50mpg from brakes?

All of the talk about perpetual motion and extremely high gas mileage reminds me of an article I read quite a few years ago. A group of college students built a gas powered vehicle that would get around 50mpg (to the best of my recollection) and had good acceleration from a stop. They used an accumulator to store hydraulic energy from braking to assist starting acceleration. I think they were using a pump to pump oil from a reservior to the accumulator to provide braking force. Then on acceleration the pressurized oil might have been routed to the transmission. I don't remember all the details anymore. I looked around on the web for a few seconds and found that there is some development in this technology. Is this feasible technology?

That's the same idea as the hybrids, except hybrids generate electricity from braking to charge the batteries. Your idea sounds like less "monkey motion", and no need for expensive battery packs. Hope someone finds some info on it and posts.

Dynamic Breaking using electrical motor appears to work well.
I'd like to see what mechanical components make up a hydraulic setup. If you find a link post it.


Off topic but on.
If I were to build a wind mill or similar energy capture device I'd probably use it to "charge" an air compressor. In turn I'd use the compressed air in either an all mechanical application or use it to power an air motor on a generator.

In my mind, (what little there is) I would think you could have a compressed air tank on your vehicle. When you brake, your vehicle could compress the air. When you need to start again, your vehicle takes the compressed air in the tank for the start or the "push".
However, like the hybrid car, all this stuff would add weight to your vehicle, a definite no-no.

At what point do you use so much energy trying to store it though,that there isn't enough useable energy left to make the whole thing feasable?

Air, when compressed, heats up dramatically. If used while hot, (or kept hot) it can return substantial amounts of the energy used to compress it. If it cools, that work is lost, the pressure drops, and that is now lost work. Not terrible, because the pressure will stay in the tank without loss, but it is not a zero sum game. JimN

ONE article is linked below. A GOOGLE search for "hydrostatic accumulator braking"
will turn up HUNDREDS more.

Don't know if any use the "fifth wheel free energy method", though!
SAVE that braking energy

Eaton is working on a similar system for trucks

The term is "regenerative braking" and it's been used for years, particularly on solar, electric, hybrid, extreme-mileage and other types of low-input vehicles. I believe some of the huge off-highway haul trucks (which often are electric-drive) have these systems as well. It's one strategy to capture energy that would otherwise be wasted. A hydraulic accumulator is one such system--there are many others. Toss "regenerative braking" into Google and you'll get a ton of hits.

That idea's been floating around for many years. (I recall my dad telling about analyzing it when he was in engineering school shortly after WWII...)

Unfortunately the concept has a bunch of problems:

1 - Complexity. You need accumulators, valving, motors, piping and controls (= added cost and weight)

2 - Control - Being able to continuously modulate braking and acceleration is a key to safe driving. Hydraulics and compressed air do not lend themselves easily to this - particularly the brake modulation part.

3 - Efficiency. Unless a way is devised to adiabatically compress/expand gases - ie. without heat transfer - in an accumulator or compressed air receiver, total system efficiency (ie. % of braking energy stored vs % of energy returned back to the wheels) for hydraulic accumulator and compressed air systems is somewhat disappointing.

4 - ALL regenerative braking systems are effective only during continuous stop-go driving. Driving at steady speed (cruising the interstate or even just running over to the next town) regenerative braking schemes REDUCE fuel mileage on account of the extra weight that is carried.

Bottom line is with current technology the small engine + battery hybrid represents the most economical regenerative braking solution for automobiles. And it's not likely to change anytime soon.

Mostly on school buses and other start/stop vehicles.

Yo Jim, that brings back ancient memories in Thermo class concerning the gas equation Pv=nrT which has to do with refrigeration and heat of vaporization n all other sorts of neat mechanical/thermo which, as an electrical engineer, was NOT my best subject grrrrrrr

If pressure is increased, then temp goes up as you explained and if the compressed air inside the storage vessel/receiver cools down then the pressure must drop and its that stored pressure that can be used to do work.

Gotta love physics

John T

Buses make sense. I was actually thinking about city transit buses as I was drafting the post. If you get stuck behind one it seems like they stop every couple of blocks.

Check out this link, UPS is running a couple of hydraulic hybrid trucks.
UPS hydraulic Hybrid trucks.

Looks like there might be some potential after all.

Seems like I remember reading in Popular Science back in the 60s about equipping a city bus with a large flywheel mounted low midship that accumulated braking energy and released it on acceleration. Sounded like a good idea to me at the time.