I didn't say that very well--here's another try. First of all, for ANY real
engine, at 90 degrees of crank rotation the piston is NOT at half of the
stroke. The stroke curve is a sine wave (or more accurately, a cosine wave.
The only way you'd have a piston halfway down would be to have an infinite
length rod. For practical rod lengths the difference in rod angle at 90
degrees is small, but at TDC/BDC even this small ratio difference has a
relatively large effect per degree of rotation, so even a relatively small
change in the ratio of rod length to stroke has an effect on acceleration.
Understand that I'm talking through my hat--I haven't taken the time to do
the math again. I looked at this a long time ago, and resolved it to my
satisfaction. Doesn't mean that I was right then, or that I'm right now. And
I'm too lazy to do it again.
As my calculus teacher used to irritatingly say "I leave the proof to you as
an exercise".
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Bill Babcock
Sent: Tuesday, June 14, 2005 8:21 AM
To: 'Richard Taylor'; 'barry rosenberg'; triumph_marx@freenet.de; 'fot'
Subject: RE: 225hp
In a word, no. At ninety degrees the piston is not necessarily halfway
through the stroke.
The effect is small for small differences in rod length, but the vertical
component of travel is equal to the cosine of the rod angle times the
stroke. For large angles, even with an infinite length rod, the rod angle
changes quickly so the piston speed is almost unaffected. But at small
angles (BDC, TDC), the difference is large, meaning the acceleration of the
piston changes substantially.
It's all more than a bit academic--long stroke engines like Triumph motors
don't gain much from rod length changes and would undoubtedly require a lot
of other changes to benefit from increased TDC dwell. In a short stroke
motor you can change engine characteristics substantially with rod length
because the ratio of rod length to stroke is bigger. Short stroke two stroke
motors get really weird because rod length changes the open duration of the
ports, which is the same as changing cams.
-----Original Message-----
From: Richard Taylor [mailto:tarch@bellsouth.net]
Sent: Monday, June 13, 2005 5:44 PM
To: Bill Babcock; 'barry rosenberg'; triumph_marx@freenet.de; 'fot'
Subject: RE: 225hp
Bill,
I still can't see it.
We agree that at TDC and BDC connecting rod length makes no difference in
piston stroke length. At 90 degrees crank rotation the piston is half way
through its stroke no matter what the length of the connecting rod. So at
the cardinal points there is no difference made by rod length. Consequently
the mystery durations must happen between the cardinal points. Right? Now
let's look at say 10 degrees of rotation of the crank shaft. The length of
the vertical vector at the crank must equal the vertical vector at the
piston no matter what the length of the rod, right? Inasmuch as there are no
horizontal components to the movement of the piston, there are no resultant
vectors other than vertical. Therefore for each vertical increment of
movement in the crankshaft there is an equal increment of vertical movement
in the piston, no matter what the length of the connecting rod.
Barry's comments this morning about the increase in internal tangent forces
for the shorter connecting rods and decrease in weight both make sense to
me. But he, too, subscribes to the "lingering piston duration" created by
shorter connecting rods at the TDC.
I appreciate you patience. Just remember. I'm the guy who put his clutch in
with the bold markings "this side to flywheel" facing the transmission.
Richard
A. Einstein taught us that our old three dimensional world can be (is)
distorted by the forth dimension of time. I just never expected to find
evidence of it in my old TR motor.
|