newbie - simple engine won't run

Hi everyone. This is my first post. I have been trolling the forums for a few months, just absorbing, learning, and checking out everyone's builds. I have been hesitant to post as everyone's engines are so impressive and mine is well...not working for one.

I am very new to machining. I have wanted to get into it for a while, but I have just pulled the trigger in the past few months. Lots to learn. In my spare time I have managed to cobble together this little engine. Unfortunately, it does not run and I was hoping some folks here might have an idea or two as to why. I have my own theories, but I was hoping some more experienced people could weigh in with a few tips.

1) there is some wiggle room between the piston and the cylinder. As in if I hold the cylinder and turn the crank shaft/flywheel, the crank/piston can rotate maybe 10 degrees individually of the cylinder. Is this causing the piston head to catch? Should the piston head be tighter inside the cylinder? There does seem to be a little grab or resistance at certain points...

You can actually see in the front image that the piston rod and the cylinder are not perfectly parallel to each other. It really is a very small amount though....

2) Could it be that the crank shaft or the pin holding the cylinder is not perfectly at straight angles and therefore adding some wobble/friction between the cylinder and the piston?

3) There is also some room between the cylinder pin and the hole guiding it through the frame. Not a lot but enough so it is a little loose. Problematic?

It seems as though if I spin the flywheel it should be free enough to spin around a few times - its does not. I'm not sure if this means anything or not.

Thanks so much for any insight. I look forward to joining the forums and a future of building stuff out of metal.






and a video if you are so inclined. Password: engine
[ame]https://vimeo.com/144004374[/ame]

I can't see your porting which is usually just drilled holes - your pivot point is very low - its normally near the middle of the cylinder.
So I suspect your ports are crossing each other with very little dwell time.
So a puff of air goes in (instead of being sustained for say 90° of overlap) as a puff and is lost to leakage before doing anything useful.

I just uploaded a dwg/pdf file of a double acting single cylinder version of my magnetically held V4 wobbler.

Grab it off the "downloads" section. (Not there yet - I presume waiting on moderator to clear files ?)

This might help you with visualising port sizes & positions.

Regards,

Ken

Thanks for the reply. I will check out the files when they clear customs.

Meanwhile I have posted an image showing where the ports are. I will make some adjustments on this Frankenstein and move the pivot point up.

Meanwhile, I take it - theoretically anyway - my other concerns about the wiggle room inside the cylinder and the pivot potentially being not quite straight are not something people worry about too much?

Thanks much

I wouldn't worry too much about the sloppy fits - it should still run with a lot of leakage.
Also with your pivot nearer the center the spring is going to help seal that running face without it being too tight.
The port holes (all 3) look like they can be maybe 50% bigger.
Get the pivot in the middle, get the faces to fit smoothly - I would guess it will run then.
Next open the ports a bit and it should get better.

Good luck.

Ken

Looking carefully at your pictures I see something that jumps out at me.
In the first pics, and your description, the piston rod is indeed not in
line with the cylinder. The give away is in the pic of the engine disassembled.

The piston is rigidly attached to the rod, as per design, but it's length is
quite a bit shorter than the diameter.

Since there is no lower guide for the rod (bottom cylinder cover or a
cross-head) the rod can tilt. This causes the piston to tilt, thus jamming
it in the bore.

Either the piston must be much longer or there needs to be a lower
rod guide of some sort.

Usually these little open ended engines have piston that is at least 1 1/2
times the diameter of the bore and the fit is quite close. There is quite
a bit of sideways friction from the porting plates and tension spring and
this force must be over come by the side load of the piston in the bore.
If the piston is too short then it will bind in the bore.

Make sense? The piston is the only bearing area working to move the
cylinder from side to side. The forces may be small but they are there
and must be countered by sufficient bearing area.

You can easily modify your piston and correct the problem by making
a new piston with a long hollow in the bottom, called a skirt, which
will make the bearing area long without changing the basic design.
Imagine the existing piston with a tubular extension on the bottom; the
solid part of the piston is in the same place as the existing one with the
skirt below that. If the cylinder is so short that the existing piston comes
flush with the bottom of the cylinder then there's a problem and you may
have to lengthen the cylinder to match.

Pete

A few points to consider:

The low pivot point means that the pivot spring has poor leverage in trying to
maintain a seal at the ports. Moving the pin higher would improve this.

With a higher pivot pin, the central area of the column, around the pivot pin, can be machined back slightly. This ensures that the pressure between the faces is concentrated at the top and bottom of the cylinder portface. Think of the heel and ball of the foot with the instep between.

A low pivot means that you get a greater angle of oscillation, and that means you can have bigger ports. Ideally, the ports should all be the same size and the gap between the inlet and outlet ports should be the same size as the cylinder port, so that there is no dwell - as soon as one closes the other starts to open. The cylinder port does not need to fully uncover the column ports at full swing.

Don't worry about a little slop in the pivot. You can fix that later.

Remember that you depend on the piston to oscillate the cylinder. Having a piston that appears to be shorter than its diameter makes its job more difficult. If possible, the piston would be better with a length one and a half times, or twice its diameter. This will reduce the pressure on the 'corners' and eliminate the tendency to jam. I don't know if you have room to do this without making other changes. With a long piston, the middle does not do much and can be reduced in diameter to reduce the mass.

With that amount of wiggle, your piston clearance is too loose.

I am a bit concerned about the crankshaft main bearing. It looks as though it is
also rather short. As it is a single bearing with an overhung crank, again I would want it to have a length at least twice its diameter. This could be achieved by making a bush to fit the column.

See if you can get hold of a copy of Model Stationary and Marine Steam Engines by KN Harris

Great tips from everyone, thank you so much. I really appreciate it.

As this engine is turning into something of a trial and error piece I am no longer concerned with the way it looks and am just hacking into it. I moved the pivot up. Still does not work. I like the idea of giving it a little concave around the pivot. I also started a longer piston and ordered a copy of Model Stationary and Marine Steam Engines by KN Harris

I am starting to wonder if - among other things - it could be a matter of my air compressor being too weak as its for airbrushing. It seems capable of pushing out 30psi or so it says....

Thanks again, I will update this post with any progress....

Well, that's why we build these simple engines as first tries. There's not
a lot of cost or time but they yield a lot of good experience. You are
going along the path most of us have traveled and the adventure
continues!!

Have fun,
Pete

learninturnin said:

I moved the pivot up. Still does not work.

Click to expand...

So you now have less movement at the top end of the cylinder. Are the ports now opening at all?

I am starting to wonder if - among other things - it could be a matter of my air compressor being too weak as its for airbrushing. It seems capable of pushing out 30psi or so it says.....

Click to expand...

30 psi should be way more than you need.

Another thing that jumps out is that your flywheel is rather small (low inertia value) since this has to carry the return / exhaust stroke it may not be big enough for the job and / or the motor will only run at very high rpm's.

Regards,

Ken

the air or steam never reach the cylinder, as the steam ports on the stand are too distant from each other; this distance should be slightly greater than the size of the cylinder steam port. when the crankshaft is near 90 ° the admission should communicate with the cylinder port, same for the other side, with the exhaust.

My first wobbler needed the connecting rod at a precise length or there was no room to the steam to enter the chamber near TDC

My double acting wobbler plans are now available in the downloads section - zip file containg dxf dwg & pdf versions - 2D only I'm afraid.

Regards,

Your first pic says it all. If you notice the rod is not inline with your cylinder. From the looks of it you should have the same angle with the cylinder and piston rod. This would cause timing issues.

The rest of the problems are probably negligible except for the piston length. If it is moving at an angle to the bore, causing binding, also this would cause your ports to be late/early, And it needs to be lengthened. Fitment of piston rule of thumb is, if you place your finger over the port , the piston should hold its place or slowly sink to bottom, and the best being if the piston springs back if you try and pull it out.

In Elmer's wobbler plan he has you make a jig to set and check steam holes. This may be something you need to check.

This was my first engine and ran out of the gate. So they are the easiest to get running. Check them or replace parts if necessary.

To help you understand a bit more about your little oscillator, you would do well to have a look at some of the designs and build notes from Elmer Verburg.
http://www.john-tom.com/
Also, have a look at http://www.panyo.com/oscillators/ You may be able to modify your existing ports without going to the trouble of plugging and redrilling what you have already done.

Dave
The Emerald Isle

You guys are amazing, thanks for this wealth of information. I will be getting back into the studio this week and making some changes.

Thanks again.

I have managed to to get back int he shop a bit and decided its better to just start from scratch on this one.

As I move forward I did have a - probably silly - question.

I bored the cylinder, and with the lathe started a piston. The piston fits into the cylinder, but rather tightly. I have to jam it (gently) up there.

My question was this: would people generally machine it to fit exactly right - in my case a few thousandths of an inch more - or sand it down to fit?

I was planning on sanding it down as its such a small amount, but I am very curious about peoples take on sanding and polishing. Would sanding and polishing ruin other tight fits such as the needed flat connection between the cylinder block and the frame?

Thanks very much

Current work on the frame, piston, and cylinder.

What you normally do with a piston and cylinder like that is to lap and polish them together.

No need for anything special, just some fine metal polish or toothpaste, put a little up the bore and gently push the piston up and down, rotating it a little as it goes in and out.

Pretty soon, it will start to go up and down very smoothly. Wash things out, add a tiny amount of oil to the piston and it should then be smooth as silk.

All it does is knock off the high spots on both components.

Best of luck

Bogs

Ahhhh, makes sense. Thanks!