mklotz
Well-Known Member
In a private communication one of the forum members asked for some insights on building a first Stirling. I suggested to him that it might be more profitable to move such a discussion to the forum where others could possibly benefit from the discussion and where more knowledgeable members than I could contribute their insights as well.
That said, here's what I consider to be the simplest design for a first Stirling.
In a Stirling the displacer motion and power piston motion must be ninety degrees out of phase with each other. If the two cylinders are aligned with each other, the crank arrangement needed to accomplish that introduces another degree of complexity that one doesn't need in a starter engine.
By mounting the two cylinders orthogonal to each other the crank arrangement becomes particularly simple. Both connecting rods can connect to the same pin on the crank disk and, for light duty demo engines, only a single ball bearing is needed to support the crankshaft.
[Aside: All Stirlings have low specific power and small Stirlings benefit immensely from any reduction in friction one can effect. While a well-crafted journal bearing would probably work here, ball bearings are cheap and most of us have some in our junk box. Since the engine is meant to demo the concept rather than model some particular engine, I recommend using a ball bearing if possible.]
Since the two cylinders are physically separated, a connection between the two must be provided so the air expanding in the displacer chamber can act upon the power piston to drive the flywheel. Here, again for simplicity of construction, I used a piece of plastic tubing. To ensure the connections are air tight, I wired them using a tool I made (copied from a commercial design) for the purpose.
(Someday, I'll make that the subject of a separate post.)
The hot side of the displacer chamber was turned from a piece of cold-rolled rod, thus avoiding the problem of sealing the end. The finned cold side is turned from aluminum.
Stirling engines come closest, of all engine types, to operating on a true Carnot cycle.
http://en.wikipedia.org/wiki/Carnot_cycle
so their effiency is equal to:
1 - Tc/Th
where Tc and Th are the cold and hot temperatures of the displacer chamber respectively. I can rewrite this expression as:
(Th - Tc)/Th
(Th - Tc) is the temperature differential of the engine and, obviously, the larger it is, the greater the engine efficiency.
For a desktop engine, we have some practical limits on what Th can be. While an oxy-acetelyne torch would provide a nice Th, we're more likely to be using something like an alcohol lamp to establish Th.
This means we're more or less limited to what we can do with Tc. Static cooling with fins, as shown here, works but, inevitably, the fins heat up and the efficiency suffers. Forced air over the fins helps and, indeed, some small Stirlings use a tiny fan driven off the crankshaft to effect such. A water jacket, either static or dynamically pumped, as used in Jerry Howell's Ringbom engine I showed, is far better but way over the top for a first engine.
The displacer for this engine is made from thin walled steel tubing with caps silver soldered into each end. Aluminum could have been used but the displacer acts as a crude regenerator (subject for another discussion) and steel, with its lower conductivity, is a better choice here).
Remember that the displacer is a loose fit in the displacer chamber - it's not a piston. As its name implies, its job is to 'displace' the air from one end of the chamber to the other. Thus, the air must be able to flow around it as the displacer moves.
Finally, with their low specific power, Stirlings are especially sensitive to friction. All parts must move easily and smoothly. Nevertheless, the power piston must form a good seal in the power cylinder and the fit of the gland where the displacer rod enters the displacer chamber must be as air tight as possible commensurate with easy movement. (Often, a bit of heavyish oil on the displacer gland will improve performance dramatically.)
Well, I've run on long enough. I hope this lengthy discourse both encourages and helps some of you to build your first Stirling.
That said, here's what I consider to be the simplest design for a first Stirling.
In a Stirling the displacer motion and power piston motion must be ninety degrees out of phase with each other. If the two cylinders are aligned with each other, the crank arrangement needed to accomplish that introduces another degree of complexity that one doesn't need in a starter engine.
By mounting the two cylinders orthogonal to each other the crank arrangement becomes particularly simple. Both connecting rods can connect to the same pin on the crank disk and, for light duty demo engines, only a single ball bearing is needed to support the crankshaft.
[Aside: All Stirlings have low specific power and small Stirlings benefit immensely from any reduction in friction one can effect. While a well-crafted journal bearing would probably work here, ball bearings are cheap and most of us have some in our junk box. Since the engine is meant to demo the concept rather than model some particular engine, I recommend using a ball bearing if possible.]
Since the two cylinders are physically separated, a connection between the two must be provided so the air expanding in the displacer chamber can act upon the power piston to drive the flywheel. Here, again for simplicity of construction, I used a piece of plastic tubing. To ensure the connections are air tight, I wired them using a tool I made (copied from a commercial design) for the purpose.
(Someday, I'll make that the subject of a separate post.)
The hot side of the displacer chamber was turned from a piece of cold-rolled rod, thus avoiding the problem of sealing the end. The finned cold side is turned from aluminum.
Stirling engines come closest, of all engine types, to operating on a true Carnot cycle.
http://en.wikipedia.org/wiki/Carnot_cycle
so their effiency is equal to:
1 - Tc/Th
where Tc and Th are the cold and hot temperatures of the displacer chamber respectively. I can rewrite this expression as:
(Th - Tc)/Th
(Th - Tc) is the temperature differential of the engine and, obviously, the larger it is, the greater the engine efficiency.
For a desktop engine, we have some practical limits on what Th can be. While an oxy-acetelyne torch would provide a nice Th, we're more likely to be using something like an alcohol lamp to establish Th.
This means we're more or less limited to what we can do with Tc. Static cooling with fins, as shown here, works but, inevitably, the fins heat up and the efficiency suffers. Forced air over the fins helps and, indeed, some small Stirlings use a tiny fan driven off the crankshaft to effect such. A water jacket, either static or dynamically pumped, as used in Jerry Howell's Ringbom engine I showed, is far better but way over the top for a first engine.
The displacer for this engine is made from thin walled steel tubing with caps silver soldered into each end. Aluminum could have been used but the displacer acts as a crude regenerator (subject for another discussion) and steel, with its lower conductivity, is a better choice here).
Remember that the displacer is a loose fit in the displacer chamber - it's not a piston. As its name implies, its job is to 'displace' the air from one end of the chamber to the other. Thus, the air must be able to flow around it as the displacer moves.
Finally, with their low specific power, Stirlings are especially sensitive to friction. All parts must move easily and smoothly. Nevertheless, the power piston must form a good seal in the power cylinder and the fit of the gland where the displacer rod enters the displacer chamber must be as air tight as possible commensurate with easy movement. (Often, a bit of heavyish oil on the displacer gland will improve performance dramatically.)
Well, I've run on long enough. I hope this lengthy discourse both encourages and helps some of you to build your first Stirling.
