Kawi2strokes.com Forum

My point is that the acceleration is not constant, even though the equation indicates that it is. The crank does slow after a power pulse (if it never did, then your engine would coast forever after you cut the ignition). It doesn't slow by much, but it does slow. Let's assume that, after the last power stroke of the 'last' cylinder the crank is turning at 5000 rpm. There is now 180 degrees for that crank to slow. Perhaps it slows to 4, 850 rpm. That's .833 rpm/degree of crank rotation. Now, cylinder #1 fires and it is trying to speed the crank back up to 5000 rpm. It now has to accelerate the crank very quickly to get back up to speed and let's assume reaches the goal.
Now we have 90 degrees of crank rotation until the next power pulse from cylinder #2, time in which the crank slows. If we assume the crank slows at the same rate (.833 rpm/degree) then the crank will only slow to 4925 rpm.
Now there is another 90 degrees of crank rotation until the final power pulse from cylinder #3, the crank slows again to 4925 rpm. So, if you are going to have to accelerate the crank a lot you want to do it with the one closest to the clutch basket to reduce the

I'll also point out that the torque does vary slightly in the equation as well since the torque an engine can produce is dependant to some degree on rpm. So I was trying to simplify the explanation by only addressing one variable, but it does involve two

A link the wikipedia page on crankshafts. Pretty good read, even talks a little about that v8 vs v6 topic. they note that cranks typically fail in two modes: bending and twisting. crank connects to a "...torsional or vibrational damper at the opposite end, to reduce the torsion vibrations often caused along the length of the crankshaft by the cylinders farthest from the output end acting on the torsional elasticity of the metal."
http://en.wikipedia.org/wiki/Crankshaft

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ride hard, ride long, ride safe!
Jason
72 h2 in a 93 Katana
03 cbr600rr
80 kz750 ltd
94 pw50

Your error is in thinking that with the 90-90-180 crank, the first firing of the group brings the crankshaft back to 5000 rpm all by itself, when if fact it takes all three to do that.

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If it surges, that's normal, upshift.

I disagree. As I read it, the idea is that it slows down and speeds up three times during each revolution. So in a way, yes it does need all three cylinders - not to get it back up to 5,000 as you say, but to maintain a max of 5,000 with the momentary fluctuations downward as discussed.

I also agree with the thinking behind firing the first one of the close group at the drive end of the crank, as this will have the least twisting effecton the crank. Now if the drive were taken centrally or at three points along the crank then the view of phasing making no difference would hold water. But not with the whole resistance to the crank's rotation being placed at one end of it.

The distance from the drive will not effect the amount of stress on the crank, it will only increase the deflection, assuming the bearings are holding the crank straight.
In fact there would be less of a stress concentration when the left cylinder fires.

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http://kawatriple.com/wtf/

Nerds!

Very interesting thoughts on this crankshaft twisting theory.
As a frame of reference, have factory race teams with other inline engine designs followed any of this drive side crank phasing concept?

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There are only two types of motor sport racing:
Nitro and everything else
(Sometimes referred to as fast and slow)

Hal, with Jason's example in which the crank just reaches 5000 rpm after the last of the close-firing pistons fires, and then it slows to 4,850 before the 1st in the group again fires, each of the three pistons then increases the rpm, and if the power pulses don't overlap, maybe it slows a little again between each, but it does not again reach 5000 until the 3rd piston finishes its input, and the contribution to increasing the rpm is essentially equal among the pistons. Just because the one after the big gap fires first doesn't mean it somehow contributes more than the others. Here's an approximation of the crankshaft RPM vs. degrees for the 90-90-180 crank, showing that each cylinder contributes an equal amount to the RPM increase when the group fires. The first cylinder in the group fires at zero degrees, and there is no extra RPM increase when it fires compared to the other two, as defined by t=Ia.



Here's the normal 120-120-120 crank, showing the same input from the pistons, and the same slope for the decrease in RPM between firings. The crankshaft does reach 5000 RPM after each firing with this configuration.

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If it surges, that's normal, upshift.

Sure looks like the first graph is vibrating more...
You might be right Z, but I'm already done my Chistmas shopping...

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http://kawatriple.com/wtf/

You don't tug on Superman's cape........ you don't spit, in the wind....... you don't pull the mask off the ol Lone ranger........ and you do't mess around with Engineer Jim!................

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Twist the throttle, tilt the horizon, and have a great time. What triples are all about...........

Jim
If you only define the answer to the question in terms of T=la, then a crank will never accelerate. Torque does change. it is dependent on the quality of the air/fuel charge in the cylinder and the angle between the con rod and the crank. I think your equation may be accurate for measuring the output of the crank, but I don't think it is completely applicable for the input

I also don't agree that the cylinders contribute equally to accelerate the crank. The power pulses do overlap (I think the only way they couldn't would be in a 180 crank...) So, the first cylinder that fires actually has to change the acceleration of the crank from a negative to a positive. The second and third cylinders only have to contribute to the positive acceleration

_________________
ride hard, ride long, ride safe!
Jason
72 h2 in a 93 Katana
03 cbr600rr
80 kz750 ltd
94 pw50