Automatic Window Opener - how does it work?

Hi friends

I'm working in a community center repair workshop once a week. One of our customers brought us an automatic window opener used for a greenhouse, and we were not able to find out how this contraption really works - but it does work, by using solar energy only.

It consists of a hollow cylinder filled with a kind of liquid petroleum/wax mix. A piston is submerged within the liquid that is not sealed towards the inside cylinder wall. This means that the pressure on both sides of the piston is always identical. On one side of the piston there is a push rod with a diameter of about 1/4 of the one of the piston. This rod goes through a well-sealed hole in the top of the cylinder, there are no leaks. When the cylinder is heated by the sun, the rod moves - rather powerfully but very slowly - outwards, and back again when it cools down.

We imagined two different, possible explanations, both based on the fact that liquids are almost incompressible:
- The areas of the two faces of the piston are different by the cross section area of the rather fat rod protruding to one side, so the forces on both faces of the piston are different, too, causing the piston to move outwards when the internal pressure rises with the temperature.
- Since the rising temperature within the incompressible liquid causes it to expand, the only way the increasing pressure can escape is in the direction of the push rod, moving it outwards.

Perhaps these two possibilities even describe the same mechanism, only with different words?

Hope anyone out there knows the correct answer for us - thanks for trying to explain!

Please feel free to ask back if I didn't describe it in a comprehensible way - if so, it is because I'm not a native speaker.

Cheers & thank you
Robert

Heffalump said:

A piston is submerged within the liquid that is not sealed towards the inside cylinder wall. This means that the pressure on both sides of the piston is always identical.

The piston limits the travel of the rod to prevent it from overextending. It is also a rod guide that keeps the rod straight and in line with the cylinder.

Wax expands when it melts, and it melts over a small temperature range. Therefore, there is a small temperature difference between the open and closed positions. Of course, there is also a lag because it takes time to melt or freeze the wax.

The force is the pressure inside the cylinder times the rod area. Or you could say that the pressure is the force divided by the rod area.

The principle is very similar that of a gas strut (search the term).

Thanks JR. I'm glad that at least our first guess was correct, then.

Don't know about the gas strut, but doesn't that have a sealed piston and different pressures on the two faces of the piston? I'm sure that it is used for totally different applications, at least.

You might look at these control valves, that use oil thermal expansion to actuate control valves.

In this case, you are using a solar energy collector (of sorts) as the driver.

I was wondering what this made me think of: This is exactly how a thermostat works on every combustion engine, even today where mechanical controls have been eclipsed by electronics.

Note in the second figure how the expansion of the wax forces the pin out of the cylinder (or, in this case, actually pushes the cylinder out against a spring since the pin is fixed).

Thanks, guys! It's interesting how one single invention can be applied to totally different subjects

Heffalump said:

Thanks JR. I'm glad that at least our first guess was correct, then.

Don't know about the gas strut, but doesn't that have a sealed piston and different pressures on the two faces of the piston? I'm sure that it is used for totally different applications, at least.

If the "piston" is not sealed, as you said in your first post, the pressure is the same within the whole enclosure. The "piston" therefore is not really a piston, but a retaining flange of the rod, as someone correctly pointed out. The force exerted is then equal to the fluid pressure times the surface area of the cross section of the rod. You can think of it as follows: the fluid pressure exerts a force "from below" onto the rod flange ("piston") equal to the fluid pressure itself times the entire area of the rod flange ("piston"). The same fluid pressure pushes in the opposite direction "from above" onto the part of the flange protruding from the rod. Thus, the forces acting on the protruding part of the retaining flange balance out, leaving a net "push" on the sole area of the rod.