Evaporative Emission Systems

EVAPORATIVE EMISSION SYSTEMS

Overview
The Evaporative Emission (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 (located in the engine compartment, in the rear of vehicle near luggage compartment area or along the frame rail) for storage until the vapors are purged to the engine for burning.

There are two types of Evaporative Emission (EVAP) Systems:
^ The Enhanced Evaporative Emission (EVAP) system.
^ The On-Board Refueling Vapor Recovery (ORVR) Evaporative Emission (EVAP) system.

Enhanced Evaporative Emission (EVAP) System

Enhanced Evaporative Emission System:

The Enhanced EVAP system (Figure 99) consists of a fuel tank, fuel filler cap, fuel tank mounted or in-line fuel vapor control valve, fuel vapor vent valve, EVAP canister, fuel tank mounted or fuel pump mounted or in-line Fuel Tank Pressure (FTP) sensor, EVAP canister purge valve, intake manifold hose assembly, Canister Vent (CV) solenoid, Powertrain Control Module (PCM) and connecting wires and fuel vapor hoses.
1. The Enhanced EVAP system uses inputs from the Engine Coolant Temperature (ECT) sensor, the Intake Air Temperature (IAT) sensor, the Mass Air Flow (MAF) sensor, the Vehicle Speed Sensor (VSS) 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 Monitor based on presence of vapor generation or fuel sloshing.
2. The PCM calculates a variable duty cycle based on the desired amount of purge vapor flow to the intake manifold for a given engine condition. The PCM can then output the duty cycle to the solenoid on 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 atmosphere using the CV solenoid, and uses the FTP sensor to observe total vacuum lost for a period of time.
3. The canister vent (CV) solenoid seals the Enhanced EVAP system to atmosphere during the EVAP leak check Monitor.
4. The PCM outputs a variable duty cycle signal (between 0% and 100%) to the solenoid on the EVAP canister purge valve.
5. The fuel tank pressure (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, fuel tank mounted fuel vapor control valve (or remote fuel vapor control valve) are used in the Enhanced EVAP system to control the flow of fuel vapor entering the engine. All of 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. The liquid/vapor fuel discriminator is part of the fuel vapor control valve assembly on Escort and Focus applications.
7. The Enhanced EVAP system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM. This can be done by pressurizing the system using Rotunda Evaporative Emission Tester kit 134-00056 or equivalent and the leak (frequency) detector included with the kit.

Hardware

EVAP Canister Purge Valve

EVAP Canister Purge Valve:

Electronic EVAP Canister Purge Valve:

The EVAP canister purge valve (Figure 100) and (Figure 101) 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 normally closed valve. The electronic EVAP canister purge valve (Figure 102) controls the flow of vapors electronically by way of a solenoid thereby, eliminating the need for an electronic vacuum regulator and vacuum diaphragm.

Fuel Tank Pressure Sensor

Fuel Tank Pressure (FTP) Sensor:

In-Line Fuel Tank Pressure (FTP) Sensor:

The fuel tank pressure (FTP) sensor (Figure 103) or inline fuel tank pressure (FTP) sensor (Figure 104) is used to measure the fuel tank pressure during the EVAP monitor test on vehicles equipped with the Running Loss-type system. Also, it is used to control excessive fuel tank pressure by forcing the system to purge.

Canister Vent Solenoid

Canister Vent (CV) Solenoid:

During the Enhanced EVAP System test monitor, the canister vent (CV) solenoid (Figure 105) 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 monitor run.

Fuel Filler Cap

Fuel Filler Cap:

The fuel filler cap (Figure 106) is used to prevent fuel spill and close the evaporative emission/fuel system to atmosphere. Some vehicles may have a Fuel Cap On Indicator Lamp (FCIL) in the instrument cluster which will illuminate when there is a failure in the vapor management system that may be due to the fuel filler cap not being sealed.

Refer to the individual component for information on the following Evaporative Emission System components: fuel vapor control valve, fuel vapor vent valve assembly and fuel vapor separator assembly.

On-Board Refueling Vapor Recovery (ORVR) Evaporative Emission (EVAP) System

On-Board Refueling Vapor Recovery Evaporative Emission System:

The basic elements forming the ORVR system (Figure 107), (Figure 108) and (Figure 109) operation are as follows when fuel is dispensed:
1. The fuel filler pipe forms a seal to prevent vapors from escaping the fuel tank, while liquid is entering the fuel tank (liquid in the one inch diameter tube blocks vapors from rushing back up the fuel filler pipe).
2. A fuel vapor control valve controls the flow of vapors out of the fuel tank (valve closes when liquid level reaches a height associated with the fuel tank usable capacity). This valve accomplishes the following:
a. Limits the total amount of fuel that can be dispensed into the fuel tank.
b. Prevents liquid gasoline from exiting the fuel tank when submerged (and also when tipped well beyond a horizontal plane as part of the vehicle roll-over protection in road accidents).
c. Minimizes vapor flow resistance during anticipated refueling conditions.
3. Fuel vapor tubing connects the fuel vapor control valve to the EVAP canister. This routes the fuel tank vapors (displaced by the incoming liquid) to the EVAP canister.
4. A check valve in the bottom of the fuel filler pipe prevents liquid from rushing back up the fuel filler pipe during the liquid flow variations associated with the filler nozzle shut-off.

Between refueling events, the EVAP canister is purged with fresh air so that it may be used again to store vapors accumulated during engine soaks or subsequent refueling events. The vapors drawn off of the carbon in the EVAP canister are consumed in the engine.

Refer to Evaporative Emissions System for information on the following Evaporative Emission System components: liquid/vapor fuel discriminator, fuel filler pipe check valve, fill limit valve assembly, fill limit vent valve assembly, fuel filler pipe flapper valve, fuel vapor control valve (fuel tank mounted), ORVR T-connector assembly and EVAP canister.