The diagram shows the two-module system. On the single-module system the IDM is integrated in the PCM.
1 Pilot injection and main injection fuel quantity
2 IDM
3 PCM
4 Fuel injector
5 CKP
6 APP
In normal driving mode, the injected fuel quantity is calculated from the following main parameters:
โข APP,
โข Engine speed.
In addition, the calculation of the injected fuel quantity is influenced by other parameters (correction parameters), such as: engine temperature and boost pressure.
1 Calculation of accelerator pedal actuation
2 Judder damper
3 Calculation unit
4 Limiter
5 Signal to the injection pump
6 Idle speed calculation
While the engine is running, the PCM uses one of the following two calculations as a basis for fuel metering:
โข idle speed,
โข accelerator pedal actuation.
Both calculations are performed continuously in parallel and independently of each other.
The values calculated from idle speed and accelerator pedal actuation are compared with each other by a calculation unit.
This calculation unit then decides which calculation (idle speed or accelerator pedal actuation) should be used as the output signal for the injection pump. The calculation unit always chooses the larger value for the injected fuel quantity.
Example: Engine cold โ the idle speed calculation yields an idle speed of 1,200 rpm and an injected fuel quantity of 7 mg. The accelerator pedal is pressed by a very small amount, and the accelerator pedal calculation provides an injection quantity of 6 mg. As the value from the accelerator pedal calculation is lower than the result for the idle speed calculation, the idle speed calculation has higher priority. If the accelerator pedal is moved further, and the accelerator pedal calculation provides a higher injected fuel quantity (> 12 mg) than the idle speed calculation as a result, then the accelerator pedal calculation takes priority.
Calculation of fuel metering when speed control system is switched on
Example: The vehicle is traveling in 5th gear at a speed of 100 km/h (62 mph) with an engine speed of 2,500 rpm. Under these conditions, the speed control system is now switched on.
Of the previously mentioned parameters, it is the idle speed calculation that determines the quantity of injected fuel required to maintain the desired speed. This means that the speed in this instance is measured via the idle control system. If load conditions change (for example if driving uphill) the system attempts to maintain the speed accordingly.
Once again, as the accelerator is pressed more, the accelerator pedal calculation assumes a higher priority again. The idle speed calculation resumes its original function until the next time the speed control system is switched on.