JP3074416B2 - Engine exhaust purification device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an engine having a lean NOx catalyst.

[0002]

2. Description of the Related Art As disclosed in Japanese Patent Application Laid-Open No. 3-25013, a transition metal is used as a catalyst for purifying NOx in engine exhaust gas in a lean air-fuel ratio region in which the air-fuel ratio is larger than a stoichiometric air-fuel ratio. Alternatively, there is known a zeolite supporting a noble metal, provided with a catalyst for reducing NOx in exhaust gas in the presence of HC in an oxidizing atmosphere, a so-called lean NOx catalyst, in the exhaust system.

[0003]

The lean NOx catalyst is for adsorbing HC and NOx in the exhaust gas component to reduce and purify NOx.
NO if the catalyst does not reach the activation temperature (300 ° C)
x cannot be purified. Further, the NOx purification rate of lean NOx has a high temperature dependency, and shows a peak change near 400 ° C. with respect to the exhaust gas temperature (inlet gas temperature). FIG.
Shows the purification rate characteristics with respect to the inlet gas temperature of the lean NOx catalyst. On the other hand, the heat-resistant temperature of the lean NOx catalyst is about 500 ° C., and if the exhaust gas temperature becomes higher than that, thermal deterioration is caused. Therefore, when the exhaust gas temperature becomes higher than a predetermined temperature, the exhaust gas flows into a bypass passage. Must be used to suppress the introduction of exhaust gas into the lean NOx catalyst. Therefore, in an engine equipped with a lean NOx catalyst and operating at an air-fuel ratio lean setting in a predetermined lean operation range, a bypass passage for bypassing the lean NOx catalyst and a bypass valve for opening and closing this bypass passage are provided as shown in FIG. It is conceivable to perform appropriate control.
That is, in the control shown in FIG. 15, when the engine oil water temperature reaches a predetermined value (elapsed time: t ₁ ), the air-fuel ratio (A / F)
From the stoichiometric air-fuel ratio (excess air ratio λ = 1) setting to the lean setting. Then, after the shift to lean, if the exhaust gas temperature is equal to or lower than the set value for starting the bypass, the bypass valve is closed and the exhaust gas is introduced into the lean NOx catalyst. However, in the case of this control, when the engine is suddenly operated on the high load / high rotation side in the lean operation range after the cold start, the exhaust gas temperature becomes higher than the bypass set value as shown in the figure, and when the engine shifts to the lean state, The temperature gradually decreases, but does not fall below the bypass set value for a while. For this reason, the lean NOx catalyst continues to be bypassed for a while after the shift to lean, and during that time, NOx cannot be purified. Therefore, in practice, such high load and high exhaust gas temperature
Lean operation cannot be performed in the high speed range. Therefore, in this high load / high rotation range, the engine does not shift even when the engine oil water temperature reaches a predetermined value, and the lean shift is performed only when the engine oil temperature deviates from the high load / high rotation range of FIG. Performing such control is considered next. In this case, as shown in FIG. 16, after the engine oil water temperature reaches a predetermined value, when the engine shifts to a lean state when the engine oil temperature deviates from a predetermined high-load / high-speed range (t ₂ ), the exhaust gas temperature rapidly decreases. After a relatively short time delay (Δt ₁ ), it becomes equal to or less than the bypass set value. When the exhaust gas temperature falls below the bypass set value, the bypass passage is closed. However, when such control is performed, the exhaust gas temperature decreases, the bypass passage is closed, and the exhaust gas starts to flow to the lean NOx catalyst. Time delay to reach (Δ
t ₂ ) increases, and during that time, NOx cannot be purified. Also, NOx purification cannot be performed during the time delay (Δt ₁ ) from the transition to the lean period to the opening of the bypass passage.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to accelerate the activation of a lean NOx catalyst without causing thermal degradation and to obtain a high NOx purification rate. I do.

[0005]

SUMMARY OF THE INVENTION The present invention is an exhaust gas purifying apparatus for an engine having a lean NOx catalyst for reducing NOx in engine exhaust gas at least in a lean air-fuel ratio region where the air-fuel ratio is larger than a stoichiometric air-fuel ratio. The exhaust gas temperature detected by the exhaust temperature detecting means detects an exhaust temperature upstream of the lean NOx catalyst, a catalyst temperature detecting means detecting the temperature of the lean NOx catalyst, and an exhaust temperature detected by the exhaust temperature detecting means. An exhaust gas introduction suppressing means for suppressing the introduction of exhaust gas to the lean NOx catalyst at a high temperature; and a set value changing means for increasing the set value as the catalyst temperature detected by the catalyst temperature detecting means is lower. I do. When the exhaust gas temperature is higher than the set value, lean NO
The above-described configuration in which the introduction of exhaust gas to the x catalyst is suppressed is changed, and warm-up detecting means for detecting warm-up of the engine is provided. The warm-up detecting means detects completion of warm-up of the engine. Until the setting, the setting value can be increased by the setting changing means. Further, the exhaust gas introduction suppressing means may be a catalyst bypass means for bypassing the lean NOx catalyst.

[0006]

After the cold start of the engine, if the exhaust gas temperature is equal to or higher than the set value even in the lean operation range, the introduction of the exhaust gas to the lean NOx catalyst is suppressed by the bypass means and the set value of the exhaust gas temperature. Is set higher when the catalyst temperature is lower, or the same value is set before the engine is warmed up compared to after the engine is warmed up.Therefore, when the catalyst temperature is low, the exhaust gas temperature is higher than when the catalyst temperature is higher. Flows to the lean NOx catalyst, which promotes activation of the catalyst.

FIG. 3 shows a lean NOx after a cold start.
3 shows the thermal degradation characteristics of the catalyst. Although the lean NOx catalyst has a higher degree of deterioration as the exhaust temperature is higher, as indicated by the white arrow in the figure, at low temperatures before the engine warm-up (the range indicated by the black arrow), even if the exhaust temperature is slightly higher, The degree of thermal deterioration does not increase significantly. FIG. 4 shows the relationship between the exhaust gas temperature and the engine warm-up time.
When the temperature of the exhaust gas is high, the warm-up time is short, so that the influence of the high-temperature exhaust gas on catalyst deterioration is small.

[0008]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a system diagram of one embodiment of the present invention. In the figure, reference numeral 1 denotes an engine, and a surge tank 3 is formed in an intake passage 2 of the engine 1. A throttle valve 4 is provided upstream of the surge tank 3, and a fuel injection valve 5 is provided downstream thereof. An inlet of the intake passage 2 is connected to an air cleaner 6, and an air flow meter 7 is provided immediately downstream of the air cleaner 6. Also, Engine 1
The exhaust passage 8 is provided with a lean NOx catalyst 9 for reducing and purifying NOx during lean operation in which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and downstream of the lean NOx catalyst 9 is NOx, HC and CO in the stoichiometric air-fuel ratio region. Is provided at the same time. The exhaust passage 2
The bypass passage 1 that bypasses the lean NOx catalyst 9
The bypass passage 11 is provided with a lean NOx
A bypass valve 12 is provided to control the ratio of the exhaust flow rate to the catalyst 9 side and the exhaust flow rate to the bypass passage 11 side.

The above-described fuel injection valve 5 and bypass valve 1
2 is controlled by a control unit 13 composed of a microcomputer. Therefore, an engine rotation signal is input from a rotation sensor (not shown) to the control unit 13, an intake air amount signal is input from the air flow meter 7, and an exhaust gas temperature signal is output from an exhaust temperature sensor 15 provided in the exhaust manifold 14. A catalyst temperature signal is input from a catalyst temperature sensor 16 provided on the lean NOx catalyst 9, and an engine water temperature signal is input from a water temperature sensor 18 provided on a water jacket 17 of the engine 1.

The control unit 13 controls the fuel injection valve 5 by calculating the fuel injection amount based on the information. Further, as shown in FIG. 6, the air-fuel ratio control is performed so as to lean transition to the engine coolant temperature (or oil temperature) when reaches the set value Tw ₁ or more for the lean transition (t _1), the exhaust gas the bypass valve 12 is closed controlled to flow the exhaust gas into the lean NOx catalyst 9 as a base if the temperature is below the bypass set value T _E, when the catalyst temperature is lower than the lean NOx catalytic purification starting temperature bypass setting value of the exhaust gas temperature T _E is performed is corrected so as to increase the predetermined amount [Delta] T, the exhaust gas is flowed to the lean NOx catalyst 9 bypass valve 12 is closed unless the corrected set value exceeds the exhaust gas temperature (inlet temperature) . At that time, the high-temperature exhaust gas flows through the lean NOx catalyst, whereby the catalyst is quickly activated, and the time delay Δt ₂ ′ before the NOx purification is started is shortened. In the case of this embodiment, as shown in FIG. 6, at the time (t ₃ ) after a lapse of a predetermined time after the shift to the lean operation, the lean operation is temporarily canceled, and the bypass valve 12 is opened.

FIG. 7 shows the air-fuel ratio (A / F) map and the exhaust gas temperature (catalyst inlet temperature) in the above embodiment, with the ordinate representing the shaft torque and the abscissa representing the engine speed. In this figure, the inside of the dotted line is the lean operation range, and the outside of the dotted line is λ = 1.
In the operating range, even in the lean operating range, the exhaust gas temperature exceeds the catalyst heat-resistant temperature (for example, 500 ° C.) in the high-load / high-rotation side portion. Therefore, when the exhaust gas temperature exceeds the allowable temperature limit, the lean NOx catalyst is bypassed. The shaded region in FIG. 8 corresponds to this lean NOx catalyst bypass region. This bypass area expands in the direction of the arrow when the setting A / F decreases. FIG. 9 is a flowchart for executing the control of the above embodiment. S11 to S18 indicate the respective steps.

In the flowchart of FIG. 9, the process is started. First, at S11, the engine coolant temperature Tw is read. Then, the engine water temperature T _W is determined whether a predetermined value T _W1 or more S12, and it returns when less than the predetermined value T _W1, when the engine water temperature T _W is higher than the predetermined value T _W1 S
Proceed to 13.

[0014] S13 reads the catalyst temperature T _C in. Then, the catalyst temperature T _C lean catalytic purification starting temperature T in S14
Determining whether _C1 below, proceeds the catalyst temperature T _C is to step S15 if less lean catalytic purification starting temperature T _C1, by adding a predetermined correction value ΔT of the exhaust gas temperature T _G in the catalyst heat resistance temperature T _E value If T _G ≦ T _E + ΔT, the air-fuel ratio A is determined at S16.
/ F lean operation is determined, and if it is lean operation, the bypass valve 12 is closed in S17. Further, when T _G > T _E + ΔT in the determination of S15, or when T _G ≦ T _E + ΔT in S15, A is determined in the determination of S16.
If it is not the / F lean operation, the process proceeds to S18 and the bypass valve 12 is opened.

If the catalyst temperature T _C exceeds the lean catalyst purification start temperature T _C1 in S14, the exhaust gas temperature T _G is compared with the catalyst heat-resistant temperature T _E in S19, and if T _G ≦ _TE is satisfied. in S20 it is judged whether the air-fuel ratio a / F or the lean operation, the closed bypass valve 12 proceeds to S17 if that lean operation, the term T _G> T _E at S19,
Alternatively, if the A / F lean operation is not performed in S20, the process proceeds to S18 and the bypass valve 12 is opened.

In the above-described embodiment, the threshold value for the bypass control determination is switched by directly looking at the catalyst temperature. Alternatively, the warm-up of the engine is detected based on the engine oil water temperature. It is also possible to control to increase the threshold value later.

FIG. 10 shows the control in another embodiment of the present invention. This embodiment is different from the above embodiment in that
Open the bypass valve from the λ = 1 operating range and lean NOx
The point is to introduce exhaust gas to the catalyst side.

The present invention can be modified in the following manner in connection with the above embodiments.

That is, FIG. 11 shows an arrangement in which two lean NOx catalysts 9a and 9b having different NOx purification characteristics with respect to a change in the air-fuel ratio A / F are arranged in series in the exhaust passage 8.
Here, the front-side lean NOx catalyst 9a is a lean NOx catalyst capable of obtaining a high NOx purification rate at the time of engine acceleration. For example, ZSM-5 is used as a base material and a Cu monometal or the like (other than Cu-Mg bimetal, Cu-Y bimetal,
A catalyst using Cu-In bimetal) as an active species corresponds to this. On the other hand, the rear-side lean NOx catalyst 9b is a lean NOx catalyst that can obtain a high NOx purification rate at the time of engine deceleration and has a slow reaction speed.
/ Ga) A metallosilicate or the like (in addition, (Al / Mn) metallosilicate) as a base material and a catalyst using Cu monometal or the like as an active species correspond to this. These two lean NOx
According to the serial arrangement of the catalysts 9a and 9b, a high NOx purification rate can be ensured both during acceleration and deceleration, and the reaction speed of the rear catalyst 9b is reduced by the front catalyst 9a. Can be supplemented by:

FIG. 12 shows a case where a parallel portion is provided in the middle of the exhaust passage 8 so that the two lean N regions having different NOx purification characteristics are provided.
This shows an arrangement in which Ox catalysts 9a and 9b are arranged in parallel, and a switching valve 19 is provided to switch the flow path by acceleration / deceleration. Thus, the catalysts 9a and 9b are accelerated and decelerated.
, A high purification rate can be stably obtained.

FIG. 13 is a flowchart according to still another modification. In this example, the exhaust gas temperature and the exhaust gas flow rate can be optimally controlled in view of the fact that the exhaust gas flow rate other than the exhaust gas temperature is the largest factor that determines the purification rate of the lean NOx catalyst. . That is, the lean NOx catalyst has an inlet temperature of, for example, 350 to 450 ° C.
, The catalyst temperature is controlled within this temperature range. On the other hand, if the exhaust gas flow rate becomes too large, the reaction time becomes insufficient, and as shown in FIG. Depending on the temperature, the purification rate cannot be improved, so that temperature control is almost meaningless. Therefore, when the exhaust gas flow rate becomes equal to or higher than the threshold value, priority is given to reducing the exhaust gas flow rate, and thereafter, the temperature control is executed. Here, means for reducing the exhaust gas flow velocity include:
For example, two catalysts having different cross-sectional areas are arranged in series and one of the catalysts can be bypass-controlled to make the catalyst cross-sectional area variable, or by narrowing the exhaust gas to make the gas flow rate passing through the catalyst variable. Or a parallel structure of exhaust pipes at the catalyst inlet and opening and closing of a part of the exhaust pipe to increase the cross-sectional area of the exhaust pipe. What you can do is available.

Next, the flow chart of FIG. 13 will be described step by step (S21 to S28). In this flowchart, the process is started, the engine speed, the intake air amount, the fuel injection amount, and the like are read in S21, and the exhaust gas flow rate is calculated in S22 based on the information to obtain the exhaust gas flow velocity. Then, in S23, the exhaust gas flow velocity V becomes equal to the threshold value _VE.
To see if more, the exhaust gas flow velocity V executes exhaust gas flow rate decrease in S24 if greater than or equal to the threshold value V _E. Further, if the exhaust gas flow velocity V in S23 has not reached the threshold value V _E, the process proceeds to S25.

In step S25, the exhaust gas temperature T is reduced to a predetermined range _TL.
Determines whether or not the ~T _H or more. Then, the exhaust gas temperature T to perform the exhaust gas cooling in S26 if more than a predetermined range T _L through T _H. Also, the term exhaust gas temperature T is determined in S25 is less than the predetermined range T _L through T _H, to see if the exhaust gas temperature T is lower than the predetermined range T _L through T _H in S27, exhaust gas temperature T There executes exhaust gas heating in S28 when that is lower than the predetermined range T _L through T _H. Also, S27
In that the exhaust gas temperature T is not lower than the predetermined range T _L through T _H is the exhaust gas temperature T is that it is within a predetermined range T _L through T _H, as it returns this case.

[0024]

As described above, according to the present invention, the activation of the lean NOx catalyst can be accelerated and a high NOx purification rate can be obtained without causing thermal degradation of the lean NOx catalyst.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between an inlet gas temperature and a purification rate of a lean NOx catalyst according to the present invention.

FIG. 3 is a characteristic diagram showing a time change of thermal deterioration of the lean NOx catalyst according to the operation of the present invention.

FIG. 4 is a characteristic diagram showing a relationship between an exhaust gas temperature and an engine warm-up time according to the operation of the present invention.

FIG. 5 is a system diagram of an embodiment of the present invention.

FIG. 6 is a time chart showing control of an embodiment of the present invention.

FIG. 7 is a region diagram showing an air-fuel ratio map and an exhaust gas temperature in one embodiment of the present invention.

FIG. 8 is a region diagram showing a lean NOx catalyst bypass region in one embodiment of the present invention.

FIG. 9 is a flowchart for executing a lean NOx catalyst bypass control in one embodiment of the present invention.

FIG. 10 is a time chart showing control of another embodiment of the present invention.

FIG. 11 is a structural diagram of a modification related to the embodiment of the present invention.

FIG. 12 is a structural view of another modification related to the embodiment of the present invention.

FIG. 13 is a flowchart for executing control of still another modification related to the embodiment of the present invention.

FIG. 14 is a characteristic diagram showing the relationship between the NOx purification rate and the exhaust gas flow velocity in relation to the modification of FIG. 13;

FIG. 15 is a time chart showing a conventional control (1).

FIG. 16 is a time chart showing conventional control (2).

[Explanation of symbols]

 Reference Signs List 1 engine 8 exhaust passage 9 lean NOx catalyst 11 bypass passage 12 bypass valve 13 control unit 15 exhaust temperature sensor 16 catalyst temperature sensor 18 water temperature sensor

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI F01N 3/28 301 F01N 3/28 301H F02D 45/00 301 F02D 45/00 301G (72) Inventor Fumihiko Saito Fuchu, Aki-gun, Hiroshima Machida Co., Ltd. 3-1, Machida Shinchi (56) References JP-A-3-74515 (JP, A) JP-A-3-225013 (JP, A) JP-A-5-231138 (JP, A) ( 58) Field surveyed (Int. Cl. ⁷ , DB name) F01N 3/20 F01N 3/24 F01N 3/28 F02D 45/00

Claims (3)

(57) [Claims] At least in the lean air-fuel ratio region where the air-fuel ratio is larger than the stoichiometric air-fuel ratio, NO in the engine exhaust gas is reduced.
An exhaust gas purification device for an engine including a lean NOx catalyst for reducing x, an exhaust temperature detecting means for detecting an exhaust gas temperature upstream of the lean NOx catalyst, and a catalyst temperature detecting means for detecting a temperature of the lean NOx catalyst. Exhaust exhaust suppression means for suppressing the introduction of exhaust gas to the lean NOx catalyst when the exhaust gas temperature detected by the exhaust gas temperature detection means is equal to or higher than a set value; and a catalyst temperature detected by the catalyst temperature detection means. An exhaust gas purifying apparatus for an engine, comprising: a set value changing unit that increases the set value as the value is lower. 2. The method according to claim 1, wherein at least in the lean air-fuel ratio region where the air-fuel ratio is larger than the stoichiometric air-fuel ratio, NO in the engine exhaust gas is reduced.
An exhaust gas purification device for an engine including a lean NOx catalyst that reduces x, an exhaust temperature detection unit that detects an exhaust gas temperature upstream of the lean NOx catalyst, a warm-up detection unit that detects a warm-up of the engine, When the exhaust gas temperature detected by the exhaust gas temperature detecting means is higher than or equal to a set value, exhaust gas introduction suppressing means for suppressing the introduction of exhaust gas to the lean NOx catalyst, and completion of warm-up of the engine by the warm-up detecting means. An exhaust gas purifying apparatus for an engine, comprising: a set value changing unit that increases the set value until the set value is detected. 3. The exhaust gas control device according to claim 2, wherein the exhaust gas introduction suppressing unit includes the lean NO.
2. A catalyst bypass means for bypassing the x catalyst.
Or the exhaust gas purification device for an engine according to 2.