JP4912189B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to a technology for reducing and purifying nitrogen oxide (NOx) in exhaust gas by using ammonia generated from an aqueous urea solution as a reducing agent in an exhaust gas purification device (hereinafter referred to as “exhaust gas purification device”) of an engine.

As a catalyst purification system for purifying NOx contained in engine exhaust, an exhaust purification device described in Japanese Patent Laid-Open No. 2000-27627 (Patent Document 1) has been proposed. Such an exhaust purification device injects and supplies a reducing agent precursor corresponding to the engine operating state to the exhaust upstream of the NOx reduction catalyst disposed in the exhaust passage, thereby catalytically reducing NOx and the reducing agent in the exhaust. Thus, NOx is purified to harmless components. Here, as the reducing agent precursor, an aqueous urea solution that generates ammonia by hydrolysis using exhaust heat and water vapor in the exhaust is used in consideration of safety and handling.
JP 2000-27627 A

  However, in the conventionally proposed exhaust purification device, since the addition flow rate of the urea aqueous solution is controlled to increase or decrease according to the NOx concentration in the exhaust gas, for example, when the unreacted urea aqueous solution or ammonia is occluded in the NOx reduction catalyst, There was a possibility that an aqueous urea solution or ammonia was excessively supplied, leading to a decrease in the utilization rate. In addition, in the urea aqueous solution injection supply control according to the instantaneous change in the NOx concentration, since there is a mechanical or electrical control delay, the urea aqueous solution injection supply (addition) is not performed at the intended timing, and the NOx in the transition period. There was also a risk that the purification rate would decrease.

  Therefore, in view of the above-mentioned conventional problems, the present invention actively controls the urea aqueous solution or ammonia occlusion amount in the NOx reduction catalyst to improve the urea aqueous solution utilization rate, the NOx purification rate, and the like. An object is to provide a purification device.

Therefore, according to the first aspect of the present invention, the engine exhaust gas purification device includes a reduction catalyst disposed in the engine exhaust pipe for reducing and purifying nitrogen oxides in the exhaust gas using ammonia generated from an aqueous urea solution. A reducing agent container for storing an aqueous urea solution, an injection nozzle for injecting and supplying the aqueous urea solution upstream of the exhaust of the reduction catalyst, and a pipe communicating the reducing agent container and the injection nozzle, and a pump and flow control A reducing agent addition device having a built-in on-off valve as a valve; ammonia concentration detecting means for detecting ammonia concentration downstream of the reduction catalyst; and nitrogen oxide concentration for detecting nitrogen oxide concentration downstream of the reduction catalyst Nitric acid detected by the detection means and the ammonia concentration detected by the ammonia concentration detection means is less than an allowable upper limit, and detected by the nitrogen oxide concentration detection means When the object density is equal to or greater than the target value, the ammonia concentration detected and the injection supply start means for starting the injection-supply of the urea aqueous solution by opening the closing valve, by the ammonia concentration detection means of the reducing agent addition device is acceptable And an injection supply stopping unit that closes the on-off valve of the reducing agent addition device and stops the injection supply of the urea aqueous solution when the value exceeds the upper limit value.

According to a second aspect of the invention, there is provided exhaust temperature detection means for detecting an exhaust temperature of the exhaust gas introduced into the reduction catalyst, and the injection supply start means is detected by the exhaust temperature detection means in addition to the conditions. When the exhaust gas temperature is higher than a predetermined temperature, the urea aqueous solution injection supply is started.

  According to the first aspect of the present invention, the ammonia concentration and the nitrogen oxide concentration in the exhaust gas downstream of the reduction catalyst are constantly monitored, and when the ammonia concentration reaches the allowable upper limit value, the injection supply of the urea aqueous solution is stopped. On the other hand, when the ammonia concentration is less than the allowable upper limit value and the nitrogen oxide concentration is equal to or higher than the target value, the injection supply of the urea aqueous solution is started. For this reason, the urea aqueous solution or ammonia occlusion amount in the reduction catalyst is actively controlled under the condition that ammonia is not released above the allowable upper limit to the atmosphere, and the urea aqueous solution utilization rate, the nitrogen oxide purification rate, etc. are improved. be able to. In addition, since it is sufficient if the injection supply of the urea aqueous solution can be controlled to be started or stopped, for example, an inexpensive on-off valve can be used, and the cost can be reduced. In the state where the urea aqueous solution injection is stopped, the nitrogen oxide purification is performed using the urea aqueous solution or ammonia stored in the reduction catalyst, so that the exhaust property does not deteriorate.

According to the second aspect of the present invention, since the exhaust temperature introduced into the reduction catalyst is added to the conditions for starting the injection supply of the urea aqueous solution, the reduction catalyst is not activated. This prevents the urea aqueous solution from being injected and supplied, and the urea aqueous solution or ammonia can be prevented from being released into the atmosphere.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 shows the overall configuration of an exhaust emission control device embodying the present invention.
A nitrogen oxidation catalyst 16 that oxidizes nitrogen monoxide (NO) into nitrogen dioxide (NO ₂ ) and an aqueous urea solution are injected along the exhaust circulation direction into the exhaust pipe 14 connected to the exhaust manifold 12 of the engine 10. An injection nozzle 18 for supplying, a NOx reduction catalyst 20 for reducing and purifying NOx using ammonia generated by hydrolyzing a urea aqueous solution, an ammonia oxidation catalyst 22 for oxidizing ammonia that has passed through the NOx reduction catalyst 20, Are arranged respectively. Further, the urea aqueous solution stored in the reducing agent container 24 is supplied to a reducing agent adding device 28 having a built-in pump and a flow rate control valve through a pipe 26 having a suction opening at the bottom thereof. The injection supply means includes the injection nozzle 18, the reducing agent container 24, the pipe 26, and the reducing agent addition device 28.

As an exhaust purification device control system, an exhaust pipe 14 located downstream of the ammonia oxidation catalyst 22 is provided with an ammonia sensor 30 (ammonia concentration detection means) for detecting the ammonia (NH ₃ ) concentration in the exhaust, and in the exhaust. A NOx sensor 32 (nitrogen oxide concentration detecting means) for detecting the NOx concentration is attached. A temperature sensor 34 (exhaust temperature detection means) for detecting the exhaust temperature of the exhaust gas introduced into the NOx reduction catalyst 20 is attached to the exhaust pipe 14 positioned between the injection nozzle 18 and the NOx reduction catalyst 20. The output signals of the ammonia sensor 30, NOx sensor 32, and temperature sensor 34 are input to a reducing agent addition control unit (hereinafter referred to as “reducing agent addition ECU”) 36 having a built-in computer. Further, the reducing agent addition ECU 36 is communicably connected to an engine control unit (hereinafter referred to as “engine ECU”) 38 via a network such as a CAN (Controller Area Network). The engine load can be read appropriately. As the engine load, known state quantities such as fuel injection amount, torque, accelerator opening, intake negative pressure, intake flow rate, etc. can be applied.

The reducing agent addition ECU 36 executes a control program stored in a ROM (Read Only Memory) or the like to electronically control the reducing agent addition device 28 in accordance with various input signals, and an appropriate amount of urea aqueous solution is injected. It is supplied to the nozzle 18. The reducing agent addition ECU 36 executes the control program, thereby realizing the injection supply start means and the injection supply stop means .

In such an exhaust purification device, the urea aqueous solution added from the injection nozzle 18 is hydrolyzed by exhaust heat and water vapor in the exhaust, and converted into ammonia. It is known that the converted ammonia undergoes a reduction reaction with NOx in the exhaust gas in the NOx reduction catalyst 20 and is purified to water (H ₂ O) and nitrogen (N ₂ ). At this time, in order to improve the NOx purification rate by the NOx reduction catalyst 20, NO is oxidized to NO ₂ by the nitrogen oxidation catalyst 16, and the ratio of NO and NO ₂ in the exhaust gas is improved to be suitable for the catalytic reduction reaction. Is done. On the other hand, the ammonia that has passed through the NOx reduction catalyst 20 is oxidized by the ammonia oxidation catalyst 22 disposed downstream of the exhaust gas, so that it is possible to suppress the release of ammonia as it is.

FIG. 2 shows a first embodiment of a control program that is repeatedly executed at predetermined time intervals in the reducing agent addition ECU 36 when the engine is started. The control program implements the injection supply start unit and the injection supply stop unit .
In step 1 (abbreviated as “S1” in the figure, the same applies hereinafter), it is determined whether or not the exhaust gas temperature detected by the temperature sensor 34 is higher than a predetermined temperature T. Here, the predetermined temperature T is a threshold for determining whether or not the NOx reduction catalyst 20 is activated, and is set to a temperature slightly higher than the activation temperature of the catalyst component, for example. If the exhaust gas temperature is higher than the predetermined temperature T, the process proceeds to step 2 (Yes), while if the exhaust gas temperature is equal to or lower than the predetermined temperature T, the process is terminated (No).

In step 2, an addition start signal is output to the reducing agent addition device 28, and the addition of the urea aqueous solution is started.
In step 3, it is determined whether or not the ammonia concentration detected by the ammonia sensor 30 is equal to or higher than the allowable upper limit value C _NH3 . Here, the allowable upper limit value C _NH3 is a threshold value that defines an upper limit value at which ammonia release into the atmosphere is allowed, and is set to a value prescribed by law or a value slightly lower than this, for example. If the ammonia concentration is equal to or higher than the allowable upper limit value C _NH3 , the process proceeds to step 4 (Yes), while if the ammonia concentration is less than the allowable upper limit value C _NH3 , the process waits (No).

In step 4, an addition stop signal is output to the reducing agent adding device 28, and the addition of the urea aqueous solution is stopped.
In step 5, it is determined whether the ammonia concentration detected by the ammonia sensor 30 is less than the allowable upper limit value C _NH3 . If the ammonia concentration is less than the allowable upper limit value C _NH3 , the process proceeds to step 6 (Yes), while if the ammonia concentration is equal to or higher than the allowable upper limit value C _NH3 , the process waits (No).

In step 6, it is determined that the NOx concentration detected by the NOx sensor 32 whether a target value C _NOx more. Here, the target value C _NOx is a threshold value that defines a target for purifying _NOx in the exhaust gas, and is set to, for example, a legally prescribed value or a value slightly lower than this. Then, while the NOx concentration to end the process if the target value C _NOx or (Yes), the NOx concentration returns to step 5 if it is less than the target value C _NOx (No).

According to such a control program, when the exhaust gas temperature becomes higher than the predetermined temperature T after the engine is started, the urea water solution having the maximum flow rate is added as shown in FIG. 3 regardless of the engine operating state. In this case, the entire amount of the added urea aqueous solution is not contributed to NOx purification, and a part of the urea aqueous solution is occluded in the NOx reduction catalyst 20 in the state of being converted into ammonia aqueous solution or ammonia. When the amount of occlusion in the NOx reduction catalyst 20 approaches a saturated state, the concentration of ammonia that could not be oxidized by the ammonia oxidation catalyst 22 gradually increases and reaches an allowable upper limit value C _NH3 . When the ammonia concentration reaches the allowable upper limit value C _NH3 , the addition of the urea aqueous solution is stopped in order to suppress the release of ammonia into the atmosphere. Thereafter, the addition of the aqueous urea solution remains suspended until the ammonia concentration is less than the allowable upper limit value C _NH3 and the NOx concentration is equal to or higher than the target value C _NOx . At this time, the urea aqueous solution is not added to the exhaust gas, but the NOx purification is performed using the urea aqueous solution or ammonia stored in the NOx reduction catalyst 20, so that the exhaust property does not deteriorate.

That is, the ammonia concentration and the NOx concentration downstream of the exhaust of the NOx reduction catalyst 20 are constantly monitored, and when the ammonia concentration reaches the allowable upper limit C _NH3 , the addition of the urea aqueous solution is stopped, while the ammonia concentration is the allowable upper limit. The addition of the urea aqueous solution is started when the value C is less than _NH3 and the NOx concentration is equal to or higher than the target value _CNOx . For this reason, under the condition that ammonia is not released into the atmosphere above the allowable upper limit C _NH3, the urea aqueous solution or ammonia storage amount in the NOx reduction catalyst 20 is actively controlled, and the urea aqueous solution utilization rate, NOx purification rate, etc. Can be improved. The flow rate control valve of the reducing agent adding device 28 only needs to be able to control whether or not the urea aqueous solution is added. For example, by using an inexpensive on-off valve, the cost can be reduced.

FIG. 4 shows a second embodiment of a control program that is repeatedly executed at predetermined time intervals in the reducing agent addition ECU 36 when the engine is started .
In step 11, it is determined whether the exhaust temperature detected by the temperature sensor 34 is higher than a predetermined temperature T. If the exhaust temperature is higher than the predetermined temperature T, the process proceeds to step 12 (Yes), while if the exhaust temperature is equal to or lower than the predetermined temperature T, the process is ended (No).

In step 12, referring to the control map in which the addition flow rate corresponding to the engine rotation speed and the load is set as shown in FIG. 5, the addition flow rate of the urea aqueous solution that matches the engine rotation speed and the load read from the engine ECU 38 is determined. decide.
In step 13, a control signal corresponding to the addition flow rate of the urea aqueous solution is output to the reducing agent addition device 28, and the addition of the urea aqueous solution is started.

In step 14, it is determined whether the ammonia concentration detected by the ammonia sensor 30 is equal to or higher than the allowable upper limit C _NH3 . If the ammonia concentration is greater than or equal to the allowable upper limit value C _NH3 , the process proceeds to step 15 (Yes), while if the ammonia concentration is less than the allowable upper limit value C _NH3 , the process proceeds to step 17 (No).
In step 15, an addition stop signal is output to the reducing agent adding device 28, and the addition of the urea aqueous solution is stopped.

In step 16, it determines the NOx concentration detected by the NOx sensor 32 whether a target value C _NOx more. Then, while the NOx concentration to end the process if the target value C _NOx or (Yes), the NOx concentration wait if it is less than the target value C _NOx (No).
In step 17, determines the NOx concentration detected by the NOx sensor 32 to or less than the target value C _NOx. If the NOx concentration is less than or equal to the target value C _NOx , the process proceeds to step 18 (Yes), while if the NOx concentration is higher than the target value C _NOx , the process returns to step 14 (No).

In step 18, as shown in FIG. 6, the control map in which the reduction rate corresponding to the NOx concentration and the exhaust temperature is set is referred to, and the NOx concentration and the exhaust temperature detected by the NOx sensor 32 and the temperature sensor 34 are adapted. Determine the reduced flow rate.
In step 19, a control signal corresponding to a value obtained by multiplying the addition flow rate of the urea aqueous solution by the flow rate reduction rate is output to the reducing agent addition device 28 to reduce the addition flow rate of the urea aqueous solution, and then the process returns to step 14.

According to such a control program, when the exhaust gas temperature becomes higher than the predetermined temperature T after the engine is started, the addition of the urea aqueous solution with the addition flow rate corresponding to the engine operation state is started as shown in FIG. In this case, the entire amount of the added urea aqueous solution is not contributed to NOx purification, and a part of the urea aqueous solution is occluded in the NOx reduction catalyst 20 in the state of being converted into ammonia aqueous solution or ammonia. When the amount of occlusion in the NOx reduction catalyst 20 approaches a saturated state, the concentration of ammonia that could not be oxidized by the ammonia oxidation catalyst 22 gradually increases and reaches an allowable upper limit value C _NH3 . When the ammonia concentration reaches the allowable upper limit value C _NH3 , the addition of the urea aqueous solution is stopped in order to suppress the release of ammonia into the atmosphere. Thereafter, the addition of the urea aqueous solution remains stopped until the NOx concentration becomes equal to or higher than the target value _CNOx . At this time, the urea aqueous solution is not added to the exhaust gas, but the NOx purification is performed using the urea aqueous solution or ammonia stored in the NOx reduction catalyst 20, so that the exhaust property does not deteriorate.

On the other hand, if the ammonia concentration is less than the allowable upper limit value C _NH3 and the NOx concentration is equal to or less than the target value C _NOx in the state where the urea aqueous solution is added, NOx purification is sufficiently performed. Thus, the flow rate of the urea aqueous solution is reduced.
That is, the ammonia concentration and NOx concentration downstream of the NOx reduction catalyst 20 are constantly monitored, and when the ammonia concentration reaches the allowable upper limit C _NH3 , the addition of the urea aqueous solution is stopped, while the NOx concentration is the target value. Addition of urea aqueous solution is started when it becomes _CNOx or more. For this reason, under the condition that ammonia is not released into the atmosphere above the allowable upper limit C _NH3, the urea aqueous solution or ammonia storage amount in the NOx reduction catalyst 20 is actively controlled, and the urea aqueous solution utilization rate, NOx purification rate, etc. Can be improved. If the ammonia concentration is less than the allowable upper limit C _NH3 and the NOx concentration is equal to or less than the target value C _NOx in the state in which the urea aqueous solution is added, the addition flow rate of the urea aqueous solution should be reduced according to the NOx concentration and the exhaust temperature. Thus, the stop of the addition of the urea aqueous solution can be made as short as possible, and for example, the occurrence of clogging in the injection hole of the injection nozzle 18 can be suppressed.

Furthermore, in the first and second embodiments of the control program, the ammonia emission amount can be monitored by the ammonia sensor 30, so the ammonia oxidation catalyst 22 is not necessary, and the system can be reduced in size and cost.
In the first and second embodiments of the control program, instead of comparing the NOx concentration downstream of the ammonia oxidation catalyst 22 with the target value C _NOx , the NOx concentration is divided by the NOx concentration at the outlet of the engine 10. The purification rate and the target purification rate may be compared. In this case, the NOx concentration at the outlet of the engine 10 may be detected directly by the NOx sensor or may be estimated from the engine speed and load. Moreover, it is good also as control which mutually replaces a part of 1st and 2nd embodiment of a control program. Furthermore, the ammonia sensor 30 and the NOx sensor 32 may be attached to the exhaust pipe 14 positioned immediately after the NOx reduction catalyst 20, that is, between the NOx reduction catalyst 20 and the ammonia oxidation catalyst 22.

1 is an overall configuration diagram of an exhaust emission control device embodying the present invention. Flowchart showing the first embodiment of the control program Time chart showing the correlation between ammonia concentration, NOx concentration and addition flow rate Flowchart showing the second embodiment of the control program Explanatory drawing of the control map in which the addition flow rate of urea aqueous solution is set Explanatory drawing of the control map in which the reduction rate of urea aqueous solution is set Time chart showing the correlation between ammonia concentration, NOx concentration and addition flow rate

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Engine 14 Exhaust pipe 18 Injection nozzle 20 NOx reduction catalyst 24 Reductant container 26 Piping 28 Reductant addition apparatus 30 Ammonia sensor 32 NOx sensor 34 Temperature sensor 36 Reductant addition ECU
38 Engine ECU

Claims (2)

A reduction catalyst disposed in the engine exhaust pipe for reducing and purifying nitrogen oxides in the exhaust gas using ammonia generated from an aqueous urea solution;
A reducing agent container for storing an aqueous urea solution;
An injection nozzle for injecting and supplying an aqueous urea solution upstream of the exhaust of the reduction catalyst;
A reducing agent addition device that is disposed in a pipe that communicates the reducing agent container and the injection nozzle, and that has a built-in on-off valve as a pump and a flow rate control valve;
Ammonia concentration detection means for detecting the ammonia concentration downstream of the exhaust of the reduction catalyst;
Nitrogen oxide concentration detecting means for detecting the nitrogen oxide concentration downstream of the exhaust of the reduction catalyst;
When the ammonia concentration detected by the ammonia concentration detecting means is less than an allowable upper limit value and the nitrogen oxide concentration detected by the nitrogen oxide concentration detecting means is equal to or higher than a target value, the reducing agent adding device Injection supply start means for opening an on-off valve and starting injection supply of urea aqueous solution;
An injection supply stopping means for closing the on-off valve of the reducing agent addition device and stopping the injection supply of the urea aqueous solution when the ammonia concentration detected by the ammonia concentration detection means is equal to or higher than an allowable upper limit;
An exhaust emission control device for an engine characterized by comprising: An exhaust gas temperature detecting means for detecting an exhaust gas temperature of the exhaust gas introduced into the reduction catalyst;
The injection supply start unit starts injection supply of the urea aqueous solution when the exhaust gas temperature detected by the exhaust gas temperature detection unit is higher than a predetermined temperature in addition to the condition . Engine exhaust purification system.

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