EFFICIENCY

in mechanics and 73 EFFIGIENCY engineering, the ratio of the useful energy given out by a machine to the energy supplied toit. HKnergy cannot be created or destroyed, but it may a ume various forms and, within limits, can be changed from any one of these forms to any other. A machine or engine is an apparatus for converting energy in some given form into energy in another a igned form. In practice it is found impo ible to convert the whole of the given energy into the form wanted, there being always a residue which is not of the right kind, and is, therefore, counted as usele . The smaller the residue the more efficient is the machine, In the machines of elementary mechanics, such as the lever or the screw, the energy supplied is work done by the power or effort, and the energy wanted is work done on the load. If E is the effort, and W the load, then if there were no friction we would have KE = Wr, where r is the velocity ratio, or ratio of the velocities of the points of application of load and effort. The relation found by experiment, however, is usually of the type EH = Wr+ OC, where Cisa constant. The efficiency is the fraction W7/E or 1 — C/E. so that it increases with the load. In heat engines, energy in the form of heat is converted into mechanical energy. Heat is taken in at the source, part of ft is changed into mechanical energy, and the remainder is rejected to the condenser. According te the second law of thermodynamics, the efficiency of such an engine has a definite upper limit whieh it cannot exceed, t being the ratio of the difference of the temperatures of the source and the condenser to the temperature of the source, these temperatures. being measured on the absolute scale, that is, from —273° C. reckoned as the zero. The efficiency of a _ steam engine is usually compared with that of an ideal engine working between the same temperatures and going through a definite periodic set of operations called the Rankine cycle. If the thermal efficiency of an actual engine is 27 per cent, and that of an ideal engine working on the Rankine cycle is 30 per cent, obviously the important figure is the ratio of 27 to 30, or 90 per cent. The performance of a steam-engine depends, not only on its thermal efficiency, but also on its boiler efficiency and its mechanical efficiency. The boiler efficiency is the percentage of the heat obtainable from the fuel consumed which is actually used in the engine; in a good boiler it may be 75 per cent. The mechanical efficiency is the ratio of the work given out at the crankshaft to the work done on the piston; in other words, it is the ratio of brake horse-power to indicated horse-power. It may perbaps be 80 per cent. ‘To arrive at the over-all efficiency, the various partial or component efficiencies must be multiplied together. In comparing one type of engine with another, what is important is aby iouely i over-all efficiency, or ratio of energy output to the theoretical energy value of the fuel employed. Thus, to take the case of marine engines, the Diesel oil-engine is inferior to the turbine and to the reciprocator in point both of thermal and of mechanical efficiency. But when the efficiency of the boilers is taken into account, the Diesel comes out very decidedly ahead of the others. Taking coal at 10,000 British thermal units per pound, and Diesel oi] at 18,000 British thermal units per pound, Mr. T. R. Wollaston has given the following figures for the number of British thermal units consumed per brake horse-power hour: steam-engine 19,000; steam turbine 21,000; gas engine; 15,000; Diesel engine 9,000. Electrical plant in general reaches a high standard of efficiency. Some figures are: transmi ion lines 85 to 95 per cent; motors and generators at full load 70 to 80 per cent from 1 tod h.p., 80 to 90 per cent from 5 te 50 h.p., and 95 per cent for large sizes. Electrical transformers are the most efficient of all machines. Their efficiency ranges from about 90 per cent in small sizes, up to perhaps 98-5 per cent for large machines at fil load. See ENERGY; INTERNAL-COMBUSTION ENGINES; STEAM-ENGINES;