The annexed diagram shows the maximum efficiency of steam at various pressures, from which it will be seen that, for a uniform increase in efficiency, we must increase the pressures at a rapidly increasing rate. 130 Curve of Pressures Curve of Efficiency 300 284 293 302 311 320 329 338 347 356 365 The maximum efficiency of the heat producing the steam may, in the same way, be found by taking the ratio obtained by using the extreme temperature of the gases in the furnace, say about 2400° F., and that of the escaping gases in the chimney, or say 2861 - 1061 about 600° T. (see p. 292), from which we get 2861 = .63. In a general way we may express the total working efficiency as follows: Efficiency of furnace and boiler,...... 10 The product of which gives 18 or nearly as the resulting efficiency. The total efficiency of the engine may be also obtained by considering the work got from the coal consumed. Thus, 1 lb. of ordinary coal by its combustion should produce about 14,500 thermal units. (See p. 278.) Let this pound of coal be consumed in one hour, then 241-6 thermal units per minute. Multiplying this by 772 (the mechanical equivalent of 203 heat) we get 186,515 foot-pounds of work per minute, and dividing by 33,000 we have 5-6 horse-power as the value which should be obtained from 1 lb. of coal per hour if we could completely utilise the energy contained in it. In the improved triple-expansion engines now used at sea we may take one and a-half pounds of coal as equivalent to one indicated horse-power, but, from the above calculation, 8-4 horsepower should be got from a pound and a-half of coal, hence the efficiency is as 1 to 84, or say between 4th and th. Further loss is sustained through the machinery driven, as in the case of a paddle-wheel or the screw-propeller. If the efficiency of the propeller be taken at, we have about th as the resulting efficiency. The late Dr. Froude gives only 37 per cent. of the indicated power as the value of the thrust of the propeller, and states the various proportions of loss of power in the mechanism as follows: Friction due to working load on engine, 143 I.H.P. Constant friction due to dead-weight and tightness of moving parts,..., ·075 "9 143 After allowing for these losses we have E.H.P., the effective horse-power, equal to the useful thrust or ship's true resistance, but in the action of the screw itself there are losses which Dr. Froude states as follows:- The proportioning of the sizes of the cylinders in the tripleexpansion engine like that for the compound (see pp. 571-580) depends on various considerations, equality of work done in each cylinder being an important point. Comparing the sizes of the cylinders of a large number of triple-expansion engines now working at sea, the average proportional arrangement of diameters is very much as QUADRUPLE-EXPANSION ENGINES.* Fig. 1 represents the cylinders and valves for a two-crank quadruple. The arrangement is the same as usual for the ordinary compound engine, the only difference being that the high-pressure cylinder is replaced by a tandem pair, which may be called the high-pressure cylinders, and the usual low-pressure cylinder by another tandem pair, which may be called the lowpressure cylinders. These tandem cylinders differ from those of tandem engines hitherto constructed in not having any valves, casings, or steam-pipes directly attached to them, the valves for distributing the steam to the upper cylinders being contained in the valve casings of the lower cylinders. This enables the upper cylinders to be lifted when required with almost as much facility as ordinary cylinder covers. The conical form of the pistons with * The Editor is indebted to Mr. Walter Brock, Dumbarton, for the drawings and description of quadruple engines. their apices towards each other, admits of a space between the cylinders without increasing the total height. This space gives access to man-holes opening into the top of the lower, and the bottom of the upper cylinders. The piston-rods between the cylinders have metallic packing like a piston, access to the junk ring being had by the doors leading into the bottom of the upper cylinders. Fig. 2 shows the cylinders for a three-crank quadruple engine, and would be the arrangement used for powers so large as to render it desirable to have two instead of one low-pressure cylinder. There are no stuffing-boxes required between the valves of the upper and lower cylinders. The piston valves for the upper cylinders also serve as balance cylinders for themselves, the lower valves, and the gear. The quadruple engine for the Jumna, built and engined in 1886 by Messrs. Denny of Dumbarton, has the cylinders of the following dimensions: 30 42 60 84 inches diameter. Stroke, 5 feet, with 160 lbs. steam pressure. By reducing in this way the variation of temperature in the cylinder, the loss from condensation of the steam is lessened and economy gained. LOCOMOTIVE ENGINES. See p. 532. Express passenger engines, with 18-inch cylinder, 7 ft. or 8 ft. driving wheels, weigh about 42 tons, and with tender, loaded, about 68 tons. The length of stroke is 24 inches, wheel base about 21 feet. Heating surface about 1000 square feet, of which about onetwelfth is in the fire-box, the rest in the tubes. The area of firegrate being about 144 square feet. Such engines carry a working pressure of 130 to 140 lbs. per square inch, and can exert a tractive force of 90 lbs. for each lb. of effective pressure on piston, and will draw a train of 17 carriages, weighing about 190 tons, at a speed of 50 miles per hour on the level. Recently the compounding of locomotives has been tried, both in France and in this country. STRENGTH OF MATERIALS. See p. 67 The ultimate strength of leather belts is about 3,200 lbs. per square inch; the working strength being about one-eighth of this. Ultimate strength of steel wire, 90 to 140 tons per sq. in. 45 The following rules give a good approximation to the strength of wire and hemp ropes : |