Alpha motor with 180 degree crank phase?

[justincorhad]

I’ve been curious lately if it would be possible to build an alpha Stirling engine with the two pistons 180 degrees out from one another. I do understand that given equal volumes that no expansion or compression could take place, but hear me out...

What if the cold side was double the volume of the hot side? I don’t know for sure if this is the ideal ratio but just for simplicity’s sake. You could achieve the double volume via doubling the stroke, or modifying the bore diameter under the wide open volume was twice that of the hot side.

So now, when our hot side is at BDC, all (most) of the working fluid is in the hot portion, taking on heat it will be forced to expand, as it expands it moves towards the cold side where it can double in volume. By the time our cold side is at BDC our hot side is now at TDC so that all (most) of the working fluid is in the cold side. Now the fluid is forced to contract, and move back toward the hot side until it takes up half its volume that it was when heated.

I don’t have any experience building these motors, I’ve read a lot about the working theories though, I’ve been challenging myself to find the “hey stupid” reason as to why this isn’t feasible, but as of yet have come up empty.

Please let me know your thoughts, and tell me what I’m missing if I am way out to lunch. You never learn as much or as well as you do when you are proved wrong in my opinion, so please prove me wrong :)

[justincorhad]

Also my thinking for this type of layout, is that the engine should naturally want to keep itself turning even from a standstill. If the hot side is at BDC and is heated, as the working fluid expands it will want to flow into the cold side in order to accommodate that expansion and take up more volume. On the flip side when the cold side is at BDC and the working fluid begins to contract it will have to flow back into the hot side in order to accommodate the contraction and take up less volume. The only issue I could possibly foresee is the engine wanting to simply rock back and forth instead of making a full revolution, but I’m sure something like that could be tuned out by tinkering with different strokes, bores, or possibly even counterweights.

[Bumpkin]

Hi Justin. You can accomplish 180 degree phasing and differing displacements with one piston — the rod is sealed and oversized to diminish displacement on that side, so both sides of the same piston work. There have been a lot of heat engines, Ericsson and many others,) that used that premise, but I think they all needed valves (or ports like the Manson engine,) to accomplish the necessary pressure change BEFORE, nut during, the volume change. (I think I’ve got that right, but now I’m going to have to go away and think for a bit.)

Bumpkin

[justincorhad]

Oh so without some sort of valving or porting to help build pressure it would not function? I would imagine the load itself would be trying to resist the engines want to turn, and in so doing allow the pressure to build before expanding into the greater volume of the cool side? Perhaps I’m missing something fundamental though

[matt brown]

Yes, this does seem possible. Check out "New Concept of Stirling Engine" from another member on pg 2 of topics near top. Follow weblink in OP (as per below) and hit "Two-Stroke Striling Engine" link on left of web page for animation...

Post by normandajc » Tue Aug 18, 2020 3:52 am
Hello,
I designed a new type of Stirling engine. I respect the Stirling cycle, but the design is radically different than the alpha, beta, gamma types. I've made a website that introduces the engine. I removed the regenerator and replaced it with a heat exchanger. I've increased the heat transfer exchange times, etc.
An academic publication will be published very soon.
A patent has been filed
http://www.cyberquebec.ca/normandajc/

If I was a tad more adept at computer stuff, I could post a sketch, but here goes the old fashioned way...

After decades around this stuff, I've never seen any diagram of such, tho it's an xlnt simple solution, albeit it does require valves (maybe that why it's ignored). As someone posted, you could use 2 (so called) double diameter pistons comprising 4 cylinders...very similar to 2 common Ericsson schemes. However, a better way to get your head around this is to consider 4 single acting cylinders on a crankshaft like a common 4 cyl in line ICE. Next, let all cyls have 3" stroke, where 2 cyls have 3" bore while 2 cyls have 4" bore. Now, let's call these 4 cyls A,B,C,D, and arrange them in line A-B-C-D and interconnect them with one way conduits and valves such that the gas flows only A-B-C-D-A (yeah, I know, scary close to typical ICE). Now, let's put the 'large' cyls on the ends, the 'small' cyls in the middle, and have bore x stroke where A=4x3, B=3x3, C=3x3, D=4x3. Now, make A & B your cold cyls, and C & D your hot cyls. If you draw this on paper (A-B-C-D, left-right) with A & C at TDC while B & D are BDC, you'll see where the regen belongs...

Now, for anyone who still thinks that valves, piston rings, low pressure, etc are what kills any/all SE schemes, simply consider that pig in your driveway (your car) which drags your butt around town despite all these downsides - especially the paltry low pressure that Mother Nature supplies (1 atm). The more you study SE, the more you'll see that they appear to be mainly a grant vehicle for weird/lame stuff that suck moola without upsetting the apple cart (status quo).

[matt brown]

BTW the Ericsson flavor closely follows previous Stirling flavor with same hot-cold-regen locations, but requires 3 cyl sizes where C & D cyl vols are modified, something like A=4x3, B=3x3, C=4x3, D=6x3. In theory, this Ericsson version could have regen as 2 counter flow "reservoirs" which would largely nix nasty dead vol issues of conv'l SE; however, neither version nixed laggardly isothermal processes...

[justincorhad]

I see what he’s doing, very interesting. Do you think it would still work with a single pair of cylinders one hot and one cold? Also no check valves just a regenerator like a normal engine? I do see what he’s trying to accomplish, he’s trying to have a constant heat exchange and give the working fluid more time to gain or lose heat, very smart, but also many many parts.

[justincorhad]

I would definitely like to build what I’m thinking of to see if it works, unfortunately I am absolutely horrendous at building things, I’ve tried many times to build the simple tin can gamma type Stirling engines and have failed miserably every single time unfortunately.

[matt brown]

I don't think 2 cylinders is ever going to do much (meaning 2 cyls with single-acting pistons vs 2 cyls with double-acting pistons which most guys would call '4' cyls). Here's 2 contrasting examples: A vs B, and both are esoteric.

A: 2 cyl with same dimensions, single-acting, interconnected but unregulated, 1 hot cyl, 1 cold cyl, pistons 180 out-of-phase. Assume there's a slow flywheel driving the gas between the 2 cyls...and yes, the gas will heat and cool as it passes between the cyls, but the net work will remain zero...and consume heat.
B: same 'eng' but with different bores (hot larger) and nothing changes EXCEPT this consumes (wastes) more heat !!!

Both A & B would have a PV plot of a singular line, where A would plot as a vertical line for isochoric heat in & out vs B would plot as a horizontal line for isobaric heat in & out; A an example of the constant volume heat capacity of a gas (aka Cv) vs B an example of the constant pressure heat capacity of a gas (aka Cp). So, just because heat is transferred (here wasted), this does not comprise a heat engine. In PV jargon, W requires an enclosed area, so always requires at least 3 processes. If you study PV stuff long enough, you'll discover that both work & efficiency favor discreet processes. The trick is marrying a valid PV cycle to a valid mech.

The net effect (W=0) is the same as an adiabatic gas spring (think manual piston bike pump with finger over outlet) where work goes in & out vs heat, but still no 'engine'. Most 2 cyl engs are very similar early atmospheric schemes, like Cayley's orig double diameter eng. The popular Ericsson eng with double diameter pistons on same conrod (thus 180 out-of-phase) adds an accumulator (LP res) to Cayley's scheme, but (1) regulation from accumulator appears iffy, and (2) gas goes thru tiny regen on way to large cyl. This co-mingling of regen and input defy req'd discreet processes, and likely why it never went anywhere (except across the web 150 yrs later).

[justincorhad]

Matt in your example of the two different engines, you said engine B would have the hot cylinder larger, that’s not correct for what I’m suggesting. I’m suggesting the cold cyl must be larger than the hot side, in order that when the smaller volume in the hot cylinder is heated and tries to expand, It will in a sense be forced to expand into the larger volume cylinder (the cold one). Would this make a difference on the PV plot do you think?

[matt brown]

Hey Justin, don't feel bad, I haven't built one yet either. My excuse is being so involved in the thermo side of this chase that I know better than running out in the garage and cobbling stuff together. However, I know where you're at, being surrounded by bogus ICE and no ECE in sight. Until recently (think IT age), more man hrs had been spent on engs than anything else, since these buggers expanded our standard of living (yep, eng R&D 'trashed' agriculture or weapons). However, despite countless man hrs & moola, a quick recap shows few 'first principles':

Watt = condensing cycle
Stirling = regenerated cycle
Otto = compression cycle
Diesel = auto-ignition cycle
Brayton = 'jet' cycle (hon mention)

Due to current ICE dominance, most guys overlook how narrow the performance envelope of ICE really are, both thermo & mechanical. Consider common ICE vs Wankel and we find that pis/cyl dominate for one reason: rings are both side & end seals (simple & effective) like a bullet in a barrel. Then ICE favor bore:stroke = ~1 and a slider crank due to dwell at ends.

[matt brown]

But Justin, how did the large 'cold' volume get into the smaller 'hot' volume ??? This sounds similar something called thermal lag which Peter Tailer tried decades ago. Starting with large_cold_volume going into small_cold_volume would raise P causing Wneg (increase in back pressure) before ever considering small...hot...volume. Colin West did an ORNL paper in late '80s that's still floating around online. I really do have to get up to snuff with my computer skills so I can poach stuff easily (but I'm a decades long hardware guy). AFAIK all 'simple' closed cycle 2 cyl schemes will have W=0, effectively a gas spring. Similarly, all simple open cycle 2 cyl schemes tend towards an atmos eng (I'd say vacuum eng, but system could be pressurized via including LP reservoir between exhaust and intake, thus increasing dP).

[matt brown]

Justin, I couldn't resist posting this esoteric eng, just to demonstrate that despite lame, it's still a "valid" eng (W>0). Consider 6' pipe with 6" diameter, and permanently sealed at both ends. Mount pipe horizontally on pivot at 3' and arrange to rotate. Rather than insulate pipe exterior, let's assume pipe in made of a nonconducting material (magic NASA high temp plastic) while each end is made of a highly conductive material. Setup heater at one end, P gauge in middle and each end, and assume 1 atm of air is sealed in pipe prior heating. Engaging heater will cause T & P > 1 atm when heater input > 'cooler' output (ambient air T at other end). If we rotate pipe, the same values will re occur, and although P may vary while rotating pipe, P will remain equal between the 3 P gauges (at middle, and each end). The point here is that while we can easily measure T & P, we don't see the gas 'moving' around, and here in a fixed volume pipe. Yep, the heat is like a collie herding sheep, and this issue sinks many ECE schemes. So, in this example, the heated end pushes more gas molecules towards the cold end, and pivoting the pipe will push the gas molecules back. Now, with a Bill Gates bank acct, if we inserted a clever little engine in the center of the pipe, we could have a heat engine, albeit a lame one. This mass issue sinks many SE, and is exaggerated by unswept volumes (think regen). All successful engines have regulated cycles, there must be something to it...

[justincorhad]

I always pictured it like this in my head. Let’s forget flywheels and reciprocating engines for a second. Imagine instead two cylinders, single acting, both attached to opposite ends of a lever, like a teeter totter, if one moves, the other moves opposite, simulating our 180 degree phase angle. Let’s say one, the “hot” is 1” bore and 2” stroke. The other, the “cold” is 2” bore, 2” stroke. So our cold cyl has more volume than the hot cyl. Now, the important part is the starting point. When assembling this system, the starting point must be with the hot cylinder at BDC and the cold cylinder at TDC. Once the cylinders are position like this only then can a connection (a hose, a tube) be made between the two cylinders, such that once connected trying to extend cold cylinder would end up pulling a partial vacuum in the system due to the volume change. Now, we’re connected, we’re at BDC in our hot and TDC on our cold, let’s apply heat to the hot cylinder, that gas is going to expand, for arguments sake let’s say that initial volume of 1” by 2” in the hot cylinder expands to where it wants to take up a bigger volume of 2” by 2”, well being that our cold cylinder is just that, it will migrate towards that cylinder, trying to occupy that larger space. As the gas shuttles into that cylinder, it will push that piston down, and via the lever the hot piston will be pulled up, occupying more of the space within the hot cylinder. Once the cold cylinder is at BDC and the hot cylinder at TDC, our total volume is now 2” by 2”, as opposed to our initial volume of 1” by 2” in the hot cylinder. This gain in volume was achieved by our heated expanded gas. Now that all our working fluid is in the cold end, and none is in the hot end, the gas will start to cool and contract. As it contracts it wants to occupy less space, if it is not allowed to occupy less space a partial vacuum would be formed. Luckily our system is made that it can indeed occupy less space, by shuttling back to our hot cylinder, thereby moving the teeter totter, bringing our total volume eventually back to our initial 1” by 2”, and then our cycle repeats itself.

As I initially said I could be missing a key piece that renders this whole arrangement useless, but I’ve yet to find a reason why it would not?

[matt brown]

Justin, I just sent PM, let me know if I failed.

Well, you got the basic double diameter 180 phased Ericsson design going on, but...

In your version, cooling is turned off in large cyl, and gas expands from small to large while heating is on in small cyl (amount will depend upon mech speed). Then when gas gets in large cyl, cooling is turned on in large cyl, and heating off in small cyl. Did I get this right ?