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Flat plane crank in a V6 engine?
Topic Started: Oct 22 2012, 08:51 PM (3,596 Views)
Carlo_Carrera
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I asked about this specifically as it applies to the new V6 F1 engine over in that tech section but no one yet knows the answer to my question so I thought I would post a more general question here.

Any of you motorheads here ever come across a flat plane crank in a V6?
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aunty dive
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Don't think a 180 degree crankshaft can be made to work in a V6. Big balance problems. A V6 has to fire every 120 degrees of crankshaft rotation in order to stay together. Some of this will help explain that:

http://www.caranddriver.com/features/the-physics-of-engine-cylinder-bank-angles-feature

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SolidEther
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aunty dive
Oct 22 2012, 09:07 PM
Don't think a 180 degree crankshaft can be made to work in a V6. Big balance problems. A V6 has to fire every 120 degrees of crankshaft rotation in order to stay together. Some of this will help explain that:

http://www.caranddriver.com/features/the-physics-of-engine-cylinder-bank-angles-feature

right...
I think what he means is a "three throw" crank (I hope).

By the way, some early generation V-6's of the 1980's were essentially V-8's with the end two cylinders omitted.
They did shake apart.
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aunty dive
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^ mmmmm ... I think he's referring to a "flat" crank. Wewillsee.
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Carlo_Carrera
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That is what I thought. I have been around race motors for a while I have never come across a flat plane crank used in a V6. It just can't be balanced well enough.

This explains why Bernie and Luca said the new Ferrari turbo V6 in 2014 spec sounded so bad. Of course it would compared to the current flat plane crank V8s. Not to mention the turbo also muffling the sound a bit.
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Carlo_Carrera
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SolidEther
Oct 22 2012, 09:12 PM
aunty dive
Oct 22 2012, 09:07 PM
Don't think a 180 degree crankshaft can be made to work in a V6. Big balance problems. A V6 has to fire every 120 degrees of crankshaft rotation in order to stay together. Some of this will help explain that:

http://www.caranddriver.com/features/the-physics-of-engine-cylinder-bank-angles-feature

right...
I think what he means is a "three throw" crank (I hope).

By the way, some early generation V-6's of the 1980's were essentially V-8's with the end two cylinders omitted.
They did shake apart.
I definitely mean a flat plane crank. Like the ones shown here.

http://www.projectm71.com/Cross_FlatPlane.htm
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aunty dive
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Quote:
 
More importantly, as a flatplane crankshaft does not require counterweights, it has less mass and a lower moment of inertia, providing higher rpm and more rapid acceleration.


This is the key.
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Carlo_Carrera
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aunty dive
Oct 22 2012, 11:47 PM
Quote:
 
More importantly, as a flatplane crankshaft does not require counterweights, it has less mass and a lower moment of inertia, providing higher rpm and more rapid acceleration.


This is the key.
Yes, it would be huge break through in high end performance if someone could design a flat plane crank that works in a V6.
Edited by Carlo_Carrera, Oct 23 2012, 09:06 AM.
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MariaFan
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SolidEther
Oct 22 2012, 09:12 PM
right...
I think what he means is a "three throw" crank (I hope).

By the way, some early generation V-6's of the 1980's were essentially V-8's with the end two cylinders omitted.
They did shake apart.
Our recips use the 3 throw crank design. Lots of counter-weight.
However, we're not interested in acceleration, simply constant mass flow.
Rotational mass - good. :thumbsup
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rr60man
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Would a W-6 solve the balance problems? Just thinking outside the box.
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Carlo_Carrera
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rr60man
Oct 24 2012, 07:49 AM
Would a W-6 solve the balance problems? Just thinking outside the box.
Maybe, I would think that the problem could also be solved by finding the perfect V angle of the cylinders to work with the crank. The F1 regs call for 90 degrees which it makes a flat plane crank not viable. Maybe a flatter angle around 120 degrees or a narrower one around 30 degrees would work.
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aunty dive
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Quote:
 
A 60-degree V-6 engine isn’t quite as successful. The rotational and reciprocating forces can’t be completely balanced because this type of V-6 is essentially two three-cylinder engines stuck together. Inline-three engines, because of their odd number of cylinders, are inherently imbalanced and will tend to rock from end to end. A flat-six engine ­manages to ­cancel the rocking because the opposing banks exactly cancel out each other’s motions. Putting two inline-threes together, end to end, to form an inline-six also works because each three-cylinder end of the engine exactly cancels the forces of the other. And since it’s basically two straight sixes joined at a common crank, the V-12 is naturally balanced regardless of its V angle.

But the 60-degree V-6 inherently shakes; the rocking motion of the inline-three can’t be canceled if the bank angle is smaller than 180 degrees. For that reason, many V-6s use balancing shafts, which are essentially additional crankshafts that use specifically weighted lobes to cancel out imbalance.
original

The firing forces, however, are balanced in modern V-6s. A V-6 fires a cylinder every time the crankshaft turns 120 degrees (720/6=120). That would imply a 120-degree angle between the banks, but that configuration is impractical for packaging reasons. The 60-degree bank angle is a good compromise for packaging, and because the firing events occur in degrees (120) that are evenly divisible by the angle of the V (60), the firing forces remain balanced.

So how do GM and Mercedes-Benz get away with 90-degree V-6s? These engines would seem to have unbalanced firing pulses because 120 isn’t evenly divisible by 90. When GM reintroduced its V-6 engines back in the mid-Seventies, it revived an early-Sixties design, which was essentially a Buick 90-degree V-8 with the two end cylinders sliced off. Because of the firing imbalance, the engine ran rough, sort of like a V-8 with two cylinders missing. To counteract this, the company developed a special crankshaft called a “split-pin” or “split-journal” unit that mounted the big ends of the paired connecting rods to crank journals that had been split and slightly offset so that the engine could achieve 120-degree firing despite its V angle.


From the link in post # 2.
Edited by aunty dive, Oct 24 2012, 09:49 AM.
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Carlo_Carrera
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So do you think a flat crank in a 120 degree cylinder set might work?
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aunty dive
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Carlo_Carrera
Oct 24 2012, 09:57 AM
So do you think a flat crank in a 120 degree cylinder set might work?
I'm afraid it wouldn't.

The only way a 180 degree crankshaft can be made to work with 6 cylinders is in an engine with a 180 degree layout - a horizontally opposed "flat" six a la Porsche.
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Carlo_Carrera
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Damn.
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Wheel_man
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aunty dive
Oct 24 2012, 12:11 PM
Carlo_Carrera
Oct 24 2012, 09:57 AM
So do you think a flat crank in a 120 degree cylinder set might work?
I'm afraid it wouldn't.

The only way a 180 degree crankshaft can be made to work with 6 cylinders is in an engine with a 180 degree layout - a horizontally opposed "flat" six a la Porsche.
Furthermore Flat plane cranks "share" journals meaning each eccentric rod journal has 2 connecting rods like in the picture bellow.

Posted Image

in a flat 6 design you would need 6 separate rod journals (180 degrees apart) so you have 3 on each side. thereby losing some of the flat plane (mass )advantage.


Here is a picture of a Porsche 911 motor, It appears they use a 120 degree crank (not flat plane)Not that you couldn't made a flat 6 that has a flat plane crank.

Posted Image

Edited by Wheel_man, Oct 26 2012, 07:27 AM.
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MariaFan
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Found this pic of a Subaru flat 6.

Posted Image

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Carlo_Carrera
Oct 22 2012, 09:26 PM
SolidEther
Oct 22 2012, 09:12 PM
aunty dive
Oct 22 2012, 09:07 PM
Don't think a 180 degree crankshaft can be made to work in a V6. Big balance problems. A V6 has to fire every 120 degrees of crankshaft rotation in order to stay together. Some of this will help explain that:

http://www.caranddriver.com/features/the-physics-of-engine-cylinder-bank-angles-feature

right...
I think what he means is a "three throw" crank (I hope).

By the way, some early generation V-6's of the 1980's were essentially V-8's with the end two cylinders omitted.
They did shake apart.
I definitely mean a flat plane crank. Like the ones shown here.

http://www.projectm71.com/Cross_FlatPlane.htm
also called a big bang crank
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