Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

[Apakrat]

Have waited for someone to pose a question about the UPS hydraulic hybrid truck as show at

http://auto.howstuffworks.com/hydraulic-hybrid3.htm,

​

I'll try to make a stab at a reasonable question.

What are the negative engineering reasons a modern automobile is not using hydraulic motors to drive the wheels?


At 74th year working on IR-One2 PhD from UHK - - -

 

[tbuelna]

EdDanzer,

Loss of efficiency at off-peak operating conditions is not a problem unique to hydrostatic hybrid drives. That is why automakers like GM have adopted a "dual mode" electric motor architecture for their electric hybrid transmission designs.

While hydraulic hybrid drivetrains may not be as "sexy" as battery-electric hybrids, hydraulic hybrids still make more sense from the standpoint of efficiency, economics, and durability. Even though the efficiency of a swashplate hydrostatic drive tends to fall off very quickly due to wear in the system, it still is much more efficient, durable and cost-effective than any battery-electric system over the typical life of an automobile.

Hydraulic hybrids are a proven, mature technology that can easily double the fuel economy of an automobile on a typical urban driving cycle, at half the production cost of a similar battery-electric hybrid system.

Personally, I don't understand the attraction of battery-electrics. Their proponents seem to cling to the hope that the battery technology that will make them truly practical is just around the corner. But as the old joke goes, "They've been saying for the past 20 years that the battery technology that will make electric cars practical is only 5 years away".

Regards,
Terry

 

[GregLocock]

"can easily double the fuel economy of an automobile on a typical urban driving cycle, at half the production cost"

I'd love to see numbers backing that up. How big are the accumulators? what is the cycle efficiency for a regen cycle , ie brake->accumulator->acceleration in the hydraulics?

As you know, an electric hybrid struggles to get a 40% improvement in mpg over the same vehicle run as pure gasoline (eg

http://www.thegoodhuman.com/2007/08/20/comparison-test-honda-civic-versus-honda-civic-hybrid/)

, without the electrical junk in it, so I find it amazing that a hydraulic system has such a huge advantage. Has this been demonstrated in real testing, or is this just back of envelope guesswork (nothing wrong with that but a factor of two sets my alarms ringing).



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EdDanzer]

The reason existing Hybrid drive trains have not given promised efficiency gains is none can capture the available braking energy.

Most braking events are of shorter duration and higher energy at low speeds than acceleration events. In my study of garbage trucks the braking event needs to absorb about 500 hp where the rubber meets the road in a few seconds at less than 20 mph. The actual launch power is less than 200 hp for a longer time period than a braking event.

Most people’s braking events are shorter than ½ the time and or distance of a full throttle acceleration event. Most electric hybrids capture less than 25% of the available braking energy.

It does not appear that near future electric drive technology will provide the needed braking energy capacity. I can assure you that swash plate hydraulic motor technology will never provide the required braking energy capacity.

Storage of electricity at even 80% of available braking event energy of even a very small car is a major cost and weight hurdle. Accumulators if properly designed and implemented will at least be affordable. The weight penalty for accumulators could be lowered if the pressure vessel is also part of the frame structure. This would require much structural redesign and analysis to be safe but may be required.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Apakrat]

a factor of two sets my alarms ringing).

My old rusty alarm quit years ago, but excerpts published by people a lot smarter than me, tend to show 2X is achievable.

Look up companies like Hybra-Drive, Innas, and Artemis for what I think is more interesting work in hydraulic drives.

http://www.innas.com/HyDrid.html

RWTH Aachen University proved that an average fuel consumption of 3.1 liter per 100 km (or 77 MPG) is possible for a mid-sized (1450 kg) passenger car.

http://www.nextenergynews.com/news08/next-energy-news6.6.08d.html

Artemis Hydraulic Hybrid Transmission Increases Fuel Economy By 2X
In confirmed third-party tests, the U.K. Energy Saving Trust, a grant supplier for the project, says Artemis’ prototype BMW 530i hydraulic series hybrid, outfitted with HEDDAT (High Efficiency Digital Displacement Automotive Transmission) technology performed with double the MPG in city tests (a start and stop environment) of the same car with a manual transmission.

http://www.hybra-drive.com/NewsGreentech.htm

The U.P.S. van has four "accumulator tanks" of 22 gallons each which can be pressurized as high as 5,000 pounds. When fully charged, the system holds 2,000 horsepower-seconds of energy, according to Benjamin M. Hoxie, engineering manager for hydraulic hybrids at Eaton, an automotive supplier that built the prototype, using technology developed by the E.P.A..


Stated differently, it could deliver 100 horsepower for about 20 seconds. In electrical terms, that is less than half a kilowatt hour — but no electric battery could absorb and deliver energy so quickly.

At 74th year working on IR-One2 PhD from UHK - - -

 

[GregLocock]

Well wadderyaknow, 30% demonstrated from actual hardware. Sure, you can cherry pick results, but that is silly.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

I doubt it will ever be worth capturing extreme braking events - 90% of braking is at 0.3 g or less. Sizing your system for more than 0.3 g seems extravagant.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tbuelna]

GregLocock,

The U.S. EPA claimed a 55% improvement overall with a modified Ford Expedition in their 2004 program.

http://yosemite.epa.gov/opa/admpres...a1385de5a863aaca85256e5100620a97?OpenDocument

You're correct, my claim of doubling fuel economy on an urban drive cycle is a bit of hyperbole. That degree of improvement has never been demonstrated with a hydraulic hybrid, and I don't have any legitimate analysis to substantiate it. But I honestly don't see why it couldn't be achieved with a drivetrain that is specifically engineered and optimized for hydraulic hybrid operation. Hydrostatic devices can have very high charge/discharge cycle efficiencies (>95%), at least until the parts wear. That is why they can be so effective in stop-and-go driving. Here's some system specs from the EPA's work:

http://www.epa.gov/OMS/technology/420f04019.pdf

As others have noted, hydraulic hybrid drive trains would suffer from some of the same limitations as electric systems, in that they are not much help for highway driving. So you still have to install a suitably sized IC engine and trans for those driving situations. And as noted, a fully capable conventional friction braking system would still be required for reasons of safety/reliability.

I'm not an expert in automotive drive trains, but the only reasons that I can see for the auto OEM's preference for battery/electrics over hydraulics is that they must feel that the electrical system gives better control, and that the hydraulic system cannot be made reliable enough over the typical life of a vehicle. From the standpoint of bang-for-the-buck, hydraulics win hands-down.

Regards,
Terry

 

[BrianPetersen]

I've yet to see a hydraulic motor in ANY application that was 95+% efficient and which was a practically small and lightweight size for its power output.

Asking a hydraulic motor to spin at an RPM comparable to drive wheel rotation speed introduces hydrodynamic losses.

But, all I've ever dealt with has been in industrial applications.

 

[EdDanzer]

Greg,
Capturing 100% of the braking energy at 100% efficiency will have a substantial benefit in city driving where the top speed is 35 mph. It takes less than 5 hp just to maintain a speed. I believe most people brake late so the speed is low, it may only be .3 g but it must be captured. The hybrid vehicles that I have driven capture very little braking energy below 25 mph. When I drove the Oshkosh truck

http://www.greencarcongress.com/2006/11/oshkosh_truck_u.html

but changing the speed of braking it was unable to capture enough energy and I had to stop to recharge the capacitors.

BrianPeterson,
Here is a link to a hydraulic motor that is very efficient at high speeds.

http://www.parker.com/portal/site/P...lt&languages=EN&parentCh=FindCatlogId#results

The difficulty designing a hybrid vehicle drive train is the wide range of speeds and torques they must operate at. To be able to handle braking loads down to 1 mph at even 60% total efficiency yet be 80% total system efficient at highway speeds has yet to be done. If it were possible many taxi cars would only need 10 hp.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[BrianPetersen]

^^ That is a fixed displacement hydraulic motor. How about a variable-displacement device of the type that would be necessary for a "CVT" operation?

At least one side of the mechanism has to be variable-displacement for this concept to work at all.

 

[automatic2]

Hydraulics, being a method of transmission, will have no advantage over the highly developed mechanical drive trains in current automotives. It has become popular in high torque vehicles due to it's flexibility.

Not to say it won't play a role, but much development is needed as hydraulics have evolved to serve industrial needs.

This is of course similar to electric motors. 'Off the shelve' components complicate our attempts to put together systems that are practicle.

Variable displacement vane motors configured as wheel hubs, brushless wheel motors, or combined motors need to be developed by the auto industry to make the next step.

 

[Apakrat]

Believe, the hydraulic fluid flow from an Accumulator, delivered in controllable amounts, to a Fixed Displacement Hydraulic Motor, would be a CVT, (Continuous Variable Transmission).

At 74th year working on IR-One2 PhD from UHK - - -

 

[BrianPetersen]

^ That won't be any more of a CVT, than a slipping clutch is.

To achieve the "controllable amount" you have to meter the fluid through some sort of variable orifice, implying pressure losses, implying a huge loss of efficiency.

You have to achieve something akin to "what is lost in force is gained in speed", i.e. "what is lost in pressure is gained in volume", and the only way to do that if you are dealing with an almost-fixed-pressure accumulator, is to use a variable-displacement motor to vary the amount of "leverage" that the almost-fixed pressure has, in order to vary the amount of torque output.

A hydraulic system that is to be as efficient as possible, cannot have any pressure-reducing valves or other conventional means of regulating flow, because those are sources of enormous efficiency losses. It ALL has to be done with variable-displacement pumps and motors.

 

[Apakrat]

Thank You, BrianPetersen. You have shown, regardless of efficiency, a hydraulic motors can be a device can inefficiently perform as a CVT.

.......... if you are dealing with an almost-fixed-pressure accumulator, ......

Believe, with just a little research, You may find that the pressure in an accumulator, as is presently being applied to an automotive vehicle, will have a substantial pressure deferential.

At 74th year working on IR-One2 PhD from UHK - - -

 

[NTROhio]

The limited amount of drive cycle analysis that I have seen did seem to overstate the benefits of the hybrid. Only velocity info was used and the effects of coasting, cornering and hills were not included. Might not be back of the envelop work but still it left a lot of room for error. I think the benefit of regenerative braking is most overstated. Traveling at 35mph as the light turns red, coasting to 25mph before braking will recover nearly 50% of the available KE and down to 17.5mph will recover 75%. The garbage truck market, i.e. large vehicles making frequent stops, will benefit the most from regen braking. Regen braking is beneficial to sedans, but is generally overstated. Of course, the more aggressive the driver the more beneficial it is.

I think the most benefit to autos comes from moderating engine load. Engine characteristics vary but what I’ve seen the peak efficiency usually occurs somewhere around 30-35% of peak power. A 150hp engine may realize peak efficiency at around 50hp. Efficiency usually drops off somewhat gradually as power increases but more sharply as power decreases. If it takes 18hp to push the car down the road the engine may output around 24hp. At that power the engine may have a peak efficiency of 22%. A CVT allows the engine to operate at peak efficiency for each power level but the engine may have an overall peak efficiency of 34%. Operating at overall peak efficiency would increase mileage by 55%, a big jump over where hybrids are in highway mileage. Even if the new drive train is only 88% as efficient as the standard drive train, the mileage still increases 36%. But this requires getting energy in and out of storage efficiently. If the in/out trip to storage is 70% efficient you’re worse off. Also, to fully moderate engine load the acceleration energy needs to come from storage. This will allow minimum engine size. A 3000lb vehicle traveling 75mph has about 950hp-secs of KE. If you have a 2000hp-sec capacity and allot 950 for acceleration, 1050 is left for operating the vehicle. At 24hp draw, it will last 43.75secs. Seems meager for an 88gallon capacity. The 88gallons is probably precharge gas volume so including the volume of the bladders and shells its approaching the size of two 55 gallon drums being hauled around in a sedan. Hydraulics ability to rapidly and efficiently store energy is enticing but the lack of storage capacity may limit it to some launch assist, partial regen and CVT functions. Can hydraulics become efficient enough to realize a net gain with these limited benefits? It would have been nice if the EPA had used the R&D money to evaluate some of the more promising hydraulic drives and left the “simple evolution” to Eaton and UPS to work out. Should at least see how far hydraulics can go instead of sticking with conventional designs because they’re more comfortable.

 

[EdDanzer]

Crankshaft engines with mechanical transmissions have poor low power (slight downhill grade) high speed (70 mph) thermal efficiency. A properly designed hydraulic motor should have a more linear efficiency over all loads and all speeds.

Variable displacement vane motors have poor efficiency at high load high speed operation as well as poor efficiency at low speed high torque operation.

There is very little public domain drive cycle information that can be used to determine drive train efficiency so determining the value of regenerative braking is not even possible. The commercial electric and hydraulic hybrids that I drove at two different HTUF meetings captured very little of the actual braking energy. The rear end and transmission absorb a reasonable amount of the lower energy braking available in a drive cycle.

Traveling at speed should not be done with electric or accumulator stored energy because of poor efficiencies compared to hydrocarbon fuels.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[tbuelna]

BrianPetersen,

Hydrostatic drives can indeed achieve very high efficiencies (>95%) and can vary speed. The most efficient type of hydrostatic drive, and thus the type employed in most automotive hydraulic hybrid systems, is a pair of swashplate axial piston pumps/motors mounted end-to-end. The displacement of each is independently variable while operating. They are mounted so that the pump discharge flows directly to the motor's inlets. Since they are both positive displacement devices pumping an incompressible fluid, increasing the pump-side discharge volume will cause the motor-side to operate at a higher speed to accommodate the incoming fluid flow rate with its lesser displacement per each piston stroke.

Losses in hydraulic systems occur due to things like flow losses in pipes and valves, friction losses in the pump mechanism and seals, and leakage. Variable displacement swash plate piston type hydrostatic drives achieve low losses by mounting the motor and pump right next to each other to eliminate piping, and they minimize friction and leakage by using extremely close tolerance piston/bore fits with no seals.

However, as others have noted, an automotive drivetrain requires a wider overall speed range than any pump can realistically achieve. So some other sort of range-extending transmission must also be used. But this problem is not unique to hydraulic hybrid systems. It is also an issue for battery electric drives. That is why many automotive companies have gone to "dual-mode" designs with their electric hybrids, that employ two separate electric motors designed for different speeds.

Here is one company's clever approach to resolving the conflicting requirements of pressure and volume in a hydraulic drive:

http://www.innas.com/IHT.html

Regards,
Terry