Flywheels and RPm

[Techno]

I was checking flywheel stuff and it re-came to me that these things purpose is either to smooth the energy delivery or store it. They aren't really storing it much on an engine but the smoothing is being done

THe question here is, if higher rpm motors use lighter flywheels would that mean they dont' perform as well or get more abuse in lower rpms than a heavier flywheel? I'm assuming the the high rpm light flywheel would equal the low rpm heavy wheel.

If the flywheel were to be completely removed would it damage the engine?-assuming the electronics were taken care of some how.

[DaveR]

I've always felt the lighter the flywheel the higher the idle rpm needs to be because it's not storing energy and doesn't want to idle. I think the biggest advantages to a light flywheel is less load on the crank, but especially a faster wind up of rpm's because there is less mass to get moving. I've never thought they added a whole lot to top end, given time to get there. I think the big rod / small rod is the same thing. Someone correct me if I'm wrong.

[Talon2.5]

dave, quick spinup lighter load on upper bearing is the way i understood it, i didnt see much difference in top rpm with my old lightwheight wheel verses my steel one, but i think it helps like i said with spinup and upper bearing life


skip

hey greg, US-1 what do you think?

[gtsmpuc]

I may be reading you wrong and I may be wrong but.... It sounds like your comparing the flywheel to lets say a harmonic balancer on a car. The only thing I know that the flywheel does is give the starter a place to make contact. The lighter the better as long as it is balanced and strong enough to stay that way. I could be completely off on this but I have a feeling I'm not, or is that indigestion. If you're looking for one JSRE kicks ass.

[us1]

DaveR pretty much said every thing I would say about it execpt for the rod stuff. Lighter rods do make more power because they are reciprocating weight. The weight of the rod has to come to a compleat stop and then go back in the other direction. At 6000 RPM that is happening 200 times per second, 100 times at TDC and 100 times at BDC.

[NPK2003]

I believe the true purpose of the lightened flywheel is not only to give a quicker acceleration of the motor mass, but also to save the crank and upper bearing assembly stress and strain. Just think about a heavy flywheel spinning 7500+ RPM on a 2.5 and you back off quickly. Flywheel wants to twist the end of the crank off. For hot boating around the lakes, I'd opt for the big rods, but not for drag races. I saw a really neat light weight flywheel for a 300x at Performance Machine display at Jasper. It was cut from billet and saved 40 (yes, 40)pounds over the cast one Merc is getting from Korea. If I had a 300x, I'd buy it for sure.

Nick~K

[us1]

You are absolutely right, that is the main reason for a light flywheel.

[B.Leonard]

Theoretically, reciprocating weight alone does not consume more power.

The increased reciprocating weight causes an increase in friction which consumes power.

There is also the factor of acceleration causing a heavy flywheel to want to explode. This could happen if the motor unexpectedly lost it's load due to a drivetrain failure or maybe the prop leaving the water if the motor wasn't rev limited.

-BL

[Techno]

Thats not exactly what I was thinking.

If you totally removed the flywheel this would be better than a light weight fly wheel by the above thinking. If starting was the only concern then a gear would be much lighter than the magnet support and all the other stuff. And even this could be eliminated by a different form of starter. So the flywheel must have some rotational/mass reason for being. removing it would do something bad but I don't know what. ( I believe drag racing cars remove them- Don't need them for this use)

I know it does 2 things, produce electricity and start the engine. I think it also smooths out the power into the crank-contiues the energy past where it would normaly end in 1/2 cycle. It takes power to turn it but also returns power when there is none.

Since the fishin motors use a heavy flywheel I was thinking that they were designed to be heavy because of the rpm zone they normally operate in.
THe hi-pos have a lighter one, for the above reasons but also at high rpms they get the added "mass" from the rotation. At higher rpms they store more energy so would perform the same as a heavier mass at a lower rpm. But at a lower rpm the wouldn't be working in the proper design zone.

Heavy mass at a low rpm = 100 units
light mass at a high rpm = 100 units

heavy mass at a high rpm = 200 units> too much
light mass at at low rpm = 50 units > too little
I was thinking that they are both a trade off if taken out of the range they work in.


BTW reciprocating mass does consume power. Not due to friction. Any increase in mass requires an increase in power to keep things the same. Actually any increase in mass requires an increase in power. If you increase a machanical componants mass you also have to increase its supports and strength, all this makes it worse than just the lone mass increase.
A good example of this is a 200 hp outboard-200 lb. driving a light boat.
Compared to an inboard that has more hp and a heavier engine(needs more hp) and increased hull strength which makes the boat heavier which means the hp has to go up, throw another engine in....

[Mark75H]

Lighter flywheels are easier to balance, especially if they have reduced diameter. If you are stuck with your flywheel carrying a starter ring gear and your diameter has to stay the same you might as well not carry any weight on the motor that you don't need, a heavy flywheel is just extra motor weight.

Flywheel out of balance will hurt acceleration more than the energy required to speed up the flywheel's mass. To accelerate the flywheel's mass at the rate that the motor would accelerate without the flywheel (on the boat with a prop) requires a tiny fraction of one horsepower.

The same out of balance would wreck havoc on your top main bearing.

The rotating mass of the flywheel (heavy or light) is minimal in relation to the restriction imposed by the load of increasing the speed of the vehicle and the rest of the rotating mass.

[fyremanbil]

Gotta throw in my $.02
Way back when I had a discussion with an engineer from GMs engine development dept. He said you'd be surprised how much connecting rods and crank journals flex as they run thru their cycles. Remember, piston engines are cyclic devices. So the gist of the conversation was that the crank doesnt run at a constant speed!!! It's accelerating and decelerating several times per revolution. I think the flywheel's there to smooth out the cycles and maintain a more constant speed. A heavy flywheel causes more crank twist but less driveline twist. It probably "feels" smoother too. The rotating weight probably doesn't make a big difference on these engines unless you continuously re-accelerate the engine ( ie. road race cars ).
Bill

[B.Leonard]

I was thinking of a pendulum but that's not the same for many reasons.

When that piston comes to the bottom of the cylinder and stops, that kinetic energy is now completely transfered into the rotating assembly.

The rotating assembly now contains exactly the same amount of energy that the piston had (with zero friction) however, that same amount cannot be transfered back into the piston without a loss from accelerating the piston back up to speed.

Although friction does play a bigger part the heavier the reciprocating mass, the system will eventually come to a stop due to the energy consumed in accelerating the piston over and over from a dead stop.

-BL

[gtsmpuc]

The magnets are for charging pruposes only. If you didn't need a charging system and made an electric starter like for Karts or Drag bikes then you don't need it. I could be wrong but I have that feeling.

[mercrazy]

i thought a light flywheel helped because the engine is jarred when the boat hits a big wave which changes the direction of the flywheel. a heavy one can't change direction as easily and snaps the crank off at the top. i saw several of these broken when the 3 litre engines first came out.
straight shift drag cars use heavy flywheels to store energy for the launch. round track cars use light ones for quick acceleration and deceleration. since a boat doesn't have a clutch and starts in gear, a light wheel will accelerate quicker. a heavy one will idle smoother, but there's a trade-off either way.
maybe a merc engineer has the real answer?

[Tony Brucato]

I think you can go too light on a flywheel. Don't forget that it does function as a harmonic damper.

Think of the load placed on a Merc V6 crank with a 1-2-3-4-5-6
firing order. I'm sure that there are some funky harmonics going on at 8000 RPM !

Eyeball engineering tells me that some rotating inertia on the top of the crank is necessary to dampen the harmonics.
I think a well balanced flywheel is more important than lightness to crankshaft longevity.

When Jay Smith lightened my new Merc flywheel he found the face of the center hub to be .020" out of square, and the balance to be out of HIS tolerance. I must say that the motor was noticeably smoother after his work, that's got to equate to less wear on the engine and more power at the prop

[dan agnew]

Guys go to a hydro race they start them with a rope on a very small hub on top of the crankshaft. The engines accelerate very
well up and down the rpm scale with no adverse engine wear or damage. The flywheel is for starting, charging, and to smooth out the power pulses.
Light is what you need. Hey this is SCREAM and fly is it not?
Danny