On the Gas-Engine Indicator- Diagram. 59 



are variable, if one drives the cone and the other the screw, 



for — : f = — , in which r is constant. 

 to' r 



We desire in conclusion to express the obligation we are 

 under to Mr. Hilger for the admirable skill and patient 

 intelligence with which he has successfully carried out our 

 idea. 



"VI. The Gas-Engine Indicator-Diagram. By Professors 

 W. E. Ayrton, F.E.S., and John Perry, M.E.* 



[Plate III.] 



THE members of this Society are probably aware that gas- 

 engines are now largely in use, and that their use is still 

 extending rapidly. There can be no doubt that gas-engines 

 would be largely used, even if they were wasteful of fuel, on 

 account of their being always ready to start or stop, and their 

 requiring so little attention ; but it is gradually becoming 

 clearer that even small specimens of this kind of engine, whose 

 history is merely beginning, are in actual fact less wasteful of 

 fuel than the largest and most carefully constructed steam- 

 engines. It is, for example, a demonstrated fact that, with a 

 Dowson's generator not larger than the boiler used in the 

 corresponding steam-engine, an Otto engine uses only l'l lb. 

 of coal per indicated horse-power per hour. It is therefore 

 not unreasonable to suppose that gas-engines will soon be 

 employed even in the propulsion of ships. 



The rapid growth of this new application of science renders 

 it necessary that help should be given to practical men to 

 enable them to use such observations as they are constantly 

 making. This paper is intended to teach such men a method 

 of obtaining information from the indicator-diagram of a gas- 

 engine. 



2. The Action in the Otto Gas-Engine. — This large model 

 which we exhibit has been constructed in the workshop of the 

 Finsbury College, to enable students to follow the motions of 

 different parts of the gas-engine. It will be seen that when 

 the piston is at the end of its stroke, only what we call the 

 clearance-space behind it is filled with fluid. This fluid is a 

 mixture of carbonic acid, water-vapour, and nitrogen, whose 

 temperature is about 410° C.f, if there was an immediately 



* Communicated "by the Physical Society. Read April 26, 1884. 

 t In our own observations at Finsbury we have not used any specially 

 contrived apparatus, as our investigations were really for the purpose of 



