New Hope for Steam Power: Testing the Efficiency

Having proven that my engine design was practical (and actually worked), the next step was to test its power output in order to determine engine efficiency. I did not have access to or the ability to build a boiler, so my only option was to run it on compressed air as a power source, and measure its power output with a DC electric motor hooked up as a dynamo.

Once again returning to the reliable scrap bin, I scrounged up some aluminum rails to use as motor and dynamo mounts. I also acquired a spare flexible coupling from another team working in the shop. The coup de grace came when I was loaned a brushed DC motor as my generator, and the use of a large electrical resistive load to test its output.

Before beginning, I measured the line pressure and flow rate of the compressor line in the shop that would serve as my power supply. This came out to about 100 psi (7 bar) and a 0.972 liters per second flow rate. Using the equation

Watts = [(V*100*ln(P))]

Where V is flow rate and P is pressure in bar, I calculated the total available power. (Approximately 189.14 Watt hours)

Next, I ran the engine and measured the output voltage and amperage at various resistances across the load bank to create a rough power curve, and compared the maximum value against the input power. The curve peaked at 48 watt-hours, which gave an approximate energy efficiency of 25.4%. This is only half the equation, however, since it does not take the Carnot efficiency of steam into account.

Because I had not included provisions for measuring the exhaust pressure of the air, I assumed that maximum energy was used and the steam exhausted out at ambient pressure. (Note: I know this is not the case, but this is only for rough estimate purposes.)

Carnot Efficiency = 1 – [(T out)/(T in)]

Assuming that the temperatures would equal those required for the input pressure (165 Celsius) and the output pressure (100 Celsius), I calculated an efficiency of 39.4%. Multiplying this by the measured energy efficiency of the engine gave me a decent estimate of the system's total efficiency.

By chance, I found a discarded AC motor in the recycle bin after I had returned the DC motor to its proper owner. Although the AC motor doesn't work yet, I had already completed my measurements and mounted it purely for display purposes. Around this time, I was getting ready to return to America. So I paid the extortionist prices to ship my parts home and prepared to embark on part 2 of the project: ironing out all the problems and proving its practicality.