Evaporative Emission (EVAP) System

EVAPORATIVE EMISSION (EVAP) SYSTEM

Overview
The EVAP system prevents fuel vapor build-up in the sealed fuel tank. Fuel vapors trapped in the sealed tank are vented through the vapor valve assembly on top of the tank. The vapors leave the valve assembly through a single vapor line and continue to the EVAP canister for storage until the vapors are purged to the engine for burning.

Enhanced Evaporative Emission (EVAP) System

Typical Enhanced Evaporative Emission System:

The enhanced EVAP system consists of a canister vent (CV) solenoid, EVAP canister, EVAP canister purge valve, fuel tank, fuel tank isolation valve (FTIV), fuel filler cap, fuel vapor control valve, fuel vapor vent valve, fuel tank pressure (FTP) sensor, intake manifold hose assembly, powertrain control module (PCM), the connecting wires, and fuel vapor hoses.

1. The enhanced EVAP system uses inputs from the cylinder head temperature (CHT) sensor, the intake air temperature (IAT) sensor, the mass air flow (MAF) sensor, the vehicle speed and the fuel tank pressure (FTP) sensor to provide information about engine operating conditions to the PCM. The fuel level input (FLI) and FTP sensor signals to the PCM are used by the PCM to determine activation of the EVAP leak check monitor based on the presence of vapor generation or fuel sloshing.
2. The PCM determines the desired amount of purge vapor flow to the intake manifold for a given engine condition. The PCM then outputs the required signal to the EVAP canister purge valve. The PCM uses the enhanced EVAP system inputs to evacuate the system using the EVAP canister purge valve, seals the enhanced EVAP system from the atmosphere using the CV solenoid, and uses the FTP sensor to measure the total vacuum lost for a period of time.
3. The CV solenoid seals the enhanced EVAP system from the atmosphere during the EVAP leak check monitor.
4. The PCM outputs a variable duty cycle signal (between 0% and 100%) and a variable current (between 0 mA and 1000 mA) to the solenoid on the EVAP canister purge valve.
5. The FTP sensor monitors the fuel tank pressure during engine operation and continuously transmits an input signal to the PCM. During the EVAP monitor testing, the FTP sensor monitors the fuel tank pressure or vacuum bleed-up.
6. The fuel tank-mounted fuel vapor vent valve assembly and the fuel tank-mounted fuel vapor control valve are used in the enhanced EVAP system to control the flow of fuel vapor entering the engine. These valves also prevent fuel tank overfilling during refueling operation, and prevent liquid fuel from entering the EVAP canister and the EVAP canister purge valve under any vehicle altitude, handling, or rollover condition.
7. The FTIV isolates the fuel tank from the rest of the EVAP system when the vapor flow is required only from the EVAP canister, and not from the fuel tank.

The enhanced EVAP system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM.