Patent Publication Number: US-2009217649-A1

Title: Exhaust system for a motor vehicle and process for regenerating a particulate filter in an automotive exhaust system

Description:
RELATED APPLICATIONS 
     This application is a U.S. National Phase application of PCT Application No. PCT/EP2006/011503 filed Nov. 30, 2006 which claims priority to German Application No. 10 2005 062 924.5 filed Dec. 29, 2005. 
    
    
     TECHNICAL FIELD 
     This invention relates to an exhaust system for a motor vehicle with a particulate filter. Furthermore, this invention relates to a process for regenerating a particulate filter in an automotive exhaust system. 
     BACKGROUND OF THE INVENTION 
     To comply with environmental regulations, the exhaust gases of motor vehicles driven by internal combustion engines must be subjected to cleaning. For reducing the particulate emissions of the exhaust gases of motor vehicles, which are driven by a Diesel engine or a lean-burning gasoline engine, suitable particulate filters are used. Such particulate filters must be regenerated from time to time by burning off the particulates accumulated on the filter surface. For this purpose, a burner is arranged upstream of the particulate filter for example, to generate the heat required for burning off by combustion of an air-fuel mixture. For igniting the air-fuel mixture, a glow plug can be used such as that known from DE 298 02 226 U1. In connection with a burner, it is also known from DE 42 42 991 A1 to use a glow plug for introducing energy into the liquid fuel. 
     Another known arrangement for regenerating a particulate filter disposes an oxidation catalyst upstream of the particulate filter, which generates the heat required for burning off the soot particulates by oxidizing an oxidizable substance present in the exhaust-gas stream. From DE 102 56 769 B4, for instance, a system is known, in which upstream of the oxidation catalyst an evaporation unit is disposed, in which the fuel is evaporated and introduced into the exhaust-gas stream. 
     In practice, however, the systems known from the prior art involve numerous difficulties, which are due to a multitude of partly contradictory system requirements. 
     For instance, the time of regeneration depends on the loading condition, i.e. the “degree of filling” of the particulate filter. If this time is chosen too early, not enough soot is present to perform a stable regeneration. If it is chosen too late, however, the particulate filter is clogged, or the combustion of soot produces very high temperatures in the particulate filter, which can lead to its destruction. 
     If the exhaust gas temperature before the oxidation catalyst is too low, the oxidizable vapour supplied cannot be converted thermally. It is condensed in the oxidation catalyst and leads to its destruction. 
     If too much fluid is introduced into the evaporation unit, and if the fluid cannot evaporate sufficiently, it enters the exhaust system in the liquid condition. If the fluid entering the exhaust system cannot sufficiently be reevaporated by the hot exhaust gases and the hot tube walls, the downstream oxidation catalyst is damaged. 
     If too much fluid is evaporated, too much energy is generated by the catalyst and the particulate filter is damaged by excessive regeneration temperatures. At the same time, fluid consumption is rising unnecessarily. 
     If too little fluid is introduced into the evaporator, the catalyst cannot produce the temperature increase of the exhaust gas necessary for the regeneration of the particulate filter. There is no regeneration of the particulate filter, but an unnecessary fluid consumption. 
     If a heating element provided in the evaporation unit is switched on too soon, power consumption rises unnecessarily. On the other hand, if the heating element is put into operation too late, the oxidizable fluid is not sufficiently evaporated, partly reaches the exhaust system in the liquid condition, and damages the oxidation catalyst. The postheating time of the heating element also determines the proper conversion of the fluid into vapour. 
     In addition, there are further influential factors which depend on the operating point of the engine, a possibly present exhaust gas turbocharger, components for exhaust gas recirculation, and many more. These factors also influence the regeneration of the particulate filter and must be considered by the regeneration system. 
     It is the object underlying the invention to solve the described technical contradictions and make the regeneration of a particulate filter safe and suitable for series production. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided an exhaust system for a motor vehicle, comprising a particulate filter upstream of which an oxidation catalyst is provided, and a regeneration device for the particulate filter. The regeneration device includes an evaporation unit for introducing a vapour generated from an oxidizable fluid into the exhaust gas stream before the oxidation catalyst. The evaporation unit includes a heating element arranged in a housing and a fluid supply with a controllable fluid pump. A control device is provided for controlling the fluid pump. By suitable control of the fluid pump, in particular in dependence on the temperatures existing at various points of the exhaust system, the difficulties known from the prior art can be solved satisfactorily. 
     In particular, the oxidizable fluid can be the same fuel which is also supplied to the internal combustion engine, whereby an additional fluid supply can be omitted. The fuel simply is withdrawn from the fuel tank of the vehicle or from the fuel return conduit. 
     The heating element advantageously is a glow plug, i.e. a mass-produced article available at low cost. 
     In accordance with a preferred embodiment, the control device is connected with the engine control or integrated in the same. In particular, the control device utilizes data present in the engine control in order to consider the same in the control of the fluid pump. 
     The object of the invention is also solved by a process for regenerating a particulate filter with upstream oxidation catalyst in an automotive exhaust system with a regeneration device, which comprises an evaporation unit for introducing a vapour generated from an oxidizable fluid into the exhaust gas stream before the oxidation catalyst. The evaporation unit includes a heating element arranged in a housing and a fluid supply with a controllable fluid pump. The process includes the following steps, which are performed periodically: The regeneration process first is started in dependence on the back pressure of the particulate filter or on the time elapsed since the last regeneration process (step a). Subsequently, the heating element is switched on, as soon as the temperature upstream of the oxidation catalyst exceeds a specified minimum value (step b). After waiting a specified preheating time for the heating element (step c), the fluid pump is switched on with a specified delivery rate (step d), and a specified pumping period is allowed to pass (step e). Subsequently, the fluid pump is operated according to specified parameters, if downstream of the oxidation catalyst a higher temperature exists than upstream of the oxidation catalyst (step f). Thereupon, a likewise specified regeneration period is allowed to pass, which starts as soon as the temperature downstream of the oxidation catalyst has exceeded a specified minimum value, wherein during the regeneration period the temperature downstream of the particulate filter is checked periodically and possibly controlled at least by influencing the introduced fuel quantity (step g). After the regeneration period has passed, the fluid pump is switched off (step h), a specified postheating time of the heating element is allowed to pass (step i), and finally the heating element is switched off (step j). Subsequently, the process starts again. Thus, the process of the invention not only makes sure that the regeneration is started at the proper time (step a) and the exhaust gas temperature before the oxidation catalyst is high enough (step b), but due to the pre-heating and postheating times, also ensures a safe evaporation of the oxidizable fluid. A precisely adapted metering of fluid, which prevents the particulate filter from being damaged or even destroyed, is achieved by the temperature-dependent control (step g). Thus, the process of the invention meets all requirements mentioned above. 
     To prevent the system from being damaged, an error can be registered, if after waiting for the specified pumping period, the temperature downstream of the oxidation catalyst is not higher than the temperature upstream of the oxidation catalyst. 
     Preferably, the process proceeds to step d) after registering the error, as long as the number of registered errors does not exceed a specified maximum value. 
     Upon exceeding the specified maximum value for registered errors, the regeneration process should be stopped and an error signal should be issued. This can include, for instance, switching on an error signal lamp, which informs the driver of the motor vehicle that repair is necessary. 
     To prevent the particulate filter from being damaged by excessive regeneration temperatures, the fluid pump is switched off in connection with the temperature control during the regeneration period, as soon as the temperature downstream of the particulate filter exceeds a specified first value during the regeneration period (step g). 
     In accordance with a first embodiment of the invention, operation of the fluid pump according to specified parameters is resumed after switching off the fluid pump during the regeneration period, as long as the specified regeneration period is not terminated and as soon as one of the following conditions occurs, which are checked periodically in the indicated order: 
     the temperature downstream of the particulate filter lies below a specified second value, 
     the temperature downstream of the oxidation catalyst falls below a specified minimum value, 
     the temperature downstream of the particulate filter no longer lies above the specified first value. 
     In this way, it is prevented that an insufficient fluid quantity is supplied to the exhaust gas stream during the regeneration period, which would impair the regeneration. 
     In accordance with a second embodiment of the invention, a proportional-integral-derivative (PID) controller is used for controlling the temperature during the regeneration period (step g), if the temperature downstream of the oxidation catalyst lies within a specified control interval. Such PID controller offers the advantage of a faster temperature control than a control by merely varying the introduced fuel quantity. As a control parameter, the temperature downstream of the oxidation catalyst, i.e directly upstream of the particulate filter, is used. 
     To prevent an “overshooting” of the PID controller caused by system-related oscillation processes, it should be checked after each control operation of the PID controller whether the temperature downstream of the oxidation catalyst still lies within the specified control interval. 
     In the variant with the PID control, after switching off the fluid pump during the regeneration period, the operation of the fluid pump according to specified parameters is resumed, if the temperature downstream of the particulate filter lies below a specified second value and the temperature downstream of the oxidation catalyst lies outside the specified control interval for the PID controller. This is of course only applicable for the time period of regeneration. 
     In addition, the current flowing through the heating element can be monitored during operation of the heating element. 
     It should be appreciated that the regeneration process only is started with running engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Further features and advantages of the invention can be taken from the following description of several preferred embodiments with reference to the attached drawing, in which: 
         FIG. 1  shows a schematic representation of an exhaust system in accordance with the invention; 
         FIG. 2   a  shows a flow diagram of a first part of the process of the invention; 
         FIG. 2   b  shows a flow diagram of a second part of the process of the invention directly adjoining the first part in accordance with a first variant; 
         FIG. 2   c  shows a flow diagram of a third part of the process of the invention, which directly adjoins the second part; and 
         FIG. 3  shows a flow diagram of an alternative second part of the process of the invention, which can replace the part shown in  FIG. 2   b.    
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  schematically shows an internal combustion engine  10  of a motor vehicle and a downstream exhaust system  12 . In particular, the combustion engine  10  is a Diesel engine. The exhaust system  12  includes an exhaust gas conduit  14 , which leads to a particulate filter  16  with upstream oxidation catalyst  18 . Upstream of the oxidation catalyst  18 , an evaporation unit  20  is provided, which includes a heating element  24 , here in the form of a glow plug, which is arranged in a housing  22 , and a fluid supply  26  with a controllable fluid pump  28 . In particular, the fluid supply  26  is a fuel conduit, and the fluid pump  28  is a fuel pump with a connection to the fuel tank of the vehicle (not shown). The fuel can also be taken from the fuel return conduit; in this case, the fuel already is preheated. 
     The evaporation unit  20  forms part of a regeneration device for the particulate filter  16 , which furthermore comprises a control device  30  for controlling the fluid pump  28 . The heating element  24  is connected with the control device  30  and can be driven by the same. The control device  30  in turn is connected with the engine control  32  or, alternatively, directly integrated in the same. 
     The exhaust system  12  furthermore includes a plurality of temperature sensors  34 , which likewise are connected with the control device  30  and determine the temperature before and after the oxidation catalyst  18  and the temperature after the particulate filter  16 . Furthermore, pressure sensors (not shown) can be provided to determine the back pressure of the particulate filter  16 . 
     For regenerating the particulate filter  16 , a process is employed, which will be described below with reference to  FIGS. 2   a  to  2   c.    
     After the start of the process (step  100 ), it is first checked in step  101  whether the internal combustion engine  10  is running. If this is not the case, there is no further activity; the process starts again. If the engine is running, the current back pressure value p DPF  of the particulate filter  16  is determined in the next step  102  by using the pressure sensors, and it is checked whether this current back pressure value exceeds a specified limit value p reg  for the regeneration. If this is the case, a regeneration requirement is detected (step  104 ). However, if the current back pressure value p DPF  lies below the specified limit value p reg , it is checked in step  103  whether the time elapsed since the last regeneration process (also referred to as “loading time” of the particulate filter) exceeds a specified limit value. If this is the case, the process likewise continues with step  104 , otherwise the process goes back to step  102 . 
     After the regeneration requirement has been detected, it is checked whether the temperature upstream of the oxidation catalyst  18 , T before     —     DOC , exceeds a specified minimum value T light-off  (step  105 ). If this is the case, the heating element (the glow plug in the present embodiment) is switched on in the next step  106 . If the temperature before the oxidation catalyst lies below the minimum value T light-off , the process goes back from step  105  to step  104 . 
     After switching on the heating element  24 , a specified preheating time is allowed to pass, in that in step  107  a count value for the preheating time first is incremented, and in step  108  it is checked whether the value of the counter for the preheating time exceeds a specified value. If this is not the case, steps  107  and  108  are repeated, until the value for the preheating time finally exceeds the specified value. Subsequently, the counter for the preheating time is reset (step  109 ), and the fluid pump  28  is switched on with a specified delivery rate (step  110 ). The delivery rate can for instance be adjusted via a pumping frequency. 
     After switching on the fluid pump (see  FIG. 2   b ), a specified pumping period is allowed to pass, in that a count value for the time to the measurement of a temperature increase after the oxidation catalyst  18  (alternatively also after the particulate filter  16 ) is incremented (step  111 ), and subsequently it is checked whether the count value for the time to the temperature measurement already exceeds a specified value (step  112 ). Steps  111  and  112  also are repeated, until the specified pumping period, which corresponds to the specified waiting period to the temperature measurement, is achieved. 
     Subsequently, the count value for the time to the temperature measurement is set to zero (step  113 ), and it is checked whether the temperature after the oxidation catalyst  18 , T after     —     DOC , is greater than the temperature before the oxidation catalyst, T before     —     DOC , as expected (step  114 ). If this is the case, the fluid pump  28  is operated according to specified parameters (step  120 ). 
     Otherwise, an error is registered, in that the value of an error counter is incremented by 1 (step  115 ), whereupon it is checked whether the error count value already is greater than a specified maximum value for registered errors (step  116 ). If this is not the case, the process is resumed with step  110 , namely switching on the pump with a specified delivery rate. However, if the error count value already exceeds the specified maximum value, the regeneration process is stopped, in that first the fluid pump (step  117 ) and then the heating element  24  is switched off (step  118 ). To inform the owner of the vehicle that repair or a system check is necessary, an error signal lamp finally is switched on (step  119 ), and the regeneration process ends with step  120 , so as not to be resumed again until after a possible repair. 
     If, as expected, a higher temperature exists downstream of the oxidation catalyst  18  than upstream of the oxidation catalyst, the fluid pump  28  is operated according to specified parameters (step  121 ), as already mentioned. Subsequently, it is checked whether the temperature after the oxidation catalyst  18 , T after     —     DOC , has exceeded a specified minimum value T reg     —     min  necessary for a successful regeneration (step  122 ). If this is the case, a count value for the regeneration period, which has a specified positive value (i.e. different from zero), is reduced (step  123 ). In the following step  124  it is checked whether the count value for the regeneration period is zero, i.e. the specified regeneration period has already been reached. However, if the temperature downstream of the oxidation catalyst  18  has not yet reached the specified minimum value T reg     —     min , the fluid pump  28  is operated further, wherein the parameters for pump operation can be varied. 
     As long as the end of the regeneration period has not yet been reached, it is checked subsequent to step  124  whether the temperature downstream of the particulate filter, T after     —     DPF , exceeds a specified first value T max . If this is not the case, i.e. if there is no risk that the particulate filter  16  becomes too hot, the process will thereupon be performed starting with step  121 , until the regeneration period is terminated, which is detected in step  124 . 
     On the other hand, if the temperature after the particulate filter  16  exceeds the specified temperature T max , the fluid pump  28  is switched off (step  126 ), in order to thus decrease the temperature existing after the particulate filter  16  (and also in the same). Subsequently, it is checked in step  127  whether the temperature after the particulate filter, T after     —     DPF , lies below a specified second value T continue , up to which a further supply of oxidizable fluid to the evaporation unit  20  is not critical. Thus, if the temperature after the particulate filter lies below T continue , the operation of the fluid pump  28  is resumed according to specified parameters, and the regeneration process is continued with step  121 , until the regeneration period has elapsed (step  124 ). Steps  121  to  124  and possibly also  125  to  127  are performed repeatedly. 
     If the temperature after the particulate filter does not lie below the specified second value T continue , the process proceeds from step  127  to step  122 , in which the temperature after the oxidation catalyst  18  is compared with the minimum value T reg     —     min  required for regeneration, without the operation of the pump being resumed. In this case, pump operation thus is not resumed before the time when in the repeated process step  125  the temperature after the particulate filter, T after     —     PDF , has decreased below the first specified value T max , whereupon the process goes to step  121 . 
     As soon it is detected in step  124  that the specified regeneration period is terminated, the count value for the regeneration period is set to a specified value, which is stored in the control device  30  (step  128 ), and the fluid pump  28  is switched off (step  129 , see  FIG. 2   c ). Subsequently, a specified postheating time of the heating element  24  is allowed to pass, in that in process step  130  a count value for the postheating time is incremented and subsequently compared with a specified value (step  131 ). As long as the count value for the postheating time does not exceed the specified value, steps  130  and  131  are performed again and again. As soon as the specified postheating time has been reached, the heating element  24 , here the glow plug, is switched off (step  132 ), and the count value for the postheating time is set to zero (step  133 ). 
     Subsequently, the counter for the time elapsed since the last regeneration process (also referred to as “loading time” of the particulate filter  16 ) is set back to zero (step  134 ), and the process goes back to the start (step  100 ). In this way, a discontinuous, periodic regeneration of the particulate filter  16  is achieved. 
       FIG. 3  shows the middle part of a process for regenerating the particulate filter  16  in accordance with a second embodiment of the invention, which differs from the above-described process of  FIGS. 2   a  to  2   c  merely in the type of temperature control during the regeneration period. The first part of the process not shown in  FIG. 3  corresponds to  FIG. 2   a,  the last part corresponds to  FIG. 2   c.  Thus, the process part as shown in  FIG. 3  merely replaces the part as shown in  FIG. 2   b.    
     The process in accordance with the second variant including step  121  proceeds analogous to the process described above. In the succeeding step  222 , it is likewise checked whether the temperature after the oxidation catalyst  18  exceeds the specified minimum value for regeneration, T reg     —     min . If this is the case, the count value for the regeneration period is reduced in the next step  223 , and it is subsequently (step  224 ) checked whether the count value for the regeneration period is zero, i.e. the regeneration period already is terminated. 
     In contrast to the process in accordance with the first embodiment, step  224  of the process is performed, if the temperature after the oxidation catalyst does not reach the minimum temperature T reg     —     min . As long as the specified regeneration period is not terminated, it is subsequently checked whether the temperature after the particulate filter  16  exceeds the specified first value T max . If this is the case, the fluid pump  28  is switched off again (step  226 ), and it is subsequently checked whether the temperature after the particulate filter lies below a second value T continue  (step  227 ). If this is the case, or if it is detected in step  225  that the temperature after the particulate filter does not exceed the specified first temperature value T max , it is subsequently checked in step  228  whether the temperature downstream of the oxidation catalyst  18  lies within a specified control interval, namely between the specified values T (look-up→PID, low) and T (look-up→PID, high). However, if the temperature after the particulate filter is not smaller than T continue , the process is continued with step  222 . 
     If the temperature after the oxidation catalyst lies within the specified control interval, a PID controller, which can be integrated in the control device  30 , is used for controlling the temperature (step  229 ), so as to bring the same to an optimum temperature value for regeneration. The PID controller offers the advantage that the desired temperature can be adjusted much faster than would be possible by merely switching on and off the fluid pump  28 . After the control operation  229 , it is checked in step  230  whether the temperature downstream of the oxidation catalyst  18  now possibly lies outside the specified control interval, i.e. whether the PID controller has controlled too much in the one or other direction. If this is the case, the process goes back to step  121 ; however, if the temperature after the oxidation catalyst still lies within the control interval, the process continues with step  231 , which corresponds to step  222 , and checks whether the temperature after the oxidation catalyst  18  lies above the specified minimum temperature for regeneration T reg     —     min . If this is the case, the count value for the regeneration period is reduced, and it is subsequently checked whether this count value is zero (steps  232 ,  233 , which correspond to steps  223  and  224 ). However, if the temperature after the oxidation catalyst lies below the required minimum temperature for regeneration, T reg     —     min , the process directly proceeds from step  231  to step  233  without reducing the count value for the regeneration period. 
     As long as the regeneration period is not terminated, the temperature after the particulate filter  16  subsequently is checked, as to whether it exceeds the specified first temperature value T max  (step  234 ). If this is not the case, the process continues with step  229 , namely the control operation by the PID controller; otherwise, the fluid pump  28  is switched off (step  235 ), and it is checked whether the temperature after the particulate filter  16  lies below the second specified value T continue  (step  236 ). If the temperature after the particulate filter  16  is smaller than T continue , there is likewise effected a control operation by the PID controller (step  229 ); however, if the temperature after the particulate filter  16  exceeds the temperature T continue , the process continues with step  231 , i.e. checks whether the temperature after the oxidation catalyst exceeds the specified minimum value T reg     —     min . 
     Thus, in the process in accordance with the second embodiment, the fluid pump  28  likewise is always switched off, as soon as the temperature downstream of the particulate filter  16  exceeds a specified first value T max  during the regeneration period. In contrast to the process in accordance with the first embodiment, the operation of the fluid pump  28  subsequently is resumed according to specified parameters, if the temperature downstream of the particulate filter  16  lies below the specified second value T continue  and the temperature downstream of the oxidation catalyst  18  lies outside the specified control interval for the PID controller. 
     Upon termination of the regeneration period, which is detected in step  233  or  224 , the counter for the regeneration period is set equal to a value specified in the control device (step  128 ), and the process for regeneration is terminated, as described already with reference to  FIG. 2   c.    
     Finally, it should be noted that none of the specified values stored in the control device  30  must be universally applicable individual values, but for each specified value a list of values can exist, from which depending on the current operating condition (current data from the engine control, currently existing temperatures at different points of the exhaust system  12 , and further parameters such as exhaust gas mass flow, etc.) the specified value corresponding to this operating condition or most suitable for this operating condition is selected. 
     LIST OF REFERENCE NUMERALS 
       10  internal combustion engine 
       12  exhaust system 
       14  exhaust gas conduit 
       16  particulate filter 
       18  oxidation catalyst 
       20  evaporation unit 
       22  housing 
       24  heating element 
       26  fluid supply 
       28  fluid pump 
       30  control device 
       32  engine control 
       34  temperature sensors 
       100 - 134  process steps 
       222 - 236  process steps