Re: Driving cars hard (high rpm's)

[Jason <jason@scirocco.org>]

At 10:45 AM 8/25/98 -0500, Ilias Glavinas wrote:

>Being electrically but not very mechanically inclined, I would love for 
>someone on the list to tell us why the forces increase exponentially as 
>opposed to linearly, or whatever else. 


Oh, it's one of those E=MC^2 things or something.  ;)

No.. seriously, it's been waay too long since I've taken a physics class...
but I will do my best. 

Your piston has to go from a complete (albeit instantaneous) stop at BDC to
a stop at TDC, right?  Because of the way the piston is connected to the
crankshaft, it has to accelerate to the speed of the crankshaft by the time
it's 1/2 way up, and then stop by the time it gets to TDC.  Since this
distance remains the same (unless you throw a rod, hehe) the acceleration
will increase exponentially as the crankshaft speed increases.  Why?  Oh,
there's some formula- I forget it- s= v1t + 1/2 at ^2 or something like
that.  ;)  The premise of the elusive formula is that, if you have to slow
down by so much in a certain distance, and you 1/2 that distance, you
quadrouple the force.  (That's why there's such a big difference in crashes
at 30mph vs 35mph (1/3 more force)).  So, in my infinite wisdom (insanity),
I mentally screwed around with that formula and came out with the idea that
the acceleration of the pistons would have to increase exponentially as the
engine speed increased.
	Damn, I wish I could remember.  Anyways, doesn't matter... someone on the
list will yell at me and give the formula.  And since each piston's
acceleration is increasing exponentially, the force exerted on the piston
is doing the same (since F=MA)... See? I remembered *something*.  

	An interesting aside for those who speak German and who have read Auto
Motor und Sport- in the Test Results of all of their Road Tests, they
mention "Mittlerekolbengeschwindigkeit", (or something to that effect),
which is the average speed of the piston (I think it's given at the torque
peak).  Using that calculation, you get a good idea of engine longevity.
Engines with lower piston speeds (long stroke, low revving engines,
generally, like american V6s) have a greater life expectancy than high
piston speed engines (short stroke, high-revvers, like BMWs, for example).
(Of course, design and materials & build quality will make the biggest
difference- but this makes a great case for BMW- with high-revving, short
stroke engines, they still outlast most... especially the low-revving,
long-stroke american engines.  That says a lot for materials and build
quality.)
	Anyhoo, I'm rambling.
	Reactions?
		Jason


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