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Harmonic Dampeners
3

Harmonic Dampeners

Harmonic Dampeners

(OP)
Can any one explain how a harmonic dampener on a V8 works? Is it tuned to the natural frequency of the crankshaft? Does it dampen the torsional twist of the crank from the power pulses? Why does one dampener work for cast/steel cranks? How can sprint car motors run with out one? Thanks!

RE: Harmonic Dampeners

It's tempting to make a joke about what a "harmonic dampener" might do (spit on you while singing?)

A torsional vibration damper is used to limit the amplitude of the torsional (twist) oscillations of the crankshaft.  

The oscillations are excited (caused) by both the firing pulses and inertia loads transmitted to the crankshaft by the reciprocating assembly (and to a much lesser extent by the timing drive and/or fuel system for high pressure injection).  These excitations are made up of many harmonic components.  The crankshaft+flywheel+rods system has a number of different mode shapes that it would like to take while vibrating, each with its own associated frequency.  Crankshaft vibration is greater when one of the harmonic components of the excitation is occuring at the same frequency as one of the crank's vibratory modes (when resonance occurs).  

Torsional vibration dampers come in four main varieties:
  1) mass-elastic-damper  <- typical rubber damper design
  2) viscous damper <- seen on large engines, also available in aftermarket for autos
  3) visco-elastic damper <- seen on large engines, probably available in aftermarket for autos
  4) mass only (absorber not damper) <- seen on aircraft engines, available on aftermarket ("Rattler" brand, for one, I believe)

Type 1 must be tuned (correct stiffness, damping, and mass) to work well with the crank system.  It will often be selected to split a big resonance hump in the running range of the engine into two smaller humps, perhaps with one of them above the running range of the engine.  It is not unlikely that changing from a cast iron to a steel crank without changing anything else will not have a large enough effect on the crankshaft vibratory mode frequencies to make a huge difference in damper tuning.  You could no doubt do better with a custom-tuned damper, but the original might be good enough.

Type 2 is "sized" rather than tuned.  It simply absorbs vibratory energy from the crank, and doesn't have the "hump splitting" effect of type one.

Type 3 must be "sized" and tuned, but is more similar in behavior to Type 2 than to Type 1.

Type 4 is a special case - it doesn't really absorb energy the way the other three do - it produces a counter-excitation at a specific engine order.  It's very useful when you have one particularly annoying excitation order to deal with, such as when you're running a fixed-speed engine.


There are lots of reasons why some motors can run without the dampers; I don't know which might be the case with sprint cars.  If an engine runs at speeds that put it below or above its major resonance humps, then a damper may be unneeded.  If the crank system mass has been significantly reduced, it's possible that some of the vib. frequencies would shift out of the running range.  If the crankshaft is strong enough that it won't break anyway, then damping the vibrations would be unneccessary (from the point of view of the crankshaft stress, anyway).  If the crank life requirement is short enough, then you might allow damaging vibrations to occur with the expectation that you'll replace the crank regularly.

RE: Harmonic Dampeners

How could I go about some type of determination as to the needs for damping on a sprint car engine?
Can the crank assembly supplier advise on this or other net info be found?
The sprint car is direct drive with an auto type flywheel for starting and use of a damper, all in a balanced assembly.
 

RE: Harmonic Dampeners

The main reason for a torsion dampener is to prevent transmitting a lot of "rattle" down the drivetrain to transmissions, etc., where it may cause trouble.  Interestingly, I've worked on a few big V8's with torsion dampeners on the front belt drive pulley too (to keep from getting a lot of vibrations transmitted into the belt driven accessories).

You can also lower the frequency of the drive system to avoid resonances, typically by putting more flywheel inertia on the drive shaft (either in front or back).  This can cause other trouble, not least of which is the expense of more metal, and precisely balanced metal at that.

I spent some time working with V-twin engines in the 20-hp range.  A few manufacturers made these with so-called "Harley" timing, where both pistons fire on the same revolution.  The resulting torque "kick" was more than 10x what we were expecting, and played havoc with pumps & blowers that the motors were to drive.  We added dampening couplers and flywheel inertia to avoid resonances with the attached equipment, and managed (eventually) to get reasonable service out of the machines.

RE: Harmonic Dampeners

This is a topic I've been stewing on for a while.  I'm no expert and will ask more questions than provide answers.
I understand the concept of harmonics and have some ideas about how it can be measured.  sadly, none within my scope.  I race a non Chevy Non V-8 so it's probable that a proper study has never been done.

So,  Thinking about it in terms of a spring/damper equation I came to realize that the inertial ring is precisely that.  Essentially analogous to a fixed ground plane in the spring /damper calcs.
So, if I'm on the right track there, is it safe to say that the weight of the inertial ring is not really a factor in damper selection as long as it is heavy enough to provide the inertia for the reaction between crank and rubber damper?  I haven't yet sorted out the reaction from too much weight but I'm still thinking.

Where I'm going is that I need to select an off the shelf Ford/Pontiac based damper to suit my six cylinder application and trying to comprehend the variables of that selection.

I think my question is, can I be overdamped with too heavy an inertial ring and am I approaching the problem from the correct direction?
Thanx in advance, Steve

RE: Harmonic Dampeners

GT6Steve

The inertia of the damper controls the frequency difference between the two resonances, and also controls how much energy can be absorbed. A rule of thumb is that the interia of the absorber needs to be 10% of the interia of the mode you are trying to control. That rather begs the question of how we work out the modal mass - for a crank take the rotational inertia of about half the crank.

"The main reason for a torsion dampener is to prevent transmitting a lot of "rattle" down the drivetrain to transmissions, etc., where it may cause trouble.  Interestingly, I've worked on a few big V8's with torsion dampeners on the front belt drive pulley too (to keep from getting a lot of vibrations transmitted into the belt driven accessories)."

No. The main reason is to stop the crank breaking from fatigue. The NVH benefits are secondary to that on most production cars.

If you think about it it is obvious that a tiny dynamic abosrber won't have much effect on a large system.

The other part of he tune is the stiffness of the rubber in the damper, and its damping characteristics.

Try and pick a damper that comes off an engine with the same length and weight of crank as your engine - that might be close enough.

Cheers

Greg Locock

RE: Harmonic Dampeners

Sorry Greg, you're the expert.  I was thinking more of torsion dampeners that are added on (by OEM/users) to commercial engines, for the purposes of reducing torsional vibration inputs to attached equipment (pumps, blowers, generators, etc.)

RE: Harmonic Dampeners

The way we describe crank harmonics (vibration) in a simplified manner is thus:

Imagine a thin section shaft welded to a large diameter and relatively high mass "flywheel". If momentary torsional loads are applied at four locations along this shaft the far end of the shaft will experience a twist relative to the flywheel end. This twist is a helix with a starting point at the flywheel. Depending on the applied loads and stiffness of the shaft/natural frequency of the shaft, etc., a considerable displacement can occur. (Angular velocity difference from end to end).

If you observe the flywheel and pulley of say a SBC V-8 on a dynamometer at WOT, full load, from say 4500 RPM to 9500 RPM with a strobe light at each end (one on the flywheel one on the pulley) one can see how much oscillation is occuring at each vibration node at each end.

A heavy flywheel will exibit little vibration as compared to the opposite end of the crank. Almost all of the crank failures we have seen occur at the flywheel end of the crank.

In a sprint car with no flywheel the relative end to end crank twist (torsional amplitude) is much reduced. Hence, maybe a harmonic dampener can be eliminated.

Will    

RE: Harmonic Dampeners

(OP)
Thanks guys!

RE: Harmonic Dampeners

A sprint car still has a load that tends to keep the output end of the crank at constant speed, whereas the other end is only restrained by the stiffness of the crank, and the inertia of the water pump.

Maybe its the lack of mass in the drive train and the flex in the tyre walls that dampens the crank in a sprint car, or maybe the crank life is not so critical as performance.

The mass of a damper will decrease acceleration performance due to the inertia of the extra rotating mass.

Regards
pat   pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Harmonic Dampeners

Hi Steve,

I have a '69 GT6+ and did 120,000km around the US in it. One of my favorite cars!

The dangerous harmonic for the GT6 crank is around 7200rpm. My cam ran out of breath at about 6500 so I never bothered changing the damper. Some people have fitted Ford 5.0 fluid dampers which seem to work. There is a guy in the UK (John Wood) who made dampers for the Triumph 6 but I just looked up his site www.classicracecraft.com and got nothing. I have an extra Weber dcoe manifold set if you need one.

cheers, derek

Join us at http://groups.yahoo.com/group/opensourcecars

RE: Harmonic Dampeners

Hello Derek,
Will you contact me direct at GT6Steve@aol.com?  Steve

RE: Harmonic Dampeners

Is tuning of a dampener necessary on diesel engines that have a max rpm of 3500? Is any good rubber dampener okay?

RE: Harmonic Dampeners

I don't think you'll absolutely have to have one as a diesel's crankshaft is already a fair bit stronger than a petrol engine's crank.

Just to be sure find a similar sized engine to yours and have a look.

Cheers

Greg Locock

RE: Harmonic Dampeners

but Greg, diesel crankshafts are still usually "near the edge" regarding stress, aren't they?  That's why they often come with the (expensive) viscous or visco-elastic dampers?


RE: Harmonic Dampeners

Oo er, perhaps I made an assumption there that I shouldn't have. When I saw a 3500 rpm red line I assumed it was automotive size, in which case the first crank resonance is likely to be well above the red line (unless it is a V12! or a two stroke).

Typical crank torsional is 300 Hz plus.







Cheers

Greg Locock

RE: Harmonic Dampeners

This dampener will be used on a new diesel engine design. It will have a new sectional crankshaft with HD needle bearings on the mains and rod journals.

RE: Harmonic Dampeners

Greg, sounds like both of our assumptions may have been wrong - I was figuring that the engine in question was likely an I6, 6L to 7L engine, which was being "tuned" for high power output (500hp+).  It's been my experience with such designs (I've only seen a couple, though) that a hi-perf damper is critical for keeping crank stress acceptably low.

kmb - I'm not sure that I understand what you mean when you say "sectional" crankshaft.  Would you mind elaborating on that?  Also, how many cylinders will this engine have?  Just one or two?  The proper way to determine the damping requirements, if there is any question about whether the crank will survive, is to perform a fairly rigorous analysis (or have it done for you).  I would suggest using a detailed FEA with both kinematic + dynamic crank loading applied to check crank stresses (@fillets, webs, oil drilling breakouts, etc).  If crank stress is acceptably low without a damper, or with a "generic" damper on the cranknose, then don't worry about it - otherwise, run a few torsional vibration analyses to help select a damper that will give acceptably low crank vibration, and/or strengthen the crank.  If you can't do this in-house, then there are plenty of consultants out there who can do it.
 

RE: Harmonic Dampeners

This crank will be used on a three, four and six cylinder in-line engine application. The crank is sectional. The wings are billet machined so that all are the exact same. The main journals as well as rod journals are also billet machined. The parts are put together by the use of induction heating or pressing. The journals are designed with a flat side that fits against a flat side in the wing. Kind of a D shaped joint. There are no welds or oil gallies. All the parts are 4340 alloy. These designs are patent pending. The bearings will be heavy duty needle bearings with inner races that are pressed over the machined journals. With the inner races pressed over the journals the needle bearing slides over the journal before the wings are pressed on the journals. The engine will use a standard main cap and rod cap arrangement. Since all parts are CNC machined,I hope the finished product is already in balance. I still want to use a dampener however, one with a serpentine belt groove for the alternator. We will be using a gear driven coolant pump.

RE: Harmonic Dampeners

the finished product is already in balance. I still want to use a dampener however

Note- the function of a torsional vibration damper has nothing to do with engine balance.

If you want to size and select a TV damper, do the TV analysis and the crank stress analysis, and assess your needs.  

If you'd like to have a consultant do that analysis work for you, there are a few good ones in the US.  I'd recommend Ricardo, Inc., because of my favorable impression of the experience level of their engineers.  http://www.ricardo.com/portal.asp&nbsp;  Many of their competitors would also be able to perform this analysis adequately, I assume.

RE: Harmonic Dampeners

I understand the dampener, but we also intend to internally balance the engine and I feel that this cnc part aproach will hopefully eleminate the need to balance the crankshaft. Other parts will need to be balanced as well. I appreciate the recomendation. I'll contact them and see what they have to say.

RE: Harmonic Dampeners

I always thought roller bearings had durability problems when subjected to shock loads from detonation.

Diesels run in a state of constant detonation?

Regards
pat   pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Harmonic Dampeners

the cylinder pressure trace from a typical diesel doesn't look all that much different (to the eye) from that of a gasoline engine - the biggest difference is that the diesel has a much higher peak pressure.  The general shapes of the curves are very similar (no step changes in pressure or anything like that).

RE: Harmonic Dampeners

These engines are all four cylcle. We will be using pilot injection and that helps with the detonation issue. The bearings are 4" od full compliment needle bearings. We are not completely sure that this will work but the force is being spread accross two of these bearings with each combustion event. 7 mains on six cylinder.
Mike

Thanks,
Mike

RE: Harmonic Dampeners

IME the pressure traces for diesel can be higher, but not necessarily so in general. For example, I try to limit the 900+hp turbo v6 SI engine I have to about 1600-1700psi max. It shows no signs of undo duress at this pressure in a 1/4 mile drags application. Yes I do have cyl pressure gear for home use :) One of the 4 cyl project turbo diesels I worked on was limited to 1800 psi max, per the mfgr. This was an emissions reduction project and we had full authority electronic control over injection timing and quantity, exh backpressure, etc, while also always monitoring cyl pressure in real time, etc (dedicated analog scope). Depending on injection timing and such, there can be a noticable step as the cylinder pressure transitions from primarily compression pressure to heat release pressure. One can also readily hear the change in how the engine "hammering" sound changes as the pressure curve changes shape. I sat there for many weeks at a dyno cell and tuned this particular engine for power targets, NOx + HC emissions tradeoffs ;) etc. And of course no, diesel combustion is not detonation :) An interesting topic, sorry to stray off the orginal damper topic for a moment...

TurboTR

RE: Harmonic Dampeners

hmmm..  I'm more used to seeing PCPs in the range of 2500-3100psi (165bar to 210bar) for modern diesels.  It's been a while since I looked at a PCP for an automotive gasoline engine, but I thought that 1750psi was pretty high for gasoline.

RE: Harmonic Dampeners

PS we also ran pilot injection on some other project diesel engines. Really an amazing quieting effect available from it- we even had an on/off switch to dramatically demo the effect. I can't wait until it catches on with all the annoyingly loud, clattery diesel p/u trucks around here, lol... And for emssions restriction to tighten on them as well <gack>. Anyway, basically pilot injection spreads the heat release curve out and greatly reduces the "hammering" clatter effect resulting from rapid heat release. Since direct diesel injection occurs right near the start of the power stroke, a diesel is constrained to getting all the fuel in and getting it burned rather quickly. It must all get into the cylinder within milliseconds in an intense, high pressure shot, depending on the load, rpm, etc of course. A port EFI engine otoh can have fuel squirting at the intake valve for 720 degrees of rotation if need be :)

TurboTR

RE: Harmonic Dampeners

I agree, 1750 is probably getting high for SI. Our engine supplier stated clearly that 1800 was the limit on this engine. 3100 psi- wow!!! If we had say a 4" bore, that would be a peak force of over 38,000 lbs pushing down on the piston (!!) The equivalent of having 11 average weight cars stacked up on top of it...

TurboTR

RE: Harmonic Dampeners

yeah, and a 4-inch bore is on the small side...

RE: Harmonic Dampeners

Like I said, even if for the wrong reason, very high point pressures on a roller bearing.

Regards
pat   pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Harmonic Dampeners

An interesting article -nice color diagrams also.

www.luk.de/publish/OR/ get.rendered?t=PubLanguage&i=12257&a=PubFile

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