Process for optimizing a removal of nitrogen oxides from the gases in an engine exhaust line according to a selective catalytic reduction

Abstract

A process for optimizing a depollution of nitrogen oxides from the gases in an engine exhaust line carried out according to a selective catalytic reduction by injection of a quantity of reducing agent into the line makes it possible to monitor a setpoint of the amount of nitrogen oxides per second at the outlet of the line. A readjustment of the setpoint is made at each completion of successive running distance intervals determined by integration of the speed over a time interval that ends as soon as a predetermined target cumulative amount of carbon dioxide released is reached, an amount of nitrogen oxides at the outlet per kilometer traveled being calculated for each interval from a cumulative amount of nitrogen oxides measured at the outlet and compared with a target amount of nitrogen oxides per kilometer for the calculation of a deviation used for the readjustment of the setpoint.

Claims

1. A process for optimizing a depollution of nitrogen oxides from the gases in an internal combustion engine exhaust line of a motor vehicle, the depollution of nitrogen oxides being carried out according to a selective catalytic reduction by injection of a quantity of reducing agent into the line making it possible to monitor a setpoint of an amount of nitrogen oxides per second at an outlet of the exhaust line, the process comprising: readjusting the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line is made at each completion of successive running distance intervals, each distance interval being determined by integration of a speed of the vehicle over a time interval that ends as soon as a predetermined target cumulative amount of carbon dioxide released in the exhaust line is reached, calculating an amount of nitrogen oxides at the outlet of the exhaust line per kilometer traveled for each interval from a cumulative amount of nitrogen oxides measured at the outlet of the exhaust line and compared with a target amount of nitrogen oxides per kilometer for the calculation of a first deviation that is used for the readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line.

2. The process as claimed in claim 1, wherein the target amount of nitrogen oxides per kilometer is dependent on at least one of the following parameters: driving style, estimation of the total weight of the vehicle, use of additional equipment in the vehicle, road profile, engine temperature or coolant temperature, vehicle speed, combustion mode and atmospheric conditions such as outside temperature, ambient pressure, wind and/or altitude.

3. The process as claimed in claim 1, wherein a cumulative amount of carbon dioxide is estimated by mapping established at least as a function of the engine speed and of a demand for injection of fuel by the driver and is compared to the predetermined cumulative amount of carbon dioxide.

4. The process as claimed in claim 1, wherein a monitoring of the nitrogen oxide emissions is carried out for evaluation windows over a predetermined running distance, the successive running distance intervals being between a twentieth and a tenth of the predetermined running distances of the evaluation windows.

5. The process as claimed in claim 1, wherein the cumulative amount of carbon dioxide released in the exhaust line is reset to zero at the beginning of each running distance interval.

6. The process as claimed in claim 1, wherein the amount of reducing agent injected is calculated according to parameters of the exhaust line and/or of combustion in the internal combustion engine taken individually or in combination such as the amount of nitrogen oxides measured or estimated upstream of the injection of reducing agent, the temperature in the exhaust line, the gas flow rate in the exhaust line and the catalysis model.

7. The process as claimed in claim 6, wherein the calculation of the amount of reducing agent is carried out in an open loop.

8. The process as claimed in claim 7, wherein, at the same time as the readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line, for a same setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line, the calculation of a second deviation between the setpoint of the amount of nitrogen oxides per second and a measured or estimated amount of nitrogen oxides at the outlet of the exhaust line per second is carried out, the amount of reducing agent injected also being corrected according to this second deviation.

9. An assembly of an exhaust line and of a depollution monitoring-control unit, the exhaust line comprising: a selective catalytic reduction system with injection of a reducing agent into the line, a monitoring-control unit receiving estimates or measurements of amounts of nitrogen oxides at an outlet of the exhaust line at least downstream of a selective catalytic reduction system, wherein the monitoring-control unit carries out a process for optimizing an amount of reducing agent injected, the monitoring-control unit comprising means for integrating a speed of the vehicle in order to determine a distance interval, means for estimating or calculating a cumulative amount of carbon dioxide released in the exhaust line, means for prior storage of target amounts of carbon dioxide and nitrogen oxides at the outlet of the exhaust line per kilometer, means for calculating a first deviation between target and measured or estimated amounts of nitrogen oxides and means for readjusting a setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line as a function of this first deviation.

10. The assembly as claimed in claim 9, wherein the line comprises at least one of the following elements: an ammonia probe, an ammonia slip catalyst, one of these elements or these elements being positioned downstream of the selective catalytic reduction system, at least one passive or active sensor of nitrogen oxides positioned downstream of the selective catalytic reduction system, a particulate filter and an oxidation catalyst when the engine is a diesel engine or a three-way catalyst when the engine is a gasoline engine.

11. The process as claimed in claim 2, wherein a cumulative amount of carbon dioxide is estimated by mapping established at least as a function of the engine speed and of a demand for injection of fuel by the driver and is compared to the predetermined cumulative amount of carbon dioxide.

12. The process as claimed in claim 2, wherein the additional equipment in the vehicle is an air conditioner.

13. The process as claimed in claim 6, wherein, at the same time as the readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line, for a same setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line, the calculation of a second deviation between the setpoint of the amount of nitrogen oxides per second and a measured or estimated amount of nitrogen oxides at the outlet of the exhaust line per second is carried out, the amount of reducing agent injected also being corrected according to this second deviation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a schematic representation of a logic diagram of an embodiment of a process for optimizing a depollution of nitrogen oxides from the gases in an internal combustion engine exhaust line according to an aspect of the present invention, and

(3) FIG. 2 is a schematic representation of two distribution curves, respectively according to an aspect of the invention as a solid line and according to the prior art as dotted lines, of monitorings of nitrogen oxides at the outlet of a motor vehicle exhaust line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) With reference to FIG. 1, the process for optimizing a depollution of nitrogen oxides from the gases in an internal combustion engine exhaust line of a motor vehicle according to an aspect of the present invention is carried out by a selective catalytic reduction with injection of an amount of reducing agent into the exhaust line.

(5) The optimization process makes it possible to carry out the catalytic, reduction by monitoring a setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line. The process according to an aspect of the invention repeats steps 1 to 7 from the prior art but adds thereto steps of readjusting the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line starting from a mode of monitoring the emissions in grams per kilometer instead of grams per second. Also taken into account is a cumulative production of CO.sub.2 in the exhaust line for the determination of the measurement interval and an amount of NO.sub.x in grams produced per kilometer according to the running conditions of the vehicle, which will be defined below, is considered.

(6) As shown in FIG. 1, the readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line is calculated in 18 and a new setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line replaces in 1 the old setpoint of the amount of nitrogen oxides at each completion of successive running distance intervals.

(7) The new setpoint may be calculated by increasing or decreasing the previous setpoint, in absolute or relative terms. Its amplitude may be based on at least some of the parameters used to set the target amount of nitrogen oxides per kilometer. The change may also be limited to one zone, given that the NO.sub.x emissions at the outlet of the exhaust line may depend on several parameters, connected to the catalysis 4 such as the temperature, or the exhaust flow rate, the NO.sub.2NO.sub.x ratio, the soot loading, the NH.sub.3 loading, etc., and/or on the same parameters used for setting the target amount of nitrogen oxides per kilometer.

(8) The expression of the distance per kilometer is not limiting but is however preferred since it is well suited to the travel of a motor vehicle.

(9) From the speed of the vehicle, referenced 10 in FIG. 1, each distance interval is determined by integration of the speed of the vehicle in a time interval, which is referenced 11.

(10) This time interval ends as soon as a predetermined target cumulative amount of carbon dioxide released in the exhaust line is reached, which is referenced 15 in FIG. 1, it being possible for this target amount of CO.sub.2 to be calibrated. The calculation of the cumulative amount of carbon dioxide actually released, referenced 14, may be obtained from the demand for injection of fuel made by the driver, referenced 13, it being possible for this injection demand to be the pressure of his/her foot on an accelerator pedal.

(11) The cumulative amount of carbon dioxide actually released may be calculated from the demand for injection of fuel made by the driver, referenced 13, by mapping established at least as a function of the engine speed and of the demand for injection of fuel by the driver 13 in the calculation step referenced 14. This amount of carbon dioxide actually released is then compared to the predetermined cumulative amount of carbon dioxide, referenced 15.

(12) The distance interval was therefore calculated in 12 from the distance calculated by integration of the distance as was carried out in 11 and of the predetermined target cumulative amount of carbon dioxide calculated in 15 stopping the integration of the distance interval and delimiting the end thereof.

(13) In reference 9, a measured or estimated amount of nitrogen oxides at the outlet of the exhaust line is calculated per kilometer traveled. This amount of nitrogen oxides is calculated for each distance interval calculated in 12 from a measured cumulative amount of nitrogen oxides at the outlet of the exhaust line which was measured in 8 according to a step known from the prior art.

(14) The measured or estimated amount of nitrogen oxides at the outlet of the exhaust line per kilometer traveled, calculated in 9, is compared in 16 with a target amount of nitrogen oxides per kilometer for the calculation of a first deviation. The target amount of nitrogen oxides per kilometer was estimated in reference 17. A first deviation was then obtained at the outlet of step 16 between the measured or estimated amount of nitrogen oxides per kilometer and the target amount of nitrogen oxides per kilometer

(15) The deviation mentioned previously in the introductory part of the present application, calculated according to a prior art process in the step referenced 7, will be subsequently referred to as second deviation so as not to be confused with the first deviation, an essential feature of an aspect of the present invention.

(16) The first deviation at the outlet of step 16 is used in step 18 for the calculation of a new setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line which replaces in 1 the old setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line used for the calculation of the amount of reducing agent to be injected.

(17) In step 17, the target amount of nitrogen oxides per kilometer may be dependent on at least one of the following parameters: driving style, estimated total weight of the vehicle, use of additional equipment in the vehicle such as air conditioning, road profile, engine temperature and in particular coolant temperature, vehicle speed, combustion mode and atmospheric conditions such as the outside temperature, the ambient pressure, the wind and/or the altitude. The use of parameters other than those mentioned above is also possible.

(18) The target amount of nitrogen oxides, advantageously expressed in grams per kilometer, may be deduced from the use of several mappings including correction factors that are dependent on the parameters mentioned above.

(19) One mode of use of the process for optimizing a depollution of nitrogen oxides from the gases in an exhaust may be carried out within the context of a real driving emissions test. In this case, according to the test, a monitoring of the nitrogen oxide emissions may be carried out for evaluation windows spreading over a predetermined running distance. The successive running distance intervals calculated during the process according to an aspect of the invention may be between a twentieth and a tenth of the predetermined running distances of the evaluation windows.

(20) This is therefore carried out with distance intervals having a smaller number of kilometers traveled than that of the evaluation windows. This enables a readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line during this test for a better adjustment of the monitoring-control of the amount of reducing agent injected during this test. An evaluation window may for example be from 10 to 20 km and a distance interval from 0.5 to 2 km. This adjustment of the distance intervals may be carried out by calibration of the target cumulative amount of carbon dioxide released in the exhaust line as a function of the evaluation windows in order to always have distant intervals smaller than the evaluation windows.

(21) The cumulative amount of carbon dioxide released in the exhaust line may be reset to zero at the beginning of each running distance interval. This is done after the end of a distance interval performed in reference 12.

(22) As was mentioned in the prior art, which is also used in an aspect of the present invention, which is referenced 3, the amount of reducing agent injected may be calculated according to parameters of the exhaust line and/or of combustion in the internal combustion engine. Without being limiting, these parameters may be, taken individually or in combination: the measured or estimated amount of nitrogen oxides upstream of the injection of reducing agent, the temperature in the exhaust line, the gas flow rate in the exhaust line and the catalysis model which is referenced as 4. Furthermore, as is shown in reference 5, the calculation of the amount of reducing agent may be carried out in an open loop.

(23) In combination and at the same time as the readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line that an aspect of the present invention carries out, for a same setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line, the calculation of a second deviation between the setpoint of the amount of nitrogen oxides per second and a measured or estimated amount of nitrogen oxides at the outlet of the exhaust line per second may be carried out. The amount of reducing agent injected may be corrected according to the second deviation. This was described in detail in the introductory part of the present application and related to steps 2, 6 and 7.

(24) Thus an aspect of the present invention, in a preferred mode, retains the correction mode disclosed by the prior art which is advantageously carried out every second or every group of several seconds but adds thereto a monitoring of the amount of nitrogen oxides per kilometer by taking into account the running conditions, hitherto not taken into consideration by the prior art, for a readjustment of the setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line.

(25) An aspect of present invention also relates to an assembly of an exhaust line and of a depollution monitoring-control unit. In this assembly, the exhaust line comprises a selective catalytic reduction system with injection of reducing agent into the line. The monitoring-control unit receives the estimates or measurements of amounts of nitrogen oxides exiting via the exhaust line at least downstream of the selective catalytic reduction system. It is preferred, but not limiting, that the amounts of nitrogen oxides be measured by an NO.sub.x sensor downstream of the SCR system substantially at the end of the exhaust line.

(26) According to an aspect of the invention, for the implementation of a process for optimizing the amount of reducing agent injected as described above, the monitoring-control unit comprises means for integrating the speed of the vehicle in order to determine a distance interval, means for estimating or calculating a cumulative amount of carbon dioxide released into the exhaust line. This calculation may be carried out according to a mapping.

(27) These means are means for prior storage of target amounts of carbon dioxide and nitrogen oxides at the outlet of the exhaust line per kilometer, means for calculating a first deviation between target and measured or estimated amounts of nitrogen oxides and means for readjusting a setpoint of the amount of nitrogen oxides per second at the outlet of the exhaust line as a function of this first deviation.

(28) The monitoring-control unit may comprise or be combined with one or more mappings, in particular a mapping established at least as a function of the engine speed and of the demand for injection of fuel by the driver, for estimating the amount of carbon dioxide released and optionally the amount of nitrogen oxides at the outlet of the exhaust line, although a measurement by a nitrogen oxide sensor is preferred.

(29) It should be kept in mind that the SCR system has an actual monitoring-control unit and that this unit can undertake the monitoring of the process according to an aspect of the invention instead of or in addition to a monitoring-control unit of the exhaust line.

(30) The line may comprise at least one of the following elements: an ammonia probe, an ammonia slip catalyst, one of these elements or these elements being positioned downstream of the selective catalytic reduction system, the excess ammonia having to be neutralized instead of being released into the atmosphere, at least one passive or active nitrogen oxide trap, at least one passive or active nitrogen oxide sensor positioned downstream of the selective catalytic reduction system, a particulate filter and an oxidation catalyst when the engine is a diesel engine or a three-way catalyst when the engine is a gasoline engine.

(31) The NH.sub.3 probe may have an additional function than the monitoring of the depollution of NH.sub.3. Specifically, the integration of an NOR sensor at the outlet of the exhaust line may take into account the potential presence of NH.sub.3 under certain conditions, owing to the fact that the NOR sensor may have a sensitivity to NH.sub.3 and may therefore distort its measurement relative to the amount of NOR. This may be achieved by addition of an additional NH.sub.3 probe that is used for the depollution of NH.sub.3 with a function of detecting NH.sub.3 useful for the calculation of the amount of NOR at the outlet of the exhaust line.

(32) Specifically, certain NOR sensors with a sensitivity to NH.sub.3 may lead to a less accurate evaluation of the NOR at the outlet of the exhaust line since it is overestimated by the presence of NH.sub.3. However, with the use of the optimization process according to an aspect of the present invention, an excess of NH.sub.3 not used for the catalysis ought to be considerably reduced by the implementation of the process according to an aspect of the present invention.

(33) FIG. 2 shows a statistical distribution Stat dist of the NOR emission controls during evaluation windows for a process in accordance with the prior art along a dotted-line curve and a process in accordance with an aspect of the present invention along a solid-line curve. A vertical asymptote symbolizes the tolerable emission limit Em Lim not to be exceeded.

(34) The application of a process according to an aspect of the present invention gives a statistical distribution Stat dist with a narrow standard deviation compared to a process from the prior art. This is expressed by a reduction in the consumption of reducing agent symbolized by the highest arrow Eco Red ag pointing to the right. The statistical distribution according to an aspect of the present invention is nullified before the tolerable emission limit Em Lim unlike the statistical distribution of the prior art. There is therefore a decrease in the risk of exceeding the tolerable emission limit Em Lim, which is illustrated by the arrow dec risk pointing to the left.

(35) The table below gives a comparison between a basic scenario and three scenarios. The first scenario corresponds to an increased production of CO.sub.2 in the flow CO.sub.2/flow+, the second scenario corresponds to an increased mass of the vehicle mass +, an increased production of CO.sub.2 in the flow CO.sub.2/flow+ and a speed that is reduced or remains constant V=. The third scenario corresponds to aggressive driving Agg driv, an increased production of CO.sub.2 in the flow CO.sub.2/flow+ and a speed that is increased or remains constant V=+.

(36) The amount of CO.sub.2 or CO.sub.2 test is given in grams per kilometer or g/km. The CO.sub.2 window gives the mass of CO.sub.2. The real CO.sub.2 is the percentage of CO.sub.2. c real NON is the calculated real amount of NO in milligrams per second or mg/sec. Real flow and real v are respectively the flow rate in the exhaust line in kilograms per hour and the speed of the vehicle in kilometers per hour, win indicating an evaluation window expressed either in kilometers or in minutes. NO.sub.x Res is the NO.sub.x result expressed in milligrams per kilometer or mg/km for the four scenarios. These results vary as a function of the conditions of each scenario.

(37) If the amount of CO.sub.2 increases over a short distance, there is no impact on the amount of NO.sub.x per kilometer. If the amount of CO.sub.2 increases as well as the exhaust gas flow rate, there is no impact on the amount of NO.sub.x per kilometer.

(38) If the average speed of the vehicle decreases, there is an increase in the amount of NO.sub.x per kilometer and therefore the amount of reducing agent calculated by the prior art to be injected is not sufficient.

(39) In the case of aggressive driving, the amount of CO.sub.2 and the exhaust gas flow rate increase. There is no impact on the amount of NO.sub.x per kilometer. If the average speed of the vehicle increases, there is a decrease in the amount of NO.sub.x per kilometer and therefore the amount of reducing agent calculated by the prior art to be injected is too high compared to the amount actually needed.

(40) TABLE-US-00001 Base mass+, CO.sub.2/flow+, V= 125 g/km CO.sub.2 test 125 g/km CO.sub.2 test 1250 g CO.sub.2 win 1250 g CO.sub.2 win 10% real CO.sub.2 12% real CO.sub.2 1.55 mg/sec c real NO.sub.x 1.55 mg/sec c real NO.sub.x 50 kg/h real flow 60 kg/h real flow 70 km/h real v 68 km/h real v 2.11 g/sec real CO.sub.2 3.03 g/sec real CO.sub.2 9.9 min win 6.9 min win 11.5 km win 7.8 km win 0.92 g NO.sub.x win 0.64 g NO.sub.x win 80 mg/km NO.sub.x Res 82 mg/km NO.sub.x Res CO.sub.2/flow+ Agg driv, CO.sub.2/flow+, V=+ 125 g/km CO.sub.2 test 125 g/km CO.sub.2 test 1250 g CO.sub.2 win 1250 g CO.sub.2 win 12% real CO.sub.2 12% real CO.sub.2 1.55 mg/sec c real NO.sub.x 1.55 mg/sec c real NO.sub.x 60 kg/h real flow 60 kg/h real flow 70 km/h real v 72 km/h real v 3.03 g/sec real CO.sub.2 3.03 g/sec real CO.sub.2 6.9 min win 6.9 min win 8.0 km win 8.2 km win 0.64 g NO.sub.x win 0.64 g NO.sub.x win 80 mg/km NO.sub.x Res 78 mg/km NO.sub.x Res