Peter---Yes, engines tend to stop in the same position, when switched
off. This does not mean that it always finds only one timing mark or
spot in a 720 deg. rotation, but rather has a stong tendency to stop at
one of three areas on the FLYWHEEL. (on a six engine. One of two areas
on a four banger)
Because the crankshaft rotates twice to complete the cycle, i.e.
pistons #1 and #6 share the same spot on the flywheel & ring gear when
firing. Or when stopping, as in this example..
This engine stopping in the same places would not be true if the
sparkplugs were removed, of course. When the rotating parts connected to
the crank coast to a stop, whichever piston in its turn has the
compression will bring things to a halt. (With the ign. key off and/ or
nothing to sustain combustion.)
I believe the piston that "stops" the engine squeezes the air above it,
and probably gets pushed back down its bore a bit. Anyone who wants to
witness this backing up could probably do so with a movie camera trained
at the fan blades.
I tested the crank position thingy today, and found the following: When
stopping the engine and marking the timing pulley < with chalk, (on 12
different events) the engine stopped within a couple of degrees of the
same mark six times. Five of the six were just after #5 cylinder had
fired, and once after #2 cylinder had fired. Both of these cylinders
share the same place on the flywheel and ring gear teeth.
Each time I stopped the engine I also removed the distributor cap and
noted the rotor/firing position. Had I made this stop test enough times,
there's no reason to think the other pistons wouldn't have showed up
equally.
Meanwhile, the other six times the crank stopped were just after #1
fired (twice), or after #3 or #4 fired. (four times)
Again, we know these last two pistons share the same spot on the
flywheel when firing.
A recent compression check on my particular engine showed them to be
within five pounds of each other. (a weak cylinder or two could cause a
discrepancy with the above "stop test". Changes to ones compression
ratio or camshaft could also move the numbers around a bit, but I think
the engine would stop very close to the same spots.
Finally, this whole exercise was more to show that moving the dowel pin
hole 90 deg. in the flywheel should move any worn ring gear teeth to a
new spot when the starter motor is first engaged. Tick Tock, Berry?
Dick
From:
plz@shaw.ca(Peter Zaborski) Date: Mon, May
From: Sally or Dick Taylor
Sent: Monday, May 21, 2007 12:37 PM
(Engines nearly always stop in the same
crankshaft position)
Dick, very interesting . . . is there a specific reason why this is so?
I'd like to know the details if you have them. Also, is this for just
the TR6 engine, inline 6 engines in general, internal combustion
engines, etc. You learn something every day!
Peter Zaborski -- CF58310 UO (sold)
WHY? (DT)
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