They aren't going away (hydraulic hybrids)
 

Green Car Congress: PSA Peugeot Citroën and Bosch developing hydraulic hybrid powertrain for passenger cars; 30% reduction in fuel consumption in NEDC, up to 45% urban; B-segment application in 2016

regards
Mech

Wonder why they are using compressed air for energy storage instead of just compressing the hydraulic fluid?

Fairly sure they are using hydraulic fluid which is compressed by nitrogen (bladder accumulator?). There issues with air being compressed beyond 500 PSI, especially if it is mixed with any combustible vapor. It tends to act like a diesel engine. Why mess with that when you can use an inert gas and have it be a fire extinguisher if ruptured.

regards
Mech

Well they can't store the energy in the liquid because liquids don't compress.

Conversly, gas doesn't work well as the working fluid in the motor/pump, precisely because it is compressible, and compression/expansion causes heating/cooling which in turn causes a great loss in efficiency.

Hydraulic accumulators can reach 99% efficiency, while you loose 15% just charging the battery on a Nissan Leaf. Just filling the "tank".

Best wheel to wheel efficiency of regeneration.
Hydraulic-close to 80% (in 2006).
Electric- close to 33% (also in 2006).

Remembering wheel to wheel, the hydraulic option allows for P&G while maintaining constant speed while gas electric hybrids can not exploit the highest levels of efficiency of an IC engine without giving away that same efficiency in conversion losses. The same applies for electric power, expecially when you use a simple direct drive that can not recover even a small percentage of regenerative energy.

It will be amusing when a decent HH car is actually built how the nay sayers will rationalize their ignorance.

rgeards
Mech

I've been wondering why these systems aren't more widely used too. Other than some delivery trucks, there really aren't any Hydraulic Hybrids in production yet that I'm aware of.

In addition to efficiency, you don't have the risk of fire that you do with high capacity batteries (see Fisker, Chevy Volt, Boeing, etc) and you don't have to rely on rare-earth elements that are just going to get more and more difficult to acquire as demand booms.

Seems like a win-win to me.

According to the article
 

"The hydraulic system, which uses compressed air for energy storage, basically comprises two hydraulic units and their pressure accumulators. PSA says the technology—called “Hybrid Air”—will be fitted on B-segment models starting in 2016"

"Unlike a gas, a liquid may not always mix readily with another liquid, will not always fill every space in the container, forming its own surface, and will not compress significantly, except under extremely high pressures."

from: Liquid - Wikipedia, the free encyclopedia


But that answers my own question the pressure require is too great.

How much range is provided with the 'Energy Storage System' and the 'Low pressure storage' tanks?

http://bioage.typepad.com/.a/6a00d83...9b15970d-800wi

What I'm wondering is how realistic those figures are, and what the state of... errh... "charge" is at the start of the NEDC and at the end of it.

The NEDC has become notorious in recent years because of the ultra-short drive cycles and the issue of letting hybrids start with a full battery, and no accounting is made of where that power came from. In this case, the EPA (flawed as it is) is superior, since "battery-only" running is tested separately.