Wind-up hybrid?

Anybody know why hybrid cars use electricity instead of simply using a wind up spring mechanism to recover the energy that would have been lost in breaking?

While commuting alone down California’s carpool lanes in my Pius I started thinking about how the engine works, and how I can drive most efficiently. I think of questions like when is the best time to start breaking? I start wondering why I can’t I (or my GPS) tell my car that I’m about to go down a long hill so empty the battery?

I’ve noticed that the hybrid battery itself does not seem to hold very much energy. For example the A/C will drain the battery when parked for more than about 30 minutes.

So, while feeling guilty about all the carbon and other environmental footprint required to give me the opportunity to daydream my way down the commuter lane, I started thinking; why did they decide to use a battery to store the kinetic energy released during braking? Yes the electric motor works about as efficiently as a generator, but I’m sure there must be quite a bit of loss in this kinetic to electrical energy conversion (what does the laws of thermodynamics say?). The high currents produced during breaking must release a substantial amount of heat, not to mention any losses in the chemical process of charging and discharging the battery itself. Wouldn’t it make sense to skip all that energy conversion and simply coil a spring? Yes some sophisticated electronics and a CVT would still be required to maintain the proper (de)acceleration, but is there some fundamental limit to the energy carrying density of a simple coil spring? This must have been thought of well before electric motors and batteries were the option of choice? Would the spring have to be too heavy to be practical?

Springs have a much lower energy density than batteries. It’s as simple as that.

I could see that as an argument why a true “spring loaded” vehicle would not work, it would not send you far on a single “wind”. But like I said, I don’t think the hybrid batetry holds that much energy either. Better to recover some energy than none.

Ford did a hybrid SUV (or was it a pickup truck?) concept car that used a hydraulic accumulator. These store energy by further compressing high pressure nitrogen gas. So clock spring-no, gas spring-yes.


Link to article about this concept truck.

A Prius has regenerative brakes and recovers energy when you take you foot off the gas.

In an extended sense of “winding things up” to store potential energy for cars flywheels have been considered for this use, but IIRC from Popular Science articles on the topic, the rotation speeds and bearing technologies necessary to hold a useful amount of energy (for powering a car) in this format exceeds existing material specifications.

Not only do springs have a low energy density (and suffer from mechanical fatigue and hysteresis), but the advantage of using a battery is that it is connected to the generator that is already there, and just running in reverse to regenerate lost energy. No extra powertrain subsystems are required.


You and your “holier than thou” attitude. :smiley:

The generator was not always there. They put it there to call it a hybrid :smack: Yes the electric motor does double duty, generator and motor, but so would a mechanical “engine” added to your gas engine also do double duty. It would “wind up” when coasting or stepping on the brake just like my Pius does, and then (magically) this spring twisting “generator” becomes a “motor” when the accelerator is pressed…

My feeling on the other post about the nitrogen gas “spring” is that along with the physical compression comes temperature elevation. Compress a gas and it heats. Guess that as long as it’s insulated properly this waste heat would be reabsorbed when the gas is decompressed. Anybody who’s used a bike pump knows how much heat is released when gasses are compressed. Mechanical springs don’t heat and cool nearly as much.

My Pius emits no smog, just a big cloud of smug instead…

Here’s one that shows alternate energy storage devices are parctical. In this case they’re showing an air “spring” capable of winding up 300KM.

Guess that as it decompresses it can also be used for air-conditioning??

Not every time. When the battery’s full it stops regenerating. Like I said 1Kft elevation drop is usually enough to take the batetry from it’s preferred ~80% charge to 100%. You can hear when this happens as it starts “engine braking” with the gas engine instaed of the quieter electric motor.

Seems to me that a flywheel would not be as good as a spring because:

  1. It needs to be heavy or spin excessively fast to store large amounts of energy, and
  2. It acts like a gyroscope so could affect the handling.

I think regenerative braking is overrated anyway - it recovers some of the energy that went into accelerating the vehicle, but that’s usually small to begin with, compared to the energy used to overcome air resistance. It may be useful on long hills, but that’s about it.

By the way, Wikipedia entry on energy density lists the energy density of mechanical springs as 0.0003 MJ/kg. The Prius has a 1310 Watt-Hour (= 4.7 MJ) battery, so you’d need a 15-ton spring to replace that 100-lb battery.

(And don’t ask me why a 1310 Wh battery can power the air conditioner for only half an hour. Maybe the cutoff point is set very high, so there’s plenty of power left to start the engine and accelerate the car up to speed.)

By the way, if you look at that same Wiki link, you’ll see that compressed air can have roughly the same energy density as batteries. Which is 3 orders of magnitude better than clock springs.

Flywheels are also right up there with batteries (better, in fact). The gyroscopic effect can be eliminated by mounting the flywheel on a gymbal. Although safety is still a concern.

Finally some proof as to why it’s impractical to store energy in a spring. Guess those toy wind-up cars suffer from cube law scaling effects.

Not if it is mounted in gimbals.

How do you handle the power coupling if you do it that way? Or are you talking about a motor/generator flywheel setup?

Yes, I think that is how the flywheel car in Popular Science did it.

Flywheel trams and buses are being developed, with one or two already in revenue service.