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Vibes in V8's with 90° and 180° cranks


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#1 Fondles

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Posted 12 October 2013 - 11:16

I know that a 90° V8 with a conventional 90° crank has very few first and second order vibrations. And the same block but with a 180° crank does have some uncancelled vibes but they are usually low enough to not damage anything.
What about 72° V8's ? Does the flat-plane crank make things worse or no real difference?



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#2 bigleagueslider

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Posted 13 October 2013 - 03:24

A 90deg V8 using a "cruciform" (2 plane or 90deg) crankshaft can be made to run with very low dynamic vibration if the counterweighting is properly distributed.  A 90deg V8 using a "flat" (single plane or 180deg) crankshaft will always suffer from a secondary force imbalance, unless some form of balance shaft device is used.  The secondary force imbalance condition created by using a flat crank in a 90deg V8 can cause serious problems due to vibration.  And the vibration problem gets exponentially worse as the engine stroke length is increased (recall Cosworth's DFL engine).  The flat plane cranks used in most F1 engines did not present too much of a vibration problem due to the fact that they had very short stroke dimensions.



#3 gruntguru

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Posted 13 October 2013 - 23:52

I'm sure Manolis will be along soon to answer your question Fondles.



#4 Magoo

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Posted 14 October 2013 - 14:29

I know that a 90° V8 with a conventional 90° crank has very few first and second order vibrations. And the same block but with a 180° crank does have some uncancelled vibes but they are usually low enough to not damage anything.
What about 72° V8's ? Does the flat-plane crank make things worse or no real difference?

 

If you do a forum search you will find a number of decent discussion threads on this very topic... and some tiresome ones, too. 



#5 manolis

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Posted 14 October 2013 - 16:59

I know that a 90° V8 with a conventional 90° crank has very few first and second order vibrations. And the same block but with a 180° crank does have some uncancelled vibes but they are usually low enough to not damage anything.
What about 72° V8's ? Does the flat-plane crank make things worse or no real difference?

Fondles,

the 90° V8 with 180 crank (flat crank, all cranpins are at the same plane) has the same inertia torques with the conventional 90° V-8 with the cros-plane crankshaft (crankpins at 0, 90, 270 and 180 degrees) because both are even firing.

The 90° V8 with 180 crank has heavy unbalanced inertia forces of 2nd order, while the crossplane crankshaft 90° V8 is balanced as regards its inertia forces.
If you take the one bank of the V8 with the flat crankshaft, it is a conventional four inline. The inertia force of this V8 equals to 1.4 times the unbalanced inertia force of the four inline, which is strong. The only way to balance this 2nd order inertia force is by adding a pair of balance shafts rotating with double crankshaft speed.

The 90° V8 flat crankshaft has some unbalanced 2nd order inertia moments, while the crossplane 90° V8 has no unbalanced inertia moments.

On the other hand, the V8 with the plane crankshaft needs not balance webs on the crankshaft for its balancing, while the crossplane V8 needs heavy balance webs otherwise it gives a strong inertia moment (rotating) of 1st order.


Case of the V8 with 72° vee.

For even firing, you need offset crankpins; then, as even firing the V8 72° has the same inertia torques as the previous two arrangements. But you cannot have offset crankpins in a flat crankshaft.

If you use a flat crankshaft, the uneven firing V8 72° (combustion intervals: 72, 108, 72, 108, 72, 108, 72, 108 crank degrees) has significant unbalanced 2nd order inertia forces (similar in size with those of the V8 90° flat, but now they are on two planes, while in the V8 90° flat crank they are on one plane (the vertical plane to the Vee bisecting plane).
The inertia torque is of 2nd order and is ten times heavier in size than the inertia torque (4th order) of the even firing V8.
The inertia moment is a little heavier than in the V8-90° with flat crank.

If you use a crossplane crankshaft, the uneven firing V8 72° has not significant unbalanced inertia forces, it also has not significant unbalanced inertia torques, but it has a strong 1st order unbalanced inertia moments. Now the balance webs on the crankshaft cannot balance the inertia moments as in the V8 90° crossplane.

The balance program at the http://www.pattakon.com web site (Teaching section), in DOS, gives the plots of all these.

If you have a specific arrangement in mind and needs the plots in comparison to the typical V8 arrangements, just let me know.

Thanks
Manolis Pattakos

Edited by manolis, 14 October 2013 - 17:09.


#6 Fondles

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Posted 14 October 2013 - 20:10

Excellent, thanks for all that information. I'm just working out various possibilities for now.