Then the expenditure of air per stroke required in order to take up that heat during its expansion is found to be 68420 =2.0354 lbs. ; 33615 whence the following results are obtained : Foot-lbs. Waste heat per stroke equivalent to the work of Indicated work per stroke, 3896 × 2·0354 = 60490 7930 Volumes successively occupied by the air in cubic feet per 29. The case of Article 9-where the whole of the heat generated in the compressing pump is abstracted as fast as it is produced-and that of the example just described-in which the whole of that heat is at first employed in raising the temperature of the air while an equal quantity of heat goes to waste with the escaping air that has done its work-form two extremes, and between those extreme cases there may lie an indefinite number of intermediate cases, in which part of the heat generated by compression is abstracted at once, and part employed in raising the temperature of the air. It will be readily understood that in all those intermediate cases the result is the same as in the two extreme cases-the whole of the heat generated by the compression of the air goes to waste either at once or with the escaping air, and none of it is available for conversion into indicated work; nor would it be possible that it should become so available, unless it were possible for heat to be directly transferred from a colder body to a hotter body. XXVI. ON THE WORKING OF STEAM IN COMPOUND ENGINES.* 1. Principal Kinds of Compound Engines.-By a compound steam-engine is meant one in which the mechanical action of the steam commences in a smaller cylinder and is completed in a larger cylinder. Those cylinders are respectively called, for convenience, the high-pressure cylinder and the low-pressure cylinder. Two classes of compound engines will be considered-first, those in which the steam passes directly or almost directly from the high-pressure to the low-pressure cylinder, the forward stroke of the latter cylinder taking place either exactly or nearly at the same time with the return stroke of the former cylinder; and, secondly, those in which the steam, on its way from the high-pressure to the low-pressure cylinder, is stored in a reservoir, so that any convenient fraction of a revolution (such, for example, as a quarter revolution) may intervene between the ends of the strokes of the cylinders. As to the latter class of engines, reference may be made to a paper by Mr. E. A. Cowper in the Transactions of the Institution of Naval Architects for 1864, page 248. Sometimes, especially in the first class of compound engines (those without reservoirs), there are a pair of low-pressure cylinders whose pistons move together, and which act like one cylinder divided into two parts. · 2. Advantages of Compound Engines.-As regards the theoretical efficiency of the steam, the compound engine possesses no advantage over an engine with a single cylinder of the dimensions of the low-pressure cylinder, working with the same pressure of steam and the same rate of expansion. The advantages which it does possess are the following:First, in point of strength, the action of the steam when at its highest pressure takes place, in the compound engine, upon a comparatively small piston, thus diminishing the amount of the greatest straining force exerted on the mechanism and framing; secondly, in point of economy of heat and steam, in a single-cylindered engine it is necessary, in order to prevent liquefaction and re-evaporation of the steam, and consequent waste of heat, that the whole metal of the cylinder should be kept, by means of a steam jacket, at a temperature equal to that of the steam when first From The Engineer of March 11, 1870. admitted; whereas, in a compound engine, it is the smaller or highpressure cylinder only which has to be kept at so high a temperature, it being sufficient to keep the larger or low-pressure cylinder at the temperature corresponding to the pressure at which the steam passes from the high-pressure to the low-pressure cylinder. Thirdly, in point of economy of work: the whole of the force exerted by the piston rod upon the crank in a single-cylindered engine takes effect in producing friction at the bearings; whereas, in compound engines, the mechanism can be so arranged that the forces exerted by the piston rods on the bearings shall, to a certain extent, balance each other, thus diminishing the friction. When there are a pair C of low-pressure cylinders H h P Fig. 1. --A duced by the reaction or inertia of the pistons and of the masses which move along with them. The advantages which have been stated are obviously greatest with high rates of expansion. 3. Combination of Diagrams.—When the diagrams of the high and lowpressure cylinders of a compound engine are taken by means of one indicator they have the same length of base; and when arranged in the customary way for inspection they present appearances which are represented in Fig. 1 for engines without reservoirs, and in Fig. 2 for engines with reservoirs. In each Fig. A A is the atmospheric line, OB the zero line of absolute pressure, and the length OP on that line is the common length of the diagrams of both cylinders, as originally drawn. The diagram of the high-pressure cylinder is represented in Fig. 1 by CDKH, and in Fig. 2 by CD KL; that of the low-pressure cylinder, as drawn by the indicator, is represented in Fig. 1 by kih, and in Fig. 2 by ligh. In combining the diagrams of the two cylinders into one diagram, it is to be borne in mind that when the area of a diagram is considered as representing the work done by the steam on the piston at one stroke, the length of the base of the diagram is to be considered as representing the effective capacity of the cylinder: that is, the space swept through by the piston at one stroke. Hence, in order to prepare the diagram of the low-pressure cylinder for combination with that of the high-pressure cylinder, the lengths of its base, and of every line in it parallel to its base, are to be increased in the ratio in which the effective capacity of the low-pressure cylinder is greater than that of the high pressure cylinder.* (When there which one is marked hie. Let c denote the ratio OB÷OP. Then, in the case of an engine without a reservoir (Fig. 1) draw Pk perpendicular to OB, cutting all the parallel dotted lines, and on each of those lines (such as sr q) lay off sq = c'rs. A curve kqej, drawn through * Manual of the Steam-Engine and other Prime Movers, page 334. the points, such as q, thus found, will be the required boundary of the enlarged low-pressure diagram, kqejhsk, which, being joined on to the high-pressure diagram CDK H, makes the combined diagram. When the engine has a reservoir, draw Ol (Fig. 2) perpendicular to O B, and crossing all the parallel dotted lines, and on each of those lines (such as srk) lay off s k = c ́s r. A curve lkefg, drawn through the points, such as k, thus found, will be the required boundary of the enlarged lowpressure diagram, which, being joined on to the high-pressure diagram CD KL, makes the combined diagram. In a theoretically perfect |