Patent Description:
There is a rising demand for cleaner and more efficient internal combustion engines, especially diesel engines. In response to these demands, new standards continue to be proposed for reduction of exhaust emissions and in particular reduction of particulate and NOx(NO and NO<NUM>) emissions. Exhaust aftertreatment systems (EAT) in engine exhaust systems have been developed to meet these standards.

<FIG> illustrates schematically a known EAT system <NUM> incorporated into an exhaust system <NUM> for a diesel engine. Exhaust gases from the diesel engine flow through the exhaust system in the direction of arrow X, which indicates the direction of flow of the gases from upstream to downstream. The EAT system <NUM> includes a diesel oxidation catalyst (DOC) <NUM> and a diesel particulate filter (DPF) <NUM> to reduce particulate mass and number emissions. The DPF may be a catalysed soot filter (CSF) in which the DPF has a catalytic coating typically of precious metal. Downstream of the DOC and DPF is a selective catalytic reduction (SCR) system for NOx reduction. The SCR system includes a dosing module <NUM> for introducing a diesel exhaust fluid (DEF) in to the exhaust flow, a mixer <NUM> for mixing the DEF in the exhaust gas and a SCR catalyst <NUM> in a SCR unit <NUM> downstream of the mixer. The DEF is a reducing agent which reacts in the SCR catalyst <NUM> to reduce NOx levels in the exhaust. Urea (which may be a urea water mixture) is commonly used as the DEF. Urea vaporises in the exhaust gases, decomposing to produce ammonia (NH<NUM>) and carbon dioxide (CO<NUM>). The NH<NUM> reacts with the SCR catalyst to convert NOx and NH<NUM> into nitrogen gas (N<NUM>) and water vapour (H<NUM>O). As is known in the art, the SCR system may also comprise an ammonia slip catalyst (ASC) <NUM> for oxidising ammonia (NH<NUM>) in order to remove excess levels of NH<NUM> not fully utilised in the SCR process from the exhaust gases. The ASC is conveniently incorporated in the SCR unit <NUM> downstream of the SCR catalyst <NUM>. The DOC and DPF are also relevant for proper functioning of the SCR process. They oxidize NO to NO<NUM>, therefore increasing the NO<NUM>/NOx ratio. This improves efficiency of NOx conversion in the SCR process, provided the ratio does not significantly exceed <NUM>/<NUM>.

The EAT system <NUM> includes an electronic control system <NUM> including one or more electronic control units (ECU), indicated schematically at <NUM> in <FIG>. The, or each, ECU comprises programmable processing means and memory and is configured and programmed to control and regulate operation of the EAT system in accordance with a number of pre-defied protocols depending on the operating conditions. The electronic control system <NUM> is typically part of an engine management system. The functions of the ECU <NUM> may be carried out by a single ECU or by number of separate ECUs interconnected as part of a network, which may be a CAN bus system, for example.

The electronic control system <NUM> also includes a number of sensors for monitoring various operating parameters of the EAT system. These include:.

The sensors provide inputs for use by the ECU <NUM> in regulating operation of the EAT system in a variety of operating conditions. Control of the EAT system is carried out using a combination of mathematical modelling to predict operating conditions and parameters at various locations within the system and closed loop feedback control, both using inputs from the various sensors. Inputs from other sensors associated with the engine and/or the exhaust gas system upstream of the EAT system may also be utilised. These may provide data relating to engine state, speed, load and temperatures, for example, as inputs to the EAT control system.

During operation, the DPF <NUM> may become filled by particulates (referred to herein as soot loading) which can affect correct operation of the engine and EAT system. Excess soot is oxidised in a process known as regeneration. Soot is oxidised by NO<NUM> reduction with the DPF in a temperature range in the order of <NUM>-<NUM> degC. This occurs during normal operation of the vehicle and is therefore termed passive regeneration. However, if the soot loading becomes too high, a more aggressive regeneration is required in which the soot is oxidised at a DPF temperature in excess of <NUM> degC. This is termed active regeneration as the process is actively triggered when the soot loading in the DPF exceeds a predetermined limit. For regeneration purposes, the temperature of the DPF is determined principally using the second temperature sensor <NUM>.

It is known to monitor soot loading of the DPF using two approaches. In a first method, monitoring of the soot loading in the DPF is carried out by the ECU <NUM> using a model based approach in which three EAT models are used:.

A further sensor based monitoring of the soot loading in the DPF <NUM> is also carried out using inputs from, amongst other things, the DPF pressure differential sensor <NUM>, the EGT measured downstream of the DOC by second temperature sensor <NUM>, the EGT upstream of the SCR unit <NUM> measured by third temperature sensor <NUM>, and the soot loading as modelled by the DPF model. This method compares the measured differential pressure over the DPF catalyst against a threshold value. If the sensor value exceeds the threshold value, this may trigger an active regeneration of the DPF.

Dosing of the DEF is controlled by the ECU <NUM> and may be controlled using a model-based DEF dosing control strategy. DEF dosing is allowed only when the engine is running and the DEF dosing strategy is adjusted according to the particular operating state of the engine. The main target of the dosing control is to reach the required tailpipe emissions of NOx while minimizing NH<NUM> slip and excessive use of DEF.

The SCR DEF dosing control model utilizes in particular the two NOx sensors <NUM>, <NUM> and the third and fourth temperature sensors <NUM>, <NUM>, located up and downstream of the SCR unit <NUM> to provide temperature data. Chemical reactions for the NOx components (NO and NO<NUM>) in the EAT system are different. Therefore accurate modelling of NOx conversion requires differentiation between NO and NO<NUM>. The upstream NOx sensor <NUM> cannot differentiate between NO and NO<NUM>. However, the DOC and DPF models provide separate concentrations for NO and NO<NUM> which are used as input for the SCR model. Accordingly, accurate modelling of the DOC and DPF are required both for monitoring the soot loading of the DPF and DEF dosing control.

<CIT> discloses a further example of a known exhaust gas aftertreatment system for a diesel engine having a SCR system downstream of a DOC and a DPF.

A problem with known EAT systems is the amount of space that is required to accommodate the system on a vehicle. This is a particular issue for agricultural vehicles such as tractors where it is becoming more difficult to find space for an exhaust aftertreatment system in addition to a simultaneous need to accommodate an increasing number of other systems and components that customers require.

<CIT> discloses an exhaust gas aftertreatment system according to the precharacterizing portion of claim <NUM>.

It is an objective of the present invention to provide an alternative EAT system comprising an oxidation catalyst, a diesel particulate filter, and a selective catalytic reduction system which occupies less space than the known EAT systems.

It is a further objective of the invention to provide an alternative method of operating an EAT system comprising an oxidation catalyst, a diesel particulate filter, and a selective catalytic reduction system which enables the EAT system to occupy less space than the known EAT systems.

According to a first aspect of the invention, there is provided an exhaust gas aftertreatment (EAT) system as set out in claim <NUM>. Further optional features of the EAT system are set out in the claims dependant on claim <NUM>.

In the EAT system according to the first aspect of the invention, the OC and PF are positioned adjacent one another providing a shorter package for the OC and PF than in the known system where the OC and PF are spaced apart to enable the pressure difference across the PF and the EGT between the OC and PF to be measured directly.

The EAT system is preferably provided on a vehicle or machine which may be an off-road vehicle or machine. The EAT system may be provided on an agricultural vehicle or machine, such as a tractor, combine harvester or the like, or on an industrial or construction vehicle or machine (which may be mobile or static), or on a generator.

The PF may have a catalytic coating and may be a CSF.

The EAT system may be configured to receive exhaust gases from a diesel engine. In which case, the OC may be a diesel oxidation catalyst (DOC) and the PF may be a Diesel Particulate Filter (DPF). Alternatively, the EAT system may be configured to receive exhaust gases from an internal combustion engine which burns an alternative fuel such as hydrogen. In this case, the OC and PF may be adapted to the type of fuel burned.

The EAT system may also comprise a mixer for mixing the EF in the exhaust gases downstream of the EF dosing module and upstream of the SCR catalyst.

The SCR system may comprise an ammonia slip catalyst (ASC). The ASC may be provided as a separate catalytic coating zone after the SCR catalyst coating in an SCR unit or it may be provided in a separate unit downstream of the SCR.

In accordance with a second aspect of the invention, there is provided a method of controlling an EAT system according to the first aspect of the invention as set out in claim <NUM>. Further optional features of the method are set out in the claims dependent on claim <NUM>.

The method may comprise subtracting the modelled pressure drop from the measured pressure drop to derive the pressure drop across the PF. In other words, the pressure drop occurring in any part of the system between the first and second measurement positions outside of the PF is modelled and subtracted from the measured pressure drop.

The method may comprise mathematically modelling the temperature of the exhaust gases entering or in the PF. Where the EAT system has an electronic control system including at least one programmable electronic control unit (ECU) configured and programmed to control and regulate operation of the EAT system in accordance with a number of pre-defied protocols depending on the operating conditions using a combination of mathematical modelling and closed loop feedback control, the control system may include a mathematical model for predicting soot loading of the PF and the method may comprise using the modelled temperature value as an input to the PF predicted soot loading model.

Embodiments of the invention will now be described, by way of example only, with reference to the remaining accompanying drawings, in which:.

The drawings are provided by way of reference only and will be acknowledged as not necessarily to scale. Like reference numbers are used to represent the same features, or features which perform substantially the same function, in each of the embodiments.

<FIG> illustrates schematically an agricultural machine <NUM>, especially in the form of a tractor. The agricultural machine <NUM> comprises a chassis <NUM>, a cab <NUM>, a front axle <NUM> and a rear axle <NUM>. The agricultural machine has an engine compartment <NUM> housing an internal combustion engine (indicated schematically at <NUM>) which selectively provides drive to the front and rear axles <NUM>, <NUM>. The internal combustion engine <NUM> in this example is specifically a diesel engine which burns diesel fuel with atmospheric air and produces power and waste gases. The waste gases are commonly referred to as exhaust gases and are rich in particulates and nitrous oxides NO and NO<NUM>, referred to collectively as NOx. In an alternative embodiment, the engine may be configured to burn other types of fuel such as hydrogen, for example.

<FIG> illustrates schematically an embodiment of an EAT system <NUM> in accordance with an aspect of the invention which is incorporated into part of an exhaust system <NUM> for the agricultural machine <NUM>. Exhaust gases enter the exhaust system <NUM> from the diesel engine <NUM> in the direction of arrow X, which indicates the direction of flow of gases through the exhaust system <NUM>. The arrow X indicates a direction from upstream to downstream.

The EAT system <NUM> is provided for reducing particulate and NOx emissions in the exhaust gases emitted to atmosphere. The EAT system is similar to that of the known system described above in relation to <FIG> and includes a DOC <NUM> and a DPF <NUM> downstream of the DOC. The DOC and DPF are configured to reduce particulate mass and number emissions. The DPF may have a catalytic coating and may be a CSF in which the DPF is coated with precious metal. Downstream of the DOC and DPF is an SCR system for NOx reduction. The SCR system includes a dosing module <NUM> for introducing a DEF into the exhaust flow, a mixer <NUM> for mixing the DEF in the exhaust gas and a SCR catalyst <NUM> downstream of the DEF dosing module and the mixer. The DEF is a reducing agent which reacts on a catalytic coating in the SCR unit <NUM> to reduce NOx in the exhaust gases. The DEF may be urea or urea-water solution which vaporizes and decomposes to produce ammonia (NH<NUM>) and carbon dioxide (CO<NUM>). The NH<NUM> reacts on the catalytic coating in the SCR to convert NOx and NH<NUM> into nitrogen gas (N<NUM>) and water vapour (H<NUM>O). The SCR catalyst <NUM> is provided in a SCR unit <NUM>. The SCR system also includes an ammonia slip catalyst (ASC) <NUM> for oxidising NH<NUM> in order to remove excess levels of NH<NUM> not fully utilised in the SCR process from the exhaust gases. The ASC may be provided as a separate catalytic coating zone after the SCR catalyst coating <NUM> in the SCR unit <NUM> or it may be provided in a separate unit downstream of the SCR unit.

The EAT system <NUM> according to the first embodiment is similar the prior art EAT system <NUM> illustrated in <FIG> and described above, except that the sensor layout is modified to enable the DOC <NUM> to be positioned closer to the DPF <NUM> with minimal spacing between them. This reduces the length of the DOC and DPF package. The sensor layout is modified such that the second temperature sensor <NUM> is moved downstream of the DPF and the pressure differential sensor <NUM> configured to measure the pressure differential across the combination of the DOC and DPF between a first measurement position 140a upstream of the DOC <NUM> and a second measurement position 140b downstream of the DPF <NUM>. As illustrated, the DOC <NUM> and DPF <NUM> may be located in a common housing <NUM>. A spacing in the region of <NUM> to <NUM>, and more preferably in the region of <NUM>, is provided between the DOC <NUM> and DPF <NUM> to allow for tolerances. The DPF <NUM> may be removable to allow for it to be taken out for cleaning and/or replacement.

Exhaust gases are directed from the engine into the common housing <NUM> through a first exhaust conduit section <NUM>. The upstream NOx sensor <NUM> and the first temperature sensor <NUM> are located between the engine and the DOC <NUM>. They can be located in the first exhaust conduit section <NUM> as shown. Alternatively, one or both could be located in an inlet chamber of the DOC <NUM>. In an embodiment, exhaust gases exiting the DPF <NUM> and the housing <NUM> are directed to the SCR unit <NUM> through a second or intermediate exhaust conduit section <NUM>. The second and third temperature sensors <NUM>, <NUM>, the DEF dosing module <NUM>, and the mixer <NUM> are all mounted to the intermediate exhaust conduit section <NUM>. The second temperature sensor <NUM> is located proximal the outlet of the DPF <NUM> and housing <NUM>, upstream of the DEF dosing module <NUM> and the mixer <NUM> and could be mounted to an outlet end of the DPF <NUM>. The third temperature sensor <NUM> is located proximal to the inlet to the SCR unit <NUM>. In an embodiment, the exhaust gases exiting the SCR unit <NUM> are directed to atmosphere through a third exhaust conduit section or tailpipe <NUM>. The fourth temperature sensor <NUM> and the downstream NOx sensor <NUM> are both mounted to the tailpipe <NUM> close to the SCR unit <NUM>. The fourth temperature sensor <NUM> and the downstream NOx sensor are typically mounted close together. Whilst the fourth temperature sensor <NUM> is shown as being positioned upstream of the downstream NOx sensor <NUM>, this order can be reversed.

The DOC <NUM>, DPF <NUM>, SCR catalyst <NUM>, and ASC <NUM> can be of any suitable types known in the art and so will not be described in detail.

The change in sensor layout to enable the DOC and DPF to be positioned closer together requires a modification in the control system and methods of controlling the EAT system.

In general, the electronic control system <NUM> is constructed and functions in a similar manner to that of the prior art control system <NUM> as described above, to which the reader should refer. Accordingly, the sensors provide inputs for use by the ECU <NUM> in regulating operation of the EAT system in a variety of operating conditions. Control of the EAT system is carried out using a combination of mathematical modelling to predict operating conditions at various locations within the system and closed loop feedback control, both using inputs from the various sensors. Inputs from other sensors associated with the engine and or exhaust gas system upstream of the EAT system may also be utilised. These may provide data relating to engine state, speed, load and temperatures, for example, as inputs to the EAT control system.

As in the prior art EAT system, monitoring of the soot loading in the DPF is carried out using a model based approach utilising three EAT models:.

A "sensor based" monitoring of the soot loading in the DPF <NUM> is also carried out. In the prior art system, the sensor based monitoring compares a measured differential pressure across the DPF catalyst against a threshold value. However, the modified sensor layout in the EAT system <NUM> does not allow direct measurement of the pressure differential across the DPF. To compensate for this, the EAT modelling includes an additional model to predict the pressure drop occurring in any part of the system between the first measurement position 140a and the second measurement position 140b but outside of the DPF. Typically this would require modelling a pressure drop occurring between the first measurement position 140a and the inlet face of the DPF <NUM> (including the pressure drop across the DOC <NUM>) and modelling of a pressure drop occurring between the outlet of the DPF and the second measurement position 140b. The output from this model is used together with the measured pressure drop across both the DOC and DPF obtained from the differential pressure sensor <NUM> to derive a pressure drop across the DPF. For example, the modelled pressure drop can be subtracted from the pressure drop measured by the pressure differential sensor <NUM> to derive the pressure drop across the DPF. This derived value for the pressure drop across the DPF is used as part of the sensor based monitoring of the soot loading in the DPF in place of the directly measure pressure differential used in the prior art method. The derived value for the pressure across the DPF and the threshold value are compared periodically. If the derived pressure value exceeds the threshold value, this may trigger an active regeneration of the DPF. The threshold value may be obtained from a look up table or a model of predicated pressure value for the operating conditions.

A further input required for secondary monitoring of the soot loading of the DPF is the EGT of the gas entering the DPF. Since this also cannot be measured directly with the modified sensor layout, the EAT modelling is further modified to generate a modelled temperature value. This might be calculated from the EGT measured at the first temperature sensor <NUM>, using a thermal model of the DOC to predict the DOC outlet temperature. Alternatively, the EGT of the gas entering the DPF can be calculated using the EGT measured by the first and second temperature sensors <NUM>, <NUM>, taking into account relevant effects such as heat transfer and thermal losses to the ambient, e.g. allowing for the higher heat capacity between the first and second temperature sensors <NUM>,<NUM>. The modelled temperature value can also be used as an input to the DPF model and can also be used to control the temperature of the DPF during regeneration, especially active regeneration. Indeed, various models which form part of the EAT control system may require recalibration and/or additional modelling to take account of the fact that the second temperature sensor <NUM> is measuring the EGT downstream of the DPF.

<FIG> illustrates a further embodiment of the EAT system <NUM>' according to the invention. This system is substantially the same as the embodiment <NUM> described above in relation to <FIG> except that it comprises only one intermediate temperature sensor <NUM> for sensing the EGT between the DPF <NUM> and the SCR unit <NUM>. In the embodiment of <FIG>, the temperature sensor <NUM> immediately downstream of the DPF <NUM> is omitted. The EAT system <NUM>' is operated and controlled in a similar manner to the previous embodiment <NUM> except that the control system is modified to take account of the fact that the EGT is only being measured in three places. This may require the creation of one or more mathematical models to generate one or more virtual temperature signals based on the measured EGT inputs from the three available temperature sensors <NUM>, <NUM>, <NUM> and taking into account relevant effects such as heat transfer, thermal losses to the ambient and chemical reactions. In addition, or alternatively, this may require recalibration of the various EAT models and/or control protocols as required to take account of the reduced number of measured EGT inputs. In a further alternative arrangement, which can be adopted in either of the embodiments shown in <FIG>, the SCR outlet temperature sensor <NUM> can be left out and replaced by a modelled value.

The EAT system <NUM>' of <FIG>, and the modified version of the EAT system <NUM> in <FIG> with no SCR outlet temperature sensor <NUM>, have the benefit of reduced cost as fewer temperature sensors are used. However, measured EGT values are more accurate than modelled values.

Whilst embodiments of an EAT system and method in accordance with the invention have been described in relation to a mobile agricultural machine <NUM> in the form of a tractor, it should be understood that the inventive EAT system and method can be applied to a range of vehicles and machines (whether static or mobile) which use an internal combustion engine and especially a diesel engine. This includes, without limitation, tracked vehicles, combine harvesters, industrial and construction vehicles, and generators. Furthermore, whilst the invention has been described in relation to an EAT configured to receive exhaust gases from a diesel engine, the principles may be applied where the EAT system is used in respect of an engine which burns an alternative fuel such as hydrogen. In this case, the EAT may have an oxidation catalyst (OC) upstream of and adjacent to a particulate filter (PF) which are adapted to the type of fuel burned in place of the DOC and the DPF. Typically, the OC and PF would be constructed and function in similar manner to a DOC and DPF but would be configured for use with the particular type of fuel. Similarly, the DEF may be referred to simply as an exhaust fluid (EF) or a reducing agent where the engine is not a diesel engine.

Claim 1:
An exhaust gas aftertreatment (EAT) system (<NUM>; <NUM>') for receiving exhaust gases from an internal combustion engine (<NUM>), the EAT system comprising:
a particulate filter (PF) (<NUM>);
an oxidation catalyst (OC) (<NUM>) upstream of and adjacent to the PF;
a pressure differential sensor (<NUM>) configured to measure a pressure differential between a first measurement position (140a) upstream of the OC and a second measurement position (140b) downstream of the PF and wherein there are no sensors for determining a parameter of the exhaust gases located between the OC and the PF;
characterized by:
a selective catalytic reduction (SCR) system downstream of the PF, the SCR system including an exhaust fluid (EF) dosing module (<NUM>) for introducing an EF into the exhaust gas flow downstream of the PF and a SCR catalyst (<NUM>) downstream of the EF dosing unit;
an upstream NOx sensor (<NUM>) for detecting NOx levels in the exhaust gases upstream of the OC;
a downstream NOx sensor (<NUM>) for detecting NOx levels in the exhaust gas downstream of the SCR catalyst;
an upstream OC temperature sensor (<NUM>) for sensing the exhaust gas temperature (EGT) upstream of the OC; and
at least one intermediate temperature sensor (<NUM>, <NUM>) for sensing the EGT downstream of the PF and upstream of the SCR catalyst;
and wherein:
a) the EAT system has two intermediate temperature sensors, one (<NUM>) downstream of the PF and upstream of the EF dosing module and one (<NUM>) downstream of the EF dosing module and upstream of the SCR catalyst; or
b) the EAT system has a single intermediate temperature sensor (<NUM>) located downstream of the EF dosing module and upstream of the SCR catalyst.