more fuel pressure

[Scot A. Sealander]

tom cloud wrote:

> While I understand about the 'adaptive learning', don't most engines,
> at WOT, go to a speed density type mode which utilizes pre-set parameters
> while ignoring the EGO?

Well.....  If it is a speed density type, it stays speed density (that is, 
it still uses manifold pressure, RPM, air temp to determine load.  If it is 
MAF equipped, it still uses the MAF for load.....

I think you are misunderstanding closed loop/open loop.  At normal operating 
temp and conditions, the ECM runs closed loop.  All this means is that it is 
using feedback from the O2 sensor.  All closed loop means is "with 
feedback", all open loop means is "without feedback".

The air flow (load) is determined the same way, whether it is operating in 
closed loop or open loop.  So if it is MAP equipped, it uses manifold 
pressure, if it is MAF equipped, it uses the MAF.  It does this at either 
extreme of the sensor, or anywhere in between.

The ECM's job is to match fuel flow to airflow (by weight or mass) at the 
desired AFR.  Usually, this is 14.7 to one. (Keep in mind that the O2 sensor 
just happens to transition at this AFR.)

Simply put:

Injector PW = Amount of air in cylinder * Inverse AFR term * Injector 
constant to make it work, or:

PW = Load * IAFR * Injk

What happens is that the system may not be perfect, and may have some error 
in it.  For instance, if someone jacks the fuel pressure up, then the normal 
injector constant will be wrong, and it will deliver more fuel than 
intended. So we can add a correction term for trimming the fuel equation.

PW = Load * IAFR * Injk * closed loop trim

Now we need a method for feedback to make the trim value correct fuel to the 
right value.  This is where the O2 sensor plays a part, in closed loop mode. 
 If the trim value is slightly above a neutral value, the O2 sensor should 
show a rich value, if it is slightly below neutral trim, the O2 sensor 
should be a lean value.  The ECM has a strategy to do this dithering of the 
trim value to determine if it is delivering the right amount of fuel.  If it 
isn't, it will attempt to correct it. I will leave it up to you to figure 
this out.....

So if the O2 Sensor does not flip rich/lean, the ECM knows which direction 
it needs to correct, and will correct for that.  It has both short term and 
long term correction factors.

So now:

PW = Load * IAFR * trim * short term corr * long term correction

Notice that to be in closed loop, the AFR term has to be at 14.7.  When the 
engine starts, the engine is in open loop, and the AFR term can be used just 
like the choke on a carburator. Set it rich, and it will deliver more fuel 
at start, and this value can then be decayed as it warms up.

For WOT, the 14.7 value is much too lean for the engine, and the ECM will go 
open loop (just meaning no feedback), and the AFR will be set richer for 
best power. This AFR usually comes from some tables that have temp and RPM 
corrections to this AFR.  Now with it being in PE, the O2 sensor is not 
useful for the correction strategy, so the trim will be set neutral, and 
that part of the code will be skipped.  But the O2 sensor is still sampled, 
and shows up in the ALDL stream, so ypu can see the value.



> If so, wouldn't the net effect be stock
> performance during normal operation (except idle, where there might be
> more petrol than the system can adapt for), and improved performance
> at WOT?

Not necessarily.  Does the engine need more fuel at WOT? ;-) I have seen 
various schemes on how to correct while in PE (power enrichment). The 
correction factors are used to add fuel if the correction is to add fuel.  
If the correction is normally removing fuel PW, the correction may or may 
not be used.....

So go get an ECM and figure it out!

I did not mean to ramble this long.  It may not be entirely clear, but hope 
that is useful.

Scot Sealander   Sealand at clarityconnect.com