WO2006066043A1

WO2006066043A1 - Control for an engine exhaust treatment system

Info

Publication number: WO2006066043A1
Application number: PCT/US2005/045505
Authority: WO
Inventors: Zhixin Hou (Jason); Wayne M. Wagner; Wenzhong Zhang; Edward A. Steinbrueck; Theodore G. Angelo; Ted J. Wiegandt; Mike D. Anderson
Original assignee: Donaldson Company, Inc.
Priority date: 2004-12-15
Filing date: 2005-12-15
Publication date: 2006-06-22

Abstract

The present disclosure relates to methods for controlling the introduction of a reactant into an exhaust system. In certain embodiments, the mean temperature of a catalyzed substrate within an exhaust system is used as a control parameter.

Description

CONTROL FOR AN ENGINE EXHAUST TREATMENT SYSTEM

This application is being filed on 15 December 2005, as an International Patent application in the name of Donaldson Company, Inc., a U.S. national corporation, applicant for the designation of all countries except the US, and Zhixin (Jason) Hou, Wayne M. Wagner, Wenzhong Zhang, Edward A. Steinbrueck, Theodore G. Angelo, Ted Wiegandt, and Mike Anderson, all citizens of the U.S., applicants for the designation of the US only, and claims the benefit of U.S. Provisional Patent Application Serial No. 60/636,480, filed December 15, 2004, which application is hereby incorporated by reference in its entirety.

Technical Field

The present disclosure relates generally to diesel exhaust systems. More particularly, the present disclosure relates to systems and methods for controlling diesel emissions.

Background

Vehicles equipped with diesel engines typically include exhaust systems that may have diesel particulate filters for removing particulate matter from the exhaust stream. With use, soot or other carbon-based particulate matter to accumulates on the diesel particulate filters. As particulate matter accumulates on the diesel particulate filters, the restriction of the filters increases causing the buildup of undesirable back pressure in the exhaust systems. High back pressures decrease engine efficiency. Therefore, to prevent diesel particulate filters from becoming excessively loaded, diesel particulate filters should be regularly regenerated by burning off (i.e., oxidizing) the particulates that accumulate on the filters. Since the particulate matter captured by diesel particulate filters is mainly carbon and hydrocarbons, its chemical energy is high. Once ignited, the particulate matter burns and releases a relatively large amount of heat.

Systems have been proposed for regenerating diesel particulate filters. Some systems use a fuel fed burner positioned upstream of a diesel particulate filter to cause regeneration (see U.S. Patent No. 4,167,852). Other systems use an electric heater to regenerate a diesel particulate filter (see U.S. Patent Nos. 4,270,936; 4,276,066; 4,319,896; 4,851,015; and British Published Application No. 2,134,407). Detuning techniques are also used to regenerate diesel particulate filters by raising the temperature of exhaust gas at selected times (see U.S. Patent Nos. 4,211,075 and 3,499,260). Self regeneration systems have also been proposed. Self regeneration systems can use a catalyst on the substrate of the diesel particulate filter to lower the ignition temperature of the particulate matter captured on the filter. An example self regeneration system is disclosed in U.S. Patent No. 4,902,487.

In addition to particulate filters for removing particulate matter, exhaust systems can be equipped with structures for removing other undesirable emissions such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). Catalytic converters are typically used to remove CO and HC. NOx can be removed by structures such as lean NOx catalysts, selective catalytic reduction (SCR) catalysts and lean NOx traps. Lean NOx catalysts are catalysts capable of converting NOx to nitrogen and oxygen in an oxygen rich environment with the assistance of low levels of hydrocarbons. For diesel engines, hydrocarbon emissions are too low to provide adequate NOx conversion, thus hydrocarbons are required to be injected into the exhaust stream upstream of the lean NOx catalysts. SCR' s are also capable of converting NOx to nitrogen and oxygen. However, in contrast to using HCs for conversion, SCR's use reductants such as urea or ammonia that are injected into the exhaust stream upstream of the SCR's. NOx traps use a material such as barium oxide to absorb NOx during lean burn operating conditions. During fuel rich operations, the NOx is desorbed and converted to nitrogen and oxygen by catalysts (e.g., precious metals) within the traps.

In general it is common practice to use exhaust conditions as inputs for exhaust fueling control. Exhaust gas temperature is commonly used to determine fueling rates, and may even be used to determine the start of a regeneration event. During steady state operation, the average oxidation catalyst temperature will be similar to the exhaust gas temperature and reasonable control of fueling rate may be achieved based solely on exhaust gas temperature. During transient operation, however, the average oxidation catalyst temperature and exhaust gas temperature vary significantly. Thus, control based on exhaust gas temperature will not be sufficient for the majority of exhaust emissions control applications.

Summary

One inventive aspect of the present disclosure relates to a method for controlling operation of an engine exhaust treatment system using the mean temperature of a diesel oxidation catalyst as a primary control parameter.

Brief Description of the Drawings

Figure 1 schematically illustrates an exhaust treatment system adapted to be controlled by methods in accordance with the principles of the present disclosure;

Figure 2 schematically shows a second exhaust treatment system adapted to be controlled by methods in accordance with the principles of the present disclosure;

Figure 3 schematically shows a third exhaust treatment system adapted to be controlled by methods in accordance with the principles of the present disclosure; and

Figure 4 schematically shows a fourth exhaust treatment system adapted to be controlled by methods in accordance with the principles of the present disclosure.

Detailed Description

One inventive aspect of the present disclosure relates to a technique for varying the rate at which fuel is dispensed/delivered into the transient flow of an exhaust system. The technique involves using a mathematical model representative of the exhaust system to determine fuel delivery rates suitable for achieving desired results taking into consideration the operating conditions of the system on a real time basis. By using a mathematical model, the fuel delivery rate can be quickly modified in response to variations in the operating conditions of the exhaust system without requiring a large amount of testing as might be required by a strictly empirical modeling approach. To enhance the speed and flexibility of the mathematical model, the model preferably relies upon a relatively small number of inputs (e.g., provided by sensors or other inputs) determined to have the most substantial effect on the operating conditions of the exhaust system. The effects of other variables can be incorporated into the model. Thus, the system can effectively operate with a fewer number of input sources.

Another inventive aspect of the present disclosure relates to a system for regenerating a diesel particulate filter. The system includes a fuel supply device positioned upstream from the diesel particulate filter. A controller controls the rate fuel is dispensed by the fuel supply device. The controller interfaces with input sources that provide data representative of characteristics of the exhaust gas being conveyed through the exhaust system. Based on the characteristics of the exhaust gas, the controller causes the fuel supply device to dispense fuel into the exhaust stream at a rate sufficient to cause the controlled regeneration of diesel particulate filter. In one embodiment, the fuel supply device is positioned upstream from a catalytic converter (diesel oxidation catalyst, DOC) that is positioned upstream from the diesel particulate filter. The diesel particulate filter may or may not include a catalyst. The desired fuel injection rate is preferably selected such that when the fuel combusts within the catalytic converter, the temperature of the exhaust gas exiting the catalytic converter and traveling to the diesel particulate filter is in the range of 500 to 700⁰C. In a more preferred embodiment, the temperature of the exhaust gas exiting the catalytic converter is in the range of 550 to 65O⁰C. In a most preferred embodiment, the gas exiting the catalytic converter is about 600⁰C.

In a preferred embodiment, the above-described controller uses a mathematical model to determine the appropriate fuel injection rate for achieving a temperature at the diesel particulate filter that is suitable for causing regeneration of the diesel particulate filter without damaging the diesel particulate filter. For example, the controller can use a model based on a transient energy balance equation for a control volume that includes the DOC. By accessing a relatively small amount of data from the exhaust system (e.g., exhaust temperature entering the control volume, exhaust temperature exiting the control volume, and mass flow through the control volume), the controller can use the model to determine the appropriate rate for fuel to be injected into the system to achieve the desired regeneration temperature. Preferably, the model can take into account the effects of fuel preparation (e.g., fuel vaporization efficiency), DOC performance (e.g., DOC hydrocarbon conversion efficiency) and DOC thermal responses (e.g., DOC energy transfer rates).

Another aspect of the disclosure relates to the use of the value TD_OC (i.e., the mean temperature of the oxidation catalyst) as a control parameter. The T_DOC value is preferably calculated using a transient energy balance equation, but other means may also be used. T_DOC, along with exhaust flow speed, allows for the determination of oxidation catalyst fuel conversion efficiency. Conversion efficiency, in turn, may be used as a parameter for precise control of fuel delivery rate to the oxidation catalyst. T_DOC is also a key parameter for determining if the state of the system (exhaust gas plus oxidation catalyst) is appropriate for the initiation of a particulate filter regeneration event. Proper use of T_DOC as a control variable allows the fuel penalty associated with exhaust fuel injection to be minimized while maintaining tight control over oxidation catalyst temperatures and the regeneration process. In alternative embodiments, the fuel injector can inject fuel directly into the diesel particulate filter without having a preheating process provided by combustion within an upstream catalytic converter. In such embodiments, the fuel ignites with the catalyst on the diesel particulate filter thereby causing oxidation of the particulate matter on the filter.

I. Example System

Figure 1 illustrates an exhaust system 20 having features that are examples of inventive aspects in accordance with the principles of the present disclosure. The system includes an engine 22 (e.g., a diesel engine), a fuel tank 24 for supplying fuel (e.g., diesel fuel) to the engine 22, and an exhaust conduit 26 for conveying exhaust gas away from the engine 22. The system 20 also includes a catalytic converter 28 (i.e., DOC) and a diesel particulate filter 30 positioned along the conduit. The catalytic converter 28 is preferably positioned upstream from the diesel particulate filter 30. The system further includes a fuel supply device 32 and a controller 34 for controlling the rate in which fuel is dispensed (e.g., injected or sprayed) into the exhaust stream by the fuel supply device 32. In one embodiment, the fuel supply device may include a fuel injector and one or more spray nozzles. The fuel supply device 32 preferably inputs fuel at a location between the catalytic converter 28 and the engine 22. Preferably, the fuel supply device 32 inputs fuel to the conduit 26 at a location immediately upstream from the catalytic converter 28. In one embodiment, fuel is supplied to the exhaust stream at a location within 36 inches of the catalytic converter 28. In another embodiment, the fuel is supplied at a location within 12 inches of the catalytic converter.

The fuel supply device 32 is used to spray fuel from the fuel tank 24 into the exhaust stream traveling through the conduit 26 at a location upstream from the catalytic converter 28. The fuel supplied by the fuel supply device 32 combusts within the catalytic converter 28 thereby generating heat. The heat generated by combustion of fuel within the catalytic converter 28 preferably raises the temperature of the exhaust gas exiting the catalytic converter 28 to a temperature above the combustion temperature of the particulate matter accumulated on the diesel particulate filter. In this manner, by burning fuel in the catalytic converter, sufficient heat is generated to cause regeneration of the diesel particulate filter.

Preferably, the rate that I fuel is dispensed into the exhaust stream is also controlled to prevent temperatures from exceeding levels which may be detrimental to the diesel particulate filter.

It will be appreciated the catalytic converter 28 and the diesel particulate filter function to treat the exhaust gas that passes through the conduit 26. Other structures for treating the exhaust gas such as mufflers for attenuating noise, SCR catalysts, lean NOx catalytic converters and NOx traps/absorbers can also be provided along the conduit 26.

II. Controller

The controller 34 functions to control the rate that fuel is dispensed by the fuel supply device 32 a given time to cause regeneration of the diesel particulate filter 30. The controller 34 interfaces with a number of sensing devices or other data inputs that provide data representative of the exhaust gas traveling through the conduit 26. For example, the controller 34 interfaces with a first temperature probe 36 positioned upstream of the catalytic converter 28 and a second temperature probe 38 positioned between the catalytic converter 28 and the diesel particulate filter 30. The controller 34 also interfaces with first, second, third and fourth pressure sensors 40-43. The second pressure sensor 41 is located at a venturi 44 positioned within the conduit 26 at a location upstream from the catalytic converter 28. The first pressure sensor 40 as well as the first temperature probe 36 are located upstream of the venturi 44. The third pressure sensor 42 is located between the catalytic converter 28 and the diesel particulate filter 30. The fourth pressure sensor 43 is located downstream of the diesel particulate filter 30.

The venturi 44 has a known cross-sectional area A_e, and the pressure readings from pressure sensors 40, 41 allow a mass flow rate through the conduit 26 to be determined. It will be appreciated that other types of mass flow sensors other than the venturi and pressure sensors could also be used. Mass flow can also be determined by other means such as accessing data from an engine controller that is indicative of the operating condition of the engine, measuring engine operating characteristics or through the use of a mass flow sensor positioned at the engine intake. Example engine operating characteristics include engine speed (e.g., rotations-per-minute), manifold absolute pressure and manifold temperature. Other engine characteristics include the displacement amount per engine rotation as well as the volumetric efficiency of the engine. Based on the operating conditions of the engine, the mass flow can be calculated or estimated.

As described below, the controller 34 can use information provided from the pressure sensors 40-43, temperature probes 36, 38 or other inputs to determine the rate that fuel should be dispensed into the exhaust gas stream to regenerate the diesel particulate filter 30 in a controlled manner. In one embodiment, the controller accesses data from the pressure sensors 40-42, and also accesses temperature data from the probes 36, 38. The venturi 44 allows mass flow through the system to be determined. The accessed data is preferably input by the controller into a mathematical model of the actual exhaust system. By using the mathematical model, the controller determines the appropriate rate for dispensing fuel to raise the exhaust gas temperature reaching the diesel particulate filter to a level conducive for regeneration without exceeding a temperature that would be detrimental to the diesel particulate filter.

To promote a controlled and efficient regeneration of the diesel particulate filter 30, it is desirable for the temperature of the exhaust gas exiting the catalytic converter 28 to have a target temperature in the range of 500 to 700⁰C, as indicated above. Thus, the rate that fuel is dispensed upstream of the catalytic converter 28 is preferably selected so that upon combustion of the fuel within the catalytic converter 28, the exhaust gas exiting the catalytic converter is within the target temperature range. ' The controller 34 can also be used to determine when the diesel particulate filter is in need of regeneration. Any number of strategies can be used for determining when the diesel particulate filter should be regenerated. For example, the controller can regenerate the filter 30 when the pressure sensors indicate that the back pressure exceeds a predetermined level. The controller 34 can also regenerate the filter 30 at predetermined time intervals. The controller can also be programmed to delay regeneration if conditions of the exhaust system are not suitable for regeneration (e.g., if the exhaust flow rate or exhaust temperature is not suitable for controlled regeneration). For such an embodiment, the controller can be programmed to monitor the operating conditions of the exhaust system and to initiate regeneration only when predetermined conditions suitable for regeneration have been satisfied.

III. Diesel Particulate Filter

The diesel particulate filter 30 can have a variety of known configurations. An exemplary configuration includes a monolith ceramic substrate having a "honey-comb" configuration of plugged passages as described in United States patent No. 4,851,015 that is hereby incorporated by reference in its entirety. Wire mesh configurations can also be used. In certain embodiments, the substrate can include a catalyst. Exemplary catalysts include precious metals such as platinum, palladium and rhodium, and other types of components such as base metals or zeolites.

The diesel particulate filter 30 preferably has a particulate mass reduction efficiency greater than 75%. More preferably, the diesel particulate filter has a particulate mass reduction efficiency greater than 85%. Most preferably, the diesel particulate filter 30 has a particulate mass reduction efficiency equal to or greater than 90%. For purposes of this specification, the particulate mass reduction efficiency is determined by subtracting the particulate mass that enters the filter from the particulate mass that exits the filter, and by dividing the difference by the particulate mass that enters the filter.

IV. Catalytic Converter The catalytic converter 28 can have a variety of known configurations. Exemplary configurations include substrates defining channels that extend completely therethrough. Exemplary catalytic converter configurations having both corrugated metal and ceramic substrates are described in United States patent No. 5,355,973, that is hereby incorporated by reference in its entirety. The substrates preferably include a catalyst. For example, the substrate can be made of a catalyst, impregnated with a catalyst or coated with a catalyst. Exemplary catalysts include precious metals such as platinum, palladium and rhodium, and other types of components such as base metals or zeolites.

In one non-limiting embodiment, the catalytic converter 28 can have a cell density of at least 200 cells per square inch, or in the range of 200-400 cells per square inch. A preferred catalyst for the catalytic converter is platinum with a loading level greater than 30 grams/cubic foot of substrate. In other embodiments the precious metal loading level is in the range of 30-100 grams/cubic foot of substrate. In certain embodiments, the catalytic converter can be sized such that in use, the catalytic converter has a space velocity (volumetric flow rate through the DOC/ volume of DOC) less than 150,000/hour or in the range of 50,000- 150,000/hour.

V. Determination of Fuel Dispensing Rate In accordance with an inventive aspect of the present disclosure, a control equation for determining the rate for fuel to be dispensed into the system by the fuel supply device 32 can be derived using a transient energy balance equation for a given control volume. In the present disclosure, a control volume CV is selected that includes an upstream end 50 positioned upstream from the venturi 44, and a downstream end 51 positioned between the catalytic converter 38 and the diesel particulate filter 30.

The transient energy balance equation is applied to the control volume as follows: (A) (B) (C) (D) (E)

where

A is the time rate of change of energy within the control volume CV per unit volume;

B is the net energy flow per unit volume carried by the mass flow leaving the control volume CV;

C is the net energy flow per unit volume carried by conduction entering through side walls 53 of the control volume CV;

D is the heat release rate per unit volume of fuel injected by the fuel supply 32 and combusted at the catalytic converter 38; and

E is the heat energy transfer rate per unit volume between the catalytic core of the catalytic converter 28 and the mass flow.

In solving the equation, it will be appreciated that:

Sf = η_vapη_c h_tm _f Heat release rate of fuel injected (2)

S DOC ⁼ ~h-A_D0C (T_gas — T_DOC ) Heat transfer rate between gas and DOC (3)

S DOC = —Γ [PDOC^VDOC^C _PDOC ^TDOC ) Rate of energy change in DOC (4) Then, combining (1), (2) and (3), ignoring term C in (1)' and integrating over the control volume:

since

we arrive at the following expressions when solving for the required fuel mass flow

^~ T_D0C )\

(7)

Further, equating (3) and (4):

^mDOC^Cp_DOC V DOC_n ⁷OOC_n-, ) _ _{u λ} (_T _ T \

_A , ~ ^HΛDOC V gas ^l DOC_n., ) (8)

we obtain the mean DOC temperature

hA_D0CAt

¹ DOC_n V gas ^DOC_n ., J⁺ ^DOC_n._t (9) m_nnrc_n

where

Symbols:

' For this application, the value C is presumed to be relatively small and therefore can be ignored. For other applications, it may be desirable to include the C value. A_DOC = DOC flow exposed surface area

[m²] c_p = specific heat

[J/kg-K] h heat transfer coefficient between exhaust gas and DOC

[W/m² K] Ji₁ = fuel lower heating value

[J/kg] k = thermal conductivity [W/ms] m = mass

[kg] rh = mass flow rate of exhaust gas unless indicated otherwise

[kg/s] ih_j- = required mass flow rate of fuel

[kg/s] P = pressure

[Pa]

Q = volumetric flow rate [πrVs]

R = ideal gas constant for exhaust gas

[J/kg-K] S_f = heat release rate of fuel injected

[W] S DOC ^{= neat} transfer ^rate between gas and DOC

[W] T = exhaust gas temperature

[K] T = mean gas temperature in DOC [K]

T_D0C = mean temperature of DOC substrate

[K] t = time [s] u = velocity [m/s] V = volume

[m³]

X = coordinate in the axial direction

[m]

X₁ = stands for x, y, z for i = 1, 2, 3

[m]

At = computational time step [S]

P = density

[kg/m³]

I vap = fuel vaporization efficiency

⁷Ic = DOC fuel conversion efficiency

Suffixes:

CV = control volume

DOC = DOC des = desired (target) value f = fuel n = current time n-1 = previous time

1 = upstream location of control volume

2 = downstream location of control volume

By using formula (9), the current temperature of the DOC (Toocn) can be calculated. By inserting the T_DOCΠ value into the formula (7) along with other sensed data and known constants, the controller 34 can estimate the fuel dispensing rate required to be supplied into the exhaust stream via the fuel supply device 32 to cause the temperature of the exhaust gas exiting the catalytic converter 28 to equal the target temperature T₂ ^ or to be within a target temperature range. The formula (7) can also be used to construct data matrixes that are used by the controller to determine the fuel injection rate required to achieve a given target temperature when the exhaust gas has a given set of characteristics as determined by sensors or other inputs.

Attached hereto at Appendix 1 is a chart showing calculated fuel mass flow rate values for example operating conditions. Initially, the fuel flow rate is selected to ramp-up the exhaust temperature exiting the control volume from the beginning temperature to the target regeneration temperature. Once the target temperature is reached, the fuel mass flow is selected to maintain the target regeneration temperature. As shown at Appendix 1, the only sensed/variable data used by the controller includes the temperature at the control volume inlet and outlet (provided by probes 36, 38), the pressure at the control volume inlet and outlet (provided by pressure sensors 40 and 42) and the exhaust mass flow rate (provided by pressure reading at the venturi 44 or other means). In addition to the variable data, the controller also uses a number of constant values that are preferably stored in memory. For example, system constant values specific to the control volume (e.g., the volume, mass and surface area of the DOC) can be stored in memory. Other data stored in memory includes application inputs, target temperatures, gas constants for the exhaust gas, and mapped data saved in look-up tables relating to fuel/operating characteristics at given temperatures and pressures such as fuel vaporization efficiency, DOC conversion efficiency and DOC heat transfer coefficient.

VI. TDOC

The fuel conversion efficiency of the DOC depends largely on the temperature of its substrate surface. Specifically, there is a "light-off DOC surface temperature, below which fuel conversion is very poor and hence fuel injection should be avoided. Fuel conversion efficiency above the light-off is also a function of the DOC surface temperature. Therefore the DOC surface temperature is one of the key control variables. Our model-based control system employs a calculated DOC surface temperature, T_DOC> as a key control parameter. T_DOC is calculated by keeping track of the moment-to-moment "internal convective heat transfer" within DOC channels. It takes into account the instantaneous gas flow temperature, the DOC channel surface area, and the substrate mass and heat capacity. The instantaneous convective heat transfer coefficient is assessed based on the flow environment. A model that was validated for the DOC substrate exposed in diesel exhaust was used to obtain the convective heat transfer coefficient. The use of T_DOC provides a practical and effective way to initiate and control the fuel injection. The concept is universal and easily implemented on any engine exhaust system, DOC material or catalyst formulation.

In the preferred embodiment, a control volume is selected such that a fuel supply point, a portion of the exhaust gas flow upstream of the oxidation catalyst, and the oxidation catalyst are within the control volume. The transient energy balance equation is then applied to this control volume, resulting in the equation,

^■

where h is the heat transfer coefficient between the exhaust gas and oxidation catalyst, A_DOC is the oxidation catalyst exposed surface area, At is the computation time step, rnooc is the mass of the oxidation catalyst, c_pDOc is the specific heat of the oxidation catalyst, and T_gas is the average exhaust gas temperature into the oxidation catalyst. In the preferred embodiment, T_DOC is used in combination with the transient energy balance equation to determine the amount of fuel needed to achieve a desired exhaust gas temperature exiting the oxidation catalyst. This theoretical fueling rate is then modified by the oxidation catalyst conversion efficiency, which is a function of both T_DOC and the exhaust gas flow speed through the oxidation catalyst. In general, it is expected that high values of T_Doc and low exhaust gas flow speeds correspond to high conversion efficiencies, while low values of T_DOC and high exhaust gas flow speeds correspond to lower conversion efficiencies. In one embodiment a conversion efficiency table is empirically developed based on the multiplicative fueling rate adjustment factor that is applied to the theoretical fueling rate in order to achieve a desired gas temperature exiting the oxidation catalyst.

In the present invention, the time-step nature of the calculation allows T_DOC to be determined very quickly. Thus, even during the most transient operation the invention described herein has the advantage of providing an equation that can be used to calculate the average oxidation catalyst temperature during all modes of operation. This allows for increased efficiency, decreased fuel consumption, and precise control over the timing of a regeneration event. T_DOC is also an important parameter for the initiation of a regeneration event. In the preferred embodiment T_DOc is greater than 240-280 degrees centigrade in order to initiate a regeneration event. In another embodiment, T_DOC is greater than 220-250 degrees centigrade before regeneration is initiated. Temperature may vary depending upon the application and catalyst. If regeneration is initiated while T_DOC is lower than the temperatures stated above, the oxidation catalyst conversion efficiency will be poor, resulting in an increased fuel consumption impact, and hydrocarbon slip through the oxidation catalyst. The hydrocarbon slip may appear as white smoke from a vehicle's tailpipe. Thus, calculating T_DOC is an integral step of the regeneration control in the preferred embodiment.

In another embodiment T_DOC is not used by itself to determine whether or not to initiate a regeneration event. In this embodiment T_Doc is instead compared to the average exhaust gas temperature upstream of the oxidation catalyst. Only when both the exhaust gas temperature and T_DOC are greater than 240-280 degrees centigrade is a regeneration event enabled.

VII. TcAT

It will be appreciated that many of the aspects described above are applicable to any type of exhaust system having single or multiple substrates which are catalyzed to promote a specific reaction with a reactant introduced into the exhaust stream up stream from the substrate or substrates. In such cases, the reaction efficiency at the catalyzed substrate/substrates often depends largely on the substrate temperature, making this temperature a key control parameter. For instance, in the case of urea based selective catalytic reduction (SCR), urea is introduced to a catalyzed substrate for the reduction OfNO_x to N₂ and O₂. In this case, the substrate temperature may be calculated or measured to assist in predicting the amount of reductant (urea) required to maintain efficient NO_x reduction, while minimizing pass-through of unconverted reductant. The method described previously for the calculation of the DOC substrate temperature can be modified for use with SCR or any other system where reactant is introduced to a catalyzed substrate. The generalized mean temperature equation for a catalyzed substrate is:

where the subscript DOC has been replace by CAT, describing general catalyst substrate parameters.

The above equation for mean catalyzed substrate temperature may be used for any system in which a reactant is introduced into exhaust passing through a catalyzed substrate where the efficiency of the ensuing reaction at the substrate is a function of substrate temperature. For example, the equation may be used in "active" DPF systems of the type shown at Figure 1, in which fuel is reacted over an oxidation catalyst to create heat. The equation can also be used for NO_x reduction systems where a reductant is introduced into an exhaust stream. For example, the equation can be used to assist in accurately determining the amount of urea/ammonia to be injected into the exhaust stream of a urea/ammonia based SCR system. Figure 2 shows an example SCR system 200 having a substrate 202 catalyzed to promote a reaction between NO_x and urea. A urea doser (e.g., injector) 204 is positioned upstream of the substrate 202. The mean temperature of the substrate 202 can be used as a parameter for controlling the rate at which urea is introduced into the system. The system also includes temperature sensors 206-208, a pressure sensor 210, a DOC substrate 212, a DPF substrate 214, and a hydrocarbon doser 216 (e.g., an injector). The temperature sensors 206-208 provide data for allowing the mean temperatures of the substrates 202, 212 to be calculated (e.g., by a controller that interfaces with the sensors). The mean temperature of the DOC substrate 212 can be used as a parameter for determining the rate/amount of fuel introduced into the system to promote the efficient regeneration of the DPF substrate 212. In other embodiments, another DOC substrate can be positioned upstream from the substrate 202 for promoting the conversion of NO to NO₂. Increased NO₂ will assist in the low temperature reduction of NO_x at the substrate 202.

Figure 3 shows an example NO_x trap system 300 having a NO_x trap 302. A first hydrocarbon injector 304 is positioned upstream of the NO_x trap 302. The mean temperature of the NO_x trap substrate can be used as a parameter for controlling the rate at which hydrocarbon is introduced into the system by the injector 304. The system also includes temperature sensors 306-309, a pressure sensor 312, a first DOC substrate 314, a second DOC substrate 316, a DPF substrate 318 and a second hydrocarbon injector 320. Similar to previous embodiments, the sensors can interface with a controller that controls the injection rates of the injectors 304, 320. The temperature sensors provide information that facilitates calculating the mean temperatures of the first DOC substrate 314 and the NO_x trap 302. The mean temperature of the NO_x trap 302 can be used as a parameter for determining the dosing rate of fuel introduced by the first injector 304, and the mean temperature of the first DOC 314 can be used as a parameter for determining the dosing rate of fuel introduced into the system by the second injector 320.

It will be appreciated that two of the NO_x trap systems of Figure 3 can be placed in parallel and used to treat exhaust flow from a diesel engine. A diverting valve arrangement can be used to alternate flow between the first and second parallel lines. While exhaust flow is directed to one of the lines, the other line can be dosed with hydrocarbon to reduce the stored NO_x that has been adsorbed on the NO_x trap. Figure 4 shows an example of a lean NO_x catalyst system 400 having a lean NO_x substrate 402. A first hydrocarbon injector 404 is positioned upstream of the substrate 402. The mean temperature of the substrate 402 can be used as a parameter for controlling the rate at which hydrocarbon is introduced into the system by the first injector 404. The system includes temperature sensors 406-408, a pressure sensor 410, a DOC substrate 412, a DPF substrate 414, and a second hydrocarbon injector 416. The sensors can interface with a controller that controls the injection rates of the system. The mean temperatures of the substrates 402 and 412 can be used as parameters for determining the rate/amount of fuel introduced into the system upstream of each of the substrates 402, 412. For example, the mean temperature of the substrate 202 can be used to determine the rate of hydrocarbon injected into the exhaust stream by the first injector 404 for efficiently reducing NO_x at the substrate 402 to acceptable levels. The temperature of the DOC substrate 412 is used as a parameter for determining the amount of hydrocarbon injected by the second injector 416 so as to increase the temperature of the DOC to a temperature suitable for causing efficient and controlled regeneration of the DPF substrate 414.

The mean temperature of the catalyzed substrate can also be used as a parameter for determining when conditions are not suitable for injecting a reactant. For example, in an example urea based SCR system, if the mean temperature of the catalyzed SCR substrate is below 180 degrees C, the system may prevent reactant (e.g., urea) from being injected into the system. By way of another example, for a lean NO_x catalyst system or a NO_x trap, if the mean temperature of the catalyzed substrate is below 200 degrees C, the system may prevent hydrocarbon fuel from being injected into the system. Thus, in a system with any type of catalyzed substrate, the mean temperature of the substrate can be used to set a minimum threshold at which the injection of reactant is permissible.

It will be appreciated that for NO_x removal systems (e.g., lean NO_x catalysts (i.e., hydrocarbon based SCR systems), NO_x traps and urea/ammonia SCR systems), the amount of reductant introduced into the exhaust system is dependent upon the amount OfNO_x in the exhaust, and the efficiency of the reduction reaction at the catalyzed substrate. The amount OfNO_x in the exhaust can be determined through the use OfNO_x sensors or based on engine operating parameters (e.g., lookup tables or maps). The efficiency of the reduction reaction is dependent upon the temperature of the substrate. Thus, once the amount of NO_x in the exhaust and the temperature of the substrate have been determined, the amount of reductant needed to be introduced into the system to remove the desired amount OfNO_x from the exhaust stream or to desorb accumulated NO_x from a NO_x trap can readily be calculated from known equations or determined by referring to empirically generated dosing maps or look-up tables.

APPENDIX 1

Fuel Injection Control Example

Variable Symbol Unit Regen Time = l Time = 2 Time = 3 Time = 4 Time = 5

Start Time = O

System constants

Volume, V m^A3 2 68E-02 2 68E-02 2 68E-02 2 68E-02 2 68E-02 2 68E-02 control volume

Mass, DOC M DOC kg 2 7₅0 2 750 2 750 2 750 2 750 2 750

Surface area, A_D0C m^Λ2 20 0 20 0 20 0 20 0 20 0 200

DOC flow exposed

Application inputs

Lower h_l MJ/k 43 2 43 2 43 2 43 2 43 2 43 2 heating g value, fuel

Time step Δt sec 1 O 1 O 1 0 1 0 1 0 1 0

Constant

Gas R J/kg- 28ό 7 286 7 286 7 286 7 286 7 286 7 constant, K exhaust gas

Sensor read Variable Symbol Unit Regen Time = 1 Time = 2 Time = 3 Time = 4 Time = 5

Start Time = O

Temperature, T_l deg C 2S_{5 0} 2_{55 0} 2S₅ O 25₅ O 255 O 300 0 CV inlet Temperature, T_2 deg C 250 0 350 0 450 0 550 0 550 0 550 0

CV exit Pressure, CV PJ Pa 70000 7₅000 80000 85000 85000 120000 inlet

Pressure, CV P_2 Pa 60000 65000 70000 75000 75000 100000 exit

Mass flow m dot,l kg/s 0 160 0 160 0 160 0 160 0 160 0 200 rate, before injector

Target

Temperature, T_2des deg C 350 0 450 0 550 0 550 0 550 0 550 0 CV exit

Mapped data

Evaporation η_vap 0 90 0 90 0 90 0 90 0 90 0 95 efficiency, fuel

Conversion 0 50 0 60 0 70 075 0 75 0 85 efficiency,

DOC

Heat transfer h_DOC W/m 100 100 10 0 100 10 0 100 coefficient, ^Λ2-K gas-DOC

Calculated data

Temperature, T_cv,n degC 2525 3025 3525 4025 4025 4250 CV average Pressure, CV P_cv,n Pa 1078000 1083000 1088000 1093000 1093000 1123000 average Density, CV p_cv,n+l kg/m 715E-01 656E-01 607E-01 564E-01 564E-Ol 561E-Ol average ^Λ3 Specific C_p,exh, J/kg- 10353 10457 10569 10687 10687 10741 heat, CV n K average Temperature, T_cv,n+1 deg C 302 5 352 5 402 5 402 5 402 5 4250 CV average @ n+l

Pressure, CV P_cv,n+1 Pa 1078000 1083000 1088000 1093000 1093000 1123000 average @ n+1

Density, CV p_cv,n+l kg/m 749E-01 495E-01 358E-01 294E-Ol 294E-01 293E-Ol average @ ^Λ3 n+1

Specific C_p,exh, J/kg- 10457 10569 10687 10687 10687 10741 heat, CV n+1 K average @ n+1

Unsteady 16523 -18208 -38236 -52262 -52262 -53969 term

Mass flow m_dot,2 kg/s 0 160 0 161 0 162 0 162 0 162 0203 rate, CV exit Specific C- J/kg- 1035 8 1035 8 1035 8 1035 8 1035 8 10452 heat, CV _p,exh,l K inlet Specific C_p,exh, J/kg- 1056 3 1080 2 1 104 2 1 104 2 1 104 2 11042 heat, CV exit 2 K Convective 17788 8 38148 9 59497 8 60108 3 60161 5 643147 term

Temperature, T_gas deg C 252 5 302 5 352 5 402 5 402 5 4250 means gas thru DOC Temperature, T DOC degC 2525 2562 2634 2737 2832 2936 DOC substrate Variable Symbol Unit Regen Time = l Time = 2 Time = 3 Time = 4 Time = 5

Start Time = O

Specific C_p,DO I/kg- 974 3 976 7 981 3 987 7 993 3 999 2 heat, DOC C K substrate Heat sink O O 92₅3 ₅ 17819 9 25758 1 23861 5 26285 0 term

Mass flow m_dot,f kg/s 1 OOE-03 1 95E-03 2 70E-03 2 77E-03 2 7OE-O3 2 44E-03 rate, fuel

Claims

What is claimed is:

1. A method for regenerating a DPF located downstream from a DOC device, the method comprising: determining the mean temperature of the DOC; and regenerating the DPF only when the mean temperature of the DOC exceeds a given temperature value.

2. A method for regenerating a DPF located downstream from a DOC device, the method comprising: determining the mean temperature of the DOC; and using the mean temperature of the DOC to determine when regeneration of the DPF is appropriate.

3. A method for introducing a reactant into the exhaust stream of a vehicle exhaust system, the vehicle exhaust system including a substrate, the method comprising: determining the mean temperature of the substrate; and introducing the reactant into the exhaust stream only when the mean temperature of the substrate exceeds a given temperature value.

4. The method of claim 3, wherein the substrate is catalyzed to promote a reaction of the reactant at the substrate.

5. The method of claim 4, wherein the reactant includes a hydrocarbon fuel, and wherein the hydrocarbon fuel is injected into the exhaust stream at a location upstream from the substrate.

6. The method of claim 5, wherein the substrate comprises a diesel oxidation catalyst.

7. The method of claim 5, wherein the substrate comprises a lean NOx catalyst.

8. The method of claim 5, wherein the substrate comprises a NOx trap.

9. The method of claim 3, wherein the substrate comprises an SCR substrate, and wherein the reactant comprises a reductant.

10. The method of claim 9, wherein the reductant includes ammonia or urea.

11. A method for introducing ammonia or urea into the exhaust stream of a vehicle exhaust system, the vehicle exhaust system including an SCR substrate, the method comprising: determining the mean temperature of the substrate; and using the mean temperature of the SCR substrate as a parameter for controlling the introduction of ammonia or urea into the exhaust stream.