CHAPTER XVIII.

ARRANGEMENT OF THE CYLINDERS OF COMPOUND, TRIPLE AND QUADRUPLE EXPANSION ENGINES.

THE type used for modern engines in the mercantile and Royal navies is either the triple or quadruple expansion engine, by which with high-pressure steam considerable advantage is gained over the earlier types of engines as regards economy, in reduction of heavy stresses on the machinery, and in other respects.

We will first describe the reasons for the superiority of the old compound over the simple engine, and the arrangement of cylinders in the former, as this will explain the similar reasons which, with increased steam pressure, subsequently led to the abandonment of the compound type and the introduction of triple, and also large numbers of quadruple expansion engines. Only one quadruple expansion engine above the steam pinnace size has so far been fitted in the Navy, viz. that of No. 90 first class torpedo boat, but their use in the mercantile marine is gradually extending.

The principal difference between the mechanism of the old simple expansion engines and that of triple expansion and other stage expansion engines is in the arrangement of the cylinders, the other parts being generally the same. In the simple expansion engine the steam enters each cylinder direct from the boilers, and at the end of each stroke is exhausted direct into the condenser. In the compound engine the steam from the boilers is only admitted direct to the smaller or high-pressure cylinder, and at the end of the stroke in that cylinder, instead of passing direct to the condenser, the steam enters a larger cylinder, called the low-pressure cylinder,' in which the expansion is completed, after which the steam passes as before to the condenser.

As will have been gathered from Chapter XIV., the compound engine has now been generally superseded by the triple expansion engine, in which the steam from the boilers is admitted to the highpressure cylinder, from whence it is led to a larger cylinder called the intermediate-pressure cylinder,' in which it expands further and performs more work. On being exhausted from the intermediate cylinder, it is conducted, as in the previous case, to a still larger cylinder, called the 'low-pressure cylinder,' in which its expansion is completed, and on being discharged from this cylinder it proceeds to the condenser.

Two-cylinder compound engines. Two principal types of twocylinder compound engines used to be fitted. The type shown in Fig. 184, generally known as the tandem' type, has certain advantages

and was largely adopted, especially for engines of large power. Two pairs of cylinders were fitted, so that large powers were obtained without introducing castings of extraordinary complexity. It was also the readiest form to which a simple engine could be converted.

This sketch shows the cylinders of a horizontal tandem compound engine with return connecting rods, numerous examples of which are still running in the Navy. In this type of engine a defect was that the clearance spaces in the high-pressure cylinders were very great, which decreased the expansive efficiency and caused considerable waste of steam.

The simpler and more usual arrangement was that with the high

and low-pressure cylinders placed side by side, the pistons acting on cranks at right angles to each other.

Three-cylinder compound engines. The ordinary three-cylinder compound engine is simply a modification of the type just described, but instead of a single low-pressure cylinder, two cylinders are used, the steam on exhausting from the high-pressure cylinder to the receiver being conducted to two low-pressure cylinders.

The three-cylinder type of compound engine was used when the power was so great that the employment of a single low-pressure cylinder would be inexpedient on account of its unwieldy dimensions, so that the division of the work between two low-pressure cylinders is

preferable. The angles at which the cranks of three-cylinder compound engines were placed with respect to each other were very varied. In the majority of cases they were set at equal angles of 120°, but various other arrangements of cranks were common, depending on the opinion of the makers as regards regularity of twisting moments, and distribution of steam. It is, however, doubtful if any of these variations possessed any practical advantage over that of placing the cranks at equal angles with each other.

Definition of the term 'receiver.'- By the term 'receiver' is to be understood in the case of a compound engine the whole of the space between the high-pressure piston when at the end of its stroke, and the back of the low-pressure slide-valve or valves, comprising the volumes of the steam and exhaust passages of the highpressure cylinder, the exhaust pipes from the high-pressure cylinder to the low-pressure valve casings, and the low-pressure valve casings themselves.

In the case of a triple expansion engine, the space between the highpressure piston at the end of its stroke and the intermediate slide-valve is called the 'intermediate receiver,' and that between the intermediate piston at the end of its stroke and the low-pressure slide-valve the 'lowpressure receiver.'

Capacity of receivers.-Large reservoirs or receivers for the steam between the cylinders were usually fitted to the first compound engines, but experience proved that they were not required, all that was found necessary being a comparatively large exhaust pipe from the eduction orifice of the high-pressure cylinder to the steam inlet of the low-pressure cylinder, the volume of the exhaust passage and pipe from the highpressure cylinder and the low-pressure valve casing being sufficient to allow for the compression that takes place between the release from the high-pressure cylinder and admission to the low-pressure cylinder. Similar remarks apply to the receivers of triple and quadruple expansion engines, and the volumes of these spaces which are necessary for other reasons are found to be sufficient for receivers. Most modern engines are made in this way.

The capacity of the receivers is immaterial so far as the total power of the engines is concerned, its effect being shown on the back pressure line of the diagram from the preceding engine, which becomes more nearly straight, and on the admission line of the diagram from the succeeding engine, which becomes more nearly parallel, to the atmospheric line as the volume of the receiver is increased.

Influence of size of cylinder on the power of stage expansion engines. The power of any stage expansion engine, working at any given rate of expansion, depends entirely on the dimensions of its lowpressure cylinders, and is not affected by the size of the high-pressure cylinder, which must only be regarded as carrying out one stage in the expansion. The capacity of the low-pressure cylinder or cylinders of such an engine requires to be the same as that of the whole of the cylinders of a simple expansion engine of the same power working at the same initial pressure of steam and total ratio of expansion. Neglecting for the moment the complicating effects of clearance and compression, this will be easily seen from the consideration that since the initial pressures and the ratios of expansion are the same, the final pressures

and volumes must be identical in the two cases. In the simple engine the whole of the steam at the end of the expansion fills all the cylinders, whilst in the compound engine it is contained by the low-pressure cylinders only. Consequently the capacity of the low-pressure cylinders of the compound engine must be equal to the capacity of all the cylinders of the simple expansion engine.

Mechanical advantages of compound and triple expansion engines. A great advantage of the stage expansion engine, so far as its mechanism is concerned, is the facility with which it allows high rates of expansion of steam to be carried out without bringing excessive stresses on the framing. As an example, if we consider the cases of two engines working at the same number of revolutions, one simple, the other compound, each supplied with steam of 60 lbs. initial pressure, and developing 2,100 I.H.P. with a total rate of expansion of 8 times, we shall find that whilst the maximum turning moment in the case of the compound engine is 960 inch-tons, it would be 1,250 inch-tons in the engine with simple expansion, or more than 30 per cent. greater, the mean moment and therefore the horse-power being the same in the two engines.

In consequence of the greater uniformity of twisting moment, the shafting and framing may be made lighter in the compound than in the simple engine, and much greater steadiness of motion may be obtained, and more efficient action of the propeller in the water expected. The great variations of pressure to which the shafts of simple engines are exposed when worked at high rates of expansion appear to produce the same effect on the material that vibration does, viz. to cause the structure to become crystalline. Several cases of broken shafts in engines of this class were attributed to the excessive intermittent stresses brought on them.

In the compound engine, although the steam is expanded 8 times when developing full power, it can be expanded still more when working at reduced powers, whereas in the non-compound engine, the steam being expanded in a single cylinder, it cannot be expanded much more than 8 times, whatever the reduction in the power may be. This results from the necessary mechanical arrangements, and is altogether independent of any loss of efficiency that would ensue from liquefaction, &c., when attempting to carry out a high rate of expansion in a single cylinder.

The superiority of the stage expansion engine is further demonstrated as the engine becomes worn. When the slides and pistons begin to leak, the loss in the simple engine is much greater than in the compound, in consequence of the greater difference of pressure in the former.

The steam leaking past the piston in the simple engine goes direct to the condenser without doing any useful work, whilst in the compound engine the steam leaking past the high-pressure piston does useful work in the low-pressure cylinder before passing to the condenser, and the amount of leakage in the low-pressure cylinder is reduced on account of the considerably smaller difference of pressure on the two sides of the piston in that cylinder.

Use of expansion valves and independent expansion fittings.— Since the cylinders of triple expansion and compound engines provide

in themselves for a considerable amount of expansion, special cut-off or expansion valves are now dispensed with, thus reducing the complexity and number of parts, as compared with the simple engine, in which expansion valves, suitable for early cut-off, are a necessity when high-pressure steam is used.

Many of the early compound engines, however, were fitted with expansion valves on the high-pressure cylinder, and some had expansion valves on the low-pressure cylinder also, in order to regulate the proportionate amount of work done by the two cylinders and equalise the stresses on the machinery. Without this valve, or some equivalent, at very low powers, as in warships on ordinary service, the work done in the low-pressure cylinder becomes very small. By setting the lowpressure expansion valve to an early cut-off, the pressure in the receiver, which forms the back pressure in the high-pressure cylinder, would be increased, so that the work done in that cylinder would be diminished and that in the low-pressure increased, and the power would consequently be more equally divided between the two cylinders.

Separate expansion valves are not now fitted to the cylinders, but to allow of adjustment in the points of cut-off, the reversing arms of the engines are now fitted with sliding blocks to enable the slide-valves to be linked up independently of the high-pressure valve, so as to vary the amount of expansion (see Chapter XV.).

Triple expansion engines generally. The arguments which prove the superiority of the ordinary compound engine over the simple expansion engine when the working steam pressures were increased from 30 to 60 lbs. per square inch, also explain the superiority of the triple expansion engine over the compound engine for steam pressures above 120 lbs. per square inch. The principal gain in each case is the increased economy due to the greater amount of expansion conveniently obtained, and the reduction of the variation in temperature of the cylinders, which decreases the loss from liquefaction. Further, as previously mentioned, there is a more regular turning moment on the shafting, and a great reduction of maximum stresses on the engine and framework.

Some of the forms in which the triple expansion system has been carried out are illustrated in Figs. 185 to 195.

The arrangement shown in Fig. 185, has the high-pressure and intermediate pistons on the same rod, with the low-pressure acting on a separate crank at right angles to the other. Though convenient in some cases, this cannot be considered altogether satisfactory, as the stresses on the crank-pins would be very unequal.

In Fig. 186, each of the cylinders is fitted over a separate crank, the high, intermediate, and low-pressure cylinders being arranged in succession. This is the usual arrangement of triple expansion engines of moderate power both in the Royal Navy and mercantile marine. The cranks are arranged at equal angles with each other, although other arrangements have sometimes been fitted. The direction of revolution when going ahead may be either with the high pressure in advance of the intermediate, or the reverse.

Four-cylinder triple expansion engines. For large powers, especially with quick-running engines, the low-pressure cylinder becomes so large as to require to be divided into two parts, although this generally