A Triple Expansion steam engine in miniature

[gbritnell]

A couple of my favorite engines have to be the Beam type and a Triple Expansion marine style. I have built several beam engines over my career but never a Triple so using the Stuart Compound Launch engine as a starting point I began designing my own. I used the Stuart dimensions for the H.P. and I.P. cylinders then using the size ratio of these 2 I calculated the dimension for the L.P. cylinder. Once I had these sizes I started drawing but at .1/2 scale.
The cylinders, valve chests and covers are made from cast iron. The heads and other steel parts are made from 1144 steel. The shafts and stanchions are made from Stainless Steel. All the brass parts are made from 360 brass. The majority of the small bolts are M1.2x.25 (that's .047 diameter with a .078 hex.
gbritnell

[gbritnell]

Some additional photos

[gbritnell]

A video also.
https://youtu.be/KWF7jRd-3eU

[mattthemuppet]

wow, just wow!

[Mcgyver]

very nice work as always George. The triple is a challenging project, what did it take you, about 6 weeks?

[Joel]

Glad to see you back George.
That is really nice. Such a tiny scale must have added considerably to the challenge!

[Sparky_NY]

Wow, nothing sort of fantastic ! Built in a couple months is amazing as well ! I would love to see more of the machining in progress, I could learn a lot !

[lynnl]

wow, just wow!

Nay! ...Double Wow! Even Triple Wow. I don't have a penny on hand right now, but can't be more than 3" long.

Does that little thing actually run? ...on steam? Would be impressive enough even if just a static model.

[Sparky_NY]

Nay! ...Double Wow! Even Triple Wow. I don't have a penny on hand right now, but can't be more than 3" long.

Does that little thing actually run? ...on steam? Would be impressive enough even if just a static model.

Sure does run, and very weil ! Engine revs up like a indy car ! Watch the video linked in post #3

[old mart]

Beautiful engineering, I'm surprised that running on live steam doesn't cause condensation problems in the low pressure cylinder due to the very small size especially before the engine warms up.

[garyhlucas]

Very nice job. I especially like the helpful giant hand. I noticed you give it a slight rotation. Is there a dead spot in the rotation of this type of engine or is it just so small it has a bit of trouble overcoming static friction?

[Mcgyver]

[QUOTE=garyhlucas;n1837714 I noticed you give it a slight rotation. Is there a dead spot in the rotation of this type of engine or is it just so small it has a bit of trouble overcoming static friction?[/QUOTE]

The cranks on a triple expansion are 120 degrees apart so no dead spot....but the steam enters each cylinder successively (thats why they get progressively larger)....so when run on air its really only the high pressure cylinder than is making it rotate

[toolznthings]

Another beautiful build, George !

[lynnl]

....so when run on air its really only the high pressure cylinder than is making it rotate

I'm suspecting that as a retired AF meteorologist I should understand why the difference in steam vs dry air. But the reasoning escapes me.

[gbritnell]

Thanks everyone for the gracious comments!
OK, here's how the expansion steam engine works. Steam is an expansive gas. When you see a steam engine operating (locomotive) you see the exhausting steam vapor coming from the engine. That steam still has heat and energy otherwise it would be water and you wouldn't see it.
Steam is admitted into the high pressure cylinder, it pushes the piston, one way then the other. The steam that moved the piston is now moved out of the exhaust port. Let's say the high pressure cylinder has 1 sq. in of surface area and the steam is at 100 lbs. per sq. in when admitted. That means there is 100 lbs. of push on the piston. Now the exhaust steam has lost some of it's energy. I don't know the exact numbers but for this explanation it doesn't matter.
Let's say it's 85 lbs. To get the same amount of pressure on the crankshaft with less steam pressure you have to increase the sq. inches proportionately.
Now the 3rd expansion is just a repeat of the second.
Some large steam engines were quadruple expansion.
Compressed air has no expansion to it. If you fill a cylinder with pressurized air then cut the supply off there is no remaining pressure. It has no expansive quality. Now you open the exhaust port and move the piston toward the other end of the cylinder. There's basically nothing except the volume of air being moved by the piston so therefore there is nothing or not enough pressure to equalize the work effort on the crankshaft.
In the video I had to turn the flywheel to get the engine off of dead center so it would start running and as was mentioned only running on the H.P. cylinder.
gbritnell