Abstract:
A method of heating an exhaust gas in an exhaust after treatment system includes selecting a heating mode between a plurality of heating modes based on an engine load and a status of a component of the exhaust aftertreatment system. The method further comprises heating the exhaust gas by operating the electric heater in the selected heating mode, and operating the electric heater in a passive regeneration heating mode to heat an exhaust gas to a predetermined temperature to increase NO2 generation when an engine load is less than or equal to approximately 25%.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of application Ser. No. 14/800,338, filed on Jul. 15, 2015, which is a divisional of application Ser. No. 13/773,176, filed on Feb. 21, 2013, which claims the benefit of 61/601,923, filed on Feb. 22, 2012. The disclosures of the above applications are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to exhaust aftertreatment systems for diesel engines, and more particularly to electric heating and control to provide assisted heating in the exhaust aftertreatment systems. 
       BACKGROUND 
       [0003]    The background description provided herein is for the purpose of generally presenting the context of the disclosure and may not constitute prior art. 
         [0004]    Diesel engines have been used in a variety of applications such as locomotives, marines and engine-generators. The U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), as well as other regulatory agencies around the world, impose strict limitations on the contents of emissions from diesel engines, such as particulate matter (PM), hydrocarbon (HC) and NOx. Accordingly, exhaust aftertreatment systems have been employed and generally include a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and an SCR (Selective Catalytic Reduction of NOx) to treat the exhaust gas and to control emissions to atmosphere or the outside environment. 
         [0005]    Various chemical reactions occur in the DOC and SCR to convert harmful nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbon (HC) into N 2 , CO 2  and water. The DPF is designed to remove diesel particulate matter (PM) from the exhaust gas. Normally these chemical reactions would take place at high temperatures. With the use of catalysts, the chemical reactions can occur at much lower temperatures. Sufficient energy in the form of heat, however, must still be supplied to the catalysts to expedite the chemical reactions. Therefore, performance of the exhaust aftertreatment system is highly dependent on the temperature of the exhaust gas, which carries the desired energy and heat to the catalysts. The normal temperature of the exhaust gas, however, does not always meet requirements for the desired chemical reactions. When the normal exhaust temperature is lower than the target temperature, the exhaust aftertreatment system cannot effectively treat the exhaust gas, resulting in higher emissions to the outside environment. 
         [0006]    One method of increasing the exhaust gas temperature is through injecting hydrocarbon upstream from a DOC either in the exhaust pipe or inside the cylinder during the exhaust stroke. This method increases fuel consumption and also changes composition of the exhaust gas. For example, when fuel injection is injected in the exhaust, NO 2  generation in the DOC is significantly reduced. NO 2  is an effective reagent for passive regeneration of DPF at much lower temperature range. Therefore, the reduced NO 2  generation adversely affects the passive regeneration of the DPF. 
       SUMMARY 
       [0007]    In one form, a method of heating an exhaust gas in an exhaust after treatment system includes selecting a heating mode between a plurality of heating modes based on an engine load and a status of a component of the exhaust aftertreatment system. The method further comprises heating the exhaust gas by operating the electric heater in the selected heating mode, and operating the electric heater in a passive regeneration heating mode to heat an exhaust gas to a predetermined temperature to increase NO2 generation when an engine load is less than or equal to approximately 25%. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The components in the figures are not necessarily to scale. In the drawings: 
           [0009]      FIG. 1  is a schematic view of an engine system including a heating module constructed in accordance with the teachings of the present disclosure; 
           [0010]      FIG. 2  is a schematic view of a heating module constructed in accordance with the teachings of the present disclosure; 
           [0011]      FIG. 3  is a graph showing relationship between concentration of NO 2  and catalyst temperature; 
           [0012]      FIG. 4  is a schematic view of an electric heater; 
           [0013]      FIG. 5  is a graph showing a heating strategy for operating the electric heater; 
           [0014]      FIG. 6  is a table showing the properties of the exhaust gas at different engine loads; and 
           [0015]      FIG. 7  is a schematic view of another form of an engine system including a heating module constructed in accordance with the teachings of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following description is merely exemplary in nature and is in no way intended to limit the present invention, its application, or uses. It should also be understood that steps within a method may be executed in different order without altering the principles of the invention. 
         [0017]    Referring to  FIG. 1 , an engine system  10  generally includes a diesel engine  12 , a generator  14 , a turbocharger  16 , and an exhaust aftertreatment system  18 . The exhaust aftertreatment system  18  is disposed downstream from a turbocharger  16  for treating exhaust gases from the diesel engine  12  before the exhaust gases are released to atmosphere. The exhaust aftertreatment system  18  includes a heating module  20 , a DOC  22 , DPF  24 , and an SCR  26 . The heating module  20  includes an electric heater  28  disposed upstream from the DOC  22 , and a heater control module  30  for controlling operation of the electric heater  28 . The exhaust aftertreatment system  18  includes an upstream exhaust conduit  32  that receives the electric heater  28  therein, an intermediate exhaust conduit  34  in which the DOC  22  and DPF  24  are received, and a downstream exhaust conduit  36  in which the SCR is disposed. 
         [0018]    The DOC  22  is disposed downstream from the electric heater  28  and serves as a catalyst to oxide carbon monoxide and any unburnt hydrocarbons in the exhaust gas. In addition, the DOC  22  converts harmful nitric oxide (NO) into nitrogen dioxide (NO 2 ). The DPF  24  is disposed downstream from the DOC  22  to remove diesel particulate matter (PM) or soot from the exhaust gas. The SCR  26  is disposed downstream from the DPF  24  and, with the aid of a catalyst, converts nitrogen oxides (NOx) into nitrogen (N 2 ) and water. A urea water solution injector  27  is disposed downstream from the DPF  24  and upstream from the SCR  26  for injecting urea water solution into the stream of the exhaust gas. When urea water solution is used as the reductant in the SCR  18 , NOx is reduced into N 2 , H 2 O and CO 2  in the following reaction: 
         [0000]      4NO+2(NH 2 ) 2 CO+O 2 →4N 2 +4H 2 O+2CO 2  
 
         [0019]    The electric heater  28  provides assisted heating of the exhaust gas flowing in the exhaust conduits  32 ,  34 ,  36 . The generator  14  is connected to the diesel engine  12  to drive the diesel engine  12  during engine startup as an option and to supply electricity to the electric heater  34  during normal engine operation. The heater control module  30  strategically controls the electric heater  28  in different heating modes to facilitate both active and passive regeneration of the DPF  24 . 
         [0020]    Regeneration is the process of burning and removing the accumulated particulates matters from the DPF  24 . Regeneration can occur passively or actively. Passive regeneration can occur in normal engine operation when the temperature of the exhaust gas is sufficiently high. Active regeneration can occur based on a monitored DPF condition or based on a predetermined timing schedule by introducing very high heat to the exhaust aftertreatment system  10 . Active regeneration can be achieved by proper engine control management to increase the exhaust temperature through late fuel injection or injection during the expansion stroke. Active regeneration can also be achieved through assisted heating by an electric heater. Active regeneration requires much more heat than passive regeneration and thus subjects the ceramic structure of the DPF  24  to the risk of cracking and decreases catalytic coating life time. 
         [0021]    Referring to  FIG. 2 , the heater control module  30  strategically controls operation of the electric heater  28  based on an engine load and a status of the DPF  24  to provide assisted heating in both active and passive regeneration of the DPF. The heater control module  24  may be a part of an engine control unit (ECU) (not shown) or external to the ECU. The ECU controls operation of the diesel engine  12 , a fuel injection system (not shown), among others, and acquires and stores various parameters relating to engine operating conditions, including but not limited to, exhaust temperature, diesel engine load, flow conditions (air flow and air pressure etc.) The heater control module  30  receives inputs from the ECU to make the proper determination how to operate the electric heater  28 . The control module could also receive information from stand alone after treatment control systems. 
         [0022]    The heater control module  30  includes a heating mode determination module  62  and a heater operating module  63  including a passive regeneration heating module  64  and an active regeneration heating module  66 . The electric heater  22  can be operated in two operating modes: passive regeneration heating mode and active regeneration heating mode. The heating mode determination module  62  determines a desired heating mode based on an engine load and the status of the DPF  24 . When the DPF  24  is actively regenerated, the desired heating mode is the active regeneration heating mode. When the DPF  24  is not actively regenerated and the engine load is low, for example, at 10%, the desired heating mode is the passive regeneration heating mode. The heating mode determination module  62  may include a heating strategy that specifies the correlation among the heating modes, duration, engine loads and the desired exhaust temperature rise. The heating mode determination module  62  also determines when the electric heater  28  should be turned on or off during normal engine operation. In response to the determination of the heating mode determination module  62 , the heater operating module  63  operates the electric heater  28  accordingly. 
         [0023]    In the passive regeneration heating mode, the electric heater  28  is controlled to heat the exhaust gas to a predetermined temperature which allows for optimum NO 2  generation in the DOC  22 . NO 2  is an effective reactant for passive regeneration of DPF  24 . Increasing NO 2  generation can facilitate passive regeneration of DPF  24 . In the active heating mode, the electric heater  28  is controlled to heat the exhaust gas differently to reduce exhaust temperature gradient across the exhaust conduits. When the temperature gradient is reduced, the active regeneration can be accomplished more efficiently. 
         [0024]    When the heating mode determination module  62  determines that the passive heating mode is desired, the passive regeneration heating module  64  then controls the electric heater  28  to heat the exhaust gas to a predetermined temperature. The passive regeneration heating module  64  calculates and determines the desired temperature rise based on an exhaust temperature and the predetermined temperature. The exhaust temperature may be obtained from the input from the ECU, temperature sensors. The predetermined temperature depends on the properties of the catalysts in the DOC  14  and is set to allow for optimum NO 2  generation. 
         [0025]    Referring to  FIG. 3 , the NO 2  concentration at the outlet of the DOC  14  is dependent on the temperature of the exhaust gas. For a BASF DOC catalyst, the NO 2  concentration is relatively high when the catalyst temperature is in the range of 300 to 460° C., particularly in the range from 320 to 380° C. Therefore, the predetermined temperature is set to be in the range of 300 to 460° C., and preferably in the range from 320 to 380° C. When the electric heater  28  heats exhaust gas to the predetermined temperature, an optimum amount of NO 2  is generated to facilitate passive regeneration of the DPF  24 . With the extensive passive regeneration of DPF, the particulate matter is accumulated on the DPF at a lower rate, thereby reducing the frequency for active regeneration. As a result, the likelihood of DPF ceramic cracking and degradation of the catalysts due to high heat associated with active regeneration (generally in the range of 500 to 650° C.) are reduced. 
         [0026]    Referring back to  FIG. 2 , when the DPF  24  is actively regenerated, the desired heating mode is the active regeneration heating mode. The active regeneration heating module  66  controls the electric heater  28  to provide differential heating to the exhaust gas. The electrical heater  22  generates more heat along the periphery of the electric heater and less heat at the center of the exhaust conduit. 
         [0027]    The exhaust conduit generally has a relatively higher temperature along the central axis of the conduit and a relatively lower temperature proximate the conduit wall. To ensure effective active regeneration across the DPF  24 , the exhaust gas proximate the exhaust conduit wall also needs to be heated to the desired active regeneration temperature. Due to the temperature gradient across the cross section of the exhaust conduit, the exhaust gas proximate the center of the exhaust conduit is unnecessarily overheated, subjecting the center portion of the DPF  24  to higher heat and higher risk of cracks. By operating the electric heater  28  to reduce the temperature gradient, less heat is required to heat the exhaust gas to the desired active regeneration temperature. Therefore, the likelihood of overheating at the center of the DPF and the accompanying problems is reduced. 
         [0028]    Referring to  FIG. 4 , an exemplary embodiment of the electric heater  28  is shown to have a low watt density zone  40  proximate the center and a high watt density zone  42  along the periphery of the electric heater  28 . The electric heater  28  can provide differential heating across the exhaust conduit. 
         [0029]    The electric heater  28  is powered by the generator  14 . The generator  14  drives the diesel engine  30  during engine startup. After the diesel engine  30  starts to operate on its own, the generator  14  is driven by the diesel engine  14  to generate electricity to power other electronics or electrical devices. The heating strategy allows for use of available electricity generating capacity when it is not needed to power the other electrical and electronic systems during low engine load operation. 
         [0030]    Referring to  FIG. 5 , the heating mode determination module  62  includes a heating strategy which specifies the correlations among the heating modes, the exhaust temperature rise, the engine loads. As shown in the exemplary diagram, when the engine load is low and the DPF backpressure is in the range of medium to high, the target exhaust temperature rise (delta) would be low and the electric heater  28  is operated in the passive regeneration mode. For example, the electric heater  22  is in the passive regeneration heating mode when the diesel engine  12  is operating near low load conditions such as 10% load. The electric heater  22  demands less electric power from the generator  14  because the desired temperature rise (delta) is less than that for active regeneration and because less exhaust gas is generated from the diesel engine  30  due to the low engine load. 
         [0031]    As the engine load continues to increase, for example, from 10% to 25%, to 50%, to 75%, the electric heater  28  is turned off. Active regeneration of DPF may be initiated when the engine load is low or according to a predetermined schedule to benefit from heating lower exhaust mass flow. When the DPF is actively regenerated, for example, at an engine load of 25%, the electric heater is turned on and operated in the active regeneration heating mode to provide differential heating. When the active regeneration is completed and the engine load starts to increase, the electric heater  28  is turned off. 
         [0032]    Referring to  FIG. 6 , the table illustrates the exhaust contents for different load conditions. As shown, when the diesel engine is operated under the 10% load condition, the exhaust gas exhibits the lowest exhaust flow (1925 cfm) and the highest available specific NOx (6.8 g/bhp-hr) among the 5 load conditions for a gen-set type of large diesel engine. For example, had the exhaust temperature been raised from 235 C (455 F) to a temperature that is within the DOC&#39;s NO2 generation sweet temperature window of 320 to 380° C., the DOC downstream of the heater will generate maximum amount of NO 2  due to higher available NOx under this load engine condition. NO 2  passively oxidizes the particulate matter loaded DPF downstream of the DOC at its maximum rate. Additionally, the delta T rise is only 85° C. which will minimize energy consumption in comparison with an active regeneration which will have a delta T as high as 350° C. 
         [0033]    For the 10% load condition on this Gen-set with a flow of 81.6 kg/min, it will require 121 KW energy input to heat the exhaust and have a delta T rise of 85° C. It will need 450 KW to heat the exhaust up to 550° C. at 25% load condition with a flow of 137.3 kg/min. 
         [0034]    For the notch 1 condition on a GE locomotive engine with a flow of 54.8 kg/min, it will require 73 KW energy input to heat the exhaust and have a temperature rise (delta) of 76° C. up to 355° C. It will need 315 KW to heat the exhaust up to 607° C. at the same notch 1 condition. 
         [0035]    With the extensive passive regeneration, the accumulation of the soot and PMs on the DPF  24 , as well as the backpressure of the DPF, are reduced. As a result, the active regeneration periods and frequencies can be significantly reduced, thereby enhancing durability of the expensive DPF. The electric heating strategy of the present disclosure may replace the fuel-injection-based active regeneration. 
         [0036]    Referring to  FIG. 6 , the heating module  20  of the present disclosure applies to all diesel engines which can generate electricity while in operation, preferably to those non-EGR diesel engines having high engine-out NOx at lower duty cycles. As shown, the heating module  20  can be applied to a catalyzed DPF only exhaust system, as well as an exhaust aftertreatment system  50  that includes DOC  52  and DPF  54  without SCR. 
         [0037]    The heating module  20  of the present disclosure has at least the following benefits: 
         [0038]    1. Utilizing available electricity generating capacity when it is not needed for other operations on a diesel-generator or a marine engine or a locomotive at low load to assist in passively regenerating the DPF as part of the engine&#39;s emission control system. 
         [0039]    2. Reducing the frequency of diesel fuel injection based active regeneration and hence enhancing fuel economy of the engine operation. 
         [0040]    3. Reducing DPF operational soot loading through heating assisted passive regeneration to minimize overall operational backpressure. 
         [0041]    4. Reducing risks of DPF cracking caused by soot overloaded runaway regenerations through heating assisted passive regeneration. 
         [0042]    5. Improving exhaust aftertreatment system&#39;s performance through delivering more uniform exhaust temperatures across the system&#39;s inlet face. 
         [0043]    Additionally, the present disclosure may include methods of heating portions of the gas flow in a more indirect matter. For example, the system could sense cooler portions within the gas flow cross section and provide heat where needed to provide a more even temperature distribution and compensate for heat losses. In addition, for systems that require more electricity than is available to regenerate the entire gas stream cross-section, the system may regenerate in certain sections or zones at different times. These alternate forms of the present disclosure would also have a corresponding heater type that supports zone heating across the cross-section of gas flow, such as, by way of example, layered heaters or modular heat trace heaters such as those disclosed in pending U.S. application Ser. No. 11/238,747 titled “Modular Layered Heater System” and in U.S. Pat. No. 7,626,146 titled “Modular Heater Systems,” both of which are commonly assigned with the present application and the contents of which are incorporated by reference herein in their entirety. 
         [0044]    The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since modifications will become apparent from the following claims.