io Walker, The Polytropic Curve 



tical of any advantage to be derived from any attempt to 

 operate an engine on the maximum efficiency cycle as obtained 

 from formula (6). The writer, however, would quote the 

 following- paragraph from Sir Dugald Clerk's book, " The 

 Gas, Petrol and Oil Engine," Vol. i, in which he says (p. 67) 

 that " if " a cycle " gives a higher efficiency in theory it will 

 do so in practice provided the practical losses do not increase 

 with improved theory." Engineers may judge whether an 

 engine with a given compression ratio will have more or 

 feAver practical losses than another of higher compression 

 ratio. Experience, at least, in this respect, is all in favour of 

 the engine with the lower compression ratio. 



It will be observed that in the cycle of Fig. 2 the expansion 

 and compression lines tend to coincide by parallel approach as 

 r tends to the value 14, so that whether p is equal to or greater 

 than unity, the limiting efficiency will always be given by (7). 

 This is clearly borne out by the full lines, as plotted. 



Attention may be drawn to the fact that these full line 

 curves indicate the path along which improvement in present- 

 day internal combustion engine efficiencies may be obtained. 

 Reduction of the high compression ratio of the Diesel engine, 

 in the manner indicated in Fig. 2, with combustion of fuel at 

 constant volume as well as constant pressure, are the lines 

 along which it appears experiment should be directed in order 

 to determine whether, to the foregoing results theoretically 

 deduced, corresponding practical results can be obtained. 



Nothing further remains to be said about these full line 

 graphs except to refer to the chain line which has been drawn 

 through the maximum efficiency point of each of these curves. 

 This has been done for the sake of comparison with some 

 curves drawn for the cycle followed in the well-known Uniflow 

 type of steam engine. The cycle of this engine corresponds 

 closely to that of Fig. 2. The writer was only made aware of 

 the existence of these results some considerable time 

 after the deduction of the formulae leading to the curves 

 of Fig. 3, and the surprisingly good agreement he found 

 between his own and the uniflow steam engine curves was 

 naturally gratifying, arguing, as it did, for the practical value 

 of formula (6). The uniflow steam engine curves referred to 

 are given in Fig. 7. These are taken from Prof. J. Stumpf's 

 book on ''The Uniflow Steam Engine." 4 



Now, the efficiency of the cycle of Fig. 2 has been deduced 

 on the assumption that the working fluid is a perfect gas. In 



4. Constable and Co., 1912. 



