Evaporative Emission System

Evaporative Emission (EVAP) System

Evaporative Emission (EVAP) System:

The Evaporative Emission (EVAP) system (Figure 1) is used to absorb fuel vapors from the fuel tank to reduce the amount of hydrocarbons emitted into the atmosphere. In a hot soak or refueling condition, fuel vapor pressure inside the fuel tank increases. In order to depressurize the fuel tank the excess air (which was drawn into the fuel tank as the fuel level decreased) and fuel vapor mixture in the fuel tank pass through the vacuum cut valve into the EVAP canister and out the EVAP canister vent valve. The EVAP canister contains activated carbon which collects the fuel vapors to prevent them from being emitted into the atmosphere with the escaping air. The fuel vapors are stored in the EVAP canister until they can be consumed by the engine during normal engine operation (i.e. not decel, idle, low engine coolant temperature, starting, and Wide Open Throttle (WOT)). During normal engine operation the Powertrain Control Module (PCM) commands the EVAP canister purge valve ON which opens the EVAP canister purge valve. When the EVAP canister purge valve is open, intake manifold vacuum is applied to the EVAP canister which draws in fresh air and fuel vapors from the EVAP canister into the intake manifold. The PCM uses various sensor inputs to calculate the desired amount of EVAP purge flow. The PCM meters the purge flow by varying the duty cycle of the EVAP canister purge valve input signal.

The Evaporative Emission (EVAP) system monitor is a self-test strategy within the Powertrain Control Module (PCM) that tests the integrity of the EVAP system. When a fault occurs, the EVAP system monitor is reset to NO and a Diagnostic Trouble Code (DTC) is set in the PCM memory. After the DTC is repaired the vehicle drive cycle must be completed to reset the monitor in preparation for Inspection and Maintenance (I/M) testing. The PCM monitors the EVAP system for leaks, electronic EVAP components for irrationally high or low voltage levels sent to the PCM, and the EVAP system for proper operation. The EVAP system monitor uses the positive and the negative pressure leak test methods to test and activate the EVAP system. The positive pressure leak test uses fuel tank fuel vapor pressure (when the fuel tank temperature is sufficient) to test the system. During the positive pressure leak test the EVAP canister purge valve is closed (OFF), the EVAP canister vent valve is closed (ON), the Manifold Absolute Pressure/Barometric Pressure (MAP/BARO) solenoid is ON (MAP/BARO sensor connected to barometric pressure) and the vacuum cut bypass valve is open (ON). The positive pressure test passes if the EVAP pressure sensor indicates a rise in EVAP pressure and the pressure holds until the EVAP canister vent valve is commanded open. The negative pressure leak test uses intake manifold vacuum to test the system. During the negative pressure leak test the EVAP canister purge valve is open (ON), the EVAP canister vent valve is closed (ON), and the MAP/BARO solenoid is OFF (MAP/BARO sensor connected to manifold absolute pressure). The negative pressure test passes if the EVAP pressure sensor indicates a decrease in EVAP pressure equal to the pressure indicated from the MAP/BARO sensor and the pressure holds until the EVAP canister vent valve is commanded open.

Evaporative Emissions Canister Vent Control Valve

EVAP Canister Vent Control Value:

The evaporative canister (EVAP) vent control valve (Figure 2) is located on the EVAP canister. The Powertrain Control Module (PCM) energizes the solenoid when it is necessary to close the canister vent. The EVAP canister vent valve is used only for diagnosis of the EVAP system and usually remains open. When this vent is closed under normal purge conditions the EVAP system is depressurized and allows EVAP system leak diagnosis.

EVAP Pressure Sensor

EVAP Pressure Sensor:

The EVAP pressure sensor (Figure 3) is an input to the Powertrain Control Module (PCM) and is used only for on-board diagnosis of the EVAP emission system. The sensor is located in the purge line of the EVAP canister and senses changes in pressure in the EVAP canister purge line.

Evaporative Emission System Test Port

EVAP System Test Port:

The test port (Figure 4) is located under the hood near the cowl and is used to perform leak testing on the EVAP system.

EVAP Canister Purge Valve

EVAP Canister Purge Valve:

The Evaporative Emission (EVAP) canister purge valve (Figure 5) is controlled by the Powertrain Control Module (PCM). The EVAP canister purge valve controls the EVAP canister purge. When the EVAP purge valve is on (ground supplied by the PCM), vacuum is supplied to the EVAP canister and purging of vapors in the EVAP canister is allowed. When the EVAP purge valve is off (open signal from the PCM) the solenoid is released to prevent purging of the fuel vapors from the EVAP canister.

Evaporative Emission Canister

EVAP Emissions Canister:

The Evaporative Emission (EVAP) canister (Figure 6) is a storage device for fuel vapors that are emitted by the fuel tank in hot soak conditions. When the vehicle is at normal operating conditions, the vapors are purged from the canister. The EVAP canister is controlled by the Evaporative Emission (EVAP) canister purge valve. This valve sends vacuum to the EVAP canister which allows the passage of the fuel vapors into the intake manifold where they are mixed with air and burned in the engine. By storing the fuel vapors and purging them into the engine at a later time, hydrocarbon emissions are reduced, and fuel efficiency is increased.

Vacuum Cut Valve

Vacuum Cut Valve:

The vacuum cut valve (Figure 7) is a one-way check valve which allows fuel vapors to enter the EVAP purge line, but prevents intake manifold vacuum from being applied to the fuel tank.

Vacuum Cut Bypass Solenoid

Vacuum Cut Bypass Solenoid:

The vacuum cut bypass solenoid (Figure 8) is installed in parallel with the vacuum cut valve on the EVAP purge line between the EVAP canister and the fuel tank. The vacuum cut bypass solenoid generally remains closed. The vacuum cut bypass solenoid is open when on (ground supplied by the PCM). When open, the vacuum cut valve is bypassed to apply intake manifold vacuum to the fuel tank. The vacuum cut bypass solenoid is used only for EVAP system diagnostics.

Water Separator

Water Separator:

The water separator (Figure 9) is attached to the EVAP canister vent control valve inlet. When the EVAP canister vent control valve is open, air is drawn into the EVAP canister through the water separator. The water separator draws water vapor from the incoming air and then vents it back to the atmosphere.

Manifold Absolute Pressure/Barometric Pressure Sensor

MAP/BARO Sensor:

The Manifold Absolute Pressure/Barometric Pressure (MAP/BARO) sensor (Figure 10) is an input to the Powertrain Control Module (PCM). The sensor is used in conjunction with a MAP/BARO solenoid that either supplies manifold vacuum (MAP function) to the MAP/BARO sensor or opens the sensor to atmospheric pressure (BARO function). The PCM uses the MAP/BARO data to control the EVAP emissions system.

MAP/BARO Solenoid

MAP/BARO Solenoid:

The MAP/BARO solenoid (Figure 11) switches air flow passage between manifold vacuum and atmospheric pressure upon demand from the PCM. When voltage is supplied from the PCM, the MAP/BARO solenoid turns "ON". When "ON" the MAP/BARO sensor monitors barometric pressure (BARO); when the MAP/BARO solenoid is "OFF" manifold vacuum is monitored (MAP).