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I have a question about engines that has been on my mind for a while can anyone answer this? What would happen if a engine didn't have a fly wheel?
 



You put a heavier flywheel on to make the bikes power more tame or to take some of the spastic acceleration on the power band out of a 2 stroke for example, so if you flat out took it off, I guess it'd just have pretty intense acceleration/deceleration.

The engine would take less time to get to a higher rpm and vice versa, since it'd be turning a few hundred grams less than it was designed to. Not sure what effect it'd have on the engines longevity. Flywheels are there for a reason, so it's probably a good idea to leave it there.

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I don't think a single-cylinder 4T engine would keep running.

The flywheel has energy transferred to it in the form of torque during the power stroke. The energy is thus stored as the rotational speed of the flywheel. The rotational speed of the flywheel is converted to torque on the mechanical load of the exhaust stroke. So what I'm saying is if there was no flywheel then what is going to make the engine turn through the exhaust stroke? In the case of an inline 6 cylinder 2T there is enough power strokes all the way through the cycle, same with some radial engines, but not a single cylinder engine. 

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Flywheels can have various purposes, controlling the power delivery, starting, power transfer, timing, etc.

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4 hours ago, DEATH_INC. said:

I ran My GSXR750 without one when I was racing it, makes it a bit more responsive. But you had to charge the battery between every race, as without the flywheel there was no alternator....

Well there ya go, it does run. I guess the weight in the crank is enough to get it through the exhaust stroke. 

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2 hours ago, DirtShow said:

Well there ya go, it does run. I guess the weight in the crank is enough to get it through the exhaust stroke. 

The GSXR750 is a four cylinder engine. 

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The GSXR750 is a four cylinder engine. 


Yeah true. Can somebody try this on their 4 stroke dirt bike and report back with results? I'm now oddly curious as to what would actually happen

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Well since a good majority of bikes don't have batteries, I don't think they would run without one. I believe the flywheel is there to produce a spark with the magneto. Without it, how would you get spark given there isn't another power source? It may have worked on the GSXR do to A. It having multiple cylinders, so for every quarter turn of the crank at least one piston fired. A typical dirt bike, single cylinder, wouldn't work I think. B. the GSXR has a battery, that would run the electronics required to make a spark. But how would it stay in time?

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All a flywheel does is be a mass used to store and release engine energy. It is not a 'must have' and many race cars and road racing bikes do not have one. They also have to idle at 3,000 or higher (momentum) to stay running. Not having a flywheel enables faster acceleration and deceleration of the engine. It does not make the car or bike faster or slower, it only changes the power delivery.

You do not have to have a flywheel to generate electrical power nor do you have to have a battery. A flywheel usually is used for the effect the mass has. So instead of making it just plain iron, why not include magnets (which are heavy) and use it to create electrical power. To determine when to fire, all that is needed is a sensor to detect the crank position.

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11 hours ago, LukeYZ426F said:

The GSXR750 is a four cylinder engine. 

Thanks for pointing that out. I skimmed that sentence and read it as "I ran some motocross bike without one when I was racing".
Not used to seeing road racers on this forum :ride::cripple:

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12 hours ago, ASP1227 said:

Well since a good majority of bikes don't have batteries, I don't think they would run without one. I believe the flywheel is there to produce a spark with the magneto. Without it, how would you get spark given there isn't another power source? It may have worked on the GSXR do to A. It having multiple cylinders, so for every quarter turn of the crank at least one piston fired. A typical dirt bike, single cylinder, wouldn't work I think. B. the GSXR has a battery, that would run the electronics required to make a spark. But how would it stay in time?

 

Probably uses waste spark and doesn't bother with tracking the crank

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9 hours ago, William1 said:

. It does not make the car or bike faster.....................

Yes it does, which is whole reason race vehicles run lighter flywheels. :)

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14 hours ago, ASP1227 said:

Well since a good majority of bikes don't have batteries, I don't think they would run without one. I believe the flywheel is there to produce a spark with the magneto. Without it, how would you get spark given there isn't another power source? It may have worked on the GSXR do to A. It having multiple cylinders, so for every quarter turn of the crank at least one piston fired. A typical dirt bike, single cylinder, wouldn't work I think. B. the GSXR has a battery, that would run the electronics required to make a spark. But how would it stay in time?

you are correct for the most part, a street bike will often have a crank position sensor that does not use the flywheel. sometimes a rotor on the opposite side of the crank and sometimes a cam based pickup. Most street bikes use the battery to charge the coils for ignition and the flywheel is used only to charge the battery.

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19 hours ago, Wild Alaskan said:

you are correct for the most part, a street bike will often have a crank position sensor that does not use the flywheel. sometimes a rotor on the opposite side of the crank and sometimes a cam based pickup. Most street bikes use the battery to charge the coils for ignition and the flywheel is used only to charge the battery.

Correct, the GSXR has a sensor on the other end of the crank.

It also fires every 1/2 stroke, not quarter. (asp1227)

A single will run without a flywheel too, if it has something to time it. The crankshaft and clutch etc usually have enough mass to get it round to the firing stroke again, though it may not idle at 1000 rpm.

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8 hours ago, Bermudacat said:

Yes it does, which is whole reason race vehicles run lighter flywheels. :)

No, race vehicles run a light flywheel to enable engine rpms to change rapidly during shifting. it does not make the vehiclke faster. All a flywheel does is store energy. Whily it can affect acceleration by a tiny bit, it also stores and releases that same energy, netting out as a zero effect.

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My bet:

On a single cylinder engine, with already very lightweight components, it likely won't start.
And if you did manage to get it going it would likely die very easily at low RPM.

The flywheel's purpose is to maintain rotational momentum to keep the engine turning between power strokes.
No flywheel on a single cylinder engine = it will likely come to a halt on its own during the compression stroke as there is not enough forward momentum to push the piston through that stroke before it fires again.

As far as "racing" engines go, those are almost always multi-cylinder engines which idle at a high RPM.
A flywheel isn't necessarily needed in that scenario to "keep things going", thus it becomes more of an uneeded burden on the engine, as it makes changing engine speeds rapidly more difficult.
Power wise William1 is correct, the net output of the engine remains the same regardless of flywheel weight.
But... it changes the behavior of how that power is delivered.

Think of a very large semi truck engine, which has an enormous rotational weight and takes a relatively long time to speed up and slow down.
But... it's still perfectly capable of creating 1,000+ HP. That type of engine is designed for more constant power delivery, VS a racing or motorcycle engine where fast engine speed changes are very desirable and a requirement.

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7 hours ago, William1 said:

No, race vehicles run a light flywheel to enable engine rpms to change rapidly during shifting. it does not make the vehiclke faster. All a flywheel does is store energy. Whily it can affect acceleration by a tiny bit, it also stores and releases that same energy, netting out as a zero effect.

If you want to get really technical it will slow you down ever so slightly because its absorbing energy when you need it (accelerating) and then just wasting it during deacceleration. The IT490 is a great example of this, the crank and flywheel are very heavy so it revs very slowly compared to a modern bike, to the point it can be a struggle to get the snap to pop the front wheel over obstacles, the engine taking a little longer to rev down when off the throttle and on the brake offers zero benefit

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The crank should have enough mass in the counterweights to keep the engine running, depending on the bike.  A 250 two stroke probably would.  I don't know about a small bore like a 65 or 85cc.  A lot of bikes don't have much weight on the flywheel anyway, just enough magnet to keep the CDI juiced up and a trigger for the ignition system.

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