Mark Wrathall wrote (of the Moroso spiral muffler):
Has anyone got any exhaust to work that has anything welded inside, away from
cooling, subjected to the full blast of the rotary exhaust? I got the
impression from Ed and Tracy's efforts, that a simple resonator works well
enough, but you need either thick or expensive material. Judging by some of the
simpler designs which have been beaten by the rotary (the fishtails spring to
mind), this looks way to complex to survive. Bet it sounds cool though:)
Now that we've solved the cooling "problems" of the Mazda Wankel <grin>, I've
been pondering the issue of the exhaust. There appear to be a 2 aspects to the
exhaust that are undesirable:
1. too hot
2. too noisy
First, the heat. There's only two ways to address that problem: either make the
exhaust gases cooler to begin with, or shield the components that you don't want
to get hot.
There is some evidence (but not conclusive) that unburned hydrocarbons are
contributing to the EGT. We need to be looking at how to ensure that those
hydrocarbons burn where they make useful energy, not the unwanted exhaust heat.
Paul, I think your idea about squirting high pressure air in the "squish" area of
the chamber is interesting. Basically, you have proposed one way of stratifying
the intake charge. Unfortunately, the engineering work to make a workable
product seems a bit aggressive.
Another way of keeping the unburned hydrocarbons in the engine is by changing
from peripheral exhaust ports to side exhaust ports. This is what the Mazda RX-8
Renesis engine is doing, and I (like everyone else here) is waiting with bated
breath for it. Until that engine is available we're stuck with the existing 13B
engines, and I don't see us changing them to side exhaust.
A third way to keep the unburned hydrocarbons in the engine is to put fewer in
there to begin with. This means running very aggressively lean. We would need
to look at ways to reduce the BSFC, as a very lean engine with a low BSFC would
still produce the same usable HP, but with a lower gph and a cooler exhaust.
Once the exhaust gases are hot, there are a few things we can do to shield the
stuff under the cowl. I like the Jet Hot coating, but I think that we would need
to use it in a slightly different manner than has recently been shown. The
exhaust manifold and piping needs to be coated on the inside *only*. The
objective is to keep the heat in the gas, but once some of the heat has escaped
to the manifold or pipes, we should not hinder their ability to shed that heat.
If the manifold & pipes were kept cooler, then we wouldn't be looking at Inconel
with lust in our eyes...
We should also be looking at thin, lightweight metal shields (aluminum?) to
shroud the exhaust manifold & pipes (cut out radiant heating of the cowl area),
and then ducting air between the shroud and the manifold/pipe to carry away the
heat. Ideally, the cooling airflow would enter at the point furthest from the
engine, and exit at the exhaust header (maintain semi-constant delta T).
Finally, noise. Assuming that the noise problem is really the exhaust note (and
not the prop tips or some such), then a low back pressure muffler is in order.
Once we get the exhaust cooled, lots of muffler designs become possible, as the
extreme temperature is major contributor to muffler destruction. The other major
contributor is the sonic waves in the exhaust caused by the rapid opening of the
peripheral exhaust ports. Again, since the Renesis has side exhaust ports which
presumably open more gradually, hopefully this will be less of an issue with that
engine, but until we all have one of those engines... One possible way to
address the sonic wave is to divide and conquer: put a separate muffler on each
port. Each muffler would be subjected to half the sonic energy and half the
total exhaust heat load, at the cost of twice the weight. (Hey, I didn't say it
was a *good* way :-)
Russell Kent
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