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I did a google search on "tuned exhaust" and got lots of hits.
I'm including a few paragraphs from the first article I'm reading.
Might be better explanations but I haven't had time to eyeball them
yet. A quick reading of the first few paragraphs leads me to believe
the article is talking specifically about two-cycle operations, but I
guess the theory is the same. Just wouldn't be as concerned about a
second reflected (positive pressure) wave?
Anyway, some additional explanation for the inquiring mind.
Calculating this stuff sounds like great fun.
At the top of the search was:
http://microcarproject.tripod.com/html/tuned_exhaust_system.htm
The Basic Process:
When the exhaust port cracks open, gases still under a considerable
pressure burst out into the exhaust tract, forming a wave front that
moves away at a high speed down the port and headed for less confined
quarters. After travelling a comparatively short distance, this wave
reaches the first part of the expansion chamber proper, which is a
diffuser (commonly called a megaphone). The diffuser's walls diverge
outward, and the wave reacts almost as though it had reached the end
of the system and is, reflected back up the pipe toward the cylinder
with it's sign inverted. In other words, what had been a positive
pressure wave inverts, to become a negative pressure wave. The big
difference between the action of the diffuser and the open end of a
tube is that the former returns a much stronger and more prolonged
wave; it is a much more efficient converter (or inverter) of wave
energy.
As the initial wave moved down the diffuser, the process of inversion
continues apace, and a negative pressure wave of substantial amplitude
and duration is returned. Also, overlayed on this the effect of
inertia on the fast-moving exhaust gases, and the total effect is to
create a vacuum back at the exhaust port. This vacuum is very much
stronger than one might suppose, reaching a value of something like
minus-7 psi at its peak. Add that to the plus-7 (approximately) psi
pressure in the crankcase working to force the fresh charge up through
the transfer ports and this will help understand how the transfer
operation is accomplished in such a very short time. Obviously,
too, this combined pressure differential of almost one atmosphere
is very helpful in sweeping from the cylinder the exhaust residue
from the previous power stroke. It's all like having a supercharger
bolted on over at the engine's intake side-but without the mechanical
complication.
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Paul Davis
INTP
Lancair Legacy builder
pdavis@bmc.com
Phone 713-918-1550
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Oh, I have strong opinions, but a thousand reasoned opinions
are never equal to one case of diving in and finding out.
--Lazurus Long, Time Enough For Love, pg 9
Arguments are repl Aviation/Lancair/List /home/pdavis/Mail/drafts/332
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