cylinders on the first engine.
Figure 19 shows a later model without any bolts holding down the
cylinders.

Pistons: The pistons used in the 1929 engine had one compression ring
and one oil scraper ring above the piston pin, and one oil scraper ring
below it. There were three grooves, two above the piston pin, and one
below it.[17] Pistons used in 1930 had two compression rings, one oil
scraper ring above the piston pin, and one oil scraper ring below it.
There were four grooves, three above the piston pin, and one below
it.[18] The 1931 pistons had one compression ring above the piston pin,
and one compression ring and four oil scraper rings below it. There were
four grooves, one above the piston pin, and three below it.[19]

[Illustration: Figure 34.--Modified pistons after endurance run. U.S.
Navy test, 1931. (Smithsonian photo A48325D.)]

Combustion Chamber: In 1931 the contour of the cylinder head was changed
slightly. This improved the combustion efficiency to the extent that the
stroke of the fuel pumps could be decreased about 15 percent. The
specific fuel consumption then decreased about 10 percent. In addition
the compression ratio was reduced from 16:1 to 14:1.[20]

These changes were designed to eliminate smoke from the exhaust at
cruising speed, and to reduce it at wide-open throttle.

Valves: A two-valve-per-cylinder model was built, but not put into
production. It featured more horsepower (300), a higher rate of
revolutions per minute (2000), and a better specific fuel consumption
(about .35 lb/hp/hr).[21]

Capt. Woolson designed the production model with a single large valve
for each cylinder. This was done in order to shorten the development
period, for it is easier to design a single valve which serves both the
intake and exhaust functions than one valve for each function. Not only
are there fewer parts, but more important, there are no heat-dissipating
problems. Although the single valve is heated when it releases the
exhaust gases, it is immediately cooled by the incoming air of the next
cycle. This cooling advantage is not shared by a valve which only passes
exhaust gases.[22]

Cylinder Head: Ribs were added to increase its rigidity (compare fig. 32
with fig. 33).

Engine Size: A 400-hp model was developed in 1930. It was not put into
production.[23]

Comments

Comments of Aeronautical Engineers: These comments appeared in
_Aviation_ for February 15, 1930, just a month be