Fuel Tank Evaporative Emission Control (Purge Control)
The control of the evaporative fuel vapors (HC) from the fuel tank is important for the overall reduction in vehicle emissions. The evaporative system has been combined with the ventilation of the fuel tank, which allows the tank to "breath".
The overall operation provides:
- An inlet vent, to an otherwise "sealed" fuel tank, for the entry of air to replace the fuel consumed during engine operation.
- An outlet vent with storage canister to "trap and hold" fuel vapors that are produced/developed from the evaporation of fuel in the tank, when the vehicle is stationary.
The canister is then "purged" by a control system that uses the engine vacuum to "pull" the fuel vapors into the combustion chamber. This "cleans" the canister and allows it to be "recycled".
Like any other form of combustible fuel, the introduction of these vapors on a running engine must be controlled. The ECM is assigned the job of controlling the purge vapors as an output function.
The purging of the HC vapors are controlled by an electric purge valve. The design and operation of the purge valve varies depending on the engine control systems as follows:
M1.x & M3.x Systems = Valve Sprung Open Power activated Closed,
M5.x & MS 41.x Systems = Valve Sprung Closed, Power activated Open,
The differences in these valves is due to changes in emission requirements.
M1.X & M3.X PROGRAMMING
Programming for the operation of the purge valve provides control for both the amount and time of purge. The valve is powered closed (ground applied) during open loop operation (shortly after start up and full throttle) and is de-energized (open) during closed loop.
- The amount of purge is also controlled based on engine speed and load. This allows only a proportional amount of purge vapors to enter the running engine.
- The amount of purge is controlled by modifying the output control signal to the valve. Both the frequency and duty cycle are increased through a timed cycle. This allows a proportional amount of purge vapors to enter the intake based on the volume of air flow.
- When the cycle is completed, the valve is powered closed (purge off) for 5 minutes and the cycle repeated.
- The purge valve is also powered closed for 3 to 5 seconds after the engine is switched off to prevent run on (dieseling).
- Starting with M3.3, the purge function became adaptive. When the purge valve is open the additional combustible vapor is detected by the O2 sensor(s). The ECM compensates for the additional fuel vapor by reducing the injection time of the injectors.
M5.X PROGRAMMING
These Systems utilize a purge valve that is sprung closed and powered open. This type of purge valve is required for OBD II compliancy and evaporative system leak detection tests. This type of valve also prevents vapors escaping into the atmosphere when the vehicle is not in use. The valve will not open under fuel vapor over pressure.
The O2 sensors detect the additional rich mixture when the DME opens the purge valves. The valves are cycled periodically during engine operation.
Full Throttle = Purge Valve fully open. Solenoid pulsed at 95% duty cycle. Engine conditions are low intake manifold vacuum.
Warm Idle = Purge Valve slightly open.
Decel/Cold Idle = Purge valve closed. No solenoid activation. Engine conditions are high intake manifold vacuum.
The duty cycle of the purge valve solenoids may vary between 0 and 100% depending on engine operating conditions.
MS 41.X PROGRAMMING
Like the M5.x system, the MS 41 .x purge valves are also normally sprung closed and powered open (same part). This ensures that the valve is closed when the engine is switched OFF, preventing vapors from escaping into the atmosphere.
The MS41.x Control Module regulates the operation of the purge valve through a duty cycle control. The valve is cycled open based on engine RPM and Lambda regulation.
- Purge control is shut down with an engine temperature < 67 °C and during decel fuel cutoff.
- Above an engine temperature of 67 °C purge control is operational depending on engine speed and load.