VincentG wrote: ↑Sun May 14, 2023 2:12 pm
Could you explain what you mean by the 90 degree phase still being present?
By that I mean:
The displacer position is no longer only function of the crankshaft angle but a function of the crankshaft angle and the deformation of the flexure.
If you take any point on the connecting rod as reference, well that reference has not moved between before and after modification with the flexure.
I.E. the phasing define by the crankshaft itself is unchanged.
But definitely, the position of the displacer is now different, but looking at the position graph that I posted earlier, the part that gets "trimmed" off will always symmetrical to TDC or BDC.
Changing the bottom and top surface of the displacer (i.e. making the displacer taller) allow you to adjust the top time off and the bottom time off independently. On my position graph posted earlier, I made them somewhat equal, but they don't have to be equal. Here is a better one:
I see what you mean now. Maybe a 180 crank could work with less spring movement though probably less dwell time(unsure without plotting that out)but with the 90 degree crank, the displacer starts to move when the crank throw is about 90 degree from vertical so connecting rod speed is at its max.
matt brown wrote: ↑Sun May 14, 2023 2:20 pm
I've been sitting on a mech that can run a clean cycle for 20 yrs, but didn't solve various regen issues until last year. This mech can run any cycle, but there's better ways to run Ericsson and Otto. I think most guys have bought into Stirling since it's nearly the only ECE discussed. I just don't see how even an optimized Stirling can produce realistic power when limited by 'isothermal' input and regen issues. In the end, I think the only ECE contenders are small Otto, mid-sized Ericsson, and large Brayton which is very similar to current ICE.
Are you talking about a test setup with linear actuators to control both pistons independently?
What regenerator issues are you referring to? Regenerators are a pretty interesting engineering challenge. Everything that makes them good in one way are also things that make them bad in other words. We both agree that typical SE (with cranks or similar mechanical apparatus) will force gas through the regenerator and expansion space during compression (generator and compression space during expansion). A good mechanical setup allowing for independent control of the piston and displacer, as well as flow control with solenoid valves should allow to really isolate those issues.
With a perfect mechanical setup allowing for the theorical cycle to be ran, what are the regenerator issues that you are still seeing?
VincentG wrote: ↑Sun May 14, 2023 2:37 pm
I see what you mean now. Maybe a 180 crank could work with less spring movement though probably less dwell time(unsure without plotting that out)but with the 90 degree crank, the displacer starts to move when the crank throw is about 90 degree from vertical so connecting rod speed is at its max.
Yeah, all I am saying is when playing with those 2 new "time-off parameters" you might find that the 90-degree may not be ideal anymore.
Thinking about it further, you are also slightly increasing compression ratio. That is, if you increase the height of your displacer and keep everything else the same, you are effectively reducing the volume of your displacer cylinder. Not by much probably, but still worth mentioning.
For sure, its actually a significant jump as compared to the original displacer and somewhere i figured it out. It also would benefit from much more power piston displacement. I've reached the practical limits of the model though so this winter I'll start on a bigger platform.
matt brown wrote: ↑Sun May 14, 2023 2:20 pm
I've been sitting on a mech that can run a clean cycle for 20 yrs, but didn't solve various regen issues until last year. This mech can run any cycle, but there's better ways to run Ericsson and Otto. I think most guys have bought into Stirling since it's nearly the only ECE discussed. I just don't see how even an optimized Stirling can produce realistic power when limited by 'isothermal' input and regen issues. In the end, I think the only ECE contenders are small Otto, mid-sized Ericsson, and large Brayton which is very similar to current ICE.
Are you talking about a test setup with linear actuators to control both pistons independently?
What regenerator issues are you referring to? Regenerators are a pretty interesting engineering challenge. Everything that makes them good in one way are also things that make them bad in other words. We both agree that typical SE (with cranks or similar mechanical apparatus) will force gas through the regenerator and expansion space during compression (generator and compression space during expansion). A good mechanical setup allowing for independent control of the piston and displacer, as well as flow control with solenoid valves should allow to really isolate those issues.
With a perfect mechanical setup allowing for the theoretical cycle to be ran, what are the regenerator issues that you are still seeing?
My ECE interest is power producing, so I mainly focus on conventional piston-cylinder-crank mechs. The Stirling cycle is challenging within this limitation, but not impossible to solve. There are several alpha schemes that solve typical SE phasing issues, but I've never seen any of these online. My main point of previous post was that regen volume is over rated while phasing issue is under rated.
You guys are focusing on dwell for gamma and doing an xlnt job, but this type of dwell is mainly limited to a displacer with balanced force. The down side to any displacer design is that regen is more difficult to achieve than with alpha where gas flow is self regulated (forced) due to positive displacement. My endless rant against the Stirling cycle is that regen heat is greater than input heat, and usually multiples of input. Even with ideal phasing this regen 'load' (ideal regen/ideal input) remains a major efficiency issue that coupled with regn cost/complexity and impossible isothermal processes just doesn't make sense. Maybe acceptable as DIY 300-600k cycle with 600rpm and free input source (where my alpha thinking ends), but I doubt any serious solution.
My ECE interest is power producing, so I mainly focus on conventional piston-cylinder-crank mechs. The Stirling cycle is challenging within this limitation, but not impossible to solve. There are several alpha schemes that solve typical SE phasing issues, but I've never seen any of these online. My main point of previous post was that regen volume is over rated while phasing issue is under rated.
You guys are focusing on dwell for gamma and doing an xlnt job, but this type of dwell is mainly limited to a displacer with balanced force. The down side to any displacer design is that regen is more difficult to achieve than with alpha where gas flow is self regulated (forced) due to positive displacement. My endless rant against the Stirling cycle is that regen heat is greater than input heat, and usually multiples of input. Even with ideal phasing this regen 'load' (ideal regen/ideal input) remains a major efficiency issue that coupled with regn cost/complexity and impossible isothermal processes just doesn't make sense. Maybe acceptable as DIY 300-600k cycle with 600rpm and free input source (where my alpha thinking ends), but I doubt any serious solution.
I see.
I wasn't going to talk about it until the parts are here, but I am retrofitting one of those 2-cylinder compressors into an alpha SE test bed for regenerator analysis. I say retrofitting and I mean retrofitting, I am only keeping the crankcase, crankshaft and connecting rods. Everything else has been redesigned and is being made right now.
The whole engine is going to be equipped with pressure sensor and thermocouples, and the electrical heat load will also be monitored/controlled.
These will be non pressurized tests, and may decide to make a new crankcase if I want to get the tests further with pressurization.
Although alpha's really different from Gamma/Beta configurations, the main reason for this test is to evaluate regenerator materials/properties.
Primarily, I think most regenerators have way too much thermal mass. The thermal capacity of the regenerator should not exceed the thermal capacity of the working volume of the gas(or at least by any more than needed to ensure enough surface area). From there, the focus should be on maximizing the temperature change of the regenerator material through each cycle. Maybe start with the time the gas spends traveling through the regenerator, even at the expense of added bends in the port to increase length, or doubling back on the flow path if you have a shape restriction. Think ICE long runner(and folded) intake manifold here. Then, the material should be the best compromise of high surface area to low thermal mass and have a low specific heat to ensure max delta T between regenerator and the gas. Tungsten comes to mind here with its extremely low specific heat. I don't think thermal conductivity may play a huge role here, given the fine gauge of wire or sheet stock used to fabricate the element.
One night while, as Matt would say, in an altered head state, I was thinking of SE efficiency and I pictured a cartoonish animation of an ideal heat engine. You may picture it as a snake that ingests a volume of hot air. The warm bubble of air turns the small traveling expanded section of snake red as it passes through, while the moment it moves through, the previously red hot section instantly cools down and progressively the heat is dissipated into the work output of expanding the snake.
What I took from that thought is the ideal heat engine has an infinitely low thermal mass and specific heat. We can't get there, but I think it's an easier ideal to target than the ideal Stirling cycle.
I'm glad I'm not the only one questioning why literature says the best regenerator would have low thermal conductivity and high specific heat.
I kind of get why high specific heat and high density would be useful, but I only see this useful as a mostly practical parameter: making the regenerator smaller. There is a finite amount of heat that the renegerator needs to retain, any more than that and it's wasted mass/volume/etc.
But thermal conductivity, I don't see how high thermal conductivity would be a bad thing. Perhaps, they mean as to not "conduct" the heat away to a housing, and ultimately through the environment? not sure, I looked and never found compelling data showing why thermal conductivity was bad.
That's one of the things I am going to try. I have samples of very similar density and heat capacity of materials with widely different thermal conductivity, so hopefully the testing will be conclusive enough to learn something.
I am trying to get all samples into a pressure drop test. Air compressor -> test rig -> flow meter. Test rig is a housing with a cylindrical sample of known diameter and length, in which compressed air is going through. it has a pressure sensor before and after to measure the pressure drop across the sample. By logging the data in real time and varying the input pressure, I'll be able to gather precious Pressure Drop Vs Flow Rate data
You mentioned pressure drop, I forgot to add to exchange the added length of regenerator with a much higher porosity to decrease flow restriction. My take on the thermal conductivity is for one thing what you said, but also I think given the short cycle times we are dealing with, you want the heat to stay as surface level as possible even considering a small guage wire. I think a lower thermal conductivity keeps the heat energy more readily available for the next cycle.