Evaporative System Monitoring

Evaporative System Monitoring

General Description
The monitoring uses a Leak Detection Pump (LDP). The LDP is an electrically/vacuum actuated device that will pressurize the evaporative emission system for the purpose of detecting leaks and verifying canister purge valve.

Leak detection
The leak detection is performed by means of two main phases:
- tank system over-pressurizing
- leak magnitude measurement

During the leak detection, the canister purge valve and the canister vent valve (CVV) are closed.

The ECU (Engine Control Module) causes the pump diaphragm to cycle at fixed frequency and for a fixed stroke number. During the pumping air is drawn from outside into the fuel tank system, the system pressure increases.

Once the tank system over-pressure phase is finished the leak measurement phase starts. diaphragm stroke is limited by the top of the diaphragm chamber and a position defined by a reed switch level. The over-pressure slows the diaphragm movement. The pump cycle duration depends on the tank pressure. So the evolution of this time shows the over-pressure level.

If there is a leak, the cycling time or "pulse interval" stabilizes at a rate, which compares to the leakage loss.

If there is no leak in the system the cycling time or "pulse interval" becomes longer.

The "pulse interval" is measured by the ECU, which determines whether or not the leak exceeds a defined threshold. Several "pulse interval" measurements are carried out to secure the test.







Canister purge valve check
When the tank system is tight or the leak measured is smaller than a defined threshold the same approach of the leakage detection is used. The purge valve is opened and each time the reed switch level is reached the LDP performs a pump stroke in order to maintain the pressure in the tank system.

If the canister purge valve is not blocked the cycling time or "pulse interval" becomes shorter. In this case the purge valve operates correctly (no blocked).

If the canister purge valve is blocked in a closed position or the connection tube canister/valve is pinched the cycling time or "pulse interval" remains long.

The "pulse interval" is measured by the ECU, which determines whether or not the purge flow exceeds a defined threshold. Several pumping cycles are carried out to secure the test.







Evaporative monitoring Block Diagram