342 80 1 and to T up to the point showZ ETA forward the sam.: until close to the de passage is opened, mis sphere, there is no before, and its piston 20,2 instantaneously make te expe i 3. 66, is a diagram from the top side of a pist 1 marine and limited to an mencement of oke. In these arge mass to be servoir of power the travel of the rly advantageous. additional weight se, compensated by developing a given t of when estimating nder engines. or is worked entirely by urther modification into y Messrs. STEWART and ally acts on both pistons off a Compound Engine. f the line of expansion; at f the education, such as in figure 66, to escape into the atmosphere. From listance from it to the atmospheric line re op side of the piston during its upwards iagrams from non-condensing engines the wn at the top, and it is well to draw the e engineer may be able to see at a gla , and to compare with it the amount of ploathe back pressure resisting the motio pressure of the atmosphere, but it r monly in non-condensing engines 5 lbs. above the atmospheric line, andress altogether. nd then, in like manner, we e engine (see Rule CXXXII, y the sum of the two. a diagram from the high pressure Le bottom of the diagram representing idicating that the exhaust steam has a of this pressure can be easily measured ressure upon the high pressure piston, undergoes on being exhausted into the of this jacket, or receiver, the greater is on of pressure, or back pressure upon the of paramount importance in a compound Sloped by both cylinders, inasmuch as the piston, the less will be the power developed wer will be developed by the low pressure. xhaust at, or under the atmospheric line, the less will be the power of the low pressure ea of the jacket, or receiver, should be such ertain portion of the stroke, and exhausted that will enter the low pressure cylinder y 80, power to the high pressure cylinder, m from the low pressure cylinder of a comort distance the piston is acted upon by steam ve the atmospheric line), this steam is the high to the amount of the back pressure; the low ve invariably a "throttled look." This is owing apon, and consequent great expansion, the initial, are the advantages fund a be in most of the large samship companies have had the enge ginie egne existing in der ships replaced by ecmpornd engines In this class of engine the seen from the bler Et ectas the small finder at a andica, vieriixde from one-third to one-half the stroke, and the seem having pressed the first pisson to the end of the stroke in the manner of a high pressure piston, escapes or exhausts into a jacket, or into a low pressure wing, whence is then enters into another cylinder of larger dimensions, and completes in work there by expanding to the end of the strike, being fly condensed in the usual manner. The steam is expanded by this two-itud operation to about six or eight times its origizal volume The pressure urging the first or high pressure piston, is consequently the difference of pressure between the steam in the boiler and that in the second cylinder; and the pressure urging the second or low pressure cylinder piston, is the difference of pressure between the steam on the eduction side of the high pressure cylinder and that of the vapour in the condenser. There will be a small difference between the pressure in the communicating parts of the high and low pressure engines, just as there is a small difference between the vacuum in the cylinder and that in the condenser. But, in well constructed high pressure engines this difference will not sensibly detract from the power. The arrangements of the compound engine which have been from time to time brought into use are various. In one arrangement the position of the low pressure cylinders is side by side, as for those of the ordinary kind; in some cases annular cylinders are used, viz., the high pressure cylinder within that of the low pressure. Another arrangement is that in which the high pressure or small cylinder is erected on the top of the large or low pressure cylinder, having a fly wheel to prevent sticking on the centre. A third arrangement has the smaller cylinder at the fore end of the larger. A fourth example consists of four cylinders, the two high pressure above the low, with the double cranks. The best and most convenient arrangement is the two cylinders, connected each to its crank, and having the cranks from 90° up to 135°, with starting valves. Of course there is no theoretical gain in the use of two cylinders. The economical advantages of expansion is the same, whether developed in a single cylinder or a double cylinder engine. When, however, expansion is carried to any great extent in a single cylinder, there is a great irregularity of pressure at different parts of the stroke. The initial pressure at the beginning of every stroke produces violent concussion through the whole of the machinery, and in proportion to the degree of expansion used, the evil will increase. When it is wished to attain a high degree of expansion in marine and other rotatory engines, with a single cylinder, the amount is limited to an extent of three or four times, from the great strain at the commencement of the stroke, compared with the mean force of the entire stroke. In these engines the initial blow is not absorbed by the inertia of a large mass to be put in motion at the beginning of the stroke, forming a reservoir of power for the steam while expanding through the remainder of the travel of the piston. Compounded engines are, in this case, peculiarly advantageous. Of course, steam engine economy almost always involves additional weight and expense in the engine. This is, however, in this case, compensated by the reduced number of boilers and furnaces required in developing a given amount of power-a fact which is not seldom lost sight of when estimating the relative advantages of compounded and single cylinder engines. In the ordinary compound engine the larger cylinder is worked entirely by the exhaust steam from the smaller; but there is a further modification into what is called the continuous expansion principle, by Messrs. STEWArt and NICHOLSON, in whose arrangement the steam partially acts on both pistons at the same time. (See Figs. 80 and 81). 310. To calculate the power from a Diagram off a Compound Engine.We first determine by a diagram and a computation, such as in figure 66, the power exerted by a high pressure engine, and then, in like manner, we determine the power exerted by the low pressure engine (see Rule CXXXII, pages 343-344). The total power is obviously the sum of the two. Explanatory Diagrams, Figs. 68 and 69, represent a diagram from the high pressure cylinder of a compound engine. It will be seen that the bottom of the diagram representing the exhaust line is above the atmospheric line, thus indicating that the exhaust steam has a pressure above the atmospheric pressure (the amount of this pressure can be easily measured off the scale); it represents the back or resisting pressure upon the high pressure piston, and is due to the amount of expansion the steam undergoes on being exhausted into the jacket, or receiver; therefore, the larger the area of this jacket, or receiver, the greater is the amount of expansion and consequent diminution of pressure, or back pressure upon the high pressure piston, but this back pressure is of paramount importance in a compound engine; in this way it equalizes the power developed by both cylinders, inasmuch as the greater the back pressure on the high pressure piston, the less will be the power developed by the high pressure engine, but the greater power will be developed by the low pressure engine; and should the high pressure engine exhaust at, or under the atmospheric line, the greater will be its power, consequently, the less will be the power of the low pressure engine; therefore, in compound engines, the area of the jacket, or receiver, should be such as when the high pressure steam is cut off at a certain portion of the stroke, and exhausted into this jacket, or receiver, the pressure of steam that will enter the low pressure cylinder shall be such as will develop in it equal, or nearly so, power to the high pressure cylinder, otherwise the engines are badly designed. The other diagram represents the usual form from the low pressure cylinder of a compound engine. It will be seen that for a short distance the piston is acted upon by steam above the pressure of the atmosphere (or above the atmospheric line), this steam is the high pressure exhaust, and is nearly equivalent to the amount of the back pressure; the low pressure diagrams of compound engines have invariably a "throttled look." This is owing to the large area of piston the steam acts upon, and consequent great expansion, the initial, or entering pressure, being low. |