X-Mozilla-Keys:                                                                                 

Fred wrote:

Paul:

I queried a friend who raced rotary engines years ago and is currently flying
a Lycoming in his Lancair.  His comments follow.  The comments about EGT were
news to me.  Are they still valid?  I ask because high temperatures are death
on metal lifetimes, and I wonder if turbos could be a problem.

The combustion chamber geometry requires late ignition
timing (25 degrees vs 40 degrees) so gasoline engines require inconel exhaust
components. Turbocharging gasoline engines is problematic due to these high
EGTs."

Paul Yaw wrote:

The rotary actually needs more timing advance than a piston engine
because of its low combustion rate.  A piston engine will have its best
leverage ratio (Point of maximum torque tranfer.) at about 12 to 15
degrees after TDC. (Archie probably knows the exact number for specific
engines.)  If you fire the plug at 40 BTDC, you have approximately 55
degrees of lead.  The point of maximum torque transfer on a rotary is 45
degrees after TDC.  If you fire the plug at 25 degrees BTDC which is
pretty common for best power WOT, you have approximately 70 degrees of
lead.  The confusion is in looking at the timing based on TDC.  The high
EGT's are a result of this slow combustion.  A fair portion of the
charge is still burning when the exhaust port opens.

(Paul L. wrote)

Paul Yaw and Tracy Crook have been doing some work on ignition timing
so I am not sure if 25 degrees advance is the real limit on timing.

That number certainly comes up a lot.  A high VE engine will normally be
happy with less advance, but that 24 to 26 (Leading) range gets it done
almost every time.

A turbo aircraft engine would probably require an inconel exhaust
manifold and inconel turbine which is not much extra money these days.
Chuck Harbert is flying a turbo three rotor with those.
Those are also what I plan to use on my engine.

As I mentioned (To Paul L.) on the phone the other day, firing both
plugs at the same time will drop the EGT's by a few hundred degrees at
the expense of some power.  The motor that I built for the bush plane
had an EGT (Before the turbo) of 1670F at 43" manifold pressure, 6,500
rpm, 260 hp.  I would GUESS that running 100LL (Rather than 92 pump gas)
and the increased advance that it requires would lower them
substantially.

PY

 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org