Torque question.

[achachm]

Hello everybody new member here. I am starting to build Stirling engines and having great fun. In my last gamma type engine, I was playing with the displacer piston stroke, noticed that less stroke leads to more RPMs, but the engine takes more time to start, so I think it has less torque, is this correct? What other factors will affect torque? What will happen if I change the power piston stroke, it will also have an effect on torque and speed? Thanks in advance!

[Ian S C]

Thinking more of the power piston, a short stroke indicates high revs, and low torque, while the reverse is true with a long stroke, but if you work it out both motors will have the same total power. The high speed motor can gain torque by gearing the speed down, vice versa, the low speed motor just needs gearing up to obtain the higher revs, the real trick is to match the rev for the load that the motor is ment to operate. I usually favor a fairly short stroke, I feel that a short stroke with a long con rod gives the least friction of the piston in the cylinder. Once I'v got the power cylinder capacity sorted, it's time to design the displacer using the capacity ratio of 1 : 1.5. If the same diameter it's easy, if the power stroke is 20 mm, the displacer stroke is 30 mm, it's when you change the displacer's diameter that you need your maths.

The design I use for displacers is; length three X the diameter. All the dimensions and ratios can change a small amount.
I try to be careful with the materials used, hot cap, and displacer stainless steel. Power piston cast iron, cylinder cast iron or steel.

Ian S C

[MikeB]

I would say that this is a question of finding the 'sweet spot'.
If you think about it, if you have a displacer stroke of zero, then the working fluid doesn't get moved from hot to cold to hot, so no energy can be obtained, but at the other extreme, if you have an incredibly long displacer (and stroke) then you waste a lot of energy just in moving the air, and the piston itself. The maths required to calculate this is 'significant' so you are probably best to (a) do what the experts suggest and follow the existing rules of thumb, (b) experiment as much as you can.

[Ian S C]

This is what happens when you use the wrong material for a displacer, I'm a slow learner, did it twice on the same motor. It did work, but apart from not being suitable as far as heat conduction, aluminium looses strength when you run the motor's hot cap at red heat.

Ian S C