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Methods and systems for passively purging a hydrocarbon trap in an engine intake in a vehicle are disclosed. In one example approach, a method comprises, in response to an ambient temperature decrease during an engine off condition while a fuel tank is sealed from atmosphere, delivering fuel stored

Methods and systems for passively purging a hydrocarbon trap in an engine intake in a vehicle are disclosed. In one example approach, a method comprises, in response to an ambient temperature decrease during an engine off condition while a fuel tank is sealed from atmosphere, delivering fuel stored in a hydrocarbon trap in an intake of the engine to a fuel vapor canister coupled to the fuel tank in an emission control system.

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1. A method for a vehicle with an engine, comprising: in response to an ambient temperature decrease during an engine off condition while a fuel tank is sealed from atmosphere and from a fuel vapor canister via an isolation valve between the fuel tank and the fuel vapor canister, delivering fuel sto

1. A method for a vehicle with an engine, comprising: in response to an ambient temperature decrease during an engine off condition while a fuel tank is sealed from atmosphere and from a fuel vapor canister via an isolation valve between the fuel tank and the fuel vapor canister, delivering fuel stored in an adsorbent in a hydrocarbon trap in an intake of the engine to the fuel vapor canister coupled to the fuel tank in an emission control system by opening the isolation valve and providing vacuum in the fuel tank to the hydrocarbon trap via the fuel vapor canister, wherein delivering fuel stored in the hydrocarbon trap to the fuel vapor canister comprises closing a canister vent valve in a vent path of the fuel vapor canister, opening a throttle in the intake of the engine, and pulsing a purge valve open and closed in order to cause airflow through the throttle to desorb hydrocarbons bound to the adsorbent. 2. The method of claim 1, further comprising closing the purge valve, closing the throttle, opening the vent valve, and closing the isolation valve in response to an amount of fuel vapor delivered from the hydrocarbon trap to the fuel vapor canister greater than a threshold. 3. The method of claim 1, further comprising in response to an amount of vacuum in the fuel tank reaching a vacuum threshold while sealed from atmosphere during the engine off condition, providing vacuum from the fuel tank to the hydrocarbon trap to deliver fuel stored in the hydrocarbon trap to the fuel vapor canister. 4. The method of claim 1, wherein delivering fuel stored in the hydrocarbon trap to the fuel vapor canister is performed in response to an amount of fuel vapor stored in the fuel vapor canister less than a threshold. 5. The method of claim 1, wherein intake and exhaust valves in cylinders of the engine are positioned in a closed state while delivering fuel stored in the hydrocarbon trap to the fuel vapor canister. 6. The method of claim 1, wherein the hydrocarbon trap is coupled in an intake manifold of the engine, and wherein the adsorbent includes activated carbon. 7. The method of claim 1, further comprising in response to a fuel vapor purging event while the engine is in operation, purging fuel vapors stored in the fuel vapor canister to the intake of the engine. 8. The method of claim 1, wherein the engine off condition follows a vehicle key-off event. 9. The method of claim 1, wherein the vehicle is a plug-in hybrid electric vehicle. 10. A method for a vehicle with an engine, comprising: in response to an ambient temperature decrease during an engine off condition while a fuel tank is sealed from atmosphere and from a fuel vapor canister via an isolation valve between the fuel tank and the fuel vapor canister, delivering fuel stored in an adsorbent in a hydrocarbon trap in an intake of the engine to the fuel vapor canister coupled to the fuel tank in an emission control system by opening the isolation valve and providing vacuum in the fuel tank to the hydrocarbon trap via the fuel vapor canister, further comprising:determining a time of day of a most recent engine-off event;waking a vehicle controller at a pre-determined time following the engine-off event, the pre-determined time based on the time of day of the most recent engine-off event; anddetermining an amount of vacuum in the fuel tank following waking the vehicle controller, and in response to the amount of vacuum determined at a selected time being greater than a threshold, providing vacuum from the fuel tank to the hydrocarbon trap to deliver fuel stored in the hydrocarbon trap to the fuel vapor canister. 11. A method for a hybrid vehicle with an engine, comprising: in response to an amount of vacuum in a sealed fuel system of the vehicle reaching a vacuum threshold during an engine off condition, selectively providing vacuum from the fuel system without activating a vacuum pump to a hydrocarbon trap in an intake of the engine to deliver fuel stored in activated carbon in the hydrocarbon trap to an activated carbon fuel vapor canister in an emission control system, wherein the fuel system is sealed from atmosphere via an isolation valve between the fuel system and the fuel vapor canister and delivering fuel stored in the hydrocarbon trap to the fuel vapor canister comprises closing a canister vent valve in a vent path of the fuel vapor canister, opening the isolation valve, opening a throttle in the intake of the engine and opening a purge valve in a purge line between the fuel vapor canister and the intake of the engine to provide vacuum to the hydrocarbon trap. 12. The method of claim 11, further comprising closing the purge valve, closing the throttle, opening the vent valve, closing the isolation valve, and maintaining the engine off in response to an amount of fuel vapor delivered from the hydrocarbon trap to the fuel vapor canister greater than a threshold. 13. The method of claim 11, further comprising determining the amount of vacuum in the fuel system during the engine off condition at a selected time based on a time of day, and in response to the amount of vacuum determined at the selected time being greater than a threshold, providing vacuum from the fuel system to the hydrocarbon trap to deliver fuel stored in the hydrocarbon trap to the fuel vapor canister. 14. The method of claim 11, wherein the vehicle is a plug-in hybrid electric vehicle and the hydrocarbon trap is coupled in an intake manifold of the engine upstream of a cylinder intake valve of the engine. 15. A hybrid vehicle system, comprising: a hydrocarbon trap including an adsorbent and situated in an intake of an engine;a fuel vapor canister coupled to the intake of the engine via a purge line, the fuel vapor canister comprising the adsorbent;a canister vent valve in a vent path of the fuel vapor canister;a fuel tank coupled to the fuel vapor canister;a fuel tank isolation valve between the fuel tank and the fuel vapor canister;a purge valve in the purge line;a throttle in the intake of the engine; anda controller configured to: at an engine-off event, determine a load of the hydrocarbon trap based on an engine run-time since a most recent engine-on event;responsive to the hydrocarbon trap load being greater than a threshold, close the fuel tank isolation valve to seal off the fuel tank from atmosphere;determine an amount of vacuum in the sealed fuel tank during an engine off condition; andin response to the amount of vacuum in the sealed fuel tank during the engine off condition greater than a threshold: close the canister vent valve;open the isolation valve;open the throttle; andopen the purge valve to provide vacuum to the hydrocarbon trap to deliver fuel stored in the hydrocarbon trap to the fuel vapor canister. 16. The system of claim 15, wherein the controller is further configured to close the purge valve, close the throttle, open the vent valve, and close the isolation valve following delivery of fuel vapor from the hydrocarbon trap to the fuel vapor canister. 17. The system of claim 15, wherein determining the amount of vacuum in the sealed fuel tank during the engine off condition is performed at a selected time based on a time of day. 18. The system of claim 15, wherein the hybrid vehicle system is a plug-in hybrid electric vehicle system, and wherein the adsorbent is activated carbon.