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efficient steam engines


David Schaffner

What is the most efficient of the steam engines and does somebody still make them? Does anybody make one that uses a working gas other than steam?


If you want a standard (reciprocating, saturated, non-condensing) steam engine design, then a poppet valve uni-flow will probably work as well or better than anything.

Of course, you can get a bit more efficient by compounding, superheating, adding a preheater to your feedwater, etc. You'd also want to do other things to eliminate as much friction as possible(Ball bearing mains, roller crosshead,etc)

An "oversquare design with a large bore and shorter stroke (and large ports) will result in a nice high speed engine, then again, simply adjusting the cut off for maximum expansion at a given load will help too.

A multi-stage turbine would USUALLY be even more efficient than this, but it isn't what most people think of when you say "steam engine"

Yes, you can use other working fluids, compressed air is often used with models for cleanliness sake -- but then they aren't really "steam" engines anymore, are they?

Dean Alling

I know you said steam engine but in a steam driven "system" it is the boiler that will be the "most" wasteful. Second will be the exhaust steam if it is not re-used somehow. Other than that I agree with the last post. The governor should be of the "variable cut off" type rather than a throttling "flyball" type for best efficiency.

Jim Conte

My favorite is Skinner Unaflow. They were the last of the marine recips before turbines took over. They were standard on the WWII liberty ships and Jeep Carriers ( baby ASW convoy escort flattops ). The Badger Carferry out of Ludington, MI is still operating with two. The Steeple compound has a little unusual steam flow, but it is capable of using superheated steam and competes well against a turbine up to about 5,000 Hp. Skinner was in business for 130 years 1869 - 1999 in Erie, PA. They built thousands of engines in almost every imaginable shape and size.

Joe W K in NH

Uniflows & Locomobiles

What everyone is saying about poppet valve uniflow engines is probably true EXCEPT the uniflow has it's greatest advantage when exhausting to a condenser. When exhausting to atmosphere an auxiliary exhaust valve (other than the usual uncovered by the piston annular exhaust passage) is usually used. Some of the inherent uniflow advantages (i.e. linear cylinder temperature differentials, uniflow steam flow in the cylinder, cylinder re-compression) are given up by this auxiliary valve. Best practice uses this auxiliary valve only in the period when condenser vacuum being established or when the extra fuel penalty for it's use can be written off in maintaining production (as when the condenser is out of service for repairs.)

So far most have talked about the engine. Another fuel economy step change occurred in the early parts of the last century (about 1914!) with the invention by the Germans of the "Locomobile." Not related to the American steam car of the same name, the German Locomobile engine was a combined boiler - steam engine unit that included many heat saving improvements including... 1. Compound or possibly a triple engine - this could be in part a poppet valve engine but was probably NOT a uniflow which is a convention that came along a little bit later and more commonly in America. 2. Highly superheated steam to the hp cylinder 3. Steam reheat on the steam going to the LP or IP AND LP cylinders, if they were used. 4. Feedwater heating by interstage feedwater heating (today known with turbines as extraction or "bleed steam.") Items 2,3,4 were reasons why the uniflow principle was not used. The uniflow engine application tended to be American, single cylinder, no re-heating of steam possible, and with feedwater heating of only the most basic kind. 5. Economizer 6. Some advanced Locomobile units may even have "air preheaters" using a flue gas to furnace air tubular heat exchanger.

These were all pretty advanced engineering principles applied early and pre-dated similar heat saving measures that were used in American power plants 30 or 40 years later. But these improvements allowed the Locomobile engine the unprecedented fuel economy of less than 1-lb coal per horsepower hour which was phenomenal for it's time and size..

Lenz of Germany was one of the German manufacturers of Locomobiles. Lenz poppet valve engines were imported in small numbers to this country but Lenz had a lot more to offer such as the Locomobiles that we didn't use. A typical Locomobile engine unit was 150KW in electrical output.

A "G. Miller" wrote extensively on the German Locomobile in the 1912-1918 time range in "POWER" magazine. He describes several plant installations of German origin, including some in South America (and he decries the wasteful American common power plant performance of the period of 5 lbs coal/hp-hr.)

Terrell Croft in his seminal work "Steam Engine Principles and Practice" describes briefly something of the German pioneering in the Locomobile and an American made "Buckeye" version that was available for a short time in this country in the 1920s.

The locomobile idea didn't catch on in the US. The combination of cheaper fuel after WW1 combined with the rise of central station power, increases in steam pressures and temperature, improved steam turbines, and the general gains from "economy of scale" that the centralized power plants afforded all served to make the complicated Locomobile financially less attractive and possibly unnecessary. (What one gained in efficiency can be easily lost in unit availability when the relatively complicated locomobile units were operated or maintained incorrectly by unfamiliar Americans.)

Hope this helps. The Locomobiles were an engineering marvel of their time.

Best, Joe W

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