Emission control device for internal combustion engine

Abstract: A plurality of catalysts (7), (9) are disposed in an engine exhaust passage (13). At least the catalyst disposed downstream of these catalysts comprises a multi-layered catalyst (7) which is covered in a catalyst layer (24a) on the surface and an HC absorbent layer (24b) on an inner layer with respect to a catalyst carrier (23). A ratio of the downstream multi-layered catalyst (7) (catalyst heat capacity/catalyst surface area) is set so that it is greater than the same ratio for the upstream catalyst (9). In this way, temperature increase in the upstream catalyst (9) is advanced, the temperature gradient in the downstream catalyst (7) is increased and temperature rises in the HC absorbent layer are reduced. As a result, the HC emission period approximates the activation period of the catalyst layer (24a). In this way, during low temperature vehicle operation, it is possible to synchronize the timing of HC emission by the HC absorbent layer and catalyst activation and therefore to achieve extreme reductions in HC emitted to the outside atmosphere.

Description: The present invention relates to an emission control device for an internal combustion engine comprising an upstream catalyst disposed in an exhaust passage of said engine, a downstream catalyst disposed in the exhaust passage downstream of said upstream catalyst, which acts as a multi-layered catalyst, the surface of a catalyst carrier of said multi-layered catalyst being covered with a catalyst layer and an HC absorbent layer being provided on an inner layer. Devices are well known in the art which control exhaust emissions by the oxidization of HC and CO and reduction of NO x components present in exhaust gas emitted from an internal combustion engine by use of a catalyst. However emission control performance is relatively low when a catalyst is not sufficiently activated as, for example, in a low temperature gas environment produced immediately after cold-starting an engine. Attempts have been made to absorb HC components from low temperature exhaust gas emissions by using an HC absorbent material which absorbs HC from the exhaust gases thus preventing such components from being emitted into the atmosphere. However such HC absorbent materials release bound HC when the temperature exceeds a certain level and thus can only be used in a low temperature exhaust gas environment. Furthermore the temperature at which HC release is initiated is generally lower than the activation temperature of the catalyst and so when the temperature of the exhaust gas exceeds the temperature at which HC release is initiated but is lower than the activation temperature of the catalyst, HC is released into the atmosphere. The European Patent Application EP 0 716 877 A1, corresponding to Tokkai-Hei-8-224449/ discloses an emission control device of the above kind wherein a catalyst converter comprises a lower layer comprising an HC absorbent material and an upper layer formed from a catalyst which are formed on a honeycomb-shaped catalyst carrier. In this way, HC permeating the catalyst layer at low temperatures is absorbed by the HC absorbent material on the lower layer. When such bound HC is released from the HC absorbent material due to rises in temperature, the HC is oxidized by the action of the catalyst when passing through the upper catalyst layer. However even in this case, the HC absorbent material releases bound HC at temperatures lower than the catalyst activation temperature for example at 150-200°C. As a result, if the catalyst is not activated, released HC is not oxidized and unavoidably released directly into the atmosphere. Besides, EP 0 661 098 A2 discloses various exhaust purification systems and combinations of HC-absorbent units and catalyst units. However, no multi-layered catalytic units are disclosed. As described above in connection with EP 0 716 877 A1, such an emission control device still holds the problem that hydrocarbon (HC) are emitted into the atmosphere during a time period when an inner HC absorbent layer has reached a temperature lower than the catalyst activation temperature while the catalyst is not yet activated to transform the released HC components, It is, therefore, an object of the invention to provide an emission control which minimizes the emission of HC into the atmosphere. For an emission control device for an internal combustion engine of the above kind, this object is solved in an inventive manner in that the emission control device further comprises: a catalyst temperature control mechanism which makes a large temperature gradient between said surface catalyst layer and said inner HC absorbent layer of said downstream multi-layered catalyst in cross section, and which increases a ratio of a catalyst heat capacity with respect to a catalyst surface area of said downstream multi-layered catalyst, that is to say, [catalyst heat capacity] / [catalyst surface area], so that said ratio is greater than the same ratio of said upstream catalyst. By the combination of the features of providing a large temperature radiant between the two layers of the downstream multi-layer catalyst and additionally setting a specific relationship between the two specific ratios of the upstream catalyst heat capacity and the downstream catalyst heat capacity, the satisfying result can be achieved that almost the entire HC components are prevented from being released into the atmosphere. Further preferred embodiments are subject to the subclaims. In the following the invention will be explained in greater detail by means of embodiments thereof taking reference to the drawings, wherein: Figure 1 is an overall schematic view showing an embodiment of the present invention, Figure 2 is a partial enlarged cross sectional view of a catalyst of the first embodiment, and Figure 3 is a partial enlarged cross sectional view of a catalyst of another embodiment, The preferred embodiments of the invention are explained below with reference to the accompanying drawings. Figure 1 shows the overall layout of the device. A throttle valve 2 is disposed downstream in the air-intake 12 of an engine 11. A fuel injection valve 3 which injects fuel is provided in the engine 11. A three-way catalyst 9 is provided upstream in the exhaust passage 13 which reduces NO x and oxidizes HC and CO components in the exhaust gas. A multi-layer catalyst 7 which comprises a three-way catalyst which has an HC absorbent function is disposed further downstream. As explained below, during low temperatures at which the catalyst is not activated, HC present in exhaust gas is bound. When temperatures rise thereafter, reduction and oxidization of exhaust gas is performed which includes the oxidization of HC released from the catalyst. The air-fuel ratio of the exhaust gas is controlled by a controller 8 to an air-fuel ratio required by each catalyst 7, 9 above, that is to say, to a stoicheometric air-fuel ratio. In that regard, the following signals are input into the controller 8, namely an air intake amount signal from an air intake amount sensor 1 disposed in the air intake passage 12, a throttle aperture signal from the throttle valve 2, an exhaust gas air-fuel ratio signal from an air-fuel ratio (oxygen concentration) sensor 5 which is disposed in the exhaust gas passage 13, and an engine rotation speed signal. A fuel injection amount is calculated on the basis of these signals so as to equal a stoicheometric air-fuel ratio and an injection amount signal is output to the fuel injection valve 3 in synchronization with the engine rotations. The air-fuel ratio of the engine is feedback controlled during normal running to equal a stoicheometric ratio. However depending on certain operating conditions such as cold-starting the engine, open control is performed to set the air-fuel ratio to a richer ratio than a stoicheometric ratio. Lastly an optimal ignition signal is output from the controller 8 to an ignition plug 4 depending on the operating conditions. Figure 2 is an enlarged cross section of each catalyst. The three-way upstream catalyst 9 comprises a catalyst carrier 21 which is formed from a honeycomb-shaped body on which cross sections of the exhaust gas passage are displaced in series in a grill shape. The surface of the carrier 21 is covered with a catalyst layer 22 which is formed from a catalyst. Exhaust gas flows from the upstream to the downstream of the passage 25 through the grill. At this time, the exhaust gas comes into contact with the catalyst layer 22 with the result that HC and CO are oxidized and NO x components are reduced. Within the downstream multi-layered catalyst 7, a catalyst carrier 23 is similarly disposed in series in the grill, the surface of the catalyst carrier 23 has an interior passage 26 and is covered with a catalyst and a HC absorbent layer 24. The surface layer comprises a catalyst layer 24a of a fixed thickness. The lower (inner) layer comprises an HC absorbent layer 24b which is formed from an HC absorbent material. Each layer is formed in a porous shape having a plurality of microscopic holes. HC exhaust gas components are absorbed on the HC absorbent layer 24b by permeating the catalyst layer 24a when the catalyst temperature is low. However as is clear from the drawings, the catalyst carrier 23 of the downstream multi-layered catalyst 7 has a low cell number (grill opening concentration) and a high board width in comparison with the catalyst carrier 21 of the upstream three-way catalyst 9. The surface area of the catalyst carrier 23 which partitions the exhaust gas passage is also small in comparison with the catalyst carrier 21 of the upstream catalyst 9. More precisely, the catalyst carrier 21 for the upstream three-way catalyst 9 is a ceramic honeycomb-shaped catalyst carrier having a board thickness of 65µm and a cell concentration of 900/square inch. The catalyst carrier 23 for the downstream multi-layered catalyst 7 is a ceramic honeycomb-shaped catalyst carrier having a board thickness of 270µm and a cell concentration of 200/square inch. Using a metal honeycomb-shaped catalyst carrier as a catalyst carrier for the downstream multi-layered catalyst 7 which has a heat capacity per unit area which is approximately 2.6 times that of ceramic enables the use of a carrier with a cell concentration of 200/square inch and a thickness of 50µm. Furthermore the thickness of the catalyst layer 24a of the multi-layer catalyst 7 is thicker in comparison with the catalyst layer 22 of the three-way catalyst 9. Preferably the thickness of the catalyst layer 24a is more than three times the thickness of the catalyst layer 22. In other words, the ratio Rc of the catalyst heat capacity to the catalyst surface area, that is to say [catalyst heat capacity]/[catalyst surface area] = Rc , is adapted to be greater in the downstream multi-layer catalyst 7 than in the upstream three-way catalyst 9. In this way, a catalyst temperature control mechanism is composed which increases the temperature gradient of the surface and inner layers in the catalyst cross section of the downstream multi-layer catalyst 7. The operation of the invention will be discussed below. The exhaust gas flows to the downstream multi-layered catalyst 7 after passing the upstream three-way catalyst 9. The temperature of the exhaust gas in the upstream section is originally higher than the downstream section. Thus there is a tendency for temperature to rise to that degree. Moreover the Rc ratio of [catalyst heat capacity]/[catalyst surface area] of the upstream three-way catalyst 9 is small. That is to say, since the heat absorbing surface area is large with respect to the unit heat capacity and the catalyst temperature rises quickly, the upstream three-way catalyst 9 quickly reaches an activation temperature after the engine is activated. In contrast, since the downstream multi-layered catalyst 7 has a larger Rc ratio of [catalyst heat capacity]/[catalyst surface area] than the upstream side and the catalyst surface area which contacts with the exhaust gas is relatively small with respect to the unit catalyst heat capacity, the catalyst temperature does not rise quickly. Furthermore since the exhaust gas temperature further downstream is also relatively low, temperature rise in the downstream multi-layer catalyst 7 displays a tendency to be slower than the upstream catalyst. When exhaust gas temperature is low immediately after engine starting and the upstream three-way catalyst 9 is not activated, HC exhaust gas components reach the downstream multi-layered catalyst 7 without being oxidized by the upstream three-way catalyst 9. HC components permeate the surface catalyst layer 24a on the downstream catalyst and are absorbed by the HC absorbent layer 24b on the inner layer. As temperature in the exhaust system rises after engine activation, even though the upstream three-way catalyst 9 approaches an activation temperature, the temperature of the downstream multi-layered catalyst 7 remains relatively low. Therefore the temperature of the HC absorbent layer 24b is enabled to successfully absorb HC components in the period until a temperature is reached at which absorbed HC begins to be released for example 150-200°C. The Rc ratio of [catalyst heat capacity]/[catalyst surface area] is larger in the downstream multi-layered catalyst 7. Precisely, there is a large temperature gradient between the deep HC absorbent layer 24b and the surface catalyst layer 24a since the thickness of the catalyst layer 24a on the surface side for example is large in comparison with the upstream catalyst. That is to say, the temperature of the HC absorbent layer 24b is low in comparison with the surface temperature of the catalyst layer 24a and the temperature difference is large. The temperature range at which the HC absorbent layer 24b begins emitting HC is lower than the temperature range at which the catalyst layer 24a is activated. Therefore since the temperature gradient is steep, when a temperature is reached at which the inner HC absorbent layer 24b begins to emit HC, or before that time, the temperature of the catalyst layer 24a can reach an activation temperature. During the period after engine activation while the temperature of the exhaust gas system is rising, the catalyst layer 24a of the multi-layered catalyst 7 approaches an activation temperature. At this time, the temperature of the HC absorbent layer 24b also rises. However since the temperature gradient is large, the HC emission initiation period approximates the catalyst activation period. Thus even if absorbed HC is emitted from the absorbent layer 24b, such HC will be oxidized while passing through the already activated catalyst layer 24a on the surface and HC emission to the atmosphere will be prevented. When the temperature of the upstream and downstream catalysts exceeds an activation temperature, HC components are oxidized by the catalyst and HC emission can be prevented even without the HC absorbent action of the HC absorbent layer 24b. Since the Rc ratio [catalyst heat capacity]/[catalyst surface area] of the upstream three-way catalyst 9 is small, early activation is possible due to rapid rises in temperature. As a result, the temperature increase of the downstream multi-layered catalyst 7 is advanced to the degree to which the temperature of the exhaust gas is raised due to the heat of reaction. Thus since the temperature of the catalyst system as a whole is rapidly raised, sufficient exhaust gas emission control performance is displayed within short periods after engine activation. To construct a catalyst temperature control mechanism which increases the temperature gradient of the downstream multi-layered catalyst 7, the present invention is adapted to increase the Rc ratio [catalyst heat capacity]/[catalyst surface area] in the downstream multi-layered catalyst 7 more than the upstream three-way catalyst 9. However it is possible to achieve the same effect even if the invention is arranged as disclosed below. That is to say, the downstream multi-layered catalyst 7, in comparison with the upstream three-way catalyst 9 may: (1) increase the relative catalyst heat capacity by increasing the thickness of the catalyst layer. (2) reduce the relative passage surface area by reducing the cell number of openings on the catalyst carrier. (3) reduce the relative surface area and increase the relative heat capacity by increasing the thickness of the catalyst carrier. (4) use a metal with a higher heat capacity than the upstream ceramic to manufacture the catalyst carrier. The above elements may be used independently or in combination. Now with respect to the embodiment of the present invention as shown in Figure 3, an HC absorbent layer 22b comprising an HC absorbent material is provided on the inner side of the catalyst layer 22a so that the updtream three-way catalyst 9 is identical to the downstream multi-layered catalyst 7. Thus in the upstream three-way catalyst 9, HC will be absorbed by the HC absorbent layer 22b and HC emitted due to rises in temperature will be oxidized by the catalyst layer 22a on the surface. However even if a portion of the HC components flow downstream without being oxidized, such HC components will be absorbed by the downstream multi-layered catalyst 7 which has a lower temperature than the upstream section. Thus as HC components are released as the temperature of the downstream multi-layered catalyst 7 rises to equal that upstream, they are processed by the surface catalyst layer 24a. In this way, HC emission to the outer atmosphere is reduced by the application of a two-stage emission control process due to two catalysts. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teaching. The scope of the invention is defined with reference to the following claims.

Claims: An emission control device for an internal combustion engine (11) comprising: an upstream catalyst (9) disposed in an exhaust passage (13) of said engine (11), a downstream catalyst (7) disposed in the exhaust passage (13) downstream of said upstream catalyst (9), which acts as a multi-layered catalyst (7), the surface of a catalyst carrier (23) of said multi-layered catalyst (7) being covered with a catalyst layer (24a) and an HC absorbent layer (24b) being provided on an inner layer, characterized by further comprising: a catalyst temperature control mechanism which makes a large temperature gradient between said surface catalyst layer (24a) and said inner HC absorbent layer (24b) of said downstream multi-layered catalyst (7) in cross section, and which increases a ratio of a catalyst heat capacity with respect to a catalyst surface area of said downstream multi-layered catalyst (7), that is to say, [catalyst heat capacity] / [catalyst surface area], so that said ratio is greater than the same ratio of said upstream catalyst (9). An emission control device for an internal combustion engine according to claim 1, characterized in that said catalyst temperature control mechanism increases a covering thickness of a catalyst layer of said downstream multi-layered catalyst (7) so that said catalyst layer (24a) is thicker than a covering thickness of a catalyst layer (22) of said upstream catalyst (9). An emission control device for an internal combustion engine according to claim 2, characterized in that said downstream catalyst layer (24a) is more than three times that of said upstream catalyst layer (22). An emission control device for an internal combustion engine according to at least one of the claims 1 to 3, characterized in that said catalyst temperature control mechanism is adapted to reduce an exhaust passage surface area of said catalyst carrier (23) for said downstream multi-layered catalyst (7) so as to be smaller than a passage surface area of a catalyst carrier (21) for said upstream catalyst (9). An emission control device for an internal combustion engine according to claim 4, characterized in that a cell concentration of said catalyst carrier (23) for said downstream multi-layered catalyst (7) is smaller than a cell concentration of said catalyst carrier (21) of said upstream catalyst (9). An emission control device for an internal combustion engine according to claim 5, characterized in that said cell concentration of said catalyst carrier (23) for said downstream multi-layered catalyst (7) is less than 1/2 that of said cell concentration of said catalyst carrier (21) of said upstream catalyst (9). An emission control device for an internal combustion engine according to claim 5 or 6, characterized in that said cell concentration of said catalyst carrier (21) for said upstream catalyst (9) is greater than 500 per square inch. An emission control device for an internal combustion engine according to claim 7, characterized in that said cell concentration of said catalyst carrier (23) for said downstream catalyst (7) is less than 250 per square inch. An emission control device for an intemal combustion engine according to at least one of the claims 1 to 8, characterized in that said catalyst temperature control mechanism is adapted so that a thickness of a catalyst carrier (23) of said downstream multi-layered catalyst (7) is greater than a thickness of a catalyst carrier (21) for said upstream catalyst (9). An emission control device for an internal combustion engine according to at least one of the claims 1 to 9, characterized in that a catalyst carrier (21)of said upstream catalyst (9) comprises a ceramic honeycomb-shaped catalyst carrier with a thickness of less than 90µm. An emission control device for an internal combustion engine according to claim 10, characterized in that said catalyst carrier (23) of said downstream multi-layered catalyst (7) comprises a ceramic honeycomb-shaped catalyst carrier with a thickness of greater than 150µm. An emission control device for an internal combustion engine according to at least one of the claims 1 to 11, characterized in that a catalyst carrier (21) of said upstream catalyst (9) comprises a ceramic honeycomb-shaped catalyst carrier and said catalyst carrier (23) of said downstream multi-layered catalyst (7) comprises a metal honeycomb-shaped catalyst carrier. An emission control device for an internal combustion engine according to claim 12, characterized in that a thickness of said downstream multi-layered ceramic honeycomb-shaped catalyst carrier (23) is more than 40µm. An emission control device for an internal combustion engine according to at least one of the claims 1 to 13, characterized in that said upstream catalyst (9) is a three-way catalyst. An emission control device for an internal combustion engine according to at least one of the claims 1 to 14, characterized in that said upstream catalyst (9) is a multi-layered catalyst having an HC absorbent layer (22a).

Inventors: KACHI NAOKI, NISHIZAWA KIMIYOSHI, SATO RITSUO, TSUZUKI MIKIO, KACHI, NAOKI, NISHIZAWA, KIMIYOSHI, SATO, RITSUO, TSUZUKI, MIKIO
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1999
Main classification: B01D53
IPC codes: F01N3, B01D53, F01N7