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Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)

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


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  - - -

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Noise, unfamiliarity, maybe efficiency, noise, noise, cost, long payback, noise, noise.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

I'd like to slightly contradict Greg.

It is possible to make a silent hydraulic system, but then you have to substitute six 'cost' and one 'weight' for five 'noise'.

Plus you have to add 'range', which you can improve with better 'efficiency', which adds 'cost'.

Hydraulic hybrids seem a natural for garbage trucks, and for urban deliveries with very short inter-stop distances.  Not so much for suburban routes.

Additionally, every part of this statement from the HowStuff site is just flat false:
"Another payoff is the efficiency of the hydraulic components themselves. Because the components are lightweight and use simple mechanics, they're easy to build, maintain and repair. "

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

"Additionally, every part of this statement from the HowStuff site is just flat false:"

I wouldn't say they are false, depends on your point of view. - easy to build, maintain and repair:Yes for a maintenance shop that is already used to rebuilding hydraulic components, such as those used on garbage trucks.
- components are lightweight and use simple mechanics: Yes considering the power hydraulics can transmit in a relatively small package, and the basic designs have been around for many years. And don't even try to compare this fairly simple system vs. a generator/battery large enough for a large refuse truck!
- efficiency of the hydraulic components: if you consider the total system efficiency of a generator/battery pack/electric motor the efficiencies are probably similar to the hydraulic system over time.

A regenerative hydraulic system makes sense on large vehicles that make frequent stops. The added weight penalty gets relatively smaller as the system power goes up.

ISZ

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

So we agree that maintenance and repair of hydraulics is a specialized operation.

Production of the hydraulic components and repair parts is even more specialized, and much more capital- intensive, partly because of the difficulty of producing the nominally simple internal geometries to extreme levels of precision.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)

I wanna Thank those responding to my query.  I have known for years, many people instantly jump on the "smear tactic wagon" on any thought of different applications to historic methods.

IceStationZebra made absolute sense, but the last paragraph was deceiving.

Quote:

"A regenerative hydraulic system makes sense on large vehicles that make frequent stops. The added weight penalty gets relatively smaller as the system power goes up."
1.  The weight of 4 hydraulic motors will probably weigh less than one single electric motor of the Torque of one of the hydraulic motors.

2.  The weight of a battery may be, in the future, less than a simple 5 gallon Accumulator hydraulic system with Reservoir, cooling system, valves, lines, etc.

3. A hydraulic system would not require a large +200 internal combustion engine.

Large horsepower automotive engines installed in modern cars are never needed for more than a few seconds at a time.  Automotive engines are not designed to deliver such power on a continuous basis and an absolute over design for urban stop and go traffic.  (Somewhere, a US Dept. of Trans. web site, which I cannot readily find again, showed almost 3 miles driven in urban areas for every mile driven on a US highway.)

My old '95 Ford Windstar weighing 4400 lbs., with handicap additions, including elec. wheelchair, and two passengers, calculates a need of slightly less than 27 HP** to maintain a speed of 70 mph on a level highway.(**Provided I did not screw up the decelerations calculation based on loss of 10 mph in 10 seconds as an average of several measured timings.)

The HowStuff article, shows UPS testing is evidently using available shelf hydraulic items.   It does not elaborate on essentials such as the internal combustion engine being controlled by a Hydraulic Governor and regulated by the Accumulators' pressure differential.

Info is not given about the size of the Accumulator, Hydraulic Motors, or of the internal combustion engine.  Under normal operation, the engine driven pump would be in by-pass mode when the Accumulator was at max psi, or would add to a low pressured Accumulator while waiting for a green light.

The HowStuff article totally blew it about the electrical workings.  Almost any Hydraulic Hybrid will still need a 12 volt system to operate the thingys necessary for comfort plus actuating certain hydraulic/pneumatic devices.

Also, let state outright that I believe there is a gap in existing shelf item hydraulic motors between small units suitable for lawn mowers and larger units for Caterpillar Tractors.  None existing that I have found are precisely  suited for adaptation to family sized automobile.  

Thank Ya'll for allowing the rant about one a favorite subject.

   

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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

" None existing that I have found are precisely  suited for adaptation to family sized automobile"

Exactly.  

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)

Existing hydraulic motors limit the top speed to about 83 mph.  This makes auto sales almost impossible.

One solution is to adapt existing hydraulic motors, built structurally for high torque, fitted  a transmission/differential arrangement, which stages increasing rpm delivery to the drive axle.  These already exist on construction equipment.  

My belief is, a pump/transmission manufacturer like Eaton, could easily mold and machine a smaller version for family size automobiles.  (These little modern thingys a MAN has to fall into, and equipped with hand gripes to use for arms to enable climbing out, are not family size automobiles.)

Pump/transmission manufacturers, like Eaton, are not in the business of automobile assembly, typical of the so called big three, GM, Ford, and Chrysler.  But, someone, as an independent, assemble a usable configuration.  It may be adapted to cause an embarrassment to Nitro Dragsters when the 15-hp Briggs and Straton 2 cycle competes in their 1/4 mile elapse time.

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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Has B&S ever made a 2 cycle?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)
oops
blush

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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Hydraulic systems can be made both quiet and durable.  Most every automobile has a low pressure hydraulic system in both its power steering and automatic transmission.  Newer diesel engines have a very high pressure (>20ksi) hydraulic system with a piston pump, in their common rail fuel systems.  Automotive hydraulic systems would benefit with regards to reliability if they employed the design practices currently used in aircraft hydraulic systems (ie. sealed fluid circuits, parts designed for on-condition service life, use of HUMS, integrated system designs with a minimum of joints).

The big upside to hydraulic hybrids versus battery-electric, is their lower production cost and ability to recapture a much greater amount of braking energy.  The drawback is the limited amount of stored energy potential for a given installation weight.

Until electric battery technology makes some huge improvements, hydraulic hybrids still make more sense overall though.

Regards,
Terry

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)



Typical of existing Eaton hydraulic system

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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Well that's a pretty picture which pretty much defines why it won't happen.

Pump motor is in a package critical part of the car, you'll need a new trans tunnel/bulkhead stamping and a crash program. It is alongside the first cat and so will need protection from 700 deg C heat.

The two accumulators displace room from the exhaust system and the fuel tanks.

The rear drive assy is volume feasible, it looks like it'll add a 40-50 kg mass penalty in order to provide good NVH, and will necessitate a rear crash program.

All these problems are solvable, given a new platform and time.
 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)

What new is needed for [p] "protection from 700 deg C heat"[/p]different than that presently used in most of the present mass produced automobiles using a hydraulic motor, (torque converter), to rotate the modern day automatic transmission?

Will agree, the accumulator location is in proximity of a 2 to 3 inch diameter tail pipe routing of an  exhaust system. But, blocking the entire path? Dah!

Quote (:GregLocock):


All these problems are solvable, given a new platform and time.
These problems have been functionally solved and time is now being used to physicaly operate practical working models.

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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Good question and good comments. When the EPA/UPS truck first came out I remember reading an article containing component info but I can't find it now. Relying on my less than perfect memory I think the main engine driven pump has a capacity of 130cc/rev and the three pump/motors on the differential are 85cc/rev each. The third unit was added to the differential because UPS has a fairly steep hill climb requirement. The system is rated for a maximum working pressure of 5000psi. The pump/motors are rated for 7000psi operation. The pumps were developed and designed for use in this drive, but I don't know at what point in the process the pump/motors were sized in relation to the selection of the UPS package delivery truck as the demonstration vehicle. The UPS truck has two high pressure and two low pressure accumulators (i.e. reservoirs), each with a 22gallon capacity. Also, someone in the EPA did claim the design is adaptable to a sedan. They claim that the drive would add about $600 to the cost of a sedan if produced in mass quantities. I'm not sure if they meant the cost of production or sticker cost but I assume sticker cost since they used the number in discussing how long it would take for the consumer to recover the cost.

So why don't we have hydraulic drive sedans? The criticism I hear most about hydraulic drives is that they are too bulky. They are not referring to the pump/motors as being bulky but to the energy storage units. The four large accumulators on the truck provide enough capacity, with moderate efficiencies, to brake the truck from less than 40mph to 0mph or about from 65 to 53mph. That's less energy than is contained in a single 12Volt Pb-acid deep discharge battery with a 100minute reserve capacity rating. A sedan is much lighter than a Class 6 truck but it is also much harder to create space in a sedan design. I think the lack of energy storage capacity causes hydraulics to be seen as having a limited ability to moderate engine load. So in general, conventional hydraulic drives do not provide enough advantages to balance their shortcomings. But hydraulics can get energy in and out of storage rapidly and efficiently so it's useful when the primary load profile is frequent power surges or includes a lot of low-speed and high-torque operation. I think the EPA spent a lot of time and money bringing nothing new forward. Their main claim to doing something innovative seems to be their pump/motor design, but I agree it looks like an off-the-shelf design. The patent seems to be more or less design options rather than something new and/or unique and the design doesn't do anything to overcome the shortcomings of conventional hydraulic drives. Look up companies like Hybra-Drive, Innas, and Artemis for what I think is more interesting work in hydraulic drives. I think the EPA and Eaton do more to inhibit technology than to innovate. I enjoy indulging in a rant every now and then myself.
 

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Using hydraulics like the EPA, Eaton and other axial piston systems for vehicle drive is a simple evolution of hydraulics. Hydraulic component manufactures spend very little on new design R&D so don't expect any great technology from them.

I do believe hydraulic could become the drive of the future for all types of vehicles if properly implemented. Capturing and reusing 80%+ of the available braking energy can be done with hydraulics cost effectively compared to electric drives.

Just because the car companies have chosen electric drives as their method of choice should be a warning based on current conditions.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Nice find on the Tucker "Flowing power" advertisement, I enjoyed it. It also makes a good point. The torque converter was applied to autos as early as 1947 but did not come into wide use until it was "properly implemented", i.e. placed between the engine and transmission and a stator mounted on an over-running clutch was added to tighten the coupling. Probably back in the 60s or 70s, then for decades nearly every car on US roads had a torque converter.

So I'd like to turn the question around and ask: How do hydraulic drives need to evolve to become useful in autos?
 

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

Hydraulic drives of the torque-converter variety will always have high losses when they are doing the torque-converting (modern automatics have improved their efficiency by locking up i.e. bypassing the torque converter except when starting off from a standstill or momentarily during gear changes).

Honda has an interesting CVT in the new DN-01 motorcycle. It has a swash-plate hydraulic pump and motor in a single housing. Interestingly, it's arranged in a way that as the road speed increases and the required drive ratio thus decreases, more of the drive goes through a direct mechanical path (the hydraulics effectively "lock up") to improve efficiency. But this doesn't lend itself to regenerative braking, etc.

To become useful in an auto, the efficiency and operating speed of positive-displacement hydraulic pumps and motors needs to improve - by a lot. Commercial hydraulic pumps and motors for industrial applications have a rather lousy efficiency compared to what's needed in an automotive transmission.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

To make hydraulic drives suitable for vehicle drive train the pump/motor needs to have the same thermal efficiency regardless of load or speed demand. Current swash plate designs have low efficiencies at low speeds and are do not great efficiencies at high speed low load.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

"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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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
www.danzcoinc.com
www.dehyds.com

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)

Quote (GregLocock):

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.

Quote (NTROhio):

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

Quote:

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.

Quote:

Artemis Hydraulic Hybrid Transmission Increases Fuel Economy By 2XIn 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.

Quote:

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  - - -

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

GregLocock,

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

http://yosemite.epa.gov/opa/admpress.nsf/b1ab9f485b098972852562e7004dc686/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

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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/PARKER/menuitem.734d2913eb3c11da13576f849420d1ca/?vgnextoid=d195f87357e28110VgnVCM10000048021dacRCRD&vgnextfmt=default&vgnextcatid=1157654&literaturetype=Catalogs&searchAttr=searchResult&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
www.danzcoinc.com
www.dehyds.com

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

^^ 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.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

(OP)
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  - - -

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

^ 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.

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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

Quote:

.......... 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  - - -

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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.
 

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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
www.danzcoinc.com
www.dehyds.com

RE: Diesel-Electric hybrid vrs Gasolene-Hydraulic hybrid?

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

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