FR3128975A1 - Method and system for purging a canister of a combustion engine equipped with at least one exhaust gas recirculation circuit - Google Patents

Abstract

This method for purging a canister of a motor vehicle internal combustion engine equipped with at least one exhaust gas recirculation system at the intake, comprises the following steps: detection of the need for purging; - calculation of a purge flow setpoint Qp and a pressure setpoint Pp allowing the flow Qp to be purged; - calculation of the minimum pressure Pcol_mini in the intake manifold to ensure the engine torque setpoint C; - adjustment of the motor to lower the pressure in the manifold to the level of the set point Pp, within the limit of the minimum pressure Pcol_mini which ensures the realization of the motor torque C. Figure for the abstract: Fig 4

Description

Method and system for purging a canister of a combustion engine equipped with at least one exhaust gas recirculation circuit

The present invention relates to the fuel supply circuits of internal combustion engines which comprise a fuel vapor suction tank, in particular gasoline.

Prior techniques

On a motor vehicle equipped with an internal combustion engine, more particularly a spark-ignition (gasoline) engine, the fuel vapor suction tank, known to those skilled in the art by its English name “canister” , includes a carbon filter and is configured to collect fuel vapors escaping from the tank when the vehicle is stopped or operating under severe conditions.

Conventionally, the canister is connected to the engine inlet by a purge pipe fitted with a valve, the opening of which controls the passage of fuel-laden vapors to the engine inlet. To avoid the appearance of vapor leaks into the atmosphere when a certain predetermined limit filling rate is exceeded, canister purge phases are carried out by opening the purge solenoid valve.

A minimum purge flow is required to allow the evacuation of a sufficient quantity of fuel vapors in order to reduce the filling rate of the canister.

It is also known that on a controlled-ignition engine, the depression of the engine intake manifold, generally regulated by means of a throttle valve of the engine to obtain a certain mass flow of air allowing the production of the engine torque, causes pumping losses that are induced by the pressure difference between the intake manifold plenum and the exhaust manifold.

There represents the pressure-volume diagram and characterizes the operation of a conventional four-stroke cycle engine. The pumping losses correspond to the hatched area 2 which represents the work consumed by the engine, unlike the hatched area 1 which represents the work supplied by the engine. According to the conventional cycle of representing the stages undergone by the gases in a cylinder of the engine, the stage of admission (intake time) corresponds to the segment AB, the compression to the segment BC, the combustion to the segment CD, the expansion to the segment DE and the escapement to segment EA.

To reduce the losses by pumping and thus reduce the fuel consumption of the engine, it is known to increase the pressure of the intake manifold while admitting into the engine the same quantity of air in mass flow rate necessary for the production of the same given torque as in an engine operating according to the conventional cycle, represented in the .

A first method of reducing pumping losses by increasing the pressure in the intake manifold, shown schematically in the and known as the Miller cycle, consists of closing the intake valves before bottom dead center, acronym “BDC”. The quantity of air in the cylinder is thus admitted, not up to BDC but only up to the position of the piston corresponding to the instant of closing of the inlet valve, materialized by point B in the . Unlike the conventional cycle where the management of the quantity of air admitted into the cylinders is obtained by varying the opening of the throttle body, in the Miller cycle the throttle body is left open and the management of the amount of air intake is carried out mainly by imposing the moment of closing of the intake valves. The air is thus admitted into the cylinder only on the segment AB, the segments of the cycle BB' and B'B'' corresponding respectively to an increase or decrease in volume of the cylinder with the valves closed. The other portions of the Miller cycle are similar to those of the conventional cycle represented in the . In the , the segments B′′C, CD, DE and EA correspond respectively to the stages of compression, combustion, expansion and exhaust of gases from an engine cylinder.

This results in a higher manifold pressure for the Miller cycle than for the conventional cycle, because the throttle body is left mostly open, even at low loads. By increasing the intake pressure, the amount of work consumed by the engine is reduced, thus the area of the hatched zone 2 of the is smaller than the area of hatched area 2 of the .

A second method of reducing pumping losses by increasing the pressure in the intake manifold, shown schematically in the and known as the Atkinson cycle, involves closing the intake valves after BDC. In this case, the intake valves remain open for part of the rise of the piston after BDC, up to point B' in the . Thus, air is admitted up to BDC corresponding to point B on the , that is to say over the entire stroke of the piston, then part of this admitted air is then pushed back into the intake manifold during the ascent of the piston towards the top dead center, acronym "PMH as long as the intake valves remain open. The quantity of air admitted for combustion is therefore determined by the moment when the inlet valves close, materialized by point B' in the .

In the Atkinson cycle, as in the Miller cycle, the throttle body is left open and the management of the quantity of air admitted is carried out mainly by imposing the moment of closing of the intake valves. The other portions of the Atkinson cycle are similar to those of the conventional cycle depicted in the . In the , the segments B′C, CD, DE and EA correspond respectively to the compression, combustion, expansion and exhaust times of the gases from an engine cylinder.

This results in the Atkinson cycle having a higher pressure in the intake manifold than in the conventional cycle, because the throttle body is left mostly open, even at low loads. By increasing the intake pressure, the amount of work consumed by the engine is reduced, thus the area of the hatched zone 2 of the is smaller than the corresponding area of hatched area 2 of the .

A third method used to reduce pumping losses by increasing the pressure in the intake manifold is to take exhaust gases and send them to the intake, a process known by the acronym "EGR" for "exhaust gas recirculation” in Anglo-Saxon terms. The introduction of a neutral gas, which does not participate in the combustion in the cylinders, makes it possible to increase the pressure in the manifold without increasing the load. Indeed, in the case where an EGR flow is introduced into the engine upstream of the throttle body, the degree of opening of the latter does not control the air flow alone, but the total flow in the engine. which is equal to the sum of the air flow and the EGR flow. It is therefore possible to obtain an identical given air mass flow rate, either by admitting only air at a first pressure value, or by admitting air and a proportion of EGR at a second pressure value higher than the first one. It should also be noted that such a partial recirculation of the exhaust gases at the intake can be combined with a Miller or Atkinson cycle.

The problem with the three methods described, namely the Miller cycle, the Atkinson cycle or the EGR recirculation, is that by increasing the pressure in the intake manifold of the engine, they decrease the ability to bleed the canister in the air intake circuit of the engine when the point of reintroduction of fuel-laden vapors is located downstream of the throttle body, because the pressure difference between the canister, which is at atmospheric pressure by its setting at the free air, and the intake manifold, is greatly reduced and no longer allows the circulation of a sufficient flow.

We know from the state of the art engines equipped with active purge systems, integrating a venturi or a pump to suck the vapors towards the intake. These systems are rather used in cases where the point of reintroduction of fuel-laden vapors is upstream of the throttle body, and they have the disadvantage of requiring piloting for the pump and the installation of additional parts.

In view of the foregoing, the object of the invention is to improve the purging capacity of the canister, without however making use of active purging systems.

In view of the foregoing, the subject of the present invention is a process for the passive purging of the canister of a motor vehicle internal combustion engine equipped with at least one exhaust gas recirculation system at the intake.

The process includes the following steps:

• detection of a purge need;
• calculation of a purge flow setpoint and a pressure setpoint Pp allowing the flow to be purged;
• calculation of the minimum pressure in the intake manifold to ensure the engine torque setpoint;
• adjustment of the engine to lower the pressure in the manifold to the level of the setpoint, within the limit of the minimum pressure which ensures the achievement of the engine torque.

The invention aims to use a so-called passive purge system, which uses the natural depression of the engine intake manifold to suck fuel vapors from the canister when necessary. It will be noted that purging by a passive system as targeted by the invention is only possible if the pressure in the engine intake manifold is lower than the pressure in the canister.

For example, the engine operates on a conventional cycle.

Advantageously, the engine is equipped with a variable valve timing system and operates according to an asymmetrical cycle of the Miller or Atkinson type.

Advantageously, the need for purging is determined when the value of the filling rate of the canister reaches a predetermined threshold.

Advantageously, the engine adjustment includes a drop in the EGR flow rate setpoint.

For example, engine tuning includes steering the valve timing system from Miller or Atkinson cycle engine operation to conventional cycle operation.

According to a second aspect, the subject of the invention is a passive purge system for the canister of a motor vehicle internal combustion engine equipped with at least one exhaust gas recirculation system at the intake.

The passive purge system comprises means for detecting a need to purge the canister, means for calculating a purge flow setpoint and a pressure setpoint, means for calculating a minimum pressure in the intake manifold to ensure an engine torque setpoint and means for adjusting the engine to lower the pressure in the manifold to the level of the setpoint, within the limit of the minimum pressure which ensures the achievement of the engine torque.

Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

,

And

, which has already been mentioned, represent respectively the pressure-volume diagram of a cylinder of a four-stroke internal combustion engine of a motor vehicle according to a conventional, Miller or Atkinson cycle;

very schematically illustrates an example of the structure of an internal combustion engine of a motor vehicle equipped with a canister purge control system according to the invention; And

illustrates a flowchart of the canister purge process, according to one embodiment of the invention.

Detailed description of at least one embodiment

In the example shown in the , the internal combustion engine 10 comprises, without limitation, three cylinders 12 in line, a fresh air intake manifold 14, an exhaust manifold 16, a turbo-compression system 18, a timing system variable 50 inlet valves 51 and exhaust valves 52.

The cylinders 12 are supplied with air via the intake manifold 14, or distributor, itself supplied by a pipe 20 provided with an air filter 22 and the turbocharger 18 of the engine 10.

The turbocharger 18 essentially comprises a turbine 18a driven by the exhaust gases and a compressor 18b mounted on the same axis or shaft as the turbine 18a and providing compression of the air distributed by the air filter 22, with the aim of increase the quantity (mass flow) of air admitted into the cylinders 12 of the engine 10.

The internal combustion engine 10 comprises an intake circuit Ca and an exhaust circuit Ce.

The intake circuit Ca comprises, from upstream to downstream in the direction of air circulation:

- the air filter 22 or air box;

- a flow meter 26 disposed in the intake duct 20 downstream of the air filter 22 to measure the actual value of the air flow entering the engine 10;

- an air inlet valve 28;

- The compressor 18b of the turbocharger 18;

- a throttle body 30 or a gas inlet valve in the engine;

- a heat exchanger 32 configured to cool the admission gases corresponding to a mixture of fresh air and recirculated gases after their compression in the compressor 18b;

-pressure and temperature sensors 33 for measuring the pressure and temperature in the intake manifold 14; And

- the intake manifold 14.

The compressor is associated with a bypass circuit equipped with an inlet relief valve 55 which opens in the event of sudden closing of the throttle body 30, to prevent the compressed air, located between the compressor 18b and the throttle body 30, does not pass through the compressor 18b and does not degrade it, when for example, the driver of the vehicle suddenly lifts his foot from the accelerator pedal.

The exhaust circuit Ce comprises, from upstream to downstream in the direction of circulation of the burnt gases:

- the exhaust manifold 16;

- the turbine 18a of the turbocharger 18; And

- A system 40 for depolluting the engine combustion gases.

As regards the exhaust manifold 16, the latter recovers the exhaust gases resulting from combustion and evacuates them to the outside, via a gas exhaust duct 34 leading to the turbine 18a of the turbocharger 18 and by an exhaust line 36 mounted downstream of the turbine 18a.

The engine 10 further comprises a partial recirculation circuit 38 of the exhaust gases at the intake, called the “EGR” circuit (“exhaust gas recirculation” in Anglo-Saxon terms).

This circuit 38 is here a low pressure exhaust gas recirculation circuit, called “EGR BP”. It is connected to the exhaust line 36, downstream of said turbine 18a, and in particular downstream of the gas pollution control system 40 and returns the exhaust gases to the fresh air supply pipe 20, upstream of the compressor 18b of turbocharger 18, in particular downstream of flowmeter 26. Flowmeter 26 only measures the flow of fresh air alone.

As illustrated, the recirculation circuit 38 comprises, in the direction of circulation of the recycled gases, a cooler 38a, a filter 38b, and a "V EGR BP" valve 38c configured to regulate the flow of exhaust gases at low pressure. The "V EGR BP" valve 38c is arranged downstream of the cooler 38a and of the filter 38b and upstream of the compressor 18b.

It will be noted that the air intake valve 28 can also be used to force the circulation of a low pressure exhaust gas flow in the EGR circuit BP in the case where the depression between the exhaust circuit and the intake circuit would be insufficient. In this case, closing the valve 28 would create a depression downstream, capable of sucking gas from the EGR circuit BP.

The engine is associated with a fuel circuit comprising, for example, fuel injectors (not referenced) injecting gasoline directly into each cylinder from a fuel tank (not shown).

Furthermore, the engine comprises an electronic control unit 70 configured to control the various elements of the internal combustion engine from data collected by sensors at various locations of the engine.

The electronic control unit 70 comprises a calculation module 72, a measurement module 73 and a control module 74.

In the spark-ignition engine, the engine speed-load operating point is adjusted by the engine computer 70 by adjusting in particular a quantity of air, a quantity of EGR recirculation gases BP, and a quantity of fuel. By "quantity" is meant here a mass flow rate.

The air flow and the flow of EGR BP recirculation gases can be set to set values by the engine computer 70 by adjusting the position of the throttle body 30 and the boost pressure of the turbocharger 18, on the one hand, which controls the total gas flow in the engine, and on the other hand that of the "V EGR BP" valve 38c of the recirculation circuit 38. If the engine is at an operating point without exhaust gas recirculation, the air flow is obtained directly by adjusting the throttle body.

The engine 10 comprises a passive circuit 62 for purging fuel vapors from the canister 60, equipped with a solenoid valve 61 and opening out at a point of the intake circuit Ca located downstream of the throttle body 30.

Canister 60 purge solenoid valve 61 is located on purge circuit 62, between canister 60 and the outlet point. Controlled by the computer 70, the solenoid valve 61 allows the recycling of the fuel vapors contained in the canister 60.

The computer 70 is able and predisposed to determine the filling rate of the canister 60 and to control the opening of the solenoid valve 61, in case of need for purging. For example, the filling rate of the canister 60 can be determined from the analysis of the flow of fuel to be injected by the fuel injectors during the regulation of the richness of the air-fuel mixture in closed loop at richness 1 by forcing a start of purging.

We will now describe with reference to the a method 80 for purging the canister 60 for internal combustion engines for automobiles, which reuse EGR flow rates at the intake.

Such a method is notably implemented by the computer 70 from the measurements delivered by the various sensors of the engine and by controlling the various elements of the engine.

The method 80 comprises a preliminary step 81 of nominal operation which corresponds to the operation of the motor 10 outside of any purge constraint of the canister 60. The motor 10 operates on a given speed-load operating point according to a torque setpoint corresponding to a vehicle acceleration setpoint determined according to the depression of the accelerator pedal by the user. From this torque setpoint and the engine speed, the computer 70 defines an engine torque setpoint C to be obtained in order to obtain this acceleration. From the engine torque setpoint C the computer 70 determines an air flow setpoint, a fuel flow setpoint, an EGR flow setpoint and a variable valve timing setpoint 50.

The computer 70 adjusts various actuators of the engine 10 to obtain this adjustment, which aims to minimize the fuel consumption of the vehicle and which does not take account of the needs for purging the canister. For example, the computer 70 adjusts a degree of opening of the throttle body 30 and the position of the valves 50 to adjust the overall gas flow Qmot in the engine and it adjusts the degree of opening of the "V EGR BP" valve 38c to adjust the EGR gas flow Qegr, the air flow Qair being obtained using the following equation:

(1)

During the following step 82, the computer 70 detects a need to purge the canister. When the canister has to be purged, the computer 70 determines a flow rate Qp of vapors to be evacuated to prevent the canister from saturating and fuel leaks from occurring into the outside atmosphere (step 83).

During the following step 84, the computer 70 calculates the pressure set point Pp not to be exceeded in the intake manifold and which is sufficient to obtain the purge flow rate Qp determined in step 83. The computer 70 uses pre-programmed maps contained in its memory, which connect the pressure in the intake manifold Pcol, the atmospheric pressure Pext and the purge flow rate Qp. The reuse of this model allows the computer 70 to determine the pressure setpoint Pp as a function of the purge flow rate setpoint Qp and the atmospheric pressure Pext.

During the next test step 85, it is checked whether the pressure measured in the intake manifold Pcol is less than or equal to the pressure setpoint Pp determined in the previous step. If this is the case, the suction of the fuel vapors from the canister takes place naturally, without any additional intervention being necessary and the method returns to step 81 of nominal operation. If the pressure measured in the intake manifold Pcol exceeds the pressure set point Pp, the pressure in the intake manifold must drop to allow the canister to bleed.

It should be noted that the method always gives priority to achieving the engine torque setpoint C and never proceeds, with a view to purging the canister, to adjusting the engine to lower the pressure in the manifold to a pressure lower than the minimum pressure to ensure the engine torque C. For this purpose, the computer 70 determines the minimum pressure in the intake manifold, necessary to ensure the engine torque setpoint C (step 86). The computer 70 uses an engine air filling model, preprogrammed and contained in its memory, which makes it possible to determine the value of the minimum pressure of the intake manifold to meet the torque setpoint of the engine C. The total gas flow Qmot entering the engine can be determined using this filling model, from a filling efficiency value and the values of the pressure Pcol and the temperature Tcol prevailing in the intake manifold which can be measured by the pressure and temperature sensors 33. The term "filling" is defined as being equal to the ratio between the mass of air sucked in and the mass of air that could have entered by considering only the total volume of the cylinders. The filling formula is expressed by the following equation:

(2)

In which :

designates the dimensionless, volumetric efficiency;

Qmot designates the total mass flow actually entering, in kg/s;

N designates the speed, in revolutions/min;

Cylinder designates the cylinder capacity of the engine, in ^m3 ;

Pcol designates the pressure in the intake manifold, in Pa;

Tcol, designates the temperature in the intake manifold, in K;

R denotes the ideal gas constant for air equal to approximately 287.058 .

In all cases, the value of the return depends on the speed N and the pressure in the intake manifold Pcol. If the engine is equipped with a variable valve timing system, particularly at the intake, the performance also depends on their position.

Equation (2) links a possible value of pressure in the intake manifold to a value of total mass flow, via a value of volumetric efficiency which can also take on several possible values, in particular depending on the timing of the valves. The computer 70 identifies among the plurality of possible values, a minimum pressure value Pcol_mini in the intake manifold which makes it possible to ensure the air flow Qmot corresponding to the engine torque C requested, on the condition that the EGR flow is zero and that the position of the valve timing system 50 ensures the maximum filling of the cylinders 12.

During step 87, the computer 70 compares the value of Pcol_mini with the pressure setpoint Pp.

If the minimum pressure value Pcol_mini in the intake manifold which makes it possible to achieve the engine torque setpoint C is greater than the pressure setpoint Pp, this means that the computer 70 cannot carry out the purge while respecting the setpoint of motor torque C. In this case, the priority is to ensure the motor torque setpoint C, the method goes back to step 81 of nominal operation and the purge is carried out later, when the motor torque setpoint has decreased.

If the minimum pressure value Pcol_mini is less than or equal to the pressure setpoint Pp, the computer 70 begins the adjustment step 88 starting by reducing the EGR rate, until the desired pressure is reached in the manifold. admission. To do this, it is possible, for example, to gradually lower the EGR flow setpoint while maintaining the air flow setpoint Qair. This translates into progressive closures of the EGR valve 38c so as to obtain the lower EGR flow rate required with, in parallel, the progressive closing of the throttle body 30 so as to obtain the total engine flow setpoint Qmot, which is more low because of the drop in the EGR flow set point.

During this phase, the computer does not modify the position of the valve timing system 50. This progressive closing of the throttle body 30 is accompanied by a drop in the pressure Pcol in the intake manifold. The computer 70 continues to gradually lower the EGR flow rate setpoint until the pressure value Pp is reached in the manifold which allows passive purging of the canister 60.

The drop in the EGR rate is very generally sufficient to bleed the canister. However, in the event that this is not sufficient, for example when the pressure Pcol does not reach the value Pp when the EGR flow rate is zero, or alternatively, when the EGR rate reaches a minimum threshold value , the computer 70 drives the valve timing system 50 to move away from Miller or Atkinson cycle engine operation and approach conventional cycle operation. For example, on an engine which operates at nominal setting according to the Miller cycle, the computer 70 can close the intake valves 51 later before bottom dead center and in parallel close the throttle body 30 further so as not to modify the gas flow Qmot entering the engine. For example, on an engine which operates in nominal setting according to the Atkinson cycle, the computer 70 can close the intake valves 51 earlier after bottom dead center and in parallel close the throttle body 30 further so as not to modify the gas flow Qmot entering the engine.

Process 80 stops with the passive purge of canister 60.

Claims (7)

Method for purging a canister of a motor vehicle internal combustion engine equipped with at least one exhaust gas recirculation system at the intake, characterized in that it comprises the following steps:
- detection of a purge requirement;
- calculation of a purge flow set point (Qp) and a pressure set point (Pp) allowing the flow rate (Qp) to be purged;
- calculation of the minimum pressure (Pcol_mini) in the intake manifold to ensure the engine torque setpoint (C);
- adjustment of the motor to lower the pressure in the collector to the level of the setpoint (Pp), within the limit of the minimum pressure (Pcol_mini) which ensures the realization of the motor torque (C). A method according to claim 1, wherein the motor operates on a conventional cycle. A method as claimed in claim 1, wherein the engine operates on an asymmetrical Miller or Atkinson type cycle, said engine being equipped with a variable valve timing system. Method according to any one of Claims 1, 2 or 3, in which the need for purging is determined when the value of the filling rate of the canister reaches a predetermined threshold. Method according to any one of Claims 1, 2 or 3, in which the engine adjustment comprises a reduction in the EGR flow rate setpoint. A method according to claim 5, wherein the engine tuning comprises steering the valve timing system from Miller or Atkinson cycle engine operation to conventional cycle operation. System for controlling the purge of a canister for an internal combustion engine of a motor vehicle equipped with at least one exhaust gas recirculation system at the intake, characterized in that it comprises means for detecting a need to purge the canister, means for calculating a purge flow rate setpoint (Qp) and a pressure setpoint (Pp), means for calculating a minimum pressure (Pcol_mini) in the collector of intake to ensure an engine torque setpoint (C) and engine adjustment means to lower the pressure in the manifold to the level of the setpoint (Pp), within the limit of the minimum pressure (Pcol_mini) which ensures the realization of the torque motor (C).