Area for air passage b/t displacer cylinder and powerpiston

[depasha]

hi Ian
tell me about what the area required for the passage of air move between the powerpiston cylinder and displacer cylinder
my engine parameters are displacer cylinder dia=61mm
piston cylinder dia =42mm
my some experiance is that if area of passage of air is small so the powerpiston move to and fro hardly .
thanks in advance.

[Ian S C]

depasha, gap of 1 mm or a fraction less, up to 2 mm is OK between the displacer and its cylinder, and a similar gap at each end.
Take for example the power cylinder, and choose the stroke you would like for the power piston, say 30 mm, find the volume. Now with the displacer the swept volume should be 1.5 times the volume of the power cylinder, I'll leave you to work it out, I'm too lazy/ don't like maths, there's probably an easy formula, but I tend to do it by hit and miss, find area, select a hight, try, then try again. One thing it teaches one that you should have taken more notice a school maths classes. Ian S C

[oswaldwark]

mm is OK between the displacer and its cylinder, and a similar gap at each end.
Take for example the power cylinder, and choose ????

[Aviator168]

What kind of engine is this depasha? alpha, beta or gamma.

[Ian S C]

It's either a BETA, or GAMMA, if a BETA the question answers it's self as the displacer and power piston occupy the same cylinder, that leaves the GAMMA motor, if its a parallel cylinder one, and you design it well the two cylinders over lap, so that a section of the head end of the power cylinder breaks into the displacer cylinder. If the two cylinders are apart from one another, the two cylinders can be joined by a tube or two, 3/8" / 10 mm bore for your motor, or two or three tubes 1/4" / 6 mm bore, this araingement would do for a V type motor also. A duct can sometimes be drilled in the motors structure instead of tubes.
Ian S C

[ishalkhan]

My opinion is really different.
If the heater chamber's length is 2/3 rd of the Length, then the cooling capacity wont be enough!!!
Also not working well the regenerator in it.
The basic drawings are said 50/50 percent of volume hot side/cold side, but it is no good enough too.
The heat separation is better in a longer cylinder, but then should be heat up only 1/3 rd of Length. I think.

[Aviator168]

I agree with ishalkhan. Depending on compression. In general, the contact area of the cooler should be at the very least 1.5 times that of heater and long length separation between the two to lessen the effect of thermal short.

[Ben]

Does the same rule of thumb, -displacer swept volume should be 1.5 times the volume of the power cylinder- apply for Low Temperature Differential Stirling engines?

Nevermind, got it guys thanks!

[Ian S C]

Ben, no, I have not calculated it but my LTD has a power cylinder of 1" dia, and a displacer dia of 6", the stroke of both is 1/2".
Ian S C

[Ben]

Thanks Ian, I adjusted the volume swept for both pistons to about 10:1 ratio, but this required changing my piston volumes. does drive piston weight affect the engine performance, especially for LTD engines?

[Ian S C]

Ben, The weight of all moving parts should be a light as possable.
On my LTD motor, the foam displacer is 1/2" thick x 6" diameter( maybe a little less, there is about 1/4" clearance in the chamber, and about 1/8" clearance top and bottom. It wasn't built to any plans, and no drawings were made. Running speed is around 120rpm on a bowl of boiling water.
Ian S C