Fuel System Monitoring (HFM-System)
Fuel System Monitoring (HFM-System)General Description
Mixture Pilot Control
The air flow sucked in by the engine and the engine speed are measured. These signals are used to calculate an injection signal. This mixture pilot control follows fast load and speed changes.
Lambda-controller
The ECM compares the Oxygen sensor signal upstream the catalyst with a reference value and calculates a correction factor for the pilot control.
Monitoring function description
Adaptive pilot control
Drifts and faults in sensors and actuators of the fuel delivery system as well as unmeasured air leakage influences the pilot control. The controller corrects amplitudes increases. If there are different correction values needed in different load speed ranges, a certain time passes until the correction is complete. The correction values will be determined in two different ranges.
Fuel trim
The basic air/fuel ratio control using the signal from the front O2 sensors(s) is corrected by an adaptation calculation. This adaptation results in a factor which is applicable for the whole working range. A further trim control based on the signal(s) from the rear O2 sensor(s) is correcting the adaptation factor. Therefore this trim control is working in the same way in the whole range. If the trim control reaches the allowed limit the fault code for fuel delivery trim control is set. Any deviation from the characteristic curve of oxygen sensor upstream catalyst due to poison will be detected by the control loop downstream catalyst.
Chart(s) and flowchart(s)
Lambda deviations in range 1 are compensated by an additive correction value multiplied by an engine speed term. This creates an additive correction per time unit.
Lambda deviations in range 2 are compensated by multiplication of a factor.
A combination of all two ranges will be correctly separated and compensated.
Each value is adapted in its corresponding range only. But each adaptive value corrects the pilot control within the whole load/speed range by using a linear interpolation formula. The stored adaptive values are included in the calculation of the pilot control just before the closed loop control is active.
Diagnosis of the fuel delivery system
Faults in the fuel delivery system can occur which cannot be compensated for by the adaptive pilot control.
In this case, the adaptive values exceed a predetermined range.
If the adaptive values exceed their plausible ranges, then the MIL is illuminated and the fault is stored.
Flow Chart Adaptive Pilot Control Monitor:
Flow Chart Trim Control Monitor:
Fuel Pressure Monitoring
General Description
Low fuel pressure system
The low pressure system is the fuel delivery system to the high pressure system. In order to keep the delivery pressure in a certain range the fuel pressure is controlled via fuel pressure sensor. The low pressure system has no direct impact on Air/Fuel mixture and emissions.
High fuel pressure system
The electrical controlled, mechanical-driven high pressure fuel pump provides the injection pressure. The fuel pressure sensor and the control valve in the high pressure system control the high pressure between approximately 3 and 11 MPa. The target pressure depends mainly on engine speed and torque request.
OBD monitors are electrical faults, out of range checks, rationality checks on the low and high pressure systems.
Monitoring function description
A. Strategy description
The pinpointing-strategy is based on the fact that incorrect pressure information will cause Lambda deviations due to the pressure based calculation of the injection time.
Incorrect pressure information can only be caused by positive or negative offset-faults of the high pressure sensor.
However faults of the pump control valve also influence the pressure control activity but they will be completely corrected (Lambda controller deviation ~ 0) through the calculation of the corresponding injection time.
Through combination and evaluation of the following characteristics therefore a differentiation of faults and drifts between sensor and actuator is possible:
- pressure controller deviation
- lambda controller deviation
Pinpointing schematic for High pressure system
B. Explanations
Pressure controller deviation
Depending on deviation between target and actual pressure, the pressure control system becomes active.
Total pressure control output consisting of proportional value, integral value and adaptation value. The total pressure control output influences the high pressure control valve in order to adjust the nominal pressure value.
Lambda controller deviation
Lambda controller deviation determines the factor of the Lambda-controller including the additive and multiplicative correction values of the fuel trim.