Evaporative Emission System Control

EVAP SYSTEM CONTROL

EVAP System Control:

Overview
EVAP stands for Evaporative Emission Control System. The purpose of the system is to recover vapor evaporated from the fuel in the tank and prevent its escape to atmosphere.

The fuel vapor passes through a hose system (1) from the fuel tank through a roll-over valve (2) to a cannister (3) which prevents vapor release to the atmosphere.

Carbon Filter
The fuel vapor from the tank passes through a bed of active carbon in the cannister. The active carbon binds the fuel, so clean air is emitted through the atmospheric coupling at the top of the cannister. Depending on the carbon volume in the cannister, 80 - 140g (0.18 - 0.31lbs) of fuel can be bound in the carbon.

Roll-Over Valve
The roll-over valve closes if the car tilts sideways by more than 45°, preventing fuel leakage in the event of an accident.

EVAP Valve
The EVAP valve (4) is positioned between the cannister and the engine's intake pipe and it regulates the flow of fuel vapor from the cannister to the engine.

It is coupled to 12 Volts at the main relay and to the control module. The control module opens the valve by grounding the cable. The EVAP valve is normally shut when the engine is switched OFF, opening only when the control module wants to purge the cannister. When purging, the valve is first pulsed slightly, followed by a slight increase in the rate of pulsation so that the mixture from the cannister increases up to a level which varies depending on engine load and rpm, among other factors. Owing to the negative pressure in the intake manifold, the fuel vapor is sucked out of the cannister into the engine's intake manifold. At the same time, air is taken in through the duct in the top of the filter.

The EVAP valve is controlled by the ECM.