Evaporative Emissions System: Description and Operation
Evaporative Emissions
EVAP System
1 Charcoal canister vent hose
2 Atmospheric vent pipe
3 Vapor separator
4 Charcoal canister
Evaporative Emissions (EVAP) Purge Valve
the EVAP purge valve is located on the LH side of the engine in the line between the charcoal canister and the inlet manifold. the EVAP purge valve is part of the EVAP control system and is used to control the extraction of fuel vapor stored in the EVAP canister.
the EVAP control system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank. the system comprises fuel cut off valves, a vapor separator, a two way valve, vent lines, the canister and the purge valve.
the ECM controls the amount of vapor drawn from the charcoal canister by controlling the length of time the purge valve is open. It controls the length of time it is open by supplying the purge valve with a PWM voltage. Control is used to maintain the required level of emissions, as a hydrocarbon vapor level of 1% can affect the air/fuel ratio by as much as 20%.
the ECM can diagnose faults with the purge valve and will store the related fault codes, along with details of the engine speed, battery voltage and intake air temperature. the driver may notice the following effects if the purge valve fails in the open position:
- the engine may stall periodically when returning to idle
- the engine may suffer from poor idle quality
Secondary Air Injection
the ECM controls the secondary air injection which is used to quickly heat the catalytic converters for emission legislation compliance.
the ECM controls the vacuum vent valve and the air injection pump relay. the air pump operates at a start temperature of between 9°C and 50°C. the pump continues to operate for a maximum of 2 minutes when the engine speed drops to idle.
Note that the secondary air valve is always open when the air pump is operating
the secondary air injection system is used to limit the emission of carbon monoxide (CO) and hydrocarbons (HC) that are prevalent in the exhaust during cold starting of a spark ignition engine. the concentration of hydrocarbons experienced during cold starting at low temperatures are particularly high until the engine and catalytic converter reach normal operating temperature. the lower the cold start temperature the greater the prevalence of hydrocarbons emitted from the engine.
There are several reasons for the increase of HC emissions at low cold start temperatures, including the tendency for fuel to be deposited on the cylinder walls which is then displaced during the piston cycle and expunged during the exhaust stroke. As the engine warms up through operation, the cylinder walls no longer retain a film of fuel and most of the hydrocarbons will be burnt off during the combustion process.
the SAI pump is used to provide a supply of air into the exhaust ports in the cylinder head onto the back of the exhaust valves, during the cold start period. the hot unburnt fuel particles leaving the combustion chamber mix with the air injected into the exhaust ports and immediately combust. This subsequent combustion of the unburnt and partially burnt CO and HO particles help to reduce the emission of these pollutants from the exhaust system. the additional heat generated in the exhaust manifold also provides rapid heating of the exhaust system catalytic converters. the additional oxygen which is delivered to the catalytic converters also generate an exothermic reaction which causes the catalytic converters to 'light off' quickly.
the catalytic converters only start to provide effective treatment of emission pollutants when they reach an operating temperature of approximately 250°C (482°F) and need to be between temperatures of 400°C (752°F) and 800°C (1472°F) for optimum efficiency. Consequently, the heat produced by the secondary air injection "after burning" reduces the time delay before the catalysts reach an efficient operating temperature.
the engine control module (ECM) checks the engine coolant temperature when the engine is started and it it is above -9°C but below 75°C, the SAI pump is started. Secondary air injection will remain operational for a period controlled by the ECM. the SAI pump operation can be cut short due to excessive engine speed or load.
Air from the SAI pump is supplied to the cylinder head via a metal pipe which splits the air flow evenly to each bank.
At the same time the secondary air pump is started the ECM operates a SAI vacuum solenoid valve which opens to allow vacuum from the reservoir to be applied to the non return valve on the metal delivery tube on the engine. Secondary air is injected into the inner most exhaust ports on each bank.
When the ECM breaks the ground circuit to de-energize the SAI vacuum solenoid valve, the vacuum supply to the SAI non return valve is cut off and the valve is closed to prevent further air being injected into the exhaust manifold.
At the same time as the SAI vacuum solenoid valve is closed, the ECM opens the ground circuit to the SAI pump relay to stop the SAI pump.
A vacuum reservoir is included in the vacuum line between the intake manifold and the SAI vacuum solenoid valve. This prevents changes in vacuum pressure from the intake manifold being passed on to cause fluctuations of the secondary air injection solenoid valve. the vacuum reservoir contains a one way valve and ensures a constant vacuum is available for the SAI vacuum solenoid valve operation. This is particularly important when the vehicle is at high altitude.
Secondary air injection system components
the secondary air injection (SAI) system components are described below:
Secondary air injection (SAI) pump
the SAI pump is attached to a bracket at the front LH side of the engine compartment and is fixed to the bracket by three studs and nuts. the pump is electrically powered from a 12V battery supply via a dedicated relay and supplies approximately 35 kg/hr of air when the vehicle is at idle in Neutral/Park on a start from 20°C (68°F).
Air is drawn into the pump from the "clean" side of the air cleaner. the air is delivered to the cylinder head on each side of the engine through a metal pipe.
the foam filter in the air intake of the SAI pump provides noise reduction and protects the pump from damage due to particulate contamination.
If the secondary air injection pump malfunctions, fault codes may be stored in the ECM diagnostic memory, which can be retrieved using 'TestBook'/T4
Secondary Air Injection (SAI) Pump Relay
the secondary air injection pump relay is located in the E-Box. the engine control module (ECM) is used to control the operation of the SAI pump via the SAI pump relay. Power to the coil of the relay is supplied from the vehicle battery via the main relay and the ground connection to the coil is via the ECM.
Secondary Air Injection Non Return Valve
the secondary air injection non return valve is located on the steel air delivery tube at the front of the engine. the valve is controlled by the ECM via the vacuum vent solenoid.