[Farmall] Farmall 504 diesel engine

[soffiler at myeastern.com]

Hi Dean!

I'll take a crack at this.  I may jump around a bit between
single-cylinder and 4-cylinder inline engines for the sake
of illustration of the concepts.

>From the standpoint of engine dynamics, there are at least
two types of imbalance (actually there's more but these are
generally the most significant):  primary imbalance and
secondary imbalance.

Primary is the basically the mass of the pistons moving
linearly.  For example, a single-cylinder engine attempts to
counteract primary imbalance with weights on the crankshaft,
so, as the piston is travelling upward the weights are
simultaneously travelling downward.  If the weights are
worked out just right, it can have perfect primary balance
at TDB and BDC.  Everywhere else is a compromise because the
piston slides linearly while the crankshaft weights are
rotating.  Single-cylinder engines shake as anyone who's
pushed a lawnmower knows.

To counteract primary imbalance, you can add a second
cylinder in-line such that one piston moves upward while the
other moves downward.  Primary balance is now satisfied but
a couple of new problems are created:  one, the engine now
wants to "rock" (hopefully you can picture this) and two,
due to the 4-stroke cycle, which requires 720 degrees of
crankshaft revolution to complete one cycle, you are going
to get TDC on each cylinder just 180 degrees apart so you
get bang-bang-zip-zip bang-bang-zip-zip and so on.  Does
"John Deere" come to mind?

To fix the problems mentioned below, you can lash two of
these two-cyinder engines together in-line, and timed so
they cancel the tendency to rock and to fill in the two
"zips".  End result:  inline four-cylinder has perfect
primary balance.

Which brings us to secondary imbalance.  This is basically
created by the mass of the connecting rods, which follow an
odd path - the bottoms are rotating while the tops are
moving linearly.  The center of mass of the conrods is
following an odd, oval-ish shape. The conrods' center of
mass reach their maximum outward swing with the pistons
exactly at mid-stroke, in other words the crankpins at 90
and 270 or horizontal if you prefer.  You could say, that
like the single cylinder that has perfect primary balance
only at TDC and BDC, the inline four has perfect secondary
balance only at mid-stroke.  Of course the inline four does
have "matched" pairs, where for every conrod's motion, there
is another whose motion is opposite.  The issue that gives
rise to secondary imbalance is that oval shaped motion.  If
it was circular, the inline four would have perfect
secondary balance too.  But being oval, it turns out that
everywhere except mid-stroke there are imbalance forces
created by the ovality.  I'd need to draw out a bunch of
vectors to explain this any better, so, I hope I'm doing it
adequate justice with words alone here.

It turns out that if you do plot out the imbalance forces
you find they are rotating around that oval at twice engine
speed.  Thus cancelling them out requires a counterbalancer
that is rotating at twice engine speed.

Why would the diesel need a balancer while the gas engine
does not?  I don't know for sure but I suspect it is related
to the fact that the diesel uses a longer stroke and heavier
components which does two things:  the oval is bigger, which
alone makes the imbalance forces bigger, and on top of that
the heavier conrods make the imbalance forces even bigger
still.  This diesel engine design must have crosses some
fine line, and the decision was made to add the
counterbalancers.

Hope this makes some kind of sense.

Best regards,
Steve O.


From: Dean Vinson <vinsond at voyager.net>
 
> Just out of curiousity, does anybody know why the balancer
> wasn't used  on the gasoline engine?  My loose and pretty
> un-schooled understanding  is that four-cylinder engines
> are inherently unbalanced at higher RPMs  due to the
> differing travel distance of the connecting rods on the 
> upstroke vs downstroke, or something like that, while
> six-cylinder  engines are inherently balanced.  But as I
> think about it, it seems the  connecting rod physics would
> be the same regardless of number of  cylinders.  And
> either way, assuming the diesel and gas engines are both 
> four-cylinder, I'd have thought they'd both have an equal
> need for a  balancer.
> 
> Dean Vinson  --  Dayton Ohio
> <http://my.voyager.net/~vinsond/>
> 
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