a575@mindlink.UUCP (Michael G. Henders) (09/23/90)
> stebbins@sisler.ucr.edu (John Stebbins) writes: > <regarding electric brakes> > involved here so this may be absurd, but I was thinking that a whopping big > capacitor may take care of part of this problem and have the side > benifit of giving you more readily available energy for your next > acceleration. The physical size of such a capacitor may be larger than > the proposed vehicle for all I know. Its just a thought. > > John Stebbins > stebbins@ucrmath.ucr.edu Well, easy enough to see... Let's say 1500 kg of car, at 15 m/s (ie, about 3300 pounds, doing around 34 mph); kinetic energy is 0.5*Mv^2, or around 170kJ. Since capacitive energy storage is 0.5*CV^2, let's look at a 1.0 mF (*milli*Farad) cap.; we need it to run at about 18.5kV. Ooops! :-) I don't think it'll fly. How big would a 1 mH, superconducting, 18.5 kA inductor be? Seriously, now...inquiring minds want to know! :-) :-) Mike Henders a575@mindlink.UUCP
cook@stout.atd.ucar.edu (Forrest Cook) (09/24/90)
>[ lots of interesting stuff about electric cars and relative efficiencies ]
.
Instead of saying it can't be done and then whining when the Japanese do it :-)
....
How about electric car designs that brake by using the motor to charge back
into the batteries? Such a system could probably save several tens of percents
in efficiency, especially for city drivers. Are there any special tricks
involved in doing this? I can see how it would be possible to slow down
by connecting higher voltage windings from the motor/generator to the battery,
but is such a system able to come to a complete stop and use all of the
braking for recharge? Perhaps a PWM system would do most of that.
--
^ ^ Forrest Cook - <<< We all P for IP, We all BM for IBM >>>
/|\ /|\ cook@stout.atd.ucar.edu WB0RIO (This posting is my OPINION)
/|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cookstebbins@sisler.ucr.edu (john stebbins) (09/24/90)
In article <8581@ncar.ucar.edu>, cook@stout.atd.ucar.edu (Forrest Cook) writes: |> |> >[ lots of interesting stuff about electric cars and relative efficiencies ] |> . |> Instead of saying it can't be done and then whining when the Japanese do it :-) |> .... |> How about electric car designs that brake by using the motor to charge back |> into the batteries? Such a system could probably save several tens of percents |> in efficiency, especially for city drivers. Are there any special tricks |> involved in doing this? I can see how it would be possible to slow down |> by connecting higher voltage windings from the motor/generator to the battery, |> but is such a system able to come to a complete stop and use all of the |> braking for recharge? Perhaps a PWM system would do most of that. There are a couple of problems you didn't address. First is batteries charge slower by a factor of 10 or more than they discharge. Second is that most people break much faster than they accelerate. This compounds the first problem. I haven't calculated the amount of energy that is involved here so this may be absurd, but I was thinking that a whopping big capacitor may take care of part of this problem and have the side benifit of giving you more readily available energy for your next acceleration. The phisical size of such a capacitor may be larger than the proposed vehicle for all I know. Its just a thought. John Stebbins stebbins@ucrmath.ucr.edu
schriste@uceng.UC.EDU (Steven V. Christensen) (09/24/90)
cook@stout.atd.ucar.edu (Forrest Cook) writes: >.... >How about electric car designs that brake by using the motor to charge back >into the batteries? Not really electronics, but... I read an article somewhere about some MECHANICAL engineers who built a car with some outrageously high efficiency. It seems that there was a small gas motor which constantly ran at low power, to keep a hydraulic container "charged" (i.e. pressurized). This energy was used when you accellerated from a stop (where most of the energy was needed), then reclaimed (in part) when you stopped (pressurized the tank again). So it was like a wound-up rubber band, springing you forward... Steven >-- > ^ ^ Forrest Cook - <<< We all P for IP, We all BM for IBM >>> >/|\ /|\ cook@stout.atd.ucar.edu WB0RIO (This posting is my OPINION) >/|\ /|\ {husc6|rutgers|ames|gatech}!ncar!stout!cook -- Steven V. Christensen U.C. College of Eng. schriste@uceng.uc.edu For the adventurous: svc@elf0.uucp
vail@tegra.COM (Johnathan Vail) (09/24/90)
In article <8581@ncar.ucar.edu> cook@stout.atd.ucar.edu (Forrest Cook) writes: >[ lots of interesting stuff about electric cars and relative efficiencies ] . Instead of saying it can't be done and then whining when the Japanese do it :-) .... How about electric car designs that brake by using the motor to charge back into the batteries? Such a system could probably save several tens of percents in efficiency, especially for city drivers. Are there any special tricks involved in doing this? I can see how it would be possible to slow down by connecting higher voltage windings from the motor/generator to the battery, but is such a system able to come to a complete stop and use all of the braking for recharge? Perhaps a PWM system would do most of that. The problem is that the efficiency is low and the braking effect is generally not good enough to get rid of "real" brakes. I saw a documentary about an Australian design that used braking on buses to charge a hydraulic cylinder. Then that energy was used to provide acceleration. This had the advantage of re-using some of the otherwise lost energy of braking but the added acceleration allowed a much smaller engine to be used, furthering the savings. The system was designed as a add-on that could be retrofitted to existing buses at competitive prices. "The death of God left the angels in a strange position." _____ | | Johnathan Vail | n1dxg@tegra.com |Tegra| (508) 663-7435 | N1DXG@448.625-(WorldNet) ----- jv@n1dxg.ampr.org {...sun!sunne ..uunet}!tegra!vail
grege@gold.GVG.TEK.COM (Greg Ebert) (09/25/90)
Although somewhat 'lo-tech', I remember some of the electric-powered mail
cars used high-speed flywheels (hence lower mass) for short-term energy
storage. The motivation for this was letter carriers who took their
vehicle on routes with curbside mailboxes, which entailed a lot of
start-stop driving. They were manufactured by Garrett (now Allied-Signal).
As someone mentioned, batteries tend to have rather long charge cycles.
They correctly mentioned that a capacitor would probably be too large to
serve as an energy reservoir. However, when HV technology improves, they
might be feasible. The energy stored in a capacitor is proportional to
the square of the voltage (1/2CV^2), but the volume (weight) of a
capacitor is more linear with respect to voltage (and capacitance), so
I would expect that using high-voltage capacitors might cut the mustard
for fast-braking. Some simple calculations:
Assume a 2000Kg vehicle (quite heavy ~4500lb) moving at 33 m/sec (~70MPH).
The total energy is about 1 megajoule. At 10,000 volts, you could dump
this into a 20,000uF capacitor.
Imagine tapping into this when you stomp on the accelerator !!!
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"I was, BANNED in the USA" - 2 Live Crew #######paul@hpldola.HP.COM (Paul Bame) (09/25/90)
> / hpldola:sci.electronics / stebbins@sisler.ucr.edu (john stebbins) / 10:12 pm Sep 23, 1990 / > In article <8581@ncar.ucar.edu>, cook@stout.atd.ucar.edu (Forrest Cook) writes: > |> > |>>[lots of interesting stuff about electric cars and relative efficiencies] [deleted] > |> . > |> How about electric car designs that brake by using the motor to charge back > |> into the batteries? Such a system could probably save several tens > of percents > |> in efficiency, especially for city drivers. [deleted] > There are a couple of problems you didn't address. First is batteries > charge slower by a factor of 10 or more than they discharge. Second > is that most people break much faster than they accelerate. This compounds > the first problem. I haven't calculated the amount of energy that is > involved here so this may be absurd, but I was thinking that a whopping big > capacitor may take care of part of this problem and have the side > benifit of giving you more readily available energy for your next > acceleration. The phisical size of such a capacitor may be larger than > the proposed vehicle for all I know. Its just a thought. > John Stebbins > stebbins@ucrmath.ucr.edu > ---------- Some experimental city busses use flywheel energy storage and supposedly use regenerative braking to great advantage. I can't remember references but it seems like probably a Popular Science article several years ago. -Paul "Spice is the Variety of Life" paul@hpldola.hp.com N0KCL
jgk@osc.COM (Joe Keane) (09/27/90)
In article <1488@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Greg Ebert) writes: >The energy stored in a capacitor is proportional to >the square of the voltage (1/2CV^2), but the volume (weight) of a >capacitor is more linear with respect to voltage (and capacitance), so >I would expect that using high-voltage capacitors might cut the mustard >for fast-braking. But if you increase the voltage rating the capacitance goes down. The volume, weight, cost, etc. of a big capacitor is proportional to the energy it can store, pretty much independent of the voltage rating. It does go up below say 20V because the thicknesses required get pretty small. Theoretically, the energy stored in an electric field is `1/2*epsilon*E^2*V' where where `epsilon' is the permittivity, `E' is the electric field density, and `v' is the volume. The first two are determined by the dielectric. You can chop up the volume in different ways to get different voltage ratings and capacitances, but the energy stays the same.