JP2001207834A - Exhaust emission cleaning device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To feed a sufficient supply of a reducing agent for NOx cleaning to a downstream NOx catalyst when an exhaust passage carries a honeycombed NOx catalyst 22a with a low peak temperature of NOx cleaning activity on the upstream side and the above honeycombed NOx catalyst 22b with a high temperature on the downstream side. SOLUTION: The passage resistance of the upstream NOx catalyst 22a to an exhaust gas flow is made lower than that of the downstream NOx catalyst 22b, so that a larger amount of a reducing agent within the exhaust gas is guided to the downstream NOx catalyst 22b after blow-by of the upstream NOx catalyst 22a wherein it is not subjected to oxidation.

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 an apparatus for suppressing the emission of NOx (nitrogen oxide).

[0002]

2. Description of the Related Art As a catalyst for purifying an exhaust gas of an engine, HC (hydrocarbon), CO (carbon monoxide) and the NOx in the exhaust gas are simultaneously and very effectively purified in the vicinity of a stoichiometric air-fuel ratio. Possible three-way catalysts are known.
However, since the diesel engine is operated at a fairly lean air-fuel ratio (for example, A / F ≧ 18 and the oxygen concentration is 4% or more), the three-way catalyst cannot reduce NOx in the exhaust gas. When the air-fuel ratio is lean, the oxygen concentration in the exhaust gas becomes considerably high. Therefore, even with a NOx purification catalyst (for example, a catalyst in which Pt as a catalyst metal is supported on zeolite), NOx is sufficiently reduced and purified. It is difficult. This point is the same in a gasoline engine having an operating range where the air-fuel ratio is lean.

On the other hand, Japanese Unexamined Patent Publication No. Hei 9-317524 discloses that NOx is provided in a low temperature region upstream of an exhaust passage of an engine.
A first catalyst device suitable for reducing and purifying NOx is disposed, and a second catalytic device suitable for reducing and purifying NOx in a high-temperature region is disposed downstream of the first catalytic device to generate engine output near the top dead center of the compression stroke. NOx purification by performing main fuel injection during the expansion stroke or exhaust stroke, and performing post-fuel injection to increase the amount of hydrocarbon supply to the catalyst during the expansion stroke or the exhaust stroke, and then controlling the fuel injection amount according to the catalyst temperature. Is described as being performed effectively.

In other words, in view of the fact that when the amount of HC in the exhaust gas increases due to the post-fuel injection, the catalyst temperature rises due to the heat of the HC oxidation reaction in the catalyst, the NOx purifying activity of the first and second catalytic devices is considered. When the temperature is lower than the peak temperature, the post-fuel injection amount is increased and the NO
x purification is performed, and when the temperature is higher than the peak temperature, the amount of post-fuel injection is reduced to suppress a rise in the temperature of the catalyst, and to purify NOx as close to the peak temperature as possible.

[0005]

As described above, controlling the degree of increase of the reducing agent effective for NOx purification based on the catalyst temperature is effective in maintaining the catalyst at a temperature near the activity peak. However, when a plurality of catalysts are arranged in series in the exhaust passage, if the upstream catalyst oxidizes and consumes the reducing agent in the exhaust gas, the increasing amount of the reducing agent is controlled based on a predetermined standard. However, the reducing agent is not supplied to the downstream catalyst as expected. For example, NO
When a NOx catalyst having a peak x purification activity is disposed and a NOx catalyst having a peak activity in a high-temperature region is disposed downstream, the amount of the reducing agent is increased by increasing the amount of the reducing agent until the temperature of the upstream catalyst reaches an activity peak. Can be activated early, and the NOx purification rate at the peak temperature can be increased. However, even if the temperature of the upstream catalyst becomes higher than the peak temperature and the NOx purification rate decreases, the consumption rate of the reducing agent by the upstream catalyst does not decrease, so that the reducing agent is supplied to the downstream catalyst as desired. However, the effect of the increase in the reducing agent is not reflected on the NOx purification of the downstream catalyst.

On the other hand, if the amount of the catalyst metal carried on the upstream catalyst is reduced, the amount of the reducing agent in the exhaust gas that is oxidized and consumed by the upstream catalyst is reduced. Although the agent is supplied, there is a problem that the upstream side catalyst is easily deteriorated by heat. That is, when the amount of the supported catalyst metal is large, the sintering is reduced when exposed to a high temperature, and the degree of dispersion on the support material is reduced.However, if the sintering proceeds to some extent, the sintering does not proceed any further. , The catalytic activity does not decrease significantly,
When the supported amount is small, the degree of dispersion upon sintering is greatly reduced or buried in the support material even if the support material is initially highly dispersed, and the catalytic activity is greatly reduced.

In view of the above, according to the present invention, a catalyst for oxidizing and consuming a reducing agent is disposed upstream of an exhaust passage, and a catalyst reacting with the reducing agent (causing a catalytic reaction in the presence of the reducing agent) is disposed downstream thereof. In this case, the increased amount of the reducing agent passes through the upstream catalyst and is supplied to the downstream catalyst in a relatively large amount without decreasing the amount of the catalyst metal carried on the upstream catalyst. The purpose of the present invention is to make the reactivity with the reducing agent effective.

[0008]

In order to solve the above-mentioned problems, the present invention provides a honeycomb-type monolith catalyst for an upstream catalyst and a downstream catalyst, and forms an exhaust gas through the upstream catalyst so that exhaust gas can easily pass therethrough. It was done.

That is, the present invention provides an engine body and a catalyst disposed in an exhaust passage of the engine body and reacting with a reducing agent in exhaust gas, for example, a catalyst for purifying NOx in exhaust gas by reaction with the reducing agent. A downstream catalyst coated on a honeycomb-shaped monolithic carrier, a reducing agent increasing means for increasing the amount of the reducing agent, and a catalyst for oxidizing the reducing agent in a portion of the exhaust passage upstream of the downstream catalyst. An upstream catalyst formed by coating a honeycomb-shaped monolithic carrier is disposed, and the upstream catalyst is formed so as to have a shorter contact time with exhaust gas than the downstream catalyst. .

The fact that the contact time with the exhaust gas is short means that the reducing agent in the exhaust gas flows to the downstream catalyst without being oxidized by the upstream catalyst. Therefore, when the amount of the reducing agent in the exhaust gas is increased by the reducing agent increasing means, the reducing agent reaches the downstream side catalyst more, and this downstream side catalyst can be effectively used. In addition, since it is not necessary to reduce the amount of the supported catalyst metal in order to suppress the oxidation consumption of the reducing agent by the upstream catalyst, the problem of thermal deterioration of the upstream catalyst can be avoided.

In order to form the upstream catalyst such that the contact time with exhaust gas is shorter than that of the downstream catalyst, for example, the upstream catalyst has a passage cross-sectional area (cell opening area) which is smaller than that of the downstream catalyst. The sum of the areas may be large and the contact area with the exhaust gas may be small. Alternatively, the cross-sectional area of the catalyst and the number of cells may be the same, for example, the cross-sectional shape of the catalyst may be the same, and the length of the upstream catalyst may be shorter than the length of the downstream catalyst.

As the reducing agent increasing means, for example, when a fuel injection valve for directly injecting fuel into the in-cylinder combustion chamber of the engine body is provided, after the main injection for injecting fuel for obtaining the required output, By performing the post-injection of injecting the fuel in the expansion stroke or the exhaust stroke, it is possible to adopt a method of increasing the amount of HC as the reducing agent in the exhaust gas.

[0013] Alternatively, the fuel for obtaining the required output is divided into a plurality of injections near the top dead center of the compression stroke with the injection pause interval (time from the end of the previous injection to the start of the next injection) of about 50 to 1000 μs. In the case of performing the divided injection, a reducing agent increasing means is employed in which the amount of HC in the exhaust gas is increased by changing the injection mode so as to increase the number of divisions or to increase the injection pause interval. You can also.

Alternatively, it is possible to employ a reducing agent increasing means for increasing the amount of HC in the exhaust gas by retarding the timing of injecting the fuel for obtaining the required output, for example, by about 10 ° CA to 20 ° CA. it can. In that case, the pilot injection before the main injection may be executed. In this pilot injection, about 1/20 to 1/10 of the fuel injection amount for obtaining the required output is injected immediately before the main injection, specifically, before the top dead center of the compression stroke. As a result, the pressure rise in the combustion chamber due to the rise of the piston causes a fire to be formed in the combustion chamber before the main injection, and the temperature in the combustion chamber becomes considerably high (premixed combustion). Therefore, even if the main injection timing is delayed, for example, after the top dead center of the compression stroke, favorable diffusion combustion can be generated without impairing the ignition of the main injection fuel. In a gasoline engine, a reducing agent increasing means for increasing the amount of HC in exhaust gas by retarding the ignition timing can be employed.

Further, according to the present invention, in the exhaust gas purifying apparatus for an engine as described above, the catalyst for purifying NOx in the exhaust gas by a reaction with a reducing agent is coated on the honeycomb-shaped monolithic carrier. It is a catalyst.

The upstream catalyst may be a so-called three-way catalyst or an oxidation catalyst. However, if this catalyst is a NOx catalyst, it is advantageous for NOx purification. Thus, while the NOx catalyst is employed as the downstream catalyst, the temperature at which the catalytic activity relating to NOx purification of the upstream NOx catalyst peaks is lower than the temperature at which the catalytic activity relating to NOx purification of the downstream NOx catalyst peaks. If you adopt a lower one,
That is, when the upstream side is a low-temperature NOx catalyst and the downstream side is a high-temperature NOx catalyst, it is advantageous in purifying NOx by effectively utilizing both the upstream and downstream catalysts.

That is, when the engine is in a low-rotation, low-load operating state, the exhaust gas temperature is low. At this time, the exhaust gas flow rate is low, and the space velocity is low (low). Even when the above-described configuration is employed in terms of time, NOx is relatively efficiently reduced by the reducing agent in the exhaust gas in the upstream NOx catalyst.
As the engine load increases, the exhaust gas temperature increases. However, at this time, the exhaust gas flow rate increases and the space velocity increases (increases), so that the above-described blow-by easily occurs in the upstream NOx catalyst. As a result, the amount of the reducing agent in the exhaust gas flowing to the downstream NOx catalyst without being oxidized increases, and NOx is efficiently reduced by the downstream NOx catalyst.

Further, according to the present invention, in the exhaust gas purifying apparatus for an engine as described above, the reducing agent increasing means supplies fuel so as to generate afterburn from a fuel injection valve having an injection port facing a combustion chamber of the engine body. It is characterized in that the amount of the reducing agent in the exhaust gas is increased by injection in an expansion stroke or an exhaust stroke.

That is, the afterburning here means that a part of the post-injected fuel is mixed with the burned high-temperature gas and thermally decomposed or oxidized, thereby increasing the volume of the exhaust gas. Therefore, the space velocity with respect to the upstream catalyst is increased, and it is easy to blow through to the downstream catalyst, and a catalytic reaction can be caused in the downstream catalyst by effectively utilizing the increased amount of the reducing agent.

Therefore, for example, a low-temperature NOx catalyst is employed on the upstream side, a high-temperature NOx catalyst is employed on the downstream side, and the downstream high-temperature NOx catalyst exhibits a higher catalytic activity than the upstream low-temperature NOx catalyst. It is preferable to execute the above-described post-injection when the temperature becomes equal to or higher than the predetermined temperature, or to increase the post-injection amount.

That is, at low temperatures where post-injection is not performed or when the post-injection amount is small, the exhaust gas volume does not increase or is small due to after-burning. NOx can be efficiently reduced in the catalyst using the reducing agent in the exhaust gas. Thus, at high temperatures, afterburning becomes remarkable due to the execution of the postinjection or an increase in the postinjection amount,
As a result, the volume of the exhaust gas increases, and the reducing agent blows through the upstream low-temperature NOx catalyst and passes through the downstream high-temperature NOx catalyst.
A large amount of the NOx catalyst reaches the Ox catalyst, and the NOx catalyst on the downstream side having high activity can be effectively used to reduce NOx.

When the engine body is a diesel engine, post-injection for injecting fuel in the first half of the expansion stroke after main injection for injecting an amount of fuel corresponding to the required output near the top dead center of the compression stroke from the fuel injection valve. , The amount of the reducing agent in the exhaust gas may be increased.
As a result, afterburning occurs in the combustion chamber, and the exhaust gas volume can be increased.

[0023]

As described above, according to the present invention, the upstream-side catalyst and the downstream-side catalyst are honeycomb monolith catalysts, and the upstream-side catalyst has a shorter contact time with the exhaust gas than the downstream-side catalyst. Since the reducing agent increasing means is combined with such a configuration, the reducing agent in the exhaust gas flows to the downstream catalyst without being oxidized by the upstream catalyst, so that it is easy to cause blow-through, and the reducing agent increasing means reduces the amount of the reducing agent in the exhaust gas. When the amount of the reducing agent is increased, the exhaust gas can be purified by blowing the reducing agent through the downstream catalyst, and the amount of the catalyst metal carried to suppress the oxidation consumption of the reducing agent by the upstream catalyst. Therefore, the problem of thermal degradation of the upstream catalyst can be avoided.

[0024]

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

FIG. 1 shows an overall configuration of an exhaust gas purifying apparatus A for a diesel engine according to an embodiment of the present invention, and 1 is an engine body of a multi-cylinder diesel engine mounted on a vehicle. The engine body 1 has a plurality of cylinders 2 (only one is shown), and a piston 3 is reciprocally fitted into each of the cylinders 2. The combustion chamber 4 is formed. In addition, an injector (fuel injection valve) 5 is disposed substantially at the center of the upper surface of the combustion chamber 4 with the injection hole at the tip end facing the combustion chamber 4.
The injection holes are opened and closed at a predetermined injection timing for each cylinder so that fuel is directly injected into the combustion chamber 4.

Each of the injectors 5 is connected to a common common rail (accumulator) 6 for storing high-pressure fuel.
A high-pressure supply pump 8 driven by a crankshaft 7 is connected to the common rail 6. The high-pressure supply pump 8 operates so that the fuel pressure in the common rail 6 detected by the pressure sensor 6a is maintained at a predetermined value or more. A crank angle sensor 9 for detecting a rotation angle of the crank shaft 7 is provided.
Is composed of a plate to be detected (not shown) provided at the end of the crankshaft 7 and an electromagnetic pickup arranged to face the outer periphery of the plate, and the electromagnetic pickup is provided around the entire periphery of the plate to be detected. A pulse signal is output in response to the passage of the projections formed at predetermined angles.

An intake passage 10 supplies intake air (air) filtered by an air cleaner (not shown) to the combustion chamber 4 of the engine body 1. A surge tank (not shown) is provided at a downstream end of the intake passage 10. Each passage branched from the surge tank is connected to each cylinder 2 by an intake port.
Are connected to the combustion chamber 4. The surge tank is provided with an intake pressure sensor 10a for detecting a supercharging pressure supplied to each cylinder 2. The intake passage 10 includes, in order from the upstream side to the downstream side, a hot film type air flow sensor 11 for detecting a flow rate of intake air taken into the engine body 1, and a blower 12 driven by a turbine 21 to compress intake air. And an intercooler 13 for cooling intake air compressed by the blower 12 and an intake throttle valve (intake air amount adjusting means) 14 for reducing the cross-sectional area of the intake passage 10. The intake throttle valve 14 is a butterfly valve provided with a notch so that intake air can flow even in a fully closed state, and is similar to an EGR valve 24 described later.
The magnitude of the negative pressure acting on the diaphragm 15 is adjusted by an electromagnetic valve 16 for negative pressure control, whereby the opening of the valve is controlled. Further, the intake throttle valve 14 is provided with a sensor (not shown) for detecting its opening.

An exhaust passage 20 for exhausting exhaust gas from the combustion chamber 4 of each cylinder 2 is connected to the combustion chamber 4 of each cylinder 2 via an exhaust manifold. The exhaust passage 20 includes, in order from the upstream side to the downstream side, an O2 sensor 17 for detecting the oxygen concentration in the exhaust gas, a turbine 21 rotated by the exhaust gas, HC, CO, and N in the exhaust gas.
A catalytic converter 22 capable of purifying Ox is provided. Further, sensors 18 and 19 for detecting exhaust gas temperature are provided at an inlet and an outlet of the catalytic converter 22, respectively.

As shown in FIG. 2, the catalytic converter 22 has two NOx catalysts 22a and 22b arranged in series on the upstream side and the downstream side in the exhaust gas flow direction. The catalyst layer is formed on the wall surface of each cell of a cordierite monolithic carrier having a honeycomb structure having a large number of cells (through holes) extending parallel to the direction.

The monolithic carriers of the NOx catalysts 22a and 22b have the same cross-sectional area, length and wall thickness constituting each cell as a whole, but the cell 33a of the upstream NOx catalyst 22a is shown in FIG. ), The downstream NOx catalyst 22
As shown in FIG. 3B, the cell 33b of b has a larger diameter in the former cell 33a than in the latter cell 33b. Reference numeral 34a is a catalyst layer of the NOx catalyst 22a, and 34b is a catalyst layer of the NOx catalyst 22b.

That is, the NOx catalysts 22a and 22b
Although the cross-sectional area of the monolithic carrier is the same, the cell opening area per unit cross-sectional area is larger in the former than that in the latter, so that the passage cross-sectional area is larger in the former than in the latter. On the other hand, the passage resistance when the exhaust gas flows is smaller in the former than in the latter. Further, since the lengths of both monolith carriers are the same as described above, the former has a smaller contact area with the exhaust gas than the latter. Therefore, the contact time with the exhaust gas at the same exhaust gas flow rate is shorter in the former than in the latter.

As the monolith carrier of the upstream NOx catalyst 22a, the number of cells per area of 6.45 cm ² is 200 to 200.
As the monolithic carrier of the downstream NOx catalyst 22b, one having approximately 400 cells per 6.45 cm ² may be used.

Each of the catalyst layers of the NOx catalysts 22a and 22b is formed by carrying a catalyst powder obtained by carrying Pt on zeolite to dryness by a spray-drying method on a carrier with a binder. The amount of Pt carried per liter was 4 g for the former and 0.3 g for the latter.
More than 5g. For this reason, both NOx catalysts 22a,
22b is the NO when each is disposed alone in the exhaust passage.
As shown in FIG. 4, the temperature characteristics at which the catalyst activity peaks are different from each other as indicated by reference numerals 22a and 22b in the x purification temperature.
In 2b, it appears on the high temperature side. Therefore, the former N
It can be said that the Ox catalyst 22a is of a low temperature type and the latter NOx catalyst 22b is of a high temperature type.

FIG. 4 shows simulated exhaust gas having an oxygen concentration of 10% and N
The Ox purification characteristics were examined. As is clear from the figure, these catalysts 22a and 22b exhausted when the air-fuel ratio A / F was leaner than the stoichiometric air-fuel ratio (for example, A / F ≧ 18). It has a function as a NOx reduction catalyst for reducing and purifying gaseous NOx, and also functions as a three-way catalyst near the stoichiometric air-fuel ratio.

An exhaust gas recirculation passage (hereinafter referred to as an EGR passage) 23 for recirculating a part of the exhaust gas to the intake side is branched from a portion of the exhaust passage 20 upstream of the turbine 21. The end is connected to the intake passage 10 downstream of the intake throttle valve 14. Near the downstream end of the EGR passage 23, an exhaust gas recirculation amount control valve (exhaust gas recirculation amount adjusting means: hereinafter referred to as an EGR valve) whose opening degree can be adjusted.
A part of the exhaust gas is recirculated to the intake passage 10 while adjusting the flow rate of the exhaust gas in the exhaust passage 20 by the EGR valve 24.

The EGR valve 24 is of a negative pressure responsive type, and a negative pressure passage 27 is connected to a negative pressure chamber of the valve box. The negative pressure passage 27 is connected to a vacuum pump (negative pressure source) 29 via a negative pressure control electromagnetic valve 28. The negative pressure passage 27 is controlled by a control signal (current) from an ECU 35 described later. By opening and closing 27, the negative pressure for driving the EGR valve in the negative pressure chamber is adjusted, whereby the opening of the EGR passage 23 is linearly adjusted.

The turbocharger 25 is a VGT (Variable Geometry Turbo), to which a diaphragm 30 is attached, and a solenoid valve 3 for negative pressure control.
By adjusting the negative pressure acting on the diaphragm 30 by 1, the cross-sectional area of the exhaust gas passage is adjusted.

Each of the injectors 5, high-pressure supply pump 8, intake throttle valve 14, EGR valve 24, turbocharger 25
And the like are configured to be operated by a control signal from a control unit (Engine Control Unit: hereinafter referred to as ECU) 35. On the other hand, the ECU 35 receives an output signal from the pressure sensor 6a, an output signal from the crank angle sensor 9, an output signal from the pressure sensor 10a, an output signal from the air flow sensor 11, and an output signal from the O2 sensor 17. An output signal, an output signal from the temperature sensors 18 and 19, an output signal from the lift sensor 26 of the EGR valve 24, and an accelerator opening for detecting an operation amount (accelerator opening) of an accelerator pedal (not shown) by a driver of the vehicle. At least an output signal from the degree sensor 32 is input.

The fuel injection amount and the fuel injection timing by the injector 5 depend on the operating state of the engine body 1 and the NO.
x is controlled according to the state of the catalysts 22a and 22b, and the common rail pressure by the operation of the high-pressure supply pump 8,
That is, the fuel injection pressure is controlled, and in addition, the intake air amount is controlled by operating the intake throttle valve 14, the exhaust gas recirculation amount is controlled by operating the EGR valve 24, and the turbocharger 2 is controlled.
5 (VGT control).

(Fuel Injection Control) The ECU 35 stores in the memory a fuel injection amount map in which the optimum fuel injection amount Qb experimentally determined according to changes in the target torque and the rotation speed of the engine body 1 is recorded. Electronically stored and provided. Then, based on the target torque obtained based on the output signal from the accelerator opening sensor 32 and the engine speed obtained based on the output signal from the crank angle sensor 9, the main injection amount Qb is obtained from the fuel injection amount map. Is read, and the excitation time (valve opening time) of each injector 5 is determined based on the main injection amount Qb and the common rail pressure detected by the pressure sensor 6a.
By this main fuel injection control, an amount of fuel corresponding to the target torque of the engine body 1 is supplied, and the engine body 1
Is operated in a state where the average air-fuel ratio in the combustion chamber 4 is considerably lean (A / F ≧ 18).

During normal operation (when the change in accelerator opening is small), the NOx catalyst 22
In order to supply a reducing agent component for promoting NOx reduction purification to the fuel tanks a and 22b, a small amount of fuel (for example, main fuel) is added during the retardation of the main injection timing and the expansion stroke or the exhaust stroke after the main injection (main fuel injection). The post-injection (injection amount of several% or less) is appropriately performed according to the temperature of the NOx catalysts 22a and 22b.

That is, as shown schematically in FIG. 5, the fuel injection control at the time of the steady operation is performed, the catalyst temperature (the catalytic converter 2
2) T is the temperature T1 (T1 is the upstream NOx catalyst 2).
2a), the pilot injection and the main injection timing are retarded until the temperature reaches the temperature T1 to T1.
At T3 (T3 is a temperature at which the NOx purification rate of the downstream NOx catalyst 22b exceeds a peak and decreases by a predetermined amount), post-injection is performed.

The amount of post-injection is between the temperatures T1 and T2 (T2 is the value when the sufficient amount of reducing agent is supplied to the downstream NOx catalyst 22b.
2a), the temperature T is set to decrease as the catalyst temperature T increases in the latter half of the period (temperature at which the NOx purification rate becomes higher than 2a)
At the temperature T3, the post-injection amount is set to zero so that the latter decreases between 2 and T3 as the catalyst temperature T increases. The reason why the post-injection amount at the temperature T2 is larger than that at the temperature T1 is that HC is oxidized and consumed by the upstream low-temperature NOx catalyst 22a.
This is to supply as much HC as possible to the Ox catalyst 22b.

Hereinafter, the contents of the control will be specifically described based on the control flow shown in FIG. This control is executed at every predetermined crank angle.

First, in step S1 after the start, a crank angle signal, an air flow sensor output, an accelerator opening, a temperature sensor output, and the like are read. Subsequent step S2
, The main injection amount Qb is read from the fuel injection amount map based on the target torque obtained from the accelerator opening and the engine speed obtained from the crank angle signal. The fuel injection amount map records the optimum injection amount Qb experimentally determined according to changes in the accelerator opening and the engine speed. The main injection amount Qb is determined as the accelerator opening increases and the engine It is set to increase as the rotation speed increases.

The main injection timing Ib is set near the top dead center of the compression stroke. For example, with BTDC 5 ° CA (crank angle) as a reference, the more the injection amount Qb, the more the advance, and the smaller the injection amount Qb, the more the retard. Is done. Further, based on the engine water temperature, when the water temperature is low, the main injection timing Ib is retarded by a predetermined amount and the warm-up operation is performed.

In the following step S3, the catalyst temperature T is estimated based on the outputs of the temperature sensors 18 and 19 on the inlet and outlet sides. That is, the average value of the exhaust gas temperatures detected by the temperature sensors 18 and 19 is estimated as the catalyst temperature T. The catalyst temperature T may be estimated based on the current operating state and operating history of the engine.
The estimation may be made based on the operation state and the operation history and the exhaust gas temperature on the inlet side or the exhaust gas temperature on the outlet side. Further, a temperature sensor is provided inside the converter 22 to detect the catalyst temperature T. You may make it.

In the following step S4, it is determined whether or not the catalyst temperature T is lower than the temperature T1. When the temperature is lower, the routine proceeds to step S5, where the pilot injection amount Qe and its injection timing Ie are set, and the main injection timing Ib is set. The retard is performed by a predetermined amount d. In step S4, the catalyst temperature T
Is determined to be equal to or higher than the temperature T1, step S
The program proceeds to 6, where the post-injection amount Qp and its injection timing Ip are set based on the catalyst temperature T. For the post-injection amount Qp, a table corresponding to the characteristic shown in FIG. 5 is created in advance, electronically stored in a memory, and read from the table. That is,
The post-injection amount Qp is the catalyst temperature T in the latter half between the temperatures T1 and T2.
Is set to be zero at the temperature T3 so that the catalyst temperature T decreases as the catalyst temperature T increases even in the latter half between the temperatures T2 and T3 so that the temperature decreases. The post-injection timing Ip is, for example, 90 ° CA from the top dead center of the compression stroke.

After setting the pilot injection or the post-injection based on the catalyst temperature T, the process proceeds to step S7,
If the pilot injection amount Qe is set, pilot injection is performed when the pilot injection timing Ie has been reached (steps S8 and S9), and main injection is performed when the main injection timing Ib has been reached (steps S10 and S11). Then, when the post-injection is set, the post-injection is performed when the post-injection timing Ip is reached after the main injection has been performed for the cylinder in which the post-injection is to be performed (steps S12 to S14).

The post-injection may be performed every time the main injection is performed on all cylinders. However, when the main injection is performed on each cylinder in a predetermined order, for example, once every five main injections You may make it perform thinning out, such as performing injection. For example, when there are four cylinders A, B, C, and D and main injection is performed in this order, after performing post-injection on cylinder A first, the following cylinders B, C, D, and A The post-injection is not performed (thinning), and the post-injection is performed for the next B cylinder.

Therefore, if the fuel injection control is performed as described above, the temperature is reduced to the temperature T1, that is, the low-temperature type N
Until the NOx purification activity of the Ox catalyst 22a reaches a peak, the amount of HC as a reducing agent in the exhaust gas increases due to the retard of the main injection timing Ib. In this case, the main injection timing I
Since the amount of HC is increased by the retard of b, there is no large increase in the exhaust gas volume. Therefore, since the space velocity is not so high in the upstream low-temperature NOx catalyst 22a, the amount of HC flowing through the low-temperature NOx catalyst 22a is small. Therefore, in the low-temperature NOx catalyst 22a, reduction and purification of NOx by HC easily proceed, whereby early activation of the NOx catalyst 22a is achieved, and unpurified N2 is reduced.
The amount of Ox emitted into the atmosphere is reduced.

During the period from the temperature T1 to the temperature T2, that is, when the NOx purification activity of the upstream low-temperature NOx catalyst 22a is higher than that of the downstream high-temperature NOx catalyst 22b even when the peak temperature T1 is exceeded, The amount of HC in the exhaust gas increases by the post-injection. Also in this case, since the degree of increase in HC is low, there is no large increase in the exhaust gas volume due to afterburning. Therefore, the reduction and purification of NOx by the increased HC in the low-temperature NOx catalyst 22a proceeds efficiently, and the amount of unpurified NOx discharged into the atmosphere decreases. Further, in this case, since the post-injection amount decreases as the catalyst temperature T increases, it is avoided that the temperature of the low-temperature NOx catalyst 22a increases due to the heat of oxidation reaction of HC and the activity thereof sharply decreases.

When the temperature exceeds T2, that is, when the NOx purification activity of the high-temperature NOx catalyst 22b downstream of the low-temperature NOx catalyst 22a on the upstream side becomes higher, the post-injection amount increases. The volume increase amount increases. In addition, the upstream low-temperature NOx catalyst 22
“a” has a small passage resistance when the exhaust gas flows. Therefore, the HC in the exhaust gas blows through without being oxidized and consumed by the upstream NOx catalyst 22a, and flows into the downstream high-temperature NOx catalyst 2a.
2b. As a result, NOx purification in the high-temperature NOx catalyst 22b proceeds efficiently, and unpurified NOx
Is reduced into the atmosphere. Further, when the catalyst temperature further rises beyond the activation peak temperature of the high-temperature NOx catalyst 22b, the post-injection amount decreases. Therefore, the temperature of the high-temperature NOx catalyst 22b becomes too high due to the heat of the oxidation reaction of HC. Thus, a rapid decrease in the activity can be avoided.

In the above embodiment, the main injection timing retard and the post-injection are employed as means for increasing the reducing agent. However, multi-stage injection (split injection) is employed as the main injection, and at least the number of injections and the injection timing are determined. One of them may be changed.

In the multi-stage injection, the main injection fuel is divided into a plurality of injections so as to continue the combustion of the fuel in the combustion chamber near the top dead center of the compression stroke as illustrated in FIG. It is. The valve opening time of each injection is 800μ
It is preferable that the injection suspension time (the time from when the injection hole of the injector 5 is closed to when it is next opened) Δt be 50 to 1000 μs or less. The second injection is preferably performed after the top dead center of the compression stroke. The basic operation of this split injection is as follows.

The fuel injected from the injection hole of the injector 5 spreads into the combustion chamber 4 while forming a spray having a conical shape as a whole, and repeatedly breaks up into small oil droplets by friction with air. The fuel evaporates from the surface of the fuel cell to generate fuel vapor. At that time, since the fuel is divided and injected, the ratio of the premixed combustion by the initially injected fuel is relatively reduced, so that the combustion pressure and the combustion temperature do not excessively increase in the early stage of the combustion. , NOx generation is reduced.

Since the injection suspension time Δt is set to 50 μsec or more, the fuel oil droplets injected later hardly catch up with the fuel oil droplets injected earlier. In particular, if the second injection is performed after the top dead center of the compression stroke, the fuel injected in the second injection immediately burns, the pressure in the combustion chamber 4 increases greatly, and the viscosity of the compressed air increases. The oil droplets of the injected fuel are immediately decelerated and do not catch up with the oil droplets of the previously injected fuel. Since the valve opening time of each time is set to approximately 800 μs or less, the fuel injection amount of each time is small,
Since the recombination of oil droplets during the fuel spray is also minimized, for example, by increasing the fuel pressure to increase the ejection speed of the fuel, the atomization of the fuel and, consequently, the vaporization and atomization are sufficiently promoted. Thus, the mixing state of the fuel vapor and the air can be greatly improved. Since the injection stop time Δt is set to 1000 μs or less, the fuel injected by each injection is not interrupted so that the next injected fuel starts burning before the combustion of the previously injected fuel ends. Burned.

In short, by performing the main injection in a divided manner, the combustion state of the injected fuel can be made extremely good, and the improvement of fuel consumption and suppression of smoke generation can be realized. Further, although the injection end timing is relatively late, the fuel intermittently injected during that period is well vaporized and atomized and diffusely burns as described above. The combustion state is not deteriorated, but rather, the pressure of the combustion chamber 4 is maintained at a high level for a relatively long time, so that the expansion force of the combustion gas is transmitted to the piston 3 very effectively. Fuel efficiency can also be improved by improving efficiency.

Thus, in the case of the multi-stage injection, the HC amount in the exhaust gas increases as the number of divisions increases as compared with the case where the fuel is injected at a time, and the exhaust gas in the exhaust gas increases as the injection pause time Δt increases. Increases the amount of HC.

FIG. 8 shows the results of an investigation on the effects of the number of divisions of the main injection and the injection suspension time Δt on the amount of HC in the exhaust gas. This is the case in which a quantity of fuel corresponding to the target torque of the engine 1 is collectively injected from the top dead center of the approximately compression stroke (hereinafter, referred to as collective injection), and the fuel is injected in two equal parts (hereinafter, referred to as 2 For each of three equal injections (hereinafter referred to as split injection) (hereinafter referred to as three-split injection), the injection pause time Δt is changed, and the crank angle at the end of injection and the exhaust gas HC in
It is a study of the relationship with quantity. In two-split injection, Δt
= 350, 400, 700, 900 μsec, and in the three-split injection, Δt = 400, 550, 700,
It examined about 900 microseconds.

According to the figure, the batch injection has a larger amount of HC in the exhaust gas than the split injection, and the split injection has a larger HC amount in the exhaust gas in the three-split injection than in the two-split injection. In addition, in the two-split injection and the three-split injection, the longer the injection stop time Δt, the larger the amount of HC in the exhaust gas.

Therefore, when the catalyst temperature T is lower than T1, two-split injection is performed with Δt set to 400 μsec, for example.
When the catalyst temperature T becomes T1 to T2, a split injection in which the HC amount is increased by at least one of the increase in the number of divisions and the increase in Δt is performed, and when the catalyst temperature T rises to T2 or more, the final stage of the division is performed. The injection amount may be increased. As a result, a large amount of HC can be made to flow to the downstream NOx catalyst 22b by increasing the exhaust gas volume by making the afterburn remarkable, and the catalyst can be effectively used for NOx purification.

In the above embodiment, the fuel injection control is performed by giving the catalyst temperature T as the temperature of the upstream NOx catalyst 22a and the temperature of the downstream NOx catalyst 22b. However, the upstream NOx catalyst 22a And NO on the downstream side
Control may be performed by separately detecting or estimating the temperature of each of the x catalysts 22b.

When the catalyst temperature T is rising, the control state is switched by using a temperature slightly higher than T1 and T2 as a threshold value, and when the catalyst temperature T is falling, the control state is switched from T1 and T2. It is preferable to switch the control state using a slightly lower temperature as a threshold value in order to prevent hunting of control.

In the above embodiment, a NOx reduction catalyst for purifying NOx in an oxygen-excess atmosphere was employed as the downstream catalyst. However, NOx was absorbed in an oxygen-excess atmosphere, that is, the engine was operated at a lean air-fuel ratio. When NOx in the exhaust gas is absorbed and the oxygen concentration in the atmosphere decreases, for example, the NOx is released when operated near the stoichiometric air-fuel ratio, and the NOx is reduced and purified by the reducing agent passing through the upstream catalyst. The absorption catalyst may be a downstream catalyst.

A temperature sensor is arranged downstream of the downstream catalyst. When a reducing agent is supplied to the downstream catalyst, an oxidation reaction (catalytic reaction) occurs and the temperature of the catalyst rises as expected. By monitoring whether or not the function of the downstream catalyst as an oxidation catalyst has been deteriorated, it is possible to diagnose whether or not the reducing agent is used at the time of the diagnosis, as in the above-described embodiment. You may make it pass through an upstream catalyst and supply it to a downstream catalyst.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of an exhaust gas purification device for a diesel engine according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a catalytic converter.

FIG. 3 is a sectional view showing a cell structure of an upstream NOx catalyst and a downstream NOx catalyst.

FIG. 4 is a graph showing the relationship between the exhaust gas temperature and the NOx purification rate of a low-temperature NOx catalyst and a high-temperature NOx catalyst.

FIG. 5 is a graph showing the relationship between catalyst temperature and reducing agent increase control.

FIG. 6 is a flowchart showing a processing procedure of fuel injection control.

FIG. 7 is a time chart showing injection timings of fuel batch injection and multi-stage injection.

FIG. 8 is a graph showing the effect of the number of divisions of main injection and the injection suspension time Δt on the amount of HC in exhaust gas.

[Explanation of symbols]

 A Exhaust gas purification device 1 Diesel engine 2 Cylinder 4 Combustion chamber 5 Injector (fuel injection valve) 20 Exhaust passage 22 Catalytic converter 22a Upstream NOx catalyst 22b Downstream NOx catalyst 34a Catalyst layer of upstream NOx catalyst 34b Catalyst of downstream NOx catalyst Layer 35 ECU (control unit)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) F02D 41/38 B01D 53/36 B (72) Inventor Takahiro Kurokawa 3-1, Fuchu-cho, Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. (72) Inventor Tomomi Watanabe 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima F-term (reference) 3G091 AA02 AA10 AA11 AA18 AA28 AB02 AB03 AB05 AB06 AB09 BA01 BA03 BA04 BA10 BA14 BA15 BA19 BA32 BA33 CA13 CA18 CB02 CB03 CB06 CB07 CB08 DA01 DA02 DA03 DA05 DB10 DB13 EA00 EA01 EA05 EA06 EA07 EA16 EA17 EA18 EA21 EA23 EA30 EA31 EA34 FA02 FA04 FA12 FA13 HA14 FA11 FB02 FB03 FC07 FC10 HA37 HA38 HA39 HA42 HA47 HB03 HB05 HB06 3G301 HA01 HA02 JA25 MA11 MA18 MA23 MA26 NE01 PA04Z PA07Z PA11Z PB08Z PE01Z PE03Z 4D048 AA06 AB0 2 AB03 AC02 BB02 CA01 CC32 CC38 CC45 CC48 DA01 DA02 DA08

Claims (5)

[Claims] 1. An engine body, a downstream catalyst in which a catalyst arranged in an exhaust passage of the engine body and reacting with a reducing agent in exhaust gas is coated on a honeycomb monolithic carrier, and an amount of the reducing agent is increased. Reducing agent increasing means, at a portion of the exhaust passage upstream of the downstream catalyst,
An upstream catalyst formed by coating a catalyst for oxidizing the reducing agent on a honeycomb-shaped monolithic carrier is disposed, and the upstream catalyst is formed so as to have a shorter contact time with exhaust gas than the downstream catalyst. An exhaust gas purification device for an engine, comprising: 2. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the upstream catalyst has a larger passage sectional area and a smaller contact area with exhaust gas than the downstream catalyst. Engine exhaust purification device. 3. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the upstream side catalyst has a catalyst for purifying NOx in exhaust gas by a reaction with a reducing agent coated on a honeycomb monolithic carrier. An exhaust gas purification device for an engine. 4. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the reducing agent increasing means is provided after a fuel injection valve having an injection port facing a combustion chamber of the engine body. An exhaust gas purifying apparatus for an engine, wherein a reducing agent in exhaust gas is increased by injecting fuel in an expansion stroke or an exhaust stroke so as to cause burning. 5. The exhaust gas purifying apparatus for an engine according to claim 4, wherein the engine body is a diesel engine, and the reducing agent increasing means meets a required output near a top dead center of a compression stroke from the fuel injection valve. An exhaust gas purifying apparatus for an engine, wherein the amount of reducing agent in exhaust gas is increased by injecting fuel in the first half of an expansion stroke after an amount of fuel has been injected.