Compression cycle.. worthwhile?

Discussion on Stirling or "hot air" engines (all types)
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tmk
Posts: 28
Joined: Tue May 01, 2007 9:37 pm

Compression cycle.. worthwhile?

Post by tmk »

I'm still just learning about stirling engines, and i'm curious as to the need for the compression cycle.

As I understand it, the compression cycle takes power that you have previously generated (at < 100% efficiency) and uses it to compress/expand air (at < 100% efficiency) which either heats it while it's being cooled, or cools it while it's being heated.

It seems like this extra compression/expansion is more a side-effect of the flywheel design than an integral part of the engine.

I would picture the ideal operation being that the compression/expansion would push or pull the piston, and then some of that extra energy would move both pistons simultaneously (ie unloaded.. no pressure) to move the air between chambers... in the case of a beta stirling, it would move the displacer.

Do i have it wrong? why the compression cycles?

-tmk
alpha stirling
Posts: 35
Joined: Sun Apr 29, 2007 9:24 am

Post by alpha stirling »

Under ideal conditions there is little actual pressure rise inside a stirling engine under the compression stroke. The compression stroke happens when the piston is moving inwards, but in the meanwhile the air temperature in the engine is decreased (by most of the air being on the cold side). This actually decreases the power needed for the compression, but doesn't eliminate it totally (nothing is perfect in our world).

The compression is actually beneficial to powerfull stirlings: When the air is compressed at the end of the compression stroke, it fits in smaller space. When the compressed air meets the hot heat exhanger, the expansion will have much more kick to be turned into work by the piston. Furthermore, If you pressurise your engine (ie. under normal conditions the working fluid has...say 10 bars overpressure, the power output of the engine will be much higher compared to a non-pressurised version.

So the compression is one important part of the stirling cycle, and by eliminating it you'll eliminate your engine's chances of running...
alpha stirling
Posts: 35
Joined: Sun Apr 29, 2007 9:24 am

Post by alpha stirling »

There are also devices called ringbom stirling engine, which has no mechanical means of moving the displacer piston. The displacer is swayed back and forth by the pressure difference between the displacer cylinder and the athmosphere...
tmk
Posts: 28
Joined: Tue May 01, 2007 9:37 pm

... what if

Post by tmk »

alpha stirling wrote:The compression stroke happens when the piston is moving inwards, but in the meanwhile the air temperature in the engine is decreased (by most of the air being on the cold side).
What if the timing was changed slightly, and we could move the displacer quickly at set points in the cycle, rather than a fixed offset from the power piston..

Eg when the power-piston is at/near its highest point, we move the displacer to push the air to the cold side. At this point the air will contract, and *pull* the power piston down, gaining energy instead of spending it compressing the gas.

I realize that the compression/expansion does 'give back' later in the cycle, but i think some energy is lost in the process.

-tmk
alpha stirling
Posts: 35
Joined: Sun Apr 29, 2007 9:24 am

Post by alpha stirling »

This (I quess) Have been tried by several groups. I quess one japanese group actually got a working engine. I have no information about the performance...

I also have lost the document where I had all of the equations to describe the stirling cycle in mathematical ways. Would have been possible to test this idea in theory... maybe.

I quess this question will stay unanswered until someone builds a prototype engine with measurable properties...
Cartech
Posts: 97
Joined: Mon Oct 30, 2006 11:26 pm
Location: Northern Nevada

Post by Cartech »

Correct me if I'm wrong but I think a Stirling that is running has almost no compression since it is basically a one stroke engine cycle. Hot pushing out and cold drawing back in. However, a cold, not running Stirling will have compression when it's spun by hand due to it being a sealed system. You will actualy have a vacuun and compression force at this time unless you engine leaks. If you remove your power piston connecting rod and heat your displacer you will see the power piston move up and down (remember "free" piston Stirling's?) when you spin the flywheel by hand. This is actually a good test if your having trouble getting your Stirling to run for the first time. So my point is, you should have compression with a cold engine and if you don't, it may not run at all when heated.
alpha stirling
Posts: 35
Joined: Sun Apr 29, 2007 9:24 am

Post by alpha stirling »

The terms "Compression" and "compression" are amazingly similar. In the internal combustion engine compression really means compressing air and fuel mixture into high pressure to allow it to burn efficiently. In the stirling engine, however, compression occuer when the working fluid is cooled, making it actually contract. The compression here doens't mean the rise of pressure unlike in the internal combustion engines. On the contrary, in stirling engine compression means that the working fluid's VOLUME decreases, pulling the power piston inwards...
tmk
Posts: 28
Joined: Tue May 01, 2007 9:37 pm

ahh.. i must have misunderstood

Post by tmk »

I was under the impression that the '90degrees out of phase' orientation of the power piston vs the displacer actually was meant to cause some 'real' compression, for example take this animation:

http://www.grc.nasa.gov/WWW/tmsb/stirli ... nim_6.html

The displacer is shown to be stationary while the air is on the hot side, but at the same time, the power piston is moving downwards which (in a well sealed engine) should cause a vacuum and a cooling effect, which the hot side is supposed to counteract by adding heat and causing the gas to expand, equalizing the pressure.

Same deal with the cold cycle, only in reverse (power piston compresses = heats/dissapated by cold)

I saw one article that said the ideal stirling should be isothermic, ie the working gas temperature never changes. This would certainly require compression/expansion.

Cheers,
-tmk
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