EP1006270B1 - Control method of the purging of nitrogen oxides in a catalytic converter for exhaust gases of an internal combustion engine - Google Patents

Description

The present invention relates to a method of ordering the nitrogen oxides purge from a jar exhaust gas treatment catalytic internal combustion engine, said pot absorbing such oxides during operation in air / fuel mixture poor of said engine. More particularly, the invention relates to such a process of the type according to which order a purge of the pot by a temporary increase fuel richness of the air / fuel mixture.

To reduce air pollution by exhaust gases from internal combustion engines, especially those who propel vehicles automobiles, the exhaust systems of these gases from a catalytic converter having the function of transform the harmful chemical species contained in these gases in less harmful, even harmless, species.

This is how we know a catalytic converter called "three-way" or "trifunctional", because it provides a triple function of oxidation of unburnt hydrocarbons (HC), oxidation of carbon monoxide ( CO) to CO ₂ and reduction of nitrogen oxides (NOx) to nitrogen gas.

The concern to reduce gas emissions and the greenhouse effect, in particular carbon dioxide CO ₂ emissions, has provoked interest in an internal combustion engine capable of operating with an air / fuel ratio called "lean", that is to say greater than that of a stoichiometric mixture. However, when a conventional three-way catalytic converter is used to depollute the exhaust gases of such an engine operating in a lean mixture, we observe a very low efficiency of the pot in reducing nitrogen oxides (NOx).

To overcome this drawback, we propose in the European patent n ° 560 991 to use a catalytic converter trifunctional further comprising means adsorption of nitrogen oxides present in gases exhaust, when the air / fuel mixture from engine is poor.

To prevent this adsorption from causing long saturation of the adsorption capacity of the pot catalytic, the aforementioned patent proposes to switch periodically the richness of the air / fuel mixture at a value corresponding to a stoichiometric mixture or rich. This switching causes desorption of the nitrogen oxides adsorbed in the pot then their reduction by unburnt hydrocarbons and carbon monoxide present in the pot due to the increase in richness of the mixture. The duration of the switching needed to desorb and reduce nitrogen oxides stored in the catalytic converter is proportional to the mass of nitrogen oxides stored.

Care must be taken to ensure that pot does not significantly disturb the torque delivered by the engine, so as to provide comfort for passengers in the vehicle. In this regard, the various strategies for command described in the aforementioned patent are not satisfactory because, combined with such an order in couple, they would cause emission peaks polluting species (carbon monoxide, hydrocarbons unburned) during transient phases, especially during of the return in lean mixture at the end of a purge, because the response time of certain actuators used such that, in particular, the motorized and controlled throttle valve by a computer, which is used to regulate the air flow entering the engine.

The object of the present invention is precisely to provide a method for controlling the oxide purge of nitrogen from a catalytic converter for gas treatment exhaust from an internal combustion engine, which allows to maintain, during and after a pot purge phase, previous engine torque, all reducing engine fuel consumption during this purge and the amount of chemical species pollutants contained in the combustion gases of the engine.

a) on commande une purge par un accroissement temporaire de la richesse en carburant dudit mélange en réduisant progressivement le débit d'air entrant dans le moteur de manière à optimiser la consommation en carburant du moteur à la richesse fixée, en réduisant brusquement, à une valeur prédéterminée, l'angle d'avance à l'allumage du mélange air/carburant, pour maintenir la constance du couple délivré par le moteur, et en asservissant ensuite une remontée dudit angle d'avance à ladite réduction du débit d'air, jusqu'à une deuxième valeur prédéterminée optimisant la consommation en carburant du moteur en fonction de la richesse et du débit d'air fixé pendant la purge,

b) on arrête la purge en ramenant les valeurs de la richesse du mélange air/carburant, du débit d'air et de l'angle d'avance à l'allumage dudit mélange, aux valeurs respectives antérieures au déclenchement de la purge,

   ce procédé étant remarquable en ce que, à l'étape b), on abaisse d'abord la richesse du mélange air/carburant à une valeur intermédiaire entre les richesses de purge et de mélange pauvre, avec commande concomitante d'une remontée progressive du débit d'air et d'une remontée brusque de l'angle d'avance à l'allumage à une troisième valeur prédéterminée fonction de ladite richesse intermédiaire, on asservit ensuite une décroissance dudit angle jusqu'à une quatrième valeur prédéterminée pendant la remontée du débit d'air, et on rétablit enfin la richesse du mélange, le débit d'air et l'angle d'avance à l'allumage aux valeurs respectives antérieures à la purge.This object of the invention is achieved, as well as others which will appear on reading the description which follows, with a process for controlling the purging of nitrogen oxides from a catalytic converter for treating gaseous gases. exhaust of an internal combustion engine, said pot absorbing said nitrogen oxides when said engine operates in an air / fuel mixture lean in fuel, said method being of the type according to which

a) a purge is controlled by a temporary increase in the fuel richness of said mixture by gradually reducing the air flow entering the engine so as to optimize the fuel consumption of the engine to the fixed richness, by abruptly reducing to a predetermined value, the angle of advance on ignition of the air / fuel mixture, to maintain the constancy of the torque delivered by the engine, and then slaving a rise in said angle of advance to said reduction in air flow, up to a second predetermined value optimizing the fuel consumption of the engine as a function of the richness and of the air flow rate fixed during the purge,

b) the purge is stopped by reducing the values of the richness of the air / fuel mixture, of the air flow rate and of the angle of advance on ignition of said mixture, to the respective values prior to the initiation of the purge,

this process being remarkable in that, in step b), the richness of the air / fuel mixture is first lowered to an intermediate value between the purge and lean mixture richnesses, with the concomitant control of a progressive rise in the air flow and a sudden rise in the ignition advance angle to a third predetermined value as a function of said intermediate richness, a decrease in said angle is then enslaved to a fourth predetermined value during the rise of the air flow, and finally restores the richness of the mixture, the air flow and the ignition advance angle to the respective values prior to the purge.

Patent FR 2 772 428 (RENAULT) describes a process according to the preamble of claim 1.

As will be seen later in detail, by ordering thus simultaneously the air flow entering the engine and the angle of advance at ignition of the air / fuel mixture power supply to this motor, the consistency of the torque delivered by the engine without causing overconsumption and the pollution peaks mentioned in preamble to this description.

• la figure 1 représente schématiquement un moteur à combustion interne équipé pour assurer la mise en oeuvre du procédé suivant l'invention, et
• la figure 2 rassemble des graphes utiles à la description du procédé suivant l'invention.

Other characteristics and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which:

• FIG. 1 schematically represents an internal combustion engine equipped to ensure the implementation of the method according to the invention, and
• FIG. 2 brings together graphs useful for describing the process according to the invention.

We refer to Figure 1 of the accompanying drawing where we schematically represents one of the cylinders of an engine 1 internal combustion, conventionally provided with a piston la, an inlet valve 1b and a valve exhaust lc. The exhaust valve lc controls the passage of combustion gases leaving the engine, in a exhaust line 2 on which a pot is mounted trifunctional catalytic 3 of the type described in the preamble of this description.

Fuel is supplied to the cylinder using a fuel injector 4 opening into the cylinder, in the case of an "injection" engine direct ", or from an injector 5 conventionally spraying the inlet valve 1b, as shown in line interrupted in Figure 1.

The air flow entering the cylinder through this valve 1b is regulated by an air intake circuit comprising a "motorized" throttle valve 6 controlled by a digital computer 7, as it is now classic. The computer controls, in addition to the butterfly 7 and the injector 4 or 5, a spark plug 8 so to adjust the angle of advance at ignition of the mixture air / fuel introduced into the cylinder.

We now refer to Figure 2 of the drawing attached to describe the following purge process the invention. This is implemented while the engine is supplied with lean air / fuel mixture. We knows that then the catalytic converter 3 is charged with oxides nitrogen. We know, for example from the patent application French n ° 97 15775 filed on December 12, 1997 by the Applicant, an expert system designed to assess continually the pot’s effectiveness in adsorption of nitrogen oxides and to trigger a purge of the pot, when this efficiency becomes insufficient.

FIG. 2 shows the development, respectively, of the richness R of the air / fuel mixture, of the air flow rate D entering the engine and of the angle of advance of ignition, from d 'An instant t ₁ of triggering a purge, instant determined by said expert system for example, these changes being determined by the purge control method according to the invention, as described in detail below.

At time t ₁ , the computer controls a sudden increase in the quantity of gasoline injected into the engine and therefore in the richness of the air / fuel mixture, so that it is suitable for purging the pot of oxides of nitrogen, this richness then passing from the previous value R ₀ corresponding to a lean mixture, to a value R ₁ greater than that (R ₂ ) corresponding to the stoichiometry.

Simultaneously, according to the invention, the computer 7 controls a gradual reduction in the air flow entering the engine from the previous value D ₀ , and an abrupt reduction in the ignition advance angle α, which then goes from the previous value α ₀ to a value α ₁ .

The controlled reduction in air flow, concomitant increase in wealth, allows this flow to be adapted to the required wealth, and therefore to ensure a functioning optimal engine in terms of consumption in fuel and therefore also from the point of view of limitation of the production of polluting species caused by combustion of the air / fuel mixture.

The sudden reduction in the angle of advance α to the value α ₁ is calculated to compensate for the sudden increase in engine torque which would otherwise result from the sudden increase in the value of the richness of the air / fuel mixture up to the value R ₁ . The engine torque is thus stabilized at its value prior to the purge, for the benefit of passenger comfort.

However, it is observed that certain actuators of the engine control system have a response time that does can be overlooked. This is particularly the case with the system mechanical component that constitutes the motorized throttle valve 6, which can open or close instantly. The phenomenon of "wall wetting" by the injected fuel is also not immediately reversible. Other phenomena such as filling / emptying the engine with air have a large time constant. These times of response and delays cause, in phases purge transients, changes in engine torque which can cause driving inconvenience if the ignition advance is not controlled by the air flow.

According to the invention, these changes in torque are eliminated by gradually increasing the angle α of ignition advance, from the value α ₁ to a value α ₂ corresponding to optimum operation of the engine at the air flow rate D ₁ and at the setpoint richness R ₁ during the purging, this progressive rise in the angle of ignition advance α being slaved to the reduction of the air flow so as to follow it over time. The response time of the air intake circuit is thus fully taken into account by the method according to the invention, which makes it possible to limit the fuel consumption of the engine during the purge as well as the emissions of polluting species, without disturb the engine torque.

Controlling the angle α to the air flow can be obtained simply by suitable programming of the computer 7.

When the expert system determines that the catalytic converter 3 has regained its effectiveness, it commands, at time t ₂ , the stopping of the purge, that is to say the restoration in the long term of the values R ₀ , D ₀ and α ₀ of the richness, the air flow rate and the ignition advance angle respectively.

We could return to these values by a strategy symmetrical to that applied at the start of the purge. It would then be necessary to wait, the air flow being master, for it to return to its initial value (D ₀ ) while the advance α, controlled by the air flow, ensures the conservation of the torque. If the pot 3 is completely purged at time t ₂ , pollution peaks (by unburnt hydrocarbons and nitrogen oxides) would be observed after time t ₂ , until the flow D ₀ is restored due to the time of air intake system response.

According to the invention, this difficulty is overcome by returning to the lean mixture by passing through a mixture of richness of value R ₂ intermediate between R ₁ and R ₀ , the value R ₂ corresponding to a stoichiometric mixture, for example, richness value for which the three-way catalyst treats polluting emissions.

Simultaneously, the computer controls the throttle valve 6 so as to increase the air flow entering the engine until gradually returning to an air / fuel mixture of richness R ₀ , therefore to a "lean" mixture.

At time t ₂ , according to the invention, the computer suddenly raises the ignition advance angle α to the value α ₃ , so as to compensate for the sudden reduction in the wealth observed at this time, α ₃ corresponding to the optimum advance value when the air / fuel mixture is of stoichiometric composition. After the instant t ₂ , the advance is again controlled by the air flow until it returns to the value D ₀ , at the instant t ₃ . At this instant, the advance α and the richness R are reduced to the values α ₀ and R ₀ respectively, corresponding to the lean mixture.

This strategy of evolution of the richness R and of the angle α of ignition advance in the time interval [t ₂ , t ₃ ] makes it possible to adapt more closely, thanks to the servoing used, these quantities at variations in the air flow D during this time interval, this flow being dependent on a response time of mechanical origin which does not affect the changes in the quantities R and α.

It now appears that the present invention achieves the goal, namely to ensure the purge of nitrogen oxides from a catalytic converter, without disturbance of the torque delivered by the engine and without overconsumption of fuel and pollution peaks transients in engine exhaust.

Claims (3)

1. Method for controlling the purging of nitrogen oxides (NOx) from a catalytic converter (3) for treating the exhaust gases of an internal combustion engine (1), said converter (3) absorbing said nitrogen oxides when said engine (1) is operating on a fuel-lean air/fuel mixture, said method being of the following type, wherein

   a) purging of the converter (3) is controlled by a temporary increase in the fuel richness (R) of the mixture by progressively reducing the flow rate of air (D) entering into the engine such as to optimise the engine fuel consumption at the richness fixed, by rapidly reducing the angle of advance ignition (α) of the air/fuel mixture to a pre-determined value (α₁) in order to maintain the constancy of the torque delivered by the engine (1), and by then controlling an increase in said angle of advance ignition (α) again to said reduction in the flow rate of air (D) to a second pre-determined value (α₂) optimising the fuel consumption of the engine (1) according to the richness (R₁) and of said air low rate (D₁) feed during the purge,

   b) the purge is terminated by returning the values of the richness (R) of the air/fuel mixture, of the air flow rate (D) and of the angle of advance ignition (α) of said mixture to the respective values (R₀, D₀, α₀) prior to the initiation of the purge,

   this method being characterised in that at step b) the richness of the fuel/air mixture is firstly reduced to a value (R₂) that is intermediate between the purging richness (R₁) and lean mixture richness (R₀) with concomitant control of the air flow rate (D) and rapid increase again of the angle of advance ignition (α) to a third pre-determined value (α₃) according to said intermediate richness, a decrease in said angle (α) to a fourth pre-determined value is then made during the increasing of the air flow rate, and lastly the richness of the mixture, the air flow rate and the angle of advance ignition are returned to the respective values (R₀, D₀, α₀) prior to purging.
2. Method according to claim 1, characterised in that said intermediate value (R₁) of the richness (R) corresponds to a stoichiometric air/fuel mixture.
3. Method according to any one of claims 1 and 2, characterised in that the purge is Fixated and terminated under the control of a system for evaluating the load factor of the catalytic converter (3) in nitrogen oxides.

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