Quantity control when idling, function

 The quantity control in the idle function is described using engine 926.9 (6 cylinders) as an example.

G07.15-A018-79

General

The delivery quantity must be constantly adapted to the operating status of the engine when at idle.

Function

If the load take-up from the engine changes, for example, or if there are higher fuel requirements (due to low outside temperatures), the engine management (MR) control unit adjusts the injection quantity (delivery quantity over the delivery angle) within the prescribed limits.

The following input factors also have an influence on calculation of the injection quantity required for idle:

 Engine speed

 Air mass

 Engine temperature

 Fuel temperature

The smoke emission limitation and altitude correction can also influence the injection quantity as well as the start of actuation of the single injection pumps for cylinders 1 to 6 at idle.

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 designed primarily so that the established limits for the air/fuel ratio λ (lambda) are not exceeded or undershot. To do this, the exhaust gas recirculation rate, the start of actuation, and the actuation period (injection quantity) of the single injection pumps for cylinders 1 to 6 are adapted. The engine management (MR) control unit derives the altitude correction via the atmospheric pressure sensor integrated in the engine management (MR) control unit.

Smooth running control

The smooth running control compensates the delivery quantity differences between the individual single injection pumps for cylinders 1 to 6.

Development of torque on the various cylinders can differ due to tolerances and wear of the single injection pumps for cylinders 1 to 6. This leads to rough engine running, which means that the turn rate of the crankshaft will always be slightly faster and slower. One particularly feels this when idling. The smooth running control of the engine management (MR) control unit can react to rough running since it actuates every single injection pump for cylinders 1 to 6 individually. It balances out the rotational speed fluctuations through slightly different injection quantities for the individual cylinder until all cylinders develop the same torque.

Exhaust gas recirculation (EGR)

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 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.

Exhaust gas aftertreatment (only for engines with BlueTEC)

The EATS is active immediately after the engine start during the whole engine operation. It ensures that the emitted emissions in the exhaust are reduced to meet the limits established for the respective emissions standard. A selective catalytic reduction (SCR) is employed here.