JP2666508B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine having a so-called lean NOx catalyst in an exhaust system.

[Conventional technology]

Recently, in order to improve fuel efficiency, the development of a lean burn (lean burn) internal combustion engine that burns at an air-fuel ratio in a lean region has been promoted, and a part thereof has been put into practical use. Since NOx cannot be purified by a conventional catalyst in a lean air-fuel ratio region, a low NOx region has become an issue for a lean burn internal combustion engine, and a catalyst capable of reducing NOx even at a lean air-fuel ratio has been attracting attention.

As a catalyst for reducing NOx even at a lean air-fuel ratio,
-130735 and Japanese Patent Application No. 63-95026 teach a catalyst (contained in a lean NOx catalyst) consisting of a zeolite carrying a transition metal and reducing NOx in the presence of HC in an oxidizing atmosphere. .

[Problems to be solved by the invention]

However, even when a lean NOx catalyst is installed in the exhaust system of an internal combustion engine, depending on the operating state of the engine, for example, in a light or medium load region such as when accelerating from idling or when climbing a slope, lean NOx
There is a problem that the NOx purification rate of the x catalyst decreases, and it becomes difficult to suppress the amount of NOx emission to the atmosphere within the regulation value.

The present invention relates to a lean engine that is operated by an internal combustion engine.
It is an object of the present invention to suppress a decrease in the NOx purification rate of a NOx catalyst by using fuel vapor from a fuel tank and always maintain a high NOx purification rate of a lean NOx catalyst.

[Means for solving the problem]

As shown in FIG. 1, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, which achieves the above object, comprises a zeolite supporting a transition metal or a noble metal and provided in an exhaust system 4 of an internal combustion engine 2 in an oxidizing atmosphere. During ~
A lean NOx catalyst 6 for reducing NOx in the presence of HC; evaporative fuel pipes 12 and 14 for guiding evaporative fuel from a fuel tank 8 to an evaporative fuel adsorption canister 10 and a lean NOx catalyst upstream exhaust pipe 4a; Operating state detecting means 18 for detecting the operating state
And the operating state detected by the operating state detecting means 18,
HC deficiency determining means 20 for determining whether or not HC in the exhaust gas flowing into the lean NOx catalyst 6 is insufficient for the amount of HC required for NOx reduction by the lean NOx catalyst 6 in the operating state. When the HC insufficiency determining means 20 determines that the engine is in the HC insufficiency operation state, the evaporative fuel is introduced into the lean NOx catalyst upstream exhaust pipe 4a. And a switching valve 16 provided for the evaporated fuel pipes 12 and 14, which is switched to be introduced into the fuel cell 10.

[Action]

As shown in FIG. 9, the NOx reduction mechanism by the lean NOx catalyst 6 is presumed to be a reaction between NOx and a part of HC in the exhaust gas, an active species generated by partial oxidation. As shown in FIG. 8, as the amount of HC increases, the amount of active species also increases,
x Purification rate is improved.

The amount of HC and the amount of active species in the exhaust gas depend on the operating state of the engine. That is, regarding the air-fuel ratio, the sixth
As shown in the figure, in the air-fuel ratio region on the leaner side than the stoichiometric air-fuel ratio, the HC amount gradually decreases and the NOx purification rate decreases until the air-fuel ratio at which the torque fluctuation starts to increase sharply. As for the exhaust gas temperature, as shown in FIG. 7, when the catalyst temperature (correlation with the exhaust gas temperature) becomes higher than a certain temperature, direct oxidation of HC proceeds, so that the NOx purification rate decreases.

At the time of light load such as at the time of steady running or moderate acceleration, the air-fuel ratio is set to be super lean of 20 to 24, and the exhaust gas temperature is relatively low. In this region, the amount of HC is large as shown in FIG. Moreover, since the direct oxidation of HC does not proceed due to the relatively low temperature, the amount of active species generated is large, and there is no problem in the NOx purification rate. Therefore, the HC deficiency determination means 20 determines that the vehicle is not in the HC deficient operation state, and the switching valve 16 is switched to the canister 10 side, so that the conventional operation is performed.

On the other hand, at the time of sudden acceleration from idol, light like at the time of pitching,
At the time of medium load, the air-fuel ratio is set to 16 to 19, and the exhaust gas temperature is relatively high or high. In this region, the HC amount is small as shown in FIG. In addition, when the exhaust gas is at a high temperature, the direct oxidation of HC proceeds, and the amount of active species generated decreases. In this region, the HC deficiency determination means 20 determines that the operation is in the HC deficient operation state, and the switching valve 16 sets the lean NOx upstream exhaust pipe 4.
Switch to a side. As a result, the fuel vapor from the fuel tank 8 is introduced into the lean NOx upstream exhaust pipe 4a, and the shortage of HC is eliminated.

In this way, the HC concentration of the exhaust gas is always kept high.

〔Example〕

 Hereinafter, embodiments according to the present invention will be described.

As shown in FIG. 2, a lean NOx catalyst 6 is provided in the exhaust system 4 of the internal combustion engine 2, and a three-way catalyst 22 is provided downstream thereof. Reference numeral 8 denotes a fuel tank for storing fuel into a fuel injection valve (not shown). Evaporated fuel is guided to an evaporative fuel adsorption canister 10 through an evaporative fuel pipe 12, and is prevented from leaking to the atmosphere.

Evaporative fuel pipe connecting the fuel tank 8 and the canister 10
From the middle of 12, another evaporative fuel pipe 14 becomes lean NOx
It extends to the exhaust pipe 4a upstream of the catalyst and removes evaporated fuel
It can be introduced into the catalyst upstream exhaust pipe 4a. A switching valve 16 is provided at a branch of the fuel vapor pipe 14 from the fuel vapor pipe 12 to introduce the fuel vapor from the fuel tank 8 into the canister 10 and to provide a lean NOx catalyst upstream exhaust pipe 4.
It is possible to switch between introduction to a. In this embodiment, the evaporated fuel is introduced into the lean NOx catalyst upstream exhaust pipe 4a when the switching valve 16 is ON, and is introduced into the canister 10 when the switching valve 16 is OFF.

Switching of the switching valve 16 is performed in response to an engine operation response. In order to detect the engine operation state, in a first embodiment described later, the air-fuel ratio for detecting the air-fuel ratio is provided upstream of the lean NOx catalyst 6 and upstream of the opening of the evaporative fuel pipe 14 to the lean NOx upstream exhaust pipe 4a. A sensor 24 and an exhaust gas temperature sensor 26 for detecting an exhaust gas temperature are provided. In a second embodiment described later, an HC sensor 32 for detecting the HC concentration is desirably provided downstream of the lean NOx catalyst 6. Reference numeral 28 denotes a crank angle sensor built in the distributor 34, which detects and outputs a crank angle for interrupting the calculation in FIGS. 4 and 5 described later. In the above, the air-fuel ratio sensor 24 and the exhaust gas temperature sensor 26 constitute the operating state detecting means 18 described in FIG.
Constitutes the operating state detecting means 18 in the second embodiment.

In FIG. 2, an engine control computer (ECU) 30 controls the operation of the engine,
It also controls ON / OFF of the switching valve 16. The control system shown in FIG. 2 shows only those necessary for turning the switching valve 16 on and off. ECU30
As shown in FIG. 3, a central processor unit (CPU) 30a for executing an operation, a read-only memory (ROM) 30b as a read-only memory, a random access memory (RAM) 30c for temporarily storing data, a digital signal input Input interface 30d, an A / D converter 30e for converting an analog signal to a digital signal, and an output interface 30f for outputting an output signal. The output of the crank angle sensor 28 is input to the input interface 30d, and the outputs of the air-fuel ratio sensor 24, the exhaust gas temperature sensor 26, and the HC sensor 32 are output to the A / D converter 30e. Further, a command from the ECU 30 to the switching valve 16 is output from the output interface 30f.

FIG. 4 and FIG. 5 show an arithmetic routine that is stored in the ROM 30b and read out by the CPU 30a to execute ON / OFF of the switching valve 16. FIG. 4 includes a routine for indirectly determining whether or not HC is insufficient from the engine operating state, and will be described as a first embodiment. FIG. 5 includes a routine for directly determining whether or not HC is insufficient based on the concentration of HC in the exhaust gas, and will be described as a second embodiment.

In the first embodiment, as shown in FIG.
Then, the air-fuel ratio ABF is read. Subsequently, in step 102, the exhaust gas temperature TEX is read. Subsequently, in steps 103 and 104,
The air-fuel ratio ABF is the low-side air-fuel ratio ABF1 (for example, air-fuel ratio = 16)
It is determined whether or not the air-fuel ratio is between ABF2 and the high-side air-fuel ratio ABF2 (for example, air-fuel ratio = 19). Since the air-fuel ratio region of ABF1 to ABF2 is a region where HC is small and NOx is large as shown in FIG. 6, the air-fuel ratio region is a region where HC is insufficient. It is introduced into the catalyst upstream exhaust pipe 4a.

When it is determined in steps 103 and 104 that the air-fuel ratio ABF is not in the range of ABF1 to ABF2, as shown in FIG. 6, the region is a region where the HC amount is large, but even in this case, the exhaust gas temperature is too high. If too much, the direct oxidation of HC and the oxidation of active species to CO and CO ₂ proceed in FIG. 9 and the number of active species that effectively react with NOx decreases, so whether such a shortage of active species occurs. Proceed to step 106 in order to determine In step 106, if the exhaust gas temperature TEX is higher than the predetermined temperature TEX1, the NOx purification rate decreases as shown in FIG. 7, so the routine proceeds to step 105, in which the switching valve 16 is turned on, and the evaporated fuel is turned into a lean NOx catalyst. It is introduced into the upstream exhaust pipe 4a.

If the exhaust temperature TEX is equal to or lower than the predetermined temperature TEX1 in step 106, the direct oxidation of HC does not proceed, so the process proceeds to step 107, where the switching valve 16 is turned off, and the evaporated fuel is introduced into the canister 10.

In the above, steps 103, 104, and 106 constitute the HC deficiency determining means 20 described in FIG. 1 for indirectly determining whether or not the operating state is insufficient for HC in the first embodiment.

In the first embodiment, the air-fuel ratio ABF
Is read from the output of the air-fuel ratio sensor 24. In general, in a lean burn internal combustion engine, a target air-fuel ratio is determined based on an operating state (engine speed NE, intake pipe pressure PM) to perform fuel injection control. So in such a case,
Instead of the actual air-fuel ratio detected by the air-fuel ratio sensor 24, a target air-fuel ratio determined from the operating state may be read in step 101.

Next, a second embodiment will be described with reference to FIG.
In FIG. 5, in step 201, the HC concentration VHC, which is the output of the HC sensor 32 (the operating state detecting means 18 of the second embodiment), is read. Subsequently, in step 202, the current HC concentration VHC is
It is determined whether the concentration is lower than a predetermined HC concentration V0. Step 202
Constitutes the HC shortage judging means 20 described in FIG. 1 in the second embodiment. If VHC <V0 in step 202, it means that HC is insufficient, so the process proceeds to step 203, the switching valve 16 is turned on, the evaporated fuel is introduced into the lean NOx catalyst upstream exhaust pipe 4a, and if VHC is not smaller than V0, the process proceeds to step 204. Then, the switching valve 16 is turned off, and the fuel vapor is introduced into the canister 10.

 Next, the operation will be described.

When the vehicle is in a light or medium load state such as when accelerating from idling and when climbing a slope, the switching valve 16 is turned on, and the fuel vapor from the fuel tank 8 is introduced into the lean NOx upstream exhaust pipe 4a. In such a case, since the torque is required, the engine is operated at an air-fuel ratio of 16 to 19, and the amount of the evaporated fuel is 5 to 20% of the amount of fuel supplied to the internal combustion engine 2 at that time. 4 will be introduced. Therefore, the N of the lean NOx catalyst 6 in FIG.
In the Ox purification rate characteristics, the amount of HC in the exhaust gas increases,
x Purification rate is improved.

In a light load state such as during steady running or moderate acceleration, the switching valve 16 is turned off, and the evaporated fuel is not introduced into the exhaust system 4,
Introduced into canister 10.

In a high load state, the operation is performed with the air-fuel ratio adapted to the stoichiometric air-fuel ratio, and the switching valve 16 is OFF. At this time, since the three-way catalyst 22 works effectively, the emission is purified by the three-way catalyst 22.

〔The invention's effect〕

 According to the present invention, the following effects are obtained.

An evaporative fuel pipe 14 for guiding the evaporative fuel from the fuel tank 8 to the lean NOx catalyst upstream exhaust pipe 4a, and an evaporative fuel for the canister 10
Switching valve 1 for switching between and lean NOx catalyst upstream exhaust pipe 4a
6, since the HC insufficiency determining means 20 for determining whether or not the operating state is insufficient for HC is provided, the switching valve 16 sets the lean NOx catalyst upstream exhaust pipe 4 when the HC insufficiency means 20 determines that the operating state is insufficient for HC.
By switching to the a side, the evaporated fuel is introduced into the exhaust system 4, the shortage of HC can be eliminated, and the NOx purification rate of the lean NOx catalyst 6 can always be kept high. As a result, the emission of NOx into the atmosphere can be kept sufficiently low. Also, since the lean air-fuel ratio region can be used freely as needed,
Excellent fit for drivability.

Further, it is not necessary to always store the fuel vapor in the canister 10, so that breakthrough of the canister 10 can be prevented.

Further, by introducing the evaporated fuel into the exhaust pipe upstream of the lean NOx catalyst 6, the evaporated fuel can be purified, and the emission of the evaporated fuel can be sufficiently suppressed at the same time as the emission of NOx is suppressed.

[Brief description of the drawings]

FIG. 1 is a basic control system diagram of an exhaust gas purification device for an internal combustion engine according to the present invention, FIG. 2 is a control and device system diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention, and FIG. 2 is a block diagram showing the configuration of an ECU in the exhaust gas purification device of FIG. 2, FIG. 4 is a control flow diagram according to the first embodiment of the present invention, FIG. 5 is a control flow diagram according to a second embodiment of the present invention, 6 is an air-fuel ratio-NOx, HC, torque fluctuation characteristic diagram, FIG. 7 is a catalyst temperature-NOx purification rate characteristic diagram of a lean NOx catalyst, FIG. 8 is an HC concentration-NOx purification rate characteristic diagram of a lean NOx catalyst, FIG. 9 is a block diagram showing a NOx reduction mechanism of a lean NOx catalyst. 2 ... internal combustion engine 4 ... exhaust system 4a ... lean NOx catalyst upstream exhaust pipe 6 ... lean NOx catalyst 8 ... fuel tank 10 ... canister 12, 14 ... evaporative fuel pipe 16 ... switching valve 18 ... Operating state detection means 20 HC shortage determination means 22 Three-way catalyst 24 Air-fuel ratio sensor 26 Exhaust gas temperature sensor 28 Crank angle sensor 30 ECU 32 HC sensor

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location F01N 3/36 F01N 7/00 A 7/00 F02D 45/00 301G F02D 45/00 301 312H 312 368G 368 F02M 25/08 B F02M 25/08 DP B01D 53/36 101B (72) Inventor Soichi Matsushita 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A 61-242622 (JP, A) JP-A-63-283727 (JP, A) JP-A-2-1295941 (JP, A) JP-A-62-251415 (JP, A) JP-B-51-29249 (JP, B2) JP 51-29250 (JP, B2)

Claims (1)

(57) [Claims] 1. A lean NOx catalyst provided in an exhaust system of an internal combustion engine and made of zeolite carrying a transition metal or a noble metal and reducing NOx in the presence of HC in an oxidizing atmosphere, and evaporating fuel vapor from a fuel tank. An evaporative fuel pipe leading to a fuel adsorption canister and a lean NOx catalyst upstream exhaust pipe; an operating state detecting means for detecting an operating state of the internal combustion engine; and an operating state detected by the operating state detecting means. H in exhaust gas flowing into the catalyst
HC insufficiency determining means for determining whether the operating state is insufficient for the amount of HC required for NOx reduction by the lean NOx catalyst, and evaporative fuel when the HC insufficiency determining means determines that the operating state is insufficient. And a switching valve provided for the evaporative fuel pipe, which switches to introduce evaporative fuel to the canister when the HC insufficiency determining means determines that the operation is not in the HC insufficiency operation state. An exhaust gas purification device for an internal combustion engine, comprising: