Abstract:
A method for regenerating a diesel particle filter ( 18 ) of an exhaust system in which an exhaust gas flow from a diesel engine is fed through the diesel particle filter ( 18 ). The system further includes at least one of a burner ( 34 ) for heating up the exhaust gas flow, a controllable air supply ( 36 ) and a metering device ( 40 ) for introducing hydrocarbons into the exhaust gas flow being fed to the diesel particle filter ( 18 ). The method includes registering at least one of an exhaust gas temperature and an oxygen content of the exhaust gas at least one of upstream and downstream of the diesel particle filter ( 18 ), selecting and activating at least one of the burner ( 34 ), the air supply ( 36 ) and the metering device ( 40 ), and controlling the selected component as a function of the registered at least one of exhaust gas temperature and oxygen content.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to diesel particle filters. 
         [0002]    Diesel particle filters (DPF) for filtering the exhaust gas from diesel engines are known from the market, a diesel particle filter needing to be regenerated at specific intervals, for example every 200 km to 500 km in motor vehicles. This is done by heating the diesel particle filter to a point at which the particles deposited therein can burn. 
         [0003]    The use of a flame burner, which directly heats an exhaust gas flow, or an injection of hydrocarbons into the exhaust gas flow, to perform the required heating are also known. Both of these possible methods only function optimally in a specific temperature range, which on the one hand should have a specific minimum temperature in order to effectively remove the particles, and on the other should not exceed an upper threshold value, so as not to damage the diesel particle filter. Moreover the standard commercial filter materials have different admissible upper temperatures. For example, the material cordierite is substantially more sensitive than, say, silicon carbide (SiC). A usual method in the prior art is therefore to measure the temperature in the exhaust gas flow and to regulate a regeneration process of the DPF on the basis of this. 
         [0004]    The so-called Arrhenius equation establishes a general correlation between a reaction rate and a temperature: 
         [0000]        k=A· exp(− E   A   /R·T ), where
       k=constant of the reaction rate;   A=constant;   E A =activation energy;   R=general gas constant; and   T=temperature.       
 
         [0010]    For example, as a rough reference value the equation yields an approximate doubling of the reaction rate for a temperature increase of 10° K., prompting a corresponding requirement to regulate the temperature as precisely as possible when regenerating the diesel particle filter. 
       SUMMARY OF THE INVENTION 
       [0011]    The method according to the invention has the advantage that an oxygen content of an exhaust gas can be regulated within a wide range during a regeneration of a diesel particle filter (DPF) of an internal combustion engine (diesel engine), allowing a proper regeneration on the one hand and preventing any damage (thermal burn-out) of the DPF on the other. 
         [0012]    The invention proceeds from the consideration that in a regeneration of the DPF, for example in a motor vehicle, a series of important variables determines the regeneration process, and that there are various means or devices of an exhaust system for influencing these variables. In particular it is possible to state a general correlation between an oxygen content and the reaction rate: 
         [0000]        r=k·c   1   x   ·c   2   y   · . . . ·c   n   z , where       r=reaction rate;   k=constant of the reaction rate;   c i =concentration of the “i” components; and   x,y,z=exponents.         
         [0017]    According to this equation the reaction rate in a regeneration of the diesel particle filter increases with rising oxygen content. A regeneration with rising oxygen content of the exhaust gas therefore has to be performed particularly sensitively or gently, in order not to damage the exhaust system, in particular the diesel particle filter. Usual oxygen contents varying as a function of a prevailing operating range of the diesel engine are approximately 8 percent to 18 percent, whereas an oxygen content of an ambient air is approximately 21 percent. 
         [0018]    To this end it is proposed to perform the regeneration of the DPF so that an exhaust gas temperature and/or an oxygen content of the exhaust gas is/are detected, preferably upstream and/or downstream of the DPF in the direction of flow. This provides important variables of the exhaust gas, in order to be able to regulate the regeneration effectively. 
         [0019]    A definite design of the exhaust system in each case is furthermore important, having regard to the available means for influencing said variables. For example, a burner, an air supply connected thereto or an independent air supply, and a metering device for injecting fuel into the exhaust system may be present in the exhaust system of a motor vehicle. In this case the air supply may be embodied as an engine-driven, controllable air pump. The purpose of the fuel injection as an alternative or supplement to the burner is to raise the exhaust gas temperature to a point at which a regeneration of the diesel particle filter can ensue within a reasonable time. According to the invention it is also possible—depending on a prevailing exhaust gas temperature and a prevailing oxygen content of the exhaust gas—to use the air supply also as a sole means of performing a regeneration of the DPF. 
         [0020]    From these available possibilities a means of performing or assisting the regeneration can then be selected in each case. The means are advantageously selected, operated and controlled as a function of the previously determined exhaust gas variables. 
         [0021]    In a first case, in partial load operation of an internal combustion engine, that is to say of a diesel engine, for example, the oxygen content is comparatively high, at 14 percent, for example. It is therefore proposed, according to the invention, to select the injection of fuel into the exhaust system (so-called catalytic combustion) as the means, since as a consequence of the previously relatively high oxygen content a subsequent excessive reduction of the oxygen content is not very probable. For a catalytic combustion it is beneficial if the exhaust gas temperature upstream of the DPF (or upstream of a diesel oxidation catalytic converter connected to the DPF on the inlet side) is at least approximately 300° C. Depending on an actual oxygen content of the exhaust gas, this process may or should be supported by an air supply. 
         [0022]    In a second case, in full load operation of the internal combustion engine the oxygen content in the exhaust gas flow is comparatively low, at 8 percent, for example. If the injection of fuel into the exhaust system were now selected as the means, although the exhaust gas temperature can be raised to regeneration temperature, the oxygen content in the exhaust gas may fall considerably. If, on the other hand, a flame burner (burner) is selected for heating the exhaust gas, it is likewise possible that the oxygen content will be reduced. Compared to the catalytic combustion, however, this reduction turns out to be significantly less. In full load operation of the internal combustion engine, therefore, a heating of the exhaust gas by the burner is a preferred and advantageous choice. According to the invention the burner affords the further advantage of even being able to increase the overall oxygen content through a possibly increased setting of the air supply. 
         [0023]    In general the burner can be used for heating the exhaust gas over a wide range of the exhaust gas temperature and/or the oxygen content, and where necessary also to support the catalytic combustion. Conversely it is likewise possible to use the catalytic combustion to supplement the burner, should the burner not be designed for the full requisite output, for instance, or if the burner cannot be regulated or controlled fully or rapidly enough. In simultaneous operation of the burner together with the catalytic combustion (that is to say the injection of hydrocarbons), it may be necessary to introduce air additionally via the air supply, and more air than would be necessary for operation of the burner alone. 
         [0024]    In both cases according to the invention the exhaust gas mass flow, the exhaust gas temperature and/or the oxygen content of the exhaust gas can be detected upstream and/or downstream of the DPF and used for control purposes. For example, the quantity of fuel to be injected in catalytic combustion can be controlled as a function of the exhaust gas temperature and the oxygen content. Similarly the air supply and the output of the burner can be adjusted as a function of the exhaust gas temperature and the oxygen content. In this case it is advantageously possible to take account of a correlation between the exhaust gas mass flow, the temperature and the oxygen content required at any one time, so that the regeneration of the DPF takes place in a reasonable time without damaging the filter material. 
         [0025]    The invention furthermore allows for the arrangement of a diesel oxidation catalytic converter in series with the diesel particle filter and for the fact that in the method the exhaust gas temperature and/or the oxygen content is registered upstream and/or downstream of the diesel oxidation catalytic converter and taken into account in steps (b) and (c). In this way the diesel oxidation catalytic converter is advantageously incorporated into the regeneration of the DPF by taking account of its influence on the exhaust gas temperature and/or the oxygen content. 
         [0026]    The method can be more flexibly deployed when the means to be used in each case act upon the exhaust gas flow upstream of the diesel oxidation catalytic converter and/or upstream of the diesel particle filter, or are incorporated in these. Better account can thereby be taken of each type of exhaust system or special requirements relating to the regeneration of the DPF. 
         [0027]    In an important development of the method the available means is the air supply. The oxygen content of the exhaust gas can advantageously be varied directly by controlling or regulating the air supply, so that the process of regenerating the DPF can be optimized. The air supply can therefore be suitably adjusted—either alone or in addition to the other means—to the oxygen content of the exhaust gas. In this case the air supply may be integrated into the burner and if necessary operated independently of the latter, or the air supply may be designed as a separate air pump. 
         [0028]    In a further development of the method the available means are the burner and the air supply. The burner can thereby be used to supplement the air supply, in order to carry out and to control or regulate the regeneration of the DPF. This means that it is advantageously possible, for example, to widen the temperature range of the exhaust gas (exhaust gas temperature) in which a regeneration of the DPF is possible or advisable. 
         [0029]    In yet another development of the method the available means are the metering device and the air supply. This makes it possible to perform a regeneration of the DPF without the aid of the burner, and yet at the same time to adjust the oxygen content of the exhaust gas. 
         [0030]    The possible scope is maximized if the available means are the burner, the air supply and the metering device. Measured directly at an outlet of the internal combustion engine, this advantageously allows a regeneration of the DPF to be performed over an especially large temperature and oxygen content range of the exhaust gas. On the one hand by determining the exhaust gas mass flow, the exhaust gas temperature and/or the oxygen content, and on the other using the means described to vary the exhaust gas temperature and the oxygen content, it is possible both to keep the time needed for regeneration short and to effectively prevent any damage to the DPF. 
         [0031]    In addition it is proposed that during a regeneration of the diesel particle filter (DPF) the means is to be used successively in different combinations and/or at different levels and in different quantities. This relates in particular to the air supply. The regeneration of the DPF can thereby be modulated, so to speak. For example, the regeneration can be started gently by initially keeping the oxygen content comparatively low and thereafter steadily increasing it. An improvement can thereby advantageously be achieved in burning the DPF clear. If in addition the exhaust gas temperature is also increased, the reaction rate also increases and the regeneration time is correspondingly reduced. 
         [0032]    Furthermore an apparatus is proposed, which comprises means for registering or detecting an exhaust gas mass flow, an exhaust gas temperature and/or an oxygen content of the exhaust gas upstream and/or downstream of the diesel oxidation catalytic converter and/or the diesel particle filter. Important prerequisites are thereby established for detecting the variables relevant to the method according to the invention. In particular the exhaust gas temperature and the oxygen content can advantageously be registered at points of the exhaust system essential for the method and the working of the diesel oxidation catalytic converter and the DPF can thus also be monitored during the regeneration. 
         [0033]    In addition it is proposed that the apparatus comprises a burner, in particular a flame burner, an air supply and/or a metering device for introducing hydrocarbons. The means to be used for the method are thereby provided, in order to perform the regeneration of the DPF according to the invention in addition to a variation of the exhaust gas temperature by also varying the oxygen content. 
         [0034]    The apparatus is of more compact construction if the air supply is structurally integrated into the metering device. This advantageously combines two of the means to be used in one module and therefore saves space. 
         [0035]    In a development of the apparatus the burner and the metering device for introducing hydrocarbons are structurally integrated. This gives the apparatus an especially compact construction whilst at the same time saving costs. 
         [0036]    The apparatus for performing the method according to the invention functions particularly efficiently if the air supply is a blower. The oxygen content can thereby be set to particularly high levels, the oxygen content of the ambient air in borderline cases to some extent being capable of reaching approximately 21 percent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  shows a flow chart of a method for regenerating a diesel particle filter of an internal combustion engine; 
           [0038]      FIG. 2  shows a first simplified exemplary embodiment of an exhaust system of the internal combustion engine having a burner and an air supply; 
           [0039]      FIG. 3  shows a second simplified exemplary embodiment of an exhaust system having a device for injecting hydrocarbons and an air supply; 
           [0040]      FIG. 4  shows a third simplified exemplary embodiment of an exhaust system having a burner, an air supply and a device for injecting hydrocarbons; and 
           [0041]      FIG. 5  shows a fourth simplified exemplary embodiment of an exhaust system having a burner, a device for injecting hydrocarbons integrated therein and an air supply, together with a diagram representing important variables defining the method. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]      FIG. 1  shows a flow chart for executing the method with a computer program of a control and/or regulating device. In the drawing in  FIG. 1  the program runs substantially from top to bottom. 
         [0043]    The procedure for a regeneration of a diesel particle filter (DPF) is begun in a start block  100 . In a succeeding block  102  a temperature of the exhaust gas is measured upstream of the DPF or is determined from other, available operating variables. In a succeeding block  104  an oxygen content of the exhaust gas is measured upstream of the DPF, for example by means of an exhaust gas probe (lambda probe), or is determined from other, available operating variables. A mass flow of the exhaust gas can be detected or measured in a further block  106 . An initial state of the exhaust gas can be determined by means of one or more of these three detected variables. 
         [0044]    As a function of one or more of these variables and where necessary of other specific defaults—for a “modulated” regeneration of the DPF, for instance—at least one means to be used is then selected in block  108 , selection being made from available means formed by a flame burner, a device for injecting hydrocarbons and an air pump. 
         [0045]    In block  110  of  FIG. 1  the actual regeneration of the DPF is controlled or regulated by the available means selected in block  108  and according to the specific defaults. For this purpose block  110  may in any particular case activate, deactivate or control the level of the flame burner, the device for injecting hydrocarbons and/or the air pump. At the same time block  110  receives the current values of the variables detected in blocks  102  to  106 , in order to regulate the regeneration of the DPF, so that this can run within a predefined regeneration time and without damaging the DPF or a diesel oxidation catalytic converter. 
         [0046]    In block  112  it is enquired whether conditions exist for either terminating the DPF regeneration in the succeeding end block  114  or continuing it in block  110 . 
         [0047]    In the succeeding  FIGS. 2 to 5  the flame burner is referred to as a burner  34 , the device for injecting hydrocarbons as a metering device  40  and the air pump as an air supply  36 . 
         [0048]      FIG. 2  shows a simplified diagram of an exhaust system  10  of the internal combustion engine (not shown further) for performing the method for regenerating a DPF (diesel particle filter). In  FIG. 2  the exhaust system  10  comprises, from left to right in the direction of flow: a pipe system  12 , through which an exhaust gas mass flow  14  flows, an oxidation catalytic converter  16  (“diesel oxidation catalytic converter”), a diesel particle filter  18  and an outlet  20 , which leads to further devices of the exhaust system  10 , for example to a silencer, which are not shown. One or more temperature sensors  22 ,  24  and  26  and one or more lambda probes  28 ,  30  and  32  are in each case arranged upstream of the oxidation catalytic converter  16  and upstream and downstream of the diesel particle filter  18 . 
         [0049]    It will be pointed out in  FIG. 2  that not all the temperature sensors  22 ,  24  and  26  and lambda probes  28 ,  30  and  32  may be needed for controlling or regulating the regeneration. Similarly the order of the diesel particle filter  18  and the oxidation catalytic converter  16  may be reversed, or the oxidation catalytic converter  16  may possibly not be contained in the exhaust system  10 . 
         [0050]    In the drawing a burner  34  and an air supply  36  are arranged above the pipe system  12 . Here the burner  34  is embodied as a flame burner and introduces a generated flame jet and/or a generated air or hot air into the pipe system  12  via a connection  38 . The air supply  36  is embodied as a blower. 
         [0051]    In a regeneration of the diesel particle filter  18  an initial state of the exhaust system  10  and of the exhaust gas is first detected by means of one or more of the temperature sensors  22 ,  24  and  26 , the lambda probes  28 ,  30  and  32  and possibly taking account of a current exhaust gas mass flow  14 . On the basis of this it is determined whether and at what level the burner  34  and/or the air supply  36  each need to be actuated in order to perform the regeneration of the diesel particle filter  18 . For example, the burner  34  is activated and a quantity of air is delivered via the air supply  36 , such that the burner  34  can be operated and an oxygen content of the exhaust gas upstream of the diesel particle filter  18  lies within predefined limits. If the exhaust gas flowing in the pipe system  12  is already at a temperature suitable for the regeneration of the diesel particle filter  18 , the burner  34  can be at least temporarily deactivated and only the air supply  36  activated in order to adjust the oxygen content of the exhaust gas. 
         [0052]      FIG. 3  shows a diagram of an exhaust system  10  comparable to  FIG. 2 , the difference compared to  FIG. 2  being that a metering device  40  (HC injector) for injecting hydrocarbons and the air supply  36  are connected to the connection  38 . 
         [0053]    In a regeneration of the diesel particle filter  18  an initial state of the exhaust system  10  is again first detected, as explained. On the basis of this it is determined whether and at what level the metering device  40  and/or the air supply  36  are each to be actuated. Again it is a question of adjusting or regulating both the temperature and the oxygen content of the exhaust gas upstream of the diesel particle filter  18  within predefined limits, in order that the regeneration can be performed within a predefined time and in a manner that spares the diesel particle filter  18 . 
         [0054]      FIG. 4  shows an arrangement combining  FIGS. 2 and 3 . The burner  34 , in turn supplied with ambient air by the air supply  36 , is arranged on the connection  38 . The metering device  40 , for injecting hydrocarbons into the exhaust gas is arranged downstream of the connection  38 . 
         [0055]    The regeneration of the diesel particle filter  18  is performed according to the arrangements represented in  FIGS. 2 and 3 , the available means to be used, comprising the burner  34 , the air supply  36  and the metering device  40  being greater, so that the regeneration can be performed even more flexibly and precisely. If the burner  34  and the metering device  40  are operated simultaneously, it is still possible to ensure, through a suitable adjustment of the level of the air supply  36 , that an oxygen content (lambda value) of the exhaust gas remains within predefined limits. 
         [0056]    The means represented in  FIG. 4  can, where appropriate, make it even easier to perform the regeneration according to predefined profiles. For example, the regeneration may be “modulated” by initially performing this at a comparatively low exhaust gas temperature and a comparatively low oxygen content, and thereafter steadily shifting to higher exhaust gas temperatures and higher oxygen contents. An improvement can thereby be achieved in burning the diesel particle filter  18  clear. 
         [0057]      FIG. 5 , in the upper part of the drawing, shows an arrangement of an exhaust system  10  similar to  FIG. 4 , this being of especially compact construction in that the metering device  40  is structurally integrated into the burner  34 , thereby at the same time saving costs. 
         [0058]      FIG. 5 , in the lower part of the drawing, shows a diagram with an abscissa  46 , which gives a length coordinate to the same scale as the upper part of the drawing in  FIG. 5 , and an ordinate  48 , on which temperatures and oxygen contents of the exhaust gas are entered according to the five curves represented in the drawing. 
         [0059]    A first curve  50  shows a qualitative profile of an exhaust gas temperature along the pipe system  12  and the oxidation catalytic converter  16 , whilst the burner  34  is in operation. A second curve  52  shows a similar profile of the exhaust gas temperature whilst the burner  34  is not in operation, but air is merely being blown in by means of the air supply  36 . A third curve  54  shows a qualitative profile of an oxygen content of the exhaust gas whilst both the burner  34  and the metering device  40  are in operation and a lambda value of the exhaust gas is set to approximately one by means of the air supply  36 . A fourth curve  56  shows a profile of the oxygen content of the exhaust gas whilst only the air supply  36  and the metering device  40  are in operation. A fifth curve  58  shows a profile of the oxygen content of the exhaust gas whilst only the metering device  40  is in operation 
         [0060]    It will be seen from the curves  54  and  56  how the air supply  36 , during the operation of the burner  34  and/or the metering device  40 , can keep the oxygen content of the exhaust gas at the inlet into the oxidation catalytic converter  16  and the particle filter  18  sufficiently high despite an inevitable fall. It will be seen from the curve  58  by contrast how, in operation without the air supply  36 , the oxygen content of the exhaust gas at the inlet into the oxidation catalytic converter  16  and the diesel particle filter  18  falls suddenly and sharply to an insufficient value. 
         [0061]    The air supply  36  therefore advantageously serves to ensure that a regeneration of the diesel particle filter  18  can be performed with an optimum oxygen content of the exhaust gas at any given time.