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Here's an
interesting question. Considering the exhaust heat of the rotary is so much
higher than a Lycoming, if doing a side-by-side comparison of the two with equal
fuel burn rates, wouldn't the Lycoming have more engine heat to remove from
under the cowl than the rotary?
I think this one is a no-brainer, but my
mother often said the same about me...
Ernest,
Not necessarily.
The conventional wisdom:
The Wankel has a higher exhaust temp
because of uncombusted hydrocarbons (due to the flat combustion
chamber) and possibly a lower expansion ratio (depending on which engine you may
be comparing it to). So the Lycoming may make more power for the same
fuel flow with less heat rejection. The Wankel also has a lot more surface area
for the combustion chamber and rotors so the heat rejected to the
coolant will be more. You never get something for nothing and giving up all
those valves, push-rods, lifters, ad nauseum, comes with a price. The
price is slightly higher cooling load and egts.
Just to confuse the issue and tickle some
nuerons:
Now that is the conventional wisdom, and if you
were to compare the wankel to 4 cyl engine of 1.3 L displacement you would
conclude that the surface area is greater and the bearing area is greater etc.
If you were to compare it to a 3.9 L 6 cyl engine at 2/3 the rpm (which is
a much more accurate comparison IMNSHO) you might find the surface areas and
bearing areas are not quite so different after all. This is like saying
that a single cylinder 302 in^3 engine has less surface area than a V8. DUH!!!.
I don't recall anybody at the drag races with a 302 briggs in their muscle
car.
So even if you say the frictional losses are
actually fictional losses, the lack of complete combustion and lower
expansion still makes for higher egt and less efficient
operation.
From a cooling drag standpoint this is not good.
You have to ingest more air and you have a lower temperature gradient to work
with than with a Lycoming. This is all very academic, but in practice, I'm not
sure it makes a hill of beans. Most aircraft cooling systems are so far from
optimum that you will never get an apples to apples comparison, plus props,
airframes etc are all different. The other issue is the rotary (normally
aspirated fixed pitch) can be run LOP at most practical power settings without
damage. This is not true of the Lycoming. So this makes up for some of the
difference. Now if you have identical airframes with ideal cooling set ups for
both types of engines running LOP at the same flight condition I think you would
find the Lycoming may have a very slight advantage in BSFC.
How much fuel can you buy for the price of top end
overhaul on a 180hp Lycoming? Or for that matter the difference in the purchase
price of the rotary vs. the Lycoming?
Monty
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