EP2038524A1

EP2038524A1 - Method of controlling the consumption of urea for a processing system for nitrogen oxides

Info

Publication number: EP2038524A1
Application number: EP07788963A
Authority: EP
Inventors: Arnaud Audouin
Original assignee: Peugeot Citroen Automobiles SA
Current assignee: PSA Automobiles SA
Priority date: 2006-07-11
Filing date: 2007-06-05
Publication date: 2009-03-25
Also published as: WO2008006984A1; FR2903728A1; FR2903728B1

Abstract

The invention relates to a method of controlling the consumption of urea intended to be used in a selective catalytic reduction (of SCR type) processing system for nitrogen oxides, installed in the exhaust line of the engine of a vehicle comprising a urea reservoir, the method comprising the following steps: (12) a coefficient of correction of the quantity of urea to be injected in the system is determined as a function of at least one parameter, said parameter(s) being selected from the group including: the quantity of urea consumed in the vehicle since a reference date, the distance traveled by the vehicle since a reference date and the volume of the urea reservoir installed in the vehicle, and (16) this coefficient of correction is applied to a quantity of urea that is predetermined (14) as a function of the speed of the engine, called uncorrected quantity, so as to obtain the quantity of urea to be injected, called corrected quantity.

Description

METHOD FOR CONTROLLING UREA CONSUMPTION FOR A SYSTEM

TREATMENT OF NITROGEN OXIDES

The present invention relates to a method for controlling urea consumption for use in selective catalytic reduction ("SCR") nitrogen oxide aftertreatment systems. The principle of an SCR system is to chemically reduce the nitrogen oxides, for example emitted by a Diesel type engine, by introducing a reducing agent into the exhaust line of the engine, upstream of a specific catalyst in which the chemical reaction takes place. Such a system is intended to allow diesel engines to meet the emission standards of nitrogen oxides which are increasingly stringent.

It is known in stationary industries to carry out the reduction of nitrogen oxides by ammonia according to a SCR type reaction. The major difficulty of this process, when it is to be applied to engines installed in vehicles, lies in the storage of ammonia in the vehicle, since the ammonia necessary for the reaction must be brought into the exhaust. For this purpose, several concepts have been proposed, allowing a safe storage of ammonia under form of solid urea, liquid urea in aqueous solution or in the form of ammonium carbamate.

In the current state of the art, it seems unrealistic to embark in a vehicle enough urea to achieve the full durability of the post-treatment system, since the volume to be expected is much too important. It has therefore been proposed to periodically fill the urea reservoir, for example at each emptying of the vehicle. In this case, the sizing of the tank is performed according to the planned emptying intervals and the average consumption of urea by the vehicle in question.

However, for the same vehicle, the urea consumption between two oil changes will not necessarily be the same, since, for example, some users use their vehicle in driving conditions generating an above average consumption of urea. Because of these differences, it is currently not possible to demonstrate that no user will be driven with a vehicle whose urea tank is empty. However, such a condition is required by certain regulatory authorities, and it is therefore necessary to remedy this drawback.

The invention solves this problem by proposing a method for controlling the consumption of urea for use in a selective catalytic reduction nitrogen oxide treatment system, called SCR, installed in the exhaust line of the engine. of a vehicle comprising a urea tank, the method comprising a plurality of successive steps.

First, it is determined, as a function of at least one parameter, a correction coefficient of the amount of urea to be injected into the system, the parameter (s) being chosen from the group comprising : the amount of urea consumed in the vehicle since a reference date, the distance traveled by the vehicle since a reference date and the volume of the urea tank installed in the vehicle. Then, this correction coefficient is applied to a predetermined amount of urea according to the engine speed, called the uncorrected amount, so as to obtain the amount of urea to be injected, called corrected amount.

The invention is intended to be integrated into the control strategies of a SCR-type nitrogen oxide post-treatment system. These control strategies, embedded on the vehicle in a specific SCR calculator or in an engine ECU, break down into two parts: a control module of the injection of urea, intended to determine the amount of urea to be injected into the vehicle. exhaust at every moment, and an onboard urea control module whose role is to ensure this injection, including managing the urea reservoir. Thus, the implementation means of the invention are added to the existing elements in the urea injection control module. This module thus comprises three sub-modules: the control of urea consumption, a calculation of the quantity of urea to be injected, intended to determine the quantity of uncorrected urea, and a closed-loop control of the quantity of urea. to inject.

The role of the different sub-modules will be described in detail below.

The major advantage of this method is to control the consumption of urea accurately to anticipate excessive consumption and ensure that any vehicle equipped with a system implementing such a process retains a quantity of urea sufficient to ensure current emission levels. This allows manufacturers to ensure that their vehicles meet the standards in force regarding gaseous emissions from vehicles.

The first step of the process consists in determining a correction coefficient reflecting the vehicle behavior in terms of urea consumption since a reference date. For example, in one embodiment of the invention, the date of the last emptying performed on the vehicle is used as the reference date; it will be possible to calculate the amount of urea consumed in the vehicle and the distance traveled by the same vehicle since the last emptying.

To determine the amount of urea consumed since a reference date, different methods can be used. Thus, in one embodiment, this amount is calculated by cumulating the quantities of urea injected at each moment since that date. In another embodiment, the method uses the level of urea remaining in the vehicle urea tank to correct and refine the amount of urea previously calculated.

The correction coefficient thus determined is, for example, a multiplying coefficient of less than 1, which is applied to the amount of uncorrected urea.

This amount of uncorrected urea is determined in a sub-module for calculating the amount of urea to be injected. Preferably, it corresponds to the amount of urea necessary to effect a total reduction of the nitrogen oxides emitted by the engine at a given instant. This quantity may be known since, for a given engine, the mass of nitrogen oxides released for a given engine torque and speed is known. Knowing the chemical reaction that takes place between urea and these nitrogen oxides, we can then determine the amount of urea necessary for a total reaction, an amount that we will correct using the coefficient determined above.

For example, if a vehicle consumes more than the average amount of urea in the first 1000 kilometers after emptying, the system will determine a relatively high correction factor, so that the maximum amount of urea injected is limited. . So the reaction of Chemical transformation of the nitrogen oxides will take place only in part. However, this limitation is such that the gaseous emissions of the engine comply with the standards in force concerning acceptable emission levels.

Certain parameters other than the engine torque and engine speed, such as engine parameters, process system parameters or parameters reflecting the gaseous composition of the exhaust gases can influence the reaction and it may therefore be useful to take them into account. counts for calculating the amount of uncorrected urea. Thus, according to the embodiments, the calculation is performed as a function of at least one parameter, this parameter (s) being chosen from the group comprising: a value of engine speed, a value of engine load, a catalyst temperature value, and a ratio between the amount of nitrogen oxides at the inlet to the treatment system inlet and the amount of nitrogen dioxide present in the vehicle exhaust gas, input of the treatment system.

In another embodiment, it is determined, for each vehicle, its state in terms of urea consumption, at a given instant, according to its consumption of urea since the last emptying and the distance traveled since the last emptying, or the distance to go to the next. The determination of this state makes it possible to classify the vehicle in a limitation level which can be used thereafter to determine the correction coefficient.

In this embodiment, the levels of urea injection limitation used are predetermined according to at least one parameter. This (these) parameter (s) may be the amount of urea consumed in the vehicle since a reference date, the distance traveled by the vehicle since a reference date, and the volume of the urea reservoir installed in the vehicle. the vehicle. These limitation levels will be described in detail later using a graph.

For each of the levels of limitation, it may be useful to determine at each moment the reduction to be applied to the amount of urea injection. Thus, in one embodiment, for each limitation level, the correction coefficient is determined as a function of the engine speed and the engine load, also called engine torque. This determination may in particular be carried out using a preestablished cartography. An example of mapping will be described later.

As previously described, this correction coefficient is then applied to the amount of uncorrected urea in order to obtain a corrected amount of urea.

The consumption of urea in a vehicle depends, of course, on the speed of the engine, but can also be a function of other parameters, internal or external to the engine. To better control the urea consumption of the vehicle, it can sometimes be useful or even necessary to take these parameters into account. For this purpose, in one embodiment, the calculation of the value of the correction coefficient takes into account at least one parameter chosen from the group comprising: the ambient temperature, the altitude and the water temperature of the engine.

Once the calculation of the amount of corrected urea has been carried out, this quantity can be transmitted to a submodule responsible for controlling the onboard urea and / or the control of the injection.

The invention can also be applied to the treatment of nitrogen oxides of a lean gasoline engine. The invention also relates to a system of post ^¬ nitrogen oxide treatment using a method as defined above.

Other features and advantages of the invention will become apparent with the description of some of its embodiments, this being done with reference to the drawings in which: FIG. 1 represents a block diagram of the module for calculating the quantity of of a system using a method according to the invention, Figure 2 shows a graph for distinguishing different levels of limitation used in a process according to the invention, - Figure 3 is a map scheme / motor for determining a correction coefficient in a method according to the invention, Figure 4 is an example of integration of a method according to the invention in the control strategies of a post-processing system, and Figure 5 is an example of operation of a motor using a method according to the invention.

FIG. 1 represents the functional architecture of a sub-module for calculating the quantity of urea to be injected, which can be used in an embodiment of the invention.

This sub-module consists of 3 blocks 10, 12 and 14 which respectively correspond to a determination of the level of limitation, a determination of the reduction of the quantity to be injected and a calculation of the quantity of urea to be injected.

The process implemented in this sub-module comprises several steps. In a first step, block 10 determines the level of limitation corresponding to the current state of this vehicle. For this, one can, for example, use a graph such as that shown in Figure 2.

On this graph are represented lines allowing to delimit different levels of limitation.

The abscissa is the distance traveled by the car in kilometers, and the ordinate is the amount of urea consumed. The extent of this graph is limited in abscissa by a number of kilometers 26 corresponding to the interval between two oil changes, and in ordinate by the volume of the tank

28. Indeed, the urea tank being filled with each emptying, the system is reinitialized each time; moreover, it is not possible to consume more urea than the quantity that can be loaded into the tank.

The graph of FIG. 2 comprises three lines delimiting four levels of limitation, numbered from 0 to 3. The tanks of the vehicles using a method according to the invention are of a size such that they contain the quantity corresponding to the average consumption of vehicle urea multiplied by the scheduled emptying interval. As long as the consumption of the vehicle remains lower than this average consumption, represented by the curve 20, the vehicle is in the limitation level 0.

If the vehicle, for whatever reason, consumes a large quantity of urea for a short distance, then the consumption may exceed this curve 20 and the vehicle will therefore be in the level of limitation 1. Similarly, if the consumption exceeds threshold curves 22 or 24, it will pass respectively in the limitation levels 2 and 3.

This evolution from one level to another can also take place in the other direction since, for example, a vehicle in a limitation level may decrease its consumption and thus return to one of the threshold curves shown in Figure 2.

After determining the level of limitation, it is determined in block 12 of Figure 1, the reduction to be applied to the amount of urea to be injected. This reduction is generally in the form of a correction coefficient, preferably strictly less than 1.

To determine this correction coefficient, it is possible to use a cartography such as that represented in FIG. 3. This cartography represents the operating torque of an engine, in Newton meters, as a function of the revolutions per minute regime. For each pair of values, the map includes, as an indication, the mass of nitrogen oxides released by the engine, in grams per hour, this mass can be determined using the scale 34. We see appear in this graph curves 30, 31, 32 and 33 respectively corresponding to the limitation levels 0, 1, 2 and 3 determined previously. The determination of the correction coefficient as a function of this mapping is done as follows: at a given moment, the point of operation of the vehicle is placed on the map at this moment, then we look at where is located in relation to the curve corresponding to the level of limitation in which the vehicle is at that moment; if the point is below the curve, the correction coefficient will be equal to 1, if the point is above the curve, the correction coefficient will be 0.

In this Figure 3 is also shown a curve 35 corresponding to an engine operating cycle such as that used by regulatory authorities to test vehicles. This curve is found below all the curves corresponding to the levels of limitation, which means that even when the vehicle is in the limitation level 3, enough urea is injected to comply with the standards in force.

The mapping of Figure 3 is shown here by way of example, one can just as well use another map, for example according to the type of vehicle on which the invention is used.

Preferably, the correction coefficient is not calculated from the operating characteristics of the engine using a mathematical law, but rather calibrated according to the use of the vehicle. Thus, for example, one can choose to limit the injection of urea on the points of heavy loads and high regime, because these points are highly urea consumers, while not limiting or that the injection at other points Operating.

Once this correction coefficient has been determined, a multiplier 16 (FIG. 1) is used to multiply the coefficient by the amount of uncorrected urea calculated in block 4. Thus, at the outlet of the system, the quantity of urea to Corrected injection, ie the actual quantity that will be fed to the exhaust in order to react with the nitrogen oxides emitted by the engine.

FIG. 4 represents a possible integration of the invention in the control strategies of a SCR-type nitrogen oxide post-treatment system. FIG. 4a shows an SCR control strategy 40 which consists of two modules: a urea injection control module 41, intended to determine the quantity of urea to be injected into the exhaust at each instant, and a module control of urea embedded 42 whose role is to ensure this injection, including managing the urea tank. FIG. 4b describes, in more detail, the urea injection control module 41. This module generally comprises a sub-module 44 for calculating the quantity of urea to be injected and a sub-module 45, intended to carry out a control in closed loop of the amount of urea to be injected. Preferably, a method according to the invention will be integrated in the module 41 in the form of a submodule 43 for controlling the consumption of urea. The three sub-modules 43, 44 and 45 have means for exchanging information with each other, in particular for transmitting urea quantities and / or correction coefficient values.

In Figure 5 are shown two curves for seeing the effect of a method according to the invention on the operation of an engine, for two types of users.

An "average" customer is a customer who uses his vehicle in such a way that the consumption of urea remains below average. It can be seen in the graph 5a that the urea consumption curve 50 never exceeds the first threshold curve, and the vehicle never leaves the limitation level 0. Thus, there is never a problem of excessive consumption, and it is therefore never necessary to limit the amount of urea injected.

On the other hand, in the case of a "severe" customer, the consumption of urea is much too important, and it is therefore necessary to control it to ensure that the urea reservoir of this customer will not be empty before the next tank drain.

It can indeed be seen in FIG. 5b that from the beginning of use of the vehicle after emptying, the customer has consumed a quantity of urea such as it is in the level of limitation 2, since the Urea consumption 52 is above the first two threshold curves. In this case, the correction coefficients determined will be relatively low, and the system will inject only little urea in the exhaust. This graph shows the interest of such a system since, rapidly, the urea consumption curve returns to the limitation level 1 and then to the limitation level 0, which corresponds to a "normal" use of the vehicle.

Claims

1. A method for controlling the consumption of urea for use in a selective catalytic reduction nitrogen oxide treatment system, SCR, installed in the engine exhaust line of a vehicle comprising a reservoir of urea, the process comprising the following steps: determining (12), as a function of at least one parameter, a correction coefficient of the quantity of urea to be injected into the system, the parameter (s) being selected from the group consisting of: the amount of urea consumed in the vehicle since a reference date, the distance traveled by the vehicle since a reference date and the volume (28) of the urea reservoir installed in the vehicle. vehicle, and applying (16) this correction coefficient to a predetermined amount of urea (14) depending on the engine speed, called uncorrected amount, so as to obtain the amount of urea to be injected, called corrected amount e.

2. Method according to claim 1 wherein the date of the last emptying performed on the vehicle is used as reference date for calculating the distance traveled and / or the quantity of urea consumed.

3. Method according to claim 1 or 2 wherein the calculation (14) of the amount of uncorrected urea is performed as a function of at least one parameter, the (the) parameter (s) being chosen (s) in the group comprising: an engine speed value, an engine load value, a catalyst temperature value, and a ratio between the amount of nitrogen oxides at the inlet of the treatment system and the amount of nitrogen dioxide present in the exhaust of the vehicle, at the entrance of the treatment system.

4. Method according to one of the preceding claims wherein the amount of urea consumed since a reference date is calculated by adding the amount of urea injected at each moment since that date.

5. Method according to one of the preceding claims wherein the calculation of the amount of urea consumed since a date of reference is corrected by using the level of urea remaining in the urea tank.

6. Method according to one of the preceding claims wherein, to determine the correction coefficient, is used predetermined limitation levels of urea injection according to at least one parameter, this (these) parameter (s) ) being selected from the group consisting of: the quantity of urea consumed in the vehicle since a reference date, the distance traveled by the vehicle since a reference date, and the volume of the urea reservoir installed in the vehicle .

7. Method according to one of the preceding claims wherein for each level of limitation, the correction coefficient is determined according to the engine speed and the engine load, also called engine torque.

The method of claim 7 wherein the determination of the correction coefficient is performed using pre-established mapping.

9. Method according to one of the preceding claims wherein takes into account, for the calculation of the value of the correction coefficient, at least one parameter selected from the group consisting of: ambient temperature, altitude and, temperature d engine water.

10. Method according to one of the preceding claims, the method being carried out by a urea injection control module (41), this module also performing a calculation (44) of the amount of uncorrected urea, and a closed loop control (45) of the amount of uncorrected urea.

11. Method according to one of the preceding claims wherein the amount of uncorrected urea is the amount necessary for a total reduction of nitrogen oxides, and wherein the value of the correction coefficient is greater than or equal to 0, and strictly less than 1.

12. A selective catalytic reduction nitrogen oxidation aftertreatment system using a process according to one of the preceding claims.