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HEAVY-OIL ENGINES FOR MARINE PROPULSION. 233 
Mechanically there are other advantages peculiar to the Diesel type, 
such for example as absence of an igniting device—absence of a carburetor, 
or vaporizing device, all of which are delicate affairs and give the greater 
part of the trouble experienced with explosive engines. 
A complete Diesel engine requires certain auxiliaries. These are the 
air compressor, the fuel pumps, a lubricating pump, and a water circulating 
pump. ‘The last two mentioned auxiliaries are also found on all engines of 
the Otto cycle. 
The air compressor is usually built in two stages, though some designers 
prefer three. Two stages seem ample as the maximum air pressure used 
for spraying the oil into the cylinders is seldom in excess of 1,000 pounds 
per square inch. In engines of a few hundred horse-power it is customary 
to use only one compressor, on the same bedplate with the engine, and 
driven from a crank on the main shaft. There are no novel features in 
connection with the design of these compressors. 
The fuel pumps are always attached to, and driven by the main engine. 
It is the usual practice to supply a separate pump for each cylinder, but it 
is possible to operate two or more cylinders from one pump. . 
The governing of the engine is accomplished by controlling the amount 
of fuel delivered by the fuel pumps to the spray valves. In this way the 
fuel is practically measured out for each stroke by the pump, and very 
accurate control is possible. There is no throttle valve employed on these 
engines. The amount of air taken in and compressed on each stroke is the 
same. At half load, for example, only about half the amount of fuel is 
delivered. This fuel burning in an excess of oxygen is completely consumed 
and imparts all its heat to the contents of the cylinder. Hence, the engine 
is efficient at reduced loads, and is as flexible as any type of internal com- 
bustion engine. Governing of these engines is merely a question of fly-wheel 
and number of cylinders. A recent test of a two-cylinder stationary engine, 
having a coefficient of fluctuation of one-ninetieth showed a variation in 
speed of 2 per cent. from full load to no load. 
The relative advantages of four-cycle and two-cycle marine engines may 
be summed up as follows: 
Economy.—The four-cycle has about 8 to 10 per cent. less fuel con- 
sumption than the two-cycle. Taking an average case, a four-cycle engine 
would use only 0.46 pounds per horse-power-hour, while a corresponding size 
two-cycle engine would consume 0.50 pounds of fuel per horse-power-hour. 
This is the principal advantage of the four-cycle over the two-cycle, but 
advocates of the latter type ee this CeeciaeA in fuel consumption will 
soon be reduced. 
