Purging, basic function

Overview

This document contains information on:

 General

 Function requirements

 Function

General

When the fuel tank is being ventilated, fuel vapors escape. These must not get into the outside air. The arising vapors are temporarily stored in the activated charcoal canister and subsequently supplied to the combustion process. In order to prevent the activated charcoal canister being overloaded with fuel vapors, a regular purge is necessary.

Function requirements

 Engine management ON (circuit 87M)

 Engine start before at least 60 to 80 s

 Coolant temperature > 40 °C

 Engine not in overrun mode

 Engine not in the lambda adaptation phase

Function

P47.30-2332-79

Purge control

The fuel tank is ventilated via the vent and bleed valves, the vent and bleed lines, and the activated charcoal canister.

The fuel vapors flow to the activated charcoal canister via vent and bleed lines. The fuel vapors are absorbed by the activated charcoal in the activated charcoal canister, and, when purging is initiated by the combustion engine control unit, they are extracted into the charge-air distributor and finally fed into the combustion process.

The vacuum in the charge-air distributor is used to extract the fuel vapors in the charge-air distributor. To regulate the regeneration quantity, the purging switchover valve is actuated at the ground end by a pulse width modulated signal by the combustion engine control unit. The purge quantity is determined by constantly opening and closing the purging switchover valve with on-times of varying durations.

P47.30-A013-76

Wide open throttle purging

In full-load operation, fuel vapors are extracted via the full-load operation check valve and the full load purge line. The required vacuum is generated by a Venturi nozzle. The fuel vapors are then routed over the wide open throttle purge line, ATL and the charge air cooler to the charge air distributor.

Purging when idling

In idle mode, the fuel vapors stored in the activated charcoal canister are extracted by the existing vacuum through the purging switchover valve, the idle mode check valve, and the idle mode purge line behind the throttle valve. The fuel vapors are then fed into the combustion process.

 The idle speed control prevents the purging from causing a change in rpm when the engine is idle. The fuel/air mixture is leaned out appropriately depending on the load condition of the activated charcoal canister.