8 THE INTERNAL-COMBUSTION ENGINE 



such a system are in getting good combustion at all running speeds and loads and the elimina- 

 tion of the shock in the cylinder, which is apt to occur with the sudden rise in pressure 

 when fuel is injected. The advantage claimed is that compressor troubles are entirely elimi- 

 nated with a correspondingly higher mechanical efficiency obtained than with air injection. 

 The fuel consumption, often erratic, however, under the most favorable conditions, is no bet- 

 ter than the air injection full Diesel. It is also questioned if an oil pressure from 2,500 to 

 4,000 pounds per square inch is more to be preferred than an air compressor and its corre- 

 sponding air system having a pressure of 850 to 900 poimds. 



The Diesel electric drive has been suggested by some, using direct current supplied by 

 several high-speed engine sets working in series. The sponsors of this system apparently 

 have either taken their cue from the turbine electric drive or are more familiar with the elec- 

 tric end than the shortcomings of the high-speed Diesel engines. To reduce the weight of 

 the engines and space occupied sufficiently to compensate for the additional electrical equip- 

 ment involved, the engines must necessarily be high speed and of the trunk-piston type. The 

 engines would correspond in design to that halfway between land and submarine practice. 



The disadvantages of such a system compared with the direct drive are as follows : 



1 . Loss in reliability in the prime movers. 



2. More major overhauls, as lifting of cylinder covers and drawing of pistons, due 

 to poorer combustion and passage of lubricating oil from crank case. 



3. About 30 per cent more fuel per knot, 15 per cent chargeable to higher fuel con- 

 sumption of engines and at least 15 per cent electrical losses with the small-size generators 

 used. 



4. Lighter grade and more expensive fuel oil required. 



5. Much higher lubricating oil consumption associated with high speed and trunk pistons. 



6. Possible vibration troubles associated with high revolutions. 



7. Necessity for using objectionably large motors of the commutator type for transmis- 

 sions of power from engines to propellers. 



8. Greater complication of controls and more expert knowledge required of the engi- 

 neer personnel. 



9. Tendency to overwork the personnel with the frequent overhauls required with high- 

 speed engines when operated continuously at full power. 



10. Danger of short circuits. 



11. Short life of high-speed engines compared with slow-speed engines. 



12. Higher first cost and maintenance charges. 



With the higher consumption of fuel and lubricating oil per indicated horse-power 

 combined with the electrical losses involved and the better quality of fuel oil required, the 

 total expenditure for these items will be about 50 per cent more than with the direct-drive 

 system. 



The Diesel electrical system has been specially recommended for converting existing 

 steamers. It is believed by the authors that this can be far better accomplished by using 

 a long stroke engine turning at the low revolutions required. Work has been done along 

 these lines by Doxford and Cammell-Laird, using the doubled opposed piston, and also Bur- 

 meister & Wain have developed a line of long stroke engines, with a stroke bore ratio of 

 two to one, specially adapted to single-screw vessels. These latter engines are to be recom- 

 mended for new vessels, as well, of 5,000 tons deadweight and less, on account of less engine- 

 room personnel required, important in small vessels, and the dispensing with one shaft alley, 

 which is also important in this size ship. 



