e engine, the
transfer ports being formed in the main casting. The openings of these
ports were controlled at both ends by the pistons, and the location of
the ports appears to have made it necessary to take the exhaust from the
bottom of one cylinder and from the top of the other. The carburetted
mixture was drawn into the scavenging cylinders, and the usual
deflectors were cast on the piston heads to assist in the scavenging and
to prevent the fresh gas from passing out of the exhaust ports.

VI. THE TWO-STROKE CYCLE ENGINE

Although it has been little used for aircraft propulsion, the
possibilities of the two-stroke cycle engine render some study of
it desirable in this brief review of the various types of internal
combustion engine applicable both to aeroplanes and airships.
Theoretically the two-stroke cycle engine--or as it is more commonly
termed, the 'two-stroke,' is the ideal power producer; the doubling of
impulses per revolution of the crankshaft should render it of very much
more even torque than the four-stroke cycle types, while, theoretically,
there should be a considerable saving of fuel, owing to the doubling of
the number of power strokes per total of piston strokes. In practice,
however, the inefficient scavenging of virtually every two-stroke cycle
engine produced nullifies or more than nullifies its advantages over the
four-stroke cycle engine; in many types, too, there is a waste of fuel
gases through the exhaust ports, and much has yet to be done in the way
of experiment and resulting design before the two-stroke cycle engine
can be regarded as equally reliable, economical, and powerful with its
elder brother.

The first commercially successful engine operating on the two-stroke
cycle was invented by Mr Dugald Clerk, who in 1881 proved the design
feasible. As is more or less generally understood, the exhaust gases of
this engine are discharged from the cylinder during the time that
the piston is passing the inner dead centre, and the compression,
combustion, and expansion of the charge take place in similar manner
to that of the four-stroke cycle engine. The exhaust period is usually
controlled by the piston overrunning ports in the cylinder at the end
of its working stroke, these ports communicating direct with the outer
air--the complication of an exhaust valve is thus obviated; immediately
after the escape of the exhaust gases, charging of the cylinder occurs,
and the fresh gas may be