Abstract:
The present invention relates to a method for regenerating a particle filter ( 10 ) arranged in the exhaust line ( 12 ) of an internal-combustion engine, notably of diesel type, wherein the clogging state of the filter is evaluated, this state is compared with a threshold value, then, if this threshold value is exceeded, a fluid and a fuel are mixed together, this mixture is subjected to catalytic combustion to generate the hot gases required for regeneration of the filter and regeneration of the filter is carried out by means of hot gases flowing through said filter and whose temperature is sufficiently high to provide combustion of the particles retained in this filter. According to the invention, prior to mixing the fluid and the fuel, the temperature of the fluid is raised up to the catalytic combustion light-off temperature.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for regenerating particle filters, notably those intended for particles present in the exhaust gases of internal-combustion engines, and to an installation using such a method.  
         [0002]     It notably concerns the sphere of managing fouling of a filter arranged in the exhaust line of an internal-combustion engine, notably of diesel type, and through which flow exhaust gases carrying particles such as carbon-containing particles or soots.  
         [0003]     Such engines generate particularly large amounts of particles and their exhaust lines are more and more often equipped with filters which retain these particles with very high filtration efficiencies close to 100%.  
         [0004]     However, such filters have to be periodically regenerated in order to prevent clogging by fouling. In fact, clogging leads to an increase in the back pressure at the exhaust, which has the effect of increasing the fuel consumption of the engine. In the extreme case of total filter clogging, the result can be serious engine dysfunctioning or even complete engine failure, and/or destruction of the filter.  
       BACKGROUND OF THE INVENTION  
       [0005]     Regeneration of a particle filter sometimes occurs naturally when the temperature of the exhaust gases has reached the level required to burn the particles present in this filter.  
         [0006]     However, under certain engine running conditions, the exhaust gas temperature is not sufficient to provide regeneration of the filter and it is then necessary to artificially initiate combustion of the particles when fouling of the filter has reached a certain threshold.  
         [0007]     It can consist in increasing the temperature of the filter above 550° C., generally by temporarily raising the air/fuel ratio of the exhaust gases flowing therethrough without a ratio 1 being reached, and in obtaining an oxidizing composition of these gases to achieve combustion of the particles retained in this filter.  
         [0008]     This involves the major drawback of increasing the fuel consumption.  
         [0009]     Another technique consists, as described more in detail in European patent EP-0,341,832, in arranging a catalyst for oxidizing the nitrogen monoxide (NO) upstream from the filter. This catalyst oxidizes the nitrogen monoxide contained in the exhaust gases to nitrogen dioxide (NO 2 ) and this nitrogen dioxide is then used to allow combustion of the particles trapped on the filter at a temperature ranging between 280° C. and 400° C.  
         [0010]     This technique requires a diesel fuel with a very low sulfur content (of the order of 50 ppm) to maintain a sufficient conversion efficiency of the oxidation catalyst so as to obtain a large amount of NO converted to NO 2 .  
         [0011]     Other techniques involve a chemical process wherein organometallic additives, such as cerium for example, are added to the diesel fuel so as to obtain combustion of the particles present in the filter at a temperature close to 400° C. to 450° C.  
         [0012]     Using such additives is quite costly and requires a particular device for feeding these additives notably into a diesel fuel tank.  
         [0013]     It is also well known to heat these exhaust gases by means of additional devices arranged in the exhaust line and upstream from the filter, such as burners or resistors, as described more in detail in patents FR-2,753,393 and FR-2,755,623 filed by the applicant.  
         [0014]     In this configuration, it is necessary to provide a high amount of heat energy to the exhaust gases, either by burning a large amount of fuel when using a burner, or by using a high electric power in the case of resistors.  
         [0015]     The major drawback thereof is that it significantly increases the fuel consumption of the engine and decreases the driving comfort.  
         [0016]     The present invention aims to overcome the aforementioned drawbacks by means of a method and of a device allowing to reach regeneration temperatures very rapidly while minimizing consumption.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention thus relates to a method for regenerating a particle filter arranged in the exhaust line of an internal-combustion engine, notably of diesel type, a method wherein the clogging state of the filter is evaluated, this state is compared with a threshold value, then, if this threshold value is exceeded, a fluid and a fuel are mixed together, this mixture is subjected to catalytic combustion to generate the hot gases required for regeneration of the filter and regeneration of the filter is carried out by means of hot gases flowing through said filter and whose temperature is sufficiently high to provide combustion of the particles retained in the filter, characterized in that, prior to mixing the fluid and the fuel, the temperature of the fluid is raised up to the catalytic combustion light-off temperature.  
         [0018]     Advantageously, the temperature of the fluid can be raised by heating said fluid by means of a heating resistor.  
         [0019]     Air and/or the exhaust gases of the engine can be used as the fluid.  
         [0020]     The exhaust gases can be circulated around a catalytic element used for catalytic combustion in order to raise the temperature of said element.  
         [0021]     An additive can be added to the fuel to lower the combustion temperature of the particles.  
         [0022]     The invention also relates to a installation for filtering exhaust gases from an internal-combustion engine, notably of diesel type, with a filtration unit comprising at least one filtration zone including a filter cartridge through which the exhaust gases of the engine flow, and a catalytic combustion device allowing to generate hot gases required for regeneration of at least one of said cartridges, characterized in that it comprises an element for preheating the fluid flowing through the catalytic combustion device.  
         [0023]     The catalytic combustion device can comprise a line connected to the filtration unit and carrying a catalytic element and a fuel injection device.  
         [0024]     Preferably, the preheating element can comprise a resistor.  
         [0025]     The catalytic combustion device can comprise a means for pumping the fluid intended to flow through the catalytic element.  
         [0026]     The filtration installation can comprise a distribution compartment arranged upstream from the cartridge and carrying an inlet for the exhaust gases from the engine and an intake for the hot gases coming from the catalytic combustion device.  
         [0027]     The distribution compartment can comprise a valve shutoff means for controlling the inflow of exhaust gases from the engine and the intake of hot gases.  
         [0028]     The filtration installation can comprise a line for heating the catalytic element.  
         [0029]     The catalytic element can comprise a catalyst element for catalytic combustion and catalytic element for oxidation of the exhaust gases.  
         [0030]     The catalytic element can be impregnated with a catalytic formulation allowing to educe the nitrogen oxides content of the exhaust gases. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0031]     Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying figures wherein:  
         [0032]      FIG. 1  is a diagram showing the particle filter regeneration installation according to the invention,  
         [0033]      FIG. 2  illustrates an example of embodiment of the filtration unit of the installation of  FIG. 1 ,  
         [0034]      FIG. 3  is a sectional side view along line  3 - 3  of  FIG. 2 ,  
         [0035]      FIG. 4  is a view similar to  FIG. 3  showing a regeneration mode,  
         [0036]      FIG. 5  illustrates a variant of the element of the installation of  FIG. 4 ,  
         [0037]      FIG. 6  is a sectional side view of a variant of the example of  FIG. 2 , and  
         [0038]      FIG. 7  is a sectional side view of another variant of the example of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0039]     In  FIG. 1 , the installation comprises a filtration unit  10 , notably a particle filter, arranged on an exhaust line  12  of an internal-combustion engine, more particularly a diesel type engine.  
         [0040]     The exhaust gases flow through this unit along a path symbolized by arrows  14  (gas inflow) and  16  (gas outflow) in  FIG. 1  and divided into at least two filtration zones, here three zones  18 ,  20 ,  22 , preferably substantially identical. The installation also comprises a catalytic combustion device  26  allowing to generate hot gases sent through a line  24  to this filtration unit.  
         [0041]     This catalytic combustion device comprises an outside air delivery pump  28  and, in the direction of circulation of this air along a line  30  connected to connecting line  24 , a device  32  for preheating the air circulating in line  30 , a device  34  for injecting fuel into the line and a catalytic element  36  referred to as catalyst in the description hereafter.  
         [0042]     The preheating device is preferably a resistor arranged within the line, between the pump and fuel injection device  34 , powered by a battery or by a supercapacity, and swept by the air circulating therein under the effect of the pump. The fuel injection device can be a pump injection nozzle connected to the fuel circuit this engine is usually equipped with. The catalyst is of oxidation catalyst type for oxidizing the fuel contained in the fuel mixture flowing therethrough and it allows to heat very rapidly the air contained in this mixture, and thus to deliver hot gases to line  24 . This catalyst can be in form of a monolith consisting of a corrugated metal strip wound round itself, thus forming a cylindrical assembly called “honeycomb”. The size of this cylindrical assembly depends on the volume of the exhaust gases flowing therethrough in order to limit the back pressure. This catalyst can also consist of a cordierite monolith or of a filtering element made of silicon carbide for example, impregnated with an oxidation catalytic formulation.  
         [0043]     In  FIG. 2 , filtration unit  10  comprises an exhaust gas inlet manifold  38  connected to exhaust line  12  into which the exhaust gases are fed. Several detectors, more precisely a pressure detector  40  (upstream pressure detector) and a temperature detector  42  (upstream temperature detector), are arranged in this manifold. The manifold opens through inlets  44 ,  46 ,  48  into filtration zones  18 ,  20 ,  22  which comprise each, downstream from these inlets, a distribution compartment  50 ,  52 ,  54  provided upstream from a filter cartridge  56 ,  58 ,  60  and into which opens, through bypasses of line  24 , a hot gas intake  62 ,  64 ,  66 . The outlets of the cartridges lead to an outlet manifold  68  connected to exhaust line  12  and which also comprises several detectors, such as a pressure detector  70  (downstream pressure detector) and a temperature detector  72  (downstream temperature detector).  
         [0044]     Each distribution compartment comprises a valve shutoff means  74 ,  76 ,  78  allowing to control the exhaust gas inflow and/or hot gas intake  62 ,  64 ,  66 .  
         [0045]     The valve shutoff means are controlled by one or more actuators (not shown) independently of one another, but without closing simultaneously all the inlets  44 ,  46 ,  48  of compartments  50 ,  52 ,  54 .  
         [0046]     By way of example, as illustrated by FIGS.  3  to  7 , the valves comprise each two shutoff means linked to one another, a first means, referred to as plate  80 , allowing to open or to close the exhaust gas inlet and a second means, referred to as slide  82 , allowing to open and to close the hot gas intake. The plate and the slide are arranged in such a way that the exhaust gas inlet and the hot gas intake of a single compartment cannot be closed simultaneously. Preferably, the inlet and the intake are arranged orthogonally in relation to one another so that the plate and the slide are also arranged orthogonally. Motion of these shutoff means is controlled by a rod  84  subjected to a translation displacement under the action of any known means, such as a jack, an electromagnet, . . .  
         [0047]     Furthermore, partitions  86  are provided to isolate zones  18 ,  20 ,  22  from one another and to delimit compartments  50 ,  52 ,  54 .  
         [0048]     In the case of the present description, the terms “upstream” and “downstream” refer, for the filtration unit, to the circulation of the exhaust gases from inlet manifold  38  to outlet manifold  68  whereas, in the case of the catalytic combustion device, circulation of the air is considered from pump  28  to intakes  62 ,  64 ,  66 .  
         [0049]     During operation, a control unit (not shown) such as an engine computer the engine is usually equipped with determines the position of valves  74 ,  76 ,  78  according to the various engine running parameters.  
         [0050]     As illustrated in  FIG. 2  by way of example, for average engine loads or for average exhaust gas flow rates (of the order of 200 to 400 m 3 /h), valves  74  and  76  may be controlled by the control unit so that they open exhaust gas inlets  44  and  46  while closing intakes  62  and  64  through slides  82  whereas valve  78  closes exhaust gas inlet  48  through plate  80 . This advantageously allows to adjust the filtration volume to the volume of the gases flowing through the filtration unit.  
         [0051]     By way of example, the installation is considered to be in the loading state as shown in  FIGS. 2 and 3 , i.e. cartridges  56 ,  58 ,  60  are not saturated with the particles or soots contained in the exhaust gases. In this case, all of the exhaust gases flows, through inlets  44  and  46 , from manifold  38  to compartments  50 ,  52 , then through cartridges  56  and  58  so that the major part of the particles contained in these gases is retained by these cartridges and eventually reaches outlet manifold  68  to be discharged through exhaust line  12 . In this configuration, resistor  32  is not supplied with fuel, no fuel is fed into line  30  through injection nozzle  34  and catalyst  36  is at ambient temperature.  
         [0052]     Periodically, generally every 200 km or every two working hours, a period that may be modified according to the conditions of use, an estimation of the clogging degree of the filtration unit is performed. More particularly, clogging by the particles of each cartridge  56 ,  58 ,  60  is examined.  
         [0053]     Therefore, starting from the configuration of  FIG. 2 , the control unit controls the valves in such a way that a single exhaust gas inlet is open. Thus, from the example shown in  FIG. 2 , valve  76  is actuated to close exhaust gas inlet  46  and only inlet  44  is open. All of the exhaust gases thus flows through this inlet  44 , then circulates in cartridge  56  and flows out into line  12  through outlet manifold  68 . By means of upstream and downstream pressure detectors  40  and  70  respectively, a pressure drop is calculated by the control unit, then compared with a value table contained in this unit. If the value of this pressure drop is lower than a threshold value of this table, the unit controls the valves in such a way that this clogging degree examination is repeated on next cartridge  58  by opening inlet  46  and by closing inlet  44 . Similarly, if the pressure drop of cartridge  58  is lower than the threshold value, the operation is repeated on cartridge  60 .  
         [0054]     If the threshold value of the pressure drop of one of the cartridges is reached, for example cartridge  56  as illustrated in  FIG. 4 , then valve  74  closes exhaust gas inlet  44  by means of plate  80  and releases hot gas intake  62  so as to start regeneration of this cartridge. Of course, the control unit will best adjust the volume of exhaust gas to be treated by the other cartridges by controlling the valves associated therewith so that at least one of inlets  46  and  48  is open.  
         [0055]     Simultaneously with the closing of inlet  44 , the unit starts pump  28  which circulates air in line  30  and sends an electric current through resistor  32 . The resistor thus heats the air in this line so that it reaches a temperature close to 250° C., which is the catalyst light-off temperature. This temperature of the air is constantly monitored by a temperature detector  88  provided in line  30  downstream from catalyst  36 . As soon as this temperature is reached, fuel is injected into line  30  and upstream from catalyst  36  through injection nozzle  34 . By catalytic reaction, the air/fuel mixture flowing through this catalyst consumes and the hot gases coming from this catalyst reach a temperature above 550° C., which is necessary and sufficient to ensure combustion of the particles present in the cartridge to be regenerated. These hot gases then enter compartment  50  through intake  62 , flow through cartridge  56  to bum the particles retained therein and flow out through manifold  68  so as to be discharged into exhaust line  12 .  
         [0056]     The cartridge regeneration rate is controlled by the control unit which manages the amount of fuel fed into line  30  as well as the flow rate of the air circulating therein by means of pump  28 . Similarly, this unit will stop supplying resistor  32  if necessary.  
         [0057]     Once regeneration of cartridge  56  is completed, the control unit actuates the valves so that the regeneration installation goes back to the configuration preceding this regeneration, as illustrated in  FIG. 2 . If necessary, the control unit will control the valves as described above to start regeneration of another cartridge.  
         [0058]     Advantageously, an additive may be mixed with the fuel prior to injecting it into line  30  through nozzle  34  in order to lower the reaction temperature of the catalyst.  
         [0059]     Preferably, as can be seen in  FIG. 5 , during regeneration of cartridge  56  and depending on the upstream and downstream temperatures detected by detectors  42  and  72  and transmitted to the control unit, this control unit may control valve  74  so as to temporarily open intake  44  by removing plate  80  from its seat while leaving intake  62  open. This has the effect of allowing exhaust gases through this cartridge, thus preventing too great a heat exchange for this cartridge during regeneration thereof.  
         [0060]     The example shown in  FIG. 6  is a variant of  FIG. 2  and therefore comprises the same reference numbers for the parts common to the two figures.  
         [0061]     In this variant, catalytic combustion device  26  comprises the same elements as those described in connection with  FIG. 1  (pump, heating resistor, fuel injection nozzle and catalyst). In this example, catalyst  36  is arranged as close as possible to the cartridges so that the flow of the hot gases between this catalyst and the cartridges is minimized and thus the thermal losses of these gases are limited.  
         [0062]     Advantageously, this catalyst may be bathed in the exhaust gases which thus transmit part of their heat energy to this catalyst, which minimizes the electric power supplied to the resistor while decreasing the time required for this catalyst to reach its light-off temperature. More precisely, a line  90  has an inlet  92  which starts at inlet manifold  38  and an outlet  94  which opens into one of the compartments, here compartment  50  and upstream from cartridge  56 . Line  30  runs substantially orthogonally across this heating line  90  whose transverse dimension is such that the transverse dimension of line  30  is included therein. The catalyst is arranged in the region of line  30  that runs across heating line  90  so that the exhaust gases coming from manifold  38  surround by sweeping the part of line  30  comprising catalyst  36  and transmit their calories to this catalyst.  
         [0063]     Preferably, outlet  94  of line  90  is not closed by slide  82  of valve  74  so that the exhaust gases permanently circulate from the inlet manifold to the compartment by bathing constantly the section of the line carrying the catalyst.  
         [0064]     Advantageously, in order to limit the thermal inertia of this catalyst, it may be separated into several elements, a first element instead of catalyst  36  to ensure catalytic combustion of the fuel mixture circulating in line  30  and a second catalytic element  36   b  arranged upstream from cartridge  56  and downstream from hot gas intake  62 , whose function is to oxidize the unburnt hydrocarbons (HC) and the carbon oxides (CO) present in the exhaust gases and/or in the hot gases resulting from catalytic combustion.  
         [0065]     Operation of the filtration unit comprising the elements described in connection with  FIG. 6  is identical to that described in connection with FIGS.  1  to  5 , with the additional advantage that catalyst  36  is at a temperature substantially close to the temperature of the exhaust gases. This allows catalyst  36  to be operational more rapidly by means of heating line  90  and to limit the electric power to be sent to the resistor to heat the air circulating in line  30 .  
         [0066]     The embodiment illustrated in  FIG. 7  is a variant of  FIG. 6  and therefore comprises the same reference numbers as this figure.  
         [0067]     This variant differs from  FIG. 6  in that the catalytic combustion device comprises no air circulation pump.  
         [0068]     In this case, line  130  carrying resistor  32 , injection nozzle  34  and catalyst  36  starts at inlet manifold  38  and opens onto hot gas intake  62  as described above in connection with FIGS.  1  to  5 .  
         [0069]     Thus, during the cartridge regeneration operations, resistor  32  is supplied (in cases where the exhaust gas temperature is not sufficient to bring catalyst  36  to its light-off temperature) and heats the exhaust gases that flow therethrough. As soon as this temperature is reached, as measured by detector  88 , the injection nozzle feeds fuel into line  130 , downstream from the resistor and upstream from the catalyst, and it stops the power supply to the resistor if necessary. The exhaust gases circulating in this line contain enough oxygen for the fuel mixture flowing through catalyst  36  to be oxidized and to provide, at the catalyst outlet, hot gases which are thereafter fed through intake  62  into compartment  50 , then flow through cartridge  56 .  
         [0070]     Of course, the present invention is not limited to the embodiment examples described and encompasses any equivalent and variant.  
         [0071]     Notably, either catalyst  36  or cartridges  56 ,  58 ,  60  may be impregnated with a catalytic formulation allowing the NOx present in the hot gases or in the exhaust gases to be reduced.