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

[Dustin Laurence]

On Tue, Feb 21, 2006 at 01:17:23PM -0800, David Lawyer wrote:
> On Mon, Feb 20, 2006 at 08:00:35PM -0800, Dustin Laurence wrote:
> > On Mon, Feb 20, 2006 at 03:59:35PM -0800, David Lawyer wrote:
> 
> You're of course right if we require the same avenge speed.  But as a
> practical matter, one can't use long coasting periods and maintain the
> same average speed without significantly exceeding the speed limit.
> So I'm comparing apples and oranges in a sense.

No need to qualify; in *every* sense you are comparing apples and
oranges.  It amazes me to find that you are advocating something that
pretty much nobody but you cares about, and I don't see the point of
discussing it.  Don't be surprised when no one cares *at all* about a
driving method that can be bettered by the simple, age-old advice to
*drive slower*.  You have to do better than *drive slower* for your idea
to be of any interest.

It would also be good for it not to be liable to get you pulled over on
suspicion of drunk driving, as oscillating between v and v/2 is likely
to.

> Again, inspect the curves and you'll find that it is nearly as
> efficient to operate at widely varying rpm.

Only because you are not looking at the curves of engines designed to
operate at one optimal speed.  Such engines are tuned with a very narrow
power band.  One of the big reasons that spark-ignition engines are
dominant for automobiles is precisely that they allow a wider power band
than diesel.  That is from an engine design textbook (which also
mentions that the number-one overriding design criterion for automobile
engines is low initial cost, sadly, not ultimate cost or efficiency).

> torque and then coasting doesn't abuse the machinery.

I spoke of the technique that actually *does* save gas, which is a much
more rapid oscillation so as to maintain nearly constant speed.  Your
way doesn't do it often enough to be much of a problem (that is just a
guess), but I wasn't speaking of that because it's irrelevant and
doesn't do anything anyone else cares about.  And it introduces new
problems; for example the catalytic converter isn't up to temperature
all the time, which introduces more pollution (and I think lowers
efficiency, but I'd need to check).

And you have to figure your periodic roadside drunk driving tests into
your average speed. :-)

> I'm not comparing them at the same average speed.

Perhaps I am guilty of trying to find an interpretation of your idea
that makes some sense to other people's driving habits.  Again, I can't
think of an idea less relevant or practical, since (at least at highway
speeds) it can be bettered by a simpler, safer, and easier technique:
*slow down*.  It also can already be automated by many modern cars: just
set the cruise control to avoid wasting gas with needless speed changes.
In fact, since I'm something of a leadfoot I do this if I am not in a
hurry just to keep the truck's speed down to 65.  I get a couple more
MPG if I set it there but if I did it manually I'd usually find myself
creeping up and have to continually slow down again.

I'm getting better mileage than your idea *and* I'm more comfortable.
Sounds like a win-win situation to me.

> ...No need for me to
> read your derivation since I'm well aware of this principle.

Too bad--I was kind of looking forward to dusting off my calculus of
variations and showing that Lagrange's equations for this case reduce to
something like dv/ds = 0. :-)

> The problem with doing it rapidly is the loss of and angular momentum
> of the engine and the need to create this momentum again when starting
> up the engine.  All this is energy wasted.  So long coasting phases
> reduce this waste.

You really don't get it.  Holding the average speed constant is the
heart of the problem to anyone but, I am amazed to find, you.  It's fine
for you to have your own special way to slow down, but if you don't hold
the average speed constant then I can't for the life of me figure out
why someone would care.  I'd only care if I were behind you on the
freeway, and then I'd get around you as quickly as possible before you
caused an accident.

Actually, that is a very important consideration I didn't even think of:
your effect on other traffic.  I shudder to think how many people you
slow down by your system.

> Thanks for discussing this with me.

You're welcome, but I find we were discussing two different problems.  I
just can't work up much interest in yours, I'm sorry.

> ...I think I'll write an article
> named "The Case for Coasting" and explain this better.  I can't just
> say to recover KE by coasting without explaining it in detail.

But again, if you explain what you do mean you lose all your readers.  I
don't know where you want to publish, but no reviewer would approve of
it if you don't explain why your idea is better than *slow down*.

> So it implies that hybrids need to coast too in order to improve their
> efficiency.

No, it implies you have the wrong constraints.

> question then remains: Would it same enough energy to justify the
> increased cost of the hybrid?

That's an interesting question regardless of the driving technique
involved.  Be sure to include the social cost of making Greenland the
Bahamas of the Arctic in your calculations. :-)

Dustin