[peppercorn (OP)]

Would a swash-plate hydraulic pump recharging the pneumatic accumulator work best?

There has to be a compromise between cost, efficiency & weight.  Is a swash-plate pump the right type?


Alternatively, since any usable vehicle needs an electric system would a generator driving a motor coupled to a high speed (but also high pressure[??]) pump be more suited. Then potentially a smaller or no alternator would be needed on the engine.

[lyner]

I have a suspiscion that a pneumatic accumulator might not be too efficient compared with a battery / capacitor energy storage system. It would need to operate at very high peak temperatures and would good insulation to prevent heat loss (for an adiabatic cycle).

[peppercorn (OP)]

The advantage is you've got lots of 'free' waste heat if combined with a heat engine as primary power source.

Hydraulic hybrids have already found some success on delivery vans & heavier vehicles which commonly operate stop-start journeys.

Here's an example:
www.boschrexroth.com/country_units/america/united_states/en/news_and_press/trade_show_information/a_downloads/Hydrostatic_Regenerative_Braking_Brochure.pdf

The pdf claims hydraulics is better suited to regenerative braking than electric systems.

[lyner]

For short term storage, I can see some advantage. You can avoid generator - charge - discharge - motor losses. You just need a good pump / motor instead. Unlike a battery, a pneumatic store won't age.

[peppercorn (OP)]

Unlike a battery, a pneumatic store won't age.

Is assume that's due to cooling of the compressed air between runs.
This is an issue, but I would have thought the exhaust gas heat should replenish this fairly quickly.

[lyner]

The 'free heat' idea has only so much going for it - the useful  temperature difference is not great so the thermodynamic efficiency doesn't give you very good value - else you could run a steam engine off the exhaust manifold! The only thing I know which is run off exhaust gases is a turbo charger.

Re the "aging": I was referring to the fact that battery performance goes downhill after a certain number of charges and it needs protection from total discharge etc..

[peppercorn (OP)]

else you could run a steam engine off the exhaust manifold!

BMW built a prototype recently that did just that:
http://www.gizmag.com/go/4936/


Obviously there is a pay-off between extracting energy from exhaust gases and effecting the gas velocity needed to help the engine breath, but as a traditional exhaust pipe is designed to keep the gas speed up whilst relying on excess heat being radiated along its length then can't the same be true if the heat (many hundred of degrees centigrade at the manifold) is carried usefully away? BMW obviously think so.

In the case of on-board compressed air storage heating the expanding air allows it to do more work.

[lyner]

Hm.
I would need more than a journalist's version of how it works before I'd believe that BMW are actually saying what he's implying.
Calling it a steam engine may be a bit of an 'exaggeration'. Steam locomotives used to consume a lot of water in order to get a working condenser system and there are no 'cool' elements in a car cooling system. How can the cooling be achieved in order to make the external combustion part have a reasonable efficiency (temperature difference), I wonder? A stirling cycle would have the same thermodynamic limitations.
However, there is such a lot of wasted thermal energy that it may well be worth while clawing back some of it in a useful  form, albeit inefficiently.
Let's wait to see when there's more proper info.
As you say, BMW are unlikely to be involved in a complete bummer.

[peppercorn (OP)]

Here's a slightly more detailed article with pretty thermal-imaging pictures (Ooo):
http://paultan.org/2005/12/11/bmw-turbosteamer

...there are no 'cool' elements in a car cooling system. How can the cooling be achieved in order to make the external combustion part have a reasonable efficiency (temperature difference), I wonder?

I guess that means the system would work better on very cold days, eh!

The advantage of having a 'cold' source on-board is one of the reasons I think the application of a compressed air reservoir has some mileage [he he -sorry!].

[lyner]

In the link they refer to an "expansion unit". Presumably this is a turbine. If it's on the crankshaft, then it needs to operate over a range of revs. Turbines only work well at high (constant) revs (usually 10's of k/minute, rather than a few k. I wonder how well that will function in the flexible rev range of a piston engine. Even with a reduction gear, the speed variation must still be a factor.
Let's face it, there is only one example (afaik) of a gas turbine being used for automotive power and that was fitted into a Rover in the early 70's (?? ish). Great on the Motorway - a nightmare in town.

These factors must have been considered by BMW but the article is still too sketchy to get an idea of the details.

I still don't see where the re-gen idea comes into this. That would require even one more motor in the system. Or are you just proposing the reservoir as a possible refrigeration source?

[peppercorn (OP)]

I still don't see where the re-gen idea comes into this. That would require even one more motor in the system. Or are you just proposing the reservoir as a possible refrigeration source?

I have to say I'm not mad about the bolt-on 'steam-engine' idea as a solution anyway.
I just wanted to point out that expanding gas does offer a cold source that can be combined with a heat engine in a number of permutations.