Abstract:
An aftertreatment device for reducing nitrogen oxides (NOx), particulate matter (PM), hydrocarbon (HC), and carbon monoxide (CO) generated by a compression-ignition (CI) engine. In this device, lean exhaust air generated in the CI engine is converted to rich exhaust air, and energy used for the conversion is recycled using an energy recovery device. The result rich exhaust air then pass through an oxidation catalyst, where NOx is reduced with CO and HC.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part application of the co-pending U.S. patent application Ser. No. 12/080,752, filed on Apr. 5, 2008, now abandoned entitled Engine Aftertreatment System with Exhaust Lambda Control, to which priority is claimed under 35. U.S.C. 120. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     Not Applicable 
     FIELD OF THE INVENTION 
     The present invention relates to systems and methods for reducing exhaust emissions from internal combustion engines, more particularly, to apparatus and methods for reducing Nitrogen Oxides (NOx), Carbon Monoxide (CO), Hydrocarbon (HC), and Particulate Matter (PM) from compression ignition engines. 
     BACKGROUD OF THE INVENTION 
     Internal combustion engines are subject to limits for exhaust emissions. In addition to improving in-cylinder designs, using Exhaust Gas Recirculation (EGR), and better controlling combustion, an aftertreatment device is normally needed for reducing pollutants, which include Nitrogen Oxides (NOx), Carbon Monoxide (CO), Hydrocarbon (HC), and Particulate Matter (PM), to required levels. In spark ignitions (SI) engines, fuel and air can be pre-mixed stoichiometrically, therefore, not much PM is seen in exhaust air, while CO, HC, and NOx are major pollutants. However, in a compression ignition CI engine, due to heterogeneous fuel-air mixing, PM and NOx are major components in its pollutants, while CO and HC are relatively insignificant. 
     In CI engines, PM and NOx emissions have strong relations to peak combustion temperature. High peak combustion temperature decreases PM generation while increases NOx emission, and low peak combustion temperature affects emissions reversely. Consequently, in using EGR for adjusting peak combustion temperature, a tradeoff needs to be made between PM level and NOx emission. When both of PM and NOx need to be controlled, normally, two methods are used with an aftertreatment device. One is tuning NOx emission low, and using a high efficiency filter for removing PM. The other one is tuning PM level low, and using lean NOx removing technology, such as urea/ammonia Selective Catalytic Reduction (SCR), Lean NOx Trap (LNT)/NOx Absorber (NAC), and Lean NOx Catalyst (LNC), for controlling NOx emission. In the first method, since PM level is high, the filter needs to be regenerated periodically. The regeneration normally is achieved by heating up the filter to 400° C. to 600° C., and the heating energy is provided by burning fuel in a combustion device, such as an oxidation catalyst or a burner. Fuel penalty for filter regeneration depends on engine operating conditions and NOx emission level. When a low NOx emission level is required, e.g. according to US2010 standard, NOx emission cannot be over than 0.2 g/bhp.hour, fuel penalty could be a limiting factor for using the particulate filter method. 
     The other method needs to remove NOx from lean exhaust air. As oxygen, NOx is also an oxidant. Therefore, a selective environment must be created more favorably for reactions reducing NOx, since oxygen concentration is much higher than that of NOx. Among all technologies used in reducing NOx in lean exhaust air, SCR has the highest conversion efficiency, and thus is used broadly. However, a difficulty in developing selective catalyst is that there exists a tradeoff between conversion efficiency and selectivity. A catalyst with high selectivity normally has poor conversion efficiency. As a result, to have high selectivity, a device with a large volume is needed when high conversion efficiency is required. 
     Though SCR technology needs not dosing fuel, the hydrolysis of urea, which is used in generating ammonia for SCR reactions, is endothermic and needs extra energy. If this energy is provided by burning more fuel in engine, this fuel penalty could be 3% of total engine fuel consumption, depending on operating conditions. Additionally, urea is consumed in reducing NOx. The overall cost of urea consumption and fuel penalty for urea hydrolysis is comparable with cost of fuel penalty in using particulate filter. Combining the particulate filter method and lean NOx reducing method could achieve the best aftrertreatment performance. However, the cost is system complexity and fuel penalty. 
     Different from that in CI engines, in SI engines, when air-fuel ratio is controlled at stoichiometric level, NOx could have a higher or comparable concentration as oxygen. As a result, even in an oxydation catalyst without good selectivity, reductant is able to remove NOx from exhaust. This type of catalyst usually is called three-way catalyst, since it uses CO and HC as reductant in removing NOx, consequently, all three pollutants are removed from exhaust. 
     Compared to SI engines, the lean combustion nature of CI engines creates lean exhaust air, which causes the difficulties in using reductants in exhaust air to reduce NOx. Accordingly if the lean exhaust of a CI engine is converted to rich exhaust, an oxidation catalyst can be used to reduce NOx with reductants. It is a goal of the present invention to provide a means for reducing NOx and other pollutants in lean exhaust air by converting the lean exhaust air to rich exhaust air without significantly sacrifing fuel economy. Furthermore, it is a goal of the present invention to use solely fuel in exhaust air aftertreatment. 
     BRIEF SUMMARY OF THE INVENTION 
     In the present invention, a new technology of reducing exhaust pollutants in a CI engine is developed. In this technology, oxygen is firstly removed and then an oxidation catalyst is used for reducing NOx, CO, and HC from exhaust air. 
     Normally, due to the lean combustion nature, air-fuel ratio in CI engines cannot be stoichiometric. In one embodiment of this invention, oxygen left in exhaust air is removed by using a fuel reactor in which fuel injected during expansion (in-cylinder late injection) or provided by a dedicated doser reacts with oxygen. The fuel reactor act as an air-fuel ratio controller, which adjusts the lambda value of the exhaust air close to 1, thus an oxidation catalyst can be used for effectively reducing NOx, CO and HC. Compared to SI engines, CI engines have better fuel economy: usually CI engines are 30% or more efficient than SI engines. Therefore, it is not economic if the dosing fuel is just used for reducing pollutants from exhaust though comparatively there could also be around 6% fuel penalty or equivalent fuel penalty when using other types of aftertreatment devices such as LNT and SCR. 
     Heat generated in exhaust lambda control needs to be recovered. Both turbines and heat exchange devices are energy conversion devices that can be used for energy recovery and the energy recovery efficiency determines overall fuel penalty. Ideally, if the energy recovery efficiency is higher than engine efficiency, there will be no fuel penalty in using the fuel reactor. 
     In another embodiment, oxygen in exhaust air is removed by using an oxygen sorption device, through which oxygen is separated from exhaust. The result rich exhaust air then passes through an oxidation catalyst where NOx, CO and HC are reduced. Once the oxygen sorption device reaches a saturation level, a regeneration process is triggered. During the regeneration, oxygen adsorbed and/or absorbed in the device is removed and the device is ready for the next sorption process. A wheel structure and/or a valve-controlled structure can be used for continuous operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an aftertreatment system with an oxygen-removing device; 
         FIG. 2 a    illustrates an embodiment of the aftertreatment depicted in  FIG. 1  with a fuel reactor for exhaust lambda control and a turbine as an energy recovery device; 
         FIG. 2 b    shows the controllers in the aftertreatment system of  FIG. 2   a;    
         FIG. 2 c    is a block diagram of a control system for controlling lambda level in the aftertreatment system of  FIG. 2   b;    
         FIG. 3  shows the aftertreatment system of  FIG. 2  further including a heat exchanger; 
         FIG. 4  depicts an aftertreatment with multiple fuel reactors and multiple turbines; 
         FIG. 5 a    is a block diagram of the aftertreatment system depicted in  FIG. 2  further including a soot filter system; 
         FIG. 5 b    illustrates an embodiment of the soot filter system in  FIG. 5   a;    
         FIG. 5 c    shows a heat exchanging device in the soot filter system of  FIG. 5 b    further including bypass valves; 
         FIG. 5 d    shows a fuel doser in the soot filter system of  FIG. 5 b    further including an electrical heater for heating dosing fuel; 
         FIG. 5 e    shows a fuel doser in the soot filter system of  FIG. 5 b    further including an electrical heater for heating exhaust air mixed with dosing fuel; 
         FIG. 5 f    shows a pulse controller for controlling electrical heaters; 
         FIG. 5 g    shows a fuel doser in the soot filter system of  FIG. 5 b    further including a fuel burner for heating exhaust air mixed with dosing fuel; 
         FIG. 6  illustrates another embodiment of the aftertreatment depicted in  FIG. 1  with an oxygen sorption device; 
         FIG. 7  shows the oxygen sorption device of  FIG. 6  with a wheel structure; 
         FIG. 8  shows an aftertreatment of  FIG. 6  controlled with valves. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As depicted in  FIG. 1 , an engine system includes an engine  101 , an oxygen-removing device  102  and an oxidation catalyst  103 . The oxygen-removing device  102  is used for enriching exhaust air emitted from the engine  101 . The result exhaust from the device  102  has a very low oxygen concentration. In the oxidation catalyst  103 , NOx in the rich exhaust reacts with CO and HC, and thereby the pollutants are removed. 
     An embodiment of the oxygen-removing device is shown in  FIG. 2 a   . A fraction of the exhaust air from an engine  201  goes back to intake manifold trough an EGR system. The rest of the exhaust air goes into a fuel reactor  202 . Therein HC fuel provided in in-cylinder late injection or through a fuel doser  203  reacts with oxygen in the lean exhaust emitted from the engine  201 . The heated exhaust air then passes through a turbo-charger including a turbine  204 , where heat energy in the exhaust air is partially recovered and used in compressing fresh air. The result exhaust air from the turbine  204  goes through a catalyst  205 , where NOx reacts with CO and HC, and the treated air is emitted to ambient or to a soot filter (not shown in  FIG. 2 ) for further removing PM. In the system, for better controlling the HC concentration in reduing NOx, or actively controlling the regeneration of the soot filter, an optional extra doser  206  can be installed between the turbine  204  and the catalyst  205 . In addition to compressing fresh air, when the turbo-charger is replaced with a turbo-generator, in which the turbine  204  is used to drive an alternator, the recovered energy can be converted to electric energy. The turbo-generator is especially useful in a hybrid vehicle. 
     The reactor can also improve aftertreatment performance at cold-start. When engine starts, the exhaust pressure and temperature is not enough to effectively drive turbo-charger. As a result, large amount of PM could be generated. The reactor can be used for increasing the exhaust temperature and thus improves the transient performance of the turbo-charger and burns PM in exhaust air. 
     To effectively remove NOx, HC, and CO from exhaust air with an oxidation catalyst, the exhaust air lambda value needs to be controlled within a narrow window (Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, 1988, Page 654-657). Low lambda value facilities NOx removing, however will cause a low efficiency oxidizing HC and CO, while higher HC and CO conversion efficiency is obtained with lower NOx conversion efficiency at high lambda values. To control the exhaust lambda value at a target value λ t , the fueling rate can be calculated using the following equation: 
                     m     fuel   .       =       (       m     fresh   .         AF   0       )     *     (       1     λ   t       -     1     λ   1         )               (   1   )               
where λ 1  is the lambda value of the engine; m {dot over (f)}uel  is the mass flow rate of fuel injection in exhaust lambda control, and AF 0  is the stoichiometric air fuel ratio.
 
     For accurately controlling the exhaust air lambda value, a feedback control can be used for a system with a lambda/oxygen sensor installed upstream the catalyst  205 , as shown in  FIG. 2 b   . In such a system, through signal lines  221 , a lambda/oxygen sensor  210  is connected to a fuel dosing controller  220 , which also controls the dosers  203  and  206  through signal lines  222  and  223  respectively. In generating fuel dosing commands, the exhaust air lambda value, λ s , is monitored by using the lambda/oxygen sensor  210 , and the engine lambda value and exhaust air flow rate value can be obtained either using physical sensors (not shown in  FIG. 2 b   ), or calculated using engine fueling rate m ėf  and fresh air flow rate m f{dot over (r)}esh , provided by an engine controller  230  with the following equation: 
     
       
         
           
             
               
                 
                   
                     λ 
                     1 
                   
                   = 
                   
                     
                       
                         m 
                         
                           fresh 
                           . 
                         
                       
                       
                         
                           
                             m 
                             ef 
                           
                           . 
                         
                         ⁢ 
                         
                           AF 
                           0 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In the dosing controller  220 , a variety of feedback control schemes can be used for controlling fuel dosing rate. An example of feedback control scheme for controlling the doser  203  is shown in  FIG. 2 c   . In this control scheme, the fueling rate m ėf  together with the exhaust air flow rate m ėxh  and the target lambda value A are used by a feed-forward controller  245  to calculate a baseline for the dosing rate to reach the target lambda value according to equations (1)-(2). The target lambda value λ t  is also compared with the lambda sensing value λ s  provided by the lambda/Oxygen sensor  210 , and the error, λ t -λ s , is used by a feedback controller  240  to calculate a compensation value, which is added to the baseline value generated by the feed-forward controller  245 . The sum value is then output to a driver  250  as a dosing command, and the driver  250  controls the doser  203  to generate the commanded dosing rate. 
     In the feedback controller, a gain scheduling method can be used to adjust the dynamic performance according to the fresh air flow rate and the engine fueling rate. For example, when a PID controller is used, the Proportional, Integral, and Derivative gains of the controller can be functions of the fresh air flow rate and the engine fueling rate. Also, to decrease the overshoot caused by the feedback controller, the feedback controller can further be enabled or disabled by the error λ t -λ s , i.e., when the error is large, only the feed-forward controller provides the dosing rate command, and when the error is decreased below certain value, the feedback controller is enabled to correct the error together with the feed-forward controller. 
     As a byproduct of the exhaust air lambda control, heat is released during the combustion of dosing fuel in the reactor and exhaust temperature increases. If the overall temperature gained by exhaust is T g , then when fueling rate in lambda control is small compared to exhaust mass flow, we have the following equation:
 
 T   g   =m   {dot over (f)}uel *LHV/( C   p   *m   ėxh )  (3)
 
where LHV is the low heating value of fuel; C p  is the specific heat at constant pressure, and m ėxh  is the exhaust mass flow,
 
 m   ėxh   =m   ėf   +m   {dot over (f)}uel   +m   f{dot over (r)}esh   (4)
 
     Based on equations (1), (3), and (4), when the value of fresh air flow, the exhaust temperature increase across the reactor is 
     
       
         
           
             
               
                 
                   
                     T 
                     g 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           1 
                           
                             λ 
                             t 
                           
                         
                         - 
                         
                           1 
                           
                             λ 
                             1 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       
                         
                           λ 
                           t 
                         
                         ⁢ 
                         LHV 
                       
                       
                         
                           C 
                           p 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               
                                 λ 
                                 t 
                               
                               ⁢ 
                               
                                 AF 
                                 0 
                               
                             
                             + 
                             1 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     According to the equation (5), with a given target lambda value, the temperature gained by the exhaust air is determined by the engine lambda value. When the engine lambda value is low, a high temperature can be generated. Consequently, the engine lambda value needs to be carefully controlled, otherwise, a complex and expensive reactor and turbo that can work at high temperature are needed. In addition to tuning EGR fraction, a heat exchanger or multi-stage turbine can be used for lowering the temperature at turbine inlet. 
     As depicted in  FIG. 3 , a heat exchanger  301  is used in between the fuel reactor  202  and the turbine  204  for decreasing the temperature of the exhaust air passing through it. A heat pump (not shown in the figure) can be used with the heat exchanger  301  for recovering the heat energy. 
     Another method for lowering the turbine inlet exhaust temperature is using multi-stage turbines. As shown in  FIG. 4 , a second stage turbine  403  is positioned downstream from the turbine  204 . In between the turbine  403  and the turbine  204 , a fuel reactor  402  is used for further lambda control, and a doser  401  can be used for flexibly controlling the temperature of exhaust air passing through the turbine  403 . Since the lambda value downstream from the turbine  403  is controlled by both of the reactors  202  and  402 , the exhaust temperature upstream from the turbine  204  can be decreased by distributing the lambda control in between the reactors  202  and  402 , i.e., the lambda value downstream from the reactor  202  can be controlled higher, resulting a lower exhaust air temperature upstream and then downstream from the reactor  202 , and the fuel reactor  402  is used for further controlling the lambda value down to the stoichiometric level. More turbines can be used for flexibly distributing heat generated in lambda control, if engine back pressure, cost and recover efficiency allow. 
     The exhaust air with lambda controlled at stoichiometric level flows into an oxidation catalyst, where HC and CO in the exhaust react with NOx and generate N 2 , CO 2 , and H 2 O. To remove PM in the exhaust air, referring to  FIG. 5 a   , a soot filter system  502 , which includes a DOC (Diesel Oxidation Catalyst) and a DPF (Diesel Particulate Filter), is installed in between the turbine  204  and an oxidation catalyst  503 . 
     Normally the soot filter system  502  needs to be regenerated after the amount of PM deposited in the DPF exceeds a certain level. During regeneration, the exhaust lambda value at the inlet of the soot filter  502  cannot be controlled below 1.0, otherwise, soot in the filter is not able to be effectively removed, since oxygen in the exhaust is not enough for soot oxidation. To have an uninterrupted deNOx operation, a doser  501  can be used for further controlling lambda during filter regeneration, in which the fuel injected from the doser  501  reacts with the oxygen left in the regeneration in the front area of the catalyst  503  for lowering lambda to stoichiometric level. 
     Through turbines, heat energy is recovered into mechanical energy or electric energy. When the energy recovery efficiency is η r , we can define the fuel penalty r p  as the ratio of the net fuel loss in lambda control and the overall fueling, i.e.: 
                     r   p     =       m       f   .     ⁢   uel       *         η   e     -     η   r           m       f   .     ⁢   uel       +       m   ef     .                   (   6   )               
where η e  is the engine energy efficiency. According to equations (1), (2) and (6), the fuel penalty can be calculated in using the following equation:
 
     
       
         
           
             
               
                 
                   
                     r 
                     p 
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         - 
                         
                           
                             λ 
                             t 
                           
                           
                             λ 
                             1 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           η 
                           e 
                         
                         - 
                         
                           η 
                           r 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The equation (7) shows that the fuel penalty actually is determined by the engine out exhaust lambda value and the difference between the energy recovery efficiency and the engine efficiency. As an example, if λ 1 =1.4, and λ t =1.0, then to have a fuel penalty of 5%, which is normally the value of an DPF system, assuming engine energy efficiency is 40%, the required energy recovery efficiency will be only 22.5%. If a turbine system has an energy recovery efficiency higher than 40%, there will be no fuel penalty. 
     In the system of  FIG. 5 a   , heat energy can also be recovered using a heat exchanger when regenerating the soot filter system  502 . As depicted in  FIG. 5 b   , in the soot filter system, an air compressor  530 , which is controlled by a dosing controller  560  through signal lines  561  has its inlet (low pressure side) fluidly connected to the outlet of the turbine  204  via a passage  511 , and its outlet (high pressure side) fluidly connected to the shell inlet of a heat exchange device  513  through a passage  512 . The shell outlet of the heat exchange device  513  is fluidly connected to a DOC  518  through a passage  514 , on which a temperature sensor  517  is mounted and linked to the dosing controller via signal lines  564 . A fuel doser  516 , which is controlled by the dosing controller  560  through signal lines  562 , is also installed on the passage  514  upstream the temperature sensor  517  for delivering fuel to exhaust air flowing through the DOC  518 . A DPF is positioned downstream the DOC  518  for removing particulate matters in exhaust air, and a delta pressure sensor  522  is used to measure the pressure drop across the DPF and report sensing values to the dosing controller  560  via signal lines  563 . A temperature sensor  519 , positioned in between the DOC  518  and the DPF  520  and connected the dosing controller  560  through signal lines  565 , and another temperature sensor  521 , positioned downstream the DPF  520  and connected to the dosing controller  560  through signal lines  566 , are used to monitor the temperature at the inlet and the outlet of the DPF respectively. Exhaust air passing through the DPF is introduced back to the tube inlet of the heat exchanger  513  through a passage  515 , and the tube outlet of the heat exchanger  513  is fluidly connected to a passage  523  to release exhaust air to the catalyst  503  ( FIG. 5 a   ). 
     In this system, PM deposited in the DPF  520  increases its restriction to exhaust air flow, resulting in higher pressure drop across the DPF. When the restriction indicated by the pressure drop, which is measured using the pressure sensor  522 , is higher than a threshold, a regeneration process is triggered. In the regeneration, after the light-off temperature of the DOC  518  is reached, hydrocarbon fuel is delivered by the doser  516  into a lean exhaust air. In the DOC  518 , the hydrocarbon fuel reacts with oxygen in the lean exhaust air releasing heat energy, and the heated exhaust air passes through the DPF  520 , burning off the PM inside it. The exhaust air then goes back to the heat exchanger  513  through the passage  515 . Therein the exhaust air in the tube loses heat energy to the exhaust air in the shell and goes out into the catalyst  503  ( FIG. 5 a   ) through the passage  523 . 
     During the regeneration process, the DOC inlet temperature and outlet temperature measured by using the temperature sensors  517  and  519  are used in determining the amount of dosing fuel needed to increase the exhaust air temperature to a target value for effectively oxidizing the PM in the DPF, and the DPF outlet temperature measured by using the temperature sensor  521  is used for adjusting the fuel dosing rate to avoid overly heating the DPF. To decrease energy consumption, the heat exchanger  513  needs to have a high heat exchanging efficiency, and thus a long resident time of the exhaust air in the heat exchanger, resulting in increase of pressure drop across the heat exchanger. The pressure drop across the heat exchanger  513  and DPF may cause a high backpressure to the engine, deteriorating fuel economy. To decrease the effects of pressure drop across the heat exchanger  513  and the DPF  520 , the exhaust air compressor  530  is used to control the engine backpressure. The exhaust air compressor  530  provides a pressure increase, which compensates the pressure drop caused by the exchanger  513 , the DPF  520 , and the catalyst  503  ( FIG. 5 a   ). When pressure increase equals to the pressure drop, then the pressure at the passage  511  is ambient pressure, and thereby the backpressure to the engine is the same as that in a system without after-treatment devices. In addition to being positioned downstream from the turbine  204  as shown in  FIG. 5 b   , the exhaust air compressor  530  can also be positioned upstream the turbine  204  or combined with the turbine  204 . 
     When the doser  516  is used solely for regenerating the DPF  520 , the heat exchange device  513  is only needed during a regeneration. To decrease the engine backpressure and energy cost in controlling the engine backpressure, the heat exchange device  513  can be bypassed in normal operations. Referring to  FIG. 5 c   , in the exchange device  513 , through a passage  553 , the air flow in which is controlled by a valve  551 , the shell inlet  555 , which is connected to the high pressure outlet of the exhaust air compressor  530  through a passage  512  ( FIG. 5 b   ), is fluidly coupled to the shell outlet  557 , which is connected to the DOC  518  through a passage  514  ( FIG. 5 b   ). Similarly, the tube inlet  556 , which is connected to the DPF  520  through a passage  515  ( FIG. 5 b   ), is fluidly coupled to the tube outlet  558 , which is connected to the passage  523  ( FIG. 5 b   ), through a passage  554  and a control valve  552 . The control valves  551  and  552  are controlled by the dosing controller  560  via signals lines  567  and  568  respectively. In normal operations, the valves  551  and  552  are open, since the restriction to exhaust air flow in the passages  553  and  554  is lower than that in the shell and tube in the heat exchanger  513 , the pressure drop is lowered. When a regeneration is triggered, the valves  551  and  552  are closed, and the exhaust air flows through the shell and tube in the heat exchanger  513  with heat energy being transferred from the exhaust air downstream from the DPF to that upstream from the DOC. 
     To effectively oxidize fuel in the DOC  518 , the exhaust air temperature needs to be higher than the DOC light-off temperature, which is normally around 250° C., otherwise, unburned fuel may slip the DPF. The light-off temperature limit causes the system unable to start dosing for low temperature exhaust air even if a regeneration is triggered. To solve this problem, a positive feedback process can be introduced by momentarily heating the dosing fuel or the DOC  518  with a second heating device in addition to the exhaust gas heating device which includes the doser  516  and the DOC  518  to “jump start” the heating process. In an exemplary system, as shown in  FIG. 5 d   , an electrical heater  570 , controlled by the dosing controller  560  through signal lines  571 , can be used as the second heating device to heat the fuel released by the doser  516  to a temperature higher than the light-off temperature. The heated fuel is then partially oxidized in the exhaust air and enters the DOC  518 , where it is fully oxidized, releasing more heat and warming up the exhaust air and the DOC. Through the heat exchanger  513  ( FIG. 5 b   ), the released heat is transferred back to the exhaust air entering the DOC  518 , which is further warmed up with dosing fuel being burned in exhaust air with higher temperature. When the heat energy released in the DOC  518  equals to the heat loss to the exhaust air, the DOC temperature is then able to be hold above the light-off temperature, and the electrical heating is turned off. To decrease the effects of heat loss from the heated fuel to the exhaust air and DOC  518 , the electrical heater can be positioned in front of DOC with exhaust air passing through, as depicted in  FIG. 5 e   . In this case, a higher heating power is needed. To decrease the burden on batteries, a pulse driving signal can generated to the heater by charging and discharging a capacitor. As shown in  FIG. 5 f   , inside the dosing controller  560 , a MOSFET switch  576  controlled by a CPU  575  connects a power source (a battery  580 ) to a capacitor array  578  through a voltage control circuit  577 , which is used to provide appropriate voltage to the capacitor array  578 . An IGBT switch  579 , which is also controlled by the CPU  575 , is used to connect the capacitor array  578  to the electrical heater  570  through signal lines  571 . In normal operations, the two switches  576  and  579  are at OFF state, disconnecting the capacitor array  578  to the battery  580  and to the electrical heater  570 . When a regeneration process is triggered and the exhaust flow temperature is low, the heater control is activated. The switch  576  is firstly energized ON, charging the capacitor array  578 . When the capacitor voltage is higher than a threshold, upon a firing command, the switch  576  is turned OFF and the switch  579  is latched ON, discharging the capacitor array through the electrical heater  570 . With the high discharging current, the heater temperature increases, heating dosing fuel and exhaust air pass through it. The heated dosing fuel is then further oxidized in the DOC  518  ( FIG. 5 b   ), releasing more heat there. Before the DOC  518  is cooled down by the exhaust air, another heating pulse is generated. Thereby the DOC temperature increases, until it reaches light-off temperature, and then the heater control is de-activated. During the heater control, fuel dosing, which is normally controlled with a PWM method, needs to be synchronized with the heating pulse, i.e., a heating pulse is generated when a dosing pulse is generated, to avoid unburned HC slipping the DOC  518  and the DPF 520  ( FIG. 5 b   ). If the battery  580  is able to provide enough power for high exhaust flow, or only low electrical power is required, e.g., when a regeneration is only required at some special engine operating modes with low exhaust air flow, the electrical heater  570  can be used without dosing fuel in regenerating the DPF  520 . In this case, a smaller DOC or no DOC is required. 
     In addition to electrical heaters, fuel burners can be used for heating exhaust air as well with low temperature exhaust air. Referring to  FIG. 5 g   , in such as system, a fuel burner  590  is positioned in between the shell outlet of the heat exchanger  513  and the DOC  518 . The fuel burner  590  has an air blower  581  fluidly connected to a fresh air supply, and a fuel pump  582  fluidly connected to a fuel supply. The air blower  581  and the fuel pump  582  are controlled by the dosing controller through signal lines  585  and  586  respectively. A glow plug  583  controlled by the dosing controller  560  through signal lines  587  is used to ignite dosing fuel. As the electrical heater  570  ( FIG. 5 d    and  FIG. 5 e   ), the fuel burner  590  can be used for either boosting up the exhaust temperature temporarily for the DOC  518  to reach and sustain light-off temperature, or for directly regenerating the DPF  520  ( FIG. 5 b   ) without dosing fuel to the DOC. When a fuel burner is used for temporarily heating the DOC  518  to jump start a DOC combustion, the doser  516  is needed to delivery fuel to the DOC. In applications using a fuel burner directly providing high temperature exhaust air, the doser  516  is not required. The DOC  518  can be used for operations with exhaust air temperature higher than the light off temperature, when the fuel burner  590  acts as a fuel doser with glow plug de-energized off, or a smaller DOC or no DOC is needed when the fuel burner  590  is used as the only heating means. Just since normally a fuel burner needs a fresh air supply to avoid generating too much PM during combustion, an extra amount of fuel is needed for heating the fresh air supply, resulting in higher energy cost. Compared to electrical heaters, fuel burners need a more complex control system for delivering fuel and air supply in combustion control, and therefore, have higher device cost. 
     Referring back to  FIG. 1 , in addition to combustion, oxygen sorption devices can also be used in the oxygen removing device  102 . As shown in  FIG. 6 , in such as system, an oxygen sorption device  602  is connected to a turbo-charger  601 . Exhaust air flows through the device  602 , where oxygen in the exhaust flow is absorbed and/or adsorbed and thereby, lambda is controlled to stoichiometric level. The result exhaust air then flows into a catalyst  603 , therein NOx reacts with the HC and CO in the exhaust and then is reduced. Hydrocarbon level in the exhaust can be controlled by either using in-cylinder late injection, or using an external doser  605 . The clean rich exhaust processed by the catalyst  603  is emitted to ambient, and a fraction of this exhaust is fed back to the oxygen sorption device  602  for device regeneration. To decrease the energy consumed in regeneration, a valve  604  is used for controlling airflow. 
     The structure of an embodiment of the oxygen sorption device  602  is depicted in  FIG. 7 . This device includes a rotating apparatus  701  driven by actuator  702 , a working area  703  and a regeneration area  704  both having oxygen sorption materials. Firstly the working area  703  in the device  602  is in the exhaust stream absorbing and/or absorbing oxygen from exhaust air, and thus the lambda is controlled at the stoichiometric level. When the oxygen sorption material in the working area  703  reaches its saturation level, the actuator  702  is energized and drives the rotating apparatus  701  moving the working area  703  to the position of the regeneration area  704  and turning the regenerated area  704  into the exhaust stream for oxygen sorption. The oxygen sorption material in the regeneration area (previous working area) is then regenerated in the rich air fed back from the outlet of the catalyst  603  (the rich air flow rate is controlled by the valve  604 ). The process repeats for continuous oxygen level control. 
     A variety of materials can be used for absorbing and/or adsorbing oxygen. Among them, perovskite-related oxides has a good oxygen sorption capacity at temperature range of 200° C. to 400° C., and can be regenerated at temperature at 600° C. [Kusaba, H., Sakai, G., Shimanoe, K., Miura, N., Yamazoe, N., Solid State Ionics, 152-153 (2002)689-694]. Extra energy is needed in regenerating the oxygen absorption material and in rotating the device. This part of energy contributes to the overall fuel penalty for exhaust aftertreatment. 
     In addition to the rotating device, a valve-controlled system can also be used for removing oxygen in exhaust air. In such a system, as depicted in  FIG. 8 , two oxygen sorption devices: devices  802  and  804  are used together with two control valves  801  and  803  for oxygen level control. At beginning, the control valve  801  is off and the control valve  803  is on. Exhaust flow from the turbocharger  601  passes through the device  804  and has oxygen removed therein. The result exhaust then goes into the catalyst  603  and NOx is reduced by HC and CO. An HC doser  805 , which in  FIG. 8  is positioned in between the oxygen removing devices and the catalyst, can be used for flexibly controlling the reactions. When the device  804  is saturated, the control valve  803  is shut off and the control valve  801  is turned on. The device  802  is then used for passing exhaust air through and the device  804  is regenerated for next cycle. The two oxygen control devices work alternatively in continuous oxygen level control. 
     For better removing NOx, referring to  FIG. 1 , the catalyst  103  may include an LNT. In this system, when the lambda is not controlled at the stoichiometric level during some transient operations, the LNT then is able to remove NOx in exhaust air. When lambda is back to stiochiometric level, the LNT is regenerated by dosing with HC. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments which are presented in this description for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow. It is noted that equivalents for the particular embodiments discussed in this description may practice the invention as well.