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Ii6 THE SUBMARINE TORPEDO BOAT
between them being that in the two stroke cycle engine
the exhaust of the burnt gases and the intake of the charge
of fresh air take place at practically the same time when the
piston is near the bottom of the stroke. In some designs
the scavenging air is admitted at the top of the cylinder
through valves in the cylinder head and so blows the burnt
gas down and out the exhaust ports at the bottom of the
cylinders. Those parts are uncovered by the piston as it
nears the bottom dead center. Other designs do away
with all valves but the air starting and fuel injection valves
in the cylinder heads. In this case the scavenging air
enters through ports in the side of the cylinder, which are
uncovered by the piston shortly after the exhaust ports
situated on the opposite side of the cylinder. With the
two stroke cycle, then, the valve gear is much simplified
and a great deal of very exasperating valve trouble is
done away with, but on the other hand the scavenging
air for this type must be injected under pressure, usually
about nine pounds, which necessitates the addition of a
low pressure air compressor and greatly complicates the
mechanism of the machine. The economy of the two
stroke cycle is also much lower than in the four stroke
cycle.
The primary cause of the serious difficulties which are
to be met with in the Diesel engine is the excessive tem-
peratures which are generated in its cylinders, — the
maximum temperature reached being about 3,000° F.
'This high temperature together with the high pres-
sure in the cylinder imposes two distinct conditions
which must be met by the designer in calculating the
stress upon the walls. These conditions apply also to the
cylinder heads and pistons. It is quite conceivable that