Wall wetting model for accel enrichment / cold start etc.

[Davidv]

Hey,

I think this might have been suggested before. But once you start to understand the wall wetting concept, it starts to explain a lot of other concepts which are described seperately in the current software. 

Example - accel enrichment compensates for changes in wall wetting, but in a fairly crude manner. 

Cold start cranking enrichment is essentially compensating for fuel film build up over the initial cranking period on a cold engine, and the long time period of "tau" on a cold motor.

Fuel cut on decel doesnt really go into this, but if there was awareness of the fuel film depleting during this period then "accel enrichment" would naturally increase again after decel fuel cut. 

It feels like if this was modelled many of the other enrichment features etc could be done away with or minimised. 

Here's a great article on it.

http://www.megamanual.com/ms2/xtau.htm

 

[mapper]

Hi agree that there is some space for improvements on Accel enrich. 

If you enrich enought that you don't see a lean spike at the begining of rappid TP change, AFR's goes usually to low. Even if you set decay rate to 100% and enrich only on one event. 

I can only explain that with fuel film which is built up during extra accel enrichment injection which is then sucked aways on the follow engine cycles, because flow goes up with RPM. 

Some kind of asynchron enrichment may help to built up a fuel film. Or a full fuel film modell. 

[mapper]

Another thing which I would like is a 3D Accel Cold correction table so it can be spanned with Engine running time or charge temperatur or a modelled intake manifold temp. 

The reason is simple. Just after cold startup, it needs much more accel enrichment, than after 1-3min running time of the engine.  From a tuning perspective, you need to add a lot of accel enrichment just after the start of the engine so throttle response is good. But after 1-3min running ECT is still similar, but manifold ports have got some temp into it. Now the big Accel enrichment just flushes the cylinders and oil film down.