four-stroke cycle engine of equal dimensions. This
result was not achieved, and, with the improvements in construction
brought about by experiment up to 1912, the output was found to be only
about fifty per cent more than that of a four-stroke cycle engine of the
same size, so that, when the charging cylinder is included, this engine
has a greater weight per horse-power, while the lowest rate of fuel
consumption recorded was 0.68 lb. per horse-power per hour.

In 1891 Mr Day invented a two-stroke cycle engine which used the crank
case as a scavenging chamber, and a very large number of these engines
have been built for industrial purposes. The charge of carburetted air
is drawn through a non-return valve into the crank chamber during the
upstroke of the piston, and compressed to about 4 lbs. pressure per
square inch on the down stroke. When the piston approaches the bottom
end of its stroke the upper edge first overruns an exhaust port, and
almost immediately after uncovers an inlet port on the opposite side of
the cylinder and in communication with the crank chamber; the entering
charge, being under pressure, assists in expelling the exhaust gases
from the cylinder. On the next upstroke the charge is compressed into
the combustion space of the cylinder, a further charge simultaneously
entering the crank case to be compressed after the ignition for the
working stroke. To prevent the incoming charge escaping through the
exhaust ports of the cylinder a deflector is formed on the top of the
piston, causing the fresh gas to travel in an upward direction, thus
avoiding as far as possible escape of the mixture to the atmosphere.
From experiments conducted in 1910 by Professor Watson and Mr Fleming
it was found that the proportion of fresh gases which escaped unburnt
through the exhaust ports diminished with increase of speed; at 600
revolutions per minute about 36 per cent of the fresh charge was lost;
at 1,200 revolutions per minute this was reduced to 20 per cent, and at
1,500 revolutions it was still farther reduced to 6 per cent.

So much for the early designs. With regard to engines of this type
specially constructed for use with aircraft, three designs call for
special mention. Messrs A. Gobe and H. Diard, Parisian engineers,
produced an eight-cylindered two-stroke cycle engine of rotary design,
the cylinders being co-axial. Each pair of opposite pistons was secured
together by a rigid connecting rod, connected to a pi