The Miser Stirling Engine

Stirling engines come in all kinds of configurations. One kind is called the "low temperature differential" (LTD) engine. Characteristically, they employ a large displacer and a small power cylinder.

One such LTD engine is the "Miser," a Jerry Howell design.

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This picture shows the power cylinder side of the Miser engine I built in January and February of 2002. I didn't want to make any mistakes while machining the parts for it, so I took my sweet time, about five weeks.

File size for the enlarged view is 89kb, but the download time may be worth the wait.

I got really lucky on this little engine. It ran on the first try.

I wasn't satisfied, however, because the 1/16" drill rod that supports the displacer was visibly curved (the way it arrived from the supplier) and it caused some binding. Nevertheless, it would run for 46 minutes while sitting on a coffee mug of boiling hot water.

After some fine tuning and balancing, it ran for 2-hours 23-minutes under the same conditions as the above 46-minute test. When it finally quit the room temperature was 85 and the water in the mug was 95. But, here is the kicker: The mug only occupied 30-percent of the engine's hot side area. The other 70-percent of the hot plate was exposed to the same room temperature as the cold side. Furthermore, the water level in the mug was down 1/2" from the top. There's no way the hot plate was actually 95.

The actual delta-T at the time it quit was much less than 10 F. When I compare it to Dr. Senft's ultra-low temperature difference engine, the P-19, I'm satisfied.

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This view shows the displacer actuating crank and the crosshead, a well thought out Jerry Howell touch.

Most of the materials came out of the local scrap yard. The flywheel started out as a corroded and scarred chunk of -inch aluminum plate. The scrap pile didn't yield any brass, so I had to buy it new. The only things that came ready-made were the ball bearings and the fasteners.

Mr. Howell specified "truly flat" aluminum plate for the top and bottom of the displacer chamber; but, can you imagine finding such a thing in a salvage pile? I took the flattest I could find, then mounted it to the lathe faceplate with double-stick tape. A few cycles of light skim cuts on first one side and then the other gradually yielded an acceptably flat plane. The engine doesn't know that it's using 0.110-inch thick plate instead of 0.125."

The main problem was getting the aluminum unstuck from the faceplate. Hint: A hot air gun does the job nicely. Any other method will likely damage the newly turned surface.

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So, that's my first attempt at building a Miser. Never in my wildest fantasies did I expect it to run on the heat from the palm of my hand. It depends on how warm a hand is, of course, but the Miser will generally start running after holding it for two or three minutes.

Its slowest sustained speed is about 12 rpm. So far, the highest observed "hand warmth" speed is about 81 rpm, but that was after holding it only five minutes. It was still accelerating, but my arm was getting tired.

My biggest surprise came after a run on "hand warmth." After being placed on an insulated surface it continued to run for another 5-minutes and 14-seconds, just from the heat in the aluminum of the bottom plate.

Check back later on and you may find construction tips and more pictures, hopefully sharper than these first two.

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Revised -- 1/18/07