Yes, of course, if you take out friction, hills will have no net effect. I
didn't realize you eliminated that from your model. Peri
-----Original message-----
From: Michael Ross <[email protected]>
To: Electric Vehicle Discussion List <[email protected]>
Sent: Fri, Nov 22, 2013 18:38:55 GMT+00:00
Subject: Re: [EVDL] EVLN: Kia Soul EV goes global
andaimsfor120-miledrivingrange
Peri, this is just physics. It is a conservation of energy thing.
Disregarding friction losses - here is no change in energy if you roll
down a hill - the sum of kinetic energy and gravitational potential energy
is unchanged from when you were at the top and unchanged when you get to
the next top. You require less to go down the hill and more to go up. At
any point along the trip there is conservation of energy - when you reach
the starting elevation the speed is the same as when you began - just like
the level ride.
The net difference between start and finish is only the losses to friction,
the two excursions are more or less the same whether you are on flat land
or hills - if you back off and don't let the thing speed up going down. If
the losses to friction are the same then there is no difference hilly
versus flat.
Now you can't have much of a hill in length or incline before simply
backing off has your speed climbing, then you are losing more energy moving
through the air. But that is because of the speed not because you we on a
hill directly.
On Fri, Nov 22, 2013 at 9:38 AM, Peri Hartman <[email protected]> wrote:
You are right that constant speed eliminates the effects of air drag (an
other mechanical restance effects). But then the question goes back to
drive train efficiencies.
How are you going to maintain constant speed? In an ideal course, I
suppose
your downhills could be designed so that the force of gravity exactly
matches the resistive forces (i.e. wind resistance). Thus zero power and
zero regen.
But even then, going uphill, you will need to use more power than you
would
on the level. How is that going to be equally efficient?
Peri
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf
Of Michael Ross
Sent: 22 November, 2013 4:51 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] EVLN: Kia Soul EV goes global
andaimsfor120-miledrivingrange
The levelness is the point of interest for me. Two identical EVs, one
goes
around a flat circuit or out and back, it does not matter. The other goes
on a hilly route of equal length. I will give the the extra inches from
climbing, but give me that the inclines are not very great, and you can
simply maintain a constant speed on the inclines by simply backing off or
adding power. Constant speed washes out the effects of increased air
drag.
There is no difference of any significance, to the power needed on either
route. You get every bit of kinetic energy back on the downhill runs
that
you converted to gravitational potential energy on the uphill runs. You
end at the same point with the same potential energy as you started.
When you start tallying up the heat losses, the greatest by far is the
heat
generated pushing through the air. Rolling resistance is very small in
comparison. On bicycles the heat generated jiggling things around,
sliding
straps, and chafing fabrics, are on the same order. You can actually save
a little by lowering tire pressure, but not too much.
I will grant for making a comparison between vehicle ranges and
performance
that a flat route is appealing, but it won't change the results that much
unless the down hill speeds are significantly different. DIfferences in
ambient wind, driving style, and so on, are harder to control and add up
to
significant noise in a study like this. You can only get results that are
dependable by a valid, statistically significant set of runs, because the
noise is too great.
I have done drive cycle testing on cars, and read a fair amount about air
drag testing on streamlined bicycles. Human powered vehicles are
particularly concerned with the minutia of energy loss in vehicles, as the
power source is limited to only 250W or so. The main thing to carry away
is minimized air drag, by design or drive style. You may not even notice
the losses due to vibration of too high tire pressure on an EV as the use
of energy is profligate in comparison.
Variations in the efficiency of motors at different speeds and loads is
probably noticeable, I would anticipate a sweet spot for each particular
EV. VFDs are probably a nice way to minimize this and improve efficiency.
They do a good job on HVAC systems, FWIW.
On Fri, Nov 22, 2013 at 3:16 AM, Martin WINLOW <[email protected]> wrote:
> What about the energy losses of (effectively) constantly having to
change
> direction on a circuitous course (or any course - especially a twisty
> mountainous one - lots of hair-pin bends)? This would manifest itself
as
> the tyres scrubbing on the road surface giving off heat, noise and wear
on
> the rubber/road. Not big but still significant. The losses on the
outward
> trip would be aded to those of the return. They would not cancel out
like
> a change in elevation would... or am I missing something? MW
>
>
> On 21 Nov 2013, at 01:09, Cor van de Water wrote:
>
> > Hi Michael,
> > Since this is a theoretical experiment, we might just as well focus on
> > that theory and indeed, at first blush is appears that the theory says
> > that if you have a closes course, your *potential* energy level is
> > identical at finish as when starting, so there is no net build-up or
> > release of energy other than the friction and kinetic energy, since
any
> > height gain or loss must be made up by the end of the trip.
> >
> > However, an electric motor works different than an ICE. Where the ICE
> > motor has a weird efficiency change (weirder for petrol as for Diesel
> > engines and the Prius' Atkinson cycle is again slightly different than
a
> > regular petrol ICE) the efficiency of the electric drivetrain is in
> > theory perfectly linear falling with the amount of power delivered,
> > since losses go up squared with the current (force) delivered.
> >
> > Result is that if half the drive causes you to use double the current
> > and half the drive you can maintain the speed downhill at zero
current,
> > your total *loss* will have doubled. It is 4x as large during half the
> > drive, compared to the 100% level ride.
> >
> > So, in theory, diving over hills and mountains at the same speed
should
> > consume more power than driving the same speed and distance on a level
> > road.
> >
> > In an electric drivetrain, loss may be at or above 30% at full
throttle,
> > while at half throttle it is typically at or below 10% for the same
> > reason.
> > So, the total energy difference is not very big, you may go from the
> > energy consumed at level road to an increase of around 8% for the
> > example drive half uphill and half downhill. (level would be 92%
energy
> > to the wheels, 8% loss, for the doubled loss in the hills you would
have
> > 16% total loss so the energy consumption rises by 8%)
> >
> > Hope this clarifies,
> >
> > Cor van de Water
> > Chief Scientist
> > Proxim Wireless Corporation http://www.proxim.com
> > Email: [email protected] Private: http://www.cvandewater.info
> > Skype: cor_van_de_water Tel: +1 408 383 7626
> >
> >
> > -----Original Message-----
> > From: [email protected] [mailto:[email protected]] On
> > Behalf Of Michael Ross
> > Sent: Wednesday, November 20, 2013 4:36 PM
> > To: Electric Vehicle Discussion List
> > Subject: Re: [EVDL] EVLN: Kia Soul EV goes global and
> > aimsfor120-miledrivingrange
> >
> > Cor, this is really a theoretical topic. Why should a level ride be
> > easier
> > than a hilly ride if it is a circuit?
> >
> > I disagree about regen saving anything except when used for braking,
> > then
> > you are trading waste heat for some charging.. If you use regen to
slow
> > down on hills you are causing an inefficiency.
> >
> > If you are on a hill that is too steep for safe coasting and brakes
must
> > be
> > used, then regen is better than heating your brake rotors.
> >
> > If you take all this out of the thought experiment, then hilly and
level
> > are equal.
> >
> >
> > On Wed, Nov 20, 2013 at 7:22 PM, Cor van de Water <[email protected]>
> > wrote:
> >
> >> Michael,
> >> How often do you encounter a steep road that is completely straight
in
> >> the mountains? All the roads in the surrounding mountains that I know
> >> have sharp turns, for which you will *definitely* want to slow down.
> > So
> >> except if you are in roller-coaster country, having regen will save
at
> >> least part of the energy from the downhill run when you need to slow
> >> down for the next turn...
> >>
> >> BTW, it may be that the information is skewed due to the redefinition
> > of
> >> electric vehicle by some manufacturers. I have noticed that my Prius
> > can
> >> get better mileage going over a large hill than when doing a steady
> >> speed in flat terrain. This is probably similar to the "Pulse and
> > glide"
> >> hypermiling technique. This has of course nothing to do with pure
> >> electrics, but since GM calls their plug-in hybrid an EV, that may be
> >> where the confusion comes from...
> >>
> >> Regards,
> >>
> >> Cor van de Water
>
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Tell me what it is you plan to do
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To invent, you need a good imagination and a pile of junk.
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*Warren Buffet*
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(919) 550-2430 Land
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