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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.
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
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)
You DO realize there is a middle ground between hyper-miling and hyper-aggressive driving, right? A sensible driver can get pretty good gas mileage by easing off a bit without creating a huge line of angry eighteen-year-olds with an anger problem. The sensible driver might miss a light now and then, but most of the time they will get there right behind the angry aggressive drivers. They also arrive safely and more relaxed. If it is possible, you might give it a try.