The result of injection-rate calculation when the car is on the move is the total quantity of fuel to be injected into each individual cylinder.
The required quantity is basically determined by the maximum from the driver’s request and the cruise control. Under certain circumstances, engine-speed-dependent injection-rate limitation can take effect at this stage.
As regards external intervention in the injection-rate control process, signals influencing the injection rate and carried by the CAN bus can limit or increase the optimum injection quantity.
The idle-actuator injection rate is included in the calculation and the smooth-run controller adds a cylinder-specific correction to compensate for tolerances and ensure smooth engine operation.
Allowance for injection-rate drift (DIS service functions, DDE)
The tester can be used to compensate for the drift in injection rate caused by wear and tear on the injection components over their service life. A certain quantity is either added to or subtracted from the drift characteristic, as applicable.
Note, however, that the values in the electronic control unit must not be changed except in accordance with instructions issued through the hotline service or in Service Information.
Overrun fuel cut-off
If a certain time expires without registration of a load request (requested fuel injection = 0), the injection quantity is ramped down to zero.
The manipulated variable output for the injection quantity is zero until coasting ceases.
Start-quantity calculation supplies the calculated injection quantity for the engine starting procedure. This quantity makes no provision for driver’s commands or cylinder-specific corrections and its sole purpose is to ensure optimum cold starting.
The engine start phase ends when engine speed exceeds a cranking speed which depends on the coolant temperature.
Pre-supply pressure monitoring
The fuel pressure in the line leading to the high pressure pump is registered by the pre-supply pressure sensor.
The supply pressure setpoint for the unaffected low pressure system is determined as a function of the average engine speed.
A differential pressure value is then obtained by subtracting the current pre-supply pressure.
This differential pressure is plotted against speed in a characteristic map to obtain a limitation for the injection quantity. This limitation is not applied unless the pre-supply pressure is below the setpoint.
By way of compensating insufficient pre-supply pressure, more fuel has to be made available at the high pressure pump. This increase is achieved by reducing the injection quantity. The result of reducing the injection quantity would be an increase in rail pressure at constant speed. The pressure control valve, however, opens the fuel return to a greater extent and more fuel flows back in the direction of the tank. More fuel is now available at the distributor and can be drawn off by the booster pump. As a result, the presupply pressure also increases and the pressure