May io, 1889.] 



SCIENCE 



355 



THE WESTINGHOUSE COMPOUND ENGINE. 



The American public is probably now fully prepared to accept 

 compounding as the one and only road to the highest fuel economy 

 in steam-engines. Compounding is almost universal among Eu- 

 ropean manufacturers, extending down to engines of the smallest 

 size, and has been forced upon them by the close margin of manu- 

 facturing protit there obtaining. The larger profits and freer 

 methods which have ruled in this country, and particularly the 

 great complication and prohibitive cost which follow the com- 

 pounding of the ordinary automatic engine, have led to the almost 

 universal adoption of the single cylinder. 



It is not proposed to enter into a treatise upon compounding. 



the terminal pressure increases ; which means, that, when the 

 steam is finally thrown away, it still has in it, say, twenty pounds 

 of available pressure above the atmosphere, or thirty-two pounds 

 above effective vacuum, which is a dead waste that ought to be 

 preserved, and converted into work. If, now, we seek to lower the 

 terminal pressure in order to waste less exhaust pressure, we not 

 only cut down the power of the engine enormously, but at once in- 

 troduce the element of excessive internal condensation in the cylin- 

 der, — a most insidious and fatal enemy of economy. Internal 

 condensation is due to the fact that the immediate internal surfaces, 

 of the cylinder, cylinder-heads, and piston, are subjected at each 

 stroke to a wide fluctuation of temperature, ranging from, say 330", 

 the temperature of the steam admitted from the boiler, to 212°, the 



WESTINGHOUSE COMPOUND ENGINE, FRONT VIEW. 



Every one knows that to compound an engine a second cylinder of 

 three or four times the piston area is added, called the low-pressure 

 cylinder, into which the exhaust steam of the first or high-pressure 

 cylinder, instead of being thrown away, is passed, and made to 

 yield a further amount of work. The additional work thus ob- 

 tained is roughly proportional to the mean effective pressure in the 

 low-pressure cylinder, multiplied by the difference in area of the 

 two pistons. By this means the power of the engine is increased, 

 and the steam, when finally exhausted, is at a pressure so low that 

 less unused work remains in it. The maximum possibilities of 

 economy are thus secured. But why cannot the same result be 

 reached by further expansion in a single cylinder? A single cylin- 

 der, in the performance of its work, must choose between the two 

 horns of a dilemma. 



It has been found in practice that there is a certain load which is 

 the most economical in a single cylinder. If the load increases. 



temperature of the exhaust. The earlier the cut-off, the lower the^ 

 terminal pressure and corresponding temperature, and the greater 

 the amount of steam required to re-heat the surfaces : hence the- 

 greater the condensation. Hence any considerable departure in 

 either direction from the rated power of a single-cylinder engine 

 means a sacrifice of economy, — waste of exhaust pressure if over- 

 load, and loss from condensation if under-load. The compound 

 engine, therefore, economizes by getting additional work out of the 

 exhaust steam, which would otherwise be wasted ; and by dividing 

 the fluctuations of temperature between two cylinders, compelling: 

 one-half the variation to take place in each cylinder, thereby re- 

 ducing internal condensation in the ratio of the squares, namely, tO' 

 one-quarter of that due to a single cylinder. 



For the Westinghouse compound engine, figures of which illus- 

 trate this article, it is claimed that it not only exceeds the economi- 

 cal performance of any single-cylinder engine, but of any other 



