USING THE HEAT IN THE STEAM
FIG. 47.—WATT’S BALL GOVERNOR
Another of Watt’s inventions which has proved of highest importance in steam engineering was the cut off. In his first engine the piston was subjected to the full boiler pressure throughout its stroke, and the steam that poured out of the exhaust port of the cylinder at the end of the piston stroke was almost as hot as that which entered the cylinder at the beginning of the stroke. The heat that goes out in the exhaust represents just so much wasted energy. Watt realized this and so he invented a valve which would cut off the flow from the boiler before the piston had completed its stroke. Then the steam back of the piston would continue to expand because of the heat within it and would keep on pushing the piston. Of course, the pressure would gradually diminish and there would be less power in the stroke than if the full boiler pressure were pushing the piston all the way, but this loss of power is offset by the saving in steam and in the fuel used to heat the steam. The point at which the cut-off takes place depends largely upon the pressure of the steam. If the steam is cut off when the piston has made only one-fifth of its stroke, one-fifth as much steam will be used at each stroke as would be the case if the steam were used nonexpansively. However, in actual practice the expansion of the cut-off steam instead of being five times would be only about four times, because of the clearance that must be allowed between the piston and the end of the cylinder. If steam of 100 pounds absolute pressure is used, the average pressure throughout the piston will be only about 57 per cent of the full pressure of the steam in the boiler, but each pound of steam will actually do .57 × 4 = 2.28 times as much work as it would if used nonexpansively. All sorts of valve gear have been invented to admit steam quickly and cut it off at the proper point to produce the most efficient result.
In low-pressure cylinders in order to prevent loss of heat through the wall of the cylinder, the latter is steam-jacketed. In other words, there is an outer casing surrounding the cylinder and between this casing and the cylinder steam is admitted to keep the cylinder walls hot.
In order to make full use of the heat in steam it is, in some engines, sent through a series of two, three, and even four cylinders. The exhaust from one cylinder goes into a second larger cylinder. From here after doing work on a piston it discharges into a third still larger cylinder and from that may be led into a fourth cylinder. The cylinders must be progressively larger to allow for the expansion of the steam.
The ordinary steam engine labors under the disadvantage of having to start and stop its pistons at the end of each stroke. Every body possesses inertia, whether it be moving or at rest. If it be at rest, it takes much more energy to set it in motion than to keep it moving. In fact it would keep on moving without further expenditure of energy were there no friction and no forces acting against it. In order to stop the body energy must be expended to overcome its inertia. The more rapidly a body is started and the more quickly it is stopped, the more work must be done in overcoming its inertia. In a steam engine not only the piston but other parts connected to it may be required to reciprocate several hundred times per minute. A great deal of energy is uselessly expended in starting and stopping these parts.
Many efforts have been made to produce a rotary engine in which the piston rotates instead of reciprocating, thus doing away with the work of overcoming inertia. However, there are serious obstacles to the construction of such an engine, and as yet no truly efficient and practical rotary engine has been built.