On Sat, 25 Jun 2005 13:35:35 +0200, Thomas Borchert
wrote:
Uhm, actually, no. Excess fuel is slowing the burn front, which in turn
cools down the burn process, which in turn reduces EGT. Now, lean of
peak, you have excess air cooling the exhaust gas.
Close, but not quite according to Deakin. The excess fuel causes the
mixture to burn more slowly, but what this does is delay the Peak
Pressure Point from occuring too close to the piston being at Top Dead
Center.
Either side of the precisely correct fuel/air mixture and the
combustion process occurs more slowly, but the actual combustion
process (the flame inside the combustion chamber) is not much less
hot.
The PPP has to occur with the piston at around 12 degree ATDC (After
Top Dead Center) in order for the maximum push on the piston with
minimum strain on the bearings and connecting rod. The closer to TDC
the piston is when the PPP occurs, the more pressure and strain and
more HEAT is produced.
The PPP doesn't happen instantly. The mixture has to be ignited a
certain number of degrees before TDC so that by the time the piston is
at the proper position, the combustion has reached it's maximum
pressure. For most fixed timing engines, that setting is around 26 to
28 degrees BTDC.
So the magnetos are set there. But this is a setting that requires
full rich in order for the PPP to occur at 12 degrees ATDC. If the
mixture is leaned for takeoff while the airplane is at or near sea
level, the combustion process speeds up. Because the combustion
process speeds up, the PPP begins to occur closer to TDC, which is a
bad thing.
So the additional fuel, or overly rich mixture, doesn't produce a
cooler flame front, it simply slows down the combustion so that it can
occur at the proper place. This produces acceptible cylinderhead
temperatures.
Leaning past the stoichiometrically correct mixture also slows down
the burning process (which keeps the engine cool), but this time,
you're using a lot less fuel while doing it.
And the above is just for takeoff. When cruising you usually slow the
engine down. Slowing the engine down brings the PPP very close to
TDC. So you have to either cruise rich or very lean in order for the
PPP to be where it needs to be to prevent long term damage to the
engine.
Corky Scott
|