the entrance of the steam in following up the piston. Its true explanation was found to be the earlier release of the waste steam, and consequent diminution of resistance. As sometimes ths of an inch, or even an inch "lead" was given in passenger engines, it was decided to try the effect of opening the exhausting passage earlier by the same amount, while the steam-port should still be made to open only at the turn of the stroke. An engine was chosen for the experiment. Its original valve resembled fig. 53. Placing the valve on the ports, so as to allow the exhausting passage to be ths of an inch open, the steam-port would, at the same time, beineh open. This space, therefore, was closed by adding to the length of the valve, at each end,inch. The eccentric was, of course, shifted on the axle to correspond with the alteration, and the engine with the altered valve (Fig. 54) was again set to work. The amount by which the valve at each end overlaps the steam-ports, when placed exactly over them, is technically termed the lap. The lap of the valve being them 3ths of an inch, the exhausting passage was about ths of an inch open when the stroke was finished. The engine was made the subject of several experiments. With passenger trains the saving in fuel was very considerable, the consumption, while running, being only about 25 lbs. of coke per mile, with loads of five to eight cars, and the speed was considerably improved. It here becomes necessary to refer to the consumption of the Liverpool and Manchester engines before and at the time we speak of, in order to form a just conception of the position arrived at. In 1836 and 1837, larger engines were gradually introduced to replace the smaller class, which had become insufficient for maintaining the higher rate of speed then demanded; and their increased consumption of fuel was commensurate with their increase of size. For an idea of the general effect attendant upon their introduction, the following table, showing the coke consumed in several consecutive years, may be consulted. 7,907 tons (gross) coke. 11,561 trips in 1836, 12,063 trips in 1837, 9,876 tons (gross) coke. 12,953 trips in 1838, 10,816 tons (gross) coke. Thus, during three years, when the change went on, although the work done increased only in the proportion of 100 to 136, without material difference in the magnitude of the loads, in 1839 and 1840 the average consumption attained its maximum, being about 49 lbs. per mile, gross, with passenger trains averaging seven cars, and 54 lbs. per mile with freight trains averaging sixteen burthen cars. 40 lbs. net consumption with passenger trains, was moderate for such an engine used for the experiment; and the performance of the engine when altered, being under 30 lbs. net, was naturally considered favourable. This result was evidently obtained from the earlier exhaustion of the steam. Whereas previously the opening of the exhaustion passage was contemporaneous with the termination of the stroke; now it took place before, and was already ths of an inch open at the end of the stroke. A portion of the steam could by that time escape, and the back pressure was diminished. The valves of two engines, Nos. 10 and 12, were next altered to have gths of an inch lap-No. 10 in January, No. 12 in June, 1841. During the last quarter of the year 1841, the gross consumption of No. 10 was 36 lbs. per mile, and that of No. 12 40 lbs. per mile, and the net consumption about 30 and 33 lbs. No. 12 Valve, with ths of an inch Lap. Valve with ths of an inch Lup. Feb'ry 9, 1841, 10 trips of 30 miles, 88 3 0 3.11. The product is the inside diameter of the boiler in inches. Example.-Required the inside diameter of the boiler for a locomotive engine, the diameter of the cylinders being 15 inches. In this example we have, according to the rule, inside diameter of boiler = 15 x 3.11 = 46.65 inches, or about 3 feet 10 inches. Length of boiler.-In the Northern and Eastern Counties Railway the length of the boiler is 8 feet; while in the North Midland Counties Railway, in the Great Western Railway, and in the Hartlepool Railway, the length of the boiler is 8 feet. In the Belgian railways the length of the boiler is 8 feet 2 inches. And in the Bordeaux and La Teste railway the length of the boiler is 8 feet 9 inches. In Stephenson's locomotive engines, the length of the boiler is between 11 and 12 feet. In this country the length is from 10 to 14 feet. Diameter of steam dome inside.-It is obvious that the diameter of the steam dome may be varied considerably, according to circumstances; but the first indication is to make it large enough. It is usual, however, in practice, to proportion the diameter of the steam dome to the diameter of the cylinder; and there appears to be no great objection to this. The following rule will be found to give the diameter of the dome usually adopted in prac RULE.-To find the diameter of the steam dome.Multiply the diameter of the cylinder in inches by 1.43. The product is the diameter of the dome in inches. Height of steam dome.-The height of the steam dome may vary. Judging from practice, it appears that a uniform height of 21 feet would answer very well. Diameter of safety-valve.-In practice the diameter of the safety-valve varies considerably. The following rule gives the diameter of the safety-valve usually adopted in practice. RULE.-To find the diameter of the safety-valve.Divide the diameter of the cylinder in inches by 4. The quotient is the diameter of the safety-valve in inches. Example.-Required the diameter of the safetyvalves for the boiler of a locomotive engine, the diameter of the cylinder being 13 inches. Here, according to the rule, diameter of safety-valve = 13 ÷ 4 = 3 inches. A larger size, however, is preferable, as being less likely to stick. Diameter of valve spindle.-The following rule will be found to give the correct diameter of the valve spindle. It is entirely founded on practice. RULE. To find the diameter of the valve spindle.-Multiply the diameter of the cylinder in inches by 076. The product is the proper diameter of the valve spindle. |