GB2555483A

GB2555483A - Exhaust gas treatment method and apparatus

Info

Publication number: GB2555483A
Application number: GB1618407.9A
Authority: GB
Inventors: Hartland Jonathan
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2016-11-01
Filing date: 2016-11-01
Publication date: 2018-05-02
Anticipated expiration: 2036-11-01
Also published as: WO2018082952A1; GB2555483B; GB201618407D0

Abstract

A controller 8 for controlling a lean burn gasoline engine to reactivate a passive NOx adsorber (PNA) 5 and a three-way catalyst (TWC) 6 comprises a processor 10 for outputting an engine control signal ECS1 to control lambda of the gasoline engine, and a memory device 11 coupled to the processor. The engine control signal includes a rich signal component to set lambda to less than one 1 for reactivating the TWC and a stoichiometric signal component to set lambda substantially equal to one 1 for reactivating the PNA. The PNA is thereby reactivated with a stoichiometric exhaust gas which will not simultaneously reactivate the TWC, so that any NOX released by the PNA gets adsorbed by the TWC. The stoichiometric signal component may be disposed after the rich signal component, or the two signal components may be juxtaposed. The stoichiometric component time period may be longer than the rich component time signal. Also claimed is a vehicle having a lean burn gasoline internal combustion engine with both a PNA and TWC, a vehicle comprising the controller of the previous aspect, and a method of controlling lambda of a lean burn gasoline engine to reactivate a PNA and a TWC.

Description

(54) Title of the Invention: Exhaust gas treatment method and apparatus Abstract Title: Exhaust gas treatment method and apparatus (57) A controller 8 for controlling a lean burn gasoline engine to reactivate a passive NOx adsorber (PNA) 5 and a threeway catalyst (TWC) 6 comprises a processor 10 for outputting an engine control signal ECS1 to control lambda of the gasoline engine, and a memory device 11 coupled to the processor. The engine control signal includes a rich signal component to set lambda to less than one 1 for reactivating the TWC and a stoichiometric signal component to set lambda substantially equal to one 1 for reactivating the PNA. The PNA is thereby reactivated with a stoichiometric exhaust gas which will not simultaneously reactivate the TWC, so that any NOX released by the PNA gets adsorbed by the TWC. The stoichiometric signal component may be disposed after the rich signal component, or the two signal components may be juxtaposed. The stoichiometric component time period may be longer than the rich component time signal. Also claimed is a vehicle having a lean burn gasoline internal combustion engine with both a PNA and TWC, a vehicle comprising the controller of the previous aspect, and a method of controlling lambda of a lean burn gasoline engine to reactivate a PNA and a TWC.

t₀ t, t₂ t₃

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to tl t₂ t₃

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FIG. 2C

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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FIG. 1

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FIG. 2A

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FIG. 2B

FIG. 2C

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FIG. 3

FIG. 4

EXHAUST GAS TREATMENT METHOD AND APPARATUS

TECHNICAL FIELD

The present disclosure relates to exhaust gas treatment method and apparatus. More particularly, but not exclusively, the present disclosure relates to a controller, method and apparatus of treating exhaust gases emitted from an internal combustion engine, such as a lean burn gasoline engine. The present disclosure also relates to a vehicle, such as an automobile, comprising an internal combustion engine.

BACKGROUND

Lean gasoline combustion can deliver up to 10% improvement in brake specific fuel consumption (BSFC). However, a dual purpose emissions aftertreatment system is required to fulfil the existing functions of the three-way catalyst (TWC) and also to control emissions in a lean environment. These requirements are similar to those of diesel engines. Due to the lean nature of diesel exhaust gas, removing pollutants is much more challenging than for gasoline engines and the aftertreatment system is more complex. Reducing the complexity, and hence cost, of the resultant combined emissions control system is key to the adoption of lean gasoline combustion. Current solutions to lean NOx conversion include lean NOx traps (LNT) and selective catalytic reduction (SCR). The SCR injects urea into the exhaust gases which is converted to ammonia. Alternatively, ammonia may be produced by an upstream LNT or TWC.

A newer technology for treating emissions from a diesel engine is a Passive NOx Adsorber (PNA). PNA technology has been disclosed as being suitable for cold starts of a diesel engine, and also for gasoline engines during a cold start phase.

It is against this backdrop that the present invention has been conceived.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to a controller, to a vehicle and to a method as claimed in the appended claims.

According to a further aspect of the present invention there is provided a controller for controlling a lean burn gasoline engine to reactivate a passive NOx adsorber (PNA) and a three-way catalyst (TWC); the controller comprising:

at least one processor for outputting an engine control signal (ECS1) to control lambda (λ) of the gasoline engine; and a memory device having instructions stored therein and coupled to the at least one processor;

wherein the engine control signal (ECS1) comprises:

a rich signal component to set lambda (λ) to less than one (1) for reactivating the TWC; and a stoichiometric signal component to set lambda (λ) substantially equal to one (1) for reactivating the PNA.

The PNA and the TWC form part of an exhaust gas treatment system connected to the gasoline engine. Lambda (λ) is the ratio of the actual air/fuel ratio (AFR) to the stoichiometric air/fuel ratio (AFR_sto/_c/,). By controlling lambda (λ), the temperature and/or composition of the exhaust gases from the gasoline engine may be controlled. The controller is configured to control lambda (λ) in order to reactivate the TWC and the PNA. The controller has particular application in controlling a lean burn gasoline engine which is configured, during normal operation, to maintain lambda (λ) greater than one (1), i.e. to maintain lean operation. The at least one processor outputs the engine control signal to reactivate the TWC and the PNA. The rich signal component of the engine control signal sets lambda (λ) to less than one (1) to provide rich operation of the gasoline engine in order to reactivate the TWC. The stoichiometric signal component sets lambda (λ) substantially equal to one (1) to provide stoichiometric operation of the gasoline engine in order to reactivate the PNA. The stoichiometric operation of the gasoline engine typically increases the temperature of the exhaust gases causing an increase in the temperature of the PNA which, at least in certain embodiments, releases stored NOx.

When used in conjunction with a selective catalytic reduction (SCR), the PNA can reduce the amount of urea that is consumed during lean operation, particularly at low exhaust gas temperatures. The amount of NOx that is emitted from a lean burn gasoline engine during lean burn combustion can be significantly higher than a diesel engine. These higher emissions might otherwise result in significant urea consumption, thereby increasing the frequency that the on-board urea storage container must be refilled.

Within the engine control signal, the stoichiometric signal component may be disposed after the rich signal component. In use, the TWC may be reactivated before the PNA. This resulting staggered reactivation of the TWC and the PNA ensures that the TWC can treat any NOx purged during reactivation of the PNA.

The stoichiometric signal component and the rich signal component may be juxtaposed in said engine control signal. The stoichiometric signal component may follow at least immediately after the rich signal component. In use, the TWC and the PNA may be reactivated sequentially with little or no delay between the reactivation cycles.

The engine control signal may comprise first and second lean signal components to set lambda (λ) of the gasoline engine to greater than one (1). The lean signal components of the engine control signal set lambda (λ) to greater than one (1) to provide lean operation of the gasoline engine; this corresponds to normal operating conditions of the gasoline engine. The first lean signal component may be disposed before said rich signal component and the second lean signal component may be disposed after said stoichiometric signal component.

The rich signal component has a first time period and the stoichiometric signal component has a second time period. The second time period may be longer than the first time period. The first time period may be relatively short to provide a rich spike in the exhaust gases to reactivate the TWC.

The first time period and/or the second time period may be predefined. Alternatively, the first time period and/or the second time period may be determined dynamically. For example, the first time period may be determined in dependence on an available oxygen storage capacity of the TWC; and/or the second time period may be determined in dependence on an available NOx storage capacity of the PNA. The available oxygen storage capacity of the TWC may be modelled. Alternatively, or in addition, the available NOx storage capacity of the PNA may be modelled.

The second time period may be determined in dependence on a determined temperature of the PNA. The temperature of the PNA may be modelled or measured.

The at least one processor may be configured to output the engine control signal to control lambda (λ) of the gasoline engine in dependence on a reactivation control signal. The reactivation control signal may be issued by a reactivation control unit. The reactivation control unit may determine the available oxygen storage capacity of the TWC and/or the available NOx storage capacity of the PNA. The reactivation control signal may be output when the available oxygen storage capacity of the TWC is below a defined oxygen storage threshold; and/or the available NOx storage capacity of the PNA is below a defined NOx storage threshold. The at least one processor may output the engine control signal upon receipt of the reactivation control signal or when suitable conditions for reactivating the TWC and the PNA are identified.

The TWC may be incorporated into a three-way lean NOx trap (TWLNT).

The instructions stored on the memory device may be non-transitory. When executed, the instructions cause the at least one processor to perform the method(s) described herein.

According to a further aspect of the present invention there is provided a vehicle comprising a controller as described herein.

According to a further aspect of the present invention there is provided a vehicle comprising a lean-burn gasoline internal combustion engine and an exhaust system, the exhaust system comprising a passive NOx adsorber (PNA) and a three-way catalyst (TWC). At least in certain embodiments, the PNA may be optimised to operate in a temperature window suitable for enabling NOx control when the gasoline engine is running both lean and stoichiometrically.

The exhaust gas temperatures are typically lower during lean operation of the gasoline engine than during rich operation. In use, the PNA stores NOx when the gasoline engine is operating lean (and the exhaust gas temperatures are lower). The PNA may subsequently desorb the stored NOx into the TWC at the higher temperatures present when the gasoline engine is operating under stoichiometric conditions.

The PNA may be disposed upstream of the TWC in said exhaust system.

The exhaust system may comprise a lean NOx trap (LNT) and/or a selective catalytic reduction (SCR) device downstream of the PNA and the TWC. It has been determined that NOx may be present in the downstream exhaust gases under certain operating conditions, for example as lambda transitions between lean conditions (λ>1) and/or stoichiometric conditions (λ=1) and/or rich conditions (λ<1). In use, the LNT and/or the SCR device may capture NOx in the downstream exhaust gases.

The TWC may be incorporated into a three-way lean NOx trap (TWLNT).

The vehicle may comprise a controller as described herein.

According to a further aspect of the present invention there is provided a method of controlling lambda (λ) of a lean burn gasoline engine to reactivate a passive NOx adsorber (PNA) and a three-way catalyst (TWC); the method comprising:

setting lambda (λ) to less than one (1) to reactivate the TWC; and setting lambda (λ) substantially equal to one (1) to reactivate the PNA.

The method may comprise setting lambda (λ) to less than one (1) to reactivate the TWC and then setting lambda (λ) substantially equal to one (1) to reactivate the PNA. The method may comprise setting lambda (λ) to less than one (1) to reactivate the TWC and then immediately setting lambda (λ) substantially equal to one (1) to reactivate the PNA. The reactivation of the TWC and the PNA may thereby be performed with little or no intervening time interval.

The method may comprise setting lambda (λ) greater than one (1) before reactivating the TWC and after reactivating the PNA.

The method may comprise setting lambda (λ) to less than one (1) to reactivate the TWC for a first time period; and setting lambda (λ) substantially equal to one (1) to reactivate the PNA for a second time period. The second time period may be longer than the first time period.

The method may comprise determining an available oxygen storage capacity of the TWC and determining the first time period in dependence on the determined oxygen available storage capacity of the TWC. The method may comprise determining an available NOx storage capacity of the PNA, wherein the second time period is determined in dependence on the determined available NOx storage capacity of the PNA.

The method may comprise determining a temperature of the PNA and determining the second time period in dependence on the determined temperature of the PNA.

Any control unit or controller described herein may suitably comprise a computational device having one or more electronic processors. The system may comprise a single control unit or electronic controller or alternatively different functions of the controller may be embodied in, or hosted in, different control units or controllers. As used herein the term “controller” or “control unit” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide any stated control functionality. To configure a controller or control unit, a suitable set of instructions may be provided which, when executed, cause said control unit or computational device to implement the control techniques specified herein. The set of instructions may suitably be embedded in said one or more electronic processors. Alternatively, the set of instructions may be provided as software saved on one or more memory associated with said controller to be executed on said computational device. The control unit or controller may be implemented in software run on one or more processors. One or more other control unit or controller may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller. Other suitable arrangements may also be used.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 shows a schematic representation of a vehicle having an exhaust system in accordance with an embodiment of the present invention;

Figures 2A-C illustrate the control strategy for reactivating the exhaust gas treatment devices of the exhaust system shown in Figure 1;

Figure 3 shows a first variant of the exhaust system shown in Figure 1; and Figure 4 shows a second variant of the exhaust system shown in Figure 1.

DETAILED DESCRIPTION

A vehicle 1 in accordance with an embodiment of the present invention is illustrated in Figure 1. The vehicle 1 comprises an internal combustion engine 2 having an exhaust system 3 for exhausting gases to atmosphere. The internal combustion engine 2 is a lean burn gasoline engine in the present embodiment.

The exhaust system 3 comprises an exhaust conduit 4 for conveying exhaust gases emitted from the internal combustion engine 2. The exhaust system 3 comprises aftertreatment devices for controlling emissions from the internal combustion engine 2. The exhaust gas treatment devices comprise a Passive NOx Adsorber (PNA) 5, a Three-Way Catalyst (TWO) 6 and a Selective Catalytic Reduction (SCR) device 7 disposed in series in the exhaust conduit 4. In use, the PNA 5, the TWO 6 and the SCR device 7 are operable to reduce NOx gases, such as Nitric Oxide (NO), present in the exhaust gas. The PNA 5 may comprise a NOx adsorber, for example comprising a noble metal, such as Palladium (Pd). The TWO 6 may comprise a NOx adsorber, such as Palladium (Pd) and/or Cerium (IV) Oxide (CeO2). In the present embodiment, the SCR 7 is a urea SCR 7 operable selectively to inject urea into the exhaust gases.

An engine control unit 8 is provided for controlling operation of the internal combustion engine 2 in response to driver inputs. A reactivation control unit 9 is provided for triggering reactivation of the exhaust gas treatment devices. The engine control unit 8 comprises a first processor 10 coupled to a first memory device 11. The first processor 10 is configured to implement the first set of non-transitory computational instructions stored on said first memory device 11. When executed, the first set of computational instructions cause the first processor 10 to implement an engine control strategy for controlling operation of the internal combustion engine 2. The reactivation control unit 9 comprises a second processor 12 coupled to a second memory device 13. The second processor 12 is configured to implement the second set of non-transitory computational instructions stored on said second memory device 13. When executed, the second set of computational instructions cause the second processor 12 to determine the available NOx storage capacity of the PNA 5 and/or the available oxygen storage capacity of the TWC 6 and to output a reactivation control signal RCS1.

The first processor 10 is configured to output an engine control signal ECS1 to control lambda (λ) of the internal combustion engine 2. Lambda (λ) of the internal combustion engine 2 is the ratio of the actual air/fuel ratio (AFR) to the stoichiometric air/fuel ratio (AFRsto/cft). During normal operation, the internal combustion engine 2 operates in a lean burn mode such that lambda (λ) of the internal combustion engine 2 is greater than one (λ>1). In the present embodiment, the engine control signal ECS1 is configured to control lambda (λ) to reactivate the PNA 5 and the TWC 6. The first processor 10 is configured selectively to modify lambda (λ) of the internal combustion engine 2 to perform a reactivation event. By modifying lambda (λ), the temperature and/or the composition of the exhaust gases can be controlled so as to reactivate the exhaust gas treatment devices. For example, the engine control signal ECS1 may be modified selectively to increase or decrease the temperature of the exhaust gases; and/or to adjust the air/fuel composition of the exhaust gases. By way of example, the engine control unit 8 selectively controls lambda (λ) to cause the internal combustion engine 2 to operate in a rich burn mode (λ<1) or close to stoichiometric (λ=1).

The second processor 12 models the available NOx storage capacity for the PNA 5 and the available oxygen storage capacity of the TWC 6, for example in dependence on known operating conditions of the internal combustion engine. Alternatively, or in addition, the second processor 12 may receive one or more signal from a lambda sensor or an oxygen sensor disposed in the exhaust system 3 to determine an operational state of one or more of the PNA 5 and the TWC 6. The second processor 12 is configured to trigger reactivation of the PNA 5 and the TWC 6 in dependence on the determined available NOx and oxygen storage capacity. When the determined available NOx storage capacity of the PNA 5 decreases below a predetermined NOx storage threshold and/or the available oxygen storage capacity of the TWC 6 decreases below a predetermined oxygen storage threshold, the second processor 12 outputs the reactivation control signal RCS1. The reactivation control signal RCS1 is published to a communication network, such as a CAN bus, and is read by the engine control unit 8.

The first processor 10 is configured to control lambda (λ) to reactivate the PNA 5 and the TWC 6 in dependence on said reactivation control signal RCS1. Different operating conditions are required to reactivate each of the PNA 5 and the TWC 6. The PNA 5 is exposed to stoichiometric exhaust gases whereas the TWC 6 is exposed to rich exhaust gases. Thus, operating the internal combustion engine 2 in a rich burn mode reactivates the TWC 6. The engine control signal ESC1 is configured initially to decrease lambda (λ) to a value less than one (1) such that rich exhaust gases are supplied to the TWC 6 (i.e. to supply exhaust gases comprising a greater proportion of fuel). The PNA 5 desorbs NOx when exposed to exhaust gases having a higher temperature. The temperature of the exhaust gases may be increased by operating the internal combustion engine 2 at stoichiometric or substantially stoichiometric conditions. The PNA 5 desorbs NOx into the TWC 6 and/or the SCR 7 while the internal combustion engine 2 is operating at stoichiometric conditions.

Figures 2A-C are schematic representations of the reactivation strategy implemented by the engine control unit 8. Figure 2A is a first plot 100 showing exhaust gas temperature; Figure 2B is a second plot 110 showing lambda (λ) of the internal combustion engine 2 (as controlled by the engine control signal ECS1); and Figure 3 is a third plot 120 schematically representing the NOx stored in the PNA 5 and the TWC 6.

With reference to Figures 2A-C, the internal combustion engine 2 is initially operating in a lean burn mode (t=tO). In dependence on the reactivation control signal RCS1, the engine control unit 8 outputs the engine control signal ECS1 to initiate a reactivation event. As shown in Figure 2B, the engine control signal ECS1 decreases lambda (λ) (t=t1) to a value less than one (1) such that the internal combustion engine 2 operates in a rich burn mode. The resulting rich exhaust gases expelled through the exhaust conduit 4 reactivate the TWC 6, as illustrated by the continuous line shown in Figure 2C. The engine control signal ECS1 maintains rich operation of the internal combustion engine 2 for a first reactivating time period T1 (t1 to t2). The duration of the first reactivating time period T1 may optionally be controlled in dependence on the determined available oxygen storage capacity of the TWC

6. For example, the engine control unit 8 may be configured to reduce the first reactivating time period T1 if the determined available oxygen storage capacity of the TWC 6 is high; and to increase the first reactivating time period T1 if the determined available oxygen storage capacity of the TWC 6 is low. The engine control signal ECS1 then increases lambda (λ) to approximately one (1) (t=t2) such that the internal combustion engine 2 operates under stoichiometric conditions. The engine control signal ECS1 maintains lambda (λ) substantially equal to one (1) for a second reactivating time period T2 (t2 to t3) in order to increase the temperature of the exhaust gases. The elevated exhaust gas temperatures increase the temperature of the PNA 5, causing NOx to desorb from the PNA 5, as illustrated by the dashed line in Figure 2C. The PNA 5 is thereby reactivated. The engine control signal ECS1 increases lambda (λ) to a value greater than one (1) (t=t4) coincident with or after reactivation of the PNA 5 is complete. The engine control unit 8 may be configured to modify the second reactivating time period T2 (t3 to t4) in dependence on the determined available NOx storage capacity of the PNA 5. For example, the engine control unit 8 may be configured to increase the second reactivating time period T2 when the determined available NOx storage capacity of the PNA 5 is low; and to decrease the second reactivating time period T2 when the determined available NOx storage capacity is high. In a variant, the engine control signal ECS1 may maintain lambda (λ) substantially equal to one (1) during the second reactivating time period T2 (t2 to t3) to maintain the temperature of the exhaust gases at least substantially constant.

It will be understood that the PNA 5 is operative to store NOx at the relatively low exhaust gas temperatures which occur when operating the gasoline internal combustion engine 2 in a lean burn mode. The PNA 5 stores NOx when the internal combustion engine 2 operates lean, and then desorbs NOx into the TWC 6 at higher exhaust gas temperatures, for example when the internal combustion engine is operating stoichiometrically. The PNA 5 may be configured to operate in a temperature window suitable for enabling NOx control when running both lean and stoichiometrically. It will be understood that the NOx desorbed from the PNA 5 is abated by the TWC 6 under stoichiometric conditions.

The engine control unit 8 is configured to control the internal combustion engine 2 to provide the required operating conditions to reactivate the PNA 5 and the TWC 6. It will be understood that various changes and modifications may be made to the embodiment described herein. The engine control unit 8 may be configured to control operation of the internal combustion engine 2 in dependence on the determined available NOx storage capacity of the PNA 5 and/or the available oxygen storage capacity of the TWC 6. For example, the reactivation control unit 9 may output the determined available NOx storage capacity of the PNA 5 and/or the available oxygen storage capacity of the TWC 6. The duration of the rich and stoichiometric operation respectively of the internal combustion engine 2 may be controlled in dependence on the determined available storage capacity.

As described herein, the reactivation control unit 9 determines when the PNA 5 and the TWC 6 should be reactivated. The reactivation control unit 9 may, for example, output a reactivation control signal RCS1 when the available NOx storage capacity of the PNA 5 decreases below a predetermined NOx storage threshold; and/or the available oxygen storage capacity of the TWC 6 decreases below a predetermined oxygen storage threshold. It is not necessary that the PNA 5 and/or the TWC 6 are completely deactivated; rather the reactivation control unit 9 may output the reactivation control signal RCS 1 when the PNA 5 and/or the TWC 6 are partially deactivated. The effectiveness of the PNA 5 and the TWC 6 may be determined in dependence on signals received from one or more sensor disposed in the exhaust system 3. The one or more sensor may comprise an oxygen sensor and/or a NOx sensor. Alternatively, or in addition, the performance of the PNA 5 and/or the TWC 6 may be modelled, for example in dependence on operating conditions of the internal combustion engine 2.

A first variant of the exhaust system 3 in accordance with an aspect of the present invention will now be described with reference to Figure 3. Like reference numerals are used for like components. The exhaust system 3 is modified to replace the SR device 7 downstream of the PNA 5 and the TWC 6 with a lean NOx trap (LNT) 14. The LNT 14 is provided to capture NOx in the exhaust gases downstream of the PNA 5 and the TWC 6. As described herein, the engine control unit 7 is operative to control lambda (λ) to reactivate the PNA 5 and the TWC 6. As lambda (λ) transitions, for example between lean conditions (λ>1) and/or stoichiometric conditions (λ=1) and/or rich conditions (λ<1), the efficacy of the PNA 5 and the TWC 6 may temporarily decrease. The LNT 14 is provided in the exhaust system 3 to abate NOx in the exhaust gases to reduce emissions.

A second variant of the exhaust system 3 in accordance with an aspect of the present invention will now be described with reference to Figure 4. Like reference numerals are used for like components. The exhaust system 3 has been described herein as comprising a PNA 5 and a TWC 6. The exhaust system 3 may be modified such that the TWC 6 is incorporated into a so-called three-way lean NOx trap (TWLNT) 15. The TWLNT 15 is a single catalyst that has both TWC and LNT functionality. The TWLNT 15 may, for example, comprise a one or more catalyst, such as Platinum (Pt) and/or Palladium (Pd). The LNT may comprise Barium (Ba). By way of example, the LNT may comprise Barium Carbonate (Ba(CO3)2)) which stores NOx as Barium Nitrate (Ba(NO₂)3))· The engine control unit 8 is configured to reactivate the TWLNT by setting lambda (λ) to less than one (1) to provide rich exhaust gas which converts the trapped NOx, as per normal LNT operation. By way of example, the hydrocarbons in the exhaust gas reduce the Barium Nitrate (Ba(NO₂)₃) and release Nitrogen (N₂). The TWLNT would be disposed downstream of the PNA 5 in the exhaust system 3. An example of the operation of the TWLNT 15 would be to enable higher temperature lean operation. Under lean conditions, the PNA 5 would desorb the trapped NOx which would be captured by the TWLNT 15. The exhaust system 3 may optionally comprise an SCR device 7 or a LNT 14.

It will be appreciated that various modifications may be made to the embodiment(s) described herein without departing from the scope of the appended claims.

Claims

CLAIMS:

1. A controller for controlling a lean burn gasoline engine to reactivate a passive NOx adsorber and a three-way catalyst; the controller comprising:

at least one processor for outputting an engine control signal to control lambda (λ) of the gasoline engine; and a memory device having instructions stored therein and coupled to the at least one processor;

wherein the engine control signal comprises:

2. A controller as claimed in claim 1, wherein the stoichiometric signal component is disposed after the rich signal component in said engine control signal.

3. A controller as claimed in claim 2, wherein the stoichiometric signal component and the rich signal component are juxtaposed in said engine control signal.

4. A controller as claimed in any one of claims 1, 2 or 3, wherein the engine control signal comprises first and second lean signal components to set lambda (λ) of the gasoline engine to greater than one (1); the first lean signal component disposed before said rich signal component and the second lean signal component disposed after said stoichiometric signal component.

5. A controller as claimed in any one of claims 1 to 4, wherein the rich signal component has a first time period and the stoichiometric signal component has a second time period; the second time period being longer than the first time period.

6. A controller as claimed in claim 5, wherein the first time period is determined in dependence on an available oxygen storage capacity of the TWC.

7. A controller as claimed in claim 5 or claim 6, wherein the second time period is determined in dependence on an available NOx storage capacity of the PNA.

8. A controller as claimed in any one of claims 5, 6 or 7, wherein the second time period is determined in dependence on a determined temperature of the PNA.

9. A vehicle comprising a controller as claimed in any one of the preceding claims.

10. A vehicle comprising a lean-burn gasoline internal combustion engine and an exhaust system, the exhaust system comprising a passive NOx adsorber and a three-way catalyst.

11. A vehicle as claimed in claim 10, wherein the PNA is disposed upstream of the TWC in said exhaust system.

12. A vehicle as claimed in claim 10 or claim 11 comprising a controller as claimed in any one of claims 1 to 8.

13. A vehicle as claimed in any one of claims 9 to 12 comprising a lean NOx trap and/or a selective catalytic reduction device downstream of the PNA and the TWC.

14. A vehicle as claimed in any one of claims 9 to 13, wherein the TWC is incorporated into a three-way lean NOx trap.

15. A method of controlling lambda (λ) of a lean burn gasoline engine to reactivate a passive NOx adsorber and a three-way catalyst; the method comprising:

16. A method as claimed in claim 15 comprising setting lambda (λ) to less than one (1) to reactivate the TWC and then setting lambda (λ) substantially equal to one (1) to reactivate the PNA.

17. A method as claimed in claim 16 comprising setting lambda (λ) to less than one (1) to reactivate the TWC and then immediately setting lambda (λ) substantially equal to one (1) to reactivate the PNA.

18. A method as claimed in any one of claims 15, 16 or 17, wherein lambda (λ) is greater than one (1) before reactivating the TWC and after reactivating the PNA.

19. A method as claimed in any one of claims 15 to 18 comprising setting lambda (λ) to less than one (1) to reactivate the TWC for a first time period; and setting lambda (λ) substantially equal to one (1) to reactivate the PNA for a second time period; the second time period being longer than the first time period.

20. A method as claimed in claim 19 comprising determining an available oxygen storage capacity of the TWC and determining the first time period in dependence on the determined available oxygen storage capacity of the TWC.

21. A method as claimed in claim 19 or claim 20 comprising determining an available NOx storage capacity of the PNA, wherein the second time period is determined in dependence on the determined available NOx storage capacity of the PNA.

22. A method as claimed in any one of claims 19, 20 or 21 comprising determining a temperature of the PNA and determining the second time period in dependence on the determined temperature of the PNA.

23. A controller substantially as herein described with reference to the accompanying figures.

24. A vehicle substantially as herein described with reference to the accompanying figures.

25. A method substantially as herein described with reference to the accompanying figures.

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Application No: GB1618407.9