<<  First of all I don't quite get this temperature thing.  From what I'm
told at normal operating rpm for airplanes the fuel consumption per horse
power is roughly the same as certified piston bangers.  I'm also told that
the combustion chamber for rotaries is slightly "colder" than the piston
engine.  Here's where I get lost: If they burn the same fuel at approximately
the same rate per horse power and have a colder combustion chamber, why is
the EGT higher?  In the certified engine as we lean the fuel mix the CHT's
get hotter, and as we richen the fuel mixture it gets colder.  If it's true
that the piston engine is marginally more efficient than a rotary, then the
rotary must be spewing more unburned fuel into the exhaust system.  So if
it's the excess fuel causing such high exhaust temperatures as some have
hypothesized, why isn't the rotary a much cooler engine under high power
settings? It has less friction, it breathes better, in fact it does a lot of
things better. !
  --  So why is it so hot relative to the piston engine?  My slide rule is
outmatched here, someone please help me out (In words simple enough for an
old guy with Alzheimer's to understand:-) >>
Robin

Lynn E. Hanover wrote:
The rotor motor isn't all that bad. One of many features overlooked is the
late ignition timing that adds to the amount of fuel still burning as the
exhaust port opens. There is no poppet valve to slow the mixture and absorb
some of the heat. Also the rapid exit allows less time for the engine to pick
up some of the heat for the water to deal with. So, no matter what else went
on inside the engine, the exhaust gasses would be hotter than a piston
engine. In order for the fuel mixture to affect the operating temperature of
the engine, it would have to be very rich indeed. We run the race engine as
rich as possible to accomplish this to some small degree. The highest
internal temps and the best power, occur rich of peak EGT. The rotor temps
are about 400 degrees, and this is not a lot cooler than a piston engine. The
difference is the surface area involved and the poor shape that results in
flame quench near the apex seals. This results in even slower flame travel
and unburned fuel in the chamber. 

Notice the two spark plugs. The engines are
very tolerant of ignition advance. For racing, we run 27 degrees with both
plugs firing at the same time. For lower RPMs this would be too much, but
maybe 25 or 24 would be OK. Running the engine just lean of peak EGT will
reduce stress on the engine, lower water temp. Typical piston engine leaning
procedure is to lean until is runs rough, then richen up until it smoothes
out. This is very dangerous. Uneven distribution may have you running way
over lean on one cylinder and at ideal mixture (for max power) on another. If
the richer cylinder has not been equipped with a cylinder head temp lead, you
can be melting that one while thinking you are going easy on the engine. The
rotor motor is less sensitive to this problem. It will run better the leaner
it gets until it misfires. It will be putting out less and less power, but
the temps will all come down, as well as the fuel consumption. The fuel
mixture leaner than max power burns slower than the ideal mixture. This
exactly the same result as increasing the octane rating of the fuel. (slower
burn rate and less likely to detonate). Paul Yaw's web site is the best on
this information, if you want to get into the weeds on this.

Lynn E. Hanover
lehanover@aol.com

 
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