Temperature/preessure change while running?

[steedspeed]

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

[Alfista]

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]

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]

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]

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]

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]

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]

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:

https://youtu.be/VcULP9Y-01A

Greetings

Ralf

[steedspeed]

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]

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]

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]

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 https://www.youtube.com/watch?v=O-INJEtLeZU 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 https://www.youtube.com/watch?v=0-rFOD75hwY 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.