JP7613239B2 - Engine stop control device - Google Patents

Description

The present invention relates to a stop control device that is applied to an engine that has cylinders, an output shaft that rotates by receiving energy from combustion in the cylinders, an intake passage through which the intake air introduced into the cylinders flows, an exhaust passage through which exhaust gas discharged from the cylinders flows, and an EGR passage that connects the intake passage and the exhaust passage, and that stops the engine when a predetermined automatic stop condition is met.

One example of the above 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 the intake passage, an EGR valve (EGR control valve) provided in the EGR passage, and a control unit that controls the throttle valve and 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 EGR valve when the engine stops due to the injection stop (fuel cut). With this 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.

JP 2004-100497 A

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 conditions immediately before the fuel cut is performed, it is expected that the engine will stop with an excessively large amount of EGR gas trapped in the intake passage. In such a case, the proportion of fresh air introduced into the cylinder when the engine is restarted will be too low, and there is a risk that the ignition of the fuel will not be ensured.

The present invention was made in consideration of the above circumstances, and aims to provide an engine stop control device that can ensure good ignition performance when restarting the engine.

In order to solve the above-mentioned problems, the present invention provides a stop control device applied to an engine having 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 connecting the intake passage and the exhaust passage, comprising an injector that supplies fuel to the cylinder, an EGR valve that is provided in the EGR passage so as to be able to open and close, a throttle valve that is provided in the intake passage upstream of a connection between the EGR passage and the intake passage so as to be able to open and close, a determination unit that determines whether an automatic stop condition for automatically stopping the engine is established, and each part of the engine including the injector, the throttle valve, and the EGR valve. and a control unit that controls the injector 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 oxygen concentration adjustment control to control the throttle valve and the EGR valve so that the oxygen concentration in the intake passage downstream of the connection portion increases to a predetermined target value between the time when the fuel cut is executed and the time when the rotation of the output shaft stops. When the oxygen concentration at a specific time after the automatic stop condition is satisfied is less than a predetermined threshold value that is smaller than the target value, the opening of the throttle valve during the oxygen concentration adjustment control is increased compared to when the oxygen concentration is equal to or greater than the threshold value (claim 1).

Before the automatic stop condition is met, 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. For this reason, if measures to increase the final intake oxygen concentration are not taken, there is a risk that the ignition of the fuel will be impaired when the engine is subsequently restarted, making the restart operation unstable. In contrast, in the present invention, oxygen concentration adjustment control is performed to increase the final intake oxygen concentration to a target value before the engine is completely stopped, so that ignition at the time of restart can be ensured well and the restart operation can be stabilized.

However, for example, if the intake air contains more EGR gas than expected at the time of fuel cut, the throttle valve opening after fuel cut must be controlled to be more open, otherwise the final intake oxygen concentration may not be increased to the target value. In contrast, in the present invention, if the final intake oxygen concentration at a specific time after the automatic stop condition is established is less than a predetermined threshold, the throttle valve opening during oxygen concentration adjustment control is relatively increased, so that the final intake oxygen concentration can be appropriately increased to the target value regardless of the operating state before fuel cut, and good ignition performance can be ensured at restart.

Preferably, the oxygen concentration adjustment control includes a first control for opening the throttle valve so that the oxygen concentration increases, and a second control for closing the throttle valve when the oxygen concentration increases to the target value, and when the oxygen concentration at the specific time is less than the threshold value, the opening of the throttle valve during the first control is increased compared to when the oxygen concentration is equal to or greater than the threshold value (claim 2).

According to this configuration, the oxygen concentration adjustment control is performed by opening the throttle valve once and then closing it, so that the final intake oxygen concentration can be raised to a target value and then maintained in that state. This means that the inert EGR gas can be maintained in the intake until the start of restart. This appropriately suppresses the combustion temperature at restart, so that the amount of NOx generated can be suppressed while ensuring ignition at restart. In addition, if the final intake oxygen concentration at the specific time is less than the threshold value, the throttle valve opening is increased during the control (first control) that opens the throttle valve, so that the final intake oxygen concentration can be appropriately raised to the target value by this opening adjustment.

In the above configuration, more preferably, the first control is a control that opens not only the throttle valve but also the EGR valve (claim 3).

This configuration reduces the amount of fresh air flowing in compared to when the EGR valve is not opened, and prevents the final intake oxygen concentration from rising sharply. This makes it possible to accurately identify the point in time when the final intake oxygen concentration reaches the target value, thereby improving the accuracy of concentration adjustment.

In the above configuration, more preferably, in the first control executed when the oxygen concentration at the specific time is equal to or greater than the threshold value, the opening of the throttle valve is set to be substantially the same as the opening of the EGR valve, and in the first control executed when the oxygen concentration at the specific time is less than the threshold value, the opening of the throttle valve is set to be greater than the opening of the EGR valve (claim 4).

With this configuration, the opening degree of the throttle valve and the EGR valve can be appropriately set according to the conditions of the final intake oxygen concentration at the specific time, and the final intake oxygen concentration can be reliably increased to the target value.

As described above, the engine stop control device of the present invention can ensure good ignition performance when restarting the engine.

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. FIG. 2 is a functional block diagram showing a control system of the engine. 5 is a flowchart showing the first half of an automatic stop control for automatically stopping the engine. 10 is a flowchart showing a second half of the automatic stop control. 4 is a time chart showing an example of time changes of each state quantity when the automatic stop control is executed. 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 shown in FIG. 5 .

<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 oil, 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 for recirculating a part of the exhaust gas flowing through the exhaust passage 40 to the intake passage 30.

The engine body 1 is an in-line multi-cylinder type having multiple cylinders 2 (only one of which is shown in FIG. 1) arranged in a direction perpendicular to the paper surface of FIG. 1. The engine body 1 comprises a cylinder block 3 in which multiple 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 multiple pistons 5 housed in each cylinder 2 so as to be able to slide back and forth. Note that 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.

A combustion chamber 6 is formed above the piston 5 of each cylinder 2. Each combustion chamber 6 is a space defined by the underside of the cylinder head 4, the side 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 burned 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.

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 a central axis in response to the reciprocating motion (up and down movement) of the pistons 5.

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.

The cylinder head 4 has an intake port 9 and an exhaust port 10 that communicate with the combustion chamber 6, each formed for each cylinder 2. The cylinder head 4 is also 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 that it injects fuel from the center of the ceiling surface of the combustion chamber 6 toward the crown surface 5a of the piston 5, for example.

The cylinder head 4 is fitted with 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. 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.

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 multiple branch pipes arranged in a direction perpendicular to the paper surface of FIG. 1) that branches out corresponding to the multiple cylinders 2 in the downstream portion close to the engine body 1. This intake manifold 30a is connected to the cylinder head 4 so as to communicate with the intake port 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.

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 (compressor 51, described in detail later). The surge tank 34 is a tank that provides space for evenly distributing the intake air to each cylinder 2, and is connected to the upstream end of the intake manifold 30a.

An airflow sensor SN3 and an intake pressure sensor SN4 are arranged in the intake passage 30. The airflow sensor SN3 is arranged 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 arranged in the surge tank 34, and detects the pressure of the intake air passing through the surge tank 34, i.e., the intake pressure.

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 multiple branch pipes arranged in a direction perpendicular to the paper surface of FIG. 1) that branches out corresponding to the multiple cylinders 2 in the 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 port 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.

The exhaust passage 40 is provided with a catalytic converter 41 that purifies the exhaust gas. The catalytic converter 41 incorporates an oxidation catalyst 42 that oxidizes and detoxifies the CO and HC in the exhaust gas, and a DPF (diesel particulate filter) 43 that collects particulate matter contained in the exhaust gas.

An exhaust _O2 sensor SN5 is attached to the exhaust passage 40. The exhaust _O2 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, i.e., the exhaust oxygen concentration.

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.

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

The compressor 51 is an impeller that rotates in conjunction with the rotation of the turbine 52 to compress and send out (supercharge) the intake air. The compressor 51 is disposed in the intake passage 30 between the air cleaner 31 and the throttle valve 32.

The high-pressure EGR device 60 is a recirculation device for recirculating a portion of the relatively high-pressure exhaust gas before it flows 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 electrically operated valve that can adjust 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 "EGR valve" in the present invention, respectively.

The location where the upstream end (exhaust passage 40 side) of the first EGR passage 61 is connected to the exhaust passage 40 is the 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 the 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.

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 that can adjust 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.

The location where the upstream end (exhaust passage 40 side) of the second EGR passage 71 is connected to the exhaust passage 40 is 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 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.

<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 operations, memories such as ROM and RAM, and various input/output buses.

Information detected by each of the engine sensors is input to the ECU 100. For example, the ECU 100 is electrically connected to the crank angle sensor SN1, water temperature sensor SN2, air flow sensor SN3, intake pressure sensor SN4, and exhaust _O2 sensor SN5. Information detected by each of the sensors SN1 to SN5, that is, information such as the crank angle, engine speed, engine water temperature, intake air flow rate, intake pressure, and exhaust oxygen concentration, is sequentially input to the ECU 100.

In addition, ECU 100 also receives detection information from various sensors equipped in the vehicle. In this embodiment, the vehicle is equipped with a vehicle speed sensor SN6, an accelerator sensor SN7, and a brake sensor SN8. Vehicle speed sensor SN6 is a sensor that detects the vehicle speed, which is the traveling speed of the vehicle, accelerator sensor SN7 is a sensor that detects the accelerator opening, which is the opening of an accelerator pedal equipped in the vehicle, and brake sensor SN8 is a sensor that detects whether or not the brake pedal equipped in the vehicle is being operated (brake operation). Detection information (vehicle speed, accelerator opening, and brake operation information) from each of the sensors SN6 to SN8 is also sequentially input to ECU 100.

The ECU 100 controls each part of the engine while executing various judgments and calculations based on the input information from each of the above 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 each of these devices based on the results of the above calculations, etc.

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" in this invention.

The automatic stop control unit 102 is a control module that automatically stops a running engine when certain conditions are met. The restart control unit 103 is a control module that restarts an engine that has been stopped by the automatic stop control unit 102. The judgment unit 101 is a control module that performs various judgments and calculations necessary to execute the control by the automatic stop control unit 102 and the restart control unit 103.

<Automatic stop control>
Next, the automatic stop control for automatically stopping the engine during operation 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.

For example, in a vehicle whose only power source for running is essentially an engine (a so-called engine vehicle), the automatic stop condition may be met when multiple conditions are met, including (i) the vehicle is essentially stopped, (ii) the accelerator pedal is in an off state, and (iii) the brake pedal is in an on state. In this case, the determination unit 101 determines whether the vehicle speed is essentially zero, whether the accelerator opening is essentially 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, if all of these determinations are YES (i.e., if all of the above conditions (i) to (iii) are met), it is determined that the automatic stop condition is met.

On the other hand, in vehicles that also use a motor as a power source for driving, i.e. so-called hybrid vehicles that are capable of EV driving using only the motor, there are cases where the driving force of the engine is not required even while the vehicle is driving, 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 motor from the vehicle speed detected by the vehicle speed sensor SN6 and the accelerator opening detected by the accelerator sensor SN7, etc., and determines whether or not engine output (positive output torque that contributes to driving) is required based on various conditions including the calculated target torque. Then, if it is determined that engine output is not required, it determines that the automatic stop condition is established.

Regardless of which of the above patterns the automatic stop condition is based on, the engine load is not high immediately before it 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, the premise for the automatic stop condition to be satisfied is that the engine is 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. Therefore, 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 61b), and the value is lower than the target value Dx of the oxygen concentration used in step S9 described later.

When the above step S1 is judged as YES and it is confirmed that the automatic stop condition is satisfied, 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 drives the throttle valve 32 to close 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 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 this state is maintained in the step S2. In other words, in the step S2, control is executed to close the throttle valve 32 while maintaining the first EGR valve 62 in an open state.

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.

Next, the determination unit 101 determines whether the permission conditions for oxygen concentration adjustment control are 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 downstream of the first EGR outlet 61b, which is the connection between the first EGR passage 61 and the intake passage 30 (mainly the surge tank 34 and the intake manifold 30a), to a predetermined target value, and refers to the control of steps S5 to S10 described below. The permission conditions for the oxygen concentration adjustment control are conditions that determine whether or not such adjustment to the target value of the oxygen concentration 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 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".

Specifically, in step S4, the fulfillment of the permission conditions for oxygen concentration adjustment control is determined based on the engine water temperature and atmospheric pressure. For example, the determination unit 101 determines that the permission conditions are fulfilled 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 fulfilled. In other words, the determination unit 101 permits the execution of oxygen concentration adjustment control from step S5 onwards, which will be described later.

On the other hand, 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, the ignition quality required for restarting the engine may not be ensured, and it is necessary to prioritize the ignition quality 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 final intake oxygen concentration described above increases to a value that can sufficiently ensure ignition.

If 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 each of the throttle valve 32 and the first EGR valve 62 so that their opening degrees are predetermined intermediate opening degrees (step S5). The intermediate opening degree is an opening degree that is neither fully closed (0%) nor fully open (100%), and is an opening degree that essentially 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.

Specifically, in step S5, the openings of the throttle valve 32 and the first EGR valve 62 are set to approximately the same value. For example, the openings of the throttle valve 32 and the first EGR valve 62 can each be set to approximately 30%. The opening of the second EGR valve 72 can be set to an appropriate value, for example, to a specific intermediate opening determined separately from the throttle valve 32 and the first EGR valve 62.

Next, the determination unit 101 determines whether the current oxygen concentration condition corresponds to a predetermined 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 inspired oxygen concentration is greater than or equal to a predetermined first threshold value D1.
(y) The detected value of the exhaust oxygen concentration is equal to or greater than a predetermined second threshold value D2.

In the above condition (x), the estimated value of the final intake oxygen concentration is an estimated value of the oxygen concentration in the intake air present at a specific location (e.g., the surge tank 34) in the intake passage 30 downstream of the first EGR outlet portion 61b, and is estimated by calculation from various conditions including, for example, the exhaust oxygen concentration detected by the exhaust _O2 sensor SN5 and the engine operation history (e.g., the opening data of the first and second EGR valves 62, 72 in the most recent specific period). In the above condition (y), the detected value of the exhaust oxygen concentration is the exhaust oxygen concentration detected by the exhaust _O2 sensor SN5. In addition, 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). The first threshold value D1 of the final intake oxygen concentration corresponds to the "predetermined threshold value" in the present invention.

If the above step S6 is judged as YES and it is confirmed that the current oxygen concentration condition corresponds to the assumed concentration condition, that is, if 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 threshold value (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 gradually decrease as the engine speed gradually decreases after the fuel cut. If the ratio of the amount of decrease in the opening of each valve 32, 62 to the amount of decrease in the engine speed is defined as the opening change rate, the opening change rate is predetermined 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 approximately the same value.

On the other hand, if the result of the above step S6 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 (i.e., the opening degree set in the above step S7) in the case where the current oxygen concentration condition falls under the assumed concentration condition, 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, which is 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, which is 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).

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 or above a predetermined target value Dx (step S9). The target value Dx of the final intake oxygen concentration used here is set to a value larger than the first threshold value D1 described above, but smaller than the oxygen concentration in the atmosphere, that is, the intake oxygen concentration when all the intake air is fresh air.

If the above step S9 returns 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 of the throttle valve 32 decreases to a position equivalent to a fully closed position.

Next, the automatic stop control unit 102 adjusts the opening 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., the 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 step S11 is started, the opening of the first EGR valve 62 is increased from the opening immediately before. In other words, since the introduction of new intake air is basically stopped by the closing of the throttle valve 32 in step S10, in order to keep the intake pressure at a weak negative pressure in this state, it is necessary to increase the opening 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 of the first EGR valve 62 from the engine speed while referring to a map or the like, and calculates the target opening of the first EGR valve 62 by correcting the determined basic opening using the pressure detected by the intake pressure sensor SN4. In step S11, the opening of the first EGR valve 62 is controlled according to the target opening calculated in this way, and 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).

Then, the determination unit 101 determines whether the engine speed has fallen below 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 future negative pressure of the intake pressure to be adjusted to an appropriate level. Specifically, the reference speed Nx is set to a value that allows the minimum value Py (see FIG. 5(d)) of the intake pressure that is significantly negative after step S13 described below 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.

If the above step S12 returns 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 a fully closed position. Here, the throttle valve 32 is already in a fully closed state through the control of the above step S10. Therefore, from step S13 onwards, both the throttle valve 32 and the first EGR valve 62 are in a fully closed state, which causes the intake pressure to become negative.

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).

If the above step S14 returns 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 or to an opening 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 avoiding excessive cranking resistance when restarting the engine.

As can be seen from the control flow from step S9 onwards, in the engine of this embodiment, the phenomenon in which the final intake oxygen concentration rises to or above the target value Dx and the phenomenon in which the engine speed falls to below the reference speed Nx usually occur in that order. However, for example, if the engine speed when the automatic stop condition (S1) is met is significantly lower than expected, or if the intake oxygen concentration when the automatic stop condition is met 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 explained.

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).

If the above step S17 returns YES and it is confirmed that the engine speed has fallen below the reference speed Nx, that is, if 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 reaches or exceeds 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 being fully closed.

Next, the determination unit 101 determines whether the engine has completely stopped, i.e., whether the engine speed has essentially dropped to zero (step S19).

If step S19 returns 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 special case described above, the throttle valve 32 is closed before the final intake oxygen concentration rises to the target value Dx, so if the engine is restarted in this state, there is a high possibility that the necessary 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.

<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 the 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 the surge tank 34, chart (e) shows the change over time of the opening degree of the throttle valve 32 and the first EGR valve 62, and chart (f) shows the change over time of the intake oxygen concentration in the surge tank 34 (i.e., the final intake oxygen concentration).

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, the opening of the throttle valve 32 is first reduced to 0% (fully closed) (chart (e)). This corresponds to the control of step S2 in FIG. 3.

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)). In addition, 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. In the example of FIG. 5, the opening degree of the first EGR valve 62 is set to a value greater than α% before time t0 when 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%.

At time t2, which is delayed from the execution time t1 of the fuel cut, 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 judgments 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 particular 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.

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 fully closing the throttle valve 32, the increase in the final intake oxygen concentration is essentially stopped, and the value is maintained near the target value Dx (chart (f)). In addition, by increasing the opening of the first EGR valve 62, the intake pressure is adjusted so that it does not drop significantly, and the value is maintained at a value that does not fall below the reference pressure Px (chart (d)).

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. This control causes both the first EGR valve 62 and the throttle valve 32 to be 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 in which the throttle valve 32 or the first EGR valve 62 is opened.

Some time after time t4 (in this case, just before the engine comes to a complete stop), 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.

At time t5, delayed from time t4 when the engine speed drops to the reference speed Nx, the engine speed drops 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., close to full opening) that is greater than both the α% and β% described above.

In the above-described operation example of FIG. 5, the control for opening both the throttle valve 32 and the first EGR valve 62 between time t1 and time t3 corresponds to the "first control" in the present invention. Also, the control for closing the throttle valve 32 at time t3 corresponds to the "second control" in the present invention.

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 step S6 above 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.

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. On the other hand, the first EGR valve 62 is driven once in the opening direction at time t3, and is then closed at time t4.

<Action and effect>
As described above, in this embodiment, in response to the establishment of the automatic engine stop condition, a fuel cut (step S3 in FIG. 3) is performed to stop fuel injection from the injector 15, and the throttle valve 32 and the first EGR valve 62 are controlled so that 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 61b, rises to the target value Dx during the period from the fuel cut to the complete stop of the engine. Specifically, the control for adjusting the oxygen concentration includes a control for opening the throttle valve 32 immediately after the fuel cut (see time points t1 to t3 in FIG. 5 or FIG. 6), and the opening degree of the throttle valve 32 in the open state is made smaller when the assumed concentration condition including the final intake oxygen concentration being equal to or greater than the first threshold value D1 is met (step S7 in FIG. 3) than when the assumed concentration condition is not met (step S8). In other words, when the final intake oxygen concentration when the throttle valve 32 is opened is less than the first threshold value D1, the opening degree of the throttle valve 32 is made larger than when the final intake oxygen concentration is equal to or greater than the first threshold value D1 (more specifically, when the condition that the exhaust oxygen concentration is equal to or greater than the second threshold value D2 is also satisfied). This configuration has the advantage of ensuring good ignition performance when restarting an engine that has automatically stopped.

Before the automatic stop condition is met, exhaust gas is recirculated through the EGR passages 61, 71 (EGR operation), so the final intake oxygen concentration is significantly lower than the oxygen concentration in the atmosphere. For this reason, if no measures are taken to increase the final intake oxygen concentration, the ignition of the fuel may be impaired when the engine is subsequently restarted, and the restart operation may become unstable. In contrast, in this embodiment, the throttle valve 32 and the like are opened immediately after the fuel cut, and the final intake oxygen concentration is increased to the target value Dx before the engine is completely stopped, so that ignition at the time of restart can be ensured well and the restart operation can be stabilized.

However, for example, if more EGR gas than expected is contained in the intake air at the time of fuel cut, the throttle valve 32 must be opened further after fuel cut to prevent the final intake oxygen concentration from increasing to the target value Dx. In contrast, in this embodiment, if the final intake oxygen concentration is less than the first threshold value D1 when the throttle valve 32 is opened, the throttle valve 32 is opened relatively larger (see time points t1 to t3 in FIG. 6), so that the final intake oxygen concentration can be increased appropriately to the target value Dx regardless of the operating state before the fuel cut, and good ignition performance can be ensured during restart.

In addition, in this embodiment, as a control for raising the final intake oxygen concentration to the target value Dx, the throttle valve 32 is once opened and then closed, so that the final intake oxygen concentration can be raised to the target value Dx and then maintained in that state. That is, the throttle valve 32 is opened immediately after the fuel cut, so that fresh air rich in oxygen flows into the intake passage 30 downstream of the throttle valve 32, thereby raising 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 inert gas EGR gas can be maintained in the intake air until the start of the restart, so that the combustion temperature in each cylinder 2 can be prevented from rising excessively during the restart, which would increase the amount of NOx produced. In this way, the configuration of this embodiment in which the throttle valve 32 is opened and then closed can suppress the amount of NOx produced while ensuring ignition during the restart.

In addition, in this embodiment, when the throttle valve 32 is opened immediately after a fuel cut, the first EGR valve 62 is also opened (step S5 in FIG. 3), so the amount of fresh air flowing in can be reduced compared to when the first EGR valve 62 is closed rather than opened, and a sudden rise in the final intake oxygen concentration can be avoided. This makes it easier to estimate the final intake oxygen concentration, so the point in time at which the final intake oxygen concentration reaches the target value Dx can be accurately identified, improving the accuracy of concentration adjustment.

Furthermore, in this embodiment, in the control for opening both the throttle valve 32 and the first EGR valve 62 (step S5 in FIG. 3), the magnitude relationship of the opening degree of each valve 32, 62 is changed depending on the condition of the final intake oxygen concentration at the start of the control (steps S7, S8). Specifically, when the final intake oxygen concentration is equal to or greater than the first threshold D1 (more specifically, when the condition that the exhaust oxygen concentration is equal to or greater than the second threshold D2 is also satisfied), the opening degree of the throttle valve 32 and the first EGR valve 62 are set to be approximately the same (see time points t1 to t3 in FIG. 5), while when the final intake oxygen concentration is less than the first threshold D1, the opening degree of the throttle valve 32 is made larger than the opening degree of the first EGR valve 62 (see time points t1 to t3 in FIG. 6). With this configuration, the opening degree of each valve 32, 62 can be appropriately set according to the magnitude of the final oxygen concentration when the throttle valve 32 and the first EGR valve 62 are opened, and the final intake oxygen concentration can be reliably increased to the target value Dx.

<Modification>
In the above embodiment, after the fuel cut, it is determined whether or not both the condition that the final intake oxygen concentration (estimated value) is equal to or greater than the first threshold value D1 and the condition that the exhaust oxygen concentration (detected value) is equal to or greater than the second threshold value D2 are satisfied (step S6), and the throttle valve 32 is controlled so that the opening degree of the throttle valve 32 when the result of the determination is NO (step S8) is larger than the opening degree when the result is YES (step S7). However, the opening degree of the throttle valve 32 may be adjusted only depending on the intake oxygen concentration without considering the exhaust oxygen concentration. In this case, the opening degree of the throttle valve 32 is increased or decreased depending on whether the final intake oxygen concentration is equal to or greater than a predetermined threshold value. In other words, the throttle valve 32 is controlled so that the opening degree of the throttle valve 32 when the final oxygen concentration is less than the threshold value is larger than the opening degree when the final oxygen concentration is equal to or greater than the threshold value.

In addition, in the above embodiment, the determination of whether or not to increase the opening of the throttle valve 32 from the normal opening was made based on the magnitude of the final intake oxygen concentration at the start of control to open the throttle valve 32 (and the first EGR valve 62). However, the time at which the final intake oxygen concentration is checked to determine whether or not such an increase is necessary (the specific time in the present invention) may be any time between the establishment of the automatic stop condition and any time during the opening operation of the throttle valve, and various modifications are possible to that extent.

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 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. However, the final intake oxygen concentration may be detected directly by a sensor. For example, a sensor capable of detecting oxygen concentration may be attached to the surge tank 34, and the final intake oxygen concentration may be detected by the sensor.

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.

In the above embodiment, an example of applying the present invention to a diesel engine that burns fuel containing diesel oil by pressure ignition has been described, but the engine to which the present invention can be applied is any engine that is capable of EGR operation that recirculates exhaust gas from the exhaust passage to the intake passage, and the present invention may be applied to engines other than diesel engines. For example, the present invention can also be applied to a gasoline engine that is capable of burning fuel containing gasoline by spark ignition and is equipped with an EGR device.

1: Engine body 2: Cylinder 7: Crankshaft (output shaft)
15: injector 30: intake passage 32: throttle valve 40: exhaust passage 61: first EGR passage (EGR passage)
61b: First EGR outlet (connection between the EGR passage and the intake passage)
62: First EGR valve (EGR valve)
101: Determination unit 102: Automatic stop control unit (control unit)

Claims (4)

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 an oxygen concentration adjustment control to control the throttle valve and the EGR valve so that the oxygen concentration in the intake passage downstream of the connection portion increases to a predetermined target value during the period from when the fuel cut is executed to when the rotation of the output shaft stops,
an engine stop control device characterized in that, when the oxygen concentration at a specific time after the automatic stop condition is satisfied is less than a predetermined threshold value which is smaller than the target value, the opening degree of the throttle valve during the oxygen concentration adjustment control is increased compared to when the oxygen concentration is equal to or greater than the threshold value. 2. The engine stop control device according to claim 1,
the oxygen concentration adjustment control includes a first control of opening the throttle valve so that the oxygen concentration increases, and a second control of closing the throttle valve when the oxygen concentration increases to the target value,
An engine stop control device characterized in that, when the oxygen concentration at the specific time is less than the threshold value, the opening degree of the throttle valve during the first control is increased compared to when the oxygen concentration is equal to or greater than the threshold value. 3. The engine stop control device according to claim 2,
2. An engine stop control device according to claim 1, wherein the first control is a control for opening not only the throttle valve but also the EGR valve. 4. The engine stop control device according to claim 3,
In the first control that is executed when the oxygen concentration at the specific time is equal to or higher than the threshold value, an opening degree of the throttle valve is set to be substantially equal to an opening degree of the EGR valve,
An engine stop control device, characterized in that, in the first control that is executed when the oxygen concentration at the specific time is less than the threshold value, the opening degree of the throttle valve is made larger than the opening degree of the EGR valve.