mark kibort

you think a piston and intake charge know that the engine is accelerating?
all it knows is that its going up and down at a faster or slower speed. if something happens at 6000rpm that is different than 3000rpm, then you have a point, but there are no dependant dynamics as long as the fuel flow is based on the proper air mass flow and the spark is correct for the appropriate piston and intake air mass flow speed.

now, you are talking about emmisions. sure more NOx is a result of lean-of-stoich. thats not the issue. the point is, lean of stoich, no detonation can occure, because combustion temps are much lower, because there is not enough fuel to create the same heat as the same power setting reached at a rich of stoich level. lean of stoich is not optimum for power, we all know this. the point is, there is no need to worry about your engine if it goes way lean of stoich. the problem is if it is near stoich. there is a danger range, and thats the issue. going way lean is not an issue, if your engine can run, then it will probalby be safe and have less power, and possibly run rough, but the egts and chts will be low.

mk

mark kibort

nah, if anything, this is an old wives tail. Plus, think about it, if it was true and you were getting better heat transfer (based on your comment) then CHTs would go up even further, and they do not. both CHTs and EGTs go down lean of peak.

keep in mind, the graphs are talking about up to about 80% of max HP. remember that max hp is based on manifold pressure. (called "MP" on the graphs) if you were able to reach lower manifold pressure, then you could have close to 100% hp. you could do this easily with a turbocharger , as this is commonly done in aircraft. eitherway, for any relative setting, lower egts will be possible for a given power setting at lean of stoich. however, you always will be able to reach higher hp at rich of stoich.

Again, this discussion is about what happens under a real lean condition. as Doc says, nothing.

MK

edit: if you look at the graphs, for different manifold pressures and power settings, you can see the same HP creates two different CHTs, but the EGTs are near the same. look at the 78% power setting compared to the 58% power setting. CHTs are about 50 degrees cooler on the lean side of stoich (peak), yet the hp created is the same near 200.

docmirror

Yep Mark, that is keerect. LOP operation is the safest, all other things being equal. Worst spot on the F/A ratio graph is just slightly rich of peak EGT. Add in problems with ignition, maybe a glowing ember or burned valve and you have the recipe for Really Bad Things to happen.

The emmision problem is a balancing act between NOx and ***. That another reason that the F/A ration runs on a variance schedule. And yes, we don't have any stinkin' EPA nimrods sniffing airplane tailpipes.

Doc

mark kibort

great set of posts by the way. Good information.

Mk

Larry Velk

It seems that science, specifically chemistry, physics, and meteorology need to be studied formally in order to be a pilot. I bet the Air Force requires some formal study of these topics. I bet even the Air National Guard requires this study. I bet lawyers may have avoided these topics in college. I bet singers and actresses who dropped out of high school don't have any idea what fuel air weight ratios are. I bet even the worst Air National Guard jet pilot knows more about science than the best lounge singer who left formal education in 9th grade.

worf928

MK I have included quotes from you to provide context. I have edited them for brevity only and not to twist your words

First: If you go full throttle and your a/f mixture is stoich you most-likely have a broken WOT switch. Possibly a fuel delivery problem because ...

... the engineers that designed the 928 motor (and other motors) understand that 1) stoich operation is not the best a/f ratio for WOT acceleration/power and 2) that stoich operation at the highest possible (for stoich a/f) is not the safest (by far) a/f ratio.

Among other things - as you have posted - temps after steady-state stoich operation are higher than for other a/f ratios. This tidbit is important because the energy required to ignite (or explode) a nicely-mixed stoich a/f mixture is 10 times less than that required to ignite a lean or rich mixture.

Now why is that important? Because ...

... while the pistons are not intelligent the system does exhibit transient operational characteristics. (If you don't believe that then explain why the 928 motor has fuel pressure dampeners and a regulator hooked to an inversely-proportional-to-load signal (i.e. manifold vacuum.))

To illustrate: A motor is operating at 50% power at stoich under steady-cruise. Now, you floor the go pedal. The throttle plate opens and within a fraction of a second the a/f mixture is very lean. The engine management system (whether it's a 'dumb' carburator or an intelligent computer-controlled multi-point injection system) attempts to compensate by adding fuel to match the increased air flow. Now, you have at least double the a/f charge in the cylinder. The density of the a/f charge is greater, pressure is therefore greater, temperature of the charge as it compresses is higher (PV=nRT) and if the a/f ratio is still stoich the high chamber temperature (due to the previous heat soak from stoich steady-cruise) might just be high enough to explode the charge before the ignition system has a chance to ignite it.

Now, on the flip side if the a/f charge in this transient going-to-WOT state is nicely-rich, it requires >10 as much energy to ignite (due in part to a cooling effect from the added fuel) and helps to prevent the a/f charge from exploding. On the gripping-hand the same is true for an overly lean mixture except that...

... in a going-to-WOT state it is very difficult to get a lean a/f charge properly mixed. It is the irregularities in charge mixture of a lean a/f ratio during transient throttle opening conditions that increases the tendency of the charge to pre-ignite (explode.)

I have, above used the terms 'ignite' and 'explode' and I have not used them interchangeably. The flame speed of a nicely-mixed a/f charge when ignited by spark is about 5000 f/s (IIRC) but if the charge explodes the speed can approach 20,000 fps. It is this 4x flame speed and the consequent pressure wave that is the direct cause of the destructive effect of 'knocking.'

mark kibort

I think you are missing the point again. quite simpley, there is very little chance of dentonation lean of stoich, even less of a chance of detonation rich of stoich as you have mentioned. thats the only point. this is becuase the CHT's are much lower at a given power setting or producton when you are lean of stoich. it doesnt matter if you are accelerating or at high or low rpm.
Again, the point is, lean of stoich is safe for operation . too lean of stoich will give you a rough running engine.

see my inserts below with the >>>>>>>>>>>>>>>>>>>

MK



se[QUOTE=worf928]MK I have included quotes from you to provide context. I have edited them for brevity only and not to twist your words



First: If you go full throttle and your a/f mixture is stoich you most-likely have a broken WOT switch. Possibly a fuel delivery problem because ...



... the engineers that designed the 928 motor (and other motors) understand that 1) stoich operation is not the best a/f ratio for WOT acceleration/power and 2) that stoich operation at the highest possible (for stoich a/f) is not the safest (by far) a/f ratio.
>>>>>>>>>>>>>>>>we all agree here. it is just that the 928 engineers have put the mixture for WOT a little too close to stoich for racer comfort and max power thats why a RRFR is used to "fatten"things up a bit for increased safety and power.

Among other things - as you have posted - temps after steady-state stoich operation are higher than for other a/f ratios. This tidbit is important because the energy required to ignite (or explode) a nicely-mixed stoich a/f mixture is 10 times less than that required to ignite a lean or rich mixture.
>>>>>>>>> we agree here as well .

Now why is that important? Because ...



... while the pistons are not intelligent the system does exhibit transient operational characteristics. (If you don't believe that then explain why the 928 motor has fuel pressure dampeners and a regulator hooked to an inversely-proportional-to-load signal (i.e. manifold vacuum.))
>>>>>>>>>>>>>now you are streaching it. just after idle manifold pressure, the vacuum signal is gone and the fuel regulators provide pressure based on fuel flow demands of the engine. I run with out this vacuum line attached in my race car. the system is binary. they pull back fuel pressure for fuel economy at idle and thats it. off idle, then pressure is dictated by the fuel demands and diaphram spring pressure return system.

To illustrate: A motor is operating at 50% power at stoich under steady-cruise. Now, you floor the go pedal. The throttle plate opens and within a fraction of a second the a/f mixture is very lean.
>>>>>>>actually the air has to pass the MAF or AFM and it responds all most as fast as it takes for the air to enter the engine. ive tested this on the dyno. a lot of times the spike is due to the O2 sensor being able to react in the tail pipe.
either way, its not much of an issue

The engine management system (whether it's a 'dumb' carburator or an intelligent computer-controlled multi-point injection system) attempts to compensate by adding fuel to match the increased air flow. Now, you have at least double the a/f charge in the cylinder. The density of the a/f charge is greater, pressure is therefore greater, temperature of the charge as it compresses is higher (PV=nRT) and if the a/f ratio is still stoich the high chamber temperature (due to the previous heat soak from stoich steady-cruise) might just be high enough to explode the charge before the ignition system has a chance to ignite it.
>>>>>>yes, if the mixture gets caught under a sudden WOT to be exactly stoich (unlikely) then you could have a big ping! this is not reality. you never hear about racers with sudden WOT activity EVER having a pinging problem unless their engines have some serious tune issues. i think you are a little off in the time it takes for carburators and AFMs, MAFs to react to increased air flow. its cause and effect. there is already a high vacuum in the intake under decel. then, suddely, you go WOT, air moves to differential pressure. as it moves, it passes through the AFM. air cant move through the AFM unless the barn door is deflected. fuel flow is triggered in milliseconds upon its deflection. same in a carburetor and with a MAF. take a holley 4 barel and blip the throttle. if it didnt have fuel flow, you would have the bog-backfire you get if there is a fuel flow issue based on the venturi pull and accelerator pump of fuel upon WOT!


Now, on the flip side if the a/f charge in this transient going-to-WOT state is nicely-rich, it requires >10 as much energy to ignite (due in part to a cooling effect from the added fuel) and helps to prevent the a/f charge from exploding. On the gripping-hand the same is true for an overly lean mixture except that...

... in a going-to-WOT state it is very difficult to get a lean a/f charge properly mixed. It is the irregularities in charge mixture of a lean a/f ratio during transient throttle opening conditions that increases the tendency of the charge to pre-ignite (explode.)
>>>>>>this is speculation. and it would be an issue with the articles and folks that have operated lean of stoich at high power settings for better fuel economy


I have, above used the terms 'ignite' and 'explode' and I have not used them interchangeably. The flame speed of a nicely-mixed a/f charge when ignited by spark is about 5000 f/s (IIRC) but if the charge explodes the speed can approach 20,000 fps. It is this 4x flame speed and the consequent pressure wave that is the direct cause of the destructive effect of 'knocking.'
>>>>>> hopefully, we all know the differnce of the explosion and ignition for controlled combustion.[/QUOTE]

worf928

It does very much matter if the load on the engine is changing rapidly. That is my point. Other than for steady-state cruise it's all about the transients. No control system can respond so quickly as to keep all operating conditions optimum. Not in this universe. And not, especially, with the 1970s era technology of our cars.

I cannot make heads or tails of the rest of your reply. Makes my head hurt trying to separate out what you've written.

a4sfed928

Mark,
I don't think the Fuel pressure regulator is a two state system as you say. The whole reason for the regulation is to compensate for different manifold pressures or to state it in another way to maintain the same delta P under any load condition between the fuel rail pressure and the engine manifold inlet pressure.

mark kibort

that is not the purpose of the vacuum line. its binary. the diaphram holds a constant pressure, regardless of fuel flow to the engine. manifold inlet pressure has no bearing on fuel pressure in the rails, that is dictated by the fuel regulator based on fuel flow through the injectors.

Mk

mark kibort

This is not true. you can prove this on the dyno.
the rate of change is relatively slow, compared to the responsivness of the system, especaily those like Ljet and LHjet.

define optimum. as you accelerate, the mass flow demands are changing in a very slow fashion. look at fuel air mixture control for brake dynos vs rolling dynos. there is really no difference. want proof. go push on a flapper vane in a AFM while the car is running. push it in real fast..... the car will flood and die. if you rev the engine, watch the vane move in proportion to air flow movement, to the exact volume response that the device is supposed to be metering.

again, what we are talking about is a real lean condition, and that in a real lean condition, there is less heat generated, transfrerd to the heads and there is a reduced chance of detonation.

If there was a problem, wouldnt you think that a car like Andersons run at 13:1 run for hours at a time, shifted at WOT hundreds of times, would have an issue with pre-ignition? especially at near 500rwhp!!?? well, it doesnt.

again, a brake dyno is a good proof that your concerns are not valid.
the car is WOT, there is a steady state measurement of power, and then with some frequency, the rpms are allowed to climp in a quick fashion or a very slow , stepped fashion. i all types of these tests, fuel air ratios are constant, or at least the same as a dynmaic , WOT run similating at 3rd or 4th gear run.

the rest of my reply makes perfect sense. read it again when your head stops hurting and ask away. You even mis-state the way a fuel regulator works. it is not tied to intake manifold pressure, exept for at idle, after idle, there is no vacuum as the fuel pressure in the rails is determined by the diaphram spring pressure to maintain set fuel pressure at most all rpms and loads.

mK

docmirror

Dave, this is a common misconception that I too shared for a long, long time. The quench cooling of the cylinder by unburnt fuel is so very small on modern auto engine as to be unmeasurable. This region you describe, is exactly where knocking occurs. Under high manifold pressures, but increasing RPM. One of the best tests for knocking is to warm the engine fully, then accelerate from low speed in third gear, and listen closely.

I said before that it is not possible to get detonation in LOP. I should clarify, that nothing is impossible, but it's much, much less likely LOP than at peak EGT or rich of peak. There is no added fuel cooling effect. If there were, then knock would be inconsistent with the very conditions we see it in, at high MP and low RPM.

Obviously, a lean-burn Diesel engine detonates. But if we restrict our discussion to a spark-fired, Otto cycle, reciprocating engine, then knock will not occur at F/A ratios of around 0.062 and leaner, given suitable rated fuel for the C/R.

Doc

Larry928GTS

Correct.


Yes it is.

No it's not.

Only if the manifold pressure is constant and the fuel pump is big enough.


Yes it does.

No it's not.

BrianG

This is all very disturbing, especially for those of us who thought we had this all figured out, and who are currently engaged in remapping the A/F maps for our supercharged cars.

Let's take this conversation practical for a moment, then.

In terms of the A/F map for a boosted 928, it was going to be my goal to strive for stoich across the range of normal cruise-power cells with a rapid drift toward 17:1 at part-throttle, coast-down power settings, and a rapid drift toward 12.6:1 at make-power power settings.

It is easy enough to determine the line of differentiation (cruise power) that I was going use as the baseline from which to make the above stated A/F ratio shifts. With the information presented in this thread, it would seem that the target A/F ratio at this "cruise" line ought to be far leaner than stoich. Does this argue toward a simple shift in the A/F mapping target for cruise, or have you guys just gone and made my life FAR more complicated that I thought it was going to be??

mark kibort

larry,

Where did you get your information and whats up with the "binary" responses?

I can summarize your response with one word. "wrong"

Here is absolute proof. why is it, i dont even run my vacuum line on the fuel regulators, yet fuel air ratios are kept constant under varied loads and rpms?

Ill provide some help here. if you measure the vacuum that is provided on the intake side of the throttle plate, you will see that at idle it provdes vacuum. as soon as you rev the engine, even at part throttle, you can see that there will be no vacuum. y ucan see this effect, if you have a fuel pressure gauge too. look at it. when you rev the engine or go WOT, the fuel pressure goes to a higher setting. on my old 84 928 race car it was 52psi and that was when it was maxed out, but at idle it was near 46psi.

Trust me, the fuel pressure is controlled by the spring pressure of the diaphram based on fuel demands of the engine. the higher the demands, the less fuel returned to the fuel tank. thats how it works.

any comments?

MK