Stirlingengineforum.com

Hi,

Have been busy with work and haven't post much lately. Want to play devil's advocate a bit. I understand if you pressurize the engine, you also need to pressurize the crank case. Here is my problem. When the engine is running, the pressure in the engine will always be greater than the pressure in the crank case. So what is the point? If a pressurized crank case does increase efficiency, what is the optimal pressure in the crank case in relation to the engine, say while the engine is cold? Just like to hear what you guys think.

Hi Aviator 168, I'm thinking, thing will come along slowly. First, with the pressure elevated above the piston, you would soon find that it would become equal to the pressure below the piston, theres got to be a leak there for a start. If the whole motor is pressurised the load on the bearings is lightened and the action of the motor cushoned. the next problem is of course the crankshaft bearings, best way round that is to fit a generator/ alternator inside the crankcase. You then just need a battery to motor over the generator for starting, or better still make a free piston motor, it should be self starting, I'v got an unpressurised one, sometimes it starts, sometimes it needs a bit of a hand. Ian S C

Ian,

Yes. If there are leaks between the piston and the cylinder wall, the pressure in the crankcase will eventually arrive at the mid-point of the hot and cold pressure, which is ideal. I have no problem sealing the crankcase. Just that electricity is not my desire output. I have been looking into magnetic couplings lately. Their efficiency can be as high as 99%. Another thing. How do you lubricate the bearings? I am thinking of using graphite powder or some kind of self-lubricating bearing.

would it not be better to put a snifter valve into the cold side of the piston? Then the equalization will be almost instant?

sam

Aviator and Ian,
As always I try to answer trying to understand before all the question, and is not so easy for me.
I'm sorry if I'm wrong.
In pressurize an engine does not happen anything special, except that the gas is in greater quantity, and is more dense.
In fact we live in a pressurized environment, .... at atmospheric pressure.
Having gas in larger quantities is a great advantage, since the heat content is proportional to the mass (weight) of gas (four times higher pressure means more gas four times); about "dense": the increase in viscosity would be negative as a loss of friction of the gas in motion, but I think that, for this energy loss, is not very significant. The stress on mechanical parts should be greater, since, just the engine produces more power (power is approximately proportional to pressure, pressure should be triple, triple the power), so bearings and connecting rods must be more robust.
Given that (unlike the internal combustion engine) the differences in pressures that drive the engine is limited a little sizing believe that a strengthening between upstream and downstream of the piston engine is sufficient.
One factor that may have a bit of importance is that a little power is lost in the compression of gases in the crankcase when the displacer and the piston engine are closer to the crankshaft. This compression is avoided when there are more engine cylinders (typically four, 90° offset. ... but do not get groped...).
I thought, for the motor with magnetic coupling, for use as a "diaphragm" between the pressurized and outside by a cylinder of fiberglass epoxy material, that is among the most robust non-magnetic, for facing of the magnets.
For lubrication: lubrication I think have to be to avoided at all, I found data for plain (not roll) bearings (needle of steel Teflon-coated) which have good wear resistance without lubrication (the relation is lubricated 5 m/sec; unlubricated 2 m/sec) ; only limitation so is the scrolling speed of the parties, this can be achieved by decreasing the diameter of the crank, possible considering the fact that the stresses beared aren't so many high. Sintherized bronze-graphite or good bronze plain may be is sufficient too.

I'm sorry that are in Italian:
Not lubricated dry bearings: composed by layers of steel, synterized bronze (porous), and PTFE
see:
http://www.cgboccole.it/cg_boccole.pdf

Ferraccio, my original question was why the crankcase has to be pressurized (if the cylinder is pressurized) to make it a stirling. After reading all the related posts in the forum, I still can't think of any advantage for pressurizing the crankcase. I understand that when the gas in the engine gets cooled, people want the pistons to be pulled up (actually pushed by the pressure in the crankcase). But this could be done by the flywheel and the energy spent doing that will be recouped during the hot gas cycle.

If you use double acting cylinders you don't have to pressurize the crankcase just the cylinders . If you can get ahold of the book Stirling Cycle Engines by Andy Ross it touches on the subject and shows some pics of double acting 4 cylinder engines in the 140 horsepower range.

As skunkworks says, a snifter valve in the piston, or as some motors have, a transfer port between bottom dead center and the the crankcase.
pressurising the crank case is no different than the case of an atmospheric engine with an open crank. The pressure is the total pressure, the small amount of gas in the working side of the motor has its pressure elevated by heat to over come the crankcase pressure, asthe pressure drops, the crankcase pressure returns the piston to TDC. Ian S C

So the point of having the crank pressurized is to keep the cylinders pressurized, not anything with the operation of the engine.

I believe the primary reason for pressurizing the crankcase is to reduce the requirements on the piston's sliding seal by moving the high pressure seal to the shaft's rotary seal. Rotary seals are easier to design and/or more effective than sliding seals.

You see that I had not understood.
My answer is simple and even obvious. any moving parts can not keep up the pressure for more than a few tens of seconds.
In short after a few seconds the engine returns to work in atmosferic pressure.
It is accentuated in the case of alternating movements, as the piston, a little 'less on the rotating parts, like the shaft, but it is still present.
This is particularly dangerous (as you can understand) into the turbines of nuclear power plants, where you CANNOT avoid gas leaks from the rotation axis, even when the gas can be radioactive. The losses are minimal, but always there are.
The only way we have of pressurizing the engine is locked in a closed box. The only seals that are just as they always fixed.
All Stirling engines are "pressurized", those elements are "pressurized" to atmospheric pressure, and the engine runs for "pressure changes" with respect to atmospheric.
In what I call really like the pressure of the pressurized engine is much higher (in the closed box) and then the gas is denser, denser gas in this place the "pressure changes" (relatively to pressurization) that make in the engine more efficiency.

For the rest: the increase of loads on the pistons (more power = more torque = more load), the increased viscosity (friction to the flow of gas), internal back pressure in a closed volume, these are just facts of an analysis (I hope as complete) overview of the situation.
Ian has understood this perfectly, and also gave an intelligent solution possible, .....but we go in hard.
Then: I was not seeking a solution. Only highlights the elements, the solution is not the flywheel.

The importance (often very small) of each factor has been well defined by me.

I do not know which elements have been established on comments by others, and do not share them.
Ferraccio

The flyweel is not a manufacturer or supplier of energy, is only an energy storage unit, you can not think to earn what you lost.

(This is just to see what happens ...)

The Philips Bungalow Set, the '50 years Stirling.
http://www.youtube.com/watch?v=M9UKu-AP02k
is a type of "dynamically pressurized" Stirling engine, (NOT closed in a box).
The pressurization when the engine is stopped vanishes rapidly, the compressed gas escapes from the seals of the piston and then the motor shaft, which goes outside.
To maintain the pressurization dynamically, you need a compressor, which is contained in the engine, and that absorbs power from the engine (I think around 20%), the compressor is too complex and needs to be lubricated and cooled, greatly complicating engine.
To start the engine, under pressurization, (when the engine is off is at atmospheric pressure) there is a reservoir air pressure builds up and allows you to make some attempts to start.

With the Phillips engine the tubes forming the cage that the motor is mounted in are the air reservior. Oh, by the way, it was a Phillips engineer that thought up the name Stirling for hot air engines. Ian S C