194 STEAM ENGINES 



cylinder 20 in. in diameter, and a stroke of 15 in. would be 

 referred to as an 11" and 20" XI 5" compound engine. In 

 the same way, a 14", 22", and 34"X18" triple-expansion 

 engine would be one in which the diameters of the cylinders 

 are 14 in., 22 in., and 34 in., and the stroke is 18 in. 



Cylinder Ratios. The cylinders of compound and multiple- 

 expansion engines increase in diameter from the high-pressure 

 to the low-pressure end, and it is customary to refer to their 

 relative sizes by means of cylinder ratios. As all the cylinders 

 have the same length of stroke, the volumes of the several 

 cylinders are in proportion to the areas of the cylinders, and 

 therefore in proportion to the squares of the diameters. The 

 area of the high-pressure cylinder is taken as unity, and the 

 other areas are referred to it, and the ratios of these areas, or 

 the ratios of the squares of the diameters, are called the 

 cylinder ratios. For example, a triple-expansion engine having 

 cylinders 12 in., 20 in., and 34 in., in diameter will have the cyl- 

 inder ratios of 12* : 20" : 34=, or 144 :400 : 1,156, which reduces 

 to 1 : 2.78 : 8.03; that is, the intermediate cylinder is 2.78 times 

 as large as the high-pressure cylinder, and the low-pressure 

 cylinder is 8.03 times as large as the high-pressure cylinder. 

 If there are two cylinders to one stage of expansion, as, for 

 example, two low-pressure cylinders, the sum of their areas 

 must be used in rinding the cylinder ratios. Thus, if there 

 had been two 24-in. low-pressure cylinders instead of one 34-in. 

 cylinder, in the foregoing case, the cy Under ratios would have 

 been 12* :2Q2 : 2X24 2 , or 144 : 400 : 1,152, which reduces to 

 1 : 2.78 : 8. 



Horsepower of Compound Engines. The indicated horse- 

 power of a compound or triple-expansion engine may be cal- 

 culated from the indicator diagrams in exactly the same manner 

 as with any simple engine, considering each cylinder as a simple 

 engine and adding the horsepowers of the several cylinders 

 together. In taking the indicator cards from a compound 

 engine, the precaution of taking the cards simultaneously 

 from all cylinders must be observed, especially when the engine 

 runs under a variable load, because, otherwise, an entirely 

 wrong distribution of power may be shown, and there may also 

 be a great variation between the indicated horsepower really 



