Solar Heat Engines

Measuring a Stirling engine with 1 watt power output and 1.5% efficiency

The following is a transcript with still photos of the previous video on the same subject. I’ve included a few more notes about the engine, my model 3F.

This is a gamma configuration Stirling engine. I’m powering it with the heat from a 70 watt light bulb so that I can accurately measure the heat input.

The vertical shaft on the left connects to the power piston and the shaft on the right connects to the displacer. The power piston has a 1.25 inch diameter and a 2.5 inch stroke.

You can see the top of the displacer cylinder but the rest of it is covered up by the large metal cylinder packed with insulation. The displacer has a 1.75 inch diameter and a 2.5 inch stroke. The displacer cylinder has a 2 inch diameter and is approximately 12 inches long. The insulating cylinder around the displacer is 8 inches in diameter.

The thermometer measures the temperature near the hot end of the displacer cylinder. It reads 400 Fahrenheit or about 200 Celsius. In the original video the resolution is so poor you can’t read the thermometer. I’ve placed a better resolution photo of the thermometer next to it so you can see the approximate reading was 400 F (although it’s just at room temperature in this photo).

The torque measuring setup shown here is detailed in a previous post. Adjustment screws on the wood torque arm are set to squeeze the rotating shaft to achieve the desired torque value which is read on the scale below. Note that the torque force is transmitted through pins in the wood arm set at a 3 inch horizontal distance that makes the horizontal position of the arm support position on the scale uncritical.

I’m measuring the torque output of the engine with this setup. The scale shows a torque of 68 grams on a 3 inch arm or 68g x 3 inches / 454 (g/pound) =.45 pound-inches of torque. I lift the torque arm briefly to the check the zero reading.

This tachometer displays the engine speed in cycles per second times ten. So 33 means 3.3 hz or 198 rpm. More information on using a bicycle speedometer as a tachometer can be found in the low-cost tachometer post.

The 70w halogen light bulb is hidden up inside the insulating cylinder, but you can see the end of the socket for it here.

The watt meter shows the light is actually using 69 watts.

Here you can see the counter balance weights for the displacer and the crankshaft. The crankshaft drives the lever connecting to the power piston directly. The piston lever pivots freely on the horizontal shaft. A second connecting rod on the crankshaft drives an arm fixed on the horizontal shaft. The horizontal shaft then drives the displacer lever. This mechanism provides the 90 degree phase shift between the displacer and power piston.

The measurements shown are used to compute power and efficiency as follows:

Torque x 2PI = work per revolution
.45 lb-in x 2PI = 2.83 in-lb/revolution

(work/revolution) x revolutions/sec = work/sec
2.83 in-lbs/revolution x 3.3 revolutions/sec = 9.33 in-lb/sec

9.33 in-lbs/sec = .778 ft-lb/sec = 1.05 watts

Efficiency = power out/power in = 1.05 w/ 69 w = 1.52%

The measurements I’ve shown compute to 1.05 watts power output and an efficiency of 1.5%