All About Engines

116 All About Engines tion for starting immediately the steam valve is opened. With high speed engines provided with a flywheel opposite cranks may be used with- out disadvantage. The cranks of an engine with three cylinders are almost invariably set at angles of 120 degrees, as in Fig. 68, because this arrangement facili- tates starting, gives a more regular turning effort, and minimises vibra- Fig. 68.—Three cranks ,. at 120° tlOn. The Admission and Release of Steam Having seen how an engine is constructed to avoid loss of steam by leakage, and to work without shaking itself to pieces, we have now to inquire how the admission and release of steam are effected ; for upon this the economy of modern engines largely depends. Of the 14,000 units of heat which are produced by every pound of coal, about 75 per cent., or 10,500, are transferred to the water. Many boilers will not do as well as this; others claim to transfer 85 per cent, under favourable conditions, but the figure we have taken is near enough for our purpose. A further loss occurs on the way to the engine, for however carefully the steam pipes are covered it is impossible to prevent all loss, and the longer the pipe is, or the more exposed to cold winds, the greater this will be. So that if 10,000 units reach the engine we may consider ourselves fortunate. As each unit of heat, when converted into work,