marcjero
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« on: June 27, 2018, 12:59:27 AM »
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Hello,
the car is a 2003 Smart roadster (ecu bosch meg 1.1)
It uses 2 narrowband o2 sensors (1 before and 1 after the kat)
I know that during closed loop STFT and LTFT values are continuously adjusted according to o2 sensor reading.
But what is happening at wot (open loop). Is the LTFT fuel adjustment still applied ?
I wonder this because if LTFT goes very low (let's say about -20%) the engine could run lean at wot without any adjustement.
Is it possible to check this behavior simply monitoring LTFT value at WOT (using an OBD2 tool) : if the LTFT value drops to 0 at WOT I guess this means that the fuel adjustment is actually off. Does it make sense ?
Thanks for sharing.
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« Last Edit: June 27, 2018, 01:02:16 AM by marcjero »
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woj
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« Reply #1 on: June 27, 2018, 02:54:57 AM »
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To the best of my understanding of ME-s, yes, LTFT is applied in open loop (and I know for certain it is on certain other ECU brands and technologies). When you think about it - that's the whole point. To cover for light/mid/high loads, there are two kinds - additive and multiplicative. This is also one of the reasons why you have to have your fuelling sorted out when tuning. It's not uncommon to use LTFT for first step fuel tuning, and some folks go even more nuts - use LTFT to adjust fuelling for Ethanol based fuel mixtures.
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marcjero
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« Reply #2 on: June 27, 2018, 07:42:48 AM »
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use LTFT to adjust fuelling for Ethanol based fuel mixtures Yes I planned to do this as well. Why do you think it's wrong ? |
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woj
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« Reply #3 on: June 27, 2018, 08:03:37 AM »
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Apart from the fact that the fuel alone does not make up for a proper Ethanol tune, the typical operation range for LTFT on most ECUS is +/- 25%, to run E85 you need ~+40%, for E100 more than 50%, you do the math. Also, when you hit the limit of LTFT you will get a DTC most likely.
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marcjero
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« Reply #4 on: June 27, 2018, 08:18:27 AM »
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Except maybe if you calibrate the injectors in order to have about -16% trim with fuel and +16% with E85.
It won't be optimal however.
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woj
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« Reply #5 on: June 27, 2018, 08:45:33 AM »
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That I have not thought of to be honest, nice one. The absolute range is indeed 50%, which might just serve the need.
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marcjero
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« Reply #6 on: June 27, 2018, 02:01:27 PM »
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I will require larger injectors and I think I can calibrate the new injectors with the FGAT0 table.
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overspeed
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« Reply #7 on: June 29, 2018, 06:08:58 AM »
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some ECU has more than one LTFT based on load and RPM (lets say something like low rpm- low load, low rpm-high load and high rpm-low load, high rpm-high load style).
as LTFT are there to deal with aging (and leaking) of entire engine operation (considering the original file is "perfect" for a new engine with sensors) they are aplied of entire operation for the particular learned load, rpm.
Marelli SFS (software flexible system)is something like this, Brazil has ethanol E100 and E27, ECU is calibrated with E67 from its base, everytime you start engine with a diference greater than 20 liters on tank than before (it has some other triggers like activit on knock sensor with rich moisture over certain point, os number of non-sucess start) it start adjusting K factor (LTFT aplied on entire range), when K factor stays stable (again a bunch of triggers) it start working like a regular ECU, but LTFT limit is lower than 25% (can´t remember exactly the value) if LTFT is great than this threshold and no activity on knock ECU start K factor again as it shold be with much ethanol on moisture.
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woj
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« Reply #8 on: June 29, 2018, 06:42:25 AM »
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Aha, so that's how it works. I have seen it mentioned elsewhere here (most likely by you even), but there were no details. The only problem I can see (especially in cold climates), that before the K factor is adjusted and you filled the "opposite" fuel, the engine runs like crap before the lambda correction kicks in properly. Also, I can imagine, a WBO would be much better solution in this case to get the ball-park of the correction quicker.
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overspeed
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« Reply #9 on: June 29, 2018, 12:04:48 PM »
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And you are absolute rigth about this "problem", in the begining the strategy was incomplete and we had a lot of problens when customer for example, didn´t run time enough to validate oxygen probe and begin K calculation when filling another fuel blend .
Now there is some variants, for example if you have some unsucessful cold starts it will not look at actual K and start trying new substitute values.
Another thing, cold start with E100 will not fire all the injectors, it will spin the engine before to gain RPM and start injection one by one
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