So I was just working on a map, ill describe it because im at work and i cant upload the image, for increased city fuel economy .

basically i leaned out the area between 1500 and 4000 rpm at a load of 90 to 160. I didnt lean it out proportionally to the stock map, but i focused more to the bottom right of that grid, so say ~+.3 in the upper left and up to ~+1.0 to the bottom right. My AFR guage went from reading dead rich in that range to about 2/3 of the way back up to the stoich reading.

what do you guys think, is it a worthwhile effort, i figure that to be the area of the map of daily city driving.

also any thoughts on timing adjustment or other ideas?

 

Load the map to savefile.com it's free and i'll take a look at it.

I can't really picture what you are saying.

So your target range is between 15.0 and 16.0?


The one thing i'm not sure of with this softwarwe is if it actually modify's the closed loop fuel trims. The computer uses multiple inputs (o2 voltage, air temp, colant temp, and maf voltage to keep a constant 14.7)


Can anyone verify if the closed loop trims can be modified.

 

I'll load the map up when i get home tonight, it'll clarify what im trying to do.

As far as the open and closed loop, i have a theory. I was monitoring my AFR guage and i noticed that it cycles rich to lean every second or so, but only under a contstant load i.e. staying at say...0psi for an extended period of time(over a second or two) which leads me to believe that it is in closed loop at that time.

Then when you apply a changing load i.e. spooling up. The AFR guage stays dead on in a certain position, generally on the rich side. which to me suggests that it has moved into the maps that we are able to edit. Hence even if you could modify closed loop, thats not where most of the fuel is being consumed.

does that make sense or am i all off?

 

you've got the right idea but it's a bit more involved, the rich condition is the computer adjusting for the increased airflow, typically OEM ECU's will only be in open lood during startup (beacuse of the o2's not being at operating temp) and at WOT.

 

really? because i did a little conrolled experiment. I drove the car b4 the map i made and under partial throttle moving from 0 to .5 bar at starting 1500 rpm, the AFR read full rich. and then AFTER i reflashed i did the same thing and the afr moved closer to stoich, hovering around 2/3 of the way from rich to stoich. Is it possible that it went open loop??

either way gas prices are killer, theres gotta be a way to improve fuel economy under partial throttle!

 

Are you using a wideband or narrow band 02?

If it does modify the closed loop tables then you are correct it would make sense. What i am questioning is if it does modify them.

 

unfortunately its a narrow band, also part of the reason why im not goin too crazy with leaning it out.

it would be great to know exactly how the open/closed loop works in these cars and moreover what the software can to about it.

 

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There are five OBD-II fuel modes:

1. Open loop - Has not yet satisfied conditions to go closed loop
2. Closed loop - Using O2 sensors as feedback for fuel control
3. Open loop - Due to driving conditions
4. Open loop - Due to detected system fault
5. Closed loop - Fault with at least one O2 sensor - may be using single O2 sensor for fuel control.

The usual condition in mode 1 is that the O2 sensors are not hot enough - most O2 sensors these days are electrically heated and are ready within a few minutes of a cold start.
Mode 2 is the desired mode for you vehicle to be in.
MOde 3 is entered during power enrichment and deceleration fuel cutoff (manual cars). Although I have heard rumours (in this forum I think) that GM plan to build a PCM that stays in closed loop up to 100% throttle.
Mode 4 and 5 - fairly obvious and is usually acompanied by the illumination of the CEL.

Closed loop:
Petrol combustion engines are most clean burning when the air to fuel ratio (AFR) is 14.7.

Don't forget this fuel mode palava is all about emission control not about power. Most everyone on a race track will run richer than 14.7 - see the flames/black smoke from the exhaust <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" /> I digress.

The O2 sensors in the exhaust system detect the presence of oxygen in the exhaust gas. Too much oxygen and your car is running lean - too little oxygen and your car is running rich.
The signals from the O2 sensors are fed back into the PCM so that the PCM can make adjustments to the injector base pulse widths (IBPW) i.e. adjustments to how much fuel is squirted each time the injectors fire.

Because the O2 sensors "sense" the AFR "after the fact" the PCM can not use the current O2 signal for the current IBPW adjustment not in real-time anyway. Instead, the PCM "remembers" the last few O2 readings and calculates an estimated IBPW adjustment. This is called short term fuel trim and is designed to compensate for short term differences in engine operation - usually due to variations in the completeness of the fuel combustion.

The short term fuel trims are also averaged over a longer time period and "remebered" for certain RPM v's MAP or MAF values. These averages are called long term fuel trim - this is what is referred to as "learning". The PCM "learns" the best IBPW adjustments for certain RPM v's MAP or MAF values. These long term fuel trims are designed to allow your PCM to slowly alter it's IBPW to keep the AFR at 14.7 when long term conditions change - like engine wear, climate and fuel quality.

Tuning for closed loop mode is pretty much making adjustments to your base fuel tables (the base pulse widths are calculated from these fuel tables) so that your short and long term fuel trims approach zero. The closer you can get your fuel trims to zero the less the PCM is compensating.

Open loop:
Is quite simple - the feedback from the O2 sensors is NOT used - either because the O2 sensor values are invalid (mode 1 and 4) or the diving conditions require a non stoichiometric combustion - i.e. the PCM does not "care" about the AFR:
- Power enrichment = you've mashed your throttle to the floor.
- Decel fuel cutoff = you're coasting and the injectors have been closed, your engine is just a big air pump.

I guess tuning up the PE curve(s) would be about the only place I can think of for open loop operation.
I believe also that above about 70% throttle the PCM stops "learning" i.e. it doesn't update the long term fuel trims - not sure if it stops using the short term fuel trims though.

 

The scenario that shiftdsm gave would switch briefly to open loop, depending on how much throttle was given. There are a lot of variables that go into open-loop, but an acceleration would most likely switch to the open-loop fuel tables, which are the ones that we can tune.

Although, I too want to know if the fuel tables that we can do have anything to do with closed loop. Closed loop just uses the o2 feedback for a stoich burnm, but are these tables only open-loop tables, or are these tables reference during closed-loop also in some way?

I would imagine they would have to be, otherwise you wouldn't be able to tune for, let's say a 255 fuel pump. That pump overruns the stock regulator, increasing base FP, making your car richer during open-loop, and would push your fuel trims pretty negative in closed loop. If altering these tables are purely open-loop tuning, then you could never 'fix' your fuel trims to be back around 0.

Does anyone have the definitive answer on this?


Thanks,
Eric

 

Trims do seem to have a little bit of an affect on the open loop operation.. Since when you start the car it runs open loop for some time before the O2 sensors are up to temp, and the trims are what alters the fueling for the car to idle correctly given the conditions.

There is a table (actually 2 of them open loop high load, and open loop low load) that determines the transition from closed loop to open loop, it bases it on throttle position voltage... From 0-3500 rpm (approx) its set to about 3.01 volts stock, at 4000rpm its 2.5v and from there up, its about 1.8v dropping to 1.7v

What that means is at the 3v point, the car drops out of closed loop into open loop maps up till 3999rpm, we know that this transition point is approximately 30% (even though mathematically it should be about 60%) But as the voltage drops, the transition point also drops, so at higher RPM, the point is lower..

The maps at the lower load points are used for reference when the ECU is reset, or the ECU for one reason or another cannot get into closed loop. The less the trims have to adjust, the easier the car will cold start, run, also trims that no longer go out of range will no longer trigger CEL codes for Rich or Lean..

The trims compensate for minor differences in the car's tune, and attempts to alter it slightly. I have been told (though I cannot confirm this) that the trims do have a slight affect on open loop maps, even at higher load, but I cannot say for sure where it no longer has any affect..

Hope this info helps..

 

From my experience with DSMLink and the DSM ECU, the trims have no effect on open-loop fueling. They have an effect in the transition from closed-loop to open loop, only when you are one extreme of the trim.

For example, if you were pegged at +12.5 on the LTFT, then the instance where you swtiched from closed loop to open loop, you would immediately lose that 12.5% of fuel that the ECU was adding, and it would look up the fuel based on the open loop tables. On the other extreme, if you were pegged at -12.5 trim, the transition to open-loop would cause you to run richer at the transistion, since the ECU is no longer taking out the 12.5% of fuel.

I'm not positive if this is identical in the Evo or not, but this is my experience with the DSM ECU.


Eric

 

That makes sense, i'm going to play around with it tomorrow and i'll post my results.

 

I do know what your saying for the most part is true, but I also have data from startup and idle in open loop that LTFT's affect fueling in that region, All the data I have from the higher RPM's is invalid since I run a UTEC. I'll be able to answer this for sure in a week or so when I finish doing a good base tune for my car that I'll be mapping the UTEC on top of (turning off Open Loop Fueling for the time being since I can now control the items that necessitated OLF in the first place)