The final commercial evolution of the Uniflow engine occurred in the USA during the late 1930s and 1940s by the Skinner Engine Company with the development of the Compound Unaflow Marine Steam Engine. This engine operated in a steeple compound configuration and provided efficiencies approaching contemporary diesels. Many lake freighters and car ferries on the Great Lakes were so equipped, a few of which are still operating. Notable among these is the SS Badger. The Casablanca class escort carrier, the most prolific aircraft carrier design in history, used two 5-cylinder Skinner Unaflow engines.
In small sizes (less than about 1000 horsepower), reciprocating steam engines are much more efficient than steam turbines. White Cliffs Solar Power Station used a three-cylinder uniflow engine with "bash" type admission valves to generate about 25 kW electrical output.
The single-acting uniflow steam engine configuration closely resembles that of a two-stroke internal combustion engine, and it is possible to convert a two-stroke engine to a uniflow steam engine by feeding the cylinder with steam via a "bash valve" fitted in place of the spark plug. As the rising piston nears the top of its stroke, it knocks open the bash valve to admit a pulse of steam. The valve closes automatically as the piston descends, and the steam is exhausted through the existing cylinder porting. The inertia of the flywheel then carries the piston back to the top of its stroke against the compression, as it does in the original form of the engine. Also like the original, the conversion is not self-starting and must be turned over by an external power source to start. An example of such a conversion is the steam-powered moped, which is started by pedalling.
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