Evaporative Emission (EVAP) Systems

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 fuel tank, fuel tank isolation valve (FTIV), fuel filler cap, fuel vapor control valve, fuel vapor vent valve, EVAP canister, fuel tank pressure (FTP) sensor, vapor management valve (VMV), intake manifold hose assembly, canister vent (CV) solenoid, powertrain control module (PCM) and 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 can then output 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.
8. The enhanced EVAP system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM.

Hardware

Canister Vent (CV) Solenoid

Canister Vent (CV) Solenoid:

During the EVAP leak check monitor, the CV solenoid seals the EVAP canister from atmospheric pressure. This allows the EVAP canister purge valve to obtain the target vacuum in the fuel tank during the EVAP leak check monitor.

EVAP Canister Purge Valve

Electronic EVAP Canister Purge Valve:

The EVAP canister purge valve, also known as vapor management valve (VMV), is part of the enhanced EVAP system that is controlled by the PCM. This valve controls the flow of vapors (purging) from the EVAP canister to the intake manifold during various engine operating modes. The EVAP canister purge valve is a normally closed valve. The electronic EVAP canister purge valve controls the flow of vapors electronically by way of a solenoid, thereby eliminating the need for an electronic vacuum regulator and vacuum diaphragm. 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.

Fuel Tank Isolation Valve (FTIV)

Fuel Tank Isolation Valve (FTIV):

The FTIV is a PCM controlled solenoid which isolates the fuel tank from the rest of the EVAP system. The FTIV is a normally open valve allowing the flow of vapors from the fuel tank to the electronic EVAP canister purge valve and the EVAP canister. Whenever it is desired to isolate the fuel tank from the rest of the EVAP system, the PCM provides a variable duty cycle signal (between 0% and 100%) to the solenoid which controls the FTIV operation.

Fuel Tank Pressure (FTP) Sensor

Fuel Tank Pressure (FTP) Sensor:

The FTP sensor is used to measure the fuel tank pressure during the EVAP leak check monitor.