Operating a Stirling Engine Inside a Pressure Vessel

[stuffandthings]

I will be doing a science project on the effect of air pressure on the efficiency of a Stirling engine.
I don't have the mechanical skills to build a pressurized Stirling engine, so I have decided to operate a regular LTD Stirling engine inside a pressure vessel. I will use a tachometer to measure the rpm of the engine.
I have confirmed on another post that this will increase the pressure of a Stirling engine.

So I have several questions on doing this.
Firstly, will a pressure of about 87 psi (6 bar) crush a regular Stirling engine, or will the external pressure inside the Stirling engine balance out?
I am using a new Cornelius keg (rated for 130 psi) as a pressure vessel. It has a opening of 4 3/4 inches (but an actual diameter of close to nine inches on the inside) and a height of 25 inches, and the Stirling engine I plan to build should fit in easily.

Would putting a cup of almost-boiling water below the Stirling engine inside the pressure vessel increase the pressure noticeably?
How would I get the Stirling engine moving? Could I place the Stirling engine while it is working and then increase the pressure? Or would that take too long and cause the heat in the heat source to dissipate?

Thanks.

[Ian S C]

What are you going to heat the engine with? Gas, or spirits, and how will you manage a fire inside the pressure chamber. With a pressurised engine, the engine is the pressure chamber. Ian S C

[stuffandthings]

I'm heating a ltd Stirling engine with hot (almost-boiling) water
You would have to especially make a pressurized engine right?
A tutorial for a small ltd Stirling engine stated to glue together three pieces of metal to form the pressure chamber. I know the pressure in the two ends of the engine changes due to the contraction and expansion of the working gas, but would the mean pressure still be atmospheric pressure?

[MSOEStirlingTeam]

So I have several questions on doing this.
Firstly, will a pressure of about 87 psi (6 bar) crush a regular Stirling engine, or will the external pressure inside the Stirling engine balance out?
I am using a new Cornelius keg (rated for 130 psi) as a pressure vessel. It has a opening of 4 3/4 inches (but an actual diameter of close to nine inches on the inside) and a height of 25 inches, and the Stirling engine I plan to build should fit in easily.

Unless your engine is perfectly sealed, no. The pressures will balance out, as you have said. What you are doing is actually very clever, and one of the better ways to achieve higher operating pressures in your engine. Essentially, you are increasing the atmospheric pressure that your engine is operating at. The reason that this increases the power of your engine is a result of the equation for the work output of these engines: [Work]=[mass of gas]*R*([High Temp]-[Low_Temp])*ln([Max Volume]/[Min Volume]). If that is out of your league, don't sweat it! The most important thing to you is the first part, mass. As you increase the pressure, you are squishing more air molecules into a fixed space, increasing the mass of air that is inside your engine. With more mass in your engine, voila! More work! And for safe measure: [Power]=[Work]*[Revolutions per Minute]*([1 Minute]/[60 seconds per minute]).

Would putting a cup of almost-boiling water below the Stirling engine inside the pressure vessel increase the pressure noticeably?
How would I get the Stirling engine moving? Could I place the Stirling engine while it is working and then increase the pressure? Or would that take too long and cause the heat in the heat source to dissipate?

The ideal gas law is your friend here: PV=mRT. You can use it to figure out how much the pressure will increase if you start increasing the temperature inside your keg, provided you know how large it is. My guess, though, is that it won't affect it too much. Your water could cool off quickly, so it may be advisable to find a way to constantly heat your engine. Maybe a hand warmer (either chemical or electrical)? If you go with the water, don't forget that the boiling point changes with temperature!

Good luck on your project, I hope this helps somewhat!

[Ian S C]

I cannot imagine an LTD (low temperature) motor running as a pressurised unit, one of these motors has only enough power to turn it over, and no more. I would suggest an electric heating element, wrapped around the hot cap. To estimate the power of the motor, you need to (a) record the rpm (b)measure the torque, either in inch /ounces, or the metric equivalent, by doing this you can get a fairly accurate measure of the power of the motor in Watts or parts there of, but you need more power than a low temp motor.
As you increase the pressure in the motor you will find it requires more heat put in to maintain the heat at the hot cap, or if you start at atmospheric pressure, and the hot cap is at red heat, you will find that as the pressure rises, the red colour becomes duller on the same heat input.
I would recommend that you find and down load(it's free) Andy Ross's book "Making Stirling Engines", this will give you an insight to the development of Stirling Engines, and the effect of pressurisation, and the effect of modifications to motors after they have been built. Ian S C

[stuffandthings]

Ian S C

Would the crankshaft simply not move in a pressurized atmosphere?

MSOEStirlingTeam

I know increasing ambient temperature would introduce other inefficiencies due to Carnot's law, but would increasing temperature have the same effect? By making particles move faster, it also would cram more particles into one area.
I have seen a formula similar to the one you posted on a research paper, and it said pressure instead of mass.

[Ian S C]

Don't get bogged down by so called theories, stick to the basic ratios, use the most suitable materials you can get, keep the friction to a minimum, and build a hot air engine, then start experimenting. Ian S C