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What are the advantages and disadvantages of the cylinder being tall with a short diameter compared to the same volume cylinder being short and large diameter? LTD stirlings are usually short and fat. If you want a compact engine to run at higher temps where is a good stopping point to how short you can make the cylinder .I suppose a short cylinder is going to have higher rpms due to the shorter stroke. Heat transfer must play a part in shorter cylinders .Are there other problems?

fullofhotair, short stroke, high revs, lower torque, long stroke, lower revs, higher torque, you pays yer money, and yer takes yer choice, the power total should in theory be the same. Ian S C

fulofhotair, LTD's use large diameters for large and simple heat exchange areas. HE is crucial for power in SE's. However, short and fat causes high conduction loss (and radiation loss in a hollow displacer, if no baffles), especially at high TD. For starters, look at engines which actually "go", they probably have their proportions about right - for their TD.

Compactness might favour proportions which keep the air nearest to a sphere. However that would reduce the surface area - where HE occurs - exacerbating the most crucial design feature for power. There are five HE's in series, each dropping some temperature: outside of head, inside of head, regenerator, inside of cooler and outside of cooler. By far the most important is the regenerator, and it could even be counted twice because its exchange is in both directions per cycle.

If you're after compact power, I'd say start with the regenerator and design the engine round it!

jester.

Jesterthought
Some of the faster engines Ive seen on utube use the displacer as the regenerator. They have wire mess filled with rolled steel wool. What I see as taking up more space than it needs to is the mechanism that converts linear motion to rotary. This is usually half the engine or more.

On compact mechanism, I've posted toan other thread of yours: sinple, compact linear to rotary.

If you are looking to shorten a motor, driving the displace via a Scotch yoke works quite well, my first motor, a V pattern GAMMA. Ian S C

The most crucial design feature for power. There are five HE's in series, each dropping some temperature: outside of head, inside of head, regenerator, inside of cooler and outside of cooler. By far the most important is the regenerator, and it could even be counted twice because its exchange is in both directions per cycle.