introduced either through a valve in the
cylinder head or through ports situated diametrically opposite to the
exhaust ports. The continuation of the outward stroke of the piston,
after the exhaust ports have been closed, compresses the charge into
the combustion chamber of the cylinder, and the ignition of the mixture
produces a recurrence of the working stroke.

Thus, theoretically, is obtained the maximum of energy with the minimum
of expenditure; in practice, however, the scavenging of the power
cylinder, a matter of great importance in all internal combustion
engines, is often imperfect, owing to the opening of the exhaust ports
being of relatively short duration; clearing the exhaust gases out of
the cylinder is not fully accomplished, and these gases mix with the
fresh charge and detract from its efficiency. Similarly, owing to the
shorter space of time allowed, the charging of the cylinder with the
fresh mixture is not so efficient as in the four-stroke cycle type; the
fresh charge is usually compressed slightly in a separate chamber--crank
case, independent cylinder, or charging pump, and is delivered to
the working cylinder during the beginning of the return stroke of the
piston, while in engines working on the four-stroke cycle principle a
complete stroke is devoted to the expulsion of the waste gases of the
exhaust, and another full stroke to recharging the cylinder with fresh
explosive mixture.

Theoretically the two-stroke and the four-stroke cycle engines possess
exactly the same thermal efficiency, but actually this is modified by a
series of practical conditions which to some extent tend to neutralise
the very strong case in favour of the two-stroke cycle engine. The
specific capacity of the engine operating on the two-stroke principle is
theoretically twice that of one operating on the four-stroke cycle, and
consequently, for equal power, the former should require only about half
the cylinder volume of the latter; and, owing to the greater superficial
area of the smaller cylinder, relatively, the latter should be far more
easily cooled than the larger four-stroke cycle cylinder; thus it should
be possible to get higher compression pressures, which in turn should
result in great economy of working. Also the obtaining of a working
impulse in the cylinder for each revolution of the crankshaft should
give a great advantage in regularity of rotation--which it undoubtedly
does--and the elimination of the op