Explain Evaporative emission control system.

Explain Evaporative emission control system.
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EVAP system Operation: When the engine is running, stored fuel vapours in fuel tank are purged from the canister whenever the throttle has opened past the purge port and coolant temperature is above 54 °C. Fuel vapors flow from the high pressure area in the canister, past check valve in the canister, through the vacuum switching valve (ECM controlled- duty cycle controlled), to the low pressure area in the throttle body. ECM uses engine speed, intake air volume, coolant temperature, and oxygen sensor information to control EVAP operation. Atmospheric pressure is allowed into the canister through a filter located on the bottom of the canister. This ensures that the purge flow is constantly maintained whenever purge vacuum is applied to the canister. When coolant temperature falls below 35°C, the vacuum switching valve prevents purge from taking place by blocking the vacuum signal to the check valve at canister. Under other conditions, as fuel is drawn from the tank, a vacuum may be created in the tank. This is prevented by allowing atmospheric pressure to enter the tank through the check valve in the charcoal canister or fuel tank cap check valve. The EVAP system is designed to limit maximum vacuum and pressure in the fuel tank. 

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