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Methods and systems are provided for operating a fuel vapor recovery system having a fuel tank isolation valve coupled between a fuel tank and a canister. Fuel vapors are purged from the fuel tank to a canister buffer over a plurality of purge pulses. The pulses are adjusted based on the buffer capa

Methods and systems are provided for operating a fuel vapor recovery system having a fuel tank isolation valve coupled between a fuel tank and a canister. Fuel vapors are purged from the fuel tank to a canister buffer over a plurality of purge pulses. The pulses are adjusted based on the buffer capacity, a purge flow rate, and a fuel tank pressure to improve control of canister loading and reduce air-to-fuel ratio disturbances.

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1. A method of operating a fuel vapor recovery system, comprising, purging fuel vapors from a canister to an engine intake to reduce a stored fuel vapor amount in the canister; andintermittently purging fuel vapors from a fuel tank to the canister to increase a stored fuel vapor amount in a canister

1. A method of operating a fuel vapor recovery system, comprising, purging fuel vapors from a canister to an engine intake to reduce a stored fuel vapor amount in the canister; andintermittently purging fuel vapors from a fuel tank to the canister to increase a stored fuel vapor amount in a canister buffer, the canister including an adsorbent therein, wherein the adsorbent has a buffer region, a duration and interval of the intermittent purging based on a determination of the stored fuel vapor amount in the buffer distinct from a remainder of the adsorbent. 2. The method of claim 1, wherein the duration is further based on a fuel tank pressure. 3. The method of claim 2, wherein the fuel vapor recovery system includes a purge valve coupled between the canister and the engine intake, and an isolation valve coupled between the fuel tank and the canister, and wherein purging fuel vapors from the canister to the engine intake includes opening the purge valve and purging with the isolation valve closed. 4. The method of claim 3, wherein intermittently purging fuel vapors from the fuel tank to the canister includes intermittently opening the isolation valve. 5. The method of claim 4, wherein the duration of the intermittent purging is decreased and the interval between consecutive purgings is increased as the stored fuel vapor amount in the buffer increases. 6. The method of claim 5, wherein the duration of the intermittent purging is increased as the fuel tank pressure decreases to maintain a mass of released fuel vapors. 7. The method of claim 6, wherein purging fuel vapors from the canister to reduce the stored fuel vapor amount in the canister includes purging fuel vapors from the canister to reduce the stored fuel vapor amount in the canister buffer below a threshold. 8. The method of claim 7, wherein intermittently purging from the fuel tank includes initiating intermittently purging only if the stored fuel vapor amount in the canister buffer is below the threshold. 9. The method of claim 8, wherein intermittently purging from the fuel tank further includes intermittently purging only if the fuel tank pressure is above a first, lower threshold. 10. The method of claim 9, further comprising, if the fuel tank pressure is above a second, higher threshold, intermittently purging from the fuel tank with the purge valve closed. 11. The method of claim 1, wherein purging fuel vapors from the canister to reduce the stored fuel vapor amount in the canister includes purging fuel vapors from the canister to empty the canister. 12. The method of claim 1, wherein intermittently purging fuel vapors from a fuel tank wherein the intermittent purging includes a plurality of consecutive purge pulses, wherein the duration of the intermittent purging includes a duration from a beginning to an end of each purge pulse, and wherein the interval of the intermittent purging includes an interval from the end of a purge pulse to the beginning of an immediately following purge pulse. 13. A method of operating a fuel vapor recovery system including a fuel tank coupled to a canister through an isolation valve, comprising, purging fuel vapors from the canister to an engine intake until a stored fuel vapor amount in a canister buffer is below a threshold, the canister including an adsorbent therein, wherein the adsorbent has a buffer region; andpulsing the isolation valve to purge fuel vapors from the fuel tank to the canister to increase the stored fuel vapor amount, a duration of each pulse, and an interval between consecutive pulses adjusted based on each of a buffer capacity, purge flow rate and a fuel tank pressure at an onset of the pulsing, a duration and interval of the purging based on a determination of the stored fuel vapor amount in the buffer distinct from a remainder of the adsorbent. 14. The method of claim 13, wherein purging fuel vapors from the canister includes closing the isolation valve and opening a purge valve coupled between the canister and the engine intake. 15. The method of claim 13, further comprising, estimating a ramp-out rate of the fuel vapors based on the pulsing of the isolation valve and a filtered value of a stored amount of vapors in the buffer when concluding the pulsing, and adjusting a fuel injection to the engine based on the estimated ramp-out rate. 16. The method of claim 13, wherein the adjustment includes, as the buffer capacity decreases, decreasing the duration of each pulse and increasing the interval between consecutive pulses; andas the fuel tank pressure increases, decreasing the duration of each pulse. 17. The method of claim 13, wherein pulsing the isolation valve includes opening the isolation valve only if the stored fuel vapor amount is below the threshold. 18. The method of claim 13, wherein the fuel tank pressure is estimated by a pressure sensor positioned in the fuel tank or between the fuel tank and the isolation valve, and wherein the buffer capacity is based on an air-to-fuel ratio feedback from an oxygen sensor and/or a hydrocarbon sensor coupled downstream of the canister. 19. An engine system, comprising, an engine including an intake;a fuel tank;a canister coupled to the intake through a first valve and coupled to the fuel tank through a second valve, the canister including a buffer, the canister including an adsorbent therein, wherein the adsorbent has a buffer region;a pressure sensor coupled to the fuel tank for estimating a fuel tank pressure;an exhaust gas sensor coupled downstream of the canister for providing air-to-fuel ratio feedback, a capacity of the buffer estimated from the air-to-fuel ratio feedback; anda controller with computer readable instructions for, opening the first valve to purge fuel vapors from the canister and increase the buffer capacity; andwhen the buffer capacity is higher than a threshold capacity, intermittently opening the second valve to purge fuel vapors from the fuel tank to the canister buffer, a duration of each opening and an interval between consecutive openings based on the buffer capacity, purge flow rate and the fuel tank pressure at an onset of the intermittent opening, a duration and interval of the purging based on a determination of the stored fuel vapor amount in the buffer distinct from a remainder of the adsorbent. 20. The engine system of claim 19, wherein the controller is configured to, decrease the duration of each opening while increasing the interval between consecutive openings as the buffer capacity decreases, and decrease the duration of each opening as the fuel tank pressure increases above a first threshold pressure; andopen the second valve for a duration while closing the first valve in response to the fuel tank pressure increasing above a second, higher threshold pressure, wherein the buffer is within the canister such that during canister loading, fuel vapors are first adsorbed within the buffer, and then when the buffer is saturated, further fuel vapors are adsorbed in the canister and during canister purging, fuel vapors are first desorbed from the canister before being desorbed from the buffer.