80 THE THERMODYNAMICS OF THE MARINE OIL ENGINE. 



gine a blower giving a pressure of lo pounds be used on the intake duct and the air 

 be heated, the speed can be reduced very materially. 



At this point it may not be out of place to call attention to the fact that the 

 internal-combustion engine is simply a heat engine. Up to the point of encounter- 

 ing lubricating troubles in the engine the cylinders should be kept as hot as possi- 

 ble. This is true whether extreme slow speed or maximum efficiency is required. 



We may now discuss the advantages of fuel valve control as to timing, etc. 



In the present oil engine the duration of the fuel injection does not take into 

 account the amount of fuel to be fed or the speed of the engine. In one particular 

 instance reported in the engineering press the fuel feed continued for 46 degrees of 

 one revolution of the shaft. This engine was two-cycle, and hence the fuel valve 

 was open for practically 12 per cent of the total running time. If an engine be 

 slowed down to half speed, considerably less fuel will be used than when the engine 

 is running at full speed. Notwithstanding this, the fuel valve is open for the same 

 length of time. This necessarily results in a waste of injection air, the thermo- 

 dynamics of which will be taken up later. If the fuel be injected early in the stroke, 

 a rise in pressure can be obtained which will increase the efficiency of the engine. If 

 the fuel be fed into the cylinder a considerable time after the end of the compression 

 stroke, it is possible to reproduce in the oil engine the same result as is obtained in 

 the explosion engine by retarding the spark. Thus it seems to be highly desirable 

 to be able to control at will both the timing of the injection and the duration of the 

 fuel feed. 



The method of compensating the theoretical loss encountered in reducing the 

 pressure of compression next calls for consideration. 



If the compression pressure is reduced, the fuel injection can be hastened and 

 combustion completed earlier in the stroke. If high compression pressure is used, 

 then the injection of the fuel must be slow, as is now done in the Diesel engine in 

 order that extreme pressure may be avoided. If the fuel be injected rapidly and 

 timed for the end of high compression pressure, results are to be expected similar 

 to those shown in Card 13 of Plate 25. At this point it might be interesting to note 

 that our experimental work never resulted in a combustion pressure of over 900 

 pounds per square inch. However, upon investigation it was found that the in- 

 dicator was jammed and that 900 pounds was the limit which the indicator could 

 record with a spring of 400 pounds to the inch. A relief valve was then fitted in 

 order to avoid dangerous pressures. 



A high-compression pressure means a small clearance and a relatively large 

 number of expansions. The large number of expansions is very desirable, but the 

 full effect is necessarily lost through the slow fuel injection. Why this is so is shown 

 on Plate 24. Assume that the card is divided into four similar cards by means of 

 the adiabatic lines as shown. Each card may be considered separately. In the 

 case of card D the average clearance is 16.1 per cent of the whole cylinder con- 

 tents; hence card D has only 6.2 expansions. Card A, on the other hand, has 11.34 

 expansions. If the fuel be injected rapidly into a cylinder whose clearance gives 



