Short term fuel trim
This is the control of the Injection Pulse Time Open over the mid to upper range of engine operation. When the engine operates at normal or higher load or at higher engine speeds, larger volumes of fuel and air are needed. In order to maintain a Lambda = 1 in these conditions, the ECU monitors the O2 sensor and calculated load (see figure 2) and compares the values against the optimal value for the fuel injection pulse width stored in the drive map. If this base fuel injection pulse width value does not yield a Lambda = 1 at the O2 sensor for the measured air mass, the computer increases or decreases the pulse width by a percentage (%) determined by the difference in Lambda from optimal. These percentages have been computed by the engineers at the factory from extensive dynamometer testing and are stored in a "weighted STFT value array(1)" in the drive maps. When the STFT reaches the limit of its adjustment it will cause corresponding decrease or increase to the Long Term Fuel Trim. If the correction to the base value exceeds +25% or -25% for longer than 10 seconds a DTC is set for rich or lean stop for STFT.
- Calculated Load = Current Air Mass/Maxiumum Air Mass * Atmospheric Pressure/Current Barometric Pressure
(todo: make into equation. is current barometric pressure actually MAP for FI cars?)
Short Term Fuel Trim in general makes very quick and small temporary changes to the fuel being delivered
to the engine. Long Term Fuel Trim makes slower more permanent changes. Each change in the Long
Term Fuel Trim is equivalent to a change of the Short Term Fuel Trim over it's entire range. The idea of this
being that when the Short Term hits it's upper/lower limit, it resets back to the beginning, and moves the long
term TRIM up or down by one count. The Short Term continues to change very quickly, and if it hit's it's limit
again, it increments/decrements the Long Term again. This continues until the Long Term has added enough
fuel to compensate for the problem or until the long term has hit it's own limit. When the later occurs the
Air/Fuel ratio cannot be maintained at Lambda=1 and a "Lambda Control" DTC would normally be set and
in later injection systems a "LTFT at rich/lean stop" fault.
Once a LTFT DTC is set, depending on the calibration, the ECU usually defaults to Open Loop (O2 sensor
not on line) the ECU determines fuel delivery based on all sensor inputs (except oxygen sensor) and
predetermined internal "drive maps".
During Closed Loop, the input from the Oxygen sensor(s) is used by the ECU to calculate fuel delivery
adjustments or Adaptations. If the Oxygen sensor(s) indicate a lean condition, the Adaptation values will be
above 0. If the oxygen sensors indicate a rich condition, Adaptation values will be below 0. Adaptation
values that are between +10% and -10% of the base injection pulse width are an indication that the ECU is
maintaining proper fuel control.
If the ECU drops into Open Loop for what ever reason, you will notice that the long term fuel trim adaptation
value will show 0.0 ms. This is because the ECU is no longer looking at the O2 sensor, and therefore can't
make any adjustments to the fuel delivery. It must rely only on the fuel curve that has been programmed into
the drive map. This is a good reason for having the fuel curve as close to perfect as possible.
Notes
O2 readings have no control over ECU activity during open loop operation. In order to change fueling during open loop, the entire fuel map would have to be changed, or tools like Lemmiwinks can be used.
[STFT]s can be monitored using [VAG-COM] channel 033.