# electric-hydrostatic hybrid question



## Coley (Jul 26, 2007)

After using a hydrostat driven skid loaders, I don't have any desire to run a hydro in an EV. 
Electric motors run best at higher rpm and the drag of the pump would, to me, be a waste of battery power.

The added weight alone would be a detriment to mileage or speed.

They are good on an ICE, but I doubt the edge sought for an ev.


----------



## Duncan (Dec 8, 2008)

Hi Steve
You will need a quantity of power to travel your distance
80 miles guess at 250 watthours/mile equals 20 Kwhrs

You will need to start off with 20 Kwhrs stored either in batteries, fuel or hydrostatic accumulators

Using batteries to run an electric motor to pump fluid to drive the drive motors is very inefficient


----------



## ga2500ev (Apr 20, 2008)

stevet47 said:


> I am researching the feasibility of building an electric-hydrostatic hybrid. Pure EV wont work for me because I drive ~80miles a day with no opportunity to recharge mid-trip.


Why do you presume that a pure EV won't work. Check out the Red Beastie as an example:

http://www.evalbum.com/037

120 miles on the highway. And that is with lead acid batteries.

I'll leave the hydrostatic discussion out. IMHO if you need 80 miles of range, simply get enough batteries to get 80 miles of range. Because as others have already pointed out in this thread, that no matter what you motive force is, you need enough energy to drive it. So throwing in two extra systems (gas and hydrostatic) simply to get range is infinitely more complicated than simply having enough batteries to make it where you need to go.

ga2500ev


----------



## stevet47 (Feb 27, 2009)

ok, so maybe I was wrong in thinking this would be a good application for an electric motor. , but it seems at least ga2500ev is thinking I would have a gas engine, electric motor, and hydrostatic drive, I wouldn't, it would just be electric and hydrostatic.


Yes, this is more complicated than pure EV, but let me explain it this way....
In an EV the motor must be sized based on the top speed you want to reach and how fast you want to accelerate. Batteries must then be sized based on the motor's draw while both accelerating to that top speed, and maintaining it.

In the electric-hydrostatic scenario, the electric motor only has to be sized to be capable of maintaining top speed, NOT accelerating. The batteries can then be sized based on the motor's draw while maintaining that constant speed, NOT based on the draw while accelerating, because the electric motor wont be doing the accelerating.

The transmission generally used in an EV conversion is then replaced by the hydrostatic drive (which will weigh about the same as an auto trans... I know, most use manual), so this is minimal added weight, and although it it slightly less efficient than a manual transmission, it provides an infinitely variable gear, acting as the accelerator and eliminating the need for an expensive EV controller. It also provides regenerative braking (which captures 80% of energy, EV's only capture ~30%, correct?), which is where the accelerating comes into play. All vehicle acceleration is done via the hydostatic system (with fluid pressure provided by regenerative breaking when possible, and the electric motor when not possible).

So again, the electic motor and batteries only have to be sized in order to maintain...say..., a 65mph top crusing speed in a 1900lb car, with a Cd of .29, the hydrostatic system takes care of the rest.

So it would be:
Batteries----> Small Electric Motor----> Hydraulic Pump----> Hydraulic Motor.

As opposed to:
Batteries----> Large Electric Motor----> Transmission.

So yes, I am adding 1 extra step, which seems like a waste, but when you take into consideration the reduced strain on the electric system, and therefore its size, it seems to make sense.


----------



## gor (Nov 25, 2009)

sounds good - in theory, for university research project with OEM donated parts, with research and manufacturing facilities; or if you have deep pockets
there is no ideal IVT or CVT - everything on the market now has it's limitations - and fare from ideal - how fare - depends on complexity, costs, etc
Parts available on the market, for DIY projects - components used by industry, and for hydraulics - it mostly from heavy-duty (earth moving) equipment -available hydroaccumulatotors, 4 example, are very heavy; 
hydraulics system should be designed and fit like a glove - no step left, no step right (guys who racing mowers - don't use hydrodtranies - they love good-old mechanical, which can be modified easily to fit any demands) 
also actual built - usually more heavy than on paper, and less effective than it seemed; advantages might be not that significant, and might not worth the efforts 
but anyway, good luck


----------



## CNCRouterman (May 5, 2008)

Hello Stevet47,
I think you are miss understanding the advantages of the electric motor and the accumulating losses you would experience with the addition of hydraulics.

In your model you will have an electric motor and a battery pack.

To size the pack you need to consider total energy requirements, continuous demand, and peak demand. Based on a previous post, it looks like you need around 20 KwHr worth of batteries (I think that is a bit optimistic, but lets use that value for now).

Remember that an electric motor is rated at its CONTINUOUS hp. Meaning, as many members here can attest to through personal experience, an electric motor can reliably output significantly more than its rated power for short periods of time, or sometime for fairly long periods of time, depending on how far you push. Therefore, the size of the electric motor you would choose for your application is really no different than you would choose if you eliminated the hydraulic portion.

If you found that you needed 10 hp to move your vehicle in a steady state at the cruising speed, then:
motor size = 10hp divided by the efficiency of the hydraulic circuit. If the hydraulic circuit is 80% efficient, then you would take 10hp/0.80=12.5hp electric motor. A 10hp electric motor could output 12.5 hp for a while, perhaps for the full commute, but you would still be using 12.5 hp instead of 10 hp, or 12.5 x 746watts/hp=9325 watts instead of 7460 watts. Your battery pack will also have to grow from 20 KwHr to 25 Kwhr. Hills and acceleration really do not change the outcome, the hydraulic circuit will always be in the loop and will always be consuming power, and a rather significant percentage.

Yes if you draw heavier amps from a lead battery pack you do pay for it in reduced capacity due to Peurket' Law, but I suspect that in your application, you will find that the losses inherent to integrating a hydraulic accumulator and hydraulic motor in series with your electric motor drive will be much more a liability than a benefit.

As for the size of the motor, remember that a 10hp electric motor may have a peak power output 3 times as much (for accelerating or hills), or even a lot more depending on the motor and your controller.


----------



## stevet47 (Feb 27, 2009)

Thanks for the crash course. Obviously I was looking at this the wrong way. This idea is going into the trash can. I think I will still pursue the same idea, but with a gas engine instead of an electric motor. This has already been shown to work by some other people... and even if it doesn't give me the result I am hoping for, the money spent on it will be a lot less than with the electric motor/batteries/etc.


----------



## CNCRouterman (May 5, 2008)

You do recognize that your IC/Hydraulic accumulator concept is really not much different than a series IC/Electric Hybrid right?

The whole principle is to use a relatively low output constant power source to feed a high capacity storage device of some sort that then can feed a high output final drive.

With electric, you have generator losses, storage losses, and power stage losses, which can add up. Hybrids tend to suffer from a pure efficiency viewpoint. That said, IC engines are notoriously innefficient in terms of potential (chemical) energy to usable kinetic (mechanical) energy. I do not have efficiency figures for hydraulics, but the folks on this website can definitely help you out with real world efficiencies for charging, DC motor efficiency, typical controller efficiency, and probably gen set efficiencies. I vaguely recall participating in a discussion regarding a series hybrid and a gen set, I think I looked up the specific fuel consumption numbers during that conversation and found that steady state series hybrids are not the best choice from a pure economy standpoint.

Hybrid systems do however have significant advantages in some areas, it is for this reason that you have to clearly define your mission goals for the project. As you have already concluded, a series hybrid demonstrates several specific capabilities that are appealing to you, and that is what counts. Hybrid electric will have a range limited only by available fuel, essentially the same as a conventional IC powered car. The series hybrid CAN be quite efficient (compared to a conventional IC version), and can offer other advantages, you just have to keep you eyes open to the disadvantages and decide if the balance tips in the direction you want.

I encourage you to evaluate, and actually document your mission priorities for your project. Then evaluate those priorities and categorize them into a couple of groups, such as: Mission critical (MUST do), Mission enabling (Should do, but not critical to mission), and Want to do (Luxuries like cup holders, built in microwave, Duke of Hazard horns, etc). Once you have your mission defined, documented, and all the features categorized, you can start running numbers to see what fits, what does not fit, and what things can fit with some re-engineering here or there.


----------



## stevet47 (Feb 27, 2009)

Dukes of Hazzard horns are most definitely Mission Critical.


----------



## automd (Feb 5, 2010)

stevet47 said:


> Does this make any sense. What are your thoughts?


I agree with Steve. Simply get enough batteries to get 80 miles of range


----------



## thevenun (Oct 31, 2012)

These train commercials say they move one ton of freight 500 miles on one gallon of diesel......could we scale that down for a car?


----------



## PaulS (Sep 11, 2012)

A train runs on nearly "frictionless" tracks, has a very low coefficient of drag, and a minimum of power potential. 
If you used a car that was one passenger wide and a block long, ran on poly-urethane tires, and used a motor rated at 1/8 hp per pound you might get similar results.
It would take you a mile or more to get to speed and you would need to be able to travel several hundred miles non-stop and then take another mile or so to bring the rig to a stop but yes, it could be scaled down. 
It would never be practical on the roads that we drive today.


----------



## Ziggythewiz (May 16, 2010)

Don't forget to ban all other traffic.


----------

