kenamond
DSM Wiseman
- 3,226
- 62
- Feb 15, 2006
-
Los Alamos,
New_Mexico
I'm bored, so...LTFT and STFT:
The ECU has a closed-loop and open-loop mode.
The ECU has a fuel map. This tells it how much fuel to injecto for every combination of airflow and rpm. The MAF and CPS tell the ECU the airflow and rpm constantly, so it uses these to determine the base fuel to inject.
When the car is in open-loop mode (WOT), it injects fuel from the map and that's it.
When the load is lower, it is in closed-loop mode. In this mode, the ECU uses the front O2 sensor to stay right at a stoichiometric AFR (14.7). The O2 sensor will read 0.5v at stoich. The ECU looks for an alternating signal that oscillates above and below 0.5v. If the O2v stays above 0.5v, the car is running richer than stoich and the ECU notices this. If it stays below 0.5v, the car is running leaner than stoich. The ECU uses a short-term fuel trim (STFT) to make small adjustments to the fuel read from the base fuel map. If the O2v is rich, the STFT will store a number that tells the ECU to remove fuel and how much to remove. The ECU is constantly using this STFT to tweak the AFR in closed loop mode. If it gets it right, the O2v will go back to oscillating around 0.5v. If conditions are right, the ECU enters learn mode. This is where it memorizes the STFT values by storing them in a long-term fuel trim (LTFT). I've heard there are two trims from some and three trims from others. I don't know for sure. Anyway, there may be a LTFT-Lo, LTFT-Mid, and LTFT-Hi. If airflow is low, it stores the STFT value in the LTFT-Lo, etc. After it memorizes the STFT value, it resets it.
So in closed-loop mode, the ECU reads the fuel map, then adds in the STFT+LTFT corrections and injects. It monitors the O2v signal and corrects via the STFT. When it enters learn mode, it stores the STFT into the appropriate LTFT. Repeat. Typically, the LTFT values will stabilize and the STFT will hang out at the reset value (the ECU has "tuned itself". But that's only for closed-loop and learn modes. WOT/open-loop mode is blind map reading as far as fuel injection goes.
I'm not sure what triggers the ECU (1g or 2g) to enter learn mode. I'm under the impression that it's usually in learn mode during cruise/idle, but I don't know any of the specifics.
If anyone wants to add to or correct what I said, fire away. I think it's close though and should give you a good idea about trims.
The ECU has a closed-loop and open-loop mode.
The ECU has a fuel map. This tells it how much fuel to injecto for every combination of airflow and rpm. The MAF and CPS tell the ECU the airflow and rpm constantly, so it uses these to determine the base fuel to inject.
When the car is in open-loop mode (WOT), it injects fuel from the map and that's it.
When the load is lower, it is in closed-loop mode. In this mode, the ECU uses the front O2 sensor to stay right at a stoichiometric AFR (14.7). The O2 sensor will read 0.5v at stoich. The ECU looks for an alternating signal that oscillates above and below 0.5v. If the O2v stays above 0.5v, the car is running richer than stoich and the ECU notices this. If it stays below 0.5v, the car is running leaner than stoich. The ECU uses a short-term fuel trim (STFT) to make small adjustments to the fuel read from the base fuel map. If the O2v is rich, the STFT will store a number that tells the ECU to remove fuel and how much to remove. The ECU is constantly using this STFT to tweak the AFR in closed loop mode. If it gets it right, the O2v will go back to oscillating around 0.5v. If conditions are right, the ECU enters learn mode. This is where it memorizes the STFT values by storing them in a long-term fuel trim (LTFT). I've heard there are two trims from some and three trims from others. I don't know for sure. Anyway, there may be a LTFT-Lo, LTFT-Mid, and LTFT-Hi. If airflow is low, it stores the STFT value in the LTFT-Lo, etc. After it memorizes the STFT value, it resets it.
So in closed-loop mode, the ECU reads the fuel map, then adds in the STFT+LTFT corrections and injects. It monitors the O2v signal and corrects via the STFT. When it enters learn mode, it stores the STFT into the appropriate LTFT. Repeat. Typically, the LTFT values will stabilize and the STFT will hang out at the reset value (the ECU has "tuned itself". But that's only for closed-loop and learn modes. WOT/open-loop mode is blind map reading as far as fuel injection goes.
I'm not sure what triggers the ECU (1g or 2g) to enter learn mode. I'm under the impression that it's usually in learn mode during cruise/idle, but I don't know any of the specifics.
If anyone wants to add to or correct what I said, fire away. I think it's close though and should give you a good idea about trims.