It was only when surface condensation and the compound principle were adopted, with improved boilers, and superior modes of raising steam and of more effectually applying its power, that the marine engine made any substantial advance. Thirty or forty years ago the usual pressure in a marine boiler seldom exceeded from 3 to 4 lbs. above that of the atmosphere, and, consequently, one of its most necessary fittings was a safety-valve opening inwardly, and called a “vacuum valve,” so as to prevent the boiler collapsing if the steam pressure should chance to fall below that of the atmosphere,[462] but now the usual working pressure is 60 lbs., and 300 lbs. is the pressure to which many men of science think we are now advancing.
In a condensing engine, the effective pressure on either side of the piston is the steam boiler pressure plus the weight of the atmosphere due to the vacuum produced on the opposite side thereof.[463]
Ratio of speed to power.
The boilers for this description of engine, being supplied with water from the sea, required frequent “blowing out” in order to prevent incrustation, and keep the water at a safe and regular density. But this “blowing out” process, which occasioned a very considerable loss in fuel, was to a great extent overcome by the introduction of the surface condenser, which produced fresh water; and this water is pumped back into the boiler to be again and again evaporated and condensed, thus dispensing with feeding from the sea. When the marine engine arrived at so comparative a stage of perfection, the public demanded increased speed, and when steam navigation was extended to distant stations, where fuel was costly, it became a matter of the greatest importance to still further economize its consumption; but considering that the speed of a steam-ship in relation to the power of the engines is subject to a ratio peculiarly its own (to double the speed of a ship the engines have to exert eight times the power necessary for the slower rate), the energies of the engineering world were severely taxed to obtain a greater speed on a less consumption. Higher pressures were introduced, and the principles of expansion more thoroughly worked out. It was known that, when steam from the boiler was cut off after the piston had traversed any desired portion of the cylinder’s length, its expansive energy still enabled it to exert a considerable, though a necessarily decreasing, motive force upon the piston: that is to say, if steam of 50 lbs. absolute pressure were cut off at one-half the stroke, its elastic energy at 910ths of the stroke would be 28 lbs., while the mean of its force throughout the whole of the stroke would be 42 lbs.: in other words, if the whole volume of steam in the cylinder, at the initial pressure, produced 50 lbs. per square inch, one-half of that volume, used expansively, would produce 42 lbs. per square inch.
To more effectually work out these principles and utilize the steam at high pressures, the compound engine was introduced, and is now, almost universally, adopted in the steamers of the mercantile marine.
The Compound Engine
The following woodcut shows an ordinary pair of direct acting inverted cylinder compound engines, as usually fitted in screw steamers.[464] It will be seen that they consist of two steam cylinders, one of small, and the other of large diameter. The steam from the boiler, at a high pressure, enters the small cylinder, and, thence, at the end of the stroke, passes, through an intermediate receiver, into the large cylinder acting upon its piston entirely by its expansive force. At the conclusion of its double work, it passes into the surface condenser, and is there condensed into fresh water, producing the vacuum effect in the large cylinder.
COMPOUND SURFACE ENGINE.
The distinctive difference between the simple and the compound engine is that, in the former, the work of the steam is begun and ended in the same cylinder, whereas, in the latter, it is begun in the small or high pressure cylinder and completed in the large or low pressure one; the work obtained in the small cylinder with the high pressure, and consequently the hotter steam, should be about equal to that in the large one with the lower pressure and cooler steam: in fact, it is the aim of engineers in designing a compound engine to proportion the cylinders and arrange the details of effecting the admission, expansion, and eduction of the steam, so that its pressure may be thoroughly utilized and as much work as possible obtained from it. Some engineers consider the simple engine to be more economical than the compound engine with the same pressure and total expansion; but I am informed, by those who have had opportunities of witnessing the performance of engines made on this principle, that, after a thorough trial in large ocean-going steamers, the anticipated results were not obtained from them, and that they were, consequently, replaced by compound engines.