[SGVLUG] OT: Hybrid efficiency (was:New Linux Lug)

[Dustin Laurence]

On Mon, Feb 20, 2006 at 10:16:54PM -0800, David Lawyer wrote:

> On Mon, Feb 20, 2006 at 10:08:47AM -0800, Dustin Laurence wrote:

> > Which lists the additional weight for a Prius (a fuzzy concept since
> > there is no direct-drive Prius) as 200 lb.  A better test is for the
> > same car, and Ars Technica lists the 2003 Civic hybrid as 200 lbs
> > heavier than it's direct-drive twin.
> What about the payload?  Is it the same for both vehicles?

If I use your logic, I can put half a ton of sand in the non-hybrid and
claim that it's a valid comparison because "I'm not holding weight
constant." :-)

> "http://www.hybridcars.com/flywheels.html" Hybrid Cars - Flywheel
> Power to Increase Hybrid Efficiency says"

Ah, flywheel energy storage.  It generated a lot of page counts in
Popular Mechanics in the '80s, but still doesn't seem to be
cost-efficient.  It's too bad, it was always theoretically attractive
and I liked it.  Nice link, thanks.

It also adds considerable weight, just like batteries do.  Not the least
for armoring the flywheel enclosure so that in case of a crash or gross
failure it doesn't get loose.  I recall that you can do better on that
by not using solid flywheels (which tend to separate into a small number
of what amount to armor-piercing projectiles), but it still didn't
really make it workable.  I don't recall anymore but I think energy
extraction from

Maybe we'll see it in a practical car someday.  If sufficiently capable
it would be useful as energy storage in almost any kind of car, hybrid
or not.  That article is pretty optimistic, but then so were all those
articles back in the '80s.

The thing I like the most is that alternative drivetrains are now "out
of the bag" and pretty much mainstream.  It might even help pure
electrics eventually, but the storage problem still makes hybrids the
only solution you can reasonably sell right now.

The current rage for Priuses amuses me because I remember talking to a
member of the Caltech electric vehicle club like fifteen years ago.  I
told him that I regarded hybrids as more practical than pure electric
and he admitted as much, but said that the then-current California
regulations only counted tailpipe emissions and not the emissions from
the fixed power plant used to charge an electric.  I did, and do, think
that was inutterably stupid.  They must have fixed that at some point.

>    The four energy transformations undermine the overall level of
>    efficiency. For example, if the motor/generator operates at 80%
>    efficiency under peak load, in and out, and the battery charges and
>    discharges at 75% efficiency at high power, the overall efficiency over
>    a full regenerative cycle is only 36%, almost the same as the figure
>    Toyota quotes for the Prius II.
> > 
> > In any case, 50% is by no means "low"; it's better than the efficiency
> > of the engine in the first place.
> 
> No, the energy was created by a 25% efficient engine.  With a 36%
> conversion efficiency the overall efficiency is only 9% (.36 x .25).

Rather than point out that the calculation isn't at all relevant to how
hybrids work (when you're accelerating under heavy load there is no need
to go through the battery charging cycle at all, you drive the motors
directly), I will simply point out that it is demonstrably false because
hybrids get better mileage in real-world tests.  That would not be the
case if your calculation were correct.

> I implied that there would be a change in driving habits.  Coasting is
> for both level, downhill, and in some cases uphill.

Then in order to criticise hybrids, you must allow the hybrid driver to
drive optimally for his vehicle or you're simply positing a straw man,
which in this case would imply driving steady at your average speed (on
level ground).  On the other hand if you want to discuss the merits of
your driving scheme for regular direct-drive cars you must again do
better at the same average speed to make anyone at all interested.

> The old cars didn't need power brakes

Having owned one, sure they did.  I didn't mind driving it, but my wife
did because she's fairly small and the pedal pressure required to work
unassisted brakes was a bit uncomfortable. :-)  We could still make them
that way if we wanted, but the fact that no one does should tell you
something.

> > Consider the extra emissions and the greater number of cars you will
> > consume over your life and the emissions involved in constructing them
> > before you pat yourself on the back.
> 
> Why would more cars be needed?

This was still assuming that you meant the technique that actually might
save gas at the same average speed, rapidly cycling the gas pedal.  In
that case you'll wear out the drive train faster.  In your case that's
probably not an issue, at least until you discover the hard way which
parts need oil pressure to receive proper lubrication (not sure if this
will be a problem, really).

> > > ...Even if one coasts (on level) only until half
> > > the cruising speed is reached, KE recover is 75% since KE is
> > > proportional to v^2.
> > 
> > I am not sure what you even mean by this, but to the extent it makes
> > sense you have it backwards.  At v/2 you only have a fourth of the
> Typo.  You mean V^2.

No, I mean that at v/2 you have one quarter the KE left that you had
at v.

> As I previously explained, wind resistance would be less.

But it is inarguably made still less by driving slower at a steady
speed, thus your argument isn't relevant to anyone.  If you don't want
to hold speed constant at all then I'll just compare your scheme against
a car putting along slowly at it's optimum efficiency, no matter how
slow that is.  See how pointless such a comparison would be?

> > > True, but what about the poor efficiency of KE recovery?

It doesn't matter, because the KE recovery of a direct drive engine is
zero.  Or do you not envision hitting the brakes to stop at stop lights?
If not, then (in addition to your rather daunting legal bill) you're
optimizing for a world that doesn't exist and never will exist.

What you call recovery by coasting is simply no such thing.  You are
simply dissipating the energy more slowly, not recovering anything.  You
would have done better not to have accelerated as much in the first
place (talking highway speeds here).

> > Nonsense.  A trivial disproof: you cannot coast up to a higher speed in
> > general, so you must use the engine to speed up.  In between optimal
> > points you are working the engine both hard and suboptimally.
> Not really.  It's a little suboptimal but much less than the 64% loss
> in the regenerative braking cycle.

You're arguing a straw-man that doesn't exist.  Nobody hits the brakes
unless they *have to*, so it's not a question of brakes vs. no brakes
(which I'm beginning to suspect you might be meaning).  If the car is
direct-drive, then that energy is lost to heat and the efficiency is
zero.  If the car is a hybrid you can get a portion of it back by using
the existing drive train.  It's "free" in that sense, and must be
compared with zero efficiency in your case.  And since the car is
already moving, the efficiency must be calculated starting with the
conversion from KE to chemical energy during regenerative braking, not
the initial fuel conversion to get up to speed (since we'd have done
that regardless).  That is exactly how the web site you quote above does
the calculation, and for good reason.

> > This is ridiculous, sorry.  If you push the gas pedal to the floor you
> > are using a lot of extra gas for a small improvement in torque.

> Look at the curves.

They say that at some point before flooring it you aren't getting any
more power, so you don't want to quite tromp all the way down.  Which is
what I said.

Not as much of an issue with modern cars with a computer that, I think,
will cut back on the throttle.  On an old vehicle with a carb, well,
it's like pouring fuel on the ground.  I know, I was a foolish kid with
a carb once.  But still in my fairly modern 4Runner you actually lose
power when tromping all the way down the last little bit (the computer
throttling back, I suspect).  Then I know it's time to downshift.  This
isn't anything theoretical; I can test it any time I want, especially on
a good steep climb at highway speeds.

> the supplying of an enriched mixture for maximum power.  So if cars
> were designed only to provide this enrichment when told to do so by
> the driver, then the best efficiency would be a maximum torque (I
> think).

So now you are telling me that pushing the pedal to the floor *is so*
optimal, since we use vehicles designed for this to be the case?  Does
"tautology" mean anything to you?  Silly me, I thought we were talking
about real cars.  You can always make a statement true by re-defining
everything in it, but that doesn't mean you're talking sense.

> No, it does need to be heavier but I read that they were intentionally
> made lighter at great expense.

Sure, but for a different reason.  The target market is (well, was)
early adopters who were willing to pay extra for increased efficiency.
It makes sense that those customers would be willing to pay extra for
increased efficiency whether it's obtained by weight reduction or
altering the drive train.  You could do it with small econoboxes too,
but those are aimed at much as those with little available money as at
anyone else.  Hybrids turned out to be "sexy" enough to sell simply
based on their being hybrids, which lets you play a lot of extra tricks.

This is why I mentioned the case of things like the Civic hybrid,
because it's the same car and so we can factor out the effects of
different design tradeoffs.

> What counts for efficiency is engine torque, not torque at the wheels.

Nonsense.  Torque at the wheels is the only thing that actually makes
you go anywhere, and nobody actually cares about crankshaft torque
except to brag about it to their friends.  Or is this another
apples-and-oranges situation where you're comparing a direct-drive car
of considerably less performance to a hybrid of considerably more?  That
would again be not interesting to anyone else in the world--what people
care about is efficiency at some level of performance they are willing
to live with (I'd love to settle for the performance of a Chevelle 454
SS, but that's a different conversation altogether :-).

> > That said, the most efficient forms of transportation for a single
> > person have two wheels because of weight and wind resistance.  They
> > brake much better, too.
> Well, my old bicycle with rim brakes worn smooth wouldn't stop in the
> rain on a steep hill.

Well, I'm glad you made it out alive, but I don't see how that's
relevant.  Look at the braking times and distances of a performance
motorcycle sometime and you'll see what I mean.

> The drag coefficient for a bicycle is extremely high, almost 1.

Sure, but since the speeds are lower it doesn't matter so much.  I
don't know where you're going with that, though.

The drag coefficient isn't that interesting; it is the drag coefficient
multiplied by the frontal cross section.  That is one reason (along with
others such as the very low weight) motorcycles get good mileage, even
though few of them are really designed for efficiency (I think we call
the latter type "mopeds" and "scooters").  High drag coefficient, but
the cross section is far less than that of a car so the product is still
good.

> result is that aero drag for a bicycle is significant, even though the
> speed is much lower than an auto.  More efficient is the pedal
> railroad train (my invention)

But if you're willing to go to that kind of trouble you need to
compare not to an ordinary bike but to one with an aero shell.
Otherwise it's apples and oranges again.

> Such bike riding can be pretty dangerous.

Not nearly as bad as riding a bike on SoCal freeways, though.

> ...But I do it on an old
> 3-speed road bike.

Um, remember, I said *dirt bike*, not *mountain bike*.

Dustin