Injection regulation, function

S07.10-0056-79

Function requirements for CDI injection control, general points

 Circuit 87 (Engine management ON)

 Engine running.

CDI injection control, general

The CDI control unit (N3/28) allocates the required fuel quantity in each case to the engine.

The CDI control unit calculates the fuel quantity and the characteristic map-dependent injection period and fuel pressure in the rail on the basis of the following sensors and signals:

 Engine oil temperature, engine oil temperature sensor (B1)

 Left hot film mass air flow sensor (B2/6) for suctioned air mass

 Left intake air temperature sensor (B2/6b1) for intake air temperature

 Right hot film mass air flow sensor (B2/7) for the suctioned air mass

 Right intake air temperature sensor for intake air temperature (B2/7b1)

 High pressure fuel pressure sensor (B4/6) for fuel pressure

 Boost pressure sensor (B5/1) boost pressure

 Camshaft Hall sensor (B6/1)

 Coolant temperature sensor (B11/4) for coolant temperature

 Charge air temperature sensor (B17/8) for temperature of the charge air

 Pressure sensor downstream of air filter (B28/5)

 Accelerator pedal module (B37/4) for engine load request from the drive

 Accelerator pedal module for quick or slow accelerator pedal actuation (during acceleration),

 Fuel temperature sensor (B50)

 Crankshaft Hall sensor (B70) for engine speed

 Oxygen sensor upstream of catalytic converter (G3/2)

 Temperature sensor upstream of diesel particulate filter (B19/9)

 DPF differential pressure sensor (B28/8)

 Atmospheric pressure sensor in CDI control unit, atmospheric pressure for altitude adaptation

Function sequence for CDI injection control

The injection control is described in the following steps:

 Function sequence for fuel pressure control

 Function sequence for preinjection

 Function sequence for main injection

 Function sequence for post injection

 Function sequence for injection quantity correction

Function sequence for fuel pressure control

The fuel pressure control is subdivided into the following control strategies:

 Pressure regulating valve regulation

 2-controller concept regulation

 Quantity control valve regulation

Pressure regulating valve regulation

The fuel pressure in the rail is regulated by the pressure regulating valve (Y74) after each engine start. At this time, the quantity control valve (Y94) is deenergized and therefore fully open so that the maximum fuel quantity is delivered to the fuel high-pressure pump.

Pressure regulating valve regulation takes place under the following conditions:

 Up to 5 s in neutral after every manual engine start

 Up to a fuel temperature of 20 °C

 Pressure regulating valve regulation causes the cold fuel to be heated rapidly by the fuel being forced at high pressure through a narrow gap in the pressure regulating valve.

2-controller concept regulation

If the conditions for the pressure regulating valve regulation are no longer fulfilled, the fuel pressure in the rail is jointly regulated by the pressure regulating valve and the quantity control valve at v  12 km/h or in overrun mode at v > 12 km/h.

Quantity control valve regulation

If the conditions for pressure regulating valve control and 2-controller-concept control are not being met, the fuel pressure in the rail is controlled by the quantity control valve.

The pressure regulating valve is closed in a controlled manner and thus performs a retention function.

 The advantage of quantity control valve regulation is that the fuel system high pressure pump only needs to compact the fuel which the quantity control valve lets through to the fuel system high pressure pump as required by the current situation. This relieves the fuel system high pressure pump and reduces fuel consumption. The fuel system high-pressure circuit is unpressurized after switching off the engine.

Function sequence for preinjection

The objective of preinjection is to reduce combustion noise and exhaust gas emissions. This is achieved by injecting fuel one or two times before the main injection starts to make the combustion run smoother.

The start of actuation of the fuel injectors (Y76) for preinjection is calculated by the CDI control unit operating point dependent. In addition, the ambient condition and the engine temperature, the last beginning of actuation of the main injection and the on-board electrical system voltage determined by the on-board electrical system battery (G1) are taken into account.

The preinjection is suppressed by the CDI control unit if one of the following conditions exists:

 Preinjection timing point is exceeded.

 The calculated pre-injection quantity is too low.

 Detected engine speed is too high.

 Main injection quantity is too low.

 Fuel pressure in rail is too low

 The engine is switched off.

Function sequence for main injection

The main injection is performed following preinjection and generates power and torque. Main injection is controlled by the start of actuation (injection timing point) and the actuation duration (injection period).

The main injection is suppressed by the CDI control unit if one of the following conditions exists:

 The fuel temperature limit is exceeded.

 Engine speed > 5000 rpm

 Fuel pressure in rail is too low

 Engine is in overrun mode.

 External interference in the quantity control is present, e.g. as a result of the Electronic Stability Program ESP®

 The engine is switched off.

Function sequence for post injection

The post injection serves to increase the exhaust gas temperature and support the conversion process of the exhaust gas components in the diesel oxidation catalytic converter and the regeneration of the diesel particulate filter (DPF).

For this purpose, the load condition of the DPF is recorded by the differential pressure sensor (DPF). The exhaust temperature is further increased and the regeneration process is triggered by the 2nd post injection. The soot particles in the exhaust gas are then burnt subsequently.

The post injection is suppressed by the CDI control unit if one of the following conditions exists:

 Preinjection timing point is exceeded.

 Detected engine speed is too high.

 The calculated post-injection quantity is too low.

 Main injection quantity is too low.

 Fuel pressure in rail is too low

 The engine is switched off.

Additional function requirements for injection quantity correction

 Engine speed between 1200   and3000 rpm (Decel mode or normal driving operation)

 Engine oil temperature > 80 °C

 Fuel temperature 30 to 50 °C

Function sequence for injection quantity correction

Injection quantity correction is subdivided into 2 subareas:

 Zero quantity calibration

 Quantity mean value adaptation

Zero quantity calibration

Due to friction and the associated wear during opening and closing of the fuel injectors a change in injection quantity takes place over the runtime. This change in injection quantity can be corrected by adaptation of the actuation duration (zero quantity calibration).

This actuation duration corresponds to a defined injection quantity. The difference between the new and nominal actuation duration is used for injection quantity correction.

Zero quantity calibration is conducted for various prescribed injection pressures in deceleration mode. The fuel injectors are actuated one after the other in this phase using a calibration value and the respective engine speed evaluated. If the respective engine speed deviates from the stored specified value then the calibration value is adapted and stored in the CDI control unit.

Quantity mean value adaptation

Quantity mean value adaptation is a teach-in function of the CDI control unit which is used to adjust the exhaust gas recirculation rate so that the emission levels do not deteriorate due to the tolerances of the fuel injectors, the left hot film MAF sensor and the right hot film MAF sensor.

To do this, the CDI control unit reads in the following signals:

 Left hot film mass air flow sensor

 Right hot film mass air flow sensor

 Oxygen sensor upstream of catalytic converter

The CDI control unit changes the exhaust gas recirculation rate via the exhaust gas recirculation positioner (high pressure circuit) (Y27/9) and the throttle valve actuator. The residual oxygen content is monitored by the oxygen sensor upstream of catalytic converter.