CALCULATION OF WORK DONE IN A COM

POUND ENGINE.

In the working of a compound engine, where the small cylinder exhausts into the large one, the work done in a stroke depends on the size of the large cylinder, and is the same as that which would be performed in a single cylinder of the same content, by expanding to the same extent from a like initial pressure.

This proposition is easily proved, and any one may satisfy himself that it is approximately true by examining a well-formed indicator diagram as taken from a compound engine.

Referring to Fig. 15, which is from an engine having a high-pressure cylinder 18 inches in diameter, with a 6 feet stroke, and a low-pressure cylinder of 36 inches in diameter, with a stroke also of 6 feet, the number of revolutions being 34 per minute.

Since the lengths of stroke are the same, and the areas of the pistons are as 1 to 4, it follows that the indicator diagram marked A, as taken from the high-pressure cylinder, would be reduced to the same scale as that from the lowpressure cylinder marked B, if we supposed the diameter of the latter cylinder to be 36 inches and the stroke feet, or one-fourth of that which it really is.

This result is set out in Fig. 15.

The diagram marked A is reversed in position and repeated on the right hand side by measuring off a series of horizontal lines, such as cd, and making c d equal 4 CD in every case.

In this way the upper shaded area represents the work done in the high-pressure cylinder as it would appear on the scale adopted in the lowpressure cylinder. The bottom shaded area is merely a repetition of the area B.

When the two diagrams are put together it will be seen that the two portions of the expansion curves fit very fairly or run into one, and that the expansion commenced above d is carried on throughout the stroke.

It will be noticed that there is a little want of similarity between this diagram and Fig. 16. Here the steam line in b is horizontal at first, and then slopes downwards.

That it is horizontal at all is owing to some peculiarity in passing the steam from one cylin

der to the other, as there should be the slope of an expansion curve throughout. But any devia

tion from theoretical proportions does not affect the general inference to be drawn from the two diagrams when viewed together, and we see that the expansion which has occurred in the highpressure cylinder might very well have taken place in the low-pressure cylinder, as something which preceded the actual expansion therein.

ENGINES WITH CRANKS AT RIGHT ANGLES.

For many purposes it is enough to have an engine with a single steam cylinder, or an equivalent engine, with a pair of cylinders acting as one only, but on the other hand, there are numerous instances where two engines should be placed side by side and work cranks at right angles to each other.

This is particularly the case in applying steampower to flour mills or to cotton mills, where it is of consequence to preserve the rotative pres

sure on the crank as nearly uniform as possible, and to maintain a smooth and even motion, or again in marine engines, for convenience of starting in any position, the same rule would hold; and before proceeding further it may be useful to point out the reason for the greater uniformity of rotative pressure which is a consequence of working with a pair of cranks at right angles.

That the variations of tangential pressure on the crank of a direct-acting engine are represented by the vertical lines on a diagram similar to that shown by the dotted curve bf in Fig. 17.

Putting a series of such curves end to end, we obtain a graphical indication of the fluctuations of tangential pressure during the working of an engine with one cylinder.

The force is zero at dead point, and rises to fe, its greatest value, after which it sinks again

to zero.

But if there be a pair of cranks at right angles, a second series of diagrams of rotative pressures must be superposed upon the first series, as shown

by the second set of dotted curves, whereof one portion is marked be, and the final result is exhibited by the upper line, not dotted, which is obtained by adding together the pairs of ordinates at each point. For example:

NM+M R M S.

A B+A B=A C.
E F+0=E F.

The greater uniformity of rotative force is apparent, and it would be improved by cutting off at half stroke in each cylinder, for then the curve be would be hollowed out and reduced, while the part bf would be unaffected, and the upper resultant wavy line would become more nearly horizontal.

By proceeding in this manner it is easy to set out a diagram of the rotative pressure upon the cranks of any pair of engines working under given conditions.

In applying these principles to direct-acting engines, where two cranks at right angles are to be connected with the cylinders, there are different methods for adoption, each of which has its advocates. One plan very commonly met with has been to place the high and low pressure cylinders in pairs, with their axes in the same straight line, so that one piston rod serves for both. Thus, in marine engines, with the cylinders vertical, there may be

(1) The high-pressure cylinder above the lowpressure cylinder.