Fuel Pump: Description and Operation

FUEL PUMP



General:
An in-tank pump is used in the Boxster. This provides advantages with regard to noise behavior and residual fuel suction behavior since, on the one hand, it is installed in the fuel tank (noise damping) and, on the other hand, it is attached directly to the specially designed base of the fuel tank (suction behavior) by means of a bayonet ring.

Construction:



The pump is designed as a two-stage system. The first stage functions according to the flow-wheel principle (1). The fuel is forced-flow ventilated by a special channel (2) as it is drawn from the fuel tank to remove any gas and vapor bubbles. It is then led into the reservoir chamber (4) via an ascending pipe (3). The precisely defined cross-section of the overflow pipe (5) produces slight overpressure in the reservoir chamber which prevents the formation of a gas bubble mixture and also ensures the precharging of pressure for the main stage. The pressurization of the fuel and the preceding ventilation process are necessary to guarantee a good hot-feed behavior of the system.
The second stage of the fuel pump (main feed pump) is configured as an internal gear pump (6) and pumps the fuel out of the reservoir chamber (7), through a filter, and then into the fuel filter and the fuel manifold pipes. The returned fuel is led back to the reservoir chamber via a return pipe (8). This completes the fuel circuit. When the engine is switched off, the pressure retention valve (9) closes the pipe loop and ensures that the necessary overpressure to prevent the formation of vapor bubbles.

Actuation:
To ensure that the required fuel pressure and the necessary amount of fuel are provided, the DME control module actuates the fuel pump for approx. 1 second whenever the ignition is switched on (pump priming).
To trigger another pump priming operation, the running engine must be switched off and the ignition then switched on again.
In addition, the fuel pump is activated by the DME control unit whenever the engine speed exceeds 15 rpm.

Safety Functions:
When the engine has been started, the airbag control unit sends a signal to the DME control module, the frequency and pulse duty factor of which is defined as "airbag not triggered". When the airbag is triggered, the pulse duty factor changes this signal. This change causes the DME control unit to switch off the fuel pump.