Hydraulic vs Electric Hybrid Drive 1

[swall]

I just saw on the ASM website that one of the makers of hydraulic motors is working on a hybrid van for UPS that would utilize hydraulic drive instead of electric. This is out of my field, but it strikes me that a hydraulic drive would be more compact, but would fall short on the energy storage aspects compared to electric. What do you guys think?

 

[MikeHalloran]

Hydraulic hybrids should do just fine on very short stop/go driving cycles, like delivery trucks and garbage trucks.

The latter, especially, travels maybe 30m between stops, so it only needs to store enough energy to go that far in order to be effective at reducing fuel consumption, and especially brake consumption.



Mike Halloran
Pembroke Pines, FL, USA

 

[kenre]

Interesting! always wondered why someone didnt do a hydrostatic driven car. I operated a few machines with it and thought about it. If fitted into a car it would allow the engine to run at a constant speed, which could be a real efficiency advantage.

Ken

 

[MikeHalloran]

Hydraulic hardware is expensive, especially as it gets larger or the pressure gets higher.

A hydraulic hybrid stores energy in compressed gas, which doesn't have the density of chemical storage, so the range on discharge is limited.

Two garbage truck brake jobs, normally needed several weeks apart, will more than pay for a hybrid conversion.




Mike Halloran
Pembroke Pines, FL, USA

 

[SomeYahoo]

Forgive my ignorance, but when you say "A hydraulic hybid stores energy in compressed gas" is that the gas remaining in the tank as hydraulic fluid is regeneratively (word?) pumped into the tank?

If so, could one use a cylindrical tank with a spring/piston setup to store additional energy/achieve additional power density?

Just a thought...

 

[patprimmer]

The spring in the cylinder is normally compressed nitrogen gas. It is called an accumulator.

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[swall]

A spring accumulator using a high force compression spring instead of nitrogen gas has also been used. Somewhat cheaper and you don't have to worry about the nitrogen leaking out after 10 years.

 

[patprimmer]

Nitrogen is quite cheap.

It is also a lot lighter than steel, and has an infinite fatigue life.

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[swall]

Pat--always more than one way to skin a cat. My previous employer was a brake system supplier and we used accumulators on ABS as well as hydraulic brake boosters. We had three types. One was a heavy wall steel sphere with a diaphragm with N2 behind the diaphragm. The second type was an aluminum cylinder with aluminum piston and teflon ring seals plus a rubber o-ring with N2 at the piston head. The third type had a piston in the bore of the ABS housing and a rod like extension of the piston pushed on a spring to store the energy. The spring was contained in a stamped metal can attached to the body with 2 self tapping bolts. This latter type was used on the "low cost" ABS.In the 1990 time frame, it had a cost advantage. The heavy walled sphere type never leaked; the aluminum cylinder type exhibited enough leakage such that some were replace in a recall campaign. Can't say how well the spring type fared, as I had left the company as they were being introduced.

 

[patprimmer]

I was thinking about accumulators as used on a hydraulic press.

As you say, horses for courses, and one reason we are here is to explore different possibilities.

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[EdDanzer]

An example of why people are looking at hydraulic hybrids for heavy vehicles.

A 3 axle garbage truck weighing 42,000 lbs going 25mph stopping to 0mph in 5 seconds will require 638 hp to stop. This is 18.3 gallons of oil at 5000 psi to store. For an electric vehicle that would be 476 kw for 5 seconds input to batteries or ultra-capacitors.
It is possible to buy accumulators to do this without too much of a weight penalty. I don’t think you can haul enough batteries or ultra-capacitors let alone payload to do this.
What does not exist are hydraulic pump/motors, and valves to do this.

 

[Fabrico]

What does not exist are hydraulic pump/motors, and valves to do this.

I doubt that. We had trash trucks 17 years ago with constant running, direct drive hydraulic pumps that took almost the full power of large Mack Maxidyne engines while packing trash. Concrete trucks also have large constant running pumps. Who knows what else is out there?

There's not much difference between hydraulic pumps and motors. Maybe everything is there but the big accumulator?

 

[patprimmer]

I think large injection moulding machines run some big pumps and valves.

I saw one where a slim guy could crawl inside the main hydraulic oil line. They had a tanker outside to supply the oil for an oil change.

Regards

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[EdDanzer]

The pump/motor problem is similar to electric drives, the torque requirement at low speeds is very high. So if you have a motor with enough power at highway speed (70mph max) it is very small compared to a pump/motor with 600 hp at 20 rpm.
It seem that individual wheel motors will be the best way to collect braking energy, rather than trying to use all wheel drive connected to a large pump motor, or a multi-speed transmission and pump/motor. 600 hp / 6 wheel motors = 100 hp per motor.
Generally speaking pumps have different valve timing and shape than motors for best efficiency.

 

[Fabrico]

The pumps I was speaking of were both gear and vane types, not much to time. Motors are also often gear or vane types.

Another consideration is the very powerful hydraulic retarders built into the big Allison and ZF transmissions. Why could'nt a similar motor be built in?

 

[MikeHalloran]

Retarders are meant to waste energy.

Hybrids can't afford to.



Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

Ah, well they don't seem to be alone in that function.

The point was that retarders prove that a large and powerful hydraulic motor could easily exist as an integral component of the input or output section of a typical heavy-duty automatic tranmission. This would also lend itself quite well to re-generative braking and/or energy storage.

Retarders are generally rated at about twice the engine's horsepower. This means that structural, torsional, thermal, size/space/weight, requirements are already in place.

http://www.allisontransmission.com/servlet/DownloadFile?Dir=publications/pubs&FileToGet=SA2953EN.pdf

 

[MikeHalloran]

Look closely; the retarder shown in the Allison transmission is not a hydraulic pump, it's a turbine, much like the driving turbine in a torque converter. It generates huge flows, but not much pressure. You need beaucoup pressure to store energy in an accumulator.



Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

MikeHalloran, it would be most helpful if you could take the time to read posts more carefully. None of mine stated anything about a retarder being a hydraulic pump, or using a retarder for anything.

I am quite familiar with the construction of transmission retarders. A proper hydraulic motor/pump unit could easily exist where retarders do now. I would not be at all surprised if the OEMS are already looking into it.

 

[EdDanzer]

Fabrico,
The components used in a garbage truck compaction system, or a concrete truck are nothing like what is required for a drive train.
Gear or vane pumps and motors do not operate over a very wide speed range and are not very efficient if operated as both a pump and motor.
Most hydraulic hybrid development is based on axial piston pump motor technology.
The limitation of using a transmission driven regeneration system, electric or hydraulic, are you limit the amount of energy that can be recaptured, increase the complexity of the control system, and create additional wear on the transmission and rear end when decelerating at full load.

Swall,
This link will provide more information about hybrid trucks.

http://www.calstart.org/programs/htuf/?p=programs