HIGH-PRESSURE ENGINE.

479

from § 372. Assuming the temperature of the steam to remain constant during the expansion, the following table, calculated by an application of Boyle's law, exhibits the relative amounts of work

Fig. 323.-High-pressure Engine with Vertical Cylinder, working Expansively and without
Condensation.

A, steam-pipe through which the steam arrives from the boiler. Z, valve-chest. B, slide-valve. C, cylinder. GG, guides fixed to the cylinder and to the frame of the engine at K and H. EF, connecting-rod. J, crank. VV, fly-wheel. L, eccentric governing the slide-valve. N. eccentric of the exhaust-pump P. D, outlet pipe for the steam. M, lever of throttle-valve, regulated by centrifugal governor.

Expansive working is often combined with the superheating of steam, that is to say, heating the steam after it has been formed, so as to raise its temperature above the point of saturation. This increases the difference of temperatures to which, according to the second law of thermo-dynamics, the maximum efficiency is proportional; and experience has shown that an actual increase of efficiency is thus obtained.

376. Form and Arrangement of the Several Parts.-As regards their mechanism, the arrangement of steam-engines is considerably varied. In stationary condensing engines, the beam and parallelogram are usually retained; but the arrangement of high-pressure non-condensing engines is generally simpler. The piston-rod frequently travels between guides, and drives the crank by means of a connecting-rod. The cylinder may be either vertical or horizontal, or even inclined at an angle. An engine of this kind is represented in Fig. 323.

Oscillating Engines. The space occupied by the engine may be lessened by jointing the piston-rod directly to the crank without any connecting-rod. In this case the cylinder oscillates around two gudgeons, one of which serves to admit the steam, the other to let it escape. The distribution of the steam is effected by means of a slide-valve whose movements are governed by those of the cylinder. Oscillating engines are very common in steam-boats, and usually produce an exceedingly smooth motion.

377. Rotatory Engines.-Numerous attempts have been made to dispense with the reciprocating movement of a piston, and obtain rotation by the direct action of steam. Watt himself devised an engine on this plan in 1782. Hitherto, however, the results obtained by this method have not been encouraging. Behren's engine, which we now proceed to describe, is one of the most promising of the rotatory engines.

2

Fig. 325 is a perspective view of the engine, and Fig. 326 a crosssection of the cylinders, showing the mode of action of the steam. C and C' are two parallel axes, connected outside by two toothed wheels, so that they always turn in opposite directions. One of these axes is the driving-shaft of the engine. These two axes are surrounded by fixed collars c and c', which fit closely to the cylindrical sectors E and E'; these latter, which are rigidly connected with the axes C, C', are capable of moving in the incomplete cylinders A and A', and act as revolving pistons. In the position represented

in the figure, the steam enters at B, and will escape at D; it is acting only upon the sector E, and pushes it in the direction indicated by the arrow; the shaft C is thus turned, and causes the shaft C to turn

in the opposite direction, carrying with it E', to which it is attached. After half a revolution the sector E' will be in a position corresponding, left for right, to that which E now occupies; it will then be urged by the steam, so as to continue the motion in the same direc

tion for another half-revolution, when the two sectors will have resumed the position represented in the figure.

380. Boilers.-There are many forms of boiler in use. That which

is represented in Fig. 327 is the favourite form in France, and is also extensively used in this country, where it is called the French boiler, or the cylindrical boiler with heaters. The main boiler-shell A is cylindrical with hemispherical ends. BB are two cylindrical tubes called heaters, of the same length as the main shell, and connected with it by vertical tubes d, d, of which there are usually three to each heater. A horizontal brick partition, a little higher than the centres of the heaters, extends along their whole length; and a vertical partition runs along the top of each heater, except where interrupted by the vertical tubes. The flame from the furnace is thus compelled to travel in the first instance backwards, beneath the

heaters; then forwards, through the intermediate space between the heaters the vertical tubes and the main shell; and lastly, backwards, through the side passages CC, which lead to the chimney. By thus compelling the flame to travel for a long distance in contact with the boiler, the quantity of heat communicated to the water is increased.

The level of the water is shown at A in the left-hand figure. The

relative spaces allotted to the steam and the water are not always the same; but must always be so regulated that the steam shall arrive in the cylinder as dry as possible, that is to say, that it shall not carry with it drops of water. Before being used, boilers should always be tested by subjecting them to much greater pressures than they will have to bear in actual use. Hydraulic pressure is commonly employed for this purpose, as it obviates the risk of explosion in case of the boiler giving way under the test.

381. Boilers with the Fire Inside.—When it is required to lessen the weight of the boiler, without much diminishing the surface exposed to heat, as in the case of marine engines, the method adopted is to place the furnace inside the boiler, so that it shall be completely surrounded with water except in front. The flame passes from the furnace, which is in the front of the boiler, into one or two large tubes, leading to a cavity near the back, whence it returns through a number of smaller tubes traversing the boiler, and finally escapes by the chimney.

382. Bursting of Boilers: Safety-valves. Notwithstanding the tests to which boilers are subjected before being used, it too often happens that, owing either to excessive pressure or to weakening of the boiler, very disastrous explosions occur.

Excess of pressure is guarded against by gauges, which show what the pressure is at any moment, and by safety-valves, which allow steam to escape whenever the pressure exceeds a certain limit.

Various kinds of manometer or pressure-gauge have been described in Chap. xiv. That which is most commonly employed in connection with steam-boilers is Bourdon's (§ 126).

A thermometer, specially protected against the pressure and contact of the steam, is also sometimes employed, under the name of thermo-manometer, on the principle that the pressure of saturated steam depends only on its temperature.

The safety-valve, represented in the upper part of Fig. 327, consists of a piece of metal, having the form either of a truncated cone or of a flat plate, fitting very truly into or over an opening in the boiler. The valve is pressed down by a weighted lever; the weight and the length of the lever being calculated, so that the force with which the valve is held down shall be exactly equal to the force with which the steam would tend to raise it when at the limiting pressure. In movable engines, the weighted lever is replaced by a spring, the tension of which can be regulated by means of a screw.