A Description of the Differential Expansive Pumping Engine

Excerpt from A Description of the Differential Expansive Pumping Engine: With Useful Notes

If this engine were to be made double-acting and employed to lift water on the steam stroke it would be expedient, for practical as well as theoretical reasons, to limit the piston speed to 4 feet per second, making it necessary, as the author has already shown, to provide a moving mass of 600 tons, but with the compound engine, working under similar conditions, only a very small moving mass would be necessary, because the variation in the effective pres sures during the stroke would be comparatively very little. An expansion of eight times in a single cylinder would give a variation of 8 to 1, but in a compound differential engine the force is nearly constant. The Cornish engine is a very costly machine; it is only single acting, and it requires a great moving mass to make it practicable to employ a high degree of expansion; nor can it be made double acting without either making it necessary to increase the mass beyond a practical limit or to employ a low ratio of expansion involving a great waste of fuel.

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