I think the cheapest, lightest, and most reliable way to
create vacuum
for your steam gauges is with an exhaust bung.
The exhaust bung would enter the exhaust pipe at a small
angle, say 20
degrees, pointed towards the exit, and extend into the exhaust gas
stream about 1/3 of pipe diameter. This creates a venturi,
and LOTS
of vacuum.
<snip>
IMPORTANT NOTE: you will have to regulate the vacuum (with a
regulator), but it will work very well. (Almost TOO well.)
So:
lightweight - a few ounces of bung material and tubing
reliable - as long as the engine is running (same as vacuum
pumps)
durable - no moving parts
cheap - tens of dollars with no annual maintenance
How's THAT for Experimental Aviation?
- Vince Orton
Wow that might be a great idea pulling on the pistons. Super
cheap
turbo compound. How do they keep the oil vapor in there? For
drag racing I guess they don't care.
<snip>
For use as a vacuum pump to drive the instruments it sounds
great.
These are also called "air ejectors" if you want to look
them up on
Google. A simple venturi might generate the -5 psi required
for vac
instruments without all the back pressure.
Paul Lamar ...No rotor no motor.
<snip>
Paul,
Modern street motorcycles have used this technique for some
years now.
They run at very high RPM's of course. Kawasaki was the first
to use this
system to my knowledge. (In the late 80's)They have had a
exhaust reed
valve system with a small orifice to the crankcase to pull vacuum.
<snip>
If your instrument will tolerate oil you're OK. Otherwise be sure
the crankcase is NOT connected to the vacuum generator.
Bill Jepson
I can see how it would reduce the windage HP loss of pushing
air out
of the cylinder with the piston going down and up into the
cylinder
with the piston going up. The rotary does not have that problem
fortunately so I don't see a need for vacuum in the crankcase.
Paul Lamar ...No rotor no motor.
*************************************************************************
ACRE -
Wow. This thread has gone far afield.
Forget crankcases. Forget oil vapor. Forget H.P. increases.
Use the exhaust bung to SIMPLY create vacuum, regulate it, and
use it to
drive your steam gages. That's it.
IMPORTANT NOTE: This will create TOO MUCH VACUUM. Something on the
order of 15 psi, if memory serves. You WILL need to regulate it.
The point was that with this solution, you will not need to
worry about
mechanical problems, shearing shafts, pump maintenance, pump
weight, and
all the other things that go along with a mechanical solution.
Yes, smog pumps are cheap, but an exhaust bung in cheaper, more
reliable, more durable, less maintenance, and ... LIGHTER.
- Vince Orton
Vince what do you mean by an "exhaust bung"? Do you have a dwg?
Paul Lamar ...No rotor no motor.
How much vacuum is generated during engine idle? Consider a long
'slam dunk' power off descent through the clouds from 12k to sea level.
Matt-
Answering my own query, I suppose one could arrange for an exhaust
throttle for such situations. The throttle would route exhaust through
a venturi (restriction). Diesels use them to generate braking force..
Matt-
The successful use of any venturi system assumes that the inlet is under
greater pressure than the outlet. Thus it has a small taper to create the
restriction followed by a more gradual longer taper to recover the gas flow
speed. The lowest pressure in the venturi is just a wee bit past the
narrowest point. OK we all know this and that these devices are commonly
stuck to the outside of all vintage aircraft cowlings. I am unclear how
just sticking a tube in a gas stream will pull enough vaccumn to power the
instruments. Perhaps the effect is more like blowing across the top of a
open bottle?
Doug in Japan
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