A Stirling theory question on average pressures

Discussion on Stirling or "hot air" engines (all types)
Post Reply
omblauman
Posts: 6
Joined: Thu Apr 12, 2018 4:01 am

A Stirling theory question on average pressures

Post by omblauman » Sat Apr 21, 2018 9:37 am

In a Stirling engine there will always be small but finite leaks in the work piston seal. In steady state the average internal pressure will unavoidably reach an equilibrium with the steadier carter pressure.
In an atmospheric engine the average internal molecular density will be lower than atmospheric in inverse proportion to the ratio of the average internal to external temperature .
I calculated this effect for my engine which is described, unfortunately in Italian, in this web page
https://sites.google.com/view/stirlingdiriva/home
Anyway the main characteristics, for what matters here, is hot T 190 C and cold T 35 C, with a very large displacer of 800 cc.
According to my calculations this effect corresponds to a negative pressurization of roughly 30%, a 30% reduction of the atmospheric molecular density.
It should imply a proportionally lower engine power.
Did i say anything wrong? Does anybody know if this effect has been already described somewhere else?
Thanks Oscar

PS Incidentally the connected manometer doesn't seem to indicate this at all, see the youtube video at Quite the opposite the manometer seems to indicate that the average internal pressure is consistently higher than the external, the 0 reading on the scale. The pressure gauge might read incorrectly at these temperatures though.

Alfista
Posts: 91
Joined: Thu Feb 18, 2016 9:14 pm

maintaining a balance of vacuum and pressure during warmup

Post by Alfista » Sat Apr 28, 2018 6:28 am


I recently read an article having to do with the snifter valve. They have been used on a variety of engines. The article in question was looking at a Heinrici engine, many of which used the snifter valve. Over time, an engine, whether working or at rest will equalize with atmospheric pressure or come close to it but the valve helps to keep the pressure / volume relationship slightly more in the positive territory during warmup. That is to say, if the cycle leans more into vacuum than pressure, then it will not run as well or produce as much power. I have read that LTD engines are even more susceptible to this problem, perhaps because of the low specific power output but I am not sure about that ... Of course, a large LTD engine could have a rather large volume of air and relatively good sealing which could also have something to do with it.



omblauman
Posts: 6
Joined: Thu Apr 12, 2018 4:01 am

Re: A Stirling theory question on average pressures

Post by omblauman » Sat Apr 28, 2018 2:19 pm

Thanks Alfista, very interesting.Do you have a reference for the snifter valve article?
To my intuition the nearest the average pressures of the carter and the engine are the easier should be for the engine to run.
By easier I mean reduce the torque fluctuation, make it rely less on the flywheel to complete the cycle. A snifter valve applied to the work cycle would shift it to higher pressures as you say so essentialy slightly pressurize the cycle hence making it more powerful. It almost looks like moderate leaks could be beneficial to make it to run more smoothly. A perfectly sealed engine starting from cold would find itself with an imbalance of averages once hot.

Blaf
Posts: 30
Joined: Fri Mar 02, 2018 2:50 am

Re: A Stirling theory question on average pressures

Post by Blaf » Sun Apr 29, 2018 1:58 am

This video imho nicely demonstrates the effect of a snifter valve.
[youtube][/youtube]

Ian S C
Posts: 2137
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: A Stirling theory question on average pressures

Post by Ian S C » Sun Apr 29, 2018 3:08 am

I tried a snifter valve on a couple of my engines, but found there was little if any power gain, so I gave up. Some people have found quite a bit of power gain, maybe my machining is too sloppy.
Ian S C

omblauman
Posts: 6
Joined: Thu Apr 12, 2018 4:01 am

Re: A Stirling theory question on average pressures

Post by omblauman » Sun Apr 29, 2018 3:51 am

About snifter valves nobody seems to indicate what the pressure threshold for opening is and what is the negative excursion of the engine pressure. An ideal snifter valve, that is a valve which would open with an infinitesimal negative pressure difference and of negligible inertia, seems to me that it would tie down the minimum work pressure to the pressure the othe rside of the valve. In atmospheric cases like the one of the video, that would be minimum P = atmospheric P, that is an average pressure higher than what one would expect in a case where the average pressure is dictated by the leaks. Has anybody checked all this to see if it’s true?

Post Reply