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
If you choose to use the turbo to absorb some of the energy in the
exhaust you can use the turbo outlet air to cool the header pipe between
the exhaust port and the turbo.
Remember that any air drawn into the cowl adds to cooling drag.
Mark LaPierre
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