THE THERMODYNAMICS OF THE MARINE OIL ENGINE. 81 



a compression pressure of 275 pounds, then the number of expansions will be 8, or 

 the same as the average of this Diesel card. 



We may now call attention to some of the benefits to be derived from reduced 

 compression pressure. 



In the engineering press the idea of reducing the compression pressure and at 

 the same time of making up for this lower compression temperature by means of a 

 higher temperature of the incoming air has been opposed, the great objection to this 

 innovation being that the volumetric efficiency would be decreased ; in other words, 

 that the power of the cylinder would be reduced, making the engine more bulky. 

 There are two different methods for using the reduction in compression. The 

 original idea was to save fuel by a process similar to compounding. In other words, 

 when only a partial load was called for, a partial cylinder of air would be com- 

 pressed. If this were done by closing the admission valve at the middle of the suc- 

 tion stroke there would be formed a vacuum in the cylinder during the rest of the 

 stroke and maintained during the first half of the compression stroke. A charge 

 which was compressed in 4 inches of the stroke would be expanded for 8 inches. In 

 this case volumetric efficiency will not suffer, whereas, in the present method of 

 governing the engine at partial loads, the efficiency is handicapped by the presence 

 of an unnecessary excess of air. 



In the other case, where it is desirable to have a large plant installed with cyl- 

 inders say 60 to 72 inches in diameter, it is practically necessary to reduce the 

 compression pressure for reasons of strength. Without going into the practical im- 

 possibility of a large, high-pressure type of engine, let us assume that it is possible. 

 Also let us assume a duplicate of this engine in everything except that the clearance 

 is made larger ; this will reduce the compression pressure. In the first case the cyl- 

 inder contents will be equal to the piston displacement plus about 7 per cent, and in 

 the other case it will equal the same piston displacement plus about 15 per cent. 

 We have already seen that, after the Diesel has run for awhile and warmed up, the 

 temperature of the charge compressed is much above normal; and from PV = RT 

 it is certain that as the temperature rises the specific volume will increase and the 

 weight of air in a certain space will decrease. If, on the other hand, some scaven- 

 ging arrangement were to be attached to each engine, more weight of air could be 

 used in the low-pressure engine; consequently more fuel could be used and more 

 power could be developed. Thus this main objection to the low-pressure type falls 

 to the ground. 



When the writer began this work he was not aware that the Diesel engine 

 after a run could operate on a reduced compression pressure. He supposed that 

 heat would have to be added to the incoming air in order to enable an ignition 

 temperature to be reached. The theory that no preheat is needed to enable the fuel 

 to be ignited at lower pressures, other than that preheat which is absolutely unavoid- 

 able in a normal engine, is sustained by actual experiments. 



The economies possible in the injection air is the last point for discussion pre- 

 taining to the present type of engine. 



