Gallery, Projects and General > How do I??
Dimensions for slow runner
bogstandard:
Roger,
If I was going to design a long stroke engine, I would give a ratio of stroke = double the bore. If you start to go for longer strokes, the cylinders can get inordinately long. That is the ratio of my long stroke oscillators, and I think my paddleducks engine.
When you see say model beam engines running, you will notice that the flywheel speeds up and slows down on each revolution. It is more noticeable when the engine is running slowly. This is because most of them don't have a balanced cylinder, so as the piston rod is pushed out, it goes out faster than when it is going in. This is caused by differential piston areas. The side of the piston with the rod on, has less surface area for the steam to act upon than the other side of the piston without the rod. To balance it out, you would make a cylinder with a rod and gland at both ends, but only connect one end to the flywheel. That will really make the engine run a lot smoother.
John
John Hill:
--- Quote from: bogstandard on July 22, 2009, 04:30:36 PM ---
If I was going to design a long stroke engine, I would give a ratio of stroke = double the bore. If you start to go for longer strokes, the cylinders can get inordinately long.
--- End quote ---
Wait till the nutating engine is complete, the piston and cylinder assembly is almost twice as long as would be expected due to the valve sleeve needing to be as long as the stroke.
rleete:
--- Quote from: bogstandard on July 22, 2009, 04:30:36 PM ---If I was going to design a long stroke engine, I would give a ratio of stroke = double the bore.
--- End quote ---
Thanks. I'll start there.
--- Quote from: bogstandard on July 22, 2009, 04:30:36 PM ---To balance it out, you would make a cylinder with a rod and gland at both ends, but only connect one end to the flywheel. That will really make the engine run a lot smoother.
--- End quote ---
That makes sense. I've seen the same effect in pneumatic cylinders for automation I've designed. It's usually compensated for by installing individually adjustable restrictors on the exhaust ports. The "extra" rod would add more visual appeal, too.
bogstandard:
It is also the way the gas struts work on your boot (trunk) lid. Charge up the whole cylinder with pressure, and because of differential areas, the strut will always extend by itself. More pressure for a heavier lid, light pressure for a lightweight lid.
Depending on the design of your valve gear, you can sometimes get away with throttling the exhaust to get a real slow runner, it is much more controllable than throttling the inlet.
I don't know why, but sometimes it will work with slide valves, sometimes not. You would need to try it after it is made.
The other way would be to put an adjustable friction device on the output side, say on the flywheel rim. It would take very little friction pressure to make it go as slow as you want.
I've just had a thought about using a double ended cylinder.
For a very quirky effect, have both rod ends anchored, and have the cylinder running up and down the rods. You would need flexible pipework, but that wouldn't pose much of a problem.
Or
Again a double ended cylinder, laying horizontal, but driving two flywheels, one at either end of the engine, but it could still be controlled by just one set of linkages.
I'll keep all the other stupid ideas to myself.
Bogs
Bernd:
The stern wheeled paddle boats used long throw pistons. There is one called the California Cutoff engine. It can be seen here: http://www.agelessengines.com/s-boat.htm
Might give you some insight to long throw pistons.
Bernd
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