Maech 7, 1902.] 



SCIENCE. 



ysi: 



unit of time will be the same for all. 

 Efficiency remains the same while the 

 quantity of working fluid circulating will 

 vary inversely as the quantity of its 

 'latent' heat of vaporization. 



In the actual engine, conditions external 

 to the cycle determine the relative desir- 

 ability of available fluids and extra-ther- 

 modynamic circumstances control. Since 

 the machine has weight and is subject to 

 friction; since the material of which it is 

 composed is always a good conductor of 

 heat; since the cost of production of power 

 and that of purchase of machinery is a 

 controlling factor; these and other, minor, 

 conditions affecting the problem of the 

 real engine compel the engineer to seek a 

 working fluid that shall best combine mini- 

 mum cost of fluid and of mechanism, maxi- 

 mum heat-storing power, maximum ' mean 

 effective pressures ' and minimum liability 

 to heat-wastes other than thermodynamic. 

 He is limited by pressures difficult to con- 

 trol safely and permanently in the steam- 

 engine, by temperatures beyond his range 

 of satisfactory operation in the gas-engine 

 and by costs and risks, often very serious, 

 with other fluids; while he finds it im- 

 practicable to utilize the lower portion of 

 the temperature-range in the steam-engine 

 and, in vastly greater degree, finds a sim- 

 ilar limitation in the standard gas-engine. 



No known fluid, up to this time, at least, 

 has been found to satisfactorily combine 

 the manageable range of temperature with 

 the safe range of pressure in the heat-en- 

 gines so as to serve as the ideal working 

 substance. On the whole, steam has 

 proved thus far best. 



By combining fluids in series, however, 

 it is at least ideally possible to secure this 

 coincidence of available pressure- and tem- 

 perature-ranges. Thus, it has often been 

 proposed to utilize the rejected heat of the 

 gas-engine in an accessory steam-engine, to 

 thermodynamically transform the waste- 



heat of the latter by a secondary apparatus 

 employing one of the more volatile liquids 

 in a cycle of low temperature-range. 

 Wellington attempted to find a series of 

 fluids, to be worked in a number of engines 

 in series in this manner, years ago, and 

 every aspect of the case has been carefully 

 studied, and usually more or less thor- 

 oughly investigated experimentally by en- 

 gineers, physicists and inventors. 



Nothing new in this branch of the sub- 

 ject has been discovered recently. It is, 

 however, usually the fact that the earlier 

 inventors have commonly reported a meas- 

 urable gain by the substitution of the more 

 volatile substance for steam in heat- 

 engines. It has seldom been large ; but has 

 been sufficient to make it evident to engi- 

 neers that the problem involved is not to be 

 absolutely ignored. It seems probable that 

 most of these available fluids possess some 

 advantage in one if not both of two ways 

 — higher mean effective pressures at low 

 ranges of cycle temperatures and less free- 

 dom of heat-exchange with the metal of the 

 cylinder.* As the former permits ex- 

 pansion to a lower limit of temperature, 

 as the gain by extending the range down- 

 ward is comparatively large and as wastes 

 by internal heat-exchanges are usually 

 serious, in the common forms of engine, 

 the engineer is interested in watching de- 

 velopments in the use of other fluids than 

 steam, though not as yet expecting much 

 progress through their use in the simple 

 engine. 



With the ' waste-heat engine ' of the 

 ' binary- vapor system ' the case is different. 

 There exists a very large defect in the 

 standard steam-engine in its inability to 

 utilize the temperature-range of its cycle 

 below about 50° C. (122° F.), while it is 

 usually true that at least one half, and 

 often much more than one half, of the heat- 



* Thurston's 'Manual,' Vol. I., pp. 172-704, 

 911-921. 



