JP4576762B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an engine, and more particularly to an exhaust emission purification device including a catalytic converter in which a three-way catalyst layer and an HC adsorption layer are formed on a catalyst carrier.
[0002]
[Prior art and problems to be solved]
A catalyst converter having a multilayer structure in which an HC adsorbent and an oxidation catalyst or a three-way catalyst are stacked on a catalyst carrier is known as an improvement in exhaust emission performance during cold engine operation. Unburnt fuel discharged from the engine during cold operation with insufficient catalyst activity is temporarily adsorbed to the HC adsorption layer, and when the temperature of the catalytic converter rises to some extent, HC is desorbed from the adsorption layer and becomes the surface layer side. Oxidized by the three-way catalyst layer. However, since the desorption temperature of HC from the HC adsorption layer is generally lower than the activation start temperature of the three-way catalyst layer, part of the desorbed HC is discharged without being oxidized in the process of activating the catalyst. End up.
[0003]
On the other hand, Japanese Patent Laid-Open No. 11-82003 discloses that ignition timing retard and rotation increase control are performed after the start of HC desorption from the HC adsorption layer until the three-way catalyst reaches the activation temperature. Has been proposed in which the exhaust temperature is raised to promote activation of the three-way catalyst. However, even with such exhaust temperature control, desorption of HC from the HC adsorption layer until the three-way catalyst is activated cannot be suppressed, so there is room for improvement from the viewpoint of HC emission performance. .
[0004]
The present invention has been made paying attention to such a problem, and an object of the present invention is to provide an exhaust emission control device that can reliably desorb HC from the HC adsorption layer when the engine is started.
[0005]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided an exhaust purification apparatus in which a catalytic converter having an HC adsorption layer formed between a catalyst carrier and a three-way catalyst layer covering the carrier is interposed in an engine exhaust passage. Heat transfer suppression means for suppressing heat transfer from the layer to the HC adsorption layer is provided. Also, when the three-way catalyst layer does not reach the activation temperature, such as when the engine is cold started, the exhaust temperature is set so that the three-way catalyst layer reaches the activation temperature before the HC adsorption layer reaches the HC desorption start temperature. And an exhaust temperature control means for controlling.
[0006]
In the second invention, an intermediate layer made of a heat insulating material is formed between the three-way catalyst layer and the HC adsorption layer as the heat transfer suppressing means of the first invention.
[0007]
The third invention applies a configuration in which the thickness of the HC adsorption layer is larger than that of the three-way catalyst layer as the heat transfer suppressing means of the first invention.
[0009]
According to a fourth aspect of the present invention, a configuration in which an interlayer contact area with respect to the three-way catalyst layer is reduced as the heat transfer suppressing means in the first to third aspects of the invention.
[0010]
According to a fifth aspect of the present invention, after the engine is started, the exhaust temperature control means according to the first aspect of the present invention is controlled by at least one of ignition timing retard control for delaying the ignition timing of the spark ignition engine and speed increase control for increasing the speed The exhaust temperature is increased.
[0011]
In a sixth aspect of the invention, the exhaust temperature control means of the first or fifth aspect of the invention controls the exhaust temperature so that the three-way catalyst layer is maintained at a temperature near the activation temperature after reaching the activation temperature. To be configured.
[0012]
In a seventh aspect of the invention, the exhaust temperature control means of the first or fifth aspect of the invention controls the exhaust temperature so as to delay the HC adsorption layer from reaching the desorption start temperature after reaching the activation temperature. To be configured.
[0013]
[Action / Effect]
According to each invention below the first invention, the heat transfer from the three-way catalyst layer to the HC adsorption layer is suppressed by the heat transfer suppressing means as shown in the second to fifth inventions, for example. It is possible to raise the temperature of the three-way catalyst layer to the activation temperature before the HC adsorption layer reaches the HC desorption temperature by raising the exhaust gas temperature during warm-up operation after starting. Thereby, exhaust emission performance can be improved by reliably suppressing HC emission under low temperature conditions.
[0014]
As the heat transfer suppressing means, as shown in the second invention, an intermediate layer made of a heat insulating material is formed between the three-way catalyst layer and the HC adsorption layer, so that the heat from the three-way catalyst layer is directly HC. Do not transmit to the adsorption layer. Alternatively, as shown in the third invention, the thickness of the HC adsorption layer is made larger than that of the three-way catalyst layer. This increases the heat capacity of the HC adsorption layer, making it difficult for the temperature to rise due to heat transfer from the three-way catalyst layer and making it easier for the three-way catalyst layer to rise in temperature. The three-way catalyst layer can be quickly raised to the activation temperature. In addition, as shown in the fourth aspect of the invention as a heat transfer suppressing means, the heat transfer from the three-way catalyst layer to the HC adsorption layer can also be suppressed by reducing the interlayer contact area with the three-way catalyst layer. .
[0015]
As control for increasing the exhaust temperature for activation of the three-way catalyst layer, control of either or both of ignition timing retard and rotation speed increase is executed as shown in the fifth aspect of the invention. If the ignition timing is delayed, the exhaust gas temperature increases with an increase in exhaust heat loss. Further, if the number of revolutions is increased, the amount of combustion gas per unit time or the exhaust gas flow rate increases, so that the exhaust temperature rises.
[0016]
As shown in the sixth or seventh invention, after reaching the activation temperature, the temperature of the three-way catalyst is maintained near the activation temperature, or the temperature of the HC adsorption layer reaches the desorption start temperature. By suppressing the ignition timing retard and control of the increase in the rotational speed so as to delay the HC, the HC desorption rate can be reduced by suppressing the temperature increase of the HC adsorption layer due to excessive heating. The frequency with which the catalytic noble metal of the original catalyst layer undergoes an oxidation reaction increases, and the HC purification rate can be further improved.
[0017]
Embodiment
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 or FIG. 2 shows a schematic configuration of the gasoline engine according to the present embodiment. In the figure, reference numeral 1 denotes a controller having a function of controlling the fuel injection amount, ignition timing, and exhaust temperature of the engine 2. An air flow meter 4 and a throttle valve 5 for detecting the intake air amount are provided in the intake passage 3 of the engine 2, and a fuel injection valve 6 is provided in the intake passage 3. Fuel is supplied to the fuel injection valve 6 at a constant pressure from a fuel supply system (not shown), and an amount of fuel corresponding to the valve opening time is injected. The fuel injection amount calculated by the controller 1 is calculated as an injection pulse width corresponding to the valve opening time of the fuel injection valve 6.
[0018]
7 is a crank angle sensor for detecting the rotation angle of the engine crankshaft and the engine speed, 8 is a water temperature sensor for detecting the engine cooling water temperature, and 9 is an air-fuel ratio sensor for detecting the oxygen concentration in the exhaust gas in the exhaust passage 10. It is. Reference numeral 11 denotes a catalytic converter interposed in the exhaust passage 10, which includes an HC adsorption catalyst having a structure in which an HC adsorption layer and a three-way catalyst layer are stacked on a catalyst carrier. A catalyst temperature sensor 12 detects the temperature of the three-way catalyst portion. As shown in FIG. 2, a configuration in which another catalytic converter 13 made of a three-way catalyst is interposed on the upstream side of the HC adsorption catalyst 11 can also be applied. An ignition plug 14 ignites the fuel at an ignition timing set based on a signal from the controller 1.
[0019]
A configuration example of the catalyst layer of the catalytic converter 11 is shown in FIG. In the embodiment of the catalyst shown in FIG. 3, the surface of the honeycomb catalyst carrier 21 is coated with an HC adsorption layer 22 made of zeolite or the like, and further a three-way catalyst layer 23 is coated thereon. As shown in the figure, the HC adsorption layer 22 is thicker than the three-way catalyst layer 23, thereby increasing the heat capacity of the HC adsorption layer 22 so that the temperature does not easily rise. . FIG. 4 shows another embodiment, in which an intermediate layer 24 made of a heat insulating material is formed between the HC adsorption layer 22 and the three-way catalyst layer 23 covering the surface of the honeycomb catalyst carrier 21. By preventing direct heat transfer from the three-way catalyst layer 23 to the HC adsorption layer 22, an increase in the temperature of the HC adsorption layer is suppressed. As the heat insulating material constituting the intermediate layer 24, for example, alumina or ceria that does not contain a catalyst noble metal can be applied to make it difficult to transfer heat.
[0020]
In order to further suppress heat transfer from the three-way catalyst layer 23 to the HC adsorption layer 22, the HC adsorption layer 22 is formed of a material having a larger specific heat, or the three-way catalyst layer is formed by pores or voids having a porous structure. It is effective to reduce the area of contact with the interlayer.
[0021]
Next, engine control executed by the controller 1 under the above configuration will be described. The controller 1 is composed of a microcomputer and its peripheral devices. As an operation state signal, an intake air amount signal from the air flow meter 4, a rotation speed signal from a crank angle sensor 9, a water temperature signal from a water temperature sensor 10, an air-fuel ratio sensor. 11 inputs an oxygen concentration signal and the like, calculates the fuel injection amount and the ignition control amount based on these signals, and controls the exhaust gas temperature using the water temperature signal and the catalyst temperature signal from the catalyst temperature sensor 14.
[0022]
5 and 6 show a first embodiment of the exhaust gas temperature control, FIG. 5 is a flowchart showing the processing contents of the exhaust gas temperature control, and FIG. 6 is a timing chart according to the processing. A symbol S in FIG. 5 represents a processing step. This process is periodically executed by the microcomputer in the controller 1.
[0023]
In this exhaust control, first, at step 11, the coolant temperature at the time of starting the engine is detected by a signal from the water temperature sensor 8, and then at step 12, the detected water temperature is compared with a preset reference value, thereby increasing the temperature of the exhaust gas. That is, it is determined whether or not the temperature condition is to heat the catalyst. When the water temperature is higher than the reference value, the current control routine is terminated without performing the subsequent processing. When the water temperature is lower than the reference value, an ignition timing retard is performed in step 13 to retard the ignition timing to near the stability limit. The ignition timing at this time is obtained, for example, by searching a table set in advance so as to give the retard amount from the engine speed and the water temperature.
[0024]
After the ignition timing retard, in step 14, based on the signal from the catalyst temperature sensor 12, the three-way catalyst temperature of the catalytic converter 11 is detected, and then in step 15, the three-way catalyst temperature is determined in advance as an activity determination. If the temperature does not reach the activation determination temperature, the process returns to step 13 to continue the ignition timing retard.
[0025]
Based on the control, as shown in FIG. 6, the exhaust temperature after the cold start increases and the catalyst quickly reaches the activation temperature. In the figure, Tc and Th are the three-way catalyst layer temperature and the HC adsorption layer temperature according to the structure of the present invention, and Tha is the HC adsorption layer temperature by the catalyst of the conventional structure without heat insulation. As described above, since the HC adsorption catalyst is configured to suppress heat transfer from the three-way catalyst layer to the HC adsorption layer, the three-way catalyst temperature rapidly increases as seen in the characteristic line Th. Even so, the HC adsorption layer is kept at a relatively low temperature, and the three-way catalyst temperature rises to the activation temperature t2 before reaching the HC desorption temperature t1. For this reason, the discharge amount of the unburned HC component is suppressed as shown by the characteristic line H in the figure. On the other hand, conventionally, the HC adsorption layer temperature has reached t1 before reaching t2, so that HC has been discharged as indicated by the characteristic line Ha.
[0026]
In addition, Tc ′ and Th ′ in the figure respectively represent the temperature change characteristics of the three-way catalyst layer and the HC adsorption layer when the ignition timing retard is continued after the three-way catalyst temperature reaches t2. In this case, HC desorption starts when Th reaches t1 (Ho), but at this point, the three-way catalyst layer is already activated to oxidize the desorbed HC, so that HC emission is suppressed. The However, if the ignition timing retard is terminated when the activation temperature of the three-way catalyst is reached as in the present invention, the temperature rise of the HC adsorbing layer becomes moderate due to the subsequent decrease in the exhaust gas temperature, and the HC desorption after t1 is correspondingly increased. Since the separation speed is reduced, the conversion efficiency by the three-way catalyst layer is improved, and the HC emission amount can be further reduced. After reaching the activation temperature, the three-way catalyst layer does not decrease in temperature as long as the operation is continued even if the ignition timing retard is terminated, so that the HC treatment can be performed without any problem.
[0027]
7 and 9 show another embodiment relating to the exhaust gas temperature control of the present invention. FIG. 7 is a flowchart showing the processing contents of the second embodiment, and FIG. 9 is a flowchart showing the processing contents of the third embodiment.
[0028]
The difference from the first embodiment will be described. In FIG. 7, until the three-way catalyst layer temperature reaches the activation determination temperature, control is performed to increase the engine speed as the processing of Step 13 to Step 16, and the activation temperature is set. After reaching, the rotation speed is restored. As control for increasing the number of revolutions, for example, control for increasing the amount of idle air is performed. FIG. 9 shows the control of both the ignition timing retard and the engine speed increase as the processing of step 13 to step 16.
[0029]
FIGS. 8 and 10 show a timing chart based on the control shown in FIG. 7 and a timing chart based on the control shown in FIG. The meaning of the reference numerals in the figure is the same as in FIG. As shown in the figure, these controls can also effectively suppress the discharge of HC by raising the temperature of the three-way catalyst layer to the activation temperature before the HC adsorption layer reaches the HC desorption temperature.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of an exhaust purification apparatus according to an embodiment of the present invention.
FIG. 2 is an overall configuration diagram showing another configuration example of the exhaust emission control apparatus according to the embodiment of the present invention.
FIG. 3 is a cross-sectional view showing a laminated structure of the first embodiment of the catalyst according to the present invention.
FIG. 4 is a cross-sectional view showing a laminated structure of a second embodiment of the catalyst according to the present invention.
FIG. 5 is a flowchart showing a first embodiment of exhaust gas temperature control according to the present invention.
FIG. 6 is a timing chart according to the first embodiment of exhaust gas temperature control.
FIG. 7 is a flowchart showing a second embodiment of exhaust gas temperature control according to the present invention.
FIG. 8 is a timing chart according to a second embodiment of exhaust gas temperature control.
FIG. 9 is a flowchart showing a third embodiment of exhaust gas temperature control according to the present invention.
FIG. 10 is a timing chart according to a third embodiment of exhaust gas temperature control.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Controller 2 Engine 3 Intake passage 4 Air flow meter 5 Throttle valve 6 Fuel injection valve 7 Crank angle sensor 8 Water temperature sensor 9 Air-fuel ratio sensor 10 Exhaust passage 11 Catalytic converter (HC adsorption catalyst)
12 Catalyst temperature sensor 13 Catalytic converter (three-way catalyst)
14 Spark plug 21 Catalyst carrier 22 HC adsorption layer 23 Three-way catalyst layer 24 Intermediate layer

Claims (7)

In an exhaust purification device in which a catalytic converter in which an HC adsorption layer is formed between a catalyst carrier and a three-way catalyst layer covering the carrier is interposed in an engine exhaust passage,
The catalyst converter is provided with heat transfer suppression means for suppressing heat transfer from the three-way catalyst layer to the HC adsorption layer,
Exhaust temperature control means for controlling the exhaust temperature so that the three-way catalyst layer reaches the activation temperature before the HC adsorption layer reaches the HC desorption start temperature when the three-way catalyst layer does not reach its activation temperature An exhaust emission control device for an engine characterized by comprising:   The engine exhaust gas purification apparatus according to claim 1, wherein the heat transfer suppression means is an intermediate layer made of a heat insulating material provided between the three-way catalyst layer and the HC adsorption layer.   The engine exhaust gas purification apparatus according to claim 1, wherein the heat transfer suppression means has a configuration in which the thickness of the HC adsorption layer is larger than that of the three-way catalyst layer.   The engine exhaust purification device according to any one of claims 1 to 3, wherein the heat transfer suppression means has a small interlayer contact area with the three-way catalyst layer.   The exhaust gas temperature control means is configured to increase the exhaust gas temperature by at least one of ignition timing retard control for delaying the ignition timing of the spark ignition engine and rotation speed increase control for increasing the rotation speed. An exhaust emission control device for an engine according to 1. The exhaust gas temperature control means according to claim 1 or claim later reached the vicinity of the active temperature three-way catalyst layer is configured to control the exhaust gas temperature so as to maintain the temperature near the active temperature 5 An exhaust purification device for an engine according to any one of the above. The exhaust gas temperature control means according to claim 1 or claim since reached the vicinity of the active temperature and is configured to control the exhaust gas temperature to be delayed that HC adsorbing layer reaches the desorption start temperature 5 An exhaust purification device for an engine according to any one of the above.

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