double acting twin alpha outboard

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danskor
Posts: 2
Joined: Sun Jan 21, 2007 8:33 am
First Name: Daniel
Last Name: Skorupka

double acting twin alpha outboard

Post by danskor »

I need advice on a power producing Stirling engine of my
design. I am working on a partly new design for an engine to power a
boat via a lower mid-range size outboard.
It is an adaptation on the standard "alpha" configuration.
Instead of having a hot cylinder and a cold cylinder, it will have
the cylinder ends that face away from the crankshaft forming the hot
ends; and the cylinder ends facing the crankshaft forming the cold
ends. The engine will be an inline twin with cranks opposed at 90
degrees. The hot end at the top of one cylinder will be linked by a
heater, regenerator, and a cooler to the cold end at the lower end
other cylinder, and vice versa. This keeps both hot ends away from
the crankcase and the oil inside it. Another beneficial effect of
this arrangement is that only small shafts protrude into the
crankcase. This means there is far less force acting against the
upstroke of the piston, thus eliminating the need to pressurise the
crankcase. A pressurised crankcase obviously must contain a pressure
high enough to counteract the pressure of the working gas on the
other side of the cylinder. This has two negative repercussions.
First; this means compressing the crankcase air or gas on the
downstroke and then expanding it on the upstroke. Doing so consumes
energy that would otherwise be useful work. Second; it means
heavier parts need to be used in making the crankcase that now must
contain whatever pressure the working gas is at. This adds excess
weight. I think this will be a solution to both these problems, but
I am not yet entirly sure. Another thing that might happen; this
time if ignored, possibly a major problem is that I don't know which
way the crankshaft will rotate. Looking at my drawings it appears
it might operate counterclockwise if built with a crank offset that
would result in clockwise crankshaft rotation. In an outboard motor
this will be a serious issue of saftey, as when a skipper attempting
to move foreward instead sends the boat backwards. To be absolutely
sure, I intend to make a proof of concept model.

My heat exchangers also have some major modifications. The
cold end heat exchanger is a set of tubes with a screw thread tapped
into the inside of them. Idealy this would quadruple the surface
area per length of tube, but a standard tap gives only 75 percent of
the full "V" of the thread. Thus the factor must be brought down to
a still impressive triple the surface of a stock tube. My engine
uses a total of 120 cm of tubing in the cold end exchangers. Had
they been stock tubes; I would have needed 360cm of equal diameter
tubing. That's about 12 feet. The hot end exchanger is, to my
knowlege, completely unique. While it has less surface area to
volume ratio than the cold end exchanger it still has over 5.3
square cm per cubic cm. The working gas flows between panels in
which groves have been milled. Some of the working gas is caught in
the groves and forms a stationary vortex that is very low in drag
but allows better transfer of heat per unit volume than a flat
surface which is curently used in many power producing engines. Each
exchanger will have 12 of these vortex groves. They are spaced so
the working gas flows efficiently from one to the next, picking up
more heat with each grove that it flows thorugh.

The regenerator is close to normal, but even it has some
origional features. One feature that has probably already been used
by other builders is the material it uses. Most of todays designs
seem to use wire mass or foil. I will be using micro-ribbon steel.
It resembles fine steel wool but is in fact tiny ribbons of steel
that are as thin on their wide axis as a hair, and even thiner in
thickness. This material is used as a substitute for wire wool, as
it holds dried detergent better than wire wool. Unfortunetly it
usually comes preimpregnated with solid detergent, which is very
hard to remove. However it can be easily obtained in a plain form at
some hardware stores. The improvement I have made is to mount the
regenerator pad into its space diagonally, and have the working
fluid pass through it on its thinest axis without
having to suddenly change direction.

My current plan uses 2 crankshafts taken from commercial
mowers of about 8 hp with displacments of about 180-190 cc. These
will be connected end to end 90 degrees opposing. The aluminum from
the cylinders will be melted and recast into new cylinders of
similar bore and stroke, but having thinner walls, more and larger
metric head bolts, higher compression ratios, and no cooling fins.
The piston rods will be made of high speed drill rod and the
connectng rods will be stock. The hot end exchanger will be made of
high temp steel. An origional idea I have for a reusable set of
gaskets including the head gaskets; is to make them from fireproof
silicone rubber resin. This is not normal silicone or rubber. It is
used as a mold for casting metal so it should stand the heat. Also,
to improve strength, I will use fiberglass to make a tough
composite. The working gas will be air pressurised to a cold
pressure between 200 and 250 psi. On the occasion the speed will
have to be instantly changed, a valve connected to both regenerators
and operated by a twist grip on the tiller will be opened or closed
to admit or restrict airflow between the two pressurised sections.
It will burn PVO (pure vegetable oil) or B-100 biodiesel

Total cold end heat exchanger volume: 41.8 cc
Total cold end heat exchanger internal surface area: 678 square cm
Total hot end heat exchanger volume: 64.5 cc
Total hot end heat exchanger surface area: 343 square cm
Total regenerator space volume: 126.75 cc
In cylinder dead volume: 77 cc
Total connecting tubes internal volume: 12 cc
Total displacment: 760 cc

I need advice on whether certain methods described are
feasible. I have or will have sources for all aformentioned
materials. I need to know what problems that I might run into when
constructing this engine, or possibly operating it. I would like to
hear ideas for improvement. I would find a calculator that will
handle the unusual specifications and configuration of this engine
and still give an accurate answer very useful in determining which
outboard engine model to convert to Stirling.

Daniel C. Skorupka
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