balancing.

[treed2 at wsu.edu (T. Reed)]

I'm gonna jump in here, against my better judgement..

> > So, if I understand this correctly, greater inertia helps you to maintain a
> > constant speed better and because of this your throttle is more constant and
> > therefore you get better mileage?
>
> To be more specific, this should read ---"greater inertia helps you to
> maintain a constant speed more easily, and therefore if your throttle is
> constant, the inertia of a weighted flywheel will be higher than a lilghtened
> flywheel, therefore more mpg."

This is similar to the argument that has come up several times on the
vortex - that cruise control hurts fuel economy because it is constantly
modulating the throttle, trying to maintain a fixed speed through varying
terrain.

My personal opinion is that it is in poor taste to make broad
generalizations about the cruise control issue -- everyone's driving style
is different and an conculsive scientific analysis is not really possible
with so many variables. "it depends" would be my answer.

I do agree, however, that modulating the throttle expends more energy
than holding it constant over the same amount of time and the same stretch
of road.

So as I understand it, David's argument is that the lumpier idle resulting
from a lightened flywheel results in modulation of the air flow plate (ie.
same effect as modulation of the throttle) and a decrease in fuel economy.

I would tend to agree with this if the idle was excessively lumpy.. but
I have a lightened flywheel and my idle is pretty close to rock solid.
If other people have a rough idle it is my /opinion/ that it is due to
something else in combination with the flywheel.

I'd say this falls under the 'negligible, if any' category. Kinda like
that magnetic fuel saver thing.

Furthermore, the lumpy idle only occurs at.. well, idle. So whenever
you're moving at a constant speed, the air flow through the engine stays
as constant as the position of your accelerator pedal.. regardless of
flywheel weight. At a constant speed, there are no acceleration forces
acting on the flywheel or your car, just frictional ones. Mass only
influences acceleration and deceleration, not constant rotation. There is
a negligible effect on frictional force due to the difference in mass.

I'll stop here since I'm kinda repeating what Dan already said.


Don't take Dan's arrogance personally. In the world of engineering,
mistakes often mean people will die, and it will be 'your' fault. Think
about all the things in your daily life that were designed by engineers
that could kill you if they were designed improperly-- cars, airplanes,
missile silos, power transmission towers, electrical appliances, bridges,
dams, nuclear power plants, etc. In the engineering world, mistakes are
not tolerated(*), and when they happen, fellow engineers are quick to
point them out.

(*) = this sounds a little gung-ho. mistakes do make it in to many
designs, but you get my point.. it's essential that there be as few as
possible.

There is so much misinformation circulating in the non-engineering world
about engineering topics that I think it begins to drive engineers a
little crazy. Imagine going to a car show full of Honda drivers where
everyone is saying "Sciroccos are slow and heavy, they don't handle well
and they always catch on fire when they crash". You'd probably feel
compelled to speak up and try to correct them. It's the same thing going
on when people pipe up on the list to correct those who make misinformed
statements. Personal attacks they are not..

> > flywheel may represent a significant amount of the rotational inertia of the
> > engine, but not a significant amount of the inertia (both linear and
> > rotational) of the whole car, especially in the higher gears where the engine
> > is only turning 3-3.5 times faster than the wheels.
>
> True, but the whole car does not push itself along, the engine does...  And if
> you improve the ability of the engine to maintain a given speed, then IF it
> maintains a given speed, the efficiency is higher...  BTW, the whole car does
> not have any rotational inertia to speak of, unless it is rolling end over
> end...  :-)

The car *does* 'push itself along', it has mass and speed.. therefore
forward momentum. At a constant speed, the engine only generates force
to counteract the frictional forces that would otherwise slow the car
down. The net force on the car at a constant speed is zero.

> > >From a purely physics point of view, baring frictional effects, if a mass is
> > at a constant velocity (linear or rotational) it takes no energy to maintain
> > that velocity.
>
> Do you engineer rockets or something?  We are on earth, where friction is
> king, and gravity its' gueen...  If that were true, I could coast from
> Oklahoma City, to Dallas without any trouble...  Even if I do not include
> friction, there is still gravity, prevailing winds, etc...  I am speaking of a
> real world scenario here, or trying to...

It is standard scientific practice to abstract the real world to the ideal
(in this case frictionless) case in order to better understand the forces
at hand, THEN make adjustments to model the real case.

> Clearly it takes less energy to accelerate a lighter mass, but
> > to maintain a constant speed requires no energy input so there is no
> > difference between a light flywheel and a heavy one.
>
> Ummm, like, what?!  We are talking about cars, not space ships...  It does
> take less energy to accelerate a lesser mass, which is why we are talking
> about all this mess...  To maintain a given speed DOES take energy, as this is
> the real world we live in, not space...

To maintain a given speed in and of itself does not take energy. To oppose
frictional (ie. surface friction as well as fluid and air resistance),
magnetic, gravitational forces, etc -- energy is needed.

> An engineer of what?!?  Perhaps instead of thinking about cars, you should

It really doesn't matter what kind of engineer he is; if he went to an
accredited school and received an engineering degree then he has more than
just an idea of what he is talking about. Engineering school forces you to
abandon 'conventional wisdom' and develop olympic-athlete-like critical
thinking skills.

[ Aside:  Critical Thinking - The disciplined ability and willingness to
assess evidence and claims, to seek a breadth of contradicting as well as
confirming information, to make objective judgments on the basis of well
supported reasons as a guide to belief and action, and to monitor one's
thinking while doing so (metacognition). The thinking process that is
appropriate for critical thinking depends on the knowledge domain (e.g.:
scientific, mathematical, historical, anthropological, economic,
philosophical, moral) but the universal criteria are: clarity, accuracy,
precision, consistency, relevance, sound empirical evidence, good reasons,
depth, breadth and fairness.  ]

> instead work on them , and experience a few things...  You seem a bit too far
> removed from working on them daily, as I do...  And are you 'implying' that
> because you are an engineer, and I am not, that I am wrong?  I may only have
> two diplommas on my wall, but let's face it...  The most novel invetions come
> from those who were not trained to 'know better'...

Working on cars, while fun and definitely educational, unfortunately does
not make you any better at understanding the physics behind them. Let me
clarify as that sounds a little too broad - by physics I mean the mpg
issue we're discussing. I'm not talking about the motion of pistons and
valves.. as there is certainly no better way to understand them than to
work on an engine. As we know from reading the Bentley the layout of
things is much clearer when they're actually in front of you and not just
on the page of a book.

> I applaud you on that, if that is what you are attempting to do...  I just
> hope that you are looking for the 'truth' in its broadest terms, and not just
> in yours...

Engineering is not meteorology.. when applied correctly, it's about as
close to 'truth' as you can get. Scientists sometimes get to ignore the
real world, but it is the playpen in which all engineers are locked. They
don't get to design and build things for an "ideal" world!

My 2 cents.. please take note that this e-mail is meant to have a
light-hearted, informative tone. I'm not trying to instigate anything
here.

Long live VWs!

-Toby