Engine model 3d is up and running!

Engine model 3d has actually been running for over a week now, but I finally am taking time to start getting this website up-to-date. You can see more photos in the photo pages.

I’m in the process of performance testing the engine to see how closely its power output agree with my simulation. There are many sinks for losing power including: bearing friction on the rotating shaft, sliding friction on the piston and displacer shaft, compression leakage around the piston and displacer shaft seal, air friction on the air moving around the displacer, and air friction on the spinning flywheel.

My basic and inexpensive instrumentation for this operation includes a stopwatch and a bicycle speedometer used as a tachometer. Using engineering computations and these two instruments I can derive the friction based on how long it takes the unpowered engine to spin down. Here’s how it works.

A flywheel stores energy proportional to the square of its speed (RPM). You can compute the actual energy stored in the flywheel at any RPM. Selecting two RPMs, a high and a low value, you compute the energy difference between the two RPMs. You spin the engine up unpowered to just above the high RPM and start your stopwatch when the RPM drops to the high RPM. The unpowered engine spins down and you stop the time when it reaches the low RPM. You already know the flywheel work difference between the two RPMs (ft-lb, joules, or whatever energy units you want). Dividing by the time you measured on the stop watch gives you the power (ft-lbs/sec, joules/sec, etc.) or rate at which flywheel work is being used up in friction.

A massive flywheel helps you make the measurements accurately and more easily because the flywheel will slow down more gradually. That’s the reason you see the club with massive bolts swinging on the flywheel above. This setup is for under 100 RPM use (don’t use it for high speeds or it could disintegrate with potential for bodily injury). You probably want the spin-down time to be at least 5 seconds and preferably 10 or more to minimize the timing measurement error. You’ll want to make enough spin down measurements so that you can see what the variation is due to timing and RPM measurement accuracy. You can also average the results to get a more accurate results.

The flywheel spin down test can be used with engine components starting with the flywheel and the main bearings it rides on. You add components one at a time, repeating tests to see the power loss due to each component. Using this method you can measure say the piston sliding friction instead of just knowing the friction of the entire engine.

I’ll be adding material for those without the technical background to make the flywheel computations. I’ll also post the method for using a bicycle speedometer as a tachometer. It’s inexpensive and really simple. I use it to measure the RPM on my variable-speed lathe too.