----- Original Message -----
From: Rusty
To: amphicar-lovers@yahoogroups.com
Sent: Saturday, October 30, 2004 7:15 AM
Subject: [amphicar-lovers] Airflow
I was tinkering with my Amphi Friday ,had it running at a fast idle for
about 20 min and I noticed that when I put my Hood down it affects the
performance of the engine. It seems that when the hood is sealed (down)the
fan creates a vaccume in the carb intake on top of the radiator and my
engine ,starves for air, and the rpm decreases slightly,, when I raise the
hood it runs better,, I took off the small hose that connects the air
cleaner to the fan schroud and close the hood and it doesnt do this. The
fan really moves a lot of air and I was wondering if anyone elce noticed
this. I am always trying to tweek the carb and this is the latest of my
findings. I had recently put a new carb on and the engine would run rough
after I was out a while,,when I would open the hood it would smooth out.
Its too bad there isnt a easy way to get fresh air other than from by the
fan. Just an observation,,Russ
The amount of air that goes through an auto engine is quite amazing. In an
Amphi, with 1147 cc of displacement, there is a movement of 1147 cc's of air
every two crank revolutions (because the engine is a four-stroke design). At
3500 RPM, that works out to be slightly more than 2 million cc's every minute,
or about 71 cubic feet per minute or 1.2 cubic feet per second. (It makes you
appreciate what a big job the air filter has to do too.)
If we generously assume that the Amphi engine compartment has about 10 cubic
feet of volume, then there is a complete air change every 8 seconds!
The best thermodynamic efficiency is achieved when you have the least wasted
heat in an engine. Heat discarded through a radiator is wasted energy, so is a
hot exhaust discharge. Further, cool air is denser than hot air, so it's
advantageous to pull cool air through a carburetor so as to get the maximum
amount of oxygen and charge into the cylinders. Your engine will work better
with 50F inlet air than with 200F air.
(I'm ignoring other factors, like fuel atomization and carburetor icing, etc.)
Anyway, this brings us to the design trade-offs made in the Amphi. We don't have
a convenient big hole under the engine, where cooling air can be discarded.
Every bit of cooling air has to be drawn in through the engine cover, and then
exhausted there too. The directional louvers and the gasket around the radiator
shroud do their best to keep cool air away from hot air. But louvers are air
flow restrictors, and there is a pressure drop across the louvers.
(I'm also ignoring the very slight "scoop" effect that louvers facing into the
airflow give you; Amphis are not exactly high-speed machines, and the most
critical cooling requirement is in the water mode, where an Amphi is putting out
maximum horsepower at 6 MPH or so, and there might be a 5 MPH breeze in any
direction, even from astern! BTW, I also suspect that an Amphi travelling at
highway speeds, and with the top up, creates a low-pressure zone behind the rear
window and in the center, just the worst location, since that's where the engine
is trying to suck air in through the intake louvers.)
The pressure drop across the intake louvers also acts to "strangle" the air flow
into the carburetor, due to the air inlet pipe being placed between the intake
louvers and the radiator. It would have been much better for the air intake to
have its own louvers in the engine cover. (Or possibly have a shroud to isolate
it from the rest of the radiator intake.)
I don't know, but the idea of drawing air from the cool side of the radiator
might have been more of a safety function than a thermodynamic consideration,
since any possible engine backfire would be directed out of the engine
compartment. Another important feature is that the Amphi design keeps water out
of the air intake fairly well. Even a swell over the engine cover would break up
to a cascade into the radiator plenum, with only a modest amount of water drawn
into the air inlet tube. Combined with the air cleaner element, this keeps the
engine running pretty well.
If you wanted to get obsessive about airflow efficiency, you could build your
own manometer out of a board, some plastic tubing and some food dye in water.
Tack the tubing, in a "U" shape, to the board and fill a few inches with colored
water. Snake one end of the tube into a volume where you want to measure
relative pressure or vacuum, and then read the pressure change in inches of
water. This is most useful for things like pressure drop across a radiator
(where you put one end of the tube in front of the radiator and the other end to
the rear side), but it would work just as well for vent louvers.
I would be surprised if Amphi louvers were grossly inadequate, but you might be
onto something interesting. The louvers shouldn't affect the performance of the
engine as much as you are observing. Let us know what you find out.
Ed
El Cajon, CA USA
67 Rust Guppy
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