JP4494201B2 - Spark ignition engine including a three-way catalyst containing NOx storage components - Google Patents

Description

  The present invention controls the air / fuel ratio of the engine so that it travels at a stoichiometric air / fuel ratio under normal driving conditions and travels on the lean side of the stoichiometric air / fuel ratio during a specific portion of the engine speed / load map. The present invention relates to a spark ignition engine including an engine control device programmed as described above and an exhaust mechanism including a catalyst. In particular, the present invention relates to such engines where the catalyst is a three way catalyst (TWC) that includes a NOx storage component.

Heterogeneous catalysts capable of simultaneously converting nitric oxide (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC) in the exhaust gas from a stoichiometrically operated spark ignition combustion engine are It is called a former catalyst (TWC). NOx reduction occurs easily on the TWC when the air-fuel ratio is on the stoichiometric rich side, whereas the CO and HC reactions are inhibited by insufficient oxygen (O ₂ ). On the lean side, the CO and HC conversion rates are high, but NOx reduction becomes difficult due to the excess of oxidizing species. Therefore, effective ternary conversion occurs within a relatively narrow air-fuel ratio tolerance. In practice, an oxygen sensor is used to detect the lambda composition of the exhaust gas upstream of the TWC and adjust the air / fuel ratio accordingly to equilibrate the exhaust gas.

A typical TWC is platinum (Pt) and / or palladium (Pd) as an oxidation catalyst and rhodium (Rh) as a reduction catalyst and oxygen on a suitable high surface area oxide support such as alumina (Al ₂ O ₃ ). A storage component (OSC), for example, a ceria-zirconia mixed oxide. Various small amounts of base metal promoters, stabilizers and hydrogen sulfide inhibitors can also be included. For more details, see International Patent No. WO 98/03251 (included here as a reference).

As a result of controlling the air / fuel ratio using the detected oxygen concentration, there is a time delay associated with the adjusted air / fuel ratio. This creates confusion around the control set point. For example, in a rich operation, it is necessary to provide a small amount of O ₂ to consume unreacted CO and HC. Conversely, if the exhaust gas becomes slightly oxidizing, it is necessary to consume excess O ₂ . One development in the TWC technology that has been adopted to reduce emissions problems associated with disruption is to incorporate an O ₂ storage component into the TWC composition. This component adsorbs (or absorbs) O _{2 in} a lean environment and releases it in a rich environment, thereby effectively extending the time that the exhaust gas is at the set point. When a larger amount of HC fuel is required to maintain the air-fuel ratio as in acceleration, the fuel is supplied by adjusting the fuel injection period, for example.

More recently, there is a movement to drive a gasoline combustion engine, such as a gasoline direct injection engine, lean. The reasoning is that, by running at a lean side of stoichiometry, improved fuel economy (thus, reduces the emission of CO ₂₎ that is to. The first problem in pushing this strategy is that the lean environment prevents NOx reduction in the TWC. One technique that has been developed to address this problem is referred to variously as NOx absorber / catalyst, lean NOx trap (LNT) or NOx trap, and is based on acid-base washcoat chemistry. In this method, during lean running conditions, NOx is adsorbed (or absorbed) and stored in the catalyst washcoat and released under rich operation. The released NOx is converted to nitrogen by catalysis as in TWC.

A typical NOx trap composition includes Pt and Rh on a support of a high surface area oxide, such as Al ₂ O ₃ , and a NOx storage component, such as barium oxide (BaO, eg European Patent EP 0 758 713, incorporated herein by reference. See the description))). Generally, the content of NOx storage component in the NOx trap washcoat can be up to 50% by weight or even higher.

  A major problem with the use of NOx trap technology is that complex control of the engine needs to be done very carefully in order to regenerate rich NOx storage components. In addition, many feedback sensors for controlling NOx trap function and NOx storage capacity, such as sensors that estimate stored NOx from the engine using stored engine maps, and NOx absorption efficiency of NOx storage components are Since it depends on temperature, a NOx trap temperature sensor is used.

The TWC technology, using BaO or lanthanum oxide minor _{(1~3%) (La 2 O} 3), are known to stabilize the gamma _-Al 2 _{O 3} sintered during high temperature aging Yes. Base metal promoters such as barium (Ba), cerium, lanthanum, magnesium, calcium and strontium can also be used (see the above-mentioned International Patent No. WO 98/03251).

In our International Patent No. WO 99/00177, which is hereby incorporated by reference, we have a catalytic converter comprising a catalytic component capable of storing NOx for a lean burn internal combustion engine, such as a direct injection gasoline engine. Is disclosed. In one embodiment, the catalytic converter is supported on a first inner layer comprising a first platinum group metal (PGM), such as Pt, and a NOx storage component, such as Ba, and a non-Al ₂ O ₃ support. And a supported layered catalyst having a second outer layer comprising a second different PGM, eg, Rh, and optionally an OSC, eg, a mixed oxide of ceria and zirconia.

  Japanese unexamined patent publication No. 5-317,652, which is incorporated herein by reference, describes a substrate and a catalyst comprising an alkaline earth metal compound and Pt supported on the substrate. ing. This description states that the car frequently accelerates and decelerates when traveling in urban areas. As a result, the air-fuel ratio can frequently change to a fuel rich side from a more steady state, for example, idling, within a range of values close to the stoichiometric point. For example, in order to reduce fuel consumption during urban driving conditions, the gasoline engine is operated on the fuel lean side, for example up to 23: 1 (wt./wt.) Air-fuel ratio. This catalyst adsorbs (absorbs) NOx on the alkaline earth metal during lean running conditions, and releases and reduces the stored NOx by utilizing the natural fluctuation of the air-fuel ratio to the rich side, thereby causing alkaline earth. It is designed to regenerate the NOx storage capacity of similar metal compounds.

US Pat. No. 5,575,983 (included here as reference) shows that the NOx absorption capacity of the catalyst of Japanese unexamined patent publication No. 5-317,652 is poisoned by sulfate. Is described. To address this, this patent comprises Pt or Pd supported on lithium stabilized Al ₂ O ₃ and rare earth elements including alkali metals, alkaline earth metals and lanthanum (La). A catalyst is proposed.

  We have lean burn gasoline engines other than direct-injection gasoline engines, such as those described in Japanese unexamined patent publication No. 5-317,652 and US Pat. No. 5,575,983. It is not widely used in the automobile industry, and one reason for this is thought to be the difficulty in suppressing NOx emissions to meet existing and future emissions regulations.

  Here we are very surprising that a spark ignition engine with a TWC containing a NOx storage component, such as a port fuel injection gasoline engine, runs lean without the need for expensive and complex control mechanisms. It has been found that it can be operated so as to receive the benefits of improved fuel economy under conditions.

  In one aspect, the present invention provides an engine that runs at a stoichiometric air / fuel ratio during normal driving conditions and runs on the lean side of the stoichiometric air / fuel ratio during a particular portion of the engine speed / load map. An engine control device programmed to control the air-fuel ratio of the engine, and an exhaust mechanism comprising a catalyst (the catalyst comprises a three-way catalyst (TWC) containing a NOx storage component) A spark ignition engine, wherein the engine controller determines the amount of NOx in contact with the TWC in response to data input from the sensor means during lean driving, thereby the residual NOx storage capacity of the TWC And is further programmed to return the air-fuel ratio to stoichiometry when the residual NOx storage capacity is lower than a predetermined value, and is operated continuously in stoichiometric mode. Compared to ignition engines, many of the NOx by the engine cycle to provide an engine, characterized by being configured to substantially prevent from being released into the atmosphere.

  Here, “engine cycle” means the time between key on and key off.

  The present invention takes advantage of the advantage that the composition of the exhaust gas of a spark ignition engine operated at a stoichiometric air-fuel ratio naturally fluctuates to a rich lambda value upon acceleration, for example, and regenerates the NOx storage component. Under the condition of λ = 1, NOx stored on the NOx storage component is not released, and the NOx storage component is not regenerated, that is, the rich side condition of lambda = 1 is required. We have also devised a catalyst that facilitates the regeneration of NOx storage components in TWC, but this catalyst is used in the preferred embodiment of the present invention.

The present invention provides a number of very substantial advantages. One such advantage is that, according to the present invention, a vehicle driven by a spark ignition engine travels while saving fuel over a similar vehicle operated continuously at substantially stoichiometric conditions. can do. Such efficiency increasing, it is possible to reduce the CO ₂ emissions in the legal test cycle for a motor vehicle. Reduction of CO ₂ emissions in the statutory test cycle, it means that the CO ₂ emission is reduced in the driving conditions of the "real world". Therefore, it can be said that the automobile of the present invention is more “preferred for the environment”. Furthermore, in countries where taxation (so-called “green tax”) is imposed by the amount of CO ₂ emitted by automobiles, such as the UK, the tax burden on consumers is reduced.

  A second such advantage is that existing vehicles including spark ignition engines can benefit from the present invention by retrofitting certain components. This replaces the existing TWC with a TWC that contains sufficient NOx storage components and allows the engine controller to (i) engine on the lean side of the stoichiometric air / fuel ratio during a particular portion of the engine speed / load map. And (ii) in lean driving, in response to data input from the sensor means, determine the amount of NOx in contact with the TWC, thereby determining the residual NOx storage capacity of the TWC, and the residual NOx When the storage capacity is lower than a predetermined value, the air / fuel ratio is returned to stoichiometry, thereby allowing for more during the engine cycle compared to a spark ignition engine operating continuously in stoichiometric mode. This is accomplished simply by replacing the engine controller programmed to substantially prevent NOx from being released into the atmosphere.

  In a particularly preferred embodiment, a specific part of the engine speed / load map is engine idling. This is a particularly advantageous setting because it provides a security mechanism for regenerating the NOx storage component. This can happen to the engine after idling, but the engine is only accelerated and subsequently the exhaust gas in contact with the TWC is only temporarily rich, after which the engine controller returns the exhaust gas to equivalence. Because. Even if the engine switch is turned off following idling, NOx is stored up to the key on, and the engine is subsequently accelerated. Therefore, this preferable setting takes advantage of the advantage that the composition of the exhaust gas naturally fluctuates to the rich side during acceleration and regenerates the NOx storage component. For similar reasons, certain parts of the engine speed / load map can also include low speed running where the NOx level exhausted from the engine is up to 10 times, for example, 5 times or 2 times higher than during engine idling.

  A very significant advantage of this preferred approach is that it does not require complex and expensive sensors and control devices to meet exhaust emission regulations that are currently an obstacle to NOx trap adoption.

  Which can be performed alone or mechanically / electronically to enter data into the engine controller and determine the amount of NOx in contact with the TWC, and from it the residual NOx storage capacity of the NOx storage component, alone or mechanically / electronically It can be used in such combinations. Many of the sensor means necessary to gather this information are already contained in the engine and / or the vehicle in which the engine is mounted and are used by the engine controller to control the engine and / or other functions of the vehicle. Has been. This is one of the reasons why the present invention can be employed by retrofitting an engine control device to a vehicle with a TWC containing NOx storage components.

  Such detection data that can be used to monitor the residual NOx storage capacity in the TWC of the present invention includes a lean operation that is predetermined or foreseeed by sensing the state of a suitable clock means. Elapsed time from start of air, air flow or manifold vacuum on TWC, ignition timing, engine speed, throttle position, eg exhaust gas redox composition using lambda sensor, preferably linear lambda sensor, amount of fuel injected into engine, automobile If an exhaust gas recirculation (EGR) circuit is included, the position of the EGR valve and the amount of EGR detected thereby, the engine coolant temperature, and if the exhaust mechanism includes a NOx sensor, upstream and / or downstream of the TWC The amount of NOx detected is mentioned. When using a clock embodiment, the predicted time can subsequently be adjusted in response to data input.

  The spark ignition engine may be any engine that can operate at a stoichiometric air-fuel ratio during normal operating conditions. In one embodiment, the engine can be driven by gasoline and the engine may be port fuel injection or direct injection. If desired, the engine can use alternative fuels such as liquid petroleum gas (LPG), natural gas (NG), hydrocarbon mixtures including methanol, ethanol or hydrogen gas. The present invention can be used with all grades of sulfur-containing fuels, but particularly good efficiencies are obtained with grades containing less than 50 ppm by weight of sulfur, most preferably less than 10 ppm by weight of sulfur.

  The present invention can be used with any known TWC composition provided that it contains sufficient NOx storage components to perform the desired function.

  A typical TWC composition comprises at least one PGM selected from the group consisting of Pt, Pd, Rh, ruthenium, osmium and iridium and combinations of two or more thereof.

  The prior art discloses many NOx storage components, any of which can be used in the present invention. Typical NOx storage components include alkali metals such as potassium or cesium, alkaline earth metals such as magnesium, calcium, strontium or Ba, rare earth metals, lanthanide group metals, preferably La, or any two or more thereof. Any viable combination, for example a mixed oxide.

  A common component of state-of-the-art TWC is OSC, which can also be used effectively in the TWC of the present invention. In fact, since OSC supports HC combustion at the stoichiometric point and its slightly richer side, it is common knowledge to those skilled in the art that the NOx trap composition does not contain OSC. This property goes against the requirements of a mechanism that includes a NOx trap composition that regenerates NOx storage components using reducing chemical species obtained from air-fuel ratio adjustment, such as HC in the exhaust. Thus, the presence of OSC in the NOx trap composition will increase the fuel consumption to regenerate the same amount of NOx storage components as compared to the NOx trap composition without OSC.

Known OSCs include manganese-based compounds supported on optionally stabilized ceria, perovskite, NiO, MnO ₂ , Al ₂ O ₃ containing mixed oxides (International Patent Application No. PCT / GB01, incorporated herein by reference). / 05124), mixed oxides of manganese and zirconium (see International Patent No. WO99 / 34904, which is incorporated herein by reference), Pr ₂ O ₃ or combinations of two or more thereof. The ceria stabilizer may be zirconium, lanthanum, aluminum, yttrium, praseodymium or neodymium.

  Preferred TWCs for use in the present invention include the first PGM, preferably Pt, and NOx storage components in the first inner layer, and the OSC and second PGM, preferably Rh, in the second outer layer. Become.

This formulation is advantageous for the following reasons. During stoichiometric running, the Rh / OSC component is active for NOx reduction and other reactions, while the Pt component is active for oxidation reactions. Under oxygen-rich conditions, Rh is relatively inert, Pt is active against NO, HC and CO oxidation, and the formed NO ₂ is stored as nitrate in the adsorbent.

  Subsequently, returning to lambda = 1, the OSC component in the second layer prevents the stored NOx from “seeking” for reducing gas, so NOx remains stored as nitrate. Rh is active against NOx reduction by CO, and Pt is active against HC and CO oxidation.

  Due to the acceleration, the gas mixture moves rich and reduces the OSC material, so that the nitrate becomes thermodynamically unstable and the stored NOx is released. The released NOx is then reduced by the Rh layer containing excess reductant.

  According to another aspect, the present invention provides an automobile comprising the engine of the present invention.

  The TWC is typically located at one or both of two locations in the vehicle, depending on the intended purpose: a close-coupled location where the TWC is located as close as possible to the exhaust manifold, and an underfloor location. The The reason for placing the TWC in the approach position is to control the exhaust immediately after the cold start, because many of the controlled exhausts are exhausted immediately after the cold start during the legal test cycle. It is. By placing the TWC close to the engine, the catalyst comes into contact with the hot exhaust gas immediately after the key on, thus reaching the cold activation temperature for CO and HC oxidation earlier than the colder underfloor position. However, when the exhaust mechanism rises to a temperature at which the three-way conversion is efficiently performed, the underfloor catalyst takes on much of the exhaust gas treatment burden. During this time, the TWC at the approach position is exposed to very high temperatures, for example up to 1000 ° C. In fact, some automobile manufacturers require that the in-situ TWC be tested at a catalyst bed temperature of at least 970 ° C. to 1010 ° C. for 50 hours. At such temperatures, the catalyst may lose activity because materials lose their surface area due to sintering, migration of active species into the pores, and interaction of components. Thus, the TWC in the close position is expected to lose some of its activity compared to the unused catalyst. The NOx storage component may also lose its NOx storage capacity because the surface area is reduced by this high temperature aging.

  It is not preferred for the present invention that the NOx storage capacity is reduced due to high temperature aging, particularly with the TWC placed in the close position, but the benefits of the present invention are still obtained if the ratio of NOx storage activity is maintained. Thus, in one embodiment of the present invention, the unused TWC includes sufficient NOx storage components to maintain sufficient NOx storage capacity, for example, even after high temperature aging at an approach location.

  According to another embodiment, the present invention provides an engine control device for a spark ignition engine comprising an exhaust mechanism comprising a TWC containing a NOx storage component, the engine control device being adapted for normal driving conditions. The engine air / fuel ratio is controlled so that the engine travels on the lean side of the stoichiometry during a specific portion of the engine speed / load map. In response to data input from the TWC to determine the amount of NOx in contact with the TWC, thereby determining the residual NOx storage capacity of the TWC and, if the residual NOx storage capacity is lower than a predetermined value, the air / fuel ratio. Compared to a spark ignition engine that is programmed to return to stoichiometry and operates continuously in stoichiometric mode, more NOx is released into the atmosphere during the engine cycle. It is set so as to substantially prevent from being released to.

  According to yet another aspect, the present invention comprises a spark ignition engine that travels at stoichiometric air-fuel ratio during normal travel conditions (the engine comprises an exhaust mechanism comprising a TWC that contains NOx storage components). ) In which the engine air-fuel ratio is controlled to run on the lean side of the stoichiometry during a specific part of the engine speed / load map, and during the lean running operation, the sensor means In response to the input data, determine the amount of NOx that comes into contact with the TWC, thereby determining the residual NOx storage capacity of the TWC and chemistry the air / fuel ratio when the residual NOx storage capacity is lower than a predetermined value. More NOx is released into the atmosphere during the engine cycle as compared to a spark ignition engine comprising a stoichiometric step and continuously operated in stoichiometric mode. It is set so as to substantially prevent the provides methods.

  For a better understanding of the present invention, the following examples are given by way of illustration only with reference to the accompanying drawings.

Example 1-Total hydrocarbon low temperature activity Three catalyst washcoats were tested. Comparative catalyst A is a state of the art Pt / RhTWC supported on a heat stable high surface area support at a ratio of 5Pt: 1Rh and a total noble metal loading of 60 gft- ³ .

Catalyst B is a TWC containing the NOx storage component of the present invention supported on an equivalent support. The catalyst comprises a first inner layer of high surface area Al ₂ O ₃ impregnated with Pt and NOx storage components such as BaO, and a second outer layer of mixed oxide OSC impregnated with Rh. The Pt: Rh ratio and total noble metal loading are equal to catalyst A.

Comparative catalyst C is a NOx trap composition comprising a high surface area Al ₂ O ₃ mixed oxide support impregnated with Pt, Rh and NOx storage components. The ratio of Pt: 1Rh was 6: 1 and the total noble metal loading was 70 gft ⁻³ .

Each washcoat was applied on a 4.66 × 6 inch (11.9 × 15.2 cm) ceramic substrate, 400 cells / in ² ((cpsi) 62 cell cm ⁻² ), wall thickness 0.15 mm, resulting The coated substrate was hydrothermally aged at 800 ° C. for 5 hours under 10% O ₂ /10% H ₂ O / balance nitrogen.

  These catalysts were attached to an exhaust mechanism of an engine mounted on a work table of a 4-cylinder, 2.0 liter port fuel injection type controlled by a Bosch ME7 controller. The catalyst temperature was increased by adjusting a heat exchanger installed in front of the exhaust line catalyst.

  The results for HC low temperature activity (the temperature at which the reaction is catalyzed to 50% efficiency) are shown in FIG. 1, from which it can be seen that the HC low temperature activation temperature for catalyst B is similar to that of Comparative Example A. It can also be seen that the low temperature activity of the comparative catalyst C is about 30 ° C. higher than that of the comparative catalyst A.

  This result shows that the TWC containing the NOx storage component (Catalyst B) is very similar in terms of HC activity compared to the state of the art TWC (Comparative Catalyst A) despite the presence of the NOx storage component. It has shown that it has activity. The performance of the NOx trap (comparative catalyst C) is not as good as that of catalyst B or comparative catalyst A despite the high Pt loading.

Example 2
The same engine as in the confused lambda scan example 1 was used and the catalyst inlet was at a temperature of 450 ° C. Lambda scan uses 10% confusion (cycle between 10% lower than lambda = 1, ie 0.147 lambda lower value to 10% higher than lambda = 1 (1.147 lambda) value) and frequency 1Hz. went. These conditions are related to the TWC located in the exhaust mechanism (which includes a lambda sensor that feeds back to the engine controller to maintain the lambda = 1 condition upstream of the catalyst inlet). A choice was made to simulate the exhaust gas composition at the inlet.

  The results are shown in FIGS. Although the NOx trap (comparative catalyst C) has more Pt, the lambda scanning performance is inferior (low conversion), and the TWC (catalyst B) containing the NOx storage component is TWC (comparative catalyst). It can be seen that the same performance as in A) is exhibited.

Example 3
In the engine test bench test room, a DC dynamometer was attached to a Mitsubishi direct injection engine obtained from a 4-cylinder, 1.8-liter, 1997-made automobile calibrated for the Japanese market. The catalyst substrate produced according to Example 1 was mounted at an approximate position of about 30 cm from the engine exhaust manifold. The substrate volume is 22% of the engine sweep volume (ESV). The concentration of NOx, EC, CO ₂ and O ₂ was measured using two rows of MEXA (Motor Exhaust Gas Analyzer) 9500 sensors, and the gas concentrations upstream and downstream of the catalyst were measured continuously. The temperature at the catalyst inlet was measured with a thermocouple.

  The engine was operated from two sets of maps, one for homogeneous mode and the other for lean stratified mode. The basic map for the timing and duration of ignition and injection first records data from the vehicle's ECU containing the same model engine as the engine used for bench testing, and then reverses processing based on this information ( reverse engineering). The engine was operated in a range of engine speeds and loads in homogeneous mode, and a supplemental map to the basic map was created for the best emission of NOx, CO and HC emissions under λ = 1 operation. The lean layered mode was mapped by adapting the torque achieved in the homogeneous mode at the same speed and load requirements.

  Prior to testing, the engine was fully warmed in idling condition. The engine was then operated in homogeneous mode so that the inlet temperature of the closely placed catalyst was 300 ° C. The operation was then switched to lean stratified operation and the EGR valve position was adjusted until the NOx exiting the engine was 300 ppm. This EGR valve position was recorded and called the lean set point. The engine was returned to homogeneous mode and the EGR valve was closed. A rich set point was obtained by increasing the fuel injector pulse width to obtain a lambda of 0.80. A series of lean / rich cycles were performed as follows. In lean mode, the EGR valve was adjusted to the lean set point position until the NOx efficiency of the mechanism dropped below 75%. The engine was then returned to homogeneous mode for 15 seconds, with the injector duration at the rich set point. This cycle was repeated 5 times and the results obtained in each cycle were recorded. Attach the exhaust mechanism to the engine and follow the above procedure to collect data in a lean stratified mode at a catalyst inlet temperature of 300 ° C, which is a typical catalyst inlet temperature for the closely located TWC in the exhaust mechanism for port fuel injection during idling. It was.

  Table 1 shows the NOx storage efficiency in which each of Comparative Catalysts A and C and Catalyst B stores NOx, and in particular how efficiently each catalyst stores NOx 30, 40, 50 and 60 mg. For example, the NOx trap (Comparative Catalyst C) stores NOx 60 mg with 97% efficiency, ie, 97% of NOx in contact with the catalyst, whereas the WCO (Comparative Catalyst A) stores NOx 60 mg with 9% efficiency. During the time required to store 60 mg of NOx, 91% of the NOx that comes into contact with the catalyst slips through.

Table 1

NOx storage efficiency
Comparative catalyst C Catalyst B Comparative catalyst A
NOx stored (NOx trap) (NOx storage component (TWC)
(mg) TWC included)
30 98% 77% 18%
40 98% 73% 16%
50 97% 68% 12%
60 97% 64% 9%

  The results of Examples 1, 2, and 3 show that the TWC containing the NOx storage component maintains the TWC performance despite the inclusion of the NOx storage component, and the NOx storage capacity is higher than the state-of-the-art TWC composition. It is several times higher.

FIG. 1 is a graph showing the relationship between HC conversion after aging and temperature of TWC including the NOx storage component of the present invention, TWC of the current state of the art, and NOx trap. FIG. 2 is a graph showing the relationship between lambda and CO, HC and NOx conversion% in TWC containing the NOx storage component of the present invention. FIG. 3 is a graph showing the relationship between the conversion rate% of CO, HC and NOx and the lambda in the current state of the art TWC. FIG. 4 is a graph showing the relationship between CO, HC and NOx conversion% and lambda in the NOx trap composition.

Claims (12)

A spark ignition engine having an exhaust mechanism,
The exhaust mechanism comprises a three-way catalyst and an engine control device,
The three-way catalyst comprises a first inner layer comprising at least one platinum group metal and a NOx storage component, and a second outer layer comprising at least one platinum group metal and an oxygen storage component. Is,
The engine controller controls the air / fuel ratio of the engine, runs at stoichiometric air / fuel ratio during normal driving conditions, and stoichiometric air / fuel ratio during a specific portion of the engine speed / load map. Programmed to run on the lean side, the specific part of the engine speed / load map is idling conditions,
During lean travel operation, in response to data input from the sensor means, determine the amount of NOx in contact with the three-way catalyst, thereby determining the residual NOx storage capacity of the three-way catalyst; and Programmed to return the air-fuel ratio to stoichiometry when the residual NOx storage capacity is lower than a predetermined value;
During the stoichiometric running, in the three-way catalyst, the platinum group metal in the second outer layer reduces NOx, and the oxygen storage component in the second outer layer stores NOx . An engine that is designed to substantially prevent NOx from being released into the atmosphere.   2. The engine according to claim 1, comprising a timepiece, and the data input from the sensor means includes a predetermined time elapsed from the start of lean driving. The sensor means comprises:
Manifold vacuum,
Ignition timing,
Engine speed,
Throttle position,
Lambda values detected upstream and / or downstream of the TWC,
The amount of fuel injected into the engine,
The engine comprises an exhaust gas recirculation circuit, the position of the exhaust gas recirculation circuit valve,
Coolant temperature of the engine,
The engine according to claim 1 or 2, wherein at least one of the amounts of NOx detected upstream and / or downstream of the three-way catalyst is detected.   The engine according to any one of claims 1 to 3, which is a port fuel injection engine or a direct injection engine.   The engine according to any one of claims 1 to 4, wherein the fuel is a hydrocarbon mixture or hydrogen gas including liquid petroleum gas, natural gas, methanol, ethanol. The at least one platinum group metal is selected from the group consisting of platinum, palladium, rhodium, ruthenium, osmium, iridium, and a combination of any two or more thereof. The listed engine.   The NOx storage component is an alkali metal selected from potassium or cesium, an alkaline earth metal selected from magnesium, calcium, strontium or barium, a rare earth metal selected from lanthanide group metals, or any of these The engine according to any one of claims 1 to 6, comprising a combination of two or more kinds. The three-way catalyst is stabilized ceria, perovskite, NiO, MnO ₂ , a manganese-based compound supported on an alumina-containing mixed oxide, a mixed oxide of manganese and zirconium, Pr ₂ O ₃ and these two types The engine according to any one of claims 1 to 7, comprising an oxygen storage component selected from the group consisting of the above combinations.   The engine of claim 8, wherein the ceria stabilizer is zirconium, lanthanum, aluminum, yttrium, praseodymium or neodymium.   A vehicle comprising the engine according to any one of claims 1 to 9. An engine control device for a spark ignition engine provided with an exhaust mechanism,
The exhaust mechanism comprises a three-way catalyst;
The three-way catalyst comprises a first inner layer comprising at least one platinum group metal and a NOx storage component, and a second outer layer comprising at least one platinum group metal and an oxygen storage component. Is,
Control the air / fuel ratio of the engine, drive at stoichiometric air / fuel ratio during normal driving conditions, and drive on the lean side of the stoichiometric air / fuel ratio during specific parts of the engine speed / load map. And the specific part of the engine speed / load map is the idling condition, and
During lean travel operation, in response to data input from the sensor means, determine the amount of NOx that contacts the three-way catalyst;
Thereby determining the residual NOx storage capacity of the three-way catalyst and, when the residual NOx storage capacity is lower than a predetermined value, programmed to return the air-fuel ratio to stoichiometry,
During the stoichiometric running, in the three-way catalyst, the platinum group metal in the second outer layer reduces NOx, and the oxygen storage component in the second outer layer stores NOx . An engine control device designed to substantially prevent NOx from being released into the atmosphere. An exhaust gas treatment method for a spark ignition engine that travels at stoichiometric air-fuel ratio during normal driving conditions,
The engine comprises a three-way catalyst;
The three-way catalyst comprises a first inner layer comprising at least one platinum group metal and a NOx storage component, and a second outer layer comprising at least one platinum group metal and an oxygen storage component. Is,
An exhaust mechanism comprising the three-way catalyst containing the NOx storage component; and
Controlling the air / fuel ratio of the engine to run on the lean side of the stoichiometric air / fuel ratio during a specific portion of the engine speed / load map , wherein the specific portion of the engine speed / load map is an idling condition; and ,
During lean travel operation, in response to data input from the sensor means, determine the amount of NOx in contact with the three-way catalyst, determine the residual NOx storage capacity of the three-way catalyst, and the residual NOx If storage capacity is lower than a predetermined value, Ri Na comprise returning the air-fuel ratio to stoichiometry,
During the stoichiometric running, in the three-way catalyst, the platinum group metal in the second outer layer reduces NOx, and the oxygen storage component in the second outer layer stores NOx . An exhaust gas treatment method designed to substantially prevent NOx from being released into the atmosphere.

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