1 Watt Stirling engine driving high-power LED

First let me say I will display this engine and generator at the Maker Faire in San Mateo, CA on May 21, 22 so you can see it running if you are there.

The above video shows a one-watt Stirling engine driving a one-watt LED. Conversion efficiency is not too high so I’m only driving the LED with about 0.38Watts (120mA at 3.2V). In the video the hot end of the displacer is covered up by a lot of insulation with a metal can over it and the cold end has some extra cooling fins. The photo below shows the bare engine. You’ll notice I recently added a diagonal brace to stiffen against the torque that was wracking the frame.

Engine 3F showing displacer cylinder uncovered

The stepper motor used as a generator can get up to nearly 50% efficiency although at my power and RPM level it was operating at 38-40% efficiency. This conversion efficiency includes the diode bridges that rectify the AC voltage to DC. I was going to publish my test results for this stepper motor as I have in an earlier post on another stepper motor, but the company I bought it from, Jameco, no longer sells it. The motor was a surplus overrun item and they ran out. When I find a good replacement I’ll post the data on it. I believe the motor is still available (KP35FM2-044) from the manufacturer (Japan Servo) but probably costs over $20, not the $6 I paid for it.

I could also use an ordinary DC electric motor for power generation. DC motors with brushes can be reasonably efficient and don’t require rectification from AC. The main drawback for me is that my engine develops maximum power around 200 to 250 rpm and the DC motors I’ve tested don’t generate useful voltage and current until about 2500 rpm or higher. The stepper motor, a type of brushless motor develops good power around 600 rpm. This is much closer to my engine RPM and is connected by a 1:2.8 belt drive to match my engines power and rpm. The belt, incidentally, is a large o-ring with a 1/8 inch cross section and seems to work quite well. I need to measure the transmission efficiency on it but haven’t yet. My indirect measurements seem to indicate an efficiency of between 80-100%.

Ordinary brushless DC motors may be more efficient than stepper motors but I haven’t found a good source for them yet that is reasonably inexpensive and matches my power needs.

Electric heat source

For a heat source I use .035 inch stainless steel wire driven by a current usually in the 4-6 amp range. Stainless steel has poor electrical conductivity (for a metal) and about 50 inches uses 4.5 amps at 13.2 VDC in the video, delivering 59W to the engine. The wire is wrapped around the displacer with a layer of fiberglass cloth used as electrical insulation underneath to prevent electrical shorting to the displacer and another layer outside the stainless steel wires, then 3 stainless steel hose clamps to lock it all in place. Finally the hot end of the displacer is wrapped with fiberglass insulation to minimize heat loss to the surrounding air.

I use the above electrical heating method for two reasons: First, I can operate the engine without an open flame which is convenient for running the engine all day long at the Maker Faire. Second, it’s much easier to measure input power using electrical heat than it is to measure power when burning alcohol or propane, especially at the low power levels my engine requires. The only way to measure power levels in those cases that I have used is to measure the fuel consumed per hour and compute the heat content of the fuel. It’s time consuming to measure a few grams an hour to make accurate readings.

If you are interested in using the electrical heating method I use, be certain to work with safe voltages, probably 24V or less. While it’s okay to transform down AC to 24V or less, you are flirting with electrocution if you use the method I did using 120VAC. If the wire gets too hot, it will soften or melt the glass cloth and the wires can short to the engine. 12V auto battery chargers used within their normal current ranges work quite well. The biggest disadvantage to this method of heating is that my engine takes about 15 minutes to come up to full operating temperature. A propane torch can easily provide more than 20 times the 59 watts and heat the engine up in less than one minute.

Power computations

For those interested in making their own power measurements that might not be comfortable with the math involved, here’s what you need to do:
(1) To compute a rotary engine’s power output at any rotational speed you only need to know the torque and the rotational speed.

(2) In the video you can see the wood torque arm clamped to the main shaft and prevented from rotating by a beam that pushes against a scale. The clamping force is adjusted until the engine is stable at the rpm I want to measure the power at. The scale measures in grams and the torque arm transmits the force at exactly 3 inches so I measure the torque in gram-inches by multiplying the two numbers together:

63 grams x 3 inches = 189 gram-inches torque

There are 454 grams in one pound so:
189 gram-inches torque x 1 lb/454grams = 0.42 pound-inches of torque

(3) To convert from pound-inches of torque to inch-pounds of work, you need to multiple by 2pi:

0.42 pound-inches torque x 6.28 inches/revolution = 2.62 inch-pounds /revolution.

(4) Power is the work done per unit of time so we need to convert the inch-pounds/revolution to inch-pounds/sec. Multiplying the above result by the revolutions/sec give the power output in in-lbs/sec. If you’ve measured revolutions per minute (rpm) you’ll need to divide that by 60:

rpm x 1 minute/(60 seconds) = revolutions per second.
In the video I measured 3.4 hz (revolutions/sec)

2.62 in-lbs/rev x 3.4 rev/sec = 8.9 in-lbs/ sec

The engine is doing the work equivalent to lifting a one pound weight up (against gravity) 8.9 inches every second. Converting to other power units:
Watts are probably the most universal measure of power although most people in the US are familiar with horsepower (1 hp = 746 watts). Another convenient measure is foot-pounds/second (1 hp = 550 ft-lbs/sec).

Converting to other power units:
8.9 in-lbs/sec x 1 ft/12inches = .74 ft-lbs/sec x 1 hp/(550 ft-lbs/sec)= .0013 hp x 746w/1 hp = 1.01w