Abstract:
A device for eliminating nitrogen oxides in an exhaust line of a lean-burn internal-combustion engine ( 1 ) a NO x  trap ( 3 ) for trapping the nitrogen oxides, a system for regenerating the nitrogen oxides when the NO x  trap is saturated, a hydrocarbon treating catalyst ( 2 ) placed upstream from the NO x  trap ( 3 ), a hydrocarbon injector ( 4 ) placed upstream from hydrocarbon treating catalyst ( 2 ), a probe ( 7 ) for measuring the gas mixture strength. 
     The hydrocarbon treating catalyst ( 2 ) is a partial (or controlled) hydrocarbon oxidation catalyst that cooperates with the NO x  trap ( 3 ) for trapping the nitrogen oxides and that allows to obtain, at the outlet thereof, gases with a low oxygen (O 2 ) concentration and with high carbon monoxide (CO) and hydrogen (H 2 ) concentrations. 
     A control unit is furthermore provided for recording and processing data coming from various detectors and/or stored in order to carry out effective regeneration of the NO x  trap ( 3 ) without disrupting the smooth running of the engine.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of treatment of gases discharged at the exhaust of lean-burn spark-ignition engines and of diesel engines. 
     Such engines emit a certain number of pollutants that have to be eliminated, which must be done all the more efficiently as standards are becoming increasingly severe, notably in industrialized countries. 
     Examples of the most numerous pollutants that have the most harmful effect on the environment are nitrogen oxides. 
     BACKGROUND OF THE INVENTION 
     It is well-known to eliminate this type of pollutant by passing the exhaust gases through catalysts (referred to as DeNO x  catalysts) intended for nitrogen oxides conversion. This requires postinjection of reducers such as hydrocarbons for example. The known catalysts being active within a given temperature range, several catalysts having different formulations, i.e. different activity ranges, can be placed in the catalytic muffler. The sphere of action of the catalytic elements is thus extended. 
     By way of example, the formulations used for low temperatures are of the Platinum/Alumina or Platinum/Zeolite type. The temperatures for which these catalysts are the most active range from 200° C. to 250° C. 
     Catalysts referred to as “High-temperature” catalysts are generally active between 300° C. and 500° C. These are for example Copper-Zeolite type catalysts. 
     However, in this context, a problem arises when the exhaust gases are not, in the catalyst(s), within a temperature range for which conversion of the nitrogen oxides is sufficient. 
     However, the global efficiency of such aftertreatment systems remains limited. By way of example, the efficiency on nitrogen oxides of a Platinum/Alumina catalyst with postinjection of gas oil is commonly below 50%. 
     There are also catalysts on which the nitrogen oxides are adsorbed in various forms. By way of example, the NO x  can be stored as nitrates or enter an oxide structure. These catalysts are commonly referred to as “NO x  traps”. 
     The “nitrate” type NO x  trap is a catalyst that allows nitrogen oxides to be stored at the surface thereof, in an oxidizing environment. It generally consists of a precious metal deposited on or close to a basic mass which is generally an oxide or a mixture of alkaline oxides, alkaline-earth oxides or rare earths. In lean mixture (excess oxygen), the NO (NO x =NO+NO 2 ) that is predominantly present in exhaust gases is oxidized by the precious metal, thus forming NO 2 . This NO 2  migrates to the catalyst surface where it is adsorbed on the oxide and forms a nitrate. These nitrates are stable in an oxidizing environment over a very wide temperature range. In order to desorb these nitrates from the surface of the catalyst, a high temperature is required in the oxidizing environment or a reducing mixture has to be made. 
     Since exclusively thermal regeneration of the NO x  trap does not allow treatment of the nitrogen oxides redischarged at the exhaust, a second catalyst (continuous DeNO x  type for example) is then necessary to reduce them. 
     Patent application EP-A1-0,540,280 describes thermal regeneration of this type, with a NO x  trap, provided with a gas heating system, followed by a nitrogen oxides reduction catalyst. Both catalysts are mounted on a line bypassing the main exhaust line. According to this document, a valve system allows to decrease the GHSV (ratio of the flow of gas to the volume of catalyst expressing the contact time between the gases and the catalyst) during trap emptying stages. The NO x  conversion coefficient of the nitrogen oxides reduction catalyst is thus improved. However, with this configuration, the part of the gas stream flowing through the main line does not flow through the NO x  reduction catalyst. 
     Regeneration through gas mixture strength control allows to reduce the desorbed NO x  by three-way type catalysis, by depositing a suitable noble metal on the catalyst (rhodium for example). 
     With a gasoline engine running with a lean mixture, transition from lean to rich is compatible with its function mode; on the other hand, with a diesel engine, it is more difficult to obtain a mixture strength above 1. 
     A known implementation consists in injecting hydrocarbons into the exhaust line upstream from the catalyst when the nitrates have to be desorbed from the NO x  trap. U.S. Pat No. 5,201,802 illustrates an embodiment of this type. However, although this method allows to obtain mixture strengths momentarily above 1, the gaseous mixture obtained contains high concentrations of oxygen, which is disadvantageous for regeneration. 
     Another known process consists in reinjecting exhaust gases at the intake at very high rates and in controlling the mixture strength at the engine intake. Document EP-A1-0,829,623 discloses such a process. 
     The latter strategy has the drawback of disrupting the smooth running of the engine and of making engine control more complex. 
     SUMMARY OF THE INVENTION 
     The present invention comprises regeneration of a NO x  trap essentially based on variation of the mixture strength of the exhaust gases, and which does not disrupt the smooth running of the engine. 
     Regeneration of a NO x  trap includes here both NO x  emptying and reduction, the latter being carried out by the NO x  trap. 
     More precisely, the object is to allow regeneration of the NO x  trap by decreasing the oxygen concentration and by increasing the carbon monoxide CO and hydrogen H 2  concentrations in the exhaust gases upstream from the NO x  trap. CO and H 2  are, by definition, good reducers as they result from partial oxidation of postinjected hydrocarbons. The carbon monoxide acts both on NO x  emptying and reduction. 
     The object of the present invention thus is a device for eliminating nitrogen oxides in an exhaust line of a lean-burn internal-combustion engine, comprising a means for trapping the nitrogen oxides, a means for regenerating said nitrogen oxides when the trapping means is saturated, a hydrocarbon treating means arranged upstream from the nitrogen oxides trapping means, a hydrocarbon injection means placed upstream from the hydrocarbon treating means, a means for measuring the gas mixture strength. 
     According to the invention, the hydrocarbon treating means is a partial (or controlled) hydrocarbon oxidation catalyst that cooperates with said nitrogen oxides trapping means that allows to obtain, at the outlet thereof, gases with a low oxygen (O 2 ) concentration and with high carbon monoxide (CO) and hydrogen (H 2 ) concentrations. The device according to the invention further comprises a means for recording and for processing data received from the various detectors and/or stored so as to allow effective regeneration of the NO x  trap without disrupting the smooth running of the engine. 
     According to an embodiment of the invention, the hydrocarbon injection means, the hydrocarbon treating means and the NO x  trapping means are arranged in this order and in series in relation to the direction of circulation of the gases in the exhaust line. 
     According to a particular embodiment of the invention, the hydrocarbon injection means, the hydrocarbon treating means and the NO x  trapping means are arranged in the main exhaust line itself. 
     According to another possibility, the hydrocarbon injection means, the hydrocarbon treating means and the NO x  trapping means are arranged in a line bypassing the main exhaust line, the device according to the invention then comprising a means for modulating the flow of the gases between said bypass line and the main line. 
     Without departing from the scope of the invention, a hydrocarbon injection means, a hydrocarbon treating means and a nitrogen oxides trapping means can thus be arranged both in the bypass line and in the main line, with a means for modulating the flow of the gases between said bypass line and the main line. 
     Furthermore, the device according to the invention can comprise at least one temperature detector that can be placed upstream from said hydrocarbon treating means. 
     The device according to the invention can also comprise a means for measuring the amount of NO x  trapped in the trapping means, downstream from the latter. 
     Furthermore, a second temperature detector can be placed in the exhaust line, downstream from the hydrocarbon treating means. 
     In particular, the means for measuring the exhaust gas mixture strength can be placed downstream from the NO x  trapping means. 
     Without departing from the scope of the invention, the means for measuring the exhaust gas mixture strength can be placed between the hydrocarbon treating means and the NO x  trapping means. 
     Furthermore, the exhaust line can comprise a gas preheating means placed upstream from the hydrocarbon treating means. 
     The present invention also relates to a process for eliminating nitrogen oxides in an exhaust line of a lean-bum internal-combustion engine, characterized in that it consists in: 
     trapping the NO x  in a suitable means, 
     injecting hydrocarbons into the exhaust line according to various engine running parameters and to the state of saturation of a NO x  trapping means, 
     partly oxidizing the hydrocarbons in a specific means so as to obtain a maximum amount of CO and H 2 , 
     regenerating said NO x  trapping means by means of the hydrocarbon oxidation products, notably CO and H 2 . 
     According to an aspect of the invention, the amount of NO x  stored in the NO x  trapping means is monitored. 
     According to another aspect of the invention, the temperature of the gases upstream and/or downstream from the hydrocarbon treating means is monitored. 
     Furthermore, the gas mixture strength upstream and/or downstream from the NO x  trapping means is monitored. 
     Said additional hydrocarbons are advantageously injected when the NO x  trapping means is saturated, the temperature (T 1 ) of the exhaust gases is above a threshold value (T R ) for which the hydrocarbon treating means is active, so that the exhaust gas mixture strength (λ) is greater than or equal to a given mixture strength (λ R ) that triggers regeneration of the trapping means. 
     More precisely, said hydrocarbons are injected for a length of time (d R ) that is shorter than a predetermined stored time (d RMAX ). 
     According to an advantageous aspect of the invention, the exhaust gas flow is modulated between a main line and a line that bypasses said main line. 
     The exhaust gases are thus furthermore heated prior to being partly oxidized in said bypass line. 
     In particular, regeneration is stopped according to the information provided by a detector such as a mixture strength probe placed downstream from the NO x  trapping means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages, features and details of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying drawings wherein: 
     FIG. 1 is a diagram of an exhaust line according to an embodiment of the invention, 
     FIG. 2 is an organization chart of the data required for the process according to the invention, 
     FIG. 3 is a flowchart relative to a regeneration process for a NO x  trap according to the first embodiment of the invention, 
     FIG. 4 is a diagram of an exhaust line according to .a second embodiment of the invention, 
     FIG. 5 is an organization chart of the data according to the second embodiment of the invention, and 
     FIG. 6 is a flowchart relative to the regeneration process for a NO x  trap according to the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the embodiment of the invention illustrated in FIG. 1, the exhaust line of engine  1  essentially comprises: a first catalyst  2  hereafter referred to as controlled oxidation catalyst, intended for partial oxidation of hydrocarbons to carbon monoxide (CO) and hydrogen (H 2 ). 
     A second catalyst  3  is placed in the main exhaust line downstream from first catalyst  2  in relation to the direction of propagation of the gases in the exhaust line. This second catalyst  3  is a “NO x  trap” whose regeneration mode (emptying and reduction) is based on the gas mixture strength. 
     Furthermore, a hydrocarbon injection means  4  is provided for postinjection. Means  4  is placed upstream from catalysts  2 ,  3 . This postinjection can be performed by the injection system of the engine, if this is possible: a common-rail type injection system, well-known to specialists, can thus perform such a postinjection at the exhaust. A specific injection system, secondary to injection in the engine itself, can also be provided without departing from the scope of the invention. 
     Furthermore, at least one temperature detector can be necessary for implementing the invention. 
     In the embodiment of FIG. 1, two temperature detectors  5 ,  6  are provided: one,  5 , upstream from controlled oxidation catalyst  2 , and the second,  6 , between catalyst  2  and NO x  trap  3 . 
     Detector  5  allows to know if the temperature of the gases is sufficient to oxidize the hydrocarbons injected during the regeneration phase of NO x  trap  3 . 
     Temperature detector  6  gives the temperature at which the NO x  are stored under lean mixture conditions and it allows, during regeneration, to know the temperature variation due to hydrocarbon postinjection. 
     A NO x  detector  8  intended to evaluate the amount of NO x  stored, at a given time, in NO x  trap  3  can be arranged downstream from NO x  trap  3 . 
     Finally, a mixture strength probe  7  can be placed between the first and the second catalyst; this mixture strength probe  7  is preferably situated downstream from NO x  trap  3 . 
     An exhaust line whose structure has been described above globally works as follows: 
     When the engine runs under lean mixture conditions, catalyst or NO x  trap  3  stores all or part of the NO x  discharged by engine  1 . The fraction of NO x  stored on the catalyst at the time t depends both on the state of saturation of NO x  trap  3  (i.e. what it has been able to store until time t) and on the temperature, flow rate, NO x  concentration, exhaust gas mixture strength conditions. The amount of NO x  stored on the catalyst can be estimated either from data from engine maps and from a mathematical model, or possibly by means of NO x  detector  8  placed downstream from NO x  trap  3 . When the amount of NO x  stored on catalyst  3  reaches a threshold value S, the regeneration procedure is activated. The amount of hydrocarbons to be postinjected in order to momentarily obtain a mixture strength above 1 in the exhaust line is determined according to the parameters obtained from the engine maps, such as the exhaust gas mixture strength and flow rate. Proportional mixture strength probe  7  placed for example downstream from controlled oxidation catalyst  2  can be used therefore in order to control the postinjection amount. Furthermore, temperature detector  5  placed upstream from first catalyst  2  indicates if the temperature of the gases that reach controlled oxidation catalyst  2  is sufficient and thus if postinjection has to be activated. Controlled oxidation catalyst  2  then consumes the oxygen contained in the exhaust gases, thus forming notably carbon monoxide (CO) and hydrogen (H 2 ), two elements that favour regeneration of NO x  trap  3 . 
     FIG. 2 is a summary of the parameters used according to the invention. It can be seen that engine  1  supplies two types of data: the engine speed (N) and the position of the accelerator pedal (α). From these data, a map C stored for example in an electronic control unit allows to determine other parameters such as the mixture strength R at the engine outlet, the NO x  ratio at the engine outlet and the flow rate at the exhaust. 
     Other data, measured by temperature detector(s)  5 ,  6  or NO x  detector  8  allow to determine, during the NO x  storage phase, the amount of NO x  stored (Q NO x ) at a given time (t=n) if they are associated with the first data. 
     A mathematical model is used instead of or in addition to NO x  detector  8  to determine the NO x  concentration in the gases. 
     If a detector  8  is used in addition to the mathematical model it is then possible to permanently adjust the parameters of the model so as to follow the real behaviour of the catalysts in the exhaust line. Aging of the catalysts can thus be taken into account for example. 
     In the regeneration phase, map C provides the same data as in the storage phase which, associated with the temperature and mixture strength measurements, allow to determine the flow rate of hydrocarbons to be injected at the exhaust. 
     FIG. 3 is a flowchart of the various stages leading, or not, to regeneration of NO x  trap  3 . 
     The amount of NO x  (Q NO x ) stored in the NO x  trap is permanently monitored. If this value reaches a certain threshold S stored in an electronic control unit, the temperature T 1  of the gases upstream from first catalyst  2  is checked; if this temperature is higher than a threshold T R  that corresponds to the minimum temperature for which first catalyst  2  is active, the mixture strength λ of the gases downstream from first catalyst  2  is checked. 
     The amount of postinjected hydrocarbons is linked with the difference between the mixture strength R of the gases at the engine outlet and the threshold strength λ R  required for regeneration. 
     At the end of the regeneration procedure, hydrocarbon injection is stopped, then the NO x  storage procedure is started again. 
     FIG. 4 relates to a second embodiment of the invention that differs from the first one in that injection means  4 , catalysts  2 ,  3  and the various temperature  5 ,  6 , mixture strength  7  or NO x    8  detectors are no longer arranged in main exhaust line  10 , but in a bypass line  9 . A valve or any other means intended to modulate the main flow of the exhaust gases coming from engine  1  is placed at the source of bypass line  9 . 
     This configuration allows, in comparison with the first one, to limit the flow of gases passing through the catalysts during regeneration of the trap. The amount of hydrocarbons to be postinjected in order to reach a mixture strength above 1 is thus decreased. 
     During the stage of NO x  storage on the NO x  trap, all of the exhaust gases flow through the line comprising the catalysts. Hydrocarbon postinjection is not activated. 
     Estimation of the amount of NO x  stored on the catalyst is similar to that described above. When the regeneration phase is activated, part of the gases is bypassed thanks to the valve means. The flow passing through the catalysts is thus limited, which allows to decrease the GHSV (Gas Hourly Space Velocity=Flow of gas/Catalyst volume). The amount of postinjected hydrocarbons depends on the exhaust gas mixture strength (provided by the engine data maps) and on the bypassed flow fraction. The fraction of flow X in bypass line  9  depends on the total gas flow and on the position of valve  11  at the head of bypass line  9 . As before, a proportional mixture strength probe placed downstream from the controlled oxidation catalyst or from NO x  trap  3  can be used to control the amount of postinjected hydrocarbons. 
     According to yet another embodiment of the invention, the exhaust line can also comprise an exhaust gas heating means  12  placed, according to FIG. 4, in bypass line  9 , just upstream from controlled oxidation catalyst  2 . 
     The working principle of this embodiment of the invention is globally the same as that described above. 
     The advantages of the third embodiment of the invention can be summarized as follows: 
     As mentioned before, hydrocarbon postinjection and therefore NO x  trap  3  regeneration depend on the temperature level of the gases passing through catalysts  2  and  3 . Thus, if this level is insufficient, regeneration of trap  3  cannot be activated, to the detriment of the global efficiency of the system. It may therefore be interesting to associate with the previous sytem a device  12  allowing to heat the gases in order to raise the temperature level during regenerations when it is too low. Temperature detectors  5 ,  6  respectively placed upstream and downstream from controlled oxidation catalyst  2  allow the gas heating system to be managed. Bypass line  9  is interesting on two accounts: it allows to decrease the amount of hydrocarbons to be postinjected in order to reach a mixture strength above 1 and it allows to limit the energy to be consumed for heating the gas fraction flowing through catalyst  2 . 
     In general, the interest of the invention is to consume, during regeneration of the NO x  trap, the oxygen contained in the exhaust gases by the hydrocarbons postinjected on controlled oxidation catalyst  2 , and to form carbon monoxide CO and hydrogen H 2  upstream from NO x  trap  3 . A mixture with a strength above 1, poor in oxygen and rich in CO and H 2  is thus obtained upstream from NO x  trap  3  during hydrocarbon postinjection, which favours both nitrates desorption and reduction of the desorbed NO x  on means ( 3 ). A notable improvement in the NO x  trap regeneration phase is thus obtained.