234 HEAVY-OIL ENGINES FOR MARINE PROPULSION. 
Heat Conditions.—The four-cycle engine has the advantage as regards 
the cooling of pistons and cylinders, but loses some advantage by having 
an exhaust valve in the head exposed to hot exhaust gases. 
Turning Moment.—The two-cycle is much superior, requiring a very 
much smaller fly-wheel. Six-cylinder high-speed engines require no fly-wheel. 
Reversibility —The two-cycle is again superior, first, because the same 
valve gear may be used both in the ahead and astern directions, thereby 
simplifying the construction; second, because for a given engine twice the 
number of cylinders may be brought into action; and third, because of less 
inertia, due to reduction in weight of fly-wheel, the engine will respond 
quicker. 
Weight—The two-cycle naturally has an advantage. 
Space.—Again the two-cycle has the advantage, due to its nature. 
It seems to be the general opinion of authorities on the subject, that 
for marine purposes, the two-cycle is superior to the four-cycle engine. For 
marine purposes, the superior economy of the four-cycle engine has to be 
balanced against the even turning moment, the simplicity, the reversibility, 
the lightness and the compactness of the two-cycle engine. If, as many 
engineers confidently expect, the economy of the two-cycle can be made the 
same as the four-cycle, there will be no room for argument. 
Considering the present development of the two types, the question, 
as applied to stationary engines, assumes an entirely different aspect. Here 
we are not greatly concerned with questions of weight, space and reversi- 
bility. This gives the four-cycle engine, due to its superior efficiency, the 
leading place. Consequently, practically all stationary oil engines at present 
are of the four-cycle type. It remains to be seen whether they will maintain 
this position. If the economy of the two-cycle engine is improved, as many 
expect, the question for land installations will then hinge upon the cost of 
manufacture. When this time arrives it is probable that the two-cycle 
engine will become the favorite for land, as well as sea. 
Going further into the development of oil engines, we come to double- 
acting types. There are now in successful operation, horizontal, four-cycle, 
double-acting, tandem engines developing 500 horse-power per cylinder. 
Their use is at present limited to land installations. 
The vertical, two-cycle, double-acting engine for marine purposes is 
the latest development. Several engines have recently been completed and 
tested. The smallest of these develops 300 horse-power per cylinder, and 
the largest 2,000 horse-power per cylinder. 
A natural advantage of the double-acting engine is that it permits a 
reduction in weight per horse-power. An alternative advantage is that for 
