Exhaust-gas recirculation consists of three functions:
• Setpoint calculation
• Air-mass calculation
• Open-loop or closed-loop control and monitoring
Setpoint calculation provides the setpoint and air mass calculation provides the actual value for open-loop/closed-loop control.
The EGR setpoint is a function of:
• Engine speed
• Injection quantity
• Air temperature
• Water temperature
• Atmospheric pressure
Various characteristic maps are used to compute a setpoint from these values.
The hot-film air-mass flow sensor (HFM) supplies the DDE with a signal which is proportional to the mass air flow rate. The HFM incorporates an air temperature sensor.
The calculated air mass is checked against the engine speed for plausibility.
If an error is detected, the system uses a substitute air mass value derived from the averaged speed on the basis of
a characteristic map.
An air mass value corrected on the basis of boost pressure, atmospheric pressure and air temperature is made available for further computations.
Open-loop or closed-loop control and monitoring
The EGR rate is controlled by specifying the air mass. The controller’s output signal is used for pulse-width-modulated activation of the electropneumatic pressure converter (EPDW).
The EGR valve is monitored for occurrence of the following error states:
• Short circuit to battery voltage (Ub)
• Short circuit to ground or breaks in wiring
Injected-quantity limitation is implemented as a substitute function (see section on injection-rate control).
If two or more errors occur the state with the priority code is stored in memory and the measures appropriate to this state are implemented. In the final analysis this can result in shutdown of the exhaust-gas recirculation system.
Shutdown of the EGR system is always triggered if:
• EGR controller is defective (deviation out of range)
• Engine idles for a prolonged period of time
• Hot-film air-mass sensor is defective
• EGR electropneumatic pressure converter is defective (deviation out of range)
• Boost pressure sensor is defective
• Engine speed is below idle speed
• During engine start
• Cold/hot shutdown
• Engine speed is above upper setpoint
• Car coasting
(cutoff or engine drag moment reduction)