Читать книгу Locomotive Engine Running and Management - Angus Sinclair - Страница 71

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When the links are notched close towards the center, the travel of the valves is so short that they close the steam-ports shortly after the beginning of the stroke, at six, nine, or twelve inches of the piston’s travel, as the case may be, permitting the steam to push the piston along the remainder of the stroke by its expansive power. Steam at a high pressure is as full of potential energy as a compressed spiral spring, and is equally ready to stretch itself out when the closing of the port imprisons it inside the cylinder; and, by this act of expanding, it exerts immense useful energy, which would escape into the smoke-stack unutilized if the cylinders were left in communication with the boiler till the release took place. Suppose, for instance, that a boiler pressure of ten tons is exerted upon the piston from the beginning to the middle of the stroke, and is then cut off. During the remainder of the stroke, the steam will continue to press upon the piston with a regularly diminishing force, till, at the end of the stroke, if release does not take place earlier, it will still have a pressure of five tons. The work performed by the steam during the latter part of the stroke is pure gain, due to its expansive principle. If the steam is cut off earlier, at a third or fourth of the piston travel, the gain will be correspondingly great. With the slide-valve link-motion used on locomotives, the steam can not be held to the end of the stroke; but the principle of expansion holds good during the period the steam is held in the cylinders after the cut-off.

The observing engineer of any experience does not require to have the advantages of working his engine expansively impressed upon his attention. His fuel-record has done that more eloquently than pen can write.