Air Compressor Pump Engine ????

Discussion on Stirling or "hot air" engines (all types)
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Gnigma
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Joined: Sun Aug 17, 2008 2:13 am
Location: New Mexico

Air Compressor Pump Engine ????

Post by Gnigma » Fri Aug 22, 2008 11:35 pm

I've been lurking here for a little while, and I may be covering the same ground many of you have already tread. After reading a good many threads here, I have been "bitten" by the bug, and I want to try to make an engine. My skills at fabrication are limited, but, being a Tennessee farm boy by birth, my skills at improvising are quite good. I have no desire to build an engine from scratch--- right now I'm in school full time as well as working full time, so it would just take too long.
However, cleaning the garage, I got inspired. I have a two cylinder air compressor pump. The cylinders are 90 degrees apart, with the connecting rods on a common pin. It's got a rather heavy flywheel, too. In other words, it looks like a basic alpha configuration for a Stirling engine. I'm going to take the heads apart, remove the reed valves, and plug the intake and output ports. Then I'm going to use some tubing to connect the two cylinders, inserting a heat exchanger. I should be able to heat one cylinder and make it run.
The cylinders are cast iron sleeves with ringed pistons that are pretty air tight, or it wouldn't compress air. It even has splash lubrication for the undersides of the pistons. I know it won't be a walk in the park, but I'm willing to try it. Thoughts? Suggestions/criticisms are welcome.

Gnigma
Posts: 2
Joined: Sun Aug 17, 2008 2:13 am
Location: New Mexico

Post by Gnigma » Tue Aug 26, 2008 8:52 am

No thoughts? On the one hand, that's a bit daunting. I didn't think I was the only one who has ever had this idea... unlikely. On the other hand, if no one's ever tried it, it might be easier than it seems... also unlikely. I had to order another flywheel for my pump, but that'll give me time to figure out the other modifications it needs to turn it into an engine. I've seen a website where a guy turned a one-cylinder lawnmower engine into a Sterling, and I think my pump will be easier to convert. My thoughts on Sterling engines in general are that while you can build a model-sized engine from a kit, there are no kits for power-producing engines. And , while the Whispergen units are available in other parts of the world, it looks like it will be years before they will be available here. And they are expensive. The applications for low to moderate sized engines, from home power generation to use of exhaust heat for automobiles are largely being ignored. I don't expect this to be the answer, but, if this can be accomplished, maybe it'll be a step closer to a commercially viable, not-as-polluting small engine.

Cartech
Posts: 99
Joined: Mon Oct 30, 2006 11:26 pm
Location: Northern Nevada

Compressor conversion

Post by Cartech » Wed Aug 27, 2008 9:26 pm

I have read that a compressor conversion has been done and it read like it was fairly common. I'm not sure if it was gamma or alpha and I'm not sure if it was refering to an automotive AC compressor or an air compressor. I don't have much to offer you here but if I remember right, the drawback was due to heavy construction there is considerable friction, therefore it was not ideal for a Stirling conversion. It should work though. I have a small 2 stroke engine I will one day attempt to convert to a gamma type. Good luck and let us know how it goes.

thrundar
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Post by thrundar » Fri Nov 07, 2008 10:00 am

Converting a 2 stroke? I have 2 old weed eater engines that I'd like to convert. However I have no idea how to go about it. I did see one site where this guy is converting a briggs and stratton 3 HP into a stirling, but all it had was pictures and no details. That seems to be the common problem. Very few plans with details.

Cartech
Posts: 99
Joined: Mon Oct 30, 2006 11:26 pm
Location: Northern Nevada

Post by Cartech » Fri Nov 07, 2008 1:53 pm

Thrundar,

The two stroke I have is also from a small weed eater. I was going to make some sort of displacer assembly much like the guy did for his Briggs and Stratton. I would then use the spark plug hole as a port and JB weld the ports on the side of the cylinder closed. I may remove the rings and it will need some sort of crank case vent. It will also have lubercation issues to deal with. I did read about a guy that made an alpha type using two engines (small two strokes) but that he was unable to get it to run.

rammstein
Posts: 16
Joined: Sun Oct 26, 2008 2:13 pm
Location: the netherlands

Post by rammstein » Mon Nov 17, 2008 3:16 pm

I just took my v-type compressor apart to look for the possibility to make a v-type stirling and I think it is possible, only replace the high friction bearings and piston seals and bring the rotating mass down.
I`m just wondering why there are so few pictures on the net of a v-type engine. I`ve seen only 1 working.....
I`m learning here !

rammstein
Posts: 16
Joined: Sun Oct 26, 2008 2:13 pm
Location: the netherlands

Post by rammstein » Thu Nov 20, 2008 2:53 pm

futher down the line of searching on V-type alpha stirling engines I only got patent sites and drawings in theory, not working models.
Can someone direct me in the right way or is this design just not working?

Very helpfull is the manuscript -manufacturing and testing of a v-type stirling engine- on the net, but there has to be more information?
I`m learning here !

RichD
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Sounds possible to me

Post by RichD » Wed Nov 26, 2008 3:00 am

Hi,
I've drawn up some basic plans to build an Alpha V twin and have considered the same approach (air compressor). I haven't built it yet, but I will eventually. My plan however doesn't use an air compressor, I plan to use 1/4"x6"x6" box steel for the crankcase, 2" pipe flanges and 2" brass nipples/caps for cylinders, and copper tubing for heater/cooler between the cylinders.

I've sort of stalled out on the plan until I decide how to pressurize the crankcase and seal it. Kicking around ideas though.

Need to order some 2.25" OD graphite to turn down for pistons, and finalize the crankshaft, connecting rods, ball or roller bearing choices...on and on.

I've already bought the 2" brass pipe nipples and was surprised to see they have a mirror finish on the ID and both measure perfectly round with no taper (and no variation from one nipple to the other). I mean I was really amazed at how perfect the bores look and measure. Didn't even have any burrs on the ends either. I've since looked at smaller brass pipe nipples in the hardware store and they look perfect inside as well.

The 2" cast steel flanges were really warped so I faced them true to the pipes in a steady rest in my lathe. 2" Brass pipe is really expensive though at $45.00 per 6" long nipple.

At least with your plan most of the work is already done. I've thought more than a few times that you could just use the compressor crankase, gut the pistons, lengthen the con rods and bolt on your own brass cylinders/graphite pistons outboard of the compressor heads. You might even be able to reuse the old pistons pressed/bolted into some graphite round stock machined to the right size.

One other thing I'm stuck on now is whether the cylinders should be the same diameter hot/cold. You are sure correct about the lack of information regarding the Alpha V twin configuration.

I'll be watching to see what you come up with.
Rich DeMartile

rammstein
Posts: 16
Joined: Sun Oct 26, 2008 2:13 pm
Location: the netherlands

Post by rammstein » Wed Nov 26, 2008 2:19 pm

Last night I did some calculations on my v-type compressor with 63mm piston (2.5 inch) and stroke off 30mm (1.3/16) with a java based calculator
from Khirala and got a wopping 16 Watt as a LTD.
I got 900+ watt with 15 bar pressure, 1200 degrees C using helium as gas.

Offcource this is in a somewhat ideal world, so I designed all kinds of metal fins inside,piston shapes, valves, delayed TDC and BDC, introduced some waterdrops and positioned the engine in a way that the cool droplets always runs to the hot plate (remember 1200 degrees) to get flash steam every 3/1 rev, a expansion fase powered coldside water return to hot side system and so on.
My brains were spinning at top speed.

After I designed the perfect engine in theory, I took a beer and wished I lived in the year 1800, because I realised I designed a modern steam engine with tube steam boiler, dual acting pistons and the well known steam engine valves :).......
I`m learning here !

Franco
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Re: Air Compressor Pump Engine ????

Post by Franco » Mon Apr 27, 2009 8:34 am

The idea to use an air compressor as an alpha stirling is sound.

There was a paper written recently by some researchers in Argentina. They took a 3 HP V-type air compressor and kept almost all the original parts such as: crankcase, flywheel, crankshaft, connection rods, pistons, piston rings and cylinders. They then added a regenerator, heater, and cooler.

They then attached an electric motor to the flywheel and ran the engine in reverse. This was done basically to obtain data on the thermal performance. Running a Stirling engine in reverse results in the hot side exchanger getting colder and the cold side exchanger getting warmer.

The paper states that it would not have run as a Stirling motor (i.e. heat engine) in which heat is added at the hot exchanger and heat is removed at the cold exchanger. This is due to low performance level of the design they had. I think that this had most to do with the fact that they didn't optimize the heater, cooler, and regenerator dimensions. And maybe they didn't install proper seals either. The seals for an air compressor, which are basically just metal piston rings if I'm not mistaken, won't work for a Stirling engine. The working gas would leak out quickly, and maintaining a high internal pressure is necessary for high power. An air compressor doesn't need to have as good a sealing around the pistons, since it only needs to maintain pressure (usually inside a tank), and timed valves prevent air from leaking out. I could be wrong but I think this is how it works.

To seal around the pistons you can use seals like PTFE (Teflon) seals which are durable and should be readily available. Make sure you have an additional seal to keep the oil from entering the engine as that will foul the inside of the heat exchangers (heater, cooler, and regenerator).

You mentioned heating the outside of one of the cylinders to make the engine run. That only works for very small model engines where the heat exchange surface is large relative to the internal cylinder volume. For larger high-power engines you have to heat the hot side exchanger which consists of many tubes with a small inner diameter, maybe 2-5 mm. The length of the tubes is typically 80-100 times the inner diameter. So if the inner diameter is 2 mm the tube length is around 160-200 mm.

Tubes (passageways) with a small inner diameter is the only way to allow rapid heat exchange for larger engines.

You probably need a few dozen tubes for your heater and cooler (I'm guessing). That will make the fabrication process difficult no doubt, especially since the more tubes you have the more difficult it is to make them airtight (no gas leaking out).

For the regenerator use 400 mesh or something similar. 400 mesh means "400 wires per inch".

This should get you started.

It will be a lot of work, but it's a good opportunity to get familiar with these engines, no matter the outcome.

Franco

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