JP6950651B2 - Internal combustion engine system - Google Patents

Description

The present invention relates to an internal combustion engine system.

In recent years, the adoption of compression self-ignition operation, which is operated by a combustion method that self-ignites the air-fuel mixture, has been studied in gasoline engines. However, when the compression self-ignition operation is performed with a low load, the exhaust temperature drops. The catalyst bed temperature tends to decrease. On the other hand, in Patent Document 1, in an engine capable of compression self-ignition operation, when the catalyst is determined to be in the inactive state, the operation of the variable valve mechanism is changed to activate the catalyst. , It is described that the control for increasing the amount of gas remaining in the combustion chamber is performed. According to Patent Document 1, this control enriches the air-fuel ratio of the combustion chamber and increases the carbon monoxide concentration in the exhaust gas. As a result, the decrease in the catalyst bed temperature is suppressed, and the catalyst is activated.

Japanese Unexamined Patent Publication No. 2004-257331 JP-A-2002-099776

During low-load operation, the ignitability may decrease due to a decrease in the temperature inside the cylinder and a decrease in the amount of air. In addition, the ignitability in rich combustion is low. Therefore, as in Patent Document 1, when the control for enriching the combustion chamber is performed during low-load operation, a situation may occur in which misfire occurs due to a decrease in ignitability.

The present invention has been made in view of the above problems, and provides an improved internal combustion engine system capable of improving ignitability in rich combustion operation at a low load.

The present invention is an internal combustion engine system, which includes an internal combustion engine, an EGR passage, a NOx storage catalyst, and a control device. The EGR passage is a passage that connects the exhaust passage and the intake passage of the internal combustion engine and returns a part of the exhaust gas to the intake passage. The NOx storage catalyst is placed in the EGR passage. The control device is configured to be able to switch between operation with the first air-fuel ratio and operation with the second air-fuel ratio richer than the first air-fuel ratio and the stoichiometric air-fuel ratio.

Further, when the control device detects a request for switching from the operation based on the first air-fuel ratio to the operation based on the second air-fuel ratio, the amount of NOx in the cylinder is increased according to the amount of NOx released from the NOx storage catalyst. presume. Here, the amount of NOx in the cylinder is the amount of NOx introduced into the cylinder of the internal combustion engine in this combustion cycle. Then, until the estimated in-cylinder NOx amount reaches the required NOx amount, the target air-fuel ratio in this combustion cycle is within the range of the air-fuel ratio that can be ignited under the in-cylinder NOx amount and the in-cylinder temperature. Then, set the air-fuel ratio on the rich side of the target air-fuel ratio in the previous combustion cycle. Here, the required NOx amount is the NOx amount required for ignition under the second air-fuel ratio and the in-cylinder temperature.

According to the internal combustion engine system of the present invention, when the target air-fuel ratio is switched from the first air-fuel ratio to the second air-fuel ratio richer than the first air-fuel ratio, the shift of the target air-fuel ratio to the rich side is It is done within the range that can be ignited. Here, the ignitable range is a range corresponding to the in-cylinder NOx amount and the in-cylinder temperature in the current combustion cycle, and the target air-fuel ratio is set to the rich side within a range not exceeding this range. As a result, ignition can be ensured even when switching to a rich air-fuel ratio.

A NOx storage catalyst is installed in the EGR passage, and the exhaust gas after combustion is configured to pass through the NOx storage catalyst installed in the EGR passage. When the floor temperature of the NOx storage catalyst rises due to the passage of high-temperature exhaust gas due to rich combustion, the amount of NOx desorbed from the NOx storage catalyst increases. As a result, the amount of NOx in the cylinder in the next combustion cycle can be increased. As a result, the ignitable range in rich combustion is expanded to the rich side. Therefore, the target air-fuel ratio in the next combustion cycle can be set to a value shifted to the richer side. Therefore, it is possible to switch to the rich air-fuel ratio more quickly while suppressing misfire when switching to the rich combustion operation.

It is a figure which shows typically the whole structure of the internal combustion engine system of embodiment of this invention. It is a figure which shows the operating region which the rich combustion mode is selected in the embodiment of this invention. It is a figure for demonstrating the relationship between the amount of NOx in a cylinder required for stable ignition, the air-fuel ratio, and the intake air temperature. It is a timing chart for demonstrating the control of the Embodiment of this invention. It is a figure which showed the routine of the specific control executed in embodiment of this invention by the flowchart.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each figure, the same or corresponding parts are designated by the same reference numerals to simplify or omit the description.

FIG. 1 is a diagram schematically showing an overall configuration of an internal combustion engine system according to an embodiment of the present invention. The internal combustion engine system of the present embodiment includes a multi-cylinder engine (hereinafter, simply referred to as an engine) 2 having a plurality of cylinders. There is no limit to the number and arrangement of cylinders. An intake passage (not shown) communicates with each intake port of each cylinder, and an exhaust passage 4 communicates with each exhaust port. A first NOx catalyst 6 is installed in the exhaust passage 4. The first NOx catalyst 6 is a NOx storage reduction type catalyst, and is installed for the purpose of reducing and purifying NOx in the exhaust gas. As the first NOx catalyst 6, a catalyst having a function of occluding or adsorbing NOx, such as an NSR (NOx Storage Reduction) catalyst and a passive NOx adsorbent (PNA), can be used. In addition, in this specification, "occlusion" and "adsorption" are not distinguished, and "occlusion" includes the form of adsorption and "adsorption" includes the form of occlusion.

The system of FIG. 1 has an EGR passage 10 for recirculating a part of the exhaust gas from the exhaust passage 4 to the intake system. The EGR passage 10 connects the exhaust passage 4 on the upstream side of the exhaust from the NOx catalyst 6 and the intake system of the engine 2. A second NOx catalyst 12, which is a NOx storage catalyst, is installed in the EGR passage 10. The second NOx catalyst 12 may be any catalyst that occludes or adsorbs NOx and desorbs the adsorbed NOx due to an increase in the floor temperature of the catalyst. As the second NOx catalyst 12, an NSR (NOx Storage Reduction) catalyst, a passive NOx adsorbent (PNA), or the like can be used.

The system of FIG. 1 includes an ECU (Electronic Control Unit) 20. The engine 2 and various sensors attached to the vehicle are connected to the ECU 20. These sensors include an air-fuel ratio sensor 22 installed on the upstream side of the NOx catalyst 6 in the exhaust passage 4 and a NOx sensor (not shown) installed in the intake system.

The ECU 20 has at least an input / output interface, a ROM, a RAM, and a CPU. The input / output interface is provided to take in sensor signals from various sensors and output operation signals to the actuator included in the engine 2. Various control data including various control programs and maps for controlling the engine 2 are stored in the ROM. The CPU reads the control program from the ROM, executes it, and generates an operation signal based on the captured sensor signal.

The ECU 20 controls the engine 2 by selecting a combustion mode according to the operating conditions or operating conditions of the engine 2. The combustion mode selected includes a rich combustion mode in which the in-cylinder air-fuel ratio is temporarily changed to a stoichiometric air-fuel ratio or an air-fuel ratio richer in fuel than the stoichiometric air-fuel ratio.

FIG. 2 is a diagram showing an operating region in which the rich combustion mode is selected in the present embodiment. The region outside the region surrounded by the broken line A in FIG. 2, that is, the region where the rotation speed of the engine 2 is high or the operation region where the load is high is the operation region in which the rich combustion mode is selected and the rich combustion operation is performed. In this operating region, the target value of the in-cylinder air-fuel ratio is set to a rich air-fuel ratio as compared with the air-fuel ratio in the region surrounded by the broken line A.

Further, the rich combustion operation is performed, for example, when the NOx of the first NOx catalyst 6 is desorbed or reduced, or when a regeneration process for desorbing and removing the occluded SOx is required. In the present embodiment, the conditions for performing the rich combustion operation are that the operating region specified by the rotation speed and the load of the engine 2 is in a predetermined operating region, and NOx reduction of the first NOx catalyst 6 is required. In some cases, it can be set as appropriate. The ECU 20 detects a request for rich combustion operation when the set conditions are satisfied.

By the way, the low load region surrounded by the broken line B shown in FIG. 2 is a region having low ignitability because the in-cylinder temperature is low. Therefore, if an attempt is made to immediately switch the in-cylinder air-fuel ratio to the air-fuel ratio required in the rich combustion operation in this operating region, a misfire may occur.

On the other hand, in the present embodiment, when the operation region is in such a rich combustion difficult region (broken line B) and a request for rich combustion operation is issued, the amount of NOx in the cylinder is estimated. By gradually shifting the air-fuel ratio to the rich side according to the estimated amount of NOx in the cylinder, it is possible to switch to the rich combustion operation while suppressing misfire. Hereinafter, a specific description will be given.

FIG. 3 is a diagram for explaining the relationship between the amount of NOx in the cylinder required for stable ignition, the air-fuel ratio, and the intake air temperature. By mixing NOx and HC in the intake gas before rich combustion, the ignitability during rich combustion can be improved. As shown in FIG. 3, the amount of in-cylinder NOx required for reliable ignition increases as the in-cylinder air-fuel ratio is rich, and the intake temperature is lower when it is higher than when it is higher. More.

According to the relationship shown in FIG. 3, the required NOx amount required for ignition can be calculated based on the required air-fuel ratio and the current intake air temperature. On the contrary, the range of the air-fuel ratio that can be ignited can be calculated based on the amount of NOx in the cylinder introduced into the cylinder and the intake air temperature.

In the present embodiment, the ECU 20 calculates the required air-fuel ratio finally required in the rich combustion operation and the required NOx amount according to the current intake air temperature for each combustion cycle, and in the cylinder in the current combustion cycle. The amount of in-cylinder NOx to be introduced is estimated, and the limit value on the rich side of the ignitable air-fuel ratio is obtained based on the amount of in-cylinder NOx and the intake air temperature. Then, until the amount of NOx in the cylinder reaches the required amount of NOx, control is performed to set the executed air-fuel ratio, which is the target value of the air-fuel ratio in the current combustion cycle, to the calculated limit value on the rich side.

FIG. 4 is a timing chart for explaining the control executed by the ECU 20. In the timing chart of FIG. 4, the horizontal axis represents the combustion cycle. In the example shown in FIG. 4, a request for rich combustion operation is issued in the first combustion cycle. At this time, the operating region is in the rich combustion difficult region.

The required air-fuel ratio is the required air-fuel ratio (second air-fuel ratio) in the rich combustion operation. Then, the required NOx amount is calculated according to the required air-fuel ratio and the current intake air temperature. However, at this point, the amount of NOx in the cylinder has not increased. The running air-fuel ratio, which is the target air-fuel ratio in this combustion cycle, is set to the richest air-fuel ratio in the ignitable range according to the current amount of NOx in the cylinder and the intake air temperature, and the set running air-fuel ratio is set. Combustion is performed at the fuel ratio.

In the first combustion cycle, combustion is performed at the actual air-fuel ratio shifted to the rich side slightly from the air-fuel ratio (first air-fuel ratio) before the start of switching to rich combustion operation, but the exhaust temperature is increased due to the enrichment of the air-fuel ratio. Rise. Since the heated exhaust gas is introduced into the intake system through the EGR passage 10, the intake air temperature rises in the second combustion cycle. As a result, the required NOx amount in the second combustion cycle is reduced.

Further, as the exhaust temperature rises, the floor temperature of the second NOx catalyst 12 installed in the EGR passage 10 rises, and NOx is released from the second NOx catalyst 12. As a result, the amount of NOx in the cylinder increases in the second combustion cycle.

As the intake air temperature rises and the amount of NOx in the cylinder increases, the range of air-fuel ratio that can be ignited expands to the rich side. Therefore, the executed air-fuel ratio of the second combustion cycle is set to a value shifted to the rich side from the executed air-fuel ratio of the first combustion cycle, and combustion is performed.

In this way, the executed air-fuel ratio is shifted to the rich side within the range where ignition is possible, and the amount of NOx in the cylinder eventually exceeds the amount of NOx in the cylinder in the fourth combustion cycle. After that, in the fifth combustion cycle, the air-fuel ratio becomes the required air-fuel ratio required in the rich combustion operation, and the switching to the rich combustion operation is completed.

FIG. 5 is a flowchart showing a specific control routine executed by the ECU 20. In the routine of FIG. 5, first, in step S102, it is determined whether or not a rich combustion operation is required. The requirement for rich combustion operation is required, for example, when reduction of NOx occluded in the first NOx catalyst 6 and regeneration treatment for desorbing and removing occluded SOx are required. If it is determined in step S102 that there is no rich combustion request, this process ends once.

On the other hand, if it is determined in step S102 that there is a request for rich combustion operation, the process then proceeds to step S104. In step S104, it is determined whether or not the current operating region is in the rich combustion difficult region. This determination is made based on the operating range specified by the current intake air temperature, in-cylinder temperature, engine speed and load.

In step S104, if the current operating region is not in the rich combustion difficult region, the process proceeds to step S106, the target air-fuel ratio is immediately set to the required air-fuel ratio, and the operation is switched to the rich combustion operation.

If it is determined in step S104 that the current operating region is in the rich combustion difficult region, the process proceeds to step S110. In step S110, the required NOx amount is calculated. The required NOx amount is calculated according to the map according to the current intake air temperature and the required air-fuel ratio. This map defines the relationship between the air-fuel ratio, the in-cylinder temperature, and the in-cylinder NOx amount as shown in FIG. 3, and the relationship is obtained in advance by an experiment or the like and stored in the ECU 20.

Next, the process proceeds to step S112, and it is determined whether or not the NOx storage amount of the second NOx catalyst 12 is equal to or greater than a predetermined amount. Here, the predetermined amount is a value for determining whether the amount of NOx required for promoting ignition is in a state where it can be released from the second NOx catalyst 12. Specific values are set in advance and stored in the ECU 20. The NOx storage amount of the second NOx catalyst 12 is an estimated value, and the estimation method is not limited.

When it is determined in step S112 that the NOx storage amount of the second NOx catalyst 12 is less than the predetermined amount, the current process is temporarily terminated. That is, the combustion mode cannot be switched to the rich combustion mode, and combustion in the current combustion mode is continued.

On the other hand, if it is determined in step S112 that the NOx storage amount of the second NOx catalyst 12 is equal to or greater than a predetermined amount, the process then proceeds to step S114. In step S114, the executed air-fuel ratio is set and combustion is performed. The effective air-fuel ratio set here is the limit value on the rich side of the range that can be ignited based on the current amount of NOx in the cylinder and the intake air temperature.

Next, the process proceeds to step S116, the in-cylinder NOx amount in the next combustion cycle is calculated, the value of the in-cylinder NOx amount is updated, and the required NOx amount is set based on the current intake air amount. Recalculated and updated. The amount of NOx in the cylinder is obtained from the amount of NOx released from the second NOx catalyst 12 and the amount of NOx contained in the fresh air that merges with the EGR gas and is introduced into the cylinder. The amount of NOx released from the second NOx catalyst 12 can be estimated from, for example, the amount of change in the floor temperature of the second NOx catalyst 12. Further, the amount of NOx in the fresh air can be calculated from, for example, the detected value of the NOx sensor installed in the intake system. Alternatively, the amount of NOx in the fresh air may be an estimated value. The estimation method does not matter.

Next, the process proceeds to step S118, and it is determined whether or not the amount of NOx in the cylinder is equal to or greater than the required amount of NOx. If it is determined in step S118 that the in-cylinder NOx amount is smaller than the required NOx amount, the process is returned to step S114, and the executed air-fuel ratio is again adjusted according to the current in-cylinder NOx amount and the intake air temperature. It is set and combustion is performed at this air-fuel ratio.

On the other hand, if it is determined in step S118 that the amount of NOx in the cylinder is equal to or greater than the required amount of NOx, the process proceeds to step S106 and is switched to the rich combustion operation at the required air-fuel ratio.

As described above, in the present embodiment, when a request for rich combustion operation is issued and the operating region is a region where rich combustion is difficult, until the amount of NOx in the cylinder reaches the required amount of NOx. A transient operation is performed in which the target air-fuel ratio is gradually shifted to the rich side within the ignitable range. As a result, misfire can be suppressed when switching to the rich combustion operation. Further, in this system, the second NOx catalyst 12 is installed in the EGR passage, and the target air-fuel ratio is gradually shifted to the rich side within the range where ignition is possible, so that NOx is released into the cylinder from the second NOx catalyst 12. Be supplied. By increasing the amount of NOx supplied into the cylinder in this way, the ignitable range can be expanded. As a result, it is possible to quickly switch to rich fuel operation while suppressing misfire more effectively.

It should be noted that when the number, quantity, quantity, range, etc. of each element is referred to in the above-described embodiment, the reference is made except when explicitly stated or when the number is clearly specified in principle. The invention is not limited in number. Further, the structure and the like described in this embodiment are not necessarily essential to the present invention unless otherwise specified or clearly specified in principle.

2 Engine 4 Exhaust passage 6 1st NOx catalyst 10 EGR passage 12 2nd NOx catalyst 22 Air-fuel ratio sensor

Claims (1)

With an internal combustion engine
The EGR passage that connects the exhaust passage and the intake passage of the internal combustion engine,
The NOx storage catalyst arranged in the EGR passage and
A control device configured to be able to switch between operation with the first air-fuel ratio and operation with a second air-fuel ratio richer than the first air-fuel ratio and the stoichiometric air-fuel ratio.
With
The control device is
When a request to switch to operation based on the second air-fuel ratio is detected,
The amount of NOx in the cylinder, which is the amount of NOx introduced into the cylinder of the internal combustion engine in this combustion cycle, is estimated according to the amount of NOx released from the NOx storage catalyst.
Until the amount of NOx in the cylinder reaches the required amount of NOx required for ignition under the second air-fuel ratio and the current temperature in the cylinder.
The target air-fuel ratio in this combustion cycle is within the range of the air-fuel ratio that can be ignited under the amount of NOx in the cylinder and the current in-cylinder temperature, and the air-fuel ratio on the rich side of the target air-fuel ratio in the previous combustion cycle. Set to,
An internal combustion engine system characterized in that it is configured in this way.

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