dwell on displacer

Discussion on Stirling or "hot air" engines (all types)
tew
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dwell on displacer

Post by tew » Sun Jul 19, 2015 10:29 am

Has anyone put a dwell on the displacer shaft of a stirling engine. I tried it and seemed to improve speed about 10%. But it makes an awful racket. Advantage is that the displacer stays at end of cycle longer. For example as power piston moves to top of cylinder the air goes to the cold end of displacer longer and not heating air and working against itself. Displacer using my design still catches up to power piston about 40 degrees past the top of stroke. Any thoughts?

burnit0017
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Re: dwell on displacer

Post by burnit0017 » Sun Jul 19, 2015 10:42 am

Hi, I made a small steam engine using a scotch yoke and it had a similar effect. Interesting idea. Do you have a video? Maybe a scotch yoke will make less noise. Thanks for posting.

Ian S C
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Re: dwell on displacer

Post by Ian S C » Mon Jul 20, 2015 4:07 am

I'v used a Scotch Yoke on two motors, any improvement was diminished by the increase in friction in the displacer drive, some improvements could have been made. No increase in noise was noted.
Ian S C

staska
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Re: dwell on displacer

Post by staska » Wed Jul 22, 2015 5:53 am

In theory it adds up to 20% of power for gamma, in practice it showed close to 12% increased power. Dwell was set to 50%

tew
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Location: missouri,usa

Re: dwell on displacer

Post by tew » Sat Aug 01, 2015 4:26 pm

Thx for the replies. I still haven't had time to look up a scotch yoke but I will. I merely slotted the yoke at the middle of the displacer shaft. Bad part is the noise but its interesting how the displacer is slung out and in really making the travel time on the displacer minimal. And minimal is good except for trying to time it.

Bumpkin
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Re: dwell on displacer

Post by Bumpkin » Sun Aug 02, 2015 1:34 pm

Hi. The thing that always bothers me about these whang-bang systems of longer dwell, is that it takes energy to accelerate the displacer each time it moves instead of the normal crankshaft connection where deceleration returns the energy to the flywheel. An eccentric gear drive or roller cam or other kinematics can solve that problem, though with a bit of added friction and complexity. I don't doubt there is some added performance potential per static displacement, but part of the displacement equation is time, (RPM) and it seems these systems would have to lose as much in speed as they could gain in thermal transfer. I'm open (sometimes even eager) to being proven wrong though. :smile:
Bumpkin

Ian S C
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Re: dwell on displacer

Post by Ian S C » Tue Aug 04, 2015 3:00 am

One advantage that can be useful, the displacer drive can be made more compact, eliminating the need for a connecting rod on the end of the displacer rod.
Ian S C

tew
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Location: missouri,usa

Re: dwell on displacer

Post by tew » Tue Aug 04, 2015 10:05 pm

I probably should have said upfront that my engine runs at less than 300 rpm's. I wouldn't try slotting the yoke on anything running much faster. By the way I run my engine off a heat lamp.

Ian S C
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Re: dwell on displacer

Post by Ian S C » Wed Aug 05, 2015 2:08 am

My first motor was a V type GAMMA motor, power cylinder 3/4" dia x 3/4" stroke. A simple meths burner, and a speed of 1000rpm, I think the Scotch Yoke would have been OK at twice that speed. With the gas torch the speed was quite a bit more (unmeasured).
Ian S C

Bumpkin
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Re: dwell on displacer

Post by Bumpkin » Thu Aug 13, 2015 9:37 am

----------------------------------------------------------------
Hi guys. I think we got to talking cross-purposes. Ian, I totally agree that a scotch yoke can run fast and smooth. I might have misunderstood, but I believe the slotted yoke tew mentions is for a different purpose - to allow some crank rotation without the displacer moving at all for an extended duration at top and bottom dead center. Someone else mentioned that back in the earlier pages of the forum.

Anyway, the scotch yoke mention got me thinking about how it gives pure sinusoidal motion that a normal connecting rod would need to be infinitely long to accomplish. The thing I find interesting is that this points out how a very short connecting rod with increased deflection angle (and side-load issues) could actually increase dwell time to an extent.

Bumpkin

Ian S C
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Re: dwell on displacer

Post by Ian S C » Fri Aug 14, 2015 2:05 am

I see what you mean Bumkin. There was something a good many years ago in the "Model Engineer " magazine, it had a graph of the path of a displacer using a scotch Yoke, and an ordinary con rod, and I don't think there was too much difference.
Ian S C

vamoose
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Re: dwell on displacer

Post by vamoose » Sat Aug 15, 2015 3:45 pm

Here's that older thread which might have something of interest

Variable Phasing
http://www.stirlingengineforum.com/view ... 1616&hilit

tew
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Location: missouri,usa

Re: dwell on displacer

Post by tew » Sat Aug 15, 2015 11:34 pm

Thx for the thread vamoose on variable phasing. My displacer crank can be adjusted by hand and like Ian I didn't see much dif. between 85 and 105 degrees. But that was static timing. I thought with the dwell a larger throw on the crank would be required but because it slings the displacer piston out and in as soon as it runs out the slack I still got a full throw with the same crank throw and about the same timing. I know my dwell device is very crude and not practical but I thought the principal and the results were worth sharing in case someone would want to pursue it.

Ian S C
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Re: dwell on displacer

Post by Ian S C » Sun Aug 16, 2015 2:30 am

All ideas, and changes are worth looking at. I think the system of varying the phase angle is worth more, and easier than changing the dwell time of the displacer.
Ian S C

Tom Booth
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Re: dwell on displacer

Post by Tom Booth » Thu Jun 16, 2016 9:41 pm

I've been thinking of another approach to the problem. Two displacers, one above the other moving more-or-less independently.

During the Hot phase, the cold side of the chamber would be completely covered by the second displacer and vice versa.

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