4cyl. crank damper

[SMOKEY44211]

All of the 4 cyl. engines I have run across operate without the use of a crank dampener. V-6 some do some don't. V-8's universally have one. I understand the dampeners function but fail to recognize why it appaerantly not needed on a 4cyl.

 

[ivymike]

short crank with lower-order primary excitation (and plenty of main bearings - if you get a 4cyl with 3 mains, it'll probably have a damper)

 

[GregLocock]

Not true anyway. Isuzu Piazza had a crank damper, at least at the development stage.

Generally you only fit a crank damper if you are worried about NVH (they are clearly audible) or more usually if you are running into excitation of crank torsion and bending modes by firing order. Since 8s tend to have longer cranks and so lower resonant frequencies, and more firing cylinders so higher frequency exctation, they need TV dampers.






Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

eg

http://www.allpar.com/fix/headgasket-2.html

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

yeah, I think all the 4-bangers I've ever owned have had them too... I took the question to mean "how can you get away with not having one" and then added the bit about the mains based on the "vibrating pretzel" lesson Rod taught me a few years back.

 

[R1100S]

I've seen plenty of harmonic balancers on 4 bangers. I think the reasons for leaving them off is economics, and the fact that in econo-boxs no one minds a little extra vibration.

 

[WilliamH]

A four cylinder INLINE engine designed in the last 30-40 years or so will not benefit from a torsional crank dampner. Reason is the crank will be built quite robustly (very little harmful rotational harmonics).

However, this same engine will exibit violent vibration modes, NOT due to crank harmonics, but imbalance in other areas (planes) such as horizontal plane imbalances.

Mitsubishi fixed this with their "balance shaft" system. This is composed of two counter weighted shafts rotating in the crank main bearing plane.

Cheaply built engines (spildly crankshafts) can benefit, or need a dampner. The Ford "Model A" is an example. An extremely cheaply built engine with a very weak crankshaft. Put almost ANY viscous dampner on the crank and main bearing life will be extended by thousands of miles.

 

[Deltona]

I run a Fiat/Lancia twincam 4 cylinder in my car, that has a crank damper, balancer (Lanchester shafts) and a very strong crank on 5 main bearings.....

 

[ivymike]

As noted above, harmonic cranksoakers can have a beneficial impact on NVH (and reportedly on the life of front-end-driven equipment). Cranksoakers have little-to-nothing to do with balance.

 

[GregLocock]

"A four cylinder INLINE engine designed in the last 30-40 years or so will not benefit from a torsional crank dampner. Reason is the crank will be built quite robustly (very little harmful rotational harmonics)."

Kind of flies in the face of the evidence? Do you think we fit them for fun?





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[WilliamH]

"Kind of flies in the face of the evidence? Do you think we fit them for fun?"

Not for fun, but possibly to off-set poor (flexible) basic crank design?? If you have accounts in your design office, "... you got problems".

Case in point:

The Porsche 944 2.5L & 2.7L inlne fours had no crank dampners. The main bearings (five) on these engines run with no wear for over 300K miles. The crank shaft design on these engines makes big block Ford & Chevrolet's look anemic. The engines did use Mitsubishi style "balance shafts".

Another point that should be addressed is the fact that the flywheel end of the crank can have a dampening effect also. This varies as to just what is in the assembly (clutch & damper springs/rubber, etc).

 

[GregLocock]

The flywheel has very little effect on the problems for which you fit a TV damper on an I4, as it is nodal for the resonant modes of interest.

All engineering designs are compromises. I am astonished to hear that the 'correct' solution is ALWAYS to make the crank bigger, as opposed to fitting a TV damper. If you think about the development life of a typical engine you'll probably agree that over time there are good reasons why the main bearing and big-end diameter tend to be constrained.

Your case in point proves little. Many I4 engines have TV dampers, and at least on some engines they are there for good reasons.

Given that 98% or more of automotive manufacturers go broke, I think that an awareness of costs is not a bad approach.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[anyoldname]

Well said Greg,

Every I4 (and probably every other configuration too) I've seen go through our consultancy has had a TV damper on it. We've worked on a really wide range of cranktrain design and development programmes, some of which have really pushed the crank design and they've all required a damper...... not just to help FEAD durability, which isn't as important on race applications.

Every new cranktrain for production requires TV testing (supported by analysis) to optimise the damper.

As for the statement that the crank is always being made bigger and heavier to solve problems, perhaps I need to suggest throwing away all the expensive FEA work we do and just start adding lots more material on instead.