All About Engines

The Modern Reciprocating Engine IT9 travelled in one stroke would be 4 inches, or one-third of a foot, and at 150 revolutions there would be 300 strokes per minute, so that the total distance the valve travelled in a minute would be 100 ft. The work done per minute, therefore, would be 180,000 ft.-lb., which is at the rate of nearly 5| horse-power. True, the engine would probably yield 200 horse- power, so the loss would be less than 3 per cent., but even that is undesirably high. An engine which needs that amount of power to move one of its own valves is like a man whose joints are stiff with rheumatism! It is one thing to discover a defect, and another thing to remedy it. The calculation we have made shows, however, that if the travel, or the area, or the steam pressure be reduced, the power required will be propor- tionately less, and the travel is reduced to half the dis- Fig. 70.—Double-ported valve tance by the double-ported valve shown in Fig. 70. In this form it will be observed that each steam port has two openings into the steam chest, and that there are corresponding openings in the valve. The construction of this valve is difficult to show in a drawing, and still more difficult to describe in words in a way that will be understood; but the arrows showing the directions of the steam will assist any reader who wishes to obtain a clear notion of it.