82 THE THERMODYNAMICS OF THE MARINE OIL ENGINE. 



In the injection air we have an engine cycle within an engine cycle. Each 

 pound of air compressed and used for the injection of the fuel also does work 

 after it has entered the cylinder. This air is unnecessary for combustion in par- 

 tial loads at least. As a result its efficiency can be considered separately as if 

 the air were used in a power plant. What will be proven in regard to injection 

 air has some bearing on the general engine cycle. No good can come directly 

 from compressing air. All benefits from compression must be felt indirectly. In 

 the main engine cycle air is compressed as near as possible without radiation. Such 

 radiation as goes on is accepted, not sought for, in the present Diesel type. It is 

 intended to show in this paper that there is no Diesel cycle as such, but that there 

 is one main principle which governs all internal-combustion engines and that this 

 is the general principle which governs all heat engines. This principle is that the 

 maximum temperature or temperature range governs the efficiency of the engine. 

 This contention will no doubt be universally accepted in time at least. 



From the best information available the injection air compressor must de- 

 velop about 10 per cent of the brake horse-power of the main engine. Of this 

 lo per cent what care has been used in regard to maintaining an economical cycle? 

 The air is compressed in two or more stages to looo pounds pressure and then 

 cooled to the original temperature or lower. In the heat engine the efficiency is 

 based upon the energy of the working fluid plus the heat added and minus the 

 heat rejected, all divided by the total heat at the beginning of the working cycle. 



The total energy in the air after it has been compressed to lOOO pounds pres- 

 sure and cooled to its original temperature is exactly what it was in the beginning. 

 All that the work of compression has done is to make a part of this original 

 energy available. Our natural conception of the energy of compressed air would 

 lead us to the consideration that the energy depended upon the pressure of the air. 

 As a matter of fact the determining factor is not pressure but temperature. There 

 are several simple proofs for this statement. One is: — "Isoenergic lines are lines 

 representing changes during which the intrinsic energy remains constant * * *. 

 It will be seen later that the isoenergic and isothermal lines for a gas are the 

 same" (Peabody). When the air after having been compressed is cooled to the 

 original temperature, it is brought back to the isothermal from which compression 

 started. 



This contention can be proven mathematically. The formula for work of ex- 

 pansion to the absolute zero of temperature and pressure is W = PV -^- {k — i). 

 This formula considers a fixed quantity of air, that is, a certain weight. In the 

 formula PV = RT, considering the same weight of air, P and V will be the same 

 as in the formula for work. By substitution, the formula for work can be re- 

 written W = RT -^ (k — i). R and k are constants, and the only variable in 

 this formula is T, the absolute temperature. This new formula will give the work 

 of expansion for one pound of gas. 



The above academic consideration of the injection air is simply to bring home 

 forcibly the fact that a waste is being made of one-half of the available energy of 



