why do most models have of displacer type piston?
why do most models have of displacer type piston?
why do most model stirling engines have displacer piston (gamma type) instead of hot piston and cold piston (alpha type)?
-
SScandizzo
- Posts: 66
- Joined: Tue May 16, 2006 5:06 pm
- Location: California
Hi Ozzu,
Great question! There are probably a number of correct answers as to why most practical versions of Stirling engines have only a "cold side" piston. I'll try to address a few.
First, pistons must be in very close contact with their cylinder walls. This creates friction. Friction is bad because it wastes useful mechanical energy and therefore makes an engine less efficient.
Second, every piston must have an airtight seal. The more pistons, the more likely that air may leak in and out of the system, which will also rob the engine of power.
Third, in a two cylinder/two piston arrangement, there needs to be a passage connecting the two chambers. In theory, this connection could be very small, but in practice, it is often quite long. This additional space within the system is referred to as "dead space". Dead space is a volume within the engine that is neither efficiently heated or cooled, thus reducing the engine's potential power.
Fourth, and probably less significant, is that an alpha engine may require more time to manufacture because two pistons must be properly honed instead of one.
So why use a displacer?
In contrast to alpha engines, beta and gamma engines require only one matched piston/cylinder. The displacer "floats" in its chamber so there is less friction. Beta engines can virtually eliminate any dead space, since both the displacer and the power piston share the same cylinder, but they are more challenging to construct, so many hobbyist build gamma engines (which are basically split apart beta engines).
Hope this answers your question!
-Stefan
Great question! There are probably a number of correct answers as to why most practical versions of Stirling engines have only a "cold side" piston. I'll try to address a few.
First, pistons must be in very close contact with their cylinder walls. This creates friction. Friction is bad because it wastes useful mechanical energy and therefore makes an engine less efficient.
Second, every piston must have an airtight seal. The more pistons, the more likely that air may leak in and out of the system, which will also rob the engine of power.
Third, in a two cylinder/two piston arrangement, there needs to be a passage connecting the two chambers. In theory, this connection could be very small, but in practice, it is often quite long. This additional space within the system is referred to as "dead space". Dead space is a volume within the engine that is neither efficiently heated or cooled, thus reducing the engine's potential power.
Fourth, and probably less significant, is that an alpha engine may require more time to manufacture because two pistons must be properly honed instead of one.
So why use a displacer?
In contrast to alpha engines, beta and gamma engines require only one matched piston/cylinder. The displacer "floats" in its chamber so there is less friction. Beta engines can virtually eliminate any dead space, since both the displacer and the power piston share the same cylinder, but they are more challenging to construct, so many hobbyist build gamma engines (which are basically split apart beta engines).
Hope this answers your question!
-Stefan
-
spinningmagnets
- Posts: 64
- Joined: Sat Aug 02, 2008 7:34 pm
- Location: NW Kansas, USA
I agree with everything Stephan has said, and I would like to add one more thing.
The power-piston ring-seal is dynamic, and must seal while sliding in the cylinder. The way to get more power from the same size of engine is to go to higher pressures, but leakage past the power piston seals results in less gas mass inside to react to the heat expansion.
Putting a tiny air pump that pushes a small chunk of air into the inside during the coldest part of the stroke might help some, but it would also rob some shaft HP. (it would also need a relief valve before the check valve to prevent injecting too much air into the inside)
By having a hot piston and a cold piston in an Alpha, the problems of sealing and ring/piston lubrication are compounded by the higher temperatures in the hot cylinder.
A Gamma uses a warm power piston, halfway between the two.
Though the Beta has the coolest power-piston/rings of the three (still quite warm, though) sealing the shaft-within-a-shaft is complex.
I am waiting for someone to make a Beta with the displacer piston moved by external magnets, and the hot/cold ends are a slightly larger diameter than the power piston with light springs to center the displacer.
I'd make the displacer doughnut shaped with a fixed regenerator in the center.
The fun thing about Stirlings is the huge variety of options...
The power-piston ring-seal is dynamic, and must seal while sliding in the cylinder. The way to get more power from the same size of engine is to go to higher pressures, but leakage past the power piston seals results in less gas mass inside to react to the heat expansion.
Putting a tiny air pump that pushes a small chunk of air into the inside during the coldest part of the stroke might help some, but it would also rob some shaft HP. (it would also need a relief valve before the check valve to prevent injecting too much air into the inside)
By having a hot piston and a cold piston in an Alpha, the problems of sealing and ring/piston lubrication are compounded by the higher temperatures in the hot cylinder.
A Gamma uses a warm power piston, halfway between the two.
Though the Beta has the coolest power-piston/rings of the three (still quite warm, though) sealing the shaft-within-a-shaft is complex.
I am waiting for someone to make a Beta with the displacer piston moved by external magnets, and the hot/cold ends are a slightly larger diameter than the power piston with light springs to center the displacer.
I'd make the displacer doughnut shaped with a fixed regenerator in the center.
The fun thing about Stirlings is the huge variety of options...