Good discussion, but I have to start off with this first...
(my little rant about current American engineering mindsets)
The reply below was a typical irrelevant response from an engineer, failing
to see the simple answer and rather relying on insignificant details to
formulate his response. This is a widespread problem with engineers being
schooled in the US today, and one of the reasons that there is an influx of
foreign engineers and an exodus of engineering jobs to foreign countries.
>From what I have witnessed, as an engineering student, foreign engineers
think on a much more simplified level. Their approaches are often more
creative and can their ideas can be developed in shorter durations of time.
American universities need to get back to basics with their engineering
curriculums, rather than churning out dull graduates armed with a great
deal of theoretical knowledge but with poor creative processes.
Anyhow the point, and I believe this thread stems from Dick Taylor's
post about Free Revs. This answer of whether revving an unloaded
engine causes harm to its components is very simple and has a very
simple answer. The answer is yes, it can be detrimental to the life
of a motor. For the simple fact that with no load present (more
detail in the following paragraphs), the engine will be able to
accelerate itself and all of its components much more quickly.
As everyone should have learned in High School physics, the faster the rate
of acceleration (or deceleration) the more force experienced by the
components in motion. Things like connecting rods and piston pins will
experience the greatest forces when being run in an unloaded state,
because they experience eight instances of acceleration within the four
operating cycles of an engine. Four times within 720 degrees of rotation,
your connecting rod is accelerated from TDC towards BDC, and from BDC
towards TDC. TDC and BDC are points of 0 velocity for the piston and upper
half of the connecting rod*. Then four times your rod (and piston and other
reciprocating components) are forced to decelerate and stop (before changing
direction) at TDC and BDC.
* for those that always have thought that the connecting rod is part
of the reciprocating mass of an engine, that is not entirely true.
The rod rotates about two axis's, the big end being the one we should
pay most attention to. The upper half of the rod, depending on the
angle of the crankshaft, is the part that essentially
reciprocates... the big end is along for the ride being attached to
the crankshaft. This complicated motion exhibited by the connecting
rod, is one of the reasons that balancing rods according to their end
weights is not entirely accurate... as some of that mass is rotating
mass in application. However, balancing rods in that manner is the
most practical method of making sure all of the rods have similar
weights.
I am less concerned about the rotating mass of the engine in an unladen
state than I am with the reciprocating components. The fact that there is
less load on the engine, means that the most simple answer lies in the fact
that the engine is now capable of accelerating itself and all of its
components much more quickly than it could if there was a significant load
it had to overcome from at rest. The major things the engine still has to
deal with are internal friction, the compression produced by cylinders in
other phases of the 4 cycle setup (compression), and the force required to
accelerate its own components (overcoming inertia).
Practically speaking, revving an unloaded engine has the same results as
increasing your redline limit. Acceleration of the components occurs more
quickly because either the engine is rotating more frequently per minute
(higher RPMs), or because an engine is allowed to accelerate more quickly
as it is unloaded or particular loads have been removed or decreased.
The old adage that connecting rods always fail on the exhaust stroke is
relatively true and is exemplified through what I said above. The exhaust
stroke signifies the period of time when the connecting rod is experiencing
the least resistance to its upwards motion - the period of time in which it
undergoing the least compressive load, but tension is very high along the
rod beam.
I cannot stress enough that thinking on the most simplistic level to
solve a problem or arrive at an answer is always the best initial
approach to problem solving. This is why Newton's original laws of
motion have lasted so long without major modification or refutement.
His motion laws are so elegant in their simplicity that they can be
used to explain very complicated phenomena very concisely.
Kai
----- Original Message -----
From: "steve bridge" <slbridge@hotmail.com>
To: <6pack@autox.team.net>
Sent: Wednesday, January 21, 2004 13:18 PM
Subject: Rotating Mass Unladen
> Hello all,
> I emailed a Professor of Mechanical Engineering and asked him about this
> subject. Of course, I got more questions, but I expected that. Here is
> what we have so far, I told hime it was a TR6 in-line motor and will post
> his reply when it comes.
>
> > As for the revving, there is indeed a formula: T=I alpha or alpha
> > (rotational acceleration)= T/I where T is the torque put on the
> > crankshaft by the pressure in the cylinders and I is the rotational
> > moment of inertia.
> > Actually, there is a much more complex formula that looks at torsional
> > vibration in the crankshaft. This is what must be considered here. I
> > have seen several broken crankshafts because people changed their
> > harmonic dampers and/or flywheels without considering this torsional
> > vibration. There could be certain RPMs where the torsional vibration
> > becomes a problem.
> > The clutch and load enter into the vibration considerations as well, and
> > thus the difference between loaded and unloaded. How was the engine in
> > consideration balanced? What type of engine is it? 4,6,8? V (90
degree
> > or 60 degree or or in-line? You raise a pretty complex question.
> >
> > Dan
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