Marine Engines
213. The early steamers were fitted with paddle-wheels, and the engines used to drive them were for the most part modified beam-engines. Bell’s "Comet" (§ 21) was driven by a series of inverted beam-engine, and another form of inverted beam, known as the slide-lever engine, was for long favourite with marine engineers. In the side-lever engine the cylinder was vertical, and the piston-rod projected through the top. Form a crosshead on the rod a pair of links, one on each side of the cylinder, led down to the ends of a pair of horizontal beams or levers below, which oscillated about a fixed gudgeon at or near the middle of their length. The two levers were joined at their ends by a crosshead, from which a connecting-rod was taken to the crank above. The side-lever engine is now obsolete.
In American practice, engines of the beam type, with a braced-beam supported on A frames above the deck, are still common in river-steamers and coasters.
214. An old from of direct-acting paddle-engine was the steeple-engine, in which the cylinder was set vertically below the crank. Two piston-rods projected through the top of the cylinder, one on each side of the shaft and of the crank. They were united by a crosshead sliding in vertical guides, and from this a return-connecting-rod led to the crank.
215. Modern paddle-wheel engines are usually of one of the following types. (1) In oscillating cylinder engines the cylinders are set under the crank-shaft, and the piston-rods are directly connected to the cranks. The cylinders are supported on trunnions which give them the necessary freedom of oscillation to follow the movement of the crank. Steam is admitted through the trunnions to slide-valves on the sides of the cylinders. In some instances the mean position of the cylinders in inclined of vertical; and oscillating engines have been arranged with one cylinder before and another behind the shaft, both pistons working on one crank. The oscillating cylinder type is best adapted for what would now be considered comparatively low pressures of steam. (2) Diagonal engines are direct-engines of the ordinary connecting-rod type, with the cylinders fixed on an inclines bed and the guide sloping up towards the shaft.
216. When the screw-propeller began to take the place of paddle-wheels in ocean-steamers the increased speed which it required was at first supplied by using spur-wheel gearing in conjunction with one of the forms of engines the usual in paddle steamers. After a time types of engine better suited to the screw were introduced, and were driven fast enough to be connected directly to the screw-shaft. The smallness of the horizontal space on either side of the shaft formed an obstacle of the use of horizontal engines, but this difficulty was overcome in several ways. In Penn’s trunk-engine, still used in the navy, the engine is shortened by attaching the connecting-rod directly to the piston, and using a hollow piston-rod, called a trunk, large enough to allow the connecting-rod to oscillate inside it. The trunk extends through both ends of the cylinder and forms a guide for the piston. It has the drawback of requiring very large stuffing-boxes, of wasting cylinder space, and of presenting a large surface of metal to alternate heating by steam and cooling by contact with the atmosphere. The use of high-pressure steam is likely to make the trunk-engine obsolete.
217. The return-connecting-rod engine is another horizontal form much used in the navy. It is a steeple-engine placed horizontally, with two, and in some cases four, piston-rods in each cylinder. The piston-rods pass clear of the shaft and the crank, and are joined beyond it in a guided crosshead, form which a connecting-rod returns.
Ordinary horizontal direct-acting engines with a short stroke and a short connecting-rod are also common in warships, where the horizontal is frequently preferred to the vertical type of engine for the sake of keeping the machinery below the water-line. In horizontal marine engines the air-pump and condenser are generally placed on the opposite side of the shaft form the cylinder, which balances the weight and allows the air-pump to be driven direct.
218. In merchant ocean-steamers one general type of engine is universal, and the same type is now to an increasing extent adopted in naval practice. This is the inverted vertical direct-acting engine, generally with two or more cylinders placed side by side directly over the shaft. In exceptional cases a single cylinder has been used, with a fly-wheel on the shaft. Two, three, and four cylinders are common.
The most usual form of existing marine engine is the two-cylinder compound arrangement, with cranks at right angles or nearly at right angles, of which figs. 135, 136, 137 (pp. 518-20) show a characteristic example (the engines of the s.s. "Tartar," by Messrs John & James Thomson, Glasgow).
Fig.135. is an end elevation, fig. 136 a longitudinal section through the centre of the engines, and fig. 137 a thwart-ship section through the condenser and air-jump. The cylinders are 50 and 94 inches in diameter, and the stroke is 5 feet. Both cylinders are fitted with liners, and are steam-jacketed. Double-ported slide-valves are used on both, and the high-pressure valve has a relief ring. The crosshead bears when the engines are going ahead, with a hollow box behind the guiding surface, and cold water is kept circulating in this to prevent the guides form heating. The crank-shaft is Vicker’s steel, 17_ inches in diameter. The condenser is in the place it usually has in engines of this type,—in the lower part of the back frame, with its tubes running horizontally from end to end of the engine. There are 1400 tubes, of 1 inch diameter and 1_ inch pitch. The air-pumps are of the single-acting bucket kind, and are driven by a lever form the crosshead. Centrifugal circulating pumps are used, driven by a pair of independent small vertical engines. The link-motion is worked by steam starting and reversing gear, which appears on the left side of the engine in fig. 135. These engines work with a boiler pressure of 90 _, and indicate 3560 horse-power. Fig 134 shows, on a larger scale, the piston packing, which consists of a pair of floating rings, pressed out by a spiral spring behind them.
219. Two other arrangements of double compound (as distinguished from triple-expansion) marine engines of the inverted vertical type require notice. One is the tandem arrangement, largely adopted in the steamers of the "White Star" line. In these each crank is operated by an independent pair of compound cylinders, the high-pressure cylinder being on top of the low-pressure cylinder, with one piston-rod common to both. The valves of both are worked by a single pair of eccentrics with a link-motion; the valve-rod of the low-pressure cylinder extends through the top of its valve-chest, and is joined either directly or by a short lever with the valve-rod of the high-pressure cylinder. Generally two parts of tandem cylinders are placed side by side, one pair abaft the other, to work on cranks at right angles. In exceptionally large engines three pairs have been used, working on cranks 120° apart,1 an arrangement greatly superior to that of two cranks in uniformity of effort on the shaft. To facilitate removing the pistons form the cylinders, the large cylinder has in some cases been set above the other.
220. The other arrangement of double compound marine engine has three cylinders set in line fore and aft. The middle one is the high-pressure cylinder; the other two receive steam from it, and form together the equivalent of one large low-pressure cylinder. The three work on cranks at 120° apart. Besides securing the advantages in uniformity of effort which three cranks have over two, this form avoid the use, in very powerful engines, of a low pressure cylinder of excessive size. On the other hand, the three cylinder form takes up more space, and has a larger number of working parts. In the most powerful engines that have yet been constructed this three-cylinder arrangement is followed. The "Umbria" and "Etruria" have a 71-inch high-pressure cylinder between two 105-inch low-pressure cylinders, with a stroke of 6 feet. These engines, which were built just before the introduction of triple expansion, are supplied with steam at a pressure of 110 _ by gauge, and indicate 14,300 horse-power. In this and in the ordinary two-cylinder form of marine engine, the low pressure valve-chest and the casing of the engine between the cylinders form an intermediate receiver for the steam.
221. during the last two or three years a great advance has taken place in marine engineering by the general introduction of triple-expansion engines, and by an increase in steam pressure which the steam of triple expansion makes practicable. In 1874 the steamer "Propontis" was fitted with a set of three-crank triple-expansion engines, designed by Mr A. C. Kirk. The experiment was prevented from being fully successful by the failure of the boilers, which were of a special type. Another experiment with triple engines in the yacht "Isa" in 1877 prepared the way for their application to regular ocean service. In 1882 the steam-ship "Abedeen", with triple engines, designed by Mr Kirk, to work with steam of 125 _ pressure, supplied from double-ended steel boilers of the ordinary marine type, demonstrated the advantage and safety of the steam. Since then its use has become general in new steamers, and in many cases the older double engines are being removed to gave place to engines of the triple-expansion type, with the effect of reducing the consumption of coal by about 25 per cent.2
222. in the most common arrangement of triple-expansion engines three cylinders are ranged in line, fore and aft, working on cranks at 120° apart. Piston-valve are generally preferred, and these are not uncommonly worked by some form of radial valve-gear instead of the ordinary link-motion. An advantage of this is that the space which would be taken up by eccentrics upon the shaft is saved, and longer main bearings are in consequence possible, without spreading the engines in the fore-and-aft direction. An objectionable feature of the three-cylinder triple engine is its length; on the other hand, the high speed and high pressure which are features of modern practice make long bearings indispensable.
223. to avoid the length of the three-crank engine, Mr Brock and others have made engines of the triple-expansion type with two cranks, by putting the high and the intermediate pressure cylinders above and tandem with two low-pressure cylinders. Mr. Brock has also built four-cylinder quadruple-expansion engines of a similar form (with two cranks), and estimates that they show an economy in coal consumption of 5 per cent as compared with triple-expansion engines working with the same pressure of steam.
224. Steam-jackets are retained by some but not by all builders; where they are employed the boiler steam is usually reduced in pressure before admission to the intermediate and low-pressure cylinder jackets and to the receiver-jackets. The feed-water is frequently heated to about 200° F. by Weir’s plan of condensing in it, by common injection, a quantity of steam taken from the second receiver; this has the advantage of freeing it of air, and of preventing local chilling in the boiler. In present-day practice the boiler pressure, for a triple-expansion engine, ranges from 120 to 170 _ per square inch (by range), and it does not appear that any material increase of this is possible without a complete departure from the present type of marine boiler. On the other hand, without material increase of pressure there is little advantage in quadruple expansion.
225. Surface condensation was introduced in marine engines by S. Hall in 1831, but was not brought into general use until much later. Previous to this it had been necessary, in order to avoid the accumulation of too dense brine in the boiler, to blow off a portion of the brine at short intervals and replace it by sea water, a process which of course involved much waste of heat. By the use of surface condensers it became possible to use the same portion of water over and over again. The very freedom of the condensed water from dissolved mineral substances was for a time an obstacle to the adoption of surface condensers, for it was found that the boiler, no longer protected by a deposit of scale, became rapidly corroded through the action of acids formed by the decomposition of lubricating oil. This objection was overcome by introducing a sufficient amount of salt water to allow some scale to form, and the use of surface condensers soon became universal on steamers plying in sea water. The marine condenser consists of a multitude of tubes, generally of brass, about _ of an inch in diameter. Through these cold sea-water is made to circulate, while the steam is brought into contact with their outside surfaces. In some cases, especially in Admiralty practice, cold water circulates outside the tubes and the steam passes inside.
226. The ordinary marine engine has four pumps:—the air-pump, which is made large enough to serve in case injection instead of surface-condensation should at any time be resorted to; the feed-pump; the circulating-pump, which maintains a current of sea-water through the tubes of the condenser; and the bilge-pump, which discharge any water accumulated by leakage or otherwise in the bilge of the ship. The pumps are so arranged that in the event of a serious leak the circulating-pump can also draw its supply from the bilge. In most engines, especially those of less recent construction, the four pumps are placed behind the condenser, and are worked by a single crosshead driven by a lever, the other end of which is connected by a short link with one of the crossheads of the engine. It is now becoming common to use a small engine, distinct from the main engine, to drive the feed-pump, and to supply circulating water by a centrifugal pump also driven by a separate engine.
227. In the improvement of the marine engine two points are note-worthy,—reduction in the rate of consumption of coal per horse-power, and reduction in the weight of the machine (comprising the engine proper and the boilers) per horse-power. The second consideration is in some cases of even more moment than the first, especially in war-ships. Progress has been made, in both respects, by increase of steam pressure, and, in the second respect especially, by increase of piston speed. Fifty years ago the boilers of marine engines made steam at a pressure of about 5 _ per square inch above that of the atmosphere. By 1860 compound engines were in use with pressures ranging from 25 to 40 _. In 1872 statistics collected for nineteen ocean steamers showed that the average consumption of coal was then 2&Mac250;11 _ per H. P. per hour, the boiler-pressure 45 to 60 _, and the mean piston speed about 375 feet per minute.1 These were for the most part two-cylinder compound engines of the vertical inverted type. Nine years later statistics for thirty engines of the same type showed a consumption of 1&Mac250;83 _ of coal, a mean boiler pressure of 77_ _, and a mean piston speed of 467 feet per minute.2 In recent triple-expansion engines the pressure is as high as 165 _; a piston speed of 700 or 800 feet per minute is not uncommon in naval engines, and in some cases it has risen to over 1000 feet per minute.3 The economy is coal consumption brought about the change form double-expansion engines working at (say) 80 _ to triple engines at 160 _ or mores is variously estimated at from 18 to 25 per cent. Much of this is due simply to the increased ranged of temperature through which the working substance is carried; but it appears that the actual performance of the triple engine is better than that of the double compound in a ratio greater than that by which its ideal efficiency—as an engine using a wider range of temperature—exceeds that of the other; and this is to be ascribed to the same causes as have been already discussed in speaking of the advantage of the compound over the simple engine. Apart from its greater economy of coal, the triple engine owes some of its practical success to the mechanical superiority of three driving cranks over two.
228. The relation of weight of machinery to power developed, and the causes which affect this ratio, have recently been discussed by Messrs Marshall and Weighton,4 from whose paper the following figures are taken. Before the introduction of triple expansion and forced draught the weight of engines in the mercantile marine, including the boilers and the water in them, was 480 _ per I.H.P. In the navy this was reduced, chiefly by the use of lighter framing, with the object of minimizing weight, to 360 _. Triple-engines of the merchant type, without forced draught, are only slightly lighter than double engines; but in naval practice, where forced draught, greatly increased speed, and the use of steel for frames and working parts have combined to reduce the ratio of weight to power, a marked reduction in weight is apparent. A recent set of vertical triple engines, which with natural draught indicate 2200 H.P., and with a draught forced by pressure in the stokehole equal to 2 inches of water indicate 4400 H.P., weigh under the latter condition (along with the boilers) only 155 _ per I.H.P. In another set, in which the draught is forced by a pressure of 3 inches, and the cylinders are only 15_, 24, and 37 inches in diameter, with a stroke of 16 inches, the indicated horse-power is 4200, and the weight of engines and boilers is 136 _ per I.H.P. In these the boilers are of the locomotive type, and the mean piston speed is 1066 feet per minute. Even these light weights are surpassed in smaller engines, such as those of torpedo boats. In so far as this immense development of power from a small weight of machinery is due to high piston speed, it is secured with out loss—indeed with some gain—of thermodynamic efficiency; forced draught, however, without a corresponding extension of the heating surface, leads to a less efficient expenditure of fuel. With a given type of engine there is a certain ratio of expansion which gives a minimum in the ratio of weight to power; when this ratio of expansion is exceeded the engines have to be enlarged to an extent that more than counterbalances the saving in boiler weight; when a less ratio of expansion is used the boilers have to be enlarge to an extent that more than counterbalances the reduction of weight in the engine proper.1
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