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

[Dustin Laurence]

On Sun, Feb 19, 2006 at 07:23:48PM -0800, David Lawyer wrote:

> On Thu, Feb 16, 2006 at 04:35:09PM -0800, Dustin Laurence wrote:

> > Ah, yes, I imagine all the happy hackers writing C code to decide which
> > LUG meeting to go to, or at worst using something like octave.  Cool
> > world. :-)
> 
> There are a number of shortest-path codes available.

Grandmother.suck.eggs.  In any case, it was one of those "joke"
things....

> problem is better called the "nearest neighbor" problem.

I have a "nearest neighbor" problem of my own. (another joke)

> > It's much more effective to simply not open the door for anyone who
> > isn't riding a moped. :-)
> Aren't they a bit polluting :-).

The point is that they are very fuel efficient, and you can't reduce CO2
with a catalytic converter.  (Though, hmm, I wonder if there is another
chemical process--it's hard to crack CO2 though, and not much available
to combine it with.  There's nitrogen, but then you get cyanide....)

But again, you mistake a joke for, well, !(joke).

> > No.  It cannot *ever* do so for engines tuned for their respective jobs
> > and a hybrid that is meant to be the real thing (and not the "hybrid
> > assist" stuff some are apparently designed for).

> ...First, the hybrid engine must be
> able to achieve high speed on climbing mountains, in which case there
> will not be nearly enough energy stored in the battery for this.  So
> the engine size of a comparable non-hybrid only needs to be a little
> larger.

No, you just don't understand the tradeoff.  In simple terms, you can
maintain the same peak power in a smaller engine if you narrow the power
curve.  The downside is that you have to run the smaller engine in a
narrower RPM range to get that peak power--something that a hybrid can
do but a direct drive engine cannot.  The effect can be quite
dramatic--I have seen the curves in engine design textbooks when I was
an undergrad.

The truth is that a reciprocating engine is not the best choice for an
advanced hybrid.  Turbines reach better thermodynamic efficiency, and
the reason we don't use them yet is because if very high initial cost
and very high operating temperatures (in fact the latter is where a lot
of the efficiency comes from, so you can't reduce it).  The downside of
a turbine is that it wants to be run nearly flat-out and just won't
accept a wide range of RPMs, so they aren't very good for direct-drive.
But the power storage of a hybrid means you can run it at it's most
efficient point to charge the batteries and not otherwise.

I seem to recall there are a number of other problems with turbines, but
I think the chances are good that a fully evolved hybrid would use some
form of one.  The problems really relate to their use in a new
application (passenger vehicles), since in other areas turbines are a
very mature technology.

Until we have them, the most efficient practical choice I am aware of is
a highly tuned hybrid which runs it's engine at it's optimum RPM.

> ...The weight of the batteries and motor-generator tend to make
> the hybrid heavier.  Hence more rolling resistance and a little more
> aerodynamic drag since the body must be slightly larger.

Most of the drag at highway speeds is wind resistance, and the
difference there is essentially zero: notice that some cars can be
purchased in either configuration.  It is the same body shape and size,
thus the same drag.  In fact, probably the only effect on drag would be
underbody airflow (smoother is better), and there is a good chance that
a hybrid is better there (I suspect less ventilation is needed).  I
think I'll just look under a Prius the next time I see one.  Better
would be to go to a Honda dealer and compare a Civic hybrid and a
regular Civic side-by-side.

At city speeds rolling resistance is more of a factor but the additional
rolling resistance due to weight is again negligible.  A random web
search turns up this:

http://reviews.cnet.com/5208-7810-0.html?forumID=77&threadID=104113&messageID=1194686

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.

http://arstechnica.com/reviews/other/2003-civic-hy.ars/1

In other words, the drive train weighs about as much as an extra
passenger.  Just not a big factor, as anyone who has recorded the
mileage of his car for a while knows.  And low-speed driving is mostly
stop-and-go in actual practice, where the additional weight is requiring
additional braking, so the difference will be even smaller for the
hybrid.

> But worst of all is the poor efficiency of regenerative braking.

No, it is not "worst of all," because the direct-drive engine has no
counterpart.  If you will, the efficiency of regenerative braking for a
direct-drive car is zero, and it costs very little to add it to a hybrid
because the drive train is already there and electric motors are easy to
run as generators.  In fact it's nearly free.

As for it being "low", this is far from the case.
http://www.greencarcongress.com/2004/09/under_the_hood_.html

Lists the Accord hybrid as achieving 95% of available kinetic energy
into electricity, an improvement over the Civic's efficiency of 93.5%.
I challenge you to make a coherent case that this is "low."  It then
has to be converted back into kinetic energy, but electric motors are
good at this and it's no worse than the situation with newly generated
electricity.

> Thus the efficiency of recovery of kinetic energy of a vehicle via
> regenerative braking is poor.  Perhaps only 50%.

Reference, please, or it shall be disregarded by the court.

In any case, 50% is by no means "low"; it's better than the efficiency
of the engine in the first place.

> with a non-hybrid if one drove optimally to conserve energy?  How
> does on recover kinetic energy for a non-hybrid?  By simply coasting
> with the engine off.

This is a false comparison on many grounds.  First, you are positing a
significant change in driving habits for the direct-drive car but not
for the hybrid.  One can also save energy in a hybrid by driving more
sensibly, especially at highway speeds where so much energy goes into
pushing air.  In fact, one can do the same trick with a hybrid.  In
fact, one does; the car essentially does it automatically because the
engine need not kick on to recharge power that is not being used.  A
hybrid is better at this by design than your direct-drive car.  Also it
only does it when it helps (when you want to stop already, or going down
a hill), unlike what you apparently intend (which seems to be on level
ground).

Second, your suggestion is not a realistic method.  It is dangerous,
because you lose power steering and (after maybe a couple of pedal
presses) power brakes.  If you accidentally turn the key all the way to
off, you may also lock the steering column, which would be disasterous.
And finally, if you do this often enough on, say, long hills, you will
probably lose lubrication in the drive train.  There is a reason that
you are not supposed to tow a rear-drive car any distance without
disconnecting the drive shaft.  I'm not sure of the situation for a
front-wheel drive car, however; towed the front wheels are generally off
the ground whereas in your scenario they are freewheeling, so for
front-wheel drive we can't use the simple towing directions.

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.

> ...This way one recovers all the the kinetic
> energy (=KE), not just 50%.

This is absolutely untrue.  You still lose it as heat, just at a slower
rate and not into the brake pads and rotors.  Then you have to re-start
the engine (the most polluting thing you can do to a normal car, though
not so bad now that the engine is warm) and accelerate back to speed
(with attendant fuel cost).  Further, wind resistance will be higher
than driving steady at the same average speed, because of the v^2 nature
of fluid drag in this regime, and thus you need still more fuel.

I'd be interested to watch you do an actual test of this idea, but you
won't be abusing my car to do it.

> ...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
original KE left.  This is *NOT* "recovery," but if it were the
efficiency would be 25% not 75%.  If you mean that you "recovered" the
three-quarters of the KE you wasted into heating the roadway, drive
train, and (mostly) air, then you are very confused.

> > Non-hybrids must have a broad power curve, which greatly lowers peak
> > efficiency.
> Not at all, since the driver only powers the car at the points near max
> efficiency and most of the time coasts with the engine off.

This jackrabbiting is going to really kill your performance, as I said.
But beyond that your argument is truly fallacious.  Drivers cannot
reliably do this.  If they could, it wouldn't matter, because as I said
above the engine has poor peak efficiency to maintain drivability.  No
production car is made any other way.  If you are positing a new car
designed for this sort of driving, then you should get rid of the
driver's judgement and let the computer start and stop the engine
(basically what Cadillac years ago tried to do with individual
cylinders, which was a disaster, and what Honda at least does now with
greater success).  But if you can do that, I'll design my own hybrid for
the average speed you make and greatly exceed your performance because
I'm not jack-rabbiting and wasting a lot of extra fuel in wind
resistance and starting.

> > A true hybrid, by comparison (one where the engine generates power
> > only and is not directly connected to the wheels), can have an
> > engine tuned with a very narrow--and thus very highly peaked--power
> > curve and run at the most efficient point regardless of vehicle
> > speed.
> True, but what about the poor efficiency of KE recovery?

First, it doesn't exist, and second, it's irrelevant to the tuning of
the engine.  Everything I said about the engine is true if you
programmed the computer not to do regenerative braking.

> > This is why, in fact, there has been interest in continuously
> > variable transmissions--you can get a similar benefit without the
> > electric drive train.  
> 
> You really don't need CVT if you shift at the proper points and coast
> at the optimal points.

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.

> ...Car's aren't designed for this, especially
> ones with automatic transmission.  They need to be redesigned.

And will never be as good as a car designed properly, for the physical
reasons I mentioned above.

> ...When I
> try to get max efficiency by pushing the gas pedal to the floor, the
> AT shifts into a lower gear, decreasing efficiency.

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.

> > However, you still don't get the ability to run a smaller engine
> > that only needs to satisfy the average power demand (letting the
> > batteries take up the slack).  Non-hybrids, even with a CVT, must
> > size the engine to the *peak* demand, and this hurts average
> > efficiency.
> They can also just not supply what you call "peak demand".

Then the corresponding hybrid also can have the same peak demand and a
correspondingly smaller engine.  You can't win that race because every
improvement you make can also be made by a hybrid, for the same reasons.

> At any rate I think that *if* one had a non-hybrid car that could be
> fully controlled (so as to operate at full throttle in high gear)

You make think so, but it isn't so.

> that traffic, signals, and laws were such as to promote coasting,

Ever heard of light sequencing?

> *if( non-hybrids were designed to have bodies lighter than hybrids,
> then such non-hybrids would be more energy efficiency *if* driven
> optimally.

And if they drove at a quarter the speed of the hybrid, they would also
be more efficient.  Both are apples-and-oranges comparisons.

> ...Some of the energy saving in hybrids is that they use more
> lighter weight aluminum etc.  So in any comparison, one needs to
> design a non-hybrid this way too.

First you argued that the hybrid had to be heavier.  Have you changed
your argument?

You can by an Accord and a Civic in either configuration, for starters,
and the hybrid is slightly heavier (according to the web reference
above) and still gets better fuel efficiency.  So this is not going to
help you enough.

> I think auto manufacturers should be required to provide efficiency
> curves for their engines so that drivers can select efficient
> operating points.

Most drivers can neither select efficient operating points nor
understand the efficiency curve.  However, I've driven cars that tried
by urging you to shift early to keep RPMs low and efficiency high rather
than late to keep up torque.

> ...Why not a display on the dash that would show
> efficiency?

I believe a Prius has this (Tom can tell us for sure).  Still sure you
don't want a hybrid? :-)

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.  Pity that it's so very, very hard to walk away
from an accident.  I wouldn't mind riding a motorcycle but I do intend
to live to see Eric grow up and I also don't intend to encourage him to
do something that would also tend to lower the liklihood of that
happening.

Hey, maybe we should buy dirt bikes or something.  At least there if you
hit a tree it's pretty much your doing, not some joker who ran a red
light.

Dustin, contemplating the interesting idea of a hybrid motorcycle (but
        where to store the electricity on such a small vehicle)?