GB2564833A

GB2564833A - An after treatment system, engine assembly and associated methods

Info

Publication number: GB2564833A
Application number: GB1704583.2A
Authority: GB
Inventors: Graham Eastwood Peter; David Sumner Andrew
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2017-03-23
Filing date: 2017-03-23
Publication date: 2019-01-30
Anticipated expiration: 2037-03-23
Also published as: GB201704583D0; GB2564833B; DE102018106420A1

Abstract

A method of regenerating a diesel particulate filter (DPF) in an after-treatment system comprises regenerating the DPF in a first mode in which soot is predominantly oxidised by nitrogen dioxide (NO2), the first mode occurring in a first temperature range and being sustained in a substantially steady state for a first time period, and regenerating the DPF in a second mode in which soot is predominantly oxidised by oxygen (O2), the second mode occurring in a second temperature range hotter than the first temperature range and being sustained in a substantially steady state for a second time period. The method may further comprise inhibiting introduction of a selective catalytic reductant into a selective catalytic reduction device during at least a portion of one or more regeneration events in the first mode. The method may further comprise sensing a dilution level of engine lubricant, and prioritising the first mode of regeneration is the dilution exceeds a threshold. Also claimed is an aftertreatment system and engine assembly comprising a controller configured to carry out the above method, an aftertreatment system and engine assembly with a recirculation system, and a method and engine assembly comprising a selective bleed system.

Description

AN AFTER TREATMENT SYSTEM, ENGINE ASSEMBLY AND ASSOCIATED METHODS

Technical Field

This disclosure relates generally to an after treatment system, engine assembly and associated methods and particularly, although not exclusively, relates to the regeneration of a diesel particulate filter.

Background

A diesel particulate filter accumulates soot, discharged by an internal combustion engine. The soot may be periodically burned, i.e. the filter may be 'regenerated'. This typically requires periodic intervention by an engine management system, e.g. to inject fuel late in the engine combustion cycle. The engine out temperature is then raised and some of this fuel also leaves the engine, after which it bums in an oxidation catalyst, thus raising exhaust gas temperature further to ~600°C ('hot regeneration'). Some of the post injected fuel may drain down, past the piston rings resulting in oil dilution and potentially compromising engine lubrication.

There is little room for manoeuvre. For example, if on the one hand the post injected fuel quantity is reduced, the exhaust gas may not be hot enough. On the other hand, if the regeneration is not frequent enough, there may be too much soot captured and the combustion of the additional soot may exceed a temperature limit for the filter. This is worsened by the adoption of newer filters with thinner, more porous walls.

Statements of Invention

According to an aspect of the present disclosure, there is provided a method of regenerating a diesel particulate filter of an after-treatment system, the method comprising:

regenerating the diesel particulate filter in a first mode in which soot collected on the diesel particulate filter is predominantly oxidised by nitrogen dioxide, the first mode occurring in a first temperature range and being sustained in a substantially steady state for a first time period; and regenerating the diesel particulate filter in a second mode in which soot collected on the diesel particulate filter is predominantly oxidised by oxygen, the second mode occurring in a second temperature range hotter than the first temperature range and being sustained in a substantially steady state for a second time period.

The after-treatment system may further comprise a selective catalytic reduction device. The diesel particulate filter may be upstream of, integral with or downstream of the selective catalytic reduction device. The method may comprise inhibiting (e.g. preventing, limiting or reducing) introduction of a selective catalytic reductant into the selective catalytic reduction device during at least a portion of one or more regeneration events in the first mode.

The after treatment system may further comprise an oxidation catalyst upstream of the diesel particulate filter.

The method may comprise selectively recirculating exhaust gases from downstream of the after treatment system, e.g. downstream of the diesel particulate filter, to upstream of the aftertreatment system, e.g. upstream of the oxidation catalyst. The recirculation may occur during one or more regeneration events in the first mode. The recirculated exhaust gases may not pass through, e.g. may bypass, an engine upstream of the after-treatment system.

The after-treatment system may further comprise a Venturi, e.g. a narrowing, in the flow path at an upstream end of the after treatment system, e.g. upstream of the oxidation catalyst. The method may comprise selectively recirculating exhaust gases from downstream of the after treatment system, e.g. downstream ofthe diesel particulate filter, to the narrowing ofthe Venturi, e.g. during one or more regeneration events in the first mode.

The method may comprise selectively recirculating exhaust gases from downstream of the diesel particulate filter through a Low Pressure Exhaust Gas Recirculation duct into a compressor of a turbocharger, e.g. during one or more regeneration events in the first mode. The method may comprise selectively bleeding inlet air from an outlet or downstream of an outlet of the turbocharger compressor through a bleed duct leading to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system, e.g. during one or more regeneration events in the first mode.

The first time period may be longer or shorter than the second time period. The method may comprise periodically regenerating the diesel particulate filter in the first mode and/or periodically regenerating the diesel particulate filter in the second mode. Regenerating the diesel particulate filter in the first mode may occur more frequently than regenerating the diesel particulate filter in the second mode. A first total amount of time spent in the first mode of regeneration may be greater than a second total amount of time spent in the second mode of regeneration.

For at least one pair of regeneration events, regenerating the diesel particulate filter in the first mode may be spaced apart in time from regenerating the diesel particulate filter in the second mode, e.g. there may be a time gap between the two regeneration events. Additionally or alternatively, for at least one pair of regeneration events, regenerating the diesel particulate filter in the second mode may immediately follow or precede regenerating the diesel particulate filter in the first mode, e.g. there may be no time gap between the two regeneration events.

The first mode may occur at temperatures between approximately 250°C and approximately 500°C. In particular, the first mode may occur at temperatures between approximately 300°C and approximately 400°C. By contrast, the second mode may occur at temperatures exceeding approximately 550°C. In particular, the second mode may occur at a temperature of approximately 600°C.

The method may comprise one or more of: tracking the build-up of soot on the diesel particulate filter; tracking the oxidation of the soot during the first and second modes of regeneration; and scheduling a regeneration event in the first or second mode.

The method may comprise sensing the amount of soot build-up on the diesel particulate filter. A regeneration event may be scheduled when the soot reaches a predetermined level.

The method may comprise recirculating exhaust gases from downstream of the diesel particulate filter through an exhaust gas recirculation duct, e.g. during one or more regeneration events in the first mode.

The method may comprise sensing a dilution level of an engine lubricant. The first mode of regeneration may be prioritised over the second mode of regeneration if the dilution level exceeds a threshold value.

The method may comprise: injecting a first amount of additional fuel into an engine upstream of the diesel particulate filter in the first mode regeneration event; and injecting a second amount of additional fuel into the engine in the second mode regeneration event. The second amount of additional fuel may be greater than the first amount of additional fuel.

A controller or engine assembly may be configured to carry out any of the above-mentioned methods.

According to another aspect of the present disclosure there is provided an after treatment system for an engine comprising a diesel particulate filter. The after treatment system may be configured to carry out any of the above-mentioned methods, e.g. the after treatment system may comprise the above-mentioned controller.

The after-treatment system may further comprise a selective catalytic reduction device. The selective catalytic reduction device may be downstream of, integral with or upstream of the diesel particulate filter. The after-treatment system may be configured to inhibit introduction of a selective catalytic reductant into the selective catalytic reduction device during at least a portion of one or more regeneration events in the first mode.

The after treatment system may further comprise an oxidation catalyst upstream of the diesel particulate filter. The after-treatment system may be configured to selectively recirculate exhaust gases from downstream of the after-treatment system, e.g. downstream of the diesel particulate filter, to upstream of the after-treatment system, e.g. upstream of the oxidation catalyst. For example, the after treatment system may further comprise a Venturi in the exhaust flow path upstream of the after-treatment system, e.g. upstream of the oxidation catalyst. The aftertreatment system may comprise a duct configured to selectively recirculate exhaust gases from downstream of the after-treatment system, e.g. diesel particulate filter, to a narrowing of the Venturi.

An engine assembly may comprise the above-mentioned after treatment system. The engine assembly may comprise a Low Pressure Exhaust Gas Recirculation duct configured to selectively recirculate exhaust gases from downstream of the diesel particulate filter into a compressor of a turbocharger. The engine assembly may further comprise a bleed duct configured to selectively bleed inlet air from an outlet or downstream of an outlet of the turbocharger compressor to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system.

According to another aspect of the present disclosure there is provided a method for an engine with an after-treatment system, the method comprising:

recirculating at least some of the exhaust gas from downstream of the after-treatment system to upstream of the after-treatment system, e.g. without passing the at least some recirculated exhaust gas through the engine.

The after-treatment system may comprise a Venturi in the flow path at an upstream end of the after-treatment system. The method may further comprise selectively recirculating exhaust gases from downstream of the after-treatment system to a narrowing of the Venturi.

The method may comprise selectively recirculating exhaust gases from downstream of the aftertreatment system through a Low Pressure Exhaust Gas Recirculation duct into a compressor of a turbocharger. The method may comprise selectively bleeding inlet air from an outlet or downstream of an outlet of the turbocharger compressor through a bleed duct leading to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system.

The after treatment system may comprise an oxidation catalyst and/or a diesel particulate filter. The oxidation catalyst may be upstream of the diesel particulate filter. The method may comprise regenerating the diesel particulate filter.

The method may comprise recirculating at least some of the exhaust gas during a warm-up phase of the after-treatment system.

According to another aspect of the present disclosure there is provided an after treatment system for an engine, wherein the after-treatment system is configured to recirculate at least some of the exhaust gases from downstream of the after treatment system to upstream of the after treatment system, e.g. without passing the at least some of the recirculated exhaust gases through the engine.

The after treatment system may further comprise a Venturi in the exhaust flow path at an upstream end of the after treatment system. The after-treatment system may comprise a duct configured to selectively recirculate exhaust gases from downstream of the after treatment system to a narrowing of the Venturi.

The after-treatment system may comprise an oxidation catalyst and/or a diesel particulate filter. The oxidation catalyst may be upstream of the diesel particulate filter. The after-treatment system may be configured to selectively regenerate the diesel particulate filter.

The after-treatment system may be configured to recirculate at least some of the exhaust gas during a warm-up phase of the after-treatment system.

An engine assembly may comprise the above-mentioned after treatment system. The engine assembly may comprise a Low Pressure Exhaust Gas Recirculation duct configured to selectively recirculate exhaust gases from downstream of the after-treatment system, e.g. diesel particulate filter, into a compressor of a turbocharger. The engine assembly may further comprise a bleed duct configured to selectively bleed inlet air from an outlet or downstream of an outlet of the turbocharger compressor to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system.

According to another aspect of the present disclosure there is provided a method for an engine, the method comprising:

selectively bleeding inlet air from an outlet or downstream of an outlet of a turbocharger compressor through a bleed duct leading, e.g. directly, to a point downstream of an outlet of a turbocharger turbine.

The method may comprise bleeding the inlet air through the bleed duct to the point downstream of the turbocharger turbine outlet. The point may also be upstream of an after-treatment system comprising an oxidation catalyst and a diesel particulate filter. The oxidation catalyst may be upstream of the diesel particulate filter.

The method may comprise recirculating exhaust gases from downstream of the diesel particulate filter through an exhaust gas recirculation duct, such as a Low Pressure Exhaust Gas Recirculation Duct.

The method may comprise controlling an exhaust gas temperature at least partially by selectively bleeding inlet air through the bleed duct. The method may comprise reducing the exhaust gas temperature by selectively bleeding inlet air through the bleed duct to reduce the temperature below an upper threshold, e.g. maximum, temperature of an after treatment system downstream of the turbocharger turbine.

The method may comprise selectively bleeding the inlet air through the bleed duct during a regeneration event for a diesel particulate filter. The diesel particulate filter may be upstream of, downstream of or integral, e.g. in a single package, with a selective catalytic reduction device. The method may comprise inhibiting introduction of a selective catalytic reductant into the selective catalytic reduction device, e.g. during one or more regeneration events for the diesel particulate filter.

The method may comprise controlling a surge margin of the turbocharger compressor by selectively bleeding inlet air through the bleed duct.

According to another aspect of the present disclosure there is provided an engine assembly comprising an engine, a turbocharger, a bleed duct and a valve, wherein the valve and bleed duct are configured to selectively bleed inlet air from an outlet or downstream of an outlet of a turbocharger compressor to a point downstream of an outlet of a turbocharger turbine.

The engine assembly may further comprise an after-treatment system downstream of the turbocharger turbine. The after-treatment system may comprise an oxidation catalyst and a diesel particulate filter. The oxidation catalyst may be upstream of the diesel particulate filter. The bleed duct may connect to a point downstream of the turbocharger turbine outlet and upstream at least a portion of the after-treatment system. The diesel particulate filter may be upstream of, downstream of or integral, e.g. unitary, with a selective catalytic reduction device.

The engine assembly may comprise an exhaust gas recirculation duct (such as a Low Pressure EGR duct) recirculating exhaust gases from downstream of the turbocharge turbine (e.g. downstream of the diesel particulate filter) to upstream of the turbocharger compressor.

The engine assembly may further comprise any of the above-mentioned controllers.

To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention.

Brief Description of the Drawings

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 is a schematic view of an engine and after-treatment system assembly according to an arrangement of the present disclosure;

Figure 2 depicts the conversion rate of NO to NO2 as a function of temperature and for various initial concentrations of NO;

Figures 3(a) and 3(b) depict the regeneration temperature as a function of time for a diesel particulate filter according to two scenarios;

Figure 4 is a schematic depicting the construction of a combined diesel particulate filter and selective catalytic reduction device;

Figure 5 is a schematic view of an engine and after-treatment system assembly according to another arrangement of the present disclosure; and

Figure 6 is a schematic view of an engine and after-treatment system assembly according to a yet another arrangement of the present disclosure.

Detailed Description

With reference to Figure 1, an engine assembly for an internal combustion engine 10 of a motor vehicle according to arrangements of the present disclosure is described. Air may enter an air inlet duct 46 through an inlet 12 and then pass through an air filter 13. The air may then pass through a compressor 14a of a turbocharger 14. The turbocharger 14 may improve the engine power output and reduce emissions. Typically, the turbocharger 14 is arranged with an exhaust gas driven turbine 14b driving the compressor 14a mounted on the same shaft. A charge air cooler 16 may be provided downstream of the turbocharger compressor 14a. The charge air cooler 16 may further increase the density of the air entering the internal combustion engine 10, thereby improving its performance. The air may then enter the internal combustion engine 10 via a throttle 18 configured to vary the mass flow of air into the internal combustion engine.

As is depicted in Figure 1, the internal combustion engine 10 may comprise four cylinders 10ad, although any other number of cylinders is also contemplated. Air may flow into each of these cylinders at an appropriate time in the engine’s cycle as determined by one or more valves (not shown).

The exhaust gases leaving the internal combustion engine 10 may enter an exhaust duct 19 configured to receive exhaust gases from the engine and exhaust them via an exhaust outlet 28.

Exhaust gases within the exhaust duct 19 may pass through the turbine 14b of the turbocharger

14. An after treatment system 20 may be provided downstream of the turbine 14b, e.g. to reduce emissions from the engine exhaust. The after treatment system 20 may comprise one or more modules, such as an oxidation catalyst 20a; a selective catalytic reduction device 20b; and a diesel particulate filter 20c. Although Figure 1 shows the after treatment modules being in a particular order, it is also contemplated that the modules may be provided in any other order, for example the selective catalytic reduction device 20b may be downstream of the diesel particulate filter 20c. Furthermore, although the after treatment modules are depicted as being separate components, they may be immediately adjacent or integrated together, e.g. any number of modules may be combined in a single unit.

A first exhaust gas recirculation loop 22 configured to selectively recirculate exhaust gases from the internal combustion engine 10 back into the internal combustion engine may also be provided. The first exhaust gas recirculation loop 22 may be provided about the turbocharger 14 such that exhaust gases leaving the turbine 14b may be recirculated into the inlet of compressor 14a. (The first exhaust gas recirculation loop 22 may thus be referred to as Low Pressure Exhaust Gas Recirculation.) The first exhaust gas recirculation loop 22 may comprise a first exhaust gas recirculation duct 23, which may branch from the main exhaust flow path, e.g. exhaust gases may be diverted from the main exhaust flow path to flow through the first exhaust gas recirculation duct 23. The first exhaust gas recirculation duct 23 may branch from the main exhaust flow path downstream of the after treatment system 20 or part way through the after treatment system 20. The first exhaust gas recirculation loop 22 may further comprise a first recirculation valve 24, which may control the amount of recirculation through the first exhaust gas recirculation duct 23.

A second EGR loop 32 configured to selectively recirculate exhaust gases from the internal combustion engine 10 back into the internal combustion engine may also be provided. The second EGR loop 32 may be provided about the engine 10 with exhaust gases leaving the engine 10 being recirculated to the air inlet of the engine 10. The second exhaust gas recirculation loop 32 may comprise a second exhaust gas recirculation duct 33, which may branch from the main exhaust flow path, e.g. gases may be diverted from the main exhaust flow path to flow through the second exhaust gas recirculation duct 33. The second exhaust gas recirculation duct 33 may branch from the main exhaust flow path at a point between the engine 10 and the turbine 14b of the turbocharger. Accordingly, the exhaust gases in the second EGR loop 32 may be at a higher pressure than the exhaust gases in the first EGR loop 22. (The second exhaust gas recirculation loop 32 may thus be referred to as High Pressure Exhaust Gas Recirculation.) The second exhaust gas recirculation loop 32 may comprise a second recirculation valve 34 which may control the amount of recirculation in the second EGR loop 32.

The oxidation catalyst 20a may assist in oxidising NO in the exhaust from the engine 10. The oxidation of NO is governed by the following reversible reaction:

NO +^O₂ ^NO₂ (A)

Figure 2 depicts the conversion rate of NO to NO2 as a function of temperature for various initial concentrations of NO. It will be appreciated that as reaction A is reversible, it may proceed in either direction. A catalyst, such as Platinum, in the oxidation catalyst 20a may promote the oxidation of NO.

In addition, the selective catalytic reduction device 20b may reduce NOx present in the engine exhaust. The selective catalytic reduction device 20b may use a reductant, such as NH3, which may be obtained from urea. There are three key mechanisms: reaction B (NO alone); reaction C, (NO and NO2 in equimolar ratio); and reaction D (NO2 alone):

4/VW₃+4/VO + O₂ ^4/V₂ + 6W₂O(B)

2NH₃ + NO + NO₂ -+ 2N₂ + 3H₂O(C)

4NH₃ + 2NO₂ + O₂-* 3N₂ + 6H₂O(D)

Reaction C is believed the most efficacious of the three reactions. There is however no independent control over the NOx speciation at the entry to the selective catalytic reduction device 20b: it will vary in accordance with the engine duty cycle and the conversion efficiency of the oxidation catalyst 20a (as depicted in Figure 2)

Soot will build up on the diesel particulate filter 20c. However, the diesel particulate filter 20c may be regenerated with the soot in the diesel particulate filter 20c being oxidised according to one or more of the following reactions:

C + -O₂^ co	(E)
C + O₂ —* CO₂	(F)
C + NO₂ -+ CO + NO	(G)
C + 2NO₂ -+ CO₂ + 2NO	(H)

In previously-proposed diesel particulate filters, the retained soot is incinerated at temperatures above 550°C; which may be referred to as 'hot regeneration'. At such temperatures the oxidant is O2 and reactions E and F dominate. By contrast, reactions G and H, in which NO2 is the oxidant, are overwhelmed at such temperatures. The supply of NO2 is in any case thermodynamically restricted at such high temperatures, as explained above with reference to Figure 2; since it speedily decomposes to NO, an ineffective oxidant. However, as mentioned above, the high temperatures of such ‘hot regeneration’ are problematic not only for thermal ageing, but also for oil dilution due to the large quantities of post-injected fuel.

According to an arrangement of the present disclosure, the diesel particulate filter 20c may be regenerated in both “hot” and “warm” regeneration modes. In particular, the diesel particulate filter 20c may be regenerated in both a first (warm) mode, which occurs in a first temperature range, and in a second (hot) mode, which occurs in a second temperature range hotter than the first temperature range. In the first mode, soot collected on the diesel particulate filter is predominantly oxidised by nitrogen dioxide (i.e. via reactions G and H), whereas in the second mode soot collected on the diesel particulate filter is predominantly oxidised by oxygen (i.e. via reactions E and F).

By way of example, the first regeneration mode may occur at temperatures between approximately 250°C and approximately 500°C. In a particular example, the first mode may occur at temperatures between approximately 300°C and approximately 400°C. By contrast, the second mode may occur at temperatures exceeding approximately 550°C. In particular, the second mode may occur at a temperature of approximately 600°C.

Since the temperatures required are lower in the first mode, less additional fuel may be injected into the engine 10 in the first regeneration mode than in the second mode. Accordingly, less fuel may leak past the piston rings and dilute the oil. Furthermore, the diesel particulate filter 20c may be exposed to less thermal aging, thereby prolonging its life. Although oxidation using NO2 (via reactions G and H) may not be as fast as with O2 (via reactions E and F), it has been realised that warm regeneration may have a sufficient oxidation effect that allows hot regeneration events to be performed less frequently. In addition, unlike oxidation with O2, reactions G and H are not known for their susceptibility to thermal runaway since the NO2 levels likely to be encountered will be sufficiently dilute to rapidly dissipate heat.

In the first and second modes, the regeneration may be maintained at a steady state for first and second periods of time respectively. In other words, the regeneration may not merely pass through the first regeneration mode to reach the second hotter regeneration mode and the regeneration may be held in the first mode for the first period of time. The first time period may be longer than the second time period to maximise the regeneration of the soot at the lower temperatures. Alternatively, the first time period may be shorter than the second time period.

Regenerating the diesel particulate filter 20c in the first and second modes may occur periodically, e.g. when a controller for the engine 10 determines that a particular regeneration mode is appropriate. The first and second mode regeneration events may occur with the same or a similar frequency. Alternatively, first mode regeneration events may occur more or less frequently than second mode regeneration events. However, a first total amount of time spent in the first mode of regeneration may be greater than a second total amount of time spent in the second mode of regeneration, thereby minimising the time the diesel particulate filter is exposed to the higher temperatures. For example, the first mode may be shorter in duration, but more frequent than the second mode. Alternatively, the first mode may be longer in duration, but less frequent than the second mode. In a further alternative, the first mode may be longer in duration and more frequent than the second mode.

With reference to Figure 3(a), regenerating the diesel particulate filter in the first “warm” mode may be spaced apart in time from regenerating the diesel particulate filter in the second “hot” mode, e.g. there may be a time gap between two particular regeneration events. By contrast, with reference to Figure 3(b), regenerating the diesel particulate filter in the second mode may immediately follow or precede regenerating the diesel particulate filter in the first mode, e.g. there may be no or little time gap between two particular regeneration events. However, if the temperature ranges for the first and second modes do not abut, there may be a short time gap as the regeneration temperature changes from one temperature range to another. (The timescales in Figure 3 are exaggerated, for example the steady state region in the warm and hot modes may last much longer than indicated.)

Hie build-up of soot on the diesel particulate filter 20c may be tracked by an engine controller. For example, the level of soot on the diesel particulate filter may be estimated with a model using engine parameters to predict the amount of soot deposited on the filter. The oxidation of the soot during the first and second modes of regeneration may also be tracked, e.g. using a model based on the oxidation temperature and engine parameters. Additionally or alternatively, the soot level on the diesel particulate filter may be sensed with a sensor that sends data to the appropriate engine controller. A regeneration event may be scheduled when the soot reaches a predetermined level. For example, a first mode regeneration event may be scheduled when the soot level has reached a first threshold. A second mode regeneration event may be scheduled when the soot level has reached a second threshold higher than the first threshold.

A dilution level of the engine lubricant may be sensed with a sensor and an engine controller may monitor the dilution level of the lubricant. If the dilution level of the lubricant exceeds a threshold value, the diesel particulate filter 20c may be preferentially regenerated in the first mode as opposed to the second mode. In other words, regeneration of the diesel particulate filter 20c in the second hot mode may be prevented or limited if the dilution level of the lubricant exceeds the threshold value.

As mentioned above, two or more of the after-treatment modules may be combined into a single unit. In a particular example, the selective catalytic reduction device and diesel particulate filter may be combined into a single unit 20d. Figure 4 shows a schematic sectional view of a portion of such a combined device, which may be referred to as an SDPF or an SCRF.

The combined selective catalytic reduction device and diesel particulate filter 20d may comprise selective catalytic reduction sites 25 (e.g. comprising copper as a catalyst), which promote reactions B, C and D. The selective catalytic reduction sites 25 may be provided on filter walls 26 of the diesel particulate filter. The flow of exhaust gases is depicted by arrow 29, which indicates the passage of exhaust gases between and over the filter walls 26. As shown, soot 27 may accumulate on the walls 26 of the diesel particulate filter. The soot 27 may be periodically oxidised in the manner described above.

In the combined selective catalytic reduction device and diesel particulate filter 20d, the soot oxidation and NOx reduction chemistries occur at the same time and there are likely to be interactions between the two chemistries. The NOx reduction reactions (C and D) and sootoxidation reactions (G and H) may compete for the available NO2. It should be noted, however, that whereas equimolar NOx best prosecutes the NOx reduction (reaction C), soot oxidation requires only NO2 (reactions G and H). In a further twist, reactions G and H might rebalance the speciation in favour of or to detriment of Reaction C, which is more effective. In other words, there may be an ambivalent relationship between the two webs of reactions.

To investigate the interplay between the various reactions, an experiment has been conducted, in which various gaseous mixtures of NO and NO2 were passed over a catalyst typically used in a combined selective catalytic reduction device and diesel particulate filter (such as copper-zeolite). For each mixture of NO and NO2, the experiment was conducted with and without ammonia. The temperature was steadily ramped up over time and the soot oxidation levels were determined by measuring the ppm of CO2 produced. The results from a first temperature (400°C), which is indicative of the first “warm” regeneration mode, are presented below:

Soot oxidation (ppm CO2) at 400°C

	NH3=0ppm	NH3=500ppm
N0=500ppm, NO2=0ppm	30	30
NO=250ppm, NO2=250ppm	160	60
NO=Oppm, N02=500ppm	260	100

By contrast, the results from a second temperature (550°C), which is indicative of the second “hot” regeneration mode are presented below:

Soot oxidation (ppm CO2) at 550°C
	NH3=0ppm	NH3=500ppm
N0=500ppm, NO2=0ppm	650	750
NO=250ppm, NO2=250ppm	400	530
NO=0ppm, N02=500ppm	0	280

It is apparent from the experimental results that the NO_X speciation and NH3 reductant are both influential. In hot regeneration, NH3 promotes the soot oxidation when the NO_X is primarily NO. This is at least partly because the warm regeneration failed to make any discernible impact on the soot. For NO2 and equimolar NO_X speciation, hot regeneration is less pronounced, because much of the soot burned out during the preceding warm regeneration. In fact, when the NO_X is primarily NO2, little soot survived until the hot regeneration. This underscores the usefulness of warm regeneration. Although the warm regeneration may be protracted, say approximately 50 minutes in duration, this period must be balanced against the hot regeneration, which although shorter, is also fiercer.

The presence of NH3 reduced the soot oxidation during warm regeneration, particularly when the NO_X was NO2. However, given the advantages of warm regeneration, the introduction of a selective catalytic reductant, such as ammonia, may be inhibited (e.g. prevented, limited or reduced) during at least a portion of one or more regeneration events in the first mode. The introduction of the selective catalytic reductant may then be re-instated after the warm regeneration has finished or during a subsequent hot regeneration.

The experimental results described above accord nicely with the behaviour of the upstream oxidation catalyst 20a. For example, in the temperatures encountered during a warm regeneration, the oxidation catalyst readily oxidises NO to NO2 (see Figure 2) such that the NO_X is primarily NO2. As demonstrated in the experimental results above, in the warm temperature range the soot oxidation rate is higher when the NO_X is primarily NO2. By contrast, during hot regeneration, the balance returns to NO (again see Figure 2) and the soot oxidation rate is higher when the NO_X is primarily NO.

Similar results may apply to an after-treatment system with a separate selective catalytic reduction device and diesel particulate filter, e.g. where the selective catalytic reduction device is upstream of the diesel particulate filter. Accordingly, for such an arrangement the introduction of a selective catalytic reductant (e.g. ammonia) may also be inhibited during at least a portion of one or more regeneration events in the first mode.

When the introduction of the selective catalytic reductant is inhibited, the NO generated by reactions G and H during warm regeneration may not necessarily be reduced by reactions B or C. However, there will likely be some residual reductant stored on the selective catalytic reduction sites 25 within the wall 26, on which the soot cake sits. The residual reductant may thus help reduce NO to N2 via reactions B or C. In any case, over a drive cycle some additional NO may be tolerated, particularly if the emissions performance of the after-treatment system 20 is improved overall. However, additional or alternative arrangements for reducing the NO emissions are described below with reference to Figures 5 and 6.

Referring now to Figure 5, the engine assembly may further comprise a bleed duct 40 configured to selectively bleed inlet air from an outlet or downstream of an outlet of the turbocharger compressor 14a to a point downstream of an outlet of the turbocharger turbine 14b. For example, inlet air may be bled from an outlet mid-way through the compression process, from an outlet at the end of the compression process or a point downstream of either outlet. The bleed duct 40 may also direct a bleed flow to a point upstream of the after-treatment system 20. It will be appreciated that Figure 5 is schematic and that the bleed duct 40 may be closely coupled to the turbocharger 14, e.g. with a short duct from the compressor outlet to the turbine outlet. Alternatively, the bleed duct 40 may branch off the inlet air passage downstream of the air cooler 16.

A valve 42 may be provided in the bleed duct 40 to selectively prevent, allow or regulate flow through the bleed duct 40. Although the valve 42 is shown as being in the bleed duct 40, it is also envisaged that the valve may be provided at the inlet or outlet of bleed duct 40.

Exhaust gases from downstream of the diesel particulate filter 20c may be recirculated to upstream of the oxidation catalyst 20a, e.g. during one or more regeneration events in the first mode. In particular, a portion of the exhaust gases downstream of the diesel particulate filter 20c may flow through the LP EGR loop 22 and into the turbocharger compressor 14a along with fresh inlet air. Some of the flow out of the turbocharger compressor 14a may then bypass the engine 10 via bleed duct 40 and may be redirected to upstream of the after-treatment system 20.

An engine controller may control valve 42 to selectively permit the flow through the bleed duct 40. For example, when the controller implements a warm regeneration mode for the diesel particulate filter, the valve 42 may be at least partially opened.

In this way at least some of the NO resulting from reactions G and H may be recycled via the LP EGR loop 22 and bleed duct 40 back into the after-treatment system 20 where the NO may be readily converted by the oxidation catalyst 20a to NO2 thanks to the favourable temperatures for reaction A. At least some of this additional NO2 may then be used in either of reactions G and H to further oxidise the diesel particulate filter. Emissions of NO during a first regeneration mode of the diesel particulate filter 20c may thus be reduced. The turbocharger compressor 14a provides a convenient means of raising the pressure of the exhaust gases so that they can be recirculated.

The bleed duct 40 may be used for other purposes in relation to the above-described engine assembly or in other engine assemblies, e g. without the above-described diesel particulate regeneration modes. For example, flow through the bleed duct 40 may be used to control an exhaust gas temperature at least partially by selectively bleeding inlet air through the bleed duct. The cooler flow through the bleed duct 40 may mix with the exhaust gases coming out of the turbine 14b to reduce the temperature of the exhaust gases. This may allow the exhaust temperature to be reduced below an upper threshold, e.g. a maximum, temperature of the after treatment system 20. The after-treatment system 20 may thus be protected from excessive exhaust temperatures, e.g. during a regeneration of the diesel particulate filter.

Additionally or alternatively, flow through the bleed duct 40 may be used to control a surge margin of the turbocharger compressor 14a by selectively bleeding inlet air through the bleed duct and thereby reducing the mass flow through the compressor. In this way a surge event in the turbocharger compressor 14a may be avoided.

With reference to Figure 6, the after-treatment system 20 may further comprise a Venturi 50 in the flow path upstream of the oxidation catalyst 20a. The Venturi 50 may comprise a narrowing or throat that serves to increase the flow velocity and reduce the local pressure. The aftertreatment system 20 may further comprise a passage 52, which may branch from the exhaust duct 19 downstream of the diesel particulate filter 20c. The passage 52 may redirect at least some of the exhaust flow to the narrowing in the Venturi 50. The low pressure at the narrowing may draw flow through the passage 52. A valve 54 may also be provided in the passage 52 to selectively permit flow through the passage 52.

Exhaust gases from downstream of the diesel particulate filter 20c may be selectively recirculated to the narrowing of the Venturi 50, e.g. during one or more regeneration events in the first mode. An engine controller may control valve 54 to selectively permit the flow through the passage 52. For example, when the controller implements a warm regeneration mode for the diesel particulate filter, the valve 54 may be at least partially opened.

In this way, at least some of the NO resulting from reactions G and H may be recycled via the passage 52 back into the after-treatment system 20 where the NO may be readily converted by the oxidation catalyst 20a to NO2 thanks to the favourable temperatures for reaction A. At least some of this additional NO2 may then be used in either of reactions G and H to further oxidise the diesel particulate filter. Emissions of NO during a first regeneration mode of the diesel particulate filter 20c may thus be reduced.

As described above, the Venturi 50 may be used during a first mode regeneration of the diesel particulate filter 20c. However, the Venturi 50 may also be used in other circumstances in which it is desired for at least some of the exhaust gases to be recycled through the after-treatment system

20. Accordingly, the Venturi 50 may be provided in the above-described engine assembly or in other engine assemblies, e.g. without the above-described diesel particulate regeneration modes or particular after-treatment system arrangement. For example, the Venturi 50 may allow at least some of the exhaust gases to be recirculated during a warm-up phase of the after-treatment system. Such recirculation may reduce the time it takes for a catalyst to reach a light-off temperature. It will also be appreciated that the Venturi 50 may be provided instead of or in addition to the bleed duct 40 depicted in Figure 5.

It will be appreciated by those skilled in the art that although the invention has been described by way of example, with reference to one or more examples, it is not limited to the disclosed examples and alternative examples may be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of regenerating a diesel particulate filter in an after-treatment system, the method comprising:

2. The method of claim 1, wherein the after-treatment system further comprises a selective catalytic reduction device, and wherein the method further comprises:

inhibiting introduction of a selective catalytic reductant into the selective catalytic reduction device during at least a portion of one or more regeneration events in the first mode.

3. The method of claim 1 or 2, wherein the method further comprises:

selectively recirculating exhaust gases from downstream of the after-treatment system to upstream of the after-treatment system.

4. The method of claim 3, wherein the after-treatment system further comprises a Venturi in the flow path at an upstream end of the after-treatment system, wherein the method further comprises:

selectively recirculating exhaust gases from downstream of the after-treatment system to a narrowing of the Venturi.

5. The method of any of the preceding claims, wherein the method further comprises: selectively recirculating exhaust gases from downstream of the diesel particulate filter through a Low Pressure Exhaust Gas Recirculation duct into a compressor of a turbocharger.

6. The method of claim 5, wherein the method further comprises:

selectively bleeding inlet air from an outlet or downstream of an outlet of the turbocharger compressor through a bleed duct leading to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system.

7. The method of any of the preceding claims, wherein the first time period is longer than the second time period.

8. The method of any of the preceding claims, wherein the method further comprises periodically regenerating the diesel particulate filter in the first mode and periodically regenerating the diesel particulate filter in the second mode.

9. The method of claim 8, wherein regenerating the diesel particulate filter in the first mode occurs more frequently than regenerating the diesel particulate filter in the second mode.

10. The method of claim 8 or 9, wherein a first total amount of time spent in the first mode of regeneration is greater than a second total amount of time spent in the second mode of regeneration.

11. The method of any of the preceding claims, wherein for at least one pair of regeneration events, regenerating the diesel particulate filter in the first mode is spaced apart in time from regenerating the diesel particulate filter in the second mode.

12. The method of any of the preceding claims, wherein for at least one pair of regeneration events, regenerating the diesel particulate filter in the second mode immediately follows or precedes regenerating the diesel particulate filter in the first mode.

13. The method of any of the preceding claims, wherein the first mode occurs at temperatures between approximately 250°C and approximately 500°C.

14. The method of any of the preceding claims, wherein the first mode occurs at temperatures between approximately 300°C and approximately 400°C.

15. The method of any of the preceding claims, wherein the second mode occurs at temperatures exceeding approximately 550°C.

16. The method of any of the preceding claims, wherein the second mode occurs at a temperature of approximately 600°C.

17. The method of any of the preceding claims, wherein the method further comprises: tracking the build-up of soot on the diesel particulate filter;

tracking the oxidation of the soot during the first and second modes of regeneration; and scheduling a regeneration event in the first or second mode.

18. The method of any of the preceding claims, wherein the method further comprises: sensing the amount of soot build-up on the diesel particulate filter.

19. The method of any of the preceding claims, wherein the method further comprises: sensing a dilution level of an engine lubricant; and prioritising the first mode of regeneration over the second mode of regeneration if the dilution level exceeds a threshold value.

20. The method of any of the preceding claims, wherein the method further comprises: injecting a first amount of additional fuel into an engine upstream of the diesel particulate filter in the first mode regeneration event; and injecting a second amount of additional fuel into the engine in the second mode regeneration event, wherein the second amount of additional fuel is greater than the first amount of additional fuel.

21. An after treatment system for an engine, the after treatment system comprising a diesel particulate filter and a controller configured to carry out the method according to any of the preceding claims.

22. The after treatment system of claim 21 further comprising a selective catalytic reduction device, and wherein the after-treatment system is configured to inhibit introduction of a selective catalytic reductant into the selective catalytic reduction device during at least a portion of one or more regeneration events in the first mode.

23. The after treatment system of claim 21 or 22, wherein the after-treatment system is configured to selectively recirculate exhaust gases from downstream of the after-treatment system to upstream of the after-treatment system.

24. The after treatment system of claim 23 further comprising a Venturi in the exhaust flow path at an upstream end of the after-treatment system, wherein the after-treatment system comprises a duct configured to selectively recirculate exhaust gases from downstream of the aftertreatment system to a narrowing of the Venturi.

25. An engine assembly comprising the after treatment system of any of claims 21 to 24, the engine assembly comprising a Low Pressure Exhaust Gas Recirculation duct configured to selectively recirculate exhaust gases from downstream of the diesel particulate filter into a compressor of a turbocharger.

26. The engine assembly of claim 25, wherein the engine assembly further comprises a bleed duct configured to selectively bleed inlet air from an outlet or downstream of an outlet of the turbocharger compressor to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system.

27. A method for an engine with an after-treatment system, the method comprising: recirculating at least some of the exhaust gas from downstream of the after-treatment system to upstream of the after treatment system without passing the at least some recirculated exhaust gas through the engine.

28. The method of claim 27, wherein the after-treatment system further comprises a Venturi in the flow path at an upstream end of the after-treatment system, wherein the method further comprises:

29. The method of claims 27 or 28, wherein the method further comprises:

selectively recirculating exhaust gases from downstream of the after-treatment system through a Low Pressure Exhaust Gas Recirculation duct into a compressor of a turbocharger.

30. The method of claim 29, wherein the method further comprises:

31. The method of any of claims 27 to 30, wherein the after treatment system comprises an oxidation catalyst upstream of a diesel particulate filter and wherein the method further comprises:

regenerating the diesel particulate filter.

32. The method of any of claims 27 to 30, wherein the method further comprises: recirculating at least some of the exhaust gas during a warm-up phase of the after-treatment system.

33. An after treatment system for an engine, wherein the after-treatment system is configured to recirculate at least some of the exhaust gases from downstream of the after-treatment system to upstream of the after-treatment system without passing the at least some recirculated exhaust gas through the engine.

34. The after treatment system of claim 33 further comprising a Venturi in the exhaust flow path at an upstream end of the after-treatment system, wherein the after-treatment system comprises a duct configured to selectively recirculate exhaust gases from downstream of the aftertreatment system to a narrowing of the Venturi.

35. The after-treatment system of claim 33 or 34 further comprising an oxidation catalyst upstream of a diesel particulate filter, wherein the after-treatment system is configured to selectively regenerate the diesel particulate filter.

36. The after-treatment system of any of claims 33 to 35, wherein the after-treatment system is configured to recirculate at least some of the exhaust gas during a warm-up phase of the aftertreatment system.

37. An engine assembly comprising the after treatment system of any of claims 33 to 36, the engine assembly comprising a Low Pressure Exhaust Gas Recirculation duct configured to selectively recirculate exhaust gases from downstream of the after-treatment system into a compressor of a turbocharger.

38. The engine assembly of claim 37, wherein the engine assembly further comprises a bleed duct configured to selectively bleed inlet air from an outlet or downstream of an outlet of the turbocharger compressor to a point downstream of a turbocharger turbine outlet and upstream of the after-treatment system.

39. A method for an engine, the method comprising:

selectively bleeding inlet air from an outlet or downstream of an outlet of a turbocharger compressor through a bleed duct leading to a point downstream of an outlet of a turbocharger turbine.

40. The method of claim 39 further comprising:

bleeding the inlet air through the bleed duct to the point downstream of the turbocharger turbine outlet, the point also being upstream of an after-treatment system comprising an oxidation catalyst and a diesel particulate filter, the oxidation catalyst being upstream of the diesel particulate filter.

41. The method of claim 40 further comprising:

recirculating exhaust gases from downstream of the diesel particulate filter through an exhaust gas recirculation duct.

42. The method of any of claims 39 to 41 further comprising:

controlling an exhaust gas temperature at least partially by selectively bleeding inlet air through the bleed duct.

43. The method of claim 42 further comprising:

reducing the exhaust gas temperature by selectively bleeding inlet air through the bleed duct to reduce the temperature below an upper threshold temperature of an after treatment system downstream of the turbocharger turbine.

44. The method of any of claims 39 to 43 further comprising:

selectively bleeding the inlet air through the bleed duct during a regeneration event for a diesel particulate filter.

45. The method of claim 44 further comprising:

inhibiting introduction of a selective catalytic reductant into a selective catalytic reduction device during one or more regeneration events for the diesel particulate filter.

46. The method of any of claims 39 to 45 further comprising:

controlling a surge margin of the turbocharger compressor by selectively bleeding inlet air through the bleed duct.

47. An engine assembly comprising an engine, a turbocharger, a bleed duct and a valve, wherein the valve and bleed duct are configured to selectively bleed inlet air from an outlet or downstream of an outlet of a turbocharger compressor to a point downstream of an outlet of a turbocharger turbine.

48. The engine assembly of claim 47 further comprising an after-treatment system downstream of the turbocharger turbine, the after-treatment system comprising an oxidation catalyst and a diesel particulate filter, the oxidation catalyst being upstream of the diesel particulate filter, wherein the bleed duct connects to the point downstream of the turbocharger turbine outlet, the point also being upstream of the after-treatment system.

49. The engine assembly of claim 48, wherein the after-treatment system further comprises a selective catalytic reduction device.

50. The engine assembly of any of claims 47 to 49 further comprising an exhaust gas recirculation duct recirculating exhaust gases from downstream of the turbocharge turbine to upstream of the turbocharger compressor.