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

Abstract

This method for purging a canister of an internal combustion engine of a motor vehicle 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 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 engine to lower the pressure in the manifold to the setpoint Pp, within the limit of the minimum pressure Pcol_mini which ensures the achievement of the engine torque C. Figure for the abstract: Fig 4

Description

Title of the invention: Method and system for purging a canister of a combustion engine equipped with at least one exhaust gas recirculation circuit Technical field

[0001] The present invention relates to fuel supply circuits for internal combustion engines which comprise a reservoir for suctioning fuel vapors, in particular gasoline. Previous techniques

[0002] On a motor vehicle equipped with an internal combustion engine, more particularly a spark-ignition engine (petrol), the fuel vapor suction tank, known to those skilled in the art by its English name "canister", comprises a carbon filter and is configured to accumulate the fuel vapors escaping from the tank when the vehicle is stopped or when it is operating under severe conditions.

[0003] Conventionally, the canister is connected to the engine intake by a purge pipe equipped with a valve, the opening of which controls the passage of fuel-laden vapors to the engine intake. To prevent 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.

[0004] A minimum purge flow rate is required to allow the evacuation of a sufficient quantity of fuel vapors to reduce the filling rate of the canister.

[0005] It is also known that on a spark-ignition engine, the depression of the engine's intake manifold, generally adjusted using a throttle body of the engine to obtain a certain mass flow of air allowing the production of engine torque, causes pumping losses which are induced by the pressure difference between the intake manifold plenum and the exhaust manifold.

[0006] [Fig. 1] 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 [Fig.l] representing the stages undergone by the gases in a cylinder of the engine, the intake stage (intake stroke) corresponds to segment AB, compression to segment BC, combustion to segment CD, expansion to segment DE and exhaust to segment EA.

[0007] To reduce pumping losses and thus reduce the fuel consumption of the engine, it is known to increase the intake manifold pressure while admitting into the engine the same quantity of air in mass flow rate necessary to produce the same given torque as in an engine operating according to the conventional cycle, shown in [Fig.l].

[0008] A first method of reducing pumping losses by increasing the pressure in the intake manifold, shown schematically in [Fig. 2] and known as the Miller cycle, consists of closing the intake valves before the bottom dead center, acronym "BDC". The quantity of air is thus admitted into the cylinder, not up to the BDC but only up to the position of the piston corresponding to the instant of closing of the intake valve, materialized by point B in [Fig. 2]. 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 quantity of air admitted is carried out mainly by imposing the moment of closing of the intake valves.Air is thus admitted into the cylinder only on segment AB, the segments of the cycle BB' and B'B” corresponding respectively to an increase or decrease in the 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 [Fig.l]. In [Fig.2], the segments B”C, CD, DE and EA, correspond respectively to the stages of compression, combustion, expansion and exhaust of the gases from an engine cylinder.

[0009] This results in a higher manifold pressure for the Miller cycle than for the conventional cycle, because the throttle body is left mainly open, even at low loads. By increasing the intake pressure, the amount of work consumed by the engine is reduced, so the area of ​​the hatched zone 2 in [Fig.2] is smaller than the area of ​​the hatched zone 2 in [Fig.l].

[0010] A second method of reducing pumping losses by increasing the pressure in the intake manifold, shown schematically in [Fig. 3] and known as the Atkinson cycle, consists of closing the intake valves after BDC. In this case, the intake valves remain open for part of the piston's ascent after BDC, up to point B' in [Fig. 3]. Thus, air is admitted up to BDC corresponding to point B in [Fig. 3], i.e. for the entire piston stroke, and then part of this admitted air is then forced back into the intake manifold as the piston rises towards top dead center, acronym "TDC", as long as the intake valves remain open. The quantity of air admitted for combustion is therefore determined by the time of closing of the intake valves, shown by point B' in [Fig. 3].

[0011] In the Atkinson cycle, as in the Miller cycle, the butterfly valve 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 represented in [Fig.l]. In [Fig.3], the segments B'C, CD, DE and EA, correspond respectively to the compression, combustion, expansion and exhaust times of the gases of a cylinder of the engine.

[0012] Thus, for the Atkinson cycle, a higher pressure is obtained in the intake manifold than for the conventional cycle, because the throttle body is left mainly open, even at low loads. By increasing the intake pressure, the amount of work consumed by the engine is reduced, so the area of ​​the hatched zone 2 in [Fig. 3] is smaller than the corresponding area of ​​the hatched zone 2 in [Fig. 1].

[0013] 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 English 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, 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.It should also be noted that such partial recirculation of exhaust gases at the intake can be combined with a Miller or Atkinson cycle.

[0014] The problem with the three methods described, namely the Miller cycle, the Atkinson cycle or EGR recirculation, is that by increasing the pressure in the engine intake manifold, they reduce the capacity to purge the canister in the engine air intake circuit when the point of reintroduction of the fuel-laden vapors is downstream of the throttle body, because the pressure difference between the canister, which is at atmospheric pressure by its venting, and the intake manifold, is greatly reduced and no longer allows the circulation of a sufficient flow rate.

[0015] Engines equipped with active purge systems are known from the state of the art, incorporating a venturi or a pump to suck the vapors towards the intake. These systems are used mainly in cases where the point of reintroduction of the vapors fuel-charged valves are located upstream of the throttle body, and they have the disadvantage of requiring piloting for the pump and the installation of additional parts.

[0016] In view of the above, the aim of the invention is to improve the purging capacity of the canister, without however using active purging systems. Statement of the invention

[0017] In view of the above, the subject of the present invention is a method for passively purging the canister of an internal combustion engine of a motor vehicle equipped with at least one exhaust gas recirculation system at the intake.

[0018] The method comprises the following steps:

[0019] - detection of a need for purging; - calculation of a purge flow rate setpoint and a pressure setpoint Pp allowing the flow rate 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 setpoint, within the limit of the minimum pressure which ensures the achievement of the engine torque.

[0020] The invention aims to use a so-called passive purging 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.

[0021] For example, the engine operates according to a conventional cycle.

[0022] Advantageously, the engine is equipped with a variable valve timing system and operates according to an asymmetric cycle of the Miller or Atkinson type.

[0023] Advantageously, the need for purging is determined when the value of the filling rate of the canister reaches a predetermined threshold.

[0024] Advantageously, the engine adjustment includes a reduction in the EGR flow rate setpoint.

[0025] For example, engine tuning includes controlling the valve timing system from Miller or Atkinson cycle engine operation to conventional cycle operation.

[0026] According to a second aspect, the invention relates to a system for passively purging the canister of an internal combustion engine of a motor vehicle equipped with at least one system for recirculating exhaust gases at the intake.

[0027] The passive purge system comprises means for detecting a need for purging the canister, means for calculating a purge flow rate 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 setpoint, within the limit of the minimum pressure which ensures the achievement of the engine torque. Brief description of the drawings

[0028] 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:

[0029] [Fig.l],

[0030] [Fig.2]

[0031] and

[0032] [Fig.3], which has already been mentioned, respectively represent the diagram pressure - volume of a cylinder of a four-stroke internal combustion engine of a motor vehicle according to a conventional, Miller or Atkinson cycle;

[0033] [Fig.4] illustrates, in a very schematic manner, an example of the structure of a motor with internal combustion of a motor vehicle equipped with a canister purge control system according to the invention; and

[0034] [Fig.5] illustrates a flowchart of the canister purge process, according to a mode of implementation of the invention. Detailed description of at least one embodiment

[0035] In the example illustrated in [Fig.4], the internal combustion engine 10 comprises, in a non-limiting manner, three cylinders 12 in line, a fresh air intake manifold 14, an exhaust manifold 16, a turbo-compression system 18, a variable valve timing system 50 for the intake valves 51 and the exhaust valves 52.

[0036] 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.

[0037] 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 increasing the quantity (mass flow rate) of air admitted into the cylinders 12 of the engine 10.

[0038] The internal combustion engine 10 comprises an intake circuit Ca and an exhaust circuit Ce.

[0039] The intake circuit Ca comprises, from upstream to downstream in the direction of circulation of the air:

[0040] - the air filter 22 or air box;

[0041] - a flow meter 26 arranged in the intake pipe 20 downstream of the air filter 22 to measure the actual value of the air flow entering the engine 10;

[0042] - an air intake valve 28;

[0043] - the compressor 18b of the turbocharger 18;

[0044] - a throttle body 30 or a gas intake valve in the engine;

[0045] - a heat exchanger 32 configured to cool the intake gases cor corresponding to a mixture of fresh air and recirculated gases after their compression in the compressor 18b;

[0046] -pressure and temperature sensors 33 for measuring the pressure and temperature in the intake manifold 14; and

[0047] - the intake manifold 14.

[0048] The compressor is associated with a bypass circuit equipped with an inlet discharge valve 55 which opens in the event of sudden closure of the throttle body 30, to prevent the compressed air, located between the compressor 18b and the throttle body 30, from passing through the compressor 18b and damaging it, when, for example, the driver of the vehicle suddenly lifts his foot off the accelerator pedal.

[0049] The exhaust circuit Ce comprises, from upstream to downstream in the direction of circulation of the burnt gases:

[0050] - the exhaust manifold 16;

[0051] - the turbine 18a of the turbocharger 18; and

[0052] - a system 40 for depolluting the combustion gases of the engine.

[0053] As regards the exhaust manifold 16, the latter recovers the exhaust gases resulting from the combustion and evacuates them to the outside, via a gas exhaust duct 34 opening onto the turbine 18a of the turbocharger 18 and via an exhaust line 36 mounted downstream of the turbine 18a.

[0054] 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 English terms).

[0055] 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 depollution system 40 and returns the exhaust gases to the fresh air supply pipe 20, upstream of the compressor 18b of the turbocharger 18, in particular downstream of the flow meter 26. The flow meter 26 only measures the flow of fresh air alone.

[0056] 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 low-pressure exhaust gases. The “V EGR BP” valve 38c is arranged downstream of the cooler 38a and the filter 38b and upstream of the compressor 18b.

[0057] It will be noted that the air intake valve 28 can also be used to force the circulation of a flow of low-pressure exhaust gases in the LP EGR circuit in the case where the vacuum between the exhaust circuit and the intake circuit is insufficient. In this case, closing the valve 28 would create a vacuum downstream, capable of sucking gases from the LP EGR circuit.

[0058] 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).

[0059] 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 different locations in the engine.

[0060] The electronic control unit 70 comprises a calculation module 72, a measurement module 73 and a control module 74.

[0061] 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 BP EGR recirculation gases, and a quantity of fuel. By "quantity" is meant here a mass flow rate.

[0062] The air flow rate and the flow rate of the recirculation gases EGR BP can be adjusted to set values ​​by the engine computer 70 by adjusting on the one hand the position of the throttle body 30 and the boost pressure of the turbocharger 18, which controls the total gas flow rate in the engine, and on the other hand that of the valve “V EGR BP” 38c of the recirculation circuit 38. If the engine is at an operating point without exhaust gas recirculation, the air flow rate is obtained directly by adjusting the throttle body.

[0063] The engine 10 comprises a passive circuit 62 for purging fuel vapors from the canister 60, provided with a solenoid valve 61 and opening at a point in the intake circuit Ca located downstream of the throttle body 30.

[0064] The solenoid valve 61 for purging the canister 60 is located on the purge circuit 62, between the 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.

[0065] The computer 70 is capable and predisposed to determine the filling rate of the canister 60 and to control the opening of the solenoid valve 61, in the event of a need for purging. For example, the filling rate of the canister 60 can be determined from the analysis of the flow rate of fuel to be injected by the fuel injectors during the regulation of the richness of the air-fuel mixture in a closed loop at richness 1 by forcing the start of a purge.

[0066] We will now describe with reference to [Fig.5] a method 80 for purging the canister 60 for internal combustion engines for automobiles, which reuse EGR flows at the intake.

[0067] 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.

[0068] The method 80 comprises a preliminary step 81 of nominal operation which corresponds to the operation of the engine 10 without any constraint of purging the canister 60. The engine 10 operates at a given speed-load operating point as a function of a torque setpoint corresponding to an acceleration setpoint of the vehicle determined as a function of 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 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.

[0069] 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 into account the purging needs of 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 rate Qmot in the engine and it adjusts the degree of opening of the “V EGR BP” valve 38c to adjust the flow rate Qegr of EGR gas, the air flow rate Qair being obtained using the following equation:

[0070] Qair-Qmot - Qegr (1)

[0071] During the following step 82, the computer 70 detects a need to purge the canister. When the canister must 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).

[0072] During the following step 84, the computer 70 calculates the pressure setpoint 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 preprogrammed 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.

[0073] In 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 occurs naturally, without any additional intervention being necessary and the method returns to the nominal operation step 81. If the pressure measured in the intake manifold Pcol exceeds the pressure setpoint Pp, the pressure in the intake manifold must drop to allow the canister to be purged.

[0074] It should be noted that the method always gives priority to achieving the engine torque setpoint C and never proceeds, for the purpose of purging the canister, to an adjustment of 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 a preprogrammed engine air filling model contained in its memory, which makes it possible to determine the value of the minimum pressure of the intake manifold to meet the engine torque setpoint C.The total gas flow rate 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 drawn in and the mass of air which could have entered considering only the total volume of the cylinders. The formula for filling is expressed by the following equation: .

[0075] Q x 120 (2) yy --- ---------_______________tfl-Ui______________________________________ N x Cylinder capacity x T „ 1 XK collar

[0076] In which:

[0077] ^rdvl denotes the volumetric efficiency, dimensionless;

[0078] Qmot denotes the total mass flow rate actually entering, in kg / s;

[0079] N denotes the speed, in revolutions / min;

[0080] Cylinder capacity means the cylinder capacity of the engine, in m3;

[0081] Pcol denotes the pressure in the intake manifold, in Pa;

[0082] Tcol, denotes the temperature in the intake manifold, in K;

[0083] R denotes the mass constant of ideal gases for air equal to approximately 287.058 / kg XK '

[0084] In all cases, the value of the efficiency ^rdvl depends on the speed N and the pressure in the intake manifold Pcol. If the engine is equipped with a system of variable valve timing, especially on the intake, the ^rdvl efficiency also depends on their position.

[0085] Equation (2) links a possible pressure value in the intake manifold to a total mass flow rate value, via a volumetric efficiency value which can also take several possible values, notably depending on the valve timing. 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 rate Qmot corresponding to the requested engine torque C, on the condition that the EGR flow rate is zero and that the position of the valve timing system 50 ensures maximum filling of the cylinders 12.

[0086] During step 87, the computer 70 compares the value of Pcol_mini with the pressure setpoint Pp.

[0087] 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 perform the purge while respecting the engine torque setpoint C. In this case, the priority is to ensure the engine torque setpoint C, the method returns to step 81 of nominal operation and the purge is performed later, when the engine torque setpoint has decreased.

[0088] 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 by first reducing the EGR rate, until the desired pressure in the intake manifold is reached. For this, it is possible, for example, to gradually lower the EGR flow rate setpoint while maintaining the air flow rate setpoint Qair. This results in progressive closings of the EGR valve 38c so as to obtain the required lower EGR flow rate with, in parallel, the progressive closing of the throttle body 30 so as to obtain the total engine flow rate setpoint Qmot, which is lower because of the reduction in the EGR flow rate setpoint.

[0089] 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 progressively lower the EGR flow rate setpoint until the pressure value Pp in the manifold is reached, which allows passive purging of the canister 60.

[0090] The reduction in the EGR rate is very generally sufficient to obtain the purging of 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 controls the valve timing system 50 to move away from Miller or Atkinson cycle engine operation and closer to conventional cycle operation. For example, on an engine operating 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 more so as not to modify the gas flow rate Qmot entering the engine. For example, on an engine operating at 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 more so as not to modify the gas flow rate Qmot entering the engine.

[0091] The process 80 stops with the passive purging of the canister 60.

Claims

Claims

1. Method for purging a canister of 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 the following steps: - detecting a need for purging; - calculating a purge flow rate setpoint (Qp) and a pressure setpoint (Pp) allowing the flow rate (Qp) to be purged; - calculating the minimum pressure (Pcol_mini) in the intake manifold to ensure the engine torque setpoint (C); - adjusting the engine to lower the pressure in the manifold to the setpoint (Pp), within the limit of the minimum pressure (Pcol_mini) which ensures the achievement of the engine torque (C).

2. A method according to claim 1, wherein the engine operates on a conventional cycle.

3. A method according to claim 1, wherein the engine operates on an asymmetric Miller or Atkinson type cycle, said engine being equipped with a variable valve timing system.

4. A method according to any one of claims 1, 2 or 3, wherein the need for purging is determined when the value of the canister filling rate reaches a predetermined threshold.

5. A method according to any one of claims 1, 2 or 3, wherein the engine adjustment comprises a reduction in the EGR flow rate setpoint.

6. The method of claim 5, wherein the engine adjustment comprises controlling the valve timing system from Miller or Atkinson cycle engine operation to conventional cycle operation.

7. System for controlling the purging 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 intake manifold to ensure an engine torque setpoint (C) and means for adjusting the engine to lower the pressure in the manifold to the level of the setpoint (Pp), in the minimum pressure limit (Pcol_mini) which ensures the achievement of the engine torque (C).

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