FR3098126A1 - MEASUREMENT SYSTEM IN EXHAUST GASES OF A NON-OXIDIZED GAS FORMING STICKY MOLECULES - Google Patents

Abstract

System for measuring, in the exhaust gases of a heat engine (2) of a motor vehicle, the quantity of an unoxidized gas forming sticky molecules, which can undergo oxidation to give an oxidized gas forming non-oxidized molecules sticky, this system comprising successively in the direction of passage of the exhaust gases, a first analyzer (6) of the quantity of oxidized gas providing a reference measurement (12), then a second analyzer (8) of the quantity of gas oxidized providing a total measurement (14), equipped at the inlet with an oxidation catalyst (10) carrying out an oxidation of the non-oxidized gas. Figure 1

Description

MEASUREMENT SYSTEM IN EXHAUST GASES OF A NON-OXIDED GAS FORMING STICKY MOLECULES

The present invention relates to a system for measuring sticky gases contained in the exhaust gases of the thermal engine of a motor vehicle, comprising adherent molecules, as well as a method for using such a measuring system, and a motor vehicle equipped with this measuring system.

The heat engines of motor vehicles emit exhaust gases containing pollutants that must be treated before being released into the atmosphere.

To carry out a dosage of certain molecules contained in the exhaust gases, it is known to place on the side of the flow an analyzer taking part of these gases to lead them to a cell for counting the molecules. A proportion is obtained which makes it possible, taking into account the total flow in the exhaust line, to obtain the quantity of these molecules contained in this total flow. This process is used in particular for common gases, oxygen O2, carbon dioxide CO2, carbon monoxide CO or nitrogen oxides NOx.

In particular, diesel type engines working in excess of oxygen, emit nitrogen oxides NOx which can be treated in a selective reduction catalyst, called "Selective Catalytic Reduction (SCR)" in English, receiving a reducing agent containing ammonia NH3 to form nitrogen N2 and water H2O.

SCR selective reduction catalysts having a certain capacity for accumulation of the reducing agent depending on the temperature of the catalyst, and the degree of conversion of nitrogen oxides NOx increasing with the level of ammonia NH3 loading of this catalyst, it It is therefore difficult to accurately predict the quantity of reducing agent to be delivered into this type of catalyst.

In addition, various other operating parameters of the thermal engine can cause an uncontrolled release of ammonia NH3 accumulated in the SCR selective reduction catalyst, which flows downstream, in particular strong variations in engine load, or regeneration of a particulate filter.

It is known to have successively at the outlet of the SCR selective reduction catalyst, a first analyzer for measuring nitrogen oxides NOx followed by a second analyzer for measuring ammonia NH3.

However, since ammonia NH3 is a gas with molecules which have the property of sticking to the walls, when these molecules are emitted, the various walls between the source and the measurement cell are saturated, which gives a delay time and a distortion of this measurement disturbing the results. It then becomes difficult to accurately estimate the flow of ammonia NH3 actually present in the exhaust line.

The same problem arises for other types of gas comprising sticky molecules, such as unburned hydrocarbons HC.

To avoid these problems, a known solution, presented in particular by document FR-A1-2902139, successively comprises a first selective reduction catalyst comprising an absolute capacity for accumulation of the reducing agent received containing ammonia NH3, forming a quantity guide for the dosage of this agent, then a second selective reduction catalyst comprising an absolute capacity of accumulation which is additional.

The dosage of the supply of reducing agent is made by taking only the absolute capacity of the first catalyst forming the guide variable, without taking into account that of the second catalyst. The second catalyst constitutes an additional capacity making it possible to intercept and absorb any release of ammonia NH3 coming from the first catalyst, to then consume it during the catalytic reduction of nitrogen oxides NOx arriving on this second catalyst.

However, this installation comprising two successive SCR selective reduction catalysts adds complexity to the vehicle's pollution control system, as well as additional mass and additional costs.

The object of the present invention is in particular to avoid these drawbacks of the prior art.

To this end, it proposes a system for measuring, in the exhaust gases of a heat engine of a motor vehicle, the quantity of a non-oxidized gas forming sticky molecules, which can undergo oxidation to give an oxidized gas forming non-sticky molecules, this system being remarkable in that it comprises successively, in the direction of passage of the exhaust gases, a first analyzer of the quantity of oxidized gas supplying a reference measurement, then a second analyzer of the quantity of oxidized gas providing a total measurement, equipped at the inlet with an oxidation catalyst carrying out oxidation of the non-oxidized gas.

An advantage of this measurement system is that the second analyzer receiving the oxidized gas thanks to the oxidation by the oxidation catalyst, by comparing its measurement with that given by the first analyzer, and by making the difference, we obtain simple part of the oxidized gas which comes from the oxidation of the non-oxidized gas present in the exhaust gases.

The quantity of non-oxidized gas present in the exhaust gases is deduced therefrom, without having to lead the non-oxidized and sticky gas along a duct leading to the second analyzer, which simplifies its production by allowing away from the exhaust line.

The measurement system according to the invention may further comprise one or more of the following characteristics, which may be combined with one another.

According to one embodiment, the oxidation catalyst is a catalyst which carries out the oxidation of ammonia NH3 to nitrogen oxides NOx.

According to another embodiment, the oxidation catalyst is a catalyst which carries out the oxidation of unburnt hydrocarbons HC to carbon dioxide CO2.

The invention also relates to a method for using a measurement system comprising any one of the preceding characteristics, noteworthy in that it subtracts the reference measurement given by the first analyzer from the total measurement given by the second analyzer, to deduce therefrom the quantity of non-oxidized gas present in the exhaust gases.

In particular, the method can calculate a quantity of ammonia NH3 to be injected into a selective reduction catalyst arranged upstream of the measurement system, from the measured quantity of non-oxidized gas present in the exhaust gases.

The invention further relates to a motor vehicle equipped with a heat engine and a pollution control system arranged in the exhaust line of this heat engine, noteworthy in that it comprises a measurement system comprising the any of the preceding characteristics.

In particular, the depollution system comprising a selective reduction catalyst, the oxidation catalyst is advantageously placed at a distance from the outlet of the selective reduction catalyst which is less than 10 cm.

The invention will be better understood and other characteristics and advantages will appear more clearly on reading the description below given by way of example, with reference to the appended drawings in which:

is a diagram showing a motor vehicle engine equipped with a measurement system according to the prior art;

comprises two diagrams presenting as a function of time, with a measurement system according to the prior art, for the first measurements of a pollutant comprising non-sticky molecules, and for the second measurements of a pollutant comprising sticky molecules, like ammonia;

is a diagram showing a motor vehicle engine equipped with a measurement system according to the invention;

are diagrams showing, as a function of time, a measurement of nitrogen oxides without ammonia, with a measurement system successively according to the prior art and according to the invention;

are diagrams presenting, as a function of time, a measurement solely of ammonia, with a measurement system successively according to the prior art and according to the invention; And

are diagrams presenting a measurement of nitrogen oxides with ammonia as a function of time, with a measurement system successively according to the prior art and according to the invention.

FIG. 1 presents a thermal engine 2 emitting exhaust gases which pass through a depollution line 4 controlled by a computer, comprising an SCR selective reduction catalyst receiving from an external tank a reducing agent comprising ammonia NH3 in order to to achieve a reduction of nitrogen oxides NOx.

As close as possible to the outlet of the SCR catalyst, a measurement system according to the prior art comprises a first analyzer 6 of the quantity of nitrogen oxides contained in the flow of exhaust gases, then just after a second analyzer 16 of amount of ammonia NH3 also contained in the exhaust gas stream.

Figure 2 presents quantities Q as a function of time T, for the measurement system according to the prior art.

The first diagram shows the quantity emitted 20 of a non-sticky pollutant at time T0 forming a substantially square window, such as the usual gases including oxygen dioxide O2, carbon dioxide CO2, carbon monoxide CO, carbon dioxide nitrogen NO2 or nitric oxide NO.

We obtain at time T1, after a time shift D corresponding to the speed of advance of the gases, the measurement 22 of this quantity emitted 20 undergoing a small deformation with a rounding of the angles because of the weak diffusion of the molecules on the walls. It then suffices to readjust the phasing of the measurement 22 by removing the time phase shift D, to obtain an estimated value 24 of the quantity emitted 20.

The second diagram shows the quantity emitted 26 of a sticky pollutant at time T0 forming a substantially square slot, such as ammonia NH3.

At time T1', after a greater time shift D', a measurement 28 of this quantity emitted 26 is obtained, occurring only after the saturation of the walls, which undergoes a strong deformation with a significant rounding of the angles because of the quantity of molecules diffusing on these walls. Even after recalibrating the phasing of the measurement 28 by removing the time phase shift D', we obtain an estimated value 30 which is quite far from the actual quantity emitted 26.

It then becomes complex to accurately estimate the flow of ammonia NH3 actually present in the exhaust gases, and to adjust the setting of the flow of reducing agent delivered to the SCR catalyst.

FIG. 3 shows the heat engine 2 with an exhaust line equipped with a measurement system according to the invention, comprising, as close as possible to the outlet of the SCR catalyst, the first analyzer 6 for the quantity of nitrogen oxides, then just after a second analyzer 8 for the quantity of nitrogen oxides, comprising at its inlet an oxidation catalyst 10 which oxidizes the ammonia NH3 entering this second analyzer into nitrogen oxides NOx.

The first analyzer 6 gives a reference measurement 12 of the nitrogen oxides NOx contained in the flow of exhaust gases. The second analyzer 8 gives a total measurement 14 comprising the measurement of the nitrogen oxides NOx contained in the exhaust gases, similar to the reference measurement 12, to which is added the measurement of the nitrogen oxides NOx coming from the oxidation of ammonia NH3 carried out by the oxidation catalyst 10.

The depollution line control computer then carries out a subtraction operation by subtracting the reference measurement 12 from the total measurement 14, to obtain the part of nitrogen oxides NOx coming from the flow of ammonia NH3, and deduce therefrom precisely the quantity of ammonia present in the exhaust gases leaving the SCR selective reduction catalyst.

In particular, by placing the oxidation catalyst 10 as close as possible to the outlet of the SCR selective reduction catalyst, in particular at a distance of less than 10 cm, a very small surface area of the ducts is obtained between this outlet and this oxidation catalyst giving a better response time of the measurement, and a minimization of the surface saturated with ammonia which sticks to the walls giving a limited deformation of the measurement.

It will be noted that the exhaust gases sampled to supply the second analyzer 8, heat the oxidation catalyst 10 on the way, which makes it possible to quickly cross its initiation temperature allowing it to carry out the oxidation reaction. In addition, in the case where it is sought to obtain a faster warming up of the oxidation catalyst 10, an electric heating device can be added to it.

It will also be noted that the pipe connecting the oxidation catalyst 10 to the second analyzer 8 can be long, for example several meters, to facilitate the installation of this analyzer in the vehicle, without degrading the quality of the measurement thanks to the good fluidity of the nitrogen oxides NOx circulating in this conduit.

Figures 4, 5 and 6 present quantities Q as a function of time T, comprising in each figure for the first diagram with a method according to the prior art, a first curve of quantity emitted of nitrogen oxides NOx 40, a second curve of quantity emitted of ammonia NH3 42, a third curve of measurement of nitrogen oxides 44 given by the first analyzer of nitrogen oxides 6, and a fourth curve of measurement of ammonia 46 given by the ammonia analyzer 16.

The second diagram of Figures 4, 5 and 6 show, with a method according to the invention, the first curve of the quantity emitted of nitrogen oxides NOx 40, the second curve of the quantity emitted of ammonia NH3 42, the third curve of first measurement of nitrogen oxides 44 with the first analyzer 6, a fifth curve of second measurement of nitrogen oxides 48 with the second analyzer 8, and a sixth ammonia calculation curve 50.

Figure 4 corresponds to exhaust gases comprising a constant rate emission of nitrogen oxides NOx and without ammonia NH3.

For the first diagram, the first emitted quantity curve 40, and the third measurement curve 44 given by the first nitrogen oxide analyzer 6, are at a constant value, the second curve 42, and the fourth curve 46 given by the ammonia analyzer 16, are at a zero value.

For the second diagram, the third first measurement curve 44 given by the first nitrogen oxide analyzer 6 and the fifth second measurement curve 48 given by the second nitrogen oxide analyzer 8 are identical. The removal of the third curve 44 from the fifth curve 48 gives the zero value of the sixth ammonia calculation curve 50.

Figure 5 corresponds to exhaust gases not comprising nitrogen oxides NOx, but a flow of ammonia NH3 forming a square crenel.

The first diagram gives a fourth ammonia measurement curve 46 which is greatly out of phase and distorted with respect to the second quantity emitted curve 42, due to the sticking of the molecules on the walls of the ducts which may have a length of several meters before reach the second analyzer 16.

For the second diagram, the fifth second measurement curve 48 given by the second nitrogen oxide analyzer 8 is very slightly out of phase and deformed, due to the transformation of the ammonia NH3 closer to the outlet of the SCR catalyst, in particular at less than 10cm, which leaves little surface for the bonding of these molecules. The nitrogen oxides NOx formed by the oxidation catalyst 10 from this ammonia NH3 pass to the second analyzer 8 with very little loss.

The removal of the third curve 44 from the fifth curve 48 gives the sixth ammonia calculation curve 50 which is equal to this fifth curve.

Figure 6 corresponds to exhaust gases comprising both a constant flow of nitrogen oxides NOx and a flow rate window of ammonia NH3. We obtain for the first diagram an addition of the curves presented by the first diagrams of figures 4 and 5.

For the second diagram, the fifth second measurement curve 48 given by the second nitrogen oxide analyzer 8 represents the addition of the nitrogen oxides contained in the exhaust gases and those coming from the ammonia oxidized by the oxidation catalyst 10.

Removing the third curve 44 from the fifth curve 48 gives the sixth ammonia calculation curve 50. of ammonia NH3 with good accuracy.

In general, the same process can be applied with other sticky molecules, in particular unburned hydrocarbons HC, using the oxidation catalyst 10 at the inlet of the second analyzer 8 transforming these hydrocarbons into carbon dioxide CO2 which does not is not sticky.

The measurement system according to the invention makes it possible in a simple manner to improve the precision of the measurement of certain molecules. Especially in the case of ammonia NH3 measurement, a nitrogen oxide analyzer 8 is used instead of the ammonia analyzer 16, which is cheaper.

Claims (7)

System for measuring, in the exhaust gases of a thermal engine (2) of a motor vehicle, the quantity of a non-oxidized gas forming sticky molecules, which can undergo oxidation to give an oxidized gas forming non-sticky molecules sticky gases, characterized in that it comprises successively, in the direction of passage of the exhaust gases, a first analyzer (6) of the quantity of oxidized gas supplying a reference measurement (12), then a second analyzer (8) of the quantity of oxidized gas providing a total measurement (14), equipped at the inlet with an oxidation catalyst (10) carrying out an oxidation of the non-oxidized gas. Measuring system according to Claim 1, characterized in that the oxidation catalyst (10) is a catalyst for the oxidation of ammonia NH3 to nitrogen oxides NOx. Measuring system according to Claim 1, characterized in that the oxidation catalyst (10) is a catalyst for the oxidation of unburned hydrocarbons HC to carbon dioxide CO2. Method of use, for a measurement system according to any one of the preceding claims, characterized in that it subtracts the reference measurement (12) given by the first analyzer (6) from the total measurement (14) given by the second analyzer (8), to deduce therefrom the quantity of non-oxidized gas present in the exhaust gases. Method of use according to claim 4 for a measurement system according to claim 2, characterized in that it calculates a quantity of ammonia NH3 to be injected into a selective reduction catalyst (SCR) arranged upstream of the measurement system, from the measured quantity of non-oxidized gas present in the exhaust gases. Motor vehicle equipped with a heat engine (2) and a pollution control system (4) arranged in the exhaust line of this heat engine (2), characterized in that it comprises a measurement system according to any of claims 1 to 3. Motor vehicle according to Claim 6, equipped with a depollution system (4) with a measurement system according to Claim 2 comprising a selective reduction catalyst (SCR), characterized in that the oxidation catalyst (10) is arranged at a distance from the outlet of the selective reduction catalyst (SCR) which is less than 10cm.