IV. Cutoff, or the art of stopping early

Steam admitted for the whole stroke pushes hard the whole way — and then is thrown out still at full pressure, its work half done. Close the valve partway instead, and the trapped steam expands, pushing as it thins, until by the stroke's end it has spent nearly everything it has. You give up some power; you keep the coal. Watch the two plates as you move the lever: earlier cutoff means fewer pounds of steam for every horsepower-hour. The whole nineteenth century is engineers finding ways to cut off earlier without the engine minding.

One chapter of nine. Read the whole Primer, or work the engines themselves.