Compound triple-expansion engines are thermodynamically efficient, but bulky.

BorgCo marine engineering have now designed a much more satisfactory layout.

The three cylinders are nested in an annular design; the innermost high pressure piston is a simple conventional one, the medium and low pressure ones surround it. Heat lost from the inner cylinders is "recaptured" by the outer ones. The HP/MP cylinder is a common assembly, as is the MP/LP cylinder. Only the outermost LP cylinder has an inner bearing face only.

The valve gear is located above the cylinder heads, driven by a vertical shaft and bevel gear from the crankshaft. Variable cutoff is implemented to economize on steam at higher R.P.M. for best expansive use of available pressure.

Since the reciprocating masses are moving in antiphase about a common axis, the unit exhibits superior balance over a side-by-side layout. The pistons themselves are light alloy, the cylinders are steel.

The pistons are double-acting; the HP stage is superheated.

The HP piston has a single conventional connecting rod and gland. The MP and LP pistons have sets of connecting rods (three for the MP, linked to a triangular yoke, for for the LP on a square yoke) attached to combined yoke/crossheads with moving slide bars and fixed bearings, using pressure-lubricated roller bearings with full oil recovery.

The big ends are conventional pressure-fed white metal journals. The crankshaft is a built-up design for ease of maintenance, balanced in situ after assembly.
-- 8th of 7, Apr 04 2020

Compound steam engine https://en.wikipedi...mpound_steam_engine
Basis of design [8th of 7, Apr 04 2020]

Trunk engine https://en.wikipedi..._steam_engine#Trunk
Annular piston design [8th of 7, Apr 04 2020]

Annular Engine https://commons.wik...r_marine_engine.jpg
[bs0u0155, Apr 06 2020]

River Don engine. https://en.wikipedi...ki/River_Don_Engine
One morning, they will open the doors, and find it gone ... [8th of 7, Apr 06 2020]

Interesting. What else do you have on that Cube?
-- whatrock, Apr 04 2020


Oh, lots of stuff. Would you care for a guided tour ?
-- 8th of 7, Apr 04 2020


The S/V ratios are arbitrary, depending on diameters, and can be defined to suit. Although there's a common crankshaft, the throws - and therefore the stroke lengths - can be very different for the individual pistons.

As to sliding friction of the piston rings, at the relatively low speeds of reciprocating steam engines it's not significant.

A drawing is under consideration.
-- 8th of 7, Apr 04 2020


[+] bun now, read later.
-- FlyingToaster, Apr 04 2020


Intriguing. I'm not certain whether the surface area ratios could make sense here or not. Maybe it could be done with the right mix of throw and surface area. The hp piston needs enough area to feed the other two larger pistons at lower pressure, and their comparative diameter means much more sliding friction at the skirts. Given the necessary differences in piston size I'm uncertain balance would be as smooth as hoped, but that could be compensated.

Oiling the middle pressure cylinder adequately could be a messy maze. While a single oil jet could cool the innermost, the middle and outer would need multiple jets. Keeping their feeds away from your rods and yokes could get tricky.

Worth a patent search.

I have a strong suspicion a steam turbine is simply more efficient, but this is interesting anyway.
-- RayfordSteele, Apr 05 2020


Further down in the "Trunk engine" description is the annular engine, no pictures but mentions hi and low pressure stages, so a baked double expansion version? Found a pic <link>.

//While a single oil jet could cool the innermost,//

I don't think cooling is an issue here. The temperatures in steam engines are nowhere near problematic, instead I'd guess it's just lubrication/corrosion prevention.
-- bs0u0155, Apr 06 2020


That annular engine is a single-expansion engine with piston rods, not even double-expanding - but as a diagram of how the MP and LP cylinders would be configured, rather than the centre gudgeon pin of the trunk engine, it's excellent.

No cooling is needed or desirable, the idea is to keep all the components as hot as possible. Conventional steam engines aren't cooled ... hence the separate condenser and hotwell, plus condensate return pump.

// The hp piston needs enough area //

No, it needs enough displacement. That's a function of bore and stroke. Both can be varied, within limits, at design time.

// Oiling the middle pressure cylinder adequately could be a messy maze. //

Not at all. It's the same total-loss lubrication as for a conventional engine. If steam can get into the cylinders, so can oil.

A steam turbine is very probably more efficient if you can run at constant load (power). The more you tune it for peak efficiency, the smaller the sweet spot is; and reversing is a nightmare.

This engine is very compact, efficient compared to a single or double-expansion engine, and can reverse very quickly; vide the River Don engine, built for a plate-rolling mill. That's not a compound, but the principle is the same. <link>
-- 8th of 7, Apr 06 2020



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