What critical feature distinguishes highly efficient condensing engines from non-condensing versions?

Answer

The creation of a vacuum on the exhaust side of the piston

A condensing engine cools used steam back into water in a vessel, creating a vacuum on the exhaust side, which results in a substantial pressure differential across the piston, thereby extracting more work per stroke.

What critical feature distinguishes highly efficient condensing engines from non-condensing versions?

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The most simple and efficient steam engine in history - YouTube

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