Idle speed control, function

G07.16-3144-79

 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 CAN (CAN 4), e. g. if the idle speed should be higher in order to operate ancillary equipment at its working speed. 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.

While taking account of the idle speed specification from the drive control (CPC) control unit (A3) and the current operating status of the engine, the engine management (MCM) control unit (A4) calculates the optimum 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 quantity into the respective combustion chamber through appropriate actuation of the quantity control valve (Y642) and the fuel injectors for cylinders 1 to 6 (Y608 to Y613).

The injection quantity is constantly adjusted to the operating status 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 low outside temperatures).

Smooth running control

The smooth running control compensates for the differences in injection quantity between the fuel injectors for cylinders 1 to 6 (Y608 to Y613).

Development of torque on the various cylinders can differ due to tolerances and wear on the fuel injectors for cylinders 1 to 6 (Y608 to Y613). This leads to rough engine running, which means that the speed at which the crankshaft rotates will always be slightly faster and slower. One particularly feels this when idling. The smooth running control can respond to rough running since it actuates the fuel injectors for cylinders 1 to 6 (Y608 to Y613) individually. It balances out the rotational speed fluctuations through slightly different injection quantities for the individual cylinder until all cylinders develop the same torque.

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.

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.

Altitude correction

The altitude correction is primarily designed in such a way that the turbine speed, the compressor outlet temperature and the turbine inlet temperature of the ATL, as well as the air/fuel ratio λ (Lambda) does not exceed or drop below established 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.