JP4488239B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to a technique for purging NOx trapped in a NOx (nitrogen oxide) storage catalyst.

  Some internal combustion engines (engines) include an NOx storage catalyst that purifies exhaust gas by capturing NOx in the exhaust gas. Such a NOx storage catalyst has a function of capturing NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is lean, and in order to recover the purification performance of exhaust gas that has decreased due to the capture of the NOx, A purge process is required.

As the purge process, a lean air-fuel ratio operation and a rich air-fuel ratio operation that increases the fuel injection amount and enriches the air-fuel ratio of the exhaust gas are repeated at regular intervals, and unburned fuel (HC) or carbon monoxide (CO ) Is regularly introduced into the NOx storage catalyst as a reducing agent, and a technique for reducing and removing NOx trapped in the NOx storage catalyst is generally known (see Patent Document 1, etc.).
Thereby, accumulation | storage of NOx can be suppressed and the purification | cleaning performance of the exhaust_gas | exhaustion in a NOx storage catalyst can be maintained.
JP 2002-130019 A

However, when the NOx purge process is performed in the NOx storage catalyst as described above, if the rich air-fuel ratio operation and the lean air-fuel ratio operation are repeated in a cycle such as a long lean operation after a short time rich operation, the purification is performed. The performance may not be sufficiently maintained.
This is because oxygen in the exhaust in the lean air-fuel ratio operation before and after the rich air-fuel ratio operation consumes (oxidizes) HC and CO in the exhaust in the rich air-fuel ratio operation, and is actually used for the purge process. It is thought that HC and CO are insufficient.

In this case, methods for promoting the purge process by increasing the rich air-fuel ratio operation time or further increasing the fuel injection amount can be considered, but these methods are not preferable because they cause a reduction in fuel consumption.
The present invention has been made to solve such problems, and an object of the present invention is to provide an internal combustion engine that can sufficiently maintain the exhaust purification performance of the NOx storage catalyst while suppressing fuel consumption reduction due to purge processing. An object is to provide an exhaust emission control device.

In order to achieve the above object, according to the first aspect of the present invention, the nitrogen oxide is provided in the exhaust passage of the internal combustion engine and captures the nitrogen oxide in the exhaust under a lean air-fuel ratio atmosphere. A nitrogen oxide storage catalyst that reduces and removes in an atmosphere of a fuel ratio; an air-fuel ratio adjusting means that adjusts an air-fuel ratio of the exhaust gas of the internal combustion engine; and a purification rate of nitrogen oxide in the exhaust gas by the nitrogen oxide storage catalyst. Nitrogen oxide trapped in the nitrogen oxide storage catalyst by repeating purification rate detection means for detecting, lean air-fuel ratio operation for leaning the air-fuel ratio and rich air-fuel ratio operation for enriching the air-fuel ratio the during NOx purge process to reduce and remove the period from said start lean air-fuel ratio operation to said purification rate detected by the purification rate detecting means becomes below a predetermined value and the lean air-fuel ratio period of operation Set, and characterized in that an exhaust purifying apparatus for an internal combustion engine and a control means for controlling the air-fuel ratio adjusting means to implement the lean air-fuel ratio operation on the basis of the implementation period of the lean air-fuel ratio operation To do.

  Further, in the invention of claim 2, in claim 1, further comprising a nitrogen oxide trapping amount detecting means for detecting the amount of nitrogen oxide trapped by the nitrogen oxide storage catalyst during the lean air-fuel ratio operation period. The control means further sets an execution condition of the rich air-fuel ratio operation so as to reduce and remove the amount of nitrogen oxide detected by the nitrogen oxide trapping amount detection means, and The air-fuel ratio adjusting means is controlled to repeat the lean air-fuel ratio operation and the rich air-fuel ratio operation while performing the rich air-fuel ratio operation based on an execution condition.

  According to a third aspect of the present invention, in the first or second aspect of the invention, when the continuous operation time of the lean air-fuel ratio operation elapses for a predetermined time, the control means is responsive to the set lean air-fuel ratio operation period. First, the operation is switched to the rich air-fuel ratio operation.

  According to the exhaust gas purification apparatus for an internal combustion engine of claim 1 of the present invention, when performing the purge process of NOx, the purification period of the nitrogen oxide in the exhaust gas in the exhaust gas with the nitrogen oxide storage catalyst during the lean air-fuel ratio operation period. Since the lean air-fuel ratio operation and the rich air-fuel ratio operation are repeated while performing the lean air-fuel ratio operation based on the execution period of the lean air-fuel ratio operation, the NOx is set. When performing the purge process, the lean air-fuel ratio operation can be continued as long as the nitrogen oxide purification rate is maintained at a sufficiently high level without the nitrogen oxide purification rate being lower than a predetermined value.

  Accordingly, the lean air-fuel ratio operation is carried out for a sufficiently long period of time, and the frequency of switching between the rich air-fuel ratio operation and the lean air-fuel ratio operation, that is, the oxygen in the exhaust gas in the lean air-fuel ratio operation before and after the rich air-fuel ratio operation. The frequency of wasteful consumption (oxidation) of HC and CO in the exhaust gas in the fuel ratio operation can be reduced. Therefore, the HC and CO flowing into the nitrogen oxide storage catalyst can be efficiently used for the purge process, and rich. There is no need to increase the air-fuel ratio operation time or further increase the fuel injection amount, and it is possible to sufficiently maintain the exhaust gas purification performance of the nitrogen oxide storage catalyst while preventing a reduction in fuel consumption.

  According to the exhaust gas purification apparatus for an internal combustion engine according to claim 2, the condition for performing the rich air-fuel ratio operation so as to reduce and remove the amount of nitrogen oxide trapped by the nitrogen oxide storage catalyst during the lean air-fuel ratio operation period. And the lean air-fuel ratio operation and the rich air-fuel ratio operation are repeated while performing the rich air-fuel ratio operation based on the execution condition of the rich air-fuel ratio operation. The rich air-fuel ratio operation can be sufficiently performed based on the amount of nitrogen oxide trapped in the nitrogen oxide storage catalyst during the air-fuel ratio operation period.

Therefore, not only the lean air-fuel ratio operation is performed for a sufficiently long period, but also the rich air-fuel ratio operation is sufficiently performed, and the frequency of switching between the rich air-fuel ratio operation and the lean air-fuel ratio operation can be further reduced, thereby preventing a decrease in fuel consumption. However, the exhaust gas purification performance of the nitrogen oxide storage catalyst can be maintained more sufficiently.
According to the exhaust gas purification apparatus for an internal combustion engine of claim 3, continuous lean air-fuel ratio operation is performed regardless of the lean air-fuel ratio operation period set based on the nitrogen oxide purification rate of the nitrogen oxide storage catalyst. When the operation time has elapsed, the lean air-fuel ratio operation is switched to the rich air-fuel ratio operation. Therefore, when the predetermined time is set appropriately, the nitrogen oxide storage catalyst can be used when the nitrogen oxide purification rate does not fall below the predetermined value. The NOx purge process can be carried out while reliably maintaining the nitrogen oxide purification rate at, and the exhaust gas purification performance of the nitrogen oxide storage catalyst can be more sufficiently maintained.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of an exhaust emission control device for an internal combustion engine according to the present invention.
As shown in FIG. 1, an engine body (hereinafter referred to as an engine) 1 that is an internal combustion engine includes, for example, fuel injection in an intake stroke (intake stroke injection mode) and fuel in a compression stroke by switching the fuel injection mode. An in-cylinder injection lean burn engine capable of performing injection (compression stroke injection mode) is employed. Specifically, in the intake stroke injection mode, the engine 1 can select not only an operation mode mainly at the stoichiometric air-fuel ratio (stoichio) but also an operation mode at a rich air-fuel ratio (hereinafter referred to as a rich operation mode). In the injection mode, it is possible to select an operation mode mainly at a lean air-fuel ratio (hereinafter referred to as a lean operation mode).

As shown in the figure, the cylinder head 2 of the engine 1 is provided with an electromagnetic fuel injection valve 6 together with a spark plug 4 for each cylinder, so that fuel can be directly injected into the combustion chamber. .
An ignition coil 8 that outputs a high voltage is connected to the spark plug 4. Further, a fuel supply device (not shown) having a fuel tank is connected to the fuel injection valve 6 via a fuel pipe 7. More specifically, the fuel supply device is provided with a low-pressure fuel pump and a high-pressure fuel pump, whereby fuel in the fuel tank is supplied to the fuel injection valve 6 at a low fuel pressure or a high fuel pressure. Can be injected from the fuel injection valve 6 into the combustion chamber at a desired fuel pressure.

  An intake port is formed in the cylinder head 2 in a substantially upright direction for each cylinder, and one end of an intake manifold 10 is connected so as to communicate with each intake port. The intake manifold 10 is provided with an electromagnetic throttle valve 14 for adjusting the intake air amount. The throttle valve 14 is provided with a throttle position sensor (TPS) 16 that detects its opening.

The cylinder head 2 has an exhaust port formed in each cylinder in a substantially horizontal direction, and one end of an exhaust manifold 20 is connected to the cylinder head 2 so as to communicate with each exhaust port.
An exhaust pipe 22 is connected to the other end of the exhaust manifold 20, and a three-way catalyst 24 is interposed in the exhaust pipe 22 in the vicinity of the exhaust manifold 20. The three-way catalyst 24 has platinum (Pt), rhodium (Rh), palladium (Pd), copper (Cu), cobalt (Co), or silver (Ag) as an active noble metal on a support, Remove HC, CO and NOx.

  A NOx storage catalyst 30 is interposed in a portion of the exhaust pipe 22 downstream of the three-way catalyst 24. The NOx storage catalyst 30 is configured by, for example, supporting a NOx storage agent such as barium (Ba) or potassium (K) on a support containing a noble metal such as platinum (Pt) or palladium (Pd). While trapping NOx under an air-fuel ratio atmosphere (oxidizing atmosphere), it has a function of releasing trapped NOx under a rich air-fuel ratio atmosphere (reducing atmosphere) and reacting with HC and CO in the exhaust to reduce it. is doing.

An air-fuel ratio sensor 32 that detects the air-fuel ratio of the exhaust based on the O ₂ concentration is provided on the upstream side of the three-way catalyst 24 of the exhaust pipe 22.
Further, the NOx storage catalyst 30 is provided with a catalyst temperature sensor 36 for detecting the catalyst temperature Tcat, and NOx sensors 38 and 40 are provided upstream and downstream of the NOx storage catalyst 30, respectively. The upstream NOx sensor 38 detects the inflow amount Qi of NOx flowing into the NOx storage catalyst 30, while the downstream NOx sensor 40 detects the outflow amount Qo of NOx flowing out from the NOx storage catalyst 30.

The ECU (electronic control unit) 60 is configured to control the engine 1 and the like, and includes an input / output device, a storage device (ROM, RAM, nonvolatile RAM, etc.), a central processing unit (CPU), a timer counter, and the like. It is configured. The timer counter can measure the operation time of the engine 1 in particular.
Various sensors such as the above-described TPS 16, air-fuel ratio sensor 32, catalyst temperature sensor 36, NOx sensors 38 and 40 are connected to the input side of the ECU 60, and detection information from these sensors is input.

  On the other hand, various output devices such as the fuel injection valve 6, the ignition coil 8, and the throttle valve 14 are connected to the output side of the ECU 60. These various output devices are calculated in the ECU 60 based on detection information from various sensors. The fuel injection amount, the fuel injection timing, the ignition timing, etc., are respectively output, thereby opening / closing the throttle valve 14 at an appropriate timing and injecting an appropriate amount of fuel from the fuel injection valve 6 at an appropriate timing. At the same time, spark ignition is performed by the spark plug 4 at an appropriate timing.

  Specifically, the ECU 60 determines the fuel injection mode based on detection information from various sensors, and sets the target air-fuel ratio in the cylinder of the engine 1, and the fuel injection amount depends on the target air-fuel ratio. Is set. Note that, by setting the fuel injection amount in accordance with the target air-fuel ratio, the air-fuel ratio of the exhaust gas in the exhaust pipe 22 is adjusted in response to the target air-fuel ratio (air-fuel ratio adjusting means).

  Further, the ECU 60 is configured to periodically perform a NOx purge process in order to release and reduce NOx trapped by the NOx storage catalyst 30 during the lean operation mode. Specifically, the ECU 60 controls the fuel injection valve 6 while taking into account the catalyst temperature Tcat information from the catalyst temperature sensor 36 based on the NOx inflow amount Qi information and the outflow amount Qo information from the NOx sensors 38 and 40, It is configured to perform a NOx purge process.

Specifically, in the NOx purge process, the fuel injection valve 6 is controlled to set the rich air / fuel ratio operation (hereinafter referred to as rich operation) to a rich set air / fuel ratio R1 and the lean air to make lean. The fuel ratio operation (hereinafter referred to as lean operation) is repeated (control means).
FIG. 2 is a flowchart showing a control procedure in the ECU 60 for executing the purge process. The NOx purge process according to the present invention will be described below with reference to the flowchart.

This routine is executed with the start of the lean operation in the lean operation mode. First, in step S10, the NOx inflow amount Qi is input from the NOx sensor 38, and the process proceeds to step S12.
In step S12, the NOx outflow amount Qo is input from the NOx sensor 40, and the process proceeds to step S14.

In step S14, the NOx purification rate Rp is calculated. The NOx purification rate Rp is the ratio of the trapped amount of NOx in the NOx storage catalyst 30 to the inflow amount Qi, and is calculated by Rp = (Qi−Qo) / Qi. Then, the process proceeds to step S16.
In step S16, a lean continuation set time Tp is set. Specifically, the lean continuation set time Tp is set based on the NOx purification rate Rp calculated in step S14. Specifically, the operation time of the engine 1 from when the lean operation is started until the NOx purification rate Rp becomes a predetermined value or less is measured with a timer counter, and the operation time is set as the lean continuation set time Tp. . Then, the process proceeds to step S18.

  In step S18, it is determined whether or not the lean continuation set time Tp set in step S16 is equal to or shorter than a predetermined time T1. Here, the predetermined time T1 is appropriately set to experiments or the like regardless of the NOx purification rate Rp so that the NOx purging process is forcibly performed even when the NOx purification rate Rp does not decrease to the predetermined value. It is a value set in advance based on this. If it is determined that the lean continuation set time Tp is equal to or shorter than the predetermined time T1, the process proceeds to step S20.

  In step S20, whether or not the lean continuation time Tc, which is the operation time of the engine 1 measured by the timer counter of the ECU 60, is equal to or greater than the lean continuation set time Tp, that is, whether the NOx purification rate Rp is equal to or less than a predetermined value. Is determined. When it is determined that the lean continuation time Tc is less than the lean continuation setting time Tp, the process returns to step S10, while when it is determined that the lean continuation setting time Tp or more, that is, the NOx purification rate Rp has become a predetermined value or less. The process proceeds to step S24.

On the other hand, if it is determined in step S18 that the lean continuation set time Tp is longer than the predetermined time T1, the process proceeds to step S22.
In step S22, it is determined whether or not the lean continuation time Tc is equal to or longer than a predetermined time T1. If it is determined that the lean continuation time Tc is less than the predetermined time T1, the process returns to step S10. If it is determined that the lean continuous time Tc is equal to or longer than the predetermined time T1, the process proceeds to step S24 as described above.

In step S24, the catalyst temperature Tcat is input from the catalyst temperature sensor 36, and the process proceeds to step S26.
In step S26, the NOx accumulation amount Qt in the NOx storage catalyst 30 is calculated (nitrogen oxide trapping amount detection means). The accumulated amount Qt is calculated from the inflow amount Qi of NOx input in step S10 and the NOx purification rate Rp calculated in step S14, and further corrected by the catalyst temperature Tcat input in step S24. Specifically, for example, a value obtained by multiplying the NOx inflow amount Qi by the NOx purification rate Rp is integrated over the lean continuation time Tc, and the integrated value is multiplied by a correction coefficient set according to the catalyst temperature Tcat. Ask. Then, the process proceeds to step S28.

  In step S28, a rich operation time Tt is calculated as a purge process condition (condition for performing a rich air-fuel ratio operation). The rich operation time Tt is a time required for reducing and removing NOx of the NOx accumulation amount Qt from the NOx storage catalyst 30 in a situation where the air-fuel ratio of the exhaust gas is the set air-fuel ratio R1. Specifically, the calculation of the rich operation time Tt may be performed, for example, by reading from the map stored in advance in the storage device of the ECU 60 according to the set air-fuel ratio R1 and the accumulation amount Qt. As a processing condition for the purge process, instead of setting the rich operation time Tt while keeping the exhaust air-fuel ratio constant, the exhaust air-fuel ratio (rich air-fuel ratio) is set with the rich operation time Tt constant. May be. Then, the process proceeds to step S30.

In step S30, a NOx purge process is performed. That is, based on the processing conditions calculated in step S28, the fuel injection valve 6 is controlled to perform rich operation. Then, the process proceeds to step S32.
In step S32, the measured lean continuation time Tc is reset to zero. Then, this routine is repeatedly executed as long as it is in the lean operation mode. As a result, the rich operation and the lean operation are repeatedly performed alternately.

  As described above, in the exhaust gas purification apparatus according to the present invention, the rich operation and the lean operation are repeatedly performed in the NOx purge process. At this time, the lean continuous setting time Tp, that is, the continuous operation time of the lean operation is set to the NOx storage catalyst 30. The rich operation time Tt, that is, the continuous operation time of the rich operation, is set based on the NOx accumulation amount Qt based on the NOx purification rate Rp. Like to do. Therefore, the continuous operation time of the lean operation and the rich operation can be set to an optimum time according to the NOx purification rate Rp and the NOx trapping state.

  That is, referring to FIG. 3, when the lean operation time and the rich operation time in the NOx purge process are conventional (setting condition A), and when the NOx purge process according to the present invention is set (setting condition B). Although shown in comparison, by performing the NOx purging process according to the present invention, as shown in the figure, both the lean operation time and the rich operation time can be set to a relatively long time. .

According to FIG. 3, since the ratio between the lean operation time and the rich operation time is the same between the setting condition A and the setting condition B, the lean operation time and the rich operation time of the setting condition B are 2 of the setting condition A, respectively. It is set to double.
If the lean operation time and the rich operation time can be set to a relatively long time as described above, the frequency of switching between the lean operation and the rich operation is reduced, and the rich operation and the lean operation are switched as shown in FIG. In the rich operation time zone Tn immediately before or after the change, the exhaust gas before and after the switching between the rich operation and the lean operation is mixed in the exhaust passage 22 upstream of the NOx storage catalyst 30, and immediately before or after the rich operation time Tn. The HC and CO supplied by the rich operation are unnecessarily consumed by the oxygen in the exhaust during the lean operation, but the HC and CO are wasted during the rich operation due to the oxygen in the exhaust during the lean operation. The opportunity for unnecessary consumption is reduced.

  Specifically, according to FIG. 3, since the ratio between the lean operation time and the rich operation time is the same between the setting conditions A and B, the amount of fuel used in the purge process within the predetermined period is the same. However, in the case of the setting condition B, the number of times of switching between the lean operation and the rich operation within the same period is halved with respect to the setting condition A, so the total time of the time zone Tn is also almost halved, and the exhaust during lean operation Wasteful consumption of HC and CO by oxygen in the inside is reduced to almost half.

Therefore, in the setting condition B according to the present invention, compared to the conventional setting condition A, HC and CO that are actually used for the purge process can be sufficiently secured, and the purge process can be performed efficiently.
Thus, referring to FIG. 4, the transition of the NOx purification rate Rp in each of the above set conditions is shown in comparison, but as shown in FIG. 4, when the purge process is performed under the set condition A, While many HCs and COs are wasted by oxygen in the exhaust gas and the NOx purification rate Rp gradually decreases, the NOx purification is performed when the purge process is performed according to the setting condition B according to the present invention. The rate is kept high enough.

  As described above, according to the exhaust gas purification apparatus of the present invention, the NOx purification rate Rp is set to the predetermined value by setting the lean operation time to an optimum time based on the NOx purification rate Rp, that is, from the start of the lean operation. By setting the lean continuous setting time Tp to be sufficiently long until it becomes below, the frequency of switching between the lean operation and the rich operation is reduced, and the HC and CO are efficiently purged in the NOx storage catalyst 30. No need to increase the rich operation time or further increase the fuel injection amount for the purge process, and sufficiently maintain the exhaust purification performance of the NOx storage catalyst 30 while preventing a decrease in fuel consumption can do.

Furthermore, the frequency of switching between the lean operation and the rich operation is set by setting the rich operation time Tt sufficiently long or setting the air-fuel ratio of the exhaust gas sufficiently rich according to the NOx accumulation amount Qt in the NOx storage catalyst 30. Further, the exhaust purification performance of the NOx storage catalyst 30 can be maintained more sufficiently by reducing the number.
When the lean continuous set time Tp is equal to or shorter than the predetermined time T1, the NOx purge process is performed when the lean continuous time Tc becomes equal to or longer than the predetermined time T1 regardless of the NOx purification rate Rp. Therefore, when the NOx purification rate Rp does not decrease to a predetermined value, the NOx purging process can be performed while maintaining the high NOx purification rate Rp, and the exhaust purification performance of the NOx storage catalyst 30 can be further improved. Can be maintained.

In the above embodiment, the NOx inflow amount Qi to the NOx reduction catalyst 30 is detected by the NOx sensor 38 and the catalyst temperature Tcat is detected by the catalyst temperature sensor 36. However, these are detected by the engine speed, the fuel injection amount, etc. You may obtain | require by calculation based on the driving | running state of the engine 1.
In the present embodiment, a cylinder injection gasoline engine is used as the engine 1, but the present invention is not limited to this, and the present invention may be applied to an internal combustion engine in which the exhaust air-fuel ratio is lean, such as a diesel engine. it can.

1 is a configuration diagram of an exhaust gas purification apparatus for an internal combustion engine according to the present invention. It is a flowchart which shows the control procedure in ECU for performing a purge process. It is explanatory drawing explaining the relationship between the setting conditions in a purge process, and a NOx purification rate. It is a graph which shows transition of the NOx purification rate in each setting condition.

Explanation of symbols

1 Engine 6 Fuel Injection Valve 30 NOx Storage Catalyst 38 NOx Sensor 40 NOx Sensor 60 Electronic Control Unit (ECU)

Claims (3)

A nitrogen oxide storage catalyst that is provided in an exhaust passage of the internal combustion engine, captures nitrogen oxide in exhaust under a lean air-fuel ratio atmosphere, and reduces and removes the captured nitrogen oxide in a rich air-fuel ratio atmosphere;
Air-fuel ratio adjusting means for adjusting the air-fuel ratio of the exhaust gas of the internal combustion engine;
A purification rate detecting means for detecting a purification rate of nitrogen oxide in the exhaust gas at the nitrogen oxide storage catalyst;
During NOx purge processing for reducing and removing nitrogen oxides captured by the nitrogen oxide storage catalyst by repeating lean air-fuel ratio operation that makes the air-fuel ratio lean and rich air-fuel ratio operation that makes the air-fuel ratio rich , The period from the start of the lean air-fuel ratio operation until the purification rate detected by the purification rate detection means falls below a predetermined value is set as the execution period of the lean air-fuel ratio operation, and control means for controlling the air-fuel ratio adjusting means to implement the lean air-fuel ratio operation on the basis of the implementation period,
An exhaust emission control device for an internal combustion engine, comprising: Nitrogen oxide trapping amount detection means for detecting the amount of nitrogen oxide trapped by the nitrogen oxide storage catalyst during the lean air-fuel ratio operation period;
The control means further sets the execution condition of the rich air-fuel ratio operation so as to reduce and remove the amount of nitrogen oxide detected by the nitrogen oxide trapping amount detection means, and sets the execution condition of the rich air-fuel ratio operation as the execution condition of the rich air-fuel ratio operation. 2. The exhaust of the internal combustion engine according to claim 1, wherein the air-fuel ratio adjusting unit is controlled to repeat the lean air-fuel ratio operation and the rich air-fuel ratio operation while performing the rich air-fuel ratio operation based on the exhaust gas. Purification equipment.   3. The control device according to claim 1, wherein when the continuous operation time of the lean air-fuel ratio operation elapses for a predetermined time, the control means switches to the rich air-fuel ratio operation regardless of an implementation period of the lean air-fuel ratio operation. An exhaust gas purification apparatus for an internal combustion engine as described.

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