Mercedes-Daimler engine of 85 horse-power, with cylinders
of 5.5 diameter with 5.9 inch stroke, the cylinders being cast in two
pairs. The overhead arrangement of valves was adopted, and in later
designs push-rods were eliminated, the overhead cam-shaft being adopted
in their place. By 1914 the four-cylinder Mercedes-Daimler had been
partially displaced from favour by a six-cylindered model, made in two
sizes; the first of these gave a nominal brake horse-power of 80, having
cylinders of 4.1 inches diameter by 5.5 inches stroke; the second type
developed 100 horse-power with cylinders 4.7 inches in diameter and 5.5
inches stroke, both types being run at 1,200 revolutions per minute. The
cylinders of both these types were cast in pairs, and, instead of the
water jackets forming part of the casting, as in the design of the
original four-cylinder Mercedes-Daimler engine, they were made of steel
welded to flanges on the cylinders. Steel pistons, fitted with cast-iron
rings, were used, and the overhead arrangement of valves and cam-shaft
was adopted. About 0.55 pint per brake horse-power per hour was the
usual fuel consumption necessary to full load running, and the engine
was also economical as regards the consumption of lubricating oil,
the lubricating system being 'forced' for all parts, including the
cam-shaft. The shape of these engines was very well suited for work
with aircraft, being narrow enough to admit of a streamline form being
obtained, while all the accessories could be so mounted as to produce
little or no wind resistance, and very little obstruction to the pilot's
view.

The eight-cylinder Mercedes-Daimler engine, used for airship propulsion
during the War, developed 240 brake horse-power at 1,100 revolutions per
minute; the cylinder dimensions were 6.88 diameter by 6.5 stroke--one
of the instances in which the short stroke in relation to bore was very
noticeable.

Other instances of successful vertical design-the types already detailed
are fully sufficient to give particulars of the type generally--are
the Panhard, Chenu, Maybach, N.A.G., Argus, Mulag, and the well-known
Austro-Daimler, which by 1917 was being copied in every combatant
country. There are also the later Wright engines, and in America
the Wisconsin six-cylinder vertical, weighing well under 4 lbs. per
horse-power, is evidence of the progress made with this first type of
aero engine to develop.

II. THE VEE TYPE

An offshoot from t