Does a Light Weight Flywheel give you more power?

[Grizzly]

I have to admit i get a little Confused with this subject. It was explained to me along time ago at collage but had lots of sleep since then.

This is the Bit thats a bit Vauge, i think the Inital Torque Level does drop due to the Rotational mass being less, but same as already said the only real way to change torque all the way through the rev range is to make the crank Throw longer, If you where to Dyno the two cars with and Without a light flywheel they would be fairly Similar. BUT what you are seeing is a difference in the rotational mass Momentum, if you where to stick a heavier flywheel on where the Torque of the starter could only just Turn it would rev very slowly but the Initial weight of the Flywheel would properly keep Turn the engine over after the Ignition has been cut (until the Momentum died off) but if you used a No/Light Flywheel the engine would Die as soon as the motor was Cut, But if you take two engines and increase the Crank throw (Keeping the same Flywheel) you will see a simalr thing (the extra weight of the Rotational mass will want to keep Turning like a Flywheel)

Its the old one cylinder engine theory if the Flywheel is big enough it will effectively throw the engine over rather than it using its own Combustion, but in turn won\'t rev very high. That\'s why a Big one cylinder engine etc will only fire every 30 seconds or so but the weight of the rotating Mass is such it will provide enough Momentum to give the needed Compression for the Next Ignition. What you are seeing is No flywheel so there is very Little mass so with things like Power steering and Alt all this tries to pull the Motor back so when the engine is making its Compression for the Fire its looseing Speed making the Mass important to smooth running,Good idle and the Initial Punch, thats why you have to rev them so the mass is increased due to the RPM not Weight.

Then add to all this if you fit a heavier flywheel to say a drag car for the Initial take off Inertia, Imagine the Centrifugal force at 10,000 rpm of the heavier flywheel as opposed to 10,000 Rpm of the lighter one, it has been Known for the Flywheel to literally explode under load at High RPM.

Like anything there is a happy medium, mazda wanted a car that could be Driven round town knowing there engine was always going to have less Torque than say a Supra, so to get round it Stalling all the time they fitted a heaveir Flywheel to increase the Rotational mass, this is Ok on a Street or even drag car but you can loose a Bit for a Track car (But not all of it as Glen says you loose the Punch out of Corners)

[strange_days_uk]

Really good thread this.

Was wanting to ask the question myself.

[Adriansturgess]

Right, Imagine you are riding a push bike. A push bike has no flywheel and so can be likened to a car with an ultra light flywheel. As you approach a steep uphill section your legs immediately feel the strain and you start to slow down. The hill is very steep and very soon you come to a complete stop unable to pedal the mass of the bike any further up the hill.

Next (you guessed it) you introduce a 5kg flywheel into the drivechain of the bike and try to pedal up the same hill. Will you get any further??? Lets assume that you lose 5kg elsewhere off the bike to make up for the additional weight of the flywheel.

You find it a little slower to accelerate the  bike and so you are not travelling quite as fast by the time you hit the beginning of the uphill section. The kinetic energy stored in the flywheel eases the immediate pressure on your legs so you don\'t slow down quite as quickly at first, but the pressure still builds and you come to an abrupt halt.

How close to the hill you start will determine which system will take you further up the hill. Given a long enough run up so that you can reach maximum speed in either case, then the stored energy in the flywheel bike will take you further up the hill. But if you start right at the base of the upward slope you may fail to even get the flywheel turning at all and the bike with no flywheel will get you further up the hill.

So what is happening? Your legs have the same muscle power in each example so the torque applied through the pedals is identical in each case. In the flywheel example, as you hit the uphill slope at full speed you benefit from the torque applied to the pedals through your legs + the stored energy in the flywheel to see you up the hill. This gives you an increased effective torque at that instant in time, (because the stored energy in the flywheel is paying you back a bit of the turning force that you lost whilst you were spinning it up to speed) but as you struggle further up the hill, the inertia of the flywheel starts to work against you giving you reduced effective torque.

In the no flywheel example effective torque is consistent throughout.

Overall, the mean torque in both systems is identical.

[Adriansturgess]

^^^Apologies for laborious post....:Hammer

The more I think about it the clearer it seems that the stored energy in a flywheel will pay you back torque (at the wheels) at certain times and lose you torque (at the wheels) at other times. For instance, If you had a massive enough flywheel (say half a tonne!!!!) and got it up to speed you could turn off the engine altogether and the flywheel would continue to supply torque to the wheels and move the car forward for a while.

But I am also convinced that the mean torque with or without flywheel is the same.

[nikp]

If you had a massive enough flywheel (say half a tonne!!!!) and got it up to speed you could turn off the engine altogether and the flywheel would continue to supply torque to the wheels and move the car forward for a while.

It\'s all about the moment of inertia of a flywheel. where the weight is, not just how heavy it is. In theory, a half a ton flywheel could be made that is easier to turn than a 1kg flywheel. It\'s true - and i\'ll let you all think about that. (And therefore the stored energy will be higher in the lighter flywheel)

[Adriansturgess]

OK, so I should modify my wording from \'a half tonne flywheel\' to \'A flywheel with an enormous moment of inertia\' to be more precise.

Of course there are similar concepts in structural engineering where you can make a beam (or structure) much lighter without significantly reducing its strength by cutting out mass in all the right places.

[nikp]

Precisely!

Ooh, you\'re a structual engineer, eh!? I did Civil Engineering as a degree. I\'m not in it as a career though.

[Adriansturgess]

Yes I did study Civil Engineering and worked in the field for many years but now run an Electronics company (sounds strange I know).

[nikp]

I think where the misunderstanding has been coming from in saying that \'you lose torque\' in running a lightened flywheel, is that you lose the potential for higher amounts of stored energy that a heavier flywheel can supply. But this is not torque.

[Adriansturgess]

Agree with that, but the rotational energy stored in the flywheel can be converted back into torque in the right circumstances, surely.

[nikp]

I think so! But, when considered - the energy that that flywheel has stored does not come from no where. The engine has to supply that energy to spin it. That same engine has to supply less energy to spin a lightened flywheel.

[Adriansturgess]

Precisely!!! So while the engine is spinning up the heavier flywheel it  has less torque to supply to the driving wheels, (because it is busy storing energy -or \'potential\' torque- in the flywheel). That energy can then be converted back to torque at some point, adding a little increased torque to the driving wheels. The engine then has to spin the flywheel back up again so losing usable torque and so on.......So the mean torque figure is the same with either flywheel but a heavier flywheel gives you more variation in torque output whilst a lighter flywheel gives you less variation in torque output??

[nikp]

Perhaps we should be talking about this in terms of energy. I assume you know the laws of energy conservation: energy cannot be \'conjoured\' up from anywhere, and nor will it magically dissappear into nothingness. Therefore, whether using a light flywheel, or heavy flywheel, with the same spec engine, the energy that can be supplied by the engine will be constant.

[onelastpost]

I need a drink!

[Adriansturgess]

Perhaps we should be talking about this in terms of energy. I assume you know the laws of energy conservation: energy cannot be \'conjoured\' up from anywhere, and nor will it magically dissappear into nothingness. Therefore, whether using a light flywheel, or heavy flywheel, with the same spec engine, the energy that can be supplied by the engine will be constant.

I absolutely understand that. But I am not conjuring up energy from anywhere. My model is clear; there is just energy transfer from engine to flywheel (energy is being temporarily stored in the flywheel) and then at some point it is released again, putting that energy back into the system.