uring each cycle; these engines are
supported on bearings on both sides of the engine, the air-screw being
placed outside the front support. In the eighteen-cylinder model the
impulses occur at each 40 degrees of angular rotation of the cylinders,
securing an extremely even rotation of the air-screw.

In 1913 the Gnome Monosoupape engine was introduced, a model in which
the inlet valve to the cylinder was omitted, while the piston was of the
ordinary cast-iron type. A single exhaust valve in the cylinder head was
operated in a manner similar to that on the previous Gnome engines, and
the fact of this being the only valve on the cylinder gave the engine
its name. Each cylinder contained ports at the bottom which communicated
with the crank chamber, and were overrun by the piston when this was
approaching the bottom end of its stroke. During the working cycle of
the engine the exhaust valve was opened early to allow the exhaust gases
to escape from the cylinder, so that by the time the piston overran the
ports at the bottom the pressure within the cylinder was approximately
equal to that in the crank case, and practically no flow of gas took
place in either direction through the ports. The exhaust valve remained
open as usual during the succeeding up-stroke of the piston, and
the valve was held open until the piston had returned through about
one-third of its downward stroke, thus permitting fresh air to enter the
cylinder. The exhaust valve then closed, and the downward motion of the
piston, continuing, caused a partial vacuum inside the cylinder; when
the piston overran the ports, the rich mixture from the crank case
immediately entered. The cylinder was then full of the mixture, and the
next upward stroke of the piston compressed the charge; upon ignition
the working cycle was repeated. The speed variation of this engine
was obtained by varying the extent and duration of the opening of the
exhaust valves, and was controlled by the pilot by hand-operated levers
acting on the valve tappet rollers. The weight per horsepower of these
engines was slightly less than that of the two-valve type, while the
lubrication of the gudgeon pin and piston showed an improvement, so that
a lower lubricating oil consumption was obtained. The 100 horse-power
Gnome Monosoupape was built with nine cylinders, each 4.33 inches
bore by 5.9 inches stroke, and it developed its rated power at 1,200
revolutions per minute.

An engine of the rotary