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[DPRG] Computing back pressure in an exhaust system
Subject: [DPRG] Computing back pressure in an exhaust system
From: Chuck McManis
cmcmanis at mcmanis.com
Date: Wed Dec 5 21:00:40 CST 2007
So one of the projects I'm working on is to design a muffler for my
gas generator so that it can be run at a much lower decibel level
than it currently does. I'm researching ways to stretch and rip the
shockwave that is normally associated with the exhaust of an internal
combustion engine in such a way that the result generates a much
lower sound signature. The generator is currently in excess of 130
decibels and I'm trying to get it into the low 20's, high 10's of decibels.
So one of the things I'd like to be sure of is that my exhaust system
doesn't change the exhaust manifold pressure enough to interfere with
the engine. Basically this is the back pressure that is seen by the
engine when it is trying to push out exhaust gases. Increases in back
pressure result in less exhaust being evacuated from the cylinder in
the exhaust stroke which then results in less efficient burning in
the input stroke. I can adjust the mixture to keep emissions in line
but it means reducing the total output power of the generator.
So what I'd like to do is figure out a process whereby I can measure
how much back pressure an exhaust system has. That way I can measure
the current system, and my new system and see if I'm adding (or
ideally reducing) back pressure as seen by the exhaust manifold.
My current best idea is to build some tubing that connects from the
output of my Shop-Vac, has a way to put an exhaust system in the
middle of it and then a vertical tube with an indicator (probably a
ping pong ball) at the output. Poor ascii graphics are:
| |
Ping Pong | - |
Ball | - |
**** \ | - |
+---+ +---+ | O |
| | +-----------------+ |===|
| +---------| |------+ |
| | Muffler/Exhaust | |
| +---------| |----------+
| | +-----------------+
| Shop-Vac |
+----------+
The thought being that even if I couldn't calibrate it in SI units I
could at least get a comparison of the existing exhaust system and my
new one. I'm hoping that the ping pong ball going up to the same
level means that they have equivalent rates of flow, which translates
into equivalent back pressure.
Any thoughts on this problem?
--Chuck
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