balancing.

[mardak at cogeco.ca (Mark)]

Well said Toby...  But I don't think the "engineering talk" is going to
convince David he's wrong.  I'll try with this (very) simple analogy:

Place two identical engines on engine stands - except one has a 12 lb
flywheel, the other has an 8 lb flywheel.  Start both engines up and run
them at a constant simulated cruise speed of 4000 rpm.  Both engines
have NO load on them.  Now picture the amount of work the engine has to
do just to keep it spinning at 4000 rpm - there's friction from the
rings on the cylinder walls, friction at each bearing point, the crank
sloshing through the oil, all kinds of frictional losses in the
valvetrain + work to overcome valve spring pressure etc. etc. (try
turning a cam shaft with a wrench, then try to imagine how hard it would
be to spin it at 2000 rpm!)  There is a substantial amount of energy
needed JUST TO turn all these parts at 4000 rpm and convert up and down
motion to circular motion.

In comparison, how much energy is needed to keep JUST the flywheel
spinning once it's up to speed - whether it weigh 12 or 8 pounds, not a
hell of a lot.  We're talking fractions of a single horsepower here,
compared to a healthy percentage of the total output of the engine just
to keep everything else spinning at 4000 rpm...  Don't forget we're
talking about engines on stands here with no direct load other than
"internal losses"...  So EVEN IF your (David's) scenario were true
(which I believe it isn't) there is only a tiny fraction of a single
horsepower difference between keeping the two engines spinning at 4000
rpm with no load.  Now how in the hell is that going to make ANY
difference when those engines are in a car that's trying to poke a big
hole through the air at 75 mph at 4000 rpm.  The energy that's required
to push the car at a steady 75 mph is ASTRONOMICAL compared to what's
needed to keep an 8 or 12 lb, twelve inch diameter disk spinning.  There
is just NO WAY that a 4 pound difference in the flywheel is going to
make even close to a measurable loss in fuel mileage, on a stand or in a
car...  

Sorry for the length, it was much more concise in my head!  :)

Mark.
80 S
81 S  ABA/JH/4K - with a power robbing 8.5 lb flywheel  :)     

> -----Original Message-----
> From: scirocco-l-bounces@scirocco.org [mailto:scirocco-l-
> bounces@scirocco.org] On Behalf Of T. Reed
> Sent: October 31, 2003 9:38 PM
> To: mr.utility@highstream.net
> Cc: Dan Bubb; scirocco-l@scirocco.org
> Subject: Re: balancing.
> 
> 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
> 
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