Interesting idea. If the spring was wound at the factory to provide life-of-the-car energy, how much is that? Feynmanning forth …
Of course the energy needed to move the car is roughly the same for an ICE or an EV or a pullback. But the conversion efficiency of each of those varies. Starting with ICE as the base case …
If a typical car lasts 300K miles and goes 300 miles on a tank of gas, we can say 1000 tankfuls is a lifetime fuel supply. For a basic transportation small sedan like in the comic that gets 20-30MPG, those are probably 12-15 gallons per tankful. Being pessimistic, 15,000 gallons of gasoline.
One gallon of gasoline is ~120E6 Joules of energy. Times 15K gallons is 1.8E12J.
An EV would be more efficient in converting stored energy into motion. So might get away with 1/3rd to 1/10th as much stored energy.
A mechanical spring will be nearly 100% efficient at energy out versus energy put in. But the mechanical transmission to manage the RPMs and get them to the wheels won’t be so efficient. Probably less efficient than an EV, but better than an ICE; that chemical-to-mechanical energy conversion step is a real efficiency killer.
In any case, for any energy source & power conversion system that is life of the vehicle we’re talking ballpark 100E9 to 1E12J.
If that much energy gets out all at once it’s going to make a very impressive mess. SPROINNNNGGG!!!11!1!!!
You can also do it with a crane: roll it back, hold the wheels, pick it up, move it forward, put it down, roll it back again, and so on and so on until it starts clicking.
And I took it the other way; rewinding in the field would be soo all-fired difficult that the car had to be delivered from the factory with a lifetime energy supply.
The number of different ways to inject sense into the nonsense is one of my main measures of merit for an xkcd comic. This one scores pretty high.
Mechanical methods of energy storage are always going to be less efficient than fuels that burn oxygen; the energy density of oxygen burning fuels are increased by the fact that you don’t have to carry oxygen around. Unless you are in space or on the moon, etcetera.
I haven’t been ignoring this most excellent question. But it’s a bit beyond my mechanical engineering chops. An hour spent digging around for background knowledge, for spring designs, and materials parameters was getting me nowhere.
So I cheated and asked Google in AI mode to design one. As expected, Google sez it’s somewhere between ginourmous and impossible with current known materials. Provided herewith for your entertainment with no editing by me and no guarantees by me either.
Google AI's Opinion of a Wind-up Spring Sufficient to Power a Car for its Design Lifetime.
My prompt:
Give me a design for a torsion spring able to store 1E12 Joules of energy.
Maybe you mean better energy to mass ratio? In this context, efficiency is the fraction of stored energy that gets converted into kinetic energy. Heat engines are not particularly efficient at converting chemical energy to motion. While devices that convert stored mechanical energy to motion tend to be very efficient.
Yes, that’s right. A machine with stored mechanical energy needs to be heavy, in order to store the energy. A kilogram of spring or a kilogram of flywheel will store less energy than a kilogram of kerosine, assuming an internal combustion engine that burns atmospheric oxygen.
