the crank case of aluminium,
and ball-bearings were fitted to the crankshaft, crank pins, and the
rotary blower pump. Ignition was by means of a high-tension magneto of
the two-spark pattern, and with a total weight of 300 lbs. the maximum
output was 102 brake horse-power, giving a weight of just under 3 lbs.
per horse-power.

One of the most successful of the two-stroke cycle engines was that
designed by Mr G. F. Mort and constructed by the New Engine Company.
With four cylinders of 3.69 inches bore by 4.5 inches stroke, and
running at 1,250 revolutions per minute, this engine developed 50 brake
horse-power; the total weight of the engine was 155 lbs., thus giving a
weight of 3.1 lbs. per horse-power. A scavenging pump of the rotary type
was employed, driven by means of gearing from the engine crankshaft, and
in order to reduce weight to a minimum the vanes were of aluminium. This
engine was tried on a biplane, and gave very satisfactory results.

American design yields two apparently successful two-stroke cycle aero
engines. A rotary called the Fredericson engine was said to give an
output of 70 brake horse-power with five cylinders 4.5 inches diameter
by 4.75 inches stroke, running at 1,000 revolutions per minute. Another,
the Roberts two-stroke cycle engine, yielded 100 brake horse-power
from six cylinders of the stepped piston design; two carburettors, each
supplying three cylinders, were fitted to this engine. Ignition was
by means of the usual high-tension magneto, gear-driven from the
crankshaft, and the engine, which was water-cooled, was of compact
design.

It may thus be seen that the two-stroke cycle type got as far as actual
experiment in air work, and that with considerable success. So far,
however, the greater reliability of the four-stroke cycle has rendered
it practically the only aircraft engine, and the two-stroke has yet some
way to travel before it becomes a formidable competitor, in spite of its
admitted theoretical and questioned practical advantages.

VII. ENGINES OF THE WAR PERIOD

The principal engines of British, French, and American design used in
the war period and since are briefly described under the four distinct
types of aero engine; such notable examples as the Rolls-Royce,
Sunbeam, and Napier engines have been given special mention, as they
embodied--and still embody--all that is best in aero engine practice. So
far, however, little has been said about the development of Ge