JPH11173136A - Exhaust emission control device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate for lowering of emission control performance due to the increase of the temperature of an NOx catalyst during lean operation after rich operation of an engine when a lean NOx catalyst to purify NOx in a lean operation state in an air-fuel ratio higher than a theoretical air-fuel ratio and a catalytic converter rhodium catalyst situated downstream from the lean NOx catalyst are arranged. SOLUTION: A lean NOx catalyst 51 on the upper stream side, a catalytic converter rhodium catalyst 52, and a lean NOx catalyst 53 on the downstream side are arranged in the exhaust passage 31 of an engine 1, in the order, toward a spot situated downstream such that a distance L2 between the catalytic converter rhodium 52 and the downstream lean NOx catalyst 53 is longer than a distance L1 between the upstream lean NOx catalyst 51 and the catalytic converter rhodium catalyst 52. Even when in the rich operation state of the engine 1 and in a state that an exhaust gas temperature on the inlet side of the upstream lean NOx catalyst 51 is increased, the coefficient of NOx purification of the lean NOx catalyst 51 on the upper stream side is decreased owing to switching to a lean operation state, NOx not purified thereby is purified by the downstream lean NOx catalyst 53 wherein temperature on the inlet side is low and NOx emission control performance is high.

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, and more particularly, to a method for purifying the exhaust gas of an engine with a lean N.
It belongs to the technical field related to those provided with an Ox catalyst.

[0002]

2. Description of the Related Art Conventionally, this type of lean NOx catalyst has been well known. By arranging this lean NOx catalyst in an exhaust passage of an engine, NOx (NOx) contained in exhaust gas when the engine is in a lean operation state is known. Nitrogen oxides). However, depending on the operating region of the engine, the lean NOx catalyst may not be able to sufficiently purify NOx in exhaust gas.

Therefore, conventionally, for example, Japanese Unexamined Patent Publication No.
As shown in Japanese Patent Publication No. 70, the exhaust passage of the engine includes:
A lean NOx catalyst containing a NOx storage material and a three-way catalyst are disposed in an exhaust passage downstream of the lean NOx catalyst, respectively, and the engine is operated in a lean operating state and a stoichiometric air-fuel ratio where the air-fuel ratio is larger than the stoichiometric air-fuel ratio. By switching to the rich operation state described below and operating the engine, when the engine is in a lean operation state larger than the stoichiometric air-fuel ratio, the lean NOx catalyst on the upstream side stores NOx in the exhaust gas, and thereafter, the engine operates at the theoretical level. When a rich operation state with an air-fuel ratio or less is reached, a lean NOx catalyst is configured to release stored NOx, and the released NOx is purified by a downstream three-way catalyst.

[0004]

However, as in the case of the above proposal, a lean NO.
When the x catalyst is disposed and the three-way catalyst is disposed on the downstream side, there are the following problems. That is, the lean N
The Ox catalyst has a characteristic that the NOx purification performance is low when the temperature is in a low temperature state (about 150 ° C. or lower), and the NOx purification performance is reduced when the temperature is higher than a predetermined temperature. And
For example, when the engine switches from a lean operation state larger than the stoichiometric air-fuel ratio to a rich operation state by acceleration or the like, the exhaust gas temperature on the inlet side of the lean NOx catalyst increases accordingly. Thereafter, when the engine is brought into a steady state and returned to the original lean operation state by the end of acceleration or the like, the NOx purification rate of the lean NOx catalyst is reduced by the higher exhaust gas temperature on the inlet side, and the NOx purification performance of the exhaust gas purification device is reduced. Decrease as a whole.

[0005] The present invention has been made in view of such a point, and an object thereof is to provide a lean NOx in an exhaust passage of an engine.
By arranging the catalyst and the three-way catalyst in association with the exhaust gas temperature, the decrease in the NOx purification performance due to the temperature rise of the lean NOx catalyst during the lean operation after the rich operation of the engine is compensated for, and the overall exhaust gas purification performance is improved. It is in.

[0006]

According to the present invention, a lean NOx catalyst is provided upstream of an exhaust passage of an engine, and a three-way catalyst is provided downstream of the lean NOx catalyst. And one more lean NOx catalyst in the exhaust passage downstream of the three-way catalyst.

More specifically, according to the first aspect of the present invention, the exhaust passage of the engine which is operated while being switched between a lean operation state in which the air-fuel ratio is larger than the stoichiometric air-fuel ratio and a rich operation state equal to or lower than the stoichiometric air-fuel ratio is provided. It is premised on an exhaust gas purifying apparatus for an engine provided with a lean NOx catalyst for purifying NOx in a lean operation state in which the air-fuel ratio is larger than the stoichiometric air-fuel ratio and a three-way catalyst.

Then, an upstream lean NOx catalyst, a three-way catalyst, and a downstream lean NOx catalyst are disposed in the exhaust passage in order from the upstream side to the downstream side.

With the above configuration, when the engine switches from a lean operation state larger than the stoichiometric air-fuel ratio to a rich operation state by acceleration or the like, the exhaust gas temperature on the inlet side of the upstream lean NOx catalyst rises, and then the acceleration or other When the engine is in a steady state and returns to the original lean operation state by the end of the above, the NOx purification rate of the upstream lean NOx catalyst having a high exhaust gas temperature on the inlet side decreases. However, a similar downstream lean NOx catalyst is further disposed in the exhaust passage downstream of the upstream lean NOx catalyst, and the downstream lean NOx
Since the temperature of the exhaust gas flowing into the downstream lean NOx catalyst is low and the NOx purification performance is high because the x catalyst is separated from the upstream lean NOx catalyst to the downstream side,
NOx that could not be purified by the upstream lean NOx catalyst
Can be purified by the downstream lean NOx catalyst, so that the NOx purification performance of the exhaust gas purifying device can be improved and maintained as a whole. If the lean operation state of the engine continues, the temperature of exhaust gas flowing into the upstream lean NOx catalyst also decreases, so that the upstream lean NOx catalyst can purify NOx.

[0010] Further, since the three-way catalyst is provided in the exhaust passage between the upstream and downstream lean NOx catalysts, the upstream lean N
NOx released from the Ox catalyst and NOx increased during the acceleration operation of the engine can be purified by the three-way catalyst. Further, even when the engine is operated richly for the purpose of early warm-up of the catalyst when the engine is cold, the exhaust gas at that time can be purified.

In the invention of claim 2, the distance between the three-way catalyst and the downstream lean NOx catalyst is made larger than the distance between the upstream lean NOx catalyst and the three-way catalyst. In this case, the downstream lean NOx catalyst moves from the three-way catalyst to the upstream lean NOx.
Since it is farther than the x catalyst, the temperature of the exhaust gas to the downstream lean NOx catalyst can be lowered by that far, and the NOx purification performance of the downstream lean NOx catalyst can be enhanced.

According to a third aspect of the present invention, in the exhaust gas purifying apparatus for an engine of the second aspect, the upstream lean NOx catalyst and the three-way catalyst are housed in the same housing. This facilitates the arrangement of the upstream lean NOx catalyst and the three-way catalyst arranged at a short distance from each other. Further, by bringing both catalysts close to each other, it is possible to suppress a decrease in the temperature of the exhaust gas flowing into the three-way catalyst, and to improve the exhaust gas purification performance during the rich operation.

According to a fourth aspect of the present invention, in the exhaust gas purifying apparatus for an engine of the first aspect, the upstream lean NOx catalyst is disposed downstream of the exhaust manifold of the engine, and the upstream lean NOx catalyst and the exhaust manifold are connected to each other. An intermediate exhaust pipe for connection is interposed between them. With this configuration, the upstream lean NOx catalyst can be disposed apart from the engine by the length of the intermediate exhaust pipe, so that the temperature of exhaust gas to the upstream lean NOx catalyst can be lowered and the NOx purification performance can be improved.

According to a fifth aspect of the present invention, in the exhaust gas purifying apparatus for an engine according to the first or third aspect, the upstream lean NOx is provided.
The capacity of the catalyst is set to be equal to or less than the capacity of the downstream lean NOx catalyst. As a result, even when the engine is cold, it is possible to suppress a decrease in the exhaust gas temperature on the inlet side of the three-way catalyst with the upstream lean NOx catalyst having a small capacity, and to quickly warm up the three-way catalyst. The purification performance can be exhibited well.

According to a sixth aspect of the present invention, in the exhaust gas purifying apparatus for an engine according to the first or fifth aspect, the upstream lean NOx is provided.
The wall thickness of the carrier in the catalyst is equal to or less than the wall thickness of the carrier in the three-way catalyst. As a result, the weight of the upstream lean NOx catalyst can be made equal to or less than the weight of the three-way catalyst, and the decrease in the exhaust gas temperature on the inlet side of the three-way catalyst when the engine is cold can be achieved similarly to the fifth aspect of the invention. Thus, the purification performance of the three-way catalyst can be sufficiently exhibited.

According to a seventh aspect of the present invention, in the exhaust gas purifying apparatus for an engine according to the first aspect, the upstream lean NOx catalyst contains a NOx storage material. In this case, when the upstream lean NOx catalyst that stores NOx in the lean operation state of the engine releases the stored NOx in the subsequent rich operation state, the released NOx is purified by the three-way catalyst on the downstream side. Can be.

According to an eighth aspect of the present invention, in the exhaust gas purifying apparatus for an engine of the first aspect, the downstream lean NOx catalyst contains a NOx storage material. In this case,
Even if HC (hydrocarbon) or CO (carbon monoxide) as a reducing agent is not sufficiently supplied to the downstream lean NOx catalyst due to purification by the upstream three-way catalyst, it is not affected by the purification. NOx can be stored and purified by the downstream lean NOx catalyst.

According to a ninth aspect of the present invention, in the exhaust gas purifying apparatus for an engine according to the first or eighth aspect, the upstream lean NOx
It is assumed that the catalyst reduces and decomposes NOx under lean operation. This makes it possible to use the upstream lean NO by using HC or CO as a reducing agent discharged from the engine during lean operation.
NOx can be purified by the x catalyst.

According to a tenth aspect of the present invention, in the exhaust gas purifying apparatus for an engine of the ninth aspect, the upstream lean NOx catalyst is of a high temperature type in which the temperature at which the maximum NOx purification rate is 250 ° C. or higher. In this case, the downstream lean N
When high-temperature exhaust gas that cannot be sufficiently purified by the Ox catalyst flows in, the NOx purification performance can be enhanced by the upstream lean NOx catalyst.

According to the eleventh aspect of the present invention, in the exhaust gas purifying apparatus for an engine according to the first or fifth aspect, the upstream lean NO
the upstream lean NOx in the exhaust passage upstream of the x catalyst.
A three-way cold catalyst having a smaller capacity than the catalyst is arranged. Thus, even when the engine is operated rich when the engine is cold, the exhaust gas can be purified by quickly warming up the small capacity three-way catalyst for cold.

According to a twelfth aspect of the present invention, in the exhaust gas purifying apparatus for an engine of the first aspect, an exhaust gas recirculation means for circulating a part of the exhaust gas to the intake system is provided. Accordingly, the temperature itself of the exhaust gas discharged from the engine can be reduced by the recirculated exhaust gas by the exhaust gas recirculation means, and the NOx purification performance of the lean NOx catalyst can be further improved.

According to a thirteenth aspect of the present invention, in the exhaust gas purifying apparatus for an engine of the first aspect, the engine is a direct injection engine having a fuel injection valve for directly injecting fuel into a combustion chamber in a cylinder. In this direct injection engine, the exhaust gas temperature can be reduced by the excess air, so that the NOx purification performance of the lean NOx catalyst can be further improved.

[0023]

FIG. 1 shows an overall configuration of an embodiment of the present invention. Reference numeral 1 denotes a multi-cylinder engine mounted on, for example, a vehicle. This engine 1 has a plurality of cylinders 2, 2,. And a cylinder head 4 mounted on the cylinder block 3.
And a piston 5 reciprocally fitted into each cylinder 2. A combustion chamber 6 is defined by the cylinder 2, the cylinder head 4 and the piston 5. Reference numeral 10 denotes a spark plug mounted on the cylinder head 4 which is the upper wall of the combustion chamber 6 so as to face the combustion chamber 6.
0 is connected to the ignition circuit 11.

A fuel injection valve 14 for directly injecting fuel into the combustion chamber 6 is attached to the side wall of the combustion chamber 6 at a position where the fuel injection valve 14 does not interfere with the moving piston 5. By trapping the injected fuel in a cavity (not shown) recessed on the top surface of the piston 5, a relatively dense mixture layer is formed near the ignition plug 10. That is, the engine 1
The engine is a direct injection engine that directly injects fuel into a combustion chamber 6 in a cylinder 2.

The fuel injection valve 14 is connected to a fuel tank 16 via a fuel passage 15. In the middle of the fuel passage 15, the low-pressure fuel pump 1
7 and a high-pressure fuel pump 18 located downstream of the low-pressure fuel pump 17. A low-pressure pressure regulator 19 is disposed in the fuel passage 15 between the two fuel pumps 17 and 18, and a fuel filter 20 is disposed downstream thereof. Further, the fuel passage 15 between the fuel injection valve 14 and the high-pressure fuel pump 18 and the fuel passage 15 between the low-pressure pressure regulator 19 and the fuel filter 20 are connected by a return passage 22 provided with a high-pressure pressure regulator 21. After the pressure of the fuel discharged from the low-pressure fuel pump 17 is adjusted by the low-pressure pressure regulator 19, it is supplied to the high-pressure fuel pump 18 to further increase the pressure, and the high-pressure fuel is injected from the fuel injection valve 14 into the combustion chamber 6 of the engine 1. Supply. Also, the high pressure fuel pump 1
The pressure of the fuel supplied to the fuel injection valve 14 is appropriately adjusted by returning a part of the fuel discharged from the fuel injection valve 8 to the upstream side of the high-pressure fuel pump 18 via the return passage 22 while adjusting the flow rate by the high-pressure pressure regulator 21. Adjustments are made.

Reference numeral 25 denotes an intake passage for supplying intake air (air) filtered by the air cleaner 26 to the combustion chamber 6 of the engine 1. The downstream end of the intake passage 25 is communicated with the combustion chamber 6 via the intake valve 8. I have. In the intake passage 25, in order from the upstream side to the downstream side, a heat-sensitive air flow sensor 27 for detecting an amount of intake air taken into the engine 1, an electric throttle valve 28 for restricting the intake passage 25, and a surge tank 2.
9 are provided respectively. The electric throttle valve 28 is not mechanically connected to an accelerator pedal (not shown) and opens and closes by driving an actuator (not shown).

Reference numeral 31 denotes an exhaust passage for discharging exhaust gas from the combustion chamber 6 of the engine 1, and an upstream end of the exhaust passage 31 is connected to the combustion chamber 6 via the exhaust valve 9. In the exhaust passage 31, from the upstream side to the downstream side, in order from the upstream side, an O2 sensor 35 which is located in a portion formed in the collecting portion 32a of the exhaust manifold 32 and detects the oxygen concentration in the exhaust gas, and purifies the exhaust gas. Four catalysts 50 to 53 are provided respectively. The O2 sensor 35 is used to detect the air-fuel ratio based on the oxygen concentration in the exhaust gas, and has a characteristic that the output changes suddenly when the air-fuel ratio is at the stoichiometric air-fuel ratio.

Further, there is provided an exhaust gas recirculation device 37 for recirculating a part of the exhaust gas in the exhaust passage 31 to the intake system. The exhaust gas recirculation device 37 has an exhaust gas recirculation passage 38 whose upstream end is branched and connected upstream of the O2 sensor 35 in the exhaust gas passage 31 formed in the collecting portion 32a of the exhaust manifold 32. Reflux passage 38
Is connected to an intake passage 25 between the throttle valve 28 and the surge tank 29. Exhaust recirculation passage 3
An electric exhaust gas recirculation control valve 39 whose opening degree can be adjusted is arranged near the downstream end of the exhaust gas 8, and a part of the exhaust gas in the exhaust passage 31 is adjusted by the exhaust gas recirculation control valve 39 via the exhaust gas recirculation passage 38. Then, the air is returned to the intake passage 25.

The ignition circuit 11 connected to the ignition plug 10, the actuator of the electric throttle valve 28, the fuel injection valve 14, the high pressure regulator 21 and the exhaust gas recirculation control valve 39 are controlled by a control unit 41. Has become. The control unit 41 includes output signals of the airflow sensor 27 and the O2 sensor 35, an output signal of an accelerator opening sensor 42 for detecting an opening (depression amount) of an accelerator pedal as an engine load, and a crank of the engine 1. The output signal of the crank angle sensor 43 for detecting the rotation angle of the shaft 7 as the engine speed and the output signal of the water temperature sensor 44 for detecting the temperature of the cooling water of the engine 1 are inputted. The fuel injection amount is determined based on the map, and fuel is injected from the fuel injection valve 14 so as to achieve the fuel injection amount.

Then, as shown in FIG. 6, whether the engine 1 is operated in a lean operation state in which the air-fuel ratio is larger than the stoichiometric air-fuel ratio (λ> 1) or in a rich operation state below the stoichiometric air-fuel ratio (λ ≦ 1) Are set in advance in accordance with the engine speed and the engine load, and the engine 1 is switched to the lean operation state and the rich operation state to operate. In I), by injecting fuel from the fuel injection valve 14 immediately before the ignition timing in the compression stroke of each cylinder 2, in a stratified combustion state with an air-fuel ratio lean (λ> 1), and at a medium load rotation of the engine 1 In the region (II), the fuel injection from the fuel injection valve 14 is performed in the intake stroke, so that the air-fuel ratio lean (λ>
In the uniform combustion state of 1), and in the high-load and high-speed regions (III) and (IV) of the engine 1, the fuel injection from the fuel injection valve 14 is performed in the intake stroke, so that the air-fuel ratio rich (λ
The engine 1 is operated in a uniform combustion state of ≦ 1). In the low-load low-rotation region (I), the medium-load medium rotation region (II), and the low-load and low-rotation regions (III) in the high-load high-rotation region, the exhaust gas recirculation control valve 3 is used.
Exhaust gas recirculation (EGR) is executed by opening the valve 9.

A feature of the present invention resides in the arrangement of the four catalysts 50 to 53 in the exhaust passage 31 of the engine 1. That is, each of these four catalysts 50 to 53 has a honeycomb structure cordierite carrier 55 (opened with a large number of through holes 55 a, 55 a,... Extending parallel to each other along the axial direction (flow direction of exhaust gas). 2 and 3
), A catalyst layer is formed on the wall surface of each through-hole 55a, and is divided into three basic catalysts 51 to 53 and one auxiliary catalyst 50. The three basic catalysts 51 to 53 are an upstream lean NOx catalyst 51, a three-way catalyst 52, and a downstream lean NOx catalyst 53 arranged in the exhaust passage 31 in order from the upstream to the downstream. The upstream lean NOx catalyst 51 and the downstream lean NOx catalyst 53 purify NOx in a lean operation state where the air-fuel ratio of the engine 1 is larger than the stoichiometric air-fuel ratio.
On the other hand, one auxiliary catalyst 50 is provided with the upstream lean N
A cold three-way catalyst disposed in the exhaust passage 31 upstream of the Ox catalyst 51, and the cold three-way catalyst 50 is set to have a smaller capacity than the upstream lean NOx catalyst 51.

Specifically, the upstream end of a connecting intermediate exhaust pipe 33 having a predetermined length is connected to the downstream end of the collecting portion 32a of the exhaust manifold 32 of the engine 1, and the intermediate exhaust pipe 33
Is connected to the inlet end (upstream end) of the upstream housing 57. In the upstream housing 57, the upstream lean NOx catalyst 51 and the three-way catalyst 52 are accommodated in a so-called two-bed system in a state of being connected in series and with a predetermined space therebetween. In addition, the vicinity of the inlet end of the same upstream housing 57 (upstream lean NO
(the upstream side of the x-catalyst 51) inside the cold three-way catalyst 50
Is housed.

An outlet end (downstream end) of the upstream housing 57 is connected to an inlet end (upstream end) of the downstream housing 58, and the downstream lean is inserted into the downstream housing 58. The NOx catalyst 53 is housed.

The distance L2 between the three-way catalyst 52 and the downstream lean NOx catalyst 53 is longer than the distance L1 between the upstream lean NOx catalyst 51 and the three-way catalyst 52 (L2> L2).
1) is set.

The capacity of the upstream lean NOx catalyst 51 is set to be equal to or less than the capacity of the downstream lean NOx catalyst 53.

Further, as shown in FIGS. 2 and 3, the wall thickness D1 of the carrier 55 in the upstream lean NOx catalyst 51 (see FIG. 2) is the wall thickness D2 of the carrier 55 in the three-way catalyst 52.
(See FIG. 3) It is as follows (D1 ≦ D2).

Further, the downstream lean NOx catalyst 53
It contains Ox storage material, and has upstream lean NOx
The catalyst 51 is configured to reduce and decompose NOx under lean operation, and the upstream lean NOx catalyst 51 is
It is a high-temperature type in which the temperature at which the maximum NOx purification rate becomes 250 ° C. or higher. This lean NOx catalyst 51
For example, the catalyst layer on the wall surface of the carrier 55 is composed of zeolite, and the zeolite includes iron Fe, cobalt Co, copper C
u, a metal carrying a transition metal such as nickel Ni and a noble metal such as iridium Ir or platinum Pt, or barium sulfate B
A barium compound such as aSO4 carrying iridium as a noble metal is used. As shown in FIG. 5, the high-temperature type upstream lean NOx catalyst 51
It shows a higher NOx purification rate in a higher temperature region than the downstream lean NOx catalyst 53.

Next, the operation of the above embodiment will be described with reference to FIG. 4. The operating range of the engine 1 is in the low-load low-rotation range (I) or the medium-load medium rotation range (II) shown in FIG. When it is constant, the engine 1 is operated in a lean operation state in which the air-fuel ratio is larger than the stoichiometric air-fuel ratio. In this state, the exhaust gas temperature at the inlet side of the upstream lean NOx catalyst 51 is lower than the exhaust gas temperature at the inlet side of the downstream lean NOx catalyst 53, and the upstream lean Nx is reduced by the temperature difference.
The NOx purification rate of the Ox catalyst 51 is higher than the downstream lean NOx.

From this state, the engine 1 shifts to an acceleration operation state by, for example, depressing an accelerator pedal, and when the operation area shifts to, for example, a high load and high rotation area (III) or (IV), the engine 1 is accordingly driven. The vehicle is driven in a rich operation state in which the air-fuel ratio is equal to or lower than the stoichiometric air-fuel ratio, and the vehicle speed increases. In this state, the exhaust gas temperature on the inlet side of the upstream lean NOx catalyst 51 near the engine 1 rapidly rises and becomes higher than the exhaust gas temperature on the inlet side of the downstream lean NOx catalyst 53, but the downstream lean NOx catalyst The exhaust gas temperature on the inlet side of 53 gradually rises by the amount that the downstream lean NOx catalyst 53 is separated from the upstream lean NOx catalyst 51 to the downstream side. Then, the upstream lean NOx catalyst 5
Since the three-way catalyst 52 is disposed on the downstream side of the engine 1, even if NOx is released from the upstream lean NOx catalyst 51 or the amount of NOx increases due to the acceleration operation state of the engine 1, The increased NOx is purified by the three-way catalyst 52, and a high NOx purification rate is obtained as a whole.

Thereafter, when the acceleration operation of the engine 1 is terminated by stopping the depression of the accelerator pedal and the engine 1 is brought into a steady operation state, the operation region becomes the original low-load low-rotation region (I) or the medium-load medium rotation region (I). When the vehicle returns to the lean operation state in II), the vehicle speed becomes constant. In this state, since the exhaust gas temperature at the inlet side of the upstream lean NOx catalyst 51 is high as described above, the upstream lean NOx catalyst 51 is immediately changed with the switching of the air-fuel ratio to the lean side.
NOx purification rate decreases. However, downstream lean NO
Since the temperature of the exhaust gas flowing into the x catalyst 53 is still low and the NOx purification performance of the downstream lean NOx catalyst 53 is high, the flow of the heated exhaust gas into the downstream lean NOx catalyst 53 causes the downstream lean NOx to flow. Catalyst 5
During the time t until the temperature of the upstream lean NOx catalyst 3 becomes higher than that of the upstream lean NOx catalyst 51, NOx that cannot be purified by the upstream lean NOx catalyst 51 is converted to the downstream lean NOx catalyst 53.
Can be purified. As a result, the NOx purification performance of the exhaust gas purification device can be improved and maintained as a whole.

At this time, since the distance L2 between the three-way catalyst 52 and the downstream lean NOx catalyst 53 is larger than the distance L1 between the upstream lean NOx catalyst 51 and the three-way catalyst 52, the downstream lean NOx The catalyst 53 is upstream lean NOx
It will be separated from the three-way catalyst 52 as compared with the catalyst 51,
The temperature of exhaust gas to the downstream lean NOx catalyst 53 can be reduced by the distance, and the NOx purification performance of the downstream lean NOx catalyst 53 can be enhanced.

The engine 1 has a low-load low-speed range (I), a medium-load medium-speed range (II), and a high-load high-speed range (III), (IV). In the case of III), the exhaust gas recirculation control valve 39 of the exhaust gas recirculation device 37 is opened, and a part of the exhaust gas of the engine 1 is recirculated to the intake system through the exhaust gas recirculation passage 38. The temperature of the exhaust gas itself drops. In addition, since the engine 1 is a direct injection engine that directly injects fuel into the combustion chamber 6 in the cylinder 2 by the fuel injection valve 14, the temperature of the exhaust gas is lowered by excess air. Due to these synergistic effects, NO of the lean NOx catalyst
x Purification performance can be further improved.

If the lean operation state of the engine 1 continues, the temperature of exhaust gas flowing into the upstream lean NOx catalyst 51 also decreases, so that the upstream lean NOx catalyst 51 can purify NOx.

In this embodiment, the exhaust passage 3 between the upstream and downstream lean NOx catalysts 51 and 53 is used.
Since the three-way catalyst 52 is disposed in the engine 1, even if the engine 1 is operated rich for the purpose of early warm-up of the catalysts 50 to 53 when the engine 1 is cold, it is possible to purify the exhaust gas at that time. In addition, since the cold three-way catalyst 50 having a smaller capacity than the upstream lean NOx catalyst 51 is arranged at the inlet portion in the upstream housing 57, the rich running state of the engine 1 in the cold state can be obtained. The exhaust gas can be purified by quickly warming up the small capacity three-way catalyst 50 for cold.

Further, the upstream lean NOx catalyst 51
Since the three-way catalyst 52 and the three-way catalyst 52 are housed in the same upstream housing 57, the arrangement of the upstream lean NOx catalyst 51 and the three-way catalyst 52 arranged close to each other is facilitated. Further, by bringing both catalysts 51 and 52 close to each other, it is possible to suppress a decrease in the temperature of the exhaust gas flowing into the three-way catalyst 52, and it is possible to enhance the exhaust gas purification performance during the rich operation.

Further, since the intermediate exhaust pipe 33 for connection is interposed between the upstream lean NOx catalyst 51 and the exhaust manifold 32 of the engine 1, the upstream lean NOx
The catalyst 51 can be disposed apart from the engine 1 by the length of the intermediate exhaust pipe 33, and the upstream lean NOx catalyst 51
And the NOx purification performance can be improved.

The capacity of the upstream lean NOx catalyst 51 is equal to or less than the capacity of the downstream lean NOx catalyst 53, and the wall thickness D 1 of the carrier 55 in the upstream lean NOx catalyst 51 is smaller than the capacity of the carrier 55 in the three-way catalyst 52. With the wall thickness D2 or less, the weight of the upstream lean NOx catalyst 51
Or less, the upstream lean NOx catalyst 51 having a small capacity and a small weight even when the engine 1 is cold.
Accordingly, it is possible to suppress the decrease in the exhaust gas temperature on the inlet side of the three-way catalyst 52, quickly warm up the three-way catalyst 52, and exhibit its purification performance satisfactorily.

Further, the downstream lean NOx catalyst 53
Since it contains an Ox storage material, even if HC (hydrocarbon) or CO (carbon monoxide) as a reducing agent is not sufficiently supplied due to purification by the three-way catalyst 52 on the upstream side, the effect is affected. NOx can be purified without the need.

Since the upstream lean NOx catalyst 51 is of a high temperature type in which the temperature at which the maximum purification rate is 250 ° C. or more, when high-temperature exhaust gas that cannot be purified by the downstream lean NOx catalyst 53 flows in, Upstream lean NO
The NOx purification performance can be enhanced by the x catalyst 51.

In the above embodiment, the upstream lean NO
Although the x catalyst 51 is configured to reduce and decompose NOx under lean operation, it may contain a NOx storage material. In that case, N
When the upstream lean NOx catalyst 51 storing Ox releases the stored NOx in the subsequent rich operation state,
The released NOx can be purified by the three-way catalyst 52 on the downstream side.

In the above embodiment, the distance L2 between the three-way catalyst 52 and the downstream lean NOx catalyst 53 is larger than the distance L1 between the upstream lean NOx catalyst 51 and the three-way catalyst 52. The distances L1 and L2 can be substantially the same. However, in that the exhaust gas temperature to the downstream lean NOx catalyst 53 can be effectively reduced, the three-way catalyst 52 and the downstream lean NOx catalyst
It is preferable that the distance L2 to the upstream side L3 is larger than the distance L1 between the upstream lean NOx catalyst 51 and the three-way catalyst 52.

Further, in contrast to the arrangement of the catalysts 50 to 53 of the above embodiment, one three-way catalyst may be arranged in the exhaust passage 31 on the downstream side of the downstream lean NOx catalyst 53 in a close or separated state. In this way, when the engine 1 is operated in a rich operation state at a stoichiometric air-fuel ratio or lower, NOx released from the downstream lean NOx catalyst 53 can be reduced and purified by the additional three-way catalyst.

[0053]

As described above, according to the first aspect of the present invention, a theoretical operation is provided in the exhaust passage of an engine which is switched to a lean operation state larger than the stoichiometric air-fuel ratio and a rich operation state lower than the stoichiometric air-fuel ratio. Upstream / downstream lean NOx that purifies NOx in a lean operation state greater than the air-fuel ratio
By arranging the catalyst and the three-way catalyst located between the two lean NOx catalysts, the engine switches from the lean operation state larger than the stoichiometric air-fuel ratio to the rich operation state, and the upstream lean NOx catalyst inlet When the engine returns to the original lean operation state and the NOx purification rate of the upstream lean NOx catalyst decreases after the exhaust gas temperature on the upstream side increases, NOx that cannot be purified by the upstream lean NOx catalyst is removed from the inlet side. It is possible to purify the exhaust gas with the downstream lean NOx catalyst in a state where the exhaust gas temperature has not risen so much and the NOx purification performance is high, and it is possible to improve and maintain the NOx purification performance of the exhaust gas purification device.

According to the second aspect of the present invention, the distance between the three-way catalyst and the downstream lean NOx catalyst is set to be larger than the distance between the upstream lean NOx catalyst and the three-way catalyst. , The NOx purification performance can be improved.

According to the third aspect of the present invention, since the upstream lean NOx catalyst and the three-way catalyst which are arranged at a short distance from each other are housed in the same housing, the arrangement of these two catalysts can be facilitated. it can. Further, by bringing the two catalysts close to each other, it is possible to suppress a decrease in the temperature of the exhaust gas flowing into the three-way catalyst, and to improve the exhaust gas purification performance during the rich operation.

According to the fourth aspect of the present invention, the upstream lean N
By providing an intermediate exhaust pipe for connection between the Ox catalyst and the exhaust manifold of the engine, the upstream lean N
The Ox catalyst can be arranged away from the engine, and the exhaust gas temperature on the inlet side can be lowered to further improve the NOx purification performance.

According to the fifth aspect of the present invention, the upstream lean N
By setting the capacity of the Ox catalyst to be equal to or less than the capacity of the downstream lean NOx catalyst, even when the engine is cold, the exhaust gas temperature on the inlet side of the three-way catalyst is raised to quickly warm it up, and its purification performance is good. Can be demonstrated.

According to the sixth aspect of the present invention, the upstream lean N
By making the thickness of the support wall of the Ox catalyst equal to or less than the thickness of the support wall of the three-way catalyst, the weight of the upstream lean NOx catalyst is made equal to or less than the weight of the three-way catalyst, thereby improving the purification performance of the three-way catalyst when the engine is cold. Can be demonstrated.

According to the seventh aspect of the present invention, the upstream lean N
Since the Ox catalyst contains the NOx storage material, the upstream lean NOx catalyst that stores NOx in the lean operation state of the engine releases NO in the rich operation state.
x can be purified by the downstream three-way catalyst.

According to the eighth aspect of the present invention, the downstream lean N
Since the Ox catalyst contains the NOx storage material, the purification of the three-way catalyst allows HC and C as reducing agents to be removed.
Even if O is not sufficiently supplied to the downstream lean NOx catalyst, NOx can be stably purified by the downstream lean NOx catalyst without being affected by the insufficient supply.

According to the ninth aspect of the present invention, the upstream lean N
Since the Ox catalyst is configured to reductively decompose NOx under lean operation, the upstream lean N using HC or CO as a reducing agent discharged from the engine during lean operation.
NOx can be purified by the Ox catalyst.

According to the tenth aspect, the upstream lean NOx catalyst has a maximum NOx purification rate of 250 ° C.
The high-temperature NOx reduction type described above allows the upstream lean NOx catalyst to purify NOx that cannot be sufficiently purified by the downstream lean NOx catalyst when high-temperature exhaust gas flows.

According to the eleventh aspect of the present invention, the three-way cold catalyst having a smaller capacity than the upstream lean NOx catalyst is disposed in the exhaust passage upstream of the upstream lean NOx catalyst, so that the engine can be cooled when the engine is cold. The exhaust gas can be purified by quickly warming up the cold three-way catalyst in the rich operation state.

According to the twelfth aspect of the present invention, a part of the exhaust gas is returned to the intake system.
The NOx purification performance of the Ox catalyst can be further enhanced.

According to the thirteenth aspect of the present invention, the direct injection engine for directly injecting fuel into the combustion chamber in the cylinder reduces the exhaust gas temperature by excess air, thereby further improving the NOx purification performance of the lean NOx catalyst. Can be enhanced.

[Brief description of the drawings]

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

FIG. 2 is an enlarged sectional view taken along line II-II of FIG.

FIG. 3 is an enlarged sectional view taken along the line III-III of FIG. 1;

FIG. 4 shows lean N that changes according to the operating state of the engine.
FIG. 4 is a characteristic diagram showing a temperature of an Ox catalyst, a NOx purification rate, and the like.

FIG. 5 shows a lean NOx catalyst containing a NOx storage material,
FIG. 4 is a diagram showing NOx purification characteristics of a high-temperature and low-temperature type lean NOx catalyst that reduces and decomposes NOx under lean operation.

FIG. 6 is a diagram showing a lean operation region and a rich operation region of the engine.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 2 Cylinder 6 Combustion chamber 14 Fuel injection valve 26 Intake passage 31 Exhaust passage 32 Exhaust manifold 33 Intermediate piping for connection 37 Exhaust recirculation device (Exhaust recirculation means) 41 Control unit 50 Cold three-way catalyst 51 Upstream lean NOx catalyst 52 Three-way catalyst 53 Downstream lean NOx catalyst 55 Carrier 57 Upstream housing L1 Distance between upstream lean NOx catalyst and three-way catalyst L2 Distance between three-way catalyst and downstream lean NOx catalyst D1 Carrier for upstream lean NOx catalyst Wall thickness D2 Three-way catalyst carrier wall thickness

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/28 ZAB F01N 3/28 301F 301 B01D 53/36 ZAB 101B (72) Inventor Tadataka Nakasumi Fuchu-cho, Aki-gun, Hiroshima Prefecture No. 3-1 Mazda Co., Ltd. (72) Inventor Keiji Yamada 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. (72) Akihide Takami 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Stock In company

Claims (13)

[Claims] 1. An engine according to claim 1, wherein said air-fuel ratio is greater than a stoichiometric air-fuel ratio. In an exhaust gas purifying apparatus for an engine provided with a lean NOx catalyst for purifying NOx in an operating state and a three-way catalyst, an upstream lean NOx catalyst and a three-way catalyst are sequentially arranged in the exhaust passage from upstream to downstream. Catalyst and downstream lean NOx
An exhaust gas purifying device for an engine, wherein a catalyst is provided. 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein a distance between the three-way catalyst and the downstream lean NOx catalyst is larger than a distance between the upstream lean NOx catalyst and the three-way catalyst. Exhaust gas purification device for engines. 3. The exhaust gas purifying apparatus for an engine according to claim 2, wherein the upstream lean NOx catalyst and the three-way catalyst are housed in the same housing. 4. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the upstream lean NOx catalyst is disposed downstream of the exhaust manifold of the engine, and a connection between the upstream lean NOx catalyst and the exhaust manifold is provided. An exhaust gas purifying apparatus for an engine, wherein an intermediate exhaust pipe is interposed. 5. The exhaust gas purifying apparatus for an engine according to claim 1, wherein a capacity of the upstream lean NOx catalyst is equal to or less than a capacity of the downstream lean NOx catalyst. 6. The exhaust gas purifying apparatus for an engine according to claim 1, wherein a wall thickness of the carrier in the upstream lean NOx catalyst is equal to or smaller than a wall thickness of the carrier in the three-way catalyst. . 7. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the upstream lean NOx catalyst contains a NOx storage material. 8. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the downstream lean NOx catalyst contains a NOx storage material. 9. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the upstream lean NOx catalyst reduces and decomposes NOx under lean operation. 10. The exhaust gas purifying apparatus for an engine according to claim 9, wherein the upstream lean NOx catalyst is of a high temperature type in which a temperature at which a maximum NOx purifying rate is 250 ° C. or higher. Purification device. 11. The exhaust gas purifying apparatus for an engine according to claim 1, wherein a three-way cold catalyst having a smaller capacity than the upstream lean NOx catalyst is disposed in the exhaust passage upstream of the upstream lean NOx catalyst. An exhaust gas purifying device for an engine, comprising: 12. The exhaust gas purifying apparatus for an engine according to claim 1, further comprising an exhaust gas recirculation means for recirculating a part of the exhaust gas to an intake system. 13. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the engine is a direct injection engine having a fuel injection valve for directly injecting fuel into a combustion chamber in a cylinder. apparatus.