416.-To determine the power of a double acting engine. Let the force of Then besides the loss from uncondensed steam there is loss, Second, by the cooling in the cylinder, (art. 157,) and pipes, 1.000 ⚫007 ⚫016 .125 Fourth, by the force necessary to expel the steam through the passages, (art. 154.) ⚫007 Fifth, by the force required to open and close the valves, raise injection water, and the friction of the axes ⚫063 Sixth, by the steam being cut off before the end of the stroke Seventh, by the power required to work the air pump, (art. 354.) •100 ⚫050 .368 1632 The force of the steam being generally thirty-five inches of mercury in the boiler, the temperature of the uncondensed steam 120°, and its force 3.7 inches; hence, (35 x 632) 3.7 = 18.42 inches, or 7.1 lbs. per circular inch for the mean effective pressure on the piston.* 417.-RULE. Multiply the mean effective pressure on the piston by the square of its diameter in inches, and that product by the velocity in feet per minute, the result will be the effective power in pounds raised one foot high per minute. To find the horses' power divide the result by 33,000. Example. The diameter of the cylinder of a double engine being twenty-four inches, the length of the stroke five feet, the number of strokes per minute twenty-one and a half, and the force of the steam in the boiler thirty-five inches of mercury, or five inches above the pressure of the atmosphere, required its power. The velocity is 2 × 5 × 21 = 215 per minute, and the mean effective pressure on the piston will be 7.1 lbs. per circular inch; therefore, 71 x 24 x 215 = 879,264 lbs. raised one foot high per minute, or *This is 9.05 lbs. per square inch. 879264 26.64 horses' power. The nominal power of this engine would be only twenty horses' power by Boulton and Watt's mode of calculation, but it will be found that the nominal and real power nearly agree when the steam acts expansively, (art. 422.) The water required for the above engine, (art. 415,) will be 5 cubic feet per minute, or thirty cubic feet per hour; and (art. 190,) 30 × 8·22 = 246'6 lbs. of caking coal, or 246.6 9.2 lbs. of coal per hour for each horse power.* When an engine is of less than ten horses' power, the consumption of fuel will be greater per horse power about in the ratio given in (art. 221.) 418. This engine is applicable to every purpose for which a stationary engine is adapted, and it is only in cases where water is procured with difficulty that it is not applied. It has also been lately brought into use as a moving agent in steam vessels. (See Sect. X.) When the steam acts expansively the power is obtained with a smaller quantity of fuel, and to save fuel is the great object in every application of steam power. 419.-Double engine acting expansively. The motion of a double engine acting expansively ought to be equalized by a fly or some other method, (see Sect. VIII.) otherwise the effect cannot be perfectly obtained. To determine the point of the stroke at which the steam should be cut off, we have this proportion. As the whole force of the steam in the boiler is to 1, so is 368 times that force, (art. 416.) added to the resistance of the uncondensed steam, to the part of the stroke to be made before the steam be cut off. Thus, if the force in the boiler be thirty-five inches of mercury, and the resistance of the uncondensed vapour 37 inches, we have of the stroke. 1 35: (35 × 368 ) + 3·7 ; ; 1 : ·473 = 2.1 Mr. Watt states to the effect that 8.7 lbs. is the quantity equivalent to a horse power, but no doubt he means when working expansively. Notes on Robison, Vol. II. p. 145. 420.-To find the mean pressure on the piston of an expansive engine, the part of the stroke at which the steam is cut off being 1 n divide 2-3 times the common logarithm of n by n, and multiply the quotient by the whole force of the steam in the boiler in pounds per circular inch, the result will be the mean moving force on the piston on a circular inch. Example. Suppose the steam to be cut off at 1 2.1 of the stroke, then n = 2·1, and the logarithm of 2·1 is ·322219; consequently, and as the pressure corresponding to this point of cutting off the steam is thirty-five inches, or 13.5 pounds per circular inch, we have 13.5 × 354 = 4·8 pounds per circular inch, the mean pressure. 421.—The velocity should be found by (art. 336, or 343,) and the quantity of steam will be 1 n part of that required when the engine works at full pressure; therefore the water for steam, the fuel, injection water, will be less in the same proportion in regard to the dimensions of the cylinder, but the passages, pumps, boiler, and other proportions should be found by the rules in (art. 415,) in order that the engine may work either at full pressure or expansively as circumstances may render desirable. 422.-Taking the dimensions and force of steam of the engine given as an example, in (art. 417,) its power as an expansive engine would be 48 × 24o × 215 = 594,432 pounds raised one foot high per minute, or 594432 eighteen horses' power. At the full pressure, the fuel was 246.6 pounds; in this case * This is the same as raising 27,000,000 pounds one foot high by a bushel of coals. 117 = 18 6.5 pounds per horse power; the advantage is therefore as 65: 9.2, or as 10 : 14.* For small engines this quantity requires to be increased in the ratio given in the table, (art. 221.) 423.-The mode of cutting off the steam by giving two movements to the slide during the stroke is shewn in Plate V.; Fig. 2, shews the position of the slide when the piston is descending and the steam cut off, with the passage D to the condenser still open. Slides have the defect of requiring a separate passage to introduce the steam to expel the air from the engine at the time of starting, technically called "blowing through;" but in other respects they seem to afford the most simple and durable means of opening and closing the passages. Combined cylinder Engines. 424.—In Hornblower's engine with two cylinders the steam acts at full pressure in the one, and expansively in the other; as a single engine it is decidedly inferior to Boulton and Watt's construction in every respect, except that of the moving force being more nearly uniform, for there is the additional friction of the small piston, and it is a singular fact, that a single engine of this kind is more complex than a double one. As mine engines they appear to be nearly abandoned, and therefore it is not necessary to occupy space in describing a species which will be sufficiently understood by imagining two single engines acting on one beain, the one of which works at full pressure, and the steam which propels it acts expansively in the other cylinder during the next stroke. In both cylinders the steam has to change from the upper to the lower sides of the piston during the ascent. The ratio of the size of the expansion cylinder to the other should be determined by the same rule as for double engines of this kind, (art. 426,) and in other respects the proportions should be as for single engines. 425.-The double engine with combined cylinders. This engine will be understood most easily with a simple mode of letting on and off the steam. Let C be the small cylinder, Plate VI. Fig. 3, and D the large one, and S the place where the steam enters the pipes. The steam enters the small cylinder at a when the piston descends, and the portion below its piston passes through b, and rising in the passage c, enters the large cylinder *If we take the mean between 6-5 and 9.2 or 7·85 it is what we may expect to be the ordinary consumption of an engine with a variable resistance, when of the best kind. E E at d, while the steam passes to the condenser through e. When the motion is reversed by the slide being moved till the parts are on the other side of the passages, then similar motions take place in the reverse directions, and the vapour passes through f down a pipe to the condenser. Thus the whole apparatus is reduced to a slide box, the rod of which has only one motion for each stroke, and though it is here shewn between the cylinders for convenience, it may be placed in the angle they form when close to each other. 426.-The proportions of combined engines. The smaller cylinder should have the same proportions as for a noncondensing engine working with steam of the same force, (art. 366,) and the loss of force must be the same, that is, 0·4 of the force of the steam in the boiler. The loss of force at the piston of the large cylinder, when its power is 1, will be First, by the cooling in the cylinder and pipes ⚫016 Second, by the friction of the piston •125 Third, by the force necessary to expel the steam through the passages ⚫007 Fourth, by the power required to work the air pump ⚫050 •198 Consequently, 6 × 198·1188 = the portion of the whole power, which added to the loss in the small cylinder, the total loss is 1188 + 4 = 5188, or 52 nearly. Hence, if f denote the whole force of the steam in the boiler, 37 the resistance of the uncondensed steam, and n, the times the capacity of the large cylinder is to exceed the small one, we have If for example the force of the steam in the boiler be 120 inches of mercury, then 120 (52 x 120)+37 1.82 = n, that is, the large cylinder should be 1.82 times the capacity of the small one; if it be larger a loss of effect must necessarily ensue. 427.-The power of a combined cylinder engine is easily ascertained from the investigation, (art. 382,) by substituting the proper constant numbers. The resulting rule for the mean pressure, supposing it to be collected on the surface of the small piston, is 2.3 times the common logarithm of the number of times the large cylinder is greater than the smaller one, multiplied by the force of the steam in the boiler on a circular inch. Thus |