The cock R being now again opened, steam is admitted below the piston, which, as before, ascends, and the descent is again accomplished by closing the cock R, and opening the cock M, admitting cold water between the cylinders, and thereby condensing the steam below the piston.
The condensed steam, thus reduced to water, will collect [Pg069] in the bottom of the cylinder, and resist the descent of the piston. It is therefore necessary to provide an exit for it, which is done by a valve opening outwards into a tube which leads to the feeding cistern L, into which the condensed steam is driven.
That the piston should continue to be air-tight, it was necessary to keep a constant supply of water over it; this was done by a cock similar to M, which allowed water to flow from the pipe M on the piston.
(40.)
On this suggestion Newcomen abandoned the external cylinder, and introduced a pipe H, furnished with a cock Q, into the bottom of the cylinder, so that, on turning the cock, the pressure of the water in the pipe H, from the level of the water in the cistern N, would force the water to rise as a jet into the cylinder, and would instantly condense the steam. This method of condensing by injection formed a very important improvement in the engine, and is still used.
(41.)
When the engine is not working, the weight of the pump-rod D ([fig. 14.]) draws down the beam A, and draws the piston to the top of the cylinder, where it rests. Let us suppose all the cocks and valves closed, and the boiler filled to the proper depth. The fire being lighted beneath it, the water is boiled until the steam acquires sufficient force to lift the valve V. When this takes place, the engine may be started. For this purpose the regulating valve R is opened. The steam rushes in, and is first condensed by the cold cylinder. After a short time the cylinder acquires the temperature of the steam, which then [Pg070] ceases to be condensed, and mixes with the air which filled the cylinder. The steam and heated air, having a greater force than the atmospheric pressure, will open a valve placed at the end X of a small tube in the bottom of the cylinder, and which opens outwards. From this (which is called the blowing valve[14]) the steam and air rush in a constant stream, until all the air has been expelled, and the cylinder is filled with the pure vapour of water. This process is called blowing the engine preparatory to starting it.
When it is about to be started, the engine-man closes the regulator R, and thereby suspends the supply of steam from the boiler. At the same time he opens the condensing valve H[15]; and thereby throws up a jet of cold water into the cylinder. This immediately condenses the steam contained in the cylinder, and produces the vacuum. (The atmosphere cannot enter the blowing valve, because it opens outwards, so that no air can enter to vitiate the vacuum.) The atmospheric pressure above the piston now takes effect, and forces it down in the cylinder. The descent being completed, the engine-man closes the condensing valve H, and opens the regulator, R. By this means he stops the play of the jet within the cylinder, and admits the steam from the boiler. The first effect of the steam is to expel the condensing water and condensed steam which are collected in the bottom of the cylinder, through the tube Y, containing a valve which opens outwards (called the eduction valve), which leads to the hot cistern L, into which this water is therefore discharged.
When the steam admitted through R ceases to be condensed, it balances the atmospheric pressure above the piston, and thus permits it to be drawn to the top of the cylinder by the weight of the rod D. This ascent of the piston is also assisted by the circumstance of the steam being somewhat stronger than the atmosphere.
When the piston has reached the top, the regulating valve R is closed, and the condensing valve H opened, and another descent produced, as before, and so the process is continued. [Pg071]