DjuiinoStar Low Temperature Stirling Engine: Rotate 1 Hour on A Mug of Hot Coffee DLTD-602

This Djuiino DLTD 602 is a Magnetically Coupled (MC) Low Temperature Difference (LTD) Engine. My particular engine runs very quietly while capable of very high speeds over very hot water; 180 F gives 690 RPM. It even runs on my hand, but needs to have its top plate cooled just a bit, and the bottom plate warmed just a bit over a coffee cup. According to the pictures, this and five other apparently identical blue engines are currently offered on Amazon. Another apparently identical, but not blue, engine DWCL-01 by SunnyTech, has been on the market at least since December of 2013. The 602 uses two ball bearings on the main flywheel shaft (You can buy such tiny bearings from Amazon for a price of about $11 per packet of 10 bearings.) I have six different LTD engines, and most are noisy. They click as the displacer piston hits a top or bottom plate. I feel that the manufacturers are lax in not using jigs as well as trained assemblers to position the displacer correctly!The Stirling engine was invented by Stirling in 1816 as competition to the rotary steam engine of Bolton and Watt, invented about 30 years earlier. To those familiar with the LTD classic Stirling engine (CS), a MC engine offers two mysteries.The first mystery: while either LCD engine will run over a cup of hot coffee, the CS also runs, but in the other direction, over ice cubes. An MC engine will not run over ice cubes. (It will run with ice cubes on top.)The second mystery: over hot water, the MC engine can run in either direction, depending on which direction you nudge it. The SC runs in one direction when the bottom plate is hotter than the top, and the other direction when it's colder.Both the CS and the MC have two power strokes per revolution, one down and one up. They differ in how changes in the interior air temperature cause pressures on the power piston. I have explained how this happens in a CS engine in a customer review in Amazon of the SunnyTech LT 001. This beautiful engine has a "brass" flywheel, curved supports for the flywheel and cranks, etc. Apparently identical engines are offered by several other vendors on Amazon.With a little experiment, you can understand what makes a MC engine go. This experiment works for the 602 engine but to my knowledge does not for other MC engines. Gently place the room temperature 602 engine over a cup of very hot water, about 175 F is good. The engine will be in its usual resting state, with the power piston all the way down, and the displacer piston at the top of its cylinder, stuck magnetically to the power piston. Wait about 60 seconds. The power piston will suddenly move a little upward, then pause. Why does it do this? It's because most of the air in the displacement cylinder is near the bottom, in contact with the hot lower plate. Theair inside the engine warms, and because it is confined, it cannot expand, so its pressure must rise. This pressure on the power piston pushes it upward, rotating the flywheel.The experiment will continue below, but we note here that already we can solve the first mystery: why a MC engine will not work when placed on ice. Hot air expands, cold air contracts. The power piston started at its lowest point, hot air inside can make it move up. Cold air can not.Back to watching. After a brief pause, the piston moves up again, pauses, then repeats again and again. This is a slow motion version of the upward power stroke in a running engine. In the experiment, as the power piston arrives NEAR BUT NOT OVER its top position, suddenly things happen. The displacer disconnects from the little rod extending down from the bottom of the power piston. The displacer quickly drops to the bottom of its cylinder, pushing its air up to be in contact with the top plate. The air very quickly cools and the pressure drops. Now the room air pressure is higher than the interior pressure and forces the power piston back down, giving impetus to spin the flywheel. So the second power stroke is downward from the top. But note that in this experiment the downward stroke rotates the flywheel in the OPPOSITE DIRECTION from its upward motion.Continue watching. The downward moving power piston approaches the displacer on the lower plate, and reattaches to it. The flywheel pulls the dropping power piston through its lowest point and as it begins to rise, the displacer begins to move interior air down to the bottom plate. The newly reheated air pushes the power piston up in a new power stroke. This time there are no pauses as it rises. The engine enters a continuing oscillation back and forth, going higher each time.Soon the oscillation is large enough for the flywheel to pull the power piston over its highest point, so it descends in a downward power stroke, starting continual rotation in one direction. The engine is running, and you did not have to nudge it to get it started! The rotation direction depends on chance, which side of the oscillation is first high enough for the power piston to move over its highest point. Tilting the engine a little will influence which direction it takes. I use an toothpick between engine and cup rim to tilt it. A CS engine and most MC engines will not start without a nudge, or sometimes a vigorous spin by hand.Now for a subtle but important point. The engine leaks air in and out past the power piston and its cylinder wall. So the pressure inside the engine (averaged over a complete revolution) stays nearly equal to the room air pressure. As the engine warms up, its interior pressure rises. If some air did not leak out, the hot air interior pressure would push the power piston all the way up and hold it there, stopping the engine!You can see the leak in action! Do a second experiment. Put your room temperature 602 on the table. It will be in its usual state, with the power piston all the way down, and the displacer all the way up, stuck to the power piston. Now rotate the flywheel by hand to move the power piston to its top position and hold it there. This adds the volume of the power cylinder to the volume of the displacer cylinder, lowering the interior pressure. (The displacer will fall to the bottom of its cylinder.) Wait a full minute to allow air to leak in. Now release the flywheel. The power piston will fall to about half way down, compressing the interior air, causing it to start leaking back out. Keep watching. The piston moves further down by tiny steps. (The best way to see this is bywatching the flywheel.) When it is nearly down, the magnet in the displacer is attracted to the rod below the power piston and the displacer leaps up to attach to the power piston. The additional weight on the flywheel causes the power piston to take a bigger step down. It pauses, then moves the rest of the way down to return the engine to its resting state.The power cylinder is transparent glass. You can see a steel washer near the bottom whose hole is slightly larger than the diameter of the rod extending below the power piston. The power piston rod moves through that hole as the engine runs. I believe this washer is there to control, in some way, the making or breaking of the magnetic attachment of the displacer to the power piston.

Nice machine. Not the classic sterling engine because it uses the magnetic interface between the displacer and the power piston but some advantages: Runs quieter, turns in either direction regardless of hot or cold being on top or bottom, I can't speak for others but mine will self-start. Giving the flywheel a spin is unnecessary. Definitely goes an hour on a cup of hot water (usually longer).

Since I was a kid, when I first learned about Stirling engines, I was immediately fascinated. I also really wanted one just to be able to see them work their magic in person. Now that I'm an adult with money, I figured I might as well buy a desk toy. I have absolutely no regrets. It's pretty sturdy and it runs smooth when it's going full speed.

Fun little time waster to watch works great.

This item was sent to a relative as a gift. All I can say is what they told me which is it was appreciated and I know delivered on the date that was specified.