Читать книгу A Catechism of the Steam Engine - C.E. John Bourne - Страница 20

129. Q.--Will you describe the more important features of the locomotive engine?

A.--The locomotive employed to draw carriages upon railways, consists of a cylindrical boiler filled with brass tubes, through which the hot air passes on its progress from the furnace to the chimney, and attached to the boiler are two horizontal cylinders fitted with pistons, valves, connecting rods, and other necessary apparatus to enable the power exerted by the pistons to turn round the cranked axle to which the driving wheels are attached. There are, therefore, two independent engines entering into the composition of a locomotive, the cranks of which are set at right angles with one another, so that when one crank is at its dead point, the other crank is in a position to act with its maximum efficacy. The driving wheels, which are fixed on the crank shaft and turn round with it, propel the locomotive forward on the rails by the mere adhesion of friction, and this is found sufficient not merely to move the locomotive, but to draw a long train of carriages behind it.

130. Q.--Are locomotive engines condensing or high pressure engines.

A.--They are invariably high pressure engines, and it would be impossible or at least highly inconvenient, to carry the water necessary for the purpose of condensation. The steam, therefore, after it has urged the piston to the end of the stroke, escapes into the atmosphere. In locomotive engines the waste steam is always discharged into the chimney through a vertical pipe, and by its rapid passage it greatly increases the intensity of the draught in the chimney, whereby a smaller fire grate suffices for the combustion of the fuel, and the evaporative power of the boiler is much increased.

131. Q.--Can you give an example of a good locomotive engine of the usual form?

A.--To do this I will take the example of one of Hawthorn's locomotive engines with six wheels represented in fig. 29; not one of the most modern construction now in use, nor yet one of the most antiquated. M is the cylinder, R the connecting rod, C C the eccentrics by which the slide valve is moved; J J is the steam pipe by which the steam is conducted from the steam dome of the boiler to the cylinder. Near the smoke stack end of this pipe is a valve K or regulator moved by a handle p at the front of the boiler, and of which the purpose is to regulate the admission of the steam to the cylinder; f is a safety valve kept closed by springs; N is the eduction pipe, or, as it is commonly termed in locomotives, the blast pipe, by which the steam, escaping from the cylinder after the stroke has been performed, is projected up the chimney H. The water in the boiler of course covers the tubes and also the top of the furnace or fire box. It will be understood that there are two engines in each locomotive, though, from the figure being given in section, only one engine can be shown. The cylinders of this engine are each 14 inches diameter; the length of the stroke of the piston is 21 inches. There are two sets of driving wheels, 5 feet diameter, with outside connections.

132. Q.--What is the tender of a locomotive?

A.--It is a carriage attached to the locomotive, of which the purpose is to contain coke for feeding the furnace, and water for replenishing the boiler.

133. Q.--Can you give examples of modern locomotives?

A.--The most recent locomotives resemble in their material features the locomotive represented in fig. 29. I can, however, give examples of some of the most powerful engines of recent construction. Fig. 30 represents Gooch's express engine, adapted for the wide gauge of the Great Western Railway; and fig. 31 represents Crampton's express engine, adapted for the ordinary or narrow gauge railways. The cylinders of Gooch's engine are each 18 inches diameter, and 24 inches stroke; the driving wheels are 8 feet in diameter; the fire grate contains 21 square feet of area; and the heating surface of the fire box is 153 square feet. There are in all 305 tubes in the boiler, each of 2 inches diameter, giving a heating surface in the tubes of 1799 square feet. The total heating surface, therefore, is 1952 square feet. Mr. Gooch states that an engine of this class will evaporate from 300 to 360 cubic feet of water in the hour, and will convey a load of 236 tons at a speed of 40 miles an hour, or a load of 181 tons at a speed of 60 miles an hour. The weight of this engine empty is 31 tons; of the tender 8–½ tons; and the total weight of the engine when loaded is 50 tons. In one of Crampton's locomotives, the Liverpool, with one set more of carrying wheels than the fig., the cylinders are of 24 inches diameter and 18 inches stroke; the driving wheels are 8 feet in diameter; the fire grate contains 21–½ square feet of area; and the heating surface of the fire box is 154 square feet. There are in all 300 tubes in the boiler of 2–3/16 inches external diameter, giving a surface in the tubes of 2136 square feet, and a total heating surface of 2290 square feet. The weight of this engine is stated to be 35 tons when ready to proceed on a journey. Both engines were displayed at the Great Exhibition in 1851, as examples of the most powerful locomotive engines then made. The weight of such engines is very injurious to the railway; bending, crushing, and disturbing the rails, and trying very severely the whole of the railway works. No doubt the weight may be distributed upon a greater number of wheels, but if the weight resting on the driving wheels be much reduced, they will not have sufficient bite upon the rails to propel the train without slipping. This, however, is only one of the evils which the demand for high rates of speed has produced. The width of the railway, or, as it is termed, the gauge of the rails, being in most of the railways in this kingdom limited to 4 feet 8–½ inches, a corresponding limitation is imposed on the diameter of the boiler; which in its turn restricts the number of the tubes which can be employed. As, however, the attainment of a high rate of speed requires much power, and consequently much heating surface in the boiler, and as the number of tubes cannot be increased without reducing their diameter, it has become necessary, in the case of powerful engines, to employ tubes of a small diameter, and of a great length, to obtain the necessary quantity of heating surface; and such tubes require a very strong draught in the chimney to make them effective. With a draught of the usual intensity the whole of the heat will be absorbed in the portion of the tube nearest the fire box, leaving that portion nearest the smoke box nothing to do but to transmit the smoke; and with long tubes of small diameter, therefore, a very strong draught is indispensable. To obtain such a draught in locomotives, it is necessary to contract the mouth of the blast pipe, whereby the waste steam will be projected into the chimney with greater force; but this contraction involves an increase of the pressure on the eduction side of the piston, and consequently causes a diminution in the power of the engine. Locomotives with small and long tubes, therefore, will require more coke to do the same work than locomotives in which larger and shorter tubes may be employed.