Fuel Pressure Regulation
Overview
The fuel system consists of a demand controlled non-return system.
- Connector to fuel injector
- Fuel rail
- Fuel pressure sensor, high pressure side
- Fuel line, high pressure side
- Fuel pump with fuel metering valve, high pressure side
- Fuel pump, low pressure side
- Fuel pump control module
- Fuel line, low pressure side
- Fuel pressure sensor, low pressure side
- Injector
Low pressure side
The fuel pump is powered and governed via the fuel pump control module which receives its control signal as a PWM signal from the Engine Control Module (ECM). A fuel pressure sensor ahead of the high pressure pump measures the pressure on the low pressure side and sends the information to the Engine Control Module (ECM).
The control module calculates a requested fuel pressure. The pressure level varies depending on the engine's operating point and fuel temperature, normally 500 - 530 kPa (absolute pressure) up to 720 kPa at extreme heat. The fuel pump is regulated to this pressure level via pre-control, regulator and adaptation. Pre-control is based on the engine's operating point and the high pressure pump's work. The regulator compensates for the actual deviation from the setpoint pressure and regulates the control to the low pressure pump to obtain the correct pressure. The adaptation takes care of the long-term deviations from the pre-control and thus the system learns to take care of component tolerances and wear.
There are two types of preparatory activations of the fuel pump to obtain the correct pressure before startup.
- Wake-Up is used to raise the pressure before the engine is started by activating the fuel pump until a certain pressure level is reached as soon as the brake or clutch pedal is depressed.
- Pre-Run is used when the start button is pressed and the start sequence starts. this is done to quickly increase the pressure to its setpoint value.
HINT: The system normally maintains good pressure with the engine switched off. The above functions are only activated if the pressure is less than the limit values, i.e. almost never at hot starts.
The fuel temperature in the low pressure system is calculated in a model based on the car's fuel consumption, operating point, coolant temperature and ambient temperature. The fuel temperature is used to regulate the setpoint pressure to prevent steam formation in the fuel.
High pressure side
The high pressure pump is mounted on the vacuum pump housing.
The high pressure pump's pumping action is achieved by a reciprocating piston. The piston rolls against a cam with four lobes. The cam is driven by the exhaust camshaft, which also drives the vacuum pump. One camshaft rotation thus provides four pump strokes.
There is a fuel metering valve on the high pressure pump that regulates the effective pump stroke so that the correct amount of fuel is supplied to the fuel rail. The valve is governed by the Engine Control Module (ECM).
The fuel pressure in the fuel rail is approx. 11 MPa at idle and varies at other loads between 2 MPa to 20 MPa.
If the fuel pressure in the fuel rail exceeds 23 MPa, a safety valve opens in the high pressure pump and the fuel is released back to the low pressure side.