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Hydraulic vs Electric Hybrid Drive

Hydraulic vs Electric Hybrid Drive

Hydraulic vs Electric Hybrid Drive

(OP)
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?

RE: Hydraulic vs Electric Hybrid Drive

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

RE: Hydraulic vs Electric Hybrid Drive

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

RE: Hydraulic vs Electric Hybrid Drive

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

RE: Hydraulic vs Electric Hybrid Drive

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...

RE: Hydraulic vs Electric Hybrid Drive

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

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RE: Hydraulic vs Electric Hybrid Drive

(OP)
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.

RE: Hydraulic vs Electric Hybrid Drive

Nitrogen is quite cheap.

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

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RE: Hydraulic vs Electric Hybrid Drive

(OP)
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.

RE: Hydraulic vs Electric Hybrid Drive

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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RE: Hydraulic vs Electric Hybrid Drive

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.

RE: Hydraulic vs Electric Hybrid Drive

Quote:

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?

RE: Hydraulic vs Electric Hybrid Drive

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.

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RE: Hydraulic vs Electric Hybrid Drive

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.

RE: Hydraulic vs Electric Hybrid Drive


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?

RE: Hydraulic vs Electric Hybrid Drive

Retarders are meant to waste energy.  

Hybrids can't afford to.

Mike Halloran
Pembroke Pines, FL, USA

RE: Hydraulic vs Electric Hybrid Drive


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

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



RE: Hydraulic vs Electric Hybrid Drive

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

RE: Hydraulic vs Electric Hybrid Drive


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.

RE: Hydraulic vs Electric Hybrid Drive

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

RE: Hydraulic vs Electric Hybrid Drive


Unless you have achieved outstanding unquestionable success, you cannot be sure of what is required. The demands of delivery trucks are certainly not the same as rubbish trucks.  

I hope Calstart does a better job with hydraulics than they did with electricity or NG. They had a large facility in a Lockheed building right down the street from my work; most unimpressive.

Some concrete trucks use piston pumps and/or piston motors.

While efficiency of a transmission based system may or may not be the highest possible, complications are minimal, and certainly less than a wheel based system. Claiming that the system will cause greatly increased wear on the driveline is not being very realistic as retarders in use now create a very similar reversal of driveline torque with few ill effects. Cooling is usually done separately and driveline shock is non-existent. >>This means a motor/pump unit would operate similarly.  

Rubbish trucks do plenty of highway operation. This means efficiency of a wheel based drive system is critical. Added weight would consist of the accumulator, valving, and at least 4 extreme torque motor/pump units. With any system, it's doubtful you will completely shed the brakes anytime soon. Considering the cost of manufacturing and maintaining a wheel based drive system, performance will have to do much better than break even.    

Using a motor/pump unit in a place that already exists inside the type of transmission already used on a rubbish truck, is a simple and doable concept for stop-and-go operation. Cost for development, production, and maintenance would be minimal. Added weight would consist of the valving and accumulator.

It is just a simple idea and is not being touted as some ultimate or broad-based answer. And, as always, there is no limit as to how complicated you can get.   

As a significant side item, loading and packing trash uses nearly as much fuel as moving the truck from stop to stop. The generated or conserved energy could almost be just as well used to perform only those tasks.

 

RE: Hydraulic vs Electric Hybrid Drive

Fabrico,
Your compaction percentages are way higher than what Ricardo reports in SAE paper 2005-01-1164, at 10%.
Most information I have seen puts garbage trucks at about 2.9 MPG average.
The use of the truck has little to do with optimizing the drive train. The average gross weight, number of stops and travel speed will affect the fuel mileage most. The difficulty is making a cost effective solution. In theory if 100% of the braking energy could be recaptured and reused for starting, the only energy required would be to travel at speed. A highway truck grossing 80,000 lbs traveling 60 mph can get over 5 MPG. This means a city driven vehicle must recapture and reuse most of the braking energy.
Permodrive ( http://www.permodrive.com/ ) , Dana and the EPA are some of the people who have done driveline regeneration. I think putting the pump/motor between the transmission and motor must be more difficult than you think. Currently the DOD is paying Dana and Permodrive to build a hybrid drive for the FMTV.

RE: Hydraulic vs Electric Hybrid Drive


Rubbish trucks are not the same as rubbish trucks.
If you consider how fantastically their service, terrain, and dump distance can vary, and how many there are, it seems fairly absurd to simply average them and develop new technology from that average. For the most part, these trucks are designed, purchased, and assigned according to needs. To whatever extent practical, drive train development should do the same.    

The Permodrive is fairly close to what I have been talking about, except external and IMHO, a bit cruder. Keep in mind that only Allison and ZF make traditional automatic transmissions, so changes to them could end up being proprietary, quiet, or absent.

The motor/pump unit I mentioned would more likely be on the output end, not between the engine and transmission, as you mentioned. However, powerful retarders exist in both locations. Conversion of existing systems may lend itself well to re-generation at the rear of the trans and motoring from the front. This could significantly and beneficially affect the size, pressure, flow, and RPM requirements of the fluid motor. As mentioned, existing physical location, torque capacity, and cooling may already be in place.

In addition to pumping and driving with a single unit, the Permodrive has a wide RPM range to deal with. It looks like a good, straight forward beginning.
   



 

RE: Hydraulic vs Electric Hybrid Drive

(OP)
The Eaton/UPS experimental delivery van is reportedly able to operate "several hundred feet" on the accumulator charge. Per the October SAE magazine.

RE: Hydraulic vs Electric Hybrid Drive


Sounds encouraging.

There was something else that was around for a few decades. It was the big electric retarders. Once in a while you would see one on the driveline, mounted very similarly to the unit above. But they were much more often found on rear trailer axles. They were quite effective and reliable, but so far I can't recall who made them.

RE: Hydraulic vs Electric Hybrid Drive

Quote:

"What does not exist are hydraulic pump/motors, and valves to do this."
I'm getting a kick out of reading how an hydraulic hybrid can't be done - since we've done extensive testing on them!

The Eaton system stores sufficient braking energy to launch a refuse truck back up to about 15 mph,  and thus gives a huge reduction in brake wear on top of about a 20 - 25% reduction in fuel consumption WHILE COLLECTING REFUSE - not of course while running down the highway.

I believe the power storage is equivalent to about 500 hp-sec., or about the same as the standard diesel engine's output for 2 seconds.

RE: Hydraulic vs Electric Hybrid Drive

Rob45,
I’m very familiar with most current hydraulic and electric hybrid systems and they work over a relatively narrow rpm and power range. I just returned from the SAE Commercial Vehicle Conference. I attended last years HTUF meeting and drove several trucks, and have all the travel plans to be at HTUF November 14-15, 2006.
At the SAE meeting this week the best the Seattle hybrid busses averaged is about 27.8% better than the diesel only busses.
The goal set in the 21st Century truck project was to triple the mileage of transit busses by 2010.
In order to meet the 3X mileage dream, current swash plate driveline systems like Eaton’s, any current electric drives, or any crankshaft engines will be insufficient.
I hope to do some testing next week and present the data at HTUF about garbage truck drive cycles that gives the actual thermal efficiency during at start stop movement to back my position.
So I will be putting my money where my mouth is.

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