The evaporative emission control system prevents evaporated fuel from the fuel tank from being released into the atmosphere by having the evaporative emissions be temporarily absorbed by a charcoal canister. These emissions are later taken in and combusted after the engine warms up.
Construction
The evaporative emission control system has passages and valves among the air cleaner, intake manifold, charcoal canister, and fuel tank as shown in the illustration. These are used to open and close the VSV, etc., to allow the engine ECU to control the movement of evaporated fuel for the entire system.
REFERENCE
Monitoring
The monitoring sequence is conducted when the air temperature sensor and the water temperature sensor show nearly the same values, such as during cold engine startup. The engine ECU uses the vapor pressure sensor to continuously monitor the fuel tank pressure, and when a malfunction is detected in the pressure, a DTC (Diagnosis Trouble Code) is stored in memory and the malfunction indicator lamp is turned ON to warn the driver. The engine ECU closes the canister closed valve and opens the purge valve and pressure switching valve to apply a vacuum to the entire system. When sufficient vacuum is applied, the engine ECU closes the purge valve to close the passages throughout the system. After this, the engine ECU conducts monitoring to check for leaks as the system pressure is gradually increased to a set vacuum. The engine ECU then operates the valves in the order of canister closed valve and then pressure switching valve, and then determines as the pressure changes whether or not the VSVs are good.
Operations
Purge flow
When the engine reaches certain conditions, the engine ECU opens the VSV (for canister closed valve) while controlling the VSV (for EVAP) using duty ratio control. This causes the intake manifold vacuum to open the air inlet valve and allow the gas absorbed by the canister to be taken together with air from the air cleaner via the VSV (for canister closed valve) into the intake manifold. The engine ECU uses duty ratio control for the VSV (for EVAP) to prevent an excessive purge flow during idling and other conditions, engine failure, and emissions from worsening.