Larry Dighera wrote:

James Robinson wrote:

[snip]

It's refreshing to get the opinions on electrical vehicles of
a professional electrical engineer.

I'm more of a mechanical engineer.

I got pretty cynical about claims of energy saving after listening to
all the people who came to our company trying to sell various products
that guaranteed huge energy savings or emission reduction.

Ah. This explains your apparent cynicism.

I prefer to consider it healthy skepticism.

The problem with many of the proposals for alternative energy sources or
new sources of power for transportation is that while the basic physics
is often sound, the benefits are too often exaggerated, the full costs
are not included, or the downsides are not explained. Sometimes even the
physics is not correct.

Often this comes from enthusiasm in trying something new. People
typically want to believe that there are alternatives, particularly if
they themselves have already spent several thousand dollars on the
technology. Sometimes it is hype to attract money for a project, or to
gain political support. On occasion it is plain deceit.

Because of the above, one needs to at least make a first pass through
some ballpark numbers to see if any claims being made for breakthrough
technology are realistic. If they seem to be a big departure from what
is currently known about the subject, then alarm bells should ring to
indicate that things might be too good to be true.

It is not always easy to find good information, however, particularly if
one doesn't understand the technology. It can also be a problem deciding
if a new technology is all it is described without some history of use.
You can't dismiss it out of hand, nor can you confidently assume it will
work as advertised. That's where the skepticism needs to come in.

So, let me see if I understand your position correctly. You believe,
that from a physics viewpoint energy recovery through regenerative
braking is marginally useful at best, that the current state of
technology cannot enable the production of useful electric vehicles,
and the emerging popularity of hybrid automobiles is just a vogue
based on marketing prowess? How far did I miss the mark?

Yes, no, perhaps, in that order.

It is simple enough to work through the calculations on regenerative
braking to see that while it does provide some savings, and is somewhat
easy to implement in the control system, it does not have a huge effect
on the economics of hybrids.

The question of whether hybrids are useful depends on what considers
useful. If an economic case has to be made, then hybrids are pretty
marginal, or even uneconomic, as Consumer Reports, for one, calculated.
That doesn't mean that the technology doesn't work, just that one
shouldn't expect a return on the additional investment. There might be
better technologies, such as turbo diesels, which are popular in Europe.

Many of the encouraging economics for hybrids early on benefitted from
subsidies provided by various levels of government. Once those subsidies
disappeared, then the economics became pretty strained.

That said, as the development of the technology continues, and
improvements are made, then the economics will also improve. It doesn't
appear, though, that with the developments now being anticipated, that
the economics will ever be breathtaking.

It reminds me of a recent news item where an inventor demonstrated how he
could ignite seawater with radio waves. He was proclaiming that it was a
possible energy source for the future. Well, what he was doing was
splitting up the oxygen and hydrogen in the water, which takes a huge
amount of energy, and the two would then catch fire. In short, the
technology might work, but the economics would be hopeless. Is it a
useful technology? maybe there is some use, but not as an alternative
energy source.

So, is the enthusiasm for hybrids a flash in the pan? Perhaps. You will
note that most auto manufacturers have not jumped on the bandwagon and
built their own hybrid designs. The Germans, as an example, seem to feel
that diesels or plug-in cars are the way of the future to help reduce
greenhouse gases or improve efficiency. Are they wrong?

The graph below shows the Tesla Roadster (upper right) in a class
by itself with better acceleration than a Lamborghini Murcielago
and twice the mile-per-gallon equivalent of popular hybrids. The
highly efficient Tesla Roadster gets the equivalent of 135 miles
per gallon with an enviable 0 to 60 time of less than four
seconds.

Sorry, but when I see claims of 135 mpg for a high performance car, when
the equivalent gasoline-powered car would only get maybe 10 or 15, my
skeptic alarm bells start to ring. It takes a certain amount of energy to
accelerate a car, and there is no magic way of avoiding it. The only
difference might be in efficiency, but there isn't enough room for
improvement for their claims to be realistic. Their claims are simply too
good to be true.

And if the GM Volt (solely electrical propulsion) ever materializes,
it is also being marketed as "energy-efficient replacements for
gasoline powered cars" with "responsive acceleration" as stated he

http://www.greencarcongress.com/2007...evrolet-v.html
Comparing the fuel costs between old and new methods of
propulsion, GM estimated that driving costs in EV mode would be 2
cents per mile&mash;or 1 cent per mile if charged
off-peak—compared to about 12 cents per mile per gallon of
gasoline for a typical car today.

As I wrote earlier, the claims of 1 to 2 cents per mile just aren't
realistic. Don't you wonder why GM wasn't able to get their earlier
electric car effort to pay off? There hasn't been any major breakthrough
that will significantly change things.

As far as acceleration, which is what this discussion was about, consider
that the Toyota RAV4-EV took 18 seconds to get from 0 to 60 mph. If it
was so easy to make a car that does it in less than 1/4 of the time, why
was Toyota's engineering so conservative. I suspect it was because of the
poor economics. Obviously, Tesla isn't that concerned about economics
with a $100,000 car, and is delivering performance for the price.

Yes, motors and conductors can be overloaded for a period of time. My
assumption is that they would normally be sized in a car for typical
acceleration and the power demand at constant speed to be economic.

But there is a liquid cooling system designed in both the Tesla
Roadster, and the GM Volt, so "upsizing" may not be necessary.

That introduces extra cost and complexity into the system. I wonder what
that does to the economics and reliability? Overall, if the vehicle is
designed to accelerate at a rate closer to the typical braking rate, then
my concerns about motor size would disappear. But improved performance
also implies lower economy.

I'm unable to provide any research about the percentage of energy
recovery achieved through regenerative braking, but it appears to
theoretically very doable given the fact that the MINI QED is not
equipped with friction brakes at all, and solely relies on
regenerative breaking for deceleration; it needs to be chocked when
parked!

Otherwise, why are ventilated disc brakes even necessary?

I could speculate: For consumer acceptance. To meet government
standards. Lack of imagination. ...

Well, according to information from Toyota, the batteries will only
accept energy up to a maximum rate of 20 kWh (sic) That's obviously a
misprint, since kW is a rate, and kWh is a quantity. They meant 20 kW.

http://techno-fandom.org/~hobbit/car...s-section6.pdf

Converting that to more familiar units, that works out to 26.8
horsepower. So, the regenerative system can only accept braking energy
at the rate of 27 horsepower. Now compare that to the typical horsepower
of an engine needed to accelerate a car from 0 to 60 in 5 seconds. It
looks like a ten to one ratio, which was my earlier guess.

I also note in the above Toyota document that they claim only 30 percent
of the braking energy is recovered. They don't go into enough detail to
know whether that is simply an average number in typical driving, or if
that is the recovery rate of the charge/discharge cycle, which would mean
an even lower percentage of total energy.

There are a couple of additional features of the regeneration system on
the Prius contained in the following discussion forum:

http://www.techno-fandom.org/~hobbit/cars/b-mode.html

It says that once the batteries are 80% charged, that no additional
energy will be accepted from the regen system. It also says that the
regen system does not work below about 7 mph due to low voltage. (there
wouldn't be much energy to recover at those low speeds in any event)

My understanding is that the vast majority of
braking power at highway speeds is dissipated as heat, rather than
being recovered as energy.

Are you able to provide any evidence of the validity of that
understanding?

I wasn't able to find a cite in a quick search, but there is lots of
documentation out there, so with a diligent search one might find
something.

However, given the info above, one can make a stab at how much energy
might be recovered.

When I drive, I typically apply the brakes such that my car decelerates
at the moderate rate of about 4 to 6 miles per hour per second. Others
might be more or less aggressive, depending on the driving situation and
their patience, but it puts things into the ballpark. A panic stop could
be up to about 20 mphps.

Now let's compare the maximum braking rate, given the weight of the car,
and the limit of 27 horsepower in braking, as a comparison.

The deceleration limit to take full advantage of regeneration at various
speeds would be:

60 mph 1.8 mphps
40 mph 2.7 mphps
20 mph 5.4 mphps

If 5 mphps is the typical deceleration rate, then an shortfall in the
above would be made up by the friction brakes on the car, effectively
converting that portion of the braking energy to heat, which is lost.

So, if I apply the brakes at 60 mph to get a 5 mphps deceleration rate,
then on a Prius, I would be capturing (1.8)/5 or about 1/3 of the
available energy in braking. At 40, it would be about 1/2 of the
available energy, and below about 20 mph, I would be able to capture all
of the energy, until the system dropped out at 7 mph.

Given that most of the energy to be recovered is at high speeds, then the
above suggests that if I drove the car, I would only recover between 1/3
and 1/2 of the available braking energy.

If someone less aggressive drives the car and has a very light touch on
the brake pedal, they might be able to capture a good proportion of the
energy.

In any event, as I suspected, the amount of energy that can be recovered
from the regenerative braking system is limited. You also have to take
into account the efficiency losses in the charge/discharge cycle, meaning
that even less of the recovered energy can be used on the power side.