JP7613236B2 - 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.

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

According to the engine stop control device described in Patent Document 1, when restarting a stopped engine, intake air containing EGR gas is introduced into the cylinder, which is expected to have the effect of suppressing the amount of NOx generated by combustion during restart. However, in environments with low external air pressure, such as at high altitudes, the engine charging efficiency is low, that is, the weight of air introduced into the engine is reduced, which tends to reduce the ignition ability of the mixture. Therefore, if a large amount of EGR gas is introduced into the cylinder in this case, the mixture may not burn properly and the startability of the engine may 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 suppress the generation of NOx while ensuring startability when the engine is restarted.

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, the stop control device including 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, external air pressure detection means that detects external air pressure, a determination unit that determines whether an automatic stop condition for automatically stopping the engine has been established, and a control unit that determines whether the automatic stop condition is established by the determination unit. The device includes a fuel cut execution unit that executes a fuel cut to stop the supply of fuel by the injector when it is confirmed that the condition is satisfied, and a valve control unit that can execute concentration adjustment control to adjust the opening of the throttle valve and the opening of the EGR valve so that the final intake oxygen concentration, which is the oxygen concentration in the intake passage downstream of the connection portion, is lower than the oxygen concentration in the atmosphere between the time when the fuel cut is executed and the time when the rotation of the output shaft stops, and the valve control unit executes the concentration adjustment control when the outside air pressure detected by the outside air pressure detection means is equal to or higher than a predetermined judgment pressure, and suppresses the execution of the concentration adjustment control when the outside air pressure is lower than the judgment pressure (Claim 1).

In the present invention, when the outside air pressure is considered to be high, concentration adjustment control is executed between the execution of fuel cut and the stop of rotation of the output shaft (i.e., the engine is stopped), and the final intake oxygen concentration, which is the oxygen concentration in the intake passage downstream of the connection between the EGR passage and the intake passage, is made lower than the oxygen concentration in the atmosphere. Therefore, when the outside air pressure is high and the charging efficiency and ignitability of the mixture tend to be high, the final intake oxygen concentration when the engine is stopped and then restarted, which is the oxygen concentration of the intake air introduced into the cylinder at restart, can be made lower than the oxygen concentration in the atmosphere. Therefore, when the engine is restarted, the startability can be ensured while the generation of NOx can be suppressed.

On the other hand, when the outside air pressure is low, the execution of concentration adjustment control to make the final intake oxygen concentration lower than the atmospheric oxygen concentration is suppressed. Therefore, when the low outside air pressure tends to reduce the charging efficiency and ignition ability of the mixture, the final intake oxygen concentration at the time of engine restart can be made high, close to the atmospheric oxygen concentration. Therefore, when the engine is restarted, the ignition ability of the mixture can be increased, ensuring startability.

In the above configuration, the concentration adjustment control preferably includes a first control for opening the throttle valve so that the final intake oxygen concentration increases after the fuel cut is performed, and a second control for fully closing the throttle valve when the final intake oxygen concentration increases to a predetermined target value (claim 2).

Before fuel cut, exhaust gas is recirculated through the EGR passage (EGR operation), which tends to make the final intake oxygen concentration excessively low. In contrast, in this configuration, the throttle valve is opened after fuel cut as part of the concentration adjustment control (first control), so that a large amount of fresh air rich in oxygen can be introduced into the intake passage downstream of the throttle valve. This allows the final intake oxygen concentration to be increased toward the target value. Then, when the final intake oxygen concentration has increased to the target value, the throttle valve is closed (second control), so that the introduction of fresh air through the throttle valve is stopped and the final intake oxygen concentration can be maintained at the target value. Therefore, according to this configuration, the final intake oxygen concentration at the time of engine restart can be more reliably set to the target value when the concentration adjustment control is executed.

In the above configuration, preferably, when the outside air pressure is less than the judgment pressure, the valve control unit executes the first control and prohibits execution of the second control (claim 3).

With this configuration, when the outside air pressure is less than the judgment pressure, the first control can be used to increase the final intake oxygen concentration, and the second control is prohibited, so that the throttle valve can be kept open to introduce fresh air into the intake passage even after the final intake oxygen concentration has reached the target value. This makes it possible to more reliably increase the final intake oxygen concentration when the engine is restarted when the outside air pressure is less than the judgment pressure, ensuring good engine startability.

As a configuration different from the above configuration, the valve control unit may execute the first control and the second control regardless of the outside air pressure, and when the outside air pressure is less than the judgment pressure, the target value may be set higher than when the outside air pressure is equal to or greater than the judgment pressure (claim 4).

In this configuration, when the outside air pressure is less than the judgment pressure, the target value of the concentration adjustment control is set higher than the value when the outside air pressure is equal to or greater than the judgment pressure, and the concentration adjustment control including the first control and the second control is executed. Therefore, when the outside air pressure is less than the judgment pressure, the final intake oxygen concentration at the time of engine restart can be adjusted to a relatively high concentration suitable for starting. Therefore, with this configuration, the final intake oxygen concentration at the time of engine restart when the outside air pressure is less than the judgment pressure can be more reliably increased, and the startability of the engine can be reliably improved.

In the above configuration, preferably, the valve control unit controls the throttle valve so that the throttle valve is closed when the rotation of the output shaft stops regardless of the external air pressure, and when the external air pressure is equal to or higher than the judgment pressure, the throttle valve is kept closed even after the rotation of the output shaft stops, and when the external air pressure is lower than the judgment pressure, the throttle valve is opened after the rotation of the output shaft stops (claim 5).

According to this configuration, when the outside air pressure is equal to or higher than the judgment pressure, the throttle valve is maintained in a closed state after the engine is stopped. This prevents fresh air from being introduced into the intake passage, and ensures that the final intake oxygen concentration when the engine is restarted is the concentration when the engine is stopped, i.e., the target value. This ensures engine startability while reliably suppressing the generation of NOx when the outside air pressure is equal to or higher than the judgment pressure. Furthermore, when the outside air pressure is less than the judgment pressure, the throttle valve is opened after the rotation of the output shaft stops, i.e., after the engine is stopped, and the introduction of fresh air into the intake passage is permitted. This makes it possible to more reliably increase the final intake oxygen concentration when the engine is restarted when the outside air pressure is less than the judgment pressure, and to more reliably improve engine startability.

As described above, the engine stop control device of the present invention can ensure engine startability while suppressing the generation of NOx.

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 a first half of the automatic stop control according to the first embodiment. 5 is a flowchart showing a second half of the automatic stop control according to the first embodiment. 4 is a time chart showing an example of changes over time in each state quantity when the automatic stop control according to the first embodiment is executed; 6 is a time chart showing an example of time changes in each state quantity when the automatic stop control according to the first embodiment is performed under conditions different from those in FIG. 5 . 10 is a flowchart showing the first half of the automatic stop control according to the second embodiment. 10 is a flowchart showing a second half of the automatic stop control according to the second embodiment.

First Embodiment
<Overall engine configuration>
A stop control device according to a first embodiment of the present invention will be described. FIG. 1 is a schematic system diagram showing a preferred embodiment of an engine to which the stop control device according to the first embodiment is applied. The engine shown in this figure is a four-stroke diesel engine mounted on a vehicle as a power source for traveling. 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 that recirculate 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 a number of 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 the 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 a number of pistons 5 housed in each cylinder 2 so as to be able to slide back and forth. In this first 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 that flows through the cylinder block 3 and the cylinder head 4 to cool the engine, 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 connected to one side of the cylinder head 4 so as to communicate with the intake port 9 of each cylinder 2. 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.

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 between the air cleaner 31 and a second EGR outlet 71b (described later) 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 connected to one side of the cylinder head 4 (the side opposite the intake passage 30) so as to communicate with the exhaust port 10 of each cylinder 2. The exhaust passage 40 is provided with a catalytic device 41 that purifies the exhaust gas. The catalytic device 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 comprises a compressor 51 arranged in the intake passage 30, a turbine 52 arranged in the exhaust passage 40, and a turbine shaft 53 connecting these. The turbine 52 rotates by receiving energy from the exhaust gas flowing through the exhaust passage 40. The compressor 51 is rotationally driven by the turbine 52 to compress and send out (supercharge) the intake air. The compressor 51 is arranged in the intake passage 30 between the air cleaner 31 and the throttle valve 32.

The high-pressure EGR device 60 is a device for returning 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 first EGR inlet 61a, where the upstream end (exhaust passage 40 side) of the first EGR passage 61 is connected to the exhaust passage 40, is located upstream of the turbine 52 in the exhaust passage 40. The first EGR outlet 61b, where the downstream end (intake passage 30 side) of the first EGR passage 61 is connected to the intake passage 30, is located between the throttle valve 32 and the surge tank 34 in the intake passage 30. The first EGR outlet 61b corresponds to the "connection between the EGR passage and the intake passage" in this invention.

The low pressure EGR device 70 is a device for returning 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 second EGR inlet 71a, where the upstream end (exhaust passage 40 side) of the second EGR passage 71 is connected to the exhaust passage 40, is located downstream of the turbine 52 and the catalytic device 41 in the exhaust passage 40. The second EGR outlet 71b, where the downstream end (intake passage 30 side) of the second EGR passage 71 is connected to the intake passage 30, is located between the air cleaner 31 and the compressor 51 in the intake passage 30.

<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 sensor of the engine is input to the ECU 100. The ECU 100 executes various judgments and calculations based on the detected information and controls each part of the engine, such as the injector 15. Specifically, the ECU 100 is electrically connected to a crank angle sensor SN1, a water temperature sensor SN2, an air flow sensor SN3, an intake pressure sensor SN4, and an exhaust _O2 sensor SN5, and information detected by these sensors SN1 to SN5 (crank angle, engine speed, engine water temperature, intake flow rate, intake pressure, exhaust oxygen concentration), etc., is sequentially input to the ECU 100. In addition, in the first embodiment, a vehicle equipped with the engine is provided with a vehicle speed sensor SN6 that detects a vehicle speed (vehicle running speed), an accelerator sensor SN7 that detects an accelerator pedal opening degree, which is the opening degree of an accelerator pedal provided on the vehicle, and a brake sensor SN8 that detects whether or not a brake pedal provided on the vehicle is operated (brake operation). The vehicle is further provided with an atmospheric pressure sensor SN9 that measures the outside air pressure, which is the atmospheric pressure in the vehicle's driving environment. The ECU 100 also sequentially receives detection information from these sensors SN6 to SN9 (vehicle speed, accelerator opening, brake operation, and outside air pressure information).

The ECU 100 functionally includes a determination unit 101 that performs various determinations, a fuel cut execution unit 102 that executes a fuel cut described below, and a valve control unit 103 that controls the throttle valve and EGR valve after an automatic stop condition described below is met.

<Automatic stop control>
Next, the automatic stop control for automatically stopping the engine during operation will be described in detail with reference to Fig. 3, which is a flow chart showing a specific procedure of the automatic stop control according to the first embodiment.

When the control according to the flowchart in FIG. 3 starts, the determination unit 101 determines whether or not a predetermined automatic stop condition is satisfied (step S1). The automatic stop condition is a condition that allows the engine to be automatically stopped, 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 the off state, and (iii) the brake pedal is in the 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, that is, 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 is determined 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. In other words, 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 basic target value Dx of the final intake oxygen concentration used in step S13 described later.

When the above step S1 is judged as YES and it is confirmed that the automatic stop condition is satisfied, the valve control unit 103 fully closes the throttle valve 32 (step S2). That is, the valve control unit 103 reduces the opening degree of the throttle valve 32 to an opening degree (0%, etc.) at which gas does not substantially move through the valve. 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 fully close the throttle valve 32 while maintaining the first EGR valve 62 in an open state.

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

Next, the valve control unit 103 controls the first EGR valve 62 so that the opening of the first EGR valve 62 becomes a predetermined stop EGR opening (step S4). The stop EGR opening is set to a predetermined intermediate opening in advance and stored in the valve control unit 103. The intermediate opening is neither fully closed (0%) nor fully open (100%), and is an opening that essentially narrows the flow path while allowing gas to flow. For example, the stop EGR opening is set to about 30%. The opening of the second EGR valve 72 can be set to an appropriate value, for example, it can be set to a specific intermediate opening determined separately from the first EGR valve 62.

Next, the determination unit 101 determines whether the outside air pressure is less than a predetermined determination pressure (step S5). The outside air pressure is an index of the ignitability of the mixture. That is, when the outside air pressure is low, the engine charging efficiency is low and the weight of fresh air introduced into the cylinder 2 is reduced, resulting in a low ignitability of the mixture, whereas when the outside air pressure is high, the ignitability of the mixture is high. The detection value of the outside air pressure sensor SN9 is used for the determination in step S4. That is, in this first embodiment, the outside air pressure sensor SN9 corresponds to the "outside air pressure detection means" that detects the outside air pressure. The above-mentioned determination pressure is preset and stored in the determination unit 101.

If step S5 returns NO and it is confirmed that the outside air pressure is equal to or higher than the determination pressure, the valve control unit 103 executes concentration adjustment control (step S20). The concentration adjustment control will be described in detail later.

On the other hand, if step S5 is judged as YES and it is confirmed that the outside air pressure is less than the judged pressure, the valve control unit 103 sets the opening of the throttle valve 32 to the throttle opening for low temperature stop (step S6). The throttle opening for low temperature stop is set to a predetermined intermediate opening in advance and stored in the valve control unit 103. For example, the throttle opening for low temperature stop is set to about 50%. In this way, the throttle valve 32 is opened, and fresh air is introduced into the intake passage 30. Also, although the first EGR valve 62 is opened by executing step S4, the oxygen concentration of the EGR gas increases toward the same concentration as the oxygen concentration in the intake passage 30 because combustion in the cylinder 2 is stopped after the fuel cut. As a result, the oxygen concentration in the intake passage 30 increases after the fuel cut.

Next, the valve control unit 103 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 valve control unit 103 gradually reduces the openings of the throttle valve 32 and the first EGR valve 62 as the engine speed gradually decreases, within a range in which the throttle valve 32 and the first EGR valve 62 are maintained in an open state. At this time, the valve control unit 103 reduces the openings of the throttle valve 32 and the first EGR valve 62 at a speed that results in an appropriate value that can stabilize the intake pressure.

Next, the determination unit 101 determines whether the engine speed has fallen below a predetermined reference speed Nx (step S8). The reference speed Nx is a value smaller than the engine speed at the time of fuel cut in step S2, and is preset to a value that allows the future negative pressure of the intake pressure to be adjusted to an appropriate level and is stored in the determination unit 101. Specifically, the reference speed Nx is set to a value that causes the minimum value Py (see FIG. 5(d)) of the intake pressure that is significantly negative after step S9 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.

The valve control unit 103 continues gradually decreasing the opening degree of the throttle valve 32 and the first EGR valve 62 (step S7) while waiting for the determination in step S8 to become YES. Then, when the determination in step S8 is YES and it is confirmed that the engine speed is less than the reference speed Nx, the valve control unit 103 fully closes the throttle valve 32 and the first EGR valve 62 (i.e., closes the valves) (step S9). With both the throttle valve 32 and the first EGR valve 62 in the closed state (fully closed), the intake pressure is made negative from step S9 onwards.

Next, the determination unit 101 determines whether the engine E has completely stopped (step S10). The determination unit 101 determines that the engine E has completely stopped when the engine speed drops to substantially zero. The determination unit 101 makes this determination based on the detection value of the crank angle sensor SN1.

The valve control unit 103 waits for the judgment of step S10 to become YES while maintaining both the first EGR valve 62 and the throttle valve 32 in a closed state. Then, when the judgment of step S10 is YES and it is confirmed that the engine E has completely stopped, the valve control unit 103 opens the throttle valve 32 to a predetermined throttle opening for starting (step S11). At this time, the opening operation of the first EGR valve 62 is not performed, and the throttle valve 32 is opened while the first EGR valve 62 is maintained in a closed state (fully closed state). The throttle opening for starting is set to a predetermined intermediate opening in advance and stored in the valve control unit 103. The throttle opening for starting is set to, for example, about 50%. Note that step S11, which opens the throttle valve 32, may be executed immediately after the engine E has completely stopped (immediately after the determination in step S10 becomes YES), but in this first embodiment, it is executed a predetermined time after the engine E has completely stopped.

In this way, when the outside air pressure is less than the judgment pressure, both the throttle valve 32 and the first EGR valve 62 are open from when the fuel cut is executed until the engine speed falls below the reference speed Nx, and when the engine speed falls below the reference speed Nx, both are closed. Then, when the engine E comes to a complete stop, the throttle valve 32 is opened while the first EGR valve 62 is maintained in a closed state.

<Density Adjustment Control>
Next, the concentration adjustment control executed when the determination in step S5 is NO will be described with reference to the flowchart in Fig. 4. In the concentration adjustment control, the throttle valve and the first EGR valve 62 are controlled so that the oxygen concentration of the intake air present in the intake passage 30 downstream of the first EGR outlet 61b (the oxygen concentration in the intake passage 30 downstream of the first EGR outlet 61b) becomes a predetermined target value, and hereinafter, the oxygen concentration that is the object of this control, i.e., the oxygen concentration of the intake air present in the intake passage 30 downstream of the first EGR outlet 61b, is referred to as the "final intake oxygen concentration."

When the concentration adjustment control starts, that is, when the result of the above step S5 is NO and it is confirmed that the outside air pressure is equal to or higher than the judgment pressure, the valve control unit 103 sets the opening of the throttle valve 32 to a predetermined throttle opening for stopping (step S21). The throttle opening for stopping is preset to an intermediate opening smaller than the throttle opening for stopping at low temperatures and is stored in the valve control unit 103. For example, the throttle opening for stopping at low temperatures is set to about 50%, while the throttle opening for stopping is set to about 30%. After step S21, the throttle valve 32 is opened as in step S6 and after, and the fuel cut is performed, so that the oxygen concentration in the intake passage 30, including the final oxygen concentration, increases.

Next, similar to step S7, the valve control unit 103 gradually reduces the opening degree of the throttle valve 32 and the first EGR valve 62 in conjunction with the engine speed (step S22).

Next, the judgment unit 101 judges whether the estimated value of the final intake oxygen concentration has reached or exceeded the basic target value Dx, that is, whether the final intake oxygen concentration has risen to the basic target value Dx (step S23). The basic target value Dx is a value smaller than the oxygen concentration in the atmosphere, that is, the intake oxygen concentration when all the intake air is fresh air. The basic target value Dx is set in advance as described above and stored in the judgment unit 101.

The valve control unit 103 continues gradually decreasing the opening degree of the throttle valve 32 and the first EGR valve 62 (step S22) while waiting for the determination in step S13 to become YES. Then, when the determination in step S13 is YES and it is confirmed that the final intake oxygen concentration is equal to or greater than the basic target value Dx, the valve control unit 103 fully closes the throttle valve 32 (step S24).

Next, the valve control unit 103 adjusts the opening of the first EGR valve 62 so that the intake pressure does not fall below the reference pressure Px (see FIG. 5) (step S25). The reference pressure Px 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 S25 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 closing the throttle valve 32 in step S24, 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 S25, the valve control unit 103 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 valve control unit 103 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 S25, 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).

Next, the determination unit 101 determines whether the engine speed has decreased to below the reference speed Nx (step S26).

The valve control unit 103 continues adjusting the opening of the first EGR valve 62 (step S25) while waiting for the determination in step S26 to become YES. Then, when the determination in step S26 is YES and it is confirmed that the engine speed is less than the reference speed Nx, the valve control unit 103 fully closes the first EGR valve 62 (step S27). Here, the throttle valve 32 is already in a closed state through the control of the above-mentioned step S24. Therefore, from step S27 onwards, both the throttle valve 32 and the first EGR valve 62 are closed (fully closed), and this progresses the negative pressure of the intake pressure.

In this way, in the concentration adjustment control, in order to set the final intake oxygen concentration to the target value, control is executed (steps S21 to S23, first control) to open the throttle valve 32 after fuel cut is executed to increase the final intake oxygen concentration, and control is executed (steps S23 to S24, second control) to fully close the throttle valve 32 when the final intake oxygen concentration has increased to the target value.

After the concentration adjustment control is performed, the process returns to the flowchart in FIG. 4, and the determination unit 101 determines whether the engine has completely stopped (step S28), similar to step S10.

The valve control unit 103 waits for the determination in step S28 to become YES while maintaining both the first EGR valve 62 and the throttle valve 32 in a closed state. Then, when the determination in step S28 is YES and it is confirmed that the engine has completely stopped, the valve control unit 103 opens the first EGR valve 62 while maintaining the throttle valve 32 in a closed state, and sets the opening degree of the first EGR valve 62 to the starting EGR opening degree (step S29). At this time, the opening operation of the throttle valve 32 is not performed, and the first EGR valve 62 is opened while the throttle valve 32 is maintained in a closed state. The above-mentioned starting EGR opening degree is set in advance to a predetermined intermediate opening degree and stored in the valve control unit 103. For example, the starting EGR opening degree is set to about 50%. Note that step S29 may be executed a predetermined time after the engine E has completely stopped, but in this first embodiment, step S29 is executed immediately after the determination in step S28 becomes YES, and the first EGR valve 62 is opened.

<Example of operation of each part under automatic stop control>
5 and 6 are time charts showing an example of time changes of each state quantity when the above-mentioned automatic stop control is executed, in which (a) is a flag indicating whether the automatic stop condition is satisfied, (b) is a flag indicating whether fuel injection from the injector 15 is required, (c) is the engine speed, (d) is the intake pressure in the surge tank 34, (e) is each opening degree of the throttle valve 32 and the first EGR valve 62, and (f) is the intake oxygen concentration (i.e., the final intake oxygen concentration) in the surge tank 34. FIG. 5 is a time chart when the outside air pressure is equal to or higher than the judgment pressure. FIG. 6 is a time chart when the outside air pressure is lower than the judgment pressure. The flag indicating whether the automatic stop condition is satisfied is ON when the automatic stop condition is satisfied. The flag indicating whether fuel injection from the injector 15 is required is ON when fuel injection from the injector 15 is required, and is OFF when fuel cut is executed.

First, using Figure 5, we will explain how each state quantity changes when the outside air pressure is equal to or higher than the threshold pressure. In Figure 5, the time when the automatic stop condition is met is set to t0. As shown in Figure 5, when the automatic stop condition is met at time t0, the opening of the throttle valve 32 is first reduced to 0% (fully closed). Next, at time t1, which is delayed from time t0, fuel cut is executed.

Immediately after fuel cut is performed, the opening of the first EGR valve 62 is set to the stop EGR opening, and the opening of the throttle valve 32 is set to the stop throttle opening. In the example of FIG. 5, the stop EGR opening and the stop throttle opening are set to the same opening α%, and immediately after time t1, the openings of the first EGR valve 62 and the throttle valve 32 are set to the opening α%. As described above, the throttle valve 32 is opened to introduce fresh air into the intake passage 30, and the fuel cut is performed to increase the oxygen concentration of the EGR gas, so that the final intake oxygen concentration gradually increases after time t1.

At time t2, which is delayed from time t1 when the fuel cut is executed, the engine speed actually starts to decrease. In response, from time t2 onwards, the openings of the throttle valve 32 and the first EGR valve 62 are gradually reduced.

After that, at time t3, the final intake oxygen concentration rises to the basic target value Dx. As a result, at time t3, the throttle valve 32 is fully closed and its opening is reduced to 0%. With the throttle valve 32 fully closed, the inflow of fresh air into the intake passage 30 stops. As a result, the increase in the final intake oxygen concentration stops at time t3, and from time t3 onwards, the final intake oxygen concentration is maintained near the basic target value Dx. Also, at time t3, the opening of the first EGR valve 62 is increased to β% (>α%). As a result, the intake pressure is maintained at a value that does not fall below the reference pressure Px.

At time t4, which is delayed from time t3 when the basic target value Dx is achieved, the engine speed drops to the reference speed Nx Iman. In response to this, the first EGR valve 62 is fully closed and its opening is reduced to 0%. As a result of this control, both the first EGR valve 62 and the throttle valve 32 are closed, so that the intake pressure drops rapidly from time t4 onwards, and the engine speed also drops rapidly. Here, the intake pressure reaches its minimum value Py a short time after time t3. As described above, the reference speed Nx is set to a value such that the minimum value Py of the intake pressure (see FIG. 5(d)) falls within the predetermined target range Z, and the minimum value Py of the intake pressure achieved a short time after time t3 falls within the target range Z.

At time t5, which is delayed from time t4, the engine speed drops to zero and the engine comes to a complete stop. In the example of FIG. 5, at time t6 after time t5, as the outside air pressure is equal to or higher than the judgment pressure, the first EGR valve 62 is opened and its opening is increased from 0% to the starting EGR opening γ%. In the example of FIG. 5, the starting EGR opening γ% is set to a high opening (for example, an opening close to full opening) that is larger than both of the above-mentioned α% and β%. In this way, the first EGR valve 62 opens at time t6, and the EGR gas in the first EGR passage 61 flows into the surge tank 34. Therefore, the intake pressure increases after time t6. On the other hand, since the throttle valve 32 is maintained in a closed state even after time t6, the final intake oxygen concentration is maintained near the basic target value Dx even after time t6. Specifically, some time after the fuel cut is executed, the components of the intake air introduced into cylinder 2 and the components of the intake air discharged from cylinder 2 become almost the same, and the oxygen concentration of the EGR gas in the first EGR passage 61 becomes almost the same as the final intake oxygen concentration. Therefore, the oxygen concentration of the EGR gas in the first EGR passage 61 at time t6 is the same as the final intake oxygen concentration, that is, close to the basic target value, and even if the first EGR valve 62 opens at time t6, the final intake oxygen concentration is maintained close to the basic target value.

Next, using Figure 6, we will explain how each state quantity changes when the outside air pressure is less than the judgment pressure. Times t10 to t12 in the time chart of Figure 6 have the same meaning as times t0 to t2 in Figure 5. That is, in the example of Figure 6 as well, the automatic stop condition is met at time t10, fuel cut is executed at time t11, and the engine speed actually begins to decrease at time t12. Thus, in the example of Figure 6 as well, the opening of the first EGR valve 62 is set to the stop EGR opening α% immediately after fuel cut is executed at time t11.

In the example of FIG. 6, since the outside air pressure is less than the judgment pressure, the opening of the throttle valve 32 at time t11 is set to a low-temperature stop throttle opening δ%, which is greater than the stop throttle opening α achieved at time t1 in FIG. 5. Then, after time t12, when the engine speed starts to decrease, the first EGR valve 62 is gradually reduced from the stop EGR opening α%, as in FIG. 5, while the throttle valve 32 is gradually reduced from the low-temperature stop throttle opening δ%.

In addition, when the outside air pressure is less than the judgment pressure, the throttle valve 32 is not fully closed as the final oxygen concentration becomes a predetermined target value. In other words, the throttle valve 32 remains open even when the final oxygen concentration becomes the basic target value. As a result, fresh air continues to flow into the intake passage 30 even after the final oxygen concentration becomes the basic target value Dx, and the final oxygen concentration continues to rise. As a result, a while after time t3, the final oxygen concentration rises to a value close to the atmospheric oxygen concentration Dy. In addition, when the outside air pressure is less than the judgment pressure, the throttle valve 32 and the first EGR valve 62 are fully closed as the engine speed becomes less than the reference speed Nx. As a result, in the example of FIG. 6, both the throttle valve 32 and the first EGR valve 62 are closed at time t13 when the engine speed becomes less than the reference speed.

In addition, when the outside air pressure is less than the judgment pressure, after the engine comes to a complete stop, the throttle valve 32 is opened while the first EGR valve 62 is maintained in a closed state. Thus, in the example of FIG. 6, at time t15 after time t14 when the engine comes to a complete stop, the throttle valve 32 is opened while the first EGR valve 62 is maintained in a closed state, and the opening of the throttle valve 32 is increased from 0% to the starting throttle opening ε%. In the example of FIG. 6, the starting throttle opening ε% is set to an opening similar to the low-temperature stop throttle opening δ%.

When the throttle valve 32 opens at time t15, fresh air that has passed through the throttle valve 32 flows into the surge tank 34. As a result, the intake pressure increases from time t15 onwards. Furthermore, the final intake oxygen concentration also increases from time t15 onwards. In the example of Figure 6, as described above, the final intake oxygen concentration has risen to close to the atmospheric oxygen concentration before time t13. As a result, the final intake oxygen concentration increases slightly from time t15 onwards.

<Functions of the First Embodiment>
As described above, according to the first embodiment, when the outside air pressure is equal to or higher than the judgment pressure, the concentration adjustment control is executed, and the final intake oxygen concentration at the time of engine stop is set to the basic target value Dx lower than the oxygen concentration of the atmosphere by adjusting the opening degree of each of the throttle valve 32 and the first EGR valve 62. Therefore, when the outside air pressure is equal to or higher than the judgment pressure and the ignitability of the mixture is high and the startability of the engine is ensured, the final intake oxygen concentration at the time of engine restart, that is, the oxygen concentration of the intake air introduced into the cylinder 2 at the time of restart, can be made lower than the oxygen concentration of the atmosphere. If the oxygen concentration of the intake air introduced into the cylinder is low, the amount of NOx generated is reduced. Therefore, according to the first embodiment, the generation of NOx at the time of engine restart can be suppressed while ensuring the startability of the engine.

On the other hand, when the outside air pressure is below the judgment pressure and the ignition ability of the mixture is likely to be low, the final intake oxygen concentration when the engine is stopped is set to a concentration higher than the basic target value Dx. This increases the ignition ability of the mixture and ensures good engine startability.

In particular, in the first embodiment, when the outside air pressure is equal to or higher than the judgment pressure, the throttle valve 32 is maintained in a closed state after the engine is completely stopped. This makes it possible to prevent fresh air from being introduced into the intake passage 30 through the throttle valve 32 after the engine is stopped. This ensures that the oxygen concentration of the intake air introduced into the cylinder 2 when the engine is restarted is set to the basic target value Dx, thereby reliably reducing the amount of NOx generated.

In addition, when the outside air pressure is less than the judgment pressure, the throttle valve 32 is opened after the engine is completely stopped, so that fresh air can be introduced into the intake passage 30 through the throttle valve 32. This makes it possible to increase the final intake oxygen concentration even after the engine is stopped, and to reliably increase the oxygen concentration of the intake air introduced into cylinder 2 when the engine is restarted, thereby more reliably improving the startability of the engine.

In the first embodiment, the throttle valve 32 is opened after the fuel cut (first control). This allows oxygen-rich fresh air to flow into the intake passage downstream of the throttle valve 32, thereby increasing the final intake oxygen concentration. Therefore, the final intake oxygen concentration can be reliably increased toward an appropriate concentration in both cases where the outside air pressure is less than the judgment pressure and where it is equal to or greater than the judgment pressure. In other words, when the outside air pressure is less than the judgment pressure, the final intake oxygen concentration can be increased toward a value higher than the basic target value Dx and closer to the oxygen concentration of the atmosphere, and when the outside air pressure is equal to or greater than the judgment pressure, the final intake oxygen concentration can be increased toward the basic target value Dx.

When the outside air pressure is equal to or higher than the judgment pressure, the throttle valve 32 is closed (second control) when the final intake oxygen concentration rises to the basic target value Dx, thereby stopping the introduction of fresh air through the throttle valve 32 and maintaining the final intake oxygen concentration at the basic target value Dx. On the other hand, when the outside air pressure is lower than the judgment pressure, the second control of closing the throttle valve 32 when the final intake oxygen concentration rises to the basic target value Dx is not executed (i.e., prohibited), and the throttle valve 32 remains open even after the final intake oxygen concentration rises to the basic target value Dx, thereby lengthening the opening period of the throttle valve 32 and reliably increasing the final intake oxygen concentration.

In addition, if the intake pressure suddenly decreases immediately before the engine is stopped and its minimum value Py falls to an excessively small value below the target range Z, the engine speed will decrease too quickly, which will increase the shock (shock transmitted to the vehicle body due to the reaction of the sudden stop) that occurs when the engine is completely stopped, and the occupants may feel uncomfortable. In contrast, in the first embodiment, both the throttle valve 32 and the first EGR valve 62 are opened after fuel cut, and their openings are gradually decreased in conjunction with the engine speed. In addition, in the first embodiment, when the concentration adjustment control is performed, the first EGR valve 62 is maintained in an open state for a while after the final intake oxygen concentration reaches the target value. Therefore, a sudden decrease in intake pressure can be prevented, and the probability that the minimum value Py of the intake pressure falls within the target range Z can be increased, so that the shock at the time of engine stop can be more reliably reduced. In addition, some engines use stop position control to adjust the stop position of the piston 5 of each cylinder 2 to a position that is favorable for restarting. In such engines, the accuracy of the stop position control of the piston 5 can be improved and the restartability of the engine can be improved by preventing the engine speed from decreasing too quickly.

Second Embodiment
Next, a stop control device according to a second embodiment will be described. The stop control device according to the second embodiment differs from the first embodiment in only part of the content of the automatic stop control, and other configurations are similar to those of the first embodiment. The control content of the automatic stop control according to the second embodiment will be described below. Figures 7 and 8 are flowcharts showing the specific procedure of the automatic stop control according to the second embodiment.

In the second embodiment, as in the first embodiment, the determination unit 101 first determines whether the automatic stop condition is satisfied (step S31). Then, in the second embodiment, when the determination in step S31 is YES and it is confirmed that the automatic stop condition is satisfied, the valve control unit 103 reduces the opening degree of the throttle valve 32 to a fully closed position (0%) (step S32), and the fuel cut execution unit 102 executes a fuel cut (step S33). Furthermore, in the second embodiment, after the fuel cut, the valve control unit 103 sets the opening degree of the first EGR valve 62 to the stop EGR opening degree (step S34).

In the second embodiment, similarly to the first embodiment, the determination unit 101 then determines whether the outside air pressure is less than the above-mentioned determination pressure (step S35). In the second embodiment, if the determination in step S35 is NO and the outside air pressure is equal to or greater than the determination pressure, the same control as in the first embodiment is executed. On the other hand, if the determination in step S35 is YES and the outside air pressure is less than the determination pressure, the second embodiment executes a control different from that in the first embodiment.

Specifically, if the determination in step S35 is NO and it is confirmed that the outside air pressure is equal to or higher than the determination pressure, the valve control unit 103 sets the target value of the final intake oxygen concentration to the above-mentioned basic target value (step S36). Also, if it is confirmed that the outside air pressure is equal to or higher than the determination pressure, the valve control unit 103 sets the opening of the throttle valve 32 to the above-mentioned stop throttle opening (step S37), similar to step S21 in the first embodiment.

On the other hand, if the determination in step S35 is YES and it is confirmed that the engine water temperature is below the judgment pressure, the valve control unit 103 sets the target value of the final intake oxygen concentration to a low temperature target value that is higher than the above-mentioned basic target value (step S38). The low temperature target value is set in advance and stored in the valve control unit 103. Next, if it is confirmed that the outside air pressure is below the judgment pressure, the valve control unit 103 sets the opening of the throttle valve 32 to the above-mentioned low temperature stop throttle opening (step S39). Note that this step S39 is the same control as step S6 in the first embodiment.

After either step S37 or S39 is executed, steps S40 to S46, which are the same as steps S22 to S28 in the first embodiment, are executed. Specifically, the valve control unit 103 gradually reduces the openings of the throttle valve 32 and the first EGR valve 62 in conjunction with the engine speed (step S40). Next, the judgment unit 101 judges whether the estimated value of the final intake oxygen concentration has reached or exceeded its target value (step S41). Here, if the outside air pressure is less than the judgment pressure, the target value used for this judgment is the low temperature target value, and if the outside air pressure is equal to or greater than the judgment pressure, the target value used for this judgment is the basic target value. Then, when it is confirmed that the final intake oxygen concentration has reached or exceeded the target value, the valve control unit 103 fully closes the throttle valve 32 (step S42). Next, the valve control unit 103 adjusts the opening of the first EGR valve 62 so that the intake pressure does not fall below the reference pressure Px (step S43). Next, the determination unit 101 determines whether the engine speed has decreased to less than the reference speed Nx (step S44). Then, when it is confirmed that the engine speed is less than the reference speed Nx, the valve control unit 103 fully closes the first EGR valve 62 (step S45). Next, the determination unit 101 determines whether the engine has completely stopped (step S46).

Thus, in the second embodiment, although the target value of the final intake oxygen concentration is changed according to the outside air pressure, the basic control content until the engine is completely stopped is the same regardless of the outside air pressure. In addition, the control after the engine is completely stopped is also the same as in the first embodiment.

Specifically, when the result of the determination in step S46 is YES and it is confirmed that the engine has completely stopped, the determination unit 101 determines whether the outside air pressure is less than the determination pressure (step S47). When the result of the determination in step S47 is NO and it is confirmed that the outside air pressure is equal to or greater than the determination pressure, the valve control unit 103 opens the first EGR valve 62 while keeping the throttle valve 32 closed, and sets the opening degree of the first EGR valve 62 to the starting EGR opening degree (step S48), similar to step S29 in the first embodiment. On the other hand, when the result of the determination in step S47 is YES and it is confirmed that the outside air pressure is less than the determination pressure, the valve control unit 103 opens the throttle valve 32 while keeping the first EGR valve 62 closed, and sets the opening degree of the throttle valve 32 to the starting throttle opening degree (step S4), similar to step S11 in the first embodiment.

<Functions of the Second Embodiment>
As described above, in the automatic stop control according to the second embodiment, when the outside air pressure is equal to or higher than the judgment pressure, the same control as in the first embodiment is executed. Therefore, according to the second embodiment, like the first embodiment, when the outside air pressure is equal to or higher than the judgment pressure, it is possible to suppress the generation of NOx while ensuring the startability of the engine when it is restarted.

In addition, unlike the first embodiment, in the automatic stop control according to the second embodiment, concentration adjustment control is executed to set the final oxygen concentration to the target value even when the outside air pressure is less than the judgment pressure. However, this target value is set higher than the target value (basic target value) when the outside air pressure is equal to or greater than the judgment pressure. Therefore, in the second embodiment, when the outside air pressure is less than the judgment pressure, the final intake oxygen concentration when the engine is stopped and therefore the oxygen concentration of the intake air introduced into cylinder 2 when the engine is restarted can be set to a high concentration higher than the basic target value and close to the oxygen concentration of the atmosphere, ensuring engine startability. Therefore, the second embodiment can also ensure engine startability and suppress NOx generation. Note that other effects obtained by having a configuration similar to that of the first embodiment can also be obtained in the second embodiment.

(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 61b, is estimated by calculation. However, 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.

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: Fuel cut execution unit 103: Valve control unit SN2: Water temperature sensor (external air pressure detection means)

Claims (5)

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;
An external air pressure detection means for detecting an external air pressure;
a determination unit that determines whether an automatic stop condition for automatically stopping the engine is satisfied;
a fuel cut execution unit that executes a fuel cut to stop the supply of fuel by the injector when the determination unit determines that the automatic stop condition is satisfied;
a valve control unit capable of executing concentration adjustment control for adjusting an opening degree of the throttle valve and an opening degree of the EGR valve so that a final intake oxygen concentration, which is an oxygen concentration in the intake passage downstream of the connection portion, becomes lower than an oxygen concentration in the atmosphere during the period from when the fuel cut is executed to when the rotation of the output shaft is stopped,
The engine stop control device is characterized in that the valve control unit executes the concentration adjustment control when the outside air pressure detected by the outside air pressure detection means is equal to or higher than a predetermined judgment pressure, and suppresses the execution of the concentration adjustment control when the outside air pressure is lower than the judgment pressure. 2. The engine stop control device according to claim 1,
an engine stop control device, characterized in that the concentration adjustment control includes a first control that opens the throttle valve so that the final intake oxygen concentration increases after the fuel cut is executed, and a second control that fully closes the throttle valve when the final intake oxygen concentration increases to a predetermined target value. 3. The engine stop control device according to claim 2,
An engine stop control device, characterized in that, when the outside air pressure is lower than the determination pressure, the valve control unit executes the first control and prohibits execution of the second control. 3. The engine stop control device according to claim 2,
The valve control unit executes the first control and the second control regardless of the outside air pressure, and when the outside air pressure is less than the judgment pressure, sets the target value higher than when the outside air pressure is equal to or greater than the judgment pressure. The engine stop control device according to any one of claims 1 to 4,
the valve control unit controls the throttle valve to be in a closed state when rotation of the output shaft stops regardless of the outside air pressure, and when the outside air pressure is equal to or higher than the judgment pressure, maintains the throttle valve in a closed state even after rotation of the output shaft stops, and when the outside air pressure is less than the judgment pressure, opens the throttle valve after rotation of the output shaft stops.

Images