Patent Application
20090107117
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 regenerating the particulate filter. Regeneration typically involves elevating the temperature of the particulate filter, thereby combusting adsorbed particulates. While regeneration is exothermic 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. Thus, many particulate filter devices 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 may 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.
The above issues 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 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.
Referring to
A first side 106A of heat transfer device 106 may be coupled to a heater 108 located downstream of first side 106A of heat transfer device 106. 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 particulate filter 110 located downstream of heater 108. Particulate filter 110 may be of various types (e.g. zeolite), and may include 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. Particulate filter 110 may be coupled to a second side 106B of heat transfer device 106 located downstream of particulate filter 110. An exhaust passage 114 may couple particulate filter 110 to second side 106B of heat transfer device 106. Exhaust passage 114 maybe isolated from the external surface of particulate filter 110 to inhibit heat transfer from the heated exhaust gas exiting particulate filter 110.
Second side 106B of heat transfer device 106 may be coupled to an additional emission control device 112 downstream of second side 106B of heat transfer device 106. For example, emission control device 112 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 114 downstream of emission control device 112.
While not shown in
A heater control system may include controller 12, shown in
An engine control system may include controller 14, shown in
As described further herein, the heater control system may control heater operation based on various operating conditions.
Referring now to
If the pressure drop exceeds the threshold value in 212, the routine continues to 214, the routine measures the temperature of the exhaust stream downstream of the heater and upstream of the particulate filter. 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
If 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 106B 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 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.
Regeneration process model 320 computes a desired temperature Tdes upstream of filter 310 and downstream of heater 308 and feeds the desired Tdes to comparator 322. 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 rate coefficient to calculate Tdes. 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 322, Tdes is compared to a measured temperature, such as T1, 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 312, as may be caused by the thermal mass of filter 312. An undesirable hot spot may develop on the leading edge of filter 312 during the time delay, potentially degrading the filter or leading to an over-temperature condition during self-sustained reactions. Thus, using T1 at comparator 322 may provide beneficial control of the heater. However, as the regeneration process achieves stable operation, it may be desirable to reference T2 or T3 at comparator 322, which may provide better feedback control of the entire system. Comparator 322 generates an error signal Terr, which is read into heater controller 324. Heater controller 324 may include parameters to control heater 308. Further, heater controller 324 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 control device 12. As an example of the operability of heater controller 324, if Terr indicates that the measured temperature (as represented by T1, T2, or T3) is less than Tdes, heater controller 324 may operate the heater to bring the measured temperature within a desired range of Tdes. Alternatively, if Terr indicates that the measured temperature (as represented by T1, T2, or T3) is greater than Tdes, the heater may be ramped down or switched off by heater controller 324.
Both the heater control algorithm depicted in
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 uncombusted fuel, the concentration of various exhaust gases, such as NOx and COx, etc., all of which may affect the regeneration reaction rate.
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 recirculation 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.
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 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.
It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense because numerous variations are possible. The subject matter of the present disclosure includes all novel and non-obvious combinations and subcombinations of the various system and exhaust configurations, algorithms, and other features, functions, and/or properties disclosed herein. The following claims particularly point out certain combinations and subcombinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.
