Idle Airflow Operation


General Operation
The PCM calculates the IAC position based on a number of Airflow calculations and estimations, the final idle airflow value consists of the following two main components:
- Base Idle Airflow (Base + LTIT)
- Adaptive Airflow correction (STIT)

LTIT = Long Term Idle Trim
STIT = Short Term Idle Trim

Note: i just made these names up cos they are easier to understand than proportional blah blah, integrator slow filtered idle airflow...

The first thing to realise is that the PCM only runs the adaptive idle control routines when at idle conditions (below certain TPS and MPH limits). However, the base airflow routine is controlling the IAC position during all driving conditions (things you are aware of already, such as the Throttle Cracker, and if you set your Base Running Airflow values too high you get cruise control etc.)

Base Idle Airflow
The Base Idle Airflow is combination of looked up values from various tables within the PCM and also incorporates a Long Term Idle Trim (LTIT) correction. This airflow directly controls the IAC position when at non-idle and is the "starting point" for adaptive idle control. The base airflow consists of the sum of the following individual components:

Base Running Airflow
- this is the main Idle Airflow when in PN (A4 only) or Gear (A4 or M6)
- a table vs ECT

Startup Airflow
- additional airflow during engine startup and initial run period (decays to zero in the first few seconds of engine operation)
- a table and a few delays and decay rates

Startup Spark Retard Airflow
- airflow correction to account for startup spark retard (if used)

Fans On Airflow
- Additional airflow to account for increased engine load during cooling fan operation
- Two values depedending if one fan active or both active.

DFCO Airflow
- used to set IAC position during DFCO

Throttle Cracker Airflow
- additional airflow to open the IAC based on MPH and RPM
- zero duing idle conditions
- a table

Throttle Follower Airflow
- controls rate of closing the IAC valve during throttle closure
- zero during idle conditions
- a few tables of initial value and decay rates

Long Term Idle Trim Airflow (LTIT)
- a slow moving correction based on the adaptive idle routines (think LTFT's for fuel)
- the idea of this correction is to bring the Short Term Idle Trims (STIT) to zero
- it has +ve and -ve limits
- a calculated value

AC Airflow
- airflow correction for when the AC is on, this is a torque based calculation that estimates how much torque the AC is pulling and calculates an airflow correction to compensate.

IAC Park Airflow
- airflow used to calculate IAC position when ignition is off and engine not running
- used in place of all of the ablove
- a table

The combination of all these components is what i call the Base Idle Airflow in grams/sec.


Adaptive Idle Control
The whole point of the idle control routines is to maintain the desired Idle RPM. The PCM therefore needs to "close the loop" and use the Idle RPM error as a feedback to provide this control. The monitoring of the Idle RPM results in a Short Term Idle Trim (STIT) that provides the fast moving closed loop control of the IAC valve. Again here it is very analagous to the STFT's and feedback from the O2 sensors. Thats why i chose these names rather than the normal Proportional, Integral, Derivate stuff that makes most peoples head spin...

Okay, so the PCM has a Desired Idle RPM it is trying to achieve and it is constantly measuring the current RPM and calculating an Idle RPM error value. The PCM uses various agressive and not so agressive algorithms to control the STIT, to provide fast convergence (and also stall saver capability) but also reasonable idle stability.

During all this, the PCM is maintaining a fairly complex state machine of are we at idle, is the engine transitioning back to idle, etc. The PCM does remember a few different last known state of the STIT, such as when you turn the AC on the PCM stops updating the "ACoff STIT" and starts updating the "ACon STIT" (again here think Fuel Trim cells). The idea of this is that when you turn the AC off the PCM can quickly return to the original IAC operating point. For A4 vehicles you also have the PN/Gear dimension as well.

A good example of the STIT in action is if you have an M6, you have your foot on the brake and you partially let the clutch out and you feel the engine pull harder to try and maintain the desired idle RPM. If you were logging the IAC steps or the desired idle airflow you would see it increase. Very soon with our scanner you will be able to log the LTIT and STITs directly and see them correcting. Monitoring the LTIT and STIT is a very good tool to get your Base Running Airflow values correct, ensuring your LTITs are not maxing out on the limits and troubeshooting PN/Gear and Fan On/Off stumble etc. especially after head/cam install.


Desired Idle Airflow
So the net result, is that the PCM takes the Base Idle Airflow (including LTIT) and then adds the STIT to come up with a final Desired Idle Airflow (which generally is available as a PID for logging). Then there is a final step that takes this airflow value and translates it to the actual IAC valve position or the ETC TPS position. Maybe more on this later, but it's basically just a units transformation for the most part and the IAC and the ETC have their own control routines and state machines that effectively take this idle airflow as an input.


What else?
In addition to the Idle Airflow routines the PCM also has an RPM based idle spark correction "closed loop" operation that it uses to control the idle RPM. Since the spark advance can move much faster than the IAC, it can provide very fine control of idle speed. When logging you will see this as a jagged spark advance chart, most noticable with cams at lower idle RPMs where the spark advance generally oscillates between it's min/max allowed values as the engine "cams" at idle.