Final Results: Summarization of work for next year
A lack of time finally forced Group E into defeat against the Stirling engine. We did not achieve a working engine for what could be a number of reasons. After much diagnostic analysis of each engine section we can summarise the main possibilities for failure and the greatest chances for improvement. Firstly the possibilities for failure:
Solutions
Leaking Air:
The engine had two areas where air was escaping which rendered it useless. Although much speculation was given by each group member about these locations, a simple “bubble test” was performed which immediately dispelled any doubt. The bubble test can be viewed here as a video was taken.
- Leaking air
- Too much dead air space
- Energy losses due to friction
Solutions
Leaking Air:
The engine had two areas where air was escaping which rendered it useless. Although much speculation was given by each group member about these locations, a simple “bubble test” was performed which immediately dispelled any doubt. The bubble test can be viewed here as a video was taken.
Bubbles can clearly be seen to be leaking from both the bush and the working piston of the engine. After some work on both areas we managed to solve the leaking air from the bush. This was solved by gluing a rubber o-ring on the end of the bush. In an attempt to seal the working piston we placed o-rings in the grooves of the piston, however they were not designed for this function and so no o-rings fitted perfectly. Even after reducing the o-ring size we could not achieve the perfect balance between air tight seal and low friction. We also managed to increase the seal by applying more grease as discussed in the “friction” section of the website, but this still do not achieve a useable seal. The second bubble test shows that no bubbles escape from the bush when the engine is submerged, however the working piston still releases some, which indicates that water is entering the engine.
In order to finally solve this problem an entirely new working piston would need to be manufactured. Only then could the tolerance be improved to produce the perfect air tight seal that we seek.
Too much dead air space:
Since the engine was designed to work in two different setups the volume of air around the displacer was variable. With the regenerative substance around the regenerator the dead air space would be reduced so that the engine should function. However when the engine was heated when it was completely sealed there was no indication of the increase in temperature increasing the pressure in the hot end. This meant that the displacer did not travel through its stoke length. From this we could say that the increase in temperature was reaching equilibrium around the displacer without having to first move it, which indicates too much free air.
If the engine could have been tested as setup two as first planned, the volume of air surrounding the displacer was set (at a lower volume) and so if the heat was applied the displacer should have moved through the stroke before reaching equilibrium. Since the second displacer was never manufactured this could not be proven.
In order to solve this problem we propose that a new regenerator should be designed, which would reduce the volume of free air around the displacer. This should allow the engine to work in setup one (so long as all other factors of error are eliminated).
Energy loss due to Friction:
The nature of the regenerator increases the friction within the engine. This could be limited by redesigning the regenerator to incorporate glide rings more commonly found in automobile engines.
One positive effect of the regenerator is that it allowed kept the displacer running in a horizontal motion. With the engine in setup two, the weight of the displacer is not supported by any means so it has a tendency to hang slightly from the bush, which increase the friction levels as the displacer shaft attempts to glide through the bush.
Too much dead air space:
Since the engine was designed to work in two different setups the volume of air around the displacer was variable. With the regenerative substance around the regenerator the dead air space would be reduced so that the engine should function. However when the engine was heated when it was completely sealed there was no indication of the increase in temperature increasing the pressure in the hot end. This meant that the displacer did not travel through its stoke length. From this we could say that the increase in temperature was reaching equilibrium around the displacer without having to first move it, which indicates too much free air.
If the engine could have been tested as setup two as first planned, the volume of air surrounding the displacer was set (at a lower volume) and so if the heat was applied the displacer should have moved through the stroke before reaching equilibrium. Since the second displacer was never manufactured this could not be proven.
In order to solve this problem we propose that a new regenerator should be designed, which would reduce the volume of free air around the displacer. This should allow the engine to work in setup one (so long as all other factors of error are eliminated).
Energy loss due to Friction:
The nature of the regenerator increases the friction within the engine. This could be limited by redesigning the regenerator to incorporate glide rings more commonly found in automobile engines.
One positive effect of the regenerator is that it allowed kept the displacer running in a horizontal motion. With the engine in setup two, the weight of the displacer is not supported by any means so it has a tendency to hang slightly from the bush, which increase the friction levels as the displacer shaft attempts to glide through the bush.
