Evaporative Emissions System: Description and Operation

Detecting leakage and control of evaporation










Control system for evaporation (EVAP)
A two-way valve (3.5 bar (50.8 psi)) and the ORVR-valve in the hose to the canister are used to prevent overfilling of the tank, thus helping to prevent fuel from running into the canister.
The canister holds 2.08 litres (0.55 gallon).
Draining of the canister is controlled by the EVAP-valve, which is controlled by Engine control module (ECM).
When there is boost pressure the canister is ventilated to the turbo's inlet side. If there is no boost pressure, the canister is ventilated to the inlet manifold. This is controlled by the no-return valves.

Integrated recycling of fumes when refuelling (ORVR)
When refuelling, fuel fumes are collected in the EVAP-canister. The air that is let out flows through a small air filter (to prevent penetration of hydrocarbons) to the leak diagnostics pump. There it is released to the ambient air through another air filter.
The ORVR-valve closes when the tank is full. Some expansion capacity remains. The two-way valve, set to 3.5 bar (50.8 psi), is closed.
An extra hose that connects the tank sleeve and the tank's upper end prevents fuel from being pressed out when the fuel tank is pressurized.

Leakage diagnosis pump (LDP)
The pump is a spring and vacuum driven diaphragm pump. It is driven by a vacuum and is connected to the inlet manifold via a non-return valve.
The pump cycle is started by a short vacuum pulse that lifts the diaphragm and compresses the spring, which draws air into the space under the diaphragm through a non-return valve. At the same time, the canister's integrated breather valve closes. At the end of the pulse the vacuum is released by an electrically controlled valve and the spring force presses down the diaphragm, which pumps air into the tank-/EVAP-system through a second non-return valve. When the diaphragm is at the down stroke's end-position, the valve closes and the cycle starts again.
If there is no leak in the system, the pressure increases following a number of strokes and the system and pressure are equal. The pump stops working.
If there is a leak the "pulse interval" stabilizes at another level and Engine control module (ECM) will interpret this as a leak.