Abstract:
A Diesel Particulate Filter (DPF) system including a Venturi exhaust passage device, in which a temperature and a pressure in a high pressure passage are measured, together with a difference of pressures in the high pressure passage and a low pressure passage, while a pressure drop across a DPF is monitored. A PM amount and an exhaust flow rate, which are key parameters in DPF control, can be calculated with the measured values. With the Venturi exhaust passage device, a two-stage bootstrapping heating device with two DOCs and an electrical heater can be further used to heat exhaust gas at a temperature lower than a light-off temperature, while a flow-back passage fluidly connected to an outlet of the DPF can be used for increasing exhaust flow-rate and making PM distribution in the DPF more uniform.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
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     TECHNICAL FIELD OF THE INVENTION 
     This present application claims priority from U.S. provisional application No. 62/011,734 having the same title as the present invention and filed on Jun. 13, 2014. 
     This invention relates to an apparatus and method for controlling an exhaust gas processing system for removing particulate matters emitted from an internal combustion engine, more specifically, to an apparatus and method for controlling regenerations of DPFs (Diesel Particulate Filters) in an exhaust gas processing system of an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Environmentally harmful species in exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species that need to be removed. In lean combustion engines, e.g. diesel engines, due to their lean combustion nature, PM and NOx are two major emissions. To remove these harmful species, a variety of technologies have being used. Among them, DPF technology is effective in decreasing PM, including both particle mass and numbers, while a number of technologies, including LNT (Lean NOx Trap) and SCR (Selective Catalytic Reduction) are used for reducing NOx emissions. 
     In a DPF, trapped PM accumulates and increases engine back pressure. To avoid excessively high engine back pressure, the trapped PM in the filter needs to be controlled lower than certain amount. A process for reducing PM is also called a regeneration process. A DPF regeneration can be performed either continuously, during normal operations of the filter, or periodically, after a pre-determined amount of PM has been accumulated. Typically to ensure that a DPF can be reliably regenerated, periodical regenerations are required. And to effectively remove accumulated PM, exhaust temperature needs to be elevated to a certain level, for example, to effectively oxidize PM with oxygen, typically exhaust temperature needs to be controlled above 500° C. 
     A variety of devices, including electrical heaters, DOCs (Diesel Oxidation Catalysts), and fuel burners, can be used in heating exhaust gas in a DPF regeneration process. And to control exhaust gas temperature to a pre-determined level, the power applied to the devices, e.g. electrical current for electrical heaters, fueling rate for fuel burners, and fuel dosing rate for DOCs, needs to be controlled by an ECU (Engine Control Unit) in response to a few control parameters, such as exhaust gas flow rate, which is normally calculated with engine operating parameters, DOC temperature, and DPF temperature. These control parameters, together with a pressure drop across a DPF, which can be measured with a differential pressure sensor, are also used in estimating PM loading in the DPF, and the PM loading amount value can be further used in triggering DPF regenerations. 
     In the control parameters, normally the engine operating parameter values are obtained from engine controls. However, the engine operating parameters are not always available, and in some systems, even though the engine operating parameters are available, their applications are limited due to the limits of the system structure. For example, in applications with mechanically controlled engines, e.g. in a vehicle retrofit, ECU and the engine operating parameters are not available since engine fueling is controlled mechanically. In engine systems with multiple exhaust branches, e.g. in a high horse power engine system, even the overall exhaust flow rate can be estimated with the engine fueling rate and engine speed, exhaust flow rate in each branch is not available. In these applications, to control DPF regenerations, either more sensors, such as engine speed sensors and throttle position sensors, are installed in the engine system for obtaining the engine operating parameters, or more assumptions are used in estimation, e.g., assuming exhaust flow is equally distributed in each exhaust branch. Installing new sensors in an engine system changes system structure, causing reliability issues, while more assumptions deteriorate control performance and diagnosis capabilities. Moreover, when sensors are installed in the engine system, different engine types and applications require different sensor types, resulting in high system cost and engineering cost. 
     When a DOC is used in heating exhaust gas, due to the limit of its light-off temperature, when exhaust gas temperature is low, fuel dosing has to be disabled, since unburnt fuel leaks through the DOC and DPF, creating an emission by itself. As a result, under certain operating conditions, such as when an engine is idling or when a vehicle frequently stops and goes (e.g. a city bus), low exhaust temperature stops DPF regenerations, causing high PM load in the DPF and un-uniform distribution or mal-distribution of PM, which are major causes of thermal runaways damaging the DPF. 
     In a DPF regeneration process, when exhaust flow rate is low, due to the low thermal energy the exhaust flow carries, a DPF could be locally heated, resulting in un-uniform PM distribution in the DPF. To prevent the PM un-uniform distribution caused by low exhaust flow rate, normally in addition to a temperature threshold, a flow threshold is also applied for disabling heating the DPF. However, as that mentioned above, the flow threshold may delay or interrupt a DPF regeneration when an engine runs into low exhaust flow modes, resulting in high PM accumulation and un-uniform PM distribution, which increase the risk of thermal runaways. Additionally, when exhaust flow rate decreases significantly in a short period of time, e.g. when an engine drops to idle, less heat is carried away by the exhaust flow. In a DPF regeneration, when the exhaust flow rate drops too low, even fuel dosing is disabled, a temperature spike could still be created in the DPF, igniting loaded PM and causing a thermal runaway if there is a high PM accumulation or a PM mal-distribution. 
     To solve the problems mentioned above, it is then a primary object of the present invention to provide an apparatus to obtain key parameter values in DPF regeneration control without using engine operating parameters, so that the DPF system is able to work when these parameters are not available. 
     A further object of the present invention is to provide an apparatus to regenerate a DPF with low temperature exhaust gas flow, so that more chances can be obtained for regenerating the DPF. 
     Another object of the present invention is to provide an apparatus to increase exhaust gas flow rate when an engine operates at low exhaust flow mode, so that DPF regenerations need not to be interrupted at these operating modes. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for controlling regenerations of DPFs in an exhaust gas processing system of an internal combustion engine system. This apparatus includes a Venturi exhaust passage device which has an upstream high pressure passage, a low pressure passage, and a downstream high pressure passage. In one embodiment of the present invention, the downstream high pressure passage is fluidly connected to a heating device, which is positioned upstream from a DPF. A pressure sensor and a temperature sensor are used to measure a pressure and a temperature in the downstream high pressure passage respectively, while a first differential pressure sensor is employed to measure a difference between pressures in the downstream high pressure passage and the low pressure passage. An exhaust flow rate, which is a key parameter in controlling DPF regenerations, is calculated by a regeneration controller according to sensing values obtained from the temperature sensor, the first differential pressure sensor, and the pressure sensor. A second differential pressure sensor is used to measure a pressure drop across the DPF, and a PM amount value, which can be used for triggering a DPF regeneration process, is calculated according to a ratio between sensing values obtained from the second differential pressure sensor and the first differential pressure sensor. 
     In another embodiment of the present invention, the upstream high pressure exhaust passage is in communication to a fuel injector, and an electrical heater is fluidly connected to the upstream high pressure exhaust passage. A front DOC positioned downstream from the electrical heater is fluidly connected to the low pressure exhaust passage, and a main DOC positioned downstream from the DPF is fluidly connected to the downstream high pressure exhaust passage. When exhaust gas temperature is low, after a DPF regeneration process starts, dosing fuel is released through the fuel injector at a bootstrapping dosing rate and the electrical heater is energized. The dosing fuel together with the exhaust gas passing through the electrical heater is heated above a light-off temperature of the front DOC, where the dosing fuel is oxidized and the exhaust gas and the front DOC is exothermically heated. The heated exhaust gas then enters the main DOC and heats it. When a bed temperature of the main DOC is higher than its light-off temperature, then the electrical heater is de-energized and a normal dosing rate is generated. There is a bootstrapping process in the heating control, since when the DOCs are exothermically heated, the DOC temperatures get higher, and so is the HC conversion efficiency. Higher HC efficiency cause more HC oxidized and more heat is released in the oxidation reactions, resulting in higher DOC temperature. When the heat released by oxidizing dosing fuel is able to sustain the bed temperature of the main DOC, the electrical heating is no longer required, and since only a fraction of exhaust gas passes through the electrical heater, electrical energy required in the heating control is significantly decreased. 
     In another embodiment of the present invention, a flow-back passage fluidly connects an outlet of the DPF to the low pressure exhaust passage. Through the flow-back passage, a higher exhaust flow rate through the DPF can be obtained, which lowers un-uniform distribution of PM, especially during a DPF regeneration, since more exhaust heat energy is carried by the exhaust flow. The high exhaust flow rate also decreases resident time of dosing fuel in the heating device if it is positioned downstream from the downstream high pressure passage. Short resident time lowers response time of the heating device and improves temperature control performance. However, it also lowers HC conversion efficiency. To avoid low HC conversion efficiency, a control valve can be used to control air flow in the feedback flow passage, and exhaust gas is only allowed to pass through the flow-back passage when a low exhaust flow rate is generated by the engine. 
     In another embodiment of the present invention, a DPF system is provided with a combination of technologies disclosed in the above embodiments of the present invention. With the calculated exhaust flow rate and PM amount values, DPF regenerations can be triggered and controlled without having engine operating parameters, while a bootstrapping process can be used in heating control when exhaust gas temperature is low. Through the flow-back passage, exhaust flow rate through the DPF is increased, thereby PM distribution in the DPF is more uniform at low exhaust flow rates, and the risk of thermal runaways is decreased. Such a system can be used for applications with low exhaust flow rate and low exhaust gas temperature, e.g. in vehicles that frequently stop and go, and retrofit applications in which engine operating parameters are not available. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an exhaust gas processing system of an internal combustion engine including an exhaust gas passage device, a heating device and a diesel particulate filter; 
         FIG. 2  is a flow chart of a service routine running periodically for a timer based interrupt for calculating an amount of PM deposited in a diesel particulate filter; 
         FIG. 3  is a schematic representation of an exhaust gas processing system of an internal combustion engine including an exhaust gas passage device, a two-stage bootstrapping heating device and a diesel particulate filter; 
         FIG. 4 a    is a flow chart of a service routine running periodically for a timer based interrupt for generating dosing fuel commands in regenerating a diesel particulate filter in a system of  FIG. 3 ; 
         FIG. 4 b    shows a block diagram of a feedback control in controlling dosing fuel flow-rate in regenerating a diesel particulate filter; 
         FIG. 5 a    is a schematic representation of an exhaust gas processing system of an internal combustion engine including an exhaust gas passage device, a heating device positioned downstream from the exhaust gas passage device, a diesel particulate filter, and a flow-back passage. 
         FIG. 5 b    is a schematic representation of an exhaust gas processing system of an internal combustion engine including an exhaust gas passage device, a heating device positioned upstream from the exhaust gas passage device, a diesel particulate filter, and a flow-back passage. 
         FIG. 5 c    is a schematic representation of an exhaust gas processing system of an internal combustion engine including an exhaust gas passage device, a two-stage bootstrapping heating device, a diesel particulate filter, and a flow-back passage. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an exhaust passage  102  is fluidly connected to an exhaust passage  104  with a smaller diameter through a cone transition  153 . The exhaust passage  104  is fluidly connected to a DPF package  130  through a cone transition  152 . Inside the DPF package  130 , a heating device  110  is positioned upstream from a DPF  120 . On the exhaust passage  102 , through a probe  108 , a pressure sensor  106 , which communicates to a controller  140  through signal lines  142 , is used to detect an exhaust gas pressure in the exhaust passage  102 , and a differential pressure sensor  107 , which is electrically connected to the controller  140  via signal lines  143 , is used for measuring a difference between the pressure in the exhaust pipe  102  and that in the exhaust passage  104  through a probe  105  and the probe  108 . The exhaust temperature in the exhaust passage  102 , in between the heating device  110  and the DPF  120 , and downstream from the DPF  120 , are sensed, respectively, by temperature sensors  109 ,  111 , and  116 . The temperature sensor  109  communicates with the controller  140  through signal lines  144 , while the temperature sensor  111  is electrically connected to the controller  140  through signal lines  146 . The heating device  110  is controlled by the controller  140  via signal lines  145 , and the temperature sensor  116  communicates with the controller  140  through signal lines  149 . The pressure drop across the DPF  120  is detected by a differential pressure sensor  113  communicating with the controller  140  via signal lines  147 , while the differential pressure sensor  113  is fluidly connected to the DPF package  130  in between the heating device  110  and the DPF  120  through a probe  112 , and fluidly connected to the DPF package  130  downstream from the DPF  120  through a probe  115 . The DPF package  130  is fluidly connected to a tailpipe  119  through a transition  151 . 
     In the system of  FIG. 1 , the exhaust passage  104  and the cone transitions  153  and  152  form a Venturi structure, thereby, the volume matric flow rate Q of an exhaust air flow passing through the heating device  110  and the DPF can be detected using a differential pressure value ΔP 2  obtained from the differential pressure sensor  107 , a pressure value P 106  obtained from the pressure sensor  106 , and a temperature value T 109  provided by the temperature sensor  109 , according to the following equation: 
                     Q   =       K   Q     ⁢         Δ   ⁢           ⁢     P   2     ⁢     T   109         P   106             ,           (   1   )               
where K Q  is a constant and can be calculated using the following equation
 
                       K   Q     =           2   ⁢   R       ⁢     C   d     ⁢     A   1     ⁢     A   2             A   1   2     -     A   2   2             ,           (   2   )               
where R is the specific gas constant; C d  is the discharge coefficient; A 1  is the cross section area of the exhaust passage  102 , and A 2  is the cross section area of the exhaust passage  104 . And the mass flow rate m ƒ  of the exhaust flow can be calculated using the following equation:
 
                       m   f     =       K   m     ⁢         Δ   ⁢           ⁢     P   2     ⁢     P   106         T   109             ,           (   3   )               
where K m  is a constant and can be calculated using the equation:
 
     
       
         
           
             
               
                 
                   
                     K 
                     m 
                   
                   = 
                   
                     
                       
                         
                           2 
                         
                         ⁢ 
                         
                           C 
                           d 
                         
                         ⁢ 
                         
                           A 
                           1 
                         
                         ⁢ 
                         
                           A 
                           2 
                         
                       
                       
                         
                           R 
                           ⁡ 
                           
                             ( 
                             
                               
                                 A 
                                 1 
                                 2 
                               
                               - 
                               
                                 A 
                                 2 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In addition to exhaust gas flow rate, sensing values obtained from the sensors  106 ,  107 ,  109 , and  113  can be further used for detecting PM load in the DPF  120 . With a differential pressure sensing value ΔP 1  obtained from the differential pressure sensor  113 , at steady states, we have a relationship described with the following equations: 
                         Δ   ⁢           ⁢     P   1         Δ   ⁢           ⁢     P   2         =         f   ⁡     (     w   p     )       ⁢   Y     +     C   0         ,     
     ⁢   and           (   5   )                 Y   =         K   2     ⁢     T   109   2           (       T   109     +     C   exh       )     ⁢       Δ   ⁢           ⁢     P   2     ⁢     P   106               ,           (   6   )               
where ƒ(w p ) is a function of a particulate layer thickness w p ; C exh  is the Sutherland&#39;s constant for exhaust gas, and C 0  is a constant determined by the DPF volume V trap , the K Q  value and a constant pressure drop coefficient ξ:
 
                       C   0     =       ξ   ⁢           ⁢     K   Q   2         V   trap   2         ;           (   7   )               
K 2  is a constant, and
 
 K   2   =λK   Q   √{square root over (R)}   (8),
 
wherein λ is a constant determined by the Sutherland&#39;s constant. The function ƒ(w p ) can be a linear function:
 
ƒ( w   p )= C   1   +C   2   w   p   (9),
 
where C 1  and C 2  are constants. In applications where a PM mass load m p  is used for triggering regeneration processes, the PM mass load can be approximated linearly with the function ƒ(w p ):
 
 m   p   =C   3   +C   4 ƒ( w   p )  (10),
 
where C 3  and C 4  are constants.
 
     In the controller  140 , the PM mass load m p  can be calculated with a service routine running periodically for a timer based interrupt, as shown in  FIG. 2 . In the routine, a regeneration status is firstly checked. If the system is in a regeneration process, then the routine ends. Otherwise, a changing rate of the ΔP 1  value, d(ΔP 1 )/dt, is compared with a threshold DP_THD. If it is higher than or equal to the threshold, i.e., the differential pressure sensor  113  is in transient, the routine ends, otherwise, the Y value and 
               Δ   ⁢           ⁢     P   1         Δ   ⁢           ⁢     P   2             
value are calculated according to equations (6) and (5), and are assigned to the i-th element of vectors F and E, F(i) and E(i), respectively. The i value is then incremented and compared to a threshold NUM_THD. The routine ends if it is lower than the threshold, otherwise, the i value is reset to 0 and ƒ(w p ) and m p  values are calculated with the vectors E and F, and the routine ends thereafter. In the routine, the exclusion of transient values eliminates effects of mismatch of sensing values to the calculation of Y and
 
               Δ   ⁢           ⁢     P   1         Δ   ⁢           ⁢     P   2             
values caused by difference in sensor response time. And a variety of methods, including least squares methods, can be used in calculating the ƒ(w p ) and m p  values.
 
     In the system of  FIG. 1 , the heating device  110  is used for heating exhaust gas in regenerating the DPF  120 . A variety of heating elements, including electrical heaters, fuel burners, and DOCs can be used in the heating device  110 . With the help of the Venturi structure formed by the exhaust passage  104  and the cone transitions  153  and  152 , a two-stage bootstrapping heating device can be used in regenerating the DPF  120  with low temperature exhaust gas. Referring to  FIG. 3 , in such as system, a heating device for regenerating the DPF  120  includes a temperature sensor  308 , a fuel injector  300 , an electrical heater  305 , a front DOC  310  and a main DOC  315 . The temperature sensor  308 , the fuel injector  300  and the electrical heater  305  are in communication with the controller  140  through signal lines  345 . And the fuel injector  300  is mounted on a connection pipe  306  fluidly connected to the exhaust passage  102  and the electrical heater  305 . Upstream from the fuel injector  300 , the temperature sensor  308  is positioned on the exhaust passage  102 , while the front DOC  310  is positioned downstream from the electrical heater  305 . A connection pipe  307  fluidly connects the front DOC  310  to the exhaust passage  104 , and downstream from it, the main DOC  315  is positioned in between the temperature sensors  109  and  111 . 
     In the system of  FIG. 3 , after a DPF regeneration process starts, when exhaust gas temperature is low, the electrical heater  305  is energized on, and a bootstrapping dosing rate is generated through the injector  300 . Through the electrical heater  305 , the exhaust gas and dosing fuel are heated to a temperature higher than the light-temperature of the front DOC  310 , where the dosing fuel is oxidized and the DOC and the exhaust gas are exothermically heated. The heated exhaust gas passes through the connection pipe  307  and mixes with the exhaust gas in the pipe  104 . And the result exhaust gas then enters the main DOC  315  and heats it. When the bed temperature in the DOC  315  is higher than its light-off temperature, a normal dosing rate is generated through the injector  300 , and the electrical heater  305  is de-energized off. The bootstrapping process then completes. In the bootstrapping process, the bootstrapping dosing rate is lower than the normal dosing rate, and dosing fuel can be fully oxidized in the front DOC  310 , while after the bootstrapping process completes, not all dosing fuel can be burned in the front DOC  310 , and the unburnt fuel is further oxidized in the main DOC  315  and the DPF  120 . The exhaust flow rate through the electrical heater  305  is only a fraction of that in the exhaust passage  102 . Therefore, electrical energy needed in heating the exhaust gas is significantly decreased. 
     The control of the bootstrapping process can be realized with a service routine running periodically for a timer based interrupt. Referring to  FIG. 4 a   , in such a routine, a regeneration status is examined first. The routine ends if the system is not in a regeneration process. Otherwise, a bed temperature of the main DOC  315 , T DOC , is compared to a threshold LF_THD. If it is lower than the threshold, then a power value P_btstrap is set to a variable P_eh, which is used to control the power applied on the electrical heater  305 , and a fuel dosing rate Dc_btstrap is set to a variable Dc controlling the fuel dosing rate through the injector  300 . The routine ends thereafter. If the T DOC  value is not lower than the threshold LF_THD, then the routine ends after the variable P_eh is set to zero, and the variable Dc is set to a normal dosing value of Dc_normal. 
     In the routine of  FIG. 4 a   , the T DOC  can be calculated using a linear combination of temperature sensing values T 109  and T 111  obtained from the temperature sensors  109  and  111  respectively:
 
 T   DOC   =W   1   *T   109   +W   2   *T   111   (11),
 
where W 1  and W 2  are constants. And the P_btstrap value can be calculated using a function of the calculated exhaust mass flow rate m ƒ  and a sensing value T 308  obtained from the temperature sensor  308 :
 
 P _btstrap= g ( m   ƒ   ,T   308 )  (12),
 
where g( ) is a function that can be realized with a lookup table with inputs of the m ƒ  and T 308  values. The bootstrapping dosing rate can also be determined by the temperature T 308  and the calculated exhaust mass flow rate m ƒ :
 
 Dc _btstrap= h ( m   ƒ   ,T   308 ),
 
where h( ) is also a function that can be realized with a lookup table. Both of the lookup tables for calculating the P_btstrap and Dc_btstrap values can be populated with experimental results obtained with different exhaust temperatures and flow rates.
 
     In calculating the normal dosing rate Dc_normal, a PID control can be used with a temperature sensing value T 111  obtained from the temperature sensor  111  in its feedback loop. An exemplary control scheme is depicted in  FIG. 4 b   . In this control, a target temperature value T trgt  is calculated in a block  330  with a temperature sensing value T 116  obtained from the temperature sensor  116  and the calculated particulate load value m p . Then a control error value is calculated by subtracting the T trgt  value with the T 111  value, and the T trgt  value is further used in a block  335  for calculating a feed-forward control value together with the T 308  value and the m ƒ  value. The control error value together with the m ƒ  value is used in a PID control block  340  for calculating a feedback control value, which is then added to the feed-forward control value, and the result value is passed through a limit block  345 , where the normal dosing rate Dc_normal is generated after dosing rate limits being applied with the T 109  value and an air-to-fuel ratio value λ in engine control. 
     In the system of  FIG. 1 , the Venturi structure can also be used for increasing exhaust flow in regenerating the DPF  120 . Referring to  FIG. 5 a   , a connection pipe  118  is fluidly connected to the exhaust passage  104  and a control valve  117 , which is also fluidly connected to the tailpipe  119  through another connection pipe  131 . The control valve  117  is controlled by the controller  140  through signal lines  151 , and the exhaust gas flow in the connection pipes  118  and  131  is controlled by energizing and de-energizing the control valve  117 . In a DPF regeneration process, when the calculated exhaust flow rate m ƒ  is low, the control valve  117  is energized open. Under a pressure in between the tailpipe  119  and the exhaust passage  104 , exhaust gas flows back to the exhaust passage  104 , resulting in a higher exhaust flow rate passing through the heating device  110  and the DPF  120 . High exhaust flow rate brings more heat energy to the DPF, thereby PM mal-distribution is reduced, and the limit of exhaust flow rate can be lowered to allow more regeneration chances. Furthermore, higher exhaust flow rate also decreases system response time in temperature control, resulting in better control performance. 
     High exhaust flow rate in the system of  FIG. 5 a    also decreases resident time of exhaust flow in the heating device  110 . If a DOC is used in the heating device  110 , low resident time may lower HC conversion efficiency. In the system of  FIG. 5 a   , low HC conversion efficiency can be avoided by de-energizing the control valve  117  when the exhaust flow rate m ƒ  is too high. Another way to avoid low HC conversion efficiency is positioning the heating device  110  upstream from the Venturi structure. Referring to  FIG. 5 b   , in such as system, a heating device  210  is fluidly connected to the cone transition  153  upstream from the exhaust passage  104 . Upstream from the heating device  210  is a connection pipe  233 , on which a temperature sensor  220  and a pressure sensing probe  204  are mounted. The connection pipe  233  is fluidly connected to an exhaust passage  232  with a smaller diameter through a cone transition  231 , and another pressure sensing probe  201  is mounted on the exhaust passage  232 . The pressure sensing probes  201  and  204  are fluidly connected to a differential pressure sensor  202 , which is in communication with the controller  140  through signal lines  221 , while a pressure sensor  203 , which communicates to the controller  140  through signals lines  222 , is fluidly connected to the pressure sensing probe  204 . The temperature sensor  220  is electrically connected to the controller  140  through signal lines  223 , and the heating device  210  is controlled by the controller  140  through signal lines  245 . In this system, since high exhaust flow rate in the DPF  120  does not affect resident time in the heater  210 , the control valve  117  is not required to shut off the exhaust flow-back path at high exhaust flow rates. 
     Referring to  FIG. 3  and  FIG. 5 b   , the heating device in the system of  FIG. 3  and the flow back device in the system of  FIG. 5 b    can be used together for increasing regeneration performance with low temperature and low flow rate exhaust gas, which is normally generated by an engine operated at low torque modes, such as in idling. Referring to  FIG. 5 c   , in such as system, a main DOC  515  is positioned in between the temperature sensors  220  and the cone transition  153 , and a connection pipe  507  fluidly connects the exhaust passage  232  to a front DOC  510 , which has an electrical heater  505  positioned upstream. The exhaust passage  232  is fluidly connected to an exhaust passage  502 , which has a larger diameter, and the electrical heater  505  is fluidly connected to the exhaust passage  502  through a connection pipe  506 , which has a fuel injector  500  mounted. Upstream from the connection pipe  506 , a temperature sensor  508  is mounted on the exhaust passage  502 , while the temperature sensor  508 , the injector  500 , and the electrical heater  505  are in communication with the controller  140  through signal lines  545 . A control scheme of  FIG. 4 b    and a control algorithm of  FIG. 4 a    can be used for controlling the electrical heater  505  and the fuel injector  500 . 
     While the present invention has been depicted and described with reference to only a limited number of particular preferred embodiments, as will be understood by those of skill in the art, changes, modifications, and equivalents in form and function may be made to the invention without departing from the essential characteristics thereof. Accordingly, the invention is intended to be only limited by the spirit and scope as defined in the appended claims, giving full cognizance to equivalents in all respects.