Twin Engine with Least Crankshaft Speed Variation?

[SmithSwede]

In the thread "My Problems with 6-Speed V7s, the always interesting and informative Pete Roper made the comment that the rotation speed of the crankshaft of a V twin Guzzi is going to fluctuate a fair bit, and thus when the clutch is engaged, all the other rotating parts in the transmission that are coupled with the crankshaft will be subjected to this variation in rotation speeds, leading to various rattly noises.   

Pete's point was that the V Twin Guzzi cannot have an even firing order, and that the energy created by the firing of cylinder No. 1 must be used to compress the charge for cylinder No. 2.   All of which leads to variations in crank speed.

I get all that, and it's obviously correct.  But it seems to me that you can also have an even firing order, with potentially worse variation in the crank speed.   After all, a BWM airhead or other flat twin has an even firing order.   But the crankshaft is asked to come to a dead stop when the two cylinder simultaneously arrive at top dead center, and then again when they are both at bottom dead center.

At least the Guzzi's crank is never asked to come to a dead stop, although it is certainly slowed down in compressing the next cylinder charge.  This makes me think that firing order is only part of the issue; another issue is cylinder arrangement. 

So I'm wondering which design has the least variation in crank speed?  The BWM flat twin or the Guzzi V twin? 

More generally, assuming we are only talking about twin engine designs, what combination of firing order and cylinder arrangement gives the minimum amount of crank speed variation?

[Antietam Classic Cycle]

The weight of the flywheel attached to the crank would smooth out the variations, it would seem to me. The heavier, the better? If so, old Loop Guzzis with their 13 lb. flywheel and 4 lb. ring gear really smooth it out! 

[oldbike54]

 Prescott , you are the physicist  Actually crankshaft phasing doesn't mean much in this discussion , as what really happens in any design is the rods and pistons in any design must stop for a very brief period and then reverse their direction of travel , correct ? Charlie is right , flywheel weight and crankshaft mass and weight are the real determining factors in how much speed variation happen in rotating or moving components .

 Dusty

[rodekyll]

RX2

[Garp]

After all, a BWM airhead or other flat twin has an even firing order.   But the crankshaft is asked to come to a dead stop when the two cylinder simultaneously arrive at top dead center, and then again when they are both at bottom dead center.

At least the Guzzi's crank is never asked to come to a dead stop, although it is certainly slowed down in compressing the next cylinder charge.

The Piston may come to a complete stop, however briefly, but the crankshaft is still spinning

[Rough Edge racing]

 Variation in crank speed is also present in V8 car/truck engines and shows up as "gear rattle" when a manual transmission is in neutral or the engine being lugged in too high a gear......
   Many older parallel twins have a 360 degree firing order ....I don't know if I buy into the 90 degree twin cylinder energy of one cylinder being used to compress the other cylinder air fuel mixture as something different than any other firing order.....

[pyoungbl]

I thought the inline 8 was the smoothest running engine design, next smoothest was the inline 6 or V 12.  Of course a radial would be pretty smooth too.  If we only consider twin cylinder engines I cannot see a way around the fact that you have two relatively big bangs to accelerate the crankshaft while everything else going on is decelerating the crankshaft.  A large flywheel will dampen all this commotion.

Just for grins, take a look at a 1 cylinder Lister diesel....huge flywheel!
www.youtube.com/watch?v=eYB_-ATJgFs

[SmithSwede]

I'm not describing my question well.

I realize that any piston must stop at the top of its stroke and again at the bottom of its stroke.  But on the V twin Guzzi engine, when one cylinder is "stopped" at the top (or bottom) of its stoke, the other piston is in mid-stroke.  So one piston is at minimum velocity/energy (stopped), while the other is at max velocity/energy (mid-stroke).    The Guzzis pistons just "swap" the energy back and forth.

A flat twin, like a BMW airhead, has both pistons coming to a dead stop at the same time.   That would seem to add more twisting force to the crank, even though the firing intervals are evenly spaced.   

Put a different way, on the BWM, when both pistons are stopped, the only way for the crank can keep rotating is to take some of the stored momentum out of the flywheel.    The BWM thus "robs" the flywheel of the energy required to re-start two pistons from a dead stop.   Whereas on the Guzzi, when one piston is stopped, the other is moving as its highest speed.  Less "robbery" of the flywheel. 

In effect, on the Guzzi, one of the pistons, being in motion, is acting like a temporary flywheel. 

[oldbike54]

 I understand what you are saying Prescott , and it does sound right , only you aren't thinking in terms of 360 degrees . The overall acceleration and deceleration during a complete cycle should average out the same . There might be some VERY small difference , but I ran all of this by my gear head physics prof buddy , we did the math , or rather he did ( ) . The difference would be in the range of .0001% . The miracle of the IC engine is that it takes linear motion and turns it into rotational force 

 Dusty

[garbln]

Guzzi's and BMW's both have clunky trans more because they are engine speed trans.  Most other bikes usually have geared down trans inputs that helps a lot.   

[KiwiKev]

A rotary engine would overcome all this  should be silky smooth - lol


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[Murray]

Onan flat twin gen set, frequency is set by the motor/alternator speed its typically regulated fairly heavily.

[Huzo]

I'm not describing my question well.

I realize that any piston must stop at the top of its stroke and again at the bottom of its stroke.  But on the V twin Guzzi engine, when one cylinder is "stopped" at the top (or bottom) of its stoke, the other piston is in mid-stroke.  So one piston is at minimum velocity/energy (stopped), while the other is at max velocity/energy (mid-stroke).    The Guzzis pistons just "swap" the energy back and forth.

A flat twin, like a BMW airhead, has both pistons coming to a dead stop at the same time.   That would seem to add more twisting force to the crank, even though the firing intervals are evenly spaced.   

Put a different way, on the BWM, when both pistons are stopped, the only way for the crank can keep rotating is to take some of the stored momentum out of the flywheel.    The BWM thus "robs" the flywheel of the energy required to re-start two pistons from a dead stop.   Whereas on the Guzzi, when one piston is stopped, the other is moving as its highest speed.  Less "robbery" of the flywheel. 

In effect, on the Guzzi, one of the pistons, being in motion, is acting like a temporary flywheel.

From a physics point of view mate try this. Over TWO full revolutions, the total input of work done by the pistons and the total loss of energy due to work done, is the same regardless of cylider configuration. One piston con rod combination, is oblivious to what the other is doing. Total work done/energy lost is the same. Great thread BTW...

[Huzo]

Ignore the whole bloody lot. Soon we'll be on electric bikes with torque curves as flat as a shit carters hat ! And smooth as cat...droppings!

[SmithSwede]

Interesting topic, at least to me.  I'm going to keep pondering.

Seems to me this issue of crankshaft speed variation is analogous to the issues related to rear wheel traction and theory of cross-plane cranks.   As I understand it, a cross plane crank (which a V twin Guzzi obviously is) will theoretically disturb the rear wheel less, thereby increasing traction upon corner exit.

Why?  Because a cross plane crank engine is not "robbing" the rear tire of momentum each time the crank comes to a dead stop at TDC and BTDC the way a flat plane crank does.

But the rear wheel is responding to the speed fluctuations of the crankshaft, since it is connected to the crank.  Thus if the rear wheel is turning more smoothly with a cross plane crank, then necessarily the crankshaft if turning more smoothly as well.

[oldbike54]

 OK Prescott , you have the "tire" thing in regards to the crossplane crankshaft firing order reversed . The reason why a crossplane crankshaft delivers better traction is because the tire has more time to recover after each power pulse . Every time the engine hits the tire deforms , and the longer interval between hits that a crossplane design creates gives the tire more time to recover . Ducati used the 90 degree design to great advantage in SB for years , and the newer V4 designs also mimic a twin . Back in the day all of the "hot" Triumph and BSA dirt track engines were twingled for the same reason , even the fastest XR 750 HD flat track race engines were twingled until about 1990 when the AMA outlawed twingleing in a effort to make other race motors more competitive . Even after going back to a natural architecture the narrow angle XR still had a huge advantage traction wise over more modern wide angle twins . Of course some other factors were involved , one of them being a falling torque curve that the XR had , max torque at 5,500 RPMS , with peak HP at 9,000 RPM's , giving a nice wide powerband . It took mighty Honda 10 years to figure that one out 

 Dusty