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Unformatted text preview: t o current minimum values of less than 0.40. This improvement has been
a chieved partly through improved design and partly because improved
fuels have allowed higher compression ratios (from about 4.0 in 1903 to
p resent values, up to 8.0.
Best weight per horsepower in 1903 was that of the Manly engine,
a t 2.6 lb/hp. This figure was reduced to 1 in 1935 and has gone slightly
b elow that since (see table 1).
O ne of the most remarkable improvements has been in reliability and
r educed maintenance. The very early aircraft engines were overhauled
after every flight. The approved overhaul period for the best modern
t ransport piston engines is now as high as 2,600 hours.
F urther improvements in piston engines would have been made had it
n ot been for the introduction of turboprop and turbojet engines, which
v irtually put an end to intensive development of the large piston engine.
J et engines are also used for certain categories of small aircraft.
F igure 72 shows the piston-engine family as it has developed, finally
c ulminating in the V-12 liquid-cooled engines as represented by the
R olls-Royce and Packard Merlin, and the 18-cylinder air-cooled radials,
b y the Pratt & Whitney R-2800 and the Wright 3350, of which the
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- Winter '14
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