Intro: Candle Power Stirling Engine
I am an engineer who has never before built an engine. I decided to use my time at Pier 9 to address this, and built the engine in front of you.
The engine is a Stirling Engine, which runs on an externally applied temperature gradient. My goal was to have the engine extract work from a tea light, turn a generator, and power a fake tealight.
Unfortunately, it is not efficient enough to run off a single candle, so for demonstration purposes it is running off a bank of hidden batteries. The next version is in process though and will hopefully achieve my goal.
Step 1: Background
A Stirling engine contains a fixed quantity of working fluid. By pushing the fluid, air in this case, between two chambers of different temperature, the fluid can be expanded and contracted thermally. The mechanical linkages are arranged to extract mechanical work from this thermal expansion and contraction.
Step 2: Engine Anatomy: Heat Exchangers
The engine on display is actually two separate engines that are 180 degrees out of phase. This balances vibration when the engine is running. Heat is transferred from the heat pipes (the copper tubes) through heat exchangers to the working fluid within the body.
One heat exchanger (the one over the candle) carries heat in to the engine, while the other (over the electric light) carries heat out.
Step 3: Engine Anatomy: Linkages
The linkages create a 90* relationship between the pairs of pistons. These linkages are deliberately over complicated for aesthetic reasons, but you can still clearly see the 90* relationship if you look at the angle between the rods directly touching the crankshaft.
Step 4: Engine Anatomy: Flywheel and Pistons
The engine only provides work for part of its cycle, so a flywheel is necessary to store energy to carry the engine through the non-productive part of its cycle. I chose brass for this because it's heavy and provides a color contrast to the aluminum.
The pistons and cylinder liners are also deliberately made of mixed materials. Aluminum on aluminum sliding surfaces tend to eat themselves. If you're interested in the phenomenon, look up "galling". To mitigate the problem I designed this issue with brass cylinder liners. They also add visual interest since they match the flywheel.
Step 5: Engine Anatomy: Motor/Generator
The original intent was to capture work from the engine using a small electric generator and use the power to run a small electric candle. However I couldn't get that working on this version and decided to run the generator as a motor to provide motion to convey the intent.. The motor is salvaged from a PC case fan, as those were the motors with specs best matched to my engine targets.
Step 6: Previous Versions
This is the third version of this engine, each significantly closer to my intent than the last.
The first failed with design issues. It featured machined heat exchangers, poorly aligned linkages, and poor overall construction.
The second failed with mechanical issues. My mechanical inexperience led me to design everything with aluminum on aluminum sliding surfaces and poorly proportioned shrink-fits. The friction in the pistons was extremely high.
The third, on display, is mechanically sound but failed thermodynamically. I overestimated the rate of heat transfer through the heat pipe and in to the working fluid.
Step 7: What's Next?
The 4th version is under development and will integrate what I've learned with version 3 - mostly with more realistic estimates for heat transfer and with better piston sizing.
I'll be blogging about it on moosh.im . Thanks for reading!