JP2687654B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust emission control device for an internal combustion engine having an exhaust system provided with a so-called lean NOx catalyst.

[Prior Art] Recently, in order to improve fuel efficiency, a lean burn (lean burn) internal combustion engine that burns at an air-fuel ratio in a lean region has been developed, and a part thereof has been put into practical use. Since NOx cannot be purified with conventional catalysts in the lean air-fuel ratio range, reduction of NOx has become an issue for lean-burn internal combustion engines, and catalysts that can reduce NOx even with lean air-fuel ratios are drawing 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 if a lean NOx catalyst is attached to the exhaust system of an internal combustion engine, depending on the operating state of the engine, the engine may be in an acceleration state above a predetermined value, such as during acceleration from idle. In the light and medium load regions, there is a problem that the NOx purification rate of the lean NOx catalyst decreases.

It is an object of the present invention to suppress a decrease in the NOx purification rate of a lean NOx catalyst, which occurs depending on the operating state of an internal combustion engine, by using EGR gas, and to always maintain a high NOx purification rate of a lean NOx catalyst.

[Means for Solving the Problems] An exhaust gas purifying apparatus for an internal combustion engine according to the present invention, which achieves the above object, is a transition metal or noble metal provided in an exhaust system 4 of an internal combustion engine 2, as shown in FIG. In an oxidizing atmosphere consisting of zeolite loaded with
The lean NOx catalyst 6 that reduces NOx in the presence of HC and the exhaust system 4 and the intake system 8 of the internal combustion engine are provided.
Exhaust gas recirculation device 12 equipped with an EGR valve 10 and operating state detection means 14 for detecting the motion state of the internal combustion engine 2.
And the internal combustion engine 2 based on the output of the operating state detecting means 14.
Is an acceleration state determination means 16 for determining whether or not the acceleration state is greater than a predetermined acceleration value that is greater than the acceleration value during steady running or slow acceleration and less than the acceleration value during acceleration from idle, and the acceleration state determination Air-fuel ratio setting means 18 for increasing the torque by setting the target air-fuel ratio to a relatively rich region within the lean side region from the stoichiometric air-fuel ratio when the means 16 determines that the acceleration state is equal to or higher than the predetermined acceleration value, and the acceleration state determination When the means 16 determines that the acceleration state is equal to or higher than the predetermined acceleration value, the EGR valve 10 is opened and EGR gas is introduced into the intake system 8 to generate HC in the combustion chamber to generate N in the lean NOx catalyst.
EGR ON means 20 for improving the Ox purification rate is provided.

[Function] As shown in FIG. 9, the mechanism of NOx reduction by the lean NOx catalyst 6 is presumed to be the reaction of NOx with the active species generated by partial oxidation of a portion of HC in the exhaust gas.
Therefore, as shown in FIG. 7, as the amount of HC increases, the amount of active species also increases and the NOx purification rate improves.

The amount of HC in the exhaust gas changes depending on the engine operating state, for example, the air-fuel ratio, and also changes depending on whether EGR is ON or OFF. For example, as shown in FIG. 8, when EGR is ON, a part of the exhaust gas is introduced into the intake system, so the fresh air component in the intake air is reduced, and therefore the oxygen amount is reduced, and Complete combustion is impaired and HC in the exhaust gas increases.

When the vehicle is in an acceleration state equal to or higher than a predetermined acceleration value, such as during acceleration from idle, the acceleration state determination means 16 determines the engine operating state as an acceleration state, and the air-fuel ratio setting means 18 makes the air-fuel ratio leaner than the stoichiometric air-fuel ratio. A relatively rich region within the side region, for example,
Set the air-fuel ratio to 16-19. In this region, since the amount of HC is small, the NOx purification rate tends to decrease. However, the EGR ON means 20 turns EGR ON to intentionally increase HC and increase the NOx purification rate. Therefore, the NOx purification rate of the lean NOx catalyst 6 does not decrease.

In this way, the HC concentration in the exhaust gas is always kept high, and the NOx purification rate of the lean NOx catalyst 6 is kept high.

[Embodiment] FIG. 2 is an overall schematic view showing an embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention. In FIG. 2, a pressure sensor 26 is provided in the intake passage 8 of the internal combustion engine 2.
The pressure sensor 26 is of a semiconductor type that directly measures the intake air pressure and generates an output signal of an analog voltage proportional to the intake air pressure. The pressure sensor 26 is included in the operating state detecting means 14 described in FIG. This output signal is provided to the AD converter 101 with a built-in multiplexer in the control circuit 40. The distributor (not shown) includes a crank angle sensor 36 whose axis generates a reference position detection pulse signal every 720 ° converted into a crank angle, and a reference position detection pulse signal every 30 ° converted into a crank angle. A crank angle sensor 38 for generating is generated. The pulse signals of the crank angle sensors 36, 38 are supplied to the input / output interface 102 of the control circuit 40, and the crank angle sensor 3
Outputs 6 and 38 are CPU (Central Processor Unit) 10
Supplied to interrupt terminal 3.

Further, the intake passage 8 is provided with a fuel injection valve 22 for supplying pressurized fuel from the combustion supply system to the intake port for each cylinder.

A water jacket (not shown) of the cylinder block of the internal combustion engine 2 is provided with a water temperature sensor 34 for detecting the temperature of the cooling water. The water temperature sensor 34 generates an electric signal of an analog voltage according to the temperature THW of the cooling water. This output is also supplied to the A / D converter 101.

The actuator of the EGR valve 10 has a built-in step motor controlled by the output from the input / output interface 102 of the control circuit 40.

In the exhaust system 4 of the internal combustion engine 2, an oxidation catalyst 24 that purifies HC and CO at a high rate is provided downstream of the lean NOx catalyst 6. A three-way catalyst may be used instead of the oxidation catalyst 24.

Further, 32 is an exhaust gas temperature sensor for detecting the exhaust gas temperature in the lean NOx catalyst 6, the output of which is supplied to the A / D converter 101 of the control circuit 40.

The control circuit 40 is configured as, for example, a microcomputer, includes an A / D converter 101, an input / output interface 10
2, ROM (read-on memory) 104, RAM outside the CPU 103
(Random access memory) 105, Backup RAM10
6. A clock generation circuit 107 and the like are provided.

Further, in the control circuit 40, the down counter 108, the flip-flop 109, and the drive circuit 110 include the fuel injection valve 22.
Is to control the That is, when the fuel injection amount TAU is calculated in a routine described later, the fuel injection amount TAU is preset in the down counter 108 and the flip-flop 109 is also set. As a result, the drive circuit 110 starts energizing the fuel injection valve 22. On the other hand, when the down counter 108 calculates a clock signal (not shown) and finally its borrow-out terminal becomes "1" level, the flip-flop 109 is set and the drive circuit 110 causes the fuel injection valve 22 to operate. Stop energizing. That is, the fuel injection valve 22 is biased by the above-mentioned fuel injection amount TAU, and accordingly, the amount of fuel corresponding to the fuel injection amount TAU is sent to the combustion chamber of the internal combustion engine 2.

The CPU 103 generates an interrupt when the input / output interface 102 receives the pulse signal of the crank angle sensor 38 after the A / D conversion of the A / D converter 101 is completed.
When an interrupt signal from 7 is received, and so on.

Intake air pressure data PM of pressure sensor 26, cooling water temperature data
THW and exhaust gas temperature TEX are fetched by an A / D conversion routine executed at predetermined time intervals or at predetermined crank angle intervals, and data PM, THW, and TEX in RAM 105 are updated at predetermined time intervals. Further, the rotation speed data NE is calculated by the interrupt of the crank angle sensor 38 every 30 ° CA, and RA
It is stored in a predetermined area of M105.

The operation of the control circuit of FIG. 2 will be described with reference to FIGS. 3, 4, and 6.

FIG. 3 shows an EGR control routine, which is executed every predetermined time. That is, in step 301, the CPU is read from the RAM 105.
The intake pipe pressure PM is continuously supplied to 103, and in step 302, an increment ΔPM of the intake pipe pressure PM calculated this time from the intake pipe pressure PM1 calculated last time is calculated. In step 303, the intake pipe pressure PM calculated this time is stored as PM1 for the next calculation, and step 3
Go to 04. In step 304, it is determined whether the engine is in an accelerating state by determining whether ΔPM is larger than a predetermined ΔPMO. Therefore, step 304 constitutes the acceleration state determination means 16 described in FIG. In the acceleration state, that is, when ΔPM> ΔPMO, proceed to step 305 and set EGR to O
N (EGR valve 10 is turned on), the EGR flag XEGR is set to 1 in step 306, and the process returns. Here, step 305 is the first
It constitutes the EGR ON means 20 described in the figure. When it is determined in step 304 that the vehicle is not in an accelerating state, that is, ΔPM> ΔPMO
If not, proceed to step 307 and turn off the EGR (EGR valve 10 OF
F), the EGR flag XEGR is set to 0 in step 308, and the process returns.

The routine of FIG. 4 may be inserted between the step 306 and the return in the routine of FIG. FIG. 4 shows that the EGR amount is calculated based on the EGR sensor 30 and the A / F sensor 28 so as to maximize the NOx purification performance of the lean NOx catalyst 6 and not impair the drivability. It is a control flowchart of EGR for controlling the obtained present EGR rate to a target EGR rate precisely.

The EGR sensor 30 and the A / F sensor 28 are, for example, oxygen concentration sensors, and are read from the RAM 105 in step 401. In step 402, the target EGR rate is calculated. For example, as shown in FIG. 5, the target EGR rate according to the exhaust temperature TEX is obtained from the map. In step 403, the current EGR rate is calculated. This is obtained as follows.

However, Ga: new intake air amount Ge: intake EGR amount In the intake pipe, from the equation of state of gas,
Equation (3) holds.

Substituting equations (2) and (3) into equation (1), Me ≒ Ma = 28, Then, the equation (4) becomes the following equation (5).

However, C ₀₂ is the O ₂ concentration in the intake pipe and is detected by the EGR sensor 30,
▲ O ^ex ₂ ▼ is detected by the exhaust A / F sensor 28. Therefore, the EGR rate ε can be calculated from the equation (5).

Then, in step 404, it is determined whether or not the current EGR rate is higher than the target EGR rate. If it is large, the procedure proceeds to step 405 to reduce the EGR valve step number, and if it is small, the procedure proceeds to step 406 to determine the EGR valve step number. Increase. Thus, the EGR rate approaches the target EGR rate.

FIG. 6 shows a fuel injection amount calculation routine, which is executed every predetermined crank angle, for example, every 360 ° CA. Step 601
Then, the intake air pressure data PM and the rotational speed NE are read from the RAM 105, and the basic injection amount TP is interpolated by the two-dimensional map stored in the ROM 104. Next, at step 602, it is judged if the engine is in the lean operating condition, for example, if the water temperature THW is not less than a predetermined value. As a result, if it is the lean operation condition, the process proceeds to step 603, and if it is not the lean operation condition, the process proceeds to step 608, the lean correction coefficient KLEAN is set to 1.0, and the process directly proceeds to step 609. KLEAN = 1.0 is the theoretical air-fuel ratio equivalent value.

The steps 603 to 607 will be described. In step 603, it is determined whether the EGR is ON by the EGR on flag XEGR. As a result, if the EGR valve is operating (XEGR = "1"),
The coefficient KLEANPM is calculated in step 604. If the EGR operation is stopped (XEGR = "0"), the coefficient KLEANPM is calculated in step 605.
Is calculated.

In step 604, the coefficient KLEANPM is interpolated by the one-dimensional map shown in step 604 stored in the ROM 104 based on the intake air pressure PM.

As can be seen by comparing the curves shown in steps 604 and 605, the value of KLEANPM is E during EGR operation (step 604).
Greater than the GR operation is stopped (step 605).
That is, it is slightly richer during the EGR operation.

In step 606, the rotational speed data NE is read from the RAM 105, and the coefficient KLEANNE is interpolated by the one-dimensional map shown in step 606 stored in the ROM 104.

In step 609, the above two coefficients KLEANPM, KLEANNE
More lean correction coefficient KLEAN is calculated by KLEAN ← KLEANPM ・ KLEANNE. In the above, steps 604, 606, 607, especially step 606 are EGR.
Is a means for setting the air-fuel ratio A / F to 16 to 19 and corresponds to the air-fuel ratio setting means 18 described in FIG.

In step 609, the final fuel injection amount TAU is set to TAU ← TP ・ K
Calculate with LEAN and α + β. Note that α and β are correction amounts determined by other operating state parameters. Then
In step 610, the injection amount TAU is set in the down counter 108 and the flip-flop 109 is set to start fuel injection.

As described above, when the time corresponding to the injection amount TAU has elapsed, the flip-flop 109 is reset by the borrow-out signal of the down counter 108, and the fuel injection ends.

 Next, the operation will be described.

At light load conditions such as steady running and slow acceleration, the air-fuel ratio
The engine is operated with the A / F set to 20 to 24, the EGR valve 10 is turned off, and the EGR gas is not introduced into the intake system 8. This state has a lot of HC,
Since the lean NOx catalyst 6 is working sufficiently, there is no problem even if EGR gas is not introduced.

In light and medium load conditions where the vehicle is accelerating above a specified value, such as when accelerating from idle, torque is required.Therefore, the air-fuel ratio A / F is set to 16 to 19 and the EGR valve 10 is turned on. At EG
R gas is introduced into the intake system 8. In this state, the amount of HC is small without introduction of EGR gas, so the lean NOx catalyst 6 does not work sufficiently. However, since EGR is turned on, the proportion of fresh air in the intake air decreases, resulting in an oxygen deficiency state, the combustion in the combustion chamber is slightly sacrificed, and the unburned content increases,
As a result, HC in the exhaust gas increases and N in the lean NOX catalyst 6
Ox purification rate is kept high.

In the above-mentioned high load range, the stoichiometric air-fuel ratio is satisfied, and purification is performed by the oxidation or the three-way catalyst 24 downstream of the lean NOx catalyst 6.

[Effects of the Invention] As described above, according to the present invention, when the internal combustion engine is in the acceleration operation state of a predetermined value or more, the air-fuel ratio is made slightly richer with the introduction of EGR to reduce NOx. The effective HC can be increased, and as a result, NOx emission can be constantly reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention, FIG. 2 is a system diagram of an exhaust gas purification device of an internal combustion engine according to an embodiment of the present invention, and FIG. 3 is control of an EGR control routine used in the present invention. A flow chart, FIG. 4 is a control flow chart that can be inserted in the middle of the control flow of FIG. 3, FIG. 5 is a map chart used to obtain the target EGR rate of the flow of FIG. 4, and FIG. Fig. 7 is a control flow chart of the fuel injection amount calculation routine used in Fig. 7, Fig. 7 is a HC supply amount-NOx purification rate characteristic diagram, Fig. 8 is an EGR amount-HC supply amount characteristic diagram, and Fig. 9 is a NOx reduction of a lean NOx catalyst. It is a block diagram showing a mechanism. 2 ... Internal combustion engine 4 ... Exhaust system 6 ... Lean NOx catalyst 8 ... Intake system 10 ... EGR valve 12 ... Exhaust gas recirculation device 14 ... Operating state detection means 16 ... Acceleration state determination means 18 ... … Air-fuel ratio setting means 20 …… EGR ON means 22 …… Fuel injection valve 24 …… Oxidation catalyst or three-way catalyst 26 …… Intake pipe pressure sensor 28 …… Air-fuel ratio sensor (A / F sensor) 30 …… EGR sensor 40 ... Control circuit

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F02D 45/00 301 F02D 45/00 301G F02M 25/07 550 F02M 25/07 550J (72) Inventor Soichi Matsushita 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor Shinichi Takeshima 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (72) Inventor Toshiaki Tanaka Toyota City, Aichi Prefecture Town No. 1 Toyota Toyota Motor Corporation (56) Reference JP 61-268845 (JP, A) JP 63-100919 (JP, A)

Claims (1)

(57) [Claims] 1. A lean NOx catalyst, which is provided in an exhaust system of an internal combustion engine and is made of zeolite supporting a transition metal or a noble metal, for reducing NOx in the presence of HC in an oxidizing atmosphere, and an exhaust system and an intake system of the internal combustion engine. An exhaust gas recirculation device provided with an EGR valve, an operating state detecting means for detecting a motion state of the internal combustion engine, and the internal combustion engine running normally or on the basis of the output of the operating state detecting means. An acceleration state determination means for determining whether or not the vehicle is in an acceleration state greater than a predetermined acceleration value greater than the acceleration value during slow acceleration and less than the acceleration value during idle acceleration; When it is determined that the acceleration state is equal to or more than the value, the target air-fuel ratio is set to a relatively rich region within the lean side region from the stoichiometric air-fuel ratio to increase the torque, and the acceleration state determination unit is set to the predetermined acceleration value or more. EGR on means for opening the EGR valve when it is determined to be in the upper acceleration state, introducing EGR gas into the intake system, generating HC in the combustion chamber, and improving the NOx purification rate of the lean NOx catalyst, An exhaust emission control device for an internal combustion engine, characterized by: