Function [2 of 4]

Fuel pressure regulation





General
Fuel pressure regulation for demand controlled fuel pumps (DECOS - DEmand COntrolled fuel Supply) means that the fuel pressure is controlled steplessly by varying the output of the fuel pump. The design of the system allows a greater maximum pressure (approximately 6.5 bar) in the fuel pump. This pressure is used in extreme situations, such as heavy engine load for example.
The following components are used for fuel pressure regulation:
- engine control module (ECM) (4/46)
- fuel pump control module (4/83)
- fuel pressure sensor with fuel temperature sensor (7/156)
- fuel pump with by-pass valve (6/33).
The time taken for the engine start procedure can be reduced by rapidly increasing the pressure in the fuel rail when the engine control module (ECM) receives a signal about the position of the ignition switch from the central electronic module (CEM).
The engine control module (ECM) is better able to calculate the injection period because the signal from the fuel pressure sensor provides information about the fuel pressure and actual fuel temperature. This particular improves the cold starting characteristics of the engine.
The advantages of varying the output of the fuel pump so that it is not always at full power are:
- the total power consumption of the fuel pump (FP) is reduced, reducing the load on the power supply system
- the service life of the fuel pump (FP) is increased
- fuel pump noise is reduced.

Control
The engine control module (ECM) attempts to maintain a fuel pressure of 480 kPa (absolute pressure). A pulse width modulation (PWM) signal from the engine control module (ECM) to the fuel pump (FP) control module requests an increase or decrease in pressure. The fuel pump control module then operates the fuel pump unit to obtain the desired pressure using a pulse width modulation voltage on the ground lead. The fuel pump (FP) can be controlled steplessly by changing the pulse width modulation (PWM) signal. Only that pressure which is required at that specific time will then be released to the fuel rail/injectors. The value of the pulse width modulation (PWM) signal is a measurement of the operational load of the fuel pump (FP) (% duty, 100% = maximum pressure).
The engine control module (ECM) continuously monitors the fuel pressure using the signal from the fuel pressure sensor. This allows the desired fuel pressure to be achieved. If necessary, the signal to the fuel pump control module can be changed so that it corresponds to that required to achieve the requested fuel pressure.

By-pass valve
When the injectors are closed because of too high pressure (during engine braking for example) there is a pressure peak. The by-pass valve in the fuel pump (FP) is used to even out the pressure peak. The opening pressure of the valve is approximately 6.5 bar.
The by-pass valve also functions as a non-return valve, ensuring that the fuel pressure in the system is maintained when the engine is switched off.
There is high pressure before the engine is started. This high pressure means that the valve in the by-pass valve opens and the system is "flushed".

Passive safety
For safety reasons, the engine control module (ECM) shuts off the fuel pump (FP) if the supplemental restraint system module (SRS) detects a collision.

Throttle control




To ensure that the correct throttle angle is reached, the engine control module (ECM) controls the throttle shutter in the throttle unit (6/120), mainly using the signal from:
- accelerator pedal (AP) position sensor (7/51)
- clutch pedal sensor (7/123) via central electronic module (CEM) (4/56)
- stop lamp switch (3/9)
- the throttle position (TP) sensor on the electronic throttle unit (6/120)
- brake pedal sensor (7/124) via brake control module (BCM) (4/16).
Additional signals and parameters are used to ensure optimum throttle control. By example by compensating for:
- the load from the air conditioning (A/C) compressor
- the load from the transmission depending on the selected gear mode (automatic)
- engine coolant temperature (ECT).
In a combustion engine, the difference between the minimum and maximum airflow is considerable. The smaller air flows need more thorough regulation (for example during idle air trim), so the throttle position (TP) sensor signal 1 is amplified approximately 4 times in the engine control module (ECM) before it reaches the Analog/Digital converter in the engine control module (ECM). This means that there are three, two real and one fictitious, input signals available to the engine control module (ECM). These signals are used to determine the position of the throttle disc and to control the throttle motor to the correct throttle angle.
Because the signal is amplified, it reaches its maximum value at approximately a quarter of maximum deployment.
The engine control module (ECM) primarily uses the signal from throttle position (TP) sensor 1 as a measurement of throttle opening. The signal from throttle position (TP) sensor 2 is mainly to check that throttle position (TP) sensor 1 is working. The engine control module (ECM) then uses the signal to calculate a throttle angle (actual value). This is the actual throttle angle. The value for the actual throttle angle is used by those functions in the engine control module (ECM) which depend on this information so that the throttle can be correctly regulated.
There is an adaptation (learning) in the engine control module (ECM) so that the control module can calculate how the damper motor needs to be controlled. See "Adaptation of the electronic throttle unit" below. This adaptation occurs automatically when necessary. The engine control module (ECM) moves the throttle disc to the different positions and reads off and registers the actual values from the throttle position (TP) sensors.
The throttle angle is regulated so that the actual angle (actual value) is the same as the angle calculated by the engine control module (ECM) (desired value). The engine control module (ECM) also uses the values that were stored during adaptation of the throttle angle, and the actual signals from the throttle position (TP) sensor.
The damper motor is deployed by the integrated power stage in the engine control module (ECM) using a pulse width modulation (PWM) signal. The torsion from the opening and return springs in the electronic throttle unit is also used. If the engine control module (ECM) detects a fault in the electronic throttle unit so that the throttle disc cannot be controlled, the springs in the throttle unit will turn the throttle disc to the limp home position (return position). This return position is calibrated to provide a throttle angle large enough to allow the car to be driven to a workshop, although with considerably reduced driveability.

Throttle angle
The engine control module (ECM) also monitors the throttle unit signals from the throttle position (TP) sensors, to ensure that these signals are within the parameters and correspond to the same throttle angle.
If the difference between the signals exceeds a set limit the maximum value is selected as the actual value (which means that the throttle disc is controlled down). If the engine control module (ECM) detects a fault in both throttle position sensors, the electronic throttle unit power stage is switched off. The throttle switches to limp home mode (return position). Diagnostic trouble codes (DTC) are stored in the engine control module (ECM) if faults are detected in the throttle position (TP) sensors.

Adaptation of the electronic throttle unit
Adaptation of the electronic throttle unit is carried out automatically when requested by the engine control module (ECM). Adaptation is carried out to check the function of the electronic throttle unit, and, if necessary, to update the values obtained from the throttle position (TP) sensors etc. during regulation, as these values can change somewhat over the service life of the throttle unit. Diagnostic trouble codes (DTC) are stored in the engine control module (ECM) if faults are detected in the electronic throttle unit.