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Target for This Month: A 3D Printed Engine

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SillyOldDuffer09/06/2022 19:25:37
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Been busy developing my SolidEdge CAD skills by tackling various parts and assemblies. After replicating Jason's current engine in cyberspace, I've left him to build a real one. Don't want to get into a race because I reckon he shapes metal about twenty times faster than me!

Thought I try something a bit different: with forum help can I design an engine that can be 3D printed and run on compressed air. There are interesting advantages and disadvantages.

Advantages:

  • Plastic can be printed in almost any shape with internal cavities and passages that cannot be made with a lathe, mill or even casting. (3D printing is like casting on steroids!) It makes previously impossible constructions possible.
  • All the skill is in the CAD design; the parts are made by machine.

Disadvantages:

  • Accuracy is limited compared with metal bashing; about 0.1mm on my hobby printer, and that's optimistic!
  • Finish tends to be poor, with ridges and blobs.
  • Limited to low temperatures - less than 100C.
  • Plastic's not very strong: the design might have to pay much more attention to stresses and strains than an equivalent metal engine. There's a strong incentive to reduce cost and print time by leaving holes in a thin structure wherever possible
  • It's at the limit of my understanding

First attempt was this double acting horizontal mill engine, very much based on metal models such as the PottyMill.

3dhorizontal.jpg

Main interest is in the design of the piston, which is coned for strength and supported at both ends so it can float to reduce friction, and eliminate the need for a cross-head, maybe. The cylinder is generously honeycombed with passages to open up airflow, as is the valve. The valve is oversized compared with steam because there's less energy in compressed air at the same pressure. Don't like it - too conventional, too big, and too flimsy.

Rethink is this single action engine, which I think has more possibilities:

3deng.jpg

Section:

3dengsection.jpg

CAD has already saved me from a mistake. I have the rotary valve driven by a cranked rod, which is no good because it can flip as it goes over the top, completely ruining the timing:

3dengcrossed.jpg

Now my next job is replace the cranked wheels with sprockets and design something like a bicycle chain that can be 3D printed.

Lots of cause for concern already; friction is sky-high, there are no fasteners (I''m thinking plastic clips), the plastic isn't optimally proportioned, the valve can't fail to leak and the conrod has magically installed itself on the crankshaft.

Already thinking a 3D/metal hybrid would be a much better bet but I'll persist with printed plastic for the time being.

Comments and suggestions welcome! Yes it could be a step too far!

Dave

JasonB09/06/2022 19:43:09
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Single acting will be more prone to failing if friction is high as you are not pushing the piston back the other way as you do with a double acting. As your plastic flywheel won't have much energy to return the piston you could be in trouble.

Simple piston valve may be easier driven by a push rod that runs against an eccentric cam.

Race would be a bit of a no contest as I've already finished that enginedevil

Edited By JasonB on 09/06/2022 19:48:40

DiogenesII09/06/2022 20:47:33
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Is Jason thinking along 'Husky' lines - could be a plan..

..I was wondering about timing admission via a port on the crank, perhaps something like a monobloc (& oversquare?) 'River Queen'..?

..are you allowing yourself glue?

Edited By DiogenesII on 09/06/2022 20:48:11

JasonB09/06/2022 20:52:37
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No you need even more energy in the flywheel for across flow like the husky.

Something simple with the piston valve opening an inlet and then as that is closed an exhaust port will be opened, timed so the piston does not have to compress any trapped air at the top of it's return stroke.

The single acting "Woody" would be the sort of thing

woody valve.jpg

Edited By JasonB on 09/06/2022 20:56:26

John Olsen09/06/2022 22:44:32
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NSU made a motorcycle years back that had the cam driven by connecting rods. but of course there were two of them with the cranks at right angles. Since it was a four stroke, there still had to be a two to one reduction gear.

John

JasonB10/06/2022 06:51:47
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Going back to your problem of the rotary valve going over the top, if you reduced the throw of the pin on the crankshaft so that you just get about 1/4 turn of the valve then that will be enough movement and it can't go too far. Same can be done with a double acting version like my Rocking Valve Filer & Stowe engine.

DiogenesII10/06/2022 07:00:03
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Posted by JasonB on 09/06/2022 20:52:37:

No you need even more energy in the flywheel for across flow like the husky.

Something simple with the piston valve opening an inlet and then as that is closed an exhaust port will be opened, timed so the piston does not have to compress any trapped air at the top of it's return stroke.

The single acting "Woody" would be the sort of thing

woody valve.jpg

Edited By JasonB on 09/06/2022 20:56:26

Yes - I get it - that's very economical of movement / friction..

..interesting to read Woody again, that was a lot of offsets & handle-work..

SillyOldDuffer10/06/2022 11:03:10
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Diogenes : yes to glue, though slightly reluctantly! I'm open to other port arrangements too.

John: two cranks, yes

Jason's got me thinking.

  • Single action means my flywheel needs to store enough energy to return the piston against a mass of friction and it's small and light.
  • Sprockets are easy to 3D print, but when I got to roughing out the chain, I realised it's relatively heavy and full of bearing surfaces. Not a problem for polished metal, I suspect bad news for plastic links.
  • I like the idea of rocking the valve by reducing the throw and it's a simple modification worth trying
  • I also see advantages in a cam
  • The 'Woody' valve is lower friction than my design

My first reaction to Woody was 'too complicated' but I'd reverted to thinking about making it in metal rather than printing it!

This will keep me quiet for a while...

Many thanks

Dave

Dave S10/06/2022 12:56:25
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As your printing it it is no more effort to make a twin or even 3 cylinder engine.

This simplifies the double acting issue by making it a sum of singles.

something like a 3 cylinder radial might work nicely.

Have each cylinder somewhat like a CO2 engine - input valve at the top, exhaust through a slot at the bottom.

vale can be a simple ball and seat or some sort of print in place flap, triggered by a pin extension on the piston.

wouldn't be a 'traditional' engine, but then isntt that the point?

Dave

duncan webster10/06/2022 13:37:54
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Posted by Dave S on 10/06/2022 12:56:25:

.........

Have each cylinder somewhat like a CO2 engine - input valve at the top, exhaust through a slot at the bottom.

......

Dave

This implies uniflow, which means lots of compression unless large clearance volume. You need a heavier flywheel for this. Proper exhaust valve better in my view

Dave S10/06/2022 16:26:34
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Yep uni flow is what I meant. Given a 3D printed piston and 3d printed cylinder there is likely to be quite a poor seal, so the compression isn’t likely to be good.
Multiple power cylinders should help with the light flywheel issue

Dave

JasonB10/06/2022 17:11:02
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You could just do an oscillator and do away with the need for a valve. Easy enough to sand the two faces flat if print quality is not good. Double acting only needs an extra hole or two over a single acting one.

SillyOldDuffer10/06/2022 17:59:34
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Not much done - an afternoon of interruptions. However, I did manage to do Jason's rocking valve*, which doesn't get crossed over, increase the diameter of the flywheel and add exhaust holes at the bottom of the cylinder. I also realised the original crankcase had no vents, so the piston would have to compress air inside it in order to move. Now it's got big holes in it, which also saves plastic!

3drocking.jpg

Dave S' two cylinder suggestion is tempting. As can be seen it wouldn't be difficult to arrange the rotary valve and crankshafts to plug into each other creating an inline engine. Uniflow is harder to do, so I'd just run any add-on cylinders timed to be usefully out of phase with the others.

Dave

* Apart from the timing. My SolidEdge skills are unreliable, and the Relationships went haywire!

SillyOldDuffer10/06/2022 22:05:14
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Well. if this engine runs at all, it's not difficult to chain two, or more, of them together.

3dengpaired.jpg

JasonB11/06/2022 06:51:13
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A Vee or opposed layout may be better as you could run with just the same two bearings thus halving the friction on the crank. And don't forget if you do link them as shown then stagger the crank pins.

lee webster11/06/2022 08:05:59
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Dave,

You said " Finish tends to be poor, with ridges and blobs. " Are you planning to print using FDM printer? Like my poor old Ender3? If so, what filament will you use? I do like the idea of a plastic printed engine!

Many years ago I had an Airfix (I think it was Airfix) plastic beam engine powered by a small electric motor. It ran, and it squeeked like mad! I miss that little engine.

SillyOldDuffer11/06/2022 10:30:59
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Posted by lee webster on 11/06/2022 08:05:59:

Dave,

You said " Finish tends to be poor, with ridges and blobs. " Are you planning to print using FDM printer? Like my poor old Ender3? If so, what filament will you use?

...

Yes to FDM, I have an Ender3-Pro. Does a reasonable job and I should have put a scale on the 'ridges and blobs' comment, because they don't matter for many purposes. This is the worst bit of the cotton-reel example, which - to save time - was printed in low quality mode:

dsc06604.jpg

Low quality is 0.28mm. 0.12mm High quality, is roughly twice as smooth provided the printer and filament are tickety-boo.

Low quality plastic finish is OK for most of the engine apart from the bearing and sliding surfaces. Unfortunately sanding and polishing them might still not be good enough. I don't know what the coefficient of friction of 3d printed plastic is.

A hybrid engine would be better, such as designing in off-the-shelf roller bearings rather than scraping plastic on plastic, but I'm trying to stay pure plastic hoping I can make it work. I'll try ABS first because that's what I have, but combinations of other plastics would be superior.

Another constraint: the printer's platform limits the size of the engine's biggest part. Have to keep an eye on that, unless I pay to have it printed professionally! While 3D-CAM opens many useful doors, the designer has to keep a close eye on production limitations; it's not difficult to CAD impossible to build assemblies!

Before I go further I need to confirm SolidEdge will work with my printer! Should be OK, SE has an entire 3D section, but I haven't looked at yet and the proof is in the pudding.

Unfortunately out today to fix my daughter's plumbing problem. When the kids were little, my life was dominated by buying them new shoes; then I became a teenage taxi-service; now they've left home I've been promoted to Bank of Dad and all-purpose handy-man. The latter has ruined my Alpha-male reputation - conclusive proof I don't know what I'm doing...

Dave

martin haysom11/06/2022 11:09:01
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Posted by lee webster on 11/06/2022 08:05:59:

Dave,

Many years ago I had an Airfix (I think it was Airfix) plastic beam engine powered by a small electric motor. It ran, and it squeeked like mad! I miss that little engine.

i made one of them too

lee webster11/06/2022 18:33:03
115 forum posts
10 photos

Dave,

I have 3D printed parts from SolidEdge using my Ender3. No problem.

SillyOldDuffer12/06/2022 15:54:51
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Posted by lee webster on 11/06/2022 18:33:03:

Dave,

I have 3D printed parts from SolidEdge using my Ender3. No problem.

Good news thanks, I'm getting close to printing the engine at half scale to see if it really does fit together. Might even run, though scaling down increases friction disproportionally .

 

3dengexploded.jpg

All seems to fit together apart from the conrod and crankshaft: one of them needs to come apart so the two can be assembled together.

Spent a good deal of today investigating a Solid Edge assembly-relationships problem. I designed this throttle valve, which animates OK except the flap rotates through the tube rather than the software recognising the tube is a physical obstacle.

throttleinnards.jpg

I've had several fights with Solid Edge where assembly relationships didn't work as expected, this latest being highly baffling because there are only two parts in it! They are related by an axial alignment between the through side hole and the tap shaft, and by a mate between the shoulders. The joint is set free to rotate. The handle's supposed to stop turning when the elliptical throttle plate hits the tube wall, and it didn't. A mystery because much more complicated relationships in the model work this way correctly. Very frustrating!!!

Until now, the main issue has been me getting relationships wrong due to not quite understanding how SE works. This one wasn't obvious - eventually identified the problem was a tiny geometry error in the through hole. I created one side manually and patterned the other from it, supposedly a 180 degree mirror image of the hole and shroud. Not sure why but the two sides were slightly misaligned, so the tap shaft was out of alignment across the tube. Tiny but enough to confuse Solid Edge's boundary calculations. Lesson learned: if relationships won't bind or behave strangely, double check the part geometry. Geometry has to be right, not nearly right.

Dave

 

Edited By SillyOldDuffer on 12/06/2022 15:55:40

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