Stirlingengineforum.com

Hi all,
I was trying to figure out dimensions and relations of a beta type stirling design and noticed from drawings and animations that there always seems to be an overlap of displacer piston and working piston. I know there is no mechanical need for this, so the reason must be of a thermodynamic nature. What do I miss?

I was wondering the same question and for the same reason. I'm fairly certain, although not quite entirely sure, that it has something to do with "dead air". I'm new to this whole Sterling engine design so I've researched as much info as I could, so far, and what I've read seems to indicate that the less dead air space the better. But then there's the question "why is it called dead air"?

There was a paper published in 1983 by a team hired by NASA to do research on the Stirling engine and the paper indicated that the Piston and Displacer "almost touch" at one point in their strokes. The paper didn't indicate why, they were using that statement to show why the math calculations are sound.

The paper was quite indepth as to the mathematics involved in a Stirling engine design but didn't give any dimentions as to phyisical shape of the engine. I guess it's the reader's responsibility to use the math to determine the size and shape of it.

The paper is here if you wish to read it.

Thank you for pointing me on that paper, the answer is right on page 74 and as I understand it correctly it is a clever way of actively reducing dead air space, because both pistons occupy the same room at a given time in the cycle, space that would otherwise have to be filled with part of the working gas.
Dead air space in general seems to be a major concern in designing stirlings. I was quite shocked to discover in the same paper that the GM stirling GPU-3 engine setup was an allmost exact copy of what I had in mind! My idea would not be as compact and sofisticated ofcourse, having a longer displacer piston and machined heater tubes. I still might get away with that, but I can see now my regenerator and cooler are so much larger that the passive dead space would have killed all power, if it would run at all. Will have to reconsider dimensions or completely change to another setup.

grz;
Leo

I suggest that if you have not done so, it would be a good idea, before you build a complex motor, build a simple one to a conventional design, there's nothing like having hands on practical experience to fully understand the workings of these machines.
With the BETA / coaxial motor, as much as possible of the space is used in the working of the motor, and waist space is eliminated.
Because the power piston is on a crank 90* from the crank that the displacer is on, at one stage the two are going to be within a mm or so from each other, and then they will be at the other extreme.
The main dimension is the ratio between the displacer and power piston, 1.5 : 1, the other one is the length to diameter ratio of the displacer of 3 : 1. Ian S C