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Posted for Gary Casey <glcasey@adelphia.net>:
From
Regarding the subject of low power descents adversely affecting the
piston/cylinder clearance I have always found a flaw in that argument. The
idea proposed is that removing the heat source (going to low power) will
cause the cylinder to contract around the piston. However, the flow path for
the heat is from combustion INTO the piston and then out through the skirt to
the cylinder and finally to the surrounding air. Seems to me if I reduce the
heat input the first thing to cool will be the piston and then the cylinder,
not the other way around. How could the piston possibly stay hot and the
cylinder decide to cool off? The only problem I've seen from prolonged
low-power operation is from too much oil getting on the plugs, causing a
fouling problem. This is caused by a low pressure in the cylinder,
encouraging oil to flow above the rings combined with a lack of combustion in
the cylinder that will burn the oil. This effect is a problem during ground
idle as well, so if you don't like idle descents then you must really not
like idling on the ground. In theory, the best way to do a low power descent
is to leave the throttle open (naturally aspirated engine) and pull the
mixture. The manifold pressure will remain high, reducing the volume of oil
pulled into the cylinder and the engine will be perfectly happy just pumping
air. I don't have the nerve to do this as I don't like shutting the engine
off in flight (what happens if the mixture cable falls off, I guess), but it
would make the engine happy. Incidentally, all modern car engine controls
shut off the fuel during a deccel and there has been no known issues.
There was also a question about "backfiring", more technically correctly
called "after-firing": It is commonly caused by an intermittent very slow
combustion caused by excessively rich or lean mixtures combined with a low
cylinder pressure. One cylinder might misfire, depositing a load of unburned
fuel in the exhaust system. The metal is probably not hot enough to light
the fire, but another cylinder might have a "slow burn", creating an open
flame in the exhaust, lighting the unburned mixture. It's no big deal, but
some people don't like the noise (but we, as kids, liked it a lot).
Gary Casey
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