Dead Reckoning

[James A. Landau]

In a message dated 7/26/01 12:59:03 AM Eastern Daylight Time, pds at VISI.COM
writes:

> At 07:06 PM 7/25/2001 -0700, A. Maberry wrote:
>  >Used in timing automobile engines. The timing is determined by the
>  >uppermost travel (top dead center) of the number one piston (which equals
>  >the point of maximum compression in the number one cylinder. It's
>  >certainly been a long time since I tried to tune my own car, so
>  >someone please correct me if I am mistaken.
>
>  Allen is correct about the involvement of the number one piston, but I
>  think that the practical use of TDC is in reference to the position of a
>  "timing mark" on a pully, gear, or flywheel connected to the crankshaft at
>  the moment that spark plug fires.  TDC and BDC are indeed 12:00 and 6:00.

"dead center" goes back to steamboats, long before the internal combustion
engine was invented.  If you have a one-cylinder steam engine driving the
paddlewheel(s) of your steamboat, and you stop the boat:  if the drive rod to
the paddlewheel is to either side of the axle of the paddlewheel (that is, if
the drive rod is anywhere except the 12 o'clock and 6 o'clock positions) then
if you let some steam into the cylinder the drive rod will proceed to rotate
the paddlewheel.  By choosing which side of the cylinder to let the steam
into, you decide in which direction the paddlewheel will rotate.

But what if the cylinder stops with the drive rod in "dead center" (either 12
or 6 o'clock)?  You let some steam into the cylinder.  The piston will try to
move but the drive rod is unable to rotate the paddlewheel and instead the
piston will try to drive the drive rod into the axle.  Hence in "dead center"
the paddlewheel cannot rotate, the boat cannot move, and an obvious
hypothesis is that the "dead" in "dead center" means "immobile, immovable".

Quite possibly the usage "dead center" was carried over from steamboats to
automobile engines.  However, if your steam or internal combustion engine has
more than one cylinder, the "dead" part does not mean "immobile" but rather
is a somewhat arbitrary way of describing the position of the number 1
cylinder.

Also "top" or "bottom" as in "top dead center" is somewhat arbitrary, since
if you have a V, slant, pancake, radial, or rotary engine then "top dead
center" is not at the vertical.

What happens to the one-cylinder steamboat stuck in dead center?  The crew
gets out levers and manhandles the cylinder out of dead center.

(My source for the steamboat info is some maritime museum, possibly one in
San Francisco Bay area.)

            - Jim Landau
              Systems (not mechanical) engineer
              FAA Technical Center (ACT-350/BCI)
              Atlantic City Airport NJ 08405 USA