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Will it coast LONGER or shorter?

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Kevin O. Pulver

Guest
I just bought a 28" clutch pulley for my 12HP Champion/Hercules. Will the extra weight make it coast longer or shorter time periods between firings? On one hand, it seems something heavier would take longer to coast down, on the other hand, it seems something heavier would also gain less speed from each single power stroke when it hits. So it seems like it might almost make no difference at all. Is there a voice of experience out there? THanks Kevin
 
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Roy Pasini

Guest
There's a simple way to find out. Run it with and without the pulley and count RPMs. Don't waste your time theorizing.
 
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Mac Leod

Guest
It should coast longer because once it gains momentum it will do a better job at keeping it, objects in motion tend to stay in motion until acted upon by an outside force (friction). Larger objects with more mass will be less effected by out side forces. It will also take longer to pick up speed because objects at rest stay at rest until acted on by an out side force (engine fireing. This is why hit n miss engines have larger fly wheels, to maintain speed. The clutch should allow for more coast time but only trying it will tell

My thoughts

Mac Leod
 
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Patrick McNallen

Guest
I think that it would gain less speed from each of the power impulses, but coast longer due to having more mass in motion. It should be able to run slower, too, since more rotating weight would help to keep it from stalling on the compression stroke. I have a 20 HP 2-cycle with no clutch and lightweight steam engine flywheels. It accelerates fast, and each power impulse causes it to speed up noticably. It coasts down fast though. It won't run at a really steady speed, even though the cylinder is in good shape. I'm sure that heavier flywheels or a big heavy clutch would even it out.
 
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Mac Leod

Guest
My friend, you have it right, wish I could have made it that clear...more mass moving will take longer to gain speed and it will take longer to loose speed.

Mac Leod
 
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Tom Cwach

Guest
It won't change coasting time. Same rotational drag, same power input. The RPM may be more steady with the extra weight. I have a 12hp Economy with a 30" Edgemont clutch pulley. Coasts the same 36 revolutions between hits clutch in or out. If you have the pulley out and stop the pulley the engine will hit twice to get back up to speed when you engage the clutch then goes back to single hits.
 
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Patrick McNallen

Guest
Tom: There is no power input when the engine is coasting.
 
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Tom Cwach

Guest
There is power input when the engine comes off latchup and makes a power stroke, once in every 30 revolutions or so, your results may vary. There is rotational drag from the bearings. piston friction drag,air pumping drag from the coasting cylinder, and some air drag on the flywheels while coasting and during the power stroke. Clear enough?
 
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Mac Leod

Guest
But the inertia will help over come this because of the extra mass moveing around. Notice how tops have a large heavy rim, reduce the mass of it and the top will not spin as long.

Mac Leod
 
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Tom Cwach

Guest
Laws of mass and inertia www.glenbrook.k12/gbssci/phys/newlaws/u2l1b.html
 
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Mac Leod

Guest
Tom I can not get the url to work, are you sure it is right?

Mac Leod
 
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Tom Cwach

Guest
my mistake. www.glenbrook.k12.il.us/gbssci/phys/newlaws/u2l1b.html it's the top of googles' search for inertia.
 
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Mac Leod

Guest
Tom, got it to work...thanks, it further backs my opinion that a more massive fly wheel will increase coast time once up to speed, and I quote "the more massive the object, the more that object tends to resist changes in its state of motion.". This state of motion would be spinning, more mass helps resist changes due to drag and friction. It may take more time to get up to speed but should coast longer.

Thanks again

Mac Leod
 
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Tom Cwach

Guest
A larger mass will take longer to come to a complete stop, but the question is if it would coast longer between hits(power strokes). The flywheels store the energy from the power stroke. Given the same power input a heavier mass won't gain as much speed as a lighter mass. The engine rpm won't vary as much between hits but the time between power strokes will be the same. As stated earlier I have engines with clutch pullies and have checked coasting revolutions and it does not change with the clutch engaged or not but I can also tell if it is disengaged because the engine will gain more rpm when it fires.
 
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Mac Leod

Guest
If it takes longer to stop then it must take longer to slow down... a hitnmiss governer causes the enging to fire when the fly wheels reach a certian low speed, the longer it takes to slow down means it has that much longer to spin.

Mac Leod
 
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David Greenwalt

Guest
I think what is not mentioned here is that the governor is not controlled by the speed of the flywheels, it is controlled by the weights and/or springs on the governor. You can take the same engine and run the flywheels at the same speed, but add weights to the governor and the engine will fire sooner. A heavier flywheel may take longer to come to a complete stop when the engine quits because of the mass, but when firing and given both have the same governor on them, running at the same RPM, the engine will fire when the weights controlling the governor tell it to, regardless of the mass of the flywheels. The reason these old engines have such large flywheels is not to control the governor, but to counterbalance the weight of the piston, connecting rod, and crank so that the piston will come back up to fire. Otherwise it would only fire once and the piston would stay down. David.
 
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Dave Showerman

Guest
Just a thought, But the further out from the center of the flywheel, The greater the effect.

Another thought, What percentage of the total rotating mass I.E. Flywheels does the clutch assy. represent? Dave in Holt.
 
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Mac Leod

Guest
Thought the governer is not controled by the speed of the fly wheels they do have an effect on the Gov... when the engine looses momentum the gov allowes the engine to fire, the longer the engine coasts because of added mass the longer it takes for the gov to allow fireing. more mass means longer coasting and higher speed longer causing the engine to not fire untill it hits the low fireing speed.

Mac Leod
 
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