ENGINE | 642.8 in MODEL 222 |
Function requirements for CDI injection regulation, general points
Circuit 87 (engine timing ON)
Engine running
CDI injection regulation, general
The CDI control unit (N3/9) allocates fuel quantities to the engine as required.
Based on a performance map, the CDI control unit calculates the fuel quantity and rail pressure on the basis of the following sensors and signals:
– engine oil temperature sensor (B1)
– left hot film MAF sensor (B2/6), for the aspirated air mass
– left intake air temperature sensor (B2/6b1)
– right hot film MAF sensor (B2/7), for the aspirated air mass
– right intake air temperature sensor (B2/7b1)
– high pressure fuel pressure sensor (B4/6)
– boost pressure sensor (B5/1)
– camshaft Hall sensor (B6/1)
– coolant temperature sensor (B11/4)
– charge air temperature sensor (B17/8)
– pressure sensor downstream of air filter (B28/5)
– accelerator pedal sensor (B37), engine load request from the driver
– accelerator pedal sensor, accelerator pedal actuation fast or slow
(for acceleration)
– fuel temperature sensor (B50)
- crankshaft Hall sensor (B70), engine speed
– oxygen sensor upstream of catalytic converter (G3/2)
- temperature sensor upstream of diesel particulate filter (B19/9)
– temperature sensor upstream of turbocharger (B19/11)
– differential pressure sensor DPF (B28/8), load condition of the diesel particulate filter
– altitude pressure sensor in CDI control unit, atmospheric pressure for altitude adaptation
Function sequence for CDI injection regulation
The injection regulation is described in the following steps:
Function sequence for rail pressure regulation
Function sequence for preinjection
Function sequence for main injection
Function sequence for post injection
Function sequence for injection quantity correction
Function sequence for rail pressure regulation
Rail pressure regulation is subdivided into the following regulation strategies:
Pressure regulating valve (Y74) regulation
2-control concept regulation
Quantity control valve (Y94) regulation
Pressure regulating valve regulation
The rail pressure is regulated by the pressure regulating valve after each engine start. At this time the quantity control valve is de-energized and therefore fully open so that the maximum fuel quantity is delivered to the high-pressure pump.
Pressure regulating valve regulation takes place under the following conditions:
– up to 5 s in idle after each 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-control concept regulation
If the conditions for pressure regulating valve regulation are no longer fulfilled, the rail pressure for v 12 km/h or in decel mode for v > 12 km/h is regulated jointly by the pressure regulating valve and the quantity control valve.
Quantity control valve regulation
If the conditions for the pressure regulating valve regulation and the 2-control-concept-regulation are not fulfilled the rail pressure is regulated 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 high-pressure pump only needs to compact the fuel which the quantity control valve lets through to it as required by the current situation. In this way the high-pressure pump is relieved and the fuel consumption reduced.
The fuel high-pressure circuit is unpressurized after switching off the engine.
During the automatic engine stop a fuel pressure of about 250 bar is maintained in the fuel high-pressure circuit.
Function sequence for preinjection
The objective of preinjection is to reduce combustion noise and exhaust 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 exceeded
- calculated preinjection quantity is too low
- detected engine speed too high
- main injection quantity is too low
- rail pressure is too low
- engine is switched off
Function sequence for main injection
The main injection is performed immediately after 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:
- fuel temperature limit exceeded
– engine speed > 4500 rpm
- rail pressure is too low
- engine is in the deceleration mode
– external intervention in quantity control is active, e.g. by the Electronic Stability Program (ESP)
- engine is switched off
Function sequence for post injection
Post injection serves to increase the exhaust temperature as well as to support the conversion of the exhaust gas components in the oxidation catalytic converter and to support DPF regeneration.
For this purpose, the load condition of the diesel particulate filter DPF is detected by the differential pressure sensor DPF (). The 2nd post injection further increases the exhaust temperature and the regeneration process is triggered. 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 exceeded
- detected engine speed too high
- calculated post injection quantity too low
- main injection quantity is too low
- rail pressure is too low
- engine is switched off
Additional function requirements
Injection quantity correction
Engine speed between 1200 rpm and 3000 rpm (deceleration mode or normal driving mode)
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 time. This change in injection quantity can be corrected by adapting 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 (EGR rate) so that the emission levels do not deteriorate due to the tolerances of the fuel injectors, LH hot film MAF sensor and RH hot film MAF sensor.
To do this, the CDI control unit reads in information from the following sensors:
- left hot film MAF sensor
– right hot film MAF sensor
– oxygen sensor upstream of catalytic converter
The CDI control unit alters the EGR rate via the exhaust gas recirculation actuator (Y279) and the throttle valve actuator (M16/6). The residual oxygen content is monitored by the oxygen sensor upstream of catalytic converter.
Referenced documents
- gf0716p9997ogs Overview of system components for common rail diesel injection (CDI)