Afm Spring Adjustment - Bothersome Thoughts

[Captain Obvious]

I've not gotten to that part of the circuitry yet, and I may never get there. but I'm not buying the 4500 RPM limit either. It takes much more air and fuel to run WOT up a steep hill at 4500 RPM than it does sitting still no load in neutral at 4500 RPM in the driveway. There's no way you would want the same amount of fuel injected under both of those conditions.

 

I won't deny it if someone shows me the circuitry or the test results, but until then... I'm not buying it.

[Mark Maras]

There is no way you'd get close to 4500 under load with the same amount of fuel that is used free revving to 4500. When my fuel filter partially plugged, the engine would easily free rev to the redline but under load all it could muster was 45 mph in 2nd, 3rd & 4th gear.

[Chickenman]

Sorry, I've not made the basic premise and explanation clear enough. BTW, this is not all some " theory " I've made up by myself. It all documented in various Technical books and SAE papers on how Vane style AFM's operate. 

 

The Vane style AFM is a way of calculating engine Load or VE. Similar to what a MAP sensor does.  Which incidentally, is why in the MR2 Turbo article the Vane Style AFM was replaced with a Device ( VPC ) that emulates a MAP sensor and why I included that article in the discussion.

 

You're not understanding the 4,500  RPM (sic)  limit correctly. When the AFM is said to " Max out " at 4,500 rpm... it is NOT maxing out purely because of any RPM limit . It's maxing out from AIRFLOW Limit because you ARE under WOT, Engine load is high and Engine RPM is high.  The result of this " combination of factors " is that AFM Flap is open 100%. Thus indicating maximum engine Load.  A Vane type AFM is a Load calculating device. Similar to a Manifold Absolute Pressure sensor. MAP's will indicate full engine Load well before maximum engine RPM. The Vane type AFM does exactly the same thing.

 

It just so happens that these conditions occur at approximately 4,500 rpm.... Above that RPM , if you're still going up a long hill at WOT for example, the AFM will indicate 100%  engine Load from 4,500 rpm all the way up to the Red Line. The ECU will calculate fuel accordingly. So if you keep your foot in it up the Hill, the ECU see's 100% Engine Load, increasing RPM and provides maximum fuel enrichment.

 

The actual figure could be 4,500 rpm, 4,527 rpm, 4,6058 RPM, 4,800 RPM  or  any figure in between. 4,500 rpm is just the arbitrary figure given, that indicates when the AFM flap will hit the 100% value due to airflow, when engine is under high throttle and high load conditions.

 

Of course the AFM is not going to Max out when you wing it in the Driveway with no Load. I thought that was well understood.

 

I hope this clarifies things.

Edited March 24, 2015 by Chickenman

[FastWoman]

Chickenman, you can hit 4500 RPM under little load, with the throttle between the TPS's cruising range (above off-idle and below "full"), with the engine not sucking in much air, and with the AFM somewhere in its midrange.  Or you can hit 4500 under heavy load, with pedal to the metal, engine sucking in all the air it can, and AFM fully deflected.  The AFM doesn't measure RPM, such that it pegs out at 4500.  Otherwise the ECU would use the spark signal, and no AFM would be designed into the EFI.

 

Think of it another way:  Let's say you go down the road in 2nd gear, steady speed, with your engine running about 4500 RPM.  People might look at you funny, but let's just say you do it.  You'll have the accelerator pedal depressed somewhat, but not a lot, so as to maintain the same speed.  Now let's say you depress the pedal a bit more -- not enough to engage the TPS's "full" position.  (Heck, let's just say you don't even have your TPS plugged in.)  When you depress the pedal a bit more, you accelerate.  Right?  How does that happen if fuel and air delivery remain the same at 4500 RPM?  Wouldn't you keep going at the same speed?

 

I'm inclined to believe the 4500 RPM peg-out occurs at WOT.  Maybe airflow from that point is somewhat linear'ish, so delivering the same metered dose per revolution works well enough.  Or perhaps mixture errors above 4500 at WOT are not so significant that the engine will be damaged, and efficiency past the declining leg of the power curve isn't deemed that important.

[Chickenman]

FW...kindly read my Post #15 again. You just repeated what I said in Post #15 and thought I had made abundantly clear in earlier posts. I never said ANYTHING about the fuel and air requirement remaining the same above 4,500 RPM. That is totally incorrect. Edit: AFM flap hitting 100% at 4,500 RPM ( approximately ) is a transition event that occurs when certain Parameters are met . I hope that makes it clearer.

 

 

I just explained ( to the best of my abilities ) how the AFM senses load and VE. It's a COMBINATION of Factors that just HAPPENS to occur at around 4,500 ( sic ) RPM when engine is under FULL LOAD, WOT  and Higher RPM's. 

 

Please forget all this light rpm cruise and driveway stuff. That is not what I'm talking about at all. 

 

I'll try one more explanation..... but it's getting late and I'm getting tired.

 

1: Vane style AFM's all lose accuracy as they near 100% opening. This is purely due to the mechanical limitations of the design. It is a well documented fact. Engineers who deal with instrumentality recognize that Analog measuring devices are always the most accurate in the middle sweep of their range. That is also a fact.

 

2: Because the engineers know that the Vane style AFM is not accurate enough near 100 % opening range, they have to design a work around. They also want good sensitivity at lower air-speeds to provide good drive-ability and fuel economy at part throttle and cruise.

 

Therefor they design the AFM slightly on the small size, relative to the engines theoretical air displacement. This gives good resolution of AFM values at part throttle and cruise...just where they need it.

 

3: Once you get to 4,000 RPM ( approximately ) and go full power. IE: 3rd or 4th gear climbing a long hill at WOT... the engine wants a fuel curve of 12 to 12.5 to 1. This is a Full Power Enrichment Curve and it is pretty constant for any 4 cycle, normally aspirated Petrol engine.

 

IE: So you downshift to 2nd and mat it to pass the Logging truck up the 15% grade. Wham..  4,000+ RPM  AFM meter flap goes to 100% open indicating 100% maximum engine load. TPS hits WOT contacts. ECU says  to the Fuel injectors " Open the flood gates boys. We need to RICHEN things up big time and make some POWER!! " AFM stays at 100 % open and ECU maintains a Full power enrichment curve based on 100% load signal from AFM and adjusted per rising RPM.

 

Now you're past the logging truck, pulling 6,000 rpm in 4th  and you roll out of the throttle, but still maintain 6,000 rpm.

 

As you roll out of the throttle the Volumetric Efficiency of the engine decreases dramatically due to the partially closed Throttle Blade and Airflow through AFM decreases. Hypothetically it may only be 70% open and WOT contact is now open. ECU sees this and reduces fueling accordingly, while still maintaining a safe AFR, as the engine is still twisting 6,000RPM.

 

4: Scenario 2; You have lots of space and leave it in 3rd to pass the Logging truck up the same hill. L28's have lots of grunt. 3,000 rpm, TPS indicates WOT but AFM only shows 60%. ECU says to injectors... Hey boys we need some more fuel...but don't go crazy yet. This stage is often called Partial Power Enrichment. As RPM's rise so does the AFM opening amount and the amount of Fuel required.  Once the  AFM hits 100%, the WOT switch is still closed and RPM's rise past a certain point, then the ECU will demand Full Power Enrichment based on a hard coded Full Power mapping. IDC will rise with RPM till Redline.  . 

 

Tired now.. going to bed.

Edited March 24, 2015 by Chickenman

[FastWoman]

The analog nature of the 280z ECU would require a frequency reference or frequency to voltage reference and a comparator tuned to flip at 4500 rpm for the input from the AFM flap. It would be interesting to see if this exists in the circuitry.

 

There's an ECU flow chart in the '75 through '78 FSMs.  It shows the spark signal triggering a multivibrator circuit.  Apparently the base width of the output is modulated by the AFM voltage and altitude switch.  So the AFM output is primary to the formation of the base pulse.  That pulse is then lengthened by correction signals deriving from the TPS, CTS, and ATS, as well as a correction for battery voltage.  At no point is there any sort of frequency-to-voltage conversion.  I feel certain that would be reflected in the flow chart, as other details of lesser significance are indicated (e.g. ignition wave shaping).

[FastWoman]

Chickenman, yes, I see we're saying more or less the same thing, at least re post 15.  I can't explain why I read it differently.  <shrug>

[Chickenman]

Chickenman, yes, I see we're saying more or less the same thing, at least re post 15.  I can't explain why I read it differently.  <shrug>

Thanks, sometimes it is difficult to express in words what you are visualizing in your head. It's why talking face to face is such a better method of communication than E-Mails. You can immediately correct any mis-communications.

 

BTW, I edited part of Post #6 as I made a confusing statement regarding the ECU reading of AFM values at WOT. It still senses them at 100% Flap opening, but of course they cannot rise above 100%. The ECU just reads this as a 100% Load value.

[Captain Obvious]

I'm more confused by the blanket statements from post #4. That's what started all the discussion:

 

And if memory serves me correct the AFM only provided mixture measurement and correction up to about 4,500 rpm. After that the ECU calculates IDC entirely based on RPM, TPS signal and CTS.

 

I'm guessing what you really meant to say was:

The AFM only provides mixture measurement and correction until it maxes out full open. And if the Z is like a lot of other cars, a full open maxed out AFM could occur as low as 4500 RPM if the throttle is full open.

 

And if the AFM maxes out full open, beyond that point the ECU calculates IDC entirely based on RPM, TPS signal and CTS.

 

Note that I've not tested this for confirmation, so I cannot attest as to the accuracy of the statement, but it sure makes more sense to me and I'm thinking it's at least a lot closer to what you really meant? 

[FastWoman]

It makes sense to me too.  I had always wondered where the 4500 RPM thing came from -- sort of in the same way as the 10% of your brain thing.  When people grab hold of these numbers, the original context is often lost, and the numbers seem to develop a life of their own!  Anyway, the AFM pegging out at 4500 RPM at WOT makes sense.

 

Having thought about this a bit more now, I bet the mix gets richer and richer above 4500@WOT.  Air flow is probably approaching some asymptote dictated by Reynolds numbers, such that air delivery per engine rotation declines, and yet the same dose of fuel is probably delivered with each engine rotation.

Edited March 25, 2015 by FastWoman

[240260280z]

Someone just has to flow an AFM to see where the door/pot top out and then convert that CFM to BSFC, HP.

If it turns out to be less than a Z motor puts out then we can put it to rest. Case closed.

I think the same can be done with SU carbs... add more weight or more spring pressure spring so that the pistons rise and top out in the dome at max hp or max torque or some other point. More fuel will be drawn in a non-CV way after they top out (i.e. richer).

Nothing new under the sun.

[Chickenman]

 

I'm more confused by the blanket statements from post #4. That's what started all the discussion:

 

 

I'm guessing what you really meant to say was:

The AFM only provides mixture measurement and correction until it maxes out full open. And if the Z is like a lot of other cars, a full open maxed out AFM could occur as low as 4500 RPM if the throttle is full open.

 

And if the AFM maxes out full open, beyond that point the ECU calculates IDC entirely based on RPM, TPS signal and CTS.

 

Note that I've not tested this for confirmation, so I cannot attest as to the accuracy of the statement, but it sure makes more sense to me and I'm thinking it's at least a lot closer to what you really meant? 

 

Yes..that is more like what I was trying to say.