JP3956738B2 - NOx purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a NOx purification device that purifies NOx in exhaust gas of an internal combustion engine, and more particularly to a NOx purification device for an internal combustion engine in which an exhaust gas reducing agent supply device is disposed upstream of a reduction catalyst provided in an exhaust system.
[0002]
[Prior art]
NOx in exhaust gas discharged from an internal combustion engine is purified by a NOx purification device. In particular, a NOx purification device used in a diesel engine places a selective reduction type SCR catalyst in its exhaust system, and a urea is disposed upstream of it. Water is supplied as a reducing agent so that NOx can be purified under an oxygen-excess atmosphere. The NOx purification rate in this NOx purification device can be rendered harmless by reducing NOx with high efficiency in a region where the temperature of the urea SCR catalyst exceeds about 350 ° C.
Here, urea water is hydrolyzed and thermally decomposed as shown in formula (1) to release NH ₃ .
[0003]
_{(NH2) 2CO + H 2 O} → 2NH 3 + CO 2 ···· (1)
Further, it is known that the denitration reaction between NH ₃ and nitrogen oxide on the SCR catalyst is performed by the following reactions (2) and (3), respectively.
[0004]
4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ 0 (2)
2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ 0 (3)
Such urea water (or ammonia) addition type SCR catalyst has a higher NOx purification rate as the amount of ammonia adsorbed on the catalyst increases, for example, as shown by a solid line in FIG. In order to obtain a purification rate, it is preferable to control the ammonia adsorption amount high. On the other hand, there is a limit to the ammonia adsorption amount that can be adsorbed to the SCR catalyst. For example, as shown in FIG. 7, the limit value (solid line n) depends on the catalyst temperature. Here, when ammonia as the NOx reducing agent is excessively added, excess ammonia exceeding the adsorption amount limit value (solid line n) is discharged, and ammonia slip occurs. In consideration of the environment, in order to prevent ammonia slip, it is necessary to control the ammonia adsorption amount to be within the limit (the region between the solid line n and the broken line). In particular, the catalyst temperature changes rapidly. The necessity for such a vehicle catalyst is high.
[0005]
In such a NOx purification device, the ammonia adsorption amount (urea water addition amount) in the SCR catalyst is controlled to be within an appropriate range. In this case, the ammonia consumption amount at the catalyst can be calculated from the NOx emission amount from the engine, the temperature of the SCR catalyst and the NOx purification rate from the map, and the ammonia adsorption amount can be calculated from this and the amount of added reducing agent.
In addition, in a catalyst that converts nitrogen oxides in exhaust gas, the amount of reducing agent added to the catalyst is increased when the harmful substance exceeds the allowable amount, and the amount of reducing agent is reduced if the amount is below the reducing agent. No. 11-512799 gazette discloses a method for metering the amount of supply.
[0006]
[Problems to be solved by the invention]
By the way, as described above, it is possible to calculate the ammonia consumption amount in the catalyst from the NOx emission amount from the engine and the NOx purification rate from the map, and it is possible to calculate the ammonia adsorption amount from the added reducing agent amount. Since the calculation is open loop control, an error is likely to occur.
In particular, when the catalyst temperature is equal to or lower than a predetermined value, for example, as shown in FIG. 2 to be described later, the slope (change amount) of the NOx purification rate with respect to the temperature change is large. easy.
[0007]
For this reason, when the engine operating state changes and the exhaust gas temperature increases or decreases, the catalyst temperature changes and the NOx purification performance also changes.
Further, as shown in FIG. 3 described later, since the ammonia adsorption amount limit is high when the catalyst temperature is equal to or lower than a predetermined value, a larger adsorption amount calculation error may be allowed.
[0008]
For this reason, if the operating time in a state where the temperature of the SCR catalyst having a high ammonia adsorption amount limit is lower than a predetermined temperature (for example, 350 ° C.) is longer than the predetermined time, the calculation error of ammonia consumption is integrated and the ammonia adsorption amount The calculation error becomes larger. If the calculation error of the ammonia adsorption amount becomes large, the ammonia adsorption amount may exceed the limit value of the adsorption amount, and ammonia slip may occur, or the ammonia adsorption amount may decrease, resulting in failure to obtain the initial NOx purification performance. was there.
[0009]
The present invention is based on the above-described problems, and NOx purification of an internal combustion engine that can prevent ammonia slip and NOx purification performance from being reduced even when the operation of the SCR catalyst at a temperature equal to or lower than a predetermined temperature exceeds a predetermined time. An object is to provide an apparatus.
[0010]
[Means for Solving the Problems]
The invention according to claim 1 is a NOx catalyst provided in an exhaust system of an internal combustion engine for selectively reducing NOx in exhaust gas by adsorbing ammonia, a reducing agent supply means for supplying ammonia or urea water to the NOx catalyst, and the NOx The catalyst temperature detecting means for detecting or estimating the catalyst temperature of the catalyst or a parameter correlated with the catalyst temperature, and the time during which the NOx catalyst temperature is lower than the predetermined temperature is determined in accordance with the detection or estimation result of the catalyst temperature detecting means. Adsorption amount decrease promoting means is provided for controlling to decrease the adsorption amount of ammonia adsorbed on the NOx catalyst when the time is longer than the time.
Here, when the operation in which the time during which the NOx catalyst temperature is equal to or lower than the predetermined temperature becomes longer than the predetermined time and the ratio of the error in the amount of ammonia adsorbed or consumed increases continues, the ammonia adsorbed on the NOx catalyst is removed. Control to decrease. In this way, the ammonia adsorption amount is returned to the zero side, and the calculation result of the ammonia adsorption amount is reset to eliminate the calculation error, and the subsequent calculation of the adsorption amount and consumption does not include the error. Thus, in determining the amount of ammonia added, it is possible to prevent the ammonia slip and the NOx purification performance from being continuously reduced.
[0011]
Preferably, the adsorption amount decrease promoting means may control to reduce the amount of urea water added by the reducing agent supply means. In this case, the reduction process by the urea water supply device can be easily controlled, and the subsequent calculation error of the adsorption amount of ammonia can be surely eliminated, so that ammonia slip and NOx purification performance can be easily prevented.
Preferably, the adsorption amount decrease promoting means may be controlled to increase the temperature of the NOx catalyst. In this case, the exhaust gas temperature is increased by increasing the amount of fuel to the engine, retarding the fuel injection timing, etc., thereby increasing the temperature of the NOx catalyst, thereby forcibly reducing the ammonia adsorption amount. Thus, it is possible to prevent errors from being included in the calculation of the subsequent adsorption amount so that ammonia slip and NOx purification performance are not continuously reduced.
[0012]
According to a second aspect of the present invention, in the NOx purification device for an internal combustion engine according to the first aspect, the adsorption amount decrease promoting means stops the supply of ammonia or urea water, and the adsorption amount of ammonia adsorbed on the NOx catalyst It is characterized by decreasing.
In this case, the ammonia adsorption amount of the NOx catalyst is once reset to zero, and further, the calculation result of the ammonia adsorption amount is reset, so that the subsequent calculation error of the ammonia adsorption amount is eliminated, and the ammonia slip and NOx purification performance is improved. Decline can be easily prevented.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an NOx purification device for an internal combustion engine as one embodiment of the present invention will be described. The internal combustion engine NOx purification device (hereinafter simply referred to as NOx purification device) is mounted on an exhaust system 2 of a diesel engine (hereinafter simply referred to as engine) 1 mounted on a vehicle (not shown).
The engine 1 includes an engine control device 3 (in the figure, an engine ECU 301 that constitutes a main control unit) 3, and a NOx purification device is provided in the exhaust system of the engine 1. An engine ECU 10 of the engine control device and an exhaust gas control device (hereinafter simply referred to as an exhaust system ECU 4) constituting a control unit of the NOx purification device are a CAN (Controller Arreer Network) mutual communication system (hereinafter referred to as a control system communication line). 5) (hereinafter simply referred to as a CAN communication line). The CAN communication device enables mutual communication between the engine ECU 10 and the exhaust system ECU 4, and in particular has an advantage that a connection line used for mutual data communication is relatively simplified.
[0014]
In FIG. 1, an engine 1 equipped with a NOx purification device includes an intake system that guides intake air to a combustion chamber (not shown), and a fuel control system that adjusts fuel injection amount, injection pressure, injection timing, and the like.
The engine ECU 10 includes a fuel injection amount setting unit 10a and a fuel injection timing setting unit 10b that set a fuel amount and an injection timing so that a required output can be obtained.
[0015]
Various sensors such as an accelerator pedal depression amount sensor 14 for detecting an accelerator pedal depression amount (accelerator depression amount) Acc and a crank angle sensor 15 for calculating an engine speed Ne are connected to the input side of the engine ECU 10 and output. On the side, the fuel that drives the on-off valve 8 that opens and closes the fuel passage 6a that connects between the common rail 6 that stores high-pressure fuel supplied by a high-pressure fuel pump (not shown) and the fuel injection nozzle 7 provided for each cylinder. Various devices such as an injection driver 9 and a metering unit for controlling the fuel supply amount of a high pressure pump (not shown) are connected.
[0016]
Thus, for example, the required fuel injection calculated according to the engine speed Ne calculated based on the crank signal θc detected by the crank angle sensor 15 and the accelerator pedal depression amount Acc detected by the accelerator opening sensor 14. Based on Q, the opening timing and opening period of the on-off valve are variably adjusted by the fuel injection driver 9, and the metering section of the high-pressure fuel pump is feedback-controlled according to the fuel pressure in the common rail 6.
[0017]
The NOx purification device provided in the exhaust system of the engine 1 includes an SCR catalyst 17 that is a NOx catalyst mounted in the middle of an exhaust pipe 16, an urea water addition nozzle 18 disposed upstream thereof, and an upstream side of the addition nozzle 18. The NOx sensor 19 for outputting the NOx concentration Snoxf, the catalyst temperature sensor 22 for outputting the temperature Tg of the SCR catalyst 17, and the exhaust system ECU 4 constituting the control unit of the NOx purification device are provided. Here, the catalyst temperature sensor 22 constitutes a catalyst temperature detecting means for detecting the catalyst temperature of the NOx catalyst. Alternatively, the catalyst temperature may be estimated from exhaust temperatures before and after the catalyst. Further, an estimated value of the catalyst temperature may be calculated in consideration of parameters correlated with the catalyst temperature, for example, the operating state of the engine (fuel amount, engine speed), the operating time for each predetermined operating range, and the outside air temperature.
The exhaust gas flowing out from the engine 1 passes through the exhaust manifold 25 and the exhaust pipe 16 provided with the NOx catalytic converter 27, and is discharged to the atmosphere through a muffler (not shown).
[0018]
The NOx catalytic converter 27 includes a ceramic catalyst carrier having a honeycomb structure (not shown) in a casing, and a catalytic metal (for example, vanadium) for functioning as the SCR catalyst 17 is supported on the carrier. Alternatively, the NOx catalytic converter 27 may have a honeycomb structure formed of a catalyst metal (for example, vanadium) for functioning as the SCR catalyst 17 and may be casingd. The SCR catalyst 17 can selectively reduce NOx in the exhaust gas with ammonia (NH ₃ ). Here, the SCR catalyst 17 can promote the reaction between NH ₃ and nitrogen oxides by performing the reactions of the above-described formulas (2) and (3) in the ammonia adsorption state described later. The catalyst temperature-target purification rate characteristic of the SCR catalyst 17 is shown in FIG. 2, and the catalyst temperature-ammonia adsorption amount characteristic is shown in FIG.
[0019]
A urea water supply device 29 is attached as a reducing agent supply means for supplying urea water into the exhaust passage 24 upstream of the SCR catalyst 17. The urea water supply device 29 includes an addition nozzle 18 that sprays urea water toward the upstream opening side of the NOx catalytic converter 27, an injection pipe 31 connected to the addition nozzle 18, and an air tank 32 at the upstream end of the injection pipe 31. And a compressed air control valve 33 provided in the vicinity of the air tank, a urea water supply part 37 for supplying urea water at a position downstream of the compressed air control valve 33, and an upstream of the urea water supply part 37, for storing urea water. A urea water tank 35 and an exhaust system ECU 4 constituting these control means.
[0020]
The exhaust system ECU 4 has a large number of ports in its input / output circuit, can input detection signals from the NOx sensor 19 and the catalyst temperature sensor 22, etc., and sends control signals to the compressed air control valve 33 and the urea supply unit 37. In addition, data can be exchanged with the engine ECU 10 via the CAN communication line 5.
The exhaust system ECU 4 includes an input / output interface 401, a storage unit 402, a non-volatile memory 403 for battery backup, and a central processing unit 404, and particularly has a NOx purification processing function.
[0021]
The exhaust system ECU 4 of FIG. 1 basically includes addition control means A0 that drives the urea water supply device 29. In particular, as the adsorption amount decrease promoting means A1, when the time SCR1 in which the SCR catalyst temperature Tg is equal to or lower than the predetermined temperature tgα is equal to or longer than the predetermined time ctα according to the detection or estimation result of the catalyst temperature detecting means (temperature sensor 22) The ammonia adsorption amount A _NH3 adsorbed on the SCR catalyst 17 is decreased, that is, the addition amount Durea is decreased, for example, controlled to zero. Here, the predetermined temperature tgα is set to 350 ° C., for example, but this value varies depending on the catalyst characteristics. An appropriate value is selected for the predetermined time ctα so as not to cause a problem due to a calculation error of the ammonia adsorption amount.
[0022]
Next, the addition control processing of the engine ECU 10 and the NOx purification device of FIG. 1 will be described using the NOx purification processing routines of FIGS. 4 and 5 and the catalyst characteristic maps of FIGS.
Here, when the engine 1 of the vehicle (not shown) equipped with the NOx purification device is driven, the engine ECU 10 determines whether or not related devices and sensors appropriately executed in a plurality of control systems, for example, the fuel control system are normal. Check whether the self-check result is normal, and if normal (OK), send a control signal to the fuel injection driver 9, the metering unit, etc. according to each input value of the related sensor described above, and execute the control Then, the sensor output obtained at that time is also transmitted to the exhaust system ECU 4.
[0023]
On the other hand, the exhaust system ECU 4 repeats the NOx purification process control of the NOx purification process routine of FIGS. 4 and 5 for each predetermined control cycle simultaneously with the turning on of the engine key. Here, key-on is confirmed in step s1, and when step s2 is reached, the SCR catalyst temperature Tg, the NOx emission amount Unox, the fuel amount Qf from the engine ECU 10, the engine speed Ne, and other data are taken in.
[0024]
When step s3 is reached, the NOx purification rate ηb is calculated based on the SCR catalyst temperature Tg, and step s4 is reached.
In Step s4, the target adsorption amount A _NH3 (n) is calculated from the SCR catalyst temperature Tg, and Step s5 is reached.
In step s5 derives the consumption _{C NH3} ammonia based on NOx emissions Unox and NOx purification rate? B.
[0025]
When step s6 is reached, the target adsorption amount A _NH3 (n) and the consumption amount C _{NH3 of} ammonia are taken in, and using this value and the previous adsorption amount A _NH3 (n-1), the following equation (4) , NH ₃ addition amount B _NH3 is calculated.
B _NH3 = A _NH3 (n) -A _NH3 (n-1) + C _NH3 (4)
When reaching step s7, it is determined whether or not the intake air amount decrease promoting control is being executed, that is, if the execution flag F _R = 1, the process proceeds to step s14, and if F _R = 1, the process proceeds to step s8.
Next, when reaching step s8, it is determined whether or not the SCR catalyst temperature Tg exceeds a predetermined temperature (for example, 350 ° C.), and if it exceeds (No), the process proceeds to step s9, and if it is less (Yes), step s10 is reached.
[0026]
When step s10 is reached at a temperature lower than the predetermined temperature (350 ° C.), the time for entering this state is integrated by the counter CT1 here. In step s11, it is determined whether or not the accumulated time CT1 (or duration) is equal to or longer than the predetermined time ctα. If not, the processing in steps s9 and s12 is performed.
If it is determined in step s11 that the accumulated time CT1 is equal to or longer than the predetermined time ctα, in step s13, a flag F _R = indicating that the adsorption amount decrease promotion control is being executed is set to 1, and further, step s14. At this time, the current addition amount output Durea is determined to be zero.
[0027]
When step s15 is reached, the time when the addition amount output Durea is zero is accumulated by the counter CT2. When reaching the step s16, if the accumulated time for maintaining the urea water addition amount = 0 is less than the predetermined time ctβ, the process proceeds to the step s12, and the urea water supply unit of the urea water supply device 29 is configured to stop the urea water addition. 37 is controlled. On the other hand, if the accumulated time CT2 is equal to or longer than the predetermined time (ctβ), the process proceeds to step s17, assuming that the operation for reducing the ammonia adsorption amount of the SCR catalyst to zero is completed, and the counter value CT2 is cleared. , further, in step s18, the execution flag _{F R} is set to 0.
[0028]
Further, when the temperature exceeds the predetermined temperature (350 ° C.) in step s8, when reaching step s9, the current urea water addition amount output Durea corresponding to the NH ₃ addition amount B _NH3 is determined, and in step s12 the urea water addition amount output The urea water supply unit 37 of the urea water supply device 29 is driven in Durea, and the current control cycle is completed. As a result, the urea water supply unit 37 places urea water of the addition amount Durea on the compressed air of the injection pipe 31 from the urea water tank via the urea water pipe 34 based on the urea water addition amount output Durea, and the exhaust passage 24 from the addition nozzle 18. To supply. As a result, the SCR catalyst 17 maintains the target adsorption amount _ANH3 and efficiently detoxifies NOx.
[0029]
As described above, when the state where the SCR catalyst temperature Tg is equal to or lower than the predetermined temperature is equal to or higher than the predetermined time ctα, the urea water supply is stopped at the urea water addition amount output Durea = 0, so that NH ₃ adsorbed on the SCR catalyst 17 is stopped. Is consumed for NOx purification over time, and the adsorption amount A _NH3 of the SCR catalyst 17 is forcibly returned to zero. As a result of such processing, the subsequent ammonia adsorption amount A _NH3 surely starts from zero, and this value is reliably reset, and it is possible to eliminate the fact that errors are inherited in subsequent calculations. That is, since an error is not included in the subsequent calculation of the adsorption amount A _NH3 , it is possible to prevent an accident that ammonia slip and a decrease in NOx purification performance occur continuously.
[0030]
In the above description, when the accumulated time CT1 of the predetermined temperature (350 ° C.) or less is equal to or longer than the predetermined time ctα, the addition amount output Durea is determined to be zero, and the adsorption amount A _NH3 of the SCR catalyst 17 is forcibly returned to zero. Thus, the subsequent adsorption amount A _NH3 was designed not to include an error. Instead of this, the exhaust gas temperature may be raised by fuel control of the engine ECU 10.
[0031]
In this case, in the fuel injection amount setting unit 10a and the fuel injection timing setting unit 10b of the ECU 10, the fuel injection timing is set to be retarded to increase the exhaust gas temperature, and the basic injection amount INJb is set so as to avoid a decrease in output. After the various injection amounts INJα are added and the latest injection amount Uf is determined, when the exhaust gas temperature increase command is received, the injection amount INJ is calculated, and the injection amount increase correction amount (ΔINJ) is further added. That is, the injection amount Uf (= INJb + INJα + ΔINJ) is calculated, the fuel injection start timing is retarded by the injection amount INJ, and the fuel is injected.
[0032]
The injection timing retardation and the fuel increase amount are continued until the temperature of the SCR catalyst 17 reaches a temperature at which the ammonia adsorption amount becomes a predetermined value from the catalyst characteristic map of FIG. During this time, the exhaust gas is heated to a high temperature, the catalyst temperature of the SCR catalyst 17 is gradually increased, and the adsorption amount A _NH3 of the SCR catalyst 17 can be forcibly reduced. Even when such a process is performed, the calculation of the ammonia adsorption amount A _NH3 can be reset, and the calculation error of the ammonia adsorption amount can be eliminated.
[0033]
Further, instead of the above-mentioned retarded injection timing and fuel increase, additional injection may be performed at a time when additional fuel can be combusted in the combustion chamber (a time when no output increase occurs) in the expansion stroke after the main fuel injection. good. Again. The exhaust gas can be heated to a high temperature, and the adsorption amount A _NH3 of the SCR catalyst 17 can be forcibly reduced to a predetermined value. Even when such additional fuel injection processing is performed, the calculation of the ammonia adsorption amount A _NH3 can be reset, and the calculation error can be eliminated.
[0034]
【The invention's effect】
As described above, the present invention allows the SCR catalyst to be adsorbed to the SCR catalyst when the time during which the SCR catalyst temperature is equal to or lower than the predetermined temperature is in the operating range in which the ratio of the error in the ammonia adsorption amount increases for a predetermined time or longer. Control to reduce ammonia. In this way, the calculation of the ammonia adsorption amount is reset to eliminate the calculation error so that the subsequent calculation of the adsorption amount does not include an error. It is possible to prevent the deterioration of performance from occurring continuously.
Further, when the supply of ammonia or urea water is stopped, the ammonia adsorption amount of the NOx catalyst is once reset to zero, and the calculation result of the ammonia adsorption amount is reset, so that the subsequent calculation of the ammonia adsorption amount is performed. By eliminating the error, it is possible to easily prevent ammonia slip and NOx purification performance from decreasing.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a NOx purification device as an embodiment of the present invention, an engine ECU connected to the same device via a CAN communication device, and an engine on which these are mounted.
FIG. 2 is a characteristic explanatory diagram of a NOx purification rate map according to the exhaust gas temperature used by the exhaust system ECU of FIG. 1;
3 is a characteristic explanatory diagram of an exhaust gas-ammonia adsorption amount map used by the exhaust system ECU of FIG. 1. FIG.
4 is an upper flowchart of a NOx purification processing routine used by the exhaust system ECU of FIG. 1. FIG.
FIG. 5 is a lower flowchart of a NOx purification processing routine used by the exhaust system ECU of FIG. 1;
FIG. 6 is a characteristic diagram of ammonia adsorption amount-NOx purification rate of an SCR catalyst.
FIG. 7 is a characteristic diagram of catalyst temperature-ammonia adsorption amount of an SCR catalyst.
[Explanation of symbols]
1 Engine 2 Exhaust system 4 Exhaust system ECU
10 Engine ECU
17 SCR catalyst (NOx catalyst)
22 Catalyst temperature sensor (catalyst temperature detection means)
29 Urea water supply device (reducing agent supply means)
Tg catalyst temperature tgα predetermined temperature A0 addition control means A1 adsorption ηb target NOx purification rate _{C NH3} consumption _B NH3 NH ₃ amount of amount _{A NH3} ammonia adsorption amount decreased promoting means A2 target adsorption amount setting means Durea ammonia

Claims (2)

A NOx catalyst provided in an exhaust system of an internal combustion engine for selectively reducing NOx in exhaust gas by adsorbing ammonia;
Reducing agent supply means for supplying ammonia or urea water to the NOx catalyst;
A catalyst temperature detecting means for detecting or estimating a catalyst temperature of the NOx catalyst or a parameter correlated with the catalyst temperature;
Control to reduce the adsorption amount of ammonia adsorbed on the NOx catalyst when the time during which the NOx catalyst temperature is below a predetermined temperature is a predetermined time or longer according to the detection or estimation result of the catalyst temperature detecting means. Means for facilitating the reduction of adsorption
A NOx purification device for an internal combustion engine, comprising:   2. The NOx purification device for an internal combustion engine according to claim 1, wherein the adsorption amount decrease promoting means stops the supply of ammonia or urea water to reduce the adsorption amount of ammonia adsorbed on the NOx catalyst.

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