512 Mr. Dugald Ghrh [Feb. 22, 



volume engine shown on the other diagram with the same maximum 

 pressure the efficiency is less, being 0*48 ; but the mean pressure is 

 105 lb. per square inch, with 500 maximum. The constant pressure 

 engine shown in another figure shows still better results. Here the 

 efficiency is 0*56, and the mean pressure is 117 lb. The Carnot 

 cycle is obviously impossible from a practical point of view. It is 

 interesting to note, however, that for equal compressions, it does not 

 matter whether the Carnot cycle, constant volume, or constant pres- 

 sure engines be used ; the theoretical efficiency is the same. In the 

 constant volume engine, which you will recognise as an engine 

 operating on the modern compression system, high theoretical effi- 

 ciencies are possible, and it is interesting to note tliat for any given 

 compression ratio, tlie efficiency is the same, whatever be the maxi- 

 mum temperature above the temperature of compression — that is, 

 the cycle is a cycle of constant efficiency, given certain conditions. 

 It has been found in practice that a first-class modern engine will 

 give in indicated power 0'7 of the heat which a perfect air engine 

 AYOuld give under the same conditions of compression, proportions, 

 etc. Thus, an engine having an air engine efficiency of * 5, will give 

 0*5 X 0*7 =^ o5 per cent, of all the heat given to it in the form of 

 indicated Avork. 



The air standard has proved its utility as a guide to the engineer 

 for twenty-five years now, and in a recent report of a committee 

 appointed by the Institution of Civil Engineers on " The Standards 

 of Efficiency in Internal Comlmstion Engines," it has been definitely 

 adopted as tb.e official standard, after exhaustive tests of engines of 

 different sizes. This simplified theory of the internal combustion 

 motor has been most useful in pointing out the way to better effi- 

 ciencies, and, with its use, internal combustion motor efficiencies have 

 risen from 16 per cent, in 18.S2 to a maximum of ?>1 per cent, in 

 1906. To enable further progress to be made, however, it is now 

 necessary to know more of the actual properties of the working fluid. 

 Unfortunately no method of investigation so far applied was able to 

 determine those properties. During the past two years, however, 

 considerable progress has been made in the invention and develop- 

 ment of new means of studying the actual working fluid within the 

 gas engine cylinder, and tlie gases composing it outside the cylinder 

 in separate vessels. In the early experiments made by me, showing 

 the rising and falling curves for gaseous mixtures, and in subse(iuent 

 experiments made by Oliver in America, and by Messrs. Bairstow 

 and Alexander in this country, the knowledge acquired of the rising 

 and falling curves was only in strictness applicable to the behaviour 

 of highly heated gases in a closed vessel. No means of obtaining a 

 cooling curve in an engine cylinder had been proposed. 



At the beginning of l'.)05 I proposed a new method, and made a 

 considerable number of experiments on a 50 horse-power gas engine. 

 A section of this en^^ine is shown on the screen. Such engines give 



