Technical Description: Purging: Notes

Fig 1: Identifying System With Vapor Recycling (ORVR) And Tank Integrity Diagnosis

Courtesy of SAAB-SCANIA OF AMERICA, INC.

The illustration shows the system with vapor recycling (ORVR) and tank integrity diagnosis.

The fuel that evaporates in the tank passes through a pipe to the EVAP canister. The active charcoal in the EVAP canister will eventually become saturated as it absorbs the hydrocarbon vapor. When the engine is started, the surrounding air is drawn through the EVAP canister via a purge valve and a non return valve in the intake manifold. The petrol vapor is drawn with the air and burned in the engine. When the pressure in the intake manifold exceeds the atmospheric pressure, another non return valve opens and purging will now take place to the suction side of the turbocharger.

In an de-energized state, the purge valve is closed. It is supplied from the main relay and is controlled by a 16 Hz PWM from control module pin 27. If the tank integrity diagnosis is active, the frequency will be 8 Hz.

Fig 2: Circuit Diagram - Purging

Courtesy of SAAB-SCANIA OF AMERICA, INC.

The pulse ratio regulates the flow so that it always constitutes a specific proportion of the total flow consumed by the engine.

If the air/fuel ratio in the flow deviates from 14.7:1, the closed loop will be affected. However, it is not the task of closed loop to correct for the excess caused by the purge so the purge has a correction factor that is affected by the closed loop as soon as the purge starts. The entire closed loop deviation from 1.00 is transferred to the purge correction factor, which means that closed loop fluctuates around 1.00 (0%) even if the purge contains large amounts of hydrocarbons or consists of pure air.

When purging is not active the factor 1.00 is used and the entire fuel fault is corrected by the closed loop along with the multiplicative and additive adaptation.

The limits for purge adaptation are 0.75 and 1.25 respectively.

The following conditions must be fulfilled before purging can be activated:

Fig 3: Identifying Workflow Diagram - Mass Air Flow Sensor (Purging)

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Fig 4: Correction Factor Graph (Purging)

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Closed loop active.
No fuel adaptation in progress; this takes place for 30 seconds every 5 minutes.
Engine coolant temperature exceeds 50°C.
If the engine coolant temperature is below 5°C when the engine is started, the vehicle speed must exceed 10 km/h.
The function is active during the whole driving cycle and reduces the risk of noise being heard from the valve. The valve makes more noise when it is cold.
Battery voltage below 16V.
Tank integrity diagnosis not active.
Engine speed exceeds 770 RPM.

The diagnostic tool shows 25% when the correction factor is 1.25 and -25% when the correction factor is 0.75.