FR2935019A1 - Process of treatment of the exhaust gases of an internal combustion engine, comprises applying a system for treating exhaust gases, comprising nitrogen oxide trap charged with palladium providing catalyst for oxidation of methane - Google Patents

Abstract

Process of treatment of the exhaust gases of an internal combustion engine, comprises applying a system (1) for treating exhaust gases, comprising a nitrogen oxide (NO x) trap (3) charged with palladium providing a catalyst for oxidation of methane and it includes an alternation of a first mode of operation of the system in which the exhaust gases have excess of oxidizing agent and a second mode of operation of the system in which exhaust gases have excess of reducing agent. Independent claims are included for: (1) data support (5) readable by a computer comprising software means for the implementation of the steps of the treatment; (2) NO xtrap (3) that is designed to be mounted on an operating system of the exhaust gas of an internal combustion engine, where the trap is charged with palladium; (3) a system (1) for the treatment of the exhaust gases of an internal combustion engine, comprising a NO xtrap (3), material means (4, 5) and/or software means for implementation of the treatment process; and (4) a motor vehicle comprising the NO xtrap.

Description

The present invention relates to a method for treating the exhaust gases of an internal combustion engine, in particular an internal combustion engine driving a motor vehicle. The invention also relates to a data medium comprising software means for implementing the processing method. The invention also relates to a system for treating the exhaust gases of an internal combustion engine implementing this method and to a motor vehicle comprising such a system.

The tightening of automobile pollution control standards is pushing automobile manufacturers to consider the issue of the treatment of methane contained in the exhaust gases, especially in the exhaust gases of diesel engines. Indeed, advances in the combustion or management of post-treatment systems provide responses in terms of consumption or emission of nitrogen oxides or NOx and particles, but are sources of methane emissions. For example, HCCI (Homogeneous Charge Compression Ignition) combustion produces a lot of methane on moderately charged operating points with low levels of EGR. Also, purges of NOx traps generate large amounts of methane.

It therefore appears necessary to treat the combustion gases or exhaust gases to reduce their methane content (whose molecules are very stable and therefore very difficult to oxidize with current treatment systems). It should be noted that methane is accounted for in unburned hydrocarbons by the European regulations on the emission of gaseous pollutants. The reduction of methane emissions is all the more important because its impact on the environment is significant. It is a greenhouse gas with a global warming potential of 21 times more than carbon dioxide.

Current formulations of oxidation catalysts and even new formulations of oxidation catalysts for equipping exhaust gas treatment systems do not provide sufficient methane conversion rates, especially for diesel engines.

At the moment, the regulations in force (Euro IV) do not specify the maximum quantity of methane emissions allowed for the exhaust of an internal combustion engine. To date, the quantities of methane emitted are not impacting for the approval of vehicles where only the total content of unburned hydrocarbons is regulated.

In order to perform the methane treatment, the new platinum and palladium catalyst formulations are in theory the best candidates, with a starting temperature of the order of 350 ° C. Such a solution must however be improved because the priming temperature is not sufficient to allow the conversion of methane on the first part of the homologation cycle (corresponding to an urban use of the vehicle).

Document WO 2007/087725 discloses a process for treating the exhaust gases of an engine using methane as a fuel. The process makes it possible to reduce the level of methane in the gases released into the atmosphere. This method provides for operating the engine under lean conditions for certain operating points and operating the engine under rich mixing conditions for some other operating points. This document provides a palladium catalyst for oxidizing methane in MS \ REN0112FR.dpt poor engine operation and provides NOx reduction in rich-mix engine operation. It is a 3-way catalyst which does not contain any NOx storage function, especially in an oxygen-poor medium. Also known from US 6,165,935 a palladium catalyst for treating the methane present in the exhaust gas of an engine using methane as a fuel and operating stoichiometric or rich mixture. Also known from JP 2001 140632 is an exhaust gas treatment system of an engine fueled mainly by methane, consisting of a NOx reduction catalyst present in the exhaust gas and a catalytic converter. Tin oxide based oxidation supporting platinum or platinum and palladium for the oxidation of methane also contained in the exhaust gas.

None of these documents provides a solution for treating gasoline or diesel engine exhaust by reducing their content of nitrogen oxides, methane and other hydrocarbons having a small carbon chain.

The object of the invention is to provide an exhaust gas treatment method for overcoming the drawbacks identified above and making it possible to improve the processing methods known from the prior art. In particular, the invention provides a simple process of treatment, implementing simple means to significantly reduce the methane content and nitrogen oxides of the exhaust gas engines gasoline or diesel. The process according to the invention makes it possible to treat the exhaust gases of an internal combustion engine. It is characterized in that it applies to an exhaust gas treatment system comprising a palladium loaded NOx trap providing a methane oxidation catalyst function and comprising an alternation of a first mode of operation of the treatment system in which the exhaust gases upstream of the NOx trap have an excess of oxidant and a second mode of operation of the treatment system in which the exhaust gases upstream of the NOx trap show excess reducer.

The first and second modes of operation can be respectively obtained by supplying the internal combustion engine with a lean mixture and a rich mixture. The second mode of operation can be obtained by injecting a reducer into an exhaust pipe upstream of the NOx trap or into a cylinder of the engine. The transition from the first mode of operation to the second mode of operation can be triggered by the crossing of a first threshold by the NOx storage efficiency or the crossing of a second threshold by the lower threshold. methane conversion efficiency. The transition from the second mode of operation to the first mode of operation can be triggered by the crossing of a third threshold downwards by the amount of NOx stored in the NOx trap and / or the crossing upwards of a fourth threshold by the amount of reducer introduced into the NOx trap since the activation of the second mode of operation. According to the invention, the data carrier readable by a computer comprises software means for implementing the steps of the previously defined processing method.

According to the invention, the NOx trap is intended to be mounted on an exhaust gas treatment system of an internal combustion engine and is characterized in that the NOx trap is loaded with palladium. The NOx trap may comprise at least 2.5 g / ft 3 of palladium, and preferably at least 20 g / ft 3 of palladium. According to the invention, the system for treating the exhaust gases of an internal combustion engine is characterized in that it comprises a NOx trap defined above and hardware and / or software means for carrying out the process. previously defined treatment. According to the invention, the motor vehicle comprises a previously defined NOx trap or a previously defined treatment system.

The attached drawing shows, by way of example, an embodiment of a system for treating the exhaust gases of a heat engine and an embodiment of an exhaust gas treatment method. a heat engine.

FIG. 1 is a diagram of an embodiment of a system for treating the exhaust gases of a heat engine according to the invention.

FIG. 2 is a flow diagram of an embodiment of a method for treating the exhaust gases of a heat engine according to the invention. FIG. 3 is a diagram of the conversion rate of methane by a palladium loaded NOx trap as a function of the time spent in a lean-burn engine operating mode. An exhaust gas treatment system 1 shown in FIG. 1 makes it possible to reduce the methane content of the exhaust gases of a heat engine, in particular a thermal engine for driving a vehicle. The system mainly comprises an exhaust manifold 2, a NOx trap 3 and a means 4 for modifying the reducing concentration upstream of the NOx trap. The NOx trap has the particularity of being loaded with palladium. The palladium load may be greater than about 2.5 g / ft 3 (about 0.085 g / L) and preferably greater than about 20 g / ft 3 (about 0.7 g / L), with 1 ft = 0.3048 m. This presence of palladium makes it possible to convert the methane (but also other small hydrocarbon molecules having, for example, a carbon chain comprising less than 6 carbon atoms) present in the exhaust gas. The tests carried out show that a palladium loaded NOx trap can satisfactorily provide the NOx treatment function while allowing the conversion of methane to more than 50%. Alternatively, the NOx trap has the particularity of being loaded with platinum. The means 4 for modifying the reducing agent concentration can control the introduction of reducing agents (such as hydrocarbons, hydrogen) into the exhaust line or late fuel injections into the combustion chamber of the engine. MS \ REN0112EN.dpt The system also comprises a computer 5 for controlling the means 4 for modifying the reducer concentration. The computer comprises means for implementing the treatment method according to the invention, that is to say to govern the operation of the treatment system according to the treatment method according to the invention and, in particular, in accordance with the method of the invention. execution of this method described below. In particular, it comprises means for implementing the various steps of the treatment method. In particular, it may comprise means for measuring the time, means for estimating the conversion rate of methane by the NOx trap. The treatment system may also comprise means for measuring the temperature of the exhaust gas and / or the NOx trap, means for determining the operating point of the engine (in particular its speed, its load, its mode rich / poor combustion). This information is for example transmitted to the computer via conductors 6, 7, 8.

The principle of the treatment method according to the invention is based on periodic excursions in rich mode (excess of reducing agent in the exhaust gas upstream of the NOx trap) making it possible to increase the amount of methane treated by the catalyst for the same temperature.

An embodiment of the treatment method according to the invention is described below with reference to FIG. 2. In a first step 110, the processing system is initialized. In particular, the amount of NOx already stored in the NOx trap is initialized.

Next, step 120 is made in which it is determined whether NOx treatment is a priority or whether methane treatment is a priority. It can be determined whether treatment of NOx or methane is a priority in determining the operating point of the engine. In a first operating zone of the engine, treatment of NOx can be defined as a priority and in a second operating zone the treatment of methane can be defined as a priority. If NOx treatment is determined to be a priority, step 130 is taken. If methane treatment is determined to be a priority, step 150 is taken.

In step 130, the storage efficiency of NOx in the NOx trap is estimated. Then, in a test step 140, it is tested whether this efficiency is lower than a first threshold value. If this is not the case, it is looped on step 130. On the contrary, if this is the case, we go to a step 170 in which we increase the concentration of the reductants in the exhaust pipe upstream of the trap to NOx. This is enabled by the action of the means 4 for modifying the reducer concentration. This high reductant concentration is then maintained to the desired NOx storage level.

In step 150, the time during which the treatment system has operated in a lean mixture (excess of oxidants) upstream of the NOx trap is determined. From this time and other possible parameters, such as the NOx trap temperature and the exhaust gas flow rate, and using a model, the efficiency of the methane oxidation reaction is determined. the NOx trap. Then in a test step 160, it is tested whether this efficiency is lower than a second threshold value. If this is not the case, we go to step 150. On the contrary, if this is the case, we go to the step 170 described above. MS \ REN0112EN.dpt In step 170, the amount of NOx stored in the NOx trap and the amount of reductant delivered to the NOx trap are further determined.

In a test step 180, it is tested whether the amount of NOx stored in the NOx trap is less than a third threshold. If this is not the case, we go to step 170. If this is the case, we go to a step 190.

In the test step 190, it is tested whether the amount of gearbox sent into the NOx trap since the beginning of step 170 is greater than a fourth threshold. If this is not the case, we go to step 170. If this is the case, we go to a step 200.

In step 200, the concentration of the reducing agents in the exhaust pipe upstream of the NOx trap is reduced (return to the nominal combustion mode). This is enabled by the action of the means 4 for modifying the reducer concentration. This reduced reducer concentration is then maintained and looped on step 110.

In a first variant embodiment of the treatment method, only one of the two test steps 180 or 190 may be implemented to decide on the end of the intake phase of the reducer in high concentration in the NOx trap. The test step 180 is used when the step 170 has been triggered following the test step 140 and the test step 190 is used when the step 170 has been triggered following the test step 160.

In a second variant of the processing method, the steps 130, 140 and 150, 160 can be implemented simultaneously in at least one operating zone of the engine. Step 170 is then implemented as soon as one of the conditions of steps 140 and 160 is achieved. MS \ REN0112EN.dpt The first and second variants can be combined.

In the various variants described, the frequency of the operating excursions of the treatment system in an operating mode with a high concentration of reducer varies according to the operating points of the engine and the emission treatment (NOx or CH4) which is defined as priority.

In the case where the treatment of NOx is a priority, the strategy of the operating excursions of the treatment system in a mode of operation with a high concentration of reducing agent is adapted to the NOx storage capacity of the NOx trap used (for example, the strategy described in EP 1 212 527) can be used.

On the contrary, in the case where the treatment of methane is a priority, the strategy of the operating excursions of the treatment system in an operating mode with a high concentration of reducing agent is adapted to allow a good conversion efficiency of the methane. In this case, the strategy is based on the calculation of methane conversion efficiency. This conversion efficiency is, among other things, dependent on the temperature, the gas flow rate, the time spent in lean mode and of course the properties of the NOx trap, in particular its palladium content. The curves in FIG. 3 represent the evolutions of the methane conversion efficiency of a palladium loaded NOx trap, as a function of the operating time in a lean mode, for three different temperatures.

MS \ REN0112EN.dpt The described treatment method allows continuous oxidation of methane at low temperatures (above 250 ° C). The solution is based on rapid activation of the catalytic reaction of oxidation of methane and / or promotion of the intrinsic capacities of the NOx trap by modifying the surface state of the NOx trap and more particularly the palladium contained in the NOx trap.

The reducing agents introduced in high concentration make it possible to modify the surface state of the NOx trap and in particular palladium which will have to be partially reduced. Periodical excursions in rich mode make it possible to lower the activation temperature of the methane oxidation catalyst function of the NOx trap by modifying the surface state of the palladium which makes it more active with respect to methane.

It has been observed that in order to achieve optimum efficiency of the NOx trap catalyst function, periodic mode trips with a high reductant concentration upstream of the NOx trap are required. They allow the NOx trap to recover its maximum capacity to process methane. It is also noted that the treatment of methane can be carried out only in a lean fashion to have the amount of oxidant (oxygen) sufficient to the reaction. The passage in rich mode (with excess of reducing agent) allows the passage of the palladium contained in the catalyst from the metallic state to the reduced state. This property of palladium to better convert the methane in lean mode after the treatment system has worked periodically in rich mode brings many benefits: -for the engines of the spark-ignition type such as 30 gasoline engines, natural gas engines or LPG engines, an adaptation of the control of the wealth MS \ REN0112FR.dpt (increase / decrease of the frequency of evolution around the wealth 1) makes it possible to improve the quantity of methane converted by the NOx traps. for engines of the compression-ignition type operating in a lean mixture such as, for example, conventional diesel engines or operating in a homogeneous mixture, the so-called lean burn, HCCI or PCCI gasoline engines, the improvement of the performance of the NOx trap to treat the Low temperature methane requires the implementation of control strategies of the engine calibration to allow operation in rich mode periodically.

The invention is of great interest for engines equipped with a NOx trap. Indeed, the NOx trap intrinsically requires an alternation of rich and poor modes, the NOx being stored in the NOx trap during the lean mode and reduced during the rich mode.

In the case of lean burn engine applications (stratified charge), the reactivation of the palladium can take place during purges of the NOx trap made with a richness greater than 1 on a periodic basis. The NOx trap is in this configuration, placed upstream of an oxidation catalyst. In this configuration, the NOx treatment function is then dissociated from that of the methane treatment.

In the description, several terms are used to designate the same mode of operation of the processing device. Indeed, the rich mode corresponds to a mode of operation in which the reductant concentration in the exhaust gas upstream of the NOx trap is high, that is to say that there is an excess of reducing agent. Such a mode is called rich mode because it can be obtained by a MS running engine with a rich mixture, that is to say with an excess of fuel.

Similarly, the lean mode corresponds to a mode of operation in which the concentration of reducing agent in the exhaust gases upstream of the NOx trap is low, ie there is an excess of oxidant . Such a mode is called lean mode because it can be obtained by operation of the engine fed with a lean mixture, that is to say with an excess of oxygen.

MS \ REN0112FR.dpt

Claims (10)

Claims: 1 Process for the treatment of the exhaust gases of an internal combustion engine, characterized in that it applies to an exhaust gas treatment system (1) comprising a NOx trap (3) charged with in palladium providing a methane oxidation catalyst function and in that it comprises an alternation of a first operating mode of the treatment system in which the exhaust gases upstream of the NOx trap have an excess of oxidant and a second mode of operation of the treatment system in which the exhaust gas upstream of the NOx trap has an excess of reducing agent. 2. Treatment process according to claim 1, characterized in that the first and second modes of operation are respectively obtained by supplying the internal combustion engine with a lean mixture and a rich mixture. 3. Treatment process according to claim 1 or 2, characterized in that the second mode of operation is obtained by injecting a reducer in an exhaust pipe upstream of the NOx trap or in a cylinder of the engine. 4. Treatment method according to one of the preceding claims, characterized in that the transition from the first operating mode to the second operating mode is triggered by the crossing down of a first threshold by the NOx storage efficiency or by crossing down a second threshold by the efficiency of methane conversion. MS \ REN0112FR.dpt 5. Treatment method according to one of the preceding claims, characterized in that the transition from the second operating mode to the first operating mode is triggered by the crossing down of a third threshold by the amount of NOx stored in the NOx trap and / or by crossing up a fourth threshold by the amount of gearbox introduced into the NOx trap since the activation of the second mode of operation. 6. Support (5) of data readable by a computer comprising software means for implementing the steps of the processing method according to one of the preceding claims. 7. NOx trap (3) for mounting on an exhaust gas treatment system of an internal combustion engine, characterized in that the NOx trap is loaded with palladium. 8. NOx trap (3) according to the preceding claim, characterized in that the NOx trap comprises at least 2.5 g / ft3 of palladium, and preferably at least 20 g / ft3 of palladium. 9. System (1) for treating the exhaust gas of an internal combustion engine, characterized in that it comprises a NOx trap (3) according to claim 7 or 8 and material means (4, 5 ) and / or software implementation of the treatment method according to one of claims 1 to 5. 10. A motor vehicle comprising a NOx trap according to claim 7 or 8 or a treatment system according to claim 9. MS \ REN0112EN.dpt