Head: as in, my melon on my shoulders. (not the exhaust kind).

I've been doing some data logging recently and with that comes a whole universe of new things that I have yet to learn and understand. Thus, why I come here.

It is my understanding that when reading about fuel trims, the short term (hereon out referred to as STFT) is what is responsible for the instant feedback to the ECU to make sure the injectors are doing what they are supposed to be doing.

The long term (LTFT) is a "less volitile" data table that the STFT uses to steady itself.

The majority of this information is grabbed from the O2 sensors, which sense whether the exhaust has excessive oxygen (lean) or excessive hydrocarbons (rich)

The ultimate goal is to have a 0% trim on the STFT which means that we're sitting at a Lambda of 1.0, Or a Stoichiometric condition. A positive trim % means the engine is running lean and a negative % means the engine is running rich.

From what I understand, when the engine is at Idle or at cruising speed, the STFT should easily be sitting at 0%.

So that was a lot of preface to a very simple question:

What in the heck is going on with my STFT? This doesn't seem normal to me especially at idle.


(Click for full image)

Here are the conditions of which this data was read:

• Engine at Idle
• Engine at normal operating temp.
• LTFT: +9% (lean)
• Ambient Temp: 37F
• IAT at start: 75.3F
• IAT at end: 89.5F
• Avg RPM : 783
• Calculated Engine Load: 25%
• Total run time: 146 seconds

Ideas? Thoughts? Suggestions? Am I an idiot?

3..2..1..discuss.

 

Well first off, very few engines (and not including ours) can run a stoichiometric burn. Immense strain on the engine. A/F for that would be ~14.7:1, above meaning lean, below meaning rich. Using this context, an engine can be considered running lean-er or rich-er from a baseline state. Our engines have to, and should, run right around the 11:1 range - this is our baseline state; they will never approach stoich or you'll melt/blow a piston

On a stock Cooper S engine/fuel map, AFRs should become leaner through the top end, but be quite rich overall...it will never even approach a 14.7:1 AFR through the RPM range.

As far as your charting goes, and I'll be honest...I have no idea how the hell to functionally understand what I am looking at. Sorry I could only chime in regarding a smaller portion of your question. To try and answer the bigger portion simply, my understanding is that short term fuel trims are under constant change in regards to readings from the O2 sensor(s), generally within a range specified by long term, editable maps. In this understanding, I am not sure if there is anything you can do, nor anything to worry about.

And I could be completely wrong.

- Matt

 

Interesting. I never knew that it was as low as 11:1.

So from what you wrote I can conclude that the trim bases it's necessity from a pre-defined AFR as opposed to a stoichiometric burn?

Also, you mentioned that the stock map (which I am still running) is generally tuned to run rich. Should it be a cause for investigation that overall I'm running on the lean side?

 

i was just thinking the same thing!

 

Sorry, my mind is kinda blown tonight...can you rephrase this portion? Sorry again.

I don't know your numbers, but you could very well still be on the tolerably "rich" side of a baseline state. Suppose you're at 10:1 idle (according to O2 sensor readings...I don't have the actual, firm numbers offhand)...your car may be pulling back a little fuel within an acceptable range map range...maybe to 10.3:1. Still rich, just lean...er.

Should it be cause for investigation? I wouldn't imagine so...you could check yourself through one of the general OBDII diagnostic tools (Scangauge, DashDyno, etc), or go so far as to get a wideband setup. The general tools will provide you with a basic idea of where your AFRs lay...they won't provide enough resolution should you want someone to tune your car (no wideband), but enough to see if you have a cause for worry.

And again, I am no expert. Just read a lot.

- Matt

- Edit -

https://www.northamericanmotoring.co...air+fuel+ratio

See post #19 there for an idea of tuned AFR's. Rest of the thread is good.

 

heh... don't worry about it. You answered it without trying.


Now this brings up an interesting point that I didn't know. I always thought there was a hard-coded AF rate that was "optimal" which is programmed for the injectors. Then the O2 sensors would sneak in and monitor what the injectors were actually doing (among a bazillion other variables such as IAT, MAP, etc) and fine tune (or trim) the AF rate with a percentage of the optimal rate based on what was coming out of the exhaust.

But if I understand this right... the "optimal" AFR is constantly changing for the injectors and then.... well wait... if the optimal AFR (ha... I just typed ARF... it's late...) is a variable instead of a constant... what's the point of the trim then?

I'm actually running my logs from OBDII right now. I have yet to find a tool I really like (without breaking the bank). I'm also, as a hobby, learning all this stuff to continue my development on my own OBDII/CAN software. (baby steps... baaaaby steps)

Same here. I appreciate your knowledge with this.

 

Sorry, you are correct...I mis-posted semantically in my prior statement...I will edit...like I said my mind is blown tonight .

Car turns over, and begins giving a set amount of fuel based on engine load, RPMs, readings from airflow sensor, etc.
Exhaust starts coming out, AFRs are read from O2 sensors in exhaust.
ECU interprets AFRs deeming them high or low according to programming (engine load, readings etc). ECU then tells injectors to provide more or less fuel, to provide for a now estimated "optimal" burn.
Process starts all over essentially, re-reading engine load, RPMs, airflow sensor and whole thing repeats, albeit very very quickly, many times a second.

The optimal AFR is, in the short term, constantly changing, as the injectors pump more/less fuel and the mass of air is more/less. The reading is never perfect, and will always overshoot/undershoot its ideal mark, and is always changing in response to the rest of the system. Also, the ECU is what controls everything...O2 sensors (and other monitors) provide feedback, and the ECU hands out instructions to the injectors and the like. One giant feedback driven loop, using "rough" suggestions (fuel/load/timing maps) as a guide.

But in the short term, this is all irrelevant...the corrections are very very minor, and very, very often. Usually they work to move the stock long-term maps from a richer to a leaner state.

I think that puts us on the same understanding page ?? I wish Jan or some other expert could chime in and shut up my possible ignorance.

- Matt

 

To answer your original question. STFT will always look like the graph you posted. This is because they do exactly what you said when you described STFTs. They are constantly reading exhaust and compensating whether you are idling or cruising or at WOT. I'm not positive, but I had heard somewhere that when the ECU gets a signal from an O2 sensor it tends to overcompensate so that it can determine a percentage of how far off from desired AFR. That is also how LTFTs are figured.

 

I would figure the system would be constantly changing due to variables such as temperature, engine temp, the quality of the current spurt of gas, etc.

I guess I was just shocked that the delta for my run was 7% at idle. I mean, compensation is one thing, but my ECU isn't a Hummer owner (get it? ba-da-dump. only kidding)

Speaking of... just what exactly is the trim a percentage of?

 

Ben,

Reading off the OBDII port may almost be too slow to accurately monitor whats going on with the ecu and engine. It's sample rate just isnt fast enough for you to get accurate enough rates. That isnt the ecu's fault but the speed of the OBDII port. Your best bet is to weld in another o2 bung and install a wideband a/f guage with data logging capabilites.

As for my expirences with reading my wideband, at idle it stays around 14.7/1, but it is always moving around....15.2...14.1 and so on similar to your graph. As you mentioned all the variables such as iat, engine load, manifold pressure, & etc weighs into your final a/f ratio. This is where the "Closed Loop" system works so well & open loop is well just that....

As your engine load increases, ie accelaration, the A/F needs of a FI engine changes then that of a NA engine. There is an increase in combustion pressure via compressed air, which leads to the ratio's being skewed from the normal 14.7/1. You have effectively more air in the same amount of volume, read density, so you will need to add more fuel to compensate, thus the ideal numbers of 11/1 depending on how many bars of pressure the cylinders are seeing. The other item that would be helpful for monitoring would be an EGT gauge. This will allow you to view what the temps of the exhaust are. Ideally you would want to see what each cylinder's temp are, but short of installing 4 gauges, generally you shoot for the highest temp cylinder, which in our SC engines is #4.

I am all for closed loop systems, as they really do offer a way better tune & reliabilty...

 

Reading is GOOD!

 

Wow, the propellers are spinning at high speed . This is way over my head . I guess that's why I just take it to the shop when I need to anything done.

I'm super impressed with your understanding of this techy info.

 

FWIW,
My WB O2 gauge has similar numbers...

shankrabbit,
I'm not sure how you logged those numbers but the O2 data my ODBII logger receives is in volts and needs to bs scaled correctly to be read in AFR. When I've looked at it, the pattern looked similar to yours though. The narrow band O2's are basicly just telling the ECU if the burn is hi or low and the ECU then "trims" the injector cycle accordingly.

 

Couldn't agree with you more. I hate only getting 1-2 samples a second. Although I heard it is possible to get up to 10 a second, still way to slow.

Honestly, who designs a specification to run slower than molasses?

I'll have to make sure to pick one of those up after I fit my new exhaust on (once my financial adviser approves the purchase)


Maybe it's like comparing apples to oranges, but what started all of this thought in my head is that I'm doing logging on both my MINI and my wife's MINI(van)(it's not a van... it's an MPV)

Her STFT on both rails (6cyl) sits almost perfectly at zero (+/- 1%) at idle.

It was because of that my mind popped the question, "Why doesn't mine sit so pretty?"

 

because her van...errr I mean MPV is NA.....& its from Japan?

 

Are you doing a header as well? You will want to weld the o2 bung in as close to the cylinder as possible for EGT, but for the wideband, weld the bung close to the stock o2 sensor to get similar readings....fwiw.....

 

Should that make a difference at idle?

The IMAP sensor reads a vacuum which means the air is being sucked into the cylinders as opposed to "boost" which air is being forced into the cylinder.

Most of what the supercharger is doing is getting tossed out the window by the BPV, so wouldn't that almost create a sort of NA situation at idle since the cylinders are causing the air movement as in a NA engine, as opposed to the supercharger doing the air movement?

(forgive me for thinking out loud)

That's the perfect answer to my question.
Q: "What is going on here?"
A: It's not from Japan.

 

I agree with you on that and was thinking the same thing....thus the Japan comment...hehehe

 

Follow up:

It was my head!

Yup, I'm an idiot.

The crazy reading is from the first O2 sensors trim.

I found my way to the second O2 sensor, and guess what. Almost perfectly on 0%.

Oiy... I'm a dummy.

 

Nah....you figured it out....nothing dumb about that!

 

Isn't the second O2 sensor after the cat?

I would expect a 0% at that one as the cat should burn most HC's/O2 right?

 

From what I hear, it will read ~1 point leaner than the pre-cat sensor. That's essentially where they sniff during a dyno run, though at the tail pipe.

 

Yeah, one pre-cat, one post-cat. I think the 0% is ideal, but won't always be that way.