Fuel Pump: Description and Operation
Fuel Pressure On The Low Pressure Side.
For DFI engines, the fuel pressure on the low pressure side has been increased to approx. 80 psi/5.5 bar (this was previously approx. 58 psi/4 bar). The left or right fuel pump is operated as the main pump in order to distribute the higher load to both fuel pumps, depending on the fuel level.
The fuel pumps are activated if the level of fuel in the tank is reduced and if the engine requires more fuel:
- Fuel level > 15.8 gal. (60 liters) to 26 gal. (100 liters): When the tank is relatively full, the right fuel pump is activated; if more fuel is required (> 13 gal./50 liters/h), the left pump is activated.
- Fuel level > 4 gals/15 liters to 15.8 gals/60 liters: When the tank is half-full, the left fuel pump is activated; if more fuel is required (> 13 gals/50 liters/h), the right pump is activated.
- Fuel level < 4 gals./15 liters: If the tank is relatively empty, both pumps run continuously.
Other switching functions include:
- If the ignition was switched off for more than 30 minutes, the left fuel pump is activated for approx. 1 to 2 seconds when the driver's door is first opened in order to build up fuel pressure even before the ignition is switched on.
- Both fuel pumps are activated while starting the engine and for several seconds after starting the engine.
Fuel High Pressure System in DFI Engines
The fuel high pressure system generates an injection pressure of up to 1740 psi (120 bar) in the combustion chamber. The components of the V6 engine and the V8 engines are different.
The following pages describe the functions of the components of the V8 DFI engines.
SAFETY: Always read and follow the safety instructions in the Technical Data, when working on the fuel supply system.
Cayenne Turbo
The high pressure pump -HD- used in the Cayenne Turbo is a six-piston pump with a maximum delivery rate of approx. 58 gals./245 liters/h at 1740 psi (120 bar). It builds up pressure and ensures flow control.
The following components are integrated into the high pressure pump:
Flow control valve -1- with pressure reducing function for the fuel high pressure side, pressure control valve, bypass valve, two temperature compensators on the oil side -2- and a fuel strainer on the inlet side with a mesh width of approx. 50 micrometer. Fuel is distributed in the same way as for the V8 naturally aspirated engine via a central high pressure rail with separate lines leading to the fuel injectors.
Flow Control Valve For Fuel High Pressure
The electric control valve for the fuel high pressure side -1- is located on the fuel high pressure pump and operates as a flow control valve. The Motronic control unit regulates the delivery rate of the high pressure pump in the fuel supply to the pump via the control valve. When the engine is switched off, the fuel high pressure is reduced by an integrated pressure reducing valve. The fuel pressure sensor monitors the required fuel pressure (approx. 580 psi/40 to 1740 psi/120 bar).
- If the control valve fails, the Motronic control unit goes into emergency operation, whereby the engine can still run in a limited way with low pressure (80 psi/5.5 bar). In this case, the bypass valve in the pump opens and provides a direct route from the low pressure side to the high pressure side.
- The bypass valve is also activated for filling the empty fuel rail on new engines or following repairs in order to reduce starting times.
Pressure Control Valve
The pressure control valve is integrated into the fuel high pressure pump. This safety valve opens a connection to the fuel low pressure system if the fuel pressure in the high pressure system exceeds approx. 2030 psi (140 bar).