EP4276297A1 - Method for adjusting the partial recirculation of exhaust gases to the intake of a diesel engine - Google Patents

Abstract

The invention relates to a method for adjusting the partial recirculation of exhaust gases at the intake of a supercharged diesel engine comprising a high pressure EGR circuit and a low pressure EGR circuit. The EGR rate and distribution between high pressure EGR gas flow and low pressure EGR gas flow is defined a priori to respect a level of NOx emissions at the engine outlet. According to the invention, the distribution between the flow rates is modified step by step. of EGR gas at high and low pressure so as to reduce the flow of nitrogen oxides at the outlet of the engine exhaust circuit.

Description

Technical field of the invention

The invention relates to a method for adjusting the partial recirculation of exhaust gases at the intake of a compression ignition internal combustion engine (diesel type).

It relates more precisely to a method for adjusting the distribution of the recirculation rate of the exhaust gases at the intake of a diesel engine between, on the one hand, a partial recirculation rate at low pressure and, on the other hand , a rate of partial recirculation at high pressure of said exhaust gases.

It finds an advantageous application in the form of an on-board adjustment process in a motor vehicle equipped with a supercharged diesel engine associated with two partial recirculation circuits of the exhaust gases at the intake of the engine, respectively at low and at high pressure.

State of the art

Many modern diesel internal combustion engines, particularly those in motor vehicles, are of the type supercharged by a turbocharger and are equipped with both a low pressure partial exhaust gas recirculation circuit at the engine intake and a high pressure partial recirculation circuit of the exhaust gases at the intake, as illustrated by the figure 1 .

On the figure 1 , we have shown a diesel type engine supercharged by a turbocharger, which in this example has four in-line cylinders.

The engine is associated with an air intake circuit 2 and an exhaust circuit 3 for the engine's combustion gases. It is also associated with a fuel supply circuit of which injectors have been shown fuel 4, which make it possible to inject fuel (diesel) directly into the combustion chambers of the engine, for example from a common rail 5 which is itself supplied with fuel from in particular a tank, d a low pressure pump and a high pressure pump not shown.

Unless otherwise specified, the flow rates mentioned below are mass flow rates.

The air intake circuit 2 comprises, from upstream to downstream in the direction of air circulation: an air filter 6, a flow meter 7 capable of measuring the air flow Qair entering the engine; a compressor 8 of a turbocharger 9; an intake valve 10 for intake gases into the engine; a cooler 11 of said intake gases when they are compressed by the compressor 8; and, an intake manifold 12, or distributor 12 of the engine.

The exhaust circuit 3 comprises, from upstream to downstream in the direction of circulation of the engine exhaust gases: an exhaust manifold 13; a turbine 14 of the turbocharger 9, which is mounted on a shaft common to the compressor 8; and, a plurality of devices 15,16,17 for depolluting engine combustion gases. It may also include an exhaust valve 18.

For example, a first depollution device 15 may comprise a nitrogen oxide trap 15, which may be associated with a first oxidation catalyst.

For example, a second depollution device 16 may comprise a catalyst for the selective reduction of nitrogen oxides (also called SCR catalyst, from the acronym in English for: Selective Catalytic Reduction) associated with a particle filter.

For example, a third pollution control device 17 may include a second oxidation catalyst, in particular mounted under the body of the vehicle.

These examples (types of pollution control devices, number, and arrangement in relation to each other) are non-limiting.

• Un premier circuit de recirculation partielle à basse pression 19, en abrégé circuit EGR-LP (des acronymes en anglais EGR pour « Exhaust Gas Recycling » et LP pour « Low Pressure »), prélève une partie des gaz d'échappement du moteur en un point du circuit d'échappement 3 situé en aval de la turbine 14, ici de manière non limitative entre la sortie du deuxième dispositif 16 et l'entrée du troisième dispositif 17 de dépollution. Il les réintroduit en un point du circuit d'admission d'air 2 situé en amont du compresseur 8, plus précisément entre la sortie du débitmètre 7 et l'entrée du compresseur 9. Le circuit EGR-LP 19 comprend un filtre 21, un refroidisseur 22 et une vanne de réglage 23, dite aussi vanne EGR-LP 23, dont la position angulaire permet de contrôler le débit des gaz recyclés à basse pression Qegrlp circulant dans ledit circuit EGR-LP 19, seule ou en combinaison avec la position du volet de vannage à l'échappement 18, dont un plus grand degré de fermeture permet d'augmenter la pression en amont de la vanne EGR-LP 23.
• Un deuxième circuit de recirculation partielle à haute pression 20, en abrégé circuit EGR-HP (des acronymes en anglais EGR pour « Exhaust Gas Recycling » et HP « pour High Pressure ») prélève une partie des gaz d'échappement en un point situé en amont de la turbine 14, c'est-à-dire à la sortie du collecteur d'échappement 13, et il les réintroduit en un point du circuit d'admission 2 situé en aval du compresseur 8, plus précisément entre la sortie du refroidisseur 11 des gaz d'admission et l'entrée du collecteur d'admission 12. Le circuit EGR-HP 20 comprend une vanne de réglage 24, dite aussi vanne EGR-HP 24, dont la position angulaire permet de contrôler le débit des gaz recyclés à haute pression Qegrhp circulant dans ledit circuit EGR-HP 20, seule ou en combinaison avec la vanne d'admission 10, dont un plus grand degré de fermeture permet d'abaisser la pression en aval de la vanne EHR-HP 24. Bien entendu, dans le cas où on est amené à refermer partiellement la vanne d'admission 10, cela a aussi tendance à diminuer le débit volumique d'air admis dans le moteur, mais on maintient alors le débit massique en augmentant la pression de suralimentation et donc la masse volumique admise.

On the other hand, in the context of the invention, the exhaust circuit 3 comprises two partial recirculation circuits 19,20 of the exhaust gases at the intake of the engine:

• A first low pressure partial recirculation circuit 19, abbreviated EGR-LP circuit (acronyms in English EGR for “Exhaust Gas Recycling” and LP for “Low Pressure”), takes part of the exhaust gases from the engine in one point of the exhaust circuit 3 located downstream of the turbine 14, here in a non-limiting manner between the outlet of the second device 16 and the inlet of the third depollution device 17. It reintroduces them at a point of the air intake circuit 2 located upstream of the compressor 8, more precisely between the outlet of the flow meter 7 and the inlet of the compressor 9. The EGR-LP circuit 19 includes a filter 21, a cooler 22 and an adjustment valve 23, also called EGR-LP valve 23, the angular position of which makes it possible to control the flow rate of the recycled gases at low pressure Qegrlp circulating in said EGR-LP circuit 19, alone or in combination with the position of the exhaust valve 18, a greater degree of closure of which makes it possible to increase the pressure upstream of the EGR-LP valve 23.
• A second high pressure partial recirculation circuit 20, abbreviated EGR-HP circuit (acronyms in English EGR for "Exhaust Gas Recycling" and HP "for High Pressure") takes part of the exhaust gases at a point located in upstream of the turbine 14, that is to say at the outlet of the exhaust manifold 13, and it reintroduces them at a point of the intake circuit 2 located downstream of the compressor 8, more precisely between the outlet of the cooler 11 of the intake gases and the inlet of the intake manifold 12. The EGR-HP circuit 20 includes an adjustment valve 24, also called the EGR-HP valve 24, the angular position of which makes it possible to control the flow rate of the recycled gases high pressure Qegrhp circulating in said EGR-HP 20 circuit, alone or in combination with the inlet valve 10, a greater degree of closure of which makes it possible to lower the pressure downstream of the EHR-HP valve 24. Of course , in the case where we have to partially close the inlet valve 10, this has also tends to reduce the volume flow of air admitted into the engine, but we then maintain the mass flow by increasing the boost pressure and therefore the admitted density.

• Notamment, ladite augmentation de la température des gaz d'échappement est défavorable au compromis NOx / particules. En d'autres termes, la diminution des émissions d'oxydes d'azote est obtenue en contrepartie d'une augmentation des émissions de particules fines ou de suie (PM) qu'il faut compenser par des régénérations plus fréquentes du filtre à particules.
• D'autre part, comme le circuit EGR-HP prélève des gaz brûlés dès la sortie du collecteur d'échappement 13, le débit Qegrhp de ces gaz recyclés à haute pression (dits de manière abrégée gaz EGR-HP) ne passe pas à travers la turbine 14. C'est autant d'énergie en moins pour la suralimentation.
• D'autre part encore, le débit des gaz EGR-HP ne traversant pas les dispositifs de dépollution 15,16,17 entraîne une diminution du flux thermique (i.e. du produit du débit massique et de la température) qui est reçu par lesdits dispositifs, ce qui est défavorable à leur montée en température et à leur efficacité de traitement.

It is known that the use of a proportion of recycled exhaust gases at the intake on a diesel engine mainly makes it possible to dilute the air-fuel mixture with such gases resulting from combustion, already burned and inert, thus reducing the partial pressure of oxygen in the engine cylinders and increasing the heat capacity of the gas mixture of air and recycled gases, which thus makes it possible to lower the combustion temperature in the engine and consequently to reduce emissions. nitrogen oxides (NOx) at the engine outlet. Furthermore, the use of high pressure recirculation can also make it possible to increase the temperature of the exhaust gases, which is favorable to the rapid rise in temperature of the pollution control devices and therefore to their treatment efficiency. However, it has disadvantages:

• In particular, said increase in the temperature of the exhaust gases is unfavorable to the NOx/particle compromise. In other words, the reduction in nitrogen oxide emissions is obtained in return for an increase in emissions of fine particles or soot (PM) which must be compensated for by more frequent regenerations of the particle filter.
• On the other hand, as the EGR-HP circuit takes burnt gases from the outlet of the exhaust manifold 13, the flow rate Qegrhp of these gases recycled at high pressure (abbreviated as EGR-HP gas) does not pass through turbine 14. This is less energy for supercharging.
• On the other hand, the flow of EGR-HP gases not passing through the pollution control devices 15,16,17 leads to a reduction in the heat flow (ie the product of the mass flow and the temperature) which is received by said devices, which is unfavorable for their temperature rise and their treatment efficiency.

• Notamment, la recirculation à basse pression ne peut pas être utilisée dans toutes les conditions, en particulier pas par grand froid, car les gaz d'échappement recyclés à basse pression (dits de manière abrégée gaz EGR-LP) sont susceptibles de faire condenser la vapeur d'eau qu'ils contiennent dans le refroidisseur 22 du circuit EGR-LP. Les gaz EGR-LP peuvent aussi faire condenser la vapeur d'eau que l'air frais d'admission contient, lors de la rencontre entre l'air et les gaz EGR-LP, cette condensation pouvant endommager les aubes du compresseur 8.
• D'autre part, pour atteindre des taux de recyclage élevés, c'est-à-dire pour obtenir des débits Qegrlp de gaz EGR-LP importants relativement à la somme du débit d'air Qair et du débit de gaz recyclés, il est parfois nécessaire de procéder à la fermeture partielle du volet de vannage à l'échappement 18 afin d'augmenter l'écart de pression entre l'échappement et l'admission du moteur dans les conditions de fonctionnement du moteur où cet écart est trop faible pour que le débit Qegrlp de gaz recyclés à basse pression puisse être obtenu uniquement en ouvrant la vanne EGR-LP 21. Cela entraîne une augmentation des pertes par pompage du moteur et par conséquent de la consommation de carburant du moteur.

The use of low pressure recirculation makes it possible to overcome these disadvantages, but it also has disadvantages:

• In particular, low pressure recirculation cannot be used in all conditions, in particular not in very cold weather, because the exhaust gases recycled at low pressure (abbreviated as EGR-LP gas) are likely to cause the gas to condense. water vapor which they contain in the cooler 22 of the EGR-LP circuit. The EGR-LP gases can also cause the water vapor that the fresh intake air contains to condense, when the air and the EGR-LP gases meet, this condensation being able to damage the blades of the compressor 8.
• On the other hand, to achieve high recycling rates, that is to say to obtain significant EGR-LP gas flow rates Qegrlp relative to the sum of the air flow rate Qair and the recycled gas flow rate, it is sometimes necessary to partially close the exhaust valve 18 in order to increase the pressure difference between the exhaust and the intake of the engine under the operating conditions of the engine where this difference is too small to that the flow rate Qegrlp of recycled gases at low pressure can be obtained only by opening the EGR-LP valve 21. This leads to an increase in engine pumping losses and consequently in engine fuel consumption.

The operating conditions of the engine being very varied and the expected performances (fuel consumption, emissions of different types of pollutants, etc.) multiple and often subject to compromises, we have therefore equipped many modern diesel engines of both types EGR circuit, so as to have the greatest number of technical possibilities to adjust the engine in order to obtain these benefits.

It is also advantageous to be able to use low pressure recirculation and high pressure recirculation simultaneously to be able to take advantage of the advantages of each of the recirculation circuits 19,20.

We thus define on such an engine on the one hand a recirculation rate τegr also called "EGR rate" as the ratio of the total flow of EGR gases divided by the total gas flow (air and EGR gas) admitted into the engine, according to the following equation 1: $$\mathrm{Tegr}=\left(\mathrm{Qegrlp}+\mathrm{Qegrhp}\right)/\left(\mathrm{Qair}+\mathrm{Qegrlp}+\mathrm{Qegrhp}\right)$$

We further define a distribution rate S also known as “split rate” as the proportion of EGR-HP gas in the total flow rate of EGR gases, according to the following equation 2: $$\mathrm{S}=\mathrm{Qegrhp}/\left(\mathrm{Qegrlp}+\mathrm{Qegrhp}\right)$$

In particular, when the distribution rate is equal to 1, there are only EGR-HP gases, and when it is equal to 0, there are only EGR-LP gases.

• Pour obtenir un taux d'EGR τegr donné, on peut notamment maximiser le débit Qegrlp de gaz EGR-LP uniquement en ouvrant la vanne EGR-LP 23 mais sans avoir recours au volet de vannage à l'échappement 18, et fournir le complément de gaz recyclés sous forme d'un débit Qegrhp de gaz EGR-HP.
• A faible régime et/ou à faible charge du moteur, il est avantageux d'utiliser plutôt des gaz EGR-HP afin de maximiser la température des gaz d'échappement pour favoriser la montée en température des dispositifs de dépollution. A plus forte charge, la température est suffisante, et il devient au contraire avantageux d'utiliser plutôt des gaz EGR-LP afin de diminuer les émissions d'oxydes d'azote, qui sont plus élevées qu'à faible charge. En cas de transition brutale d'un point de fonctionnement du moteur peu chargé vers un autre qui est plus chargé, par exemple lors d'une accélération franche qui augmente la consigne de couple du moteur, il est avantageux de basculer progressivement de la recirculation à haute pression vers la recirculation à basse pression en fonction de la valeur courante du couple du moteur pendant la phase transitoire, en faisant varier le taux de répartition, plutôt que de transiter brutalement d'un type de recirculation vers l'autre, ce qui entraînerait des difficultés de contrôle du moteur (plus précisément : mauvaise adéquation momentanée entre le débit réel d'air et le débit de gaz EGR, en raison de la dynamique d'établissement plus lente du débit de gaz EGR-LP par rapport au débit de gaz EGR-HP).

For example, it may be advantageous to use a distribution rate S different from 1 or 0 in different situations.

• To obtain a given EGR rate τegr, it is possible in particular to maximize the flow rate Qegrlp of EGR-LP gas only by opening the EGR-LP valve 23 but without having to resort to the exhaust valve 18, and provide the complement of recycled gases in the form of a flow rate Qegrhp of EGR-HP gas.
• At low speed and/or low engine load, it is advantageous to use EGR-HP gases instead in order to maximize the temperature of the exhaust gases to promote the rise in temperature of the pollution control devices. At higher load, the temperature is sufficient, and it instead becomes advantageous to use EGR-LP gases instead in order to reduce nitrogen oxide emissions, which are higher than at low load. In the event of a sudden transition from an operating point of the lightly loaded motor to another which is more loaded, for example during a sharp acceleration which increases the torque setpoint of the motor, it is advantageous to gradually switch from recirculation to high pressure towards low pressure recirculation according to the current value of the motor torque during the transient phase, by varying the distribution rate, rather than suddenly transiting from one type recirculation towards the other, which would cause difficulties in controlling the engine (more precisely: momentary poor match between the actual air flow and the EGR gas flow, due to the slower establishment dynamics of the recirculation flow EGR-LP gas compared to the EGR-HP gas flow).

It is a general objective of the invention to define in all conditions of use, not only an EGR rate τegr which, in association with an air flow rate Qair and one or more fuel flow rates Qcarb (and of the corresponding phasing(s), is adapted according to a nitrogen oxide emissions level objective, but also to define the best possible distribution rate S between the flow coming from the EGR-HP circuit and that which comes from the EGR-LP circuit.

• selon le point de fonctionnement du moteur, c'est-à-dire en fonction d'un ensemble de paramètres comprenant au moins le régime N, couple C, et généralement en outre la température du liquide de refroidissement ;
• selon les conditions ambiantes dans lesquelles fonctionne le moteur, par exemple : température ambiante, pression atmosphérique ;
• en fonction du taux d'EGR τegr requis et de la capacité du circuit EGR-LP à réaliser ce taux à lui seul en augmentant le degré d'ouverture de la vanne EGR-LP 23 sans avoir recours au vannage à l'échappement, comme mentionné plus haut.

In engines known from the state of the art, this distribution rate S is defined in particular:

• according to the operating point of the engine, that is to say according to a set of parameters comprising at least the speed N, torque C, and generally in addition the temperature of the coolant;
• depending on the ambient conditions in which the engine operates, for example: ambient temperature, atmospheric pressure;
• depending on the required EGR rate τegr and the capacity of the EGR-LP circuit to achieve this rate alone by increasing the degree of opening of the EGR-LP valve 23 without having to resort to exhaust valving, as mentioned above.

However, in these engines, the value of the distribution rate S is defined to respond to the various constraints mentioned above without taking into account the efficiency of treating nitrogen oxides of the different pollution control devices, which can lead in many cases to situations not to minimize nitrogen oxide emissions released into the outside atmosphere.

Indeed, if a given EGR rate τegr leads to a given reduction in nitrogen oxide emissions at the engine outlet in an almost independent of the distribution rate S, EGR-LP gases and EGR-HP gases having practically the same effect on combustion, up to their temperature level, the same is not true of nitrogen oxide emissions by the vehicle, that is to say at the exit of the exhaust circuit 3, since the gas flow rates passing through the depollution devices, and consequently, the respective treatment efficiencies thereof, are affected by this distribution rate S.

A notable point in the differences between EGR-LP gases and EGR-HP gases is the fact that EGR-LP gases pass through all or part of the engine's pollution control devices (depending on where in the exhaust circuit where the EGR-LP gases are sampled), while the EGR-HP gases are sampled without passing through any of them. In the example of the figure 1 , the EGR-LP gases are taken between the second 16 and the third depollution device 17, that is to say, after passing through on the one hand the nitrogen oxide trap of the first depollution device 15 and the SCR catalyst of the second depollution device 16 which both have an effect of treating the nitrogen oxides included in the combustion gases coming from the engine.

Thus, if low pressure recirculation makes it possible to lower the level of NOx emissions directly at the engine outlet in the same way as high pressure recirculation, we observe the following two phenomena:
Firstly, the NOx flow rate which is seen by the post-treatment device(s) passed through is greater when using a flow rate of low pressure recycled gases EGR-LP Qegrlp than if using an equivalent flow rate of high pressure recycled gases. Qegrhp, because the total burnt gas flow rate to be treated is equal to the sum of the air flow rate Qair, the fuel flow rate Qcarb and the low pressure recycled gas flow rate Qegrlp, while in the case of recirculation at high pressure, the total burnt gas flow rate to be treated is equal only to the sum of the air flow rate Qair and the fuel flow rate.

Then, with equal NOx treatment efficiency of the device(s) concerned, that is to say assuming that the percentage of nitrogen oxides treated is identical, the emissions at the outlet of the exhaust circuit are likely to be increase.

• dans le cas où la recirculation se fait entièrement à haute pression (taux de répartition S égal à 1), les émissions de NOx à la sortie du circuit d'échappement sont égales à 80% des émissions du moteur, car le débit des NOx en sortie du moteur est multiplié par un facteur égal à 1 - 0,2 ; tandis que,
• dans le cas où la recirculation se fait entièrement à basse pression (taux de répartition S égal à 0), le débit vu par les dispositifs de dépollution est sensiblement égal au double du débit d'air (en négligeant le débit de carburant), si bien que les émissions de NOx à la sortie du circuit d'échappement sont augmentées de 60% par rapport aux émissions à la sortie du moteur, car le débit des NOx à la sortie du moteur est multiplié par un facteur égal à 2 × (1 - 0,2).

For example, if we consider an EGR rate τegr which is equal to 50% and a NOx treatment efficiency of the pollution control devices passed through 15,16 which is equal to 20% in all cases:

• in the case where the recirculation takes place entirely at high pressure (distribution ratio S equal to 1), the NOx emissions at the outlet of the exhaust circuit are equal to 80% of the engine emissions, because the flow of NOx in motor output is multiplied by a factor equal to 1 - 0.2; while,
• in the case where the recirculation takes place entirely at low pressure (distribution rate S equal to 0), the flow rate seen by the pollution control devices is substantially equal to twice the air flow rate (neglecting the fuel flow rate), if although the NOx emissions at the exhaust system outlet are increased by 60% compared to the engine outlet emissions, because the NOx flow rate at the engine outlet is multiplied by a factor equal to 2 × (1 - 0.2).

Of course, such a difference in behavior of NOx emissions at the outlet of the exhaust circuit, depending on whether high or low pressure recirculation is used, depends each time on the recirculation rate and the treatment efficiency. NOx considered.

Secondly, the NOx treatment efficiency of the post-treatment device(s) is variable, and it depends in particular on the volume flow rate of burnt gas Qech,vol passing through such a device. For the same (mass) flow rate of fresh air Qair and fuel Qcarb admitted into the engine, this volume flow rate Qech,vol increases with the use of low pressure recirculation. However, it is known that in general, the treatment efficiency of a pollution control device decreases when the hourly volume speed VVH increases. The hourly volume velocity VVH of a pollution control device is defined as the ratio between the flow rate volume Qech, incoming flight, expressed in l/h, and the volume of the catalytic cake of the device, expressed in liters. It represents the inverse of the residence time of the gases in the device, that is to say, the duration that the gases take to pass through the device. The shorter the residence time, the lower the treatment efficiency, and even more so in operating phases where maximum efficiency is not achieved.

We cannot therefore assume that the effectiveness of the pollution control device(s) passed through by the burnt gases is the same depending on whether high or low pressure recirculation is used.

To increase the NOx treatment efficiency of pollution control devices, it would be appropriate to reduce the hourly volumetric speed VVH.

Thus, if it seems wise to reduce NOx emissions at the engine outlet, in terms of concentration in the gases, by choosing an appropriate EGR τegr rate, it is also appropriate to consider this reduction with regard to the increase of the flow rate and the drop in hourly volume velocity VVH which can be generated by a choice of given distribution rate S. However, the optimum distribution rate is not constant depending on the sensitivity of the efficiency of the various post-treatment devices to a plurality of parameters, including the hourly volumetric speed, the temperature, the mass of NOx already stored in a nitrogen oxide trap, the flow rate of reductants (Adblue ^® ) injected into an SCR catalyst as well as the quantity of ammonia AS stored there, etc.

The modeling of the effectiveness of the different depollution devices being very complex and obeying non-linear phenomena, the processes according to the state of the art do not take into account the particular states of the depollution devices when defining the distribution rate. S. NOx emissions at the outlet of the exhaust system are an indirect consequence of emissions at the engine outlet, which we try to minimize, and of the exhaust flow upstream of the pollution control devices as well as the effectiveness of these. We cannot therefore achieve optimization of NOx emissions into the outside atmosphere.

Presentation of the invention

The invention aims to remedy the defects of known adjustment methods for the partial recirculation of exhaust gases at the intake of a diesel engine. It aims more precisely to improve the adjustment of the distribution rate S, so as to reduce NOx emissions at the exit of the engine exhaust circuit.

Summary of the invention

• déterminer (100) un ensemble de paramètres (N,C) représentatifs du point de fonctionnement du moteur
• déterminer (200) une valeur de débit d'air (Qair), un débit de carburant (Qcarb), un taux de recirculation (τegr) des gaz d'échappement à l'admission et un taux de répartition initial (So) entre un débit de gaz recirculés à haute pression (Qegrlp) et un débit de gaz recirculés à basse pression (Qegrlp), permettant de produire le couple moteur (C) en respectant une concentration d'oxydes d'azote [NOx]eo donnée à la sortie du moteur,

To achieve this objective, the invention relates to a method for adjusting the partial recirculation of exhaust gases at the intake of a supercharged diesel engine and associated with a high pressure partial recirculation circuit and a recirculation circuit partial at low pressure of the exhaust gases at the intake of the engine, said method comprising:

• determine (100) a set of parameters (N,C) representative of the operating point of the engine
• determine (200) an air flow value (Qair), a fuel flow rate (Qcarb), a recirculation rate (τegr) of the exhaust gases at the intake and an initial distribution rate (So) between a flow rate of recirculated gases at high pressure (Qegrlp) and a flow rate of recirculated gases at low pressure (Qegrlp), making it possible to produce the engine torque (C) while respecting a concentration of nitrogen oxides [NOx]eo given at the outlet of the motor,

• faire varier de manière itérative (1200) le taux de répartition (S) par rapport à la valeur de taux de répartition précédente (S,So) dans un premier sens de variation tant qu'une valeur de débit d'oxydes d'azote (Q_NOx,tp) à la sortie du circuit d'échappement du moteur diminue ;
• dès que ledit débit d'oxydes d'azote à l'échappement (Q_NOx,tp) augmente, faire varier de manière itérative (1400) le taux de répartition (S) dans un deuxième sens de variation contraire au premier sens de variation tant que la valeur de débit d'oxydes d'azote (Q_NOx,tp) à la sortie du circuit d'échappement du moteur diminue.

The main characteristic of the invention is that it comprises the following steps:

• iteratively vary (1200) the distribution rate (S) relative to the previous distribution rate value (S,So) in a first direction of variation as long as a nitrogen oxide flow value ( Q _NOx ,tp) at the outlet of the engine exhaust circuit decreases;
• as soon as said flow rate of nitrogen oxides at the exhaust (Q _NOx , tp) increases, iteratively varying (1400) the distribution rate (S) in a second direction of variation contrary to the first direction of variation as that the nitrogen oxide flow value (Q _NOx , tp) at the outlet of the engine exhaust circuit decreases.

Presentation of figures

• [Fig. 1] La figure 1 est une représentation schématique d'un premier exemple de dispositif de motorisation apte à la mise en oeuvre du procédé selon l'invention.
• [Fig. 2] La figure 2 est un logigramme qui représente les différentes étapes du procédé selon l'invention, selon un mode de réalisation.

The invention will be better understood on reading a non-limiting embodiment thereof, in support of the appended figures among which:

• [ Fig. 1 ] There figure 1 is a schematic representation of a first example of a motorization device suitable for implementing the method according to the invention.
• [ Fig. 2 ] There figure 2 is a flowchart which represents the different stages of the process according to the invention, according to one embodiment.

detailed description

There figure 1 has already been described above and does not call for additional comments.

There figure 2 illustrates the different stages of the process according to the invention, in a preferred embodiment thereof.

The process is iterative and it is implemented by an engine computer, in successive time steps, as long as the operating point of the engine remains sufficiently stable, that is to say, as long as the variations in speed N and of the engine torque are substantially zero.

The process begins with a step 100 of determining the operating point of the engine by the computer. A set of parameters representative of the operating point is determined including at least the speed N and the torque, and possibly additional parameters such as the temperature of the coolant (water temperature) Twater.

The method continues with a step 200 of determining an air flow rate Qair, a fuel flow rate Qcarb, an EGR rate τegr and an initial distribution rate So between gas flow rate EGR- LP and EGR-LP gas flow, which are capable, according to the state of the art, of allowing the production of engine torque C while ensuring a plurality of services already mentioned above, in particular obtaining a concentration of nitrogen oxides at the engine outlet [NOx]eo. In step 200, this concentration of nitrogen oxides at the engine outlet [NOx]eo, which also corresponds to the concentration of nitrogen oxides upstream of the pollution control devices, is determined, for example using a concentration sensor or by a model.

The method then comprises a set of iterative steps which are implemented as soon as the computer detects that the operating point (step 100) is sufficiently stabilized, and whose progress is interrupted if it detects that it is not. more. For example, to conclude that there is stability, the calculator can for example calculate the time derivative of the regime dN/dt and the couple dC/dt and note that these two derivatives are lower than respective thresholds.

The method comprises a step 300 in which, according to the invention, the value of the distribution rate S is replaced by a modified value. The first value retained corresponds to the initial distribution rate So.

The method comprises a step 400 in which the total flow rate of recycled gases Qegr, the EGR-HP gas flow rate Qegrhp and the EGR-LP gas flow rate Qegrlp are calculated, from the air flow rate Qair, the EGR rate τegr and the distribution rate S, using equations 1 and 2.

The method then comprises a step 500 of calculating the exhaust gas flow rate Qech at the inlet of the first depollution device 15, before EGR-LP gases are taken if necessary. This is the sum of the air flow Qair, the fuel flow Qcarb (which can possibly be neglected in the case of a diesel engine operating on a lean mixture) and the EGR-LP gas flow Qegrlp.

In the context of the invention, at least one depollution device capable of having an action of treating nitrogen oxides is passed through by the exhaust gases before EGR-LP gases are sampled. In the particular example of the figure 1 , this is the first depollution device 15, which comprises a nitrogen oxide trap, and the second depollution device 16, which comprises an SCR catalyst.

We will assume in the following to simplify the presentation of the process that, contrary to the case presented in figure 1 , the EGR-LP gases are taken from the outlet of the first pollution control device. However, the process is easily adaptable mutatis mutandis in the case where several successive depollution devices are crossed before the EGR-LP gas sampling point (for example two in the case of the figure 1 ).

In step 600 of the process, the flow rate of nitrogen oxides Q _NOx , ech entering the first depollution device 15 is calculated. This is the product of the flow rate Qech calculated in step 400 and the concentration of nitrogen oxides [NOx]eo determined in step 200.

In step 700, the volume flow rate of exhaust gas Qech, vol at the inlet of the first depollution device is calculated, from the (mass) flow rate Qech calculated in step 500, using the ideal gas equation and from the pressure and temperature at the inlet or in said device.

In step 800, the hourly volumetric speed in the first depollution device is deduced, by dividing the volume flow Qech,vol of step 700 by the volume of the catalytic cake of said device.

The process continues with a step 900 in which a plurality of additional parameters are determined which have an influence on the NOx treatment efficiency (reduction of NOx into dinitrogen N ₂ and water H ₂ O)) of the first depollution device . These parameters depend on the nature of the device. In the example of the figure 1 , regarding a nitrogen oxide trap, we will determine at least the temperature T and the mass of nitrogen oxides already stored in the trap. For example, in the case of an SCR catalyst, at least the temperature, the flow rate of injected reductants and the quantity of ammonia AS stored in the SCR catalyst are determined.

The process continues with a step 1000 in which the value of the processing efficiency ε _red of the first device is determined. We can in particular use a map established by tests prerequisites according to the values of the hourly volumetric speed VVH of step 700 and the various parameters determined in step 900.

In step 1100, the flow rate of nitrogen oxides at the outlet of the exhaust circuit Q _NOx , tp, also known as the “tail pipe” NOx flow rate, is determined, according to the following equation 3: $${\mathrm{Q}}_{\mathrm{NOx}},\mathrm{tp}=\left(1-{\mathrm{\epsilon }}_{\mathbf{red}}\right)\times {\mathrm{Q}}_{\mathrm{NOx}},\mathrm{ech}$$

According to the invention, the method then comprises a step 1200 in which the distribution rate S is varied relative to the value previously determined and stored in the calculator in step 300. In a first variant of the method, the distribution rate S. For example, said rate is varied by a negative constant value. In a second variant of the method, the distribution rate S is increased. For example, said rate is varied by a positive constant value.

This action has, on the one hand, an effect on the hourly volumetric speed and on the effectiveness of the depollution devices. On the other hand, it also has an action on the concentration of nitrogen oxides at the outlet of the engine [NOx]eo, and therefore on the flow rate of nitrogen oxides Q _NOx ,ech entering the first device of depollution 15. These two cumulative effects can lead, depending on the situation, namely depending on the sensitivity of nitrogen oxide emissions by the engine to the temperature of the EGR gases (depending on whether EGR- HP or EGR-LP) and depending on the sensitivity of the effectiveness of the depollution device 15 to the hourly volume speed WH, to increase or decrease the flow of nitrogen oxides at the outlet of the exhaust circuit Q _NOx ,tp.

The process continues with a step 1300 in which the new value of nitrogen oxide flow rate at the exhaust Q _NOx , tp resulting from this modification of the distribution rate S is determined, and a test is carried out which consists of checking if said flow rate Q _NOx , tp is lower than the previous value.

If this is the case, the process resumes at step 300, in which the value of the distribution rate is replaced by the new value which led to a reduction in NOx emissions. We can then continue to the next step with a new modification of the distribution rate S, which goes in the same direction of reduction or increase as in the previous step, and of an identical value, for example -0.005 or + 0.0005.

Otherwise, the method directs towards a step 1400 in which the distribution rate is varied in the opposite direction to that which had been retained iteratively in step 1200. In other words, in the first variant of the process, where the distribution rate S was reduced in step 1200, it is increased in step 1400. Likewise, in the second variant of the process, where the distribution rate S was increased at the step 1200, it is reduced in step 1400.

The process then continues with a step 1500 in which the new value of nitrogen oxide flow rate at the exhaust Q _NOx , tp resulting from this modification of the distribution rate S is determined, and a test is carried out which consists of check if said flow rate is lower than the previous value.

If this is the case, the process continues with a step 1600, in which the value of the distribution rate S is replaced by the new value having led to a reduction in NOx emissions. We can then continue to the next step with a new modification of the distribution rate S, which goes in the same direction of increase or decrease as in the previous step, and which can be of an identical value, for example + 0.005 or -0.0005.

In the opposite case, that is to say if we observe an increase in the flow rate of nitrogen oxides at the exhaust Q _NOx , tp compared to the immediately preceding value, the process directs towards a step 1700 in which we freeze the value of the distribution rate S on the immediately preceding value.

In one embodiment of the invention, the total cumulative variation of the distribution rate S can be limited to a predetermined constant value, for example +0.10 or -0.10 variation of distribution rate relative to the value initial So in both senses, i.e. both in step 1200 and in step 1400, after which the value is frozen, until the operating point of the motor is modified, which will at this time lead to defining a new distribution rate value initial So in step 300 and to resume the iterative steps of the process on this basis.

In other words, if after a succession of steps 1200 of variation of the distribution rate reaching -0.1 (case of the first variant of the process) or +0.1 (second variant of the process) relative to the value initial distribution rate So, we continue to observe a reduction in the flow rate of nitrogen oxides at the exhaust Q _NOx , tp, we stop varying said distribution rate S and we freeze it on the last value obtained . In other words, the process does not resume at step 1200.

In the same way, if after a first iteration of step 1200 of variation of the distribution rate leading to an increase in the flow rate of nitrogen oxides at the exhaust Q _NOx , tp which leads the process to continue with steps 1400 of variation of the distribution rate in the opposite direction, that is to say in the direction of increase according to the first variant of the process or in the direction of decrease according to the second variant of the process, we observe that after a succession of steps 1400 of variation of the distribution rate in said opposite direction reaching +0.1 (case of the first variant of the process) or -0.1 (second variant of the process) relative to the initial value of the rate distribution rate So, we continue to observe a reduction in the flow rate of nitrogen oxides at the exhaust Q _NOx , tp, we stop varying said distribution rate S and we freeze it on the last value obtained. In other words, the process does not resume at step 1400.

Of course, the total cumulative variation of the rate of variation S is always limited to a maximum value in the direction of decrease and in the direction of increase by the fact that the distribution rate S is always between 0 and 1, regardless of whether or not a predetermined constant value is provided for the maximum cumulative variation of the rate.

This process makes it possible, step by step and empirically, to effectively reduce nitrogen oxide emissions into the atmosphere without the need to resort to very complex modeling of the entire motorization device.

We understand from the preceding description of the steps that the invention makes it possible to empirically find a local minimum of nitrogen oxide flow rate at the exhaust Q _NOx , tp.

Claims (6)

Method for adjusting the partial recirculation of the exhaust gases at the intake of a supercharged diesel engine and associated with a high pressure partial recirculation circuit and a low pressure partial recirculation circuit of the exhaust gases at the engine intake, said method comprising: - determine (100) a set of parameters (N,C) representative of the operating point of the engine, - determine (200) an air flow value (Qair), a fuel flow rate (Qcarb), a recirculation rate (τegr) of the exhaust gases at the intake and an initial distribution rate (So) between a flow of gas recirculated at high pressure (Qegrlp) and a flow of gas recirculated at low pressure (Qegrlp), making it possible to produce the engine torque (C) while respecting a concentration of nitrogen oxides [NOx] eo given at the output of the engine, CHARACTERIZED IN THAT it comprises the following stages: - iteratively vary (1200) the distribution rate (S) relative to the previous distribution rate value (S, So) in a first direction of variation as long as a nitrogen oxide flow value (Q _NOx , tp) at the outlet of the engine exhaust circuit decreases; - as soon as said flow rate of nitrogen oxides at the exhaust (Q _NOx , tp) increases, iteratively vary (1400) the distribution rate (S) in a second direction of variation opposite to the first direction of variation as long as the nitrogen oxide flow value (Q _NOx , tp) at the outlet of the engine exhaust circuit decreases. Method according to claim 1, characterized in that the successive variations of the distribution rate (S) in the first direction of variation have identical values to each other. Method according to claim 1 or 2, characterized in that the successive variations of the distribution rate (S) in the second direction of variation have identical values to each other. Method according to any one of the preceding claims, characterized in that the modification of the distribution rate (S) in the first direction of variation is stopped if the cumulative total variation reaches a predetermined maximum constant value. Method according to any one of the preceding claims, characterized in that the modifications of the distribution rate (S) in the second direction of variation are stopped if the cumulative total variation reaches a predetermined maximum constant value. Method according to any one of the preceding claims, characterized in that the variations in the distribution rate (S) begin if the operating point of the engine is stable, and that they are interrupted as soon as it is no longer stable.

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