AutoblogGreen

Sounds familiar
http://www.autoblog.com/2006/02/14/ford-f-150-to-debut-with-hydraulic-hybrid-system/

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How much pressure are we talking about here? I'm not sure I want this on my civilian vehicle, let alone under a vehicle that is going to get shot at and IED'd.

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The storage on these is technically pneumatic, because you can't compress a liquid.

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Since the hydraulic accumulator stores power by compressing air, there are thermodynamic losses. Unless they limit the storage to modest pressure, the thermodynamic losses would be greater than with electric batteries. Moreover, reguardless of how high pressure is used, the power storage is just a fraction of what a good battery pack of equivalent size could store, limiting the regeneration and energy savings.

The far greater storage capacity of electric batteries makes electric plugin hybrids possible, but not so for hydraulic hybrids.

For applications that already use hydraulic power, such as construction machines, it makes sense. But for other applications, the additional cost and weight may not be worth the modest fuel savings.

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The hydraulic accumulator works by compressing air in a tank. There are thermodynamic energy losses, which would be greater than the energy losses for batteries, unless the pressure and energy storage is kept very low. Even at very high pressures, the amount of energy stored is just a fraction of the energy stored in a battery of equivalent size. This severely limites the amount of regeneration that can be used.

This limited energy storage makes hydraulic hybrids unsuitable for plug-in hybrid applications.

For applications that already require hydraulics, such as construction machinery, hydraulic hybrids may make sense. For automotive use, electric hybrids can achieve much higher efficiency, especially with a plug-in option.

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This idea has been around for years.
http://www.designnews.com/article/CA220671.html

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I was all with the story until the claim of improved mileage in the order of 60% and 90% reduced emissions, at which point I just groaned, rolled my eyes and dismissed it as another wishful pie-in-the-sky.

P.S.: I'm saying this from the standpoint of someone who is working for a company that has worked with the EPA on a hydraulic-drive vehicle. Look through your own article archives to see which company I'm talking about.

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Chris,
I don't believe they are using air as their fluid judging by the language and the pictures on the website. That said most pumps are on the order of 90% efficient so if you are able to store 90% of the input energy and use 90% of the output this would put the overall efficiency in the neighborhood of 81%. The only electric technology that could compete with that are capacitors.

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THank you Linton for all of your faithfully green articles.
This one doesn't seem to work so good.
Hydraulics efficiency on mid-sized or heavy vehicles doesn't seem to work for highway or coasting speeds.
What about landfill anerobic energy, manure, algae, etc.. doesn't that all add up to something? Can we get a recount?

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THank you Linton for all of your faithfully green articles.
This one doesn't seem to work so good.
Hydraulics efficiency on mid-sized or heavy vehicles doesn't seem to work for highway or coasting speeds.
What about landfill anerobic energy, manure, algae, etc.. doesn't that all add up to something? Can we get a recount?

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Fluid does not compress, it moves and transfers. The compression and storage is in the form of a gas which is on the other side of a bladder in an accumulator and compressed during breaking. This can't store large quantities of energy. It is a light weight, cheap and efficient way to capture and reuse the energy bursts of braking. This would NOT make a good plug-in, however and therefore is not well suited for passenger vehicles. Having hydraulic regenerative braking and electric drive and on one vehicle sounds complex, but maybe it should be studied. Using a good ultracapacitor seems more logical.

No one system will serve all needs. Perhaps this system is best suited or stop-start heavy vehicles like garbage trucks or delivery vehicles. By the way, there are almost ALWAYS thermal losses when energy changes states

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Gear pumps are used with viscous fluids, not air. Secondly the website states hydraulic hybrid, not pneumatic. Linton is also correct it is possible to store liquids under pressure. It just so happens that when under pressure their volume change is negligable, this is why liguids are commonly refered to as incompressible. Keep in mind that PV=nRT applies to ideal gases only.

Pressurizing an oil is exactly how hydraulics generate force, the oil is under pressure.

Yes this system is not suited to highway driving or plug in capability, but it makes electric regen braking look pathetic. It is intended to act as an acceleration aid after you come to a stop. This is very important because most fuel is used in accelerating a vehicle, not maintaining speed.

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The reason hydraulic energy storage can beat electric storage in efficiency has nothing to do with thermodynamic losses or energy density. Unless you are using expensive ultracapacitors, electric energy storage works at a very modest power rate, so you are not able to capture all of the energy. The energy you do capture can be captured efficiently, but the remainder will be dissipated as heat when the friction brakes kick in.

Stopping a typical car from 60mph over a period of 10 seconds generates an average power of over 50kW, while most hybrid battery packs can only support charge rates under about 10kW. Of course, you can improve performance by slowing gently, but people don't drive that way and it's not always practical to do so.

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Yes, if you want to be pinhead, you can compress a fluid however since the energy storage benefits are negligible who cares and it is not germane to this discussion.

The tighter and less excited the molecular structure, the harder it is to compress and the more efficient the energy transfer. As far at this system is concerned, they are referring to pressurization of the fluid which compresses the gas in the expansion tank.

The benefits are in the efficiency, speed, cost and weight over battery systems. This too may change as batteries & ultracapacitors improve. If EEStor is not BS, it will be another game changer. Only time will tell.

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Scappy#14 "Yes this system is not suited to highway driving or plug in capability, but it makes electric regen braking look pathetic. It is intended to act as an acceleration aid after you come to a stop. This is very important because most fuel is used in accelerating a vehicle, not maintaining speed."

I couldnt agree more. No system is perfect as was said before. Most fuel is used and thus emissions expelled, during acceleration. Therefore its possible that a vehicle that makes use of the hydraulic power, garbage truck for example, would benefit greatly.

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Well, the army is working with them, so they got something.

I doubt though that it is SO much more efficient than HEVs

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I still fail to see any mention of gas (air) on their website.

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To clarify how a hydraulic accumulator works: It is a pressure tank containing air (or another compressable gas) with a flexible diaphragm or bladder separating the gas from the hydraulic fluid. When the hydraulic fluid under pressure flows into the tank, it compresses the air, and that compressed air can force the fluid back out when needed. You could get a similar effect with a hydraulic piston connected to a stout spring, but that would be more expensive and heavier.

Yes, a hydraulic accumulator may be able to accept a faster charge rate than a battery, but that may not help if it quickly gets filled to capacity and cannot accept any more charge, long before braking has ended. That makes it more suitable for slow speed vehicles, like construction equipment or garbage trucks, that also uses hydraulics for other things.

Liquids (including hydraulic oil) can be put under high pressure, but any change in volume is so miniscule that for all practical purposes it is considered "uncompressable", unlike a gas that has a substantial change in volume when pressurized.

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