22). Although details are omitted, in this step S22, control is executed to increase the opening of the throttle valve 32 to a relatively high value at any timing after the fuel cut so that the above-mentioned final intake oxygen concentration increases to a value that can sufficiently ensure ignitability.

[0055] When the above step S4 is judged as YES and it is confirmed that the oxygen concentration adjustment control is permitted, the automatic stop control unit 102 controls the throttle valve 32 and the first EGR valve 62 so that their openings become predetermined intermediate openings (step S5). The intermediate opening is neither fully closed (0%) nor fully open (100%), and is an opening that substantially narrows the flow path while allowing gas to flow. The control of step S5 changes the throttle valve 32 from a fully closed state to an open state. Such opening of the throttle valve 32 has the effect of increasing the oxygen concentration in the intake air.

[0056] Specifically, in the above step S5, the opening degrees of the throttle valve 32 and the first EGR valve 62 are set to substantially the same value. For example, the opening degrees of the throttle valve 32 and the first EGR valve 62 can each be set to about 30%. Note that the opening degree of the second EGR valve 72 can be set to an appropriate value, but for example, it can be set to a specific intermediate opening degree determined separately from the throttle valve 32 and the first EGR valve 62.

[0057] Next, the determination unit 101 determines whether the current oxygen concentration condition corresponds to a preset assumed concentration condition (step S6). Specifically, the determination unit 101 determines that the oxygen concentration condition corresponds to the assumed concentration condition when both of the following conditions (x) and (y) are satisfied. (x) The estimated value of the final intake oxygen concentration is equal to or greater than a preset first threshold value D1. (y) The detected value of the exhaust oxygen concentration is equal to or greater than a preset second threshold value D2.

[0058] In the above condition (x), the estimated value of the final intake oxygen concentration refers to the estimated value of the oxygen concentration in the intake air present at a specific location (for example, the surge tank 34) in the intake passage 30 on the downstream side of the first EGR outlet portion 61b. For example, exhaust O 2 It is estimated by calculation from various conditions including the exhaust oxygen concentration detected by the exhaust gas sensor SN5 and the engine operation history (for example, the opening degree data of the first and second EGR valves 62 and 72 in a recent specific period). In the above condition (y), the detected value of the exhaust oxygen concentration refers to the exhaust O 2 The exhaust oxygen concentration detected by the exhaust gas sensor SN5. Also, the first threshold value D1 used in the above condition (x) is set to a value smaller than the target value Dx of the oxygen concentration used in step S9 described later (D1 < Dx), and the second threshold value D2 used in the above condition (y) is set to a value even smaller than the first threshold value D1 (D2 < D1).

[0059] When the determination in step S6 is YES and it is confirmed that the current oxygen concentration condition corresponds to the assumed concentration condition, that is, when it is confirmed that both the estimated value of the final intake oxygen concentration and the detected value of the exhaust oxygen concentration are equal to or greater than the thresholds (D1, D2), the automatic stop control unit 102 controls the throttle valve 32 and the first EGR valve 62 so that their openings change in conjunction with the engine speed (step S7). Specifically, the automatic stop control unit 102 controls the throttle valve 32 and the first EGR valve 62 so that their openings also gradually decrease as the engine speed gradually decreases after the fuel cut. If the rate of decrease in the opening of each valve 32, 62 relative to the decrease in the engine speed is defined as the opening change rate, the opening change rate is previously determined to be an appropriate value that does not cause a sudden change in the final intake oxygen concentration and can stabilize the intake pressure. In this embodiment, the openings of the throttle valve 32 and the first EGR valve 62 are controlled to decrease at the same rate of change in opening. In other words, in step S7, the openings of the throttle valve 32 and the first EGR valve 62 are gradually decreased while being maintained at substantially the same value.

[0060] On the other hand, if the determination in step S6 above is NO and it is confirmed that the current oxygen concentration condition does not fall under the assumed concentration condition, that is, if it is confirmed that at least one of the estimated value of the final intake oxygen concentration and the detected value of the exhaust oxygen concentration is less than the threshold value (D1 or D2), the automatic stop control unit 102 controls the throttle valve 32 and the first EGR valve 62 so that the opening degree of the throttle valve 32 is greater than the opening degree of the first EGR valve 62 (step S8). In this step S8, the opening degree of the throttle valve 32 is set to a value greater than the opening degree of the throttle valve 32 in a case where the current oxygen concentration condition falls under the assumed concentration condition (i.e., the opening degree set in step S7 above) and greater than the opening degree of the first EGR valve 62. For example, if the openings of the throttle valve 32 and the first EGR valve 62 are both about 30% at the start of step S7, then in step S8, the opening of the throttle valve 32 can be increased to about 50%, and the opening of the first EGR valve 62 can be set to 30% or a value slightly lower than that. That is, in the control of step S8 executed under conditions in which the oxygen concentration is lower than expected, the opening of the throttle valve 32 is increased compared to the control of step S7 executed under other conditions. On the other hand, the opening of the first EGR valve 62 is not increased, and is maintained at approximately the same opening (the same or a slightly lower opening).

[0061] After the control of either step S7 or S8 is started, the judgment unit 101 judges whether the estimated value of the final intake oxygen concentration has increased to a predetermined target value Dx or more (step S9). The target value Dx of the final intake oxygen concentration used here is set to a value larger than the above-mentioned first threshold value D1, but smaller than the oxygen concentration in the atmosphere, that is, the intake oxygen concentration when all the intake air is fresh air.

[0062] When the result of the determination in step S9 above is YES and it is confirmed that the final intake oxygen concentration is equal to or greater than the target value Dx, the automatic stop control unit 102 closes the throttle valve 32 (step S10). That is, the automatic stop control unit 102 drives the throttle valve 32 to close so that the opening degree of the throttle valve 32 decreases to a position equivalent to a fully closed position.

[0063] Next, the automatic stop control unit 102 adjusts the opening degree of the first EGR valve 62 so that the intake pressure does not fall below a predetermined reference pressure Px (see FIG. 5(d)) (step S11). The reference pressure Px used here is the pressure in the surge tank 34 detected by the intake pressure sensor SN4, i.e., a reference value of the intake pressure, and is set to a value corresponding to a weak negative pressure that is slightly lower than the atmospheric pressure. Since the reference pressure Px is such a value (weak negative pressure), when the step S11 is started, the opening degree of the first EGR valve 62 is increased from the opening degree immediately before. That is, since the introduction of new intake air is basically stopped by closing the throttle valve 32 in the above step S10, in order to keep the intake pressure at a weak negative pressure in this state, it is necessary to increase the opening degree of the first EGR valve 62 to promote the return of EGR gas through the first EGR passage 61. Therefore, in step S11, the automatic stop control unit 102 drives the first EGR valve 62 in the opening direction, thereby adjusting the intake pressure so that it does not fall below the reference pressure Px (so that it remains at a weak negative pressure). Specifically, the automatic stop control unit 102 determines the basic opening degree of the first EGR valve 62 from the engine speed while referring to a map or the like, and calculates a target opening degree of the first EGR valve 62 by correcting the determined basic opening degree using the pressure detected by the intake pressure sensor SN4. In step S11, the opening degree of the first EGR valve 62 is controlled according to the target opening degree calculated in this manner, so that the intake pressure in the surge tank 34 is adjusted so that it does not fall below the reference pressure Px (to a weak negative pressure).

[0064] Next, the determination unit 101 determines whether the engine speed has decreased to less than a predetermined reference speed Nx (step S12). The reference speed Nx is set to a value that is smaller than the engine speed at the time of fuel cut in step S3 above, and that allows the intake pressure to be adjusted to an appropriate level in the future. Specifically, the reference speed Nx is set to a value that allows the minimum intake pressure Py (see FIG. 5(d)), which is significantly negatively pressurized after step S13 described later, to fall within a predetermined target range Z. Note that the target range Z can be set to include, for example, 50 kPa, and the reference speed Nx can be set to, for example, about 700 to 800 rpm.

[0065] When the determination in step S12 is YES and it is confirmed that the engine speed is less than the reference speed Nx, the automatic stop control unit 102 closes the first EGR valve 62 (step S13). That is, the automatic stop control unit 102 drives the first EGR valve 62 to close so that the opening degree of the first EGR valve 62 decreases to a position equivalent to the fully closed position. Here, the throttle valve 32 is already in the fully closed state through the control in step S10. Therefore, from step S13 onwards, both the throttle valve 32 and the first EGR valve 62 are in the fully closed state, and this causes the intake pressure to become negative.

[0066] Next, the determination unit 101 determines whether the engine has completely stopped (step S14). That is, the determination unit 101 determines whether the engine speed detected by the crank angle sensor SN1 has substantially decreased to zero, and determines that the engine has completely stopped when the engine speed has substantially decreased to zero (i.e., when the rotation of the crankshaft 7 has stopped).

[0067] If the determination in step S14 is YES and it is confirmed that the engine has completely stopped, the automatic stop control unit 102 opens the first EGR valve 62 to an intermediate opening degree or an opening degree close to full opening while maintaining the throttle valve 32 in a fully closed state (step S15). This is a preparatory control for restarting the engine, and is performed for the purpose of preventing excessive cranking resistance when restarting the engine.

[0068] As can be seen from the flow of control from step S9 onwards, in the engine of this embodiment, the phenomenon in which the final intake oxygen concentration rises to or exceeds the target value Dx and the phenomenon in which the engine speed falls to below the reference speed Nx usually occur in this order. However, for example, when the engine speed when the automatic stop condition (S1) is satisfied is significantly lower than expected, or when the intake oxygen concentration when the automatic stop condition is satisfied is significantly lower than expected, the latter phenomenon may occur before the former phenomenon. Next, the contents of the control performed in such special cases will be described.

[0069] In order to check whether the above special case has occurred, when the judgment result in step S9 is NO (when the final intake oxygen concentration is less than the target value Dx), the judgment unit 101 further judges whether the engine speed has decreased to less than the reference speed Nx (step S17).

[0070] When the above step S17 is judged as YES and it is confirmed that the engine speed has fallen below the reference speed Nx, that is, when it is confirmed that a special case has occurred in which the engine speed falls below the reference speed Nx before the final intake oxygen concentration becomes equal to or greater than the target value Dx, the automatic stop control unit 102 closes both the throttle valve 32 and the first EGR valve 62 (step S18). That is, the automatic stop control unit 102 drives the throttle valve 32 and the first EGR valve 62 to close so that the opening degrees of the throttle valve 32 and the first EGR valve 62 are reduced to the extent equivalent to fully closed.

[0071] Next, the determination unit 101 determines whether or not the engine has completely stopped, that is, whether or not the engine speed has substantially decreased to zero (step S19).

[0072] When the above step S19 is judged as YES and it is confirmed that the engine has completely stopped, the automatic stop control unit 102 opens the throttle valve 32 to a predetermined intermediate opening (step S20). This is a preparatory control for restarting the engine, and is performed for the purpose of avoiding insufficient ignition when the engine is restarted. That is, in the above special case, the throttle valve 32 is closed before the final intake oxygen concentration rises to the target value Dx, so that if the engine is restarted in this state, there is a high possibility that the required ignition will not be ensured. Therefore, by opening the throttle valve 32 in advance in step S20, ignition at the time of restart is ensured.

[0073] <Example of operation of each part under automatic stop control> FIG. 5 is a time chart showing an example of the change over time of each state quantity when the automatic stop control shown in FIG. 3 and FIG. 4 is executed, where chart (a) shows the change over time of a flag indicating whether the automatic stop condition is met, chart (b) shows the change over time of a flag indicating whether fuel injection from injector 15 is required, chart (c) shows the change over time of the engine speed, chart (d) shows the change over time of the intake pressure in surge tank 34, chart (e) shows the change over time of the opening degree of each of throttle valve 32 and first EGR valve 62, and chart (f) shows the change over time of the intake oxygen concentration in surge tank 34 (i.e., the final intake oxygen concentration).

[0074] In Fig. 5, the time when the automatic stop condition (step S1 in Fig. 3) is satisfied is set as t0. In response to the automatic stop condition being satisfied at this time t0, first, the opening of the throttle valve 32 is reduced to 0% (fully closed) (chart (e)). This corresponds to the control of step S2 in Fig. 3.

[0075] At time t1, which is delayed from time t0 when the automatic stop condition is satisfied, a fuel cut is performed to stop fuel injection (chart (b)). This corresponds to the control of step S3 in FIG. 3. Furthermore, immediately after this fuel cut, the opening of the throttle valve 32 is increased from 0% to α%, and the opening of the first EGR valve 62 is also set to α% (chart (e)). This corresponds to the control of step S5 in FIG. 3. α% is set to a predetermined intermediate opening (for example, about 30%) that is neither 0% nor 100%. In this way, by opening the throttle valve 32 to the intermediate opening, the final intake oxygen concentration gradually increases after time t1 (chart (f)). Moreover, since not only the throttle valve 32 but also the first EGR valve 62 are opened, the final intake oxygen concentration does not increase so rapidly but increases at a relatively stable rate of increase. 5, the opening degree of the first EGR valve 62 is set to a value greater than α% before time t0 at which the automatic stop condition is satisfied. Therefore, when fuel is cut off, the opening degree of the first EGR valve 62 is reduced from an opening degree greater than α% to α%.

[0076] At time t2, which is delayed from time t1 when the fuel cut is executed, the engine speed actually starts to decrease (chart (c)). In response to this, the openings of the throttle valve 32 and the first EGR valve 62 are gradually reduced after time t2 (chart (e)). This corresponds to the control of step S7 in FIG. 3. Here, the execution of the control of step S7 means that the previous determinations of steps S4 and S6 were both YES. In other words, the time chart of FIG. 5 shows an example of operation when the permission condition for the oxygen concentration adjustment control is established and the oxygen concentration condition corresponds to the assumed concentration condition. In this case, since the final intake oxygen concentration is equal to or higher than the threshold value D1, there is no need to increase the final intake oxygen concentration in a hurry. Therefore, in FIG. 5, in order to moderately suppress the rate of increase of the final intake oxygen concentration, the above control is executed to gradually decrease the openings of the throttle valve 32 and the first EGR valve 62 in conjunction with the engine speed after time t2.

[0077] At time t3, which is delayed from time t2 when the engine speed starts to decrease, the final intake oxygen concentration rises to the target value Dx (chart (f)). In response to this, the opening of the throttle valve 32 is reduced to 0%, and the opening of the first EGR valve 62 is increased to β% (>α%) (chart (e)). This corresponds to the control of steps S10 and S11 in FIG. 4. By bringing the throttle valve 32 into a fully closed state, the increase in the final intake oxygen concentration is substantially stopped, and the value is maintained near the target value Dx (chart (f)). Furthermore, by increasing the opening of the first EGR valve 62, the intake pressure is adjusted so as not to drop significantly, and the value is maintained at a value that does not fall below the reference pressure Px (chart (d)).

[0078] At time t4, which is delayed from time t3 when the target value Dx is achieved, the engine speed drops to the reference speed Nx (chart (c)). In response to this, the opening of the first EGR valve 62 is reduced to 0% (chart (e)). This corresponds to the control of step S13 in FIG. 4. By this control, both the first EGR valve 62 and the throttle valve 32 are fully closed, so that after time t4, the intake pressure drops rapidly and a relatively strong negative pressure is generated in the surge tank 34 and the intake manifold 30a (chart (d)). The generation of such negative pressure increases the pumping loss of the engine, and therefore the speed at which the engine speed drops is increased compared to the case where the throttle valve 32 or the first EGR valve 62 is opened.

[0079] Some time after time t4 (in this case, just before the engine is completely stopped), the intake pressure drops to the minimum value Py and does not drop any further. Chart (d) shows an example in which the minimum value Py of the intake pressure falls appropriately within the target range Z. This means that the closing of the first EGR valve 62, triggered by the drop to the reference rotation speed Nx, is performed at an appropriate timing so that the minimum value Py of the intake pressure falls within the target range Z.

[0080] At time t5, which is delayed from time t4 when the engine speed decreases to the reference speed Nx, the engine speed decreases to zero and the engine comes to a complete stop. Then, at the subsequent time t6, the first EGR valve 62 is increased from 0% to γ%. This corresponds to the control of step S15 in Fig. 4. In the example of Fig. 5, γ% is set to a high opening (e.g., an opening close to full opening) that is larger than both of the above-mentioned α% and β%.

[0081] 5, the control of opening the throttle valve 32 at time t1 and then closing the throttle valve 32 at time t3 corresponds to the "first control" in the present invention. Also, the control of increasing the opening degree of the first EGR valve 62 at time t3 and then closing the first EGR valve 62 at time t4 corresponds to the "second control" in the present invention.

[0082] Next, an example of operation when the determination in step S6 in FIG. 3 is NO, that is, when the oxygen concentration condition does not meet the assumed concentration condition, will be described with reference to FIG. 6. The time points t0 to t6 in the time chart in FIG. 6 have the same meaning as the time points t0 to t6 in the time chart in FIG. 5 described above. However, in the example in FIG. 6, the final intake oxygen concentration at the time point t1 when the fuel cut is executed is lower than that in FIG. 5 and is below the first threshold value D1. As a result, in the example in FIG. 6, the determination in the above step S6 becomes NO, and the control in step S8 for opening the throttle valve 32 relatively widely is executed. This is why the opening degree of the throttle valve 32 from time point t1 to time point t3 in FIG. 6 is larger than that in FIG. 5. Specifically, in FIG. 6, the opening degree of the throttle valve 32 is increased from 0% to α1% after time point t1, and the increased opening degree α1 is made larger than the opening degree α2% of the first EGR valve 62 set immediately after time point t1.

[0083] The opening of the throttle valve 32 after time t1 increases the final intake oxygen concentration at a relatively large rate. As a result, at time t3, which is delayed from time t1, the final intake oxygen concentration increases to the target value Dx, and the throttle valve 32 is accordingly closed. Meanwhile, the first EGR valve 62 is once driven in the opening direction at time t3, and is then closed at time t4.

[0084] <Action and effect> As described above, in this embodiment, in response to establishment of the automatic engine stop condition, a fuel cut (step S3 in FIG. 3) is executed to stop fuel injection from the injector 15, and the throttle valve 32 and the first EGR valve 62 are controlled so that, during the period from the fuel cut to the complete stop of the engine, the final intake oxygen concentration, which is the oxygen concentration in the intake air present in the intake passage 30 downstream of the first EGR outlet portion 61b, rises to the target value Dx and the minimum value Py of the intake pressure falls within the target range Z. This configuration has the advantage of being able to ensure ignition performance at the time of restart while suppressing vibrations at the time of engine stop.

[0085] Before the automatic stop condition is satisfied, exhaust gas is recirculated (EGR operation) through the EGR passages 61, 71, so that the final intake oxygen concentration is significantly lower than the oxygen concentration in the atmosphere. Therefore, if no measures are taken to increase the final intake oxygen concentration, there is a risk that the ignition of fuel will be impaired when the engine is subsequently restarted, making the restart operation unstable. In contrast, in this embodiment, the final intake oxygen concentration is increased to the target value Dx before the engine is completely stopped, so that the ignition at the time of restart can be well ensured and the restart operation can be stabilized.

[0086] In addition, in this embodiment, the minimum intake pressure Py from fuel cut to complete stop is kept within the target range Z, so that the engine speed can be reduced at an appropriate speed after fuel cut, and vibrations when the engine is stopped can be suppressed.

[0087] For example, when the minimum value Py of the intake pressure is large, that is, when the negative pressure level of the intake pressure is small, the pumping loss of the engine is small, and the period during which the engine rotates by inertia after the fuel cut (the period from the fuel cut to the complete stop, which is the period from time t1 to time t5 in FIG. 5 and FIG. 6) is long. Here, in an engine mounted on a vehicle, a resonance band that causes resonance with the vehicle body exists on the lower rotation side than the idling rotation speed, and the vibration of the vehicle body is likely to increase in this resonance band. Therefore, if the period of the inertia rotation is long, the period during which the engine rotation speed is included in the resonance band becomes long, and there is a risk that unpleasant vibrations will be transmitted to the passengers. For this reason, it is desirable that the minimum value Py of the intake pressure is made small to a certain extent. However, if the minimum value Py of the intake pressure is made excessively small (the negative pressure level of the intake pressure is made excessively large), the speed at which the engine rotation speed decreases becomes too fast, and as a result, the shock generated when the engine completely stops (the shock transmitted to the vehicle body due to the reaction of the sudden stop) is likely to increase, and the passengers may feel uncomfortable.

[0088] In contrast, in the present embodiment, it is possible to keep the minimum intake pressure Py within the target range Z. Therefore, by setting an appropriate target range Z taking into consideration the above-mentioned circumstances, it is possible to reduce vibrations when the engine is stopped, and improve the ride comfort of the vehicle.

[0089] Specifically, in this embodiment, as control for increasing the final intake oxygen concentration to the target value Dx and keeping the minimum intake pressure Py within the target range, control for opening the throttle valve 32 and then closing it (e.g., see times t1 to t3 in Figure 5) and control for opening the first EGR valve 62 and then closing it (e.g., see times t3 to t4 in Figure 5) are executed in this order, so that the above-mentioned final intake oxygen concentration and intake pressure can be appropriately adjusted.

[0090] That is, in this embodiment, the throttle valve 32 is opened after the fuel cut, so that fresh air rich in oxygen flows into the intake passage 30 downstream of the throttle valve 32, thereby increasing the final intake oxygen concentration. In addition, the throttle valve 32 is closed (fully closed) at the point when the final intake oxygen concentration rises to the target value Dx, so that the introduction of fresh air through the throttle valve 32 is stopped, and the final intake oxygen concentration can be substantially maintained at the target value Dx. This means that the final intake oxygen concentration at the time of engine restart can be adjusted to a value higher than the concentration at the time when the automatic stop condition is established and lower than the oxygen concentration contained in the atmosphere. In other words, in this embodiment, the state in which the EGR gas, which is an inert gas, is contained in the intake air can be maintained until the start of the restart, so that it is possible to prevent the combustion temperature in each cylinder 2 from excessively increasing at the time of the restart and the amount of NOx generated from increasing. In this way, according to the configuration of this embodiment in which the throttle valve 32 is opened and then closed, it is possible to suppress the amount of NOx generated while ensuring ignition at the time of the restart.

[0091] On the other hand, the first EGR valve 62 is closed later than the throttle valve 32 (the first EGR valve 62 is kept open for a while after the throttle valve 32 is closed), so it is possible to prevent the minimum value Py of the intake pressure from becoming excessively small and falling below the target range Z. This increases the probability that the minimum value Py of the intake pressure will fall within the target range Z, thereby improving the effect of reducing vibrations when the engine is stopped.

[0092] In addition, depending on the engine, stop position control may be performed to adjust the stop position of the piston 5 of each cylinder 2 to a position favorable for restart. In such an engine, when control for relatively delaying the closing of the first EGR valve 62 is adopted as in this embodiment, the speed at which the engine speed decreases can be adjusted within an appropriate range, so that the accuracy of the stop position control of the piston 5 described above can be prevented from decreasing, and the restartability of the engine can be improved.

[0093] In this embodiment, since the opening degree of the first EGR valve 62 is adjusted so that the intake pressure does not fall below the reference pressure Px during the period from when the throttle valve 32 is closed to when the first EGR valve 62 is closed (times t3 to t4), the pressure state before the intake pressure becomes negative in earnest due to the closing of the first EGR valve 62 can be stabilized, and the behavior of the pressure drop thereafter can be stabilized. This makes it possible to keep the minimum value Py of the intake pressure within the target range Z with a high probability, thereby further enhancing the vibration reduction effect described above.

[0094] Furthermore, in this embodiment, the first EGR valve 62 is closed when the engine speed drops to the reference speed Nx (t4 in FIG. 5). Therefore, the timing for closing the first EGR valve 62 can be determined in a simple manner based on the engine speed, and the intake pressure can be subsequently reduced to an appropriate level.

[0095] <Modification> In the above embodiment, the final intake oxygen concentration, which is the oxygen concentration in the intake air present in the intake passage 30 downstream of the first EGR outlet portion 61b (the oxygen concentration in the intake air after the EGR gas is recirculated from the first EGR passage 61), is estimated by calculation, and the throttle valve 32 is closed when the estimated final intake oxygen concentration rises to the target value Dx, but the final intake oxygen concentration may be directly detected by a sensor. For example, a sensor capable of detecting the oxygen concentration may be attached to the surge tank 34, and the final intake oxygen concentration may be detected by the sensor.

[0096] In the above embodiment, in addition to the high-pressure EGR device 60 including the first EGR passage 61 connecting the exhaust passage 40 upstream of the turbine 52 and the intake passage 30 downstream of the throttle valve 32, the engine is provided with a low-pressure EGR device 70 including a second EGR passage 71 connecting the exhaust passage 40 downstream of the turbine 52 and the intake passage 30 upstream of the throttle valve 32, but the low-pressure EGR device 70 is not essential and may be omitted.

[0097] In the above embodiment, an example in which the present invention is applied to a diesel engine that burns fuel containing diesel oil by compression ignition has been described, but the engine to which the present invention can be applied may be an engine capable of EGR operation for recirculating exhaust gas from an exhaust passage to an intake passage, and the present invention may be applied to engines other than diesel engines. For example, the present invention may be applied to a gasoline engine capable of burning fuel containing gasoline by spark ignition and equipped with an EGR device. [Explanation of symbols]

[0098] 1: Engine body 2: Cylinder 7: Crankshaft (output shaft) 15: Injector 30: Intake passage 32: Throttle valve 40: Exhaust passage 61: 1st EGR passage (EGR passage) 61b: First EGR outlet (connection between the EGR passage and the intake passage) 62: First EGR valve (EGR valve) 101: Judgment section 102: Automatic stop control unit (control unit)

Claims

1. A stop control device is applied to an engine having a cylinder, an output shaft that rotates by receiving energy of combustion in the cylinder, an intake passage through which intake air introduced into the cylinder flows, an exhaust passage through which exhaust gas discharged from the cylinder flows, and an EGR passage connecting the intake passage and the exhaust passage, an injector for supplying fuel to the cylinder; an EGR valve provided in the EGR passage so as to be capable of opening and closing; a throttle valve provided in an intake passage upstream of a connection portion between the EGR passage and the intake passage so as to be capable of opening and closing; a determination unit that determines whether an automatic stop condition for automatically stopping the engine is satisfied; a control unit that controls each part of the engine including the injector, the throttle valve, and the EGR valve, When the determination unit confirms that the automatic stop condition is satisfied, the control unit executes a fuel cut to stop the supply of fuel by the injector, and executes adjustment control to adjust the opening degrees of the throttle valve and the EGR valve so that, during the period from when the fuel cut is executed until the rotation of the output shaft stops, the oxygen concentration in the intake passage downstream of the connection portion increases to a predetermined target value and the minimum value of the intake pressure falls within a predetermined target range; the adjustment control includes a first control for opening the throttle valve and then closing it to maintain the state in which the oxygen concentration has increased to the target value, and a second control for opening the EGR valve and then closing it to keep the minimum value of the intake pressure within the target range, The engine stop control device, wherein the control unit closes the EGR valve through the second control at a later time than the throttle valve through the first control.

2. 2. The engine stop control device according to claim 1, The control unit adjusts the opening degree of the EGR valve so that the intake pressure does not fall below a predetermined reference pressure while the EGR valve is in an open state by the second control.

3. 3. The engine stop control device according to claim 2, The control unit is configured to close the EGR valve through the second control when the engine speed drops to a predetermined reference speed.

Images