Part 9 (1/2)

1. The expansion of steam, and six methods of applying the principle and of equalizing the expansive power.

2. The double-acting steam-engine, in which the steam acts on each side of the piston alternately, the opposite side being in communication with the condenser.

3. The double or coupled steam-engine--two engines capable of working together, or independently, as may be desired.

4. The use of a rack on the piston-rod, working into a sector on the end of the beam, thus securing a perfect rectilinear motion of the rod.

5. A rotary engine, or ”steam-wheel.”

The efficiency to be secured by the expansion of steam had long been known to Watt, and he had conceived the idea of economizing some of that power, the waste of which was so plainly indicated by the violent rus.h.i.+ng of the exhaust-steam into the condenser, as early as 1769.

This was described in a letter to Dr. Small, of Birmingham, in May of that year. When experimenting at Kinneil, he had tried to determine the real value of the principle by trial on his small engine.

Boulton had also recognized the importance of this improved method of working steam, and their earlier Soho engines were, as Watt said, made with cylinders ”double the size wanted, and cut off the steam at half-stroke.” But, though ”this was a great saving of steam, so long as the valves remained as at first,” the builders were so constantly annoyed by alterations of the valves by proprietors and their engineers, that they finally gave up that method of working, hoping ultimately to be able to resume it when workmen of greater intelligence and reliability could be found. The patent was issued July 17, 1782.

Watt specified a cut-off at one-quarter stroke as usually best.

Watt's explanation of the method of economizing by expansive working, as given to Dr. Small,[40] is worthy of reproduction. He says: ”I mentioned to you a method of still doubling the effect of steam, and that tolerably easy, by using the power of steam rus.h.i.+ng into a vacuum, at present lost. This would do a little more than double the effect, but it would too much enlarge the vessels to use it all. It is peculiarly applicable to wheel-engines, and may supply the want of a condenser where force of steam is only used; for, open one of the steam-valves and admit steam, until one-fourth of the distance between it and the next valve is filled with steam, shut the valve, and the steam will continue to expand and to pa.s.s round the wheel with a diminis.h.i.+ng power, ending in one-fourth its first exertion. The sum of this series you will find greater than one-half, though only one-fourth steam was used. The power will indeed be unequal, but this can be remedied by a fly, or in several other ways.”

[40] ”Lives of Boulton and Watt,” Smiles.

It will be noticed that Watt suggests, above, the now well-known non-condensing engine. He had already, as has been seen, described it in his patent of 1769, as also the rotary engine.

Watt ill.u.s.trates and explains his idea very neatly, by a sketch similar to that here given (Fig. 28).

Steam, entering the cylinder at _a_, is admitted until one-fourth the stroke has been made, when the steam-valve is closed, and the remainder of the stroke is performed without further addition of steam. The variation of steam-pressure is approximately inversely proportional to the variation of its volume. Thus, at half-stroke, the pressure becomes one-half that at which the steam was supplied to the cylinder. At the end of the stroke it has fallen to one-fourth the initial pressure. The pressure is always nearly equal to the product of the initial pressure and volume divided by the volume at the given instant. In symbols,

_PV_ _P'_ = ----.

_V'_

It is true that the condensation of steam doing work changes this law in a marked manner; but the condensation and reevaporation of steam, due to the transfer of heat to and from the metal of the cylinder, tends to compensate the first variation by a reverse change of pressure with change of volume.

[Ill.u.s.tration: FIG. 28.--Expansion of Steam.]

The sketch shows this progressive variation of pressure as expansion proceeds. It is seen that the work done per unit of volume of steam as taken from the boiler is much greater than when working without expansion. The product of the mean pressure by the volume of the cylinder is less, but the quotient obtained by dividing this quant.i.ty by the volume or weight of steam taken from the boiler, is much greater with than without expansion. For the case a.s.sumed and ill.u.s.trated, the work done during expansion is one and two-fifths times that done previous to cutting off the steam, and the work done per pound of steam is 2.4 times that done without expansion.

Were there no losses to be met with and to be exaggerated by the use of steam expansively, the gain would become very great with moderate expansion, amounting to twice the work done when ”following” full stroke, when the steam is cut off at one-seventh. The estimated gain is, however, never realized. Losses by friction, by conduction and radiation of heat, and by condensation and reevaporation in the cylinder--of which losses the latter are most serious--after pa.s.sing a point which is variable, and which is determined by the special conditions in each case, augment with greater rapidity than the gain by expansion.

In actual practice, it is rarely found, except where special precautions are taken to reduce these losses, that economy follows expansion to a greater number of volumes than about one-half the square root of the steam-pressure; i. e., about twice for 15 or 20 pounds pressure, three times for about 30 pounds, and four and five times for 60 or 65 and for 100 to 125 pounds respectively. Watt very soon learned this general principle; but neither he, nor even many modern engineers, seem to have learned that too great expansion often gives greatly-reduced economy.

The inequality of pressure due to expansion, to which he refers, was a source of much perplexity to Watt, as he was for a long time convinced that he must find some method of ”equalizing” the consequent irregular effort of the steam upon the piston. The several methods of ”equalizing the expansive power” which are referred to in the patent were attempts to secure this result. By one method, he s.h.i.+fted the centre as the beam vibrated, thus changing the lengths of the arms of that great lever, to compensate the change of moment consequent upon the change of pressure. He finally concluded that a fly-wheel, as first proposed by Fitzgerald, who advised its use on Papin's engine, would be the best device on engines driving a crank, and trusted to the inertia of a balance-weight in his pumping-engines, or to the weight of the pump-rods, and permitted the piston to take its own speed so far as it was not thus controlled.

The double-acting engine was a modification of the single-acting engine, and was very soon determined upon after the successful working of the latter had become a.s.sured.

Watt had covered in the top of his single-acting engine, to prevent cooling the interior of the cylinder by contact with the comparatively cold atmosphere. When this had been done, there was but a single step required to convert the machine into the double-acting engine. This alteration, by which the steam was permitted to act upon the upper and the lower sides of the piston alternately, had been proposed by Watt as early as 1767, and a drawing of the engine was laid before a committee of the House of Commons in 1774-'75. By this simple change Watt doubled the power of his engine. Although invented much earlier, the plan was not patented until he was, as he states, driven to take out the patent by the ”plagiarists and pirates” who were always ready to profit by his ingenuity. This form of engine is now almost universally used. The single-acting pumping-engine remains in use in Cornwall, and in a few other localities, and now and then an engine is built for other purposes, in which steam acts only on one side of the piston; but these are rare exceptions to the general rule.

The subject of his next invention was not less interesting. The double-cylinder or ”compound” engine has now, after the lapse of nearly a century, become an important and usual type of engine. It is impossible to determine precisely to whom to award the credit of its first conception. Dr. Falk, in 1779, had proposed a double-acting engine, in which there were two single-acting cylinders, acting in opposite directions and alternately on opposite sides of a wheel, with which a rack on the piston-rod of each geared.

Watt claimed that Hornblower, the patentee of the ”compound engine,”

was an infringer upon his patents; and, holding the patent on the separate condenser, he was able to prevent the engine of his compet.i.tor taking such form as to be successfully introduced. The Hornblower engine was soon given up.

Watt stated that this form of engine had been invented by him as early as 1767, and that he had explained its peculiarities to Smeaton and others several years before Hornblower attempted to use it. He wrote to Boulton: ”It is no less than our double-cylinder engine, worked upon our principle of expansion.” He never made use of the plan, however; and the princ.i.p.al object sought, apparently, in patenting this, as well as many other devices, was to secure himself against compet.i.tion.