Figure 36.--Advertisement emphasizing the advantages of fire-safe fuel.
(Smithsonian photo A48848.)]
Due to the greater simplicity, it was more practical to build a large
diesel than a large gasoline engine. Large airplanes would therefore
need fewer engines if diesel powered. Smaller fuel tanks could be used
because of the greater fuel economy of the diesel, and also because of
the high specific gravity of fuel oil as compared to gasoline.
Furthermore, these smaller tanks could be placed in more convenient
locations. Not having a carburetor the engine could not backfire,
further reducing the fire hazard. The exhaust note was lower because of
the diesel's higher expansion ratio. The absence of an ignition system
permitted the diesel to operate in the heaviest types of precipitation.
Such conditions might cause the ignition system of a gasoline engine to
malfunction. The Packard diesel was flown at times without exhaust
stacks or manifolds; this was practical from a safety standpoint because
of the diesel's lower exhaust temperature due to its higher expansion
ratio. Elimination of these parts reduced the weight and cost of the
engine installation. Finally, the engine was ideal for aerobatics, since
the injectors, unlike carburetors, would work equally well whether right
side up or upside down.
An advantage peculiar to the Packard among aeronautical diesels was its
light weight. The English Beardmore "Tornado III" weighed 6.9 lb/hp, and
the German Junkers SL-1 (FO-4) weighed 3.1 lb/hp, while the Packard
weighed but 2.3 lb/hp. In fairness to the Beardmore, it was the only one
of the three engines designed for airship use, and part of its heaviness
was due to the special requirements of lighter-than-air craft. A
contemporary and comparable American gasoline engine, the Lycoming
R-680, weighed 2.2 lb/hp. To have designed a diesel aircraft engine as
light as a gasoline one was a remarkable achievement.