engines steam cylinder piston pressure cylinders type triple expansion shaft
ENGINE [STATIONARY ENGINES.
cylinder, which is enclosed within the upper part of the boiler, and the piston are of gun-metal, and work without lubrication. Steam is admitted by an ordinary slide-valve, also of gun-metal, worked by an eccentric in the usual way. The condenser stands behind the boiler; it consists of a number of upright tubes in a box, through which a current of cold water circulates from a supply-pipe at the bottom to an overflow-pipe at the top. In larger sizes of the motor the cylinder stands on a distinct frame, and the boiler has a hopper fire-box, which will take a charge of coke sufficient to drive the engine for several hours without attention. About 6 or 7 /b of coke are burned per horse-power per hour.
From the earliest clays of the rotative engine attempts have Rotary been made to avoid the intermittent reciprocating motion which an engines. ordinary piston-engine first produces and then converts into motion of rotation. Murdoch, the contemporary of Watt, proposed an engine consisting of a pair of spur-wheels gearing with one another in a chamber through which steam passed by being carried round the outer sides of the wheels in the spaces between successive teeth.' In a more modern wheel-engine (Dudgeon's) the steam was admitted by ports in side-plates into the clearance space behind teeth in gear with one another, just after they had passed the line of centres. From that point to the end of the arc of contact the clearance space increased in volume ; and it was therefore possible, by stopping the admission of steam at an intermediate point, to work expansively. The difficulty of maintaining steam-tight connexion between the teeth and the side-plates on which the faces of the wheels slide is obvious ; and the same difficulty has prevented the success of many other forms of rotary engine. These have been devised in immense variety, in many cases, it would seem, with the idea that a distinct mechanical advantage was to be secured by avoiding the reciprocating motion of a piston.2 In point of fact, however, very few forms entirely escape having pieces with reciprocating motion. In all rotary engines, with the exception of steam turbines, - where work is done by the kinetic impulse of steam, - there are steam chambers which alternately expand and contract in volume, and this action usually takes place through a more or less veiled reciprocation of working parts. So long as engines work at a moderate speed there is little advantage in avoiding reciprocation ; the alternate starting and stopping of piston and piston-rod does not affect materially the frictional efficiency, throws no deleterious strain on the joints, and need not disturb the equilibrium of tho machine as a whole. The case is different when very high speeds are concerned ; it is then desirable as far as possible to limit the amount of reciprocating motion and to reduce the masses that partake in it.
A recent interesting and successful example of the rotary Tower's type is the spherical engine of Mr Beauchamp Tower,3 which, like spherical several of its predecessors,4 is based on the kinematic relations of engine. the moving pieces in a Hooke's joint. Imagine a Hooke's joint, uniting two shafts set obliquely to one another, to be made up of a central disk to which the two shafts are hinged by semicircular plates, each plate working in a hinge which forms a diameter of the central disk, the two hinges being on opposite sides of the disk and at right angles to one another. Further, let the disk and the hinged pieces be enclosed in a spherical chamber through whose walls the shafts project. As the shafts revolve each of the four spaces bounded by the disk, a hinged piece, and the chamber wall will suffer a periodic increase and diminution of volume, between limits which depend on the angle at which the shafts are set. In Mr Tower's engine this arrangement is modified by using spherical sectors, each a quarter sphere, in place of semicircular plates, for the pieces in which the shafts terminate. The shafts are set at 135°. Each of the four enclosed cavities then alters in volume from zero to a quarter sphere, back to zero, again to a quarter sphere, and again back to zero, in a complete revolution of the shafts. In practice the central disk is a plate of finite thickness, whose edge is kept steam-tight in the enclosing chamber by spring-packing, and the sectors are reduced to an extent corresponding to the thickness of the central disk. One shaft is a dummy and runs free, the other is the driving-shaft. Steam is admitted and exhausted. by ports in the spherical sectors, whose backs serve as revolving slide-valves. It is admitted to each cavity during the first part of each periodical increase of the cavity's volume. It is then cut off and allowed to expand as the cavity further enlarges, and is exhausted as the cavity contracts. If the working shaft, to which the driven mechaniim serves as a fly-wheel, revolves uniformly, the dummy shaft is alternately accelerated and retarded. Apart from this, the only reciprocating motion is the small amount of oscillation which the comparatively light central disk undergoes.
Another rotary engine of the Hooke's-joint family is Mr Fieldhies,' in which a gimbal-ring and four curved pistons take the place of the disk. Two curved pistons are fixed on each side of the gimbal-ring, and as the shafts revolve these work in a corresponding pair of cavities, which may be called curved cylinders, fixed to each shaft.
Steam 212. Attempts have been made from time to time to devise turbines. steam-engines of the turbine class, where rotation of a wheel is produced either by reaction from a jet of escaping steam or by impact of a jet upon revolving blades. A revolving piece which is to extract even a respectable fraction of the kinetic energy of a steam jet must move with excessive velocity. In Mr C. A. Parsons's steam-turbine this difficulty is overcome and a moderate degree of efficiency is secured by using a series of central-flow turbine wheels, in the form of perforated disks, all on one shaft, with fixed disks between which are perforated to serve as guide-blades. Steam passes from end to end of the series, giving up a small portion of its energy to each, but retaining little at the end.
XII. MARINE ENGINES.
Types of 213. The early steamers were fitted with paddle-wheels, Side- long a favourite with marine engineers. In the side-lever lever engine the cylinder was vertical, and the piston-rod proengines. jected through the top. From 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 other ends by a crosstail, from which a connecting-rod was taken to the crank above. The side-lever engine I is now obsolete.
braced-beam supported on A frames above the deck, are still common in river-steamers and coasters.
Steeple- 214. An old form of direct-acting paddle-engine was engines. 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.
Oscillat- 215. Modern paddle-wheel engines are usually of one ing of the following types. (1) In oscillating cylinder engines engine' 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 is inclined instead 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 presDiagonal sures of steam. (2) Diagonal engines are direct-acting engines. engines of the ordinary connecting-rod type, with the cylinders fixed on an inclined bed and the guides sloping up towards the shaft.
navy, the engine is shortened by attaching the connecting- Trunk rod directly to the piston, and using a hollow piston-rod, engines. 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.
The return-connecting-rod engine is another hori- Return- zontal form much used in the navy. It is a steeple-engine connect- placed horizontally, with two, and in some cases four, i piston-rods in each cylinder. The piston-rods pass clear erd of the shaft and the crank, and are joined beyond it in a guided crosshead, from which a connecting-rod returns.
Ordinary horizontal direct-acting engines with a short Horizon- stroke and a short connecting-rod are also common in wartal directships, where the horizontal is frequently preferred to the eaeti?ig n vertical type of engine for the sake of keeping the machinery engines. below the water-line. In horizontal marine engines the air-pump and condenser are generally placed on the opposite side of the shaft from the cylinder, which balances the weight and allows the air-pump to be driven direct.
In merchant ocean-steamers one general type of en- Inverted gine is universal, and the same type is now to an increasing vertical extent adopted in naval practice. This is the inverted verti- engines. cal 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," Example. 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-pump. 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 guides are fitted on the side on which the crosshead hears 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 from heating.
The crank-shaft is of Vicker's steel, 17i inches in diameter. The condenser is in the place it usually has in engines of this type, - in the lower ;girt of the back frame, with its tubes running horizon. , tally front end to end of the s, y=engine. There are 1400 tubes, of The air-pumps are of the singleby a lever from 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 lb, 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.
Two other arrangements of double compound (as distin- Tandem guished from triple-expansion) marine engines of the inverted vertical vertical type require notice. One is the tandem arrangement, engines. 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 pairs 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 1200 apart,' an arrangement greatly superior to that of two cranks in uniformity of effort on the shaft. To facilitate removing the pistons from the cylinders, the large cylinder has in some cases been set above the other.
Three- 220. The other arrangement of double compound marine engine cylinder has three cylinders set in line fore and aft. The middle one is the arrange- high-pressure cylinder ; the other two receive steam from it, and feet. These engines, which were built just before the introduction of triple expansion, are supplied with steam at a pressure of 110 lb by gauge, and indicate 14, 300 horse-power.
In this and iu 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.
Triple. 221. During the last two or three years a expan- great advance has taken place in marine sion engineering by the general introduction of engines. triple-expansion engines, and by an increase in steam pressure which the system 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 by the failure of the boilers, which were of brought into general use a special type. Another experiment with until much later. Pretriple engines in the yacht "Isa" in 1877 1 i vious to this it had been avoid the accumulation ship "Aberdeen," with triple engines, de- of too dense brine in the signed by Mr Kirk, to work with steam of, 01415:#7-1() boiler, to blow off a pordouble-ended steel boilers of the II ll ordinary marine type, demonstrated the advantage and safety of the system. Since then its use has become general in new steamers, and iu many cases the older double engines are being removed to give place to engines of the triple-expansion type, with the effect of reducing the consumption of coal by about 25 per cent.2 objectionable feature of the FIG. 135. - End Elevation of Marine Engine.
three-cylinder triple engine is of water over and over again. The very freedom of the condensed I See description of tics engines of the "City of Rome," with three 46-inch water from dissolved mineral substances was for a time an obstacle and three 86-ineh cylinders, with a stroke of 6 feet, working up to 11,890 I. 11. P., Proc. Inst. Mech. Eng., 1850. to the adoption of surface condensers, for it was found that the • 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 Quad-engines of a similar form (with two cranks), and esti- ruple-exmates that they show an economy in coal consumption pansion of 5 per cent. as compared with triple-expansion engines engines. working with the same pressure of steam.
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 lb per square inch (by gauge), 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 a material increase of pressure there is little advantage in quadruple expansion.
/tirtrine 226. The ordinary marine engine has four pumps: - the air-pump, substance is carried ; but it appears that the actual performance of engine which is made large enough to serve in case injection instead of the triple engine is better than that of the double compound in a pumps. surface-condensation should at any time be resorted to; the feed- ratio greater than that by which its ideal efficiency - as an engine which discharges any tubes of the condenser; and. the bilge-pump, pump; the circulating-pump, which maintains a current of sea- using a wider range of temperature - exceeds that of the other; and water through the this is to be ascribed - of tubes, generally of brass, about i of an inch in diameter. over 1000 feet per minnte.3 The economy in coal consumption Through these cold sea-water is made to circulate, while the steam brought about by the change from double-expansion engines work- is brought into contact with their outside surfaces. In some cases, big at (say) 80 lb to triple engines at 160 lb or more is variously especially in Admiralty practice, cold water circulates outside the estimated at from 18 to 25 per cent. Much of this is due simply tubes and the steam passes inside. to the increased range of temperature through which the working water accumulated by the advantage of °the leakage or otherwise compound over the in the bilge of the simple engine. Apart ship. The pumps are Ifrom its greater ecoso arranged that in the event or a serious leak _ engine owes some of its 7., the circulating-pump practical success to the can also draw its sup- F mechanical superiority ply from the bilge. In I 'Ai of three driving cranks most engines, especi- over two.
ally those of less re- .. . via,: ..
• II _. 228. The relation of Relation behind the condenser, MIliismii-j1 iv 4 11. to power developed, to power.
and the causes which and are worked by a iiry -, Kt, 6-.1 : affect this ratio, have single crosshead driven .lia ■.
end of which is con- Ail% 111 Fifieft:IS I _, Ira .11 recently been discussed by Messrs Marshall and netted by a short link L. paper the following with one of the crossby a lever, the other gm figures are taken. Beheads of the engine.
pill, • 103- fore the introduction It is now becoming common to use a small L -1'4'41 rfp ,r.. of triple expansion and forced draught the engine, distinct from --las - : um Salt Walt ' wirils,,sillr ild weight of engines in ""TARI the main, engine to - - t t, .,...,t k including eel imercantileiding the e marine,boilers N 1, and to supply circulat- 1 rIvi and the water in them, ing water by a centri- i- II El .4.1111°“- ii B was 480 lb per I.H.P.
where forced draught, the boilers) per horse- greatly increased speed, power. The second , and the use of steel for cases of even more a I, , frames and working consideration isin some ' parts have combined moment than the first, to reduce the ratio of especially in war-ships. l weight to power, a made, in both respects, by increase of steam L_- marked reduction in weight is apparent.
- , • indicate 2200 II.P., triple engines which with natural 'draught piston speed. Fifty and with a draught marine engines made about 5 /b per square steam at a pressure of aiiiitpa x the stokehole equal to years ago the boilers of _ -, , forced by pressure in 1.1 1 (tate 4000 H. P., weigh inch above above that of the atmosphere. By 1860 135. - Longitudinal Section of Marine Engine. tion (along with the compound engines were boilers) only 155 lb per in use with pressures ranging from 25 to 40 lb. In 1872 statistics I.H.P. In another set, in which the draught is forced by a prescollected for nineteen ocean steamers showed that the average con- sure of 3 inches, and the cylinders are only 1511, 24, and 37 inches sumption of coal was then 2'11 lb per H.P. per hour, the boiler- in diameter, with a stroke of 16 inches, the indicated horse-power pressure 45 to 60 lb, and the mean piston speed about 375 feet per is 4200, and the weight of engines and boilers is 136 It per I.H.P. minute.' These were for the most part two-cylinder compound In these the boilers are of the locomotive type, and the mean piston engines of the vertical inverted type. Nine years later statistics for speed is 1066 feet per minute. Even these light weights are surthirty engines of the same type showed a consumption of 1.83 lb of passed in smaller engines, such as those of torpedo boats. In so coal, a mean boiler pressure of 77i lb, and a mean piston speed of 467 far as this immense development of power from a small weight of feet per minute.2 In recent triple-expansion engines the pressure is 11) achinery is duo to high piston speed, it is secured without loss as high as 165 lb ; a piston speed of 700 or 800 feet per minute is - indeed with some gain - of thermodynamic efficiency ; forced not uncommon in naval engines, and in some cases it has risen to draught, however, without a corresponding extension of the beating Loc. cit.
i Sir F. J. Bramwell, Proc. Inst. Mech. Eng., 1872.
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 au extent that more than counterbalances the saving in boiler weight ; when a less ratio of expansion is used the boilers have to be enlarged to an extent that more than counterbalances the reduction of weight in the engine proper.'