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hydraulic air?

JoeTheJuggler

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JoeTheJuggler
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Jun 7, 2006
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In a recent CNN article about higher efficiency automotive technology http://tinyurl.com/29snl5 was the following:

The EPA has built a modified hybrid that uses a hydraulic system, not a battery, to store braking energy. When you press the brakes, the wheels drive a pump that compresses nitrogen gas, which is inexpensive and inert. When you accelerate again, that compressed gas runs the pump in reverse to help power the vehicle.

The hydraulic-hybrid system, scheduled to begin testing in two UPS trucks this month, with another to follow next year, promises to return at least 70 percent of the braking energy back to the wheels, which would lead to a 60 to 70 percent jump in fuel economy and a 40 percent reduction in emissions.

Perhaps that's why Charles Gray, the director of the Advanced Technology Division and one of the developers of the hydraulic hybrid, can't contain his excitement about its potential.

"This is going to be the biggest revolution in automotive history," he declares. "Bigger than the assembly line."

That's yet to be seen, of course, but the hydraulic hybrid is also smaller and cheaper than conventional hybrids.
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Am I missing something, or shouldn't a system that uses compressed nitrogen gas be called pneumatic rather than hydraulic?
 
I find it odd that they think that nitrogen gas being "inexpensive and inert" is a selling point. What's the cost of the compressor and gas-storage system?
 
It may well be worthwhile, i.e. cost effective, for UPS trucks and such, but maybe not so much for a typical auto. Consider that a UPS truck stops and accelerates about 100 times that of a typical auto (I'm guessing at the 100 number that seems a good gut fit.
 
Gilmar said:
I find it odd that they think that nitrogen gas being "inexpensive and inert" is a selling point. What's the cost of the compressor and gas-storage system?
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I pulled the quote out of context. Sorry. In the article it was obvious that this was meant in comparison to the storage batteries on today's electric-gasoline hybrid cars (and maybe in passing also to hydrogen fuel cells).
 
Hydraulic/pneumatic would be a system where the wheels are driven hydraulically (those Bobcat tractors are) then, in braking, the same hydraulic motors in the wheels act as pumps, pumping the fluid into a pressure vessel filled with nitrogen. The nitrogen is inert, so vessel won't act as a diesel engine and explode, as it could with air. Then, the gas pressure pushs the hydraulic fluid back out to the wheels to run the motors. No electrics involved, just a series of valves to control fluid flow. The system would only be built big enough to store one 'braking' worth of fluid, since high pressure storage takes space.

Some UPS trucks make over 100 deliveries a day. Plus the stop and go driving to go between the stops.

Plus, UPS has an interesting business model of not depending on depreciation of vehicles. They keep them for loong times, some are over 20 years old. Lots of maintenance, engines can be swapped from gas to diesel and back over the life of the truck. So to them, the cost of a hybrid system can definitly pay them back. Plus all that maintenance is a deduction each year, the cost of a new truck would need to be written off over it's lifespan.

Plus one other advantage- no brake shoes are killed or harmed in the production of this hybrid. UPS's savings on the brake maintenance would be enough to trurn a profit, 100 stops a day, shoes must last only weeks. Unlike electric hybrids, a hydraulic system can be locked up solidly, and bring the truck to a full stop. No minimum rpms needed for the re-generation. It will need a radiator in circuit though, to cool the fluid. But the hydraulic drive system already has one, it probably needs to be twice as big for the brake heat.... or does it? energy= heat, store the enenergy, store the heat????

Plus, UPS eats starter motors, at hundreds of starts each day. None needed, just use the hydraulic accumulator to motorize the main pump to start the
diesel.

UPS is already my hero. I believe in old trucks. In 36 years of using pick ups as my daily driver, I'm only on my third truck. I get an average life span of 18 years, or 250,000 miles. Then I sell them to some other truck lover.
 
Another thought. Depending on the size of the accumulator, they would never need much of a main motor. No 200hp V8 diesel needed for acceleration, just a small motor to make up for the rolling resistance and air drag. Like 50 hp? Best engine efficiency is when the smallest possible motor is run at full power. Combine all of these advantages, without any long term battery costs (not even to start the main engine) and UPS could really save. I bet they come up with a retrofit program for their existing chassies. Then redesign their new trucks for a flat floor, end to end, for larger cubic capacity. And lower the floor too, lessen strain on the drivers, who climb in and out 100 times /day.
 
The hydraulic-hybrid system, scheduled to begin testing in two UPS trucks this month, with another to follow next year, promises to return at least 70 percent of the braking energy back to the wheels, which would lead to a 60 to 70 percent jump in fuel economy and a 40 percent reduction in emissions.
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If a 70% savings of braking energy translates to 60% increase in total efficiency, doesn't that mean that 46% of all energy is spent braking? That seems really high to me.
 
Art Vandelay said:
If a 70% savings of braking energy translates to 60% increase in total efficiency, doesn't that mean that 46% of all energy is spent braking? That seems really high to me.
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Yeah, the concept seems like it might have some benefit, but the numbers sound like they were put together by somebody who doesn't understand the information.
 
Lots of power is used in acceleration. Given one weight, w1, to given speed, s1 speed, takes x energy. 2w weight takes 2x energy. Double the speed of w1 to s2, takes 4x energy. E=MC SQUARED.

So, lots of energy needed to accelerate, can be re-stored from the de-celeration.

Remember how much better milage you get at lower speeds? 55mph anybody?

I've read that the normal size car only uses about 12 hp to roll down the road. Maybe next time you get a smog certificate, the tech can tell you at what HP the test is done?

Oh, and the dynomometer? It measures "Brake Horsepower". The roller has an adjustable brake on it. The brake has arm to a scale that reads the pounds of torque that the brake is making. The math to convert needs to use the torque, the length of the arm, and the speed in RPMs to convert to horsepower.

Brakes do get hot, fast. The chassis dyno I used 25 years ago was water cooled by a thousand gallon tank. Oh, muscle car mania, how I miss thee.
 
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I think I get it from Casebro's explanation. The CNN article only said that the gas gets compressed--I assumed the pump was directly compressing the gas, but it's actually pumping the hydraulic fluid into the storage chamber that is otherwise filled with nitrogen gas. Hydraulic fluid is uncompressible (or near enough) so the gas is what gets compressed.

But overall, it's a hydraulic system because the pump both from braking or for driving the wheels pumps fluid.

Izzat right?
 
Yeah--the article was pretty good at explaining that this system is intended for situations where ultralight and ultrastreamline isn't practical (you're moving heavy packages around in a big metal box).

I hope the UPS test does indeed result in recovering some 70 plus % of the braking energy. That'd be a big jump over what the electric-gasoline hybrids are doing.
 
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JoeTheJuggler said:
I think I get it from Casebro's explanation. The CNN article only said that the gas gets compressed--I assumed the pump was directly compressing the gas, but it's actually pumping the hydraulic fluid into the storage chamber that is otherwise filled with nitrogen gas. Hydraulic fluid is uncompressible (or near enough) so the gas is what gets compressed.

But overall, it's a hydraulic system because the pump both from braking or for driving the wheels pumps fluid.

Izzat right?
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I don't get it that way. conventional hydraulic brakes just use hydraulic components to push on the pads and rotors to dump the car's energy as heat. If you're going to turn motion into stored energy it has to be a separate system from that, and if I read the little snippet there right, it's doing it with a compressor driven by rotation. The only thing the hydraulic system would do differently is to engage the pump rather than push on pads, (though I suspect it would still have to have some mechanical backup too, for panic stops) and it would still not be the same fluid as whatever is being pumped. For a compressor to work, it must be squeezing a compressible gas, and for hydraulic brakes to work worth a damn, they must be entirely free of compressible gas.

You might call it a hydaulically actuated pneumatic brake, but it still seems as if the newfangled part is pneumatic, not hydraulic.
 
As with anybody else, when I jump to a conclusion it is towards known ground. I may have made some unwarrented assumptions.

As you mentioned, hydraulic fluid is uncompressible. So, if you have a hydraulic motor in the wheel, then shut off the out put, the fluid will be incompressible, stoppping the pump/motor. period. full stop. Unless something like a hose can't take the pressure. Anti lock system? just put a pressure relief valve in. Those Bobcat tractors are called "skid steer" for a reason. No brake pedals, just a joy stick that controls the valves.

Many hydraulic systems use an accumulator. Some accumulators use nitrogen gas, usually on one side of a piston. The piston prevents the gas from being absorbed into the fluid. Why nitrogen? Same reasons it's used in aircraft tires: large molecules, so slower to leak out. Temperature stability, and inertness.

Compressing gas is not such a good regeneration system. The compression pushs heat out of the gas, then , when you need to blow the gas back through the system, you will need to re-heat the gas. Plus the heat that was given off is energy, lost. So I think the writer of the article may have gotten confused? He is a writer, not an engineer. As I conjected, the accumulator would be the part that would compress the gas. A small quantity of nitrogen would make a constant pressure of 585 psi in the accumulator, no matter the volume of hydraulic fluid on the other side of the piston. If the circumstance tried to compress the nitrogen, it would liquify, maintaining the same pressure. That liquification pressure may be why to use nitrogen. CO2 could only make 350 psi at liqufication. I dunno where other gasses would fall. Vs prices?

That's my take.
 
I suppose there's another possibility of having a hydraulic motor at the wheel. When slowing down, it would be switched with valves to pump hydraulic fluid back against a head of pressurized nitrogen. When power is needed, you simply reverse the flow and the pump becomes a motor again. Using the same device as pump and motor is very easy in hydraulic power, and it's not hard to regulate speed with valving either.

In this case I guess it would be a hydropneumatic system. The problem I'd foresee here is that would be difficult to get the hydraulic pressure high enough using compressed gas. It might work, though, if the drivetrain is hydraulic to begin with.
 
Presumably, H2 would have an even higher pressure, but there'd be a lot of problems with it. If I understand correctly, a volume of nitrogen just at the brink of liquifying, if compressed, would become part liquid, part gas, with the former portion increasing as it is compressed, and store energy in the process. This would be reasonably close to adiabatic, and therefore not waste very much energy turning work into heat. Is this an accurate analysis?
 
Art Vandelay said:
Presumably, H2 would have an even higher pressure, but there'd be a lot of problems with it. If I understand correctly, a volume of nitrogen just at the brink of liquifying, if compressed, would become part liquid, part gas, with the former portion increasing as it is compressed, and store energy in the process. This would be reasonably close to adiabatic, and therefore not waste very much energy turning work into heat. Is this an accurate analysis?
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That's my conjecture.

Actually, quite an elegant system. No brakeing system, no starter motor, no drive lines or tranmission. No differential. So. no need for the engine to be in any particular location, move it off to the side, or under the floor opposite the accumulator. Want 4 wheel drive? Add motors to the other wheels. But wait... no need to drive any particular combination of wheels either. One, two, three wheels drive? Diagonally opposite? "Traction control" via valving.

No need for a steering box, just skid-steer like the Bobcats. Fly-by-wire, driver can be located any place, in or out of the vehicle. In a full truck, driver can lay on a stack of parcels, no need for a fixed seat. Save the driver walking back to the truck, put a joy stick on the gismo they have us sign on. Use some hydraulic capacity to adjust ride height too- high for snow, or dirt roads, low for summer in the city.
 
Looking at it, it seems like it is basicaly mechanical regenerative breaking. Still that could be very useful as depending on the exact fuctionality you could work it out to be more or less a mechanical hybrid, useing a lower power engion to store energy in the tank while not being needed for the car, and useing that energy for acceleration.

Not sure how effecient this would be compaired to similar elecrical systems though.
 
Hydraulics last LOooong time. Batteries not so long. Friction does cause losses in hydrailics, but batteries are not 100% efficient either. No hi tech materials needed, silicone, lithium, etc. No hazardous waste but oil and steel. The biggest factor I'm ignorant of is size- how big of an accumulator would it need? GPM per HP, times what, 40 seconds? Big Brown is not Don Garlitz.
 
casebro said:
Hydraulics last LOooong time. Batteries not so long. Friction does cause losses in hydrailics, but batteries are not 100% efficient either. No hi tech materials needed, silicone, lithium, etc. No hazardous waste but oil and steel. The biggest factor I'm ignorant of is size- how big of an accumulator would it need? GPM per HP, times what, 40 seconds? Big Brown is not Don Garlitz.
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But you have a high pressure ressevour in the car, and that is pretty instantly convertable to mechanical energy(as a form of explosion)
 
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