I'm an old guy, a real old guy.  It's probably no surprise that I struggle with the digital dilemma when you consider that in my first semester of college it was required that I take a full semester course in slide rule.  It must have looked kind of funny seeing a bunch of "long hairs" in tie dyed shirts and pants milling around campus with something that looked strangely akin to samurai swords hanging on their "peace belts."  As all of the slide rule warriors who toted those wooden wonders manufactured by the Pickett Corporation will tell you, they weren't exact.  --  They only got close...  Maybe that's why I'm having trouble resolving some very basic characteristics of the rotary engines (Back then it was pistons or nothing.)

    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

I am not sure about this as a reason Robin but the poppet exhaust valve in a piston
engine transfers some of the exhaust heat to the piston engine cooling system.
The rotary is run way lean of peak to compete with the fuel burn of
a Lyc/Cont. running rich of peak. The Lyc/Cont. needs to run rich of peak to avoid destroying
the exhaust valves. At the same air fuel ratio the piston engine would be more
efficient with a cooler exhaust. As the rotary is leaned out the exhaust temp comes
down just like a piston engine. There are no exhaust valves to destroy in a rotary.
Just mufflers :-)

Paul Lamar
 
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