All About Engines

132 All About Engines the cylinder barrel are alternately heated and cooled by the entering and leaving steam, and there is an unavoidable loss by condensation when steam enters the cylinder. In the Uniflow engine the piston is very long— equal, in fact, to nearly half the length of the stroke —and the exhaust port is half-way between the two ends. The opening is in the form of a number of slots which, when uncovered by the piston nearing the end of its stroke, enable the steam to escape into an annular chamber running all round the barrel. Even though these are only uncovered for a short time, they offer a large area through which the expanding steam flows rapidly, leaving only sufficient to be trapped by the returning piston to serve as a cushion at the end of its stroke. The fact that the steam enters at the ends and flows out at the centre, never reversing its direction, gives the engine its distinctive name. General Arrangements The reader may wonder why some engines are made horizontal and others vertical, and whether there is any other reason than that of space. As to the floor space required for engines of the same power, there can be no question. Figs. 80 and 81 show to the same scale a vertical and a horizontal engine, and the advantage is undoubtedly with the former. But this is not the only consideration. The wear of the cylinder liner and of the stuffing box in a hori- zontal engine is unequal; for in addition to the ordinary friction the weight of the moving parts is con-