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Prototype Volvo Flywheel Tech Uses Car's Wasted Brake Energy
cartechboy (2660665) writes "Sometimes we get carried away with sexy moonshot car tech--whereas most everyday gains are about reducing inefficiencies, piece by piece. Volvo's flywheel energy-recovery prototype is a great example of the latter--not to mention similar to one used in Formula 1 racing. The system recaptures energy that would be wasted in braking, like a hybrid does, to reduce fuel consumption by up to 25 percent. When you hit the brakes, kinetic energy that's usually wasted as heat is transferred to a "Kinetic Energy Recovery System" mounted to the undriven axle. It spools up a carbon flywheel that turns at 60,000 rpm to store the energy. When the driver hits the gas, some of the stored energy is transferred back to power the wheels through a specially designed transmission, either boosting total power to the wheels or substituting for engine torque to cut fuel consumption."
The big factor is mass. To store energy you need to spin up and down the mass. However to drive in general you want to carry less mass on the vehicle.
Factor #1: A more massive flywheel can store more energy at slower spin rates.
Factor #2: A more massive flywheel is going to be more of a load in general driving.
The optimium point of flywheel mass is going to depend on driving conditions. Really you should have at least 2 interchangeable fly wheels that you physically replace in the vehicle. One flywheel for city driving one for highway driving.
Factor #3: A spinning flywheel is one hell of an energy store. Having a stopped vehichle with a fully spun up flywheel hit could release the spinning flywheel to the detriment of pedestrians in the neighborhood.
Factor #4: Starting from a stop and attempting to corner, left or right, having a spinning flywheel is going to do gyroscopic things to the vehicle.
There are all sorts of tradeoffs and safety considerations here.
It briefly stores energy from braking and uses it to accelerate a moment later. If you don't hit the brakes, it does nothing. If you hit the brakes and stay at a low speed for five minutes, it does nothing.
When it works is when you stop (which stores energy), then go (which uses the stored energy). In other words "stop and go" traffic is EXACTLY what this is designed for.
This was discussed when flywheel KERS was added to formula one. The forces involved are not significant, and on a heavy vehicle (as opposed to an F1 car) would have even less effects.
Ultimately it is just a stop gap. Electric is so much more flexible than the complex CVT and fundamentally limited flywheel used in this. Which is why F1 all went to battery based systems.
It occurs to that this is basically a larger copy of the "friction motor" that was used in toy cars. The ones you'd spin up by rolling them on the floor , then you let go and they speed away. If you ever played with those, you know that the spinning flywheel has WAY more than enough rotational energy than required to accelerate its own mass. Those aren't going nearly 60,000 RPM either. (I think, I've never measured their flywheel speed.)
The Chrysler Patriot prototype in the early 90's had this problem. This was a vehicle that was being designed for the 24 Hours of Lemans. It had a gas turbine that ran a alternator, which powered an electric motor driving the wheels. Instead of a battery pack it used a composite flywheel to store energy. Initially the flywheel caused too much of a gyroscopic effect and it was found that you couldn't turn the car. The solution was to make the flywheel gimbaled so it could rotate as needed while the car maneuvered.
http://www.allpar.com/model/patriot.html
That seems to make sense and seems like an interesting idea. Can you express it using a car analogy?
He's getting rather old, but he's a good mouse.
I hate getting caught behind one of those "How slowly can I accelerate and still call it acceleration" types. Invariable on my commutes, it's those dickheads that do not understand that the lights are timed for NORMALS and cause a huge traffic jam behind them from stopping at every damn light. Yeah, you save a LOT of gas stopping at every light on the road instead of getting up to speed in a reasonable distance and getting the green. I have one road on my commute that has 15 consecutive lights. Pass the eco-nazis and I never have to stop. Get caught behind them and my commute time doubles.
People who don't leave adequate braking distance and accelerate as hard as possible are the reason most of the traffic jams on my morning route occur. A single light touch on the brakes gets magnified into a ripple of progressively more urgent braking until you have traffic that grinds to a stop - no obstruction required. A few large gaps help to absorb this kind of thing and would keep the traffic flowing, but the few people who seem to think that tailgating people at beyond the speed limit until they give way and let the guy overtake you - so he can do the same thing to the next guy in the fast lane going the same speed - is acceptable make everyone else so paranoid that they are missing out on a particular piece of road that hardly anyone is willing to leave any space.
If everyone drove with a little more room, then the traffic wouldn't jam up so much, and paradoxically, people would get to their destination faster. The tailgaters are just spoiling their own driving party.
Going from memory from many years back, there was a few very interesting points when I was reading about flywheel research for hybrid F1 racers
1) Something like 90%+ efficient at converting physical energy into rotational and back out
2) Decided to use carbon fiber because instead of turning into shrapnel, it disintegrates when it smashes into its cage
3) Added less weight than an extra person
4) Was able to supply 80hp for 10 seconds at max
5) Was able to quickly and efficiently capture energy, so you could slam on the breaks and get your 80hp for 10 seconds very easily
6) Increased fuel efficiency for F1 racers by 10%-20% because of lots of hard breaking followed by hard acceleration.
I'm sure other safety issues will bring down the effectiveness of these devices for regular car users, but there is a lot of margin to make it an overall win.
Actually, yes, I think we can, strangely.
Imagine you're coasting your way to the top of a hill and stopping at the top of it, with the brakes doing very little of the work in stopping you. By cresting to the top of the hill, we've effectively converted the kinetic energy you had into potential energy that can later be reclaimed when you go down the hill, and we've lost very little of that energy to heat from the brakes. That is, we can reclaim that stored energy to get a good chunk of the way back up to speed for a fraction of the fuel cost that it would have taken had that energy been lost.
In much the same way, a flywheel is capable of converting forward momentum into a form that can then be used later. You can think of it as an invisible incline under the car every time you hit the brakes, helping to bring you to a stop while storing that energy for later, and an invisible declination under the car every time you follow the braking with the accelerator, helping you get back up to speed without having to consume as much fuel.
(I'm now eagerly awaiting corrections, since I'm sure I misused terms and explained things poorly)