WO2011114028A1

WO2011114028A1 - Method for monitoring polluting emissions from a vehicle hybrid engine

Info

Publication number: WO2011114028A1
Application number: PCT/FR2011/050308
Authority: WO
Inventors: Evangelos Georgiadis; Charles Bizet
Original assignee: Peugeot Citroën Automobiles SA
Priority date: 2010-03-19
Filing date: 2011-02-15
Publication date: 2011-09-22
Also published as: FR2957578A1

Abstract

The invention relates to a method for monitoring a system for treating the Nox that are present in a hybrid engine exhaust line, in which: for a request for torque (20), greater that an initial torque (10), the predictable temperature T in the catalytic converter SCR corresponding to the request for torque (20) when the latter is fully assigned to the thermal torque is determined; said predictable temperature T is compared to a reference temperature for which the mass of the reducing agent present in the catalytic converter SCR is optimal for reducing the NOx; and part of the requested torque (20) is assigned to the electric torque if the predictable temperature T is greater than a reference temperature. The invention also relates to a device for implementing the above-mentioned method.

Description

METHOD FOR CONTROLLING POLLUTANT EMISSIONS OF AN ENGINE

HYBRID VEHICLE

The present invention claims the priority of the French application 1051987 filed March 19, 2010 whose content (text, drawings and claims) is here incorporated by reference.

The present invention relates to a method for controlling pollutant emissions of nitrogen oxides by injecting a reducing agent into the exhaust line of a gasoline or hybrid diesel engine. [ooo3] The use of fossil fuel such as oil or coal in a combustion system, particularly in a combustion engine, results in the production of significant amounts of pollutants that can be discharged through the exhaust into the environment and cause damage. Among these pollutants, especially nitric oxide or nitrogen dioxide collectively noted nitrogen oxides (called NO _x ). Nitrogen oxides are a particular problem since these gases are suspected to be one of the factors contributing to the formation of acid rain and deforestation. Additionally, NO _x are linked to health problems for humans and are a key part of the training smog in cities. The legislation imposes levels of increasing rigor for their reduction and / or their elimination including emissions from vehicles.

[ooo4] A solution to reduce the use of heat engines, and therefore the atmospheric pollutants they contribute to generate, is to use a hybrid engine that combines two modes of energy generation, a heat engine and an engine. electric machine. Even if certain running phases are then operated with the engine stopped or idle, the fact remains that this type of vehicle I always produces a significant amount of NO _x that must be processed.

[ooo5] One of own solutions to the treatment of NO _x in the exhaust of the vehicle is the selective catalytic reduction (SCR). This process is based on a selective reduction of NO _x in nitrogen (N ₂ ) and water (H ₂ O), in the presence of a catalyst specific, by the action of a reducer. This reducer is generally injected upstream of the SCR catalyst in the exhaust line. The reduction is carried out in a medium containing an excess of air. The reducing agent may be one or more hydrocarbon (s), partially oxidized hydrocarbon species, or more commonly ammonia (NH ₃ ), or a compound, such as urea, generating ammonia by chemical decomposition.

The SCR catalysts are generally devices comprising micropores, or pores, in which the reducing agent to be stored, for example ammonia, is adsorbed. For NO _x reduction to be maximum, a relatively large amount of ammonia must be adsorbed. However, it has been found that, under the effect of a high temperature, desorption of ammonia occurs competing with the adsorption of the latter, thus leading to an inadvertent release of pure ammonia into the atmosphere, at levels that may exceed regulatory requirements in this area. [ooo7] Thus, the optimum mass of reducing agent present in the SCR catalyst to effect a maximum reduction of NO _x is a function of the temperature in the SCR catalyst and this mass of reducing agent must be at the nearest, but always lower of the desorption limit from which the SCR catalyst discharges ammonia. [ooo8] The ammonia injection upstream of the SCR catalyst is managed by specific control functions and strategies, housed in a specific computer or in the engine ECU so that the mass of reductant present in the SCR catalyst is adjustable, in view of the mass of reducing agent already present and the temperature in the catalyst, to approach as closely as possible the mass of ammonia called optimal for the conversion / reduction of NO _x .

[ooo9] The problem comes from the unpredictable evolution of the temperature in the SCR catalyst. Thus, during a strong acceleration, the temperature in the SCR catalyst can go from low to high in a very short time, resulting in the desorption of ammonia. The present invention intends to remedy this problem in vehicles with hybrid drive through the control of electrical and thermal torque, especially during the phases of strong acceleration.

[Ooi i] According to the invention, it is thus proposed a control method of a nitrogen oxides treatment system (NO _x ) present in an exhaust line of a hybrid engine of a vehicle capable of providing an electric torque and a thermal torque, said system comprising means for introducing into the exhaust line a reducing agent upstream of an NO _x reduction catalyst, the reducing agent being intended to be used for chemically reducing the NO _x , during a catalytic reduction reaction called "SCR" in the reduction catalyst, characterized in that, during a propulsion phase of the vehicle, it is determined, for a torque demand greater than a couple initial, the predictable temperature T in the catalyst SCR corresponding to the torque demand when the latter is allotted to the thermal torque; this predictable temperature T is compared with a reference temperature T 0 for which the mass of reducing agent present in the SCR catalyst is considered optimal for the reduction of NO _x ; and allocating a portion of the requested torque to the electrical torque if the predictable temperature T is greater than To.

According to the invention, the optimum reducing agent mass present in the SCR catalyst is considered to be reached when the NO _x reduction is maximum while no phenomenon of desorption of the reducing agent in the SCR catalyst is observed.

According to other advantageous features of the invention:

the part of the torque attributed to the electrical torque corresponds to the additional torque, ie to the difference between the initial torque and the requested torque; the additional torque attributed to the electrical torque is exclusively attributed to said torque only during a transient phase ti from the instant t ₀ of the torque request; the transient phase lasts for at least 0.5 seconds; beyond the transitional phase t-ι, the part of the additional torque supported by the electric torque decreases to a zero value; the part of the additional torque supported by the thermal torque correspondingly increasing; the reduction of the part of the additional torque supported by the electrical torque is linear or substantially linear; the entire torque requested, that is the initial torque and the additional torque, is attributed to the engine torque when the foreseeable temperature T determined is lower than the aforementioned reference temperature To. [0014] The invention also relates to a control device for the implementation of the method described above, comprising a supervisor capable of determining the temperature in the SCR catalyst and the mass of reducing agent present in said catalyst, characterized in that the function of the supervisor is: to determine a predictable temperature T in the catalyst SCR as a function of a torque demand greater than an initial torque, this predictable temperature T being determined under the assumption that the totality of the requested torque is attributed to the thermal torque; determining whether this predictable temperature T is greater than a reference temperature To for which the mass of reducing agent present in the SCR catalyst is considered optimal for the reduction of NO _x ; and controlling the allocation of at least a portion of the requested torque to the electrical torque if the predictable temperature T is greater than the reference temperature To.

Advantageously, the supervisor checks, as a function of the mass of reducing agent and of the temperature in the SCR catalyst, as well as of the reference temperature To, of injecting a quantity of reducing agent. in the exhaust line and is able to control this injection. Other details and advantageous features of the invention appear from the detailed description given below by way of non-limiting example with the aid of the attached figures in which:

FIG. 1 shows a graph illustrating the mass of reducing agent that can be stored in the SCR catalyst as a function of the temperature in said catalyst;

FIG. 2 shows a graph identical to that of FIG. 1 in which it is noted the evolution of the efficiency of the SCR catalyst as a function of different loading / injection instructions of ammonia in the SCR catalyst, and in particular of a flat setpoint;

FIG. 3 shows a graph identical to that of FIG. 1 in which it is shown how the value of To can be determined.

FIG. 4a shows a diagram illustrating the operation without optimization strategy of the electrical / thermal torque and its consequences at the level of the SCR catalyst;

FIG. 4b shows a diagram illustrating the operation according to the method of the invention and its consequences on the SCR catalyst;

FIG. 5 shows a functional diagram of a control method of an NO _x treatment system according to the invention. For convenience, in the following, the nitrogen oxides are frequently designated by the NO _x nomenclature. Similarly, the selective catalytic reduction type catalyst is referred to as the SCR catalyst or simply SCR.

In addition, the terms "reducing agent" and "ammonia" are used interchangeably, knowing that in all cases, all the means and features described herein can be used with any other reducing agent. Ammonia used for NO _x reduction can be in any phase, liquid, gaseous or solid. By way of example, an additive consisting of a 32.5% aqueous solution of urea can be used. [Ooi 9] The structure of the nitrogen oxides treatment system and that of the exhaust line, not shown in the accompanying figures, is conventional and should not undergo any particular modification in the context of the present invention. Thus, in the usual way, the nitrogen oxides from the engine are directed to an SCR catalyst in which the chemical reduction of NO _{x takes place} . In order for this reduction to occur, it is necessary to add ammonia to the nitrogen oxides, for example contained in liquid urea. This liquid urea is usually stored in a specific tank installed in the vehicle. This reservoir is connected, via a feed pipe, to a specific injector, for injecting urea into the exhaust line of the engine, upstream of the catalyst SCR. The various elements of the exhaust line are conventionally managed by an on-board computer, calculator or supervisor, of the vehicle using different information records.

The information recorded for the management of the exhaust line include temperature measurements using temperature sensors located in the SCR catalyst or these temperatures can be obtained by modeling, from experimental data recorded. in a memory, using temperature sensors placed at other locations in the exhaust line. As regards the mass of ammonia to be injected into the exhaust line, this information is conventionally obtained by experimental modeling on the basis of the quantity of NO _x produced as a function of the vehicle traveling conditions and / or using a sensor. (s) NO _x usually located at the output of the engine. With the mass of ammonia injected into the exhaust line, the amount of NO _x produced by the engine, as well as the temperature in the SCR catalyst, the mass of ammonia present in the SCR catalyst can be determined.

The Applicant is part of the observation of the ratio between the ammonia mass optimal for maximum conversion / reduction of NO _x in the SCR catalyst as a function of the temperature in this catalyst. The expression "optimum ammonia mass" means that the quantity, or mass, of ammonia present in the SCR catalyst allows a maximum reduction of the NO _x emitted by the engine, while avoiding a desorption phenomenon of ammonia in the SCR. The evolution curve of the optimum ammonia mass in the SCR catalyst is shown in Figure 1 and note that on the one hand that curve follows substantially parallel to that, shown in dashed line, the mass of ammonia limit in the SCR before desorption of ammonia and secondly that the Optimum ammonia mass (and thus the limit before desorption) decreases with increasing temperature.

It is understood that to avoid any risk of desorption, a solution could consist in having, depending on the temperature in the SCR catalyst, a mass of ammonia in the catalyst substantially lower than the limit mass before desorption so that in case of rapid increase of the temperature in the SCR catalyst, thanks also to the stop or the reduction of the injection of ammonia in the exhaust line, the risk of desorption would be relatively low. Nevertheless, as is apparent in FIG. 2, the more the mass of ammonia present in the SCR catalyst decreases and moves away from the limit mass before desorption, the lower the efficiency of the SCR system. Thus, to optimize the reduction of NO _x (mass of ammonia close to the limit mass before desorption) and to avoid any desorption by a management of the variation of the mass of ammonia present in the SCR catalyst, it would be necessary to be able to vary this mass of ammonia very quickly.

However, in case of strong acceleration, that is to say a torque demand by the driver much greater than the initial or previous torque, the temperature usually rises very rapidly in the SCR catalyst and it It is not possible to reduce the mass of ammonia present in the SCR catalyst sufficiently rapidly if this mass of ammonia is optimal for the reduction of NO _x , that is to say (very) close to the limiting mass. before desorption. As also illustrated in Figure 2, another solution to avoid the desorption of ammonia is to inject a quantity of ammonia in the exhaust line to maintain the mass of ammonia constant in the SCR catalyst; this constant mass of ammonia being chosen such that no desorption phenomenon can occur whatever the temperature in the SCR catalyst. It is understood that, if the risk of desorption of ammonia is removed, the effectiveness of the SCR catalyst is low and much lower than the case of an optimal mass of ammonia in the SCR catalyst. Figure 3 illustrates a system according to the invention by essentially reproducing the scheme of Figure 1, that is to say, in a hypothesis favoring the efficiency of the catalytic treatment. At the initial moment, the mass of ammonia in the catalyst is optimized for the catalyst temperature

If, taking into account the demand for additional torque, the temperature of the catalyst is expected to rise to a level T. If this temperature T is greater than the temperature T ₀ for which, at iso-mass of ammonia, a desorption takes place, there is a risk of uncontrolled desorption if the transition is too fast. If on the other hand this rise in temperature can be controlled, the control of the stored mass will allow to remain along the optimal curve.

From the foregoing, it is noted that the value of T ₀ depends on the value of

But since the optimal mass and desorption risk curves are essentially parallel, it is possible to calculate T ₀ by adding a predefined difference to Tjnj _. It is also possible to choose a value in a reference table, in other words by an appropriate mapping, defined during the engine tuning phases, possibly taking into account the nature of the catalyst and the gearbox.

FIGS. 4a and 4b respectively illustrate the case where the treatment system plans to bring the mass of reducing agent into the SCR catalyst at its optimal or substantially optimal value (= maximum efficiency of the SCR) but without otherwise applying the the invention and the case where the same management of the reducing agent mass in the SCR catalyst is applied this time using the invention, namely by optimizing the electrical / thermal torque.

To illustrate the differences, in both cases, we start from an identical initial pair for which the temperature in the catalyst SCR is equal to an initial value T _ini . At time t ₀ , a new torque 20, greater than the initial torque 10, is requested by the driver. This new torque 20 has an additional torque noted 30 with respect to the initial torque 10.

In a conventional operation of a hybrid engine, during the propulsion of the vehicle using the only thermal torque, the additional torque is generally affected almost completely to the thermal torque so that the temperature increases very rapidly in the SCR catalyst during a strong acceleration, to switch to a temperature T. Evidently, this temperature T is all the more important and higher than the initial temperature

that the torque 20 is large compared to the initial torque 10. In the case illustrated in FIG. 4a, the mass of ammonia present in the SCR catalyst essentially decreases because of its consumption by the addition of a quantity more important of NO _x but also thanks to the stop / reduction of the injection of ammonia in the exhaust line. Nevertheless, this decrease of the mass of NO _x present in the SCR catalyst is relatively low and can not compete with the very rapid increase of the temperature in the SCR so that the mass of ammonia in the SCR becomes greater than the limiting mass. before desorption. As a result, as shown in this figure, during a relatively long phase, a desorption phenomenon of the reducing agent is observed in the SCR catalyst.

In Figure 4b, the same conditions being considered, the invention is applied and it is intended to control the temperature in the SCR catalyst via the control of electrical and thermal torque. Thus, at the instant t ₀ (torque request 20), the new predictable temperature T is determined in the case of an exclusive allocation of the torque 20 to the thermal torque, that is to say in the case illustrated on FIG. Figure 4a. If the foreseeable temperature T is determined higher than the reference temperature T ₀ for which the mass of ammonia is optimal in the catalyst (maximum efficiency of the SCR without however observing a desorption phenomenon), there is a risk of desorption and allocates the additional torque 30 to the electrical torque. The allocation of the additional torque 30 to the electrical torque makes it possible not to increase, or almost not, the temperature in the SCR catalyst. The additional torque 30 is ideally exclusively allocated to the electrical torque during a transient phase t ₀ -ti (of the order of a few hundred milliseconds to a few seconds, depending on the difference between T and To) and then beyond t-ι, the portion of the additional torque 30 attributed to the electrical torque decreases while the portion attributed to the thermal torque increases correlatively. Thus, for the requested torque, a final temperature T 'is obtained which is lower than the predictable temperature T. In addition, the increase in temperature is also controlled so that the final temperature T 'is reached later (lower temperature increase slope) than in the case of standard management of the hybrid engine (mode of the Figure 4a). It will be noted that, moreover, the reduction in the mass of ammonia present in the SCR catalyst is identical, or substantially identical, because the reduction / stop of the injection of ammonia into the exhaust line is ideally based on on the predictable temperature T and not on the actual temperature in the SCR. Of course, from the exclusive allocation of the torque requested to the thermal torque, the final temperature T 'in the catalyst SCR will gradually increase to the predictable temperature T insofar as the requested torque is equivalent and that the attribution to the thermal couple is exclusive. Nevertheless, by reducing the rate of increase of the temperature in the SCR catalyst by the management of the thermal / electrical torque, any phenomenon of desorption of the ammonia is avoided.

Figure 5 shows the operation of the control method according to the invention. A supervisor may be dedicated to this control but this supervisor may also consist of a computer already present in the vehicle, including the one present to ensure the complete management of the exhaust line. If a strong torque gradient is detected, the supervisor determines the risk of desorption based on the mass of ammonia in the SCR, the temperature of the latter and the expected temperature T that will result in the torque demand 20.

If there is a risk of desorption of ammonia, the temperature control strategy in the SCR catalyst is triggered, as described in connection with Figure 4b. The supervisor regularly determines whether, given the temperature detected in the SCR catalyst and the theoretical mass of ammonia present in said catalyst, the risk of desorption still exists. As soon as the risk of desorption is eliminated, due to the drop in ammonia mass and the lower / slower temperature increase in the SCR catalyst, the standard strategy of the hybrid engine is reset. Note that the temperature supervisor in the SCR catalyst can also be activated only if a strong torque gradient is recorded, for example an increase in torque of at least 50%. In the hypothesis of a torque increase not exceeding a torque increase threshold value, the supervisor not being activated, there is obviously no particular control strategy of the electrical / thermal torque, that is to say a functioning torque, because it is estimated that no risk of ammonia desorption is likely to occur.

It is understood that in hybrid vehicles, it is possible to provide a portion of the torque is dedicated to the electrical torque but up to the present invention, the allocation of at least a portion of the requested torque The electrical torque has never been based on the desorption risk management of the reducing agent in the SCR catalyst.

The invention is not limited to a particular embodiment of the exhaust line to the extent that the latter comprises at least one SCR catalyst. Furthermore, the invention is also applicable regardless of the type of hybrid engine of the vehicle as long as it can provide an electric torque and a thermal torque during the propulsion of the vehicle.

The invention makes it possible to optimize the conversion / reduction of NO _x by the SCR catalyst, especially when the exhaust line is cold and that there is a risk of observing, in case of strong acceleration, a strong gradient. temperature in the catalyst. The invention makes it possible to guarantee the absence of desorption of ammonia in the SCR catalyst and thus to dispense with an additional system for treating ammonia at the outlet of the SCR catalyst, of the "clean-up" catalyst type. In addition, because of the absence of desorption, the invention optimizes the consumption of reducing agent, where in particular reduced maintenance and / or to provide a less bulky ammonia reservoir.

Claims

1. A method of controlling a nitrogen oxide treatment system (NO _x) present in exhaust gas of a vehicle equipped with an engine and an electric machine capable of providing a thermal torque respectively and an electric torque, said system comprising, arranged in the exhaust line, a SCR catalyst for selectively reducing NO _x and means for introducing an NO _x reducing agent upstream of said SCR catalyst, characterized in that during a propulsion phase of the vehicle, the expected temperature T in the SCR catalyst corresponding to the torque demand (20) is determined for a torque demand (20) greater than an initial torque (10) when the latter is attributed in full to the thermal torque; this predictable temperature T is compared with a reference temperature T 0 for which the mass of reducing agent present in the SCR catalyst is considered optimal for the reduction of NO _x ; and allocating a portion of the requested torque (20) to the electrical torque if the predictable temperature T is greater than To.

2. Method according to claim 1, characterized in that the optimum reducing agent mass present in the SCR catalyst is considered to be reached when the NO _x reduction is maximum while no phenomenon of desorption of the reducing agent in the catalyst. SCR is not observed.

3. Method according to claim 1 or 2, characterized in that the part of the torque attributed to the electrical torque corresponds to the additional torque (30), the difference between the initial torque (10) and the requested torque (20).

4. Method according to claim 3, characterized in that the additional torque (30) attributed to the electrical torque is exclusively assigned to said torque only during a transient phase ti from the moment t ₀ of the torque request (20).

5. Method according to claim 4, characterized in that the transient phase lasts for at least 0.5 seconds.

6. Method according to claim 4 or 5, characterized in that, beyond the transitional phase t-ι, the portion of the additional torque (30) supported by the couple electric decreases to zero; the portion of the additional torque (30) supported by the correspondingly increasing thermal torque.

7. Method according to claim 6, characterized in that the reduction of the portion of the additional torque (30) supported by the electrical torque is linear.

8. Method according to any one of the preceding claims, characterized in that all of the requested torque (20), ie the initial torque (10) and the additional torque (30), are attributed to the engine torque when the foreseeable temperature T determined is less than the aforementioned reference temperature To.

9. A pollution control of _NOx for implementing the method according to any preceding claim comprising a supervisor adapted to determine the temperature in the SCR catalyst and the weight of reducing agent present in said catalyst, characterized in that the function of the supervisor is:

determining a predictable temperature T in the catalyst SCR as a function of a torque demand (20) greater than an initial torque (10), this predictable temperature T being determined under the assumption that the totality of the requested torque is attributed to thermal torque;

determining whether this predictable temperature T is greater than a reference temperature To for which the mass of reducing agent present in the SCR catalyst is considered optimal for the reduction of NO _x ; and

- Control the allocation of at least a portion of the requested torque (20) to the electrical torque if the predictable temperature T is greater than the reference temperature To.

10. Device according to claim 9, characterized in that the supervisor checks if necessary, depending on the mass of reducing agent and the temperature in the SCR catalyst and the reference temperature To, of injecting a quantity of reducing agent into the exhaust line and is able to control this injection.