FR2833308A1 - METHOD FOR PURGING A FUEL ABSORBER FOR A VARIABLE DISTRIBUTION ENGINE - Google Patents

Abstract

The invention concerns a method for bleeding a fuel vapour absorber in an intake manifold of an internal combustion engine with variable distribution, the engine operating in a first mode wherein the valve lift laws are variable and creating a slight depression in the intake manifold. The invention is characterized in that the method comprises: a step (203) which consists in estimating the load of the absorber, a step (205) which consists in comparing the estimated load with a first threshold (high_threshold); if the load is greater than the first threshold the engine switches into a second operating mode wherein the valve lift laws correspond to that of a standard engine with fixed distribution.

Description

<Desc / Clms Page number 1>

Method of purging a fuel absorber for a variable valve engine
The present invention relates to a method of purging a fuel vapor absorber which controls and regulates the manifold pressure of an internal combustion engine and variable distribution of a motor vehicle, the timing and / or the duration of emergence of some or all of the valves is (are) said variable (s) and this whatever the means or means implemented to make the distribution variable. The motor vehicle is equipped inter alia with a fuel vapor absorber, circuits necessary for purging the fuel vapor absorber, an actuator used to regulate the purging of the fuel vapor absorber, and which is located between the latter and the intake manifold of the engine on one of the circuits mentioned, of one (or more) actuator which controls (s) the lifting of the valves, a housing comprising a butterfly, which is a actuator whose purpose is to manage the air intake of the engine and an electronic computer which, among other things, determines and generates the control signals of the three actuators mentioned above.

 In order to comply with the standards concerning hydrocarbon emissions from motor vehicles, in addition to improving the tightness of bodies containing hydrocarbons, it is known that those equipped with internal combustion engines and more particularly engines spark ignition engines, each use a fuel vapor absorber. This makes it possible to store the hydrocarbon vapors from the fuel tank, referred to simply as the tank below, in order to prevent their rejection in the ambient air, whether the vehicle and / or its engine is stopped. (s) or in operation (s).

<Desc / Clms Page number 2>

 More specifically, the present invention aims to reduce and regulate punctually the pressure inside the intake manifold, by a programmed strategy in the electronic engine computer, to promote the purge of the vapor absorber of the engine. fuel.

 This object is achieved by a method of purging a fuel vapor absorber comprising: a step of estimating the load of the absorber, a step of comparing the estimated load with a first threshold, if the estimated load is greater than the first threshold, the operation of the engine switches to a second mode in which the valve lift laws correspond to those of a conventional fixed distribution engine.

 According to another characteristic, as long as the engine is operating in the second mode, the estimation step and the comparison step are performed regularly, if the estimated load is less than or equal to the first threshold, the method comprises a second comparison step. with a second threshold, if the load is lower than the second threshold, the operation of the motor switches to the first mode of operation, otherwise the second mode of operation is maintained.

 According to another characteristic, before the switching or the maintenance of the motor operation in the second mode, the method comprises a validation step consisting in checking whether the operation in the second mode of operation is not contradictory with a priority event, if such is the case of switching or keeping in the second mode of operation is postponed until the cessation of the event.

 According to another characteristic, the second mode of operation corresponds to a variable valve lift law defined

<Desc / Clms Page number 3>

par un objectif de pression collecteur déterminé et en ce que lors du q 9 basculement dans le deuxième mode de fonctionnement, une électrovanne de purge placée entre l'absorbeur et le collecteur d'admission du moteur est positionnée en pleine ouverture.

by a purpose of collector pressure determined and that when q 9 switchover in the second mode of operation, a purge solenoid valve placed between the absorber and the intake manifold of the engine is positioned in full opening.

 According to another characteristic, the collector pressure objective is determined according to a table giving the collector pressure objective as a function of the air-fuel mixture mass objective to be purged.

Other features and advantages of the present invention will appear more clearly in the following description given with reference to the accompanying drawings in which:
FIG. 1 represents a diagram of an internal combustion engine capable of implementing the method according to the invention, FIG. 2 represents a simplified diagram of a first embodiment of the method according to the invention, FIG. a simplified diagram of a second embodiment of the method according to the invention,
Before describing precisely the invention it is necessary to recall the function of the fuel ab sorber and the principle of its purging.

 The fuel vapor absorber, referred to simply as the absorber below, is a receptacle that stores the hydrocarbon vapors by means of absorption elements that are permeable, for example activated carbons. It is generally provided with three orifices, a first connection port with the reservoir, a second suction port and a third vent port.

 The connecting orifice of the absorber is connected to the reservoir by means of a permeable circuit which is called a connecting circuit. By its construction, and in the absence of leakage, the tank allows the hydrocarbon vapors to escape only to the absorber. The vent port is an opening of the absorber to the atmosphere and so the

<Desc / Clms Page number 4>

 reservoir is put at atmospheric pressure via the absorber and more particularly via the absorption elements.

 The absorber has some storage capacity. When this capacity is reached, it is said that the absorber is saturated. In order to avoid saturation and therefore the release into the ambient air of hydrocarbons via venting, it is necessary to regenerate the absorption elements. It is known that the regeneration takes place when the engine is in operation and that it is necessary to use a set of pipes, which is called purge circuit, which connects the suction port of the absorber to the engine intake manifold. The movement of the engine pistons creates a vacuum in the intake manifold, which makes it possible, by difference with the atmospheric pressure, to establish an air flow between the venting port and the exhaust manifold. admission which sweeps the absorption elements; the air takes charge of the fuel contained in the absorber and the fuel mixture thus obtained is sucked by the engine to be burned.

 The quality of the regeneration of the absorber is a function of the amount of air passing through it, and therefore, is a function of the pressure difference between the engine intake manifold and the atmosphere. Trivially, if the collector pressure is that of the atmosphere, the depression is zero and it is impossible to regenerate the absorber by this system that has been exposed.

 The fuel mixture supplied by the purge of the absorber must not induce in the engine cylinders disparities of wealth detrimental to the respect of standards on pollutant emissions by the engine. Thus, it is known that this input must be controlled and limited by a device comprising, an actuator (for example a solenoid valve) controlled by the electronic computer and which is integrated in the purge circuit, and servo means programmed in the electronic calculator that

<Desc / Clms Page number 5>

entres autres déterminent et génèrent le signal de commande dudit e 9 actionneur.

among others, determine and generate the control signal of said actuator.

 This device makes it possible to regulate the gas passage section of the purge circuit; the pressure drop of this circuit is therefore variable according to the control of the actuator. Often, the actuator has several intermediate positions between a complete closure, ie an infinite pressure drop of the purge circuit, and a maximum opening, ie a minimum but not zero pressure drop of the circuit purge. It is therefore necessary to reduce the opening of the actuator to reduce the contribution of the fuel mixture. In the following, it is appropriate to consider the general case where the actuator has several intermediate positions of openings; and that those driven by an opening cyclical ratio are assimilated to it (the opening position is given by the value of the opening duty cycle denoted RCO).

The opening of the actuator is carried out gradually by means of an opening step. To determine the opening of this device, it is known to use:
1) The load of the absorber which is estimated by the strategy, by means, for example of the information from the exhaust probe usually used for the regulation of wealth. In other words, the load of the absorber corresponds to the richness of the air-fuel mixture purged and sucked by the engine, that is to say the ratio of the mass of fuel on the air mass that come from the circuit purge.

 2) Purge objective: this is the desired mass ratio of the fuel flow purged from the absorber to the total fuel flow (fuel flow for the injector plus the purged fuel flow) admitted to the engine multiplied by 100, this purge target represents the maximum percentage of fuel that can be supplied by the purge and which is tolerated by the engine without significant damage to the wealth regulation. It is determined by means of tests. Purge goal is a

<Desc / Clms Page number 6>

 scalar that is, either defined for the entire range of use of the engine, or deduced from different values established for different operating points of the engine. The value or the table of values also called cartography is duly programmed in the electronic computer of the engine.

 3) The flow rate of the purge circuit of the absorber which is determined according to different parameters and which is duly programmed in the electronic computer of the engine.

 During motor operation, the control seeks to reach the purge objective. The actual ratio at a given instant is called the purge rate, it is less than or equal to the purge goal.

 When the absorber is charged, a low purge lens involves a small opening of the actuator that regulates the purge. The amount purged is therefore small and it is therefore difficult to empty the absorber. Moreover, if the supply of fuel vapor in the absorber by evaporation of the fuel contained in the tank is equal to or greater than the quantity which is purged at the same time, then it is impossible to empty the absorber, see, hydrocarbons escape by venting.

 The standardized test of hydrocarbon emissions by the vehicle is carried out on a complete vehicle duly representative of the series vehicle, according to a normative test composed of different stages or all of them (except for stages of pre-production). conditioning) the vehicle must not emit an amount of hydrocarbon above a threshold specified in the law.

 In practice, in order to meet the standard, it is necessary to purge the absorber sufficiently at each stage which is carried out with the engine in operation (according to vehicle speed profiles as a function of time), in order to release a capacity of sufficient storage to recover vapors from the tank during this step and in subsequent stages where the engine is stopped.

<Desc / Clms Page number 7>

Several standards exist depending on the geographical area; in general, the rolling profiles have a large share of urban pipes with a low engine load.

 In contrast to the variable valve engine, the engine whose valve lift (s) is (are) constant (given, for example, by the profile of one or more shafts) Cams) and unregulated (s), is called fixed distribution motor. A fixed distribution motor has a large manifold vacuum for low loads because the valve opening and closing settings are optimized especially for heavy loads. This major depression is beneficial for emptying the absorber. In addition, it is necessary to associate with this strong depression, as far as possible, an important purge objective so that the purge of the absorber is the most effective possible.

 One of the objectives of the use of a variable distribution is to reduce the manifold depression in cases of low load runs in order to reduce the specific consumption of the engine.

 Thus, it is known that the admission valves see exhaust are controlled by a device comprising one or more actuators, controlled by the electronic computer, servo means programmed in the electronic computer which among others determine and generate the signal controlling these actuators.

These commands vary depending on the operating point of the motor.

As a result, the opening of the throttle which manages the air intake of the engine is controlled in order to obtain the expected engine torque.

 Compared to a fixed distribution motor, for the same mass of air admitted by the engine, and the same air intake circuit of which the butterfly is part, it is known that the throttle position may be more open for a variable valve motor. Indeed, at low load, it is interesting to push back into the intake manifold a part of the air

<Desc / Clms Page number 8>

 admitted into the cylinders, by means of the use of a large delay in closing the intake valves.

 Increasing the throttle opening causes a decrease in the flow velocity in the intake manifold downstream of the throttle, or opens the bleed circuit. This flow is less turbulent. Now it is known that turbulence promotes the homogenization of gases. Thus, for the fuel supply through the purge of the absorber does not interfere with the regulation of wealth, it can be envisaged that this type of engine may require to reduce the purge objective compared to that of an engine fixed distribution.

 For heavy loads, valve and throttle controls are similar to those used on fixed-valve engines.

 Variable distribution engines appear to be unfavorable for the purge of the fuel vapor absorber, at least because of the decrease in the depression, or the decrease of the purge objective.

 However, on a variable distribution engine, it is possible to have a large collector vacuum at low load by adopting defined distribution settings for a fixed distribution engine.

 The present invention is applicable to a variable distribution engine. It uses an already known purge circuit which is associated with an already established absorber draining strategy. According to the invention, as a function of the load of the absorber which is estimated, the manifold depression is increased by using a distribution adjustment similar to that usually used for a fixed distribution engine. As this is a function of the load of the absorber, this increase of depression will be punctual (provided that the characteristics of desorption of the absorber are not binding). In addition, the increase in depression may result in a simultaneous increase

<Desc / Clms Page number 9>

 the purge purpose; the increase in depression may be used for this purpose.

 The vehicle, according to FIG. 1, uses a fuel vapor absorber 1 which makes it possible to recover the hydrocarbon emissions from the fuel tank 2 by means of a connecting circuit 3 which connects the tank 2 to the fuel port. linkage 101 of the absorber 1. The absorber 1 is provided with two other openings: an orifice of an air vent 102 and a suction orifice 103. Free air 102 is separated from the connecting port 101 by the absorption elements 104 which are permeable. The suction port 103 is connected to the intake manifold 7 of the engine 8 downstream of the throttle valve 9, by a purge circuit 4 in which is integrated a solenoid valve 6 which is an actuator controlled by the electronic injection computer 10. motor 8.

 The air intake circuit of the fixed distribution engine 8 is composed, inter alia, of a housing comprising a butterfly 9 controlled by the computer 10 of the engine 8 duly programmed, intake valves 11 and exhaust (no shown). The intake manifold 7 connects the throttle valve 9 to the various cylinders of the engine 8. The manifold pressure is known by means of the information coming from a pressure sensor 5 connected to the computer 10 and placed on the intake manifold 7 or well by reconstituting information from other sensors.

 The variable distribution is obtained by control laws of the inlet valves 11 and / or exhaust, which are controlled by one or more actuators 12. The computer 10 controls the throttle valve 9 and the actuator (s) 12, in order to achieve an air filling objective of the engine 8 and thus obtaining the desired engine torque. It is theoretically possible to obtain the same mass of air admitted for different torques [valve control law 11, opening of the throttle valve 9J, with consequent variation in the collector pressure.

<Desc / Clms Page number 10>

Par ce système de contrôle des levées des soupapes, il est aisé de 4 commander les soupapes 11 et en conséquence le papillon 9, de la même manière qu'un moteur à distribution fixe, en programmant le calculateur 10 de telle manière que la loi de distribution soit invariable. On obtient alors des réglages équivalents à ceux d'un moteur à distribution fixe et donc des dépressions collecteurs importantes pour les faibles charges.

By this valve lift control system, it is easy to control the valves 11 and consequently the throttle valve 9, in the same way as a fixed distribution engine, by programming the computer 10 in such a way that the law of distribution is invariable. This gives settings equivalent to those of a fixed distribution motor and thus large collector depressions for low loads.

Consequently, whatever the point of operation of a variable distribution engine, two extreme modes of operation can be defined:
A first mode or mode 1: according to different engine parameters, the computer 10 determines the laws, which are variable levers valves 11 and actuates by controlling one or more actuators 12. This mode favors the reduction of the consumption by increasing the collector pressure (the collector depression is low).

 A second mode or mode 2: the laws of levées of valves It are invariable and they are similar to those of a motor with fixed distribution. These laws are programmed in the computer 10 which controls one or more actuators 12 to lift the valves 11. In return the consumption at low load is greater than in variable mode, due to a lower collector pressure.

 Between these two extreme modes, it is possible theoretically to obtain different laws of levies of the valves for which a specific consumption has been obtained which increases with the reduction of the collector pressure. We call mode 3, all of these intermediate possibilities of levee laws.

 The algorithms, the variables and the constants necessary for the regulation of the motor 8, including those dedicated to the control of the purge of the absorber 1 and those dedicated to the present invention, are duly programmed in the computer 10.

<Desc / Clms Page number 11>

 A first embodiment of the invention will now be described.

 If the engine 8 is in mode 1, the present invention aims to reduce the collector pressure punctually in order to facilitate the purging of the absorber 1 of fuel vapors by strategy.

 The method according to the invention consists in using the strategy of purging the absorber established with the learning of the load of the absorber 1, and, when the motor 8 operates in mode 1, to determine the opportune moment or it It is desirable to switch to mode 2 to increase the collector depression, then the moment when it is possible to return to mode 1. This collector pressure decrease must be punctual so as not to penalize consumption. The criterion which makes it possible to choose the mode of operation of the motor 8 is the load of the absorber 1. This makes it possible to be effective on a normalized cycle, but also in cases of severe rolling or the absorber 1 is very loaded. When it is necessary to reduce the collector pressure, the method according to the invention makes the necessary request to a manager who authorizes it or not.

 According to the invention, two thresholds are used which are to be defined and programmed in the computer 10. After initialization and learning of the load of the absorber 1, if the load exceeds a first threshold called threshold high, then a request to pass in mode 2 is generated to decrease the collector pressure. This request will continue the necessary time so that the load decreases below a second threshold, called low threshold. If the load decreases below threshold, the request to decrease the collector pressure is stopped; the operation of the motor 8 then switches to the Isauf mode if it is necessary to maintain the mode 2. This case may occur if a diagnosis of a pollution control device such as a catalyst or an exhaust probe which requires stability in the engine operation. In some cases the depression prevailing in the intake manifold is used to amplify the force of

<Desc / Clms Page number 12>

 Vehicle braking is increasing the hydraulic pressure in the brake system. To increase this depression and thus increase braking, one solution is to switch the operation of the engine in mode 2 and maintain this mode time that the need for braking requires. In this case, switching to mode 1 is suspended until mode 2 is no longer needed.

permet d'éviter de changer souvent de mode de fonctionnement du moteur 8. If the load increases again and goes between high-threshold and low-low, there is no request made to decrease the collector depression. This will be the case if the load increases sufficiently to go above the high-threshold. This logic is respected unless a priority event requires to refuse the request. In this case the state of the motor 8 remains in mode 1. This event passed, a new request to reduce the collector depression can be accepted. The use of two thresholds

avoids often changing the operating mode of the motor 8.

 By way of example, a priority event is a diagnosis of a pollution control device such as a catalyst or an exhaust probe that requires stability in the engine operation. If such a diagnosis is in progress, it is expected to be completed or stopped.

 After starting the engine 8, the initialization procedures of the different strategies are executed, including those of the purging of the absorber 1, in a first step 201 (Figure 2). By default, the operating mode selected is mode 1, which corresponds to the mode where the collector pressure is high.

 If the purge of the absorber 1 is allowed in a second step 202, then the actuator 6 is open and the load of the absorber 1 is estimated in a third step 203, otherwise the second step 202 remains active.

<Desc / Clms Page number 13>

Dans une quatrième étape 204, on teste si la charge de l'absorbeur fi.

In a fourth step 204, it is tested whether the load of the absorber fi.

1 is considered learned; if it is not the case, it returns to the second step 202, otherwise it goes to a fifth step 205.

 This fifth step 205 aims to compare the load of the absorber 1 previously estimated at a threshold called high threshold; if it is higher than this threshold, then a seventh step 207 is executed, otherwise a sixth step 206 is executed.

 In the sixth step 206, a new comparison is made: the load of the absorber 1 previously estimated is compared to a threshold named only! low, with a low threshold which is less than threshold; if the estimated load is higher than this second threshold and the motor 8 is already in mode 2, then the seventh step 207 is executed otherwise an eleventh step 211 is executed.

 At the seventh step 207, a request is made to a manager in order to obtain the authorization to switch, or to maintain mode 2 in activity, in order to increase the collector depression to make it easier to purge the absorber 1. Then, if in an eighth step 208 the request is accepted, then mode 2 is activated or maintained if it is already activated in a tenth step 210, otherwise it is mode 1 which is activated or maintained s' it is already activated in the ninth step 209.

 In the eleventh step, it is checked whether the motor is already in mode 2.

If not, the ninth step is executed, otherwise a twelfth step 212 is executed. The twelfth step 212 tests whether mode 2 should remain active for the purposes of another strategy (not shown). If it is the case, the tenth step 210 is executed, otherwise the ninth step 209 is executed.

 After execution of the ninth step 209 or the tenth step 210, the second step 202 is executed again.

 This solution, which has the merit of being simple, has a choice between two modes. It's an all or nothing system.

<Desc / Clms Page number 14>

 A second embodiment of the method according to the invention will now be described.

 With the first embodiment, discussed above, if the purge lens used and the state of charge of the absorber 1 and the flow rate of the purge circuit, that the actuator 6 can not be opened in full opening, then it must be considered that the collector depression is greater than necessary. As a result, the overconsumption is greater than that associated with a manifold pressure defined in such a way that the purge objective is reached with the electromechanical system 6 open in full opening.

 The second embodiment of the invention requires controlling the collector pressure.

 The increase of the collector depression, is translated (Figure 3) by a request to switch to mode 3, accompanied by a calculation of a collector pressure target to achieve by means of the regulation of the actuator (s) 12 and the butterfly 9, in order that the supply of fuel through the purge circuit does not exceed the purge objective and that the actuator 6 is controlled in full opening permanently.

le moteur 8 est calculé au moyen de l'équation E. 1 définie par :

Masse purge obj = Obi purge x Débit carburant x 1 + 1) - fC-J J-1 & - < Charge- absorbeur

uu : Massepurgeobj = objectif de masse de mélange air-carburant à purger. The calculation of the manifold pressure objective is as follows: the maximum air-fuel mixture flow that can be purged by

the motor 8 is calculated by means of the equation E. 1 defined by:

Mass purge obj = Obi purge x Fuel flow x 1 + 1) - fC-J J-1 & - <Load-absorber

uu: Massepurgeobj = objective of mass of air-fuel mixture to be purged.

 Obj purge: previously defined purge goal.

 Fuel flow: fuel flow required to operate the engine 8.

<Desc / Clms Page number 15>

Chargeabsorbeur : charge de l'absorbeur 1 ou richesse du mélange 0 air-carburant purgé.

Chargeabsorber: load of the absorber 1 or richness of the mixture 0 air-fuel purged.

TabtepcotLfmassepurgeobj. On définie l'équation E. 2 : PCoiLobj = interpolation (Table pcoll-f masse purge obj, Masse purge obj)
Les valeurs de cette table sont issues, par exemple, de mesures de débits effectués sur un circuit de purge pour différentes pressions collecteurs, et pour un RCO de commande dont la valeur est celle définie comme étant la valeur maximum. Then, the appropriate objective collector pressure is calculated, by means of interpolations, according to the objective of air-fuel mass to be purged, in a table or a map of values noted

TabtepcotLfmassepurgeobj. We define the equation E. 2: PCoiLobj = interpolation (Table pcoll-f mass purge obj, Mass purge obj)
The values in this table are derived, for example, from flow measurements made on a purge circuit for different manifold pressures, and for a control RCO whose value is the one defined as the maximum value.

 Then, it is enough to adopt the necessary settings so that the collector pressure converges towards the collector pressure objective.

 Continuous regulation of the collector pressure, which has the advantage over all-or-nothing regulation, is thus obtained of obtaining the same purge of the absorber 1 while limiting the increase in consumption.

 The first to fifth stages and the ninth stage of the second embodiment are identical to those of the first embodiment.

 The first step (201 ') of the second embodiment differs from the first embodiment in that the solenoid valve (6) is positioned in full opening.

 The sixth step 206 'of the second embodiment differs from the sixth step 206 of the first embodiment in that it is verified that the motor is in the mode 3, in addition to checking that the load is greater than a threshold down to the seventh step 207'consisting in the development of a request to increase the depression in the collector. This request consists in passing or maintaining the mode 3. In the eighth step 208 ', if the request sent in the seventh step 207 is refused then the ninth step 209 is triggered. On the other hand,

<Desc / Clms Page number 16>

la pression collecteur à utiliser est déterminé dans une dixième étape 210' . 4 au moyen des équations E. 1 et E. 2 précédemment définies, puis active ou maintien le mode 3 s'il était déjà actif dans une onzième étape 211.

the manifold pressure to be used is determined in a tenth step 210 '. 4 using equations E. 1 and E. 2 previously defined, then activates or maintains mode 3 if it was already active in an eleventh step 211.

 After execution of the ninth step 209 or the eleventh step 211, the second step 202 is executed again.

 The present method is added to an already established absorber purging strategy and makes it possible to achieve the stated goal of reducing the collector pressure on a selective basis in order to be able to favor the purging of the fuel vapor absorber 1 by strategy and by varying, for this purpose, the (the) control (s) of the actuator (s) 12 of the valves 11 and the opening of the butterfly 9.

 Of course, the invention is not limited to the embodiment which has just been described as an example.

Claims (7)

1. A method of purging a fuel vapor absorber in an intake manifold of a variable distribution internal combustion engine, the engine operating in a first mode in which the levy laws of the valves are variable and creating a low vacuum in the intake manifold characterized in that it comprises: a step (203) for estimating the load of the absorber, a step (205) for comparing the estimated load with a first threshold, (threshold high), if the estimated load is greater than the first threshold, the operation of the engine switches to a second mode in which the valve lift laws correspond to that of a conventional fixed distribution motor.  2. Method according to claim 1, characterized in that as long as the engine is operating according to the second mode, the estimation step and the comparison step are performed regularly, if the estimated load is lower than the first threshold the method comprises a second comparison step with a second threshold (low threshold), if the load is lower than the second threshold the operation of the motor switches in the first operating mode, otherwise the second operating mode is maintained.  3. Method according to claim 1 or 2, characterized in that before the switching or the maintenance of the motor operation in the second mode the method comprises a step (208) of validation consisting in checking if the operation in the second mode of operation n is not contradictory with a priority event, if this is the case the switchover or maintenance in the second mode of operation is postponed until the cessation of the event.  4. Method according to one of claims 1 to 3, characterized in that the second mode of operation corresponds to a variable valve lift law defined by a collector pressure lens <Desc / Clms Page number 18>  determined and in that when switching to the second mode of operation, a purge solenoid valve (6) placed between the absorber and the intake manifold (7) of the engine is positioned in full opening.  5. Method according to claim 4, characterized in that the collector pressure objective is determined according to a table giving the collector pressure objective according to the air-fuel mixture mass objective to be purged.  6. Method according to claim 5, characterized in that the objective of

 mass of air-fuel mixture to be purged is defined by the formula:

 Masspurgeobj = Purpose purge x Fuel flow x 1 + 1 J Load - absorber uù: Masspurgeobj = target mass of air-fuel mixture to purge.  Obj purge: Purge goal defined beforehand.  Fuel flow: fuel flow required to operate the engine 8.  Chargeabsorber: load of the absorber (1) or richness of the air-fuel mixture purged.  7. Variable distribution internal combustion engine comprising a fuel vapor absorber, the engine operating in a first mode in which the levy laws of the valves are variable and comprising a computer 10 for implementing the method according to one Claims 1 to 6.