Talk:Car/Kinentic Vehicles Proposal

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When this says "use of power cube" does that mean this will be a hydraulic drive vehicle that gets 100mpg? I remember reading that hydraulic transfer is much less efficient than mechanical, how will the loss in efficiency be made up? - Lex Berezhny 13:47, 14 February 2012 (CET)

  • I think the best practical efficiency of high quality hydraulic pump/motors is 85%. The efficiency of a hydraulic-drive vehicle can be increased in the following ways:
    • Use hydraulics for pretty much everything; this saves weight. The drive, steering and suspension can all be hydraulic.
    • Build the hydraulic system so it recaptures energy from the suspension and braking activities. FedEx trucks were able to get 85mpg that way.
    • Integrate the largest, strongest hydraulic component (the accumulator) into the frame of the vehicle as a stressed member.
    • As much as possible, use metal tubes instead of rubber hoses.
    • As much as possible, use the highest quality pumps/motors possible.
    • Three wheels instead of four.
    • Aerodynamic body.
  • The efficiency discussion is complicated by the fact that the GVCS is supposed to be a system of systems. To a large extent, it is the efficiency of the overall system that is important. The efficiency of any one machine is much less important. Measuring the efficiency of a vehicle as compared to other vehicles is unfair, since there are dramatic efficiency gains in terms of manufacturing and maintenance that are realized by the use of modular components. Additionally, defining the vehicle in such a way that it has no (or very few) commercial equivalents (a 3-wheel 2-seater with 1000 mile range for example) means that it's difficult (if not impossible) to compare the vehicles efficiency to anything else.
  • Besides, the ultimate goal is a biomass-fueled vehicle, which means even if it's 100% efficient it's going to have a 30 gallon "tank" (or something outrageous given the low energy density of pellets) just to reach the 300 mile range that's standard for passenger vehicles.
  • What I mean is: don't worry about the efficiency. There are far more important factors in the design. - Matt_Maier 14FEB2012

Lex, that's been my experience as well--hydraulic drives are ideal for low speed high force applications, such as lifting loads and moving soil, but viscous drag etc makes them less suited to high speed low force duties...such as driving a lightweight streamlined car at highway speeds. But Matt offers some interesting ideas for improvements; here are my ideas on those ideas:

    • There aren't a whole lot of accessory tasks on the OS Car (as originally proposed) and the only need for power steering or power brakes will be if the drive system gets so heavy that normal human effort is insufficient. The suspension idea is intriguing--shock absorbers are already hydraulic and are currently designed to waste energy, not conserve it--if there's enough energy there to be worth the weight of tapping it, that should be a winner.
    • Recapturing energy from braking is a proven winner, and hydraulics are (apparently) much more efficient than batteries at that task. If the car does sufficient stop-and-go driving to justify the mass of the recovery system, then it's a winner. It seems ideal for the small package express business (though an 85 mpg FedEx truck seems beyond the current state of the art).
    • Technically, using the frame as an accumulator is ideal, but in an accident, the risk might be excessive. Might be; I don't know enough about hydraulic drive. I do know that high pressure hydraulics (e.g. diesel fuel injection) pose a risk to workers diagnosing pinholes and cracks, but if frame damage only risks the passengers getting slippery, rather than getting sliced, having the chassis serve double-duty sounds like a good idea.
    • Possibly the frame would be more useful as a tube and hose substitute than as an accumulator; at the very least as a fluid return line. (sort of like how the frame is used as the negative wire back to the battery on conventional cars).
  • All in all, the weight given to the other important (indeed more important) factors of the design will determine where efficiency factors rank, but there's a point when efficiency factors get low enough that there's no use bothering with the OS Car at all.

I'll try to find a place to list the trade-offs we need to consider before we start turning wrenches. For example, can someone provide a drawing of where the Power Cube goes in a streamlined two passenger three wheeled car? All my sketches end up either unstable in roll, or with worse aerodynamic drag than four wheeled variants. - JackMcCornack 18FEB2012

  • Technically anything can be "low speed high force" if you gear them properly :-)
  • I've been collecting research on hybrid hydraulics for a while now. Hybrid_Hydraulic_System Car/Research_Development#Design
  • My mistake. It was a UPS truck. EPA Hybrid Hydraulic Program presentation on HHV's
  • One of the reasons active hydraulic suspensions weren't popular in car racing was the danger of getting sprayed by 3000psi hot oil. At that pressure it gets injected straight into your body; right through the skin. Kind of dangerous.
  • I figured the power cube(s) would go in between the front wheels, the passengers behind them, and then a single rear wheel. The ones we have now are kind of bulky, tho. A 2 foot cube is hard to fit anywhere. But that's probably okay; I figure we'll need a new power cube model for a passenger vehicle anyway. Getting up to 70mph will probably require a redesign for significantly higher flow if not for additional reasons. - Matt_Maier

Thanks for the links, Matt, and the insight...I followed the links you gave and got some examples.

  • Technically anything can be "low speed high force" if you gear them properly :-)

True, but with a gear pump a la the Prince SP20B16 used in the Power Cube, you only get one gear, and in most machinery (including mobile agricultural machinery) you gear that gear to be 'low'. If you gear it for high speed (highway speed rather than furrow speed) then force goes down accordingly. One -could- make a car work with hydraulic drive and a gear pump, but it would be like driving a car with only one gear and a hydraulic clutch: if you geared it low you'd still be able to go fast (if you had the horsepower) but you'd be spinning the motor way outside its efficient RPM range, if you geared it high for high efficiency cruising, you'd need huge gobs of engine torque to get away from stop signs, because low RPM power recovery would be so inefficient. There's an "Overall efficiency" pressure/rpm/efficiency graph on page 9 of the <steerable hydraulic wheel motor> pdf link on the Hybrid_Hydraulic_System page; it's percentages are quite typical of fixed displacement hydraulic motor graphs, regardless of size. This graph assumes a variable displacement pump (it's a wheel motor for big equipment) so it assumes the pump will deliver variable pressure at variable gallons per minute (high pressure at low revs) but it's valid for fixed displacement pumps (such as the Power Cube) too, if you cut the pressure off at the max pressure at max RPM. So, draw a horizontal line across the graph at 2000 psi and let's gear it in a car so 100% RPM is 75 mph, and let's power the pump with the engine in my own high mileage car (32 hp diesel).

From zero to 30 mph, the hydraulic motor is working at better than 90% efficiency, which is better than my current transmission-and-differential system. However, at full power at top speed, motor efficiency drops to 60%. What's worse is, high RPM efficiency drops further when power is reduced; cruising at 60 mph (80% of full RPM) my car currently requires about 8 hp (1/4 of maximum engine power) at the crank, but with this hydraulic motor at that RPM and torque, efficiency drops to 50%...and that's not including pumping losses through the valves and hoses, or the inefficiencies of the pump itself. In cruise, this hypothetical hydraulic drive system will be less than 50% efficient at converting engine rotation to wheel rotation. The complete system efficiency is even worse, because since in order to equal the performance of a 32 horse engine with mechanical transmission, the hydraulic system will need more horsepower, which means a bigger engine, which increases weight and lowers fuel efficiency.

From the hypothetical to the documented, using the Power Cube BOM and the and web sites, that pump and motor combo can give us 12.11 GPH and 3000 psi at 24.33 hp--but let's throttle down slightly to 12 gph at 24.11 because it makes the math easier. The Wolverine CMM50 georotor motor looks like a good choice for a drive motor; it's the right pressure flow and RPM range for the Power Cube and OS Car, if we use two of them in series we can have two wheel drive and won't need a differential...(comment: actually plumbing two hydraulic motors in series simulates a solid axle: same number of revolutions on each shaft. You want to plumb them in parallel to act like a differential. ChuckH 16:56, 28 February 2012 (CET)) so cut the pressure in half ('cause there's two of them) and subtract 100 psi for back pressure and the drag of the rest of the hydraulic system valves, hoses and fittings (why 100 psi? Because I don't know exactly and deducting 100 psi makes the math so easy) and looking at the CMM50 efficiency chart on (Prince makes Wolverine) we find those motors can take in 12 GPM at 1400 PSI (each, in series) and put out 457 inch pounds of torque at 888 RPM, which is 6.44 horsepower each, total for two is 12.88 hp...24.11 hp in, 12.88 hp out = about 53% efficiency at full power. And since this car isn't going to need 12.88 wheel horsepower to cruise, hopefully it will need half that, which at those revs drops efficiency even further.

In brief (hah! about time I did something brief) the Power Cube (or any fixed displacement hydraulic pump system) is a poor power source for a road car, because of its severe performance and fuel economy handicaps. JackMcCornack 2/23/12

  • My sense is that hydraulic drive doesn't change this basic truth of getting power from an IC engine to vehicle motion: you need a wider range of speed at the wheels than the engine can handle, so you need a transmission of some kind. Porportional hydraulic valves for speed control -- as used in Lifetrac -- have too much throttling loss for an efficient application. The most common approach is a variable displacement pump (or one pump per circuit, a.k.a. "displacement control" in hydraulic drive parlance). A fairly new approach is the common pressure rail system with a flow rate transformer per circuit. IMHO it would be very good for the GVCS ecosystem in the long run to develop a simple switch-mode hydraulic transformer; however placing this on the critical path for first-prototype-car development may be too risky. ChuckH 17:15, 27 February 2012 (CET)
  • I'm glad you can do the math on it (I can't). I totally agree that the Power Cube isn't suitable for a highway-capable passenger vehicle. For numerous reasons. It seems like a good idea to consider Wikispeed's solution, which is to physically position the engine so that it can directly drive a pair of wheels. If we want to go with the 2-passenger 3-wheel configuration we could just put their engine module in front and have one dragger wheel at the back. The only reason to go for a hydraulic drivetrain is if you want to deliver power to physically separate areas, which is nice to have but isn't necessary. - Matt_Maier 23FEB2012

There are other reasons for a hydraulic drive train. With more sophisticated pumps and motors, the efficiency losses might compare well with the efficiency losses of a transmission/differential drive system, and hydraulic drive could be lighter than the mechanical drive, and it brings forth the possibility of energy storage for acceleration and braking (hydraulic regenerative braking) as described on the Hybrid_Hydraulic_System wiki page (which Matt_Maier has been keeping current). I've been doing math and research for the last couple of days, coming to a conclusion (there's not much time left for me) on the best layout for this prototype car, and I've decided four wheels is better than three...and the future hydraulic hybrid is part of why. The Power Cube itself is another reason, and though I dont think the PC is ready for a car in its current state, I think it has great potential. Besides, Marcin likes the power cube, and he's smarter than me by scads.

I posted my full length 3 vs. 4 essay on the Car/Design_Criteria discussion page, at JackMcCornack 2/25/12