Sorry, I just realized this is responding to a couple of other responses, not just Green Caveman's.
I don't disagree that using the brakes will kill the energy in the flywheel leading to total system efficiency losses. I'm right there with you on that. But how much energy is that?
I don't know how you think you're using the figure 700+W. And where do you get the idea that the flywheel stores over a mile of energy every time it spins up? There seems to be a lot of confusion over power/energy/units. Do you really think that people with clutches and flywheels are losing over a mile of range each time they come to a stop? You can test it yourself, spin the flywheel to 4000 rpm with the clutch depressed, then dump it and see how far you get. That's pretty much how much range you'll save by removing it if everything survives. <note: this last sentence is hyperbole>
(Yes, I know that accelerating a vehicle consumes far more energy than just maintaining its speed. The point was to give you an idea that there isn't much energy stored in a conventional car flywheel.)
Here are the numbers.
Firstly, watts is a unit of how much power is flowing at any given time. You need to have a time component for it to be an amount of energy. In the case of O'Zeeke's conversion, he calculated 46274 Joules of energy to spin his 33kg flywheel/clutch combination to 4000rpm. That comes out to (http://www.unitconversion.org/energy...onversion.html
): 46274 Joules = 12.854 watt-hours (700+ watt-mins, actually more like 770 watt-mins.)
Luckily, we gauge EV efficiency in watt-hours. An efficient EV may consume 200 watt-hours/mile (a very efficient EV, I might add). That's 1/15th of mile in flywheel energy consumption (5%), but only if 100% of the energy in the flywheel were bled off uselessly.
Unless you're in the habit of braking hard enough to stop the vehicle instantly, therefore insuring that 100% of the stored energy in the flywheel was converted to heat, then the larger 5% figure starts to go down to 0% (a non-braked coast to a stop).
As KiwiEV points out, there are so many other variable involved, such as a sudden braking maneuvers. Obviously, if your EV is more inefficient the effects of the flywheel will be even less. The effects of vehicle speed will play a factor since the EV will not use 200 watt-hours/mile all the time. The flywheel will be spinning at different speeds so will not always be storing the same amount of energy... etc. You stand more of chance of getting back the 0-5% energy loss by changing your driving behavior.
So yes, I agree with you, the clutch/flywheel combination will technically be more inefficient, but practically it won't be very significant, IMO.
I even machined the ring gear and a raised portion off of my flywheel, but knowing it was to save overall vehicle weight. Even then it didn't amount to much, but I had it piggybacked onto some other machine shop work I was having done.
I'd be curious to know just how much more range Craig Vinton got due to the 12lb flywheel shave (not even something like a 33kg loss), as opposed to his aerodynamic aids... or merely different driving behavior.
Of course, what do people mean when they say significant?
I realize now, that Green Caveman's use of 700+W was probably from F16bmathis' report of 771 watts in converting 46274 Joules to Watts (correct me if I'm wrong). What's confusing is that while watt usage is typically assumed to be usage for an hour, sometimes it isn't. If he then claimed that he could "generate the 771 watts in a minute or two (not verbatim)
" then he'd be generating 771 watt-minutes of energy (or 700 watt-"two mins" for the latter, to be esoteric), or divided by 60 mins, about 12 watt-hours. This only works if the 771 watt figure was quoted for a minute, though.
In a later post, his real-world experience of being able to maintain 75 watts of power on his bike generator is probably accurate. If he were able to cycle at that rate for an hour he would produce 75 watt-hours of energy, or 4500 watt/mins (75w*60mins). Assuming F16bmathis' abilities on the bike generator, the equivalent amount of energy in the flywheel could be generated by him in around 10 seconds. Now that does seem small, doesn't it?
Hmmm, that was supposed to clarify things...
OK, one more analogy. Remember that 46000 Joules? That's equivalent to about 11 nutritional calories. Now that really seems small.
Originally Posted by green caveman
Craig Vinton with his Chevy Tracker
claims that machining 12lbs off the flywheel improved his range. If the stored energy is really 700+W, then that's more than a mile of range every time you stop that without recoverying the energy. For a 3000lb car moving at 30mph that's about 25% of the kinetic energy of the car stored in the flywheel - a significant chunk, but as you say, that only comes into play when you stop it with the brakes.
Since we have a Sepex/Regen system, it should be possible to recover energy from the 5000 RPM hunk of very heavy steel even on a quick stop - so long as you remember to push out the clutch.
I still tend to think that some amount of material can reasonably, and cost effectively, be removed from the flywheel without any danger to the structure and that it would be an advantage to do that.