I'm toying with a project idea, but I've never worked with hydraulics before. The project is a simplified version of the EPA's new hydraulic-hybrid system they're testing out in a couple UPS trucks in the near future. What I want to do is recover wasted energy from braking and store the energy in a pressure tank. That energy would then be pushed back onto the axle upon acceleration. Very similar to how most gas/electric hybrids work nowadays.
My first hurdle so far is understanding how a hydraulic pump works. I intend to mechanically gear the axle to the input shaft of the pump. The pump would then draw fluid (gas or liquid, don't know yet) from a reservoir and pressurize a tank. What I would like to know is: If I then allow the flow to reverse back into the resevoir through the pump (say, during acceleration), will this in turn force the input shaft of the pump - which is connected to the axle - to rotate? And more generally, are there any serious flaws to my hypothetical design so far?
Iggy, While the fluid is not, but the gas in a hydraulic accumulator is. The accumulator is the key to the system, but to gain any significant advantage, I would think a large accumulator would be needed, and with the extra weight of the hydraulic fluid and losses through the hydraulic motor, I don't know that any real savings would be realized.
The short answer is yes, the long answer is a very qualified maybe.
A gear type hydraulic pump will work as either a pump or a motor with pretty close to the same effiency either way. Ditto for a pneumatic gear pump although the effiency will be less due to the higher leakage. I don't have actual effiency numbers handy, you will need to get that from the manufacturers spec sheets. Expect good by not anything resembling perfect numbers. The pneumatic effiency numbers will drop with pressure rise.
A pneumatic accumlator is nothing more than an air tank. The hydraulic accumulator is a tank with a diaphram in the middle and a compressed nitorgen charge in the far end. Energy density is relatively low until you get to very high pressure ie several thousand PSI. You should be able to easily compute the available energy if you assume constant temperature by using PV=NRT
One issue is that when you try regenerative braking, you will need to run your pump at a considerably higher speed than you just used for power. This is MUCH easier to do with an electrical system but it can be done with a mechanical system. Think of a small planetary gear box that is engaged when driven forward, disengaged when driven in reverse.
As described, either the pump is connected to the axle or not -- which provides only one level of braking force (hence rate of deceleration) dependant on the pressure and braking rate. A clutch could be used, but that's no different than a dissipative brake. Throttling the flow would also be a dissipative process.
In order to have both controllable braking rate and energy recovery, you'd need some sort of automatic transmission that alters pump-to-axle ratio to achieve a match between current reservoir pressure and desired rate of deceleration.
This is easier to do with electrical systems because it can be done with efficient switchmode electronics. It is possible to realize what amounts to a variable-ratio DC transformer with switchmode electronics, with efficiencies > 90%.
A hybrid electric/hydraulic system might make some sense because a pressure reservoir that can accept and release energy at the necessary rates may be considerably lighter than a similarly-capable battery. Stopping a vehicle weighing only 1500 lb from 30 mph to 0 amounts to an energy change of 6119 joules. If this deceleration is done over 10 seconds, the power flow is 6.119 KW, or about 255 amps into a 24-volt battery. It'd take a pretty good-sized battery to accept this sort of charge rate with any frequency. A volume of 541 cu in pressurized to
"Don Foreman" wrote: (clip) A volume of 541 cu in pressurized to 1000 psig contains 6119 joules. ^^^^^^^^^^^^^^^^^^ There is a huge hurdle buried in this simple satement. A tank which starts at 1000 psig, and delivers 6119 joules winds up at atmospheric pressure. How would you draw this energy from the tank in any useful way as the pressure drops to zero?
The Toyota Prius uses a very clever differential hookup which "adds" electric motor rotation, engine rotation and wheel rotation, using a computer controlled differential. The characteristics of the storage battery are very different from a compressible gas reservoir. It is as though you could pump up the tank without raising the pressure appreciably. As pointed out in an earlier post, digital control of electrical energy is established technology. There is no equivalent in fluid mechanics.
How about posting a link to this EPA experiment? I'd be interested to see where I'm wrong. That said, here I go jumping in without all the facts on the EPA project:
I'd guess that they are not storing energy IN the hydraulic system itself. Maybe use hydraulics for tight coupling of wheel energy to something like a flywheel for very temporary storage.
I think you'd loose a lot of the available energy due to heat losses in the hydraulic system. Hydraulics get HOT when run at high pressures. Think of all that friction of the fluids in the lines and all those mechanical parts going around. friction, friction, friction!!! That heat would be a good part of the energy you are trying to store. Have you ever wondered why automobiles aren't driven by hydraulic wheel motors? Ever think about the reason that automatic transmissions are always less fuel efficient than a standard transmisson? I suspect that the reason a UPS truck has a chance is that it is in constant start-stop mode in only inner city routes, maybe even within some sort of single complex. If you compress air, the compressor and the tank get hot. This heat is dissapated into the air. There goes a good part of your energy. Think of a way to store all that heat, and maybe you've got something.
This is one of those reinventing the wheel things. Hydraulic drive was tried out in various vehicles(mostly military) at the beginning of the 20th century along with electric drive, coupled with various forms of combustion engine. Didn't prove out then, may not now. There's a lot of losses associated with hydraulic drive, more than with electric drive. It's good for specific things, but I wouldn't think it'd be that great for general transport vehicles. There's been a lot more improvements in electrical equipment than with hydraulics over the last 100 years, specifically rare-earth magnets and the resulting power-to-weight improvement in generators and motors, plus the development of solid-state motor controls. The Germans had a diesel- hydraulic locomotive that one of the American railroads bought a few of('40s-'50's era), they lost their shirts with the maintenance costs, replaced them with diesel-electrics. Things haven't changed that much between then and now in the hydraulic field.
I believe the UPS truck thing is because they do a lot of short starts and stops, enough fluid could be stored to allow short moves without running the engine. A special case, like I said above.
I'm not considering using a full-out hydraulic drivetrain. Just regenerative braking hydraulic assist. Here's an article about a Ford concept they're calling "Hydraulic Launch Assist".
That's basically the fundamentals of what I want to try... on a much smaller car (my little Honda Insight only weighs 1800lbs). Ideally, I want it to be a fully passive system, so it is only activated when I manually control it. The UPS trucks, partnered with the EPA, use a moch more sophisticated system. Looks like they're using the full hydraulic drivetrain (no traditional transmission, etc). Here's a good article I found on them dated last October:
Popular Sci or Mechanics, many years ago, featured a number of alternative ways to power vehicles, one of them being a big-assed flywheel. Gyroscopic considerations notwithstanding, braking energy would be used to speed up the flywheel, and subsequent accelerations would draw energy from the flywheel. But then you have to cart that big-assed flywheel around, which costs. No free lunch, I spose. If cars, like trains, were connected to power grids, dumping that large amount of power Don calc'd would be less problematic. Big "if", tho. But mebbe not....
This was done many years ago on a VW frame, and published in Mother Earth News. You need a reversible, variable-displacement hydraulic motor, a hydraulic accumulator (or two) and a pressure-controlled variable-displacement hydraulic pump on whatever engine you will use.
The most serious flaw is the cost of the components and the inefficiency. I think you'll find electric motors with solid state controls are a lot more efficient than hydraulics. There was a time when this kind of equipment was used in anything that required variable speed, from industrial mixers and rolling mills to fire water boost pumps in tall buildings. Practically all that stuff is now done with AC motors and VFDs. You could get lucky and find suitable gear in a scrapyard, though.
As for how it works, it is very much like the hydrostatic drives used in lawn tractors, but with storage added. The motor and pump can both be vane-type devices, with the outer housing able to slide slightly in a radial direction to the motor/pump's shaft axis. By changing the position of the housing, you can change the displacement of the unit. By moving past the center, you can make the motor reversible, either to go backwards or to recover energy from the moving vehicle and store it in the accumulators.
I built most of an electric hybrid car about 20 years ago, as even then I thought it was a better way to go. Big hydraulic accumulators could store enough energy to drive you several blocks. If the accumulators hold 20 gallons, then you need a 20 gal. storage tank to hold the fluid when it ISN'T in the accumulator. A bank of trolling motor batteries might weigh a bit more, but will drive you for miles. If I ever re-do this, the current plan is to get a pair of 10-15 Hp AC induction motors (3 phase 240 V) and re-house them in an all-aluminum case, and fit them to the half-shafts of a front-wheel drive car. The motors can act as the differential, too. But, I'd need something around a 4:1 reduction gear between motor and drive shafts. Power them with a pair of off the shelf VFDs. Then, I'd need to build a Cuk (that needs an Umlaut over the u) to convert, say, a 120 V battery bank up/down to the 340 V DC needed by the VFDs. The Cuk converter is a reversible converter, it will send energy from battery to motors or motors to battery without any changes of the circuit topology, while shifting voltage up or down.
My previous hybrid attempt used a jet engine starter motor and 48 V of trolling motor batteries, and I drove it around some, it worked quite well. I never finished the adaptation of a junked Honda 750 motorcycle engine to a Stratofortress generator.
That's why you want to use a variable-displacement pump on the axle. It can be set to zero displacement to coast, or move the shuttle one way to accelerate, the other to decelerate, storing energy back in the accumulator.
I did it with a 48 V set of 90 AH trolling batteries, and the terminals got hotter than I liked. I was pulling about 250 A peak when climbing hills, and about
125 A when coasting on a straightaway. I had a field control on the jet engine starter motor, but no armature control.
It's been awhile since I worked with hydraulics, but here goes.
Depends on the type of pump. A gear pump will probably fail unless it has been retrofitted with "motor" seals on the input/output shaft. Also, if the pump has a shaft drain to the suction side, it will need to be plugged for bidectional use.
Piston pumps should operate as motors.
Same for gerotor pumps.
I don't recall for vane pumps, but they probably won't produce the needed pressure.
I would like to elaborate more, but the old memory gets foggy after 20 something years away from the industry.
For a good idea of how much energy is lost in these systems, you need only pick up an industrial supply catalog. Find an air motor or a hydraulic motor, at say, 1 horsepower output. Note the air/fluid input requirements. Now find an air compressor or a hydraulic pump that will supply those. Now look at the horsepower of the electric motor required to drive the air compressor or hydraulic pump.
You go look it up for yourself; it will have more sticking power than if I tell you what I've already seen.
Impressive, isn't it? Electricity starts to look a whole lot better...
My car is actually already an electric hybrid. But in order to make it practical, the electrical system isn't all that beefy. It only regenerates a very small percentage of the braking energy. And the electric motor isn't nearly powerful enough to get the car accelerating on its own. A few guys I know have retrofitted their Insights with more powerful electric supplements (checkout my idol, Mike Dabrowski, and his 5-wheeled Insight:
I'm more intrigued by the theoretical simplicity of the hydraulic assist design.