Let's discuss Transient Fuel

[12secSS]

Transient Fuel
The Transient Fuel Tab contains parameters related to Transient Fueling. Due to the port injection configuration of the LS1 engine, a model of fuel transfer from the injector to the cylinder is required. Fuel is not injected directly into the cylinder, but into the airstream aimed at the back of the inlet valve (and port walls) where it subsequently puddles and then boils off (evaporates) into the incoming airstream at a certain rate. The amount of fuel pudding and boiling must be accounted for to ensure optimum fuel delivery. This is particularly true during throttle transients.

During positive throttle (MAP) transients additional fuel must be injected than commanded to account for the fact that not all the fuel injected will make it into the cylinder. If additional fuel were not injected a lean condition would result as more fuel is puddling on the valve and port walls rather than entering the cylinder.

Conversely, during negative throttle (MAP) transients, less fuel needs to be injected than commanded to account for remaining fuel that is still boiling off the walls. If less fuel was not injected a rich condition would result.

Transient Fuel Qualification
Min Fuel Milligrams: Minimum injector pulsewidth used to limit transient adjust fuel.
Transient Fuel Min RPM: Disable Transient Fuel below this RPM.
Transient Fuel Max TPS: Disable Transient Fuel below this TPS if RPM is less than Transient Fuel Min RPM.

Warmup Transient Fuel Correction
Warmup Transient Correction (also known as Stomp Compensation) is used to add additional fuel to transients during engine startup conditions. This is to adjust for conditions of inlet port and valve components not being up to satisfactory temperature. ie. extra fuel puddles on a cold intake.

DFCO TPS Min: Warmup Transient Correction will not be added if TPS below this and exiting DFCO.
Fuel Delta Min Increase: Minimum commanded fuel delta to add correction (only compensate for reasonable sized transients).
Fuel Delta Limit: Negative transient fuel delta threshold to disable correction.
Initial Time Decay Mult vs. ECT: Initial value for time decay multiplier factor.
Time Decay Mult vs. ECT: Initial Time Decay Mult is multiplied by this reduction factor as the decay progresses. Smaller numbers mean faster decay.
Previous Correction Reduction vs. ECT: The previous correction value is reduced by this amount and added to the current correction calculations.

Transient Fuel Film Mass Transfer Calculation
Fuel Boiling Time vs. ECT vs. MAP: Boiling Time value
Fuel On Wall Exp Decay Mult vs. Airflow:
Fuel From Wall Stabilization:
Fuel On Wall Remainder:

Transient Fuel Compensator Gain Calculation
Fuel To Wall Impact Factor vs. Cool Temp vs. MAP:
Impact Factor gain vs. Airflow:

I fully understand the fuel transient theory, but as some others, I would like more in depth info regarding adjustment of these tables. Is there any way to "tune" these besides blindly add, subtract, multiply, divide? As it is, we can not scan these tables, only log Commanded AFR and Actual AFR and watch the lean spikes and/or rich dips. I don't mind playing around with these tables (although conservatively), but I prefer to understand the how and why of changing these values. So I ask our more knowledgeable experts here (those who look inside the PCMs ) if they can share their infinite wisdom with us.

[jimmyblue]

Looking at the names, I don't see any of them that
look like "pump shot volume" to me. Bunch of fancy
qualification and so on but which of them would you
twiddle, to make the AFR not-dip? Presuming it were
otherwise all enabled etc.

[Chris@HPTuners]

There are 2 main parts to this.

1. The Fuel Evaporation part
This is a model of how fast the fuel evaporates off the intake surfaces. Smaller numbers mean the fuel evaporates faster. What this means is that more of the fuel you inject from the injector that hits the intake surfaces makes it into the cylinder faster and the effects of any transients are shorter. This is pressure and temperature dependant process that follows an exponential decay law. Fuel boils off faster under vaccuum and at higher temperatures. There is also an airflow based gain that modifies the evaoration rate based on airflow.

2. The Extra fuel hitting the walls on each injector pulse
On each injector pulse a certain amount of fuel hits the intake surfaces and adds to the amount of fuel already there that is boiling off. Higher numbers mean more fuel hits the walls (hence less directly in the cylinder), meaning the PCM has to inject more fuel than it thinks. You'll notice that the numbers get smaller as the intake gets hotter and also bigger as manifold vacuum drops as you would expect. There is also a gain component that multiplies this value with airflow, as higher airflows mean less fuel will impact the walls etc.

You can think of this whole impact/evaporation process as a "leaky bucket", you have a bucket (the intake surfaces) that has an amount of water in it (an amount of fuel on the surfaces) that is leaking out (evaporating off the surfaces). You also have a hose that is filling the bucket at a certain rate (the fuel injected that hits the intake surfaces).

When you put it all together you have this:

1. Injector fires with the intent to inject a certain amount of fuel into the cylinder (Fdesired)
2. some of the fuel goes right into the cylinder (Fdirect)
3. some of the fuel hits the intake surfaces (Fimpact) and adds to the amount of fuel already there from previous cycles (Fwall)
4. some of the fuel that was already on the intake surfaces evaporated during the last cycle so it went into the cylinder on this cycle (Fevap), along with the fuel that already went directly in.

So the total amount of fuel that went into the cylinder (Fcyl) was:

Fcyl = Fdirect + Fevap

and

Fevap = Fwall(e-boiling time) ... an exponential decay

Fimpact = Fdesired * impact factor * gain

Fwall(new) = Fwall(old) + Fimpact - Fevap

From this you can hopefully see (if your head hasn't exploded yet) that there are 3 possible states this can be in:

steady state condition
Fevap = Fimpact then the actual fuel amount (Fcyl) equals the desired injected amount (Fdesired) so the PCM doesn't inject any extra fuel to compensate (no need to increase Fdesired)

positive MAP transient, you hit the gas
Fevap < Fimpact then Fcyl is less than Fdesired, because more fuel is adding to the fuel on the intake surfaces instead of going into the cylinder, so Fdesired must be increased by the difference between Fimpact and Fevap for a short period. This means the PCM needs to inject more fuel than it initially thinks otherwise a lean spike will occur.

negative MAP transient, you take your foot off the gas
Fevap > Fimpact then Fcyl is greater than Fdesired, because fuel is still evaporating from the intake surfaces instead and going into the cylinder, so Fdesired must be decreased by the difference between Fimpact and Fevap for a short period. This means the PCM needs to inject less fuel than it initially thinks otherwise a rich dip will occur.


The main things to mess with are the Impact factor and its gain and the boiling time constant and its gain. eg. you can see that in the stock tables as MAP increases (you hit the gas) the boiling time increases (fuel evaporates slower) and also the impact factor increases (more fuel hits the intake surfaces), both of these increases result in the PCM adding more fuel during the transient. You can also see that as MAP decreases the numbers get smaller, resulting in the PCM subtracting fuel for the transient.

Hope that helps,

Chris...

[Chris@HPTuners]

i should also add that this is not intended to be the "pumpshot". In the LS1 the pumpshot actually comes from the airflow side (TPS and MAP based predictive filtering) ie. the dynamic airmass calculation models it as an "airmass shot". This is only used when the dynamic airflow calc is running (ie. the MAF is working) and its complex as hell.

Chris...

[MikeGyver]

I think reading that gave me a brain aneurysm.

[MeentSS02]

This is some good stuff...good ideas on where to go. I'd definitely like to get mine in line...my transients usually run rich, and I'd like to get that under control.

[12secSS]

Chris

I was looking at the boiling time last night (on a turbo car) and I was tempted to increase it in the higher MAP areas, I just wasn't sure if I was going to go in the right direction. Plus, it being a turbo car (9.8 psi) I did not want to take a chance at over shooting or possibly heading the wrong direction. I will experiement on my own car first and see what the outcome is. I also looked at the impact gain, but was unsure how to modify that one ... after reading your post I understand how this needs to be changed. I take it we cannot log this, as there are no sensor to measure the Fvalues. Sounds like a trial and error thing.

Thanks again, oh great Sensei.

PS: This is going into my collection of tuning doc.'s, much like many post made by the HPT-Team.

After reading your post for the 30th time (I like to hammer the info into me ) ... these values need to be adjusted on boosted applications, since the pressure model is no longer the same (actually higher). Is it OK if the MAP level only goes to 100? On an N/A car, it would require very little change, as compared to boosted applications.

[EC_Tune]

George,

Got to play with an 03 TT Vette and BOY does the transient fueling make a difference when you nail the throttle on the dyno!

I had the VE dialed in with the eddy current (its working better than ever!!) and then went to do an inertia only run and the AFR error shot up from ~+/- .5% error to 10-15%+ at the lower end! Naturally I was thinking WTF!

Then I started altering the high MAP region of the Impact & Impact gain tables and got that down to transport delay to the wideband (~ .25 seconds) I even got it to the point where it went real rich & I had to back off a lot. That table makes a huge difference.

Ended up around 597 RWHP running 155 Kpa from 3200 RPM on up on a stock LS6 engine! Turbos RULE! LOL

I've got another TT to do (heads & cam, meth, Boost controller & Mototrons) and I think this table will help that one out a lot as well.

[12secSS]

Excellent results Doug! Glad to hear it worked out, I will have to play with those setting when I get a chance. Thanks for adding your comments. Can I ask how you went about modifying those tables, to simplify the process for others?

[EC_Tune]

I kept adding to the 100 Kpa line at operating temps until the "lean spot" went away and didn't over shoot.
I also modified the Impact Factor Gain all the way across: again until the "lean spot" went away and didn't overshoot. I don't have a preferred mathmatical way (well, multiply by 1.75 was a good start...) , just emperical data.

Wish there was stock supercharged vehicle that I could look at transient fueling on. I looked at an STS-V but they were way different. Thought the V6'ers would have something but nada...

[12secSS]

Doug,

I will send you an STS 1998 A4 covette I did (completely stock sans turbo), for your review. It put down 450 rwhp (unlocked) on a dynapack. I will send the file over to you possibly tonight, it should be good data to compare with.

[EC_Tune]

Cool! Sounds good.

[Michael@L&M Engines]

I have just taken delivery of VCM Editor and downloaded 3 - 2003 truck 5.3 stock files from the repository for examination and I find that all 3 do not have acceleration enrichment nor transient fueling parameters under the tabs.

Is this due to these VIN's not having these parameters available in HP Tuners?

Why would they not be available?

I use these tables extensively in SCT software for the Fords, which SCT generally only allows these parameters to be available to individuals that have "Calibrator" status.

For an added explaination and model of transient fuel one can refer to John Heywood's text "Internal Combustion Engine".

Michael

[BADSZ28]

Would changing the impact factor gain help fix an off idle lean condition? Seam it would. I will try. I am totally out of ideas. I am a novice and have consulted a few guys on LS1tech.com and nothing has made much of a difference.

[Hauss]

3 do not have acceleration enrichment nor transient fueling parameters under the tabs.

the ones I have tdone have that but it is called power enrichment.. is that what you are referring to???

OOPS..the ones I did were 2007's... sorry bout' that.

[EC_Tune]

If you have the IFR, VE & MAF (if applicable) dialed in perfectly, then it's a transient fuel issue. If not, start with VE/MAF/IFR.

[sikhabib]

whats an acceptale afr to have during transient conditions?
if your crusing along in 5th at 1600rpm and 35map and depress the throttle slightly so that map goes upto 75 (still not in pe), are you meant to get a rich condition before it settles back to stoich?
on my tt car when i do the above afr will dip into the low 13s and then go back to normal closed loop afr

[EC_Tune]

Sounds like you have a bit too much transient fueling. You want it to be either level (AFR wise) or a slight dip (.5 AFR or less idealy) towards rich.

[sikhabib]

on boosted applications is this table still referneced during transients after 100kpa?

[EC_Tune]

Yes, definitely. It just uses the last line of the table.