THE SUBMARINE OF TO-DAY AND TO-MORROW. 219 



The material efficiency of the Diesel engine is extremely low. A great quantity of metal 

 has to be put in the engine to take the pressures that come up instantly, and come down 

 almost on the same curve— a very small percentage of the total time, less than 7 per cent in 

 the two-cycle and 3 per cent in the four-cycle engine, and all the metal there for that brief 

 period of time. 



If a much larger mean effective pressure could be obtained so as to cut down the material 

 incidental to a given motive power, a great advance could be made. Let us hope that some- 

 one will be able to do this, or someone will make a suggestion by means of which it can be 

 done. 



Another great trouble is the storage battery. Whether an Edison battery or a lead bat- 

 tery, it is found to be very heavy as compared with the horse-power it is capable of giv- 

 ing. The lead battery has shown superior qualities in the even discharge line as compared 

 with the Edison. I believe that gives quite a different line. Of course, the trouble with the 

 Edison cells, again, is that you must have almost twice as many of them, and very much 

 less power for a given cost, so that as regards the motive power we are up against a num- 

 ber of problems. We need suggestions of improvement in this respect, and it is just such 

 discussions as this to-day that is necessary. No matter whether a man has a good idea or a 

 poor one, it is wise to get up and air it. It may lead to something better and he will cer- 

 tainly be appreciated, I am sure. 



Mr. Wm. T. Donnelly, Member: — I do not know that I can add anything to the paper as 

 it is very comprehensive, but I should like to say something relative to the fundamental prin- 

 ciples involved from a combined mechanical and dynamic engineering point of view. 



The most discouraging thing about the Diesel engine to mechanical engineers has been its 

 excessive weight per unit power delivered, and this has been brought out very clearly by Mr. 

 Sperry's statements. 



Every Diesel, and in fact every internal-combustion engine, depends upon itself for the 

 compression of air used as a source of oxygen to consume the fuel supplied by the oil. All of 

 us who are familiar with the card of a Diesel engine know how much of the area is taken up by 

 the work represented in compressing the air, and the fact needs no emphasizing that, if this 

 area could be represented by useful work, the output of the engine per unit of weight will be 

 very much greater. The question from a mechanical point of view is : It is possible to pro- 

 vide the compression of the air by an exterior device of very much less weight, so that, 

 having the air compressed, the output of the Diesel engine cylinder would be the complete area 

 of the card below the expansion line ? 



Considered from a thermo-dynamic point of view, the cylinder of a Diesel engine, heated 

 from previous explosions, compressing the air in a single stage without cooling, is a very 

 wasteful form of air compression and one that could not be tolerated in any industry demand- 

 ing a large amount of compressed air. 



As a matter of fact, submarines as now constructed have entirely independent air com- 

 pressors, capable of delivering air at a sufficiently high pressure for the Diesel engine, and 

 all submarines have, or can be provided with, tanks of large capacity to hold this air under com- 

 pression. 



Even if this method of operating a Diesel engine was used only for emergency runs — 

 that is, when the very highest speed on the surface was necessary — it would be of very great 

 importance, and, if air previously stored up could be used for this purpose, it would enable 



