[mdlug-discuss] OBD fuel monitor fine tuning

[Robert Meier]

Drew,

> With the rich-lean mix self adjusting in [a PI servoloop],
> it should matter a great deal less what is used as fuel,
> ...

It would be less sensitive than open loop mixing,
but more sensitive than a driver-controlled mixture.

Servoloops are only stable when the plant world line
(e.g. oxygen output as a function of fuel and air meter rate, ...).
When a plant goes out of bounds,
open loop system performance tends to change continuously,
and needs only a margin of tolerance from other systems for graceful failure.
When a plant goes out of bounds,
servo loop performance tends to "go to the rails",
and needs a detection and shutdown system for graceful failure.

As an analogy think of an unanchored swing set.
So long as the weight of the swinger is small enough,
the swing pendulums back and forth smoothly.
If the weight is too large and/or the displacement too far,
the swing overbalances and tips over.

Many of my friends have seen their cars "just stop for no apparent reason"
when gradual  wear of some seemingly unrelated component pushed it barely
or intermittently out of bounds.

Unfortunately "single point design" of automobiles makes determination
of the worn component, often impossible, without disassembly.

Aviation regulations require "redundancy" in aircraft,
so any loop failure produces a "discrepancy" that identifies
the loop (and sometimes the component).

Reporting,
-- 
Robert Meier

  When asked what time it is: "Do you mean now?"
     -- Yogi Berra