All About Engines

The Locomotive 2Ö7 Since 1900, however, it has become general, and there are now over 30,000 engines using superheated steam. The cylinder valves on British locomotives are either “ short D ” slide valves or piston valves, which have already been described in Chapter V. On the Continent poppet valves, such as are used on gas engines, and in America valves of the Corliss type, Fig. 152.—Joy’s valve gear are being tried. But perhaps the most important change since Stephenson’s days is in the so-called valve gear, by which admission and release of steam is governed, and reversal of motion effected. The mechanism illustrated in Fig. 144 suffered from the disadvantage that the “ lead ” was altered when the link was moved.1 The gear frequently met with in 1 As, however, the gear can be arranged to give an increasing lead as it is .. notched up,” the variable lead is regarded as an advantage by some engineers on account of the fact that at high speeds the engine generally runs on short cut- offs, and an increased lead tends to reduce " wire drawing,” which is explained in Chapter XII.