JPH11210525A - Exhaust emission purification device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve fuel consumption while maintaining good NOx purifying efficiency by gradually varying an air-fuel ratio according to a NOx release characteristic in a NOx occlusion catalyst in a rich spike time. SOLUTION: When an internal combustion is operated under layer combustion based on a lean air-fuel ratio, NOx in the exhaust is adsorbed and retained in a NOx occlusion catalyst 7 so as to be prevented from being released to the outside. In this process, an adsorption retaining quantity in the NOx occlusion catalyst 7 is estimated by means of a controller 10 from the integrated value of the NOx discharged quantity, and if it is determined that the adsorption retaining quantity reaches the predetermined value around the limit retaining ability, a fuel increment quantity is corrected, and rich spike control for the air-fuel ratio is carried out. In this way, a reduction environment is obtained, and NOx adsorbed and retained in the catalyst 7 is released and reduced. An initial value of the air-fuel ratio in the rich spike control time is maintained at a fixed value set according to the NOx adsorption retaining quantity and the like, and after a lapse of a predetermined time, fuel increment is controlled so as to be reduced in compliance with reduction of the NOx release quantity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal combustion engine provided with a NOx storage catalyst that adsorbs and holds NOx in exhaust gas during operation at a lean air-fuel ratio and desorbs and reduces NOx when switching to a stoichiometric or rich air-fuel ratio. An exhaust purification device.

[0002]

2. Description of the Related Art As disclosed in JP-A-6-10725, a NOx storage catalyst is provided in an exhaust system in order to reduce NOx contained in exhaust gas of an internal combustion engine operated at a lean air-fuel ratio. It has been known.

The NOx storage catalyst adsorbs and holds NOx contained in exhaust gas during operation at a lean air-fuel ratio, and desorbs and reduces NOx adsorbed and held when the air-fuel ratio is switched to rich. However, unlike the conventional three-way catalyst which exerts the NOx reducing action only at the stoichiometric air-fuel ratio, there is an advantage that the emission of NOx to the outside can be prevented even at the lean air-fuel ratio.

In the NOx storage catalyst, when the amount of NOx adsorbed and held reaches a certain saturated value, NOx is directly discharged. Therefore, the air-fuel ratio is temporarily switched to rich immediately before the saturated state (this is called a rich spike). ), It is necessary to carry out desorption reduction of the retained NOx to regenerate the catalyst. However, since the rich spike only unnecessarily increases the air-fuel ratio for the internal combustion engine, deterioration of fuel efficiency is unavoidable.Therefore, when the NOx storage catalyst is regenerated, the air-fuel ratio is not excessively increased. You have to do an efficient rich spike.

[0005] In the above-mentioned conventional example, the Nx
Focusing on the fact that the rate of Ox desorption reaction is high when the temperature of the catalyst is high and low when the temperature of the catalyst is low, the concentration of the rich spike (air-fuel ratio) and the maintenance time thereof are controlled in accordance with the catalyst temperature.

[0006]

Looking at the NOx desorption characteristics of the NOx storage catalyst at the time of regeneration, it is found that the maximum value is obtained immediately after the rich spike, but thereafter it gradually decreases. I have. In the NOx desorption reaction, excess HC, CO, and the like in the exhaust gas generated by making the concentration higher than the stoichiometric air-fuel ratio are used. However, immediately after the reaction, even if almost all of the supplied HC is used for the desorption of NOx, thereafter, as the desorption amount of NOx decreases, the supplied HC becomes excessive. In this case, in this case, the HC is supplied in vain, and the fuel efficiency deteriorates accordingly.

However, according to the conventional rich spike control, when the catalyst temperature is high, the air-fuel ratio is high and the maintenance time is short, and when the catalyst temperature is low, the air-fuel ratio is low (but higher than the stoichiometric air-fuel ratio). Although the time is long,
Since the air-fuel ratio during this period is maintained at a constant value, even if HC can be supplied immediately after the reaction without excess or deficiency, HC is excessively supplied for most of the period until the reaction is completed thereafter. As a result, this causes the fuel efficiency to deteriorate.

The present invention has been proposed in order to solve such a problem, and the purification efficiency of NOx is improved by gradually changing the air-fuel ratio in accordance with the NOx desorption characteristics of the NOx storage catalyst during a rich spike. The aim is to improve fuel economy while maintaining good performance.

[0009]

A first aspect of the present invention is a NOx storage catalyst that adsorbs and holds NOx in exhaust gas during a lean air-fuel ratio operation and desorbs and reduces NOx adsorbed and held during a rich air-fuel ratio operation. A means for estimating the amount of NOx adsorbed and held by the NOx storage catalyst during the lean air-fuel ratio operation, and temporarily enriching the air-fuel ratio when the adsorbed amount reaches a predetermined value to obtain NOx Rich control means for desorbing and reducing NOx. The rich control means maintains a substantially constant initial value of the rich air-fuel ratio for a predetermined time from the start of the rich control, and thereafter sets the air-fuel ratio according to the NOx desorption characteristics. Control is performed so that the degree of concentration of the fuel ratio is gradually reduced.

[0010] In a second aspect based on the first aspect, the rich control means gradually reduces the degree of enrichment of the air-fuel ratio in a quadratic or exponential manner.

In a third aspect based on the first or second aspect, the rich control means determines the initial value of the rich air-fuel ratio in accordance with the NOx adsorption holding amount and the intake air amount at the start of the rich control.

In a fourth aspect based on the first to third aspects, the rich control means corrects an initial value of the rich air-fuel ratio in accordance with a temperature representative of the temperature of the NOx storage catalyst at the start of the rich control. .

In a fifth aspect based on the first to fourth aspects, the rich control means corrects an initial value of the rich air-fuel ratio in accordance with the degree of deterioration of the NOx storage catalyst at the time of starting rich control.

[0014] In a sixth aspect based on the first to fifth aspects, the rich control means is configured to control N on the downstream side of the NOx storage catalyst.
The degree of enrichment of the air-fuel ratio is reduced according to the Ox concentration.

[0015]

In the first and second aspects of the invention, NO
NOx adsorbed and held by the x storage catalyst during the lean operation is desorbed from the catalyst and reduced by switching the air-fuel ratio to the rich side. During the desorption and reduction reactions of the NOx storage catalyst, HC or the like corresponding to the rich air-fuel ratio becomes NOx.
Used for the elimination reduction of The amount of desorption and reduction of NOx takes the maximum value at the start of the regeneration and then gradually decreases. The rich control means enriches the air-fuel ratio to a predetermined state at the beginning of regeneration, but after a predetermined time has elapsed, the degree of enrichment is gradually reduced according to the NOx desorption characteristics. N
The amount of HC required for the desorption reduction of Ox will be supplied without excess and deficiency, and the desorption of NOx at the time of regeneration will be incomplete, or the fuel consumption will deteriorate due to the excessive supply of HC. Can be avoided reliably.

In the third aspect, the initial value of the rich air-fuel ratio is determined according to the NOx holding amount at that time.
An appropriate amount of HC can be supplied. Further, even if the air-fuel ratio is the same, the supply amount (absolute amount) of HC changes in accordance with the intake air amount. For example, the supply amount of HC increases as the intake air amount increases. Therefore, optimal regeneration control can be performed by lowering the air-fuel ratio enrichment initial value as the intake air amount increases.

In the fourth aspect, the initial value of the rich air-fuel ratio is corrected according to the temperature of the NOx storage catalyst. The higher the temperature, the denser the air-fuel ratio required for the NOx desorption reaction of the catalyst.
Also, the maintenance time is shortened. Therefore, by setting the initial value of the air-fuel ratio according to the catalyst temperature in this way, optimal regeneration can be performed.

Further, in the fifth invention, the initial value of the rich air-fuel ratio is corrected according to the degree of deterioration of the NOx storage catalyst. The more the catalyst deteriorates, the lower the NOx adsorbing / holding capacity is, and the smaller the air-fuel ratio concentration required during regeneration is. Therefore, by setting the initial value of the air-fuel ratio in accordance with the degree of deterioration in this way, HC can be supplied without excess or deficiency in accordance with the condition of the catalyst, and unnecessary deterioration of fuel efficiency can be avoided.

In the sixth invention, the degree of enrichment of the air-fuel ratio is controlled in accordance with the NOx concentration downstream of the NOx storage catalyst during catalyst regeneration. In this case, by adjusting the air-fuel ratio while detecting the actual state of desorption and reduction of NOx,
It is possible to supply HC most efficiently, and NO
The best control for x purification and fuel efficiency can be performed.

[0020]

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

In FIG. 1, 1 is an engine main body, 2 is an intake passage, 3 is an exhaust passage, and a combustion chamber 4 is provided with a fuel injector 5 for directly injecting fuel and an air-fuel mixture containing the injected fuel. An ignition plug 6 for igniting is provided.

Fuel is injected from the fuel injector 5 in the latter half of the compression stroke such as when the engine is partially loaded, and a combustible air-fuel mixture layer is formed and maintained near the ignition plug. , Realizes stable stratified combustion. The mixture is switched to the stoichiometric air-fuel ratio, such as when the engine is under heavy load,
At this time, the fuel injection timing shifts to the intake stroke, a homogeneous air-fuel mixture having a stoichiometric air-fuel ratio is formed, and normal premixed combustion is performed.

The exhaust passage 3 contains N in the exhaust during the lean operation.
A NOx storage catalyst 7 that adsorbs and holds Ox is provided. When the adsorption holding amount of the NOx storage catalyst 7 reaches a predetermined state, the air-fuel ratio is temporarily switched from lean to rich, that is, a rich spike is performed and the held NOx is held.
The controller 10 switches and controls the amount of fuel injected from the fuel injector 5 in order to desorb and reduce the fuel and regenerate the catalyst.

In the drawing, reference numeral 8 denotes an exhaust gas recirculation device for recirculating a part of the exhaust gas into the intake air.

The control device 10 determines NO according to operating conditions.
The amount of NOx adsorbed and held by the x-storage catalyst 7 is predicted, and a rich spike is performed at a predetermined timing based on the predicted amount, and the air-fuel ratio at this time is gradually changed in accordance with the NOx reduction characteristics to reduce the fuel consumption. The NOx purification efficiency is controlled optimally without deterioration.

For this reason, the control device 10 includes the intake passage 2
Operating conditions from a throttle opening sensor 11 for detecting a throttle valve opening, an air flow meter 12 for measuring an intake air amount, a crank angle sensor 13 for detecting a crank angle, a water temperature sensor 14 for detecting a cooling water temperature, and the like. An air-fuel ratio sensor 1 for receiving a signal and further detecting an exhaust air-fuel ratio upstream of the NOx storage catalyst 7 in the exhaust passage 3
5 and an exhaust gas temperature sensor 1 for detecting the exhaust gas temperature immediately before the catalyst
6, a catalyst temperature sensor 17 for detecting a temperature representative of the catalyst temperature, and a NO for detecting a NOx concentration downstream of the catalyst.
A signal from the x sensor 18 or the like is input.

The above-described control executed by the control device 10 will be described in detail with reference to the flowchart of FIG.

First, at step S1, the engine speed N
e, intake air amount Q, exhaust gas temperature Ta, catalyst temperature Tc,
The cooling water temperature Tw, the NOx concentration, etc. are read, and step S
In step 2, a rich spike permission determination is made. This permission determination is based on the integration of the NOx emission amount per unit time during the lean operation, whether or not the NOx storage amount of the NOx storage catalyst has reached a predetermined value that has some margin even by the limit capacity during the lean operation. The determination is made based on a value or the like. Only when it is determined that the predetermined value has been reached, the process proceeds to step S3.

In step S3, the NOx adsorption holding amount Nmas at that time is obtained by calculation, and then in step S4, the initial value K0 of the fuel increase correction coefficient for the rich spike.
To determine. Referring to a map such as that shown in FIG. 3, a value of K0> 1.0 is calculated based on the intake air amount and the NOx adsorption holding amount at that time.

In this case, the initial value K0 of the fuel increase correction coefficient
Is set so that the air-fuel ratio becomes denser as the adsorption holding amount of NOx increases, and when the intake air amount is large, the absolute amount of the supply amount of HC increases even with the same change amount of the air-fuel ratio. The air-fuel ratio is set so as to decrease as the intake air amount increases.

In step S5, a response delay time T0 of the NOx desorption reaction at the start of the rich spike is calculated based on, for example, the catalyst temperature and the exhaust gas temperature.

In step S6, the fuel injection pulse width T
i is calculated as in the following equation.

Ti = Tp * α * K0 (T0) + Ts where Tp is a basic fuel injection pulse width calculated based on the intake air amount and the rotation speed, and α is an air-fuel ratio feedback determined based on the output of the air-fuel ratio sensor. The correction coefficient is clamped to a constant α <1.0 during lean operation and α = 1.0 during rich spike. Ts is an invalid fuel pulse width.

Then, in step S7, an increased fuel injection, that is, a rich spike, is started based on the fuel injection pulse width Ti. At this time, the air-fuel ratio becomes an air-fuel ratio which is higher than the stoichiometric air-fuel ratio by a predetermined value, and
The desorption reaction of NOx by the Ox absorption catalyst is started.

In step S8, this fuel increase correction coefficient K
Elapsed time Tn from the start of the increase correction based on 0
Is compared with the delay time T0. Until the delay time T0 elapses, the process returns to step S6, and the same increase correction value (air-fuel ratio) is maintained.

When the delay time T0 has elapsed, the process proceeds to step S9, and a fuel increase correction coefficient KnTn is calculated, for example, by the following equation.

Kn (Tn) = C * [2 * K (n-1) ²
(T (n-1))-K (n-2) ² (T (n-2))]
^1/2 , where Kn = K0 when the delay time T0 has elapsed, and C is a constant, n ≧ 2 (integer).

The fuel increase correction coefficient Kn (Tn) gradually and quadratically decreases with the passage of time in accordance with the NOx desorption characteristics from the catalyst. In this way, the calculation can be performed exponentially.

Kn (Tn) = K (n−1) (T (n−1)) / e ^A *
^{T (n-1)} where A is a constant and n ≧ 1 (integer).

Once the fuel increase correction amount Kn (Tn) that changes with the passage of time is obtained in this way, in step S10, the fuel injection pulse width Ti
Is calculated as in the following equation.

Ti = Tp * α * Kn (Tn) + Ts In step S11, the increased fuel (air-fuel ratio) is corrected by this Ti, and in step S12, Kn (T
It is determined whether n) = 1, and the process returns to step S9 until Kn (Tn) = 1, and the fuel injection amount is gradually reduced.

Then, the fuel increase correction coefficient Kn (Tn) =
When the value becomes 1, the process proceeds to step S13, in which the rich spike is terminated assuming that the catalyst regeneration has been completed, and the lean air-fuel ratio is returned to the lean air-fuel ratio. In this state, the air-fuel ratio correction coefficient α is 1.
The constant becomes 0 or less, and the air-fuel ratio becomes thinner than the stoichiometric air-fuel ratio.

Next, the overall operation will be described with reference to FIG.

During operation of the internal combustion engine in which stratified combustion is performed at a lean air-fuel ratio, NOx in the exhaust gas is adsorbed and held by the NOx storage catalyst 7, and emission to the outside is prevented. From the integrated value of the NOx emission amount in accordance with the operating conditions, the adsorption holding amount in the NOx storage catalyst 7 is predicted, and when it is determined that this reaches a predetermined value near the limit holding capacity, the fuel increase correction is performed. Thus, rich spike control of the air-fuel ratio is performed. The HC in the exhaust gas increases due to the rich spike, and a so-called reducing atmosphere is formed, so that NOx adsorbed and held by the NOx storage catalyst 7 is released and reduced.

The initial value of the air-fuel ratio during the rich spike control is a constant value (maximum value) set in accordance with the amount of NOx adsorbed and held, the amount of intake air at that time, and the like.
Supplying HC necessary for desorption of NOx in the Ox storage catalyst 7,
In other words, the value is sufficient to form the reducing atmosphere, and the same value is maintained for a predetermined delay time T0 after the start of the reaction.

After the delay time T0 has elapsed, thereafter, as shown in FIG. 4, in response to the decrease in the amount of desorbed NOx, the increased amount of fuel decreases, and the time when the desorption ends. Is controlled to end the increase correction substantially.

For this reason, the enrichment of the air-fuel ratio is appropriately set according to the NOx desorption reaction, and the air-fuel ratio is enriched by a certain amount despite the decrease in the desorption amount as in the conventional case. Unlike this, HC is not supplied excessively, and deterioration of fuel economy can be minimized.

Another embodiment is shown in FIGS.

First, in the embodiment of FIG. 5, the initial value K0 of the fuel increase correction coefficient is corrected based on the catalyst temperature at the start of the desorption reaction.

The desorption characteristics change depending on the temperature of the NOx storage catalyst 7, and the desorption required air-fuel ratio generally increases as the temperature increases.

For this reason, after calculating the initial value K0 of the fuel increase correction coefficient in step S4, in step S4-1, based on the NOx storage catalyst temperature Tc, a correction coefficient C1 (when the catalyst temperature is The higher the value, the larger the value). After the correction coefficient C1 is calculated in this manner, in step S4-2, the fuel increase correction coefficient K0 is multiplied by the correction coefficient C1, that is, K0 =
As K0 * C1, the fuel increase correction coefficient K0 = K0 when calculating the fuel injection pulse width Ti in step S6.
* Replace with C1.

As a result, the initial value of the fuel injection amount at the time of the rich spike, that is, the initial air-fuel ratio becomes higher as the catalyst temperature increases, and conversely, the degree of enrichment decreases as the catalyst temperature decreases.

Therefore, NOx which changes according to the temperature
The supply amount of HC is controlled in accordance with the NOx desorption characteristics of the storage catalyst 7, so that excess HC is supplied, thereby preventing deterioration of fuel efficiency and incomplete reaction of NOx desorption.

In the embodiment of FIG. 7, the initial value K0 of the fuel increase correction coefficient is corrected according to the degree of deterioration of the catalyst. The NOx adsorption / holding capacity changes according to the degree of deterioration of the NOx storage catalyst 7, and the air-fuel ratio required for the NOx desorption reaction changes accordingly. As the deterioration progresses, the air-fuel ratio enrichment request decreases. Become.

Therefore, in this embodiment, in step S4-1 in FIG. 7, the degree of deterioration of the catalyst is calculated from the engine operating state, for example, the intake air amount and the number of revolutions, or the integrated value of the catalyst temperature. At S4-2,
The correction coefficient C2 is calculated from a map as shown in FIG. 8 based on the degree of deterioration. The correction coefficient C2 decreases as the deterioration of the catalyst progresses.

In step S4-3, the fuel increase correction coefficient K0 is multiplied by the correction coefficient C2, that is, K0 = K
0 * C2, which is used as a fuel increase correction coefficient K0 for calculating the fuel injection pulse width Ti in step S6, K
Replace 0 * C2.

By calculating the fuel injection pulse width Ti in step S6 while including the correction coefficient C2 in the correction coefficient C2, the degree of enrichment of the air-fuel ratio during a rich spike decreases as the catalyst deteriorates. Therefore, as the NOx storage catalyst 7 deteriorates, the supply amount of unnecessary HC can be reduced, and deterioration of fuel efficiency due to excessive supply of HC can be prevented.

The embodiment of FIG. 9 detects the actual NOx concentration downstream of the NOx storage catalyst 7 during rich spike control, and further corrects the fuel increase correction coefficient in accordance with the degree of change in the NOx concentration. The rich spike air-fuel ratio is appropriately controlled.

Therefore, after calculating the fuel increase correction coefficient Kn (Tn) in step S9, in step S9-1, N
The amount of change ΔNOx in NOx concentration per unit time is calculated from the output of the Ox sensor. In step S9-2, the change amount ΔNOx is compared with a predetermined value.
In step S9-3, a correction coefficient C3 is obtained based on the change amount ΔNOx according to a map as shown in FIG.

Then, in step S9-4, using the correction coefficient C3, the fuel increase correction coefficient Kn (Tn)
Is replaced by Kn (Tn) * C3, and step S
In step 10, the fuel injection pulse width Ti is calculated based on this.

The correction coefficient C3 is set so as to increase as the NOx change amount per unit time ΔNOx is smaller. If the desorption speed of NOx is lower, the degree of enrichment of the air-fuel ratio is increased and the desorption is performed. The reaction is promoted, and if the desorption rate is high, the degree of concentration is reduced, and excess HC
Avoid the supply of

In this manner, the NOx desorption reaction can be optimized, and both improvement of exhaust performance and prevention of deterioration of fuel efficiency can be achieved.

In the above embodiment, it is of course possible to control the rich spike so as to simultaneously include all the contents of the second to fourth embodiments.

In the above embodiment, an example in which the lean air-fuel ratio operation is performed by the in-cylinder direct injection internal combustion engine has been described.
The present invention is not limited to this, and other lean combustion systems can be employed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an overall configuration common to each embodiment of the present invention.

FIG. 2 is a flowchart illustrating rich spike control according to the first embodiment.

FIG. 3 is a characteristic diagram in which an initial value of a fuel increase correction coefficient is set.

FIG. 4 is an explanatory diagram showing NOx desorption characteristics during rich spike control.

FIG. 5 is a flowchart illustrating rich spike control according to the second embodiment.

FIG. 6 is a characteristic diagram in which characteristics of a fuel increase correction coefficient are set.

FIG. 7 is a flowchart illustrating rich spike control according to a third embodiment.

FIG. 8 is a characteristic diagram in which characteristics of a fuel increase correction coefficient are set.

FIG. 9 is a flowchart illustrating rich spike control according to a fourth embodiment.

FIG. 10 is a characteristic diagram in which characteristics of a fuel increase correction coefficient are set.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 4 Combustion chamber 5 Fuel injector 6 Spark plug 7 NOx storage catalyst 10 Controller 15 Air-fuel ratio sensor 17 Catalyst temperature sensor 18 NOx sensor

Claims (6)

[Claims] 1. An internal combustion engine equipped with a NOx storage catalyst that adsorbs and holds NOx in exhaust gas during a lean air-fuel ratio operation and desorbs and reduces NOx adsorbed and held during a rich air-fuel ratio operation. Means for estimating the amount of NOx adsorbed and held by the NOx storage catalyst; and rich control means for temporarily switching the air-fuel ratio to the rich air-fuel ratio when the amount of adsorbed fuel reaches a predetermined value to desorb and reduce NOx. The rich control means keeps the substantially constant initial value of the rich air-fuel ratio for a predetermined time from the start of the rich control.
An exhaust purification device for an internal combustion engine, characterized in that the degree of enrichment of the air-fuel ratio is gradually reduced in accordance with the desorption characteristics of Ox. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said rich control means gradually reduces the degree of enrichment of said air-fuel ratio in a quadratic or exponential manner. 3. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said rich control means determines an initial value of said rich air-fuel ratio in accordance with a NOx adsorption holding amount and an intake air amount at the start of rich control. . 4. The method according to claim 1, wherein said rich control means corrects an initial value of said rich air-fuel ratio in accordance with a temperature representative of a temperature of said NOx storage catalyst at the start of rich control. An exhaust gas purification device for an internal combustion engine. 5. The internal combustion engine according to claim 1, wherein said rich control means corrects an initial value of said rich air-fuel ratio in accordance with a degree of deterioration of said NOx storage catalyst at the time of starting rich control. Exhaust gas purification device. 6. The exhaust gas purification of an internal combustion engine according to claim 1, wherein said rich control means reduces the degree of enrichment of said air-fuel ratio in accordance with the NOx concentration downstream of the NOx storage catalyst. apparatus.