GB2454313A

GB2454313A - A diesel engine exhaust aftertreatment system

Info

Publication number: GB2454313A
Application number: GB0819156A
Authority: GB
Inventors: Timothy Sung Jae Chyo; Jian Kong; Paul Joseph Tennison
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2007-10-30
Filing date: 2008-10-20
Publication date: 2009-05-06
Also published as: US20090107117A1; CN101424202A; DE102008036278A1; GB0819156D0

Abstract

A diesel engine exhaust gas aftertreatment system is described. The system includes a heat transfer device 106 comprising a first side 106A and a second side 106B, a heater 108 downstream of the first side 106A of the heat transfer device 106, a filter 110 downstream of the heater 108 and an exhaust recirculation path 114 downstream of the filter 110 leading to the second side 106B of the heat transfer device 106. The exhaust path is preferably isolated from an external surface of the filter 110 to inhibit heat transfer from the heated exhaust gas exiting the filter 110 to the external surface of the filter 110. A method of use of the system is also described and claimed.

Description

A DIESEL ENGINE EXHAUST AFTERTREATMENT SYSTEM

This invention relates to diesel engines and in particular to the treatment of exhaust gasses emitted by a diesel engine.

The effectiveness of a diesel particulate filter (DPF) in an exhaust after treatment system for a lean-burn internal combustion engine can be improved by periodically ao regenerating the particulate filter. Regeneration typically involves elevating the temperature of the particulate filter, thereby combusting adsorbed particulates. While regeneration is exothermjc and may produce enough energy to be self-sustaining after initiation, lean-burn internal combustion engines typically have an exhaust gas temperature that is lower than that required to activate regeneration.

Many particulate filter devices therefore incorporate heat sources such as catalysts or heaters to initiate the regeneration process. On the other hand, the particulate filter can be damaged at excessive temperatures. For example, catalyst particles incorporated into the particulate filter may sinter (reducing the activity of the catalyst), the structural integrity of the filter element may be damaged, etc. The inventors herein have recognized that these thermal limitations often force a trade-off between the efficiency and reliability of a particulate filter. Particulate filter devices that attempt to conserve energy by reusing waste heat from the regeneration reaction to directly heat the particulate filter risk potentially self-sustained reactions, especially without a separate cooling mechanism to manage excessive temperatures. Alternatively, particulate filter devices that do not recycle waste heat from the regeneration reaction may also lower fuel economy, leading to increased operating costs. Moreover, additional heat shielding and/or cooling may be used if the temperature of the particulate filter effluent is too high to safely pass to downstream devices, adding to manufacturing and/or maintenance costs.

It is an object of the invention to provide a diesel engine exhaust gas aftertreatment system that overcomes or minimises the disadvantages associated with the prior art.

According to a first aspect of the invention there is provided a diesel engine exhaust gas aftertreatment system comprising a heat transfer device arranged to receive exhaust gas from a diesel engine having first and second sides, a heater located downstream of the first side of the heat transfer device, a filter located downstream of the heater and an exhaust recirculation path from the filter to the second side of the heat transfer device to transfer exhaust gas from the filter to the second side.

Preferably, the exhaust path may be isolated from an external surface of the filter to inhibit heat transfer from the heated exhaust gas exiting the filter to the external surface of the filter.

The system may further comprise a heater control device to regulate operation of the heater responsive to at least one of temperature of the exhaust gas and a pressure of the exhaust gas.

The heater control device may be further responsive to an engine control unit.

The heater control device may be further responsive to an engine condition received from the engine control unit comprising at least one of engine speed, air fuel ratio, fuel injection timing and engine load.

The system may further comprise a catalyst coupled downstream of a third side of the heat transfer device.

The heat transfer device may comprise a shell and tube heat exchanger.

Substantially all of the exhaust gas entering the first side of the heat transfer device may be conducted to the second side of the heat transfer device after being conducted through at least the heater, the filter, and the exhaust path.

The heater may be a fuel-fired heater.

The heater may be located directly upstream of the filter and the heat transfer device may be located directly downstream of the filter.

The heat transfer device may have a first passageway and a second passageway, the first passageway between the first side and the second side of the device, the second side Opposite the first side, the second passageway between a third side and a fourth side of the device, the fourth side opposite the third side, a first exhaust path leading exhaust gas from the engine to the first side, the exhaust recirculation path is formed by a second exhaust path leading exhaust gas from the second side to the third side, the second path including the heater and the filter coupled downstream of the heater, a third exhaust path leading exhaust gas from the fourth side, the third path including a catalytic emission control device.

Substantially all of the exhaust gas exiting the second side of the heat transfer device may be conducted to the third side of the heat transfer device after being conducted through at least the heater and the particulate filter.

The system may further comprise a controller to operate the heater during a first mode where heat is transferred from exhaust gas in the second passageway to exhaust gas in the first passageway and to deactivate the heater during a second mode where heat is transferred from exhaust gas in the first passageway to exhaust gas in the second passageway, the controller being operable to transition the system between the first mode and the second mode responsive to temperature of the catalytic emission control device.

The controller may further transition the system among the first and second mode responsive to the temperature during regeneration of the particulate filter.

The controller may adjust operation of the heater responsive to engine operating conditions.

According to a second aspect of the invention there is provided a method of operating a diesel engine exhaust gas aftertreatment system having a heat transfer device, a particulate filter and a heater, the method comprising, during a first exhaust gas condition, conducting a first volume of exhaust gas through a first passage of the heat transfer device, operating the heater to elevate temperature of the first volume of exhaust gas exiting the heat transfer device, conducting the first volume of exhaust gas exiting the heater through the particulate filter, conducting the first volume of exhaust gas exiting the particulate filter through an exhaust passage to a second passage of the heat transfer device and heating a second volume of exhaust gas conducted from the engine to the first passage of the heat transfer device with energy transferred from the first volume of exhaust gas in the second passage of the heat transfer device and, during a second exhaust gas condition, conducting a third volume of exhaust gas through the first passage of the heat transfer device, conducting the third volume of exhaust gas through the heater without operating the heater, conducting the third volume of exhaust gas exiting the heater through a particulate filter, conducting the third volume of exhaust gas exiting the particulate filter through the exhaust passage to the second passage of the heat transfer device and cooling a fourth volume of exhaust gas conducted from the engine to the first passage of the heat transfer device via energy transferred to the third volume of exhaust gas in the second passage of the heat transfer device.

Preferably, the exhaust passage may be isolated from an external surface of the filter to inhibit heat transfer from the heated exhaust gas exiting the filter to the external surface of the filter.

The method may further comprise controlling operation of the heater responsive to at least a temperature of the aftertreatment system.

The method may further comprise routing exhaust gas from the second passage to a catalyst and adjusting operation of the heater responsive to a temperature of the catalyst.

The heater may be further adjusted responsive to at least a pressure and an oxygen concentration of the exhaust gas.

The invention will now be described by way of example with reference to the accompanying drawing of which:-Fig.lis a schematic diagram of a diesel engine aftertreatment system with waste heat recovery according to the invention; and Figs.2A, 2B, 3A and 3B illustrate high-level flowcharts and block diagrams for operating the diesel engine aftertreatment system with waste heat recovery shown in Fig. 1.

Referring firstly to Fig.l, engine 10 includes an intake manifold 100 fluidly coupled to a bank of combustion cylinders 102. The combustion cylinders 102 also communicate with an exhaust manifold 104.

A heat transfer device 106 is coupled to exhaust manifold 104. The heat transfer device 106 comprises at least two sides 106A and 106B which permit the transfer of heat between gases on each side without commingling the gases. Heat transfer device 106 may be of various kinds.

For example, the heat transfer device 106 may be a shell-and-tube heat exchanger, a plate heat exchanger, etc. and, while not shown in Fig.1, the heat transfer device 106 may incorporate baffles on one or more sides to facilitate efficient heat transfer.

The first side 106A of heat transfer device 106 is coupled to a heater 108 located downstream of first side 106A of heat transfer device 106. The heater 108 may provide heat from an external, fuel-fired flame, from the combustion of fuel injected into the exhaust stream, from an electrically-powered heating element, etc. Heater 108 may be further coupled to a particulate filter 110 located downstream of the heater 108. The particulate filter 110 may be of various types (e.g. zeolite) and includes a filter element 112 in the form of a monolith or a packed-bed, or some combination thereof. Particulate filter 110 may be inert or may incorporate catalytically active material.

The particulate filter 110 has an outlet coupled to the second side 106B of heat transfer device 106 located downstream of particulate filter 110. An exhaust passage 114 is used to couple the particulate filter 110 to second side 1063 of heat transfer device 106. The exhaust passage 114 may be isolated from the external surface of particulate filter 110 to inhibit heat transfer from the heated exhaust gas exiting particulate filter 110.

The second side 106B of heat transfer device 106 may be coupled to an additional emission control device 116 downstream of second side 1063 of heat transfer device 106.

For example, emission control device 116 may include a NOx trap, NOx catalyst, Urea SCR catalyst, or various others or combinations thereof. Emission control device 112 may be further coupled to a tailpipe 118 downstream of emission control device 112.

While not shown in Fig.1, the system may include an exhaust gas recirculation (EGR) loop, which may be fluidly coupled to exhaust manifold 104 and intake manifold 100.

The EGR loop may include a heat transfer device, such as an EGR cooler. The EGR loop may also include a catalyst such as an oxidation catalyst or various other EGR catalyst types, or combinations thereof. The system may also include a high pressure diesel fuel injection system for delivering injected fuel to combustion cylinders 102. For example, the diesel fuel injection system may be a common rail system which permits injection timing to be adjusted based on the operation of engine 10.

A heater control system includes a heater controller 12, shown in Fig.l as a microcomputer, including a microprocessor unit, input/output ports, an electronic storage medium for executable programs and calibration values in read-only memory, random access memory, keep-alive memory, and a data bus. The controller 12 receives various signals from temperature and pressure sensors 160, as well as signals from an engine control system, such as air-fuel ratio, engine speed, crank angle, fuel injection timing, oxygen concentration, etc. Controller 12 contains stored control algorithms for operating heater 108 in response to signals or combinations of signals received from various sensors and from an engine control system by operating various actuators 170.

An engine control system includes a controller 14, shown in FIG.1 as a microcomputer, including a microprocessor unit, input/output ports, an electronic storage medium for executable programs and calibration values in read-only memory, random access memory, keep-alive memory, and a data bus. The controller 14 receives various signals from sensors 140 coupled to engine 10, in addition to those signals previously discussed, including measurement of inducted air mass flow rate from mass air flow (MAF) sensor; engine coolant temperature (ECT); engine speed, throttle position (TP) from a throttle position sensor; and manifold absolute pressure (MAP) from a manifold pressure sensor. The controller 14 contains stored control algorithms for operating various devices in response to signals or combinations of signals received from various sensors coupled to engine 10. Finally, the controller 14 is operable to send various signals to actuators 150 associated with the engine 10.

As described further herein, the heater control system controls heater operation based on various operating conditions.

Fig.2A illustrates a high-level flowchart for one method of operating a heater control system in response to system parameters.

First, in 202, the heater control system reads exhaust system parameters, such as temperature and pressure, as well as engine operating conditions, such as speed, fuel injection timing, air-fuel ratio, etc. Then, at 204, the heater control system determines if the conditions are appropriate for particulate filter regeneration. For example, occlusion of the particulate filter may be reflected in the pressure drop across the particulate filter. Thus, if the pressure drop exceeds a preset threshold value, regeneration may be appropriate.

The threshold pressure drop value may be selected based on a desired magnitude of back pressure experienced by the engine, desired levels of filter loading, etc. Alternately, the engine control unit may set a flag overriding or triggering particulate filter regeneration in response to a condition or conditions, as discussed below.

If the heater control system determines that it is inappropriate to start the regeneration process, the routine skips to the end otherwise the routine proceeds to 206 and enters the heater control routine depicted in Fig.2B.

Referring now to Fig. 2B, at 210, the routine determines the pressure drop across the particulate filter 110.

Next, at 212, the routine determines if the pressure drop is greater than a preset value. This value may represent a threshold value for an acceptable level of filter fouling and may include factors accounting for measurement error, etc. The value in 212 may be different from the value in step 204 of Fig.2A; for example, a pressure drop which can trigger a regeneration cycle in step 204 of Fig. 2A may exceed the value selected at step 212 of Fig. 2B. The value selected at 212 may correspond to a range of filter conditions that are not so fouled as to trigger regeneration in 204 of Fig.2A.

If the pressure drop exceeds the threshold value in 212, the routine continues to 214, where the routine -10 -measures the temperature of the exhaust stream downstream of the heater 108 and upstream of the particulate filter 110.

Next, at 216, the routine determines if the temperature exceeds a threshold temperature. For example, the threshold temperature may be the ignition temperature of the regeneration reaction, the temperature necessary to sustain a particular regeneration reaction rate, etc. If the temperature is below the threshold temperature, the routine proceeds to 218. For example, the temperature may be below the threshold temperature at the beginning of the regeneration cycle, where the exhaust gas does not have sufficient thermal energy to activate the regeneration combustion reactions.

At 218, the routine operates the heater according to a heater operation algorithm, an example of which is described by Figs.3A and 3B. From 218, the routine then returns to 210.

If at 216 the temperature exceeds the threshold temperature, the routine proceeds to 220, where the heater is turned off, before returning to step 210. For example, the temperature may exceed the threshold temperature once the regeneration process has ignited and heated exhaust gases from the particulate filter begin transferring heat from a second side lO6B of heat transfer device 106 to a first side 106A of heat transfer device 106.

If the pressure drop does not exceed the threshold value in 212, the routine proceeds to 222. A pressure drop that does not exceed the threshold value may indicate that the regeneration process has completed, although other approaches may be used to determine the completion of the regeneration process. For example, the temperature change across the filter may decrease to below a temperature difference threshold, indicating that the reaction rate has dropped to a level indicative of minimal combustion -11 -activity. Alternatively, other approaches maybe employed which directly measure the temperature or other characteristics of the filter, such as electrical conductivity, weight, etc. At 222, the routine disables the heater and proceeds to the end of the routine.

Fig.3A illustrates a view of the aftertreatment system including a heater 308 and a filter 310 and indicates the position of multiple sensors. A heater controller such as the heater controller 12 is used to control operation of the heater 308. Sensors may include temperature sensors downstream of heater 308 but upstream of filter 310, such as Ti, temperature sensors located within filter 310, such as T2, or sensors located downstream of filter 310, such as T3.

Pressure sensors such as P1, located upstream of filter 310, and P2, located downstream of filter 310, may also be included.

Fig.3B illustrates a high-level heater control algorithm for use with the heater control routine described in Fig.2B.

Firstly, the heater control algorithm reads engine and exhaust operating parameters into regeneration process model 320. Examples of engine operating parameters include engine speed, air-fuel ratio, fuel injection timing, etc. Examples of exhaust parameters include oxygen concentration, aftertreatment system temperatures, pressures, etc. Regeneration process model 320 computes a desired temperature TIp-lLei upstream of filter 310 and downstream of heater 308 and feeds the desired T=ire to comparator 312.

For example, regeneration process model 320 may read a desired regeneration reaction rate from heater control unit 12 or from engine control unit 14. Further, regeneration process model 320 may read aftertreatment system pressure P1, an exhaust oxygen concentration, and a stored reaction -12 -rate coefficient to calculate Tt-irt.i. Regeneration process model 320 may be updated through an auto-adaptive technique over the life of filter 310 to compensate for changes in the activity of the regeneration process. For example, if filter 310 includes a catalyst, an auto-adaptive technique may help compensate for changes in catalyst activity to provide more efficient regeneration conditions throughout the life of filter 310.

At comparator 312, T-irj is compared to a measured temperature, such as Tl, T2, or T3. For example, early in the initiation of the regeneration process, there may be some undesirable delay in sensing a temperature change downstream of filter 310, as may be caused by the thermal mass of filter 310. An undesirable hot spot may develop on the leading edge of filter 310 during the time delay, potentially degrading the filter or leading to an over-temperature condition during self-sustained reactions.

Thus, using Tl at comparator 312 may provide beneficial control of the heater 308. However, as the regeneration process achieves stable operation, it may be desirable to reference T2 or T3 at comparator 312, which may provide better feedback control of the entire system. Comparator 312 generates an error signal Tern, which is read into heater controller 12. Heater controller 12 may include parameters to control heater 308. Further, heater controller 12 may be configured for proportional-integral-derivative (PID) control, wherein the parameters may be varied over different temperature domains to provide robust control over some temperature domains and refined control over other temperature domains. Further still, the parameters may be configured in advance or may be auto-tuned by heater controller 12. As an example of the operability of heater controller 12, jf T indicates that the measured temperature (as represented by Tl, T2, or T3) is less than T[)e.'lrel, heater controller 12 operates the heater 308 to bring the measured temperature within a desired range of T[e:re.i.

-13 -Alternatively, jf T. indicates that the measured temperature (as represented by Ti, T2, or T3) is greater than Tij,e, the heater 308 may be ramped down or switched off by heater controller 12.

Both the heater control algorithm depicted in Fig.3B and the heater control routines in Figs.2A and 2B may also be responsive to other conditions, such as those illustrated below. Driver demand and/or engine load may influence the frequency and incidence of filter regeneration. For example, it may be desirable for the particulate filter to regenerate after a threshold time interval has elapsed since the last regeneration cycle, or it may be desirable to delay regeneration while the engine is under a heavy load. Thus, engine control unit 14 may set a flag triggering or delaying a regeneration process. In turn, heater control unit 12 may be responsive to at least one engine condition received from engine control unit 14.

Conditions in combustion cylinders 102 may also affect the operation of the heater control system. For example, low temperature combustion conditions established by engine control unit 14, such as those where the engine is operated under very lean conditions may produce a lower engine exhaust temperature, and may require more energy input from heater 108 than conditions where more fuel is supplied to combustion cylinders 102, which can produce a higher engine exhaust temperature. As another example, regeneration process model 320 may read in other engine conditions from engine control unit 14, such as speed, fuel injection, air fuel ratio, etc., which may provide alternative estimates of the oxygen concentration of the engine exhaust, the concentration of un-combusted fuel, the concentration of various exhaust gases, such as NOx and COx, etc., all of which may affect the regeneration reaction rate.

-14 -The operating conditions that may be used and/or adjusted with regard to the above figures may include, for example, air-fuel ratio, engine speed, fuel injection amount and/or timing, exhaust and/or inlet oxygen concentration, exhaust gas recirculatjon amounts, driver demand, and/or others.

Various example operations are described herein to illustrate system coordination; however, various alternative operations may also occur due to the system structure. For example, as an additional example, under a first exhaust condition, exhaust gas entering the after treatment system may be warmed by a heater prior to entering the filter.

Such a condition may be experienced when the particulate filter regeneration process is being initiated, for example in response to a signal from an engine control unit indicating pressure across the filter has reached a threshold value. As the temperature of the particulate filter rises, the rate of the exothermic regeneration reaction may increase, increasing the temperature of the exhaust gas exiting the filter. Heat energy from the exhaust gas may then be transferred to exhaust gas entering the after treatment system in the heat transfer device.

When the temperature of the exhaust gas reaches a threshold, the heater control unit may switch the heater off, conserving fuel. Furthermore, once the regeneration process has completed, such as detected by a pressure difference measurement, the heater may remain switched off. Moreover, the heater control unit may interact with the engine control unit to vary the operation of the heater in response to changing engine loads, driver demands, etc., which may change the oxygen concentration in the exhaust stream and thus affect the regeneration process or the frequency of regeneration in a way that requires more or less input from the heater.

-15 -The specific routines described in the flowcharts and diagrams may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments of the invention io described herein, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used.

Further, these figures graphically represent code to be programmed into the computer readable storage medium in the controller.

Therefore in summary, the disadvantages and problems associated with the prior art may be at least partially addressed by, in one example, a diesel engine exhaust gas aftertreatment system. The system may comprises a heat transfer device comprising a first side and a second side, a heater downstream of the first side of the heat transfer device, a filter downstream of the heater, and, an exhaust path downstream of the filter leading to the second side of the heat transfer device, wherein the exhaust path is isolated from an external surface of the filter to inhibit heat transfer from the heated exhaust gas exiting the filter to the external surface of the filter.

In this way, a desired heater operational efficiency may be achieved while also addressing thermal degradation.

For example, the after treatment system may permit waste heat from the regeneration to be recycled without directly subjecting the particulate filter to the thermal load of the heated exhaust gases exiting the particulate filter, thereby -16 -reducing potential of self-sustained regeneration resulting in over-temperature conditions that may degrade the particulate filter or other components. Further, the temperature of the gas exiting the particulate filter may be lowered toward desired tailpipe conditions as heat is transferred to exhaust gas entering the system from the engine. Further still, because thermal energy may be supplied from at least the heater to activate the regeneration process, the engine may be operated under low-temperature combustion (LTC) conditions. Low-temperature combustion processes may generate less particulate matter, permitting the use of smaller particulate filter devices, where smaller particulate filters may reduce the risk of over-temperature, may require less fuel to heat during the initial activation, etc.

Claims

-17 -Claims 1. A diesel engine exhaust gas aftertreatment system comprising a heat transfer device arranged to receive exhaust gas from a diesel engine having first and second sides, a heater located downstream of the first side of the heat transfer device, a filter located downstream of the heater and an exhaust recirculation path from the filter to the second side of the heat transfer device to transfer exhaust gas from the filter to the second side.
2. A system as claimed in claim 1 wherein the exhaust path is isolated from an external surface of the filter to inhibit heat transfer from the heated exhaust gas exiting the filter to the external surface of the filter.
3. A system as claimed in claim 1 or in claim 2 wherein the system further comprises a heater control device to regulate operation of the heater responsive to at least one of temperature of the exhaust gas and a pressure of the exhaust gas.
4. A system as claimed in claim 3 wherein the heater control device is further responsive to an engine control unit.
5. A system as claimed in claim 4 wherein the heater control device is further responsive to an engine condition received from the engine control unit comprising at least one of engine speed, air fuel ratio, fuel injection timing and engine load.
6. A system as claimed in any of claims 1 to 5 wherein the system further comprises a catalyst coupled downstream of a third side of the heat transfer device.

-18 -
7. A system as claimed in any of claims 1 to 6 wherein the heat transfer device comprises a shell and tube heat exchanger.
8. A system as claimed in any of claims 1 to 7 wherein substantially all of the exhaust gas entering the first side of the heat transfer device is conducted to the second side of the heat transfer device after being conducted through at least the heater, the filter, and the exhaust path.
9. A system as claimed in any of claims 1 to 8 wherein the heater is a fuel-fired heater.
10. A system as claimed in any of claims 1 to 9 wherein the heater is located directly upstream of the filter and the heat transfer device is located directly downstream of the filter.
11. A system as claimed in any of claims 1 to 10 wherein the heat transfer device has a first passageway and a second passageway, the first passageway between the first side and the second side of the device, the second side opposite the first side, the second passageway between a third side and a fourth side of the device, the fourth side opposite the third side, a first exhaust path leading exhaust gas from the engine to the first side, the exhaust recirculatjon path is formed by a second exhaust path leading exhaust gas from the second side to the third side, the second path including the heater and the filter coupled downstream of the heater, a third exhaust path leading exhaust gas from the fourth side, the third path including a catalytic emission control device.
12. A system as claimed in claim 11 wherein the system further comprises a controller to operate the heater during a first mode where heat is transferred from exhaust gas in -19 -the second passageway to exhaust gas in the first passageway and to deactivate the heater during a second mode where heat is transferred from exhaust gas in the first passageway to exhaust gas in the second passageway, the controller being operable to transition the system between the first mode and the second mode responsive to temperature of the catalytic emission control device.
13. A system as claimed in claim 12 wherein the controller further transitions the system among the first and second mode responsive to the temperature during regeneration of the particulate filter.
14. A system as claimed in claim 12 or in claim 13 wherein the controller adjusts operation of the heater responsive to engine operating conditions.
15. A method of operating a diesel engine exhaust gas aftertreatment system having a heat transfer device, a particulate filter and a heater, the method comprising, during a first exhaust gas condition, conducting a first volume of exhaust gas through a first passage of the heat transfer device, operating the heater to elevate temperature of the first volume of exhaust gas exiting the heat transfer device, conducting the first volume of exhaust gas exiting the heater through the particulate filter, conducting the first volume of exhaust gas exiting the particulate filter through an exhaust passage to a second passage of the heat transfer device and heating a second volume of exhaust gas conducted from the engine to the first passage of the heat transfer device with energy transferred from the first volume of exhaust gas in the second passage of the heat transfer device and, during a second exhaust gas condition, conducting a third volume of exhaust gas through the first passage of the heat transfer device, conducting the third volume of exhaust gas through the heater without operating the heater, conducting the third volume of exhaust gas -20 -exiting the heater through a particulate filter, conducting the third volume of exhaust gas exiting the particulate filter through the exhaust passage to the second passage of the heat transfer device and cooling a fourth volume of exhaust gas conducted from the engine to the first passage of the heat transfer device via energy transferred to the third volume of exhaust gas in the second passage of the heat transfer device.
16. A method as claimed in claim 15 wherein the exhaust passage is isolated from an external surface of the filter to inhibit heat transfer from the heated exhaust gas exiting the filter to the external surface of the filter.
17. A method as claimed in claim 15 or in claim 16 wherein the method further comprises controlling operation of the heater responsive to at least a temperature of the aftertreatment system.
18. A method as claimed in any of claims 15 to 17 wherein the method further comprises routing exhaust gas from the second passage to a catalyst and adjusting operation of the heater responsive to a temperature of the catalyst.
19. A method as claimed in claim 18 wherein the heater is further adjusted responsive to at least a pressure and an oxygen concentration of the exhaust gas.
20. A diesel engine exhaust gas aftertreatment system substantially as described herein with reference to the accompanying drawing.
21. A method of operating a diesel engine exhaust gas aftertreatment system having a heat transfer device, a particulate filter and a heater substantially as described herein with reference to the accompanying drawing.