Patent Description:
Particularly in Diesel applications for CN6b and Euro 6d / Euro7 markets there will be a requirement for improved control of emissions and thus this will require more complex after treatment systems in vehicle exhausts than CN6a or Euro 6d-temp.

One typical variant consists of dual SCR catalysts (i.e. two serially located SCR units) combined with two corresponding separate urea/reductant (e.g. Adblue) dosers therefor. In the exhaust system of such system is a first SCR unit located upstream of a second (downstream) SCR unit. The second SCR unit is often referred to as an Under-Floor SCR unit (UF SCR), and the first one is often a combined Diesel particulate filter (DPF) with SCR functionality; often referred to as a SDPF. Hereinafter reference to SDPF can be interpreted as reference and interchangeable to the first (upstream) SCR unit and reference to the UF SCR can be interpreted as reference and interchangeable to the second (downstream) SCR unit
Above <NUM>-<NUM>, most of the NH3 (from urea) is oxidized in the SDPF and can then not convert NOx Adding a second doser at a colder location allows NOx conversion in a second downstream SCR (e.g. UF SCR)
The addition of a second urea injection point increases the complexity of the SCRs control: the second urea doser provides an extra degree of freedom for the control. The second SCR catalyst can be fed with NH3 slipping out of the SDPF and also by direct urea injection with the second doser.

With two independent dosers, the control of two SCRs requires new open-loop and closed-loop strategies. Each SCR has to be controlled to provide a given stored amount of NH3 for NOx conversion. Expensive NOx sensors have to be kept to a minimum number. Dual doser control needs to have commonality with single doser control and there need to be an avoidance of proliferation of controls and calibration methodologies
Such dual doser control can become much more complex than single doser. In the <NUM> publication "<NPL>) describes a dual doser control system. Several issues are linked with this kind of control. The Kalman filter employed breaks the link between the physical/chemical behavior of the actual system and the control, which makes it difficult for the calibrators to fine-tune the control based on physical/chemical observations. The NOx sensor located downstream of SDPF will operate with high levels of NH3 in the gas most of the time (NH3 slip from SDPF). This is due to fact that a SDPF requires high level of NH3 filling to perform at its highest possible efficiency. NOx sensors being cross-sensitive to NH3, the estimation of NH3 concentration and NOx concentration downstream of SDPF are very inaccurate. The signals will typically be less accurate than the delivery accuracy of state-of-the-art dosing systems. Moreover, NOx sensors greatly increase the total cost of an SCR system. Adding this extra sensor should be avoided if not absolutely required.

A further problem is the inaccuracy of the NH3 signal (and NOx) between SDPF and SCR, combined with the activation of the second doser makes it very risky to correct the dosing flow of the second doser (closed-loop): a drift of the front doser can wrongly be identified as a drift of the second doser, when closing the loop with the rear NOx sensor. In case of urea slip due to poor mixing / degraded front SCR, this becomes even more obvious that the second closed loop wrongly corrects the second doser.

The added complexity of the above control and its additional NOx sensor are difficult to justify because the second doser is mainly (/only) intended to be used in very high exhaust temperature conditions such as DPF regeneration. In normal conditions, it is actually detrimental to the global NOx performance of the system to control the rear SCR with the second doser instead of overfilling the front SCR to generate extra NH3: the SDPF performance greatly increases when it is saturated with NH3, which is the case when the front doser is used to control both SCR catalysts.

Documents <CIT> , <CIT>, <CIT> and <CIT> disclose exhaust systems with two SCR catalyst units with doser for each unit.

In one aspect is provided a method of controlling the operation of a second doser in a vehicle exhaust system, said system including a first SCR catalytic unit and a second SCR catalytic unit located downstream of said first unit, and including a first urea doser adapted to inject reductant upstream of said first unit, and where said second urea doser is located upstream of said second catalytic unit and adapted to inject urea upstream of said second catalytic unit, said method comprising the steps of.

Said desired slip fraction may be determined from the variables of the temperature of the first SCR unit and/or the soot loading of said first SCR unit when it has DPF functionality.

According to the invention, said slip fraction is defined as a/(a+b) where "a" is the desired NH3 slip and "b" is the NH3 flow to said second SCR unit provided by operation of said second doser.

The value of "a" and/or "b" may be determined from said desired slip fraction and variables including the amount of NH3 stored on said second SCR unit and the target value for NH3 stored on said second SCR unit.

The value of "a" and or "b" may be determined from said desired slip fraction and also variables including an actual computed slip flow.

The stored NH3 of said second SCR unit may be determined from a model of the second SCR unit.

The input to the model may be the total NH3 flow to the second SCR.

The method may include controlling the operation of the first doser is dependent on the computed desired slip flow "a".

The method may include determining a value of "b" and operating the second doser dependent on this variable.

The total NH3 flow may be determined from a modeled slip flow provided by a model of said first SCR unit and a determined flow of urea from said second doser.

The method of controlling may be open-loop or feed forward.

The present invention is now described by way of example with reference to the accompanying drawings in which:.

<FIG> shows an example of a dual SCR system in which embodiments of the invention can be performed. It shows a portion of a vehicle exhaust system having dual doser/SCR after- treatment. Essentially there is a first upstream SCR (e.g. SDPF ) unit <NUM> and a second SCR downstream <NUM> therefrom. There is a first urea doser <NUM> adapted to dose urea/reductant upstream of the first unit and a second urea/reductant doser/injector <NUM> adapted to inject urea/reductant upstream of the second unit (and downstream of the first unit) and shown in <FIG>, where SDPF stands for SCR on DPF. There is also located a NOx sensors <NUM> as shown which provides feedback control. Sensors <NUM> and/or <NUM> shown in the dotted line may not be present but are present in prior art systems. In the figure, the "a" represents the NH3 (slip) form the first catalytic unit and "b" represents the NH3 flow provided by the second urea doser. Typically a NOx sensor <NUM> is required to measure the NOx into the system (into the first SCR unit) and used as an input in control. Prior art system include a further NOx sensor <NUM> located between first and second SCR unit.

In aspects of the invention there is provided an alternative control method for such dual dosers (and dual SCR) where there are two SCR units each with corresponding respective dosers.

The general feed-forward part of the control of an SCR unit/doser system is known and an example of such control is found in EP Patent Application <CIT>, which describes single doser (single and dual SCR) control which is therefore not described in details in this document. The following description focuses on the invention with respect to the control for dual dosers.

In essence in example of the invention, under certain circumstances, the system is controlled in wherein the NH3 filling state of the second (downstream) SCR, in other words the control of the second doser, is controlled under certain circumstances to be based on both the NH3 slip from the front SCR unit (SDPF) and the second urea doser for the second SCR unit according to a desired slip fraction.

This desired slip fraction is computed and may be based on (e.g. bed) temperature of the first catalytic unit and/or soot loading thereof when the first unit has DPF functionality.

In terms of NOx conversion efficiency, it is usually better to control NOx conversion by providing variable e.g. extra NH3 slip provided by the front SCR unit (SDPF), and not operating the second doser. However there are some conditions when the use of the rear doser is preferable. For example, this is in cases where the front SCR unit (SDPF) temperature increases above <NUM>, more and more NH3 is oxidized in the front SCR (SDPF), meaning that the amount of NH3 available for the rear SCR decreases. Above a certain temperature which depends on e.g. the SDPF technology, most of the NH3 oxidizes, potentially to NOx, which is therefore counterproductive in terms of NOx after treatment. In prior art systems control generally then switches to using the second SCR/doser only.

According to the invention, when reaching these high temperatures, instead of the control simply switching to determining a desired NH3 (/urea) assumed to be solely provided from the second doser, the control allows the desired/demand NH3 for the second SCR to be provided as a fraction of NH3 slip from the front SCR and NH3 from the second urea doser. The control adjust the demand for the second doser so as to provide this slip fraction, the slip fraction being dependent on operating conditions of the first.

This desired NH3 slip fraction can be a function of first SCR bed temperature and/or soot load in the first SCR when it is an SDPF. The slip fraction can be found from a look-up table or MAP based on either or both these parameters.

<FIG> shows how the slip fraction can be determined. The inputs to a look-up table / MAP <NUM> are the first upstream SCR bed temperature <NUM> and/or the first SCR soot loading <NUM>. The output <NUM> is the desired slip fraction. The desired slip fraction is defined as <MAT> where "a" is desired slip (the NH3 provided from slip from the first SCR) which is designated nh3_flow_slip_dsrd, and "b" is the NH3 provided from the second urea doser, as shown in <FIG>.

The total desired NH3 flow is a + b and the values of a and/or b may be determined based on this and the determined desired slip fraction.

NH3 slip fraction also accounts for DPF soot load to enable passive DPF regeneration if the soot load is critical, allowing it to get out of critical conditions. Passive regeneration requires NOx to be present in the DPF (no SCR reaction then). An active DPF regeneration is forbidden when the filter is too heavily loaded (risks of damage).

In ideal conditions, the recommended slip fraction is <NUM> or <NUM> (i.e. exclusive use of each doser depending on temperature) but in real condition optimally, the inventor has determined that the slip fraction may be between the two, e.g. may operate or transition between <NUM> and <NUM> for optimum NOx performance and control robustness.

Methods according to the invention can still provide a very high NOx conversion efficiency at low temperatures with a <NUM>% fraction (the desired NH3 for the rear SCR is fully provided by NH3 slip control from the SDPF) and prevent NH3 oxidation issues of high temperatures by specifying low or zero % fraction at high SDPF temperatures (e.g. with zero fraction all the desired NH3 for the rear SCR control is provided by the rear doser), but allows also for "hybrid" or "mixed" operation as described above.

In examples one criteria for switching to "mixed" control (where the slip fraction is not <NUM> or <NUM>, i.e. between these values) is by using the variable of soot load in the SDPF, which allows balancing of the fraction depending on the amount of soot in the SDPF. Typically, this feature allows an earlier (colder) deactivation of the front doser to preferred/favorize passive soot oxidation in the SDPF: when NOx is converted in the first SCR unit (SDPF) by NH3, it cannot oxidize the soot. When the SDPF is critically loaded with soot, the passive oxidation of soot by NOx is highly desirable because it allows to safely reduce the soot load below critical level that allows an active DPF regen to be launched: when a DPF/SDPF is too heavily loaded, the active DPF regen is inhibited to prevent mechanical damage of the filter.

Another criteria is (bed) temperature of the first SCR unit.

In refined examples the desired extra NH3 slip (slip offset))can be limited for example to prevent excessive NH3 concentration at SDPF-out. In this case, the remainder of the limitation is provided by the second doser, in order to fulfill the total desired NH3 flow at the second SCR.

The desired extra slip may be regarded as th desired NH3 flow to be injected by first doser and slipping through SDPF to feed the second SCR catalyst. The main reason for the limitation is software: there are internal variables limited to 2000ppm, so if the NH3 slip exceeds this limit, the software erroneously models the system, so typically we calibrate such that the 2000ppm NH3 limit is not exceeded downstream of SDPF.

Effectively according to examples of the invention, the system can transition from a state where all the NH3 flowing into the second catalytic unit is provided by the first SCR/doser system to a state where a portion is provided by the second doser, or vice versa, or can operate where a portion is provided by the second doser. This portion is determined, according to examples, dependent on the state of the first unit in terms of the temperature and/or the soot loading After the portion (i.e. slip fraction) is determined, the second doser is controlled dependent thereupon. So a second doser demand is made dependent on the (desired) slip fraction.

<FIG> shows how the NH3 slip vs <NUM>nd doser fraction varies with operating conditions. The figure shows how the NH3 slip vs <NUM>nd doser fraction can behave. For demonstration purposes, the slip fraction has been calibrated to start to move from zero at relatively low temperature on the second doser, and still for demonstration purposes, the fraction smoothly transitions zero to <NUM> as control moves from providing NH3 in the second unit from SDPF NH3 slip to <NUM>nd doser dosing.

Reference numeral <NUM> (green) is the SCR bed temperature (TSE Scr_bed_temp). Ref numeral <NUM> (purple) is the P-T_Scr2_scr_nh3_flow dsrd_slip) which is an indication extra NH3 slip demand from front SDPF (and first doser). Ref numeral <NUM> (red) signal is the <NUM>nd doser mass flow (P_T_Scr_urea_mass_flow).

Before time T1, the slip fraction is <NUM> because of high SDPF temperature. The NH3 for the second SCR is then fully provided by the <NUM>nd doser. Reference numeral <NUM> (yellow) is the P_T_Ser2 scr nh3 slip fraction). When the SDPF temperature goes down, the slip (fraction) computed according to the methodology starts to increase, which allows both some urea flow dosing from the <NUM>nd doser and extra NH3 slip demand from the SDPF. At time T2, the SDPF temperature is low, which makes it optimal to control the rear SCR purely by SDPF NH3 slip. The fraction then becomes <NUM> and the <NUM>nd doser is shut-off.

<FIG> shows an example of the control according to one aspect. The figure generally shows inputs and outputs of models/look up tables and to and from physical components of an exhaust system such as the first (upstream doser) and second (downstream) dosers.

The figure is generally divided into two portions; the top portion shows the control of the first SCR unit (e.g. SDPF) SCR <NUM> and the bottom portion relates to the second SCR control (UF SCR) SCR <NUM>.

In the top portion is shown inputs to block B1 (p_t_urea dosing block). The inputs are "P-T_SCR_stored nh3 target", which is the target amount of urea for the first SCR. The second input is P_T_Scr2_scr_nh3_flow_slip_dsrd which is the desired or demand NH3 slip i.e. the NH3 going from the first to the second SCR unit. This is computed as will be described. There is also a third input which is the value of P T SCR stored NH3 which is the stored NH3 in the first SCR unit. Block B1 is thus a P T urea dosing map/model, and the output is the desired urea flow (demand quantity ) for the first doser <NUM>, i.e. P_T_scr_urea flow forward desired; in other words a demand signal for the first DCU Doser <NUM> shown by block B2, (equivalent to the doser upstream of the first SCR unit).

The output of the doser unit <NUM>/block B2 is the variable PT SCR urea mass flow of the first doser (which may be measured or estimated ) which is fed back i.e. input to block B3 which is a MAP/table or model that determines the NH3 flow through the first SCR unit "p_t_scr_nh<NUM>_flow " This is then input to a model B4 of the first SCR unit and the output of B4 is the P_T_Scrnh3_slip_model i.e. the modelled NH3 slip flowing out of the first SCR unit to the second SCR unit. The model B4 also determines the value of P _T_SCR_stored NH3 which is the stored NH3 in the first SCR unit and this is fed into the block B1.

With regard the control of the second doser, this value P_T_Scrnh3_slip_model, (modelled NH3 slip flowing out of the first SCR unit to the second SCR unit), along with the variable P_T_Scr2_urea mass flow (which is the mass flow from the second SCR doser upstream of the second SCR unit) calculated as explained hereinafter, is input to box B5. Block B5 determines the value of P_T Scr 2_nh3_ flow which is total flow of NH3 to the second SCR unit and so will comprise both slip flow and the NH3 flow to the second SCR provided by the second doser. In addition Block B5 determines and outputs the value of P TSCR2 scr slip nh3_flow that is the actual NH3 slip flow (Actually this is the same thing but P_TScr_nh3_slip_model is the concentration [ppm] while P_T_Scr2_scr_slip_nh3_flow is the mass flow [mg/s].

Actual slip may be modelled slip because it may not be measured. In embodiments a real NH3 sensor or a virtual NH3 sensor (based on a NOx sensor) can be used to bypass the P_TScr_nh3_slip_model and in this case, P_T_Scr2_scr_slip_nh3_flow becomes a "measured"NH3 flow.

The total NH3 flow to the second unit (P_T Scr 2_nh3_flow) flow is then input into a model shown by block B6 and the output is the PT_Scr2_stored nh3 which is the NH3 stored on the second SCR unit. This is input to block B7 along with the P_T_Ser2 stored target, which is the target value NH3 stored in the second SCR unit. In addition is a third input P T scr2_scr_slipnh3_flow determined from block B5 (actual slip flow).

With respect to block B7, this is where the computed desired slip fraction is computed/used.

The inputs to block <NUM> are the P_T_Scr 2_scr_slip NH3 flow which is the actual slip flow (which is modelled or measured) , P_T Scr <NUM> stored NH3 and P_T_Scr2_stred NH3 target. The slip fraction <NUM> is defined a/a+b as described above and can be considered as determined here (e.g. from first SCR unit temp/soot loading) Block B7 determines values of "a" i.e. the parameter PT_Scr_scr_nh3_flow slip_dsrd. This is output to block B1. In addition block B7 determines a value of "b" which is the desired value of NH3 flow to the second unit from the second doser. This is used to determine a values of _T Scr <NUM> urea flow desired (that is the desired urea flow demand for the second DCU urea doser 4B7 located upstream of the second SCR unit. ) This value is thus a demand input to the second doser unit doser. Typically this value is a function solely of variable "b" such as a linear function So in examples this value is k*b.

The value (a+b) is the total desired NH3 flow and may be determined (in e.g. block B7) according to operational requirements. It is to be noted that there may be more than one pair of variable of "a" and "b" which fulfil the criteria for the desired slip fraction.

In the above examples the control can be regarded as being in open loop (feed-forward control). In examples of such control, this may be defined as being no feedback in the control from the downstream NOx sensor <NUM> (and/or sensor <NUM>).

A measured signal from this unit is the P _ T Scr <NUM> urea mass flow which is the dosed urea from doser <NUM>. This variable is fed into block B5 as described.

As mentioned a further output of block B7 is the PT_Scr_scr_nh3_flow slip_dsrd or "a" in equation <NUM>, which is input as mentioned to block B1. This value is equivalent to "a" in equation <NUM>. So with reference to this value this is determined from the desired slip fraction (p_t_scr2_scr_nh3_slip fraction is computed as described above with reference to <FIG>).

Claim 1:
A method of controlling the operation of a second doser (<NUM>) in a vehicle exhaust system, said system including a first SCR catalytic unit (<NUM>) and a second SCR catalytic unit (<NUM>) located downstream of said first unit (<NUM>), and including a first urea doser (<NUM>) adapted to inject reductant upstream of said first unit (<NUM>), and where said second urea doser (<NUM>) is located upstream of said second catalytic unit (<NUM>) and adapted to inject urea upstream of said second catalytic unit (<NUM>), said method comprising the steps of
a) determining a desired NH3 slip fraction based on operating conditions of said first SCR unit,
b) controlling the operation of the second doser (<NUM>) based on said desired slip fraction,
wherein said slip fraction is defined as a/(a+b) where "a" is the desired NH3 slip and "b" is the NH3 flow to said second SCR unit (<NUM>) provided by operation of said second doser (<NUM>).