http://www.torque-online.co.uk/news/?p=3631
  --

  Paul Lamar


Paul,

Did this kart have ceramic bearings and if not when (if ever) will the AIXRO
folks add them?

I have not seen any reference to it, but do they have the liquid cooled
shaft per the paper you have linked to i.e., the "LCCR" ?

Also,
I see the Cubewano UAV Wankel is using Ceramic Coatings on the Rotor and the
epitrochoid surface, as well as claims of lower BSFC.

Any comments?

My best,
Anthony

Those questions are above my pay grade :) Send your questions directly to:

Poul Henrik Woelfle AIXRO <ph.woelfle@woelfle-engineering.com

I will say IMHO ceramic coatings on the rotor are not worth the risk of
damaging your rotor housings when it comes off.

BTW they have a new very nice web site.

http://www.woelfle-engineering.com/index.html

Paul Lamar
--

Hi there,

First, thanks to Paul for publishing the links.

Now to the questions:
The engine may have had ceramic apex seals, but there is not so much of a
difference to standard seals. The biggest difference is above 8000rpm, where
the ceramic seals produce less friction and allow for some more top-end
power. Even if it's about 3hp (that's >6% on a 45hp engine), the average
karter will not really notice the difference though.
On a side note: We hardly ever see any wear, but we do see some "running in"
with the standard seals. Therefore changing from standard to ceramic on a
used engine is no good unless the trochoid housing is exchanged as well.

We have tried ceramic rotor needle bearings, which were good for lower
bearing temperatures. These things are very sensitive though. If anything
manages to get into the bearing (dirt, etc.), the needles will dissolve and
destroy the e-shaft, rotor, seals and housing, i.e. the complete core
engine. The apex seals are much less sensitive to such occasions, because
they can wipe potential dirt or the like out the p-port exhaust. For this
reason we don't use ceramic bearings.
Ceramic coating of the rotor and housing is surely interesting, and when it
works it's great. But I agree with Paul that getting it right is a difficult
exercise, especially in aircraft.

Our trochoid housing and side plates are from aluminium and water cooled.
The E-shaft is charge cooled. Of course a liquid cooled shaft (or a
completely different rotor cooling/lubrication system) would allow for more
power, but it would also add to the cost, weight and complexity of the
system. The charge cooling system we are using works fine in the intended
power range.

Best Regards,
Poul

Let me put on my devil's advocate hat here for a few minutes with some
assumptions and questions. The devil is always in the details... especially
with a rotary engine.

Since the kart engine is limited to 10 seconds at full power for max HP,
what percentage of max HP can it run at full power for 5 minutes?

A certified aircraft engine must run at full power for 5 minutes at a time
according to the FAA.

I assume this is due to the rotor bearing temperature in the charge cooled
rotary engine.

Lets also assume for the moment that the dirt particle capable of shattering
a ceramic needle bearing is made from diamond, being the hardest particle I
can think of right at the moment.

What diameter of diamond particle relative to the diameter of a ceramic
needle causes the needle to shatter?

Can we come up with a filter that will remove particles that size and above
in the premixed oil/fuel mixture before it is added to the fuel tank?

Can we come up with a filter that will remove particles that size and above
in a filter on the engine located in the fuel/oil feed system?

What is the best place to put a thermal couple to measure the temperature of
the rotor bearing if that is in fact possible?


Paul Lamar



Dear sir with the devil's advocate hat,

the XR50 "sprint" karting engine has its suggested redline at 11000 rpm, and
the limiter now sets in at 11500rpm. Peak power of 33kW/45HP is at around
8750rpm.
The XF40 "continuous power" version, which is also used in some aircraft, is
basically the same engine, but with some modifications (side bearings, port
control, ignition and output setup, etc.). The limiter of the XF40 sets in
at 7000rpm, and we suggest to set the engine up to reach max. 6500rpm under
full load. At 6500rpm it will supply 26kW/35HP, and it will do that for five
minutes.
You are right in that the main bearing temperature is the limiting factor.
That's why we stay below the limit with our engines. We have just recently
got a message from another customer who bought an XR50 karting engine,
didn't read the manual and went flying with up to 10000rpm, and the engine
survived, but I think that was more luck than anything.

I don't think you need a diamond particle to shatter a ceramic needle. They
can carry very high loads, but they are sensitive to punctual loads. If
something gets in the way for a needle and the needle rolls over it, the
needle can crack even if it crunches the particle to pieces. And once one
needle cracks it will cause a "chain reaction", which will stop the engine
instantly. Next thing you do is throw it in the bin and get a new one. Steel
needles may be more temperature sensitive, but if something gets in the way
they will at least fail slowly, so there is some time to react (stop the
engine in a kart, reduce power and find a good spot for landing in an
aircraft). Given this and the cost of the ceramic needles we decided rather
not to use them anymore.

Apart from this the occurrence of such problems is not very high at all, but
we rather want to be on the safe side. We do suggest that customers use a
proper fuel filter, and that does help (also against carburettor failures).
But especially in karting there is often a dusty, dirty environment, and
sometimes dirt also makes its way into the engine through the air filter, or
it even gets dirty inside when people try to clean it...
All this is probably less of an issue in an aircraft, because aviation
people are generally more cautious about their engine for obvious reasons,
and that helps a lot.

A good place for a thermocouple to monitor the bearing is in the transfer
port, i.e. where the charge that has just passed the rotor and main bearing
enters the combustion chamber. All our engines come with an M5 thread in
this area, and it is recommended to use a small (=quick) thermocouple. We
call it the bearing temperature, but of course it's just a relative
indicator. The bearing temperature increases with the engine load, and if
something should be wrong it will also rise.

I hope that answers your questions. If not, put the hat on again :)

Best Regards,
Poul