Evaporative emission control system (Emission control - V8)

Evaporative emission control system

The evaporation emission control (EVAP) system is used to reduce the level of hydrocarbons emitted into the atmosphere from the fuel system. The system comprises an EVAP canister which stores the hydrocarbons from the fuel tank, pressure valves, vent lines and a purge control solenoid valve.

Fuel vapour is stored in the canister until it is ready to be purged to the inlet manifold under the control of the Engine Control Module (ECM).

A two-way valve is included in the vent line between the fuel tank and the EVAP canister in all markets except NAS.

A fuel vapour separator is fitted next to the fuel filler neck, the construction is different between NAS and ROW vehicles; the liquid vapour separator (LVS) on NAS vehicles is an L-shaped metal tube and for all other markets it is an integral part of the moulded plastic filler neck. FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

NAS vehicles have stainless steel filler necks whilst all other markets use moulded plastic filler necks. On NAS fillers, a valve closes the roll-over valve (ROV) vent line when the fuel filler cap is removed; for all other markets a pressure relief valve is fitted into the ROV vent line. FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

Four ROV's are fitted to the fuel tank, for NAS vehicles the valves are fitted inside the fuel tank and for ROW vehicles the ROV's are welded external to the fuel tank. Nylon vent lines from the ROV's connect to the liquid vapour separator allowing vapour to pass to the EVAP canister via the LVS. To prevent the canister from being overloaded (particularly in hot ambient conditions) and to prevent wastage of fuel, the vapour is allowed to condense within the LVS and flow back through the ROVs into the tank. FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

Pressure / vacuum relief valves are incorporated into the fuel filler cap which operate in the event of an evaporation system failure (e.g. blockage in the evaporation system line to atmosphere). The cap relieves fuel tank pressure to atmosphere at approximately 1.8 to 2.0 psi (12 to 14 kPa) and opens in the opposite direction at approximately – 0.7 psi (- 5kPa) vacuum. All plastic bodied fuel fillers are fitted with a tank overpressure relief valve.

A vent line flow restrictor (anti-trickle valve) is fitted to the filler pipe in the line between the tank and the canister on NAS vehicles. The purpose of the anti-trickle valve is to preserve the vapour space in the tank by blocking the vent line during the fuel filling process. The valve is operated by the action of inserting the filler gun, so that when the fuel in the tank reaches the level of the filling breather, flow cut off occurs due to fuel filling the filler pipe. FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

The breather ports from the EVAP canister are located high up in the engine bay (CVS unit on NAS vehicles with vacuum type, fuel evaporation leak detection capability; via an air filter on NAS vehicles with positive pressure type, fuel evaporation leak detection capability; snorkel tubes on ROW vehicles), to prevent water ingress during vehicle wading.

The ECM connectors and pins which are pertinent to evaporative emission control are listed in the following table: