GB2581493A

GB2581493A - Control apparatus and method

Info

Publication number: GB2581493A
Application number: GB1902239.1A
Authority: GB
Inventors: Cudmore Stephen; Hartland Jonathan
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2019-02-19
Filing date: 2019-02-19
Publication date: 2020-08-26
Anticipated expiration: 2039-02-19
Also published as: GB2581493B; GB201902239D0

Abstract

Disclosed is an engine control unit 7 for controlling a diesel engine 2 to regenerate a diesel particulate filter 6 disposed in an exhaust system 3 connected to the diesel engine 2. The engine control unit 7 has at least one processor 8 configured to control an diesel engine 2. A memory device 9 having instructions stored therein is coupled to the at least one processor 8. The at least one processor 8 is configured to control the diesel engine 2 to perform a regeneration event for regenerating the diesel particulate filter 6. The regeneration event includes repeated rich-lean cycling of the diesel engine 2 during the regeneration event. A catalyst, such as a three way catalyst 5, may be provided upstream of the particulate filter 6. The catalyst 5 may include a Diesel oxidation catalyst which is used to alternately store and release oxygen depending on the temperature of the exhaust gas. An oxygen sensor may be used to control the rich/lean cycling. The arrangement and method allow an engine system to remain around the stoichiometric limit when regenerating a Diesel particulate filter.

Description

CONTROL APPARATUS AND METHOD

TECHNICAL FIELD

The present disclosure relates to a control apparatus and method. More particularly, the present disclosure relates to an engine control unit and to a method for controlling a diesel engine to regenerate a diesel particulate filter. The present disclosure also relates to a vehicle.

BACKGROUND

It is known to provide aftertreatment systems in an exhaust system connected to an internal combustion engine to control emissions. In the case of a diesel engine, the aftertreatment systems may comprise a diesel particulate filter (DPF) operative to trap particulates from the combustion process. The DPF traps carbonaceous particulate material (such as soot) and a regeneration strategy is implemented to oxidise the carbonaceous particulate material and to regenerate the DPF. The regeneration strategy may, for example, comprise one or more regeneration event. During a regeneration event, the operating mode of the diesel engine is modified to increase the exhaust gas temperature to oxidise the carbonaceous particulate material trapped in the filter. However, whilst operating in this mode, the diesel engine tends to generate higher emissions than during normal operation. In particular, the NOx emissions may be higher during a regeneration event. In current emissions legislation a correction factor is typically added to the measured emissions to take into account increased emissions during the regeneration event and the frequency of the regeneration event. With tighter emissions legislation in Europe, China (Beijing) and the US, the emissions control during a regeneration event is becoming more important. For some legislation, high efficiency aftertreatment may be required during the DPF regeneration. However, the high temperatures during the regeneration event typically make it difficult to achieve high aftertreatment efficiency.

At least in certain embodiments, the present invention seeks to provide a control apparatus and method which overcomes or ameliorates at least some of the limitations of prior art systems.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to an engine control unit, a vehicle and a method as claimed in the appended claims.

According to an aspect of the present invention there is provided an engine control unit for controlling a diesel engine to regenerate a diesel particulate filter disposed in an exhaust system connected to the diesel engine, the engine control unit comprising: at least one processor configured to control an diesel engine; a memory device having instructions stored therein and coupled to the at least one processor; wherein the at least one processor is configured to control the diesel engine to perform a regeneration event for regenerating the diesel particulate filter, the regeneration event comprising rich-lean cycling of the diesel engine. The engine control unit implements the rich-lean cycle during the regeneration event. Thus, the engine control unit may provide substantially stoichiometric operation of the internal combustion engine during the regeneration event. By cycling between rich and lean operation of the diesel engine, the engine control unit can perform the regeneration event. The engine control unit has particular application in a vehicle having a Diesel Oxidation Catalyst (DOG) operative to convert nitrogen oxides (NOx) generated during the regeneration event. For example, the DOC may be disposed upstream of the DPF and operate stoichiometrically during the rich-lean cycling. The DOG may reduce the NOx in the exhaust gases, thereby improving the efficiency of the aftertreatment of the exhaust gases during DPF regeneration.

The engine control unit is also applicable in a vehicle having a Three Way Catalyst (TWC). A TWC is operative to combine oxygen with unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas to produce water (H2O) and carbon dioxide (CO2). The TWC is also operative to convert nitrogen oxides (NOx) in the exhaust gas. Significantly, the TWC may convert NOx generated during the regeneration event when rich-lean cycling of the diesel engine is performed. The rich-lean cycling of the diesel engine comprises cycling between lean running and rich running of the diesel engine. The TWC may store oxygen during lean running when the exhaust gas expelled from the diesel engine comprises oxygen for oxidising HC and CO in the exhaust gas. The TVVC may use the stored oxygen to oxidise the HC and CO during rich running when the exhaust gas expelled from the diesel engine is rich and there would otherwise be insufficient oxygen for oxidation. The TWC may be disposed upstream of the DPF. The TWC may operate at high temperatures to reduce the NOx in the exhaust gases during a regeneration event of the DPF. The TWC may thereby improve the efficiency of the aftertreatment of the exhaust gases.

Lambda (A) is less than one (1) for rich operation; and greater than one (1) for lean operation. The at least one processor is configured to implement the rich-lean cycling of the diesel engine by cyclically increasing lambda (A) to a value greater than one and decreasing lambda (A) to a value less than one.

The at least one processor may be configured to receive a first signal from a first oxygen sensor for determining an oxygen content of an exhaust gas in the exhaust system downstream of the diesel particulate filter and to control the rich-lean cycling of the diesel engine in dependence on said first signal. The engine control unit may thereby implement a closed loop control strategy. The first oxygen sensor provides feedback to the processor.

The at least one processor may be configured to control the rich-lean cycling of the diesel engine by transitioning from rich operation to lean operation when the first signal indicates a decrease in the oxygen content of the exhaust gas in the exhaust system downstream of the diesel particulate filter. The at least one processor may increase lambda (A) from a value less than one (1) to a value greater than one (1). The at least one processor may be configured to control the rich-lean cycling of the diesel engine by transitioning from rich operation to lean operation when the first signal indicates the oxygen content of the exhaust gas downstream of the diesel particulate filter is less than or equal to a predefined first oxygen content threshold.

The at least one processor may be configured to control the rich-lean cycling of the diesel engine by transitioning from lean operation to rich operation when the received first signal indicates an increase in the oxygen content of the exhaust gas downstream of the diesel particulate filter. The at least one processor may reduce lambda (A) from a value greater than one (1) to a value less than one (1). The at least one processor may be configured to control the rich-lean cycling of the diesel engine by transitioning from lean operation to rich operation when the received first signal indicates the oxygen content of the exhaust gas downstream of the diesel particulate filter is greater than or equal to a predefined second oxygen content threshold.

The at least one processor may be configured to control lambda (A) to implement said rich-lean cycle by generating a control signal having one of the following: a sinusoidal wave profile, a triangular wave profile, a saw tooth wave profile and a square wave profile.

The at least one processor may be configured to determine a temperature of the diesel particulate filter. During a regeneration event, the at least one processor may be configured to transition from rich operation to lean operation when the determined temperature is greater than or equal to a predefined temperature threshold. The predefined temperature threshold corresponds to a minimum temperature for oxidation of carbonaceous particulate material in the particulate filter. The at least one processor may be configured to model the temperature of the particulate filter. Alternatively, or in addition, the at least one processor may receive a temperature signal from a temperature sensor disposed in the particulate filter.

According to a further aspect of the present invention there is provided a vehicle comprising a diesel engine, an exhaust system connected to the diesel engine having a diesel particulate filter, and an engine control unit as described herein.

The exhaust system may comprise a catalyst disposed upstream of the diesel particulate filter. The catalyst may comprise oxygen storage means for storing oxygen during lean operation of the diesel engine. The oxygen storage means may be suitable for releasing the stored oxygen during rich operation of the diesel engine. The oxygen storage means may comprise Ceria; Cerium Oxide; Cerium(III) Oxide (Ce203); and Cerium(IV) Oxide (Ce02). Alternatively or in addition the catalyst may be applied to the diesel particulate filter. The catalyst may be a diesel oxidation catalyst (DOG) or a three way catalyst (TWC) According to a further aspect of the present invention there is provided a method of controlling a diesel engine to regenerate a diesel particulate filter disposed in an exhaust system, the method comprising: controlling the diesel engine to perform a regeneration event for regenerating the diesel particulate filter, the regeneration event comprising rich-lean cycling of the diesel engine.

The method may comprise implementing the rich-lean cycling of the diesel engine by cyclically increasing lambda (A) to a value greater than one and decreasing lambda (A) to a value less than one.

The method may comprise monitoring an oxygen content of exhaust gas downstream of the diesel particulate filter and implementing the rich-lean cycling of the diesel engine in dependence on the oxygen content of the exhaust gas.

The method may comprise controlling the diesel engine to transition from rich operation to lean operation in dependence on detection of a decrease in the oxygen content of the exhaust gas in the exhaust system downstream of the diesel particulate filter. The method may comprise transitioning from rich operation to lean operation when the oxygen content of the exhaust gas downstream of the diesel particulate filter is less than or equal to a predefined first oxygen content threshold.

The method may comprise transitioning from lean operation to rich operation in dependence on detection of an increase in the oxygen content of the exhaust gas downstream of the diesel particulate filter. The method may comprise transitioning from lean operation to rich operation when the oxygen content of the exhaust gas downstream of the diesel particulate filter is greater than or equal to a predefined second oxygen content threshold.

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 vehicle incorporating an engine control unit and an exhaust system in accordance with an embodiment of the present invention; and Figures 2A-C show schematic representations of lambda control signals for controlling operation of the internal combustion engine shown in Figure 1 in accordance with different embodiments of the present invention.

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 conveying exhaust gas from the internal combustion engine 2. The vehicle 1 in the present embodiment is an automobile, but the present invention may usefully be implemented in other types of vehicle.

The internal combustion engine 2 is a diesel engine which combusts diesel fuel in one or more combustion chamber (not shown). In the present embodiment, the internal combustion engine 2 is a light duty diesel (LDD) adapted to operate under lean conditions. Exhaust gases from the combustion cycle are expelled from the internal combustion engine 2 into the exhaust system 3 for treatment by aftertreatment systems (denoted by the reference numeral 4). The aftertreatment systems 4 include a catalyst 5 5 and a diesel particulate filter (DPF) 6. The catalyst may be a diesel oxidation catalyst or a three way catalyst. In the present embodiment the catalyst is a diesel oxidation catalyst (DOC). The DOC 5 is disposed upstream of the DPF Sin the exhaust system 3, i.e. the DOC 5 is positioned between the internal combustion engine 2 and the DPF 6. The DPF 6 traps carbonaceous particulate material particulates generated during combustion of the diesel fuel. As described herein, a regeneration event is performed periodically to oxidise the trapped carbonaceous particulate material.

The DOC 5 is operative to convert nitrogen oxides (N0x) in the exhaust gases expelled from the internal combustion engine as is known to someone skilled in the art. In the present embodiment the DOC 5 comprises a quantity of rhodium. The DOC 5 also comprises means for storing oxygen known as an Oxygen Storage Capacity (OSC), this may be in the form of an oxygen storage (OS) material. The OS material may, for example, comprise or consist of Ceria (Cerium Oxide; Cerium(III) Oxide (Celia); Cerium(IV) Oxide (Ce02)). The OS material stores oxygen present in the exhaust gases during lean operation of the internal combustion engine 2. The stored oxygen is released from the OS material during rich operation of the internal combustion engine 2. As described herein, the release of oxygen by the OS material can promote regeneration of the DPF 6. The OS material may also help to dampen a drop to idle event which might otherwise cause damage to the DPF 6, for example in hot conditions.

The DPF 6 collects particles of carbonaceous particulate material generated during combustion and transported through the exhaust system 3 in the exhaust gas. The carbonaceous particulate material may comprise particles of soot. The DPF 6 may be regenerated by oxidising the carbonaceous particulate material at high temperature in the presence of oxygen. The carbonaceous particulate material oxidation may, for example, require that the temperature of the DPF 6 exceeds 500°C or even 600°C.

The vehicle 1 comprises an engine control unit 7 for controlling operation of the internal combustion engine 2, including fuelling of the internal combustion engine 2. The engine control unit 7 comprises a processor 8 connected to a memory device 9. The processor 8 is configured to implement a set of non-transitory computational instructions stored on said memory device 9. When executed, the computational instructions cause the processor to implement an engine control strategy for controlling operation of the internal combustion engine 2. The processor 8 is configured to output a lambda control signal CON1 for controlling lambda (A) of the internal combustion engine 2. Lambda (A) is the ratio of the actual air/fuel ratio (AFR) to the stoichiometric air/fuel ratio (AFRstoich) and is defined by the following equation:

AFR

-

AFRstotch The operation of the internal combustion engine 2 is stoichiometric when lambda (A) is equal to one (A=1). The lambda control signal CON1 may increase or decrease lambda (A) of the internal combustion engine 2. Under normal operating conditions, the internal combustion engine 2 is adapted to operate under lean conditions, i.e. lambda (A) is greater than one (1). By varying lambda (A), the temperature and/or composition of the exhaust gas expelled from the internal combustion engine 2 may be selectively controlled.

In order to regenerate the DPF 6, the engine control unit 7 is configured to control the internal combustion engine 2 to perform a regeneration event. During the regeneration event, the internal combustion engine 2 is controlled to adjust the temperature and composition (particularly the oxygen content) of the exhaust gas. The regeneration event comprises controlling lambda (A) to perform rich-lean cycling of the internal combustion engine 2. The engine control unit 7 cycles the internal combustion engine 2 between phases of rich operation (Ad) and lean operation (A>1). Rich operation of the internal combustion engine 2 generates high temperature exhaust gases which increase the temperature of the DPF 6 to a temperature sufficiently high to enable oxidation of the carbonaceous particulate material trapped in the DPF 6. However, the oxygen content of the exhaust gases is reduced during rich operation and there may not be sufficient oxygen present to complete carbonaceous particulate material oxidation. Conversely, lean operation of the internal combustion engine 2 increases the oxygen content of the exhaust gas, but reduces the temperature of the exhaust gases. The rich-lean cycling of the internal combustion engine 2 establishes the required conditions for carbonaceous particulate material oxidation in the DPF 6. The OS material in the DOG 5 is operative to store oxygen during lean operation within the rich-lean cycle. The stored oxygen is subsequently released during rich operation within the rich-lean cycle and supplied to the DPF 6 with the exhaust gases. The OS material may promote carbonaceous particulate material oxidation within the DPF 6. The DPF 6 may optionally have a catalytic coating (and may be referred to as a coated DEW). The catalytic coating may be configured to perform at least some of the functions of the DOC 5 described herein. In particular, the catalytic coating may comprise an OS material, such as ceria, for storing oxygen during a lean operating phase.

Provided the temperature of the DREG is high enough and oxygen is available, carbonaceous particulate material trapped in the DPF 6 is oxidised. The oxidation process reduces the oxygen content of the exhaust gas and this change in the oxygen content may be detected downstream of the DPF 6 by the first oxygen sensor 10. It will be understood that carbon monoxide (CO) and/or unburned hydrocarbons (UHC) may also be oxidised in the DPF Band these processes will also consume oxygen. The engine control unit 7 may determine when carbonaceous particulate material oxidation is occurring by monitoring the oxygen content of the exhaust gas expelled from the DPF 6. As shown in Figure 1, the exhaust system 3 comprises a first oxygen sensor 10 disposed downstream of the DPF 6. The first oxygen sensor 10 comprises a Heated Exhaust Gas Oxygen (HEGO) sensor (also referred to as a lambda sensor or a "narrow-band" sensor). The first oxygen sensor 10 is adapted to monitor the oxygen content of the exhaust gas downstream of the DPF 6 and to provide feedback to the engine control unit 7. The first oxygen sensor 10 outputs a first oxygen content signal SIG1 which is used by the engine control unit 7 to implement a closed-loop fuelling control strategy to control lambda (A) of the internal combustion engine 2 during a regeneration event. In particular, the engine control unit 7 is configured to control the rich-lean cycling of the internal combustion engine 2 in dependence on the first oxygen content signal 5I01. During lean operation in the rich-lean cycle, the engine control unit 7 is configured to transition to rich operation of the internal combustion engine 2 when the first oxygen sensor 10 detects oxygen in the exhaust gas downstream of the DPF 6 (i.e. when oxygen breakthrough is detected). In order to transition from lean operation to rich operation, the engine control unit modifies lambda (A) from a value greater than one (A>1) to a value less than one (A<1). Conversely, during rich operation in the rich-lean cycle, the engine control unit 7 is configured to transition to lean operation of the internal combustion engine 2 when the first oxygen sensor 10 detects a drop in the oxygen content of the exhaust gas downstream of the DPF 6 (i.e. when oxygen is consumed by carbonaceous particulate material oxidation). Alternatively, during rich operation in the rich-lean cycle, the engine control unit 7 maybe configured to transition to lean operation of the internal combustion engine 2 when a temperature sensor 11 located in the after treatment system 4 detects that the temperature of the DPF reaches a threshold. When transitioning from rich operation to lean operation, the engine control unit modifies lambda (A) from a value less than one (A<1) to a value greater than one (A>1).

The temperature sensor in the illustrated example is located downstream of the DPF but it may alternatively be located immediately upstream of the DPF. In alternative examples two temperature sensors may be provided, one upstream of the DPF and one downstream of the DPF.

As outlined above, the processor 8 outputs a lambda control signal CON1 for controlling lambda (A) of the internal combustion engine 2. As illustrated in Figures 2A-C, the profile of the lambda control signal CON1 may be modified in different applications. With reference to Figure 2A, according to a first aspect of the present invention the lambda control signal CON1 comprises a generally sinusoidal waveform. With reference to Figure 2B, according to a second aspect of the present invention the lambda control signal CON1 comprises a generally square waveform. With reference to Figure 2C, according to a third aspect of the present invention the lambda control signal CON1 comprises a generally triangular waveform. As outlined above, the frequency and period of the phases of rich operation (A<1) and lean operation (A>1) in each of these control strategies is controlled in dependence on the first oxygen content signal SIG1.

At least in certain embodiments, the engine control unit 7 in accordance with the present invention may improve carbonaceous particulate material oxidation in the DPF 6.

In an alternative embodiment to that described above, the aftertreatment system 4 includes a three way catalyst (TWO) instead of a diesel oxidation catalyst. The remaining features of the vehicle 1 are the same as described with respect to Figure 1.

The TVVC is operative to convert nitrogen oxides (N0x) in the exhaust gases expelled from the internal combustion engine. At least in certain embodiments the TWO is adapted to operate at high temperature, for example at temperatures required to oxidise carbonaceous particulate material in the DPF 6. The TWC may thereby reduce NOx emissions during a regeneration event. The TWC stores oxygen present in the exhaust gases during lean operation of the internal combustion engine 2. The stored oxygen is released from the TWC during rich operation of the internal combustion engine 2. As described herein, the release of oxygen by the TWO can promote regeneration of the DPF 6. The TWO may also help to dampen a drop to idle event which might otherwise cause damage to the DPF 6, for example in hot conditions. The DEW 6 collects carbonaceous particulate material generated during combustion and transported through the exhaust system 3 in the exhaust gas. The carbonaceous particulate material may comprise particles of soot. The DPF 6 may be regenerated by oxidising the carbonaceous particulate material at high temperature in the presence of oxygen. The carbonaceous particulate material oxidation may, for example, require that the temperature of the DPF 6 exceeds 500°C or even 600°C.

The TWC is operative to oxide unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas; and to convert nitrogen oxides (N0x) in the exhaust gas. In particular, the TWO is operative to convert NOx generated during the regeneration event during rich-lean cycling of the internal combustion engine 2. The TWO may store oxygen during lean operation of the internal combustion engine 2 and may release the stored oxygen during rich operation of the internal combustion engine 2. The TWO may thereby ensure that the DPF 6 is supplied with oxygen during rich operation of the internal combustion engine 2. At least in certain embodiments, the TWO may enable oxidation of the HC and CO during regeneration of the DPF 6. The TWO may promote carbonaceous particulate material oxidation within the DPF 6. The DPF 6 may optionally have a catalytic coating (and may be referred to as a coated DPF). The catalytic coating may be configured to perform at least some of the functions of the TWO described herein.

It will be appreciated that various changes and modifications may be made to the engine control unit 7 described herein without departing from the scope of the present invention. For example, the engine control unit 7 may determine an operating temperature of the DPF 6. The determined temperature of the DEW may be used by the engine control unit 7 to control the rich-lean cycle during a regeneration event. For example, the engine control unit 7 may use the determined temperature to control a transition either to or from lean operation. The engine control unit 7 may, for example, transition from rich operation to lean operation when the determined temperature is greater than a predefined temperature threshold.

Claims

CLAIMS: 1. An engine control unit for controlling a diesel engine to regenerate a diesel particulate filter disposed in an exhaust system connected to the diesel engine, the engine control unit 5 comprising: at least one processor configured to control an diesel engine; a memory device having instructions stored therein and coupled to the at least one processor; wherein the at least one processor is configured to control the diesel engine to perform a regeneration event for regenerating the diesel particulate filter, the regeneration event comprising rich-lean cycling of the diesel engine.
2. An engine control unit as claimed in claim 1, wherein the at least one processor is configured to implement the rich-lean cycling of the diesel engine by cyclically increasing lambda to a value greater than one and decreasing lambda to a value less than one.
3. An engine control unit as claimed in claim 1 or claim 2, wherein the at least one processor is configured to receive a first signal from a first oxygen sensor for determining an oxygen content of an exhaust gas in the exhaust system downstream of the diesel particulate filter and to control the rich-lean cycling of the diesel engine in dependence on said first signal.
4. An engine control unit as claimed in claim 3, wherein the at least one processor is configured to control the rich-lean cycling of the diesel engine by transitioning from rich operation to lean operation when the first signal indicates a decrease in the oxygen content of the exhaust gas in the exhaust system downstream of the diesel particulate filter.
5. An engine control unit as claimed in claim 4, wherein the at least one processor is configured to control the rich-lean cycling of the diesel engine by transitioning from rich operation to lean operation when the first signal indicates the oxygen content of the exhaust gas downstream of the diesel particulate filter is less than or equal to a predefined first oxygen content threshold.
6. An engine control unit as claimed in any one of claims 3, 4 or 5, wherein the at least one processor is configured to control the rich-lean cycling of the diesel engine by transitioning from lean operation to rich operation when the received first signal indicates an increase in the oxygen content of the exhaust gas downstream of the diesel particulate filter.
7. An engine control unit as claimed in claim 6, wherein the at least one processor is configured to control the rich-lean cycling of the diesel engine by transitioning from lean operation to rich operation when the received first signal indicates the oxygen content of the exhaust gas downstream of the diesel particulate filter is greater than or equal to a predefined second oxygen content threshold.
8. An engine control unit as claimed in any one of the preceding claims, wherein the at least one processor is configured to implement said rich-lean cycle by generating a control signal having one of the following: a sinusoidal wave profile, a triangular wave profile, a saw tooth wave profile and a square wave profile.
9. A vehicle comprising a diesel engine, an exhaust system connected to the diesel engine having a diesel particulate filter, and an engine control unit as claimed in any one of the preceding claims.
10. A vehicle as claimed in claim 9, wherein the exhaust system comprises a catalyst disposed upstream of the diesel particulate filter.
11. A vehicle as claimed in claim 10, wherein the catalyst comprises oxygen storage means operative to store oxygen during lean operation of the diesel engine.
12. A vehicle as claimed in claim 11, wherein the oxygen storage means is operative to release the stored oxygen during rich operation of the diesel engine.
13. A vehicle as claimed in any of claims 10 to 12, wherein the catalyst comprises a diesel oxidation catalyst.
14. A vehicle as claimed in claim 13, wherein the oxygen storage means comprises cerium.
15. A vehicle as claimed in any of claims 10 to 12, wherein the catalyst comprises a three way catalyst.
16. A vehicle as claimed in claim 15 wherein the three way catalyst is disposed upstream of the diesel particulate filter; and/or the three way catalyst is applied to the diesel particulate filter.
17. A method of controlling a diesel engine to regenerate a diesel particulate filter disposed in an exhaust system, the method comprising: controlling the diesel engine to perform a regeneration event for regenerating the diesel particulate filter, the regeneration event comprising rich-lean cycling of the diesel engine.
18. A method as claimed in claim 17 comprising implementing the rich-lean cycling of the diesel engine by cyclically increasing lambda to a value greater than one and decreasing lambda to a value less than one.
19. A method as claimed in claim 17 or claim 18 comprising monitoring an oxygen content of exhaust gas downstream of the diesel particulate filter and implementing the rich-lean cycling of the diesel engine in dependence on the oxygen content of the exhaust gas.
20. A method as claimed in claim 19 comprising controlling the diesel engine to transition from rich operation to lean operation in dependence on detection of a decrease in the oxygen content of the exhaust gas in the exhaust system downstream of the diesel particulate filter.
21. A method as claimed in claim 20 comprising transitioning from rich operation to lean operation when the oxygen content of the exhaust gas downstream of the diesel particulate filter is less than or equal to a predefined first oxygen content threshold.
22. A method as claimed in any one of claims 19, 20 or 21 comprising transitioning from lean operation to rich operation in dependence on detection of an increase in the oxygen content of the exhaust gas downstream of the diesel particulate filter.
23. A method as claimed in claim 22 comprising transitioning from lean operation to rich operation when the oxygen content of the exhaust gas downstream of the diesel particulate filter is greater than or equal to a predefined second oxygen content threshold.