Electronically heated selective catalytic reduction (SCR) device

An exhaust gas treatment system is provided having an internal combustion engine, an exhaust gas conduit, a passive selective catalyst reduction (SCR) device, a heated SCR device, and a control module. The exhaust gas conduit is in fluid communication with and is configured to receive an exhaust gas from the internal combustion engine. The passive SCR device is in fluid communication with the exhaust gas conduit and is configured to receive the exhaust gas. The passive SCR includes a passive SCR temperature profile. The heated SCR device is in fluid communication with the exhaust gas conduit and is configured to receive the exhaust gas. The heated SCR device is located upstream of the passive SCR. The heated SCR is selectively activated to produce heat. The control module is in communication with the heated SCR and the engine and includes a control logic for determining the passive SCR temperature profile.

FIELD OF THE INVENTION

Exemplary embodiments of the invention relate to exhaust gas treatment systems for internal combustion engines and, more particularly, to an exhaust gas treatment system having a selectively heated selective catalyst reduction (“SCR”) device.

BACKGROUND

The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.

One type of exhaust treatment technology for reducing NOxemissions is a selective catalyst reduction (“SCR”) device. The SCR device usually includes a substrate, where a SCR catalyst compound is applied to the substrate. A reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be a urea solution that decomposes to ammonia (NH3) in the hot exhaust gases, and is subsequently absorbed by the SCR device. The NH3then reduces the NOxto nitrogen in the presence of the SCR catalyst.

Some types of engines tend to produce cooler exhaust temperatures, especially during engine startup and during moderate operating conditions. For example, highly efficient engines tend to have cooler exhaust temperatures. Cooler exhaust temperatures also tend to occur during low load driving as well. However, cooler exhaust temperatures tend to reduce the effectiveness of the SCR device. This is because a SCR device needs to reach a minimum operating or light-off temperature to convert the urea to ammonia, which is typically about 200° C., to effectively filter NOx. In low temperature environments, an SCR device may not efficiently clean exhaust until several minutes after an engine has been started.

One approach to increasing the effectiveness of the SCR device involves having the engine operate at a higher temperature, which in turn also raises the temperature of the exhaust gases. However, this approach involves the engine operating at a lower level of efficiency to create the hotter exhaust gas, which results in greater fuel consumption. Accordingly, it is desirable to provide an efficient approach to increasing the temperature of the exhaust gases upstream of the SCR device.

SUMMARY OF THE INVENTION

In one exemplary embodiment of the invention an exhaust gas treatment system is provided having an internal combustion engine, an exhaust gas conduit, a passive selective catalyst reduction (“SCR”) device, a heated SCR device, and a control module. The exhaust gas conduit is in fluid communication with and is configured to receive an exhaust gas from the internal combustion engine. The passive SCR device is in fluid communication with the exhaust gas conduit and is configured to receive the exhaust gas. The passive SCR includes a passive SCR temperature profile. The heated SCR device is in fluid communication with the exhaust gas conduit and is configured to receive the exhaust gas. The heated SCR device is located upstream of the passive SCR. The heated SCR is selectively activated to produce heat. The control module is in communication with the heated SCR and the engine and includes a control logic for determining the passive SCR temperature profile. The control module includes a control logic for determining if the passive SCR temperature profile is below a threshold value. The control module also includes a control logic for activating the heated SCR, where the heated SCR is activated if the passive SCR temperature is below the threshold value.

DESCRIPTION OF THE EMBODIMENTS

Referring now toFIG. 1, an exemplary embodiment is directed to an exhaust gas treatment system10, for the reduction of regulated exhaust gas constituents of an internal combustion (IC) engine12. The exhaust gas treatment system described herein can be implemented in various engine systems that may include, but are not limited to, diesel engine systems, gasoline direct injection systems, and homogeneous charge compression ignition engine systems.

The exhaust gas treatment system10generally includes one or more exhaust gas conduits14, and one or more exhaust treatment devices. In the embodiment as illustrated, the exhaust gas treatment system devices include an oxidation catalyst device (OC)18, a passive selective catalytic reduction device (“SCR”)20, a heated SCR device22, and a particulate filter device (“PF”)24. As can be appreciated, the exhaust gas treatment system of the present disclosure may include various combinations of one or more of the exhaust treatment devices shown inFIG. 1, and/or other exhaust treatment devices (not shown), and is not limited to the present example.

InFIG. 1, the exhaust gas conduit14, which may comprise several segments, transports exhaust gas15from the IC engine12to the various exhaust treatment devices of the exhaust gas treatment system10. The OC18may include, for example, a flow-through metal or ceramic monolith substrate that is packaged in a stainless steel shell or canister having an inlet and an outlet in fluid communication with exhaust gas conduit14. The substrate can include an oxidation catalyst compound disposed thereon. The oxidation catalyst compound may be applied as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. The OC18is useful in treating unburned gaseous and non-volatile HC and CO, which are oxidized to form carbon dioxide and water.

The passive SCR20and the heated SCR22may be both be disposed downstream of the OC18. In a manner similar to the OC18, the passive SCR20and the heated SCR may each include, for example, a flow-through ceramic or metal monolith substrate that is packaged in a stainless steel shell or canister having an inlet and an outlet in fluid communication with exhaust gas conduit14. The substrate can include an SCR catalyst composition applied thereto. The SCR catalyst composition can contain a zeolite and one or more base metal components such as iron (Fe), cobalt (Co), copper (Cu) or vanadium (V) which can operate efficiently to convert NOxconstituents in the exhaust gas15in the presence of a reductant such as ammonia (NH3).

The heated SCR22is an SCR unit30combined with an electrical heater32, where the electrical heater32is selectively activated and heats the SCR unit30. The electrical heater32is connected to an electrical source (not shown) that provides power to the electrical heater32. In one embodiment, the electrical heater32operates at a voltage of about 12-24 volts and at a power range of about 1-3 kilowatts, however it is understood that other operating conditions may be used as well. The heated SCR device22is located upstream of the passive SCR20in the exhaust gas conduit14. The electrical heater32provides heat to the SCR unit30to heat the SCR unit30to a minimum operating or light-off temperature. The minimum operating temperature is the temperature that is needed to convert a reductant such as, for example, urea, into NH3. In one exemplary embodiment, the minimum operating temperature is about 200° C., however it is understood that the minimum operating temperature may vary depending on NOxefficiency requirements. The heated SCR device22also provides heat to the passive SCR20to heat the passive SCR20to the minimum operating temperature as well.

An ammonia (NH3) reductant25may be supplied from a reductant supply source (not shown) and may be injected into the exhaust gas conduit14at a location upstream of the passive SCR20and the heated SCR22using an injector26, or other suitable method of delivery of the reductant to the exhaust gas15. The reductant25may be in the form of a gas, a liquid, or an aqueous urea solution and may be mixed with air in the injector26to aid in the dispersion of the injected spray. A mixer or turbulator28may also be disposed within the exhaust conduit14in close proximity to the injector26to further assist in thorough mixing of the reductant25with the exhaust gas15.

The PF24may be disposed downstream of the SCR20. The PF24operates to filter the exhaust gas15of carbon and other particulates. In various embodiments, the PF24may be constructed using a ceramic wall flow monolith filter23that is may be packaged in a shell or canister constructed of, for example, stainless steel, and that has an inlet and an outlet in fluid communication with exhaust gas conduit14. The ceramic wall flow monolith filter23may have a plurality of longitudinally extending passages that are defined by longitudinally extending walls. The passages include a subset of inlet passages that have and open inlet end and a closed outlet end, and a subset of outlet passages that have a closed inlet end and an open outlet end. Exhaust gas15entering the filter23through the inlet ends of the inlet passages is forced to migrate through adjacent longitudinally extending walls to the outlet passages. It is through this wall flow mechanism that the exhaust gas15is filtered of carbon and other particulates. The filtered particulates are deposited on the longitudinally extending walls of the inlet passages and, over time, will have the effect of increasing the exhaust gas backpressure experienced by the IC engine12. It is appreciated that the ceramic wall flow monolith filter is merely exemplary in nature and that the PF22may include other filter devices such as wound or packed fiber filters, open cell foams, sintered metal fibers, etc.

A control module40is operably connected to and monitors the engine12and the exhaust gas treatment system10through a number of sensors. Specifically,FIG. 1illustrates the control module40in communication with two temperature sensors42and44, as well as an engine out NOxsensor50located in the exhaust gas conduit14. The first temperature sensor42is situated between the passive SCR20and the heated SCR22, and the second temperature sensor44is situated downstream of both the passive SCR20and the heated SCR22. The temperature sensors42,44send electrical signals to the control module40that each indicate the temperature in the exhaust gas conduit14in specific locations. The NOxsensor50is located in the exhaust gas conduit14downstream of the engine12, and upstream of the passive SCR20and the heated SCR22. The NOxsensor50sends an electrical signal to the control module40indicating the concentration of NOxin the exhaust gas NOX1, and may be expressed in part per million (PPM).

The control module40is also operably connected to the electrical heater32of the heated SCR22and the reductant supply source (not shown). The control module40includes control logic for monitoring the temperature sensors42,44and selectively activating the electrical heater32based on the temperature of the temperature sensors42,44. Specifically, the control module40includes control logic for determining the temperature profile of the passive SCR20based on the first and second temperature sensors42,44. The temperature profile of the passive SCR20represents the overall temperature of the passive SCR device20, and is based on the first temperature T1detected by the first temperature sensor42, and the second temperature T2detected by the second temperature sensor44. The second temperature T2also represents the temperature of the heated SCR20. The control module40includes control logic for averaging the first temperature T1and the second temperature T2together, where the average of the first temperature T1and the second temperature T2is the temperature profile Tavg SCRof the passive SCR20. The passive SCR20is heated through convection. Thus, a front face46of the passive SCR device20will be heated first, and a rear face48of the passive SCR device20will be heated more slowly than the front face46. It should be noted that whileFIG. 1illustrates two temperature sensors42,44, in an alternative embodiment the temperature sensors42,44may be omitted. Instead, the control module40may include control logic for calculating the temperature profile Tavg SCRof the passive SCR20based on the operating conditions of the exhaust gas system10and the engine12, as well as the mass of the passive SCR device20. Specifically, the temperature profile Tavg SCRof the passive SCR20could be calculated based on the exhaust gas inlet temperature that is measured by a temperature sensor (not shown) located in the exhaust gas conduit14upstream of the OC18, the mass flow rate or exhaust flow (“Exh_Flow”) of the engine12, and the mass of the passive SCR unit20. The Exh_Flow of the engine12is calculated by adding the intake air mass of the engine12and the fuel mass of the engine12. The intake air mass is measured using an intake air mass flow sensor (not shown) of the engine12, which measures air mass flow entering the engine12. The fuel mass flow is measured by summing the total amount of fuel injected into the engine12per second. The fuel mass flow is added to the air mass flow rate to calculate the exhaust flow Exh_Flow of the engine12.

The control module40includes control logic for selectively activating the heated SCR22based on the temperature profile Tavg SCRof the passive SCR22. Specifically, if the temperature profile Tavg SCRof the passive SCR22is below a light-off or minimum operating temperature, then the electrical heater32is activated to heat the SCR unit30, and the SCR unit30is warmed to the minimum operating temperature. Thus, because the SCR unit30is heated to the minimum operating temperature, the heated SCR device22converts the reductant into ammonia and generally effectively lowers the amount of NOxin the exhaust gas15. The heated SCR device22converts the reductant into ammonia more quickly than a conventional SCR device that does not include an electrical heater32. Including a separate heated SCR22tends to increase light-off timing of the SCR unit30after engine start up and improves NOxefficiency when compared to an exhaust treatment system that does not include a heated SCR22. Including the heated SCR device22will also improve fuel economy of the engine12, Board Diagnostics-Second Generation (OBDII) emissions margins, and reduce the regeneration frequency of the PF24when compared to an exhaust treatment system that does not include a heated SCR22.

The control module40also includes control logic for determining the amount of reductant25dosed by the injector26, as well as control logic for activating the injector26to dose reductant. The amount of reductant25dosed by the injector26is based on the first temperature T1, the second temperature T2, the temperature profile Tavg SCRof the passive SCR20, and the concentration of NOX1in the exhaust gas15. The amount of reductant dosed by the injector26is also based on the mass flow rate or exhaust flow Exh_Flow of the engine12.

The amount of reductant15dosed by the injector26is also based on values that are stored in the memory of the control module40. Specifically, the amount of reductant25dosed by the injector26is based on the amount of catalyst (“SCR_vol1”) located in the passive SCR20, as well as the amount of catalyst (“SCR_vol2”) located in the heated SCR22. Both of the values SCR_vol1and SCR_vol2are stored in the memory of the control module40. The amount of reductant dosed by the injector26is also based on a calculated value of NOxthat is located between the passive SCR20and the PF24, (denoted as “NOx′”). The control module40includes control logic for determining the calculated value NOx′. The calculated value NOx′ is calculated based on the second temperature T2, the Exh_Flow of the engine12, and the concentration of NOxin the exhaust gas15. The calculated value NOx′ represents the amount of NOxthat is removed by the heated SCR device22.

The amount of reductant dosed by the injector26is a function of the concentration of NOx1in the exhaust gas15, the calculated value NOx′, the second temperature T2(which represents the temperature of the heated SCR20), the temperature profile SCR_vol2of the passive SCR22, the exhaust flow Exh_Flow of the engine12, the amount of catalyst SCR_vol1located in the passive SCR20, and the amount of catalyst SCR SCR_vol2located in the heated SCR22. The concentration of NOx1in the exhaust gas15, the second temperature T2which represents the temperature of the heated SCR22, the exhaust flow Exh_Flow of the engine12, and the amount of catalyst SCR_vol2located in the heated SCR22are combined together and are denoted as a first function F1. The first function F1represents the operating conditions of the heated SCR device22. The first function F1is added to a second function F2, where the second function F2represents the operating conditions of the passive SCR device20. The second function F2is based on the calculated value NOx′, the temperature profile of the passive SCR22, the exhaust flow Exh_Flow of the engine12, and the amount of catalyst SCR_vol1located in the passive SCR22. In one embodiment, the amount of reductant25dosed by the injector26is calculated by adding the first function F1to the second function F2, and can be expressed by:
Amount of reductant dosed by injector 26=F1+F2

A method of operating the exhaust gas treatment system10will now be explained. Referring toFIG. 2, an exemplary process flow diagram illustrating an exemplary process of operating the exhaust gas treatment system10is generally indicated by reference number200. Process200begins at step202, where a control module40includes control logic for monitoring the temperature profile of a passive SCR20. Specifically, referring back toFIG. 1, the control module40is in communication with two temperature sensors42and44, where a first temperature sensor42is situated between the passive SCR20and a heated SCR22, and a second temperature sensor44is situated downstream of both the passive SCR20and the heated SCR22. The temperature profile of the passive SCR20is based on the first temperature T1detected by the first temperature sensor42, and the second temperature T2detected by the second temperature sensor44. The control module40includes control logic for averaging the first temperature T1and the second temperature T2together, where the average of the first temperature T1and the second temperature T2is the temperature profile SCR Tavg SCRof the passive20. Method200may then proceed to step204.

In step204, the control module40includes control logic for determining if the temperature profile SCR Tavg SCRof the passive SCR20exceeds a minimum operating temperature. The minimum operating temperature is the temperature that is needed to convert a reductant such as, for example, urea, into NH3. In one exemplary embodiment, the minimum operating temperature is about 200° C., however it is understood that the minimum operating temperature may vary depending on NOxefficiency requirements, where the temperature profile of the passive SCR20is monitored. If the passive SCR temperature Tavg SCRexceeds the minimum operating temperature (which is denoted by a “Y” inFIG. 2), method200proceeds back to step202, where the temperature profile of the passive SCR is monitored. If the temperature profile Tavg SCRof the passive SCR20does not exceed the minimum operating temperature (which is denoted by an “N” inFIG. 2), then method200proceeds to step206.

In step206, the heated SCR22is activated. Specifically, an electrical heater32of the heated SCR22is activated and heats the SCR unit30to the minimum operating temperature. Method200may then proceed to step208.

In step208, the control module includes control logic for determining the amount of reductant to be dosed by an injector26. The amount of reductant25dosed by the injector26is based on the concentration of NOx1in the exhaust gas15, the calculated value NOx′, the second temperature T2(which represents the temperature of the heated SCR20), the temperature profile Tavg SCRof the passive SCR20, the exhaust flow Exh_Flow of the engine12, the amount of catalyst SCR_vol1in the passive SCR20, and the amount of catalyst SCR_vol2. located in the heated SCR22. Method200may then proceed to step210.

In step210, the control module40includes control logic for activating the injector26, where the injector26doses reductant25. The reductant25may be injected into the exhaust gas conduit14at a location upstream of the passive SCR20. Method200may then terminate.