Idle speed control, function

W07.16-1059-79

 The idle speed is parameterized in the engine management (MCM) control unit (A4). To be able to operate ancillaries at their working speed, the common powertrain controller (CPC) (A3) can however transmit a separate request via the drive CAN (CAN 4) if the idle speed should, for example, be higher.

Essentially, the higher engine speed is 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 engine 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.

Taking into account the idle speed input from the common powertrain controller (CPC) (A3) and the current engine operating status, the motor control module (MCM) (A4) calculates the optimal start for fuel injector control. In addition, it calculates the optimal injector control duration (injection quantity) and causes the fuel to be injected at the right point in time and in the correct quantity into the respective combustion chamber via a corresponding actuation of the quantity control valve (Y642) and fuel injectors for

cylinders 1 to 6 (Y608 to Y613).

During the entire idle mode, the injection quantity is constantly adjusted to the engine operating status. For example, if the engine load condition changes or a higher fuel requirement exists (due to low outside temperatures).

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 individual fuel injectors.

Development of torque on the various cylinders can differ due to tolerances and wear on the individual fuel injectors. This results in an irregular engine running, i.e. the turn rate of the crankshaft becomes 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

Altitude correction is primarily designed in such a way that the rotor speed, the compressor outlet temperature, and the turbine inlet temperature of the exhaust gas turbocharger as well as the air/fuel ratio λ (lambda) do not exceed or undershoot defined limits. To do this, the exhaust gas recirculation rate (EGR 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.

In order to reduce nitrous oxide emissions, the exhaust gas recirculation (EGR) is active for a certain period of time during idling mode (the EGR 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 (EGR), 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.