FUEL VAPOR CONTROL METHOD AND SYSTEM

Abstract

A method for operating a fuel system (18) including a fuel tank (20) coupled to a fuel vapor activated carbon canister (202) via a shut-off valve (110), comprising: Purging fuel vapors from the charcoal canister (202) to an internal combustion engine inlet (23) for a duration with the shutoff valve (110) open until a threshold level of fuel tank vacuum is generated, the duration being based on a purge flow rate and a fuel tank vacuum level, and Purging fuel vapors after the duration from the activated carbon canister (202) to the internal combustion engine inlet (23) with the shut-off valve (110) closed, while simultaneously applying the generated fuel tank vacuum to the fuel system (18) to identify a fuel system leak.

Description

[0001] The present application relates to fuel vapor purging and leak detection in vehicles such as hybrid vehicles.

[0002] US 5,305,724 A discloses an evaporative fuel purging device used for an internal combustion engine. A canister containing an absorbent for absorbing evaporative fuel generated in a fuel tank of the internal combustion engine is provided. A purge control valve is provided at a purge port for opening the canister to atmospheric air and is adapted to open and close the purge port. A purge control valve is provided at a purge passage connecting the canister to a part of an intake system and is adapted to purge the evaporative fuel absorbed into the canister by opening and closing the purge passage. A diagnostic device diagnoses the presence or absence of leaks in a purge system, which is sealed by closing the purge control valve and the purge control valve after the pressure reduction treatment.

[0003] Reduced engine operating times in hybrid vehicles enable benefits related to fuel economy and reduced fuel emissions. However, the shorter engine operating times can result in inadequate purging of fuel vapors from the vehicle's emission control system and insufficient time to complete a fuel system leak diagnostic operation. To address some of these issues, hybrid vehicles may incorporate a fuel tank isolation valve (FTIV) between a fuel tank and an emissions system hydrocarbon canister to limit the amount of fuel vapors absorbed in the canister. Opening or closing of the FTIV can then be adjusted based on fuel system conditions to enable fuel vapor purging or leak diagnostics.

[0004] An exemplary approach for fuel system control is demonstrated by Fujimoto et al. in US 2003 / 0 183 206 A1. When conditions are met to perform a leak diagnostic routine, the fuel tank isolation valve is closed while varying a canister purge rate between a low purge rate and a high purge rate. A change in fuel tank pressure between the high canister purge rate condition and the low canister purge rate condition is used to infer fuel system degradation.

[0005] However, the inventors of the present invention have identified potential problems with such an approach. As one example, fuel vapor purging operations may compete with the leak diagnostic routine for available time during the vehicle drive cycle. In other words, while the (higher and lower) purge rates may be sufficient to enable identification of fuel system degradation, the duration of the purge may not be long enough to allow sufficient purging of the charcoal canister. Consequently, during a subsequent drive cycle, fuel vapors may not be stored, and exhaust emissions may deteriorate. On the other hand, if the purge operation is allowed to continue to purge the stored fuel vapors, there may not be enough drive cycle time remaining to perform the leak detection routine.As a result, fuel system deterioration may not be detected in time and exhaust emissions may deteriorate again.

[0006] US 4,702,216 A shows a fuel system including a fuel tank coupled to a fuel vapor canister via a shutoff valve. In the fuel system, fuel vapors from the canister are purged to an internal combustion engine inlet with the shutoff valve open until the internal pressure in the fuel tank becomes negative. This is intended to prevent deformation of the fuel tank due to a vacuum. However, this method is not suitable for leak detection.

[0007] Some of the above problems can be at least partially addressed by the inventive method for operating a fuel system including a fuel tank coupled to a fuel vapor canister via a shutoff valve. The method comprises purging fuel vapors from the canister to an internal combustion engine intake for a duration with the shutoff valve open until a threshold level of fuel tank vacuum is generated. In this way, the vacuum generation potential of a purging operation can be advantageously used to generate the vacuum required for a leak detection routine.

[0008] If purge conditions are met and if a purge flow rate (as determined based on a canister load and engine speed-load conditions) is above a threshold rate, it may be determined that a purge operation has a vacuum generation potential. If insufficient fuel tank vacuum exists to perform a leak detection diagnostic routine (e.g., the fuel tank vacuum level is below a target level), purging is performed with the isolation valve open for a duration until the target vacuum level is reached. After the target fuel tank vacuum is reached, the isolation valve may be closed to isolate the fuel tank and initiate a leak detection routine. A fuel tank vacuum bleed rate may be monitored to identify a fuel tank leak.Optionally, purging can be continued with the shutoff valve closed so that fuel vapor purging to the engine inlet and fuel tank leak detection are performed simultaneously.

[0009] In this way, by purging fuel vapors from a canister with an open isolation valve for at least a purge duration, fuel vapor purging can be used opportunistically to reduce fuel tank pressure to a desired vacuum level, such as a vacuum level at which a pressure drop-based leak diagnostic routine can be performed. By purging with the isolation valve closed while performing a leak detection routine, both fuel vapor purging and leak diagnostics can be subsequently performed and completed within the same vehicle drive cycle. Furthermore, variation in test results between cycles can be reduced. By improving the termination frequency of both purging and leak detection operations, emissions compliance can also be better ensured. Short description of the drawings Fig. Figure 1 shows a schematic representation of an internal combustion engine and an associated fuel system. Fig. 2 shows an embodiment of the fuel system of Fig. 1. Fig. Figure 3 shows a high-level flow diagram illustrating a routine for enabling vacuum generation during charcoal canister purging for a subsequent leak detection routine. Fig. 4 shows a map for determining a vacuum generation potential of a purging operation. Fig. Figure 5 shows an example of fuel vapor purging for vacuum generation and fuel system leak detection. Detailed description

[0010] The following description relates to systems and methods for operating a fuel system such as the system of Fig. 2, which is connected to an internal combustion engine system such as the engine system of Fig. 1. During selected purge conditions, the vacuum generation potential of a purge operation ( Fig. 4) advantageously be used to draw a desired level of fuel tank vacuum. An engine controller may be configured to perform control routines such as the example routine of Fig. 3, to purge fuel vapors from a charcoal canister to an engine intake with the isolation valve open to generate a fuel tank vacuum. The isolation valve is then closed so that purging can continue while the generated vacuum is applied to identify leaks in the fuel system. Example purging operations with vacuum generation are described in Fig. 5 described.

[0011] Fig. Figure 1 shows a schematic representation of a hybrid vehicle system 6 that can derive motive power from the engine system 8 and / or an on-board energy storage device (not shown), such as a battery system. An energy conversion device, such as a generator (not shown), can be operated to absorb energy from vehicle motion and / or engine operation and then convert the absorbed energy into a form of energy suitable for storage by the energy storage device.

[0012] The engine system 8 includes an engine 10 having a plurality of cylinders 30. The engine 10 includes an engine intake 23 and an engine exhaust 25. The engine intake 23 includes a throttle 62 fluidly coupled to the engine intake manifold 44 via an intake passage 42. The engine exhaust 25 includes an exhaust manifold 48 leading to an exhaust passage 35 that directs exhaust gas to the atmosphere. The engine exhaust 25 may include one or more emission control devices 70 mounted in a closely coupled position. The one or more emission control devices may include a three-way catalyst, a lean NOx trap, a diesel particulate filter, an oxidation catalyst, etc. It is understood that other components may be included in the engine, such as a plurality of valves and sensors, as in the embodiment of Fig. 2 further elaborated.

[0013] In some embodiments, the engine intake 23 may further include a boosting device, such as a compressor 74. The compressor 74 may be configured to draw in intake air at atmospheric pressure and boost it to a higher pressure. As such, the boosting device may be a compressor of a turbocharger, with the boosted air introduced upstream of the throttle, or the compressor of a supercharger, with the throttle positioned upstream of the boosting device. Using the boosted intake air, boosted engine operation may be performed.

[0014] The engine system 8 is coupled to a fuel system 18. The fuel system 18 includes a fuel tank 20 coupled to a fuel pump system 21 and a fuel vapor recirculation system 22. The fuel tank 20 may contain a variety of fuel blends, including fuel having a range of alcohol concentrations, such as various gasoline-ethanol blends including E10, E85, gasoline, etc., and combinations thereof. The fuel pump system 21 includes one or more pumps for pressurizing fuel supplied from the fuel injectors of the engine 10, such as an exemplary fuel injector 66. Although only a single injector 66 is shown, additional injectors are provided for each cylinder. It should be understood that the fuel system 18 may be a returnless fuel system, a recirculation fuel system, or various other types of fuel systems.Vapors generated in the fuel tank 20 are conducted via a channel 31 to the fuel vapor recovery system 22 described below before being purged to the engine outlet 23.

[0015] The fuel vapor recovery system 22 of the fuel system 18 includes one or more fuel vapor recovery devices, such as one or more charcoal canisters filled with an appropriate adsorbent for temporarily capturing fuel vapors (including vaporized hydrocarbons) generated during fuel refueling operations, as well as daily vapors. In one example, the adsorbent used is activated charcoal. When purge conditions are met, such as when the charcoal canister is saturated, vapors stored in the fuel vapor recovery system 22 may be purged to the engine intake 23 by opening a charcoal canister purge valve 112.

[0016] The fuel vapor recovery system 22 further includes a vent 27 that can vent gases from the recirculation system 22 to the atmosphere when fuel vapors are stored or trapped from the fuel tank 20. The vent 27 can also allow fresh air to be drawn into the fuel vapor recovery system 22 when stored fuel vapors are purged to the engine intake 23 via a purge line 28 and the purge valve 112. A charcoal canister check valve 116 can optionally be included in the purge line 28 to prevent (increased) intake manifold pressure from causing gases to flow in the reverse direction into the purge line. Although this example shows the vent 27 communicating with unheated fresh air, various modifications may also be used. A detailed system configuration of the fuel system 18 with the fuel vapor recovery system 22 is described below with respect to Fig. 2, including various additional components that may be included in the inlet and outlet.

[0017] As such, a hybrid vehicle system 6 may have reduced engine operating times due to the vehicle being powered by the engine system 8 during some conditions and by the energy storage device under other conditions. While the reduced engine operating times reduce overall carbon emissions from the vehicle, they may also result in insufficient purging of fuel vapors from the vehicle's emissions control system. To address this, the fuel tank 20 may be designed to withstand high fuel tank pressures. In particular, a fuel tank isolation valve 110 is included in the conduit 31 such that the fuel tank 20 is coupled to the activated carbon canister of the fuel vapor recirculation system 22 via the valve.The isolation valve 110 may be maintained normally closed to limit the amount of fuel vapors absorbed into the canister from the fuel tank. In particular, the normally closed isolation valve separates the storage of refueling vapors from the storage of daily vapors and is opened during refueling to allow refueling vapors to be directed to the canister. As another example, the normally closed isolation valve may be opened during select purge conditions, such as when the fuel tank pressure is above a threshold (e.g., a mechanical pressure limit of the fuel tank above which the fuel tank and other fuel system components may experience mechanical damage), to release refueling vapors into the canister and maintain the fuel tank pressure below pressure limits.The isolation valve 110 may also be closed during leak detection routines to isolate the fuel tank from the engine intake. In one example, as shown in FIG. Fig. 3, when sufficient vacuum is available in the fuel tank 20, a shut-off valve may be closed to isolate the fuel tank, and a fuel tank vacuum discharge rate (i.e., a decrease rate in fuel tank vacuum or an increase rate in fuel tank pressure) may be monitored to identify a leak in the fuel tank.

[0018] In some embodiments, the shutoff valve 110 may be a solenoid valve, where the operation of the valve may be controlled by adjusting a drive signal to (or a pulse width from) its own solenoid (not shown). In still other embodiments, the fuel tank 20 may also be constructed from a material that can structurally withstand high fuel tank pressures, such as fuel tank pressures above a threshold and below atmospheric pressure.

[0019] One or more pressure sensors ( Fig. 2) may be coupled to the fuel tank upstream and / or downstream of the shut-off valve 110 to estimate a fuel tank pressure or a fuel tank vacuum level. One or more oxygen sensors ( Fig. 2) may be coupled to the canister (e.g., behind the canister) or may be positioned in the engine intake and / or the engine exhaust to provide an estimate of a canister load (e.g., an amount of fuel vapors stored in the canister). A purge flow rate may be determined based on the canister load and further based on engine operating conditions, such as engine speed-load conditions.

[0020] The vehicle system 6 may further include a control system 14. The control system 14 is shown receiving information from a plurality of sensors 16 (various examples of which are described herein) and sending control signals to a plurality of actuators 81 (various examples of which are described herein). As one example, the sensors 16 may include an exhaust gas sensor 126 located upstream of the emission control device, a temperature sensor 128, and a pressure sensor 129. Other sensors, such as additional pressure, temperature, air-fuel ratio, and composition sensors, may be coupled at various locations in the vehicle system 6, as shown in Fig. 2. As another example, the actuators may include a fuel injector 66, the shut-off valve 110, the purge valve 112, and the throttle 62. The control system 14 may include a controller 12. The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on an instruction or code programmed therein according to one or more routines. An example control routine is described herein with respect to Fig. 3 described.

[0021] Fig. 2 shows an embodiment 200 of the fuel system 18 including the fuel vapor recovery system 22. The fuel vapor recovery system 22 may include one or more fuel vapor containment devices, such as a fuel vapor canister 202 comprising an adsorbent. The canister 202 may receive fuel vapors from the fuel tank 20 through the conduit 31. During regular engine operation, the isolation valve 110 may be kept closed to limit the amount of daily vapors directed from the fuel tank 20 to the canister 202. During refueling operations and selected purge conditions, for example, the isolation valve 110 may be temporarily opened for a duration to direct fuel vapors from the fuel tank to the canister 202.While the example shown shows the shut-off valve 110 positioned along the line 31, in alternative embodiments the shut-off valve may be mounted on the fuel tank 20.

[0022] One or more pressure sensors may be coupled to the fuel tank 20 to estimate a fuel tank pressure or vacuum level. While the example shown shows the pressure sensor 120 coupled to the fuel tank 20, in alternative embodiments, the pressure sensor may be coupled between the fuel tank and the isolation valve 110. In still further embodiments, a first pressure sensor may be positioned upstream of the isolation valve, while a second pressure sensor is positioned downstream of the isolation valve to provide an estimate of a pressure differential across the valve.

[0023] A fuel level sensor 206 disposed within fuel tank 20 may provide an indication of the fuel level ("fuel level input") to controller 12. As shown, fuel level sensor 206 may include a float connected to a variable resistor. Alternatively, other types of fuel level sensors may be used. Fuel tank 20 may further include a fuel pump 207 for pumping fuel to fuel injector 66.

[0024] The fuel tank 20 receives fuel via a refueling line 216, which acts as a passage between the fuel tank 20 and a refueling door 229 on an external structure of the vehicle. During a fuel tank refueling event, fuel may be pumped into the vehicle from an external source through the refueling door. During a refueling event, the isolation valve 110 may be opened to allow refueling vapors to be directed into and stored in the charcoal canister 202.

[0025] The charcoal canister 202 may communicate with the atmosphere through the vent opening 27. The vent opening 27 may include an optional charcoal canister vent valve (not shown) for adjusting the flow of air and vapors between the charcoal canister 202 and the atmosphere. The charcoal canister vent valve may also be used for diagnostic routines. If included, the vent valve may be open during fuel vapor storage operations (e.g., during fuel tank refueling and while the engine is not running) so that air, after being depleted of fuel vapor following passage through the charcoal canister, may be forced out to the atmosphere.Similarly, during purging operations (for example, during charcoal canister regeneration and while the engine is running), the vent valve can be opened to allow a flow of fresh air to remove the fuel vapors stored in the charcoal canister.

[0026] Fuel vapors released from the canister 202, for example, during a purge operation, may be directed into the engine intake manifold 44 via the purge line 28. The flow of vapors along the purge lines 28 may be controlled by the canister purge valve 112 coupled between the fuel vapor canister and the engine intake. The amount and rate of vapors released by the canister purge valve may be determined by the duty cycle of a canister purge valve solenoid (not shown). As such, the duty cycle of the canister purge valve solenoid may be determined by the vehicle's powertrain control module (PCM), such as the controller 12, in response to engine operating conditions, including, for example, engine speed-load conditions, an air-fuel ratio, a canister load, etc.By commanding the charcoal canister purge valve to close, the controller can seal the fuel vapor recovery system from the engine intake.

[0027] An optional canister check valve may be included in the purge line 28 to prevent intake manifold pressure from forcing gases in the opposite direction of the purge flow. As such, the check valve may be necessary if the canister purge valve timing is not precisely tuned or the canister purge valve itself may be forced open by high intake manifold pressure. An estimate of manifold absolute pressure (MAP) may be obtained from a MAP sensor 218 coupled to the intake manifold 44 and in communication with the controller 12. Alternatively, MAP may be inferred from alternative engine operating conditions, such as mass airflow (MAF), as measured by a MAF sensor (not shown) coupled to the intake manifold.The check valve may be positioned between the charcoal canister purge valve and the intake manifold or may be positioned upstream of the purge valve.

[0028] The fuel vapor recovery system 22 can be operated by the controller 12 in a variety of modes by selectively adjusting the various valves and solenoids. For example, the fuel vapor recovery system can be operated in a fuel vapor storage mode (e.g., during a fuel tank refueling operation and when the engine is not running), where the controller 12 can open the isolation valve 110 while closing the canister purge valve (CPV) 112 to direct refueling vapors into the canister 202 while preventing fuel vapors from being directed into the intake manifold.

[0029] As another example, the fuel vapor recovery system may be operated in a refueling mode (e.g., when a fuel tank refueling is requested by a vehicle operator), where the controller 12 may open the isolation valve 110 while keeping the canister purge valve 112 closed to relieve fuel tank pressure before allowing the addition of fuel thereto. As such, the isolation valve 110 may be kept open during the refueling operation to allow refueling vapors to be stored in the canister. After refueling is complete, the isolation valve may be closed.

[0030] As yet another example, the fuel vapor recirculation system may be operated in a canister purge mode (e.g., after an emission control device light-off temperature has been reached and with the engine running), where the controller 12 may open the canister purge valve 112 while closing the isolation valve 110. Here, the vacuum created by the intake manifold of the operating engine may be used to draw fresh air through the vent 27 and through the fuel vapor canister 202 to purge the stored fuel vapors into the intake manifold 44. In this mode, the purged fuel vapors from the canister are combusted in the engine. Purging may continue until the amount of stored fuel vapor in the canister is below a threshold.In an alternative embodiment, compressed air passed through a boosting device (such as a turbocharger or supercharger) may be used for enhanced purging operation instead of fresh air at atmospheric pressure. As such, the fuel vapor recovery system 22 may require additional lines and valves to enable enhanced purging operation. During purging, the learned vapor amount / concentration may be used to determine the amount of fuel vapors stored in the canister, and then during a later portion of the purging operation (when the canister is sufficiently purged or empty), the learned vapor amount / concentration may be used to estimate a fuel vapor canister loading condition.

[0031] The inventors of the present invention have recognized that a vacuum potential is generated in the fuel system at the fuel tank and at the exit port of the activated carbon canister, which is directly proportional to the purge flow. In particular, as described with reference to the characteristic diagram of Fig. 4, at any given fuel tank pressure, the purge flow rate of a given purge operation also relates to the vacuum generation potential of the purge operation. As such, the purge flow rate for a given purge operation can be determined by the prevailing engine operating conditions (e.g., engine speed and load) and based on the canister load. However, by opportunistically trapping a vacuum in the fuel tank whenever there is a potential for it (by purging with the isolation valve open) and then closing the isolation valve when this potential has been eliminated, the vacuum potential can be advantageously used, for example, in leak detection routines ( Fig. 3). During some purge conditions, when the purge flow rate is sufficiently high, purge vapors can be purged from the charcoal canister to the engine intake with the isolation valve open to opportunistically create a fuel tank vacuum. After a sufficient fuel tank vacuum is available, the isolation valve can be closed, and the created vacuum can be applied to the fuel system to identify a leak. Purging can then continue with the isolation valve closed, allowing leak detection and purging to be performed simultaneously to improve the completion frequency of each operation. During other purge conditions, when the purge flow rate (as determined by the prevailing engine operating conditions) is not high enough to allow a vacuum potential, fuel vapors can be purged from the charcoal canister to the engine intake with the isolation valve closed.

[0032] Now with reference to Fig. 3, an exemplary routine 300 for coordinating various fuel vapor recirculation system operations based on vehicle operating conditions is described.

[0033] At 302, engine operating conditions may be estimated and / or inferred. These may include, for example, engine speed, engine load, torque demand, engine coolant temperature, exhaust catalyst temperature, canister load, fuel tank pressure, time since the last canister purge / storage operation, etc. At 304, it may be determined whether a fuel tank vacuum level is above a threshold level. The fuel tank vacuum level may be estimated by a pressure sensor coupled to the fuel tank. Here, the threshold level may be a fuel tank vacuum level required to enable a fuel system leak detection routine, such as a vacuum drop (or pressure drop)-based diagnostic routine.

[0034] If the vacuum level is above the threshold level, then the routine may proceed directly to 318, where a shutoff valve coupling the fuel tank to the fuel vapor canister may be closed. In this way, the fuel tank may be disconnected from the engine intake. Then, at 320, a leak detection routine may be initiated. In one example, the leak detection routine may be a pressure drop-based routine, where identifying a fuel system leak includes indicating a fuel system leak when a rate of vacuum drop from the disconnected fuel tank is above a threshold rate. In particular, in response to a rapid drain of fuel tank vacuum, a leak in the fuel tank may be determined and indicated by setting an appropriate diagnostic code.

[0035] If the vacuum tank level is below the threshold level, then the routine confirms at 306 whether purge conditions are met. Purge conditions may be considered met if, for example, the engine is running, an emission control device temperature has reached a light-off temperature, a canister fuel vapor load is above a threshold load, and / or a specified duration has elapsed since a previous canister loading operation. If purge conditions are met, then based on the vacuum generation potential of the purge operation, a controller may enable purging of fuel vapors with an open isolation valve from a canister to an engine intake to create a fuel tank vacuum.

[0036] Specifically, at 308, the routine includes determining a purge flow rate based on engine operating conditions such as engine speed and engine load, and further based on canister load. As such, a lower purge flow rate may be used as canister loading increases due to engine hardware limitations (e.g., injector sizing). Likewise, a higher purge rate may be applied at higher engine speed-load conditions, while a lower purge rate may be applied at lower engine speed-load conditions to reduce air-fuel ratio disturbances. The purge flow rate applied at the lower engine speed-load conditions may also be limited by throttle body size.

[0037] At 310, it may be determined whether the vacuum generation potential of the purge operation is above a threshold. As shown in the map 400 of Fig. 4, the vacuum generation potential (curve 402) of a given purge operation may be based on the determined purge flow rate of the operation (shown along the x-axis) as well as a current vacuum level (shown along the y-axis) of a vacuum reservoir coupled to the canister being purged (here, the fuel tank). In particular, as the purge flow rate increases while the fuel tank vacuum level of the fuel tank decreases, a vacuum generation potential of the purge may increase (proportional to the purge flow rate). Likewise, for a given purge flow rate (as determined based on engine operating operations and the amount of fuel vapors stored in the canister), the vacuum generation potential of the purge may increase as the fuel tank vacuum level decreases. A controller may be configured to use a map such as map 400 of Fig. 4 to assess whether the determined purge flow rate of the current purge operation (at the current fuel tank vacuum level) has sufficient vacuum generation potential. In one example, if the purge flow rate (determined at 308) is above a threshold rate, then it may be determined that the purge operation has vacuum generation potential.

[0038] If the vacuum generation potential of the purge operation is insufficient to create a fuel tank vacuum, then the routine at 312 includes purging fuel vapors with the isolation valve closed from the evaporative emission control canister to the engine intake. If, by comparison, sufficient vacuum generation potential exists, for example, if the determined purge flow rate during purge is above the threshold rate, then the routine at 314 includes purging fuel vapors with the isolation valve open from the evaporative emission control canister to the engine intake for a duration until a threshold fuel tank vacuum level is created. The duration may be based on the purge flow rate and the fuel tank vacuum level.

[0039] As such, since the purge rate is based on engine operating conditions that vary over time, there may be conditions where, when purging is initiated, the purge rate is below the threshold rate and the purge vacuum potential is below the threshold potential. Thus, purging may be initiated with the isolation valve closed. However, after a certain period of purging, the engine operating conditions may change, causing the purge rate to also change. For example, a change in engine speed-load condition may allow an increase in the purge rate. The adjusted (e.g., increased) purge rate may now be above the threshold rate and the purge vacuum potential may now be above the threshold potential. If a fuel tank vacuum is required at this time, purging may continue with the isolation valve open, at least until the desired fuel tank vacuum level is reached.

[0040] During some conditions, an initial purge flow rate may be further adjusted based on whether purging occurs with the isolation valve open (to create a fuel tank vacuum) or with the isolation valve closed. In one example, the controller may determine an initial purge flow rate of the isolation valve open purge based on engine speed and load conditions. The controller may increase the purge flow rate of the isolation valve open purge in response to the estimated fuel tank vacuum level being below the threshold level. For example, the purge flow rate may increase as the difference between the estimated fuel tank vacuum level and the threshold vacuum level increases. As another example, the controller may increase the purge flow rate regardless of the canister fuel vapor load (e.g., even though the canister load is not very high) until the threshold fuel tank vacuum level is created.As such, this may only be possible during high engine speed-load conditions where the change in scavenging flow rate does not significantly affect an engine's air-fuel ratio.

[0041] As such, it is understood that during purging with the isolation valve open, a fuel tank pressure may be below a mechanical pressure limit of the fuel tank. In other words, the isolation valve is not opened to purge fuel vapors from the fuel tank to the canister to maintain the fuel tank within pressure limits. Rather, the fuel tank pressure may already be within the mechanical pressure limits, and a fuel tank vacuum may be opportunistically created for a subsequent leak detection routine. At 315 and 313, a fuel injection amount to the engine cylinders may be adjusted based on the determined purge flow rate (for purging with or without the isolation valve open at 314 and 312, respectively).

[0042] If the canister is purged with the isolation valve closed, the routine may end when purging has ended (e.g., when the canister load has returned below a threshold fuel vapor load). If the canister is purged with the isolation valve open, the routine may continue (at least) until a threshold fuel tank vacuum level is established. In particular, at 316, after the duration of purging from the canister to the engine intake with the isolation valve open, it may be determined whether the fuel tank vacuum level has reached the target vacuum threshold level. If not, the controller may continue purging fuel vapors to the engine intake with the isolation valve open until the vacuum threshold level is reached. In one example, the controller may start a timer and verify the fuel tank vacuum level after the specified duration has elapsed.If the fuel tank vacuum target level is not reached at the end of the duration, the timer can be reset.

[0043] If, after the duration, the vacuum threshold level is confirmed, at 318-320, the routine includes purging fuel vapors from the evaporative emission control canister to the engine intake with the isolation valve closed while applying the generated fuel tank vacuum to identify a fuel system leak. As discussed above, at 318, the isolation valve may be closed to isolate the fuel tank. At 320, a rate of fuel tank vacuum leakage in the isolated fuel tank may be measured to identify a leak. For example, the controller may indicate a fuel tank leak if a rate of decrease in the fuel tank vacuum is above a threshold rate.

[0044] If purging was previously performed with the isolation valve open, the routine at 322 may optionally continue purging with the isolation valve closed. Here, the method enables purging fuel vapors from the canister to the engine intake with the isolation valve closed while simultaneously detecting a leak in the fuel system. In one example, purging may continue after closing the fuel tank isolation valve if the canister load is still above a threshold load after the duration. Here, by performing both operations simultaneously, both operations may be completed in the same drive cycle, even if limited time is available. In an alternative embodiment, purging may be completed based on the fuel tank vacuum level.For example, if the canister purge was performed for opportunistic vacuum generation and the canister fuel vapor load is below a threshold load, the purge may be terminated when the threshold fuel tank vacuum is achieved and the isolation valve is closed. The generated vacuum may be applied to perform a leak detection routine after (but not simultaneously with) the purge operation.

[0045] It should be understood that during select conditions, even if purge conditions are not otherwise met, a purge operation may be performed to create the desired fuel tank volume. During select engine speed-load conditions (such as a partial throttle condition), when the canister load is not sufficiently high to require a purge operation, fuel vapors may be purged from the canister to the engine intake with the isolation valve open at an increased purge flow rate solely to create a fuel tank vacuum. For example, at 307, in response to purge conditions not being met while an insufficient fuel tank vacuum exists, a purge flow rate may be increased to create a fuel tank vacuum.Once a sufficient fuel tank vacuum has been created (as queried at 316), the isolation valve can be closed and the leak detection routine initiated (at 318-320). As long as the engine's combustion stability is not compromised, a purge flow can be adjusted in this way to increase the amount of vacuum created, if deemed necessary.

[0046] In this way, during a first purge condition, a purge flow rate is increased in response to the canister load being above a threshold load (i.e., to reduce the canister loading), while during a second condition, the purge flow rate is increased in response to the fuel tank vacuum level being below a threshold level while the canister load is below the threshold load (i.e., even though the canister is not fully loaded, the canister is purged to create a vacuum).

[0047] The procedure of Fig. 3 is further illustrated by the exemplary rinsing with a vacuum generation operation of Fig. 5. In particular, Fig. 5 illustrates an example map 500 showing example purge operations performed with the isolation valve open or closed, as based on the vacuum generation potential of the purge operation. Map 500 shows changes in a canister fuel vapor load at curve 502, example purge flow rates and their vacuum generation potential at curve 504, the open or closed status of a fuel tank isolation valve at curve 506, and a fuel tank vacuum level (relative to a threshold level) at curve 508.

[0048] In the example shown, at t1, a canister fuel vapor load (i.e., the amount of fuel vapors stored in the canister shown at curve 502) may exceed a threshold load 503, and canister purge conditions may be confirmed. During this first purge condition, a fuel tank vacuum level (curve 508) may be below a threshold level 509. As such, the threshold level 509 may correspond to an amount of fuel tank volume required to perform a vacuum decay-based leak diagnostic routine. A purge flow rate for purging may be determined based on the canister load and further based on engine operating conditions such as engine speed and load and engine airflow. In particular, a first purge flow rate 511 that is above a threshold rate 505 may be determined.The purge flow threshold rate may represent a purge flow rate above which a purge operation may have a vacuum generation potential and below which the purge operation may not have sufficient vacuum generation potential.

[0049] In response to the higher (than threshold) purge flow rate 511, it may be determined that the purge operation confirmed at t1 has a vacuum generation potential and may generate a fuel tank vacuum. Thus, to raise the fuel tank vacuum level, purging of fuel vapors from the canister to an engine intake may be performed with the isolation valve open (FTIV, at curve 506) for a (first) duration d1 (between t1 and t2) until the fuel tank vacuum level is above the threshold level 509. The first duration may be based on the canister load, the engine load, and the fuel tank vacuum level. Thus, the first duration d1 may increase as a difference between the (estimated) fuel tank level and the vacuum threshold level 503 increases to enable a leak detection routine. At t2, the isolation valve may be closed.However, since the canister load remains above threshold load 503 (i.e., the canister is not sufficiently purged), after duration d1, purging of fuel vapors from the canister to the engine intake may continue (until t3) with the isolation valve closed. In one example, after duration d1, at t2, a leak detection routine may be initiated, where a fuel tank leak may be determined if a rate of decrease in the fuel tank vacuum level (i.e., the slope of curve 508 after t2) is above a threshold rate. Here, between t2 and t3, purging of canister fuel vapors to the engine intake with the isolation valve closed may be performed concurrently with detecting a leak in the fuel system. As such, this allows both operations to be completed within the same drive cycle.

[0050] At t4, the canister fuel vapor load may again exceed threshold load 503 and canister purge conditions may be confirmed.

[0051] During this second purge condition, the fuel tank vacuum level may also be below threshold level 509. However, the second purge flow rate 512 determined for the second purge operation may be below threshold rate 505, and it may be determined that the purge operation confirmed at t4 does not have sufficient vacuum generation potential. Consequently, purging of fuel vapors from the charcoal canister to the engine intake may be performed with the isolation valve closed for a (second) duration d2 (between t4 and t5).

[0052] In one example, purging may end at t5 after the second duration elapses (see dashed line 516). For example, if the canister load falls below the threshold load after the second duration d2 (see dashed line 526), ​​purging may end at t5. Here, the second duration may be based on the canister load and the engine load (rather than the fuel tank vacuum level), such that purging ends when the canister load is restored below the threshold load 503. In the example shown, the second duration d2 is shorter than the first duration d1.

[0053] In an alternative example, at t5, the purge rate may change due to a change in engine operating conditions while purging is occurring. For example, a higher purge flow rate may be applied due to a sudden change in engine speed, load conditions and / or engine airflow. Specifically, the purge flow rate may be increased from the lower purge flow rate 512 to a higher purge flow rate 513 in response to the change in engine operating conditions. The higher purge flow rate 513 may now be above the threshold rate 505, and the vacuum generation potential of the purge may now be above the threshold potential. Thus, fuel tank vacuum generation may now be possible.In response to the increase in purge flow rate while the fuel tank volume is still below the threshold level, the isolation valve may be opened at t5, and purging may continue with the isolation valve open, at least until the fuel tank vacuum threshold level is reached at t6. Thus, in the example shown, for the given purge operation (occurring between t4 and t6), at least a portion of the purge (between t4 and t5) may be performed with the isolation valve closed (due to the lower purge flow rate and lower vacuum generation potential of this portion of the purge), while another portion of the purge (between t5 and t6) may be performed with the isolation valve open (due to the higher purge flow rate and higher vacuum generation potential of this portion of the purge).This means that the vacuum generation potential of the purge operation can be opportunistically used to generate a fuel tank vacuum.

[0054] At t7, the canister fuel vapor load may again exceed threshold level 503, and canister purge conditions may be confirmed. During this purge condition, the fuel tank vacuum level may also be below threshold level 509. Additionally, a purge flow rate 514 determined for the purge operation may be below threshold rate 505, and it may be determined that the purge operation confirmed at t7 does not have vacuum generation potential. Consequently, purging of fuel vapors from the canister to the engine intake may be performed with the isolation valve closed for a duration between t7 and t8. At t8, the canister load may have dropped below the threshold load, and further purging may not be required. However, the purge rate may be increased to create the desired fuel tank vacuum.Specifically, at t8, the purge flow rate may be increased from purge flow rate 514 (which is dependent on the canister load) to purge flow rate 515 (which is independent of the canister load), and purging of fuel vapors from the canister to the engine intake may be performed for a duration between t8 and t9, with the isolation valve open solely for the purpose of creating a fuel tank vacuum until the threshold vacuum level 509 is reached (at t9). In one example, the sole purge flow rate used to create the tank vacuum may be a maximum purge flow rate. At t9, the isolation valve may be closed, and purging may be discontinued. In this example, purge termination may be adjusted based on the fuel tank vacuum level, with purging terminated when the fuel tank vacuum level reaches the threshold level.

[0055] It is understood that while each of the Fig. 5, when purging is performed with the isolation valve open, a fuel tank pressure may be below a mechanical pressure limit of the fuel tank. That is, the isolation valve may be opened to draw a fuel tank vacuum, but not to expel fuel vapors from the fuel tank to the canister (as may occur during select fuel tank depressurization conditions to reduce the likelihood of mechanical damage to fuel system components).

[0056] As such, the examples shown illustrate various purge conditions during which the fuel tank vacuum level is below a threshold level. It should be understood that the fuel tank vacuum level may be above the threshold level during other purge conditions, wherein purging of fuel vapors from the canister to the engine intake may be performed with the isolation valve closed.

[0057] In this way, the vacuum generation potential of a purge operation can be opportunistically used to draw sufficient fuel tank vacuum to enable fuel system leak diagnosis. By drawing a fuel tank vacuum and performing the leak detection routine under constant and uniform conditions, variability in test results between cycles can be reduced. By allowing purge and leak detection to be performed simultaneously, completion of both operations can be better ensured. Consequently, emissions compliance can be improved.

Claims

[1] A method for operating a fuel system (18) including a fuel tank (20) coupled to a fuel vapor activated carbon canister (202) via a shut-off valve (110), comprising: Purging fuel vapors from the charcoal canister (202) to an internal combustion engine inlet (23) for a duration with the shutoff valve (110) open until a threshold level of fuel tank vacuum is generated, the duration being based on a purge flow rate and a fuel tank vacuum level, and Purging fuel vapors after the duration from the activated carbon canister (202) to the internal combustion engine inlet (23) with the shut-off valve (110) closed, while simultaneously applying the generated fuel tank vacuum to the fuel system (18) to identify a fuel system leak. [2] The method of claim 1, wherein the threshold level includes a fuel tank vacuum level required to enable a fuel system leak detection routine. [3] The method of claim 1, wherein during purging, a vacuum generation potential of the purge is above a threshold, the vacuum generation potential being based at least on the purge flow rate. [4] The method of claim 1, wherein purging includes increasing the purge flow rate independent of a canister fuel vapor load until the threshold level of fuel tank vacuum is created. [5] The method of claim 1, wherein identifying the fuel system leak includes indicating the fuel system leak when a vacuum decay rate from the isolated fuel tank (20) is above a threshold rate. [6] The method of claim 1, wherein during purging with the shutoff valve (110) open, a fuel tank pressure is below a mechanical pressure limit of the fuel tank (20). [7] The method of claim 1, further comprising terminating the purging after the threshold level of fuel tank vacuum is created, and applying the created fuel tank vacuum to the fuel system (18) to identify the fuel system leak. [8] A method for operating a fuel system (18) including a fuel tank (20) coupled to an activated carbon canister (202) via a shut-off valve (110), comprising: during a first purge condition, purging fuel vapors from the charcoal canister (202) with the shutoff valve (110) open to an engine inlet (23); and during a second purge condition, purging fuel vapors from the charcoal canister (202) to the engine inlet (23) with the shut-off valve (110) closed, wherein during both the first and second purging conditions, a fuel tank pressure is within a mechanical pressure limit of the fuel tank (20), and wherein during the first condition, flushing occurs at a first, higher flush flow rate and wherein during the second condition, flushing occurs at a second, lower flush flow rate. [9] The method of claim 8, wherein during the first condition, a fuel tank vacuum level is below a threshold level and wherein during the second condition, the fuel tank vacuum level is above the threshold level. [10] The method of claim 8, wherein the second purge flow rate is based on a canister fuel vapor load and wherein the first purge flow rate is independent of the canister fuel vapor load. [11] The method of claim 8, wherein during the first condition, purging occurs for a first duration based on canister load, engine load, and fuel tank vacuum level, and wherein during the second condition, purging occurs for a second duration based on canister load and engine load, the first duration being longer than the second duration. [12] The method of claim 11, wherein the first duration increases as a difference between the fuel tank vacuum level and a vacuum threshold level for enabling a leak detection routine increases. [13] The method of claim 8, further comprising: after the first duration has elapsed during the first condition, purging fuel vapors from the charcoal canister (202) to the engine intake with the isolation valve (110) closed while simultaneously detecting a leak in the fuel tank (20). [14] The method of claim 13, wherein the detecting is based on a vacuum drop rate from the fuel tank (20) with the shutoff valve (110) closed. [15] Fuel system (18) for a vehicle (6), comprising: a fuel tank (20); an activated carbon canister (202) coupled to the fuel tank (20) via a shut-off valve (110); an engine (8) with an engine inlet (23); a pressure sensor (120) coupled to the fuel tank (20) and configured to estimate a fuel tank vacuum level; and a control system (14) with computer-readable instructions for: Purging fuel vapors from the charcoal canister (202) with the isolation valve (110) open to the engine intake (23) for a duration until the fuel tank vacuum level is above a vacuum threshold level; and purging fuel vapors from the charcoal canister (202) with the isolation valve (110) closed to the engine intake after the duration while simultaneously detecting a leak in the fuel system (18). [16] The system of claim 15, wherein detecting the leak in the fuel system (18) includes indicating the fuel tank leak when a rate of decrease in the fuel tank vacuum level is above a vacuum threshold rate. [17] The system of claim 15, wherein the control system (14) further includes instructions for determining an initial purge flow rate of the open shutoff valve purge (110) based on engine speed, engine load, and canister load; and increasing the open shutoff valve purge flow rate (110) in response to the estimated fuel tank vacuum level being below the vacuum threshold level.

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