Closed loop operation...

[FIScot at aol.com]

In a message dated 96-04-16 10:47:23 EDT, you write:

>Speaking of closed loop operation, I'm having a difficult time uderstanding 
>exactly how it works...  is it still a lookup table with a correction factor
>for the O2 sensor reading, or is it an autonomous system where just the 
>O2 sensor controls the whole thing?

Let's start with a very simple explanation.

Open loop ->  a system without feedback for error correction.

Closed loop -> a system with feedback for error correction.

In this case, the system is the fuel injection computer and associated
hardware/software, and the feedback for error correction comes from the O2
sensor.

In simplest terms, an ECU needs to do one thing, and that is match injected
fuel to the amount of air entering the engine, and doing so at the desired
air to fuel ratio (AFR).

 So fuel needed is something like:

Fuel PW =  AFR term * airflow/unit of time

If the measured airflow is dead on accurate, and the fuel delivery system
also is dead on accurate, no correction will be needed.  In the real world,
that accuracy does not exist, so there is a need to correct for some errors,
and that correction lies in the O2 sensor and the software that deals with
correction.

So the corrected equation may look like:

Fuel PW = AFR term * airflow/unit of time * correction term

When all engines first start, they are operating in open loop or 'no
feedback' mode.  The fuel PW is determined from the airflow and AFR term.  If
the ECM has also been just powered for the first time, the correction factor
will be neutral, that is, neither adding or subtracting fuel. Once the closed
loop conditions have been met, the ECM will enter closed loop or 'with
feedback mode'.

In closed loop, the desired or perfect AFR is 14.7 to one. The feedback
device, in this case the O2 sensor, just happens to flip output voltage as
the AFR travels across this 14.7 to one value. This makes it a good device
for feedback.  The AFR term is set to 14.7 in the ECM, and the fuel PW is
dithered rich/lean, so the O2 sensor should flip rich/lean with the dithered
value.  If it doesn't correctly follow the commanded rich/lean dither, the
correction term is changed until the the AFR term really represents 14.7 to
one.

Once the correction is established, the dithering will continue to happen,
making sure the AFR stays correct.  There is usually a number of locations
for correction terms, representing different engine load and engine speed
combinations.  Each of these will eventually get to the right value over
time, and as different engine conditions occur.

Now once the engine goes into open loop, such as cold starts and during power
enrichment, the engine's commanded AFR will be pretty close,using the
calculated value from the ECM airflow measuring device and desired AFR term.
 To measure the airflow in the MAF equipped engine, this is easy, as the MAF
(Mass Airflow sensor) supplies the mass of air entering the engine.  In the
case of MAP (Manifold Absolute Pressure) sensor equipped engines, commonly
called 'speed density' systems,  the airflow has to be calculated using a
volumetric efficiency (VE) table, the MAP sensor, and various corrections due
to engine and air temp, and baropress.

Notice that during cold starts, the 'choke' or needed enrichment can come
from the AFR term.  Just set the AFR to the needed value to make the engine
run at your cold start temp, then decay the AFR to stoke as it warms up.

I tried to make this fairly simple, so don't jump me if I over simplified
what is truely going on...  Just correct the part you don't like.... ;-)

Thanks, Scot Sealander   FIScot at aol.com