All About Engines

M6 All About Engines In practice there is a limit to the size, not of the cylinders which can be made, but which it is desirable to make; and since the boiler is most efficient when producing high-pressure steam, it is more satisfactory to share the temperature drop over several cylinders of moderate size. This forms a compound engine. It has two, three, or four cylin- ders, and the name “ compound ” is usually given to the first, the second and third being designated “ triple expansion” and “quadruple expansion” respectively. In the compound engine the steam first enters the high-pressure, or h.p., cylinder, and then ex- hausts into the steam chest of the low - pressure cylinder. Both cylinders have the same length of stroke, but the low-pressure, or l.p., cylinder has a larger diameter because the steam with which it is supplied, having expanded in the h.p. cylinder, has a larger volume. In the triple-expansion engine there is an inter- mediate pressure, or i.p., cylinder between the h.p. and l.p. cylinders, the steam passing through each in turn. The quadruple - expansion engine, again, has two i.p. between the h.p. and l.p. cylinders. They may be arranged vertically or horizontally, the compound engine corresponding to either of the crank arrangements shown in Figs. 65 and 66, and the triple-expansion engine to that shown in Fig. 68. The quadruple expansion engine, if horizontal, is sometimes arranged with the cylinders in pairs, tandem, driving a crank-like (Fig. 66) ; but in marine engines they are vertical and all in a row.