AS APPLIED TO MARINE PROPULSION. 7 



tor and compressor in tandem, the latter being uncoupled when in port. This was soon 

 replaced by three, and now, as in some of the larger vessels, by four, small sets having 

 generators only. The compressors on the main engines at the same time were clianged from 

 single high-stage compressors, taking their air from the maneuvering air system to inde- 

 pendent three stage compressors. For the two low-pressure air compressors, one of which 

 was always operated at sea, was substituted one motor-driven compressor for maneuvering 

 purposes only. This latter arrangement is better in that the high and low-pressure air sys- 

 tems are not interdependent and the auxiliary power plant is more flexible. From an opera- 

 ting standpoint it is also better, as, by having more than two sets and being small in size, they 

 can be overhauled in turn at sea, relieving the work of the engineers' personnel in port, 

 whose time should then be given to more important port duties. 



There has been much discussion as to whether the two or four-cycle engine is better. 

 Judged in the light of the number of motor ships in successful operation, at present the four 

 cycle has the decided advantage. 



The special claims made for the two-cycle engine is that more power can be obtained 

 per cubic space of cylinder, from having double the power strokes of a four cycle, and hence 

 less weight and space occupied ; also that the two cycle, having no exhaust valves to cause 

 trouble, can burn the lower grade oils of high sulphur content. 



The four-cycle advocates dispute these claims by saying that the two-cycle engine cannot 

 run with as high mean pressures in the cylinders as the four cycle if internal heat troubles 

 are to be avoided, and that the combustion is not apt to be as complete as in the four cycle. 

 They also point out the lower mechanical efficiency, resulting from negative work required 

 for scavenging, and hence higher fuel consumption. With the rings passing across the open 

 ports wearing conditions are not so good as with the unbroken liner surface of the four cycle, 

 and the cylinder lubricating oil consumption is much higher than the four cycle due princi- 

 pally to the oil being scraped into these ports and blown out through the exhaust. It is also 

 stated that with the long pistons required, scoring is apt to result from misalignment caused 

 from wear at crossheads and guides. The two cycle originally dominated the field of the light 

 high-speed submarine engine, but after some ten years have been spent in its development it has 

 been superseded in this field to a great extent by the four cycle. This would not have been 

 possible if there is any inherent advantage in the two cycle as to weight for a given output. 

 Moreover, in the long trade routes, which is the merchant motor ship's chosen field, it must 

 be obvious that the conceded superior economy of the four cycle will give an advantage in 

 deadweight cargo capacity more than sufficient to offset the advantage in weight of machin- 

 ery claimed, but not yet proved, by the two-cycle advocates. 



Some makers, who originally built four-cycle engines and changed to two cycle, have in 

 recent years changed back to four cycle, and others, who have always built the four cycle, are 

 known to be experimenting with the two cycle. All experienced builders are fairly well 

 agreed, however, that the Diesel engine works best at low speeds due to the nature of the 

 injection and burning of the oil, and that the large cylinder engines better bum the lower- 

 grade oils. Oil of higher sulphur content, though, as commonly used under boilers, is not 

 to be recommended, as the sulphuric acid formed in the burning not only affects adversely the 

 exhaust valves of the four-cycle engine but attacks impartially the exhaust pipes and other 

 parts of either the four or two cycle. 



Some makers have advocated the solid injection or injection of the fuel by pressure 

 alone, with which Vickers has been most successful. The chief difficrdties encountered with 



