All aluminium slow rpm air engine?

[GeeBee]

I am considering building a very low rpm air motor, I have enough Aluminium around here to build the piston and cylinder but how bad would the combination be at low rpm's ? I know it is meant to be a no no for normal engines.
The motor at the moment looks like being 15 mm bore and 75 mm stroke, heavy flywheel and ball raced shaft, I would love to get a sub 60 rpm idle any advise would be appreciated ?

 

[Blogwitch]

As long as you keep it well lubricated, although not ideal, ali in ali will work just fine.

Your main problem will be the actual height and width of the engine.

The following figures will be rough guesstimates, but they won't be very far off.

Not only will your stroke be 5 times the piston diameter, so 75mm (3"), your crank throw will also have to be the same 75mm (3"), plus a bit of room in between to take into account of the crank webs, and if double acting, the stuffing gland and if not a wobbler type, a conrod as well.

So the crank webs would need to be about 85mm (3.5") diameter at least, then a bit of space, say 30mm (1.25") if a wobbler or about 175mm (7") for the conrod swing if using a piston or slide valve (at those lengths you would need some sort of crosshead).

So say you are building a wobbler, that would be around 90mm (3.5") for the cylinder, plus 30mm (1.25") for running clearance, then the crank webs at 85mm (3.5") plus say 10mm (0.4") clearance underneath the crank.

A total overall wobbler height of roughly 215mm (8.5") , or for a valved engine, 390mm (15.5").

For a wobbler, your cylinder swing would be rather large side to side, so you might find that the port holes will be excessively large to allow continuous running, if it will run at all.

In reality, if you wanted to try it, I personally would go for a maximum of 2.5 or 3 times bore diameter, even at the higher figure, things start to get a little out of hand.

I hope this has helped your decision.


John

 

[GeeBee]

Thanks for the reply, it will be a fixed cylinder single acting engine, possibly a beam engine.

Is there any information on the best proportions for very low rpm's? the 5 to 1 was a figure i came across whilst searching for information, I am still researching.

 

[Blogwitch]

GeeBee,

What you have to take into account is the throw on the beam, maybe around 20 degrees overall throw seems to be about right, so if you make the beam 330mm (12") long, pivoted in the middle, then that would need a throw of about 30mm (1.2"). So that means for a 15mm diameter piston, you will be working on a 2 to 1 ratio. For the same length of beam, I reckon you could go to about a 3 to 1 ratio before it starts to look off scale

You would need to use a parallel motion link, or make a grasshopper type.

There is a very easy way to get slow running engines. Make them as friction free as possible, with a heavy flywheel, and run them on a very low pressure.

I would suggest you have a look at Elmer's beam engines and scale up from that.

John

 

[mklotz]

Considering the amount of time and effort you will be putting into the construction of a beam engine (or similar) I think you owe it to yourself to get the small amount of appropriate metal needed for the parts that will be sliding against each other.

 

[Duncan.e]

What if the piston used an O ring, or two - then surely the rubbing surfaces would not be an issue....

 

[Jasonb]

A 12" beam will be more than ample for a 3" stroke. As an example the Stuart beam has a 2" stroke and the beam centres are only 7" so to get your 3" stroke would only need the cts to be 10.5"

Though I would be tempted to up teh bore to 1" with a 3" stroke.

As Bogs says put a big heavy flywheel on it.

J

 

[rustyknife]

Briggs and stratton ran aluminum pistons in aluminum bores for years, probly still do to this day......cast iron rings of course...but heck, they ran forever. Hone one out .030 over, use a standard size piston with .030 over rings, and it would scream. Loose is fast