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Purging

Purging


The illustration shows a system with vapour 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 vapour. When the engine is started, the surrounding air is drawn through the EVAP canister via a purge valve and a nonreturn valve in the intake manifold. The petrol vapour is drawn with the air and burned in the engine. When the pressure in the intake manifold exceeds the atmospheric pressure, another nonreturn valve opens and purging will now take place to the suction side of the turbocharger.

In a normal state, the purge valve is closed. It is supplied from the main relay and controlled with a 16 Hz PWM from control module pin 27 but during the tank integrity diagnosis, the frequency is 8 Hz.

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 and the multiplicative adaptation 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:




• 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 16 V.

• No tank integrity diagnosis 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.

Diagnostics


• If there is an open circuit or short circuit to ground, diagnostic trouble code P0444 will be generated.

• If there is a short circuit to B+, diagnostic trouble code P0445 will be generated.

• If there is an internal leak in the valve, diagnostic trouble code P0441 will be generated.

Note

Purging adaptation can cause problems in fault diagnosis of the fuel system because the closed loop can fluctuate round or close to 0 % despite major faults such as air leaks. If an LED is connected instead of the purge valve and the purging adaptation is read with the diagnostic tool it will show the total fuel fault when the purge valve is active provided the multiplicative and additive adaptation is reset first.



Integrity diagnosis


The on-board diagnostics must be able to detect a leak in the purge system corresponding to a hole with diameter 1 mm.

For this reason, there is a differential pressure sensor fitted to the tank and a shut-off valve on the EVAP canister atmospheric connection. When the shut-off valve is closed, it should be possible to create and maintain a vacuum in the tank using the purge valve. Otherwise, the system is leaking and a diagnostic trouble code will be generated. The diagnosis is performed once per driving cycle.

The pressure sensor voltage is fed with 5 V from the control module pin 46 and is grounded from control module pin 70.

Depending on the pressure difference between the tank and atmosphere the pressure sensor passes a proportional voltage to control module pin 59.

The pressure sensor value is adapted to 0 kPa when the ignition is turned on provided the coolant temperature is under 40°C and the fuel level is under 50 litres.

The shut-off valve is open in a de-energized state. It is powered from the main relay and is controlled from control module pin 37.

Diagnostics


Pressure sensor


• If the sensor voltage is too low, diagnostic trouble code P0452 will be generated.

• If the sensor voltage is too high, diagnostic trouble code P0453 will be generated.

• If the pressure value is implausible, diagnostic trouble code P1451-P1453 will be generated.


System reaction to a fault


• Integrity diagnosis is blocked.

• Fuel adaptation and purging blocked on P1452.


Shut-off valve


• If there is an open circuit or short circuit to ground, diagnostic trouble code P1444 will be generated.

• If there is a short circuit to B+, diagnostic trouble code P1445 will be generated.


System reaction to a fault


• Fuel adaptation and purging are blocked.


Integrity


• A minor leak will generate diagnostic trouble code P0442.

• A major leak will generate diagnostic trouble code P0455.