Paul,
I am trying to get more information about this, but in the meantime
I had to wonder why the exhaust was such a problem, since there are
other 20b's flying, to say nothing of 13's, that have evidently tamed
their exhausts. Would it just have been because he was trying to fit it
into a slender Mustang cowl?
Peter G
We have at least two RV4 tandem airplanes that have flown. I own
one and the other was built by Brian Trubee.
Brian's has an aluminum cowl. Mine has SS sheet between the exhaust
and the fiber glass cowl.
There is also a scale spitfire running in Texas.
We have a bit of evidence that a too restrictive muffler will raise
the EGT's above the normal 1600 to 1700 F.
The secrete is to use thicker wall (.090") bends on the ports. Or
even thicker .125".
The mixture ratio enters into it of course.
Here is our latest muffler recommendations.
Feel free to pass this on to you contacts in ZA.
Paul Lamar ....In Parkes NSW
Forwarded message ----------------------------------------------------
Hi Peter
AS happy New Year to you.
A beautifully built 7 year labour of love ¾ P51 Mustang with a
three core 20B Mazda Rotary (Wankel) engine crashed on its third flight.
I learned something new – that exhaust temperatures can be a
problem with these rotary installations.
Here is an extract from a discussion on our local forum
www.avcom.co.za <http://www.avcom.co.za> <http://www.avcom.co.za> –
which I believe you have lost your password to!
G
The engine is a 20B with a Tracy Crook PSRU and a local engine
management system. The powerplant is popular in the US especially with
the Canard guys.
We spent a lot of time with Rob and his conversion as we are
installing a 20B into a S208, we have 68 hours of ground testing and are
still not entirely happy.
The thing with the 20B is exhaust gas temps - 1850 deg F is not
unusual on t/o. During our testing, we noted that nothing inside any
type of standard muffler can withstand that heat - we even tried
stainless steel wire....it ignited and dripped out the exhaust...the
only way to quieten these engines is by separating gas pulses from sound
waves and making the sound waves work against each other, that is done
through a big ugly exhaust system - and yes, they need to be muffled,
the exhaust note is horrific without some form of muffler (much of the
combustion is still happening when the rotor passes the exhaust port)
They are fantastic power plants and Rob did a fantastic job but I
never liked the exhaust system.
Temps is surely extremely high.
Just a question. Have you consulted with the guys that race with
rotary engines. I know that this is/was one of their biggest problems.
But a company in the USA came to the rescue with systems that can easily
handle 2000 f with ease,and still keep weight and size down.
Just to add to this.Way back ,when the rotary started to show up,
On racing tracks and drag strips,many strips banned unmuffled rotary
powerplants due to the noise. Lots of research was then done to counter
this. And as many also say,The very first thing you need to pay
attention to, if you ever want to go the rotary way, is the exhaust
system. Forget anything else that might be a obstacle. And for some non
sponsered racers, the solution was homebuilt mufflers ,filled with lavarock.
And being limited with space in a aircaft, i gues this must be sort
of the nightmare to solve.
We tried everything - the problem is weight - one can put a cast
iron exhaust on (as per the automotive arrangement) but these things
weigh a lot. We then started researching the subject and started
following Paul Lamar's writings on the subject. The lightest way to do
it is to use Inconel and separate gas pulses and sound waves through a
measured system (using nothing but space as a muffler) The idea we went
with uses a 75mm down pipe (has to be long and so exits the cowl
underside the aircraft and enter's a round chamber which increases the
diameter to 3X the input pipe, this chamber slow's the sound wave
frequency and 'confuses' them but still allows the gases to pass without
restriction.
The system works, the engine is still loud but nothing inside the
exhaust can ignite. It is ugly as sin though and certainly was not a
solution for Rob.
Still, this may have nothing to do with Rob's problem and is only
speculation at the moment......this accident really hits home, it was
such a perfect build project.
A turbocharger quietens the exhaust down quite a bit!
Rotary exhaust temps are also high because it is effectively 3
"cylinders" that share an exhaust port. Racers get the most performance
by using equal-length pipes for the manifold, but at the expense of
losing a little power, a large diameter "log" type manifold would
probably quieten things down a bit.
end forwarded message
-------------------------------------------------------------------------------------
Guys, there might be a solution that is derived from transport jet cabin
pressurization systems. The cabins are pressurized using bleed air from
the engines. The trouble is that this air is around 400 degrees or more
when it exits the compressor. To be able to use this air, they run it
through a cooling turbine. When a turbo compresses the air, the air gets
heated. When it goes from high pressure to low pressure through the
turbine, it expands and loses much of its heat energy.
So, using the turbine as an example and coupling it with normal recip
engine installations, where the turbo is never that I have found bolted
directly to the exhaust manifold at the exhaust outlet. It is always
mounted somewhere downstream in the system.
So, this is what we know: 1. air cools as it slows down and expands. 2.
There is plenty of energy in the exhaust even downstream.
So, we can place a turbo a bit downstream and let it both cool our
exhaust and quieten the engine and just dump the compressor air
overboard. If you don't want to use the turbo compressed air for the
engine, use it like an augmenter in some older twins and dump it down a
larger tube along with the exhaust. The old Twin Bonanza T-bone has such
a system if you want to look it up. Basically, the exhausts dump into a
larger tube where it expands and is cooled by additional air coming in
from the nacelles.
If you dont use the turbo for sound reduction, you can still use a scoop
and augmenter approach. Dump the hot exhaust into a larger tube, let it
expand, and it will cool itself.
My plan is this- for my turbo p-port 13b I am going to fab a header from
the exhaust to the turbo that will be mounted between the engine and the
firewall. The header will be double layer thick walled pipe. The inner
tube will carry the exhaust. The outer tube will be thinner and will be
fed air from the outside as a cooling medium. The hot air will dump
overboard, while the exhaust goes to the turbo. From here the exhaust
will discharge down and into a slightly shorter version of the tube
exhaust that some have tried on the other rotary engined planes. I plan
to use the compressed air, I want to fly high and fast and that takes
power, but with the temps of the exhaust I am relocating the turbo
farther downstream and removing some of the heat energy before it gets
to the turbo.
I know that the high exhaust temps are an issue, so basically there are
2 choices. Reduce the heat or contain it. I run turbo diesel engines in
some of the boats that I am Captain for. Some have insulated turbos,
some have bare turbos, and some have water cooled turbos. The bare
turbos run white hot under load and you can barely stand the heat in the
engine room for a short period, which is bad. The ones with insulated
turbos keep the room temps down but the turbos don't last as long. The
insulation is either a factory fitted blanket, or exhaust wrap that is a
synthetic fiber specifically made for this purpose that is available
from most large marine stores. Bare turbos have a habit of setting the
boats on fire through radiation heating, insulated turbos not so much.
The water cooled ones work best, but adapting them to our planes would
mean another radiator and cooling system. It is doubtful our engine
cooling radiators will have even close to the capacity needed to cool
the turbo too. While we do not have the whole ocean available to cool
our engines, we do have an infinite amount of air. So lets use it.
If you double tube the exhaust, force air down the tube and dump the
heated air overboard, and also insulate the exterior, you will go a
long way toward at least reducing the fire potential. The turbo is only
a medium weight penalty, so it could be used as a turbine to slow and
cool the gasses, and quieten the exhaust too. I wouldn't kick it out as
an option. You can buy a T04 turbo for around $400.00, and if you had to
throw it out for a new one every 2 to 3 years, that is still cheap for
what it is giving you.
FWIW, I have one wheeler with a LYC IO360 200HP in it, with the exhaust
dumping out under the engine in the center of the cowling. The plane
didn't have any insulation in it when I flew it, but the exhaust exit
location at my feet made it quite noisy. With that in mind, I can see
several benefits of running the exhaust out to behind the passenger
compartment if it will get the noise behind me. My next plane will
resemble a twin velocity- quiet, efficient stability.
Kevin Alderman
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