m 



.'578 



uiiroiiT — 1881. 



1; 



lit 



inconipk'lc cxpan^idu llio principiil Rourco of loss, as do usimlly otlicr 

 writers on tliurinoilyiiiuiiics. 



TlioiiiiiH Trt'd^^old, writiiij; in lS'27, who, bnt little later tlian C'arnot, 

 puts the limit to eoononiiciil exfHiiision at tho point Bubsequenlly iiidi- 

 cated and more rully dcnionst rated by Do Panilioiii', oxa<;ji;erates tlic 

 losses due to tlie pi'aetieal (conditions, bnt evidently does pereeive tlicii 

 nature and <;'enerul elleet. He also shows that under the eonditions 

 assumed the losses may bo riidueed to a minimum, so I'ar as beinjj; de- 

 pendent upon the form of the cylinder, by making tho stroke twiee tin 

 diameter. 



The limit oi' rlllcieney in heat-en<;ines, as has been seen, is (hermd- 

 dyiuimically determined by the limit of cnmpleto expansion. So well 

 is this understood, and so generally is this assumed to rejirescnt the 

 prnctieal limit, by writers unfamiliar with the operation of the steam- 

 engine, that c\cry treatise on the subjeet is largely devoted to the 

 examination of the amount of the loss due to what is nl\va3'H known as 

 'incomplete expansion ' — expansion tei'minating at a. pressure higher 

 than tho back pressure* in the (Cylinder. The causes of the practieai 

 limitation of the ratio of expansion to a very much lower value than those 

 which maximum edieiency of fluid would seem to demand have not been 

 usually considered either with care or with intelligence by wriiers 

 thorougidy familiar with tlu' dynamical treatment, apart fnma the modify- 

 ing conditions here under consideration. 



Watt, ami pi-oljably his contemporaries und successors, for many year.> 

 supposed that the irregularity of motion duo to the variable pressure 

 occurring with higli expansion was the limiting condition, and does not 

 at first seem to have realised that the cylinder-condensation discovered 

 by him had any economical bearing upon the ratio of expansion at 

 maximum efficiency, it UTidoubtedly is the fact that this irregularity 

 was the lirst limiting condition with the large, cumbrous, long-strokcil, 

 and slow-moving engines of his time. Every accepted authority fi'oai 

 that day to the present has assumed, tacitly, that this method of waste 

 has no influence upon the value of that ratio, if we except one or two 

 writers who were practitioners rather than scientific authorities. 



Mr. D. K. Clark, publishing his ' Railway jMachincr}',' in !><■'>'>. Avas 

 tho first to discuss this subject with knowledge, and with a clear under- 

 standing of tho eil'ects of condensation in the cylinder of the steam-engine 

 upon its maximum efficiency. Cornish engines, from the lieginning. 

 had been restricted in their ratio of expansion to about one-fourth as a 

 maximum. Watt himself adopting a ' cut-off ' at from one-half to two- 

 thirds. Hornblower, with his compound engine competing with tlio 

 single cylinder- engines of Watt, had struck upon this rock, and liad been 

 beaten in economy by tho latter, although using nnich greater i-atios of 

 expansion ; but Clark, a half century and more later, was, nevertheless, 

 the first to perceive precisely where the obstacle lay, and to state 

 explicitly that the fact that increasing expansion leads to increasing 

 losses by cylinder-condensation, the losses increasing in a much higher 

 ratio than the gain, is the practical obstruction in our progress toward 

 greater economy. 



Clark, after a long and arduous scries of trials of locomotive engines 

 and prolonged experiment looking to the measurement of the magnitude 

 of the Avaste produced as above described, concludes : ' The magnitude oi 

 the loss is so great as to defeat all such attempts at (>conomy of fuel and 



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