Idle speed control, function

W07.16-1140-79

 The idle speed is parameterized in the engine management (MCM) control unit (A4). The drive control (CPC) control unit (A3) can, however, send its own request via the drive train CAN (CAN 4), for example if the idle speed should be higher in order to operate ancillary assemblies at their working speed. The higher rotational speed is basically always regulated. In other words, if the idle speed required by the drive control (CPC) control unit (A3) is higher than the idle speed parameterized in the engine management (MCM) control unit (A4), then the speed required by the drive control (CPC) control unit (A3) is regulated and vice versa.

The idle speed is regulated immediately after the starting procedure or if the drive control (CPC) control unit (A3) recognizes that the accelerator pedal is not being actuated. A3A4Y608Y613While taking account of the idle speed specification from the drive control (CPC) control unit () and the current operating condition of the engine, the engine management (MCM) control unit () calculates the optimal start of actuation for the fuel injectors as well as the optimum actuation period (injection quantity) while also ensuring that the fuel is injected at the correct point in time, and in the correct quantities into the respective combustion chamber through appropriate actuation of the fuel injectors for cylinder 1 to 6 ( to ). Through appropriate actuation of the quantity control valve (Y643) one ensures that an adequate amount of fuel is available at the fuel system high pressure pump. The pressure regulating valve (Y634) is actuated in such a way that the pressure in the rail is about 900 bar. This ensures smooth engine running. The injection quantity is constantly adjusted to the operating condition of the engine during the whole idling operation - e.g. if the load condition of the engine changes or a higher fuel requirement exists (due to a low outside temperature).

To calculate the injection quantity, the engine management (MCM) control unit (A4) mainly takes account of the following input factors:

 Engine speed

 Coolant temperature

 Fuel temperature

 Load request (specified torque)

Smoke limitation, smooth running control and altitude correction in the engine management (MCM) control unit (A4) can also influence the injection quantity (period of actuation) as well as the start of actuation for the fuel injectors.

Smoke limit

The fuel quantity is limited via the smoke limitation characteristics so that virtually no output of smoke occurs.

Smooth running control

The smooth running control compensates for all of the differences in injection quantity in the different fuel injectors. Development of torque on the various cylinders can differ due to tolerances and wear on the individual fuel injectors. This leads to irregular running of the engine, that is the rotational speed of the crankshaft is slightly faster and slower. One particularly feels this when idling. The smooth running control can respond to rough running because it actuates every injection nozzle individually. It balances out the rotational speed fluctuations through slightly different injection quantities for the individual cylinder until all cylinders develop the same torque.

Altitude correction

The altitude correction is primarily designed so that the limits established for the air/fuel ratio λ (lambda) are not exceeded or gone below. To do this, the exhaust gas recirculation rate (AGR rate), the actuation begin as well as the actuation period (injection quantity) for fuel injectors for cylinder 1 to 6 (Y608 to Y613) are adapted.

Exhaust gas recirculation (EGR)

In order to reduce nitrous oxide emissions, the exhaust gas recirculation (AGR) is active for a certain period of time during idling mode (the AGR is switched off for longer periods in idling mode so that the exhaust gas recirculation cooler is not sooted up).

In the case of the exhaust gas recirculation (AGR), the suctioned out or supercharged fresh air is mixed with exhaust gas. This leads to the situation where the oxygen content of the suctioned out or supercharged air drops and its specific heat increases. One can therefore achieve a lower combustion temperature and thus reduced output of nitrogen oxides (NOx), whose proportion increases with increasing combustion temperatures. The amount of exhaust gases produced is also reduced.

Fan control

During the whole operation of the engine, that is also in the idle phase, the fan speed is controlled dependent on the coolant temperature. The fan speed is determined from the input signals (A) of the fan rpm sensor (Y616 b1). Control takes place through appropriate actuation of the fan clutch solenoid valve (Y616).

Exhaust aftertreatment

The EATS is active immediately after the engine start during the whole operation of the engine. It ensures that the emissions in the exhaust are reduced down to the limits established for the respective emissions standard. Thus different measures are taken according to the emissions standard. For the engine version EURO VI exhaust aftertreatment takes place, for example, through:

 Selective Catalytic Reduction (SCR) with an ammonia locking CAT

 Diesel oxidation catalytic converter (DOC)

 Diesel particulate filter (DPF)

Camshaft adjustment

Through shifting the timing of the exhaust camshaft for low engine rpm one can ensure that the temperature of the exhaust is increased, which improves regeneration of the diesel particulate filter.