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How STFT/LTFT are calculated?

  • guafa
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20 Mar 2020 15:44 #38496 by guafa
How STFT/LTFT are calculated? was created by guafa
Hi guys,

I have recently finished my basic PCM (no ignition/timming control. No DTC skills. Until now, only fuel control and PID's transmission to a phone through Bluetooth).

To get that oscillation in O2 sensor voltage we can see in our tools, you must decrease and increase pulse width constantly. Let's say if you have at idle a calculated pulse about 4 ms according to your VE and Map table, then you have to decrease your pulse to 3.6 ms to make air/fuel ratio to get lean and then back again to 4 ms to get it rich.

My question. How then these parameters are calculated?

Cheers and thanks in advance.

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20 Mar 2020 17:17 - 20 Mar 2020 17:34 #38498 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic How STFT/LTFT are calculated?
On engines with a normal zirconnia O2 sensor if everything is running pefectly the STF will be almost perfect upsidedown reflection of the O2 sensor output. The centre line of the O2 sensor output should be 450mv and ideally if the LTF is correct the STF will be centred on zero

The STF works directly from the O2 sensor output.

The computer continuously monitors the O2 senor output and adjust the duration it reads from the fueling map. if rich the computer immediately alters the STF which subtracts a value from the injector duration stored in the fueling, if lean it adds to the duration in fueling map.

The LTF is worked out from the average of STF

Over a longer time period the engine computer detects if the STF is on average adding more fuel subtracting fuel it will correct the average of the STF by altering the the LTF until the oscillations of STF are centred on zero and the O2 sensor output oscilations are centred on the 450mv stoichiometric.

Some manufacturers engine computers apply the LTF correction much more quickly than others and some engine computers the LTF will become stuck if the maxiumum or minimum range is reached.

The LTF is worked out on the average of the STF, the computer changes the LTF until the average + & - STF is close to zero

If the O2 sensor and engine is working normally and the STF and LTF are correct the O2 sensor will follow the normal oscilation centred around 0.450mV.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



Last edit: 20 Mar 2020 17:34 by Andy.MacFadyen.
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20 Mar 2020 18:14 #38499 by guafa
Replied by guafa on topic How STFT/LTFT are calculated?
Thanks Andy. Ok, got it.

This means in my earlier example, if i decrease pulse width from 4ms to 3.6ms, my STFT should be -10% (assuming at 4ms my O2 sensor is 450mv).

Second question. Where in the heaven did you find that info?

I'm trying to collect that valious info, but still fighting

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