Wind-up hybrid?

[Stranger_On_A_Train]

@scr4

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.

For long transits (i.e. highway driving) this is definitely true. But in stop and go traffic or in a stoplight grid, where the mode is throttle, throttle, throttle, brake, brake, brake, air resistance isn’t much of a factor (because you are moving at slow speed) and a considerable portion of energy through the drive train is then lost to heat in braking. Other losses are hydraulic losses in the torque converter (about 2-5% in modern planetary gear transmissions), tire flex (a couple percent with properly inflated tires), and lateral inertial changes from turning, plus whatever you lose to power fans, pumps, lights, spark plugs, et cetera. Essentially, if you are moving in a straight line along a level surface the distance of roughly one city block between start and stop, you can theoretically regain >80% of the energy expended (assuming that you aren’t using a power hog like the air conditioner), although realistic values are lower because the regenerator brake only absorbs a fraction of the total braking energy. On electric powered construction equipment (moving at <10 mph) regenerative braking can extend battery life by 30% or more. And for a hybrid, which already has a generator in the loop to help equalize the power demand (allowing you to use a smaller, more efficient, but less torque-y engine) it’s a gimme, because you don’t really need any extra equipment; you’re just reversing the polarity on the generator and sucking charge back into the battery.

@scr4

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.

The problem with any system involving stored energy via gas compression is that compression is a thermodynamically inefficient process; too much energy is lost in the temperature change that accompanies steep compression that is not readily recoverable. (For a demonstration of this, go to your local scuba shop and ask to feel an air tank that has just been rapidly filled; the surface will be almost too hot to touch. This is all lost energy, and it is a hell of a lot of it.)

Flywheels are very efficient and also good at moderating demand and balancing loads, which is why they’re used at some large power plants as a transient storage system, but they either have to be very heavy, or be very large, or spin extremely fast (or more likely, two of the three) in order to store a lot of energy. This makes them not so desirable for use in mobile applications. While the gyroscopic effects can be mitigated by gimbal mounting, this also increases the complexity of the device, and means that you need a big spherical volume in which to place it. I think this is impractical for an automobile, though you might use something like this on a train to recover energy lost on downhill runs.

@Smug Doug

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.

I never thought about it, but this is a great example of cube law scaling. Of course, if you were able to fill your car up with one humongous spring I suppose you might be able to get it to go the equivalent of 20 or 30 lengths, but the force to wind the spring would be enormous.

Stranger

[Q.E.D]

@Stranger On A Train

Of course, if you were able to fill your car up with one humongous spring I suppose you might be able to get it to go the equivalent of 20 or 30 lengths, but the force to wind the spring would be enormous.

Plus you’d have a hell of a time with indoor parking with that big-ass key sticking out of your roof.

[Critical1]

@TheLoadedDog

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

came here to post that. I remember seeing those on a show years ago. I imagine you could toss them on any large vehicle, but with something like a semi truck you would probably be better off without them unless the truck was seeing heavy city use (I could be wrong about this but I imagine carrying that big ass weight around on long haul drives wouldnt payout the same)

[matt]

@Mangetout

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

One system I’ve read about involves carbon-fibre flywheels spinning at up to 1000 revs/second. They’re held in vacuum on magnetic levitating bearings, and the vacuum chamber is on gimbals. The units were quite small, about the size of two fists.

Energy density beats most battery technologies, and power density is very good. Recharge cycles are unlimited. Safety is the kicker though. Crash that thing so the flywheels hit their containments, and all the stored energy comes out at once.

If you like detail, there’s a lot here:
https://www.llnl.gov/str/pdfs/04_96.2.pdf

And a more low-tech approach:
http://www.hykinesys.com/KineticEnergyStorage5.pdf

[ZenBeam]

The problem with any system involving stored energy via gas compression is that compression is a thermodynamically inefficient process; too much energy is lost in the temperature change that accompanies steep compression that is not readily recoverable. (For a demonstration of this, go to your local scuba shop and ask to feel an air tank that has just been rapidly filled; the surface will be almost too hot to touch. This is all lost energy, and it is a hell of a lot of it.)

The EPA has been testing compressed gas hybrid technology in UPS delivery trucks, apparently very successfully. See for example Recent Developments with Urban Delivery Vehicles, and the links from there. They are testing these here in Ann Arbor. I happened to be talking with a guy at the local swimming pool this summer who works there. At the time, they were road testing UPS trucks with this, optimizing them for different sorts of routes, and testing how they performed. At a later time, they were planning on evaluating them in service on real routes.

They claim:
60-70% better fuel economy in laboratory tests
40% or more reduction in carbon dioxide, the primary greenhouse gas
Ability to recoup additional cost for new hydraulic hybrid technology in less than 3 years.
and also
a net lifetime savings over this vehicle’s typical 20 year lifespan would be over $50,000

Here’s another link: EPA demonstrates hydraulic hybrid UPS delivery vehicle with a little video (warning, it starts automatically) that describes it.

[Sage_Rat]

@Stranger On A Train

I never thought about it, but this is a great example of cube law scaling. Of course, if you were able to fill your car up with one humongous spring I suppose you might be able to get it to go the equivalent of 20 or 30 lengths, but the force to wind the spring would be enormous.

Stranger

http://www.tramways.freeserve.co.uk/Tramframe.htm?http://mysite.wanadoo-members.co.uk/tramways/Articles/Clockwork.htm

[Una_Persson]

@Stranger On A Train

Flywheels are very efficient and also good at moderating demand and balancing loads, which is why they’re used at some large power plants as a transient storage system, but they either have to be very heavy, or be very large, or spin extremely fast (or more likely, two of the three) in order to store a lot of energy.

Can you think of some specific sites? I’ve been to more than 600 power plants and I’ve never seen one or heard of one at a large power plant.

[Cerowyn]

@ZenBeam

The EPA has been testing compressed gas hybrid technology in UPS delivery trucks, apparently very successfully. See for example Recent Developments with Urban Delivery Vehicles, and the links from there. They are testing these here in Ann Arbor. I happened to be talking with a guy at the local swimming pool this summer who works there. At the time, they were road testing UPS trucks with this, optimizing them for different sorts of routes, and testing how they performed. At a later time, they were planning on evaluating them in service on real routes.

They claim:
60-70% better fuel economy in laboratory tests
40% or more reduction in carbon dioxide, the primary greenhouse gas
Ability to recoup additional cost for new hydraulic hybrid technology in less than 3 years.
and also
a net lifetime savings over this vehicle’s typical 20 year lifespan would be over $50,000

Here’s another link: EPA demonstrates hydraulic hybrid UPS delivery vehicle with a little video (warning, it starts automatically) that describes it.

But aren’t you talking about burning fossil fuels? The discussion at hand was with regard to storing energy by compressing gas, and then recovering the energy by allowing it to expand.

[scr4]

@Cerowyn

But aren’t you talking about burning fossil fuels? The discussion at hand was with regard to storing energy by compressing gas, and then recovering the energy by allowing it to expand.

That’s what the “hydraulic hybrid” system does. The energy ultimately comes from fossil fuel (diesel?), but excess power from the engine and regenrative braking is stored by compressing nitrogen in a tank. Same way the battery is used on the Prius.

[zut]

@Cerowyn

But aren’t you talking about burning fossil fuels? The discussion at hand was with regard to storing energy by compressing gas, and then recovering the energy by allowing it to expand.

Where do you think the energy in hybrid vehicles comes from? It comes from, for the most part, burning fossil fuels. (In a broad general sense, it doesn’t have to, but when we talk about commercially-available hybrid vehicles, we’re talking about a gasoline or diesel engine as the primary power source.)

The “hybrid” part come in when another system is used to store and release energy as required. You can do that in a lot of ways. You’re probably at least passingly familiar with an electric hybrid like the Prius, which stores the energy in a battery. The UPS truck ZenBeam cites uses an hydraulic system, storing the energy in compressed gas. The OP of this thread suggested a mechanical spring as a storage device, and astro suggested a flywheel. From a fundamental energy transfer and storag point of view, these are equivalent systems. But they still require a primary power source to be a hybrid.

[Smug_Doug]

@zut

The OP of this thread suggested a mechanical spring as a storage device, and astro suggested a flywheel. From a fundamental energy transfer and storag point of view, these are equivalent systems. But they still require a primary power source to be a hybrid.

Seems the advantage of air as a spring is that it’s light. The air car was talking about huge pressures, though.

I’d think that air springs, or any springs for that matter (including flywheels), would best be used in a “hybrid” arrangement, coupled with a primary power source - onboard for an autonomous vehicle like a car, or stationary for things on defined routes. The requirement for an alternate power source is that the force of the spring decreases as it “un-winds”. So a pure “air powered car” might claim 300Km on a “charge”, but they fail to tell you that the last 100Km takes 10 hours :o

BTW kudos to Uncle Cecil for staring this thing. Must take half the research task off his desk.

[Chronos]

In addition to gimballing, you could also counteract the gyroscopic effects of a flywheel by having two coaxial counter-rotating flywheels. But you’ll still have gyroscopic stresses in the thing which is holding them coaxial, so you’d better make sure you have strong materials.

Gyroscopic effects (both torques and stresses) could also be somewhat allieviated by mounting the flywheels horizontally rather than vertically (i.e., with the axis vertical). Cars make a lot of turns in a horizontal plane (which would keep the axis of the flywheel vertical, hence no gyroscopic effects), but don’t generally change their other two axes very much.

[scr4]

@Chronos

Gyroscopic effects (both torques and stresses) could also be somewhat allieviated by mounting the flywheels horizontally rather than vertically (i.e., with the axis vertical). Cars make a lot of turns in a horizontal plane (which would keep the axis of the flywheel vertical, hence no gyroscopic effects), but don’t generally change their other two axes very much.

Hm, would that make the car more comfortable to ride? Take the place of anti-sway bars, for example?

But I guess the car would try to tilt to the side every time you come across a hill, and the road tilts up (or down). Probably not a good thing…