As you've found out, conventional air compressors aren't very well suited to
low speed operation. There's the cogging problem, the periodic resistance on
the compression stroke that at high speed is overcome by flywheel inertia.
Then there's the blowby problem. As you know, you can slowly turn either an
engine or an air compressor through its compression stroke and accomplish
nothing. All the air leaks past the piston and rings. Again, both machines
rely on speed to cause the compression to happen fast enough that the blowby
has little effect. Plus, most piston rings require a certain amount of
backing pressure to properly seat.
All you need for low speed compression is a suitable design. One that doesn't
rely on inertia or conventional piston rings.
Recently I became interested in big bore air rifles. Think .50 caliber with
enough energy to kill big game. Like this, for instance
http://www.quackenbushairguns.com/
The gun is pre-pressurized to from 2500 to 4500 psi. Most folks use diving
tanks but believe it or not, there is actually a hand pump available that can
generate that kind of pressure.
http://www.airgundepot.com/hand-pump-samyang.html
Of course, in the fine print for one of the rifles, it notes that from 1500 to
2000 strokes are required for the first pressurization. Yikes! Holy
exhaustion, Batman!
Not being interested in building my triceps or wearing myself out and not
being terribly interested in having to travel 50 miles to the nearest dive
shop to beg 'em to fill a tank for a non-certified non-diver, I naturally
started looking for alternatives. I momentarily considered building a slow
speed crankshaft affair that would drive that manual pump with a motor. Having
some doubts about its durability when used like that, I decided to look
further.
There are, of course, commercially made high pressure air compressors but
they're big $$$$. One of the air gun e-stores sells a ChiCom made unit for
$1995 or thereabouts, far too rich for my blood.
So I went googling. I didn't find anything related to air guns. I knew that
both divers and emergency responders also use high pressure air. Divers are
more likely to homebrew something than are firemen so I included diving terms
in my search. There I hit paydirt. Consider this homemade low speed
shop-air-drive oxygen compressor.
http://www.brianb.org/images/Scuba/Homemade/Gas Booster/gas_booster.htm
From practically zero pressure to 3400 psi in one stage! Pretty zippy. I
would have done several things differently. I would not use the lever arm but
instead would find the correct diameter air cylinder to generate the necessary
force directly. That way the two cylinders are directly coupled and there is
no bending moment. I'll probably also go with more than one stage. And since
I happen to have an electro-hydraulic power pack, I'll probably use a
hydraulic cylinder instead of air.
Anyway, here's a design of a pump that can be directly driven from an
old-fashioned farmer's wind mill. You can drive it directly from the
reciprocating motion that these windmills deliver to the ground. Actually,
the air cylinder that he used to drive his pump would probably be more
suitable. You'd want the inlet and outlet check valves to be as close to the
cylinder as possible to minimize dead space.
So how long will it take you to get one running?
