Doing some more testing, this time for volumetric expansion.
According to a combined gas law calculator, with a starting temperature of 40 degrees F and the 15cc capacity of the chamber, the final volume at 212 degrees F should be 20cc total. In testing I am getting right about 4cc (19cc total) which seems exceptional and a much better result than pressure testing would indicate. I've found the volume of my pressure testing rig is too high to get accurate results with pressure. But with volume expansion, extra "dead space" here does not have nearly as big of an impact. So I bet with a digital pressure transducer I would get better pressure results.
In fact, even with my excessively large 50cc glass syringe, I'm getting measurable expansion (just shy of 1ccc) between ice on the top plate and my 65 or so degree kitchen.
Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier
Re: Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier
Still waiting on my smaller test cylinder, so I tried some new tests with the larger 50cc syringe. By taping a 1.5lb copper bar on the 1 square inch piston, I can apply 1.5psi to the displacer chamber. On just hot water and ice, you can see in the video the chamber is easily able to set the piston into oscillation. Even with a large amount of dead air in the syringe, it has no problem lifting the weight rapidly. I ran out of memory so it cut off the part showing that the chamber can barely lift the weight with a starting charge of just 1ATM.
I would estimate that if I used a heavier weight, it would lift it equally as well, as the heavier weight will simulate an even higher charge pressure.
If I try to move the weighted piston that quickly by hand, it's not easy at all and my wrist gets tired lol. Pretty impressive I think from only 15cc and a cup of hot water.
Although the weight is just used to simulate a higher charge and buffer pressure, this arrangement of setting a large mass into oscillation could prove very practical. I'll need to get a glass pressure vessel to test at higher pressures and with other types of gas using a normal free piston.
The arrangement of a driven displacer and a free power piston seems extremely effective and simple. Obviously a linear generator is needed to take full advantage of this, adding difficulty. But the benefit is constant mechanical effectiveness from the piston. That is, there is no connecting rod/crank angle issues to limit torque near TDC and BDC like in a normal engine.
https://youtube.com/shorts/WPtcVRK35nU? ... H3OG_zNEL4
Edit: It will even set the piston into pretty good oscillation with just ice on the top and room temperature as the heat source!
I would estimate that if I used a heavier weight, it would lift it equally as well, as the heavier weight will simulate an even higher charge pressure.
If I try to move the weighted piston that quickly by hand, it's not easy at all and my wrist gets tired lol. Pretty impressive I think from only 15cc and a cup of hot water.
Although the weight is just used to simulate a higher charge and buffer pressure, this arrangement of setting a large mass into oscillation could prove very practical. I'll need to get a glass pressure vessel to test at higher pressures and with other types of gas using a normal free piston.
The arrangement of a driven displacer and a free power piston seems extremely effective and simple. Obviously a linear generator is needed to take full advantage of this, adding difficulty. But the benefit is constant mechanical effectiveness from the piston. That is, there is no connecting rod/crank angle issues to limit torque near TDC and BDC like in a normal engine.
https://youtube.com/shorts/WPtcVRK35nU? ... H3OG_zNEL4
Edit: It will even set the piston into pretty good oscillation with just ice on the top and room temperature as the heat source!
Re: Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier
I've repeated the tests with a 4lb steel weight. The chamber was just as easily able to set it into oscillation, even on just ice and room temperature on the bottom plate.
It became clear to me that the only limit to this will likely be the heat transfer rate of the aluminum plate, and that just two ice cubes could set in motion a 100lb mass!
In fact, unless I botched the calcs, my .125" thick 2.5" diameter aluminum sinks can conduct 6600 watts at just a 30k temperature difference...someone please correct me if I botched that.
With a chamber pressure of 115psi absolute(result of 100lb weighted piston), only just above 3 joules is needed to heat air 30 degrees k.
This allows a practically unlimited weight capacity to this system.
I'm using online calculators mainly....surely I must be making a mistake somewhere!
It became clear to me that the only limit to this will likely be the heat transfer rate of the aluminum plate, and that just two ice cubes could set in motion a 100lb mass!
In fact, unless I botched the calcs, my .125" thick 2.5" diameter aluminum sinks can conduct 6600 watts at just a 30k temperature difference...someone please correct me if I botched that.
With a chamber pressure of 115psi absolute(result of 100lb weighted piston), only just above 3 joules is needed to heat air 30 degrees k.
This allows a practically unlimited weight capacity to this system.
I'm using online calculators mainly....surely I must be making a mistake somewhere!
Re: Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier
I've done some expansion testing with a much smaller 10cc glass syringe. With ice on top and over boiling water, the expansion is 3.8cc, out of a possible 5cc assuming 40F(277k) and 212F(373k). The next step would be temperature sensors on the inside of the plates to see what the actual temperature reaching the gas is.
But this engine is just too small and I believe thermal performance will increase when scaled up. I've also realized the general shape and construction of these style pancake engines leaves a lot to be desired. But I'll save that for a future post.
For now I'll consider the input temperature, giving a temperature ratio of 1 to 1.25. The observed expansion ratio would give us a PP/DP volume ratio of 1 to 3.94.
So theoretically this 15cc chamber would be at the zero point with a 4cc power piston. That is again to imply that there would be no forced compression or expansion.
Based on what I observed in this thread, viewtopic.php?t=5638 that means this 15cc chamber could run quite well with a power piston of around 9cc. I may get around to trying this but for now my attention is on the 60l drum project.
But this engine is just too small and I believe thermal performance will increase when scaled up. I've also realized the general shape and construction of these style pancake engines leaves a lot to be desired. But I'll save that for a future post.
For now I'll consider the input temperature, giving a temperature ratio of 1 to 1.25. The observed expansion ratio would give us a PP/DP volume ratio of 1 to 3.94.
So theoretically this 15cc chamber would be at the zero point with a 4cc power piston. That is again to imply that there would be no forced compression or expansion.
Based on what I observed in this thread, viewtopic.php?t=5638 that means this 15cc chamber could run quite well with a power piston of around 9cc. I may get around to trying this but for now my attention is on the 60l drum project.
Re: Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier
A (very basis) control experiment for volumetric expansion was conducted.
I placed the 50cc syringe in the freezer for about 5 minutes. It was then set to 30cc and capped off.
Then I rotated it around the surface of boiling water and recorded the expansion. 39cc was the maximum I saw.
Using the ideal gas calculator and assuming a starting temperature of 30 degrees F and a final temperature of 210F, I should have seen an expansion to 41cc. 2cc short of the target.
The epoxy test engine is 15cc and under roughly the same conditions was about 1.2cc short of the target.
The test engine is expanding air into a room temperature cylinder, so expansion is lost in that regard. That and the Tc of the engine with just ice on top was likely above the Tc of the syringe after being in the freezer.
I would like to set up a more accurate test to see just how "ideal" a test rig has to be to duplicate the predicted expansion rate exactly.
The conclusion for now is that it's relatively easy to build a chamber with essentially ideal real-world performance. So once this level of expansion is reached, effort is better directed to other issues.
Response time of the chamber is likely the next question of interest.
I placed the 50cc syringe in the freezer for about 5 minutes. It was then set to 30cc and capped off.
Then I rotated it around the surface of boiling water and recorded the expansion. 39cc was the maximum I saw.
Using the ideal gas calculator and assuming a starting temperature of 30 degrees F and a final temperature of 210F, I should have seen an expansion to 41cc. 2cc short of the target.
The epoxy test engine is 15cc and under roughly the same conditions was about 1.2cc short of the target.
The test engine is expanding air into a room temperature cylinder, so expansion is lost in that regard. That and the Tc of the engine with just ice on top was likely above the Tc of the syringe after being in the freezer.
I would like to set up a more accurate test to see just how "ideal" a test rig has to be to duplicate the predicted expansion rate exactly.
The conclusion for now is that it's relatively easy to build a chamber with essentially ideal real-world performance. So once this level of expansion is reached, effort is better directed to other issues.
Response time of the chamber is likely the next question of interest.
Re: Constant volume compression/expansion-displacer chamber analysis-heat powered mechanical amplifier
Some quick and dirty response time testing until I get some kind of rapid response pressure transducer. The 10cc syringe is very light and so I don't think its inertia is impacting results much. The temperature difference here is very low. The water below the engine was cooler than the top plate due to the house heating up during the day and the water temp lagging the air temp.
I was unable to measure the driven rpm but I would estimate around 2k rpm. You can see the piston is only slightly lagging the displacer, and it's moving just as much as it was at slow speeds.
https://youtube.com/shorts/AvZ_GvFu8xE? ... faO4EBcnvt
I'll repeat test with better lighting and higher temperatures. Will the response time and piston travel be this good at higher temperature deltas?
I was unable to measure the driven rpm but I would estimate around 2k rpm. You can see the piston is only slightly lagging the displacer, and it's moving just as much as it was at slow speeds.
https://youtube.com/shorts/AvZ_GvFu8xE? ... faO4EBcnvt
I'll repeat test with better lighting and higher temperatures. Will the response time and piston travel be this good at higher temperature deltas?