Engineering reminiscences contributed to "Power" and "American machinist"

by Charles T. Porter

CHAPTER IV

Engineering conditions in 1860. I meet Mr. Allen. Mr. Allen’s inventions. Analysis of the Allen link.

Before resuming my narrative, it seems desirable to present a brief sketch of steam engineering conditions forty years ago.

The science of thermodynamics had been established on the foundation laid in the experiments of Joule, determining with precision the rate at which, through the medium of water, heat is converted into dynamical force. This science was, however, as yet without practical results. The condensation of steam in the cylinder from the conversion of its heat into mechanical energy was unregarded. The same was true also respecting the far greater loss from the changing temperatures of the surfaces with which the steam comes in contact in alternately entering and leaving the cylinder. The action of these surfaces in transmitting heat from the entering to the exhaust steam without its doing any work was imagined by very few.

In the United States economy of steam was sought only by mechanical means—by cutting off the admission of the steam at an early point of the stroke in a single cylinder and permitting the confined steam to complete the stroke by its expansion. By this means a large saving of steam over that consumed in earlier practice was effected, and with this gain the universal disposition was to rest content.

America was eminently the land of the cut-off system, an early application of which was on steamboats. The earliest device for this purpose was the elegant Stevens cut-off, which still keeps its position on the class of boats to which it was first applied, though commonly modified by the Sickles improvement. In this system the exhaust and the admission valves are operated by separate eccentrics on opposite sides of the engine, and all the valves have the amount and rapidity of their opening and closing movements increased by the intervention of wiper cams, those for the admission valves being very long and giving a correspondingly greater enlargement of opening. The valves were double poppet valves, moving nearly in equilibrium in directions vertical to their seats. This cut-off was found to be capable of improvement in one important respect. The closing motion of the valve grew slower as the valve approached its seat, and while the piston was moving most rapidly much steam passed through the ports at a lower pressure, and so a great part of its expansive value was lost. This was technically termed “wire-drawing.” To remedy this defect Mr. Sickels invented his celebrated trip cut-off. The valve, lifted by the Stevens wiper, was liberated by tripping the mechanism, and fell quickly to its seat, which it was prevented from striking forcibly, being caught by water in a dash-pot. The steam was thus cut off sharply and the economy was much improved. The pressure used in this system was only about 25 pounds, the vacuum being relied upon for the larger portion of the power.

On the Great Lakes a pressure of 60 pounds was commonly employed, and the valves were the four cylindrical rotating slide valves afterwards adopted by Mr. Corliss. What was called the cut-off was made by a separate valve located in the steam-pipe somewhere between the engine and the boiler.

On the Mississippi and its tributaries, much higher pressures were carried, condensers were not used, and the admission and release of the steam were generally effected by four single poppet valves, lifted by cams against the pressure of the steam.

On land engines Mr. Sickels’ invention of the trip cut-off stimulated inventors to a multitude of devices for working steam expansively. Of these the one of enduring excellence proved to be that of Mr. Corliss. He applied the trip cut-off to the rotating slide valve, and arrested the motion of the liberated valve by an air-cushion. This proved a satisfactory method, as the valve, moving in directions parallel to its seat, did not need to be stopped at a determinate point. Mr. Corliss applied the governor to vary the point of liberation of the valve, and so produced a variable cut-off, which effected a large saving of steam and regulated the motion of the engine more closely than could be done by a throttle valve outside the steam-chest. This was by far the most prominent of the numerous forms of automatic variable cut-offs, to all of which it was supposed that the liberating feature was essential.