The Stirling Engine Project

in this video i'm going to talk about my sterling engine design project now every techno tweaker gearhead science enthusiast who knows what a sterling engine is has probably felt that compulsion that yearning clawing obsessive desire to build one so here's mine this is my 3d printed water cooled rhombic drive sterling engine and what a machine it is so i started to realize that i had a problem when i found myself falling deeper and deeper into the youtube rabbit hole watching tin can sterling engine videos endlessly i was completely hooked thank god i don't have much of a machine shop that's the only thing that saves me from going completely insane but i knew i had to try my hand at this somehow so i thought let's try to figure out the best sterling engine design that we can make with a 3d printer and some basic tools and household items the basic premise of making a heat engine out of plastic is totally counter-intuitive but i realized that if we honor the thermodynamics of the problem there is a solution in there someplace a sterling engine functions by leveraging the heat flow from a hot place to a cold place if we use a constant flow of cooling water to cool the engine we can make a nice cold place and also prevent the flow of heat from the hot parts from melting and destroying the rest of the engine this seemed like the ticket mechanically speaking a sterling engine can take on many different physical configurations i settled on the beta configuration with a rhombic drive because it satisfies my perverse desire for symmetry and it's also really compact this allows me to make most of the parts on my 3d printer which has a maximum dimension of 20 centimeters in all three dimensions after choosing a basic engine configuration the next step is to optimize the geometry i made this kinematic model in cad and played with it for hours and hours agonizing over the best dimensions phase angle and all the other little details that make this thing work this is really the heart of the matter and i spent a lot of time fretting over this i went sniffing around at my local kitchen supply store for suitable vessels that could be converted into my sterling engine cylinder head i found this nifty stainless steel pepper shaker thing that's perfect it's 66 millimeters on the inside and has a nice domed cap i cut it down into two sections the domed cap and a simple cylinder that serves as the cold part of the engine one of the key design problems i had to solve was figuring out how to mount the hot cylinder head into a plastic assembly how do you do this without it immediately just melting at the interface well i found this material which is actually surface mount resistor feeding tape which is a really good thermal insulator and seems to resist high temperatures i wrap a strip of this tape around the base of the cylinder head and then glue it into the 3d printed retaining ring with high temperature silicone adhesive the retaining ring itself is also relieved with teeth which help to reduce the thermal conductivity between the joint and the rest of the plastic this configuration seems to do the job pretty well the lower stainless steel tube section is cooled by a 3d printed water jacket which allows a constant flow of cooling water to flow in the annular space around the lower portion of the cylinder this serves to isolate the lower parts of the engine from a lot of the thermal nastiness and also preserves the large thermal differential between the hot and cold space of the engine this improves the efficiency dramatically the water jacket and the diaphragm flange plate get glued together the cylinder is slid in and then the whole thing gets sealed up on both sides with more high temperature red silicone it's really hard to get this thing not to leak but luckily it's usually under negative pressure so it just sucks air bubbles into the cooling water the hot cap and the water cooled cylinder assembly then get screwed together i had to make some nice formed and placed silicone gaskets to make sure that the joint is airtight it's essential that it's air tight to hold compression looking up inside the cylinder head it just looks like a continuous stainless steel tube but in reality across that joint is a huge difference in temperature it's this difference in temperature that makes the air inside violently expand and contract which pushes down on the diaphragm powering the engine the displacer and power piston yoke along with the diaphragm piston are all 3d printed parts they're connected together with a minimum of metal parts which are epoxied into the 3d printed parts the displacer rod is a simple stainless steel three millimeter rod the power piston has a piece of brass tube that just fits over the displacer rod the rombic drive mechanism rolls on three millimeter ball bearings at every point these bearings are shielded and not sealed for minimum drag it's really difficult to get all these bearing fits perfect the 3d printed parts are just not the best thing for this i ended up having to crazy glue some of the bearings in just to keep them from falling out but in the end it seems to be very robust that that part hasn't broken yet the gear drive is designed as a cassette to make the assembly of the engine easy you just drop in the rhombic linkage and then push the gearbox in from the side very easy the gears themselves are gigantic to compensate for the fact that they're made from a very weak material with very low precision they turn on five millimeter stainless steel shafting supported by more shielded bearings that snap into the cassette some collars hold the shafts to keep everything from falling apart the actual rhombic crank pins are just pieces of stainless steel three millimeter rod that get epoxied into holes in the faces of these gears there's also a small magnet glued into one of the gears which is going to serve as a rpm pickup for my later r d efforts before i completed all the mechanical parts of the engine and did the final assembly i really wanted to check out to make sure that the thermodynamics of the whole idea really worked before i bothered so i assembled the whole cylinder head and displacer assembly and without connecting any of the rhombic mechanism just heated it up with a propane torch and just watched what happened i was quite pleased to see that moving the displacer piston back and forth created large changes in the internal pressure you could see that because the diaphragm violently moves in and out as i move the displacer even though there's no connection between those two parts this proves that the basic concept of the engine is sound and made me feel confident to go ahead and finish assembling all the rest and see if it would run i made sure to have ample cooling water flowing during this test to make sure that i didn't damage anything and also that the thermal difference between hot and cold space was appropriate to what it would be when the engine actually ran after the success of the thermal test i decided to open up the cylinder head and have a quick look at the displacer which is made out of a big wad of steel wool and a little mechanism that holds it all in place you can see it's a little bit discolored from the high temperatures that it experienced but it's all together in pretty decent shape during a good long test run i started to notice that the rpm was dropping and the engine seemed to be getting sluggish i stopped the test and opened up the engine what did i find my displacer piston is starting to melt at the base of it there's a 3d printed plastic part which i hoped was far enough away from the hot action to stay in one piece but alas it's melting and dying i thought i would pay homage to the aluminum can gods so i made a new displacer out of a piece of a coke can and some very thin printed circuit board material this was a total fail the circuit board material burned releasing horrible toxic fumes and totally gunked up the inside of the engine i realized now that i got super lucky with my first design attempt it's actually really a tricky part of the whole engine this displacer regenerator thing lots more fiddling to be done the water jacket cooling system also begs for a redesign it leaks and it also gets air pockets stuck inside which negatively impact the cooling the engine also vibrates too much i really need a counterbalance on the two crankshafts when i designed this i kind of ignored all of that so i've been fiddling with this sterling engine thing for weeks now but somehow i still feel compelled to fiddle with it more there's something about a sterling engine that's a perfect trap for the engineering mind on the surface it seems really straightforward mechanically there's nothing that's scary about it but as you try to optimize it try to optimize it you realize that it's this infinitely complicated soup of contradictory parameters and ideas that all work together in this deliciously complicated way it's very intellectually interesting so i have to warn you that this is extremely addictive stuff so be very careful if you try to mess around with a sterling engine so as i think of improvements for this engine and think of things i want to do to it i realized i don't know anything about the performance of this engine without a way to quantify the performance of the engine you're really just jerking off you're just going around in circles so the next step for me to really make some scientific progress is to build a dynamometer for this thing i looked around on the internet to see like what do other people do about this problem and there's a few things available but nobody really has a really sexy small engine sort of micro dynamometer so that's what i'm going to do next i'm going to build a dyno so i can close the loop on this process and really optimize my engine so as usual i hope you learned something and enjoyed my video please make comments smash the like button subscribe to my channel help me grow so i can produce more good content like this thanks again you

Channel: Leo's Bag of Tricks

Views: 162,122

Keywords: Sterling Cycle, Thermodynamics, Dynamometer, Engine Design, 3D printed, Plastic Engine

Id: eZUteOLEKz8

Length: 13min 51sec (831 seconds)

Published: Tue Mar 09 2021