. The steam-engine and other heat-motors . ia-gram from a single-valve high-speed engine. In the latter casewhere, owing to the fling of the pencil, the steam line vibrates, t See Trans. A. S. M. E. Standard Rules. CURVES AND THE WORK OF EXPANSION. 71 the maximum pressure is found by taking a mean of the vibra-tions of the highest point. The commercial cut-off, B, as thus determined is situated at anearlier point of the stroke than the actual cut-off, D, referred to. Fig. 37. Steam being elastic entirely fills an increasing volume,but its pressure diminishes, as is seen by the decreasing ordin
. The steam-engine and other heat-motors . ia-gram from a single-valve high-speed engine. In the latter casewhere, owing to the fling of the pencil, the steam line vibrates, t See Trans. A. S. M. E. Standard Rules. CURVES AND THE WORK OF EXPANSION. 71 the maximum pressure is found by taking a mean of the vibra-tions of the highest point. The commercial cut-off, B, as thus determined is situated at anearlier point of the stroke than the actual cut-off, D, referred to. Fig. 37. Steam being elastic entirely fills an increasing volume,but its pressure diminishes, as is seen by the decreasing ordinatesof the expansion curve CD. We have already seen that it isnecessary to reject the steam during the return-stroke. At thepoint D, where there is another change in the curvature, theexhaust-valve opens and the pressure rapidly falls as the pis-ton moves to the end of the stroke at E. The piston now returnsand the steam is forced out by the piston sweeping it out. As theresistance is constant the back-pressure line is parallel to the at-. Fig. 47.—Single-valve Engine. High-speed Commercial Cut-off; BCAC mospheric line XY. If the exhaust-passages had been short andample the line EF would have practically coincided with have seen how the atmospheric line was drawn. Butpressures measured from it are not absolute pressures, as wewell know that the atmospheric pressure is some poundsper square inch above zero pressure. The steam-gages used onboilers indicate not the steam-pressure in the boiler but the burst-ing pressure, which is the difference between the steam-pressureinside and the atmospheric pressure outside. Hence, to obtainthe absolute or true pressure above zero we must add the atmos-pheric pressure to all pressures that are measured above theatmosphere. The barometer gives this pressure in inches ofmercury that can be converted into pounds per square inch by 72 THE STEAM-ENGINE AND OTHER HEAT-MOTORS. by _o?r= In all localities near the
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