Displacer motion

[SScandizzo]

Hi Guys,

Thank you Darryl for creating this forum, as I have few places to bounce my ideas.

I've been thinking about how the displacer moves in its cylinder in a basic beta or gamma engine. With a circular motion driving a fixed con-rod the displacer should be moving in a sinusoidal manner (that is, speeding up and slowing down). In moving this way, the displacer spends a significant period of time in transit between the hot and cold ends which would appear to mean that heat is being bled off as part of the air is still being heated and part of the air is being cooled.

With me so far?

What would happen if there was a way to allow the displacer to remain at the ends of its cylinder for a longer portion of its cycle? Would this be a more efficient use of heat energy?

There are several ways to do this and I'd love to explore it further, but before I ran off to the workshop, I was curious to hear what others thought...

-Stefan

[stanhbaker]

Ivo Kolin in his book "Isothermal Cycle Stirling Engine," 1983? and "Stirling Motor," 1991 used a snap action displacer. Also Hubert Stierhoff has a web site with flat plate stirling engines utilizing the same action. I'll have to hunt up the web site.

[SScandizzo]

Thanks for your leads, Stan. I've also seen a mechanism using two elliptical gears to drive the displacer in an almost intermittent manner. Here's the link:

http://www.stirlingsouth.com/richard/de ... opment.htm

They claim that the initial tests have been favorable, but they stopped posting after that point.

-Stefan

[SScandizzo]

Stan,

Ivo Kolin's book "Isothermal Cycle Stirling Engine" is not only out of print, but completely unavailable! If you happen to be able to post some of its contents electronically (or by email) I'd really like to see it.

-Stefan

[Moderator]

Use a rotary displacer and hook it to the power piston 90 degrees out of phase. I would be happy to e-mail the rough drawings I have. This eliminates several moving parts, namely the diplacer piston, which robs efficiency by having to change directions. It also acts as the flywheel and output shaft. This is in the 'drawing board phase' right now, but would like help from others in building it.

[SScandizzo]

A rotary displacer doesn't provide an intermittent heating and cooling cycle: it remains sinusoidal, which is my original concern. Keep in mind that there is a good reason that the Wenkel (rotary IC engine) hasn't supplanted the standard piston models: they are more expensive to manufacture and much harder to service (albeit it requires less maintenance on average due to fewer moving parts).

-Stefan

[stanhbaker]

Something(s) to think about:
==========================
The Stirling is a Strange Beast!

The kinematics of the common beta configuration results in ~sinusoidal motion of both displacer and piston due to slider crank mechanism.

QUESTIONS:
1. What effect does this have on the operation?
2. Is this of any real importance?

Heating and cooling of the gas is going on simultaneously as well as the total volume of the system changing due to ~sinusoidal motion of piston.

QUESTIONS:
1. Are these variations detrimental to best possible operation?
2. Can any improvements be made?
====================

[SScandizzo]

I'm in the process of building a second engine with the same specifications as my test bed engine. When completed, I plan to modify the original to use an intermittent movement displacer. I also just got done building an electronic counter that allows me to measure the RPM of my engines without interfering with their performance.

As you pointed out, air is both heating and cooling simultaneously during the cycle. This is a source of inefficiency because it degrades the temperature differential of the system. Ultimately, we'll only know for sure by experimentation. The article you sent me by Kolin describes a performance increase in engines with intermittent displacement, but doesn't record imperical data to support this claim.

I'll report back when I get my second engine completed.

-Stefan

[jumpers]

Sinusodial is the desciption of a particular cycle. It DOES NOT pause sir.It goes peek to peek. Actually, it goes from zero to peek, then to zero, then to peek. The positive to negative occurs between alternations(peek to peek) .A sin wave is equal at both peeks, unless "clipped". The peeks are refered to as the max, the clipped portion is the effective. This is determinded by the losses, due to rotation/friction,and windage. DC is different, as it has no peeks. Therefore, alternating, is the cycle, and it is constent. Only the frequency changes due to speed. The "delay" in a Stirling is the result of the cycle of a Stirling. There must be this type of "delay", to aid the strokes momentom. If you have been reading/searching, you do know that the Stirling is NOT like any other engine.That's why it is called a Stirling cycle.

[SScandizzo]

Point made, Jumpers. Let's call it a "modified Stirling cycle" then? There are a number of designs (such as Ringborn engines) that don't follow the strict specifications either but are commonly labeled as Stirling engines.

-Stefan

[sarimshaikh]

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