CRANKSHAFT TORSIONAL VIBRATION

[RREY]

Hello all. I am new in this forum though I´ve been following some posts for a long time.

We are developing a 4 cyl, N/A 2.0L endurance race engine and so got designed and machined custom billet crank, rods, pistons... Once engine is ready and on the dyno, we´ve found that there is a HUGE torsional vibration from the powertrain!. I´ve neve seen this before!. It happens only at 5200-5500rpm (about 90Hz) and it is so bad that even shears the intake manifld bolts. This is obvioulsy unnaceptable for an endurance engine, specially taking into account that the peak torque is at 5900rpm.


The point is: how is possible the billet crank has such bad harmonics??. How can we move these harmonics to a (much)higher or lower rpm?. How the reciprocating weight can affect this or even compression ratio?

Thank you in advance.

 

[Dave41A]

It is unclear to me exactly what is vibrating. The original post seems to suggest that it is the engine itself that is torsonally vibrating about the crankshaft, not torsonal vibrations within the crankshaft itself (as is evidenced by the damage to the intake manifold...the latter would damage the main bearings or shear the crankshaft itself).

So, if this is the case where the engine block itself is rocking side to side violently, such vibrations will be due to resonance between the natural (torsonal) frequency of the engine block on its stand and any output torques from the engine itself.

To remedy this, I would suggest more heavily damped engine mounts or mounts with a different stiffness. Another idea is to position the same mounts differently, as their lateral separation will affect how the system behaves. Placing them further apart (giving the engine a wider "stance" when viewed from the front end) will change the natural frequency at which the engine wants to rock side to side. If you can raise this natural frequency high enough, you can take it outside of the normal operating range of the engine.

Good luck,

Dave

 

[berserkerx]

Not sure if this issue has been resolved yet. But a simple fix for a lot of i4 engines to rid of bad frequencies is to change the damper on the end of the crank. Putting a polymer based damper on the crank helps a lot.

Can I ask what engine is it? What company manufactured the engine that you are using?