Posted by Bruce Richmond on March 25, 2010, 12:29 am
The trapped air is acting as a spring while the water has mass. The
mass of the moving water/air is the inductance 'L'. The spring and
mass combine to determine the frequency that it resonates at.
Watching some of the related videos the engines that have no water or
piston make noise because their frequency is so high. The mass is
just that of the air. With either water or air you can still think of
the moving mass as a displacer. The cold chamber has a fixed volume
but accepts/rejects air depending on its pressure. The hot chamber
has varying volume. The water displaces air in the hot chamber,
forcing it into the cold head. Even when there is no water the air
can be thought of as a displacer.
On another note (yeah, there's a pun in there ;) when a pressure wave
reaches the end of a tube it reflects and goes back down the tube.
That idea is used to tune expansion chambers on 2-stroke engines. You
can change the frequency by the length of the tube and expander and
reducer cones can magnify the signal.
Posted by Morris Dovey on March 25, 2010, 4:51 am
On 3/24/2010 7:29 PM, Bruce Richmond wrote:
Ok. I think the rate at which heat is transferred into the hot head is
also a significant factor.
Hmm - I hadn't looked at it from that perspective before now. It'll take
me a while to internalize this.
I'm just beginning to think about designing for frequency - a result of
seeing the TA/Laminar engines. At first I was thinking about pushing the
frequency to whatever speed the check valves could handle, but now I'm
thinking that I might want to trade off speed for displacement in order
to not waste the momentum of the water being pumped...
I like puns, so pun away :) Oh goody - another opportunity to display my
considerable ignorance: Please speak more on pressure waves, and
expander/reducer cones. This has the feel of something I should
understand (search terms and links welcome!)
Bruce, thank you for being one of those who help me identify the things
I need to learn about. It isn't always comfortable, but progress would
be a _lot_ slower if I had /no/ clue as to what I didn't know. :-/
DeSoto, Iowa USA
Posted by Bruce Richmond on March 25, 2010, 5:31 am
Yes, increased heat increases the spring pressure in the hot chamber,
forcing air into the cold head where it cools. The displacer puls
back and draws the cool air out of the cold head into the hot chamber
to be heated again. In the process it gives the displacer a little
kick that will run down the tube and be reflected back to push heated
air into the cold head again.
Not sure but I think the frequency will be highest with no load. As a
load is applied it will drag the frequency down. Same idea as with a
solid piston Sterling. Let it run free to get max rpm, but it will
run slower under load. Put too much load on it and it will stall.
I did a google search on "expansion chambers exhaust pipe theory".
Might want to look at http://www.pmbmodelboats.com/tunedpipetheory.htm
The whole idea is to use the positive and negative pressure waves to
scavange and presurize a cylinder or chamber. The heat added in the
hot head gives each pulse an extra kick.
Posted by daestrom on March 26, 2010, 4:36 pm
Bruce Richmond wrote:
Thing about a lot of that is that pressure waves travel at the speed of
sound in the medium they're traveling in. With engine exhaust, this
means the length of the pipe is related to speed of sound and frequency
of pressure pulses (RPM).
But I think any significant mass of water column is going to have a
natural frequency that is very much lower than a typical engine speed of
a several tens of hertz. So the 'tuned pipe' length would be quite a
Posted by Bruce Richmond on March 27, 2010, 10:41 pm
It would be longer for the loser frequency if they traveled at the
same speed, but they don't. In fact the two set-ups are not directly
compareable. With the air column the pressure wave has to travel to
the end of the tube before it reverses direction. The frequency is
dictated by the length of the tube and the speed of sound. With the
water column the water rises until its momentum can no longer overcome
I haven't looked yet but I am wondering if water can be pumped at the
frequency of sound, and how low a frequency do the air tube engines