Hi Maxis,
Great job on that engine. I have followed your build log from the start as I really like that design.
Measuring speed and torque is the obvious way to measure power, but as you say, measuring the torque is the hard part. Someone recently described an eddy current brake which may work if it gives an appropriate torque range. The brake band around the flywheel type is the classic way, but I don’t know how well they work with the variation in speed each revolution of our engines. Some damping in the force measuring system may be required to get a useable force measurement.
Or use an electric motor as a generator. If you have a double ended motor so you can support the armature each end, then use a torque arm to restrain the stator, you can calculate the torque from the force on the torque arm. Again, some damping in the force measuring system may be required. I don’t know if this can be done with force sensors and electronic filtering of the output signal, but it’s a possibility.
But I think Zephyrin’s suggestion of lifting a weight is best. Use your iPad stopwatch facility to time lifting a pre-weighed mass through a pre-measured distance. Measuring the work per unit time like this averages out those speed variations so no damping is necessary. Some thought is needed about the winch design, so the wound string does not change the diameter too much. If you can arrange a long lift, this will minimise the effect of errors at the start and end of the run in both timing and operating speed. Some experimenting with the weight allows setting an appropriate range by trial and error. You could also use meccano gears to vary the winch speed, though the gears introduce friction, so possibility better to just experiment with the winch diameter.
So those are the theoretical possibilities, I suspect the need for damping of the force measurement might make the winch on a fixed mass for a timed mass the most practical. I have not made much progress on my own experimentation in that area, it’s one of those things on my list.
I am sure there will be a lot of interest in your results.
MJM460