Emission Control System Description

Evaporative Emission (EVAP) control system's role (with On-board Refueling Vapor Recovery System), is to prevent fuel vapor from emitting into the atmosphere. The vapor produced in fuel tank is stored in the EVAP canister. While filling fuel vapor in fuel tank flows to the EVAP canister through EVAP vapor control valve and is stored there. When fuel tank becomes full, a float in the vapor control valve works to close vapor passage, rejecting further refueling. When fuel tank is filled with fuel, the vapor produced in the fuel tank passes through the tank pressure control valve and flows into the EVAP canister through the refuel vapor control valve and is stored there. When fuel tank is not filled with fuel the vapor produced in the fuel tank flows into the EVAP canister through the refuel vapor control valve and is stored there. When the EVAP canister purge valve opens, fuel vapor stored in EVAP canister is drawn into the intake manifold together with air coming in through the suction filter and burned. EVAP canister purge valve is controlled by ECM according to signals from various sensors. When engine is at normal operating temperature, throttle valve is open wider than closed throttle position and engine is operating in closed loop mode, ECM controls ON/OFF (open/close) switching of EVAP canister purge valve at a constant rate of 10 times/sec. Length of EVAP purge valve On time depends on; engine speed, engine load and concentration of fuel vapor in purge line (fuel vapor concentration is calculated based on short term fuel trim and vapor purge rate) so as to prevent air/fuel mixture from being changed suddenly. The EVAP canister vent valve and the fuel tank pressure sensor are among component parts installed so that ECM diagnoses leakage of fuel vapor from EVAP control system. The EVAP canister vent valve is usually open, but is closed by ECM when checking for EVAP system leakage.