Method for selective catalytic reduction with desorption of ammonia from a cartridge in an exhaust line

Abstract

Disclosed is a method for selective catalytic reduction operating by desorbing ammonia from at least one storage cartridge in an exhaust line at the output of a motor vehicle engine, the cartridge being arranged in at least one bypass branch of a main line of the exhaust line. The exhaust gas flow rate in the bypass branch is controlled according to an estimated or measured temperature in the bypass branch and a desired amount of ammonia to be injected by desorption estimated in the exhaust line to provide a catalytic reduction of the nitrogen oxides present in the exhaust gas, a temperature of the cartridge being estimated according to the gas flow rate at the temperature estimated or measured during a given time interval and corresponding to an amount of desorbed ammonia equal to the desired amount of ammonia.

Claims

1. A method for selective catalytic reduction operating by desorption of ammonia from a storage cartridge (7) in an exhaust line (10) at a motor vehicle combustion engine (1) outlet, said storage cartridge (7) being positioned in a bypass branch (6) of a main pipe (6a) of the exhaust line (10), the method comprising: determining a desired amount of ammonia to be injected by desorption from the storage cartridge (7) into the exhaust line (10) to ensure catalytic reduction, at a location along the exhaust line, of nitrogen oxides present in the exhaust gases; controlling, via an electronic controller, a valve that diverts the exhaust gas from the exhaust line to the bypass branch in order to regulate a flow rate of the exhaust gases in said bypass branch (6) in order to bring a cartridge temperature of the storage cartridge (7) to a target temperature sufficient to desorb from the storage cartridge the desired amount of ammonia into the exhaust line (10) that ensures a catalytic reduction of the nitrogen oxides present in the exhaust gases, the cartridge temperature being estimated at the controller as a function of an amount of desorbed ammonia detected by an ammonia probe located downstream of the storage cartridge, and the valve being controlled based on said estimated cartridge temperature and a mapping, stored in a non-transitory memory of the controller, of cartridge temperatures and temperatures/flow rates of gases in said bypass branch, wherein the desired amount of ammonia is updated by repeatedly determining an amount of the nitrogen oxides remaining in the exhaust line (10) downstream of the location along the exhaust line of the catalytic reduction.

2. The method as claimed in claim 1, wherein in order to start desorption of the storage cartridge (7), the flow rate of the exhaust gases in said at least one branch is regulated by the valve so that the storage cartridge (7) reaches a predetermined desorption start temperature of 80° C. with a variation range of 15%, and at an end of life of the storage cartridge (7) when the remaining amount of ammonia in the storage cartridge (7) is equal to a predetermined percentage of an initial amount of ammonia varying from 0% to 20%, an increase in flow rate is implemented by said valve in order for the storage cartridge (7) to reach a predetermined maximum desorption temperature equal to 150° C. with a variation range of 15%, a remaining amount of ammonia in the storage cartridge (7) being estimated by the controller by subtracting, from an initial amount of ammonia of a new cartridge (7), a desorbed amount of ammonia estimated as a function of estimated cartridge temperatures stored in the memory of the electronic controller.

3. An exhaust line (10) at the motor vehicle combustion engine (1) outlet, the exhaust line (10) comprising: a main pipe (6a); a bypass branch (6) that diverges from the main pipe (6a) at a first location and rejoins the main pipe (6a) at a second location downstream of the first location; a selective catalytic reduction (SCR) catalytic converter (9) located downstream of the second location; a storage cartridge (7) positioned in the bypass branch (6), the storage cartridge configured to desorb ammonia for reduction of nitrogen oxides present in exhaust gases in the exhaust line; a valve (5) that regulates a flow of exhaust gases from the main pipe (6a) to the bypass branch (6); an ammonia probe (8) located downstream of the second location and upstream of the SCR catalytic converter; and a controller that controls the valve (5) for managing a desorption of a desired amount of ammonia from the storage cartridge, the controller configured to regulate the valve to bring a cartridge temperature of the storage cartridge (7) to a target temperature sufficient to desorb from the storage cartridge the desired amount of ammonia into the exhaust line (10), wherein the controller includes a non-transitory memory having stored therein a mapping of cartridge temperature to a temperature and flow rate of the exhaust gases in said bypass branch (6), the controller configured to apply an amount of desorbed ammonia detected by the ammonia probe to the mapping stored in the non-transitory memory in order to estimate the cartridge temperature, and to update the desired amount of ammonia by repeatedly determining an amount of the nitrogen oxides remaining in the exhaust line (10) downstream of the SCR catalytic converter.

4. The exhaust line (10) as claimed in claim 3, wherein the valve (5) is one of i) positioned at an inlet branch point of the storage bypass branch (6) on the main pipe (6a) of the exhaust line (10), the valve (5) being a three-way valve, or ii) positioned downstream of the inlet branch point of the bypass branch, the valve (5) being a two-way valve adjustable in plural opening positions between a complete opening and a complete closing of the valve (5).

5. The exhaust line (10) as claimed in claim 3, wherein the valve is positioned downstream of the inlet branch point of the bypass branch, the valve (5) being a two-way valve adjustable in plural opening positions between a complete opening and a complete closing of the valve (5), and wherein the main pipe (6a) of the exhaust line (10) comprises a back-pressure element positioned downstream of the inlet branch point and upstream of an outlet branch point of said at least one bypass branch (6).

6. The exhaust line (10) as claimed in claim 3, further comprising: a nitrogen oxides sensor (4b) being positioned downstream of the SCR catalytic converter (9) in the exhaust line (10).

7. The exhaust line (10) as claimed in claim 3, wherein the storage cartridge (7) is of elongated shape and comprises a median longitudinal passage (12) passing entirely through said at least one cartridge for the exhaust gases.

8. The exhaust line (10) as claimed in claim 4, further comprising: a nitrogen oxides sensor (4b) being positioned downstream of the SCR catalytic converter (9) in the exhaust line (10).

9. The exhaust line (10) as claimed in claim 5, further comprising: a nitrogen oxides sensor (4b) being positioned downstream of the SCR catalytic converter (9) in the exhaust line (10).

10. The exhaust line (10) as claimed in claim 4, wherein the storage cartridge (7) is of elongated shape and comprises a median longitudinal passage (12) passing entirely through said at least one cartridge for the exhaust gases.

11. The exhaust line (10) as claimed in claim 5, wherein the storage cartridge (7) is of elongated shape and comprises a median longitudinal passage (12) passing entirely through said at least one cartridge for the exhaust gases.

12. The exhaust line (10) as claimed in claim 6, wherein the storage cartridge (7) is of elongated shape and comprises a median longitudinal passage (12) passing entirely through said at least one cartridge for the exhaust gases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics, aims and advantages of the present invention will become apparent on reading the detailed description which will follow and on examining the appended drawings, given by way of nonlimiting examples and in which:

(2) FIG. 1 shows an assembly of a combustion engine and of an exhaust line exhibiting a bypass branch comprising an ammonia storage cartridge according to an embodiment of the present invention,

(3) FIG. 2 shows five curves of desorption, as percentage, of ammonia as a function of the temperature in degrees Celsius for five different formulations of storage salts in an ammonia storage cartridge which can be used in an exhaust line according to an embodiment of the present invention,

(4) FIG. 3 is a diagrammatic representation of a longitudinal section of an ammonia storage cartridge which can be used in an exhaust line according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) In that which follows, reference is made to all the figures taken in combination. When reference is made to one or more specific figures, these figures are to be taken in combination with the other figures in order to recognise the designated reference numerals.

(6) With reference to FIG. 1, there is shown a combustion engine 1 and an exhaust line 10 for discharge of the exhaust gases resulting from the combustion in the combustion engine 1. The exhaust line 10 can comprise, in the vicinity of an exhaust manifold of the engine 1, a reduction catalytic converter 2 and a particulate filter 3, for a compression-ignition engine 1, in particular a diesel engine 1 or an engine running on gas oil.

(7) In the case of a spark-ignition combustion engine, in particular an engine running on gasoline fuel or on a mixture containing gasoline, the line 10 can comprise a three-way catalytic converter and a gasoline particulate filter.

(8) An upstream nitrogen oxides probe 4a, also denoted upstream NOx probe, is positioned downstream of the particulate filter 3. Not all the characteristics relating to the reduction catalytic converter 2, to the particulate filter 3 and to the upstream NOx probe 4a are essential for the implementation of the present invention.

(9) Subsequently, the exhaust line 10 is divided into a main exhaust pipe 6a and at least one bypass branch 6. The bypass branch 6 illustrated in FIG. 1 comprises an ammonia storage cartridge 7. In FIG. 1, there is shown a valve 5 positioned at the inlet branch point of the bypass branch 6 on the exhaust line 10 upstream of the main pipe 6a.

(10) At the outlet branch point of the bypass branch 6 where it returns to the main pipe 6a of the exhaust line 10, there is shown a mixer 13 of the ammonia with the exhaust gases, in particular the exhaust gases which have passed in transit through the main pipe 6a.

(11) Finally, downstream of the outlet branch point of the bypass branch 6, an ammonia probe 8 can be positioned upstream of a selective catalytic reduction catalytic converter 9 or SCR catalytic converter forming part of an SCR system supplied by the cartridge 7. There is provided, downstream of the SCR catalytic converter 9, a downstream NOx probe 4b which makes it possible to check whether all the nitrogen oxides have been reduced. The mixer 13, the downstream NOx probe 4b and the ammonia probe 8 are not essential for the implementation of the present invention, whereas the selective catalytic reduction catalytic converter 9 is essential.

(12) The present invention relates to a selective catalytic reduction method operating by desorption of ammonia from at least one storage cartridge 7 in an exhaust line 10 at the outlet of a motor vehicle combustion engine 1, for example in an exhaust line 10 as shown in the figure, which is not limiting. There may be several bypass branches 6 from a main pipe 6a of the exhaust line 10 comprising one or more storage cartridges 7. The cartridge(s) 7 are positioned in the bypass branch(es) 6 of the main pipe 6a of the exhaust line 10.

(13) In the method according to the invention, an exhaust gas flow rate in the bypass branch(es) 6 is regulated as a function of the temperature of the exhaust gases measured or estimated in each bypass branch 6 respectively in order for the cartridge(s) 7 in each bypass branch 6 to be brought to a temperature sufficient to desorb an amount of ammonia necessary for a reduction of the nitrogen oxides present in the exhaust gases.

(14) The exhaust gas flow rate in the bypass branch(es) 6 can be regulated as a function of a temperature estimated or measured in each bypass branch 6 and of a desired amount of ammonia to be injected by estimated desorption into the exhaust line 10 in order to ensure a catalytic reduction of the nitrogen oxides present in the exhaust gases.

(15) The reaction time for the desorption of the ammonia from the cartridge is known. It is thus possible to anticipate the amount of ammonia desorbed during a predefined time interval.

(16) The temperature of each cartridge 7 can be estimated as a function of the gas flow rate at the estimated or measured temperature during a given time interval and corresponding to a desorbed amount of ammonia equal to the desired amount of ammonia.

(17) The desired amount of ammonia can be updated by estimating or measuring the amount of nitrogen oxides remaining in the exhaust line 10 downstream of the catalytic reduction, this advantageously by measurement of the downstream NOx probe 4b.

(18) The amount of ammonia desorbed can be monitored by measurement of the ammonia probe 8 upstream of the selective catalytic reduction catalytic converter 9.

(19) It is known that a storage cartridge 7 desorbs the ammonia which the cartridge 7 contains only above a minimum temperature. In order to ensure a start of desorption of each cartridge 7, the flow rate of the exhaust gases in the branch 6 containing the respective cartridge 7 can be regulated as a function of the temperature of the exhaust gases in the branch in order for the cartridge 7 to reach a predetermined desorption start temperature.

(20) This first predetermined desorption start temperature for the cartridge 7 can be 80° C. with a variation range of 15% around this desorption start temperature.

(21) At the end of life of a cartridge 7, a remaining amount of ammonia in the cartridge 7 can be estimated by subtracting, from the initial amount of ammonia contained in a new cartridge 7, a desorbed amount of ammonia estimated as a function of the temperatures and flow rates, stored in computer memory, in the branch associated with the cartridge 7.

(22) To ensure complete emptying of the cartridge 7, when the remaining amount of ammonia in said at least one cartridge 7 is equal to a predetermined percentage of the initial amount of ammonia representative of said at least one virtually empty cartridge 7 varying from 0 to 20%, an increase in flow rate is carried out in the branch containing the cartridge 7 to be completely emptied in order for the cartridge 7 to reach a predetermined maximum desorption temperature equal to 150° C. with a variation range of 15% around this maximum desorption temperature.

(23) FIG. 2 shows five curves of desorption, as percentage, of ammonia as a function of the temperature in degrees Celsius for five different formulations of storage salts in an ammonia storage cartridge which can be used in an exhaust line according to an embodiment of the present invention. The salt is a barium strontium dichloride of formula Ba.sub.xSr.sub.(1−x)Cl.sub.2 mixtures with x respectively equal to 1, 0.5, 0.25, 0.125 and 0. There is shown, on the ordinate, a desorption of ammonia, NH.sub.3 desorb, as percentage of an amount of ammonia initially contained as a function of a temperature T in degrees Celsius T° C. on the abscissa.

(24) Preferentially, formulations for which the desorption temperature is greater than 40° C. will be selected in order to avoid undesired desorption at ambient temperature.

(25) Looping with regard to the temperature necessary to release the ammonia, for example between 40-80° C. and 100-150° C., makes it possible to determine the level of filling of the cartridge.

(26) The presence of a stationary phase between 80-100° C. and 120-150° C. to release the last moles of ammonia makes it possible to further improve the accuracy of estimation of amount remaining when a cartridge is virtually empty, for example with an amount of ammonia of less than 20% of the amount of ammonia initially contained, thus signaling the need to switch to a new cartridge.

(27) The curve with x equal to 0 shows a percentage of ammonia desorption which is relatively constant at more than 80% of desorbed NH.sub.3, NH.sub.3 desorb, for temperatures of between 80 and 130° C., while such a stationary temperature phase for a relatively constant ammonia desorption does not exist for x equal to 0.5 or 0.25 or is shorter for x equal to 0.125.

(28) In another nonlimiting example which is not shown in the figures, use is made of a strontium dichloride SrCl.sub.2, which becomes strontium dichloride octamine Sr(NH.sub.3).sub.8Cl.sub.2 by absorbing ammonia or NH.sub.3 at low temperature. At a mean temperature of approximately 80° C. with a variation range of 15% around this temperature, desorption of the ammonia begins, releasing, according to the chemical reaction shown, at this temperature, seven moles of NH.sub.3. The strontium dichloride octamine then becomes strontium dichloride monoamine of formula Sr(NH.sub.3)Cl.sub.2.

(29) At a temperature of 150° C. with a variation range of 15% around this temperature, there occurs, in addition to the first chemical reaction, a reaction on the strontium dichloride monoamine of formula Sr(NH.sub.3)Cl.sub.2 to release one mole of NH.sub.3 while becoming strontium dichloride SrCl.sub.2. From this temperature, all the ammonia of cartridge 7 is released, which would not be the case between 80° C. and below 150° C., where ammonia remains in the form of Sr(NH.sub.3)Cl.sub.2.

(30) With reference again more particularly to FIG. 1, an exhaust line 10 for the implementation of a method as described above comprises a selective catalytic reduction system operating by desorption of ammonia from at least one storage cartridge 7, the catalytic reduction system comprising an SCR catalytic converter 9 downstream of the cartridge 7 which is managed by a controller 23.

(31) The cartridge(s) 7 are positioned in at least one bypass branch 6 of a main pipe 6a of the exhaust line 10, i.e. one cartridge 7 for one or for each bypass branch 6 or several cartridges 7 for one or more bypass branches.

(32) A valve 5 is present in branch 6 or for each branch and regulates an exhaust gas flow rate in this associated bypass branch 6. A means for estimating or measuring a temperature of the exhaust gases in the or each bypass branch 6 is present in order for a controller of the reduction system to manage the valve 5 for a desorption of a desired desorbed amount of ammonia in conformity with an amount of ammonia necessary to reduce the nitrogen oxides in the line 10.

(33) The controller of the reduction system can incorporate a mapping giving a temperature of the cartridge(s) 7 as a function of the temperature and of the flow rate of the exhaust gases in the bypass branch 6 which carries a cartridge 7 or cartridges 7.

(34) In a first preferred embodiment of the present invention, the valve 5 can be positioned at an inlet branch point of the associated bypass branch 6 on the main pipe 6a of the exhaust line 10. The valve 5 can be a three-way valve 5.

(35) In a second preferred embodiment of the present invention, which is an alternative to the first embodiment, the valve 5 can be positioned in the associated bypass branch 6 downstream of the inlet branch point of this bypass branch 6. The valve 5 can be a two-way valve 5 adjustable in several opening positions between a complete opening and a complete closing of the valve 5.

(36) For this second preferred embodiment, when the valve 5 is a two-way valve 5 adjustable in several positions, the main pipe 6a of the exhaust line 10 can comprise a back-pressure element 29 positioned downstream of the inlet branch point and upstream of an outlet branch point of said at least one bypass branch 6. This back-pressure element 29 can be a branch element of a line for recirculation of the exhaust gases to an air intake of the combustion engine 1.

(37) The exhaust line 10 can comprise at least two bypass branches 6 each containing one or more cartridges 7, each bypass branch 6 being associated with a respective valve 5. The valves 5 of the bypass branches 6 can be managed with different openings and closings provided that an addition of the two amounts of ammonia produced in each of said at least two branches 6 gives the desired amount of ammonia.

(38) FIG. 3 shows an embodiment of an ammonia storage cartridge 7. The cartridge 7 is of elongated shape and comprises a median longitudinal passage 12 passing entirely through it for the exhaust gases. A storage compartment 11 of cylindrical shape surrounds the median longitudinal passage 12. It is this compartment 11 which comprises the ammonia storage material, for example a strontium dichloride, which is not limiting.