All About Engines

Engines for Ships 279 smallest consumption of fuel. Moreover, since boilers produce steam more economically at high than at low pressures the boiler pressure has risen to 200 or 225 lb. on the square inch, and with these high pressures two cylinders are insufficient, so that triple- and quadruple - expansion engines are employed, though the latter are less common. It must be remembered that a four-cylinder engine is not necessarily a quadruple - expansion engine. Thus, if the boiler pressure is high the low-pressure cylinder tends to become unduly large, and fre- quently two low-pressure cylinders are used. In this case the best balancing is not obtained by having the cranks at right angles, but at such angles as to suit the arrangement of the cylinders, and to balance most effectively the rotating parts, The two low-pressure cylinders are generally placed at the ends, with the high-pressure and intermediate cylinders between them. This is the plan adopted on the White Star liner Olympic. The Modern Marine Engine Let us now consider the modern marine engine of the reciprocating type—the kind of engine which was universal from about i860 to 1890, and which is still employed for ships of moderate speed. First (see Fig. 156), there is a strong, heavy bed- plate carrying the bearings and provided with “ wells ” into which the cranks are able to dip. On the bedplate are bolted firmly the A standards which support the cylinders and provide the guides for the cross head.