March 1st, 1887.] 



SCIENTIFIC NEV/S. 



1 1 



is not compressed before being ignited, whereas in the 

 latter it is compressed to about 40 lbs. to the square inch. 

 These engines, in fact, represent two distinct types, known 

 as compression and non-compression engines, and Mr. 

 Cleric, in his work, very fully discusses their relative 

 efficiencies. We may perhaps take exception to his state- 



engine there is no compression, whereas in the Otto there 

 is, and to this fact alone is attributed much of the superiority 

 of the latter. This is doubtless true, but although Mr. 

 Clerk has described the chief advantages of compression he 

 has not stated the whole case ; for instance, he has omitted 

 to take account of the fact, that when the charge is com- 



ment that the working fluid in all gas engines is air, whereas 

 strictly speaking it is a mixture of air and of the products 

 of combustion. An excess of air may be, and usually is, 

 present, and this of course becomes heated in turn, but the 

 high pressures developed are essentially due to the explo- 

 sion or combustion of the gas. In a steam engine the 

 working fluid is steam, produced in a separate apparatus, 

 and in designing an engine the steam engineer has this great 

 advantage, that the expansive fluid with which he has to 

 work needs no mixture with air, or addition of any 

 kind. It obeys certain well-known laws, and all that 

 is necessary is to insure the engine being designed to 

 make the best possible use of the fluid. With a gas engine, 

 whether the working fluid be air, or a mixture of air and pro- 

 ducts of combustion, we have in the first place to insure com- 

 plete combustion of the gas, or there will be waste of fuel, 

 and this has to be done under somewhat difficult conditions. 

 In the next place we have to find the best conditions of 

 pressure and volume to insure the greatest development of 

 power, with the least expenditure of gas. It will thus be 

 seen that the maker of a gas engine has not only to devise 

 the necessary mechanical contrivances, but he has to pro- 

 vide very specially for the treatment of thegas with which the 

 engine is to be worked. The theoretical efficiency of a gas 

 engine, considered as a heat engine, depends on the dif- 

 ference between the initial temperature produced in the 

 cylinder when the charge is ignited, and the final tempera- 

 ture of the products of combustion escaping through the 

 exhaust. The greater the difference between these tem- 

 peratures the greater the efficiency. With this in view 

 Mr. Clerk shows that by compressing the mixture of gas 

 and air before ignition, its temperature is raised by reason 

 of the work done upon it, and consequently there is a 

 greater difference between the initial and final temperatures, 

 and a corresponding increase in efficiency. In the Lenoir 



pressed the molecules of gas are brought closer together, and 

 are then more readily ignited ; also that when so compressed 

 they are exposed to less cooling surface in the cylinder. 

 Both of these are important considerations, and should not 

 be lost sight of. 



] ^^ Exh oust Vali/e 



Fig. 2. 



Mr. Clerk describes other engines, and gives useful details 

 of their mechanical parts, but those we have selected may be 

 taken as typical of the first gas engine used for practical 

 purposes, and of the best modern engine now made. We 

 have already seen that in the Lenoir engine there is no com- 

 pression of the gas and air before ignition. The action 



