Fuel Supply
Fuel Supply
The engine is designed to provide optimum performance and fuel consumption if unleaded premium fuel with 93 octane ((R+M)/2) is used. If unleaded premium fuels with a lower octane number is used, the engine's knock controller automatically adapts the ignition timing. The maximum filling approx. 26 Gals. (100 liters), with a reserve of approx. 3 Gals. (12 liters).
SAFETY: Always read and follow the safety instructions in the Technical Data, when working on the fuel supply system.
- Fuel low-pressure system in DFI engines.
TECHNICAL MANUAL: The procedure for checking the fuel pressure and the amount of fuel delivered by the fuel pumps is described in the Technical Data.
- The low pressure system delivers the fuel from the fuel tank to the high pressure pump on the cylinder head.
- The new Cayenne vehicles have a returnless fuel system (RF).
- The demand control function of the fuel delivery rate reduces fuel heating in the tank by switching on the second fuel pump.
The fuel tank is not symmetrical. The left half of the tank has a higher volume than the right side. There is a "hump" in the middle so that both chambers are separated from each other once a certain fuel level is reached in the tank. When the tank is half full, the left fuel pump is activated because the left half of the tank has the higher volume. The fuel pumps are each supplied by one sucking jet pump whose hoses are routed diagonally and which "helps itself" to fuel from the other half of the tank.
The fuel pressure and the way in which the two fuel pumps are activated has changed compared to Cayenne vehicles up to model year 2006. On Cayenne vehicles up to M.Y. 2006, the left fuel pump was permanently activated and the right pump was only activated as required (for starting, higher delivery rate, etc.).
Functional diagram of the low pressure side in the fuel tank.
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.
Fuel High Pressure System in V8 DFI Engines
The fuel high pressure system in V8 DFI engines is made up of the these parts/components.
TECHNICAL MANUAL: Always read and observe the specifications in the Technical Data when securing all fuel lines in the high pressure area.
Fuel Temperature Sensor (on low pressure side)
At start of production, all Cayenne V8 engines have a fuel temperature sensor on the low pressure side, which will be replaced by a temperature model in the DME in later production. Depending on the amount of fuel required and the fuel temperature, the DME control unit, together with the flow control valve, regulates the amount of fuel on the high pressure side upstream of the high pressure injectors.
Fuel High Pressure Pump
The fuel high pressure pump pumps the amount of fuel required for injection and builds up a fuel pressure of up to 1740 psi (120 bar). The axial piston pump is driven by the inlet camshaft. The Cayenne S and Cayenne Turbo are equipped with different high pressure pumps.
Cayenne V8 Naturally Aspirated Engine
The high pressure pump used in the Cayenne S is a three-piston pump with a maximum delivery rate of approx. 47.5 gals./180 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 with pressure reducing function for the fuel high pressure side, pressure control valve, bypass valve, a temperature compensator on the oil side and a fuel strainer on the inlet side with a mesh width of approx. 50 micrometer. Fuel is distributed via a central high pressure rail with separate lines leading to the fuel injectors.
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.
Bypass Valve (including pressure relief valve)
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).
High Pressure Line
The high pressure line connects the high pressure pump to the fuel rail.
Central High Pressure Rail
The central high pressure rail in V8 engines is located in the engine's inner V. From here, the fuel is supplied via individual lines to the fuel injectors for cylinders 1 to 8. The high pressure rail provides the same pressure for all injectors. The volume of the high pressure rail is adapted according to the amount of fuel the engine needs (V8 naturally aspirated engine 100 cm3, V8 Turbo 150 cm3).
- The rail volume is determined by the required pressure variation behavior and the shortness of the starting time.
High Pressure Fuel Line
Fuel Injector Retainer Components
Fuel Pressure Sensor
The fuel pressure sensor is installed on the central high pressure rail under the intake system and informs the Motronic control unit about the current pressure in the fuel high pressure system. The Motronic control unit evaluates the signal and regulates the fuel pressure on the high pressure side via the flow control valve.
Fuel Injectors/High Pressure Injectors
The electromagnetically operated fuel injectors are on the intake side of the cylinder head. They are activated by the DME control unit in accordance with the firing order. Following activation, they inject fuel directly into the combustion chamber at a pressure of 580 psi (40 bar) to 1740 psi (120 bar). During this process, a vortex is created even before the fuel emerges at the valve tip.
The Cayenne S and Cayenne Turbo are equipped with different injectors, designed specifically to suit the engine's fuel requirements. The injectors can be differentiated by their part numbers and a color marking. The piston recesses in the V8 naturally aspirated engine and the V8 Turbo are also different so that the injected fuel can be whirled around and mixed perfectly with the air that is drawn in.
Fuel Injectors - V8 Naturally Aspirated Engine
At a fuel pressure of 580 psi (40 bar) and an injection time of 0.6 ms, the amount of fuel injected is approx. 5.5 mg/stroke. While at a fuel pressure of 1740 psi (120 bar) and an injection time of 5.8 ms, the amount of fuel injected is approx. 67 mg/stroke.
Fuel Injectors - V8 Turbo
At a fuel pressure of 580 psi (40 bar) and an injection time of 0.6 ms, the amount of fuel injected is approx. 7.8 mg/stroke. While at a fuel pressure of 1740 psi (120 bar) and an injection time of 6.1 ms, the amount of fuel injected is approx. 107 mg/stroke.
In addition to the amount of fuel injected and the injection time, the shape and alignment of the fuel jet is also important.
Retainers For Fuel Injectors
The four retainers -NH- for the two injectors ensure the following:
- Installation as a pre-assembled unit with injectors installed.
- Twist-lock protection for screwing on fuel lines.
- Exact installation position for aligning the fuel jet in the combustion chamber.
- Correct pretensioning of the fuel injectors in the cylinder head.
- Vibration damper for reducing the transmission of vibrations from the cylinder head to the injectors.