FPC worry? Should I?

[Jimdohc]

Red, nice write up :clap2:

I guess "relatively", "Most noticeable" & "feel" aren't great ramblings

Maybe, less perceivable, is more accurate nontechnical summary?

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[Vickstang]

I'm quite ambivalent about making this post. One the one hand, I'd like to expand/correct a general idea promulgated several posts up. On the other hand, correcting is generally very bad form in venues such as this. Based on the poster's other posts, which are generally spot on (in my opinion), he/she seems to be a decent person and will hopefully not be offended. I'm not getting into a pissing match, period. Does no good and time (for me anyway) is limited...

So, with all that out of the way, this post concerns the secondary imbalance. I will keep it as non-technical as possible, but there will be a tiny bit of 'math'. I will even simplify to the point of being technically incorrect, to get a general point across. If anyone wants to point that out and correct me, no offense taken.

First, a more 'real life' instance of rotational imbalance (this would be a primary imbalance): a washing machine where the load has gotten all fubar'ed and is plastered to one side of the drum. I think we've all seen this (or perhaps this is more an indictment of my laundry skills ). During the wash cycle, the entire machine can/will gradually walk across the floor due to the imbalance. But during the spin cycle, where the drum spools up to a higher rpm to fling water out of the clothes, the machine smooths out. But, the laundry is still in the same position on the drum. What happened? Why did the imbalance appear to lessen?

The imbalance appeared to disappear due to the fact that the force of the imbalance is applied in any one given direction for a shorter and shorter period of time as the rpm increases.

To keep things simple (and brief!) accept the following as a given:

x(t) = a*t*t/2

meaning that the displacement due to a constant acceleration equals half the acceleration multiplied by the time squared (and here's where I'm taking liberties w/ correctness -- I'm using translational dynamics yet taking about rotational dynamics). We also know:

a = F/m (Newton's second law for constant mass).

So let's look at how the displacement (of our washing machine) varies with time. Just picking force and mass values out of thin air, let's say the washing machine mass is 30 kg (approx. 66 pounds) and the force of the laundry imbalance is 10 Newtons (about 2.25 pounds). If we were to apply this force to the washing machine for one second, it would move 0.15 meters -- or about six inches. Now, assume the force of the imbalance acts for 0.1 seconds (I'm making a rough analogy to 600 rpm here). Now the washing machine will move only 0.0015 meters, or roughly 0.06 inches. Not much...

To finish up this little analogy (and before making another post, I guess, with some specific numbers for the 5.2), recall that the force of the imbalance (due to the laundry being on one side of the drum) changes direction. At one instance, the imbalance force might be pointing out the front of the washing machine. At some later time (determined by the rpm of the drum) the force will be pointing directly out the rear of the machine (for a top-loaded washing machine). So the machine is actually getting pushed and pulled by this force. Hence we have something called a harmonically excited oscillator.

The equation(s) for harmonically driven oscillation give the displacement (the motion) as a function of, time, mass, spring constant, and damping constant. And it would be asinine to go into them. Suffice to say, that as the frequency of the oscillation begins to significantly exceed something called the undamped natural frequency of the oscillator, the displacement of the oscillator becomes relatively very small depending on the characteristics of the oscillator and the driving force. And, in this case, the force is being applied in one direction for such a short period of time that the mass (the washing machine) just doesn't move much.

So, this is why an engine w/ an imbalance will appear to smooth out at higher revs -- not because the imbalance itself either disappeared or is very small.

More to come regarding the 5.2 -- a computer run finished up a bit ago so I have to do some work before submitting yet another.

Wow great post! Very informative and interesting, if some of it over my head. Please keep it up Red.

 

[Red]

Thank you folks. Much appreciated, Jim, thanks. I was quite skeptical how the washing machine abstraction would play out!

Here are a couple specifics (as I calculate them) for the 5.2 w/ a flat crank. If you want to run some numbers yourself, take a look at:

"The Internal Combustion Engine in Theory and Practice. Volume 2: Combustion, Fuels, Materials, Design" by C. F. Taylor, First Ed 1977, Revised Ed 1985.

On page 259 is equation (8-31). That's what I'm simply running numbers through. I won't given the equation, but what's important about it is that the force (that is the imbalance) increases w/ the square of the rpm. That is, if the force is, say 10 pounds at 1000 rpm, it will be 40 pounds at 2000 rpm, 160 pounds at 4000 rpm, and 640 pounds at 8000 rpm. That is, every doubling in rpm causes the force to increase by a factor of four. Another important point is that the force varies linearly w/ reciprocating mass (more below). So, doubling the mass doubles the imbalance force.

We need to know a few particulars about the engine. Let's take the stroke to be 92.7 mm, same as the 5.0, since whoever JarStang interviewed (sorry, don't remember) said the increased displacement was due to increased bore. Let's also take the connecting rod length to be that of the 5.0: 150.7 mm. I feel reasonably sure about these dimensions. Finally, we need some estimate of the mass that is considered to reciprocate w/ the piston. This will include the masses of the piston, wrist pin, rings, some part of the upper end of the con rod, a little oil, wrist pin retainers. I'm going to take this total mass to be 0.85 kg (400 gr piston, 120 for the pin, and 320 for the rings and upper part of the rod).

Finally, remember that this unbalanced secondary force acts horizontally, across the engine -- from left to right as installed in the car.

So, let's specify 8000 rpm. Care to guess what the side (aka shaking) force is for the entire engine? 10,820 pounds. Now, before everyone panics and/or says I'm full of shit, hear me out for a second.

Recall that we're dealing with a harmonically driven oscillator here (the engine being the oscillator with the 'driver' being the secondary imbalance). Pulling out a Theory of Vibration book I have (from about 114 years ago -- it was printed in 1972 and was only a few years old when I took the course!) and picking a relation that doesn't exactly model what we have here, but should be reasonably close, I get that the engine (440 pounds or 200 kg of mass) will have an amplitude of oscillation of only ~1/8". That is max left to right motion. As Jim said, in effect, BFD.

Also, to put the 10,820 pounds of force in perspective, consider the force acting on one piston, rod, crank throw due to piston acceleration at TDC. At 8200 rpm, and using the above masses and dimensions, it is 8,540 pounds. So, while 10,820 is a lot of force, it is for all eight cylinders as well.

Finally, if at some point we know or have a better estimate for the reciprocating mass, we can easily multiply 10,820 by the ratio:

(revised mass in grams)/850 grams

to have a better estimate of the secondary imbalance force.

Just thought some specifics would be of interest. I'm going to eat and return w/ one more short post where I actually comment on what worries me (and it is not the secondary imbalance) and ask what you all think of it.

 

[Red]

Here's what concerns me. In JarStang's interview, it was stated that a dual mass flywheel is used. I imagine (but could be wrong, will welcome additional understanding anyone can provide) that the "DMF" is employed to act as a large torsional damper for the crank. I believe the secondary force acts at each crank pin. If it does, the crank will be see a lot of torsional harmonic stresses. Fine --- I assume the DMF adequately handles this. My concern is w/ the DMF itself.

DMF's were put behind Powerstroke and Duramax diesels with the manual trans (I do not know if this was also done for the Cummins & manual trans). I happen to have owned both, and still own the Ford (awesome truck, btw). But, the DMF is/was pretty much universally hated on various diesel truck forums. I didn't hate mine, but .... it failed at ~120k miles (they all failed at some point, just about). I have to wonder about the reliability of this item, esp. w/ repeated 8000 rpm pulls. Without a doubt, a DMF adds more possible points of failure over a 'regular' flywheel.

Wondering if anyone else has any thoughts about this. Thanks.

 

[Trackaholic]

I have a 350z with a DMF. Now that car has a V6, which unbalanced and has a non-positive torque profile during portions of each revolution. The V8 at least gets rid of the second issue by having a greater number of power strokes per revolution (4 compared to 3). Now the DMF in the 350z is also not loved, but I think the main reason is because it is relatively large and heavy.

At one point I replaced my stock DMF with a lightweight single mass unit. This improved throttle response at the expense of low RPM smoothness. When the clutch wore out I eventually went back to the stock unit because at the time I was still using the Z as a DD and wanted the smoother performance. Nowadays I kinda wish I had kept the aftermarket unit in place.

In any event, I would guess that the dual mass unit is there to help improve smoothness in street driving. Hopefully it really is lightweight and therefore doesn't affect throttle response too badly. I'll admit to being a little surprised to see the DMF being advertised as a feature, but am willing to give Ford the benefit of the doubt on this. It is possible that it will really help the daily drivability of the car. If not, there's always the aftermarket.

Here's a link to a company that makes DMFs, that explains the reasoning behind them:
http://www.luk.de/content.luk.de/en/products/clutch_systems_new/zms_new/zms_new.jsp

-T

 

[Jimdohc]

...DMF is/was pretty much universally hated on various diesel truck forums. I didn't hate mine, but .... it failed at ~120k miles (they all failed at some point, just about). I have to wonder about the reliability of this item, esp. w/ repeated 8000 rpm pulls. Without a doubt, a DMF adds more possible points of failure over a 'regular' flywheel.

Wondering if anyone else has any thoughts about this. Thanks.

True, I does add a point of failure. I'm not too worried about it. 1 extra point out of 1,000+ won't deter me. Plus, if needed, I can live with extra cost of swapping flywheel & clutch every 100k (like a timing belt in some older cars) <-- I'm not saying it's required... just IF. Personally would consider it a maintenance cost, not a reliability issue (once again IF required). In a diesel, where reliability, longevity and saving money are all high priorities. I can understand the disappointment.

My bother-in-law's 4th gen Jetta w/ 1.8t & DMF failed at around ~160,000 miles / 6yrs. It was his first new car after college and drove it hard. DMF & dry rotted fuel pressure regulator vacuum hose were the only problems he ever had with that car. I swapped the flywheel (& vacuum hose & cleared the code) and he drove it another year with no issues. He bought a new Passat. Something in the steering column broke the second week. Never can tell.

 

[Jimdohc]

Could we calculate the permissible unbalance for a given device to maintain reasonable bearing life? Then we'd know how light to design the pistons & rods and/or how heavy the crank, flywheel & other needs to be.

-set bearing life goal
-estimate minimum piston & rod weight, give our budget
-calculate the required weight of crank, flywheel & other items.
-test
-add weight to crank, flywheel &/or others. Or spend more money on lighter pistons & rods.
-test

once unbalance is within permissible limits. design engine mounts and other devices to reduce effects on driver and prevent vibrating the bumpers off the car.

 

[Red]

Interesting about the V6, Track.

From what I've read, a DMF was put behind the diesels to eliminate the transmission of the so-called 'diesel spike' to the trans. I now have a regular flywheel; for the first month or so w/ the regular flywheel, there was a lot of very bad sounding rattle and gear noise from the ZF5. Played around with lubes some, and I think things just wore in more over the years and now it's all pretty quiet again.

I realized when I said my DMF failed at ~120k miles that that wasn't much of an indictment against its reliability. Supposedly, according to various forums, they usually failed much sooner. Take everything w/ a grain of salt...

I have to wonder if there's another reason for the DMF in this car. The car is supposed to be 'hard core' (or whatever you want to call it), so I don't see why a super smooth clutch would be needed. We're getting a flat crank, so smoothness and refinement just took an immediate hit right off the bat.

And I'm not a real fan of dropping trannies and replacing clutches and flywheels, at this point. I've done it plenty of times, on my back, and would like to put that in the past, for the most part!

 

[Red]

Jim, I don't know -- could "we"? Do you know how many "we"'s I've been part of?

Anyway, in all seriousness, I suppose "one" could, yes. I wrote a little code to compute some metrics given stroke, rod, bore, rpm, etc. I have been thinking about adding this secondary imbalance stuff to it to get total forces (to first order) acting on the crank, rods, block, etc as a function of crank angle. This would force me to understand one or two things better, which would be good.

But... I am not an automotive engineer, which is likely immediately apparent to those who are -- I'm more a fluids guy, which entails (for me) mathematics and physics. Plus, I tend to be quite busy (really busy). Plus, I've got my old Merc still in pieces in the garage that I really want to get to. The engine came back from the dyno in June, and it's just been sitting (it was fogged at the conclusion of the run, so it should be fine for a while, but still...). And I have a lot of things to do to the car before I can get the engine running.

I ramble. To the point: Yes, I suppose "we" could do what you're considering, but it will take time and effort. I don't have that much time, I'm looking for a little relaxation, and I can't be held to any sort of a schedule. Just the simple addition of the secondary forces to the little code I have might not happen for six months (and that's just a wag -- I have no idea). And bearing life is way out of my skill set (w/o having to learn it all from scratch) -- that would have to come from you.

 

[Tim Hilliard]

Interesting about the V6, Track.

From what I've read, a DMF was put behind the diesels to eliminate the transmission of the so-called 'diesel spike' to the trans. I now have a regular flywheel; for the first month or so w/ the regular flywheel, there was a lot of very bad sounding rattle and gear noise from the ZF5. Played around with lubes some, and I think things just wore in more over the years and now it's all pretty quiet again.

I realized when I said my DMF failed at ~120k miles that that wasn't much of an indictment against its reliability. Supposedly, according to various forums, they usually failed much sooner. Take everything w/ a grain of salt...

I have to wonder if there's another reason for the DMF in this car. The car is supposed to be 'hard core' (or whatever you want to call it), so I don't see why a super smooth clutch would be needed. We're getting a flat crank, so smoothness and refinement just took an immediate hit right off the bat.

And I'm not a real fan of dropping trannies and replacing clutches and flywheels, at this point. I've done it plenty of times, on my back, and would like to put that in the past, for the most part!

LOL a lot of my time was spent on my back between Super Duty's and 6.5 GM's. The Clutch repair always contained a new solid flywheel and a sprung disk Not a fan of DMF's. The springs would get wiped out and chatter like a MOFO, PITA to resurface, nothing good about them IMO. In my area they failed quickly, traffic, snow plowing. Did not last long. Saved by the aftermarket thankfully. Again a failed attempt to make bad drivers good drivers. My assumption it's in the 5.2 to isolate larger harmonics to transmission.

 

[oldlugs]

...this is new ground for ford. Should I be worried about longevity? I know testing is probably crazy good, but they have never done a FPC before...

Unless you love revving past redline (too often), I wouldn't worry. Ford hasn't made flat plane V8s for production road cars, but no doubt that they must have had some input in the manufacture of the Ford/Cosworth racing V8s of the '60s & '70s. Surely at least, they had learned something from those, which I believe were considered some of best racing V8s of their day.
Apparently Ford had commissioned Cosworth to build them, but even if Ford just used 'em for engineering studies (very likely), how could they go wrong?
A Cosworth / Ford DFV V8:

BTW - The Ford DFV made max torque at 8500 rpm, and 465 hp at 10,500. Displacement was only 3 liter!
Voodoo has a new, stiffer block casting than Coyote, and you can be sure that Voodoo's crank is made of good steel.. Keeping the revs to a conservative level should have them lasting plenty long for their intended purpose (a sports car).

 

[Red]

Tim, do you swear much when dropping trannies? Yeah, me neither.

I actually did not do the clutch job on the truck due to work demands. Was keen to see the DMF, but no way around it. Most of my experience in this area is w/ my 82 Merc -- always fooling around w/ the clutch. I think I still have a depression in my chest from that damn drain plug on the bottom of a Toploader.

I've seen many of your posts and appreciate them. We share the same views. I agree about the need for the DMF -- I 'think' it's a crank damper actually. Could be wrong, of course, and welcome correction.

I'll be gone for a while beginning tomorrow. Will check in when I can. Cheers all.

 

[Tim Hilliard]

Tim, do you swear much when dropping trannies? Yeah, me neither.

I actually did not do the clutch job on the truck due to work demands. Was keen to see the DMF, but no way around it. Most of my experience in this area is w/ my 82 Merc -- always fooling around w/ the clutch. I think I still have a depression in my chest from that damn drain plug on the bottom of a Toploader.

I've seen many of your posts and appreciate them. We share the same views. I agree about the need for the DMF -- I 'think' it's a crank damper actually. Could be wrong, of course, and welcome correction.

I'll be gone for a while beginning tomorrow. Will check in when I can. Cheers all.

LOL you didn't miss much, a flywheel with a spring loaded friction surface. Great in theory....

 

[Jimdohc]

Talked to some friends. They reminded me R34 GTR & Supra turbo had DMF. Both cars share a common Getrag 6spd (different case & ratio but same design). Without DMF drivetrain vibration takes place around 2,000rpm. R32 & 33 have the same engine as R34 but have different transmission & no DMF. They also don't have drivetrain vibrations unless ultra lightweight aftermarket flywheel is used. Simple answer, in that platform, DMF is used because of transmission.
They also said DMF can have performance advantages. If clutch disk springs are removed. Basically, moving damper mass from disk to flywheel. Lighter disk/s allows quicker shifts. For twin disk, mass of two springs sets, could be a significant. As a bonus, mass transferred to flywheel can help with clutch engagement, dampening engine vibration & allows lower idle rpm. So, if you have to have dampener mass. It's better to have on flywheel than disks. Of course, I don't know if GT350 is designed this way. Just random ramblings.

 

[RocketGuy3]

That Engineering Explained video that's been going around was indicating that FPC V8s are meant for higher end supercars and race cars that are not intended to be driven that much... saying that they have longevity concerns.

I sure hope that's not the case with the 5.2. Mustangs are made to be driven.

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