Where derivative fits in O2 control
O2 control compares the measured mixture with the target and trims fuel to close the gap. It does that with three terms:
- Proportional reacts to how big the error is right now.
- Integral builds up over time to remove any error that lingers.
- Derivative reacts to how quickly the error is changing.
In plain terms, derivative softens the proportional push as the mixture heads toward target. If the error is shrinking fast, derivative eases off the correction so it doesn't carry straight through the target and out the other side. More derivative means more of that damping.
How much to use
| Sensor type | Typical derivative | Why |
|---|---|---|
| Narrowband | Zero | Narrowband control deliberately swings back and forth across stoichiometric, so overshoot is part of how it works |
| Wideband | Zero or very small | Only needed to calm overshoot when proportional gain has been set aggressively |
Too much derivative can hold the correction back so hard that the trim never actually reaches target. If lambda creeps toward target and stops short, derivative is one of the first things to reduce.
The table
Derivative gain is a table rather than a single number, so it can vary across the operating range. It's laid out with TPS or MAP on the X axis (left to right) and RPM on the Y axis (bottom to top), and both axes can be reassigned to any ECU channel. Select the table and press F3 to edit or add axis values. When you change an axis value, NSP adjusts the cells to keep the table's shape intact.
Our approach
- Start with derivative at zero everywhere.
- Tune proportional and integral until the trim responds quickly and settles. See proportional, integral and derivative gain for the full picture.
- Only if you see the trim overshoot and ring around target in the logs, add a small amount of derivative in those areas of the table.
- If adding it makes the trim sluggish or stop short, back it off. Often the better fix is simply less proportional gain.
Also check the O2 control delay is set correctly. A trim that reacts before the sensor has seen the result of the last change will look like it needs derivative when it really needs a better delay.
