JP6579151B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to an exhaust emission control device for an engine.

  In the engine, the NOx catalyst disposed in the exhaust passage of the engine occludes NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and the exhaust gas There are some which reduce the stored NOx when the air-fuel ratio is close to the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. Patent Document 1 discloses that when NOx occluded in the NOx catalyst becomes a predetermined value or more, the amount of air is reduced to enrich the air-fuel ratio of the exhaust gas, and NOx reduction control is executed. Yes.

JP-A-10-184418

  When executing NOx reduction control (DeNOx control), the air amount is reduced and the fuel post-injection is performed to enrich the air-fuel ratio of the exhaust gas. However, it has been found that misfire may occur at the beginning of the start of NOx reduction control. In pursuit of such a cause, at the beginning of the NOx reduction control, the exhaust gas energy decreases as the air amount decreases, while the increase in the exhaust gas energy due to post injection causes a delay. Has been found to cause an undershoot that temporarily falls below the target boost pressure, and this undershoot causes a misfire.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an engine exhaust purification device that can prevent engine misfire at the beginning of NOx reduction control.

In order to achieve the above object, the following solution is adopted in the present invention. That is, as described in claim 1,
A fuel injection valve for supplying fuel to the engine;
An air amount control valve for adjusting the amount of air supplied to the engine;
An exhaust turbocharger,
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control for reducing the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of reducing the NOx stored in the NOx catalyst is executed. NOx reduction control means for
With
When the NOx reduction condition is satisfied, the NOx reduction control means turns the air amount control valve toward a set opening that is larger than a target opening corresponding to the target air-fuel ratio capable of NOx reduction. Reduce the air volume little by little by decreasing the opening little by little ,
The NOx reduction control means, when the air amount control valve reaches the set opening, starts post injection that becomes fuel injection during the expansion stroke from the fuel injection valve,
The NOx reduction control means is configured to change the opening amount of the air amount control valve from the set opening amount to the target when the exhaust gas temperature when the air amount control valve is set to the set opening amount is a predetermined temperature or lower. While the exhaust gas temperature when the air amount control valve is set to the set opening degree is a high temperature exceeding the predetermined temperature, the opening amount of the air amount control valve is changed from the set opening degree while gradually decreasing to the opening degree. Decrease to the target opening at once
It is like that.

According to the above solution, since the opening of the air amount control valve is gradually decreased until the set opening set to a larger opening than the target opening, the boost pressure is initially set at the beginning of the NOx reduction control. However, an undershoot that falls below the target boost pressure can be prevented, and misfire can be prevented.

Further , the fuel consumption can be suppressed by delaying the post injection for enriching the air-fuel ratio until the opening of the air amount control valve reaches the set opening. Also, by performing post-injection from the time when the opening of the air amount control valve reaches the set opening, the reduction in exhaust gas energy is sufficiently increased regardless of the subsequent decrease in the opening of the air amount control valve. Thus, it is possible to more reliably prevent the supercharging pressure from decreasing from the target supercharging pressure.

The NOx reduction control means reduces the opening of the air amount control valve from the set opening to the target opening when the exhaust gas temperature when the air amount control valve is set to the set opening is low. from varying little by little until time, which is preferable in order to prevent the boost pressure is reduced from the target boost pressure more reliably. On the other hand, when the exhaust gas temperature is higher than a predetermined temperature when the air amount control valve is set to the set opening, the exhaust gas energy is sufficiently large, so the opening of the air amount control valve is reduced. The target opening can be reduced to a target opening at a stroke, and the target opening can be made early.

A preferred mode based on the above solution is as follows. That is,
The set opening degree is set to be larger as the exhaust gas temperature is lower (corresponding to claim 2).
The NOx reduction control means, wherein the air quantity control valve at low temperatures where the exhaust gas temperature when the said set opening becomes the predetermined temperature or less, the opening degree of the air control valve with an increase in exhaust gas temperature Is reduced (corresponding to claim 3 ). In this case, by performing more precise control that sets the opening of the air amount control valve according to the exhaust gas temperature, the situation where the supercharging pressure falls from the target supercharging pressure can be prevented more reliably. can do.

The air amount control valve is arranged downstream of a compressor wheel in the exhaust turbocharger in the intake passage (corresponding to claim 4 ). In this case, it is preferable to accurately control the air amount by the air amount control valve.

  According to the present invention, it is possible to prevent engine misfire at the beginning of NOx reduction control.

The figure which shows an example of the engine to which this invention was applied. The figure which shows the example of a setting of the execution area | region of DeNOx control. The time chart which shows the control content of this invention. The block diagram which shows the example of a control system of this invention. The flowchart which shows the example of control of this invention.

  In FIG. 1, reference numeral 1 denotes an engine (engine body), which is an in-line four-cylinder automobile diesel engine in the embodiment. The engine 1 has a cylinder 2, a cylinder head 3, and a piston 4 as is known. An intake port 6 and an exhaust port 7 are opened with respect to the combustion chamber 5 formed above the piston 4. The intake port 6 is opened and closed by an intake valve 8, and the exhaust port 7 is opened and closed by an exhaust valve 9. A fuel injection valve 10 is attached to the cylinder head 3 so as to face the combustion chamber 5. In the embodiment, the fuel injection is of a common rail type, and extremely high pressure fuel is injected from the fuel injection valve 10.

  In the intake passage 20 connected to the intake port 6, the air cleaner 21, the compressor wheel 22 a of the large exhaust turbocharger 22, and the compressor of the small exhaust turbocharger 23 are sequentially arranged from the upstream side to the downstream side. A wheel 23a, an intercooler 24, an air amount control valve 25, and a surge tank 26 are provided. The surge tank 26 and each cylinder (the intake port 6 thereof) are independently connected by a branched intake passage 27.

  A bypass passage 28 is provided in the intake passage 20. The bypass passage 28 has an upstream end opened to the intake passage 20 between the compressor wheels 22a and 23a. Further, the downstream end of the bypass passage 28 is opened to the intake passage 20 between the compressor wheel 23 a and the intercooler 24. A control valve 29 is disposed in the bypass passage 28.

  In the exhaust passage 40 connected to the exhaust port 7, the turbine wheel 23b of the exhaust turbocharger 23, the turbine wheel 22b of the exhaust turbocharger 22, the NOx catalyst 41, A DPF 42 is connected.

  A waste gate passage 40a that bypasses the turbine wheel 22b is formed in the exhaust passage 40, and a waste gate valve 43 is disposed in the waste gate passage 40a. Further, a bypass passage 40b that bypasses the turbine wheel 23b is formed in the exhaust passage 40, and a control valve 40b is disposed in the bypass passage 40b. The downstream end of the bypass passage 40b is opened between the exhaust passage 40 and the downstream end of the turbine wheel 23b and the upstream end of the bypass passage 40a.

  The intake passage 20 and the exhaust passage 40 are connected via an EGR passage 50. The upstream end of the EGR passage 50 is opened to the exhaust passage 40 on the upstream side of the turbine wheel 23b. Further, the downstream end of the EGR passage 50 is opened between the intercooler 24 and the air amount control valve 25 in the intake passage 20.

  An EGR cooler 51 is connected to the EGR passage 50, and an EGR valve 52 is disposed on the downstream side of the EGR cooler 51. The EGR passage 50 is provided with a bypass passage 53 that bypasses the EGR cooler 51. The bypass passage 53 has an upstream end opened to the EGR passage 50 upstream of the EGR cooler 51, and a downstream end opened to the EGR passage 50 downstream of the EGR valve 52. An EGR valve 54 is disposed in the bypass passage 53.

  Next, a region in which NOx reduction control (DeNOx control) is executed will be described with reference to FIG. Note that the NOx reduction control is performed in the hatched region in FIG. 2 using the engine speed and the engine load as parameters. This DeNOx control execution region is also a region where supercharging is performed. The supercharging is performed mainly by the small exhaust turbocharger 23 at the time of low engine rotation, and at this time, the control valve 29 is closed. Further, at the time of high engine speed, supercharging by the large exhaust turbocharger 22 is performed, and at this time, the control valve 29 is opened. The supercharging pressure is controlled mainly by controlling the opening degree of the control valve 44, and is also performed by adjusting the opening degree of the wastegate valve 43 when the supercharging pressure increases.

  DeNOx control is started when the NOx occlusion amount in the NOx catalyst 41 becomes equal to or higher than a predetermined upper limit value, assuming that the NOx occlusion amount in the NOx catalyst 41 is 0, assuming that the NOx occlusion amount in the execution region shown in FIG. It is based on doing until it becomes. However, the DeNOx control may be stopped in a state where the NOx occlusion amount in the NOx catalyst 41 is reduced to a lower limit value greater than zero. The detection (or estimation) of the NOx occlusion amount in the NOx catalyst 41 can be performed by a known appropriate method. For example, a sensor that detects the occluded NOx amount is used, or data indicating the engine driver state is used. And NOx occlusion amount can be estimated.

  In the case of DeNOx control, post injection is performed in addition to main injection for obtaining necessary torque. This post-injection is performed during the expansion stroke, more specifically in the first half of the expansion stroke, so as to be burned in the cylinder in order to suppress discharge of unburned fuel from the cylinder and oil dilution suppression. Further, EGR is executed in order to prevent or suppress the generation of soot during DeNOx control. The exhaust gas recirculation by EGR execution delays the ignition of the post-injected fuel and ensures the time until the post-injected fuel is combusted (the post-injected fuel is suitable for air and fuel) Ignited in a mixed state).

  Next, with reference to the time chart of FIG. 3, the outline of DeNOx control will be described focusing on the reduction of the intake air amount and post injection. First, until the time point t1, the vehicle is in a steady operation in an area other than the execution area of the DeNOx control shown in FIG. Prior to time t1, the request flag for executing DeNOx control is set to 0, and DeNOx control is not executed. Further, supercharging is performed normally by the exhaust turbochargers 22 and 23, and the air amount control valve 25 is fully opened.

  At the time t1, the DeNOx control region shown in FIG. Accordingly, the opening degree of the air amount control valve 25 is reduced (decreased) from the fully opened state.

  The opening degree of the air amount control valve 25 is reduced in two steps as follows. That is, the first stage of opening degree reduction of the air amount control valve 25 is a gradual reduction to a preset opening degree set in advance. The time when the opening is gradually reduced to the set opening is the time t2 in FIG. This set opening is set to an opening larger than the target opening of the air amount control valve 25. That is, the air amount control valve 25 is finally lowered to the target opening corresponding to the target air-fuel ratio at the time of enrichment of the air-fuel ratio for DeNOx control. However, the air amount control valve 25 is not reduced to the target opening at once, but the opening is gradually reduced to a set opening that is larger than the target opening. The set opening degree can be set to, for example, about 60.80% of the target opening degree. The set opening may be a constant value, but may be a variable value that increases as the exhaust gas temperature decreases, for example.

  The second stage of opening degree reduction of the air amount control valve 25 is opening degree reduction from the set opening degree to the target opening degree. In the embodiment, the opening degree reduction at the second stage is feedback-controlled so that the opening degree of the air amount control valve 25 becomes an opening degree corresponding to the exhaust gas temperature. That is, the lower the exhaust gas temperature, the more easily the undershoot of the boost pressure, i.e., misfire, occurs. Therefore, the opening degree of the air amount control valve 25 is reduced as the exhaust gas temperature rises.

  In FIG. 3, when a predetermined temperature is set as the exhaust gas temperature and the exhaust gas temperature is lower than the predetermined temperature when the opening degree of the air amount control valve 25 reaches the set opening degree, the exhaust gas temperature is set according to the exhaust gas temperature. Therefore, the opening degree of the air amount control valve 25 is decreased (with an increase in the exhaust gas temperature). Then, the time when the temperature of the exhaust gas reaches the predetermined temperature is shown at time t4 in FIG. 3, and the opening of the air amount control valve 25 is set as the target opening at time t4. Although not shown in FIG. 3, if the exhaust gas temperature is higher than a predetermined temperature when the opening of the air amount control valve 25 becomes the set opening, the exhaust gas energy is sufficiently large. The opening degree of the air amount control valve 25 is immediately set to the target opening degree. The exhaust gas temperature can be regarded as exhaust gas energy. When the exhaust gas temperature is equal to or higher than a predetermined temperature, it is determined that the exhaust gas energy is sufficiently large and undershoot (misfire) does not occur. can do.

  The post-injection of fuel is started when the opening of the air amount control valve 25 reaches the set opening (the injection timing is in the first half of the expansion stroke as described above, but is performed in the second half of the expansion stroke, etc. Appropriate time can be selected during the expansion stroke). The post injection amount is gradually increased. The post-injection will be described more specifically. First, as the opening degree of the air amount control valve 25 is decreased, the supercharging pressure (supercharging pressure on the downstream side of the air amount control valve 25) is gradually decreased, and eventually the target is reached. The boost pressure is reached (time t3 in FIG. 3), and the time t3 when the target boost pressure is reached is also the time when the target air amount during DeNOx control is reached. The post injection amount is maximized at time t3 when the supercharging pressure becomes the target supercharging pressure. The post-injection amount is preferably gradually increased as the supercharging pressure decreases (or as the air amount decreases) until the supercharging pressure decreases to the target supercharging pressure. It should be noted that the start timing of the post injection may be performed simultaneously with the opening reduction of the air amount control valve 25 or prior to the opening opening reduction. In this case, the fuel consumption increases, It is preferable to start from the time when the amount control valve 25 reaches the set opening.

  As described above, the air amount is gradually decreased to the set opening degree that is larger than the target opening degree without reducing the air quantity due to the opening degree of the air amount control valve 25 to the target opening degree at once. Since the post-injection is started and the exhaust gas energy is positively increased when the set opening is reached, misfire is reliably prevented without causing an undershoot at the beginning of the DeNOx control opening. be able to. Further, by reducing the opening degree of the air amount control valve 25 from the set opening degree while observing the exhaust gas temperature (that is, the exhaust gas energy), undershoot due to insufficient exhaust gas energy (according to misfire) Can be prevented more reliably.

  In the embodiment, EGR is executed during DeNOx control. That is, by performing EGR, the post-injected fuel is burnt slowly, so that the generation of soot is prevented or suppressed. In the embodiment, in order to suppress the change in the air-fuel ratio of the exhaust gas due to the fluctuation of the EGR amount, one of the EGR valves 52 and 54 that is currently opened is opened at a constant opening ( In the embodiment, the valve is fully open) (the other EGR valve is fully closed).

  The state in which the EGR valve is opened at a constant opening is performed prior to a decrease in the opening of the air amount control valve 25. That is, when the opening of the air amount control valve 25 is decreased while the EGR valves 52 and 54 are closed (fully closed), the suction negative pressure increases rapidly (negative pressure increases), and supercharging is performed. It becomes easy to generate an undershoot (misfire accompanying this) in which the pressure is lower than the target supercharging pressure. However, if the EGR execution state is secured in advance prior to the decrease in the opening amount of the air amount control valve 25, undershoot due to an increase in suction negative pressure can be prevented.

  FIG. 4 shows an example of a control system for performing DeNOx control. In the figure, U is a controller (control unit) configured using a microcomputer. The controller U receives at least signals from various sensors S1 and SS6. S1 is an intake air amount sensor that detects the intake air amount, and is disposed, for example, on the downstream side of the air cleaner 21 in the intake passage 20. S2 is a rotation speed sensor that detects the engine rotation speed. S3 is a load sensor that detects an engine load (accelerator opening).

  S4 is an air-fuel ratio sensor (oxygen sensor) that detects the air-fuel ratio of the exhaust gas. In the embodiment, S4 is disposed on the downstream side of the DPF 42 in the exhaust passage 40. It can also be arranged immediately upstream. S5 is a temperature sensor that detects the exhaust gas temperature, and is disposed, for example, upstream of the opening position of the EGR passage 50 in the exhaust passage 40. S6 is a supercharging pressure sensor, which is disposed in the surge tank 26, for example. The controller U controls the EGR valves 52 and 54 in addition to the fuel injection valve 10 and the air amount control valve 25. Note that the target air-fuel ratio in the DeNOx control is set so that λ (excess air ratio) is in the range of 0.96 · 1.0, for example, and becomes the target air-fuel ratio using the air-fuel ratio sensor S4. Is feedback controlled.

  Next, the control contents of the controller U will be described with reference to the flowchart of FIG. In the following description, Q indicates a step. First, at Q1, it is determined based on the current engine speed and engine load whether or not it is in the execution region of the DeNOx control shown in FIG. If YES in Q1, the execution flag for DeNOx control is set to 1 in Q2 (time t1 in FIG. 3).

  After Q2, in Q3, among the EGR valves 52 and 54, the opening degree of the EGR valve that is currently opened is set to a predetermined constant opening degree (full opening in the embodiment) (the other EGR valve is all closed). From the viewpoint of increasing the exhaust gas energy, it is preferable to recirculate the exhaust gas bypassing the intercooler 51 to the intake passage 20, so that the EGR valve 52 is fully closed while the EGR valve 54 is kept at a predetermined constant level. It can also be an opening.

  After Q3, it is determined in Q4 whether or not the EGR valve has been opened to a certain opening. When the determination of Q4 is NO, the processes of Q3 and Q4 are repeated. If YES in Q4, the opening of the air amount control valve 25 is gradually reduced toward the set opening in Q5. After Q5, at Q6, it is determined whether or not the opening of the air amount control valve 25 has decreased to the set opening. When the determination of Q6 is NO, the processes of Q5 and Q6 are repeated.

  If YES in Q6, post-injection is started in Q7 in addition to the main fuel injection (time t2 in FIG. 3). After Q7, it is determined in Q8 whether or not the exhaust gas temperature is equal to or lower than a predetermined temperature. When the determination in Q8 is YES, in Q9, the air control valve 25 is gradually decreased toward the target opening. The opening degree of the air amount control valve 25 is gradually decreased in response to a gradual increase in the exhaust gas temperature. In this way, when the exhaust gas temperature is low, the exhaust gas energy is not sufficiently increased, and the air pressure control valve is likely to cause an undershoot in which the supercharging pressure is lower than the target supercharging pressure. The opening degree of 25 is gradually decreased according to the exhaust gas temperature.

  If NO in Q8, the exhaust gas temperature is high and the exhaust gas energy is sufficiently large. At this time, in Q10, the opening degree of the air amount control valve 25 is reduced to the target opening degree at a stretch.

  After Q9 or after Q10, it is determined at Q11 whether or not the NOx occlusion amount in the NOx catalyst 41 has decreased to 0 or almost 0, respectively. If the determination in Q11 is NO, the process returns to Q7. If the determination in Q11 is YES, in Q12, the DeNOx control execution flag is reset to 0 and the processing ends.

  If the determination in Q1 is NO, only fuel main injection is performed in Q13 (no post-injection).

  Although the embodiment has been described above, the present invention is not limited to the embodiment, and can be appropriately changed within the scope described in the scope of claims. For example, the invention includes the following cases. . The engine 1 may be a gasoline engine. Only one exhaust turbocharger may be provided, and a variable vane type may be used instead of a waste gate valve type. Each step or step group shown in the flowchart indicates the function of the controller U, and the name indicating the function can be attached to the name of the means so as to be grasped as a constituent requirement of the controller U. Of course, the object of the present invention is not limited to what is explicitly stated, but also implicitly includes providing what is considered preferable or substantially preferable, and can also be understood as a control method. It is.

  The present invention is suitable for application to, for example, an automobile diesel engine.

U: Controller S2: Revolution sensor S3: Load sensor S4: Air-fuel ratio sensor S5: Temperature sensor (for detecting exhaust gas temperature)
S6: Supercharging pressure sensor 1: Engine 2: Cylinder
DESCRIPTION OF SYMBOLS 10: Fuel injection valve 20: Intake passage 22: Exhaust turbo type supercharger 22a: Compressor wheel 22b: Turbine wheel 23: Exhaust turbo type supercharger 23a: Compressor wheel 23b: Turbine wheel 25: Air quantity control valve 40: Exhaust Passage 41: NOx catalyst 50: EGR passage 52: EGR valve 54: EGR valve

Claims (4)

A fuel injection valve for supplying fuel to the engine;
An air amount control valve for adjusting the amount of air supplied to the engine;
An exhaust turbocharger,
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control for reducing the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of reducing the NOx stored in the NOx catalyst is executed. NOx reduction control means for
With
When the NOx reduction condition is satisfied, the NOx reduction control means turns the air amount control valve toward a set opening that is larger than a target opening corresponding to the target air-fuel ratio capable of NOx reduction. Reduce the air volume little by little by decreasing the opening little by little ,
The NOx reduction control means, when the air amount control valve reaches the set opening, starts post injection that becomes fuel injection during the expansion stroke from the fuel injection valve,
The NOx reduction control means is configured to change the opening amount of the air amount control valve from the set opening amount to the target when the exhaust gas temperature when the air amount control valve is set to the set opening amount is a predetermined temperature or lower. While the exhaust gas temperature when the air amount control valve is set to the set opening degree is a high temperature exceeding the predetermined temperature, the opening amount of the air amount control valve is changed from the set opening degree while gradually decreasing to the opening degree. Decrease to the target opening at once In claim 1,
The engine exhaust gas purification apparatus, wherein the set opening degree is made larger as the exhaust gas temperature is lower. In claim 1 or claim 2,
The NOx reduction control means, wherein the air quantity control valve at low temperatures where the exhaust gas temperature when the said set opening becomes the predetermined temperature or less, the opening degree of the air control valve with an increase in exhaust gas temperature An exhaust emission control device for an engine characterized by reducing the exhaust gas. In any one of Claims 1 thru | or 3 ,
The engine exhaust gas purification apparatus, wherein the air amount control valve is disposed downstream of a compressor wheel in the exhaust turbocharger in the intake passage.

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