Discussion and questions related to the course Flex Fuel Tuning
Looking to confirm my suspicions, was talking flex tuning the other day specifically with the elite ecu's and was told that you cannot use 02 control and long term trims with an engine running a flex tune (with the flex sensor obviously).
now my understanding was you could, you may need a table that trims your Target O2 relative to ethanol content but other wise it will work.
his rebuttal was long term trim affect the main fuel table, which is tuned to be correct to suit several ethanol %'s, it cannot how ever tune your ethanol Scalar and correction.
so pretty much can you use a long term trim on a flex tuned car or will it simple be for ever trimming the main map with the varying ethanol content?
Can you give more background and context to the discussion? What engine do you have, what are the goals, what has been tuned on it so far? Who were you talking to, what is this person's expertise and experience?
discussion was with a local full time tuner.
engine was a 3sgte Forged with a gtx2867 and haltech elite.
pretty much after tuning the car he mentioned O2 control and long term trims cant be used on any flex tuned car. due to the ecu not being able to determine if the trim needed is because of the main fuel map being off or the ethanol content map being off.
I'm looking at the Haltech Elite software right now and there doesn't seem to be anything preventing you from running long term fuel trim and flex fuel at the same time. If somebody here can find it, please chime in.
What ethanol content map is he referring to? If you go through the help files, there are three main tables:
Fuel Composition Corr
This table allows for corrections to be made where the automatic calculation is not correct. The default for this table is to have all numbers as “0.0” and to adjust as required.
Fuel Composition Scalar
This table allows for adjustment of how the automatic correction is applied over changing ethanol content.
Fuel Composition Corr
This table allows for the Target Lambda to be changed with varying amounts of ethanol. Useful when closed-loop O2 Control is being used and the target is to be altered with changing ethanol.
So unless I misunderstand something, there is no direct interaction between long term fuel trim and ethanol. The ethanol content is read directly from a sensor; it's not estimated by fuel trims like most factory flex fuel vehicles are today. Now, maybe as part of his tuning work flow he is adjusting the fuel compensation correction table manually to reduce the fuel trims.
I don't see any reason why you can't install the flex fuel sensor, leave the target lambda compensation table alone (target same lambda regardless of ethanol %) or even adjust the target based on ethanol. Then just leave the fuel compensation tables alone and let the fuel trims handle it, including long term fuel trim. The ethanol content is directly measured. It's going to be accurate enough.
What's he's probably running into is the fact that wall film compensation (tip in fueling, X-tau, fuel film model/puddle model, whatever you want to call it) is different between E85 and E10. The Haltech Elite doesn't compensate for that. I can't think of any standalone that has ethanol based wall film compensation, but OEM flex fuel systems do (different Tip in maps based on Ethanol content). So he's probably noticing the inaccurate fueling with the E85 during transients and band aiding it with the compensation tables or VE table or whatever.
Also, as a general response to your point: he is right that it can be hard to determine whether an engine is lean because the ethanol content is actually higher or if there is something else going on. However, the whole point of the ethanol sensor is to tell you what the ethanol content is. On OEM systems that do not use a flex fuel sensor, they use three levels of fuel trim: short term, long term, and calculated ethanol % (which is basically just another fuel trim that increments based on long term fuel trim and refueling judgments). In those systems it is possible to have the control system get confused and start erroneously learning a different ethanol %, but that's why they have specific ethanol learning windows.
So for example, one system I worked on would use the fuel level sensor and a timer to set a refueling judgment flag (bit goes from to 0 to 1). When the refueling judgment is active, a timer and other conditions such as speed and engine load count start counting down the ethanol learning window. During that time, the ECU uses the long term fuel trim to calculate the ethanol percentage. Then, once the timer is up, the ethanol calculation isn't allowed to learn again until another refueling judgment. Using that approach false ethanol learning is avoided; large fuel trims are assumed to be the result of some other problem, and if the long term fuel trims stay too large for too long, the engine throws a code.