1.6.5.1 Monitoring function
For gasoline direct fuel injection a high pressure fuel control system is necessary for fuel preparation and metering as shown in Fig 1 below. The low fuel pressure from the fuel pump module within the tank is increased by the high pressure fuel pump and adjusted to a desired set-point fuel pressure.
The high pressure fuel system consists of a common fuel rail for all high pressure Piezo-injection valves, a fuel rail pressure sensor, a high pressure fuel pump with a built-in fuel volume control valve and overpressure-valve.
In dependence of engine load and engine speed, high pressure has to be adjusted to values between 5000 and 20000 kPa. Therefore the fuel pressure in the rail is measured and controlled with help of the fuel volume control valve. According to the desired fuel-mass and fuel pressure set-point value the pre-control calculates the driver-signal for the fuel volume control valve. This calculated driver-signal is additionally controlled by a closed loop control using the measured fuel pressure and the desired set-point value as input.
The high pressure system diagnosis is a rationality check. It compares the difference between the measured fuel pressure and the set-point fuel pressure. The diagnosis finds out whether the set-point value of the fuel rail pressure can be adjusted by the high pressure fuel control.
A not adjustable fuel rail pressure (too high) is detected if the measured fuel pressure is greater than the desired set-point fuel pressure with the result that the difference of these two values (set-point - measured) is negative. If the negative difference falls below a calibrated threshold for a calibrated period of time, a malfunction is detected and a maximum fault High Pressure System monitoring is set.
A not adjustable fuel rail pressure (too low) is detected if the measured fuel pressure is less than the desired set-point fuel pressure with the result that the difference of these two values is positive. If the positive difference exceeds a calibrated threshold for a calibrated period of time, a malfunction is detected and a minimum fault High Pressure System monitoring is set.
The fuel mass plausibility diagnosis checks the plausibility of the high pressure sensor signal.
The diagnosis compares the output of the lambda-controller and lambda-adaptation with the output of the closed-loop controller.
A too high pressure sensor signal is detected if the lambda-controller or the lambda-adaptation shows a rich combustion while the fuel pressure controller is below a calibrated threshold.
A too low pressure signal is detected if the lambda-controller or the lambda-adaptation shows a lean combustion while the fuel pressure controller is above a calibrated threshold.