Temperature/preessure change while running?

Discussion on Stirling or "hot air" engines (all types)
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steedspeed
Posts: 14
Joined: Sat Jan 20, 2018 1:45 pm

Temperature/preessure change while running?

Post by steedspeed » Sun Feb 04, 2018 3:03 pm

Hi everyone. I'm wondering if anyone knows what the pressure change is inside a Stirling engine while it is running?

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

formula

Post by Alfista » Sun Feb 04, 2018 7:33 pm

The formula is (T1-T2) / T2 = Pressure in bars. Dividing the result by 2 will probably give you a more realistic number :÷(

Think 2 - 5 psi. T is in kelvin. T1 is the high temperature. T2 the low

steedspeed
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Joined: Sat Jan 20, 2018 1:45 pm

Re: Temperature/preessure change while running?

Post by steedspeed » Sun Feb 04, 2018 8:36 pm

Ok. I'm thinking of building a Stirling that runs fairly slow soit has more time to heat and cool and uses the resulting torque to do work rather than high speed but low HP

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

Re: Temperature/preessure change while running?

Post by Ian S C » Mon Feb 05, 2018 2:51 am

steedspeed, with a Stirling Engine the power band is fairly flat regardless of speed, I find that the best power is around half the free running speed, which on my motors is usually about 600 to 1000 rpm.
I think in your last line you mean low revs and high torque against high revs and low torque, which ever way you do it comes out the same, just that one way you will need to gear it up, and the other, gear it down to get the revs you want.
Ian S C

steedspeed
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Joined: Sat Jan 20, 2018 1:45 pm

Re: Temperature/preessure change while running?

Post by steedspeed » Mon Feb 05, 2018 7:21 pm

Yes. I'm thinking of gearing up the flywheel so it carries the slow moving crank past TDC and keeps it going

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

Re: Temperature/preessure change while running?

Post by Ian S C » Tue Feb 06, 2018 2:24 am

I think you should aim at a crankshaft speed of 500 to 1000 rpm, the advantage of gearing up the flywheel is that you can reduce the mass of it, you'll have to do the maths, but if you double the speed the flywheel can be about 1/4 the mass, or something like that.
Ian S C

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

larger flywheel

Post by Alfista » Tue Feb 06, 2018 7:07 am

I agree in essence with all of Ian's comments. I would like to add that a better alternative to gearing up would be to make a larger diameter flywheel instead of a heavier one. Gearing up will add friction, that is certain and as such would be a better choice if the source of power were very high in torque but the regulation of that power required a high speed or some sort of finesse as in the case of a clock mechanism, or a hand-cranked flashlight. As far as gearing for the output of the engine, that is another matter. My limited experience and my reading suggests that matching the output to the load is critical for the successful and efficient application of most hot air engines since specific output is usually preciously low.


tom-rock
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Joined: Tue Feb 25, 2014 1:24 pm

Re: Temperature/preessure change while running?

Post by tom-rock » Sun Feb 18, 2018 1:30 pm

Hi steedspeed and all,

I have a pressure sensor attached to my stirling engine which measures the working pressure in real time while running.
At the moment it has a pressure difference with 10 bar meanpressure of 7 bar (8 bar min / 15 bar max pressure).

You can see it in this little video:



Greetings

Ralf

steedspeed
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Joined: Sat Jan 20, 2018 1:45 pm

Re: Temperature/preessure change while running?

Post by steedspeed » Sun Feb 18, 2018 2:46 pm

Thanks for the info guys. So Ralf. In one rotation the pressure change is how much psi? I'm not quite understanding what you mean. I'm trying to work out the force applied to the piston. I'm going to use a two 4 inch diameter pistons.

I also had another thought. Has anyone tried putting a bit of water inside a Stirling to increase the heat transfer and perhaps expansion rate?

ficoce
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Joined: Thu Apr 05, 2018 2:17 pm

Re: Temperature/preessure change while running?

Post by ficoce » Sat Apr 07, 2018 11:39 am

I'm planning a 4" diameter stroker engine, too. People rarely mention what stroke/bore ratios they use.Seems like 1:1 or less is used a lot from eyeballing things and the rpm's shown. I'm making something for a boat and 200-300 rpm, preferably less, would be really comfortable. Pressure difference depends on build tolerance - and build tolerance uses an exponential scale when it comes to time, money, and effort.

steedspeed
Posts: 14
Joined: Sat Jan 20, 2018 1:45 pm

Re: Temperature/preessure change while running?

Post by steedspeed » Sat Apr 07, 2018 2:48 pm

I have a full cnc machine shop so I'm well equipped to make good quality parts. My hope is to come up with a commercially viable generator for cabins and for camping etc.

joweizbrot

Re: Temperature/preessure change while running?

Post by joweizbrot » Sun Apr 08, 2018 8:08 am

I have a question: what is in general the average pressure in steady state running compared to the carter pressure? With reference to our primitive engine shown at we can't understand why the average pressure doesn't settle in steady state at the atmospheric pressure, =0 in the manometer shown in the video. My intuition would say so considering the ample leaks of the work piston which we observe when we pressurize the cycle gas (the carter is kept at atmospheric pressure) . See for example at where the cycle pressure is raised to roughly 2 bar absolute, 1 bar on the manometer.
Our understanding for the increased rpms (from 100 to 130) when we close the compressed air valve is the exclusion of the very large dead volume connected with the compressor line and reservoir.

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