76 THE THERMODYNAMICS OF THE MARINE. OIL ENGINE. 



When the writer wrote to the Department of the Interior suggesting correc- 

 tions in this government publication he was informed that the matter was correct 

 and an authority on thermodynamics (Ennis) was quoted. 



Without troubling to specialize on these misstatements we will attempt to 



show — 



First, that, if possible, the extreme high pressure of compression must be 

 reduced. 



Second, that to give the greatest possible efficiency under all conditions the 

 proportions of air to fuel must be kept constant regardless of load. 



Third, that to get extreme or emergency slow speed the injection of the fuel 

 and the ratio of air to fuel should be varied contrary to the condition for maximum 

 efficiency. 



Fourth, that the percentage of the stroke during which the fuel valve is held 

 open should be under the control of the operator in order that injection air losses 

 may be reduced. 



Fifth, that by reducing the pressure of compression and at the same time has- 

 tening the fuel injection the advantage of the high compression is not materially 

 impaired. 



Sixth, that by reducing the pressure of compression it is possible to obtain 

 substantially the same power with the same theoretical efficiency but with an in- 

 creased mechanical efficiency. 



Seventh, that by increasing the temperature of the injection air a saving of 

 practically lo per cent of the fuel now used may be effected without danger to the 

 plant; and in conclusion we shall ask for a brief consideration of a new type of 

 plant combining fundamentals both new and old. 



Let us, then, first consider doing. away with the high pressure of compression. 



In order best to understand the eft'ect of the high pressure of compression 

 and its relation to the cycle of the engine it is necessary to consider temperature 

 volume diagrams of the oil engine and of other types. 



On Plate 21 are shown two typical Diesel cards copied from Diesel catalogues 

 of 1898 and 1913. The only peculiar point in plotting these cards is that the end 

 of the expansion stroke corresponds with 100 per cent volume and the beginning 

 corresponds with the clearance of the engine. Plotted in this way it is possible to 

 read pressure and also percentage volume. Assuming that we begin the compres- 

 sion with air at 14.7 pounds pressure and 60° F., the specific volume will be 

 13.09, and the specific volume at any other point will be 13.09 times the percent- 

 age volume. Thus, from the formula PV = RT, the temperature of the charge 

 can be figured with a great degree of accuracy. In this formula — 



P is the specific pressure or pounds per square foot. 



V is the specific volume or the cubic feet occupied by one pound of gas. 



R is a constant (53.22 in English units for air). 



T is the absolute temperature or glass reading plus 460° F. 



By plotting the two cards shown and figuring a certain number of points the 



