Fuel Pressure Sensor/Switch: Description and Operation
Fuel High-pressure System In DFI Flat-six Engines
The fuel high-pressure system generates the injection pressure of 580 psi to 1740 psi (40 to 120 bar). The fuel is distributed to the high-pressure injectors by the high-pressure pump via the high-pressure lines to the fuel rails of cylinder banks 1 and 2. The following pages describe the functions of the components for the Carrera 3.6- and 3.8-liter DFI engines.
The fuel high-pressure system of the DFI flat-six engines comprises the following components:
NOTE: Observe the instructions in the Technical Manual and perform a tightness test when fastening all fuel lines to the fuel high-pressure system.
The illustration depicts the high-pressure pump (HD) with the structural protection. The flow control valve (MS), the temperature compensator and the pressure control valve are integrated into the high-pressure pump.
Control and temperature compensation
Depending on the amount of fuel required and the calculated 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. In the DFI flat-six engines, the fuel temperature on the low-pressure side (ahead of the high-pressure pump) is calculated by a temperature model in the DME control unit. The following information is included in the model: outside temperature, temperature of the flow control valve, operating duration, load point, tank filling level, idle period prior to vehicle start.
Fuel high-pressure pump
The fuel high-pressure pump provides the amount of fuel required for injection and the fuel pressure from 580 psi to 1740 psi (40 to 120 bar). The axial-piston pump is driven by the exhaust camshaft of cylinder bank 1. The high-pressure pump is a three-piston pump with a maximum delivery rate of approx. 47.5 gph (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 size of approx. 50 microns.
Flow control valve for fuel high pressure
The flow control valve is an electric control valve and is located on the intake side (low-pressure side) of the high-pressure pump. The DME control unit activates the flow control valve using a current of 0 to approx. 2 amperes, resulting in flow control from 580 psi to 1740 psi (40 to 120 bar) by the high-pressure control system. When the engine is switched off, the fuel high pressure is reduced by an integrated pressure-reducing valve. The (monitoring) measurement of the fuel pressure is performed by the fuel pressure sensor (approx. 580 psi to 1740 psi/40 to 120 bar).
If the control valve fails, the DME control unit goes into emergency mode, whereby the engine can still operate in a limited manner with low pressure (approx. 5.0 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 Check Engine light is activated.
The bypass valve is also activated when the empty fuel rail is filled 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).
Fuel high-pressure control
The fuel is sucked in from the low-pressure side (5.0 bar) by the pump pistons via the flow control valve and the intake valves. The pistons pump the fuel into the high-pressure system via the exhaust valves. The pressure relief and bypass valve has two functions. It protects the system against excessively high pressure and relieves the pressure from the high-pressure side when the engine is switched off.
The flow control valve has specific flow tolerances between the valves due to its design. For this reason, the characteristic while driving is adjusted to the respective valve by the high-pressure adaptation ranges 1 to 5. The DME control unit activates the flow control valve using a current of 0 to approx. 2 amperes. This maintains the fuel pressure at between 580 psi to 1740 psi (40 to 120 bar) when the engine is running.
NOTE: After replacement of the high-pressure pump (with flow control valve), the adaptation values must be reset and an adaptation test drive carried out in order to guarantee control stability. Observe the corresponding information according to the Technical Manual.
The fuel high-pressure system normally operates in flow control mode.
Pressure control is activated only when the fuel flow rates are very low near idle speed.
- Engine at idle speed
- High oil and coolant temperature
- High fuel share from tank ventilation
- Low engine load (AC off, low vehicle electrical system load)
The graph shows the characteristic spread, which is balanced out by the high-pressure adaptations 1 to 5. The respective adaptation range is 0 ± 25%.
High-pressure lines
The fuel is pumped by the high-pressure pump to the fuel rails for cylinder banks 1 and 2 via the high-pressure lines.
High-pressure rail for bank 1 and bank 2
Two high-pressure rails are used in the DFI flat-six engines (depending on the type). The fuel injectors for banks 1 and 2 are fastened to the cylinder head over the high-pressure rails, which are located under the intake system. The high-pressure rails provide the same fuel pressure to all fuel injectors. The volume of the high-pressure rails is adapted according to the amount of fuel the engine needs. It is approx 100 cubic centimeters for the 997 DFI engines.
The rail volume is determined by the permissible pressure variation behavior and the shortness of the starting time. The volume and the chokes in the high-pressure system attenuate the pressure fluctuations, but they delay attainment of the release pressure for injection when the engine is started.
Illustration of fuel rail for bank 2
Fuel pressure sensor
The fuel pressure sensor is fitted on the high-pressure rail for bank 2 and informs the DME control unit about the current pressure in the fuel high-pressure system. The DME control unit evaluates this input signal and regulates the fuel quantity on the high-pressure side via the flow control valve.
Fuel injectors/DFI high-pressure injectors
The electromagnetically operated fuel injectors are on the intake side of the cylinder heads. 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 to 1740 psi (40 to 120 bar). The fuel is given a spin even before it leaves the tip of the fuel injector (shown below) and is injected in a finely atomised, conical jet. The connection to the rails is via an O-ring connection.
According on the engine operating state (start, idling, partial load, wide open throttle), the quantity of fuel injected is distributed over one, two or three injections in the rpm range below approx. 3,200 rpm, depending on the operating point. This supports mixture preparation and therefore the engine operating behavior. When the engine is started from a cold state, some of the fuel is injected only immediately prior to ignition. This reduces the fuel quantity required for starting. Injection is released once a defined, temperature-dependent pressure has been reached (approx. 507 to 1450 psi/35 to 100 bar).
Fuel injectors
The 3.6- and 3.8-liter engines are fitted with different injectors that are designed specifically to suit the engines' respective fuel requirements. The injectors can be differentiated by their part numbers and by a color marking.
- Injection quantity at 580 psi (40 bar) fuel pressure and 0.6 ms injection time: approx 7.5 mg/stroke.
- Injection quantity at 1740 psi (120 bar) fuel pressure and 5.2 ms injection time: approx 80 mg/stroke.
- Injector-needle stroke: approx 50 microns
- The characteristic drop diameter is approx. 30 microns
- Jet-cone angle: approx. 69°
- Bend angle: approx. 15
In addition to the amount of fuel injected, the injection time and the drop size, the shape and alignment of the fuel jet are also important.
NOTE: A defective injector is detected by the misfire detection system and is not activated again