ce use.... Flight tests, until
the durability of the engine is improved, be limited to a determination
of the critical engine speeds, and to short hops in seaplanes.... It is
believed that this size engine should be made suitable for service use
before this type in a larger class is attempted." This latter statement
probably refers to the 400-hp model.

A year had passed between the making of engine 100 and 120, yet the
reliability had not improved. Although unreliability was the immediate
cause of failure, there were two design defects which would have doomed
the engine even if it had been reliable. All the Packard diesels were of
the 4-stroke cycle unblown type, yet the most successful airplane
diesels were of the 2-stroke cycle blown type.[30] The advantages of the
latter type for aeronautical use are that it is of a more compact
engine, of lower weight and greater efficiency.[31] The engine was
therefore built around the wrong cycle.

The Packard diesel of 1928 was designed to compete with the Wright J-5
"Whirlwind" which powered Lindbergh's "Spirit of St. Louis" in 1927.[32]
The specifications were within two percent of each other. The diesel
engine's fuel consumption was far less although its price was
considerably higher.

_Packard Diesel_ _Wright J-5_
_DR-980_ _"Whirlwind"_

Diameter (in.) 45-11/16 45
Horsepower 225 225
Weight (lb) 510 510
Weight-horsepower ratio 2.26 2.26
Fuel consumption (lb per hp/hr at 0.40 0.60
cruising).
Cost $4025 $3000

The advantages of lower fuel cost and greater cruising range offered by
the diesel engine would be relatively unimportant to a private pilot
flying for pleasure, but would be vital to the commercial operator using
airplanes powered by engines having several times the horsepower of the
Packard diesel. Its size, moreover, was too small for the technology of
fuel injectors.[33] The Packard Company realized that the production
engine was too small.[34] In 1930 a 400-hp version was built but was not
put into production, probably because of the unreliability of the 225-hp
model.

The fourth principal reason why the engine failed is explain