Hybrid Car – More Fun with Less Gas

Single-Piston Fluidyne

register ::  Login Password  :: Lost Password?
Posted by Morris Dovey on March 21, 2010, 12:34 am
For anyone interested in Stirling cycle engines, and fluidyne engines in
particular, I've added a web page to present a "proof of concept" design
for a single-piston in-line fluidyne at


I thought folks here on alt.solar.thermal might find it interesting...

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by daestrom on March 21, 2010, 2:35 pm
Morris Dovey wrote:

Hi Morris, always enjoy reading your musings/experiments.

So, from what I understand, the area above the regenerator is the 'cold'
side and the area below is the 'hot' side?

I can see how the expansion/contraction of air will cause the level in
the plastic tube to oscillate, but I don't see what will move the air
from the hot to cold and back.  Don't you need some sort of 'displacer'?

In your other fluidyne engines you get this by having the two water
columns rise/fall at different times/rates.  So the air is shuffled back
and forth somewhat from hot/cold.

Or is the regenerator some how free-floating and moving up/down?


Posted by Morris Dovey on March 21, 2010, 3:28 pm
 On 3/21/2010 9:35 AM, daestrom wrote:

Yes, exactly so.

No, all that's needed is for there to be a phase difference between the
expansion and contraction cycle in the hot head and the expansion and
contraction cycle in the cold head.

What we want is for the hot side expanding air to displace the water
downward faster than it expands into the cold head, and for back
pressure from the load to help push hot (expanded) air through the
regenerator so as to have a quarter-cycle delay between the hot side
cycle and the cold side cycle. Any phase difference at all will result
in oscillation, but it's that quarter-cycle difference that's golden.

Sorta true - what I found was that as I got my fluidynes running as well
as I could get 'em to run, there wasn't much visible piston motion in
the cold side. It had been in front of me the whole while but I just
hadn't been paying attention - see


to see just how blind I'd been.

Someone here on alt.solar.thermal had suggested (very much earlier and I
hadn't taken the suggestion anywhere near seriously enough) that I take
a look at thermo-acoustic engines. I did take a quick look, but it took
a while for me to consider them in terms of the gas laws.

About the time I was puzzling over the apparent uselessness of the cold
side piston, I saw the video at


and was (finally!) able to connect the dots. The engine I'm about to
build is configured a bit differently than the one in the video, but I
think I have a viable configuration - and I think that I can use the
bore/length of the regenerator to tune the displacement and operating speed.

Nope - there isn't any moving hardware. The regenerator will be brazed
in place between hot and cold heads.

The very first thing I have to try (even before I add the plastic tube)
will be to see if it'll oscillate /without/ water when I heat the hot
side with a torch... :)

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by daestrom on March 22, 2010, 9:00 pm
 Morris Dovey wrote:

Well, one rule I live by, "If it happens, it must be possible..."

So a certain speed of oscillation versus the air moving through the
orifice in the re-generater is what gives you the phase displacement.

Some sort of RC-circuit-like calculation for the orifice and cold head
storage volume versus the oscillations in pressure caused by the piston
(or water column in your case).

Perhaps if that 'RC' constant for the cold head /orifice is 'tuned' to
match the natural frequency of the water column?

But I thought the natural frequency of the water column can change if
you're pumping water against some variable head.  Of course there are a
lot of pumping arrangements where you won't have a variable head factor,
so that's not a deal-buster.


Posted by Morris Dovey on March 23, 2010, 3:32 pm
 On 3/22/2010 4:00 PM, daestrom wrote:

Heh - you'd laugh if you could see the zig-zag smile /that/ produced! :)

Sorta/kinda. It's more the rate of expansion of the air in the hot head
and (apparently) the restriction of flow to/from the cold head that's
imposed by the regenerator passage.

Again, sorta - my difficulty all along has been that "RC" is expanded to
"PVTt" (small /t/ for time).

Unless you're building a show-n-tell engine, it appears best to
completely disregard the water column (except as load) and any resonance
(or lack of resonance) associated with it, and to reduce the amount of
water in the engine as much as possible without introducing flow losses.

With a solar-powered engine, the input temperature is going to vary
considerably, so I want to limit the "fineness" of tuning (Q?) to
provide good performance over a workable range of Th and Tc values.

Hmm - it just occurred to me that the combination of cold head and
regenerator might add a bit of 'L' to the circuit...

I don't think so. In just about every practical pumping application, the
head will be essentially fixed and the water being pumped will move in
one direction only. Given the essentially non-compressible nature of
water, that wouldn't leave much room for natural frequency effects.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread