<WOB> Reply to Kibort on Flywheels, part 1, PgDn if you don't car e

["Osterbrink, Mark (AFS-Warren)" <Osterbrink@xxxxxxxxxxxxxx>]

Why, Mark Kibort is giving me lessons in automotive engineering!  What's
next, 
ethics lessons from Bill Clinton?

Well, Wile E., let's grade your little dissertation on the value of reduced
mass
flywheels:

<snip>
> >>>>>>>>>>>>>>>>yes , we are talking about a pulley . Just an
example>>>>>>>>>

Most technically competent people that I know don't have a problem
differentiating between a pulley and a flywheel.  You seem to think that
they
are the same.  They are not.  I pointed out that your "4 lbs" and "12
inches"
provided questionable thickness for a disk-shaped flywheel, and suggested
alternate dimensions that produced a mass more representative of a BMW
flywheel,
based on data for stock and lightened flywheels from the Turner Motorsports
web
site. Since you referred to a flywheel twice in your explanation and didn't
call
it a pulley until I suggested it, we will have to assume that you are unable
to
correctly define the problem. My kinematics professor would have given you a
0
for the entire problem at this point (you get credit until the first error),
but
I'll be lenient and ignore this one.  We will consider both a "4 lb, 12 inch
diameter" pulley and a flywheel, although I've never heard of a lightened
pulley
being portrayed as a performance enhancement (reduced diameter yes,
lightened no
).

<Snip> 
> I would *love* to see how you figured this out.
> What assumptions did you make?  Just pulling some numbers out of the air
for
> the flywheel of 12 inches in diameter and 1 inch thick, the power consumed
in
> your example is more like 2 to 15 hp, depending on the material and speed
> range. After looking at the weights of some BMW flywheels (11 to 25+
pounds),
> I think my estimate is a little more accurate.
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>I figured this out using simple physics. 

"I figured this out using simple physics" - Wile E., you must show your
work!
How do we know that you didn't copy from the person next to you?  Perhaps
there
was a formula or two that you could show us, along with some numbers?  It is
going to be very difficult to give you any partial credit for this! -50
points.

> REMEMBER, the HP needed to accelerate an inertial load is dependant on the
> speed of acceleration.

What are you trying to say here?  That the required power is proportional to
the
acceleration rate?  Yes, but there is more to it than that... -5 points.

> Now, Im telling you the assumptions.   4 lbs , 12" diameter pulley  (sorry
,
> not flywheel, no difference though)

Oh, I'm sorry Wile E.!  Your assumptions don't completely define the
problem!
- -20 points.

> IT IS NOT 12 HP TO ACCELERATE  A 12" DIAM. PULLEY , 4 LBS OVER 1000 RPMS
IN 3
> SECONDS

Ooops! You didn't read the problem before starting your answer. I didn't say
it
was 12 hp to accelerate a "4 lb 12 inch diameter pulley" over 1000 RPM in 3
seconds. I said it is more like 2 to 15 horsepower to accelerate a flywheel
that is closer to what is installed in a BMW (one inch thick, weighing 11 to
25
lbm). I think we'll have to take off 10 points for not reading the problem.
- -10 points.
But, actually it DOES take 12 horsepower to accelerate a disk shaped, 12
inch
diameter, 4 lbm "flywheel" 1000 RPM in 3 seconds. I will give you 5 bonus
points
if you can tell us the conditions when this is true. Must be completed by
3:00 PM on Friday, January 22, 1999 to be valid.

> The numbers were not "pulled out of the air they were used as an example
that
> in a real life application , even removing a 4 lb pulley ,will not
regester on
> a dyno.  Do you understand how a chassis dyno works.

Yes, I understand how chassis dynos work.  Both types.

> you run your car over a 12 second run over a speed range of 3000 to 6000
RPM. 
> since the most acceleration  is at the peak torque area , we look at the
> fastest 1000rpm change. at 3 seconds for  a 1000 rpm change on a 300 hp
car,
> you get less than 1 HP, YES, about a 1/2HP. Now, figure that out and tell
me
> that Im all wet.    Before you go on, figure that out.

There is nothing to figure out from what you have stated in the last
(approximate) paragraph. You have not defined a problem, just tossed out
some
numbers.  - 5 points.  But, for your example, it can be approximately 1/2 to
2/3
of a horsepower, although without you showing your work, I can only give you
1 
point. But we are talking about flywheels, not lightened pulleys. There is
much
more mass associated with a flywheel than a pulley. Consequently, one type
of
dynamometer can not register the effects of mass reduction, one can. But
whether
a dyno can measure it is irrelevant. What matters is the effect on
performance.
- -4 points

> a human being can accelerate a 4 lb , 1 foot diameter mass over 3 seconds
to
> 1000 rpm.  this means it is less than 1 hp. I hp is 550 lbs in 1 second ,
> lifted 1 foot.

Ooooh! Very good.  1 horsepower also equals about 746 W, 7.457 x 10^9
erg/second
, 2544 BTU/hour, or a six pack of Rolling Rock in 10 minutes. What does this
have to do with anything??

<Snip>
> >>>>>>>>>>>>>Yes, i will be more of a factor in low gears as the engine
revs
> faster, thus increasing the HP needed to accelerated the mass.  Yes, now
how
> much is very easy to calculate. How long are you in 1st gear. 3000 rpm to
7000
> in 1.5 second or something.  Then ,the HP is much greater, but you cant
> determine this on a chassis dyno.

Depends on the dyno.
 
> Now a performance flywheel is saving how much, 12" 4 lbs???  

No, It's saving more like 10 lbm.

> do the math.

No, Wile E.  You are supposed to do the math.  You're the genius under test.
Or
are you asking me to do the work for you?  Why Wile E. that's cheating!-2
points
for even suggesting that!  -2 points.

<Snip>
> Time is everything here.  This is my point, if you are racing someone on
the
> street from 50mph to 100 mph it will take you about 10 seonds or a little
more
> . 3seconds for 1000 rpms of speed change .  this is real life.

What you seem not to be grasping here is that reducing the Moment Of Inertia
(MOI) of the engine DECREASES the time that it takes to accelerate. The MOI
of
the engine DOES NOT subtract power; it does the equivalent off ADDING MASS
that
must be accelerated, and is quite significant in lower gears (i.e. on the
street
). Also, you seem to be fixated on 50 to 60 MPH and up.  Most street contest
I
see start at 0 and end by 60 MPH, in a range where a lightened flywheel
makes a
big difference. -8 points for not getting the concepts and grasping for
straws
to defend your position. -8 points.

------------------------------

End of bmw-digest V9 #863
*************************