Trunk engines

[JohnB]

I recently came across a reference to trunk engines as built by Penn, and fitted to warships in the 1860's and 1870's (HMS Hercules, Sultan and Neptune). These are described as "the largest single expansion engines ever built with cylinders 127in by 4.5 ft stroke, with 47in diameter trunks which reduced the effective area to 118ins bore".

So what's a trunk engine, and why were they built like that? The arithmetic suggest a 4 foot diameter piston rod which went through the top cover of the cylinder too, leaving a 118" annulus which housed the piston. Is that right?

[Bierjunge]

So what's a trunk engine, and why were they built like that? The arithmetic suggest a 4 foot diameter piston rod which went through the top cover of the cylinder too, leaving a 118" annulus which housed the piston. Is that right?

Absolutely.
But on trunk engines, that piston rod of large diameter is not solid, but a hollow tube, with the bearing of the connecting rod inside.
Thus, piston and crosshead are merged into one part (the crosshead is in fact inside the piston!) allowing a very compact solution with reduced dimensions (compared to conventional engines with external crosshead), which can be crucial for paddlewheel engines.
Oscillating engines were a different approach for the same goal.

To get a better idea. simply make a google image search for "trunk engine".

[JohnB]

Thanks Gretzschel. I now have a picture in my mind to work from.

[malcolmbeak]

I thought this drawing might help
Malcolm

[JohnB]

It certainly does. Thank you.

It's interesting to see what I guess is the air pump being driven from each piston. I can't quite make it out, but I think there is a gland where the air pump drive rod comes through the cylinder cover.

And that slide valve looks like it might be balanced. The source I found that started me off on trunk engines said that the slide valves were so large that a man could get inside them from the exhaust pipe to scrape off the tallow, then used to lubricate the cylinders.  These were some big beasts!

[AlistairD]

There is a replica, working but not stem-powered of a trunk engine on HMS Warrior at Portsmouth.
It looks like an oscillating engine, as used on the Dresden paddlers, laid on its side.

Alistair

I recently came across a reference to trunk engines as built by Penn, and fitted to warships in the 1860's and 1870's (HMS Hercules, Sultan and Neptune). These are described as "the largest single expansion engines ever built with cylinders 127in by 4.5 ft stroke, with 47in diameter trunks which reduced the effective area to 118ins bore".

So what's a trunk engine, and why were they built like that? The arithmetic suggest a 4 foot diameter piston rod which went through the top cover of the cylinder too, leaving a 118" annulus which housed the piston. Is that right?






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