David Carter wrote:

V. E. Welch wrote:

What is the failure mode for a turbocharger?  In other words if it should
fail will the engine continue to receive ram air (just without the boost)?
I am wondering if I should make provisions for an alternate air source and a
method to manually open the wastegate in the event of a turbo failure.

Vince

Paul L. responded:
My guess is the common failure is the bearings go out when
people don't let them cool down by idling the engine for
awhile. If the bearings go out in the air the engine has to
suck air through the compressor with a loss in power.
Perhaps a valve bypassing the intake would also be a wise idea.
Some turbo's have water cooled bearings which are a good idea.
If I was doing it I would bust my buns to get water cooled
bearings as a turbo on a rotary is going to naturally run hotter.

Other's have ceramic ball bearings. Check with Turbonetics on the
status of those.

Another failure mode is melting the turbine. That could cause real
problems. Get the Inconel turbine to be safe.

Paul Lamar

David Carter added:

About turbos "cooking" and "coking" after shutdown - A really simple
solution to help bearings cool down after engine shutdown was applied to
some turbo'd VW engines in KR-2s:

Simply plumb a T fitting into the pressurized oil supply line to the
turbo bearings, and put a 1 quart (your choice on size) can (call it an
oil accumulator) with a single line running out the bottom to the T
fitting.  Can should be able to stand your system's oil pressure.  Also,
you must put a flow restrictor somewhere in the line from the oil
accumulator to the T fitting - to regulate the time the oil will flow
and cool the bearings.

When you start the engine before flight, pressurized oil flows to the
turbo bearings and also up the T-fitting and line into the oil
accumulator, filling it with oil until the air above the oil is
compressed to a pressure equal to oil pressure.  Need to have enough
total oil capacity in engine to stand this "temporarily lost" oil
volume.  Nothing else happens until your flight is over.

After you shut down the engine, the oil supply to the still SPINNING,
and still HOT turbine ceases immediately (from engine oil galleys)  BUT,
the oil in your added "oil accumulator", which has been pressurized by
the engine oil pump, now flows out and baths your turbo bearings as the
rotor spins down - until accumulator can empties.

Four engineering unkowns (to me):  1)  I don't know how much of the flow
would be reverse flow back into the engine's oil pump/oil galley - can't
assume it all goes in one direction from the T to the turbo.  Given that
you knew the "back flow" rate, 2)  I don't know the flow rate needed "to
the bearings" to satisfy their needs for lub while turbo spins down, 3)
I don't know  how long it takes for the turbo to spin to a stop or to a
low enough speed that lack of oil flow ceases to be a problem for
lubrication,  and 4) I don't know if there needs to be flow for any
period of time after the rotor stops, just for cooling alone, so that
oil that eventually stagnates in the turbo won't "coke" (burn).

For a "final check" on a tentatively adquate system, after installation,
with whatever flow and flow time it gave, I'd want some way to monitor
temperature at the bearings to see if they were cooled enough by the
time the accumulator emptied.

David Carter

 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

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